A time projection chamber for detection of double beta decay

226

Nuclear Instruments and Methods in Physics Research A273 (1988) 226-239 North-Holland, Amsterdam

A TIME PROJECTION CHAMBER FOR DETECTION OF DOUBLE BETA DECAY S.R. ELLIOTT *, A.A . HAHN and M.K . MOE Physics Department, University of California, Irvine, Irvine, California 92717, USA

Received 11 April 1988 A time projection chamber is being used to study double beta decay m 82 Se . The construction, operation, and performance of this device are described 1. Introduction The search for double beta decay [1] has historically been a struggle against the obscuring effects of, background . The energy release in this extremely rare disintegration is similar to that found in the much more common decays of radionuclides such as the daughters of uranium and thorium. The vast disparity in half-lives between double beta emitters and the ever present uranium and thonum contaminants places severe demands on the purity of the source and on the background rejection capabilities of the double beta decay detector. Calorimetric techniques, particularly the use of high resolution germanium diodes [2], have been highly successful in improving sensitivity to the sharply peaked energy spectrum expected for neutrinoless double beta decay ((3(3(Ov)) . For the broadly distributed energy spectra of the two-neutrino (ß(3(2v)) and majoron (ß(3(0v, B)) modes [3], however, additional background rejection is needed . For these modes a tracking chamber in a magnetic field has distinct advantages . The two negatively charged electrons from double beta decay can be seen emerging from a common point on the source (see fig. 1) . Helices can be fitted to the tracks, and the deduced electron energies and opening angles serve to distinguish double beta decay from closely mimicking processes. Since source and detector are separate, there is freedom to choose a source with a high transition energy and short half-life. For solid-source experiments the track information comes at a price, however. The need to preserve the angles and energies of the = 1 MeV electrons can be met only by making the source very thin, which places a limitation on the mass. For (3ß(2v) and [3[3(Ov, B) this Present address : Los Alamos National Laboratory, Physics Division, Los Alamos, NM 87545, USA 0168-9002/88/$03 .50 O Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

price is clearly justified [4-6], but for (3ß(0v), the advantage is given to the calorimetric detectors with their significantly larger sources. A desirable feature available in some tracking chambers is the ability to recognize alpha particles appearing within a few hundred lLs of a triggering event. One of the most dangerous contaminants in the uranium series is 214 Bi whose beta decay is occasionally accompanied

Fig. 1. Idealized representation of a double beta decay event m the TPC. The s2 Se is contained m the inner octagon, between two sheets of aluminized Mylar stretched across the central plane of the chamber. The electron tracks are helices about the magnetic field lines, and are normally displayed m the x-z and y-z projections indicated at the bottom of the figure .

S.R . Elliott et al / Time projection chamber

by a second electron ejected from the daughter atom by the process of internal conversion . These false double beta events can be rejected if the subsequent 164 gs Zi4Po alpha decay is recorded. A cloud chamber used previously [7,8] in our laboratory for a 82 Se double beta decay search performed the alpha particle identification quite well, and yielded a collection of other tracks suggestive of double beta decay. But the device had a number of shortcomings that eroded our confidence in the result . A large dead time and low trigger efficiency lead to a very slow counting rate and poor statistics. A time projection chamber (TPC, [9]) is almost continuously sensitive, and event storage can be triggered with essentially 100% efficiency . The cloud chamber was resistant to attempts to lower the internal radon level by means of steady turnover of the gas. The TPC has no problem with continuous gas flow . The geometry of the cloud chamber permitted electrons to sneak in unseen through regions of poor visibility . The TPC is uniformly sensitive throughout its volume, and placement of the source far from the chamber walls makes unseen electrons extremely improbable . Thousands of cloud chamber photographs had to be scanned by a physicist, whereas the digital readout of the TPC turns the bulk of this job over to computer software. For these and other reasons, the TPC is a far superior double beta decay detector. Still, the TPC had to operate within a number of constraints . The need to see the alpha particle following 214Bi decay meant that the selenium source thickness had to be limited further to well below the alpha particle range. This required a source of large area if a reasonable counting rate was to be expected . Unlike TPCs used for accelerator experiments where tracks stem from a well defined interaction region, this TPC had to deal with a vertex at any random spot on the large source area. This randomness and lack of a preferred direction for the tracks meant that spatial resolution was required in all directions at all locations, and could not be focussed on a few cathode pads with preferred orientations . These considerations motivated an all-wire cathode design without the use of pads . An additional requirement was the use of construction materials largely free of radioactivity . The TPC was built with the prejudice (from the cloud chamber and from nuclear theory [1]) that the 82 Se half-life was in the neighborhood of 10' 9 yr . Severely radioactive materials such as G-10 circuit board and ceramic capacitors were avoided, but tradeoffs accepting small levels of activity in some materials were tolerated according to this prejudice . The recently published ßß(2v) half-life of (1 .1± °o a ) x 10 2° yr [6] was arrived at only after modifying the TPC in several iterations to further reduce its activity . Subsequently, an entirely new chamber has been built to eliminate all materials harboring measurable levels of radioactivity . This new chamber is now

227

operating, and the changes from the original TPC are described in a later section. 2. Source preparation A 38 g sample of elemental selenium enriched to 96 .7% in s2 Se was obtained on loan from the Oak Ridge National Laboratory . Because this material is very costly it was important to avoid losing more than a fraction of a percent. It was also important to maintain or even enhance its chemical purity . The source had to be thin and uniform on the scale of the 2' 4 Po alpha particle range . We aimed at a total source thickness of about 7 mg/cm2, equivalent to an alpha escape probability of about 80%. The technique was very similar to that used in the cloud chamber [7,8] experiment, but will be described here in somewhat greater detail . A glass box was constructed in the shape of a right octagonal cylinder 1 .2 m high and 54 cm between opposing faces. The top was closed by a glass plate containing a small gas port. The bottom was open, and a second gas port was located a few cm above the bottom edge . This edge was bonded to a thin rubber gasket . 20 cm from the bottom, on the cylinder axis, a 9 ml quartz crucible was supported in a tungsten heating coil . The coil spanned the gap between two aluminum brackets mounted to opposite sides of the box in such a way as to present the minimum cross section in the plan view . The mounting screws passed through holes in the glass walls and served as electrical contacts for heating the crucible . A large sheet of 0.575 mg/cm2 aluminized polyester film (Mylar) was stretched taut, with the aluminized surface facing downward, and glued to an octagonal ring . The glass box with about 7 g of selenium contained in the crucible, was then set upon the center of this stretched Mylar which was temporarily supported from beneath by a flat surface covered with a polyethylene film . The rubber gasket, lightly coated with vacuum grease, formed a seal between the box and the Mylar. Air in the box was first displaced with argon to prevent oxidation of the selenium during the evaporation phase. The top port was then closed and the bottom port connected to a sealed bag to avoid overpressure while heating. The evaporation of the Se was done in two steps. First a current of 30 A was passed through the coil (4-strand, 0.76 mm tungsten wire) to gently melt the Se powder in about 19 min. When the Se was entirely melted, the current was increased to 60 A to bring the Se to just below the boiling point. The evaporation took about 34 min during which the current was regulated to prevent boiling and splattering . The generous height of the box allowed the upward velocity of the vapor plume to dissipate before impacting the top plate. The vapor condensed in the

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S.R Elliott et al / Time projection chamber

argon as fine particulates . When the molten Se was nearly gone the current was reduced to avoid making the crucible unnecessarily hot. We wanted to avoid driving off any less volatile impurities that may have been left behind by distillation . When the Se had completely evaporated, the current was stopped and the suspended Se was allowed to settle out of the Ar onto the Mylar below. This settling took about 24 h, after which 92% of the original crucible charge rested on the Mylar in a smooth but delicate deposit. The remaining Se adhered to the glass walls from which it could later be recovered with a razor blade. The glass box was removed and replaced with a wooden box. Polyvinyl acetal resin (Formvar) was dissolved in chloroform at 72 mg/ml and diluted to 9 mg/ml with cyclohexanone to prevent the chloroform from attacking the Mylar. This solution was infected into the box with an atomizer in a series of one-minute shots separated by two-minute settling times. During the one-minute spraying periods a shutter covered the Se to protect it from nonumform mist. The shutter was withdrawn for the settling time. The box was propped open to air-dry the Formvar and leave the selenium securely bonded to the Mylar. This entire procedure was repeated on another octagonal aluminized Mylar film to make a second such deposit. The two deposits were then placed together face-to-face to form the source plane with the Se sandwiched between the two Mylar films, and the aluminized surfaces facing outward toward the wire planes. The space between the films was coupled to the atmosphere so a slight overpressure in the TPC would force out any trapped air and press the source into a thin flat sheet. The resulting source was homogeneous on the scale of the 7.8 MeV 214Po alpha particle range, and consisted of 5.66 mg/cmz of 82 Se, 1 .15 mg/cmz Mylar, and 0.3 mg/cmz Formvar, for a total thickness of 7.1 mg/cmz. 3. Chamber design Beta particles passing through the TPC gas left tracks consisting of positive ions and electrons (see fig. 1) . The electrons drifted to wire planes at the ends of the chamber, and avalanched to produce pulses which were recorded with sufficient spatial and temporal resolution that a digital three-dimensional reconstruction of the tracks could be created. The framework of the TPC was a closed octagonal cylinder 20 cm high, and 82 cm between opposing faces (see fig. 2) . This gas-tight structure, made of 1 .2 cm polycarbonate (Lexan) sheet, supported the wire tension on the Lexan octagons that formed the end caps of the chamber. The source was supported by Lexan rings about the center . Lexan is a clear plastic material manufactured by General Electric Corp . It has accepta-

ble gas poisoning properties and is very free of radioactivity. The source plane served as a central electrode, with the aluminized surfaces held at the proper potential to define the drift field (see fig. 3) . Electrical contact to the aluminum was made with a small dab of silver-loaded epoxy. On either side of this source plane were two 10 em drift regions which terminated at a wire grid . To insure the uniformity of the drift field created by the potential difference between the source plane and the grid wires, field shaping rings were employed . Mylar with Ni plated Cu strips spaced on 1.27 mm centers was glued on the inside of the TPC walls. Groups of four of these strips were removed so that pairs remained spaced on 7.62 mm centers. 1 MU resistors connected these pairs in series to form a voltage divider between the source plane and the grid wires. 5 mm behind the grid was a plane of alternating anode and field wires, followed after another 5 mm by a plane of cathode wires. The anode wires were 20 hum gold plated tungsten tensioned with a 40 g weight . All other wires were 75 win Be-Cu (3% beryllium) tensioned with a 225 g weight. Center-to-center spacing of neighboring wires was 2.54 mm in all three planes . Since anode and field wires alternated, the anode-to-anode wire spacing was 5.08 mm . The anode and field wires ran parallel to the grid wires, and the potentials were adjusted to ensure that the field lines from the source plane terminated only on the anodes . Thus every ionization electron drifting toward the wire planes proceeded toward an anode to produce an avalanche. The pulse induced on the anode wire gave one coordinate of the arriving electron . The cathode wires ran perpendicular to the anode wires and were connected in pairs, again giving a 5.08 mm effective spacing. A pulse was induced on cathode wires near the anode avalanche. (A detailed description of the processes responsible for the development of these pulses has been given by Charpak and Sauli [10] .) This cathode pulse gave a second coordinate of the electron . The spatial resolution for these x and y coordinates was dust the effective wire spacing, or 5 mm . The z coordinate was derived from the arrival time of the drifting electron . This time was measured with a precision equivalent also to 5 mm . A magnetic field of 700 G was applied parallel to the chamber axis by Helmholtz coils. This choice of field was an attempt to optimize the ß(3(2v) energy resolution resulting from the combined effects of scattering and the 5 mm spatial granularity. A higher field would be appropriate to search exclusively for ßß(0v) . There were 300 instrumented wires (150 anode and 150 cathode) on each side of the source plane, with a few wires under voltage but unused at the TPC edges. The sets of wire planes on opposite sides of the source

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S. R. Elliott et al / Time projection chamber 012mm COPPER BUS STRIPS INTERLEAVED WITH 16mm LEXAN-

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Fig. 2. Construction of the TPC. Lexan is transparent, and much of what is shown in the plan wall . differed by a 90° rotation about the chamber axis, so if the anodes determined the x coordinate on one side of the source, they determined the y coordinate on the opposite side . This arrangement gave a useful comparison of anode and cathode performance when a cosmicray muon traversed both sides of the chamber. It also guaranteed that the vertex determination in both xz and yz projections had an anode component for double beta events that produced a track on each side of the

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source. Since anode projections tend to be sharper, this better defined the vertex . In retrospect, these are minor advantages . The TPC was surrounded by a 10-15 cm thick lead house (see fig. 4) . This Pb house was within a 4m cosmic-ray veto system to avoid triggering on the approximately 40 cosmic-ray muons/s that passed through the TPC. This system consisted of six gas multiwire proportional counters of varying sizes, forming a box

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S. R . Elliott et al / Time protection chamber

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about the TPC. These "vetos" were 2.54 cm thick and had 20 lim gold plated tungsten wire on 2.54 cm centers spanning their length . They were operated at about 1770 V and used a 90% argon, 10% carbon dioxide gas mixture. Gas was continuously flowed through the veto system at 7 1/h. The experiment was located at sea level in the basement of the Physical Sciences building at the University of California, Irvine, USA. 4. Electronics

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Each anode wire and each cathode pair had its own circuitry channel. The wires were soldered and then glued to pads on circuit boards of Rexolite, a glass-free styrene cross-linked co-polymer selected for its radiopurity, and manufactured by Oak Materials Group, Franklin, NH, USA. This material required special handling during the etching process to prevent warping. These boards supported the 1 MQ load resistors which were bussed to positive high voltage for the anode, and to ground for the cathode. A 10 MSS series resistor to the high voltage supply limited the current to harmless levels in the event of a voltage breakdown. The 0 .0027 IiF Mylar decoupling capacitors for the anodes were also located on the Rexolite boards . The grid wires were bussed directly to ground, and the field wires to another high voltage supply . The output of these boards was transmitted by twisted pair ribbon cables to

S. R. Elhott et al / Time projection chamber

amplifier-discriminator modules which were located outside the Pb house . These 16 channel amplifier-discriminator boards had a low component density and continuous ground plane to reduce cross-talk . They employed LeCroy TRA1000 low noise amplifiers and MVL406 time-over-threshold discriminators . The MVL406 is no longer commercially available. The anode and cathode boards were similar except for the addition of a transistor stage in the cathode circuit for pulse inversion and extra gain . The pulse width was approximately 200 ns for the anode and 400 ns for the cathode. The discriminator levels of 20 mV for the anode and 75 mV for the cathode were set by software through a DAC. The TPC was operated with < 1% of the channels dead . The differential ECL output from these amplifierdiscriminator boards was transmitted, also by twisted pair ribbon cable, to latch modules contained within a CAMAC crate. At this stage the signal was converted to TTL and clocked into an 80 "time bucket" deep shift register at 1 MHz. A grand OR of all active wires in each 1 tts time bucket was formed, and any combination of the time buckets filled by this grand OR could be selected by a built-in programmable hardware trigger. Since all tracks were clocked into the shift registers the trigger merely determined which tracks were initially saved. The time t = 0 was simply the first hit to

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arrive, there being no scintillator or other fast signal . In order to allow operation of the TPC in the event of a few noisy wires, the latch modules contained circuitry which could be used with software commands to disable the signals from individual wires. A flow chart of the signal route is shown in fig. 5 . During normal operation the drift speed was 0.5 cm/~Ls which meant that ionization over the entire drift region was collected in 20 ps . The remaining 60 buckets of the shift register were used to store any additional wire hits that occurred in the following ms (to catch the 214 Po alpha particle). The normal trigger required at least one hit in each of the four 5 Rs blocks which span this 20 ps interval . After each trigger the cosmic ray detectors were checked to see if there was a coincidence within the preceding 30 [s . If so, the trigger was rejected as a cosmic-ray muon or the decay electron of a muon that stopped. All triggers interrupted a computer, and the decision to reject an event due to a veto was made by software. For acceptable triggers, the only latch modules read out were those containing wires which were hit. This helped suppress the volume of stored data . The wire data for each latch module in a given time bucket was read out in sequential reads by the computer . Each time bucket was read in turn until all wire data were copied from the latch modules into the computer .

ELECTRONICS DIAGRAM

Fig. 5. A flow chart of the detector signal pathway

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Other circuitry included an ADC which monitored the magnet current and a counter module used to compute dead time. These were also located in the CAMAC crate . 5. Data readout A Terak (DEC LSI/11 computer with a bit-mapped graphics display) was used to read the track data from the CAMAC latch modules for the approximately 2.5 triggers/s which were not vetoed. The Terak provided an on-line event display that was convenient for debugging the electronics and setting the TPC voltages . This display also included a histogram of the number of hits which occurred on each wire . Thus histogram was used to identify dead or noisy wires. Initially, all triggering events were written directly from the Terak to magnetic tape . However, due to the = 2.5 event/s data rate, a magnetic tape was filled every six hours. For an experiment of this duration (years) this was plainly unacceptable . Therefore, after the first 2000 hours we implemented an on-line stripper in another computer. The events were transferred from the Terak to an MDB LSI/ 11-23 + by means of a direct memory access link . The MDB then sorted through these events, rejecting those which were uninteresting, and writing the surviving events to magnetic tape . As a test, the original 2000 hours of unstripped data were subsequently passed through this stripper to verify that the MDB was not eliminating any good events . The dead time due to vetos and computer loading was typically 7% . A printout of scalers monitoring the TPC operation was made by the MDB every two hours. The magnetic tapes were analyzed on a VAX 11/750 . This off-line stripping left about 200 events/day to be scanned by a physicist. 6. TPC operating parameters The gas used in the TPC for the majority of the data run was a 92 .5% helium (He) and 7 .5% propane (C3H8) mixture . A high percentage of He was desirable to minimize scattering of the low energy electrons. The specified purity was 99 .999% for the He, and 99 .5% for the C3 H R . This gas was purchased premixed and the C3HR portion actually varied between 7.2 and 7.8% among gas cylinders. Other gas mixtures we experimented with were 97% He, 3% C3HR and 80% He, 20% methane (CH4 ). Though the He-CH, mixture resulted in the most dramatic alpha particle signature, the 7.5% C3 HR gave the best beta particle tracks and least trouble with breakdowns . The gas was continuously bled into the chamber at 28

1/h and exhausted to the atmosphere through 2 cm of pump oil . A slower flow rate could be tolerated without degrading track quality, but the above rate was maintained to hold down the level of 222Rn whose daughters 214 Pb and 214Bi can contribute to background . The various wire voltages were chosen to give the highest quality tracks possible without making the chamber susceptible to high voltage breakdown. The actual values of the voltages were varied slightly as necessary to counter effects of variations in the C3 HR percentage . As a numerical monitor of track quality, we used the ratio of the number of cathode wire hits to anode wire hits during a period of = 5 min. This ratio was found to be a linear function of anode voltage for any given gas cylinder . Its range of values included a window within which tracks were judged to be of acceptable quality, namely solid, but not excessively broad on the cathodes . The anode voltage was adjusted to keep this ratio near the center of the acceptable window . Thus track quality was maintained at a consistent level. The necessary anode voltage increase was about 5.5 V for each increase of 0.1 in the percentage of propane. Since the TPC was operated at atmospheric pressure, changes in the barometric pressure were also considered but showed little if any effect . The anode voltage range was centered near + 1530 V, the grid and cathode wires were held at ground, and the field wires at +340 V. At these settings the gas gain was nearly saturated. For example, an attempt to identify alpha particles by their induced pulses on the field wires failed because muons and some beta particles induced pulses that were just as large. Furthermore, the number of time buckets not containing a hit and thus leaving holes in the track was consistent with the number expected due to ionization statistics if one assumed the TPC was sensitive to two or three primary ionization electrons. The latch modules operated with a 5% dead time . This dead time, however, was responsible for only a small fraction of the empty time buckets. Operation at lower gas gain would undoubtedly be possible with additional effort to reduce electronic noise and grounding problems . The central source plane was operated near -640 V, adjusted with the aid of cosmic-ray muon tracks penetrating the source plane. When the muon trackfragments on the two sides were made to join at the central plane to appear as a single straight line, we could be sure that the drift time was the proper 20 ~Ls . If the anode voltage was lowered to = 1350 V then only alpha particles would trigger the system . One could then measure the rate of alpha activity and do a scatter plot to identify hot spots. This technique showed the solder to be an alpha source. Occasionally the TPC would develop a continuous discharge. It was noticed that once a discharge began, it

S R. Elliott et al / Time projection chamber

could usually be quenched by lowering the anode potential to ground and then reestablishing it at its normal level . One convenient symptom of these glow discharges, was the greater drop in potential across the 10 MU series resistor to the anode supply . Thus an automated correction unit was designed to monitor the anode potential. Whenever it dropped below some threshold, this correction unit would turn the anode power supply off for about 3 min. This worked well for reducing detector down time due to high voltage dis-

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7. TPC2 vs TPCI The TPC described above (hereafter referred to as TPCI) produced the data responsible for the recently reported 15,6] ßß(2v) result . There was still some background, however, which obscured the turnover of the sum spectrum and reduced the sensitivity to the majoron

Fig. 6. Construction of TPC2 . All electronics and their associated radioactivity has been banished to the outside of the lead shield . The stainless steel TPC wires continue out of the chamber where they are glued between sheets of Mylar to form cables for passage through the shield . Again, the plan view includes structure seen through the transparent Lexan

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mode . In an attempt to reduce this background further, a new TPC (refered to as TPC2) was constructed . The materials which were recognized as radioactive in TPC1 were either moved outside the Pb shield, replaced by clean substitutes, or eliminated altogether. The active materials included the Be-Cu wires, solder, resistors, ribbon cable, and cable connectors . Unlike TPCI, TPC2 was assembled in a clean-room to minimize dust deposits on the wires . Low activity stainless steel wire replaced the Be-Cu. The wire was wound by placing the supporting Lexan pieces back-toback and tumbling them - a much faster method than stringing each wire by hand as in TPC1 . As the wire came off the spool it was degreased by drawing it between a pair of cotton pads saturated with acetone, and a second pair saturated with isopropyl alcohol. The anode wires were tensioned at 0.20 N and all other wires were tensioned at 2.25 N (e .g . for the anode, a 40 g weight was suspended from a Nylon pulley hanging in the U formed by two vertical sections of the wire.) The wire spacings and diameters were the same as in TPCI, but grooves machined in a Lexan support more accurately defined the wire spacing. The higher resistance of the stainless steel had no noticable effect on the pulses . TPC2 had no circuit boards or solder within the Pb shield . All wires were glued in place by a bead of epoxy applied just behind the grooves. The epoxy consisted of eight parts Versamid 140 polyamide resin to ten parts Shell Epon Resin 826. To prevent surface discharges between closely spaced wires at different potentials, red GLPT insulating varnish was painted over the Lexan edge where the anode and field wires passed through the grooves. The assembly was baked at 55 ° C for 24 h. (TPC2 does not experience the high voltage discharges that occasionally occured in TPC1 .) The anode and cathode wires were brought through the Pb shield sandwiched between sheets of 0.1 mm Mylar (pure polyester, without antistatic additives.) The anode wires, being extremely fine, were first joined to 75 Win wire, and this heavier wire was used between the Mylar sheets (see fig. 6) . The internal electrical connections were made with conducting epoxy. Outside the Pb house the cables were soldered to G-10 circuit boards from which ribbon cables passed the signal on to the previously described amplifier-discriminator boards . Thus, all circuit components and ribbons cables were placed outside the Pb shield . Whereas the inside of the source envelope was coupled directly to the atmosphere in TPCI, the new arrangement made the coupling through 2 mm of pump oil to discourage the intrusion of radon. TPC2 is square and uses space more efficiently than the octagonal TPCI . This allowed additional Pb to be added to the shield, making it 15 cm thick all around . All of these improvements reduced by a factor of 2-3 the background due to gamma rays impinging on the

source [5,6], and reduced the trigger rate by a factor of 10 . The number of events per day requiring scanning by a physicist dropped to about 50 . The amount of radon is lower in TPC2 as well . Much of the remaining 222Rn comes from radium in the gas supply cylinders, although there is still a little entering the chamber either by diffusion from the outside air or from internal 226R a . The 222 Rn level in TPC2 is currently about 45 atoms, although efforts are being made to reduce it further. The calibration, resolution, and all efficiencies are assumed to be the same for TPC2 and TPC1 . The operating voltages are alike for both . 8. Calibration and resolution 8.1 . EnerKv calibration

The TPC produced two orthogonal projections of each helical track made by an electron in the magnetic field. These two projections were sine waves. An example is shown in fig. 7. The parameters of the helix were estimated using an algorithm developed by McKee [111, and these estimates were then used in a linearized least squares fit to the sine waves. The energies and opening angles could then be determined from these parameters . The energy calibration was based on the 976 keV internal conversion (IC) line of 207Bi . A 1 nCi source of 207Bi deposited on a 1 cm diameter piece of very thin Mylar was suspended in the TPC gas about 2 cm front the wire planes on one side . Data were recorded for electrons leaving this source and passing through the selenium source plane to the opposite side . The energy of these events was measured twice, once on either side of the source plane . The energy loss through the central source plane was shown by the difference in the mean energies for the incoming and outgoing tracks . This energy loss agreed with the prediction of the Bethe-Bloch formula. The calibration deduced from the 976 keV line was checked at 482 keV using the other IC line in 207Bi, and agreement was better than 1 .0%. Fig. 8 shows the result of these measurements . At 2-3 MeV the calibration was checked against the Compton edge from the 2.62 MeV 208Tl gamma ray from a thorium source, and against the 2.53 MeV IC line from 2o8T] decay observed following thoron (220Rn) injections (see later sections for discussion of the radioactive gas injections). Both were consistent, although their precision was not as good as achieved with the much better statistics of the 2°' Bi calibration . The effect of both the K and L conversion lines were considered in the calibration procedure. 8 2 . Energy resolution

The resolution was measured from the same 2°7Bî data. It was necessary to correct for a 0.44 gs timing lag

S R Elliott et al / Time protection chamber

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X-Z PROJECTION r

Y-Z

82 Se SOURCE

PROJECTION

Fig. 7. Two projections of the two tracks produced m the TPC by a pair of electrons. These electrons were emitted on opposite sides of the source plane by a likely 82 Se double beta decay. The track above the source and the one below each show the IT/2 phase shift between their respective x-z and y-z projections. In both cases the direction of the shift is away from the source, indicating that these tracks were indeed those of two electrons leaving the source and not a single electron that passed through it . (See figs . 1 and 2 for definition of x, y and z with respect to the magnetic field ) The x-z projection of the upper track was determined by cathode wires, as was the y-z projection of the lower track. Note how these cathode projections are more solid than the other two which were constructed from anode hits. The energies are 1 0 and 0.6 MeV for the upper and lower tracks, respectively

of the digital anode pulses with respect to the cathode

to order to achieve the best resolution. Fig. 8 shows the full width at half maximum (FWHM) to be about 200 keV at 1 MeV after this correction . (Before the correction the FWHM was 240 keV.) When the 200 keV width

of fig. 8 was corrected for the inclusion of the L line, the detector resolution was 175 keV FWHM at 1 MeV. Thus the sum energy resolution for a 2 MeV event comprised of two I MeV electrons was about 12 .4% FWHM .

Fig. 9 shows a scatter plot of momentum vs the

cosine of the angle the electron makes with the magnetic field direction for a large number of 207Bt IC electrons. The figure shows that we were unable to

deduce the energies of tracks with very small or very large angles . If an electron made an angle too large, the

SINGLE ENERGY (MeV) Fig. 8. The energy measurement of approximately 10 ° IC electrons from 207Bi The 207 131 source was located on one side of the source plane near the wires. The top figure shows the energy as measured for each track on that side. The bottom figure shows the energy measurement for each track after it has passed through the source plane.

cos Fig. 9. A plot of the momentum of IC electrons from 2°7Bî as a function of the cosine of the polar angle they make with respect to the magnetic field.

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9. Opening angle bias C U N

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COSINE OF ANGLE TO MAGNETIC FIELD

Fig 10 The calculated percentage FWHM energy resolution for single electrons at 1 MeV as a function of the cosine of the polar angle the electron makes with the magnetic field. The discontinuity arises at the angle at which an electron makes more than one complete turn in the TPC before exiting The calculation is not valid below that point

track wrapped back onto itself . This limit arose because of the 1 gs time bucket or 5 mm z direction spatial resolution . If the angle was too small, the track became too straight to observe any curvature. This limit was a result of the 5 mm wire spacing. The main contributions to the resolution width were multiple scattering in the TPC gas and the detector spatial resolution. For high cos 0 the 5 mm spatial resolution was the primary cause of the resolution width, and for low cos 0 multiple scattering carried the blame. For the data m fig. 8 the presence of the L and M IC lines was a factor contributing to the width. Other small contributions to the resolution were the 0.5% accuracy of the ADC which measured the magnet current and the energy loss as the electron traversed the TPC gas. A calculation of the cos 0 dependence of the percent FWHM to be expected for the 1 MeV 207 Bi line as a result of these contributions is shown in fig. 10 . The details of this calculation and its favorable comparison to measurement are described elsewhere [5].

To interpret observed opening angle distributions we had to understand the detector acceptance as a function of opening angle. First we limited our opening angle analysis to events which had the two electrons emitted on opposite sides of the source plane (referred to as OS events). Events with the two electrons on the same side of the source plane (SS events) had a more complicated bias (due to the software identification technique) because the two electron tracks interfered with each other. The bias was initially approximated by a simple Monte Carlo calculation . For each event of a sample of two-electron Monte Carlo events the opening angle was generated according to an isotropic distribution . The direction of one of the electrons was chosen randomly and the other was determined by the opening angle. First a restriction was placed on these events requiring that the angles represent OS events . Second, events including one or more tracks that made an angle of greater than 78 .5 ° (cos 0 < 0.2) or less than 25 .8 ° (cos 0 ? 0.9) with respect to the magnetic field were also eliminated . This cut simulated the loss of events which were either too parallel or too perpendicular to the magnetic field to be measured (see fig. 9) . The opening angle distribution of the surviving events was an approximation of the bias function . The deviation from isotropy shown by this distribution of opening angles simulated the TPC bias for OS events . Next, to find the bias actually measured by the TPC, we needed a selection of two-electron events which were drawn from an isotropic distribution . This selection was generated in two ways . One, a sample of lone electrons (not members of pairs) was used . Each electron was paired to turn with every other electron that was on the opposite side of the source . The track parameters of

15

w Q

MEASURED

10

Z 0 H v Z 05

8.3. Opening angle resolution

Although the layer of Se was very thin, the opening angle resolution width was due largely to multiple scattering of the electrons by the Se . The scattering was studied by looking at electrons which passed entirely through the TPC, penetrating the source plane. To see directly how well events preserved their opening angles, we examined a group of Moller scatters, and found that the measured opening angles approximated the Moller angle with a RMS of about 12 .5° .

- FIT

a

00 -10

Fig

-05 COSINE

00

05

(Opening Angle)

10

11 . The opening angle bias for events which have the electrons emitted on opposite sides of the source plane.

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S R Elliott et al / Time projection chamber

these artificial pairs were used to deduce the opening angles . Secondly, the candidate events themselves were used, however the pairs were broken up and the individual electrons were matched up with each opposite-side electron as before . Again the opening angle distribution given by these pairs should represent the bias function . The Monte Carlo and the two real-track methods all agree, and their result is plotted in fig . 11 . Also plotted is the least squares fit to a 5th order polynomial to the data . The Monte Carlo calculation showed that the flat plateau seen in fig. 11 was caused by the restriction of angle relative to the magnetic field and source plane. (Without this restriction, the bias would be similar to (1 - cos B) .) This bias function was used to unfold the detector bias from the data . 10 . Efficiency 10.1 . TPC trigger efficiency

The TPC trigger efficiency for muons was measured by looking for tracks in coincidence with two proportional counters placed on opposite sides of the TPC . Most coincidences between these counters corresponded to the passage of a single muon which, therefore, also passed through the TPC. The TPC trigger efficiency was found to be virtually 100% for cosmic-ray muons and it was assumed to be the same for beta particles. 10.2. Efficiency for finding lone electrons

The 214 Bi, 214 Po, ß + a cascade was exploited to measure the software efficiency for different event topologies . This was accomplished by infecting the 214 Bi progenitor, 222Rn, into the gas inflow line . The flow was then stopped and the radon in the TPC gas decayed. (The most dangerous daughters of radon are short-lived and do not permanently contaminate the TPC. However, there remains a small, low energy background contribution from the 22 yr 210 Pb residue.) The 222 R n daughter, 218 Po, is usually born with a positive charge, in which case it is attracted to the central electrode [8] where it a-decays to 214ph. The nucleus may recoil from this a-decay, but again, it will usually be attracted to the central electrode where it then remains fixed [13] while it ß-decays to 214Bi and 214po . The great majority of 214 Bi decays produced only a single electron . With the strippers left off after a radon infection, we searched the data tape for alpha particles showing up in the 1 ms delay period following each recorded trigger event. When an alpha particle was found, the associated trigger event was essentially always a single electron . The collection of single electrons

100 O

80 Y

0 0 w

rr w m z

60 40 20 0 [YL

00

i

,

i

0 51 1 1 1 0 1 .5 ENERGY (MeV)

20

2.5

Fig 12 . A comparison of the 214131 ß spectrum as measured by the TPC (circles) and as measured by a beta spectrometer (histogram) They are normalized to the total number of counts over 200 keV. found in this manner was then run through the strippers, which were blind to the alpha particle, and the surviving fraction gave the efficiency of the strippers for finding a lone electron (not a member of a pair) leaving the source plane. The resulting lone electron efficiency averaged over all energies was 38 .8 ± 4.0%. The main losses were due to backscatter of electrons from the Lexan behind the wire planes (measured to be 19 .2 ± 1 .4%), and tracks which have angles too close to parallel or perpendicular to the magnetic field direction to be resolved (measured to be 28 .8 ± 1 .4%) . To understand the energy dependence of this electron efficiency, the beta spectrum from a collection of ß + a events was compared to the 2" Bi spectrum as measured by a beta spectrometer [14] . The TPC reproduced the 214131 spectrum quite faithfully between 0.2 and 2.5 MeV as shown in fig. 12. For electrons with an energy above about 0.2 MeV the efficiency is energy independent and is 52 .5 ± 5 .4% . 10.3. Efficiency for finding delayed alpha tracks

The thickness of the Se deposit was small enough that 77% of the 214po a particles would not be trapped within the source . Thus, theoretically 77% of the twoelectron events due to 214 Bi could be removed by observation of the a. However the 214 Bi was present only on the source surface [5]. This gave rise to an additional elimination technique. When the a leaves the nucleus, it may produce some shake-off electrons that then drift to the wire planes to give a one or two time-bucket "blip" at the event vertex . Thus these shake-off electrons allow identification of the 214Bi event even when the a is absorbed by the source .

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S R Elliott et al / Time projection chamber

The only 222 Rn daughter capable of producing beta particles with an energy greater than about 1 MeV is 214Bi . Thus by looking at ßs at this energy or higher following the radon injections, we could assume that there would be a following a and look to see whether it had been recorded . The probability that one of these ßs had a recorded a or some shake-off electrons following it was measured to be 96 _+ I% . The question was how may of these as could be found without the use of the ß . After an alpha particle track was collected at the wires, there would usually follow a sequence of isolated one or two time-bucket hits that would dribble in over a period of hundreds of microseconds, up to the end of the 1 ms recorded delay interval . Since this behavior was unique to alpha particles, the "dribbles" provided a useful alpha-particle signature. Thus to search for the a of 214po we required a track at least five buckets long, delayed with respect to the primary track, and followed by at least one dribble. The mechanism responsible for the dribbles was never conclusively determined . Heavily ionizing as however seemed to be the only tracks with which the dribbles were consistently associated . Alpha tracks in the gas mixture He-CH4 made many more dribbles than the mixture He-C 3 H s , and thus our a detection efficiency was much larger in the former mixture. In addition, dribbles increased dramatically if the gas flow was stopped for a day to allow a buildup of impurities . The probability that our dribble technique would find the a without the aid of the (3 was 35 ± 3% for the He-C3Hs mixture, and 74.3 ± 0.5% for He-CH4 . With TPC2 we instigated a procedure for ignoring tracks in the 1 ms delay period if they were associated in time with a cosmic-ray veto pulse. Since most tracks in this period had been caused by the cosmic rays, the burden of saving all triggers having delayed tracks was greatly reduced. The number of events saved was small enough that the a particles could be found efficiently without the use of dribbles, so the dependence of the identification efficiency on gas composition and purity was no longer a factor . 10 .4 Efficiency for finding two-electron events

The efficiency for finding events with two-electrons was also measured following the radon injections . This efficiency was deduced in four separate ways and the weighted average, 28 .2 ± 2.3%, was used as the detection efficiency . Each method had advantages and drawbacks and thus the necessity of the differing methods as a cross check. The agreement of the four methods is reassuring . Method 1 deduced the two-electron event efficiency from the lone electron event efficiency . The advantage

of this method was the large statistics . The disadvantage was the necessity of estimating the effect of the interference between two electrons. This method gave a result of 29 .7 ± 2.1%. Method 2 used Monte-Carlo-generated double-betadecay-like events . These simulated TPC tracks were simple discretized helices. The disadvantage of this method was the necessity of correcting for the artificially high track quality of the Monte Carlo tracks and for backscattering losses . The advantages were good statistics and that the interference between the electrons' tracks was approximated . The result was 27.5 ± 1 .5% . Method 3 used actual two-electron events caused by the ß + IC sequence in 214Bi . These events were found by the delayed a decay. This was the most direct method as it included real TPC two-electron tracks, but it suffered from small statistics . The result was 25 .6+ _ 4.3% . Method 4 used the 214 Bi (3 + a rate during the radon injections along with the IC probability to estimate the number (3 + IC + a events there should be. This number was then compared to the number found. This still had a rather large error due to the low number of ß + IC + a events . The result was 32 .0 +_ 5.9%. To investigate any energy dependence of the efficiency for two electron events, the spectrum of ß + IC events from 214Bi was compared to Monte Carlo predictions (see fig. 13). This agreement implies small if any dependence on energy, especially in the higher energy ranges, i .e . greater than about a sum energy of 500 keV.

20 15

> 10 Y

O O

N Ir W

z

0 0

SUM ENERGY SPECTRUM -

5 6~ SINGLE ENERGY SPECTRUM

40

20 I

2

ENERGY (MeV)

3

Fig 13 . A comparison of the ß + IC spectra for 214 Bi as measured by the TPC (plotted points) and as predicted by the Monte Carlo (histogram) . The Monte Carlo is normalized to the total number of measured counts. There are no applied thresholds

S. R. Elliott et al / Time projection chamber

11 . Overview of the event-searching algorithms The software which was written to search for double beta decay candidates classified events into two types, namely those which had the two electrons emitted on opposite sides of the source plane (OS), and those for which both electrons were emitted on the same side of the source plane (SS) . For OS events, the direction in which each electron was traveling with respect to the source plane was all that had to be determined to identify candidates . Since the source plane separated the two cracks from each other, there was no confusion concerning the matching of the projections. For SS events however, not only did the two tracks within each projection have to be untangled, but the correct matching of the two projections needed to be determined . To untangle the tracks, each time bucket was examined in each projection for the presence of two groups of hit wires separated by a gap. For a given time bucket, the left group of wires, as one views the wire data plots, was assigned to one track and the right group of the remaining track. If no such gap was present it was assumed that the tracks "crossed over" in that time bucket, and both tracks were assigned that position . In the time buckets following the cross-over, the definition of track assignment for the left and right groups of hit wires was switched. Most tracks had some empty time buckets due to ionization statistics and a 5% dead time in the latches. Thus a false cross-over point could possibly arise due to a track containing such a hole. The time buckets which were cross-over point candidates were compared to neighboring time buckets to decide which of the two possibilities was more likely . At this point, four projections, two x-z and two y-z, had been untangled. However, there were two possible pairings and the correct one had to be determined . Both possible pairings were analyzed by the track fitting routines, and the pairing with the better fit was chosen as the correct one. If both tracks were found to be leaving the source, the event was considered a candidate. The extra complexity of the search for SS events resulted in a lower efficiency for that topology . 78 .5% of the candidate events found were OS events . The OS and SS events surviving these analyses were examined by a physicist to determine whether or not they should be considered candidates . 12 . Conclusions The time projection chamber has proved to be a useful tool for investigating double beta decay. It is especially effective in searching for modes with broad

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energy spectra (ß(3(2v) and ßß(0v, B)) to a variety of isotopes. The wealth of information supplied with each event contributes to a good understanding of background processes, and provides the data necessary to distinguish a double beta decay signal . Acknowledgements The authors gratefully acknowledge the contribution of the late Herb Chen, who brought TPC technology to our attention when we were looking for a substitute for the cloud chamber. We acknowledge invaluable discussions with Dave Nygren, Jay Marx and Nick Hadley at LBL, Herb Anderson and Wayne Kinneson at LANL, Doug Bryman at TRIUMF, Kate Morgan and Roger Boucher at CERN, and Andrus Skuja at DESY . We thank Carl Brannon, Herman Brown and Urs Herzog for major contributions to the electronics and construction of the experiment . We owe special thanks to Frederick Reines for his continuous help and encouragement throughout this work . This project is supported by the US Department of Energy, Contract no . DE AT03-76SF00010 . References

[2]

[41

[61

[8]

[III [121 [13] [14]

W.C . Haxton and G.J . Stephenson Jr ., Prog . Part . Nucl. Phys . 1 2 (1984) 409 . A recent review of double beta decay experiments is given by D.O . Caldwell, Nucl. Instr. and Meth . A264 (1988) 106. M . Doi, T. Kotani and E. Takasugi, Prog . Theor. Phys. Suppl. no . 83 (1985) 1. S.R . Elliott, A.A . Hahn and M.K . Moe, Phys . Rev . Lett 59 (1987) 1649 . S.R . Elliott, Ph .D . dissertation, University of California, Irvine (1987) unpublished . S.R . Elliott, A.A . Hahn and M.K . Moe, Phys . Rev. Lett . 59 (1987) 2020 . D.D . Lowenthal, Ph .D . dissertation, University of California, Irvine (1976) unpublished. M.K . Moe and D .D . Lowenthal, Phys . Rev C22 (1980) 2186 . R.J . Madaras and P.J . Oddone, Phys . Today 37 (1984) 36 G. Charpak and F. Sauli, Nucl . Instr and Meth. 162 (1979) 405. R.J McKee, The Time Projection Chamber, ed J.A . MacDonald, AIP Conference Proceedings No 108 (AIP, 1984) P. 242 M Atac, IEEE Trans. Nucl . Sci. NS-31 (1) (1984) 99 . L. McCarty, Master's Thesis, University of California, Irvine (1987) unpublished. H. Daniel and R. Nierhaus, Z Naturforsch . Ila (1956) 212.