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Origin and development of the TPC idea
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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Origin and development of the TPC idea David R. Nygren Department of Physics, University of Texas at Arlington, Arlington, TX 76019, United States
a r t i c l e
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Keywords: Time Projection Chamber Particle detection
a b s t r a c t The emergence of the Time Projection Chamber concept in 1974 is an important chapter in the history of particle detection advances. Recalling an era very different from today, I describe the flow of events from the moment of inspiration through prototypes to realization as a major detector for the Positron–Electron Project (PEP) at SLAC. The sense of excitement in bringing forth a genuine paradigm shift carried the collaboration forward, through struggles, victories, defeats and exhaustion, yet ultimately leading to success. © 2018 Published by Elsevier B.V.
1. Historical context The first instance of electronic detection of single subatomic particles can likely be traced to experiments by Ernest Rutherford and Hans Geiger in Manchester: An Electrical Method for Counting 𝛼-particles in Radio-Active Substances, published in the Proceedings of the Royal Society in 1908 [1]. Reading this milestone paper is rewarding, for insight into the scientific acumen of Rutherford and Geiger and for the engaging style of writing, truly a letter to colleagues. They state: ‘‘It has been recognized for several years that it should be possible by refined methods to detect a single 𝛼-particle by measuring the ionization it produces in its path.’’ After discussing difficulties encountered in attempts to measure that charge directly, they declare: ‘‘We then had recourse to a method of automatically magnifying the electrical effect due to a single 𝛼-particle. For this purpose we employed the principle of production of fresh ions by collision. In a series of papers, Townsend [2] has worked out the conditions under which ions can be produced by collisions with the neutral gas molecules in a strong electric field.’’ After describing their apparatus, (see Fig. 1) the authors provide their method for the first electronic detection of a subatomic particle (as 𝛼-particles were seen in those days): ‘‘In this way, the small ionization produced by one 𝛼-particle in passing through the gas could be magnified several thousand times. The sudden current due to the entrance of an 𝛼-particle in the testing vessel was thus increased sufficiently to give an easily measurable movement of an ordinary electrometer’’. And so was realized, in the first decade of the twentieth century, the first electronic detection of individual particles in the subatomic realm. Over a century later the path-breaking technique of electrical magnification by production of fresh ions – albeit with much further refined methods – is still prominent in many of today’s experiments in particle and nuclear physics. By 1968, a zenith had been reached through the
conception and development of the Multi-Wire Proportional Chamber (MWPC) by Georges Charpak [3] at CERN. At about the same time, the drift chamber was developed in Heidelberg, Saclay and CERN. The MWPC and drift chamber concepts in various forms revolutionized particle detection. Through their unprecedented high rate capabilities-enabled by advances in electronics—these detection techniques opened the current era of high sensitivity and statistical precision in elementary particle physics. Here I recall some of the memorable events that occurred during the invention and subsequent development of the Time Projection Chamber (TPC). My main aspiration for this highly personal memoir is to offer the young scientist a glimpse of an extraordinary moment of inspiration, collective struggles, and ultimately, success. The story begins, however, with the author in a quandary. 2. Quandary In February 1974 with this toolset of detection techniques at hand, I set myself the task to conceive a detector that would provide some substantial advantage relative to those currently in use at the Stanford Positron–Electron Accelerator Ring (SPEAR), [4] which had been brought into operation in 1972. The discovery of the 𝛹 and the subsequent November Revolution were yet to arrive. As the first Divisional Fellow in the Physics Division at the Lawrence Berkeley Laboratory,1 I was strongly motivated to do something noteworthy. I was also driven by stylistic proclivities acquired as a post-doc working for Jack Steinberger while he was a faculty member at Columbia University. 1 (from 1958, the Lawrence Radiation Laboratory (LRL), then Lawrence Berkeley laboratory (LBL) in 1971, and finally renamed Ernest Orlando Lawrence Berkeley National Laboratory in 1995, and thence, known simply as Berkeley Lab)
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2018.07.015 Received 5 March 2018; Received in revised form 5 July 2018; Accepted 6 July 2018 Available online xxxx 0168-9002/© 2018 Published by Elsevier B.V.
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From Jack, through some powerful osmotic force, I had absorbed a guiding principle: one should focus on doing only the best physics and best experiments. His disdain for efforts falling short in either regard was palpable. So, perhaps unconsciously, I committed myself to a risky path: to find something truly innovative, hopefully paradigm shifting—and if unsuccessful, be prepared to accept failure and leave the field. For about six months prior to February 1974, I had been solidly in the quadrant of failure with neither innovative nor paradigm-shifting ideas. After a long afternoon at a drafting table with pencil, straight-edge, compass and triangles, I had only an increasing pile of crumpled papers on the floor to show for my effort. I had exhausted known technical approaches. I began to see failure as an imminent possibility and decided to take some time to reflect. 3. Inspiration strikes Fig. 1. The apparatus of Rutherford and Geiger, used in the first experiment to detect subatomic particles electronically. With stopcock F open, 𝛼-particles emanating from radon in volume E could pass through a thin window into volume A, ionizing the gas present. The ionization electrons were attracted to wire B at a positive potential. The potential was sufficiently high that avalanche multiplication of several thousand was achieved. The resulting current was large enough to cause visible impulse deflection of an electrometer.
After a while my thoughts wandered to an observation made several years earlier while I was a graduate student at the University of Washington, Seattle. The UW group had undertaken to measure the magnetic moment of the 𝛴 + hyperon at the LRL Bevatron [5]. At the secondary beam energies available, the experiment required a very strong magnetic field to rotate appreciably the hyperon magnetic polarization before decay occurs. The central particle detector was a small spark chamber placed inside a 200 kG pulsed magnet (see Fig. 2). Optical reflections at the spark chamber edges permitted external photographs to be taken of each triggered pulse. The pulsed electric field of the spark chamber was thus aligned with the magnetic field. A very strong increase in spark optical brightness was observed in tracks when the magnetic field was on; strangely, the sparks were visibly much narrower in that case. This unexpected effect was beneficial for event reconstruction and taken for granted henceforward; no one thought to further pursue any underlying phenomena. But I remembered it as an interesting curiosity. At that moment in February 1974 while I was feeling my goal slipping away, it suddenly occurred to me that, instead of the traditional orientation of perpendicular magnetic and electric fields in drift chambers, perhaps a parallel configuration of electric and magnetic fields might plausibly have a beneficial effect—even for more mundane values of electric and magnetic fields. Might this unorthodox configuration suppress diffusion of electrons during drift? If so, then perhaps track images could be transported over very large distances with minimal deterioration. And, I realized, if the readout planes could capture x–y information as a function of time, and, if the time of the event was known (by some other means), the event would be captured in perfect, precise, unambiguous 3-D. Clearly this idea needed a captivating name! Thus was the Time Projection Chamber concept born on a waning winter afternoon. With this novel and alluring idea tumbling rapidly around in my mind, I sat, transfixed, imagining new possibilities until sunset. However, I had no particular expertise in the physics of gasbased detectors beyond participation in construction and utilization, under Jack’s leadership, of a set of large MWPCs for experiments in 𝐾0 decay physics at Brookhaven National Laboratory’s Alternating Gradient Synchrotron (AGS). Imagination and recollection were my real assets that day. On my way home, I stopped at the LBL Physics Library and discovered Electrons in Gases, written by J. S. Townsend [6]. The title seemed perfect; I checked it out and took it home.2 On page 20, (see Fig. 3) I discovered the following expression, describing the impact of a magnetic field B on transverse diffusion of electrons in gases:
Fig. 2. An image of an event produced with a small spark chamber (left) operating in a pulsed 200 kG field, showing tracks of a 𝛴 + hyperon decay (from [5]. A beam’s eye view of the chamber (right) shows how two orthogonal views were obtained by a 90◦ reflection of the spark images. The crucial aspect of this experiment for the author is that the electric field inducing the spark and the magnetic field were parallel, leading to brighter and narrower sparks than in the case of no magnetic field.
𝐷(𝐵) = 𝐷(0)∕(1 + (𝜔𝜏)2 )
(1)
Here 𝜔𝜏 is the dimensionless product of 𝜔, the electron cyclotron frequency, and 𝜏, the mean time between collisions of electrons with
Fig. 3. A portion of page 20 in [6], showing the impact of an imposed magnetic field on transverse diffusion. The effect is determined by the dimensionless product of cyclotron frequency 𝜔 and mean collision time 𝜏 of electrons in the gas.
2
I shall return it, someday soon.
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Fig. 4. (a) Electron–argon cross-sections showing a deep minimum at ∼0.25 eV for elastic scattering, at which point the mean collision time 𝜏 is maximized. (b) electron–molecule cross-sections, also showing a similar deep minimum around ∼0.25 eV. The appearance of inelastic vibrational excitation modes in methane provides the pathway to control electron temperature by choosing an optimum drift electric field.
atoms/molecules in the gas. D is the transverse diffusion constant: 𝐷 = (2𝑉 𝑙∕3)
(2)
where 𝑉 is the speed of electrons, and 𝑙 is the mean free path between electron–atom/molecule collisions. From (1), it is clear that, if 𝜔𝜏 can be somehow made large, say 𝜔𝜏 ≈ 10, particle track information in dimensions transverse to drift can be preserved even after a drift distance of ∼100 times greater than without the parallel fields. Drift distances of a meter or more now seemed plausible. Intuitively, the notion of electrons executing undisturbed and confining cyclotron motion between collisions was very natural, yet the implication – that a very large detector capable of precise capture of highly complex events with nothing inside the active volume except gas and fields – seemed magical. All complexity was now contained in the (still undefined) readout planes. In a semi-trance I wrote up a note, with a sketch of the now familiar cylindrical TPC [7].
Fig. 5. A ball-wire electrode made by melting the tip of a platinum wire. This electrode provided excellent avalanche gain in P10 gas and also displayed interesting Geiger-like pulses in argon with a sub-percent admixture of isobutane. The ball-wire approach for a TPC readout plane was abandoned due to the complications of fabricating a large honeycomb array.
Further exploration of this wonderful little book contained two more surprises. First, the book was published in 1948! How could such a simple idea, to exploit parallel fields, have gone unnoticed for a quarter century? Second, and even more remarkably, Townsend had published his initial work and this exact result (1) in 1912 [8]! I remain astonished by this because simple and useful ideas are usually rediscovered several times. At least this one seems to have gone completely unappreciated by the community of particle and nuclear physicists, perhaps due to the fact that such concepts fell within a distinctly different research
topic, gaseous electronics. Nonetheless, the lesson here seems familiar— that knowledge of a few things outside of one’s expertise may enable significant new opportunities.
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Fig. 6. (a) Drawing of device fabricated to explore E × B effects in argon–methane mixtures showing a source, drift region, and movable detector slot. The device was designed to fit inside an 8 inch aperture of a standard Bevatron beam magnet. (b) The device as actually built and used.
4. Conceptual stabilization
sub-mm scale even for a 1-meter drift length. With Eq. (1) and argon– methane, it seemed that Nature had provided an extraordinary gift.
Initial reaction to the idea of a ‘‘Time Projection Chamber’’ was varied. Some seasoned researchers thought it to be unlikely to be useful or even feasible, while others took a wait-and-see attitude. A few became excited to see if the idea might really work. Most of 1974 was spent trying to digest the idea and to design a path towards demonstration. I soon realized that an ideal gas mixture would be argon–methane due to the presence of a deep minimum in the electron–atom/molecule cross-section around 0.25 eV in both gases (see Fig. 4). This is the famous Ramsauer–Townsend [9] minimum, a quantum mechanical effect with a substantial macroscopic consequence for the TPC. This providential phenomenon, at just the right electron energy, provided a major boost to the value of the mean time between collisions and hence 𝜔𝜏. Yes, 𝜔𝜏 could be much greater than 1! Transverse diffusion of electrons in argon–methane mixtures could be brought down to the
Exploration began with a soda-can sized device sporting a single ball-wire detector (see Fig. 5). The idea was to fabricate a readout plane with a dense honeycomb of these devices, a sort of fakir’s bed. To construct one of these, I carefully melted the end of a platinum wire into a quasi-spherical shape and centered it within a piece of 14 -inch diameter copper tubing. This structure gave very good avalanche gain in P-10 gas, and, in argon with sub-percent admixtures of isobutane, gave beautifully uniform Geiger-like pulses. Very interesting, but fabricating a large honeycomb array soon seemed like science fiction. At about this time I learned of the existence of the detector Identification of Secondaries by Ionization Sampling (ISIS), invented by Wade Allison at Oxford University. The ISIS device [10] had been conceived, constructed and brought into operation at CERN in 1972, well before I had thought about such matters. It consisted basically of a large bag 4
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product for thousands of channels. This problem remained intractable until it was recognized that a novel integrated circuit might be applicable. Fairchild Semiconductor had recently offered a single-channel commercial device intended for use as a variable length electrical delay line. A signal applied at input would emerge 256 clock cycles later; the delay is thus determined by clock frequency. The Fairchild device was based on the charge-coupled device (CCD) structure (optical CCDs came later). So we thought, why not use this newfangled CCD thing to continuously capture waveforms at the desired 10 MHz, and then, switch to a much slower clock rate to digitize (by means of a Wilkinson commonramp ADC) only triggered events? The entire ionization history of the one-meter drift region for each signal channel could be stored within a single device. We soon discovered that the Fairchild device worked perfectly—except, unfortunately, when the clock frequency is changed abruptly! In that case, large pulse artifacts appeared at fixed intervals and ruined the captured waveform. Suddenly, we were back in a nonsolution situation, again. A visit to Fairchild down in Silicon Valley quickly showed us what was happening inside the device: it had been designed as a folded set of straight sections with diodes used to pump charge around the 180◦ turn. The diodes were injecting unwanted charge into the signal record whenever the clock frequency changed abruptly. Remarkably, Fairchild management readily agreed to change the internal layout using non-Cartesian design to curve the array segments in a serpentine fashion, thereby eliminating corners and diodes. No further charge was injected—and with no charge to us either! The new CCD devices were perfect and used successfully in more than 20,000 channels of electronics. To my knowledge, this architecture was the first major use of free-running clocked signal capture followed by triggered digitization.
Fig. 7. A sketch of the proposed TPC as shown in the PEP-4 proposal. The TPC as built had these dimensions. Each TPC sector had 192 proportional wires to provide 3-D measurements of space and time, as well as dE/dx.
of argon–CO2 equipped with an electric field to drive ionization tracks down to a one-dimensional readout plane. ISIS used discrete electronics to trigger storage of both time and signal amplitude for a limited number of samples whenever a pulse occurred. ISIS operated well, provided good 2-D images of multiple tracks, and constitutes an earlier and close relative of the TPC. Although I felt somewhat scooped, ISIS and TPC were independently conceived, and both are contributions to the art of experiment. By early 1975 a small group at LBL had formed to understand and demonstrate this suite of ideas.3 Our attention soon turned to recent developments made by Charpak’s group at CERN and away from SPEAR. PEP [4], an electron–positron collider with much higher energy than SPEAR had been approved for construction at SLAC. Proposals for major detectors for PEP were due in late 1976. The task of turning a set of novel approaches into a credible design for a major detector required us to develop simultaneously novel approaches in a wide range of mechanical, electrical and HV engineering issues, as well as quantitative understanding of electron transport in various gas mixtures in the presence of magnetic fields, field misalignment, gas impurities, avalanche gain dependencies, signal induction, low-noise electronics, signal transmission, triggering, digitization, and particle identification. The Charpak group had shown that one could determine the position of an avalanche along a wire by sensing signals induced on a set of ‘‘pad’’ electrodes by positive ions as they move away from the positively charged wire [11]. Even though the width of the induced signals on the pad plane was on the order of one cm, the centroid could be found to a small fraction of the width since many electrons contribute. This was clearly the best solution for realizing a 2-D readout plane and we adopted it quickly. In retrospect, it seems rather likely that the Charpak group would have soon discovered the TPC ideas, had they not emerged first at LBL. With diffusion suppression and pad plane centroids providing good spatial resolution transverse to the drift direction, we needed to sample these signals in time to measure properly the drift dimension z. As longitudinal diffusion would be unaffected by the presence of a magnetic field, drift speed provides the other factor needed to determine the sampling interval in time. The drift speed of electrons in argon–methane mixtures can be high, reaching 5 cm/μs or even more. A sampling interval of not more than ∼100 nanoseconds (10 MHz) was necessary to properly capture information. In 1975, the challenges of reading out this barrage of information had no obvious solution. No free-running ADC-based approach at that time had an affordable cost-performance
5. Demonstration In early 1975, we constructed another soda-can sized device to ensure our understanding of diffusion and drift in parallel and semiparallel magnetic fields (see Fig. 6). We first tried the ball-wire device again, this time as a cathodic source of electrons. Negative high voltage applied through a very high resistance induced semi-periodic breakdown, resulting in a relaxation oscillator and pulses of electrons. The electrons traversed a 15 cm drift space and arrived at the anode plane. The anode was equipped with a single wire detector operating in avalanche mode. Only those electrons arriving at the anode plane just above a narrow slit could pass through and reach the wire. The transverse impact points were determined by moving the slotted detector along one axis in the anode plane. We observed highly non-gaussian spatial distributions and belatedly realized the electron ensemble itself was the culprit: space charge forces were larger than the effects of diffusion. To avoid these space charge effects we implemented detection of single electrons. We replaced the ball-wire with an alpha-particle source placed in a small enclosed gas volume at the top. A variable electric field in that small volume allowed a bias condition to be found in which only one electron at a time was likely to emerge through a pinhole in the cathode. The entire device was designed to fit within a standard secondary beam magnet with 8-inch aperture. We were able to rotate our device to vary the angle between magnetic and electric fields to induce non-parallel conditions and observe the E × B effects. Although Townsend had worked it all out, completely, at the beginning of the century, we finally understood, in 1975, how electron trajectories in non-parallel fields behave. During this hectic early period, with our four fully engaged physicists and one superb technician, I received a phone call from a vice-president at one of the largest engineering and manufacturing firms in the US. He confided that he had learned that LBL had invented a time-projection chamber. In hushed tones, he asked me ‘‘How far have you gotten, and in which direction?’’ Unfortunately, I lacked the presence of mind to
3 Jay Marx, Peter Robrish, Marcel Urban, the author, and Ray Fuzesy, a superb technician
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Fig. 8. The early demonstration of waveform capture using the improved CCDs from Fairchild Electronics. A small shoebox size TPC with 16 CCD channels captured tracks in a secondary beam at the Bevatron. This result was a critically important validation for our proposed PEP-4 DAQ system concept.
by measuring dE/dx along the lines of ISIS. Since we expected the competition would be stiff – with then unknown performance claims – we concluded that our detector should operate with pressurized argon– methane, at 8 12 bars.4 With that gas density, unsurpassed particle ID would be obtained for all particles over most of the range of interest in momentum and angle. The parameter 𝜔𝜏 would still be large even though the mean free path l was reduced by the pressure. With our new CCDs, we then constructed a small TPC about the size of a shoebox with 16 channels and placed it in a Bevatron secondary beam. It worked! (see Fig. 8). But the deadline for a proposal to SLAC for construction of a major detector at one of the four interaction regions at the new PEP collider was December 30, 1976, and approaching fast. We had to demonstrate the TPC idea with something more convincing than devices at the scale of soda-can and shoebox. With the same secondary beam magnet at hand, we built a suitcase-sized device that would operate at design pressure and would also have the capabilities and realistic features of an actual TPC readout plane sector (see Fig. 9). In late December we installed the new system in a beam line at the Bevatron. This was our only chance to get data before the proposal had to be submitted and without convincing data, our chances would be negligible. The steel of the magnet yoke constrained the flat vessel walls under 8 12 bars—no time for tests. Everything seemed to be working fine until sparks occurred inside the chamber, and HV on the wire planes would no longer hold. Broken wires had stopped us cold. Even while the Bevatron was running, and only for us, we opened the vessel, extracted the detector and saw that the untested gas purifier had blown in some metallic dust, thereby leading to sparks. Ray Fuzesy cleaned the detector, snipped off the broken wires, and dabbed epoxy here and there. We reassembled the system with a hastily installed filter in the gas system. Two or three hours later, we reinstalled the system. It worked! We were back in business. Data was acquired, and data quality was good. The proposal for what became PEP-4 was submitted a day before deadline. The subsequent proposal presentation went off well enough,
Fig. 9. A real TPC sector, showing the 192 wires in reflection, as well as numerous pad plane rows. Faintly visible are three approximately radial lines of holes, through which 55 Fe X-rays for calibration could be activated by a remotely controlled shutter.
ask him ‘‘How much is it worth to you?’’ His disappointment with my tedious explanation of reality was clear, and the call was brief. As we persevered on several fronts simultaneously, we also developed a viable collaboration; momentum was building. Nobel Laureate Owen Chamberlain, serving on the SLAC Scientific Program Committee, resigned so he could join our effort. Soon, a detector design with an axially symmetric TPC of 2 m in diameter and two drift spaces of one meter was fixed (see Fig. 7). A superconducting magnet concept developed at LBL would provide a field in excess of 1 T. Calorimeters, muon and vertex detectors were included. In addition, to beat the anticipated competition, we decided that our TPC would not only provide detailed trajectories and accurate momenta of particles, but should also deliver excellent particle identification
4 In the absence of any clear criterion for operating pressure, the value was taken from the title of the Fellini movie: 8 12 .
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Fig. 10. (a) One of the two massive feedthrough rings under construction. These allowed the tens of thousands of signals from the TPC sectors to pass from pressurized argon–methane to the external world. These magnificent objects and other aspects of the unorthodox pressure containment worked flawlessly. (b) After the feedthrough rings were installed, wiring from the preamplifiers embedded in the TPC sectors brought signals to the feedthrough rings.
and the SLAC Scientific Program Committee seemed receptive although somewhat guarded. A few days later, I received the dreaded phone call from SLAC’s Director, Pief Panofsky, who, contrary to my expectations, cheerfully informed me that our PEP-4 proposal had been recommended by the SPC and accepted. The long period of anxiety before the call was instantly replaced by a moment of terror: now we really had to do it! We were joined in the subsequent enterprise by another collaboration, PEP-9, focused on the exploration of two-photon processes that lead to mainly forward–backward going events. We had progressed in less than three years from seemingly bizarre notions and pencil sketches to a successful proposal, against major competition from SLAC and elsewhere.
prefer that the electric field should be perpendicular to the direction we intended. To deal with this design flaw, we applied several hundred meters of wide copper tape to all the copper lines in the field cage to force the field to behave. This worked relatively well as a temporary fix. A better solution was eventually found: paint the field cage with a resistive polyurethane paint. Such a coating creates a weak current sheet that is yet strong enough to establish the desired potential gradient against the G10 anisotropies. Severe difficulties were also encountered in the production and testing of the superconducting magnet coil at LBL. Despite high priority allotted to its needs, as well as early prototype successes, the PEP-4 coil burned out fatally during its initial cold test. In shock, we tried to understand its failure and what to do, while our PEP-9 collaborators took charge and worked at SLAC to design and build a temporary warm magnet coil. SLAC had been patient during all these unfolding difficulties, but insisted on better project management. We did not really know what that meant, but soon Pief decided that maybe the Scientific Program Committee (SPC) should take another look at us, perhaps to direct us toward some other socially useful activity. Deputy spokesman Jay Marx and I sat in chairs in SLAC’s Orange room facing the SPC to answer charges. As it turned out, one of the Scientific Program Committee members, Jim Cronin, had recently been awarded the 1980 Nobel Prize in Physics. So it was decided that the Committee should go to the Velvet Turtle nearby for a celebratory luncheon. As an afterthought, Jay and I were invited to join the celebration. Jay and I felt, instead, that we were heading for the Last Supper. Our responses must have been sufficient or perhaps the lunch was especially good, as the SPC recommendation was that SLAC should continue supporting us. We were spared to live on. Pief had no animosity toward us but clearly recognized we had bitten off way more than we could chew, and simply wanted some sort of successful enterprise, however modest. Subsequently, LBL Director David Shirley intervened with additional support to affirm LBL’s commitment, and DOE’s manager for particle physics, Bill Wallenmeyer, also was prepared to stay the course. Meanwhile, the superconducting coil was rewound with substantial design changes in quench protection. The warm magnet was replaced after a long shutdown of PEP-4 outside the circulating beams. After cool-down, current was gently ramped up. Suddenly, the coil left its position of axial metastable equilibrium and moved toward one of the pole pieces. A mechanism designed to hold the coil in place after thermal contraction was found to have inadequate azimuthal rigidity and departure from circularity allowed axial motion. Another complete disassembly and redesign followed, with the inclusion of long
6. Realization The real work, now of a different nature, began. Although LBL had gone through a large contraction in support for physics during its transition to a multi-purpose laboratory, enough engineering expertise remained to get most of the work done correctly. In what follows I describe effort and events more parsimoniously, in part to minimize tedium and despair, but mainly to illustrate more fully indelible moments of success or terror as well as some interesting advances made along the way. For example, to reach a pressure of 8 12 bars would require a pressure vessel so massive as to be simply out of the question. In the spirit of the moment, we decided, instead, that a combination of a slightly thicker magnet cryostat wall, the steel end caps of the magnet yoke, and titanium cones that would mate with a small titanium tube concentric with the beam pipe would do the job. This rather desperate scheme would seem unlikely to pass today’s review processes; but it worked flawlessly from the first time of assembly through several installation and pressure cycles. Tests performed at final assembly. Furthermore, the massive and magnificent feed-through rings made at LBL supported the pressure without incident and passed all tens of thousands of signals (see Fig. 10). Finally, the TPC with twelve sectors was ready. Initial tests at SLAC of the TPC itself showed instantly that it was detecting cosmic rays, with tracks flashing across the screen as soon as HV settings were made. Success! Champagne had to be quickly procured for the occasion. But our celebration was short-lived, as tracks appeared to have strange, varying curvatures near the field cage walls. We soon recognized that our design for the electric field was flawed. We learned that on G10 copper lines of 1 mm width every 10 mm did not necessarily establish a uniform potential gradient. G10 is highly anisotropic and seems to 7
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Fig. 11. A spectacular image of the production and decay of an anti-hypertriton, captured in the STAR TPC at the Brookhaven National Laboratory Relativistic Heavy Ion Collider.
axial supports to enforce the equilibrium position. Finally, and after replacing several failed turbines in the liquid helium refrigerator, the superconducting magnet worked too. Despite having failed spectacularly in the initial provision of appropriate electric and magnetic fields – key elements of the TPC idea – and after a delay of several years, the experiment was on! We never found out how much PEP-4 really cost, nor, I suppose, did we really want to know. Remarkably, the on-going troubles at PEP-4 did not dissuade two of the four experiments at CERN’s Large Electron–Positron Collider (LEP) from choosing a TPC for their central tracker.
with thousands of reconstructed particle tracks adorn book covers and agency reports (see Fig. 11). But the TPC ideas were entirely strange in 1974 and birth pangs at PEP-4 were severe. It is comforting to see such vigor now, perhaps a conceptual middle age. What remains most vivid for me are memories of so many collaborators, engineers, technicians, graduate students, post-docs and faculty, all working together in PEP-4 and PEP-9. They made the project ultimately both a technical success and a noteworthy episode in human endeavor; there were many, and I thank them all. I regret that I cannot provide here a more detailed compilation of their sustained and creative contributions. Even with the passage of more than four decades, many of these moments remain among the most indelible and outstanding for me. I recognize that such a deeply personal and limited account does not provide a balanced historical perspective, and I regret shortcomings in this regard. I also thank the editors of this special issue for their invitation to recapture some of the flavor of a dynamic and bygone era. For a journal normally focused on purely technical advances, the inclusion of a personal memoir in this Centennial issue is greatly appreciated. I hope it may serve to remind us, once again, that while some of the best science is initiated by someone with a novel idea, often that science is done best by a small group of people who share a deep commitment and motivation to advance those ideas. Due to those sustained and heroic efforts toward the realization of PEP-4, I believe Rutherford and Geiger would have agreed that the TPC idea has earned a place in that special class of ‘‘refined methods’’.
7. Perspective We had gone from soda can and shoebox to suitcase scale, then imprudently to the 1000-ton PEP-4. Even on a logarithmic scale, that was a major extrapolation. We were exhausted. Our only technical publications for the TPC idea, until 1982 [12], were my short notes in SLAC Summer Studies of 1974 and 1975 [13] and two articles in Physics Today [14,15]. Yet, eventually, PEP-4 performance met every expectation. The magnetic field shape at the design value was indeed uniform, and momentum resolution was obtained at the promised level. To date, PEP-4 particle identification by dE/dx is the best in any large system. The timing resolution for track samples arriving at the readout plane was systematically calibrated to yield ultimately an impressive 1 ns rms, just 1% of the time sampling interval. The PEP-4 TPC worked, as expected. SLAC’s, LBL’s, and DOE’s investment had finally paid off. My motivation for describing the mix of travails and successes is to provide, beyond the sense of saga, a recognition that, even during this time of troubles, excitement for the transformative potential of the TPC idea persisted, sustained us and carried us onward. Stress levels were high and tensions sometimes higher. Yet no students defected (despite offers to do so), nor did any collaborators, and most graduate student and post-doc survivors of PEP-4 went on to excellent careers in academia and elsewhere. A genuine paradigm shift had been realized. In 1992 Georges Charpak was awarded Nobel Prize in Physics for his invention of the MWPC [16]. He included the following remark in his Nobel lecture on particle imaging: ‘‘. . . It was, however, D. Nygren who, by combining the effects of parallel magnetic and electric fields and solving formidable data acquisition problems, succeeded in creating an instrument which provides the finest images of the most complex configurations obtained in colliders;. . . ’’ Reading that passage made all the teething problems seem very distant. Over the last four decades, the TPC idea has been adapted and applied in a remarkably wide range of experimental contexts [17]. Although a breakthrough at the time, diffusion suppression in gas by magnetic field is no longer an essential element in every case. Very large liquid noble element TPCs are now prominent in searches for dark matter, neutrino-less double beta decay, and in present and future detectors for neutrinos. The TPC has indeed evolved, and these days the TPC idea is a commonplace. Iconic images of heavy-ion collisions
References [1] E. Rutherford, F.R.S., H. Geiger, An electrical method of counting the number of 𝛼-particles from radio-active substances, Proc. Roy. Soc. 81 (546) (1908) 27. http: //rspa.royalsocietypublishing.org/content/royprsa/81/546/141.full.pdf. [2] J.S. Townsend, On the conductivity of gases exposed to negative ions, Proc. Roy. Soc. 1 (sixth series, #2) (1901). See also issues of June 1902; April, September, November, 1903. [3] G. Charpak, R. Bouclier, T. Bressani, J. Favier, Č. Zupančič, The use of multiwire proportional counters to select and localize charged particles, Nucl. Instrum. Methods 62 (#3) (1968) 262–268. [4] SLAC storage rings SPEAR and PEP: see https://www-ssrl.slac.stanford.edu/ content/spear3/spear-history and https://www6.slac.stanford.edu/about/slachistory. [5] V. Cook, T. Ewart, G. Masek, R. Orr, E. Platner, Measurement of the 𝛴+ magnetic moment, Phys. Rev. Lett. 17 (1966) 223. [6] F.R.S. John Townsend, Electrons in Gases, Hutchinson’s Scientific and Technical Publications, London, 1948. [7] David R. Nygren, Proposal to investigate the feasibility of a novel concept in particle detection, LBNL internal note, February 1974; see https://inspirehep.net/record/ 1365360. [8] J.S. Townsend, Diffusion and mobility of ions in a magnetic field, Proc. Roy. Soc. A 86 (1912) 571. [9] Stephen G. Kukolich, Demonstration of the Ramsauer-Townsend effect in a xenon thyratron, Amer. J. Phys. 36 (1968) 701. http://dx.doi.org/10.1119/1.1975094. [10] W.W.M. Allison, C.B. Brooks, J.N. Bunch, J.H. Cobb, J.L. Lloyd, R.W. Pleming, The identification of secondary particles by ionisation sampling (ISIS), Nucl. Instrum. Methods 119 (1974) 499–507.
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[11] G. Charpak, F. Sauli, High-accuracy two-dimensional read-out in multiwire proportional chambers, Nucl. Instrum. Methods 113 (#3) (1973) 381–385. [12] R.Z. Fuzesy, N.J. Hadley, P.R. Robrish, Design of the multiwire proportional detectors for the PEP-4 Time Projection Chamber, Nucl. Inst. & Meth. 223 (1984) 40–46. [13] http://slac.stanford.edu/pubs/slacreports/reports04/slac-r-190.pdf.
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[14] J. Marx, D. Nygren, The time projection chamber, Phys. Today 31 (#10) (1978) 46. [15] R.J. Madaras, P.M. Oddone, Time projection chambers, Phys. Today 37 (#8) (1984) 36. [16] Electronic imaging of ionizing radiation with limited avalanches in gases. https:// www.nobelprize.org/nobel_prizes/physics/laureates/1992/charpak-lecture.html. [17] D. González-Díaz, F. Monrabal, S. Murphy, Gaseous and dual-phase time projection chambers for imaging rare processes, Nucl. Instrum. Methods. Phys. Res. A 878 (2018) 200–255.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Origin and development of the TPC idea David R. Nygren Department of Physics, University of Texas at Arlington, Arlington, TX 76019, United States
a r t i c l e
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Keywords: Time Projection Chamber Particle detection
a b s t r a c t The emergence of the Time Projection Chamber concept in 1974 is an important chapter in the history of particle detection advances. Recalling an era very different from today, I describe the flow of events from the moment of inspiration through prototypes to realization as a major detector for the Positron–Electron Project (PEP) at SLAC. The sense of excitement in bringing forth a genuine paradigm shift carried the collaboration forward, through struggles, victories, defeats and exhaustion, yet ultimately leading to success. © 2018 Published by Elsevier B.V.
1. Historical context The first instance of electronic detection of single subatomic particles can likely be traced to experiments by Ernest Rutherford and Hans Geiger in Manchester: An Electrical Method for Counting 𝛼-particles in Radio-Active Substances, published in the Proceedings of the Royal Society in 1908 [1]. Reading this milestone paper is rewarding, for insight into the scientific acumen of Rutherford and Geiger and for the engaging style of writing, truly a letter to colleagues. They state: ‘‘It has been recognized for several years that it should be possible by refined methods to detect a single 𝛼-particle by measuring the ionization it produces in its path.’’ After discussing difficulties encountered in attempts to measure that charge directly, they declare: ‘‘We then had recourse to a method of automatically magnifying the electrical effect due to a single 𝛼-particle. For this purpose we employed the principle of production of fresh ions by collision. In a series of papers, Townsend [2] has worked out the conditions under which ions can be produced by collisions with the neutral gas molecules in a strong electric field.’’ After describing their apparatus, (see Fig. 1) the authors provide their method for the first electronic detection of a subatomic particle (as 𝛼-particles were seen in those days): ‘‘In this way, the small ionization produced by one 𝛼-particle in passing through the gas could be magnified several thousand times. The sudden current due to the entrance of an 𝛼-particle in the testing vessel was thus increased sufficiently to give an easily measurable movement of an ordinary electrometer’’. And so was realized, in the first decade of the twentieth century, the first electronic detection of individual particles in the subatomic realm. Over a century later the path-breaking technique of electrical magnification by production of fresh ions – albeit with much further refined methods – is still prominent in many of today’s experiments in particle and nuclear physics. By 1968, a zenith had been reached through the
conception and development of the Multi-Wire Proportional Chamber (MWPC) by Georges Charpak [3] at CERN. At about the same time, the drift chamber was developed in Heidelberg, Saclay and CERN. The MWPC and drift chamber concepts in various forms revolutionized particle detection. Through their unprecedented high rate capabilities-enabled by advances in electronics—these detection techniques opened the current era of high sensitivity and statistical precision in elementary particle physics. Here I recall some of the memorable events that occurred during the invention and subsequent development of the Time Projection Chamber (TPC). My main aspiration for this highly personal memoir is to offer the young scientist a glimpse of an extraordinary moment of inspiration, collective struggles, and ultimately, success. The story begins, however, with the author in a quandary. 2. Quandary In February 1974 with this toolset of detection techniques at hand, I set myself the task to conceive a detector that would provide some substantial advantage relative to those currently in use at the Stanford Positron–Electron Accelerator Ring (SPEAR), [4] which had been brought into operation in 1972. The discovery of the 𝛹 and the subsequent November Revolution were yet to arrive. As the first Divisional Fellow in the Physics Division at the Lawrence Berkeley Laboratory,1 I was strongly motivated to do something noteworthy. I was also driven by stylistic proclivities acquired as a post-doc working for Jack Steinberger while he was a faculty member at Columbia University. 1 (from 1958, the Lawrence Radiation Laboratory (LRL), then Lawrence Berkeley laboratory (LBL) in 1971, and finally renamed Ernest Orlando Lawrence Berkeley National Laboratory in 1995, and thence, known simply as Berkeley Lab)
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2018.07.015 Received 5 March 2018; Received in revised form 5 July 2018; Accepted 6 July 2018 Available online xxxx 0168-9002/© 2018 Published by Elsevier B.V.
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From Jack, through some powerful osmotic force, I had absorbed a guiding principle: one should focus on doing only the best physics and best experiments. His disdain for efforts falling short in either regard was palpable. So, perhaps unconsciously, I committed myself to a risky path: to find something truly innovative, hopefully paradigm shifting—and if unsuccessful, be prepared to accept failure and leave the field. For about six months prior to February 1974, I had been solidly in the quadrant of failure with neither innovative nor paradigm-shifting ideas. After a long afternoon at a drafting table with pencil, straight-edge, compass and triangles, I had only an increasing pile of crumpled papers on the floor to show for my effort. I had exhausted known technical approaches. I began to see failure as an imminent possibility and decided to take some time to reflect. 3. Inspiration strikes Fig. 1. The apparatus of Rutherford and Geiger, used in the first experiment to detect subatomic particles electronically. With stopcock F open, 𝛼-particles emanating from radon in volume E could pass through a thin window into volume A, ionizing the gas present. The ionization electrons were attracted to wire B at a positive potential. The potential was sufficiently high that avalanche multiplication of several thousand was achieved. The resulting current was large enough to cause visible impulse deflection of an electrometer.
After a while my thoughts wandered to an observation made several years earlier while I was a graduate student at the University of Washington, Seattle. The UW group had undertaken to measure the magnetic moment of the 𝛴 + hyperon at the LRL Bevatron [5]. At the secondary beam energies available, the experiment required a very strong magnetic field to rotate appreciably the hyperon magnetic polarization before decay occurs. The central particle detector was a small spark chamber placed inside a 200 kG pulsed magnet (see Fig. 2). Optical reflections at the spark chamber edges permitted external photographs to be taken of each triggered pulse. The pulsed electric field of the spark chamber was thus aligned with the magnetic field. A very strong increase in spark optical brightness was observed in tracks when the magnetic field was on; strangely, the sparks were visibly much narrower in that case. This unexpected effect was beneficial for event reconstruction and taken for granted henceforward; no one thought to further pursue any underlying phenomena. But I remembered it as an interesting curiosity. At that moment in February 1974 while I was feeling my goal slipping away, it suddenly occurred to me that, instead of the traditional orientation of perpendicular magnetic and electric fields in drift chambers, perhaps a parallel configuration of electric and magnetic fields might plausibly have a beneficial effect—even for more mundane values of electric and magnetic fields. Might this unorthodox configuration suppress diffusion of electrons during drift? If so, then perhaps track images could be transported over very large distances with minimal deterioration. And, I realized, if the readout planes could capture x–y information as a function of time, and, if the time of the event was known (by some other means), the event would be captured in perfect, precise, unambiguous 3-D. Clearly this idea needed a captivating name! Thus was the Time Projection Chamber concept born on a waning winter afternoon. With this novel and alluring idea tumbling rapidly around in my mind, I sat, transfixed, imagining new possibilities until sunset. However, I had no particular expertise in the physics of gasbased detectors beyond participation in construction and utilization, under Jack’s leadership, of a set of large MWPCs for experiments in 𝐾0 decay physics at Brookhaven National Laboratory’s Alternating Gradient Synchrotron (AGS). Imagination and recollection were my real assets that day. On my way home, I stopped at the LBL Physics Library and discovered Electrons in Gases, written by J. S. Townsend [6]. The title seemed perfect; I checked it out and took it home.2 On page 20, (see Fig. 3) I discovered the following expression, describing the impact of a magnetic field B on transverse diffusion of electrons in gases:
Fig. 2. An image of an event produced with a small spark chamber (left) operating in a pulsed 200 kG field, showing tracks of a 𝛴 + hyperon decay (from [5]. A beam’s eye view of the chamber (right) shows how two orthogonal views were obtained by a 90◦ reflection of the spark images. The crucial aspect of this experiment for the author is that the electric field inducing the spark and the magnetic field were parallel, leading to brighter and narrower sparks than in the case of no magnetic field.
𝐷(𝐵) = 𝐷(0)∕(1 + (𝜔𝜏)2 )
(1)
Here 𝜔𝜏 is the dimensionless product of 𝜔, the electron cyclotron frequency, and 𝜏, the mean time between collisions of electrons with
Fig. 3. A portion of page 20 in [6], showing the impact of an imposed magnetic field on transverse diffusion. The effect is determined by the dimensionless product of cyclotron frequency 𝜔 and mean collision time 𝜏 of electrons in the gas.
2
I shall return it, someday soon.
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Fig. 4. (a) Electron–argon cross-sections showing a deep minimum at ∼0.25 eV for elastic scattering, at which point the mean collision time 𝜏 is maximized. (b) electron–molecule cross-sections, also showing a similar deep minimum around ∼0.25 eV. The appearance of inelastic vibrational excitation modes in methane provides the pathway to control electron temperature by choosing an optimum drift electric field.
atoms/molecules in the gas. D is the transverse diffusion constant: 𝐷 = (2𝑉 𝑙∕3)
(2)
where 𝑉 is the speed of electrons, and 𝑙 is the mean free path between electron–atom/molecule collisions. From (1), it is clear that, if 𝜔𝜏 can be somehow made large, say 𝜔𝜏 ≈ 10, particle track information in dimensions transverse to drift can be preserved even after a drift distance of ∼100 times greater than without the parallel fields. Drift distances of a meter or more now seemed plausible. Intuitively, the notion of electrons executing undisturbed and confining cyclotron motion between collisions was very natural, yet the implication – that a very large detector capable of precise capture of highly complex events with nothing inside the active volume except gas and fields – seemed magical. All complexity was now contained in the (still undefined) readout planes. In a semi-trance I wrote up a note, with a sketch of the now familiar cylindrical TPC [7].
Fig. 5. A ball-wire electrode made by melting the tip of a platinum wire. This electrode provided excellent avalanche gain in P10 gas and also displayed interesting Geiger-like pulses in argon with a sub-percent admixture of isobutane. The ball-wire approach for a TPC readout plane was abandoned due to the complications of fabricating a large honeycomb array.
Further exploration of this wonderful little book contained two more surprises. First, the book was published in 1948! How could such a simple idea, to exploit parallel fields, have gone unnoticed for a quarter century? Second, and even more remarkably, Townsend had published his initial work and this exact result (1) in 1912 [8]! I remain astonished by this because simple and useful ideas are usually rediscovered several times. At least this one seems to have gone completely unappreciated by the community of particle and nuclear physicists, perhaps due to the fact that such concepts fell within a distinctly different research
topic, gaseous electronics. Nonetheless, the lesson here seems familiar— that knowledge of a few things outside of one’s expertise may enable significant new opportunities.
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Fig. 6. (a) Drawing of device fabricated to explore E × B effects in argon–methane mixtures showing a source, drift region, and movable detector slot. The device was designed to fit inside an 8 inch aperture of a standard Bevatron beam magnet. (b) The device as actually built and used.
4. Conceptual stabilization
sub-mm scale even for a 1-meter drift length. With Eq. (1) and argon– methane, it seemed that Nature had provided an extraordinary gift.
Initial reaction to the idea of a ‘‘Time Projection Chamber’’ was varied. Some seasoned researchers thought it to be unlikely to be useful or even feasible, while others took a wait-and-see attitude. A few became excited to see if the idea might really work. Most of 1974 was spent trying to digest the idea and to design a path towards demonstration. I soon realized that an ideal gas mixture would be argon–methane due to the presence of a deep minimum in the electron–atom/molecule cross-section around 0.25 eV in both gases (see Fig. 4). This is the famous Ramsauer–Townsend [9] minimum, a quantum mechanical effect with a substantial macroscopic consequence for the TPC. This providential phenomenon, at just the right electron energy, provided a major boost to the value of the mean time between collisions and hence 𝜔𝜏. Yes, 𝜔𝜏 could be much greater than 1! Transverse diffusion of electrons in argon–methane mixtures could be brought down to the
Exploration began with a soda-can sized device sporting a single ball-wire detector (see Fig. 5). The idea was to fabricate a readout plane with a dense honeycomb of these devices, a sort of fakir’s bed. To construct one of these, I carefully melted the end of a platinum wire into a quasi-spherical shape and centered it within a piece of 14 -inch diameter copper tubing. This structure gave very good avalanche gain in P-10 gas, and, in argon with sub-percent admixtures of isobutane, gave beautifully uniform Geiger-like pulses. Very interesting, but fabricating a large honeycomb array soon seemed like science fiction. At about this time I learned of the existence of the detector Identification of Secondaries by Ionization Sampling (ISIS), invented by Wade Allison at Oxford University. The ISIS device [10] had been conceived, constructed and brought into operation at CERN in 1972, well before I had thought about such matters. It consisted basically of a large bag 4
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product for thousands of channels. This problem remained intractable until it was recognized that a novel integrated circuit might be applicable. Fairchild Semiconductor had recently offered a single-channel commercial device intended for use as a variable length electrical delay line. A signal applied at input would emerge 256 clock cycles later; the delay is thus determined by clock frequency. The Fairchild device was based on the charge-coupled device (CCD) structure (optical CCDs came later). So we thought, why not use this newfangled CCD thing to continuously capture waveforms at the desired 10 MHz, and then, switch to a much slower clock rate to digitize (by means of a Wilkinson commonramp ADC) only triggered events? The entire ionization history of the one-meter drift region for each signal channel could be stored within a single device. We soon discovered that the Fairchild device worked perfectly—except, unfortunately, when the clock frequency is changed abruptly! In that case, large pulse artifacts appeared at fixed intervals and ruined the captured waveform. Suddenly, we were back in a nonsolution situation, again. A visit to Fairchild down in Silicon Valley quickly showed us what was happening inside the device: it had been designed as a folded set of straight sections with diodes used to pump charge around the 180◦ turn. The diodes were injecting unwanted charge into the signal record whenever the clock frequency changed abruptly. Remarkably, Fairchild management readily agreed to change the internal layout using non-Cartesian design to curve the array segments in a serpentine fashion, thereby eliminating corners and diodes. No further charge was injected—and with no charge to us either! The new CCD devices were perfect and used successfully in more than 20,000 channels of electronics. To my knowledge, this architecture was the first major use of free-running clocked signal capture followed by triggered digitization.
Fig. 7. A sketch of the proposed TPC as shown in the PEP-4 proposal. The TPC as built had these dimensions. Each TPC sector had 192 proportional wires to provide 3-D measurements of space and time, as well as dE/dx.
of argon–CO2 equipped with an electric field to drive ionization tracks down to a one-dimensional readout plane. ISIS used discrete electronics to trigger storage of both time and signal amplitude for a limited number of samples whenever a pulse occurred. ISIS operated well, provided good 2-D images of multiple tracks, and constitutes an earlier and close relative of the TPC. Although I felt somewhat scooped, ISIS and TPC were independently conceived, and both are contributions to the art of experiment. By early 1975 a small group at LBL had formed to understand and demonstrate this suite of ideas.3 Our attention soon turned to recent developments made by Charpak’s group at CERN and away from SPEAR. PEP [4], an electron–positron collider with much higher energy than SPEAR had been approved for construction at SLAC. Proposals for major detectors for PEP were due in late 1976. The task of turning a set of novel approaches into a credible design for a major detector required us to develop simultaneously novel approaches in a wide range of mechanical, electrical and HV engineering issues, as well as quantitative understanding of electron transport in various gas mixtures in the presence of magnetic fields, field misalignment, gas impurities, avalanche gain dependencies, signal induction, low-noise electronics, signal transmission, triggering, digitization, and particle identification. The Charpak group had shown that one could determine the position of an avalanche along a wire by sensing signals induced on a set of ‘‘pad’’ electrodes by positive ions as they move away from the positively charged wire [11]. Even though the width of the induced signals on the pad plane was on the order of one cm, the centroid could be found to a small fraction of the width since many electrons contribute. This was clearly the best solution for realizing a 2-D readout plane and we adopted it quickly. In retrospect, it seems rather likely that the Charpak group would have soon discovered the TPC ideas, had they not emerged first at LBL. With diffusion suppression and pad plane centroids providing good spatial resolution transverse to the drift direction, we needed to sample these signals in time to measure properly the drift dimension z. As longitudinal diffusion would be unaffected by the presence of a magnetic field, drift speed provides the other factor needed to determine the sampling interval in time. The drift speed of electrons in argon–methane mixtures can be high, reaching 5 cm/μs or even more. A sampling interval of not more than ∼100 nanoseconds (10 MHz) was necessary to properly capture information. In 1975, the challenges of reading out this barrage of information had no obvious solution. No free-running ADC-based approach at that time had an affordable cost-performance
5. Demonstration In early 1975, we constructed another soda-can sized device to ensure our understanding of diffusion and drift in parallel and semiparallel magnetic fields (see Fig. 6). We first tried the ball-wire device again, this time as a cathodic source of electrons. Negative high voltage applied through a very high resistance induced semi-periodic breakdown, resulting in a relaxation oscillator and pulses of electrons. The electrons traversed a 15 cm drift space and arrived at the anode plane. The anode was equipped with a single wire detector operating in avalanche mode. Only those electrons arriving at the anode plane just above a narrow slit could pass through and reach the wire. The transverse impact points were determined by moving the slotted detector along one axis in the anode plane. We observed highly non-gaussian spatial distributions and belatedly realized the electron ensemble itself was the culprit: space charge forces were larger than the effects of diffusion. To avoid these space charge effects we implemented detection of single electrons. We replaced the ball-wire with an alpha-particle source placed in a small enclosed gas volume at the top. A variable electric field in that small volume allowed a bias condition to be found in which only one electron at a time was likely to emerge through a pinhole in the cathode. The entire device was designed to fit within a standard secondary beam magnet with 8-inch aperture. We were able to rotate our device to vary the angle between magnetic and electric fields to induce non-parallel conditions and observe the E × B effects. Although Townsend had worked it all out, completely, at the beginning of the century, we finally understood, in 1975, how electron trajectories in non-parallel fields behave. During this hectic early period, with our four fully engaged physicists and one superb technician, I received a phone call from a vice-president at one of the largest engineering and manufacturing firms in the US. He confided that he had learned that LBL had invented a time-projection chamber. In hushed tones, he asked me ‘‘How far have you gotten, and in which direction?’’ Unfortunately, I lacked the presence of mind to
3 Jay Marx, Peter Robrish, Marcel Urban, the author, and Ray Fuzesy, a superb technician
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Fig. 8. The early demonstration of waveform capture using the improved CCDs from Fairchild Electronics. A small shoebox size TPC with 16 CCD channels captured tracks in a secondary beam at the Bevatron. This result was a critically important validation for our proposed PEP-4 DAQ system concept.
by measuring dE/dx along the lines of ISIS. Since we expected the competition would be stiff – with then unknown performance claims – we concluded that our detector should operate with pressurized argon– methane, at 8 12 bars.4 With that gas density, unsurpassed particle ID would be obtained for all particles over most of the range of interest in momentum and angle. The parameter 𝜔𝜏 would still be large even though the mean free path l was reduced by the pressure. With our new CCDs, we then constructed a small TPC about the size of a shoebox with 16 channels and placed it in a Bevatron secondary beam. It worked! (see Fig. 8). But the deadline for a proposal to SLAC for construction of a major detector at one of the four interaction regions at the new PEP collider was December 30, 1976, and approaching fast. We had to demonstrate the TPC idea with something more convincing than devices at the scale of soda-can and shoebox. With the same secondary beam magnet at hand, we built a suitcase-sized device that would operate at design pressure and would also have the capabilities and realistic features of an actual TPC readout plane sector (see Fig. 9). In late December we installed the new system in a beam line at the Bevatron. This was our only chance to get data before the proposal had to be submitted and without convincing data, our chances would be negligible. The steel of the magnet yoke constrained the flat vessel walls under 8 12 bars—no time for tests. Everything seemed to be working fine until sparks occurred inside the chamber, and HV on the wire planes would no longer hold. Broken wires had stopped us cold. Even while the Bevatron was running, and only for us, we opened the vessel, extracted the detector and saw that the untested gas purifier had blown in some metallic dust, thereby leading to sparks. Ray Fuzesy cleaned the detector, snipped off the broken wires, and dabbed epoxy here and there. We reassembled the system with a hastily installed filter in the gas system. Two or three hours later, we reinstalled the system. It worked! We were back in business. Data was acquired, and data quality was good. The proposal for what became PEP-4 was submitted a day before deadline. The subsequent proposal presentation went off well enough,
Fig. 9. A real TPC sector, showing the 192 wires in reflection, as well as numerous pad plane rows. Faintly visible are three approximately radial lines of holes, through which 55 Fe X-rays for calibration could be activated by a remotely controlled shutter.
ask him ‘‘How much is it worth to you?’’ His disappointment with my tedious explanation of reality was clear, and the call was brief. As we persevered on several fronts simultaneously, we also developed a viable collaboration; momentum was building. Nobel Laureate Owen Chamberlain, serving on the SLAC Scientific Program Committee, resigned so he could join our effort. Soon, a detector design with an axially symmetric TPC of 2 m in diameter and two drift spaces of one meter was fixed (see Fig. 7). A superconducting magnet concept developed at LBL would provide a field in excess of 1 T. Calorimeters, muon and vertex detectors were included. In addition, to beat the anticipated competition, we decided that our TPC would not only provide detailed trajectories and accurate momenta of particles, but should also deliver excellent particle identification
4 In the absence of any clear criterion for operating pressure, the value was taken from the title of the Fellini movie: 8 12 .
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Fig. 10. (a) One of the two massive feedthrough rings under construction. These allowed the tens of thousands of signals from the TPC sectors to pass from pressurized argon–methane to the external world. These magnificent objects and other aspects of the unorthodox pressure containment worked flawlessly. (b) After the feedthrough rings were installed, wiring from the preamplifiers embedded in the TPC sectors brought signals to the feedthrough rings.
and the SLAC Scientific Program Committee seemed receptive although somewhat guarded. A few days later, I received the dreaded phone call from SLAC’s Director, Pief Panofsky, who, contrary to my expectations, cheerfully informed me that our PEP-4 proposal had been recommended by the SPC and accepted. The long period of anxiety before the call was instantly replaced by a moment of terror: now we really had to do it! We were joined in the subsequent enterprise by another collaboration, PEP-9, focused on the exploration of two-photon processes that lead to mainly forward–backward going events. We had progressed in less than three years from seemingly bizarre notions and pencil sketches to a successful proposal, against major competition from SLAC and elsewhere.
prefer that the electric field should be perpendicular to the direction we intended. To deal with this design flaw, we applied several hundred meters of wide copper tape to all the copper lines in the field cage to force the field to behave. This worked relatively well as a temporary fix. A better solution was eventually found: paint the field cage with a resistive polyurethane paint. Such a coating creates a weak current sheet that is yet strong enough to establish the desired potential gradient against the G10 anisotropies. Severe difficulties were also encountered in the production and testing of the superconducting magnet coil at LBL. Despite high priority allotted to its needs, as well as early prototype successes, the PEP-4 coil burned out fatally during its initial cold test. In shock, we tried to understand its failure and what to do, while our PEP-9 collaborators took charge and worked at SLAC to design and build a temporary warm magnet coil. SLAC had been patient during all these unfolding difficulties, but insisted on better project management. We did not really know what that meant, but soon Pief decided that maybe the Scientific Program Committee (SPC) should take another look at us, perhaps to direct us toward some other socially useful activity. Deputy spokesman Jay Marx and I sat in chairs in SLAC’s Orange room facing the SPC to answer charges. As it turned out, one of the Scientific Program Committee members, Jim Cronin, had recently been awarded the 1980 Nobel Prize in Physics. So it was decided that the Committee should go to the Velvet Turtle nearby for a celebratory luncheon. As an afterthought, Jay and I were invited to join the celebration. Jay and I felt, instead, that we were heading for the Last Supper. Our responses must have been sufficient or perhaps the lunch was especially good, as the SPC recommendation was that SLAC should continue supporting us. We were spared to live on. Pief had no animosity toward us but clearly recognized we had bitten off way more than we could chew, and simply wanted some sort of successful enterprise, however modest. Subsequently, LBL Director David Shirley intervened with additional support to affirm LBL’s commitment, and DOE’s manager for particle physics, Bill Wallenmeyer, also was prepared to stay the course. Meanwhile, the superconducting coil was rewound with substantial design changes in quench protection. The warm magnet was replaced after a long shutdown of PEP-4 outside the circulating beams. After cool-down, current was gently ramped up. Suddenly, the coil left its position of axial metastable equilibrium and moved toward one of the pole pieces. A mechanism designed to hold the coil in place after thermal contraction was found to have inadequate azimuthal rigidity and departure from circularity allowed axial motion. Another complete disassembly and redesign followed, with the inclusion of long
6. Realization The real work, now of a different nature, began. Although LBL had gone through a large contraction in support for physics during its transition to a multi-purpose laboratory, enough engineering expertise remained to get most of the work done correctly. In what follows I describe effort and events more parsimoniously, in part to minimize tedium and despair, but mainly to illustrate more fully indelible moments of success or terror as well as some interesting advances made along the way. For example, to reach a pressure of 8 12 bars would require a pressure vessel so massive as to be simply out of the question. In the spirit of the moment, we decided, instead, that a combination of a slightly thicker magnet cryostat wall, the steel end caps of the magnet yoke, and titanium cones that would mate with a small titanium tube concentric with the beam pipe would do the job. This rather desperate scheme would seem unlikely to pass today’s review processes; but it worked flawlessly from the first time of assembly through several installation and pressure cycles. Tests performed at final assembly. Furthermore, the massive and magnificent feed-through rings made at LBL supported the pressure without incident and passed all tens of thousands of signals (see Fig. 10). Finally, the TPC with twelve sectors was ready. Initial tests at SLAC of the TPC itself showed instantly that it was detecting cosmic rays, with tracks flashing across the screen as soon as HV settings were made. Success! Champagne had to be quickly procured for the occasion. But our celebration was short-lived, as tracks appeared to have strange, varying curvatures near the field cage walls. We soon recognized that our design for the electric field was flawed. We learned that on G10 copper lines of 1 mm width every 10 mm did not necessarily establish a uniform potential gradient. G10 is highly anisotropic and seems to 7
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Fig. 11. A spectacular image of the production and decay of an anti-hypertriton, captured in the STAR TPC at the Brookhaven National Laboratory Relativistic Heavy Ion Collider.
axial supports to enforce the equilibrium position. Finally, and after replacing several failed turbines in the liquid helium refrigerator, the superconducting magnet worked too. Despite having failed spectacularly in the initial provision of appropriate electric and magnetic fields – key elements of the TPC idea – and after a delay of several years, the experiment was on! We never found out how much PEP-4 really cost, nor, I suppose, did we really want to know. Remarkably, the on-going troubles at PEP-4 did not dissuade two of the four experiments at CERN’s Large Electron–Positron Collider (LEP) from choosing a TPC for their central tracker.
with thousands of reconstructed particle tracks adorn book covers and agency reports (see Fig. 11). But the TPC ideas were entirely strange in 1974 and birth pangs at PEP-4 were severe. It is comforting to see such vigor now, perhaps a conceptual middle age. What remains most vivid for me are memories of so many collaborators, engineers, technicians, graduate students, post-docs and faculty, all working together in PEP-4 and PEP-9. They made the project ultimately both a technical success and a noteworthy episode in human endeavor; there were many, and I thank them all. I regret that I cannot provide here a more detailed compilation of their sustained and creative contributions. Even with the passage of more than four decades, many of these moments remain among the most indelible and outstanding for me. I recognize that such a deeply personal and limited account does not provide a balanced historical perspective, and I regret shortcomings in this regard. I also thank the editors of this special issue for their invitation to recapture some of the flavor of a dynamic and bygone era. For a journal normally focused on purely technical advances, the inclusion of a personal memoir in this Centennial issue is greatly appreciated. I hope it may serve to remind us, once again, that while some of the best science is initiated by someone with a novel idea, often that science is done best by a small group of people who share a deep commitment and motivation to advance those ideas. Due to those sustained and heroic efforts toward the realization of PEP-4, I believe Rutherford and Geiger would have agreed that the TPC idea has earned a place in that special class of ‘‘refined methods’’.
7. Perspective We had gone from soda can and shoebox to suitcase scale, then imprudently to the 1000-ton PEP-4. Even on a logarithmic scale, that was a major extrapolation. We were exhausted. Our only technical publications for the TPC idea, until 1982 [12], were my short notes in SLAC Summer Studies of 1974 and 1975 [13] and two articles in Physics Today [14,15]. Yet, eventually, PEP-4 performance met every expectation. The magnetic field shape at the design value was indeed uniform, and momentum resolution was obtained at the promised level. To date, PEP-4 particle identification by dE/dx is the best in any large system. The timing resolution for track samples arriving at the readout plane was systematically calibrated to yield ultimately an impressive 1 ns rms, just 1% of the time sampling interval. The PEP-4 TPC worked, as expected. SLAC’s, LBL’s, and DOE’s investment had finally paid off. My motivation for describing the mix of travails and successes is to provide, beyond the sense of saga, a recognition that, even during this time of troubles, excitement for the transformative potential of the TPC idea persisted, sustained us and carried us onward. Stress levels were high and tensions sometimes higher. Yet no students defected (despite offers to do so), nor did any collaborators, and most graduate student and post-doc survivors of PEP-4 went on to excellent careers in academia and elsewhere. A genuine paradigm shift had been realized. In 1992 Georges Charpak was awarded Nobel Prize in Physics for his invention of the MWPC [16]. He included the following remark in his Nobel lecture on particle imaging: ‘‘. . . It was, however, D. Nygren who, by combining the effects of parallel magnetic and electric fields and solving formidable data acquisition problems, succeeded in creating an instrument which provides the finest images of the most complex configurations obtained in colliders;. . . ’’ Reading that passage made all the teething problems seem very distant. Over the last four decades, the TPC idea has been adapted and applied in a remarkably wide range of experimental contexts [17]. Although a breakthrough at the time, diffusion suppression in gas by magnetic field is no longer an essential element in every case. Very large liquid noble element TPCs are now prominent in searches for dark matter, neutrino-less double beta decay, and in present and future detectors for neutrinos. The TPC has indeed evolved, and these days the TPC idea is a commonplace. Iconic images of heavy-ion collisions
References [1] E. Rutherford, F.R.S., H. Geiger, An electrical method of counting the number of 𝛼-particles from radio-active substances, Proc. Roy. Soc. 81 (546) (1908) 27. http: //rspa.royalsocietypublishing.org/content/royprsa/81/546/141.full.pdf. [2] J.S. Townsend, On the conductivity of gases exposed to negative ions, Proc. Roy. Soc. 1 (sixth series, #2) (1901). See also issues of June 1902; April, September, November, 1903. [3] G. Charpak, R. Bouclier, T. Bressani, J. Favier, Č. Zupančič, The use of multiwire proportional counters to select and localize charged particles, Nucl. Instrum. Methods 62 (#3) (1968) 262–268. [4] SLAC storage rings SPEAR and PEP: see https://www-ssrl.slac.stanford.edu/ content/spear3/spear-history and https://www6.slac.stanford.edu/about/slachistory. [5] V. Cook, T. Ewart, G. Masek, R. Orr, E. Platner, Measurement of the 𝛴+ magnetic moment, Phys. Rev. Lett. 17 (1966) 223. [6] F.R.S. John Townsend, Electrons in Gases, Hutchinson’s Scientific and Technical Publications, London, 1948. [7] David R. Nygren, Proposal to investigate the feasibility of a novel concept in particle detection, LBNL internal note, February 1974; see https://inspirehep.net/record/ 1365360. [8] J.S. Townsend, Diffusion and mobility of ions in a magnetic field, Proc. Roy. Soc. A 86 (1912) 571. [9] Stephen G. Kukolich, Demonstration of the Ramsauer-Townsend effect in a xenon thyratron, Amer. J. Phys. 36 (1968) 701. http://dx.doi.org/10.1119/1.1975094. [10] W.W.M. Allison, C.B. Brooks, J.N. Bunch, J.H. Cobb, J.L. Lloyd, R.W. Pleming, The identification of secondary particles by ionisation sampling (ISIS), Nucl. Instrum. Methods 119 (1974) 499–507.
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[11] G. Charpak, F. Sauli, High-accuracy two-dimensional read-out in multiwire proportional chambers, Nucl. Instrum. Methods 113 (#3) (1973) 381–385. [12] R.Z. Fuzesy, N.J. Hadley, P.R. Robrish, Design of the multiwire proportional detectors for the PEP-4 Time Projection Chamber, Nucl. Inst. & Meth. 223 (1984) 40–46. [13] http://slac.stanford.edu/pubs/slacreports/reports04/slac-r-190.pdf.
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[14] J. Marx, D. Nygren, The time projection chamber, Phys. Today 31 (#10) (1978) 46. [15] R.J. Madaras, P.M. Oddone, Time projection chambers, Phys. Today 37 (#8) (1984) 36. [16] Electronic imaging of ionizing radiation with limited avalanches in gases. https:// www.nobelprize.org/nobel_prizes/physics/laureates/1992/charpak-lecture.html. [17] D. González-Díaz, F. Monrabal, S. Murphy, Gaseous and dual-phase time projection chambers for imaging rare processes, Nucl. Instrum. Methods. Phys. Res. A 878 (2018) 200–255.
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