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Construction and performance of a large area liquid scintillator cosmic ray anticoincidence detector
152
Nuclear Instruments and Methods in Physics Research A274 (1989) 152-164 North-Holland, Amsterdam
CONSTRUCTION AND PERFORMANCE OF A LARGE AREA LIQUID SCINTILLATOR COSMIC RAY ANTICOINCIDENCE DETECTOR James J. NAPOLITANO, Stuart, J. FREEDMAN, Gerald T. GARVEY §, Michael C. GREEN §§, Kevin T. LESKO +, James E. NELSON and James N. WORTHINGTON ++ Physics Division, Argonne National Laboratory, Argonne, IL 60439-4843, USA
Kenneth P. COOVER t, John W. DAWSON, William HABERICHTER and Emil P. PETEREIT High Energy Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA Received 13 July 1988
We describe details of the construction and performance of a large area cylindrical cosmic ray anticoincidence detector suitable for a medium energy neutrino experiment. The device provides 41r coverage with approximately 400 m2 of liquid scintillator . Cosmic ray muons are rejected on-line with an inefficiency of =10 -° which is improved to a level of = 3 X 10 -b or less with the application of off-line cuts. We also discuss the custom electronics used to operate and read out the device . Results from studies of the attenuation of neutral cosmic ray backgrounds by passive shielding are included .
1. Introduction Neutrino sources which use the beam dumps of medium energy proton accelerators such as the Los Alamos Meson Physics Facility (LAMPF) are especially useful because they produce copious amounts of Pe, essentially no ve , and have exceptionally well known neutrino energy distributions . However, experiments using these sources confront serious cosmic ray backgrounds. Duty factors are large and cross sections are small since neutrinos have low energy coming principally from the decays of thermalized pions and muons.
neutrino detector must be surrounded by a highly efficient, large area cosmic ray anticoincidence detector, or " shield".
LAMPF experiment E645, a collaboration of Argonne National Laboratory, California Institute of Technology, Lawrence Berkeley Laboratory, Los Alamos
National Laboratory, Louisiana State University and Ohio State University, is a study of neutrino oscillations through the appearance of ve from vP (or, in principle, from PP or Pe) [1]. An elevation view is shown in fig. 1.
Neutrinos are produced in the decays of rr' and subse-
Neutrino initiated reactions can be simulated by cosmic ray generated backgrounds. To obtain reasonable rates,
the neutrino detectors must be large. Consequently, the
* This work was supported by the US Department of Energy, Nuclear Physics and High Energy Physics Divisions, under contract number W-31-109-ENG-38. One of us (K .T .L .) acknowledges support during the latter phases of this project from Lawrence Berkeley Laboratory by the U.S . Department of Energy under contract number DEAC03-76SF00098 . Present Address: Los Alamos Meson Physics Facility, Los Alamos, NM 57545, USA. Present Address: LeCroy Research Systems, Springer Valley, NY 10977, USA. Present Address: Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA. Present Address: P.O . Box 336, Brinnon, WA 98300, USA. Deceased .
0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
'ACTIVE AND PASSIVE SHIELD
Fig. 1 . Elevation view of the experiment. The central detector sits on supports mounted inside the shield assembly . The shield assembly separates into two sections . One consists of the rear wall and the lower portion including the central detector. The other section includes the cylinder and attached wall . Both of these sections as well as the water plug move independently on rails embedded in the concrete slab . The steel and earth g/cm2 vertically . overburden amounts to roughly 3000
J.J. Napolitano et al / Large area liquid scintillator cosmic ray detector
I dE/dx
2
3
4
5
6
Relative to minimum ionizing
Fig. 2. Scatter plot of kinetic energy versus differential energy loss as measured by the central detector. Muons and electrons are both minimum ionizing whereas muons are long tracks and are thus more energetic. Protons from neutron recoil are both highly ionizing and possess high kinetic energy . To be included in the figure, events must have tracks that are reasonably well contained within the central detector . This constraint decreases the number of muons significantly. quent N+ in the LAMPF beam stop whereas ir - and P - undergo nuclear capture in the high-Z material (mostly copper). The central detector consists of = 20 tons of liquid scintillator in a volume approximately 4 x 4 x 7 m3 . The detector is sensitive to the e + from the reaction p(ve , e + )n where the detector's scintillator also serves as the proton target and the ve can only come from vw (or v~. or ve) oscillations. The scintillation counters are arranged into 40 vertical planes of 12 counters each . Each counter is a liquid scintillator-filled acrylic extrusion measuring 3 cm x 30 cm x 3.7 m with a photomultiplier tube on each end. Between the scintillator planes are one plane each of 45 vertical and 45 horizontal proportional drift tubes (PDTs) for tracking . We trigger the central detector by requiring signals in both photomultiplier tubes on scintillators in three-outof-four consecutive planes. Possible evidence for neutrino oscillation is provided by any beam-associated "electron-like" signal, where the detector can distinguish between electrons, muons, and highly ionizing protons via measurements of total energy and dE/dx as shown in fig. 2. Each plane also contains a sheet of Mylar coated with 10 mg/cm2 of Gd 203 so that the neutron from the inverse beta decay reaction can be detected through its thermalization and radiative cap-
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ture on gadolinium, the resulting gamma rays being detected in the scintillators. The use of neutron detection is very effective in reducing backgrounds but with a large loss in efficiency for the neutrino oscillation signal. We attempt to be sensitive to a very small beam-associated signal and the sensitivity of the experiment is ultimately limited by the cosmic ray background rate . In this paper we concentrate on the problem of background rejection without resorting to neutron detection. Fig. 3 displays, for different cosmic ray background rates, possible limits on the neutrino oscillation parameters sin2 2B and Am 2 . (We adopt the usual simplifying assumption of two-state neutrino mixing .) Existing limits on ve appearance from Brookhaven National Laboratory (BNL) [2] and CERN [3] are included for comparison. The largest potential cosmic ray background for E645 is from the e } resulting from the decays of muons stopping within the detector volume . The cosmic ray flux, beneath the steel and earth overburden, is approximately 130/m2 s or about 3 kHz in the central detector . This flux is composed mostly of muons, of which roughly 2% stop in the detector, and 25% of these decay to e ± which in turn trigger the apparatus . Therefore, the rate of stopping muon decay background in the detector is about 15 s -1 or 10 5 per LAMPF-day, where a LAMPF-day takes into account the typical LAMPF duty factor of = 6% . The combined inefficiency of the active shield augmented by the central detector must therefore be less than 10 -5 to achieve a background rate less than 1/LAMPF-day due to stopping-muon decay . This rejection must be achieved while minimizing dead time losses and other inefficiencies . 100
10
r 0.001
0.01
sin 2 2B
0.1
Fig. 3. Possible limits on neutrino oscillations from E645 showing the dependence on cosmic ray background . The BNL and CERN experiments are limits on (vu, ve) mixing . The sensitivity for E645 is based on (v~L, ve) mixing.
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J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
Uncharged cosmic rays can also generate backgrounds. Photons will induce electron-like backgrounds via Compton scattering or pair production processes. High energy neutrons can induce electron-like signals by virtue of recoil protons in the detector which, due to fluctuations in the energy-loss measurement, may appear as electrons. Neutrons also produce pions which in turn decay to muons and then to electrons. Important features of the shielding which attempt to minimize these backgrounds are described herein . Two previous neutrino experiments at LAMPF, experiments E31 [4] and E225 [5], also sought to observe beam-associated electron-like signals and confronted similar cosmic ray backgrounds . In E31 (a non-imaging water Cherenkov detector with a two-layer plastic scintillator cosmic ray active shield), the active shield rejects charged particles with an inefficiency of 10 -° and achieves an ultimate background rate of 350/ LAMPFday. For E225 (a scintillator and flash tube hodoscope detector with a multi-layer proportional tube cosmic ray active shield), the on-line inefficiency is 10 -° and the ultimate background rate (into 41T) is = 10/LD. In this paper, we demonstrate that the active shield for E645 achieves an inefficiency of 10 -° on-line and < 3 x 10 -6 off-line. Preliminary results [1] indicate that background rates less than 1/LAMPF-day are achievable with reasonable efficiency for neutrino oscillation detection.
ionizing particle allows us to trigger the device well above background radioactivity and PMT noise. The design described in ref. [6] was expanded into the design of the device described in this paper. The device also incorporates a layer of lead "passive" shielding within the active layer to attenuate photon backgrounds. The most severe photon background is from stopping muons decays to e ± which bremsstrahlung. Photons with energies between 30 and 50 MeV are likely to Compton scatter, contributing to the background . The passive shield prevents these photons from entering the detector . Calculations indicate that the rate of such photons impinging on the central detector would be =103/LAMPF-day in the absence of any passive shielding and consequently an attenuation factor of 10 4 is called for. The passive shield does not attenuate cosmic ray neutrons effectively given their long attenuation length (= 300 g/cm 2 ; see the appendix). To achieve an adequate shielding factor we have placed the experiment in a tunnel under = 3000 g/cm2 of steel and earth. An 8 m long "water plug" (see fig. 1) is placed in the end of the tunnel further decreasing the cosmic ray neutron rate . The effect of this shielding on the neutron rate is discussed below.
2. Design overview
The shield is essentially a 10 m long, 8 m diameter, cylindrically shaped steel superstructure, separated into two concentric "tanks". The outer tank is fitted with PMTs and contains the liquid scintillator . The inner tank contains the lead passive shielding. The cylinder is truncated on the bottom to provide a surface for mounting the central detector . A cross-sectional view of the shield is shown in fig. 4. The shield is physically separated into two parts, namely the "cylinder" and attached vertical wall nearest the beam stop (see fig. 1 .), and the "cart" (on which the central detector is mounted) with attached vertical wall . The cylinder and its attached wall are optically connected, whereas the cart and its attached wall are optically isolated . The cart is itself divided into six separate tanks due to internal support girders. On all other parts of the shield, support is provided by an "exoskeleton" of 1-beams so that dead space is avoided on all but the bottom of the shield . The majority of the steel superstructure was constructed by Pittsburgh-Des Moines Corporation [7]. The total mass is 394 tons, not including the central detector . The inner tank contains the lead passive shield . This tanks is 7 in . thick bounded by 1 in . thick steel walls. Internal support girders along the circumference of the inner tank are placed every 19 in . . The lead is in the form of a fine (= 1 mm diameter) shot and is poured
To fully cover the central detector, the active shield must have an area of about 400 m2. Consequently, to achieve an inefficiency of less than 10 -6 there can be no more than 2 cm2 of dead space. Recovery time losses cannot be tolerated and the system must always be active . The charged particle cosmic ray rate through an active shield of this area is several kHz. These are primarily through-going muons and must be reduced by several orders of magnitude to make data acquisition feasible . To reduce the rate of triggers from stopping muons, the veto signals must be several muon lifetimes long . For example, a 10 kHz cosmic ray rate with a 10 ws long veto contributes a 10% dead time to the experiment . Consequently, the active shield must not only be highly efficient, but must be operated well of the " noise" to avoid excess rate leading to excessive dead time . These considerations lead us to design an active shield composed of a continuous layer of liquid scintillator, 6 in . thick, viewed by 5 in . diameter hemispherical photomultiplier tubes (PMT) . Tests of a prototype based on this design [6] indicate that the light attenuation spreads the signal over many PMTS providing a great deal of redundancy . These tests also demonstrate that the = 30 MeV of energy deposited by a minimum
3. Construction details
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J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
PMT ASSEMBLIES
CENTRAL DETECTOR . 40 PLANES WITH 12 Scintillation Counters 45 Vertical PDT's 45 Horizontal PDT's PER PLANE
TRAIN RAILS
LIQUID/ SCINTILLATOR
10 SCALE (feet)
Fig. 4. An axial cross section view of the active and passive shield assembly .
into the inner tank during fabrication . The packing fraction of the lead is approximately 70% leading to an effective lead thickness of 5 in . Additional lead bars are attached to the inside of the passive shield to compensate for the area subtended by the steel support girders.
The 6 in. thick outer tank containing the scintillator is bounded by the 1 in . thick wall of the inner tank and a ; 6 in . thick steel outer skin . As pointed out in ref. [6],
a high reflectivity white coating is essential for good optical properties . Prior to assembling the scintillator tank, all surfaces are sandblasted and the inside of the
tank is painted with the least three coats of Nuclear Enterprises NE561
two-component reflecting
paint.
After welding the outer skin to the rest of the su-
to the inside of the cylinder wall extending two feet over the cart .
The entire apparatus moves on three pairs of train
rails. The inner pair supports the cart along with the
attached vertical wall and the central detector . The outer two pairs support the cylinder and wall . Wheel assemblies are bolted directly to the cart, and to a total of
10
load-bearing
trucks
supporting
the
cylinder
through the use of large spherical washers. There are a total of 10 wheels on the cart and 40 on the cylinder .
The wheel and truck assemblies were constructed by the Whiting Corporation [8].
The entire active shield is viewed by a total of 360
EMI 987013, five inch hemispherical photomultiplier
perstructure, touch-up painting is done along the weld
tubes (PMTS), approximately one per square meter arranged on a triangular lattice. There are 46 PMTS on
noted above, the cart serves not only
the cart, and 6 on the shelf. The PMTS view the scintil-
seams. As
as an
integral part of the active and passive shield, but is also
each of the two vertical walls, 200 on the cylinder, 62 on
the support base for the central detector. The thickness
lator through custom-fabricated hemispherical glass windows [91 that are sealed by a bolted pressure ring
lead . Passive shielding is produced by stacking lead
over plastic because of the corrosive nature of the liquid
of the scintillator tank is only five inches allowing greater structural rigidity. There is no "inner tank" for bricks and ingots on top of the cart making a four inch
thick layer. To cover a small otherwise inactive area at
the interface between the cart and the cylinder wall, a small section of active shield called the "shelf" is added
through a Viton O-ring to flanges welded to the outer skin of the scintillator tank . Glass windows were chosen scintillator, particularly at high stress points such as the O-ring surface. The PMTS themselves are attached to the windows through optically clear RTV potting com-
pound. The RTV remains quite optically clear and
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J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector CONNECTORS : SIGNAL
HIGH VOLTAGE
BLACK POLYETHYLENE LIGHT, SHIELD
BOROSILICATE GLASS WINDOW
OPTICALLY CLEAR RTV POTTING COMPOUND
Fig . 5. Schematic diagram showing details of the photomultiplier tube (PMT) assembly. holds the PMT rigidly, but unfortunately makes it very difficult to remove a PMT from the glass window. The entire PMT assembly is shown in fig . 5 . The assembly is made light tight with a black polyethylene light shield around the base of the flange, a cast aluminum housing made by Northern Aluminum Foundry [10], a black polyethylene back plate mounted on the housing, and a small amount of black RTV applied to the interface between the aluminum housing and the steel pressure ring . The EMI PMTs are reasonably reliable, although we encountered a serious problem in the early stages of the experiment . After complete installation, nearly 5% of the PMTs failed within several months due to cracking of the glass envelope around the electrical feedthroughs . Although the exact causes of this cracking is not understood, it seems to be associated with a defective lot of PMTs . After the broken PMTs were replaced, the problem has not recurred with either the other PMTs originally installed or with the replacements . We use a simple voltage divider string soldered to a printed circuit card and socket to apply high voltage to the base and extract the anode signal . The overall resistance is = 1 MSS and the typical operating voltage is = 1200 V. We apply positive high voltage and the extended photocathode surface is kept at ground potential . To avoid ground loops in this large area electronics system, the base is grounded only through the signal cable shield .
The liquid scintillator we use is BC517P from Bicron Corporation which is a dilute (= 5%) mixture of pseudocumene in mineral oil. Properties of the scintillator are listed in table 1 . The active shield holds = 10000 gallons of liquid scintillator . Scintillator is stored in a 20 000 gallon stainless steel liquid nitrogen dewar in a slightly pressurized argon atmosphere . Scintillator is transferred to the shield through a 75 m long, 2 in . diameter fill line by pressurizing the storage tank . The liquid level is monitored with pairs of float switches on each of the three separated segments, namely the cylinder (and vertical wall), the cart, and the cart wall . With the storage tank pressurized initially to 20 psi, it takes approximately four hours to fill the entire system . While in the active shield, the scintillator is continually circulated through a polishing loop at a rate of 25 gal/min. The polishing system consists of an alumina drying Table 1 Properties of Bicron BC-517P liquid scintillator Light output Mean free path (400-500 nm) Wavelength of maximum emission Refractive index Specific gravity Coefficient of thermal expansion Vapor pressure at 20 ° C Viscosity at 20 ° C
28% of anthracene >6m 425 nm 1 .47 0.85 0.00075/ ° C 0.16 mm Hg 2.10 centistokes
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector agent followed by a 10 pm mesh microfilter . A manually operated manifold allows the scintillator in any one of the three sections to be polished . This manifold also allows scintillator to be pumped between any two sections . Each of the three sections has a = 20 gallon overflow tank which is vented through a gas trap and may be emptied into a common basin to be pumped back into any of the three sections through the polishing loop. This feature proves especially useful because temperature swings in the tunnel of up to 5 ° C are common and cause a significant amount of scintillator to spill into the overflow system. Upon cooling down, this leaves areas of reduced scintillator thickness, and consequently reduced efficiency, near the top of the various sections of the active shield . 4 . Veto logic and analog readout electronics The active shield electronics serves three functions . First, it generates a veto signal for the experiment trigger processor based on the multiplicity of hit PMTs to reduce the trigger rate to a tolerable level . Second, it provides a time history readout of the PMT multiplicity on which the veto logic is based . Finally, it digitizes the individual PMT pulse heights and provides for its time history readout . The electronics is housed in a single Fastaus crate . There are 24 interchangeable Fastaus cards each of which accepts up to 16 PMT signals after being patched into two bundles of 8-conductor ribbon cable. The cylinder and shelf, the wall, and the cart each use distinct sets of cards in the Fastaus crate . The Fastaus master board, or "controller", provides all clock signals, coordinates the veto information, and prepares the analog data for readout . All synchronization is performed by, and 1/0 signals are processed through, the controller . The controller is interfaced to an LSI-11/73 host computer via CAMAC through a Standard Engineering Model 10-612 dual 24-bit 1/0 register . This read-write interface is used for all control functions including reset and changing adjustable parameters, as well as for data readout . The hardware veto is based on the number of PMTs whose amplitudes cross discriminator threshold . This threshold is adjusted by front panel potentiometers on the input cards . Each potentiometer controls a group of eight PMT inputs . In an attempt to minimize dead time by distinguishing (at some level) passing muons from those that stop in the detector, the electronics divides the signals according to the three optically isolated sections, namely the cylinder (including the attached wall), the cart, and the cart wall (hereafter referred to as simply the "wall") . If a preset number of PMT signals cross threshold on two or more of the three sections, a short veto signal is generated ; whereas if only one
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section fires, a separate long veto is issued . The length of these two signals is independently adjustable and by keeping the length of the short veto small it is possible to minimize the dead time due to passing muons which would not cause a delayed trigger. For monitoring purposes, the OR of all discriminators on a card is accessible via a rear panel NIM output . Each time a PMT'S amplitude exceeds threshold it is added to the total number of PMTS for the corresponding section via a special multiplicity bus . To reduce the maximum time to add all hits on the cylinder, the 16 cards it uses are logically separated into two groups of eight . The addition proceeds independently in each group before combining the total . This division of cylinder PMTS roughly corresponds to dividing the cylinder into two halves, one on either side of the radial axis . The multiplicities of the three sections are polled in time with 12 .5 MHz veto clock and if conditions are met, the appropriate veto signal is generated . Before a single PMT signal is added onto the multiplicity bus, it passes through a removable PROM which, if so programmed, weights that PMT as more than one, or as none . If the veto conditions are again satisfied while the present veto is still in effect, the veto signal is updated . The number of PMTs which determine whether a section fires and the length of the long and short vetoes is set through the host computer via the 10-612/ Controller . A separate history clock records the multiplicity conditions in the controller in a memory which is continually updated until receipt of the trigger signal . We make the memory circular by simply over-writing the earliest address when all address are filled . We use a 6 .67 MHz history clock and a 1K RAM giving a total of = 150 ws of history . Data is stored in a single 16-bit word for each clock tick with 4-bits per section . We read out the memory through the 10-612/ Controller when operation is halted by an input trigger signal. Operation resumes when the controller is issued a restart signal either through the CAMAC interface or through a manual button . We also record the status of the veto signals in the circular memory . Fig . 6 histograms the veto lengths generated in the history or randomly triggered events. The updating veto length is shown by the fall-off at twice the long veto length . Histories of the digitized PMT signals are recorded for later readout using flash ADCs and associated RAM memories . The PMT signals are amplified, inverted, and stretched and the resulting signal is sampled by an RCA 3300D 6-bit flash ADC using a +2 V reference level and strobed by the 6 .67 MHz history clock . On each clock tick, the result is written into a circular 1K RAM memory and the address is incremented . The pulse rise time is much less than the clock period, and the pulse falls with an exponential decay time of 650 ns . This technique yields a 150 ws pulse height history for each phototube with virtually no digitizing dead time .
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J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
I 5
VETO
I 10
LENGTH
I 15
(usec)
I 20
Fig . 6. Histogram of the lengths of hardware generated vetoes as observed using randomly triggered data . The updating na ture of the hardware is seen by the extension of the veto length out to twice the nominal long veto length . The total volume of data contained in the ADC memories for all channels is 24 X 16 X 1K 6-bit words. This would consumer nearly ]! Mbyte at readout. However, most of the data are zeros and so the controller "compacts" the data and only data above some adjustable threshold are stored in a FIFO memory for readout. It takes less than 100 ms to compact the data from a typical event. The ADC data is read out through CAMAC as 24-bit words which specify 6-bit digitized ADC values of four adjacent channels, and the slot, quadrant (i .e. one of four four-channel blocks) and RAM address of the values . We normally run with a compaction threshold of 2 which means that for a quadrant of ADC values to be stored in the FIFO, at least one of the values must be 2 or greater. Under these conditions, the average event size to the host computer is 800 16-bit words, and 99% of the events have less than 2K 16-bit words.
usefulness of the distinction between long and short vetoes for minimizing on-line dead time . The long veto rate is rather insensitive to Mwau (the multiplicity of PMT hits in the wall) for Mwall >_ 4 while the short veto rate changes very little for all values of M wall . This is not surprising since the wall subtends much less solid angle for the predominantly vertical muons than do the cylinder and cart . On the other hand, the sharp change in the short veto rate at MCan = 5 indicates that a substantial fraction of throughgoing muons have Mc_t < 4. The long veto rate reaches a plateau at a cylinder multiplicity threshold of 5. For the majority of data taking in E645, we run with standard multiplicity thresholds of 5, 4, and 3 for the cylinder, wall, and cart respectively . The overall veto rate of = 10 kHz corresponds to a flux through the shield of = 130/MZ s. This is consistent with the measured trigger rate in the central detector of 1 .6 kHz, after accounting for the fact that only cosmic rays perpendicular to the planes trigger. Consequently, we know there is no large extraneous rate with the standard multiplicity settings . We observe a = 0.3% increase in the long veto rate during the 500 [sec long LAMPF beam gate which we trace to a 5% increase in the rate in the wall which is near the open end of the tunnel (with the water plug installed) . We attribute this effect to sky shine.
M CART _2
M CART - 3
MCART -4
MCART
5. Performance To measure the hardware veto rates, we trigger the apparatus randomly and read out the multiplicity history . Since the multiplicity history records the number of hit PMTS as a function of time over a 150 Its period we can measure the hardware veto rates as a function of multiplicity condition using an off-line analysis program . Fig. 7 shows the long and short veto rates as a function of multiplicity threshold in the cylinder and in the cart, for a variety of thresholds in the wall. Notice that rather independent of the multiplicity settings, the long veto rate is considerably larger than the short veto rate . This is apparently because a large fraction of the muons stop in the considerable thickness of lead in the passive shielding. The net effect is that it limits the
CYLINDER MULTIPLICITY
( M CYLINDER )
Fig. 7. Long and short veto rates as a function of various multiplicity settings on the three optically disconnected sections of the active shield . If a long or short veto is found by the software at a particular history location, the next = 2 Its of history are excluded . The dead time associated with the high long veto rate causes the slight increase in the short veto rate from Mcarc = 2 to Mcart = 3.
J.J. Napolitano et al. / Zarge area liquid scintillator cosmic ray detector
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w w a w U
Hz O U
Q W Q
0
50
100
150
200
DISTANCE FROM PHOTOTUBE (cm)
Fig. 8 . Attenuation of light in the active shield as a function of distance from the PMT. The analog information from the flash ADCs allows us to construct more sophisticated veto conditions in software incorporating the geometric arrangement and pulse height of hit PMTS . It is also useful for studying the shield using various kinds of triggers . Using data taken with the usual three-out-of-four central detector trigger and the active shield veto off, we reconstruct tracks with the central detector and study the light attenuation properties of the shield . Fig . 8 shows the average pulse height for PMTS (corrected for track angle) in the cylinder and in the wall as a function of distance from the PMT. The attenuation is rather nonexponential but is roughly approximated by a 1/e attenuation length of 50 cm. (Based on prototype tests [6] we know that the total number of photoelectrons is on the order of 10 3 for muons near the PMT.) In our prototype tests [6] we found that the attenuation length had two components . Within 50 cm of the PMT, the prototype attenuation length was = 50 cm, but for larger distances it was = 1 .9 m. It appears that only the 50 cm attenuation length is relevant to a large device . Light reflecting from the sidewalls of the prototype may have contributed to the 1 .9 m component . By triggering randomly and searching the event history for shield pulses, we obtain raw pulse height distributions independent of the central detector. In fig . 9 we histogram the pulse height distribution for the typical PMT on the cylinder when it is accompanied by at least one neighboring PMT, when there are no neighbors, and when all neighbors are present . The figure shows a clean separation between the distributions with all neighbors and with none . However the short attenuation length washes out the separation for events with fewer neighbors since the PMT is frequently near the edge of a "cluster" of nearest neighbors .
20
40
60
PMT PULSE HEIGHT (ADC channels)
Fig. 9 . Pulse height of a typical active shield PMT . The data is obtained by searching the pulse height history of randomly triggered events . We set high voltages on the PMTS by equalizing the average random pulse height. We only consider pulses when the PMT is accompanied by at least one neighbor. We histogram the resulting average pulse height for the 360 PMTs in fig . 10a . Fig. 10b shows the average PMT rate as determined by the pulse height history . After this first order adjustment, we increase the voltage on some PMTS on the basis of a program which, using veto-on central detector triggers, tracks cosmic ray
.~
-
SAk%A i l ~
0 10 20 30 AVERAGE PMT PULSE HEIGHT (channels)
0.5 1 .0 1.5 2.0 2.5 AVERAGE PMT RATE (kHz)
Fig . 10 . Average pulse heights (a) and rates (b) (from the pulse height history) for the 360 PMTS on the active shield after gain adjustment via high voltage .
J.J . Napolitano et al / Large area liquid scintillator cosmic ray detector
160
Z ô U
N PM
Fig. 11 . Histograms of the number of PMTS in a cluster near a cosmic ray muon track for each of the four types of sections in the active shield as determined from the pulse height history. A cluster is defined to be a set of nearest neighbor PMTS . Note that the cylinder wall is optically connected to the cylinder . muons and points to areas of the shield that require somewhat higher on-line efficiency . Using veto-off cosmic ray data, we locate the PMT cluster on the shield associated with the track as determined by the central detector . We only use tracks that appear to both enter and exit the boundaries of the central detector. We histogram the number of PMTS in each cluster in fig. 11 for the cylinder, the two walls, and the cart. The number of tubes in a wall cluster is typically more than in the cylinder because of somewhat shorter overall attenuation length and because particles cross the walls at steeper angles . Clusters with one or two PMTS occur when the cosmic ray moves very close to or through a PMT. Although this results in few hit PMTS, the pulse height is very large. The number of PMTs per cluster on the cart is significantly smaller because of the cart segmentation . We see no evidence for loss of efficiency for particles passing near the sides of the individual tanks. Using dE/dx and total energy from the central detector to identify nations, we use "veto-on" data to measure the rate of muons escaping the hardware veto . For a specific set of running conditions including the standard "5, 4, 3" veto, we measure a muon rate of 0.70 s -1 . By extrapolating the tracks to the active shield, we can see if any particular sections are less efficient at detecting muons. In fig. 12 we plot the intersection coordinates of these tracks with the vertical plane defined by the (optically disconnected) wall . Most of the
points lie on a curve defined by the boundary of the wall and the cylinder . The muon rate through this region is 0.57 s -t or 81% of all muon-like triggers . The scintillator thickness of the cylinder at this boundary is only 2 in . for structural reasons reducing the net light output. We confirm this by using the active shield ADC data in an off-line analysis program and examining the pulse height in the 11 PMTS nearest this boundary. After installing specially programmed PROMS which weight each of these 11 rim PMTS as three hits instead of one in the hardware veto logic, the muon trigger rate decreases to 0.16 s -1 while the veto rate increases by only 20%. Thus we sharply reduce the major source of on-line muons while only slightly increasing the dead time . Examination of the remaining muons shows no other obvious areas of inefficiency. By comparing the residual muon rate to a veto-off muon trigger rate of 1600 s -i , we conclude that under usual operating conditions the active shield rejects muons with an inefficiency of 10 -4 using the on-line veto electronics . Over the course of long data runs, we periodically adjust high voltages on sections of the active shield, monitor the liquid scintillator levels making relevant adjustments, and experiment with different multiplicity conditions for the on-line veto . The muon trigger rate with the veto on is plotted in fig. 13 for each of many runs taken over the course of approximately six weeks during the summer of 1987 . The muon rate, being such a small fraction of the veto-off rate, is obviously very sensitive to the condition of the shield and the electronics. However, the rate of electron-like triggers, also plotted in fig. 13, is rather stable because most of them are from the decays of muons which live longer than the 10 ws veto . The rate of proton-like triggers, from cosmic ray neutron recoils, shows a steady decline as steel overburden was continually added primarily to the sides and back of the overhead area. The proton rate decreases continuously despite large fluctuations in the
z â U
-200 -300 -400
-200 0 200 400 HORIZONTAL POSITION AT WALL (cm)
Fig. 12 . Intersection points of unvetoed cosmic ray muons with the wall . Nearly all the points fall on the interface between the cylinder and the wall .
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
0.4 MUONS _ 0.3
¢+}~ }++~{} }+ +f"e°¢}}~+++ ++++++++e }ee+~~+ ++}} ++
0.2
ELECTRONS
r
s . e+ + °".° . . ."..
.1 .
+ 4+
} 0
e
00 1
*
PROTONS
0 42 DAYS
Fig. 13 . Rate of muon, electron, and proton triggers for many runs taken over a six week period. The muon rate fluctuates by large amounts since it is very sensitive to the condition of the shield . However, the electron and proton rates show no such discontinuity . muon rate since the active shield is insensitive to neutrons. Having eliminated the majority of muon triggers escaping the on-line veto by customizing the multiplicity PROMs, we examine the response of the active shield to the remaining muon-like particles . Fig . 14 histograms the number of PMT clusters in the active shield for muon-like particles which escape the on-line veto . For comparison, we also include (dashed histogram) the same distribution for normal "veto-off
103
10
I I I I I I 0
I I I I l_ 8 2 4 6 NUMBER OF CLUSTERS
Fig. 14 . Number of PMT clusters observed in coincidence with the trigger for muon-like particles which escape the on-line veto (solid histogram) and for normal veto-off muons (dashed histogram) .
muons . We see that 123 muon-like triggers out of a total of 4500 give no active shield signals. Using a veto-on trigger rate of 0 .2 s -1 (see fig . 13), this implies an unvetoed rate of muon-like triggers of = 5 X 10 -3 s - ' . The veto-off muon rate of 1600 s - ' therefore would imply an ultimate shield inefficiency of = 3 X 10 -6 assuming that all muon-like triggers are ordinary cosmic ray muons . However, it is not difficult to find processes which, at the few-parts-per-million level, could mimic muons in the central detector and give no signal in the active shield . For example, the central detector proton rate implies that cosmic ray neutrons impinge on the active shield at a rate of at least = 0 .1 s -1 . If only a few percent of these interact in the = 150 g/cm2 lead passive shielding or in the outer portion of the central detector to produce charged pions or energetic protons, this would account for the residual events we observe . It appears that the relevant cross sections are sufficiently high, although it is difficult to make accurate estimates of the rates . In any case, we consider the ultimate muon inefficiency of 3 X 10 -6 to be an upper limit. Assuming this to be the inefficiency, the background rate in our neutrino oscillation experiment due to undetected stopping muons would be =0.25/LAMPF-day . By using central detector information to improve the veto, this small inefficiency can be exploited while retaining a large detection efficiency for neutrino oscillation events . We end with a discussion of the cosmic ray neutron attenuation by the overburden . Veto-on measurements made outside of the experimental tunnel (i .e . no overburden) give a proton trigger rate of = 40 s -1 . When the apparatus is moved inside the tunnel but with the water plug (see fig . 1) empty, the rate drops to 0 .2 s -1 . Consequently the overburden attenuates the fast neutron background by 200 while the muon rate (as measured by the shield veto rate) is cut only a factor of 3 . One would expect, however, a factor of = 10 ° attenuation on the basis of measurements made of hadronic cosmic ray attenuation rates . (See the appendix .) We note, however, that the cosmic ray neutron flux is significant even to large zenith angles . Filling the water plug (800 g/cm2 of water in the horizontal direction) decreases the proton trigger rate to 0 .05 s -1 and additional shielding at large zenith angles (see fig . 13) reduces the rate further to 0 .02 s -1 for an attenuation factor of 2000 . 6. Multiple-muon detection and ultrahigh energy gamma ray astronomy We have obtained results using the active shield as a multiple-muon detector. Astrophysical sources of ultrahigh energy (E >_ 10 1^ eV) gamma rays are studied with large arrays of charged particle detectors which measure the direction and lateral extent of "air showers" produced when the gamma ray interacts with the Earth's
16 2
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
w U
ô w m z 1? a w â
0 I 2 3 4 5 NUMBER OF TRACKS IN CENTRAL DETECTOR
Fig. 15 . Average number of shield clusters as a function of the number of tracks in the central detector . The straight line assumes (Nouster = 2 x (NDet-track + 1) ; i.e. two clusters per track with one track unobserved . atmosphere [11] . Such events may be discriminated from more abundant hadronically-induced showers by identifying penetrating muons from associated qT ± decay. An air shower array of this type has been mounted at LAMPF on the grounds surrounding the target area and hence our neutrino oscillation experiment [121 . Since our active shield (as well as the central detector) is situated close to the center of the array and is beneath = 3000 g/CMZ , it serves as a suitable muon detector . Data from the central detector, triggered by the air shower array, typically show many separate but parallel tracks . Unfortunately, it is difficult to deal with the large amount of data from the central detector when operated in conjunction with the air shower experiment . Consequently it is interesting to study the use of the active shield alone as a detector of multiple muons. By acquiring some data from both the central detector and the active shield while triggering on the air shower array, we can "calibrate" the response of the active shield using the central detector to tag multiple muons. Fig. 15 plots the average number of PMT clusters in the active shield as a function of the number of tracks observed in the central detector . Also included is a straight line approximation which assumes two shield clusters per track with one track always present but unobserved in the central detector, presumably because of the reduced solid angle of the central detector . The average number of clusters saturates since for a high density of tracks, clusters tend to merge together . In any case, it is clear that some information regarding the number of muons can be deduced from the rather crude active shield information . 7. Conclusions We have built and operated a large liquid scintillator cosmic ray active shield that rejects cosmic ray muons
at a level of 10 -4 on-line. Dead time losses are approximately 10% due to the = 10 kHz veto rate and the = 10 ~.s veto length. Using digitized data from the apparatus we can reject cosmic ray muons with off-line software at a level of 3 x 10-6 or less . The device also serves as a crude "muon multiplicity" detector for a cosmic ray air shower experiment . Perhaps the biggest drawback of the final apparatus is the lack of single PMT redundancy since it is clear that cosmic ray muons which pass through a particular PMT frequently register a signal in only that PMT. Since the glass window holding the PMT projects nearly three inches into the six inch thick scintillator, the problem is largely due to the decreased scintillator thickness at the PMT location . A better design might incorporate flat instead of hemispherical PMTs . A different, albeit more expensive, approach would be to have two or more PMTS at each location . Acknowledgements We are grateful to all our collaborators on E645 . In particular, we acknowledge B. Fujikawa, Dr . J. Donahue, and Dr . V. Sandberg . We thank our contacts with the construction firms that built many of the components including : G. DiStefano, J. McCoy (PittsburghDes Moines); A Provenzano (Kopp Glass) ; W. Meyst (Northern Aluminum Foundries) ; and M. Milligan (Whiting Corporation) . Much of the PMT installation was done by R. Amrein, D. Cyborski, S. Schmidt, N. Roy, M. Madden, D. Dryer, and K. McShane. F. Bloennigen provided critical assistance during our original tune-up for data taking . On site mechanical support and engineering was provided by S. Cushing, J. Gomez, A. Kirby, J. Eddlemen, N. June, and D. Justice. J. Bittman provided considerable software support in the initial phase of the experiment . Data acquired with multiple muons was done with the help of B. Dingus, Dr . D. Nagle, and Dr . R. Cady . Finally, we acknowledge the invaluable support of G. Suazo at LAMPF for coordinating the majority of the engineering tasks associated with the project. Appendix Attenuation of neutral cosmic ray backgrounds To study the dependence of neutral cosmic rays on the amount of overhead material, we measure the rate in a shielded 10 in, long x 12 in . diameter NaI(TI) detector as a function of energy and overburden depth. The apparatus is shown in fig. 16 . The liquid scintillator counter described in ref. [61 is incorporated into the apparatus, although only the cylindrical plastic active
163
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector LIQUID SCINTILLATOR
NO VETO
MeV 15-20
WON
gw-1
61
20-30 30-40
PLASTIC SCINTILLATOR
40-50 50-60
Fig. 16. Apparatus used to measure neutral cosmic ray backgrounds as a function of overburden.
shield was used in the hardware veto . We acquire data on an event-by-event basis . We read pulse heights from all PMTs as well as the signal time relative to the Nal(TI) detector. Note that a crucial difference between the design of this system and that of the E645 shield is that here the passive shield lies outside the active shield . The apparatus is placed along with a vertical cosmic ray muon telescope on a movable stand situated in a tunnel burrowed into the side of the Los Alamos mesa. The contour of the mesa enables us to change the overburden level simply by changing the distance into the tunnel . From the telescope rate and its calculated acceptance we deduce the amount of overburden using an empirical relation [13] for the muon intensity as a function of depth from the top of the atmosphere. The agreement with the original land survey is very good. Fig . 17 shows the observed rate in the Nal(Tl) detector for different energy bins as a function of overburden depth . The data for each energy bin is fit to two decaying exponentials, with the long attenuation length component fixed to be the same for all energies . The short attenuation length varies from 322 g/cm2 for the 60-100 MeV data to 973 g/cm2 for 15-20 MeV . The longer attenuation length is = 1700 g/cm2 which is consistent with the muon attenuation as shown in the upper curve in fig. 17 . Fig . 18 shows the TDC spectrum for delayed pulses in the Nal(TI) detector relative to the triggering signal for data taken with a veto length of = 2 [,s, clearly demonstrating muon decay in the detector . By examining the time spectra of the neutral backgrounds relative to any associated signal in the liquid scintillator counter, we gain additional insight as to the source of these backgrounds . Fig . 18b plots this spectrum for the data taken with 3000 g/cm2 overburden. There should be
60-100
Fig . 17 . Attenuation of cosmic ray rates observed in the Nal(Tl) detector as a function of overburden for different energies. three components to this spectrum . One is the = 2 .2 Ws decay time of W+ whose decay positrons trigger the Nal(Tl) detector via bremsstrahlung in the lead passive shielding . Another is the = 80 ns [14] component due to W- capture in the lead . The third component is a = 1 ~Ls [14] component from muons which contribute background via capture in the surrounding rock (mostly Si02 ) . Also included in fig . 18b is a fit to three decay time components plus a constant background . Roughly
z w w ô w
I
I
I
BACKGROUND at 3000gm/Cm 2 O .I I i 0 .03 /~ s TSHORTTLONO - 0.47 ± 0.05ps
z
Fig . 18 . Time of the event in the Nal(Tl) detector relative to (a) delayed coincidence in itself and (b) the liquid scintillator detector located over the apparatus (see fig . 16). (See text for details .)
164
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
80% of the events have a lifetime of 110 ± 30 ns, consistent with lt capture on lead. A long lifetime component yielding a lifetime of = 0.5 p s may indicate capture in the rock, but is poorly fit. The fit does not,
however, allow any significant component from free W+ decay. Since lt+ decay may contribute background only through bremmstrahlung while lr capture can proceed via the emission of a - 1000 MeV neutron, we suspect that some of the neutral background at large depths
may be due to the latter process. References
[1] L.S . Durkin et al ., to be published; J. Napolitano, as presented in Selected Topics in Electroweak Interactions, (Feb. 1987) Proc . 2nd Lake Louise Winter Institute on New Frontiers in Particle Physics,
Chateau lake Louise, Canada, pp . 376-385; E.S . Smith, as presented in the 1986 Massive Neutrinos in Astrophysics and in particle Physics (Jan ./Feb, 1986) Proc . 6th Moriond Workshop, Tignes, France, pp . 287-292.
[2] L.A. Ahrens et al., Phys . Rev. D 31 (1985) 2732. [3] C. Angelini et al ., Phys . Lett . B178 (1986) 307. [4] S.E . Willis et al ., Phys . Rev. Lett. 44 (1980) 522; S.E . Willis, Ph .D . Thesis, Yale University (1979) unpublished . [5] R.C . Allen et al ., Phys . Rev. Lett . 55 (1985) 2401 . [6] Stuart J. Freedman et al ., Nucl . Instr. and Meth . 215 (1983) 71 . [7] Pittsburgh-Des Moines Corporation, 3400 Grand Ave., Neville Island, Pittsburgh, PA 15225, USA. [8] Whiting Corporation, 15700 Lathrop St ., Harvey . IL
60426, USA. [9] Kopp Glass Incorporated, P.O . Box 8255, Pittsburgh, PA 15218, USA. [10] Northern Aluminum Foundry Company, P.O . Box 1065, 502 Van Dyne Road, North Fond du Lac, WI 549935, USA. [ll] For example, see J. Wdowczyk and K.E . Turver, in : Cosmic Rays at Ground Level, ed . A.W. Wolfendale (Inst. of Physics, London, 1973). [12] B.L . Dingus et al ., Phys . Rev. Lett . 60 (1988) 1785 . [13] A.I. Barbouti and B.C . Rastin, J. Phys . G 9 (1983) 1577 . [14] T. Suzuki et al ., Phys. Rev. C35 (1987) 2212 .
Nuclear Instruments and Methods in Physics Research A274 (1989) 152-164 North-Holland, Amsterdam
CONSTRUCTION AND PERFORMANCE OF A LARGE AREA LIQUID SCINTILLATOR COSMIC RAY ANTICOINCIDENCE DETECTOR James J. NAPOLITANO, Stuart, J. FREEDMAN, Gerald T. GARVEY §, Michael C. GREEN §§, Kevin T. LESKO +, James E. NELSON and James N. WORTHINGTON ++ Physics Division, Argonne National Laboratory, Argonne, IL 60439-4843, USA
Kenneth P. COOVER t, John W. DAWSON, William HABERICHTER and Emil P. PETEREIT High Energy Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA Received 13 July 1988
We describe details of the construction and performance of a large area cylindrical cosmic ray anticoincidence detector suitable for a medium energy neutrino experiment. The device provides 41r coverage with approximately 400 m2 of liquid scintillator . Cosmic ray muons are rejected on-line with an inefficiency of =10 -° which is improved to a level of = 3 X 10 -b or less with the application of off-line cuts. We also discuss the custom electronics used to operate and read out the device . Results from studies of the attenuation of neutral cosmic ray backgrounds by passive shielding are included .
1. Introduction Neutrino sources which use the beam dumps of medium energy proton accelerators such as the Los Alamos Meson Physics Facility (LAMPF) are especially useful because they produce copious amounts of Pe, essentially no ve , and have exceptionally well known neutrino energy distributions . However, experiments using these sources confront serious cosmic ray backgrounds. Duty factors are large and cross sections are small since neutrinos have low energy coming principally from the decays of thermalized pions and muons.
neutrino detector must be surrounded by a highly efficient, large area cosmic ray anticoincidence detector, or " shield".
LAMPF experiment E645, a collaboration of Argonne National Laboratory, California Institute of Technology, Lawrence Berkeley Laboratory, Los Alamos
National Laboratory, Louisiana State University and Ohio State University, is a study of neutrino oscillations through the appearance of ve from vP (or, in principle, from PP or Pe) [1]. An elevation view is shown in fig. 1.
Neutrinos are produced in the decays of rr' and subse-
Neutrino initiated reactions can be simulated by cosmic ray generated backgrounds. To obtain reasonable rates,
the neutrino detectors must be large. Consequently, the
* This work was supported by the US Department of Energy, Nuclear Physics and High Energy Physics Divisions, under contract number W-31-109-ENG-38. One of us (K .T .L .) acknowledges support during the latter phases of this project from Lawrence Berkeley Laboratory by the U.S . Department of Energy under contract number DEAC03-76SF00098 . Present Address: Los Alamos Meson Physics Facility, Los Alamos, NM 57545, USA. Present Address: LeCroy Research Systems, Springer Valley, NY 10977, USA. Present Address: Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA. Present Address: P.O . Box 336, Brinnon, WA 98300, USA. Deceased .
0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
'ACTIVE AND PASSIVE SHIELD
Fig. 1 . Elevation view of the experiment. The central detector sits on supports mounted inside the shield assembly . The shield assembly separates into two sections . One consists of the rear wall and the lower portion including the central detector. The other section includes the cylinder and attached wall . Both of these sections as well as the water plug move independently on rails embedded in the concrete slab . The steel and earth g/cm2 vertically . overburden amounts to roughly 3000
J.J. Napolitano et al / Large area liquid scintillator cosmic ray detector
I dE/dx
2
3
4
5
6
Relative to minimum ionizing
Fig. 2. Scatter plot of kinetic energy versus differential energy loss as measured by the central detector. Muons and electrons are both minimum ionizing whereas muons are long tracks and are thus more energetic. Protons from neutron recoil are both highly ionizing and possess high kinetic energy . To be included in the figure, events must have tracks that are reasonably well contained within the central detector . This constraint decreases the number of muons significantly. quent N+ in the LAMPF beam stop whereas ir - and P - undergo nuclear capture in the high-Z material (mostly copper). The central detector consists of = 20 tons of liquid scintillator in a volume approximately 4 x 4 x 7 m3 . The detector is sensitive to the e + from the reaction p(ve , e + )n where the detector's scintillator also serves as the proton target and the ve can only come from vw (or v~. or ve) oscillations. The scintillation counters are arranged into 40 vertical planes of 12 counters each . Each counter is a liquid scintillator-filled acrylic extrusion measuring 3 cm x 30 cm x 3.7 m with a photomultiplier tube on each end. Between the scintillator planes are one plane each of 45 vertical and 45 horizontal proportional drift tubes (PDTs) for tracking . We trigger the central detector by requiring signals in both photomultiplier tubes on scintillators in three-outof-four consecutive planes. Possible evidence for neutrino oscillation is provided by any beam-associated "electron-like" signal, where the detector can distinguish between electrons, muons, and highly ionizing protons via measurements of total energy and dE/dx as shown in fig. 2. Each plane also contains a sheet of Mylar coated with 10 mg/cm2 of Gd 203 so that the neutron from the inverse beta decay reaction can be detected through its thermalization and radiative cap-
15 3
ture on gadolinium, the resulting gamma rays being detected in the scintillators. The use of neutron detection is very effective in reducing backgrounds but with a large loss in efficiency for the neutrino oscillation signal. We attempt to be sensitive to a very small beam-associated signal and the sensitivity of the experiment is ultimately limited by the cosmic ray background rate . In this paper we concentrate on the problem of background rejection without resorting to neutron detection. Fig. 3 displays, for different cosmic ray background rates, possible limits on the neutrino oscillation parameters sin2 2B and Am 2 . (We adopt the usual simplifying assumption of two-state neutrino mixing .) Existing limits on ve appearance from Brookhaven National Laboratory (BNL) [2] and CERN [3] are included for comparison. The largest potential cosmic ray background for E645 is from the e } resulting from the decays of muons stopping within the detector volume . The cosmic ray flux, beneath the steel and earth overburden, is approximately 130/m2 s or about 3 kHz in the central detector . This flux is composed mostly of muons, of which roughly 2% stop in the detector, and 25% of these decay to e ± which in turn trigger the apparatus . Therefore, the rate of stopping muon decay background in the detector is about 15 s -1 or 10 5 per LAMPF-day, where a LAMPF-day takes into account the typical LAMPF duty factor of = 6% . The combined inefficiency of the active shield augmented by the central detector must therefore be less than 10 -5 to achieve a background rate less than 1/LAMPF-day due to stopping-muon decay . This rejection must be achieved while minimizing dead time losses and other inefficiencies . 100
10
r 0.001
0.01
sin 2 2B
0.1
Fig. 3. Possible limits on neutrino oscillations from E645 showing the dependence on cosmic ray background . The BNL and CERN experiments are limits on (vu, ve) mixing . The sensitivity for E645 is based on (v~L, ve) mixing.
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J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
Uncharged cosmic rays can also generate backgrounds. Photons will induce electron-like backgrounds via Compton scattering or pair production processes. High energy neutrons can induce electron-like signals by virtue of recoil protons in the detector which, due to fluctuations in the energy-loss measurement, may appear as electrons. Neutrons also produce pions which in turn decay to muons and then to electrons. Important features of the shielding which attempt to minimize these backgrounds are described herein . Two previous neutrino experiments at LAMPF, experiments E31 [4] and E225 [5], also sought to observe beam-associated electron-like signals and confronted similar cosmic ray backgrounds . In E31 (a non-imaging water Cherenkov detector with a two-layer plastic scintillator cosmic ray active shield), the active shield rejects charged particles with an inefficiency of 10 -° and achieves an ultimate background rate of 350/ LAMPFday. For E225 (a scintillator and flash tube hodoscope detector with a multi-layer proportional tube cosmic ray active shield), the on-line inefficiency is 10 -° and the ultimate background rate (into 41T) is = 10/LD. In this paper, we demonstrate that the active shield for E645 achieves an inefficiency of 10 -° on-line and < 3 x 10 -6 off-line. Preliminary results [1] indicate that background rates less than 1/LAMPF-day are achievable with reasonable efficiency for neutrino oscillation detection.
ionizing particle allows us to trigger the device well above background radioactivity and PMT noise. The design described in ref. [6] was expanded into the design of the device described in this paper. The device also incorporates a layer of lead "passive" shielding within the active layer to attenuate photon backgrounds. The most severe photon background is from stopping muons decays to e ± which bremsstrahlung. Photons with energies between 30 and 50 MeV are likely to Compton scatter, contributing to the background . The passive shield prevents these photons from entering the detector . Calculations indicate that the rate of such photons impinging on the central detector would be =103/LAMPF-day in the absence of any passive shielding and consequently an attenuation factor of 10 4 is called for. The passive shield does not attenuate cosmic ray neutrons effectively given their long attenuation length (= 300 g/cm 2 ; see the appendix). To achieve an adequate shielding factor we have placed the experiment in a tunnel under = 3000 g/cm2 of steel and earth. An 8 m long "water plug" (see fig. 1) is placed in the end of the tunnel further decreasing the cosmic ray neutron rate . The effect of this shielding on the neutron rate is discussed below.
2. Design overview
The shield is essentially a 10 m long, 8 m diameter, cylindrically shaped steel superstructure, separated into two concentric "tanks". The outer tank is fitted with PMTs and contains the liquid scintillator . The inner tank contains the lead passive shielding. The cylinder is truncated on the bottom to provide a surface for mounting the central detector . A cross-sectional view of the shield is shown in fig. 4. The shield is physically separated into two parts, namely the "cylinder" and attached vertical wall nearest the beam stop (see fig. 1 .), and the "cart" (on which the central detector is mounted) with attached vertical wall . The cylinder and its attached wall are optically connected, whereas the cart and its attached wall are optically isolated . The cart is itself divided into six separate tanks due to internal support girders. On all other parts of the shield, support is provided by an "exoskeleton" of 1-beams so that dead space is avoided on all but the bottom of the shield . The majority of the steel superstructure was constructed by Pittsburgh-Des Moines Corporation [7]. The total mass is 394 tons, not including the central detector . The inner tank contains the lead passive shield . This tanks is 7 in . thick bounded by 1 in . thick steel walls. Internal support girders along the circumference of the inner tank are placed every 19 in . . The lead is in the form of a fine (= 1 mm diameter) shot and is poured
To fully cover the central detector, the active shield must have an area of about 400 m2. Consequently, to achieve an inefficiency of less than 10 -6 there can be no more than 2 cm2 of dead space. Recovery time losses cannot be tolerated and the system must always be active . The charged particle cosmic ray rate through an active shield of this area is several kHz. These are primarily through-going muons and must be reduced by several orders of magnitude to make data acquisition feasible . To reduce the rate of triggers from stopping muons, the veto signals must be several muon lifetimes long . For example, a 10 kHz cosmic ray rate with a 10 ws long veto contributes a 10% dead time to the experiment . Consequently, the active shield must not only be highly efficient, but must be operated well of the " noise" to avoid excess rate leading to excessive dead time . These considerations lead us to design an active shield composed of a continuous layer of liquid scintillator, 6 in . thick, viewed by 5 in . diameter hemispherical photomultiplier tubes (PMT) . Tests of a prototype based on this design [6] indicate that the light attenuation spreads the signal over many PMTS providing a great deal of redundancy . These tests also demonstrate that the = 30 MeV of energy deposited by a minimum
3. Construction details
15 5
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
PMT ASSEMBLIES
CENTRAL DETECTOR . 40 PLANES WITH 12 Scintillation Counters 45 Vertical PDT's 45 Horizontal PDT's PER PLANE
TRAIN RAILS
LIQUID/ SCINTILLATOR
10 SCALE (feet)
Fig. 4. An axial cross section view of the active and passive shield assembly .
into the inner tank during fabrication . The packing fraction of the lead is approximately 70% leading to an effective lead thickness of 5 in . Additional lead bars are attached to the inside of the passive shield to compensate for the area subtended by the steel support girders.
The 6 in. thick outer tank containing the scintillator is bounded by the 1 in . thick wall of the inner tank and a ; 6 in . thick steel outer skin . As pointed out in ref. [6],
a high reflectivity white coating is essential for good optical properties . Prior to assembling the scintillator tank, all surfaces are sandblasted and the inside of the
tank is painted with the least three coats of Nuclear Enterprises NE561
two-component reflecting
paint.
After welding the outer skin to the rest of the su-
to the inside of the cylinder wall extending two feet over the cart .
The entire apparatus moves on three pairs of train
rails. The inner pair supports the cart along with the
attached vertical wall and the central detector . The outer two pairs support the cylinder and wall . Wheel assemblies are bolted directly to the cart, and to a total of
10
load-bearing
trucks
supporting
the
cylinder
through the use of large spherical washers. There are a total of 10 wheels on the cart and 40 on the cylinder .
The wheel and truck assemblies were constructed by the Whiting Corporation [8].
The entire active shield is viewed by a total of 360
EMI 987013, five inch hemispherical photomultiplier
perstructure, touch-up painting is done along the weld
tubes (PMTS), approximately one per square meter arranged on a triangular lattice. There are 46 PMTS on
noted above, the cart serves not only
the cart, and 6 on the shelf. The PMTS view the scintil-
seams. As
as an
integral part of the active and passive shield, but is also
each of the two vertical walls, 200 on the cylinder, 62 on
the support base for the central detector. The thickness
lator through custom-fabricated hemispherical glass windows [91 that are sealed by a bolted pressure ring
lead . Passive shielding is produced by stacking lead
over plastic because of the corrosive nature of the liquid
of the scintillator tank is only five inches allowing greater structural rigidity. There is no "inner tank" for bricks and ingots on top of the cart making a four inch
thick layer. To cover a small otherwise inactive area at
the interface between the cart and the cylinder wall, a small section of active shield called the "shelf" is added
through a Viton O-ring to flanges welded to the outer skin of the scintillator tank . Glass windows were chosen scintillator, particularly at high stress points such as the O-ring surface. The PMTS themselves are attached to the windows through optically clear RTV potting com-
pound. The RTV remains quite optically clear and
156
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector CONNECTORS : SIGNAL
HIGH VOLTAGE
BLACK POLYETHYLENE LIGHT, SHIELD
BOROSILICATE GLASS WINDOW
OPTICALLY CLEAR RTV POTTING COMPOUND
Fig . 5. Schematic diagram showing details of the photomultiplier tube (PMT) assembly. holds the PMT rigidly, but unfortunately makes it very difficult to remove a PMT from the glass window. The entire PMT assembly is shown in fig . 5 . The assembly is made light tight with a black polyethylene light shield around the base of the flange, a cast aluminum housing made by Northern Aluminum Foundry [10], a black polyethylene back plate mounted on the housing, and a small amount of black RTV applied to the interface between the aluminum housing and the steel pressure ring . The EMI PMTs are reasonably reliable, although we encountered a serious problem in the early stages of the experiment . After complete installation, nearly 5% of the PMTs failed within several months due to cracking of the glass envelope around the electrical feedthroughs . Although the exact causes of this cracking is not understood, it seems to be associated with a defective lot of PMTs . After the broken PMTs were replaced, the problem has not recurred with either the other PMTs originally installed or with the replacements . We use a simple voltage divider string soldered to a printed circuit card and socket to apply high voltage to the base and extract the anode signal . The overall resistance is = 1 MSS and the typical operating voltage is = 1200 V. We apply positive high voltage and the extended photocathode surface is kept at ground potential . To avoid ground loops in this large area electronics system, the base is grounded only through the signal cable shield .
The liquid scintillator we use is BC517P from Bicron Corporation which is a dilute (= 5%) mixture of pseudocumene in mineral oil. Properties of the scintillator are listed in table 1 . The active shield holds = 10000 gallons of liquid scintillator . Scintillator is stored in a 20 000 gallon stainless steel liquid nitrogen dewar in a slightly pressurized argon atmosphere . Scintillator is transferred to the shield through a 75 m long, 2 in . diameter fill line by pressurizing the storage tank . The liquid level is monitored with pairs of float switches on each of the three separated segments, namely the cylinder (and vertical wall), the cart, and the cart wall . With the storage tank pressurized initially to 20 psi, it takes approximately four hours to fill the entire system . While in the active shield, the scintillator is continually circulated through a polishing loop at a rate of 25 gal/min. The polishing system consists of an alumina drying Table 1 Properties of Bicron BC-517P liquid scintillator Light output Mean free path (400-500 nm) Wavelength of maximum emission Refractive index Specific gravity Coefficient of thermal expansion Vapor pressure at 20 ° C Viscosity at 20 ° C
28% of anthracene >6m 425 nm 1 .47 0.85 0.00075/ ° C 0.16 mm Hg 2.10 centistokes
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector agent followed by a 10 pm mesh microfilter . A manually operated manifold allows the scintillator in any one of the three sections to be polished . This manifold also allows scintillator to be pumped between any two sections . Each of the three sections has a = 20 gallon overflow tank which is vented through a gas trap and may be emptied into a common basin to be pumped back into any of the three sections through the polishing loop. This feature proves especially useful because temperature swings in the tunnel of up to 5 ° C are common and cause a significant amount of scintillator to spill into the overflow system. Upon cooling down, this leaves areas of reduced scintillator thickness, and consequently reduced efficiency, near the top of the various sections of the active shield . 4 . Veto logic and analog readout electronics The active shield electronics serves three functions . First, it generates a veto signal for the experiment trigger processor based on the multiplicity of hit PMTs to reduce the trigger rate to a tolerable level . Second, it provides a time history readout of the PMT multiplicity on which the veto logic is based . Finally, it digitizes the individual PMT pulse heights and provides for its time history readout . The electronics is housed in a single Fastaus crate . There are 24 interchangeable Fastaus cards each of which accepts up to 16 PMT signals after being patched into two bundles of 8-conductor ribbon cable. The cylinder and shelf, the wall, and the cart each use distinct sets of cards in the Fastaus crate . The Fastaus master board, or "controller", provides all clock signals, coordinates the veto information, and prepares the analog data for readout . All synchronization is performed by, and 1/0 signals are processed through, the controller . The controller is interfaced to an LSI-11/73 host computer via CAMAC through a Standard Engineering Model 10-612 dual 24-bit 1/0 register . This read-write interface is used for all control functions including reset and changing adjustable parameters, as well as for data readout . The hardware veto is based on the number of PMTs whose amplitudes cross discriminator threshold . This threshold is adjusted by front panel potentiometers on the input cards . Each potentiometer controls a group of eight PMT inputs . In an attempt to minimize dead time by distinguishing (at some level) passing muons from those that stop in the detector, the electronics divides the signals according to the three optically isolated sections, namely the cylinder (including the attached wall), the cart, and the cart wall (hereafter referred to as simply the "wall") . If a preset number of PMT signals cross threshold on two or more of the three sections, a short veto signal is generated ; whereas if only one
15 7
section fires, a separate long veto is issued . The length of these two signals is independently adjustable and by keeping the length of the short veto small it is possible to minimize the dead time due to passing muons which would not cause a delayed trigger. For monitoring purposes, the OR of all discriminators on a card is accessible via a rear panel NIM output . Each time a PMT'S amplitude exceeds threshold it is added to the total number of PMTS for the corresponding section via a special multiplicity bus . To reduce the maximum time to add all hits on the cylinder, the 16 cards it uses are logically separated into two groups of eight . The addition proceeds independently in each group before combining the total . This division of cylinder PMTS roughly corresponds to dividing the cylinder into two halves, one on either side of the radial axis . The multiplicities of the three sections are polled in time with 12 .5 MHz veto clock and if conditions are met, the appropriate veto signal is generated . Before a single PMT signal is added onto the multiplicity bus, it passes through a removable PROM which, if so programmed, weights that PMT as more than one, or as none . If the veto conditions are again satisfied while the present veto is still in effect, the veto signal is updated . The number of PMTs which determine whether a section fires and the length of the long and short vetoes is set through the host computer via the 10-612/ Controller . A separate history clock records the multiplicity conditions in the controller in a memory which is continually updated until receipt of the trigger signal . We make the memory circular by simply over-writing the earliest address when all address are filled . We use a 6 .67 MHz history clock and a 1K RAM giving a total of = 150 ws of history . Data is stored in a single 16-bit word for each clock tick with 4-bits per section . We read out the memory through the 10-612/ Controller when operation is halted by an input trigger signal. Operation resumes when the controller is issued a restart signal either through the CAMAC interface or through a manual button . We also record the status of the veto signals in the circular memory . Fig . 6 histograms the veto lengths generated in the history or randomly triggered events. The updating veto length is shown by the fall-off at twice the long veto length . Histories of the digitized PMT signals are recorded for later readout using flash ADCs and associated RAM memories . The PMT signals are amplified, inverted, and stretched and the resulting signal is sampled by an RCA 3300D 6-bit flash ADC using a +2 V reference level and strobed by the 6 .67 MHz history clock . On each clock tick, the result is written into a circular 1K RAM memory and the address is incremented . The pulse rise time is much less than the clock period, and the pulse falls with an exponential decay time of 650 ns . This technique yields a 150 ws pulse height history for each phototube with virtually no digitizing dead time .
15 8
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
I 5
VETO
I 10
LENGTH
I 15
(usec)
I 20
Fig . 6. Histogram of the lengths of hardware generated vetoes as observed using randomly triggered data . The updating na ture of the hardware is seen by the extension of the veto length out to twice the nominal long veto length . The total volume of data contained in the ADC memories for all channels is 24 X 16 X 1K 6-bit words. This would consumer nearly ]! Mbyte at readout. However, most of the data are zeros and so the controller "compacts" the data and only data above some adjustable threshold are stored in a FIFO memory for readout. It takes less than 100 ms to compact the data from a typical event. The ADC data is read out through CAMAC as 24-bit words which specify 6-bit digitized ADC values of four adjacent channels, and the slot, quadrant (i .e. one of four four-channel blocks) and RAM address of the values . We normally run with a compaction threshold of 2 which means that for a quadrant of ADC values to be stored in the FIFO, at least one of the values must be 2 or greater. Under these conditions, the average event size to the host computer is 800 16-bit words, and 99% of the events have less than 2K 16-bit words.
usefulness of the distinction between long and short vetoes for minimizing on-line dead time . The long veto rate is rather insensitive to Mwau (the multiplicity of PMT hits in the wall) for Mwall >_ 4 while the short veto rate changes very little for all values of M wall . This is not surprising since the wall subtends much less solid angle for the predominantly vertical muons than do the cylinder and cart . On the other hand, the sharp change in the short veto rate at MCan = 5 indicates that a substantial fraction of throughgoing muons have Mc_t < 4. The long veto rate reaches a plateau at a cylinder multiplicity threshold of 5. For the majority of data taking in E645, we run with standard multiplicity thresholds of 5, 4, and 3 for the cylinder, wall, and cart respectively . The overall veto rate of = 10 kHz corresponds to a flux through the shield of = 130/MZ s. This is consistent with the measured trigger rate in the central detector of 1 .6 kHz, after accounting for the fact that only cosmic rays perpendicular to the planes trigger. Consequently, we know there is no large extraneous rate with the standard multiplicity settings . We observe a = 0.3% increase in the long veto rate during the 500 [sec long LAMPF beam gate which we trace to a 5% increase in the rate in the wall which is near the open end of the tunnel (with the water plug installed) . We attribute this effect to sky shine.
M CART _2
M CART - 3
MCART -4
MCART
5. Performance To measure the hardware veto rates, we trigger the apparatus randomly and read out the multiplicity history . Since the multiplicity history records the number of hit PMTS as a function of time over a 150 Its period we can measure the hardware veto rates as a function of multiplicity condition using an off-line analysis program . Fig. 7 shows the long and short veto rates as a function of multiplicity threshold in the cylinder and in the cart, for a variety of thresholds in the wall. Notice that rather independent of the multiplicity settings, the long veto rate is considerably larger than the short veto rate . This is apparently because a large fraction of the muons stop in the considerable thickness of lead in the passive shielding. The net effect is that it limits the
CYLINDER MULTIPLICITY
( M CYLINDER )
Fig. 7. Long and short veto rates as a function of various multiplicity settings on the three optically disconnected sections of the active shield . If a long or short veto is found by the software at a particular history location, the next = 2 Its of history are excluded . The dead time associated with the high long veto rate causes the slight increase in the short veto rate from Mcarc = 2 to Mcart = 3.
J.J. Napolitano et al. / Zarge area liquid scintillator cosmic ray detector
15 9
w w a w U
Hz O U
Q W Q
0
50
100
150
200
DISTANCE FROM PHOTOTUBE (cm)
Fig. 8 . Attenuation of light in the active shield as a function of distance from the PMT. The analog information from the flash ADCs allows us to construct more sophisticated veto conditions in software incorporating the geometric arrangement and pulse height of hit PMTS . It is also useful for studying the shield using various kinds of triggers . Using data taken with the usual three-out-of-four central detector trigger and the active shield veto off, we reconstruct tracks with the central detector and study the light attenuation properties of the shield . Fig . 8 shows the average pulse height for PMTS (corrected for track angle) in the cylinder and in the wall as a function of distance from the PMT. The attenuation is rather nonexponential but is roughly approximated by a 1/e attenuation length of 50 cm. (Based on prototype tests [6] we know that the total number of photoelectrons is on the order of 10 3 for muons near the PMT.) In our prototype tests [6] we found that the attenuation length had two components . Within 50 cm of the PMT, the prototype attenuation length was = 50 cm, but for larger distances it was = 1 .9 m. It appears that only the 50 cm attenuation length is relevant to a large device . Light reflecting from the sidewalls of the prototype may have contributed to the 1 .9 m component . By triggering randomly and searching the event history for shield pulses, we obtain raw pulse height distributions independent of the central detector. In fig . 9 we histogram the pulse height distribution for the typical PMT on the cylinder when it is accompanied by at least one neighboring PMT, when there are no neighbors, and when all neighbors are present . The figure shows a clean separation between the distributions with all neighbors and with none . However the short attenuation length washes out the separation for events with fewer neighbors since the PMT is frequently near the edge of a "cluster" of nearest neighbors .
20
40
60
PMT PULSE HEIGHT (ADC channels)
Fig. 9 . Pulse height of a typical active shield PMT . The data is obtained by searching the pulse height history of randomly triggered events . We set high voltages on the PMTS by equalizing the average random pulse height. We only consider pulses when the PMT is accompanied by at least one neighbor. We histogram the resulting average pulse height for the 360 PMTs in fig . 10a . Fig. 10b shows the average PMT rate as determined by the pulse height history . After this first order adjustment, we increase the voltage on some PMTS on the basis of a program which, using veto-on central detector triggers, tracks cosmic ray
.~
-
SAk%A i l ~
0 10 20 30 AVERAGE PMT PULSE HEIGHT (channels)
0.5 1 .0 1.5 2.0 2.5 AVERAGE PMT RATE (kHz)
Fig . 10 . Average pulse heights (a) and rates (b) (from the pulse height history) for the 360 PMTS on the active shield after gain adjustment via high voltage .
J.J . Napolitano et al / Large area liquid scintillator cosmic ray detector
160
Z ô U
N PM
Fig. 11 . Histograms of the number of PMTS in a cluster near a cosmic ray muon track for each of the four types of sections in the active shield as determined from the pulse height history. A cluster is defined to be a set of nearest neighbor PMTS . Note that the cylinder wall is optically connected to the cylinder . muons and points to areas of the shield that require somewhat higher on-line efficiency . Using veto-off cosmic ray data, we locate the PMT cluster on the shield associated with the track as determined by the central detector . We only use tracks that appear to both enter and exit the boundaries of the central detector. We histogram the number of PMTS in each cluster in fig. 11 for the cylinder, the two walls, and the cart. The number of tubes in a wall cluster is typically more than in the cylinder because of somewhat shorter overall attenuation length and because particles cross the walls at steeper angles . Clusters with one or two PMTS occur when the cosmic ray moves very close to or through a PMT. Although this results in few hit PMTS, the pulse height is very large. The number of PMTs per cluster on the cart is significantly smaller because of the cart segmentation . We see no evidence for loss of efficiency for particles passing near the sides of the individual tanks. Using dE/dx and total energy from the central detector to identify nations, we use "veto-on" data to measure the rate of muons escaping the hardware veto . For a specific set of running conditions including the standard "5, 4, 3" veto, we measure a muon rate of 0.70 s -1 . By extrapolating the tracks to the active shield, we can see if any particular sections are less efficient at detecting muons. In fig. 12 we plot the intersection coordinates of these tracks with the vertical plane defined by the (optically disconnected) wall . Most of the
points lie on a curve defined by the boundary of the wall and the cylinder . The muon rate through this region is 0.57 s -t or 81% of all muon-like triggers . The scintillator thickness of the cylinder at this boundary is only 2 in . for structural reasons reducing the net light output. We confirm this by using the active shield ADC data in an off-line analysis program and examining the pulse height in the 11 PMTS nearest this boundary. After installing specially programmed PROMS which weight each of these 11 rim PMTS as three hits instead of one in the hardware veto logic, the muon trigger rate decreases to 0.16 s -1 while the veto rate increases by only 20%. Thus we sharply reduce the major source of on-line muons while only slightly increasing the dead time . Examination of the remaining muons shows no other obvious areas of inefficiency. By comparing the residual muon rate to a veto-off muon trigger rate of 1600 s -i , we conclude that under usual operating conditions the active shield rejects muons with an inefficiency of 10 -4 using the on-line veto electronics . Over the course of long data runs, we periodically adjust high voltages on sections of the active shield, monitor the liquid scintillator levels making relevant adjustments, and experiment with different multiplicity conditions for the on-line veto . The muon trigger rate with the veto on is plotted in fig. 13 for each of many runs taken over the course of approximately six weeks during the summer of 1987 . The muon rate, being such a small fraction of the veto-off rate, is obviously very sensitive to the condition of the shield and the electronics. However, the rate of electron-like triggers, also plotted in fig. 13, is rather stable because most of them are from the decays of muons which live longer than the 10 ws veto . The rate of proton-like triggers, from cosmic ray neutron recoils, shows a steady decline as steel overburden was continually added primarily to the sides and back of the overhead area. The proton rate decreases continuously despite large fluctuations in the
z â U
-200 -300 -400
-200 0 200 400 HORIZONTAL POSITION AT WALL (cm)
Fig. 12 . Intersection points of unvetoed cosmic ray muons with the wall . Nearly all the points fall on the interface between the cylinder and the wall .
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
0.4 MUONS _ 0.3
¢+}~ }++~{} }+ +f"e°¢}}~+++ ++++++++e }ee+~~+ ++}} ++
0.2
ELECTRONS
r
s . e+ + °".° . . ."..
.1 .
+ 4+
} 0
e
00 1
*
PROTONS
0 42 DAYS
Fig. 13 . Rate of muon, electron, and proton triggers for many runs taken over a six week period. The muon rate fluctuates by large amounts since it is very sensitive to the condition of the shield . However, the electron and proton rates show no such discontinuity . muon rate since the active shield is insensitive to neutrons. Having eliminated the majority of muon triggers escaping the on-line veto by customizing the multiplicity PROMs, we examine the response of the active shield to the remaining muon-like particles . Fig . 14 histograms the number of PMT clusters in the active shield for muon-like particles which escape the on-line veto . For comparison, we also include (dashed histogram) the same distribution for normal "veto-off
103
10
I I I I I I 0
I I I I l_ 8 2 4 6 NUMBER OF CLUSTERS
Fig. 14 . Number of PMT clusters observed in coincidence with the trigger for muon-like particles which escape the on-line veto (solid histogram) and for normal veto-off muons (dashed histogram) .
muons . We see that 123 muon-like triggers out of a total of 4500 give no active shield signals. Using a veto-on trigger rate of 0 .2 s -1 (see fig . 13), this implies an unvetoed rate of muon-like triggers of = 5 X 10 -3 s - ' . The veto-off muon rate of 1600 s - ' therefore would imply an ultimate shield inefficiency of = 3 X 10 -6 assuming that all muon-like triggers are ordinary cosmic ray muons . However, it is not difficult to find processes which, at the few-parts-per-million level, could mimic muons in the central detector and give no signal in the active shield . For example, the central detector proton rate implies that cosmic ray neutrons impinge on the active shield at a rate of at least = 0 .1 s -1 . If only a few percent of these interact in the = 150 g/cm2 lead passive shielding or in the outer portion of the central detector to produce charged pions or energetic protons, this would account for the residual events we observe . It appears that the relevant cross sections are sufficiently high, although it is difficult to make accurate estimates of the rates . In any case, we consider the ultimate muon inefficiency of 3 X 10 -6 to be an upper limit. Assuming this to be the inefficiency, the background rate in our neutrino oscillation experiment due to undetected stopping muons would be =0.25/LAMPF-day . By using central detector information to improve the veto, this small inefficiency can be exploited while retaining a large detection efficiency for neutrino oscillation events . We end with a discussion of the cosmic ray neutron attenuation by the overburden . Veto-on measurements made outside of the experimental tunnel (i .e . no overburden) give a proton trigger rate of = 40 s -1 . When the apparatus is moved inside the tunnel but with the water plug (see fig . 1) empty, the rate drops to 0 .2 s -1 . Consequently the overburden attenuates the fast neutron background by 200 while the muon rate (as measured by the shield veto rate) is cut only a factor of 3 . One would expect, however, a factor of = 10 ° attenuation on the basis of measurements made of hadronic cosmic ray attenuation rates . (See the appendix .) We note, however, that the cosmic ray neutron flux is significant even to large zenith angles . Filling the water plug (800 g/cm2 of water in the horizontal direction) decreases the proton trigger rate to 0 .05 s -1 and additional shielding at large zenith angles (see fig . 13) reduces the rate further to 0 .02 s -1 for an attenuation factor of 2000 . 6. Multiple-muon detection and ultrahigh energy gamma ray astronomy We have obtained results using the active shield as a multiple-muon detector. Astrophysical sources of ultrahigh energy (E >_ 10 1^ eV) gamma rays are studied with large arrays of charged particle detectors which measure the direction and lateral extent of "air showers" produced when the gamma ray interacts with the Earth's
16 2
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
w U
ô w m z 1? a w â
0 I 2 3 4 5 NUMBER OF TRACKS IN CENTRAL DETECTOR
Fig. 15 . Average number of shield clusters as a function of the number of tracks in the central detector . The straight line assumes (Nouster = 2 x (NDet-track + 1) ; i.e. two clusters per track with one track unobserved . atmosphere [11] . Such events may be discriminated from more abundant hadronically-induced showers by identifying penetrating muons from associated qT ± decay. An air shower array of this type has been mounted at LAMPF on the grounds surrounding the target area and hence our neutrino oscillation experiment [121 . Since our active shield (as well as the central detector) is situated close to the center of the array and is beneath = 3000 g/CMZ , it serves as a suitable muon detector . Data from the central detector, triggered by the air shower array, typically show many separate but parallel tracks . Unfortunately, it is difficult to deal with the large amount of data from the central detector when operated in conjunction with the air shower experiment . Consequently it is interesting to study the use of the active shield alone as a detector of multiple muons. By acquiring some data from both the central detector and the active shield while triggering on the air shower array, we can "calibrate" the response of the active shield using the central detector to tag multiple muons. Fig. 15 plots the average number of PMT clusters in the active shield as a function of the number of tracks observed in the central detector . Also included is a straight line approximation which assumes two shield clusters per track with one track always present but unobserved in the central detector, presumably because of the reduced solid angle of the central detector . The average number of clusters saturates since for a high density of tracks, clusters tend to merge together . In any case, it is clear that some information regarding the number of muons can be deduced from the rather crude active shield information . 7. Conclusions We have built and operated a large liquid scintillator cosmic ray active shield that rejects cosmic ray muons
at a level of 10 -4 on-line. Dead time losses are approximately 10% due to the = 10 kHz veto rate and the = 10 ~.s veto length. Using digitized data from the apparatus we can reject cosmic ray muons with off-line software at a level of 3 x 10-6 or less . The device also serves as a crude "muon multiplicity" detector for a cosmic ray air shower experiment . Perhaps the biggest drawback of the final apparatus is the lack of single PMT redundancy since it is clear that cosmic ray muons which pass through a particular PMT frequently register a signal in only that PMT. Since the glass window holding the PMT projects nearly three inches into the six inch thick scintillator, the problem is largely due to the decreased scintillator thickness at the PMT location . A better design might incorporate flat instead of hemispherical PMTs . A different, albeit more expensive, approach would be to have two or more PMTS at each location . Acknowledgements We are grateful to all our collaborators on E645 . In particular, we acknowledge B. Fujikawa, Dr . J. Donahue, and Dr . V. Sandberg . We thank our contacts with the construction firms that built many of the components including : G. DiStefano, J. McCoy (PittsburghDes Moines); A Provenzano (Kopp Glass) ; W. Meyst (Northern Aluminum Foundries) ; and M. Milligan (Whiting Corporation) . Much of the PMT installation was done by R. Amrein, D. Cyborski, S. Schmidt, N. Roy, M. Madden, D. Dryer, and K. McShane. F. Bloennigen provided critical assistance during our original tune-up for data taking . On site mechanical support and engineering was provided by S. Cushing, J. Gomez, A. Kirby, J. Eddlemen, N. June, and D. Justice. J. Bittman provided considerable software support in the initial phase of the experiment . Data acquired with multiple muons was done with the help of B. Dingus, Dr . D. Nagle, and Dr . R. Cady . Finally, we acknowledge the invaluable support of G. Suazo at LAMPF for coordinating the majority of the engineering tasks associated with the project. Appendix Attenuation of neutral cosmic ray backgrounds To study the dependence of neutral cosmic rays on the amount of overhead material, we measure the rate in a shielded 10 in, long x 12 in . diameter NaI(TI) detector as a function of energy and overburden depth. The apparatus is shown in fig. 16 . The liquid scintillator counter described in ref. [61 is incorporated into the apparatus, although only the cylindrical plastic active
163
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector LIQUID SCINTILLATOR
NO VETO
MeV 15-20
WON
gw-1
61
20-30 30-40
PLASTIC SCINTILLATOR
40-50 50-60
Fig. 16. Apparatus used to measure neutral cosmic ray backgrounds as a function of overburden.
shield was used in the hardware veto . We acquire data on an event-by-event basis . We read pulse heights from all PMTs as well as the signal time relative to the Nal(TI) detector. Note that a crucial difference between the design of this system and that of the E645 shield is that here the passive shield lies outside the active shield . The apparatus is placed along with a vertical cosmic ray muon telescope on a movable stand situated in a tunnel burrowed into the side of the Los Alamos mesa. The contour of the mesa enables us to change the overburden level simply by changing the distance into the tunnel . From the telescope rate and its calculated acceptance we deduce the amount of overburden using an empirical relation [13] for the muon intensity as a function of depth from the top of the atmosphere. The agreement with the original land survey is very good. Fig . 17 shows the observed rate in the Nal(Tl) detector for different energy bins as a function of overburden depth . The data for each energy bin is fit to two decaying exponentials, with the long attenuation length component fixed to be the same for all energies . The short attenuation length varies from 322 g/cm2 for the 60-100 MeV data to 973 g/cm2 for 15-20 MeV . The longer attenuation length is = 1700 g/cm2 which is consistent with the muon attenuation as shown in the upper curve in fig. 17 . Fig . 18 shows the TDC spectrum for delayed pulses in the Nal(TI) detector relative to the triggering signal for data taken with a veto length of = 2 [,s, clearly demonstrating muon decay in the detector . By examining the time spectra of the neutral backgrounds relative to any associated signal in the liquid scintillator counter, we gain additional insight as to the source of these backgrounds . Fig . 18b plots this spectrum for the data taken with 3000 g/cm2 overburden. There should be
60-100
Fig . 17 . Attenuation of cosmic ray rates observed in the Nal(Tl) detector as a function of overburden for different energies. three components to this spectrum . One is the = 2 .2 Ws decay time of W+ whose decay positrons trigger the Nal(Tl) detector via bremsstrahlung in the lead passive shielding . Another is the = 80 ns [14] component due to W- capture in the lead . The third component is a = 1 ~Ls [14] component from muons which contribute background via capture in the surrounding rock (mostly Si02 ) . Also included in fig . 18b is a fit to three decay time components plus a constant background . Roughly
z w w ô w
I
I
I
BACKGROUND at 3000gm/Cm 2 O .I I i 0 .03 /~ s TSHORTTLONO - 0.47 ± 0.05ps
z
Fig . 18 . Time of the event in the Nal(Tl) detector relative to (a) delayed coincidence in itself and (b) the liquid scintillator detector located over the apparatus (see fig . 16). (See text for details .)
164
J.J. Napolitano et al. / Large area liquid scintillator cosmic ray detector
80% of the events have a lifetime of 110 ± 30 ns, consistent with lt capture on lead. A long lifetime component yielding a lifetime of = 0.5 p s may indicate capture in the rock, but is poorly fit. The fit does not,
however, allow any significant component from free W+ decay. Since lt+ decay may contribute background only through bremmstrahlung while lr capture can proceed via the emission of a - 1000 MeV neutron, we suspect that some of the neutral background at large depths
may be due to the latter process. References
[1] L.S . Durkin et al ., to be published; J. Napolitano, as presented in Selected Topics in Electroweak Interactions, (Feb. 1987) Proc . 2nd Lake Louise Winter Institute on New Frontiers in Particle Physics,
Chateau lake Louise, Canada, pp . 376-385; E.S . Smith, as presented in the 1986 Massive Neutrinos in Astrophysics and in particle Physics (Jan ./Feb, 1986) Proc . 6th Moriond Workshop, Tignes, France, pp . 287-292.
[2] L.A. Ahrens et al., Phys . Rev. D 31 (1985) 2732. [3] C. Angelini et al ., Phys . Lett . B178 (1986) 307. [4] S.E . Willis et al ., Phys . Rev. Lett. 44 (1980) 522; S.E . Willis, Ph .D . Thesis, Yale University (1979) unpublished . [5] R.C . Allen et al ., Phys . Rev. Lett . 55 (1985) 2401 . [6] Stuart J. Freedman et al ., Nucl . Instr. and Meth . 215 (1983) 71 . [7] Pittsburgh-Des Moines Corporation, 3400 Grand Ave., Neville Island, Pittsburgh, PA 15225, USA. [8] Whiting Corporation, 15700 Lathrop St ., Harvey . IL
60426, USA. [9] Kopp Glass Incorporated, P.O . Box 8255, Pittsburgh, PA 15218, USA. [10] Northern Aluminum Foundry Company, P.O . Box 1065, 502 Van Dyne Road, North Fond du Lac, WI 549935, USA. [ll] For example, see J. Wdowczyk and K.E . Turver, in : Cosmic Rays at Ground Level, ed . A.W. Wolfendale (Inst. of Physics, London, 1973). [12] B.L . Dingus et al ., Phys . Rev. Lett . 60 (1988) 1785 . [13] A.I. Barbouti and B.C . Rastin, J. Phys . G 9 (1983) 1577 . [14] T. Suzuki et al ., Phys. Rev. C35 (1987) 2212 .