nucleon range

Nuclear Instruments and Methods in Physics Research 224 (1984) 247-271 North-Holland, Amsterdam

TWO COSMIC=RAY DETECTORS WITH HIGH CHARGE AND ENERGY RESOLUTION GeV/NUCLEON RANGE *

247

IN THE

R o b e r t D W Y E R , Steven J O R D A N ** a n d Peter M E Y E R ** Enrico Fermi Institute, University of Chicago, Chicago, Illinois, USA Received 9 December 1983

We discuss the design and performance of two large area cosmic-ray detectors which are capable of unambiguously identifying the charge of cosmic-ray nuclei over the range helium through nickel at energies between 0.6 and 10.0 GeV per nucleon. The technique involves a combination of high resolution Cherenkov counters, scintillation counters, and MWPC hodoscopes. These components, common to both instruments, permit an accurate determination of the particle velocity over a limited, properly chosen range. Using the geomagnetic method of isotope measurement, they can determine isotopic abundances from helium through silicon. Balloon flights have been carried out with both detector systems.

1. Introduction The nuclear cosmic radiation consists of an isotropic flux of all atomic elements with a continuous energy spectrum that can be approximated by a power law. The elemental abundances, and even more those of their isotopes, are fundamental tools to illuminate the nucleosynthesis processes that produce the matter that is accelerated to cosmic-ray energies, and to unravel the effects caused by their subsequent propagation in the galaxy. Over the past years we have constructed and flown two balloon-borne cosmic-ray detector telescopes designed primarily to obtain information on isotopic abundances of cosmic-ray nuclei from He through Si ( Z = 14). This work covers a range from 1 to 10 GeV/n, using the geomagnetic method [1,2]. Since these experiments at the same time require very precise charge and energy measurements, they also yield relative elemental abundances and energy spectra over a charge range that extends to Ni ( Z = 28). To achieve these goals several principles used in the design and construction of the detector systems are novel and of interest for a wide range of charged particle experiments. This broader applicability is the motivation for this paper. Scientific results obtained with the first of these instruments have been published elsewhere. The work on mean mass measurements of the abundant nuclei in the group from B to Si ( Z = 5-14) * This work was supported in part by the Natinal Aeronautics and Space Administration under grants NGL 14-001-005 and NGL 14-001-258. ** And Department of Physics, University of Chicago. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

at 1.2 G e V / n can be found in refs. [2-5]. Relative elemental abundances are presented by ref. [6] and their implications discussed in ref. [7]. The energy spectra of the Fe and Ni nuclei from 1 to 10 G e V / n were discussed in ref. [8]. The second instrument was more recently flown near the geomagnetic equator to exploit the predicted sharp rigidity cut-off at those latitudes [9]. This is expected to provide the isotopic composition of cosmic-ray nuclei He through O ( Z = 2-8) at higher energy (5-10 GeV/n). In the following, we shall describe the experimental requirements, the design, and finally the performance of the two balloon-borne instruments.

2. Experimental requirements The requirements for an instrument capable of measuring isotopic composition using the geomagnetic field may be recognized best by briefly considering the method (for details see refs. [1] and [2]). The Earth's field acts as a rigidity filter for the incoming charged particles. It is most simply described in terms of an interval in magnetic rigidity called the penumbra: that rigidity interval below which the Earth's magnetic field allows no particles to arrive, and above which all particles arrive. Within the penumbral range, particles may or may not be transmitted since slight changes in the rigidity result in very different trajectories in the Earth's magnetic field. In general, this geomagnetic filtering depends on latitude and arrival direction. Fig. 1 shows the transmission function of the geomagnetic field calculated for two different arrival directions at Palestine, Texas and indicates the variation of this com-

248

R. DuTer et al. / Two cosmic-ray detectors

Zenith Angle=5 ° Azimuth=18oe(sOuth)

'~

4.0

4.2

llltrt [I I 4.4

4.6

418

g Zenith Angle =15= Azimuth=O °(North)

"~

oI°

4]0

I]1] Ptf]r]l]Pll

4]2

4.4

4.6

4.8

Rigidity, OV

Fig. 1. Typical geomagnetic transmission functions calculated for Palestine, Texas [10,11]. This pattern varies significantly with latitude and longitude and with arrival direction for a given location. Open areas show intervals allowed by the geomagnetic field; forbidden intervals are shaded.

plicated pattern with arrival direction at a given location [10,11]. Similar calculations at equatorial latitudes (the highest cutoffs) show a simple transmission function switching from zero to one at one cutoff rigidity with none of the penumbral structure exhibited at higher latitudes as in fig. 1. If the velocity spectrum for a single cosmic-ray isotope inside the geomagnetic field could be observed, it would accurately map this rigidity filter. The spectrum of a different isotope with higher A / Z would show this pattern shifted to lower velocities. The connection is given by R = (A/z)ttv.

(1)

R is the magnetic rigidity (in units of the nucleon rest mass), A is the nucleon number, and ~, = (1 -/~2)-1/2 where fl is the particle velocity relative to the speed of light. Contamination of a given element's velocity spectrum from neighboring elements could lead to severe systematic errors in this analysis. This determines the first experimental requirement: complete separation of adjacent element distributions. Excellent resolution of elemental abundance distributions is therefore a necessary by-product of the isotope analysis. A second requirement is very high accuracy of the velocity measurement over a specific range since all the isotopic information is contained in the shape of the velocity spectra. The range of this velocity measurement must cover the penumbral rigidity range for all values of A / Z of isotopes of interest. The latitude dependence of this penumbral range means that a given balloon flight launch location implies a specific range of cutoff rigidities. The first instrument was constructed to measure the mean atomic mass of the elements B though Si in flights conducted in Texas and Oklahoma (geomagnetic midlatitudes) and will be referred to as Instrument A in this

paper. Instrument A employed a liquid Chcrenkov counter with appropriate index of refraction t~> determine the velocities of the 3.0 6.0 GV cosmic-rays transmitted at these mid-latitudes. The second instrument, here referred to as Instrument B, was designed to respond to significantly higher energies. The initial flight of this instrument was made in Hawaii where the rigidity range of the geomagnetic cutoff is 9- 18 GV. Therefore, Instrument B uses a high pressure gas Cherenkov counter to determine the velocity distribution of these higher energy incident nuclei. Although calculations [9] predict a sharp cutoff for equatorial locations, the value of this cutoff depends on direction of incidence. This requires that Instrument B have a more sophisticated hodoscope and magnetometer system to determine the incident sky direction of all particles. The two instruments have several features in common. Both use plastic scintillation counters housed in white light integration boxes to measure the nuclear charge Z of the incident particles. The velocity measurement is made with Cherenkov radiators of different indices of refraction for different velocity ranges since the velocity resolution is best near the Cherenkov threshold. In order to minimize uncertainties due to photoelectron statistics, steps are taken to optimize the collection of low light yields from these radiators. One of the largest systematic effects that potentially broaden an individual detector's resolution is due to the distribution in angle of incidence of the traversing particles. This effect, however, can be fully corrected for from the knowledge of each particle's trajectory. Both instruments therefore have a multiwire proportional chamber hodoscope which not only permits the determination of that trajectory, but is also used to correct for small nonuniformities in the counter's response and to eliminate background events due to multiple particles.

3. Description of the instruments 3.1. Instrument A A cross section of the apparatus is shown in fig. 2. This configuration was flown on two balloon flights from Muskogee, Oklahoma in 1975. The original Instrument A did not have the plastic Cherenkov counter TO but had a thicker liquid counter T2 leading to the same total thickness in g / c m z traversed by a particle penetrating the detector telescope. This original configuration was flown twice from Palestine, Texas in September 1973 and May 1974, and was described in ref. [2]. Details on the detector specifications can be found in table 1. The main telescope scintillator and Cherenkov detectors are housed in white highly reflecting boxes each viewed by 12 RCA-4525 5" diameter photomultiplier

249

R. Dwyer et a L / Two cosmic-ray detectors

CERENKOV

COUNTER'T-O

~IMWPC A

SCINTILLATION COUNTER T-I I T IIX--f--"~'5''R MTUBES UQUID CERENKOV COUNTER T'2 II

]':":'--"--"--'-'-'--"--':.":'~7":":':~:' I~ wS:? PC

B

3.1.1. Scintillation d e t e c t o r s - T1 and T3

III ~:----"------"-"----------~---w--P--"c-------c-i

~-'/-i SCINTILLATIONCOUNTER T-3

"---t L~

2"PMTUBES

~'~'GUARD COUNTER G

I ,K ,L I 0 25 CM

Fig. 2. Schematic cross-section of Instrument A. MWPC = multiwire proportional chamber. Each MWPC measures both the X and Y coordinate of the traversing particle.

tubes (PMT). The advantages of diffuse reflecting boxes over light pipes is two-fold: the light collection is more efficient (crucial for the low light yield Cherenkov radiators) and it is more uniform (the signal from the sum of the twelve tubes exhibits smaller variations among particles traversing different areas of each detector). Almost all the light collected by the PMTs has been reflected at least once. If a is the fraction of the interior surface area of the detector box covered by the PMT faces, and/~ is the fraction of incident light absorbed by the wall surfaces on each reflection then, assuming the PMTs absorb all the light falling on them, the fraction of the emitted light which is collected by the PMTs is

/=~

(1-~0"(1-.)"=

+ ,

where the n th term in the series represents the fraction of the light collected after n reflections. In Instrument A, a = 6%. The different reflecting materials used and their corresponding absorption coefficients # will be discussed below.

(2)

In the original configuration of Instrument A (TO absent), the identical scintillation counters T1 and T3 were to provide the measurement of the nuclear charge Z of the particle. Redundant measurements before and after traversal of the relatively thick T2 Cherenkov counter were made in order to reject nuclei which had suffered a charge-changing nuclear interaction, thus confusing the velocity measurement in T2. The addition of TO does not change this; however, consistency among more detector signals can now be required. The scintillation detectors are square sheets of Pilot Y plastic scintillator 68.6 cm on a side and 1 cm thick. Pilot Y was chosen because it was thought to have less undesirable saturation of the signal from high-Z nuclei. Its light output only gradually deviated from a Z 2 response so that adjacent charges, even for high-Z, could still be unambiguously distinguished. The homogeneity of the commercially available plastic scintillator sheets was very good, leading to high uniformity of response across a single sheet. Due to the large light yield from the plastic scintillator, reflectivity of the light collecting box walls does not have to be maximal in order to keep fluctuations in the number of recorded photoelectrons below Landau fluctuations in the energy lost. The reflecting material lining the inside of T1 and T3 was an insulating beadboard made in closed-cell sheets by Dow Chemical Company [12] with an absorptivity coefficient estimated to be # = 6%. The surface of the scintillator was lightly frosted by sandblasting to

Table 1 Instrument "A" detector specifications Detector

Type

Thickness cm

TO T1 T2 T3 G MWPC,A,B,C hodoscope

Pilot 425 plastic Pilot Y plastic scintillator Dupont Freon fluorocarbon liquid type E-2 Cherenkov radiator Pilot Y plastic scintillator NE110 plastic scintillator Multiwire proportional chamber Ar-Co2 (80%-20%)

Geometry factor = 2500 cm2sr Total power requirement = 52 W Total weight of instrument including pressure shell = 1050 kg a) In early configuration (Texas balloon flights). b) In later configuraton (Oklahoma Ballon flights).

1.1 bi

g/cm2 1.3 b)

1.0

1.0

2.5 a) 1.9 b)

4.2 a) 3.2 b)

1.0

1.0

0.7

0.7

5.0 each

< 0.1

250

R. Dwyer et aL / Two cosmw-rcO, detector~

reduce total internal reflection of the light. The 1 cm thickness of the scintillator is a compromise to minimize both Landau fluctuations in the energy lost and nuclear interactions in the material of the detectors. 3.1.2. C h e r e n k o v radiators - T2 a n d TO

The liquid Cherenkov counter T2 is the sole Cherenkov counter in the original instrument configuration. Good energy resolution at or above 1 G e V / n requires a Cherenkov counter which has the lowest index of refraction in a suitable liquid. D u p o n r s Freon fluorocarbon liquid Type E-2 [13], a heavy, colorless fluid (density = 1.66 g / c m 3, molecular weight = 452.1) has an index of refraction n = 1.257 (Na D line). It is also very stable and transparent through the visible and ultraviolet where much of the Cherenkov radiation is emitted. The transparency of 2.5 cm of liquid was measured to be more than 90% from visible wavelengths down to 2300 A for the Laser Grade purity we used. A cross section of the liquid counter T2 is shown in fig. 3. The liquid volume is 69 x 69 cm 2 and 2.5 cm deep. The sides are made from a solid piece of magnesium. The botton is machined to a honeycomb pattern to reduce the grammage in the aperture of the telescope while still retaining strength to support the 20 kg of liquid. It is easy to underestimate the difficulties involved in using a liquid as one of the radiators in a balloon-borne experiment. Considerable precautions were taken to maintain uniformity in radiator thickness throughout the flight in the presence of changes in temperature and potential deviations from level suspension of the experimental package, e.g. as ballast was dropped. The nominal thickness is maintained by a 0.25 mm transparent mylar lid stretched very thighly across the top of liquid container. Connected to the main volume of the liquid

RP~

RC..

RF

Fig. 3. Cross-section of T2 liquid Cherenkov counter, Instrument A. M = main frame. B = bottom, makes O-ring seal to main frame. Both M and B are made of magnesium. Excess material is machined away to save weight without sacrificing strenght. O = O-ring groove. L = main constraining lid for liquid, 0.25 mm clear (Type S) mylar stretched very tightly. F = main lid frame which holds the tension in L. O-ring around every holddown screw. RC = reservoir holddown cover, 0.05 mm mylar epoxied very loosely onto R F = reservoir frame. RP-reservoir cover plate, painted with BaSO4 white paint. The reservoir is connected to the main volume via 20 channels around the perimeter of the main volume.

is a reservoir (see fig. 3) which is designed to take up any volume changes occurring due to temperature variations that occur during a balloon flight of two days duration. The reservoir is covered with a 0.05 mm mylar lid which is "floppy", i.e. has no tension and. in fact, has excess material so any liquid expansion will fill the reservoir volume instead of bulging the main volume mylar lid. Before flight, tests were conducted which verified that the reservoir worked as expected. We immersed the entire liquid counter assembly in water and varied the water temperature over a range exceeding that encountered in the flights. While the reservoir volume changed appreciably over this temperature range, the main radiator thickness changed by less than _+0.05 mm ( + 0.2%). To guard against tilting of the suspension axis of the gondola during flight, we measured and shimmed the liquid container until it was level with respect to an exterior flange surface on the gondola pressure shell and then suspended the gondola before flight such that this flange surface was within + 1° of being level. A t o tilt had previously been shown to cause less than 0.3% change in liquid counter thickness anywhere over its surface. During the flights, ballast, weighing up to 25% of the suspended weight of the experimental package, was strapped under the instrument's center of gravity and a check was made that the addition of the ballast hopper and changes in its weight did not change the leveling of the flange surface. As an added check that the instrument remained level during the balloon flight, a circular bubble level was photographed once a minute. To further constrain the main mylar lid and thus the liquid, four symmetrically placed posts held the lid to the nominal 2.5 cm thickness. However, since the mylar lid ruptured at the posts, spilling the liquid upon termination of the first balloon flight and impact on the ground, these posts were removed in the later flights from Oklahoma. The dominant source of signal fluctuations from the Cherenkov counter was due to the small number of photoelectrons produced in the PMTs. To increase the light output in the region of sensitivity of the PMTs a variety of wavelength shifting organic chemicals was tried in a number of ways, e.g. on the PMT faces, on the walls of the light diffusing box, on the mylar lid, and dissolved in the liquid. Although the solubility was low, dissolving a wave shifter in the liquid produced the most reliable improvement in the photoelectron yield. A variety of wavelength shifters (sometimes in combination so as to "cascade" the light into diffferent wavelength regions) was tried. Optimum was 0.9 g / l (saturated) of 2,5-diphenyloxazole (PPO) which increased the PMT output by a factor of 2. Over the wavelength range obtained by folding in the Cherenkov emission spectrum with the absorption of the wavelength shifters, the refractive index of the liquid changes

251

R. Dwyer et aL / Two cosmic-ray detectors

only very slightly so dispersion is not an important factor. The white reflecting material for T2 in addition to being on the interior box walls must also be on the inside surface of the liquid volume, in contact with the liquid. Its reflectivity must be as high as possible to collect the maximum amount of light. We chose a homemade paint made from BaSO4 powder suspended in ethyl alcohol with polyvinyl alcohol as binder [14]. Prolonged contact between the E-2 liquid and the painted surface showed no degradation of either liquid or paint. We conducted relative reflectivity tests on a variety of BaSO4 paint recipes, various millipore filter paper types and Dow beadboard and found BaSO4 to have the highest reflectivity over the range of sensitivity of the RCA-4525 PMTs. It's absorption coefficient /~ was then measured absolutely in our laboratory to be /x---4%. After final assembly of the instrument, extensive tests with sea level muons showed that - 50 photoelectrons are produced by each traversing Z = 1, fl = 1 particle. The TO plastic Cherenkov counter (see fig. 2) was added for the two 1975 balloon flights from Muskogee, Oklahoma. It consists of a square sheet of Pilot-425 lucite with wavelength shifter and has a nominal refractive index of 1.49 (Na D line). In order not to limit the instrument's opening angle or geometry factor, the dimensions of TO are 75 cm on a side, slightly larger than the 69 cm sides of the T1 and T3 scintillators. Its light diffusion box has BaSO4 paint as reflecting material, like T2. About 35 photoelectrons are produced from the passage of a Z = 1, fl = 1 particle through that radiator which is 1.1 cm thick. Its index of refraction makes it sensitive to energy measurements from 0.3 to about 3 GeV/nucleon although we used it predominantly for the measurement of nuclear charge Z.

tory with each T detector to correct for nonuniformities in response across their area to otherwise identical particles. Fig. 4 shows the cross section of one of the MWPCs. Each wire chamber A, B, and C has both an X and a Y plane of signal wires so that for an ideal trajectory 3 (X, Y) coordinate pairs are obtained. There are 64 wires per plane, spaced 1 cm apart. The stainless steel anode wires [15], 50/~m in diameter, are stretched on a square magnesium frame, tied to spring at one end and held by epoxy at the other. Each signal wire has its own amplifier and two-level discriminator. The high voltage planes are made of 50 #m mylar sheets stretched and glued across square magnesium frames. They are sprayed with a conducting paint except for a 1 cm wide gap around the frame edge. Some of the high voltage planes were sprayed with Electrodag [16], which uses silver as a conductor. Others were painted with AquaDag [16], a graphite based electrically conducting paint, which is transparent to the 55Fe 5.9 keV X-rays used to measure the gain on the wires. The gas mixture in the proportional chambers is A r - C O 2. The CO 2 proportion is (20+ 1)%. During flight a 60 1 gas cylinder provides a steady flow of - 1.5 l / h through the chambers. To prevent back diffusion the gas is vented through a 16 m coil into the gondola pressure shell. When all gas is expended the pressure in the shell builds up by less than 10% over 1 atm. Tests showed that no degradation of the gain on individual

M..

•

.

3.1.3. The M W P C hodoscope

The multiwire proportional counter hodoscope has been included in the instrument to determine the trajectories of all particles. This information is used in three ways: (1) To eliminate "background" events. At float altitude, under a residual atmosphere of 3-5 g / c m 2, interactions of the copious cosmic-ray proton and alpha-particle flux (a factor - 100 more abundant than all Z > 2 particles) produce a significant background of events which can masquerade as single, unaccompanied nuclei with Z > 2 in the T counters, but which do not show straight line trajectories in the MWPC hodoscope. For those events which have a unique straight line trajectory, this information is further used to: (2) Calculate the zenith angle of incidence on the plane parallel detectors to correct each T counter pulse height to that of normal incidence. (3) Calculate the point of intersection of the trajec-

M'-"'~

HV-~

MG

_ _- Fig. 4. Exploded view o f cross-section o f a multiwire proportional chamber ( M W P C ) for Instrument A. M G = magnesium frame for holding M = 0.05 mm stretched mylar on which is painted H V = electrically conducting paint for the high voltage planes. L = lucite pieces on wl'dch the individuals anode wires (W x = x-plane and Wy = y-plane o f wires) are strung. A t the other end (not shown) the wires are tied and epoxied to a spring anchored to the lucite. GS = brass guard strips (at

ground potential) for smooth termination of high voltage at chamber edges. PC = printed circuit board to which anode wires are epoxied.

252

R. Dw,ver eta/. / Two cosmic-ray detectors

wires occurred at this flow rate. At the time this instrument was built, our laboratory used a data transmission system which could handle a maximum rate of 700 bits per second. This constrains the information that can be recorded from the M W P C hodoscope for each event. Therefore, individual wires were not pulse height analyzed; rather, a wire was considered " s e t " if the signal on the wire was above a preset discriminator level. In scanning across each of the wire planes, the address of the first set wire was read out (6 bits) plus 2 more bits indicating whether the neighboring two wires were also set. If another wire farther along in the scan across the plane was set, its address was also read out along with the state of the next two adjacent wires. Finally, an excess hit was set if any further wires were set in that plane. In the course of analysis, events with this final excess bit set in any of the six wire planes were eliminated. This method of readout was chosen because a high Z nucleus in traversing the instrument material may generate delta-rays (knock-on electrons) which might deposit enough energy to fire a wire not associated with the trajectory of the incident nucleus. 3.1.4. A uxiliary components

Some overall features characterizing the operation of Instrument A are to be noted here. The pulse height analysis for each of the T counters was provided by 2000 channel linear analyzers. The digitization of these pulse heights took _< 500 /~s. The scan across the wire chamber planes and digitization of the address of the wires that were set took 4 p s / w i r e with scans in all six planes taking place simultaneously for a total time of 256 /~s. There was essentially no dead time for these processes. Recording of the data took 250 ms per event however. To reduce the dead time associated with this process the instrument was equipped with three buffers, in turn feeding three separate onboard tape recorders. The data going to one of the recorders (one-third of all data) were simultaneously telemetered to the ground where instrument operation was monitored. Once each minute the instrument switches into a housekeeping mode, transmitting time, temperatures, pressure, and various singles and coincidence counting rates in a 160 ms burst. Two independent systems were used to measure the pressure of the residual atmosphere during the flights. They agreed within < 0.1 mm Hg with each other throughout the flights. To be able to determine the arrival direction of each particle on the celestial sphere, a magnetic compass was photographed once a minute as part of one of the pressure measuring systems. The master coincidence mode for the flights of Instrument A was T 1 . T2. T3 - ( A , + A , . ) . ( C , + C , ) with a resulting geometry factor of 2500 cm2sr. The total power requirement for the instrument was 52 W plus 14

W for the transmitter supplied by lead acid automobile batteries flown in a separate power pack. The instrument itself is housed in a pressurized shell which mai,atains a 1 arm dry nitrogen environment. 3.2. Instrument B

The second instrument is designed to mesure isotopic composition at rigidities from 9-18 GV with the geomagnetic method. Using this method for those high rigidities requires exposure near the geomagnetic equator. The expected absence of a penumbra in the geomagnetic cutoff [9] leads to the expectation that one may observe individual isotopic abundances rather than mean masses derived from comparison with an element of known isotopic abundances as done by Instrument A. This expectation had indeed been borne out. The principles that guide the design of this instrument are very similar to those followed for Instrument A. The major difference is the choice of a gas Cherenkov counter that covers the higher energy range. The size of this detector is determined by the requirement that resolution be maintained at the high particle velocity even though the Cherenkov light yield per centimeter in a low index of refraction gas counter is much less than in a higher index liquid counter. A cross section of this instrument is shown in fig. 5 and its technical specification given in table 2. The detector was flown once from Kauai (Hawaii) on April 7, 1981. To maximize the geometry factor of the telescope which is housed in a cylindrical pressure shell, all counters (except the Cherenkov counter) have octagonal shapes. Altogether, the instrument consists of two scintillation counters (T1, T2), two Cherenkov counters (C1, C2), and four multiwire proportional chambers (PI P4), HELD{ 1.4 m. J ',,i.~ . . . . .

5"P M T ~ [16} ~

2,2

"'

,,,

Scintiltotion Ti

~Counter

~ . . . .

!. . . . .

m

-

MWPC(X,Y) ']}1 1 -

2 '

. . . . .

. . . .

Gos Cerenkov Counter CI

~--

5"PMT(24)

-. . . . .

GoS Cerenkov

Counter C2

MWPCIX,Y) Sclnt~llotion

5" PM

H6)

Fig. 5. Schemanc cross-section of Instrument B.

CounterT2

R. Dwyer et aL / Two cosmic-ray detectors

Table 2 Instrument "B" detector specifications Detector

Type

Material

Thickness cm

T1, T2 CI, C2 PI-P4

Scintillator NEll0 Cherenkov Ethylene MWPC Ar/CO 2

1.0

70 (mean) 2

g/cm 2 1.0

2.3 <0.1

Geometry Factor = 2100 cm2sr Total power requirement = 200 W Total weight = 1860 kg

each of which provides x and y coordinate information on the particle trajectory. The geometry factor is 2100 cm2sr with a maximum half opening angle of 35 o. The average inclination of particle trajectories with respect to the instrument axis is 13.7 ° . In the following, we describe individual features of the detectors. 3.2.1. Scintillation detectors T1 a n d T 2

The two telescope scintillators, placed in light integration boxes painted with high reflectance white paint (Kodak BaSO4 #6080), are each viewed by 14 RCA4525 5" photomultipliers. Two additional RCA-4522 fast photomuitipliers per box are used for time of flight measurement between these scintillators. The scintillator material consists of octagonal plastic sheets of N E l l 0 , 1 cm thick, and 107 cm across, machined fiat, and finely frosted. There is an 11.5 cm gap between the edge of the scintillator and the walls of the integration box to improve light collection uniformity from the entire surface of the scintillator. The signals from the photomultiplier tubes are summed into four groups to provide four independent measurements of the light from each scintillator. Consistency between all eight scintillator measurements is demanded for each event before final summing to determine the charge of the particle. This consistency requirement proves to be a powerful technique for background rejection. Due to the relatively large size and short distance between the two scintillators, two time of flight (TOF) measurements are needed to accurately determine the TOF between the two scintillators [17]. The lo with of the time of flight distribution for He-particles is approximately 1 ns. Since upward and downward moving particles are separated by a mean of 10 ns, this turns out to be valuable for background rejection in the measurement of particles of low Z. Other features of these detectors are similar to those of Instrument A. 3.2.2. The Cherenkov counters C1 and C2

The range of energy for which Instrument B must provide accurate velocity measurements is from 4 to 10 G e V / n . This requires a Cherenkov counter with index

253

of refraction n = 1.021. Taking into account the light collection efficiency of a container with high reflectance white walls of the required diameter, the quantum efficiency of the PMTs, and the number of PMTs, a design length for each of the two Cherenkov counters, C1 and C2, of about 70 cm is required. In such a counter we expect 70 photoelectrons per traversing singly charged relativistic particle. The use of silica aerogel [18] in this application is ruled out due to aerogel's large internal light absorption and nonuniformities. We chose ethylene at 350 psi pressure as the counter gas from candidate gases like freon, SF6, and others. Ethylene exhibits no measurable scintillation and does not condense over the ranges of temperature and pressures encountered. Ethylene's index of refraction shows no significant change from 2500 ,~ through visible wavelengths. A complete filling of gas in both counters increases the weight of the instrument by 108 pounds and represents a layer of 4.6 g / c m 2 to a traversing particle. A flammable Cherenkov gas at 350 psi needs special housing. In Instrument B it is enclosed in a cylindrical aluminum container of height 148 cm, and diameter 116 cm with the ends consisting of two torispherical titanium alloy lids, each of 2.8 g / c m 2 thickness, bolted to the cylinder by a special flange. The construction follows the ASME Boiler code [26] using a safety factor of four. Twenty-four quartz windows, 3.2 cm thick, are inserted into the cylindrical wall and each is optically coupled to a 5" RCA-4522 PMT with partially UV transparent windows (9741 glass). The optical coupling between quartz windows and PMTs is achieved by a layer of silicon oil that shows high UV transparency and represents a good match between the indices of refraction of quartz and the multiplier window material. The quartz windows take up 2½% of the counter's surface area. Painting the inside walls with a white inorganic BASO4 paint preparation [19,20] results in the Cherenkov light being collected in a nondirectional, diffusive mode. A flat, thin light baffle separates the Cherenkov counter gas optically into two separate, equal volumes producing two independent Cherenkov counters C1 and C2 of the same dimensions and index of refraction. Consistency between the two Cherenkov counter pulses can be required before they are added in the analysis. This feature is used in the background rejection. To further reduce background, the signals from the individual PMTs are summed into four groups for each counter to provide four independent measurements of the Cherenkov light. Consistency requirements in the pulse heights between the four segments eliminate cases where a secondary particle may enter the quartz window of one PM tube, thereby causing locally an unduly high pulse. The pulse heights of the eight segments of both counters are added for acceptable events. The high pressure gas Cherenkov counter is an un-

254

R. Dwver et al. / Two cosmic-ray detector*"

usual device for a balloon-borne instrument. We therefore describe some of the precautions that were taken to assure safe performance. As mentioned, the gas container was designed following the A S M E boiler code. Prior to installation, all quartz windows were mounted in a special fixture in the same manner as in the pressure tank and tested at a pressure of 700 psi, two times the maximum pressure in the vessel. The quartz windows were mounted and sealed in the pressure vessel with Teflon piston cups. Once assembled, the entire counter was hydrostatically proof tested to 525 psi and then cycled 100 times between 0 and 350 psi. Since ethylene gas is flammable, all oxygen was removed from the gondola shell and dry nitrogen flushed through the shell at all times on the ground. This prevents any possible buildup of a combustible gas mixture should there develop a small leak in the pressure vessel. Careful monitoring of ethylene contamination in the gondola shell revealed no such leak. Although the pressure vessel can easily withstand the shock at impact after parachute descent, we chose to release most of the ethylene prior to cutdown. For this purpose, a command was available to open a valve that releases the gas in the pressure vessel to the outside atmosphere. A check valve insured that 2 atm of residual gas pressure remained in the pressure tank at all times. This is to avoid the possibility that, during descent, the tank could ever reach a pressure level below one atmosphere and subsequently take in air, creating a combustible mixture. 3.2.3. The M W P C

hodoscope

Instrument B uses four multiwire proportional chambers of a novel type. Each of them permits coordinate determination of the particle trajectory in x a n d y direction with only one plane of wires. Also, for every traversing particle the energy loss in each chamber gas volume is measured. The chamber frames are octogonal in shape with the same dimensions as the scintillation counters (108 cm across). The 108 wires of 50 t~m diameter are individually suspended at 1 cm spacing on springs to give uniform tension and to prevent wire breakage in shipment or in-flight operations. The chamber lids are made of stretched 0.05 mm thick aluminized mylar sheets a distance of 2 cm apart from each other, twice the wire spacing. The aluminum is etched away for 0.4 cm near the chamber frame, and insulating lines are etched across the windows to subdivide them into 6 cm wide conducting strips. Both chamber windows are etched in the same manner but with the strips running perpendicular to each other. Positive high voltage is applied to the chamber wires and the window strips are used for the electronic readout of the pulse height. These perpendicular strips permit the gathering of simultaneous x and y coordinate and pulse height information with one chamber. The width of 6

cm for the strips was chosen to provide the accuracy in the trajectory position that is necessary to adequately correct the pulse heights in the scintillation and Cherenkov counters for pathlength variations and any nonuniformities in these counters. This strip width results in a residual pathlength uncertainty in the scintillators which is typically 0.9% and never exceeds 1.8%. The gas for the MWPCs is the same as in Instrument A, a mixture of 80%-20% of Ar and C02. During flight each chamber is continuously purged at 1.4 I/h. The gas is vented into the gondola shell through a long tube to avoid back-diffusion of the N 2 that fills the shell. 3.2.4. A u x i l i a r y

The data handling and transmission system of Instrument B is radically different from that used in Instrument A. It takes full advantage of an improved ground support system to transmit a serial data stream of 80 k b / s , two orders of magnitude greater than for Instrument A. This allows transmission of considerably more information for each event, resulting in improved background identification and rejection. As an example, a typical He nucleus traversing the instrument results in 768 transmitted bits vs. 182 bits/event for Instrument A. To achieve high efficiency and flexibility an onboard microprocessor ( D A T A General ~Nova) was used to collect, format, and transmit all data relating to each event. Every 3'~ min the microprocessor would also collect and transmit housekeeping information. This included temperatures, pressures, counting rates, instrument configuration parameters, altitude, dead-time, bus voltage, and information concerning the #Nova's own performance. The microprocessor could also send a block of diagnostic information if any anomalous conditions were detected in the instrument during flight. For the operation of the instrument, several alternate master coincidence modes could be commanded from the ground. A non-restrictive master coincidence of TI + T2 > 2M and T1 and T2 each - 1 / 2 M was found to be adequate where M is the most probable signal from a minimum ionizing particle with Z = 1. This eliminates most protons and includes all particles with charge Z > 2. When an event satisfying the master coincidence occurs, charge sensitive amplifier signals from all counters and M W P C strips are locked in by a peak sensing track and hold system. The stored counter analog signals are then multiplexed int their associated linear 10-bit ADCs, stored temporarily in a FiFo buffer, and then placed on the data bus going into the 10-bit ~ N o v a port. Simultaneously, four independent scans are ~arted for the MWPC's to find which of the 144 strips were fired above threshold. An M W P C strip is pulse height analyzed and read out only if it had a signal greater than half of that which would result from a minimum ionizing Z = 1 particle. This strip threshold is

255

R. Dwyer et al. / Two cosmtc-ray detectors

automatically raised to half the signal of a Z = 2 particle if the scintillator's signal indicates a traversing particle with Z > 3. Threshold shifting allows us to eliminate 8-rays produced by higher Z particles. An unaccompanied particle passing through the middle of one x and one y strip in each MWPC results in a total of eight strips being pulse height analyzed. Occasionally two strips will share the charge produced by a particle passing between them. This phenomenon is in fact used to more accurately define the trajectory based upon the ratio of the signals in the two strips. Because of this charge sharing and the generation of &rays, a cosmic-ray proton fires an average of 9 strips, helium and average of 12, and carbon an average of 16 strips. The energy loss in the MWPC gas increases as Z 2. Therefore, to increase the dynamic range a square root amplifier precedes each of the MWPC ADCs, resulting in a pulse height signal proportional to charge. Each chamber has a dedicated 8-bit ADC which places the address and pulse height of any fired strip into a FiFo. This information is then placed on the data but going to the 8-bit #Nova port. Every event produces a fixed amount of pulse height information from the scintillators and Cherenkov counters but a variable amount of information from the MWPCs. Therefore, the data collection system is naturally divided into two sections, each entering the #Nova through a separate port. The first port is labelled the 10-bit port because all scintillators and Cherenkov counter signals are pulse height analyzed by four independent 10-bit (1024-channel) linear ADCs. The second port is the 8-bit port since each MWPC has an 8-bit (256 channel) ADC. These two ports use direct memory

access to place information into the #Nova's memory, saving the processor overhead. Following storage of all pulse height and MWPC data, the processor accepts the time of flight, the instantaneous orientation of the instrument provided by magnetometers, time, and certain operating mode information. These data are formatted into a compact form for transmission. If the particle had a Z > 3 the data are both recorded on the flight tape recorder and telemetered to the ground in PCM encoded form on a 2.2 GHz ratio link. Particles whose Z is less than 3 are only transmitted to the ground where they are recorded. The entire #Nova data handling system uses 70 W from the instrument's 200 W power budget. For balloon flights from Hawaii, the axis of Instrument B is suspended at a fixed zenith angle of 15 ° with the ability to vary its azimuthal angle. A set of magnetometers controls a torque motor housed within the suspension system. Commands from the ground cause the experiment to be oriented into and maintain a desired azimuth angle to within a few degrees. The azimuth is chosen to point the instrument to that part of the sky that has the most desirable range of geomagnetic cutoffs. Unfortunately, a bearing failure during the first flight made this orientation system inoperative.

4. Balloon flight details Instrument A has been flown in four successful balloon flights using 20.8 million cubic foot balloons. The first two were launched in September 1973 and May 1974 from Palestine, Texas where the geomagnetic

Table 3 Balloon flight summary Instrument and flight #

Launch site (nominal cutoff vertical incidence)

A1

Palestine, Texas (4.5 GV)

A2

Palestine, Texas (4.5 GV) Muskogee, Oklahama (3.4 GV) Muskogee, Oklahoma (3.4 GV) Kauai, Hawaii (12.8 GV)

A3

A4

B1

Launch date

Duration at float (h)

M e a nresidual atmosphere (g/cm2)

Exposure factor (good data) (m2sr h)

Comments on balloon trajectory

9/29/73

41

3.8

10.3

5/07/74

36

3.6

9.0

9/23/75

44

4.7

10.5

10/01/75

33

3.6

7.8

12.4

5.1

2.6

drifted to lower geomagnetic cutoff (3.4-4.6 GV) drifted to higher cutoff (4.3-5.8 GV) range of cutoffs (3.0-4.2 GV) nearly constant cutoff ( - 3.3 GV) nearly constant cutoff

4/07/81

256

R. Dwyer et al. / Two cosmic-ray detectors 5.1.1. B a c k g r o u n d rejection

,°

C+O NUCLEI

o

:

. , " .

Palest,he Cuto!f,EI

• : Arkonsas

."

Cutoff. FI

...""

0

0.2

0.4

J~

0.6

e

0.8

1.0

Cerenkov Sign01/Z ~

Fig. 6. Cherenkov signal spectra observed with Instrument A in flights from Texas launch location. The lower velocity particles allowed at the lower (Arkansas) cutoff are evident.

cutoff rigidity for vertically incident particles is 4.5 GV. The latter two were flown from Muskogee, Oklahoma (vertical cutoff of 3.4 GV) in September 1974. Details on the flight history are given in table 3. During the flights the balloon drifted over a range of cutoffs. This is reflected in changes in the Cherenkov signal spectra. Fig. 6 shows the T2 liquid Cherenkov signal spectrum for C and O nuclei observed in the beginning and at the end of the first Texas flight (" Palestine cutoff" and "Arkansas cutoff", respectively). The change in the geomagnetic cutoff with latitude is evident. Instrument B has been launched twice from Kauai, Hawaii where the vertical geomagnetic cutoff is 12.8 GV. Because the first flight stayed within 50 miles of the launch site for 12.4 h, there was no variation in cutoff with latitude. This flight unfortunately had to be terminated prematurely because of a malfunctioning ballast system. Details of this flight are given under flight B1 in table 3. During ascent of the second flight the Winzen 28 million cubic foot balloon burst. The equipment was recovered after several days in the ocean without damage. We are currently planning further flights of this apparatus from Hawaii in Spring 1984.

5. Performance of the instruments 5.1. I n s t r u m e n t A - trajectory m e a s u r e m e n t

This section discusses the performance of the M W P C hodoscope for trajectory measurements with respect to each of its three uses mentioned in section 3.1.3: 1) Background rejection. 2) Pathlength correction due to variations in zenith angle of incidence. 3) Corrections for nonuniformity in response of the counters.

For the Texas flights the discriminators were set to accept nuclei with Z > 3. Here the M W P C hodoscope was of primary importance in selecting events due to cosmic-ray nuclei and eliminating side showers and other background. In the later flights from Oklahoma, the discriminators were set for Z > 6 and the fraction of background events was smaller. We conclude that the M W P C hodoscope was crucial for background rejection and resulted in significant improvement over earlier experiments that did not include such a hodoscope. The low discriminator setting ( Z > 3) led to a problem when a high Z nucleus such as iron ( Z = 26) traversed the instrument. To discriminate against cosmic-ray protons and alphas, each wire discriminator was set at a level about six times the signal from a Z = 1, /3 = 1 particle corresponding to a - 30 keV energy loss in a chamber. A cosmic-ray iron will lose - 5 GeV in passing through the T2 Cherenkov detector. A fraction of this energy is transported via 6-rays to fire wires not associated with the main track. Since we were limited to distinguish a maximum of six wire firings per plane, the efficiency for primary track identification was a decreasing function of Z. Table 4 shows the fraction of events for various nuclear charges that show a straight line in the M W P C hodoscope observed in data from the balloon flights. No other selection criteria have been applied to the numbers in the table. The decrease in the fraction of events yielding straight lines in data from the Texas flights is due to an increase in the number of events that fire too many wires to extract the actual particle trajectory. On the second flight from the Texas launch location, circuitry was added that delayed the scan of the wire planes by an amount proportional to the pulse height in

Table 4 Percent of all events registered during flight that show a unique straight line trajectory in the MWPC hodoscope - Instrument A Element

Texas flights ~)

Oklahoma flights h~

B C N O Ne Mg Si ('a Fe

61.4 8O.3 61.0 77.6 54.6 58.6 49.5

83.8 93.6 92.8 94.5 95.4 91,4 93.7

") Discriminator threshold set at - 6 times signal from a minimum ionizing Z = 1. 36 times signal from a b) Discriminator threshold set at minimum ionizing Z = 1.

257

R. Dwyer et al. / Two cosmic-ray detectors

the plastic scintillator. In this scheme, the signal on the wire sensing the primary particle track would be the largest for highly ionizing particles and hence would still be above the fixed discriminator threshold after other wires with spurious signals from knock-on electrons had fallen below that level. The results indicated that the system worked as planned; however, by the time Instrument B was being designed, the introduction in our laboratory of a new high speed data readout sytem allowed pulse height analysis of all wire chamber signals. This permits identification of delta rays post facto and yields the trajectory of the nucleus over a wide range of Z. In the flights from Oklahoma, we chose to focus on the higher Z elements including iron and therefore set the wire chamber thresholds at above the signal given by a minimum ionizing carbon ( Z = 6). At this level particle trajectories were clearly selected with full efficiency independent of Z from oxygen to nickel ( Z = 8-28).

different parts of the radiator, presumably due to the specific geometry of the r a d i a t o r / b o x / P M T face configuration a n d / o r surface variations in reflectivity in the box.

-y a

0,3

24

-0.I

-0.9

-0.9

48

0.9 -0A

- 0.3

-1.0

40

32

-0.6

-0.8

-0.7

-0.4

-0.2

-0.8

-0.4

+0.3

+0.1

-0.9

+o.I

56

1.0,t+0.0 +1.7 0

-0.7 + 1.6

0

+0.1

1"0.3+1.3

+0.3

+0.5

+0.7

+0.5

+0.7

+1.2 +1.6

16 -I.I 24 -1.3 32

+0.7

-1.2 40 -1.2

-I.0

-0.3

+0.2

+0.7

+0.4

+0.8

-0.6

-0.5

; +0.1

+0.7

+0.6

48

5.1.2. Pathlength correction

This correction is important since the wide acceptance angle of the telescope (43 ° edge to edge, 52 ° corner to diagonally opposite corner) results in a significant broadening of the resolution (in terms of the standard deviation). We did not make any assumption about this variation; rather it was verified in the data that the pulse height from a given radiator increased, as expected, as the secant of the zenith angle of incidence. The relative standard deviation of this distribution is 7.6% which would broaden the distribution for a given element by this fixed amount, independent of Z. For a signal response proportional to Z 2, the relative interval width for a given charge distribution is + 1, where 1 Z(Z+I)-Z(Z-1) I = 2 Z2

I

+0.3 X

16

8

0

-0.8

+1.2 F2.8

56 +0.2

-0.6

-0.8

+0.3

-0.4

-0.2]+0.5+,.Ot+l.4 +l.7 ~5.3I

+0.1

+1.5

+1.7~..4+2.c.I FI

=y 32 l

b

36

32

40

+4.5

+4.3

+4.2

+3.9

44

48

52

56

60

~-4.0+3.2 +3.C ,2.( =-1.4i.O.I -I.4 -I.7 -2.1

-3.0

36 X

+3.8

+3.4+2.~ FI.~ ~0.I-I,0-1.9 -3.3

+ 4.6

+3.E

-4.5

40 1-3.5+4.1 +3.6

(3) 48

Without including any other sources of signal fluctuations, the pathlength distribution alone would result in no element being resolved beyond nitrogen (assuming "resolved" means + 20 within the interval for that Z). The most striking demonstration of the improvement in resolution will be shown in section 5.2 on charge resolution.

-0.5 ~ ~0

44

= 1/Z.

+I.C 1.1.7

+3.1

+3,0

+1.7

+2.1

+1.0

+1.4

-3.8

F3.91+4A+3.9+2.1 ÷1.c o -I.I -2.2 -3.1 +2.4 +2.1

-3.6

-0.1

.52 -1.6 56

~2..' I-I.7 ~0.6-0.4-2.1 -3A F4.61FI +4.E

-0.9

-2.5

-1.9

-3.8

-2.8

60 -4.5

-3.5

+1.3

+1.0

+0,1

40,4

-3.2

-I.0 -2.4

-3.0

0.4 -I,I -1.4 -2,8

-1.4

1.6

-3.5

-2,8

-3.0 -2.4

FI

5.1.3. Detector nonuniformity correction

The M W P C hodoscope was used to map the response of each T counter as a function of the ( x , y ) position across the counter. This map is used to correct for nonuniformities in response of the counter as a function of the particle trajectory intersection point with the radiator. Principal among the sources of these response nonuniformities are: 1) Variations in actual radiator thickness. 2) Variations in photon collection efficiencies from the

Fig. 7. (a) Maps for correction of signal nonuniformities for entire T1 scintillator in Instrument A. x and y units are in cm. Correction factors are in percent, e.g. + 2.8 means that all pulse heights for this cell in this detector are multiplied by 1.028 in order to normalize the average pulse height for this cell to the average pulse height for the whole counter. (b) Map for one quadrant of T2 liquid Cherenkov counter, Instrument A. "FI'" indicates a cell to be flagged for elimination, e.g. the corners and the location of a support post for the mylar lid.

258,

R. Dwver et al, / Two cosmic-ray detectors

3) Variations in gain from one PMT to another. This could generate a response nonuniformity since each individual PMT preferentially samples an area of the radiator closest to it. This mapping was done both before and after the flights using sea level muons and was checked using in-flight oxygen nuclei. The extent of the mapping (number of particles required) depends on a balance of a number of factors. In this experiment, the intrinsic (before mapping correction) areal nonuniformities were already small. The range of the response before correction was _+2.3% in the scintillators, _+5.1% in the liquid Cherenkov counter, and _+14.5% in the plastic Cherenkov counter. Accumulating the pulse heights from - 106 sea level muons reduced the residual nonuniformities (after correction) to a few tenths of a percent. Approximately the same statistical accuracy was obtained using - 4 0 0 0 0 oxygen nuclei avialable during the balloon flights. We divided each counter into cells and measured the response of each of them. The size and shape of a cell was determined by two factors. If the response around a given point was changing rapidly, i.e. had a steep gradient, this dictated small cell sizes. However, particularly near the edge of a counter where fewer particles were accumulated, the information on the response function was lost in the statistical fluctuations if the selected cell size was too small. Examples of the final maps used for the T1 scintillator and the T2 Cherenkov counter are shown in figs. 7a and 7b. As mentioned previously, four posts were placed in the liquid counter to confine the lid. These posts could easily be seen as "holes" in the detector response function, thus providing a check on this correction. 5.2. Instrument A - charge measurement

To describe the methods for achieving high charge resolution we concentrate on the analysis of the Oklahoma flights that were made with Instrument A. The goal is to find the optimal combination of the measured pulse heights from the two identical scintillators T1 and T3, the liquid Cherenkov counter T2, and the plastic Cherenkov counter TO to achieve the best charge resolution. Clearly the sum of the scintillator signals (TI + T 3 ) / 2 contains information on the particle charge. The two Cherenkov counters TO and T2 provide information on both Z and velocity. If one accumulates a matrix of events where one Cherenkov pulse height is plotted against the other, events having the same Z will lie on a single straight line track for energies above both thresholds. Fig. 8 shows such a matrix of T2 vs TO for the charges 12-16. The resolution of the detectors determines whether a charge track is separated from its neighboring charge tracks. However, since the tracks are straight lines, a rotation of the matrix will produce a

representation where all information on Z is in one dimension and all information on velocity is in the other. Starting from the raw pulse heights in the two Cherenkov radiators TO and T2 this linear transformation is CZ = cos 0 TO - sin 0 T2 cc 0.7T0 - 0.3T2.

(4)

CV = sin 0 T0 + cos 0 T2 cc 0.3T0 + 0.7T2,

(5)

since in our case 0 = 23 °. TO and T2 represent the pulse height channels in the two Cherenkov radiators after normalizing so that highly relativistic nuclei of a given Z give pulse heights in identical channels for both counters. The angle 0 depends only on the two indices of refraction. tan 0 . . . . . n2-1

.

(6)

For two Cherenkov counters, the conclusion is that all the information on Z is contained in this "Cherenkov charge" function CZ and all information on velocity is in CV, the "Cherenkov velocity" function, both just linear combinations of the two pulse heights. The instrument thus provides two independent measures of Z: the average of the two scintillator pulse heights (T1 + T 3 ) / 2 and the Cherenkov charge function CZ. These two measures are complementary. The scintillator average has very good intrinsic resolution but the relativistic rise in the ionization loss has the effect of making the signals from highly relativistic nuclei with charge Z overlap with those at minimum ionization with charge Z + 1. (The increase in scintillator signal below minimum ionizing velocities is small due to the geomagnetic cutoff.) The Cherenkov charge function has no such difficulty but since it is formed as the subtraction of two independent quantities, its relative fluctuations are larger than those in the T2 or TO signals individually. Thus there exists an optimum mixture of the two which will provide the best charge resolution. After extensive computer testing, the final function selected was

TI + T 3 U= 0.73--~ + 0.27CZ.

(7)

Since the Cherenkov response is proportional to Z 2 and, due to saturation, the scintillator response is not, these proportions apply at Z = 26. At lower Z the fraction of the scintillator signals in this function is increased. Besides selecting the best function for charge measurements, we impose selection criteria that eliminate background and nuclear spallations. The first is a requirement that the ratio of the scintillator pulse heights T 1 / T 3 agree within certain limits (9% for oxygen, 5% for iron). The scintillator and the Cherenkov charge measure must also agree within limits which depend on Z. In addition to the requirement of a unique straight line in the hodoscope, these conditions help to select

259

R. Dwyer et aL / Two cosmic-ray detectors -

- .':

--

..

:"

-

' -

-

" "-":'

:'.

. . . .

t

i

!~ ~::

biii ~!

iii

:: ~il!

~.; :::

-

o,,..-o.-ilii!!!!:-: -L,:-.; ".=,:.i~'.:~ :: =:

:

:

:.:iitl

"::c: ;'L:.":: -%-.,I

~

....

.

Z'-J-':

:llilili

:

..

-. %-=-

"r, .

.

:'- "

.

-

"

.

- "- " -

"

_

::~

i-i :i

~',

::: -'"

.+.

ii! :::

,i!:;: -:~ :iii ::~:

7ili

i'.d".'::. ....

lilb]? ItS

- . :.-.::-";:::':

:::

~3~:'~

:

.

_:.:iiiii!lii~fi:~iii:;i=:--:;':iiiiii~K=" :-::;~!!~|;ili:~:-:~:H := =~ii~:ii:H~: .... tllli~llllll=~.=':.12111~

.2lltl~£~lll'~L'i':.;;'.:'.-

: . - --

-

~III'II:'I.2::

12.~II~I~'~

:-

: "

:::::;!-::

T2

~i '"

"--

:. ~:. . - " - :" - -

-

_

.:

:

_

.

.

-

. : - :

. . . .

-

~1! ::

_

/, -:~757~'~!~7:-:~-:'=

:-:

:

/ 1

4

TO

;

12

~

Fig. 8. M a t r i x of T2 liquid Cherenkov pulse height vs TO plastic Cherenkov counter pulse height for the charge range above Z = 12.

only non-spallating cosmic-ray nuclei from the sample of all events which trigger the instrument. After application of these criteria, the sum along the charge tracks yields fig. 9 for the charge range Z = 15-28. Excellent charge separation over this entire interval is obtained. For the elements boron through silicon ( Z = 5-14), the charge resolution is comparable to or better than that shown here [2]. The charge resolution varies from 0.11 charge units (1.4%) at oxygen to 0.20 charge units (0.8%) at iron. Relative abundance measurements over the energy range 1.2-2.4 G e V / n [6] lead to a matrix of charge vs. energy for the iron group (fig. 10) that separates even the track for the very rare element cobalt ( Z = 27) from the neighboring iron track.

Si HI

P

S

CI

A

K

Co

Sc

Ti

v

C~

Mn

Ee

Co

Ni

8001

600

z

400

o., 14

L ,,11,..., 15

IC

17

I~

.... 19

80

,h.., h..,Ih..ill,.,Jll,, 2i

22

?S

">4

Fig. 9. Charge histogram for Z = 1 4 - 2 8 G e V / n from Instrument A.

25

26

...... ,,.. 27

28

and energy 1.2-2.4

260

R. Dwver eta/. / Two cosrm'c-ray detectors

Mn

•

.

•

'

Fe

7 i . . . . . . .:

.

.

.

::::

.

.

.

.

.

.

.

.

.

.

.

;:-:::~

.

;

.

Co

:;i;:,'::

~::

: . ~ , ~ , ~ . ; ~ . ~ , :

: . . . , : . ~ , ~ . . ~ , "

:~

. . . .

; .

.

.

i

. :

•

.

.

.

.

.

.

.

.

.

.

.

. •

. .:

:

.

"

= . : . . : .

. . . . . . . .

:.

.

: .

.

.

:.

~ . . . . : . . :. : . . . = ,

.

= . . ~

.

.

.

.

"!:::: !i ?i :! :i ". i i : :i

"

. . . . . .

.

.

•

.

.

: .

.

.

.

.

.

~ . .

:.

•

= . . .

.

~ . . .

, , : . : . .

. . . . . .

=.=.

-

.

) = .

•

.:::;:

il ..

.

. = . . .

i:!;!!!:

25

•

:.2i:i}!ii~?~,izi~:i ili:.

,

.

'i

.

.

. . . . . . . .~...,.,,' . ~:'?T:.: : : : : :

•

Ni

=2.

:.: . : : : ? i : ii" , = . . = . .

.

26

27

28

Fig. 10. Matrix of energy (1.2-2.4 GeV/n) vs. charge for the iron group (Z = 25-28).

The observed resolution can be accounted for by known effects. For the scintillation counters, we calculated the V a v i l o v - L a n d a u distribution for several charges and found the shape and width (standard deviation) to agree with the observed distribution• The width of the distribution for a given Z broadens significantly with increasing energy from - 1 to 10 G e V / n since

more energy can go into single 8-rays which are the source of the signal fluctuations. The signal fluctuations in the Cherenkov counters will be discussed in the next section. We plot in fig. 11 the relative standard deviation o in the U function for kinetic energies between 1 and 2 G e V / n vs Z. The predominant source of signal fluctua-

R. Dwyer et aL / Two cosmic-ray detectors 2.0

I

I

the map correction is the same for all zenith angles. We cannot be sure whether there is not some small coupling, particularly in the Cherenkov radiators, where the emission is initially directional (even though both radiators contain isotropically emitting wavelength shifters). It is also possible that the pathlength a n d / o r map corrections are slightly Z dependent, e.g. different areas of the scintillator may saturate differently at higher Z. These small effects would become a concern only if the same charge separation was required in a charge range considerably above iron.

I

-~ *G 1.5 C~

g 1.0 cr)

0.5

I

'5

I

I0

I

210

15

2J5

30

5.3. Instrument A - velocity measurement

Z Fig. I I. Relative standard deviation (o/mean) in the U-function [eq. (7)] in percent vs nuclear charge Z observed in the balloon flights of Instrument A for the energy range 1.2-2.4 GeV/n.

tions in the scintillators are Landau fluctuations and, in the Cherenkov counters, Poisson fluctuations in the number of recorded photoelectrons. For both these processes the relative standard deviation should be proportional to 1 / Z (apart from saturation affecting the scintillator signal): a ( Z ) = o s / Z where o ( Z ) is the relative o for charge Z and os is the relative o for Z = 1 due to these statistical processes alone. At low Z, the figure shows that the 1 / Z dependence is indeed followed. If there were a residual systematic source of error which broadens the detector response by a fixed amount that is independent of Z, for example due to the pathlength distribution before correction by the hodoscope, this would show up in fig. 11 as an asymptotic approach to a fixed value. Both components would add in quadrature giving the observed resolution o 2 ( Z ) = ( o s / Z ) 2 + o2.

261

(8)

It is of interest to make a fit to this variation with Z to extract the value for of, the residual, "fixed", Z-independent, systematic error, of would include errors in the pathlength correction, map or nonuniformity correction, in the temperature drift correction, errors in background rejection, etc. This fit yields of = (0.7 + 0.1)% which we consider extremely low. We cannot determine the origin of this 0.7% residual effect, or else we could correct for it. However, in the course of the data analysis some suggestions offered themselves. First, the nonuniformity maps are discontinuous, made digitally on a cell by cell basis (see fig. 7). Thus there will always be an error for some events of about one-half the difference between correction factors of adjacent cells. The pathlength and the detector nonuniformity map corrections were made separately and sequentially. This assumes that the pathlength correction goes as cos 0 for all areas of all detectors and that

The velocity measurement is made by the Cherenkov counters. The instrument as flown from Texas had one liquid Cherenkov counter T2. In the Oklahoma flights we included an additional, supplementary plastic Cherenkov counter TO. The light output, C, from an idealized Cherenkov radiator depends only on its index of refraction, n, for particles of a given velocity C / Z 2 = 1 - 1/n2,8 2 = 1 - p2° 1 - 1/n 2 p2'

(9)

where normalization is to 1 at 13 = 1, p = fly is momentum per nucleon (expressed in units of the nucleon rest mass), and P0 is the threshold momentum/nucleon for Cherenkov emission given by P°2 =/3°2Yg - n 2 1- 1 '

(10)

Thus the velocity resolution of a Cherenkov radiator is best just above its threshold and gradually decreases with increasing velocity. If a detector has signal resolution limited by fluctuations in the number of photoelectrons recorded at the phototube cathodes, a measure of the maximum resolvable momentum/nucleon for a given Z is given by determining the signal which is lo below maximum signal: PmaJPo = Npt/4`

(11)

where Npe is the number of photoelectrons for a particle with fl = 1. 5.3.1. Model for light emission in the Cherenkov counter The liquid Cherenkov radiator T2 present in all our flights is the mainstay of the velocity measurement. To correctly relate the velocity of a particle to its pulse height signal, a complete model of all sources of light emission is necessary. These include: a) Cherenkov light from the primary particle. b) Cherenkov light from &rays. c) Cherenkov light in the mylar lid constraining the liquid.

262

R. Dwyer et al. / Two cosmic-ray detectors i

I

I

expected from Landau fluctuations in the number of 8-rays of sufficient energy to produce Cherenkov radiation. If ~-rays contribute to the resolution, then they contribute to the mean signal and it is important to evaluate their contribution to the counter's response. A calculation of the contribution of ~-rays to the mean Cherenkov signal was carried out by Evenson [21] for a radiator similar to ours and we have used his results. The pertinent figure from that paper is shown in fig. 13 which gives the 8-ray signal vs l / f l 2 - 1. A classical Cherenkov response corresponds to a straight line with intercept on the x-axis dependent on the refractive index. The 8-ray response is parametrized by E c which is best described as that energy 8-ray with range equal to the radiator thickness. For our geometry, E c = 6 MeV seems most reasonable. In the figure, 6-ray signals for E~ = 3, 6 and 20 MeV are shown. The 6-ray contribution can be parametrized as the sum of several classical Cherenkov contributions each from a medium with a progressively lower index of refraction. We have fitted the E~ = 6 MeV curve with two such lines (marked A and B in fig. 13). Component A has n = 1.270, the same as our radiator and component B has n = 1.063. We chose not to fit the remaining portion with a third index since the uncertainty in E~, itself does not justify it. Prior to the flights we measured the amount of residual scintillation in the liquid with an 241Am alphaparticle source and found an upper limit to its magnitude of 6% of the primary Cherenkov signal at fl = 1 for the same Z. The residual scintillation we observe in the flight data below Cherenkov threshold does not exceed this limit. Extensive tests were made to evaluate light emission from homemade BaSO 4 white reflectance paint and Eastman Kodak's white reflectance standard. Tests in our laboratory showed that the small amount of light that is emitted as a charged particle passes through a

I

1200

g 800 o

o

40O

0

- - - - 20

4O 60 Chonnel Number

,..i

80

100

Fig. 12. Pulse height spectrum measured for sea-level Z =1 particles in the T2 liquid Cherenkov counter of Instrument A.

d) Residual scintillation of the radiator or the wavelength shifter dissolved in the liquid. e) Scintillation or Cherenkov light from the BaSO 4 paint lining the light integration box and lining the liquid container. The only parameter necessary to relate a given signal to the particle velocity for component (a) the Cherenkov light from the primary, is the index of refraction [eq. (9)]. The index we used was 1.270 which includes small corrections for dispersion and for the temperature dependence of the index of refraction. This is a 1% change from the value of 1.257 given by the manufacturer. An important contribution comes from Cherenkov light generated by 6-rays (knock-on electrons). The contribution by delta-rays is evident from the asymmetric shape of the sea-level muon signal distribution in T2 (fig. 12). If the signal distribution were limited by photoelectron statistics, the distribution at our light levels ( - 5 0 photoelectrons for Z = fl = 1) should be almost indistinguishable from a Gaussian. The actual distribution shows a skewness to the high pulse side as Kinetic Energy,GeV/n I0 4 E

I

2.5

1.5

1.0

.80

,70

.60

T

I

i

I

I

l

D

20%

Ec=

_f-- 20 MeV , ~ - - 6 MeV ~ ' ~ _ ~ " / - 3MeV

~" I0%

0

I

L

I

I

I

I

0.1

0.2

0.3

0.4

0.5

0.6

t/13z-1 Fig. 13. Parametrization of 8-ray contribution to liquid Cherenkov counter. Instrument A [21].

263

R. Dwyer et al. / Two cosmic-ray detectors

Table 5 Model for components of the light emission in T2 liquid Cherenkov counter (Instrument A) Source

Index of refraction assumed

Percent of light emitted at ~ = 1

1) Cherenkov light from primary 2) Cherenkov light from 8-rays: Component A Component B 3) Cherenkov light from the mylar lid 4) Residual scintillation of the liquid or wavelength shifter 5) Cherenkov light in the BaS04 white paint

1.270

84.7

1.270 1.063 1.60

5.5 2.3 1.4

1.60

4.7 1.4

painted surface consists primarily of Cherenkov radiation. In our geometry, it would contribute an amount - 1-2% of the primary Cherenkov light. The mylar lid that constrains the liquid contributes a similar amount, namely 1.4% of the total light emission for particles with p=l. Table 5 contains the complete list of components we include in the light emission with their relative fractions at fl = 1. Fig. 14 presents the relative contributions of these components as a function of velocity. An important part of relating the Cherenkov signal to velocity is determining the channel in the measured spectrum that corresponds to the true signal for a particle having fl = 1, the so-called fl = 1 points. We have employed several methods of locating these fl = 1 points. Using the data from the balloon flights we have 100

/~uid

. ~ 50

-~ 20

o "~

I--

[

CS_roy

"B 10

Residu01

_

g_ :

Ior+point 10.5

I

I I

I 2

I 5

three different ways of obtaining a sample of particles with high enough energy so that the mean of their distribution is a measure of the fl = 1 point location. In the second Texas flight, the balloon drifted to higher cutoffs and by selecting particles arriving from the highest (eastward) cutoffs, the geomagnetic filter provides one such sample of high energy particles. Otherwise, we can use the distribution of particles in one Cherenkov counter accepting only the highest energy particles as measured in the other Cherenkov counter. Thirdly, the relativistic rise in the pastic scintillators provides a way of selecting particles with signals near the fl = 1 point (see the more detailed discussion in ref. [22]). Through extensive simulations of the Cherenkov counter response using models for the light emission and fluctuations (which will be discussed in section 5.3.2) we find that the fl = 1 point is where the measured Cherenkov pulse height spectrum has fallen to about 60% of its peak value. This assumes a power law for the cosmic-ray energy spectrum and Gaussian fluctuations about the mean. Naturally, for higher Z distributions ( Z > 14), the resolution ( o / m e a n ) is on the order of two percent or less so the fl = 1 points are immediately available to that accuracy independent of any assumptions or simulations. To summarize, we believe that, although each method of locating the fl = 1 points may have some uncertainty, the benefit of averaging the results of several independent approaches provides a more unbiased determination. For dements with Z >_ 14 we estimate the location of the fl = 1 points to be accurate to _< 0.5%, increasing to 1% at Z = 10 and 2% at Z = 6.

10

KE/n (GeV/n) Fig. 14. Components of the Response Model for T2 liquid Cherenkov counter in Instrument A. The amount of Cherenkov light vs. kinetic energy per nucleon from the primary particle in the liquid, from &rays and from the primary in the mylar and paint are shown along with the residual scintillation (assumed constant over this energy range).

5.3.2. Resolution of the Cherenkov counter

To measure the signal resolution one must determine the signal distribution from a sample of particles all having the same velocity (signal). Accepting only particles whose signal, in the absence of fluctuations, would be at the fl = 1 point provides a convenient method. This can be done approximately by techniques men-

R. Dwyer et al, / Two cosmic-rc 9, detectors

264

tioned in the previous discussion for locating the fl = 1 point. Thus one obtains limits on the resolution: the standard deviation o of the resulting distribution will be equal to or larger than the standard deviation of the parent distribution. In the same manner as discussed in section 5.2 on charge resolution, we expect the relative standard deviation o ( Z ) = o/mean to depend on Z in the following way:

o2(Z) = (oJZ) 2 + o2,

(12)

where os is the relative standard deviation for Z = 1 arising from fluctuations due to the discrete nature of a statistical process, e.g. photoelectron statistics or fluctuations in the number of &rays which emit Cherenkov radiation. The distribution obtained with sea level muons closely approximates % of is the contribution from "fixed factors", factors which contribute a fixed percentage to the fluctuations independent of Z. For a high Z element such as iron the contribution from the fixed factor is dominant in the resolution. Making a fit to data points for several charges gives the best values for o~ and o r for the signal distributions in the Cherenkov counters. The results are the following: TO plastic Cherenkov: o~ = (24.8 + 0.4)%, of = (1.2 + 0.3)%; T2 liquid Cherenkov:

o~ = (24.5 + 0.4)%,

Of (1.3 + 0.3)%. The value for T2 is derived from the Oklahoma flights when the liquid counter thickness was 1.9 cm relative to the 2.5 cm thickness in the Texas flights. The reduction in thickness of T2 for the Oklahoma flights degraded the resolution by decreasing the number of photoelectrons recorded. This was partly offset by our discovery of a procedure which significantly increased the transmittance of the liquid in the ultraviolet where much of the Cherenkov radiation occurs: repeated filtration through Attapulgus clay, designed to remove trace amounts of organic contaminants. We now describe a model for the fluctuations in the Cherenkov counters that properly accounts for the various components including their dependence on Z and ft. The low intensity of Cherenkov light emission makes fluctuations in the number of recorded photoelectrons at the photocathode a major factor. This is evident by noting that the reduction in the relative width (o/mean) of the sea level muon distribution corresponds to that expected from the increased light obtained when a wavelength shifter is dissolved in the liquid. As m e n t i o n e d b e f o r e the asymmetrical nature of this signal distribution for sea level m u o n s implies that there is a contribution d u e to d e l t a rays to the signal fluctuations. For a given velocity, fluctuations in both the number of photoelectrons and in Cherenkov emitting 6-rays will scale similarly with Z [ o ( Z ) = o ( 1 ) / Z for =

the relative standard deviation]. However, it is important to note that they will scale differently with velocity for a given Z. The velocity dependence of the photoelectron fluctuations is c~ ]rNpeO~ ~flC, where Npc is the number of photoelectrons which is proportional to the signal C, a known function of velocity [eq. (9)]. The fluctuations in Cherenkov light emitted by 8-rays depends on how many 8-rays are created in and above the radiator with energy exceeding the Cherenkov threshold and on their pathlength in the radiator. Hence this fluctuation depends in a complicated way on the geometry and mass distribution of an individual instrument. It will be worst for gas Cherenkov radiators with a high threshold since Cherenkov-emitting &rays must be energetic enough to have a range of several g/cm 2, thus correlating the Cherenkov radiation with nonlocal mass in the instrument. For T2, the liquid Cherenkov radiator, electrons with kinetic energies above 320 keV will emit Cherenkov radiation. For TO, the plastic Cherenkov counter, the threshold energy for electrons is 170 keV. These low values imply that an equilibrium distribution of 8-rays is rapidly set up in the radiators, thus weakening the dependence on the mass distribution outside or above the radiator. For T2, the liquid counter, we have relied on a calculation by Dayton et al. [23] of this velocity dependence of the fluctuations in 8-ray Cherenkov emission. They made a Monte Carlo calculation using a model that included the increase in the maximum &ray energy with particle energy, but neglected fluctuations in the &ray path length due to multiple scattering. Since the range of threshold energy &rays in T2 is < 1/10 the thickness of T2, these assumptions would seem reasonable. To parametrize their results for the liquid counter, we fit a polynomial which gives os(C ) for this contribution to the resolution where C is the Cherenkov signal normalized to 1 at/3 = 1

os(C)=a(ao+a,C+a2C2+a3C3+a4C4),

(13)

where a 0=2.874.

a ~ = 2 . 0 2 2 x 1 0 -2 ,

a 2 = 1.274 x 1 0 - 3

a 3 = -2.597 x 10-5,

a 4 = 2.510 x 10 -7. The parameter a = 1.08 is characteristic for our particular instrument and is derived from the sea level muon distribution. The effect of this dependence is most important for higher velocities (signals C is the liquid counter greater than - 0.8). Fig. 15 shows the velocity dependence of the three contributions we consider for the liquid counter for nuclei with Z = 14. The photoelectron contribution to the fluctuations is characterized by o,o(Z = 1, fl = 1 ) = 14.4% ,

U,o = 48.

265

R. Dwyer et aL / Two cosmic-ray detectors

3~-'•I "•

Z:14

J- %'.¢,~,o, . . . . . . . . . . ° 2~-

1 .6

60

'~,3e

.................... 10.2

/

~8

22.9

11.4

•

I

0.2

0.4

I

0.6 Cerenk0vSign01/Z2

I

0.8

I

1.0

I

V!:i:)::~,

:::::::::.

18.o

" ~

4 g

W -- o.e --~o.1 --11.1 ~i?12.a - - ls.e - - 199 - - 2r ~-- E

\

\

'iii:ii:ii'/

\

Fig. 15. Total relative standard deviation (o/mean) in percent from various sources of signal fluctuations in the T2 liquid counter of Instrument A vs. Cherenkov signal/Z 2 [eq. (9)]. The contributions by individual effects, i.e. photoelectron fluctuations, or , fluctuations in light from 8-rays. o8 and the "fixed" factor ot [eq. (12)] are shown in their relative proportions for a nucleus with Z = 14.

\/

..!:129 1o 6

I~-:~;:~1

/ 17.9

I 234

I

S For the &ray contribution we have taken os(Z

=

1, fl

=

1)

=

18.1%,

and the fixed factor is o r -- 1.3% independent of Z and

5.4. Instrument B - trajectory measurement

We now turn to the performance of Instrument B. To separate and identify individual isotopes by the geomagnetic method a knowledge of the particle's arrival direction is required. The azimuth and zenith angle of the particle's path gives the information that the observed particle had a rigidity equal to or greater than the cut-off rigidity associated with that direction in the sky. For example, fig. 16 shows the location in the sky over Hawaii where the cutoff rigidity is between 11.9 and 12.9 GV. The MWPC's strip width of 6 cm allows an angular uncertainty of less than 2.1 ° which corresponds to a rigidity cut-off uncertainty of less than 0.1 G V for most regions of the sky. The trajectory also provides the pathlength in the gas counter which relates the total measured light signal to the actual velocity of the particle. Fig. 17 shows the pathlength distribution in the Cherenkov counter C1 for isotropically incident helium. Using the trajectory information all events are normalized to a pathlength of (72 + 1) cm. In addition, the wire chamber hodoscope is used to accurately determine the energy loss in the scintillators and to eliminate many classes of interacting particles in a m a n n e r similar as described for Instrument A. A source of error in determining the direction of incidence comes f r o m magnetometer alignment and calibration uncertaintie~These are estimated to be less

Fig. 16. Geomagnetic cutoff rigidities in GV for selected directions of incidence at 118000 ft. over Kauai, Hawaii. The shaded region contains all directions with cutoff rigidities between 11.9 and 12.9 GV.

than 5 o. This accuracy in azimuth is compatible with the velocity resolution of the Cherenkov counters and the calculational uncertainties in our knowledge of geomagnetic cutoffs. Many of the higher Z nuclei will generate an observable 8-ray in traversing the instrument. The pulse height analysis of the MWPC strips enables us to distinguish the location of the primary particle trajectory from that of the &ray. The d E / d x resolution in the four MWPCs which permit this discrimination is shown in fig. 18.

5O00

I

I

I

I

I

I

I

70,

80.

I

90.

r 20C0

E

moo

oi20.

i

50.

i

40.

i

50.

i/

60.

i

100.

Pothlength (cm.] Fig. 17. The distribution of particle pathlengths through Cherenkov counter C1.

R. Dwyer et al. / Two cosmic-ray detectors

266

....

2000

T - -

V- .... q

I

I

T~ .....

l

[

l

Helium

1800

1600

1400 (,l') (.J .f-.

1200

(3I

1000 ~-

L iI

0

i I

800~

E

:3

z

,oo !

/

400~--

200

0

\

i I

Z

50.

F

60.

70.

'\ !

2

i .... 80.

1 90.

_

J 100.

i 110.

L_ 120.

130.

140.

t50.

Meon Signol (chonnels) Fig. 18. The mean signal distribution detected along the trajectory of helium nuclei measured on the etched strips of the four MWPCs in Instrument B. The peak of the g-ray distribution is at channel 45.

A significant fraction of events registered during the flight have consistent scintillator and Cherenkov signals indicating the passage of a single nucleus. For some of these events the M W P C hodoscope however shows a complicated interaction or shower of particles. Thus the hodoscope is essential for identifying and eliminating these events that otherwise would be accepted as a primary particle. The total number of M W P C strips that are fired proves to be a powerful selection criterion. The analysis of data obtained in the first flight of this instrument concentrates on helium. Fig. 19 displays the number of strips fired for events selected as helium by the scintillators. Helium candidates with 16 or more strips fired rarely pass all of the other selection criteria whereas candidates that show between 6 and 16 strips fired seldom fail other possible selection criteria. The firing of only six strips is acceptable since this represents an event with a trajectory highly inclined with respect to the Z-axis that traverses one X and one Y strip in each of three MWPCs. Since scintillators define the collecting geometry of the instrument, some particles incident

at large angles may miss P1 or P4 or both MWPCs. The few events that miss both P1 and P4 are rejected because of limited track information. The straight line fitting algorithm constructs all possible (X,Y,Z) points from the data and finds the best fit lines for all combinations of points. For each Combination and fit line a goodness of fit parameter is produced which is the total absolute deviation of the four M W P C data points from the fit line and is measured in cm. The line with the best fit in both the X view and Y view is accepted as the trajectory of the event. Fig. 20 shows the distribution of this goodness of fit parameter for one view. Events are well separated into one group where the fit is extremely good (sum of deviation in all four chambers less than one strip width) and a second group where the fit is poor. The second group of events is rejected. A small number of events have more than one possible trajectory that fall into the first group. These are also rejected because of the ambiguity. Detailed mapping of scintillator and Cherenkov nonuniformities was not necessary for the first sample of helium data from Instrument B since the fluctuations in

R. Dwyer et al. / Two cosmic-ray detectors I

10000

I

I

I

I

I

I

I

267 I

Helium

9OOO

8O0O

7000

6000

o. '4"-0

5OO0

1

4000

E z 5000

2000

L L_

1000

I .0

5 .0

F

L

6.0

9.0

I 12.0

I 1.5.0

Ll-~ 18.0

I 21.0

I 24.0

I 27.0

50.0

Sfrips/evenf Fig. 19. The total number of X and Y strips fired in all MWPCs for helium nuclei obtained in the first flight of Instrument B. 12000

I

---

,ooo

10500

7500-rt 6OOO ¢J

om

-,I--

0

l=

I

I

I

I

I

I

I

Helium

L

4500--

Z 3OOO

1500

0 -.0

I

2.0

4.0

L-L~ 6.0

8.0

I

10.0

I

12.0

I

14.0

]

16.0

I

18.0

20.0

Fit Porometer (cm.) Fig. 20. The goodness of fit parameter for accepted trajectories in the X view of the MWPCs. See text for definition of this parameter. Each of the 4 X-strips involved in the fit are 6 cm wide.

268

R. Dwyer et al. / Two cosmtc-ray detectors

the detector signals due to Landau and photoelectron fluctuations for helium nuclei are large in comparison to any expected nonuniformities.

5.5. I n s t r u m e n t B - c h a r g e m e a s u r e m e n t

The photocathodes of the 14 PMTs in each scintillator light integration box represent 4% of the total surface area. Since the absorptivity of the paint is also about 4%, we collect nearly 50% of the light generated by helium nuclei [eq. (2)] which the PMTs convert into approximately 8000 photoelectrons. The statistical fluctuations from this process of - 1% are insignificant when compared to Landau fluctuations. Fig. 21 displays the resolution obtained for selected He in the top scintillation counter by adding the signals from all 14 tubes. The resolution of 0.10 charge units agrees well with the calculated energy loss distribution and demonstrates that helium has no overlap with the neighboring hydrogen or lithium distributions. Particles incident at various angles have different pathlengths through the sheet of scintillator and these variations are corrected using the trajectory from the MWPCs. Saturation of the scintillator response at higher charges is not a problem since we initially concentrate on the isotopic measurement of helium only. The variation in light output with velocity at these relativistic energies is measured to be less than 0.7% over the entire energy range of interest (4-8 G e V / n ) .

5000

I -- --T

I

I

~ - - - c

i

HELIUM 4500

]

t I

4000 3500 3000 2500 2000 D

15°° !

1000 500 35 40 MEAN SIGNAL OF T1

L! 45

50 55 60 (CHANNELS)

Fig. 21. The energy loss distribution measured in scintillator T1 for He nuclei.

This lack of significant variation was confirmed by selecting a low velocity and a high velocity sample of helium nuclei with the Cherenkov counters and comparing their scintillator signal distributions. The T1 scintillator distributions for the two samples are identical whereas the T2 scintillator distribution means differ by (0.5 +_ 0.2)%, the high energy sample having the slightly higher mean. The avoidance of velocity bias in the selection requirements is crucial since the geomagnetic method employs a comparison of velocity spectra measured in the Cherenkov counters to derive isotopic ratios. Since our primary interest from the first flight of Instrument B is the element helium, and since no significant velocity dependence is observed in the scintillators, we use a simple charge function, the average of the total T1 and T2 signals. Each step of the selection process is designed to pass only those events where the likelihood of being a single helium nucleus is much higher than the likelihood that it is a background event. The charge selection process in the scintillators takes advantage of the four independent groups of PMTs in each counter. Each of the four individual pathlength corrected pulse heights for each event are compared with the mean of the other three pulse heights in that counter to derive a variance from the mean for each event. The average standard deviation for a tube group is found to be 8%, which is related to the light collection uniformity over the surface of the scintillator as seen by four equally spaced PMTs. For an event to be accepted as a helium candidate, all four groups must have a signal consistent within 2½0 of their mean. This criterion primarily eliminates events that had an accompanying particle penetrating a tube face. We found that more events were eliminated by this criterion in the bottom counter than the top, consistent with the additional mass above this counter. We next require that the sum of the four TI groups and the sum of the four T2 groups each have a signal within the range expected for a helium nucleus. Since the sum of TI and T2 is a good measure of the charge we also require this to fall within the helium range for all candidate events. Limiting this sum eliminates events where both scintillators fluctuate simultaneously too high or too low. A small number of additional background events are rejected by requiring the ratio of the two signals T 1 / T 2 to be consistent with the expected independent fluctuations in each counter. For helium we limit this ratio to the range (0.65-1.54). These selection criteria on the scintillation counter signals result in a sample of nuclei with unambiguous charge identification at the expense of efficiency. Our final sample, after scintillator and MWPC requirements have been met, consists of - 40% of the possible number of events, based upon published helium flux measurements [24,25].

R. Dwyer et aL

/ Two cosmic-ray detectors

5. 6. I n s t r u m e n t B - velocity m e a s u r e m e n t

269

This is probably due to imperfections in the white coating of the counter walls and the efficiency of light collection by the photomultipliers behind the thick quartz plates. Apparently the UV light collection was significantly reduced by a contamination of the optical coupling silicon oil by plasticizers in the O ring seal. A solution to this problem has since been found by replacing the Buna-N O-ring material with Viton synthetic rubber O rings. After the first flight we noticed that the photoelectron yield had dropped to 30 photoelectrons, presumably due to the continuing contamination of the optical coupling oil by O ring plasticizers which, at that time, was unrecognized. The corresponding light signal of approximately 120 photoelectrons from a high energy helium nucleus will have statistical fluctuations, about 1.5 times greater than the original design anticipated, but still within acceptable limits for achieving isotopic separation. Equally significant are the fluctuations in &ray generated light. Any &ray with an energy >_ 3 MeV will produce Cherenkov light in the high pressure

The two large gas Cherenkov counters incorporated in this instrument are designed to accurately measure velocities over the range of fl = 0.982 to fl = 0.996. The most important limits to the velocity resolution come from three sources: (1) the number of photons collected: (2) fluctuations in 6-ray generation; (3) pathlength variations in the detector volume. From the design of the white diffuse Cherenkov counters we expected each Cherenkov counter to produce 70 photoelectrons for a relativistic singly charged particle (muon). To test this yield we used a pulsed LED and variable neutral density filter to calibrate the counter's pulse height analyzers in terms of photoelectrons. Using a measurement of the muon distribution of signals in the counter we determined that relativistic muons generated only 40 photoelectrons in each counter. The measured photoelectron yield lies almost a factor two below the value calculated using the absorptivity of the paint, # = 4%, and the PM tube efficiency.

11.6-12.6 GV iREGION p I I I 11.9 GV for 3He

11.9 GV for 4He

~/ iiiJ.~ll++I

400

300 I

I

200

-1

¢

¢

i

100 P

c ,7 o (o

I

l"ml~

s.

I

"

u.

I

~a.

Cerenkov signal

I

24.

I

3o.

I

s6.

(arbitrary units)

I 42.

llllllllllll mm 48. 54.

..I

60.

..1 66.

]

72.

Fig. 22. The Cherenkov signal distribution for helium nuclei arriving from the crescent shaped region of the sky where the geomagnetic cutoff is between 11.6 and 12.6 GV (see fig. 16). The 3He and 4He cut-offs ad different energy (same rigidity) are indicated by the arrows.

270

R. Dwyer et al. / Two cosmic- ray detectors

gas. The mean number of 8-rays generated in each counter above 3 MeV by a relativistic He nucleus is calculated to be {N)---0.3. These 8-rays emit approximately 25% of the light that a helium nucleus produces since their range is similar to the counter dimensions. Fluctuations in the number of 8-rays and variations in their actual pathlength in the counter limit the velocity resolution by a factor comparable to that from photoelectron statistics. Pathlength differences between individual helium nuclei in the gas volume would also be an important limit to the velocity resolution if not corrected by the M W P C trajectory information. Since ethylene does not scintillate, fluctuations in the scintillation light do not limit the resolution. Fig. 22 shows the Cherenkov signal distribution for helium arriving from regions of the Hawaiian sky where the geomagnetic cut-off is between 11.6 and 12.6 GV (see fig. 16 also). The steepness of the curve on both the fl = 1 side of the distribution and the geomagnetically influenced side of the distribution reflects the velocity resolution obtainable. The separate cutoff of 4He and 3He is apparent. Based upon this observation a beam of monoenergetic helium producing a mean signal of half the fl = 1 signal would have a Cherenkov signal resolution of 12%. This is sufficient to identify the 3He and 4 He cutoffs which differ by 26%. Improvements in the performance of this counter will come through increasing the light collected by the PMTs and minimizing the effects of 8-ray fluctuations. The light collection improvement will be achieved through removing the contamination source in the optical coupling oil and possibly improvements in the reflectivity of the paint. The 8-ray contribution to the signal can in principle be identified in the analysis because it is a correlated signal in all four groups of PMTs of one Cherenkov counter. Correlation plots of one group vs another in the top counter for a small range of velocities selected by the bottom counter show correlated fluctuations between the two as well as uncorrelated fluctuations. The degree to which this knowledge can improve the velocity resolution is currently being investigated.

6. Summary In this paper we have attempted to provide a detailed description of two instruments which have recently been developed and built in our laboratory for the study of the elemental and isotopic composition of the cosmic radiation at energies above about 1 G e V / n . To achieve this goal requires special techniques that yield excellent charge resolution and velocity resolution for the nuclei under study. A combination of scintillation detectors, Cherenkov detectors and a trajectory hodoscope made of multiwire proportional chambers

and assembled in a counter telescope achieves the desired results. We described the construction, performance, and limitations of these devices and our experience from actual exposure of the instruments in balloon flights. We emphasized those aspects of the detectors that may lead to background events and errors, often of a subtle nature. It is our hope that this paper will be of use to other investigators who wish to develop instrumentation for research in releated areas. We are greatly indebted to the technical staff of the Laboratory for Astrophysics and Space Research in the Enrico Fermi Institute. In particular to G. Kelderhouse, W. Johnson, W. Hollis, L. Littleton, L. Glennie, P. Parilla, B. Lynch, and J. Fowler. We wish to thank Prof. B. Peters for much stimulation and encouragement and Dr. T. Parnell for sharing his experience in building multiwire proportional chambers for balloon-borne experiments. We thank Dr. G. Minagawa who prepared Instrument A for the Oklahoma balloon flights and carried out those flights. The staff of the National Scientific Balloon Facility ably performed launch and recovery.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11]

[12]

[13] [14]

[15]

B. Peters, Nucl. Instr. and Meth. 121 (1974) 205. R. Dwyer, Astrophys. J. 224 (1978) 691. R. Dwyer and P. Meyer, Phys. Rev. Lett. 35 (1975) 601. R. Dwyer and P. Meyer, Astrophys. J. 216 (1977) 646. R. Dwyer and P. Meyer, Proc. 16th Int. Cosmic-ray Conf., Kyoto, vol. 12 (1979) p. 97. R. Dwyer and P. Meyer, Proc. 17th Int. Cosmic-ray Conf., Paris, vol. 2 (1981) p. 54. R. Dwyer, M. Garcia-Munoz, T.G. Guzik, P. Meyer, J.A. Simpson and J.P. Wefel, ibid., vol. 9, p. 222. G. Minagawa, Astrophys. J. 248 (1981) 847. M.A. Shea, D.F. Smart and H. Carmichael. Technical Report AFGL-TR-76-0115, Air Force Geophysics Laboratory, Hanscom AFB, Mass. (1976) D.F. Smart and M.A. Shea, Proc. 14th Int. Cosmic-ray Conf., Munich, vol. 4 (1975) p. 1304. M.A. Shea and D.F. Smart, Technical Report AFCRLTR-74-0159 (26 March 1974), Air Force Cambridge Research Laboratory, Bedford, Mass. l?.eadboard (white reflecting), Frosty Foam Inc., 181 Ida St., Antioch, IL 60002, USA. Specify 2 lbs/ft ~. saw cut, no colored inclusions. E-2 Freon Fluorocarbon, E.I. Dupont Co., Freon Products Division, 1102 West St., Wilmington, Del. 19898, USA. The formulation procedure for the white paint was obtained from K. Atallah, A. Modlinger and W. Schmidt, Max-Planck Institute, Garching bei Munchen, W. Germany. Primer is VP2222 from Wacker Chemie, SWS Silicone, Sutton Road, Adrian, MI 49221, USA. The anode wires for the MWPCs were obtained from California Fine Wire Co., 338 S. Fourth St., Grover City, Ca. 93433, USA. The wire used was Ultrafinish 304 stain-

R. Dwyer et al. / Two cosmic-ray detectors

[16] [17] [18]

[19]

less steel sire extruded through a custommade diamond die, then stress relieved to remove curling. ElectroDag and Aqua-Dag, Acheson Colloids Co., Port Huron, Mi 48060, USA. C. Ward, A. Berick, E. Taglioferri and C. York, Nucl. Instr. and Meth. 30 (1964) 61. M. Cantin, M. Casse, L. Koch, R. Jouan, P. Mestreau, D. Roussel and F. Bonnin, Nucl. Instr. and Meth. 118 (1974) 177; M. Cantin, J. Engelmann, L. Koch, P. Masse, N. Lund and B. Brynak, Proc. 14th Int. Cosmic-ray Conf., Munich vol. 9 (1975) p. 3188. J.B. Shutt, J.F. Arens, C.M. Shai and E. Stromberg, Appl. Optics 13 (1974) 2218.

271

[20] S. Ahlen, B. Cartwright and G. Tarle, Nucl. Instr. and Meth. 143 (1977) 513. [21] P. Evenson, Proc. 14th Int. Cosmic-ray Conf., Munich, vol. 9 (1975) p. 3177. [22] J. Benegas, M. Israel and J. Klarmann, ibid., p. 3172. [23] B. Dayton, N. Lund and T. Risbo, Danish Space Research Institute Report (1969). [24] M. Ryan, J. Ormes and V. Balasubramanyan, Phys. Rev. Lett. 28 (1972) 985 and 1497. [25] L.H. Smith, A. Buffington, G.F. Smoot and L.W. Alvarez, Astrophys. J. 180 (1973) 987. [26] Boiler and Pressure Vessel Code, The American Society of Mechanical Engineers, United Engineering Center, 345 East 47th St., New York, NY 10017, USA.