PARTICLE DETECTORS The time proj ection chamber turns 2 5

A time projection chamber (TPC) provides a Since its birth 25 years charged to -75 kV produced a strong elec­ complete, 3D picture of the tric field (figure 2, see p41). Underthe influ­ deposited in a (or liquid) volume. It acts ago, the time projection ence of this field, ionization drifted somewhat like a bubble chamber, albeit to one of the two end caps. A solenoidal with a fast, all-electronic read-out. The TPCs chamber has developed into minimized the transverse 3D localization makes it extremely useful in a mature technology that is diffusion and bent the charged particles to tracking charged particles in a high-track- allow momentum measurement. density environment, and for identifying used in many fields, as The end caps were divided into six sectors, particles through their ionization energy loss each one containing a 183- multiwire (dE/dx). To honour the 25th anniversary of Spencer Klein describes. proportional chamber (MWPC). Drifting elec­ the TPC, a symposium was organized at the trons were accelerated in the strong electric Lawrence Berkeley National Laboratory on fields around the wires and acquired enough 17 October 2003, with workshops that kinetic energy to ionize the gas and produce included presentations on the past, present an avalanche. A single drift pro­ and future of the TPC. duced about 1000 electrons in the wire. The TPC was invented by Dave Nygren at The wire signals were sampled 10 million the Lawrence Berkeley Laboratory (LBL) in times per second to a 9-bit accuracy by an the late 1970s. Its first major application analogue storage unit based on a charge- was in the PEP-4 detector, which studied coupled device (CCD). The signals were 29 GeV eV collisions at the PEP storage then digitized at a slower rate using in­ ring at SLAC. Since then TPCs have been expensive ADCs. The wire data were used to used to study e+e" collisions at PEP, at the measure particle energy loss. Because of TRISTAN collider, at the KEK laboratory and the high gas pressure, the ionization could at the Large Electron Positron (LEP) collider be measured accurately and the dE/dx at CERN. A TPC could also be the central resolution achieved was an unprecedented detector at future e+e" linear colliders. 3%. This meant that pions, kaons and The device has also figured in a number protons could be identified over most of of experiments involving heavy-ion collisions the kinematic range. at machines such as LBL's Bevalac and Charged particles were tracked using the Relativistic Heavy Ion Collider (RHIC) at data from 15 rows of 7 x 7.5 mm2 metallic Brookhaven; and now the ALICE collabora­ pads located under the wires. When an tion is building a large TPC to study heavy- electron produced an avalanche on an ion collisions at the Large Hadron Collider anode wire, a cloud of positively charged (LHC). TPCs have also been used in a whole Fig. 1. Dave Nygren (left) and colleague ions remained in the gas. The image charge host of non-accelerator experiments. stand in the PEP-4 experiment showing that formed on the metallic pads was the TPCs eventual location. (Photo: LBNL.) then measured using a charge-sensitive TPCs in particle preamplifier. By measuring the relative The PEP-4 TPC (figure 1) was built to combine charged-particle track­ charge on several adjacent pads, the ionization could be localized ing with good particle identification by measuring the specific energy to approximately 250 |jm. These pads were also read out by loss (dE/dx) of charged particles. This 2 m long cylindrical TPC had the CCD system. an inner diameter of 40 cm and an outer diameter of 2 m, and had Later TPCs used many of the techniques pioneered by PEP-4. most of the features of newer TPCs. Some notable examples were the ALEPH and DELPHI TPCs at LEP, Charged particles from eV collisions in the centre of the TPC the TOPAZ experiment at TRISTAN and the early vertex chambers for ionized molecules in a mixture of 80% and 20% methane gas the CDF experiment at Fermilab. The ALEPH TPC at LEP was one of at 8.5 atmosphere. A central membrane (the ) that was the larger examples, measuring 3.6 m in diameter and 4.4 m in

40 CERN Courier January/February 2004 PARTICLE DETECTORS

length, with twice as many dE/dx measurements as the PEP-4 TPC. Both of the LEP TPCs used flash ADC systems instead of CCDs. TPCs have also been used in a number of smaller experiments, such as in studies of muon decay and capture. The MuCap experi­ ment at the Paul Scherrer Institute, for example, is building a 10 atmosphere hydrogen-gas TPC to measure muon lifetime.

TPCs for heavy ions With the growth of research with relativistic heavy-ion collisions in the early 1980s, TPCs found another home. The 3D picture of ion­ ization is ideal for tracking particles in high-density environments - hundreds or thousands of particles from a single collision - in which other detectors are overwhelmed by the huge multiplicity. The first large-acceptance TPC was in the Equation-of-State experiment Fig. 2. A schematic view of the PEP-4 TPC showing the drift (EOS), which studied heavy-ion collisions at energies of a few GeV volume (dark blue), wire chambers (green) and pads (light blue). per nucléon at the Bevalac. The rectangular EOS TPC measured 150 x 96 x 75 cm. Electrons drifted downwards in a uniform electric has a 4 m diameter, 4 m long TPC at its centre. The TPC for STAR field and were amplified by 3000 times in a MWPC. Data were read follows the geometry of the PEP-4 and ALEPH TPCs, but relies on out from 15 308 pads, which sensed the image charge from positive 138 000 pads that are read out by SCA digitizers for both dE/dx and ions in the same way as in PEP-4. tracking information. The system is much faster than previous exper­ One key development in using TPCs in heavy-ion collisions con­ iments: it can digitize an event containing 70 million volume elements the electronics. In the high-track-density environment many to 10 bit precision and transmit it to the data-acquisition system in pads are required and each pad detects signals from such a large 10 microseconds. Figure 3 shows an event in the STAR TPC. number of tracks that it must be read out by a waveform digitizer. The ALICE heavy-ion experiment at the LHC is built around a mam­ The CCD analogue storage units used with earlier TPCs required moth TPC, 2.5 x 5.5 m with 750 000 pads. Analogue-to-digital-con­ considerable power and difficult calibrations. They were also expen­ verter technology has matured and ALICE has replaced SCAs with sive. So EOS used a new technique, the switched array custom integrated circuits, each containing 16 x 10-bit ADCs with (SCA), developed by Stuart Kleinfelder. digital filters for tail suppression and zero-suppression circuitry. Data The EOS SCA consists of an array of 128 , each con­ can be read out 1000 times per second. nected to an input by a switch. By rapidly opening and closing the Sometimes longitudinally drifting electrons may not be optimal. switches, the capacitors can be connected to the input one by one, The collaborations for STAR and CERES (NA45 at CERN) have built forming an analogue storage unit. The sampling rate is matched to cylindrical TPCs where the electrons drift outwards radially from a the drift time of electrons across the TPC, and the capacitors are read cylindrical central cathode towards on a concentric outer out by an inexpensive (but slow) analogue-to-digital converter. This cylinder. This geometry is advantageous when tracks are parallel to scheme reduced the cost and power consumption of waveform digi­ the cylinder's axis, but it introduces many complications. The electric tizers, making TPCs a practical tool for the study of heavy-ion colli­ and magnetic fields are no longer parallel, which leads to complex sions. Packaging was integral to the success of the electronics. The electron-drift trajectories. Curved pad-planes are then required, or preamplifiers, SCAs, digitizers and multiplexers were mounted directly the idealized cylindrical geometry must be complicated. For these on the TPC, and a handful of optical fibres replaced 15 000 cables. reasons, radial-drift TPCs have a more complex structure and poorer After completing its service at the Bevalac, the EOS TPC was resolution than linear-drift devices. However, sometimes these fac­ moved to the Brookhaven Alternating Gradient Synchrotron (AGS), tors can provide a worthwhile trade-off. where it was used to study heavy-ion collisions in experiment E895 and proton-ion collisions in experiment E910, and then to Fermilab Non-accelerator applications for experiment E907 probing higher energy proton-ion collisions. TPCs are also used in many non-accelerator experiments such as By the time E907 ends, EOS will have seen more than 15 years of double-beta decay and dark-matter searches. Often these experiments service at three different laboratories. use dense media, such as liquids, where the active detector volume EOS pioneered techniques that were used in many other experi­ also serves as the experimental target (for or ) or ments. At CERN the NA35, NA36 and NA49 experiments all used a radioactive source (for double-beta decay, proton decay, etc.). TPCs to study heavy-ion collisions. NA49 took the construction of the The first laboratory observation of double-beta decay, in 1987, devices to new volumes with four huge TPCs - the largest pair meas­ by Steve Elliott, Alan Hahn and Michael Moe, used a thin layer of uring 3.8 x 3.8 x 1.3 m. The complex was read out by 182 000 82Se deposited on the central cathode of a TPC. Though very suc­ SCAs; these SCA chips had integral ADCs. cessful, this technique was limited to relatively small sample The next step up in heavy-ion collision energy was the RHIC at volumes. Most current efforts use a single material, such as liquid Brookhaven, and it was natural that TPCs would play a key role. Two 136Xe, as both a source and drift medium. One particularly ambi­ original TPC-based proposals merged into the STAR detector, which tious group, the Enriched Xenon Observatory collaboration, plans >

CERN Courier January/February 2004 41 PARTICLE DETECTORS

previously uncharged molecule. The advantage of ion drift is that the diffusion can be much smaller. One big drawback is that posi­ tively charged ions cannot induce avalanches, greatly complicating the detection of the signal. The much slower drift velocity seems to offer both advantages and disadvantages. Ion-drift TPCs have been considered for a variety of applications, including double-beta decay and dark-matter and axion searches.

Future directions The most exciting technological developments in gaseous TPCs concern electron amplification, where two new technologies are replacing wire chambers. Gas Electron Multipliers (GEMs) are plastic foils that are metal coated on both sides, with 50-100 |jm diameter holes punched in them. The metal coatings are charged to a potential difference of a few hundred volts, creating strong electric fields in the holes. Electrons drifting into the holes ionize the gas, cheating an avalanche much like that formed around anode wires in conventional chambers. Fig. 3. A side view of the tracks from a 200 GeV per nucléon GEMs have several advantages over wire chambers. They are easily pair gold-on-gold collision at RHIC, as reconstructed in the TPC supported, eliminating wire sag and instability, and can be placed of the STAR experiment. very near to read-out pads, reducing diffusion after amplification. The high hole density provides an even amplification over a large area. Positive ions generated in the wire avalanche drift naturally away from the amplification region, eliminating the build-up of space charge in the amplification region. Micromesh gaseous structure chambers (Micromegas) use a thin metal mesh instead of anode wires. The mesh can be supported a small distance above the pads. A simple wire grid above the Fig. 4. This image shows a muon-to-electron decay recorded in Micromegas produces a potential difference with the mesh, so elec­ the liquid-argon TPC that is used in the ICARUS project. The tron avalanches form in the strong electric fields around the mesh muon travels about 60 cm before decaying. The decay elements. Like GEMs, Micromegas can be placed very close to read­ produces a pronounced kink. out pads, greatly reducing diffusion. They also have the same advan­ tage as GEMs for positive-ion elimination. to use a liquid- TPC to localize double-beta decay events and Both GEMs and Micromegas have a somewhat lower gain than then insert a probe into the xenon to extract the 136Ba daughter wire chambers. However, two or three layers of GEMs or Micromegas product for detailed study. can be cascaded by placing the foils or meshes on top of each Liquid-xenon TPCs are also being used as imaging detectors to other, thereby multiplying the gains. GEMs and Micromegas are track photons with energies of a few MeV. The photon directions are beginning to replace wire chambers in some experiments, most determined by reconstructing double-Compton interactions. A liquid- notably in the COMPASS experiment at the SPS at CERN. They are xenon imager has already been used to study the galactic centre. also prominent in R&D for future linear colliders and for upgrades of The technology might also be used to search for photons from the detectors at RHIC. smuggled nuclear material. Over the past 25 years, TPCs have grown into a proven, mature Liquid-argon TPCs have been studied for many years under the and flexible technology. With these new developments the next aegis of the ICARUS (Imaging Cosmic and Rare Underground quarter century looks equally bright. Signals) project. The current T600 prototype, based on 476 tonnes of liquid argon in a volume of 275 m3, recently completed a 68 day Further reading engineering run. The collaboration's goal is a 3000 tonne detector in J AMacDonald (ed) 1984 The Time Projection Chamber AlPConf. the Gran Sasso Laboratory in Italy, which will study solar and atmos­ Proc. 108 (AIP, New York). pheric neutrinos, terrestrial neutrinos from CERN's Super Proton Jay N Marx and Dave Nygren 1978 Physics Today October 46. Synchrotron (SPS), and also proton decay. The solar study For more about the Time Projection Chamber symposium, see may be quite challenging in terms of backgrounds. www-tpc. I b I. gov/sy m posi u m/. One interesting idea being pursued by several groups is to use For more about the use of TPCs at future linear colliders, see drifting ions rather than drifting electrons in a gas or liquid. Both www-lc.lbl.gov/tpc/meeting/. positively and negatively charged ions have been considered; the latter can be formed when an ionization electron attaches itself to a Spencer Klein, Lawrence Berkeley National Laboratory.

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