Drift chambers D. C. Imrie The development of powerful particle accelerators requires concurrent developments in the apparatus used to analyse the interactions they make possible. Based upon principles which have been known for nearly half a century, drift chambers have been developed during the past ten years into versatile and sophisticated chargedparticle detectors that can have a spatial resolution better than 100 urn or an active area as large as 20 m*.
The enormous energies and intense fluxes produced by modern high-energy particle accelerators pose severe problems for the experimental physicist. Many experiments use the bending of the trajectory of a charged particle in a strong magnetic field to measure the momenta of charged secondaries produced when a high energy beam interacts with a target or with another beam of charged particles. The momentum resolution of such a measurement should, ideally, be independent of the incident energy, but this implies either that the apparatus must increase in size, or that the spatial resolution of the detectors used to reconstruct the tracks must improve as the incident energy increases, assuming, as is usually the case, that a large increase in the strength or volume of the magnetic field is uneconomic. In addition, the multiplicity increases with energy and, in stationary target experiments, the produced particles concentrate in a cone of progressively-decreasing angle around the forward direction. High-resolution detectors are needed to distinguish the closelyspaced tracks just downstream of the target. The need to detect rarer processes and to measure cross sections with much greater statistical precision than in the past has also created a demand for detectors capable of operating in intense particle fluxes, with the ability to digitise track coordinates and transfer the measurements to an on-line computer as rapidly as possible. The result of these pressures has been the development of several new types of detector and the improvement of many older techniques. Perhaps the most surprising of the innovations have been the multiwire proportional chambers and drift chambers. It is practically impossible to over-emphasise the importance of these new families of particle detectors for current high-energy physics experiments, yet they are based upon the principles of operation of the simple proportional counter of the 1930s (11. Although the multiwire proportional chamber is unrivalled for localising particle trajectories in high particle fluxes, its cost begins to be prohibitive for active areas above a few square metres, if a spatial resolution ofthe order of 1 mm is required, and it has proved difficult to improve the resolution beyond 250 Pm even in very small chambers. It is in these two directions that drift chamber developments have been so spectacular. Chambers with an active area of around 20 m2 and a spatial resolution of better than one millimetre, can now be constructed at acceptable cost. Spatial resolutions of the order of 150 pm are possible in chambers of moderate size, and. less than 20 em has been achieved in small chambers operating at high pressure. A proportional counter consists of a gas-filled metal tube a few centimetres in diameter containing a thin axial wire held at a high positive potential. When a charged particle passes through the counter it ionises a few of the gas atoms, leaving a trail of ion pairs in its wake. In a typical counter gas at one atmosphere pressure 20-30 primary ion pairs will be produced per cm of path. The electrons drift to the anode under the influence of the electric field in the counter. The field varies as+, and values as high as 10’ Vrn-i can be reached in the vicinity of the anode. In this region, electrons gain sufficient energy between collisions with gas D. C. Imrie.
6.Sc..
Ph.D..
F. Inst.
P.
Was born in 1939, and educated at University College London. After spending three years as an ICI Research Fellow in the University of London he became a Research Fellow at Harvard University (1966-68) Thereafter he was appointed to a Lectureship in the Department of Physics and Astronomy at University College London. His research interests are in experimental high-energy physics and he has spent three of the past six years at the European Centre for Nuclear Research (CERN), Geneva. 104
molecules to produce further ion pairs. The liberated electrons ionise other gas molecules and an avalanche of electron-ion pairs develops rapidly. In a typical proportional counter, a single primary electron produces an avalanche containing lo4 to lo6 electrons. The resulting anode signal can easily be shaped and amplified to provide a pulse of short duration capable of triggering standard logic circuits. Single cell drift chambers
The idea underlying the drift chamber is to use the drift time of the primary electrons to the anode as a measure of the coordinate of the incident particle. A simple drift chamber (figure 1) consists of a drift region containing a uniform electric field against which the electrons drift with, at the positive end, a proportional counter. Impressive detectors of this type were constructed by J. Saudinos et al. in 1970 121.The uniform field region was 0.5 m long, and a spatial resolution of 1.6 mm was achieved for electrons that had drifted the full length of the chamber, demonstrating that electrons could be drifted over long distances without excessive losses and that diffusion of the primary electrons did not prevent useful resolutions being obtained. Diffusion is the most important factor limiting the spatial resolution for long drift paths. If an electron drifts for a time t, then the rms displacement, dx, relative to the mean drift path, x, is given by dx = m, where D is the diffusion coefficient. The mean electron drift velocity, u, is given by ~.LE,where l.~is the electron mobility and E the electric field. Since t = x/u = x/l.& dx = d?!i%$i?, illustrating that, in a-constant drift field, the rms error in x increases as the square root of the path length. D/p is a function of the reduced field, E/p, where p is the gas pressure. It follows that dx = g(E/p) m where g is a function of E/p. Therefore, at fixed reduced field, dx w p-t, and the spatial resolution can be improved by operating the chamber at elevated pressure. A second factor limiting the spatial resolution is the granularity of the primary ionisation process; the average distance between ionising encounters for a minimum ionising particle traversing a gas at one atmosphere is of the order of 0.3 mm. This process affects the spatial resolution to a negligible extent for drift distances greater than a few mm, but is dominant at very small drift distances. Like the diffusion term, it can be reduced by operating the chamber at high pressure. The final factor affecting the spatial resolution is the precision of the drift time measurement. If the mean electron drift velocity is 50 mm ps-i, then a +3 ns jitter in the electronics leads to a spatial uncertainty of * 150 pm. It is clearly essential to employ wide bandwidth amplifiers and low threshold discriminators in drift chamber readout systems. Figure 2 shows some recent results on the spatial resolution of an atmospheric-pressure drift chamber as a function ofthe drift distance 131. Multiple-cell
drift chambers
In order to construct large-area drift chambers a multiple-cell geometry must be employed. One solution is to join together several chambers similar to the one shown in figure 1, but with a shorter drift space (figure 3(a)). Such a structure can be kept fairly thin in the direction of the incident particle, and the drift field is uniform and independently adjustable. On the other hand, a large number of potential dividers are required in order to provide the cathode wire potentials. Several other structures have been used for the construction of multicell drift chambers. The simplest of these is probably the
regions are separatedfrom each other by thin metal foils. As a consequencethe drift field is practically constant and there is no potential loss in efficiency near the end of the drift cell. Multiple-cell drift chambers have been constructed in sizes up to 4.5 x 3.5 m*. Since the anode wire spacing is, typically, a few cm, the number of readout channels, which dominates the overall cost, is not prohibitive even for very large chambers.
scaled-up multiwire proportional chamber, with thick field wires introduced between the anodes in order to remove the low field regions that would otherwise occur (figure 3(b)). A disadvantage of this arrangement is that it has to be made relatively thick in order to ensure a good efficiency for incident tracks that pass close to the field wires. In common with many simple drift chamber arrangementsthis schemehas a non-linear drift field. As a consequence the relationship between drift time and drift distance is not linear. An experimental calibration has to be obtained, unless the chamber is filled with a gas mixture such as argon/isobutane in which the drift velocity is essentially independent of the drift field and gas composition over a wide range of values,(figure 4).
The left-right
Anode wire HVI 1.1, Cathode
I
HVZI-I
I A uniform-field,
Figure 1
single-cell
embiguity
Multiple-cell drift chambers have a drift region on either side of each anode wire and it is impossible to tell, a priori, whether the incident track passedto the left or right of the anode. This leftright ambiguity can be resolvedin a variety of ways. Stacking two chambers of the type shown in figure 3(b), with the field wires of one opposite the anode wires of the other (figure 5) permits the elimination of the ambiguity and, for tracks normal to the chamber plane, provides excellent time resolution, since the sum of the drift times measuredin the two chambers is equal to the drift time for a single complete cell. However, the structure is rather thick and, for inclined tracks, an approximate knowledge of the inclination is required. A secondtechnique is to replace each anode wire by a pair of anode wires stretched side-by-side a few tens of microns apart. The left hand anode detects electrons arriving from the left hand drift space and vice versa. It is often necessary to stretch a shielding wire between the two anodes in order to minimise the electrostatic repulsion between them. It is also necessary to employ a gas mixture which absorbs photons efficiently, otherwiseit is possible for an avalanche on one anode to initiate a discharge on the second, with disastrous consequencesfor the spatial resolution since, if both anodes count, the incident trajectory must be assumedto passbetweenthem. Recently it has been realised that the avalanche does not spread around the anode wire, but tends to remain localised on
drift chamber.
Figures 3(c) and 3(d) illustrate other multicell structures. In figure 3(c), the field wires are replaced by I-section beams insulated from the cathodes, which provide better field shaping and structural strength for the assembly. In figure 3(d) the main structure contains relatively few field wires and individual drift
100
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X (mm1 Figure 2 The spatial resolution of a uniform-field drift chamber as a function of drift distance. The dashed lines are hand drawn to indicate the contributions of electron diffusion, the time resolution of the electronics, and the granularity of the primary ionisation process to the overall resolution. 105 END. 313-c
the side from which the initial electron approached.The left-right ambiguity can therefore be eliminated if the position of the avalanche can be detected. In a structure of the type shown in figure 3(b) the shapeof the signals induced on the field wires can be observed[41,while, in a chamber with wire cathodes,the signal induced on cathode wires in the vicinity of the anodescan be used to localisethe avalanche [51. Anode wire
the anode wire and to measurethe relative delay of the induced signals appearing at the two ends of the delay line. The use of a delay line simplifies the timing becausethe induced pulses travel much slower on the line than on the anode wire itself, but the line must be carefully constructed to ensure adequatecoupling to the anode and to minimize dispersion and losses. With care, a precision of about 0.1 per cent of the length of the anode wire can be achieved.
HVI (+ ] Anode wire HVI (+) I
Field wire
Field wire
Insulation I
. . . . . . .
Cathode wares
HV2 l-l
Cathodes
1
0 L cm
Figure 3(a) Multiple-cell drift chamber constructed several uniform-field single-cell units.
Measurement
of the second
by joining
coordinate
Planesof drift chambersare often stackedwith alternate planesof anode wires orthogonal (figure 6) in order to permit a particle’s trajectory to be reconstructed, but difficulties arise when more than one particle traverses the array during the resolution time, becauseof the problem of correctly assigning coordinates to each trajectory. By using sufficient planes, some with angled anode wires, it is, in principle, possible to solve the problem, even in the presence of multiple incident tracks. In practice, because it is necessary to allow for the possibility of chamber inefficiencies, multiple scattering, and finite spatial resolution, the reconstruction routines frequently generate‘ghost’ tracks and the computing time required fully to solve the combinatorial problem is very large. It is, therefore, important to attempt to construct chamberswhich provide correctly-correlated x and y coordinates from a single active wire plane. Anode
,Field wire HV2(-)
i
0 -
i-beam
HV2(-1
I
cm Figure 3(c) Multiple-cell drift chamber with field wires replaced by l-beams for structural strength and better field shaping.
In many chamber designs it is possible to divide the cathodes into strips about 1 cm wide perpendicular to the anodes I6 I. The induced signals on thesestrips allow the position of the avalanche to be determined. The induced signal spreadsover several strips, but excellent precision can be obtained by calculating the centroid of the distribution, ;= txxII I z Xl where Xi is the charge induced on the strip centred at x,. Spatial resolutions of well under 100 pm have been achieved with this technique.
wrre HVI (+I rreld wire HV2 t-1
li’
Figure 3(d) Multiple-cell drift chamber structure with cells separated by foils and the drift field produced by relatively few field wires.
Cathodes 1
0
cm
Figure 3(b) Simple multiple-cell drift chamber geometry having plane cathodes and anodes separated by field wires.
The simplest techniques are those that detect the anode signal at both ends of the anode wire and infer the position of the avalanche either from the relative amplitude or the relative delay of the two signals.The precision of such schemesis rather limited; about 1 per cent of the wire length using current division, and a few cm for the timing measurement. A conceptually similar schemeis to run a delay line parallel to 106
Large-volume
drift chambers
The drift chambers described in the previous paragraphs are essentially two-dimensional devices that provide, at most, the x -and y coordinates of the trajectory of a particle that passes approximately normal to the chamber plane. It is equally possible to arrange for the incoming particle to move parallel to the anode wire plane, roughly perpendicular to the wires, so that many coordinate measurementsmay be madeon a single track. Several ambitious detectorsof this type have beenproposedduring recent years, for use either as particle identifiers downstream of bubble chambers in hybrid experiments, or as part of a detector system surrounding the interaction region in a colliding beam experiment.
The earliest of these detectors was ISIS [71, a huge drift chamber 4 x 1.8 x 5.1 m3 designed for use with the European Hybrid Spectrometer (EHS) at CERN (figures 7 and 8). There are two large drift spacessituated on either side of a single wire plane containing 640 anode wires and 640 field wires spaced 4 mm apart. The voltage applied across the 2 m drift spacecan be as high as 200 kV. The gas usedis a mixture of argon with 20 per cent CO, at atmospheric pressure. Particles from the rapid C
1
per unit length of chamber gas can be estimated. Provided this measurement can be carried out with adequate precision it provides a means of identifying pions, kaons, and protons of known momentum in the above interval. Particle
identification
Figure 9 shows the relative ionisation energy loss in argon/5 per cent CH,, at one atmosphere, as a function of the momentum
I
I
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I
I
800
1200
1600
2000
2400
60I-
GOlU I mm I-E-‘)
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Drift field Figure 4 The electron drift velocity fraction of isobutane in the mixture.
in argon/isobutane
gas mixtures
cycling-bubble chamber, which is the target and vertex detector of EHS, passthrough the 4 x 1.8 m* face of ISIS and traverse its length approximately parallel to the detector plane. Since the anode wires are connected in pairs there are 320 coordinate measurements,each with a precision of a few mm, madeon each track. The electronic circuitry connected to each anode pair is designedto accept up to 30 signals during the resolution time. A detector of this size presents very formidable operating problems. In addition to the difficulties implied by the use of drift voltages in excessof 100 kV, the oxygen contamination in the gas must be reduced to less than 0.2 x 1W6by volume, becauseCO, catalyses the attachment of the primary electrons to residual oxygen molecules.The 2 m drift path implies positive ion clearing times between 150 and 200 ms. As a consequence, the background flux of ionising particles must be kept below 2.5 x lo4 me2 s-i, in order to prevent the positive ion density increasing to a level where it can significantly distort the drift field. The very attractive feature of ISIS and similar detectorsis that, when combined with momentum measurement, they offer the possibility of particle identification in the difficult momentum range between 5 and 70 GeV/c. The amplitude of each anode pulse is measuredand, by appropriately combining the 320 pulse height measurements,the most probable ionisation energy loss
( V cm-‘)
as a function
of the electric?ield.
The curves
are labelled
with the volume
divided by the massof a singly-charged particle ISI. Plotted in this way the ionisation lossesof all particles of interest fall on the same curve. The mean ionisation loss increasesby about 50 per cent as p/me increasesfrom 5 to 500 and flattens off at higher values. It follows that, in the momentum range 5-70 GeV/c, the most probable energy lossesof pions, kaons, and protons of the same momentum differ by between8 and 15 per cent. So, if the loss can be measuredwith a precision exceeding 2-3 per cent, a particle of known momentum can be identified with a high degree of confidence. Unfortunately, the rms fluctuation of the energy loss in one cm of gas at one atmosphereis of the order of 40 per cent, and decreasesonly slowly with sample thickness; consequently many measurementsmust be made on a particle in order to establishits identity. The large high-energy-loss tail of the ionisation distribution also causes difficulties and it is necessaryto compute the average for the lowest 30-70 per cent of the samplesin order to obtain a reliable estimateof the most probable energy loss. Drift chambers
in colliding
beam experiments
. The excellent spatial resolution of high pressure drift chambers, combined with the possibility of particle identification, make them attractive devices for use with colliding beam accelerators. The detectors can be kept compact without compromising 107
momentum resolution and the low fluxes of background particles in the vicinity of the collision region enable rather long drift paths to be used.
Particle Cathode /
.
F
. c
A
.
F
t2
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A
. .
Cathodes
F
.
The x, y, and z coordinates of tracks traversing the module are obtained from each anode. x is provided by the particular anode wire involved, y by the drift time, and z by the division of the charge signal betweenthe two ends of the wire. Tests on a single module have indicated that the spatial resolution in they direction is better than 150 pm. The total charge detected from a wire is also used for particle identification. The left-right ambiguity is resolved in a very elegant fashion by staggering alternate anode wires on either side of the median plane through a small distance of the order of the y resolution. The stagger can be neglectedin making a first fit to the trajectory in the x-y plane, but a measurementof the instantaneous slope using successivepairs of anode wires is systematically greater or less than the true slope, dependingupon whether the particle passedto the left or the right of the median plane.
.
A
.
. T
t1
‘Cathode
Figure 5 chambers
Resolution of the left-right ambiguity of the type illustrated in figure 3(b).
by stacking
two
The central cylindrical drift chambersof the JADE detector at the e’e- storage ring PETRA at DESY, Hamburg 191are an excellent example of such a system (figure 10). In this, 96 trapezoidal modules are arranged to form three coaxial, cylindrical chambers.Each element has field electrodesarranged
PartIck
I
/
Anode and field wire plane Figure
Chamber
Chamber
3 fxl
I (xl
Figure 6 Reconstruction of the trajectory of a particle of chambers with orthogonal planes of anode wires,
in a system
around its periphery to provide a uniform electric field across the module, and sixteen anodes and nineteen field wires 2.4 m long, spaced 5 mm apart, are arranged along the median plane. The complete system is mounted inside a pressure vessel containing an argon-hydrocarbon gas mixture at 4 atmospheres,which is placed within an 0.5 T axial magnetic field. 108
7
The ISIS chamber.
An equally audacious detector is the time projection chamber (TPC) designed for use with Stanford’s PEP storage rings [IO]. This detector (figure 11) is designedto operate with a mixture of 0.8 argon/O.2methaneat a pressureof 10 atmospheresin a 2.0 T axial magnetic field. Unlike the JADE chambers, the six sectors are completely unencumbered by wires, the drift field being parallel or antiparallel to the magnetic field. All detecting systems are located on the end caps. The sensitive volume is split in two longitudinally, giving a maximum drift path of 1 m. A uniform drift field of 1.5 kV m-i is provided by a central circular electrode at a potential of 150 kV, with a system of annular guard rings. Each end cap has 192 anode wires alternated with field wires. A wire grid defines the end cap region and there is a plane cathode on the outer side of the anode plane. Beneath 12 of the anode wires the cathode plane is divided into 7.5 mm squaresin order to provide a coordinate measurement along the anodes from the amplitude distribution of the induced pulses. In the TPC, x coordinates are provided by the anode wire number, y coordinates by the segmented cathodes, and. z
Figure 8 ISIS 1, a prototype of ISIS, which has the full transverse dimensions of the final version, but is only half as long. The electrode structure has been slid out of the external gas-retaining vessel, which appears in the foreground.
Ar + 5%
CHL
4 -
measurementsby the drift time. The anode pulse height is also used for particle identification. In prototype tests the spatial resolution in the y direction has been measuredto be better than 150 pm. The z resolution is only a few mm, but this is adequate because high resolution is not required in the direction of the magnetic field. Conclusions
I I I III, 1
IO
I
I
,
I I,,,,,,
102
103
, , ,,,,,
I IO‘
,
,,,,,
I 105
plmc
Figure 9 Relative ionisation energy loss in argon/O.05 CH, at NTP as a function of the momentum divided by the mass of the incident particle. The experimental data were obtained with the external particle identifier of the BEBC bubble chamber at CERN, Geneva [B].
Drift chambers have been developed to a remarkable degree during the past few years. Although the large volume detectors such as ISIS, JADE and the TPC have yet to prove themselves under experimental conditions, there seemslittle doubt that highpressure drift chambers with particle identification will be of crucial importance both at current accelerators and at future accelerators such as the proposed large European electronpositron storagerings. Cylindrical chambers can probably be increased in size by a 109
Particle
1.6
magnetic field
la1 Figure
10
The central
drift chambers
of the JADE
lb1 experiment
at PETRA.
factor of 1.5 to 2, still maintaining the spatial resolution in one dimension at around 150 pm, but the most sophisticated engineering techniques will be required to reduce systematic constructional errors to an acceptablelevel. The increasing useof
large-scale integrated circuits and the availability of relatively new components, such as fast random-access memories and charge-coupled devices, is already revolutionising the design of read-out systems.Whatever the long term developmentsmay be,
192 anode wires in total
/I
la)
Im
12 anode wires equispaced across end cap, fitted with 7.5mm2 segmented cathodes
(b)
Grid
t I
Anodes \
6 end caps at each end
110
11
The time projection chamber. (al General view (b) Arrangement of end cap detectors (4 End cap electrode structure in detail-views parallel and perpendicular to the anode wires,
TE
Grid
./
Field
wire
-T-r-r-J-v 0
to preamplifiers
Segmentedcathodes Figure
E
(cl
it seemscertain that drift chambers will continue to be essential
components of many high energy experiments in the forseeable future. Acknowledgments
It is a pleasure to thank Dr T. G. Walker of the Rutherford Laboratory, Chilton, Oxon, for severalhelpful comments.Dr W. M. M. Allison of Oxford University kindly provided the photograph of ISIS 1 (figure 8). I should also like to record my thanks to Una Campbell for her excellent work in producing the line drawings and graphs.
References
[ 11 An excellent summary of the principles of operation of multiwire proportional and drift chambers has been given by F. Sauli, CERN 77-09, (1977). Accounts of recent developments can be found in the articles by G. Charpak, Physics Today, 31, 23, 1978and J. Heintze, Nucl. Instrum. Meths., 156,227,1978.
Enduvour, IOPergamn
Now Swim~Vduma 3, No. 3,197s Prow. Plintod in Groat Britain)
121 Saudinos, J., Duchazeaubeneix, J.-C., Laspalles, C., and Chaminade, R. Nucl. Instrum. Meths., 111.77, 1973. 131 Filatova, N. A., Nigmanov, T. S., Pugachevich, V. P., Riabtsov, D. V., Vcclopianov,A. S., Sauli, F., and Atac, M., Nucl. Instrum. Meths., 143.17, 1977.
[41 Fischer, J., Okuno, H., and Walenta, A. H., Nucl. Instrum. Meths., 151,451,1978. 151 Breskin, A., Charpak, G., and Sauli, F., Nucl. Instrum. Meths., 151,473,1978.
I61 Charpak, G., Peterson, G., Policarpo, A., and Sauli, F., Nucl. Instrum. Meths., 148,47 I, 1978. 171 Allison, W. M. M., Brooks, C. B., Bunch, J. N., Cobb, J. H., Lloyd, J. L., and Flemina. R. W.. Nucl. Instrum. Meths.. 119. 499. 1974. I81 Lazeyras, P., Lehraus, I., Matthewson, R., and Tejessy, W., Proc. 1978 Nuclear Science Symposium, Washington D.C., 18-20 October 1978,(to be published). 191 Farr, W., Granz, B., Heintze, J., Heuer, R. D., Lennert, P., Nozaki, T., Riesenberg, H., and Wagner, A., Nucl. Instrum. Meths., 156,283,1978.
1101Nygren, D. R. and Marx, J. N., Physics Today, 31,46, 1978.
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