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Focal plane detector for the Oxford MDM-2 spectrometer
Nuclear Instruments and Methods in Physics Research A251 (1986) 297-306 North-Holland, Amsterdam
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FOCAL PLANE DETECTOR FOR THE OXFORD MDM-2 SPECTROMETER J.S. W I N F I E L D *, D.M. P R I N G L E , W.N. C A T F O R D **, D.G. L E W I S ***, N.A. J E L L E Y a n d K.W. A L L E N Nuclear Physics Laboratory, University of Oxford, Keble Road, Oxford OX1 3RH, UK Received 9 May 1986
A "hybrid" focal plane counter comprising a gridded ionization chamber with position sensitive proportional counters, 300 mm long, 510 ram deep and with an active height of 60 ram, is described. The detector is designed for use with the Oxford MDM-2 magnetic spectrometer and is suitable for both light and heavy ions. Typical results obtained are 0.6 mm position resolution and 1% total energy resolution. Two energy loss, a veto and two position signals are available as well as provision for height and timing signals. Techniques are described for the reduction of capacitive noise, correction of electric field nonuniformities, and the reduction of interference from positive ions on the ionization signals.
1. Introduction A heavy-ion magnetic spectrometer (the MDM-2) has been in operation since 1982 at the Nuclear Physics Laboratory in Oxford. The spectrometer itself has been described in a previous paper [1] and here we discuss the focal plane detector. The main requirements for the detector were as follows: (1) isotopic identification of heavy ions via energy-loss (AE) signals and a total energy (Etot) signal, (2) a position resolution of about 0.5 mm in order not to compromise the excellent resolving power of the spectrometer itself, and (3) the detection of light ions. The dimensions of the image at the focal surface of the MDM-2 spectrometer required a detector acceptance length of 60 cm (full focal plane) and a relatively large maximum height of 6 cm. In view of the size of such a detector, it was decided initially to build a prototype counter of one half the length of the full focal plane. The above requirements led immediately to the consideration of a detector of the type developed at Argonne [2] and Rochester [3] a "hybrid" counter which comprises a gridded ionization chamber with a split anode of plate electrodes and position sensitive proportional counters (PSPCs). Versions of these detectors have now * Present address: National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA. ** Present address: Dept. of Nuclear Physics, The Australian National University, GPO Box 4, Canberra, ACT 2601, Australia. *** Present address: Dept. of Physics, The Royal Marsden Hospital, Sutton, Surrey SM2 5PT, UK. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
been constructed at various nuclear physics laboratories, each addressing particular design problems. In our case, an important advantage owing to the MDM-2 spectrometer is that the focal plane has particles crossing at a 90 ° mean angle of incidence, so that any angle-dependent effects in the position and energy measurements are small and require no special attention. On the other hand, we needed to consider the problems raised by the large volume of the counter. Despite the comparatively small size of the electrodes in the early designs of hybrid counters [2,3], the energy resolution was limited by their capacitance to ground. Later designs succeeded in reducing this capacitance, with the result that the noise in some cases [4] was low enough for the counter to work also as a good detector for light ions. We were concerned from the outset with keeping stray capacitances to a minimum, and have been able to achieve noise levels which compare well with the smaller counters. We briefly discuss the design (sect. 2) and performance (sect. 4) of the Oxford hybrid counter. Sect. 3 is a more detailed description of some of the problems encountered with the present counter and their solutions.
2. Design and construction Fig. 1 shows a cross section of the Oxford counter with the general layout of the elements. The counter consists of a gridded ionization chamber, from which the total energy signal is obtained, with a split anode plane divided into two A E electrodes, a residual energy electrode (from which no signals are taken at present),
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J.S. WinfieM et al. / Focal plane detector for MDM-2 spectrometer POSITION MEASUREMENT
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Fig. 1. A schematic cross section of the focal plane detector.
and three proportional counters. The first two proportional counters are position sensitive, and provide signals which enable the reconstruction of the position of the particle at the focal plane and its crossing angle. The third wire counter provides a signal which may be used to veto nonstopping particles. The "dynamic range" of the counter is given by the ratio of the depth (51 cm in the present case) to the distance of the second proportional counter from the entrance window (17 cm); the Oxford counter has a very useful dynamic range of 3:1. In addition, the greater depth compared to that of the Argonne or Rochester counters enables the counter to be operated at lower gas pressures. The first proportional counter is as near to the entrance window as possible (2 cm), allowing for field nonuniformities very close to the entrance (see discussion later). Considerations of the desired angular resolution and the multiple scattering of ions led to a design with a separation of 15 cm between the first and second proportional counters; this also allows for AE anode plates of adequate depth. The large vertical image size of the Oxford MDM-2 spectrometer, together with the fact that the vertical focus is displaced downstream from the horizontal focus, necessitate a 6 cm high window to the focal plane detector. Since it was desirable to have a re-entrant window in order to keep the dead layer of gas to a minimum, the cathode to grid spacing was designed to accommodate the thickness of the window mounting as
well. The resulting 10.5 cm height makes the chances of a multiply scattered particle striking the cathode or passing through the grid very small. However, the large vertical dimension also means that field correction at the sides of the ionization chamber is important for utilization of the full length of the counter. All the detector elements are firmly bolted to two blocks of Delrin plastic. This frame is mounted on the front plate of the gas cell, through which all electrical and gas connections are made. Thus the entire body of the detector can be taken out from the gas cell without disconnecting any internal cabling. Reports on other hybrid counters [2,3] mention trouble with microphonic noise on the ionization signals arising from rotary pumps on their spectrometer systems. Despite the cantilever mounting of the present detector, microphonic noise was not a problem. As a precaution, rotary pumps in the vicinity of the focal plane were decoupled mechanically from the detector housing. Some low frequency ( = 25 Hz) noise that was associated with one rotary pump was observable on the output of the preamplifier connected to the cathode, but this was entirely filtered out by the main amplifiers. The original design for the ionization chamber called for a double grid as is common in other hybrid counters. The upper grid is intended to shield the lower (Frisch) grid from the large numbers of positive ions generated in the proportional counters. In the final design for our counter, we have replaced this upper grid by individual small grids immediately beneath the pro-
J.S. Winfield et al. / Focal plane detector for MDM-2 spectrometer
portional counters. These small grids appear to work at least as well as the former full grid (see sect. 3.3) and, unlike the latter, do not contribute significantly to the capacitive coupling of the Frisch grid to ground. The Daresbury detector [5] is also a single-gridded counter. The Frisch grid was wound from 100 /tin Be-Cu wire with a 1.5 mm pitch. A rectangular frame of G10 glass fibre supported the wires with printed circuit board along the edges for electrical contact. In order to keep the middle wires taut, jacking bolts were required on the frame. The grids beneath the proportional counters were made with the same wire but with a finer spacing (1 ram). The final design of the counter has horizontal field shaping wires around all four sides. The wires at the entrance (a double bank, with the first bank immediately next to the window and the second bank 7 mm inside the counter) were added after tests showed that field distortions were unacceptable without such corrections (see discussion in sect. 3.2). The wires are 100 t~m Be-Cu, with a vertical spacing of 7 ram. The recessed window assembly is designed to be separately removable from the gas cell front plate. The re-entrant part was constructed from packed layers of G10 glass fibre glued together; the choice of insulating material avoided bringing a conductor near the critical region underneath PC1. The glass fibre section is permanently bolted to the steel mounting flange and sealed with an O-ring. Commercial mylar windows, ranging in thickness from 5 to 25 #m, have been in standard use for the counter. These are glued with epoxy to the G10 face of the window assembly with about I cm of contact all around. This avoids the need for a clamping ring which would have been necessary if an O-ring were used to make the seal. Since the spectrometer focal plane is perpendicular to the incident ions, the PSPCs were simple to design and construct. The bodies (cathodes) are made from 400 mm lengths of rectangular aluminium bars with Ushaped grooves machined into one edge. The width of this slot is 10 mm and the anode wire is made to lie along the axis of the cylindrical section, 9 mm from the opening of the slot. The wire used was 10 # m N i - C r , which was soldered under light tension to high voltage coaxial connectors at each end of the counter. Initially, occasional trouble was experienced with wire breakages when high voltages were applied. The source of the problem was probably a large discharge caused by spikes on the voltage line when supplies with discrete thumbwheels were used. The trouble has not recurred since we changed to voltage supplies with continuous potentiometers. Many other workers (e.g. Shapira et al. [3], Erskine et al. [2], Oed et al. [6]) have shown that isobutane has two advantages over argon-methane mixtures for hybrid counter use: it has a greater stopping power and
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produces less multiple scattering of the ions for a given energy loss. Isobutane is especially sensitive to electronegative impurities, however, and precautions are needed to avoid contamination. We have therefore constructed the gas cell of the counter from stainless steal. Flexible stainless steel pipes connect from the front plate of the counter to the exterior gas handling system, thus allowing the counter to move for kinematic correction. Where insulating material is required, Delrin plastic (which has a low hygroscopic coefficient) has been used rather than nylon. In order to minimize the outgassing of impurities once the detector is filled, it was important that a low impedance pumping path be provided directly between the detector volume and the high vacuum system. A low-cost solution was to equip the gas cell with a 4 cm diameter solenoid-operated valve in the back plate in order that the interior can initially be pumped by the spectrometer high vacuum system. With these precautions it has been possible to operate the detector without the need to flow the gas. A fresh charge of isobutane in a typical heavy-ion experiment lasts about one day or more before the ionization signals show signs of deterioration. During light ion experiments, the counter has been run for up to one week without changing the gas. CP grade of isobutane (95% isobutane, 4% propane) has routinely been used in the counter.
3. Discussion of specific problems encountered 3.1. Noise due to capacitive coupling to ground
The capacitive coupling of the ionization chamber to ground may significantly contribute to the noise on the ionization signals (see, e.g. ref. [4]). We were careful to allow a large separation between the cathode and the bottom of the gas cell, but the relatively small separation (1 cm) of the double grids and their large area (40 × 53 cm 2) in the original design caused such high noise levels on the Etot signal that we eventually dispensed with the second grid. Measurements with a bridge circuit have shown that the capacitive coupling of the single grid to the anode plane is of the same magnitude as the coupling of the chamber as a whole to the (grounded) gas cell front plate. Thus it is probably difficult to further reduce the total capacitance to ground (480 pF, which corresponds to a noise level of 180 keV with Ortec 142B preamplifiers). The preamplifiers for the ionization signals are mounted outside the gas cell for ease of access and to reduce the outgassing within the counter. The additional stray capacitance due to the cable lengths is negligible. This applies especially to t h e / t E signals, for which energy-loss straggling rather than electronic noise is the major contribution to the resolution in almost all practical cases.
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J.S. Winfield et a L / Focal plane detectorfor MDM-2 spectrometer
3.2. Entrance field nonuniformity Shapira et al. [7] have shown the importance of correcting nonuniformities in the electric field distribution at the sides of the counter when the separation between the cathode and grid is significant compared to the other dimensions. We have made electrostatic field calculations for various configurations at the entrance of the counter in a manner similar to refs. [7] and [5]. These showed that either a window with horizontal bands of potential-graded strips [8] or an insulating window with external upper and lower guard plates [5] would be suitable schemes. On the basis of ease of construction, the latter approach was adopted. Subsequent tests with heavy-ion beams showed, however, that the external guard plates were not sufficient to correct the field beneath the first proportional counter (PC1) and the first A E plate (AE1). When voltages approximately equal to those of the Frisch grid and cathode were applied to the upper and lower grid guard plates respectively, the signal amplitude from PC1 increased from being undetectable to a size equal to that of PC2. However, the PC1 signal was - 2 #s late with respect to PC2 and the resolution of A E1 was significantly worse than that of AE2. Continual adjustment of the guard plate voltages was necessary to reduce these problems and it seems likely that the charging-up of the insulating window under bombardment by the ions caused the residual field nonuniformities. Yet a similar design of external guard electrodes apparently works well in the Daresbury detector [5], possibly because there the counter gas is continually flowed. The adopted solution was to insert the double bank of horizontal potential-defining wires described in sect. 2. This works well and allows for the easy replacement of the window foils unlike the striped-window design of Naulin et al. [8]. 3.3. Positive ion feedback The prevention of spurious signals on the ionization electrodes induced by the large cloud of positive ions from the PCs is a matter of some concern with hybrid counters. If the positive ions are not efficiently trapped within the proportional counter or on a shielding grid, a large slowly rising pulse will be seen a few hundred microseconds later than, and of the same polarity as, the genuine ionization pulse [5,9]. We had designed the proportional counters with an anode wire recessed 9 mm into the cathode structure. This design was intended to trap a significant portion of the positive ions before they left the proportional counter. However, tests with an a-particle source and subsequently with scattered beams from an accelerator showed that a large feedback problem was present. With the main amplifier shaping time set at 8 #s (in order to filter out high frequency noise components),
the ionization signals had poor resolution and were shifted up in amplitude when the PCs were operated at high voltage. Shorter shaping times on the amplifier reduced the problem because of the slow drift time of the positive ions which were escaping through the shielding grid beneath the proportional counter. In an attempt to stop the positive ions from reaching the region near the Frisch grid, an additional fine grid of wires was inserted in the anode plane, beneath the proportional counters. This had only a small beneficial effect: about 25% larger pulses could be obtained from the PC before the Etot resolution began to deteriorate. The lack of effect appears to stem from the recessed-wire design of the PCs. An undesirable consequence of this arrangement is that it is difficult to make the electric field strength immediately above the shielding grid significantly greater than that in the region between the Frisch grid and the shielding grid - a condition which is necessary if the shielding grid is to capture a large fraction of the positive ions [10]. In order to increase the field above the shielding grid, the proportional counters were raised 6 mm above the anode plane and the potential of the PC as a whole was increased. The grid should then have become more opaque to positive ions moving downwards but more transparent to electrons drifting upwards. In practice it was found that although the efficiency of electron collection was more than doubled, the size of the positive ion pulse on the Etot signal was only reduced by some 20% for equivalent charge collection on the PC wires. The time delay between the true ionization signal and the start of the interference varied considerably with the voltage applied to the body of the PC: for 500 V the delay was 0.3 ms but for 100 V it was 1.4 ms. No further attempts to capture all of the positive ions were made because the remaining interference could be filtered out by the main amplifier. With a shaPing time r of 4 #s the pulses had almost disappeared and at = 2 #s the effect of the positive ions was negligible. In general use, the counter has been run with 2 #s shaping times on the ionization signal amplifiers; this does not appear to significantly worsen the signal to noise ratio. A simpler overall design, without raised proportional counters, may have been possible if the proportional counters had shallow slots so that the field from the anode wire would be strong in the critical region just above the shielding grid. This would allow the ratio of the fields above and below this grid to be set to > 2 : 1, which was seen to improve the electron collection efficiency of the PCs and is thus particularly important for light-ion experiments. We conclude, however, that the positive ions are too numerous to be stoppe d entirely by shielding grids and that some filtering of the ionization signal is always necessary. Ophel [11] has come to a similar conclusion from experience with a small hybrid counter.
J.S. Winfield et al. / Focalplane detectorfor MDM-2 spectrometer 3.4. Capacitive coupling between PC body and A E plates Another problem caused by the large signals from the PCs is direct capacitive coupling to the ionization chamber electrodes. Careful routing of signal cables and the construction of anode dements with bevelled edges are standard precautions to reduce this effect in hybrid counters. Originally, the PCs were set on the anode plane with their cathodes held at ground potential and no evidence of capacitive pickup of PC signals was observed on the ionization signals. When the PCs were raised above the ground plane some fast, destructive interference was seen on the A E signals: the size of the A E signals was reduced as the PC wire voltage was raised until eventually the signal polarity appeared to be reversed. That this was a crosstalk problem rather than the drift of a positive ions was clear from the observation of delayed pulses some 300 #s later which were attributable to drifted ions. The crosstalk was eliminated by connecting 5 nF capacitors between the PC bodies and ground. As an additional precaution, the A E electrodes were shielded from the PC's by grounded copper plates.
3.5. Loss of total energy signal at sides of counter Tests in which an elastically scattered group of partides was systematically stepped across the detector showed that the energy signal began to drop in amplitude some 8 cm away from the centre. At 12 cm from the centre the peak height was about 5% lower than the value at the centre of the detector, and at 15 cm (i.e. close to the edge of the window) it had further fallen by another 5%, with a noticeable deterioration of resolution. Both effects were less pronounced when the spectrometer's horizontal aperture was reduced. Since it would appear from linearity tests of the proportional counters (see sect. 4.2) that the electrostatic field at the sides of the counter is quite uniform, we may conclude that the loss in energy signal arises because the cathode and Frisch grid do not form a complete Faraday cage. N o signals are taken from the potential defining wires around the sides of the counter and the positive ions left in the chamber after the electrons have passed through the grid will induce charge on the wires to an extent that varies with distance. The wires could be ac-coupled to the Etot signal to avoid the loss of induced charge, but this has not proved necessary for experiments so far. Alternatively, the present Eto t signal could be software-corrected if desired.
3.6. Signal reduction at localised regions of PSPC wires After long periods of use, some PSPC wires have shown a reduction in pulse height for signals corresponding to particular locations on the wire. The effect
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may be conveniently monitored via a plot of the position parameter against the signal from one end of the counter (see fig. 2). The distortions in the position spectrum alone may be subtle and less easy to detect. It seems likely that this deterioration in the wire performance is a consequence of intense bombardment of the counter - by well-focussed elastically scattered beams, for example. Deposition of the products of isobutane cracking would cause a local increase in the effective diameter of the wire (this phenomenon has been noted in ref. [12]) and hence reduce the gas gain at that location.
4. Operation and performance 4.1. Energy and energy-loss signals Rather than capacitively coupling the cathode and Frisch grid to obtain a height-independent total energy signal, our usual practice is to sum the independent signals from the cathode and grid preamplifiers before the main amplification. This method makes the cathode signal available both for a measure of the height of the ion (see sect. 4.5) and as a timing signal (sect. 4.4). The energy-loss signals are taken from the plates on the anode plane. Tests with elastically scattered 35 and 63 MeV 12C particles stopped in the detector gave a best resolution of 0.9% for the Etot signal. The resolution of the A E signals is dominated by statistical fluctuations in the ionization. A typical result, for 43 MeV 9Be in 100 Torr of isobutane, is 7.0% measured resolution to be compared with 6.3% calculated contribution from energyloss straggling [13], including a factor of 1.6 for chargeexchange straggling [2]. An example of particle identification from an experimental run is shown in fig. 3. When the detector is used in light-ion mode, in which the particles of interest are not stopped in the counter, the Etot signal becomes a measurement of the energy loss. However, we have preferred in this case to use the summed signals from the two ends of the first proportional counter for particle identification. Then, any particles which are not stopped in the counter but which strike the sides before the veto counter do not cause anomalously small ionization signals (this is not a problem in heavy-ion mode since the particles of interest will give much greater Etot signals than any lighter particles which do not stop).
4.2. Position signals We chose charge division as the method of determing the position from the proportional counters. The timing method would probably be just as suitable for a 30 cm wire length, but would not have been as simple.
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Fig. 2. A plot of the signal from the right end of PC1 against the position parameter. The focal plane was illuminated by a-particles from the 12C(12C, aX) reaction. The arrows indicate two localized regions along the wire where a marked signal reduction occurs.
Rather than using an analog divider which may introduce nonlinearities, the division of the signal amplitude from one end of the counter by the summed amplitudes from the two ends is done in the data analysis software. This generally works well, but does give rise to a potential problem to do with the division of small integers. If the digitized signals from the ends of the wires are small relative to the number of channels in the position spectrum, a significant amount of accuracy will be lost. Then in a division operation of the form P
A L CL+R
(where L and R are the integer results of digital conversion, A is the number of channels in the A D C , and C is the compression factor used in the generation of the histogram of P), certain values of P will be suppressed and other values enhanced. For example, if
A = 512 and a 512 histogram of P is made ( C = 1), for L < R < 125 a result of P = 255 cannot be obtained whereas P = 256 can result from many combinations of L and R. This effect gives rise to sharp oscillations in a histogram of P (see fig. 4). The oscillations are particularly pronounced at one half, one third and two thirds along the spectrum. We find empirically that in order to avoid this problem, the size of the digitized signals L and R should be at least A / ( 2 C ) when particles are incident at the centre of the counter. Obviously, the situation is alleviated by compressing the histogram; the consequent loss of resolution could be avoided by the use of A D C s with more channels. We presently digitize the PSPC signals with 4096-channel ADCs. The resolution of the proportional counters has been investigated both on the bench with an a-particle source and with scattered beams. In the bench test, a doublecollimated alpha source was mounted inside the counter and the resolution of PC2 was investigated (this
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Fig. 3. Plot of the energy-losssignal (AE) against the total energy signal (Etot) for ions analyzed by the spectrometer with a 48 MeV 11B beam incident on a 37C1 target. The spectrometer was set at 20 ° with a 7 msr aperture.
eliminated contribution to broadening from multiple scattering in the entrance foil). With 40 Torr of isobutane in the counter, the best resolution of 0.6 mm was observed with 1 kV bias applied to the anode wire. This was consistent with the geometrical broadening from the finite object slits and thus is an upper limit on the wire resolution. The position resolution observed during heavy-ion scattering experiments at the energies avalable from the Oxford 10 MV tandem Van de Graaff accelerator is almost always limited by target effects. On the other hand, the resolution we have seen with scattered protons or deuterons has been limited by the small signal to noise ratio from the proportional counters, a typical figure being 2 mm for 27 MeV deuterons which lost about 50 keV of energy below PC1. The best position resolution obtained, 0.6 mm, has been for the case of 27 MeV a-particles elastically scattered from a thin gold target ( - 2 btg/cm2) with a thin carbon backing. Again this is an upper limit on the resolution of the counter itself, since the beam spot size is esti-
mated to have contributed about 0.4 mm to the broadening and energy-loss straggling in the target also contributes. The linearity of the proportional counters has been investigated by stepping a collimated alpha source across the entrance window outside the detector. For both PC1 and PC2, the linearity was found to be good to within 0.9 mm across the 300 mm length seen through the window. Most of the deviations from perfect linearity arise at the extreme ends of the counters.
4.3. Timing signal Some experiments have required coincident detection of light particles in the focal plane with heavy-ion recoils detected in the spectrometer target chamber [14]. Thus, it has been necessary to derive a timing signal from the hybrid counter. The cathode and grid signals appear as soon as tlae electrons and positive ions drift apart in the ionization trail, and increase at a rate
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J.S. Winfield et aL / Focal plane detector for MDM-2 spectrometer I
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Posif ion (channe[s) Fig. 4. A spectrum (taken with a PSPC similar to the ones in the Oxford detector) of the position parameter gated on a-particles. Because the gains of the PC and amplifiers were set for the detection of 12N ions, the signals for the a-particles were only about 1/40th of the ADC full scale. The spectrum is compressed by a factor of 4 and the insert shows a detail of the central region. The oscillations in the spectrum are caused by the division of small integers (see text). I
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Time (l~S) Fig. 5. Composite spectrum of timing peaks observed when various particle types were detected at the focal plane of the spectrometer in coincidence with recoiling nuclei detected with a silicon surface-barrier counter in the target chamber. The timing signal at the focal plane is derived from the cathode of the gas counter. The timing resolution improves as the energy deposited in the focal plane detector increases.
J.S. Winfield et aL / Focalplane detectorfor MDM-2 spectrometer
characterized by the electron drift velocity. The starting time of these signals provides the only available timing references which are independent of the vertical height of the incident particle. Of the two, the cathode signal is the preferable choice since it has less capacitive and microphonic noise. The decoupled cathode signal was amplified with shaping time constants of rdiff = 10 #S and 'lint 1 #s in order to preserve the leading edge of the pulse, while filtering out some noise components. The rise time of the cathode signal is then of the order of microseconds. A discriminator on the cathode signal was used to start a time-to-amplitude converter (TAC) which was stopped by signals from a recoil coincidence detector (a silicon surface-barrier detector) mounted in the target chamber. Typical TAC peaks are shown in fig. 5. Note that the observed timing resolution (which is large relative to the contribution of - 1 6 ns from flight path differences in the spectrometer) is dependent on the energy deposited in the detector. Clearly, for the very light ions shown in fig. 5, the timing resolution is noise-limited. For heavy ions, the resolution is expected to be much improved: Erskine et al. [2] estimate a resolution of 5 ns with their small counter for 56 MeV 160 ions. =
4.4. Height signal
The vertical height of particles within the ionization chamber is conveniently measured by the drift time taken for electrons to reach the Frisch grid. A suitable arrangement is to start a TAC with the signal from a constant fraction discriminator set on the cathode signal and stop it with a signal from one of the front proportional counters. This method is now used with the Oxford counter because it is essentially independent of the energy deposited. Alternatively, the vertical position may be measured from the amount of charge induced\
2cm
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on the cathode from the positive ions remaining in the ionization chamber after the electrons have been collected. This cathode signal, divided by the total energy (cathode + grid signals) to remove the energy dependence, gives a similar resolution to the timing method. Fig. 6 shows a spectrum derived with the latter method and with a monoenergetic scattered beam. A plate with three horizontal slots, all 2.5 mm high but of different lengths, covered the entrance to the counter. The observed 4 mm fwhm of the peaks is mostly accounted for by multiple scattering and the spread in vertical angle of the incident ions (which penetrated some 20 cm into the chamber). The height signal has been used to investigate the vertical image formed by the spectrometer [1] and as a diagnostic for the counter itself.
5. Summary A hybrid counter with an acceptance of 30 x 6 cm 2, covering half the focal plane of the Oxford MDM-2 spectrometer, has been built and tested. Two energy-loss, a total energy, a veto and two position signals are available and the counter is usually run with the cathode and Frisch grid decoupled in order that height and timing signals may be obtained. Several problems experienced with the original design have been corrected by relatively small modifications. Shaping of the electric field to maintain uniformity at the edges of the ionization region was found to be of crucial importance, particularly near the entrance window and first proportional counter. It was found advantageous to use a single (Frisch) grid rather than the conventional two grids in order to reduce capacitive noise. With additional small grids beneath the proportional counters, improved electron collection was achieved, although some interference on the ionization signals due to positive ions drifting down from the proportional counters still remained. The interference is filtered out at the main amplification stage, however, and has not been a problem during experiments. Overall, the counter's performance has been very satisfactory in spite of its large dimensions. It is capable of detecting both light and heavy ions, with typical heavy-ion results of 0.6 mm position resolution and 1% energy resolution.
Acknowledgements 75
125 175 Verfics[ position (chonnels}
Fig. 6. Vertical position spectrum obtained with a three-slit aperture placed in front of the counter entrance window. The positions and relative areas of the slits are indicated beneath the peaks.
We wish to thank L.K. Fifield and D. Sinclair who worked on the early stages of this project, W.D.M. Rae and R.K. Bhowmik for helpful suggestions, and the assistance of E.F. Garman and S.K.B. Hesmondhalgh during on-line testing. Financial support from the Sci-
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ence a n d Engineering Research Council ( U K ) is gratefully acknowledged. One of us (D.M.P.) is i n d e b t e d to the Carnegie Trust for the award of a fellowship.
References [1] D.M. Pringle, W.N. Catford, J.S. Winfield, D.G. Lewis, N.A. Jelley, K.W. Allen and J.H. Coupland, Nucl. Instr. and Meth. A245 (1986) 230. [2] J.R. Erskine, T.H. Braid and J.C. Stoltzfus, Nucl. Instr. and Meth. 135 (1976) 67. [3] D. Shapira, R.M. DeVries, H.W. Fulbright, J. Toke and M.R. Clover, Nucl. Instr. and Meth. 129 (1975) 123. [4] T.R. Ophel and A. Johnston, Nucl. Instr. and Meth. 157 (1978) 461. [5] R.A. Cunningham, N.E. Sanderson, W.N.J. Snodgrass, D.W. Banes, S.D. Hoath and J.N. M0, Nucl. Instr. and Meth. A234 (1985) 67. [6] A. Oed, P. Geltenbort, F. Gonnenwein, T. Manning and D. Souque, Nucl. Instr. and Meth. 205 (1983) 455.
[7] D. Shapira, G.L. Bomar, J.L.C. Ford Jr., J. Gomez del Campo and L.C. Dennis, Nucl. Instr. and Meth. 169 (1980) 77. [8] F. Naulin, M. Roy-Stephan and E. Kashy, Nucl. Instr. and Meth. 180 (1981) 647. [9] L. Tassen-Got, C. Stephan, G. Bizard, J. Duchon, J.L. Laville, M. L'Haridon and M. Louvel, Nucl. Instr. and Meth. 200 (1982) 271. [10] O. Bunemann, T.E. Cranshaw and J.A. Harvey, Can. J. Res. A27 (1949) 191. [11] T.R. Ophel, Australian National University report no. ANU-P/848 (1982); [12] D.R. Grady and J.C. Robertson, Nucl. Instr. and Meth. 179 (1981) 317. [13] H.W. Fulbright, in: Handbuch der Physik, vol. 45, ed., S. Fliigge (Springer, Berlin, 1958). [14] W.N. Catford, D.M. Pringle, D.G. Lewis, A.E. Smith, E.F. Garman, I.F. Wright and J. Lukasiak, Nucl. Instr. and Meth. A247 (1986) 367.
297
FOCAL PLANE DETECTOR FOR THE OXFORD MDM-2 SPECTROMETER J.S. W I N F I E L D *, D.M. P R I N G L E , W.N. C A T F O R D **, D.G. L E W I S ***, N.A. J E L L E Y a n d K.W. A L L E N Nuclear Physics Laboratory, University of Oxford, Keble Road, Oxford OX1 3RH, UK Received 9 May 1986
A "hybrid" focal plane counter comprising a gridded ionization chamber with position sensitive proportional counters, 300 mm long, 510 ram deep and with an active height of 60 ram, is described. The detector is designed for use with the Oxford MDM-2 magnetic spectrometer and is suitable for both light and heavy ions. Typical results obtained are 0.6 mm position resolution and 1% total energy resolution. Two energy loss, a veto and two position signals are available as well as provision for height and timing signals. Techniques are described for the reduction of capacitive noise, correction of electric field nonuniformities, and the reduction of interference from positive ions on the ionization signals.
1. Introduction A heavy-ion magnetic spectrometer (the MDM-2) has been in operation since 1982 at the Nuclear Physics Laboratory in Oxford. The spectrometer itself has been described in a previous paper [1] and here we discuss the focal plane detector. The main requirements for the detector were as follows: (1) isotopic identification of heavy ions via energy-loss (AE) signals and a total energy (Etot) signal, (2) a position resolution of about 0.5 mm in order not to compromise the excellent resolving power of the spectrometer itself, and (3) the detection of light ions. The dimensions of the image at the focal surface of the MDM-2 spectrometer required a detector acceptance length of 60 cm (full focal plane) and a relatively large maximum height of 6 cm. In view of the size of such a detector, it was decided initially to build a prototype counter of one half the length of the full focal plane. The above requirements led immediately to the consideration of a detector of the type developed at Argonne [2] and Rochester [3] a "hybrid" counter which comprises a gridded ionization chamber with a split anode of plate electrodes and position sensitive proportional counters (PSPCs). Versions of these detectors have now * Present address: National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824, USA. ** Present address: Dept. of Nuclear Physics, The Australian National University, GPO Box 4, Canberra, ACT 2601, Australia. *** Present address: Dept. of Physics, The Royal Marsden Hospital, Sutton, Surrey SM2 5PT, UK. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
been constructed at various nuclear physics laboratories, each addressing particular design problems. In our case, an important advantage owing to the MDM-2 spectrometer is that the focal plane has particles crossing at a 90 ° mean angle of incidence, so that any angle-dependent effects in the position and energy measurements are small and require no special attention. On the other hand, we needed to consider the problems raised by the large volume of the counter. Despite the comparatively small size of the electrodes in the early designs of hybrid counters [2,3], the energy resolution was limited by their capacitance to ground. Later designs succeeded in reducing this capacitance, with the result that the noise in some cases [4] was low enough for the counter to work also as a good detector for light ions. We were concerned from the outset with keeping stray capacitances to a minimum, and have been able to achieve noise levels which compare well with the smaller counters. We briefly discuss the design (sect. 2) and performance (sect. 4) of the Oxford hybrid counter. Sect. 3 is a more detailed description of some of the problems encountered with the present counter and their solutions.
2. Design and construction Fig. 1 shows a cross section of the Oxford counter with the general layout of the elements. The counter consists of a gridded ionization chamber, from which the total energy signal is obtained, with a split anode plane divided into two A E electrodes, a residual energy electrode (from which no signals are taken at present),
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J.S. WinfieM et al. / Focal plane detector for MDM-2 spectrometer POSITION MEASUREMENT
ANGLE MEASUREMENT
..... ,.,,- AE1
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Fig. 1. A schematic cross section of the focal plane detector.
and three proportional counters. The first two proportional counters are position sensitive, and provide signals which enable the reconstruction of the position of the particle at the focal plane and its crossing angle. The third wire counter provides a signal which may be used to veto nonstopping particles. The "dynamic range" of the counter is given by the ratio of the depth (51 cm in the present case) to the distance of the second proportional counter from the entrance window (17 cm); the Oxford counter has a very useful dynamic range of 3:1. In addition, the greater depth compared to that of the Argonne or Rochester counters enables the counter to be operated at lower gas pressures. The first proportional counter is as near to the entrance window as possible (2 cm), allowing for field nonuniformities very close to the entrance (see discussion later). Considerations of the desired angular resolution and the multiple scattering of ions led to a design with a separation of 15 cm between the first and second proportional counters; this also allows for AE anode plates of adequate depth. The large vertical image size of the Oxford MDM-2 spectrometer, together with the fact that the vertical focus is displaced downstream from the horizontal focus, necessitate a 6 cm high window to the focal plane detector. Since it was desirable to have a re-entrant window in order to keep the dead layer of gas to a minimum, the cathode to grid spacing was designed to accommodate the thickness of the window mounting as
well. The resulting 10.5 cm height makes the chances of a multiply scattered particle striking the cathode or passing through the grid very small. However, the large vertical dimension also means that field correction at the sides of the ionization chamber is important for utilization of the full length of the counter. All the detector elements are firmly bolted to two blocks of Delrin plastic. This frame is mounted on the front plate of the gas cell, through which all electrical and gas connections are made. Thus the entire body of the detector can be taken out from the gas cell without disconnecting any internal cabling. Reports on other hybrid counters [2,3] mention trouble with microphonic noise on the ionization signals arising from rotary pumps on their spectrometer systems. Despite the cantilever mounting of the present detector, microphonic noise was not a problem. As a precaution, rotary pumps in the vicinity of the focal plane were decoupled mechanically from the detector housing. Some low frequency ( = 25 Hz) noise that was associated with one rotary pump was observable on the output of the preamplifier connected to the cathode, but this was entirely filtered out by the main amplifiers. The original design for the ionization chamber called for a double grid as is common in other hybrid counters. The upper grid is intended to shield the lower (Frisch) grid from the large numbers of positive ions generated in the proportional counters. In the final design for our counter, we have replaced this upper grid by individual small grids immediately beneath the pro-
J.S. Winfield et al. / Focal plane detector for MDM-2 spectrometer
portional counters. These small grids appear to work at least as well as the former full grid (see sect. 3.3) and, unlike the latter, do not contribute significantly to the capacitive coupling of the Frisch grid to ground. The Daresbury detector [5] is also a single-gridded counter. The Frisch grid was wound from 100 /tin Be-Cu wire with a 1.5 mm pitch. A rectangular frame of G10 glass fibre supported the wires with printed circuit board along the edges for electrical contact. In order to keep the middle wires taut, jacking bolts were required on the frame. The grids beneath the proportional counters were made with the same wire but with a finer spacing (1 ram). The final design of the counter has horizontal field shaping wires around all four sides. The wires at the entrance (a double bank, with the first bank immediately next to the window and the second bank 7 mm inside the counter) were added after tests showed that field distortions were unacceptable without such corrections (see discussion in sect. 3.2). The wires are 100 t~m Be-Cu, with a vertical spacing of 7 ram. The recessed window assembly is designed to be separately removable from the gas cell front plate. The re-entrant part was constructed from packed layers of G10 glass fibre glued together; the choice of insulating material avoided bringing a conductor near the critical region underneath PC1. The glass fibre section is permanently bolted to the steel mounting flange and sealed with an O-ring. Commercial mylar windows, ranging in thickness from 5 to 25 #m, have been in standard use for the counter. These are glued with epoxy to the G10 face of the window assembly with about I cm of contact all around. This avoids the need for a clamping ring which would have been necessary if an O-ring were used to make the seal. Since the spectrometer focal plane is perpendicular to the incident ions, the PSPCs were simple to design and construct. The bodies (cathodes) are made from 400 mm lengths of rectangular aluminium bars with Ushaped grooves machined into one edge. The width of this slot is 10 mm and the anode wire is made to lie along the axis of the cylindrical section, 9 mm from the opening of the slot. The wire used was 10 # m N i - C r , which was soldered under light tension to high voltage coaxial connectors at each end of the counter. Initially, occasional trouble was experienced with wire breakages when high voltages were applied. The source of the problem was probably a large discharge caused by spikes on the voltage line when supplies with discrete thumbwheels were used. The trouble has not recurred since we changed to voltage supplies with continuous potentiometers. Many other workers (e.g. Shapira et al. [3], Erskine et al. [2], Oed et al. [6]) have shown that isobutane has two advantages over argon-methane mixtures for hybrid counter use: it has a greater stopping power and
299
produces less multiple scattering of the ions for a given energy loss. Isobutane is especially sensitive to electronegative impurities, however, and precautions are needed to avoid contamination. We have therefore constructed the gas cell of the counter from stainless steal. Flexible stainless steel pipes connect from the front plate of the counter to the exterior gas handling system, thus allowing the counter to move for kinematic correction. Where insulating material is required, Delrin plastic (which has a low hygroscopic coefficient) has been used rather than nylon. In order to minimize the outgassing of impurities once the detector is filled, it was important that a low impedance pumping path be provided directly between the detector volume and the high vacuum system. A low-cost solution was to equip the gas cell with a 4 cm diameter solenoid-operated valve in the back plate in order that the interior can initially be pumped by the spectrometer high vacuum system. With these precautions it has been possible to operate the detector without the need to flow the gas. A fresh charge of isobutane in a typical heavy-ion experiment lasts about one day or more before the ionization signals show signs of deterioration. During light ion experiments, the counter has been run for up to one week without changing the gas. CP grade of isobutane (95% isobutane, 4% propane) has routinely been used in the counter.
3. Discussion of specific problems encountered 3.1. Noise due to capacitive coupling to ground
The capacitive coupling of the ionization chamber to ground may significantly contribute to the noise on the ionization signals (see, e.g. ref. [4]). We were careful to allow a large separation between the cathode and the bottom of the gas cell, but the relatively small separation (1 cm) of the double grids and their large area (40 × 53 cm 2) in the original design caused such high noise levels on the Etot signal that we eventually dispensed with the second grid. Measurements with a bridge circuit have shown that the capacitive coupling of the single grid to the anode plane is of the same magnitude as the coupling of the chamber as a whole to the (grounded) gas cell front plate. Thus it is probably difficult to further reduce the total capacitance to ground (480 pF, which corresponds to a noise level of 180 keV with Ortec 142B preamplifiers). The preamplifiers for the ionization signals are mounted outside the gas cell for ease of access and to reduce the outgassing within the counter. The additional stray capacitance due to the cable lengths is negligible. This applies especially to t h e / t E signals, for which energy-loss straggling rather than electronic noise is the major contribution to the resolution in almost all practical cases.
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J.S. Winfield et a L / Focal plane detectorfor MDM-2 spectrometer
3.2. Entrance field nonuniformity Shapira et al. [7] have shown the importance of correcting nonuniformities in the electric field distribution at the sides of the counter when the separation between the cathode and grid is significant compared to the other dimensions. We have made electrostatic field calculations for various configurations at the entrance of the counter in a manner similar to refs. [7] and [5]. These showed that either a window with horizontal bands of potential-graded strips [8] or an insulating window with external upper and lower guard plates [5] would be suitable schemes. On the basis of ease of construction, the latter approach was adopted. Subsequent tests with heavy-ion beams showed, however, that the external guard plates were not sufficient to correct the field beneath the first proportional counter (PC1) and the first A E plate (AE1). When voltages approximately equal to those of the Frisch grid and cathode were applied to the upper and lower grid guard plates respectively, the signal amplitude from PC1 increased from being undetectable to a size equal to that of PC2. However, the PC1 signal was - 2 #s late with respect to PC2 and the resolution of A E1 was significantly worse than that of AE2. Continual adjustment of the guard plate voltages was necessary to reduce these problems and it seems likely that the charging-up of the insulating window under bombardment by the ions caused the residual field nonuniformities. Yet a similar design of external guard electrodes apparently works well in the Daresbury detector [5], possibly because there the counter gas is continually flowed. The adopted solution was to insert the double bank of horizontal potential-defining wires described in sect. 2. This works well and allows for the easy replacement of the window foils unlike the striped-window design of Naulin et al. [8]. 3.3. Positive ion feedback The prevention of spurious signals on the ionization electrodes induced by the large cloud of positive ions from the PCs is a matter of some concern with hybrid counters. If the positive ions are not efficiently trapped within the proportional counter or on a shielding grid, a large slowly rising pulse will be seen a few hundred microseconds later than, and of the same polarity as, the genuine ionization pulse [5,9]. We had designed the proportional counters with an anode wire recessed 9 mm into the cathode structure. This design was intended to trap a significant portion of the positive ions before they left the proportional counter. However, tests with an a-particle source and subsequently with scattered beams from an accelerator showed that a large feedback problem was present. With the main amplifier shaping time set at 8 #s (in order to filter out high frequency noise components),
the ionization signals had poor resolution and were shifted up in amplitude when the PCs were operated at high voltage. Shorter shaping times on the amplifier reduced the problem because of the slow drift time of the positive ions which were escaping through the shielding grid beneath the proportional counter. In an attempt to stop the positive ions from reaching the region near the Frisch grid, an additional fine grid of wires was inserted in the anode plane, beneath the proportional counters. This had only a small beneficial effect: about 25% larger pulses could be obtained from the PC before the Etot resolution began to deteriorate. The lack of effect appears to stem from the recessed-wire design of the PCs. An undesirable consequence of this arrangement is that it is difficult to make the electric field strength immediately above the shielding grid significantly greater than that in the region between the Frisch grid and the shielding grid - a condition which is necessary if the shielding grid is to capture a large fraction of the positive ions [10]. In order to increase the field above the shielding grid, the proportional counters were raised 6 mm above the anode plane and the potential of the PC as a whole was increased. The grid should then have become more opaque to positive ions moving downwards but more transparent to electrons drifting upwards. In practice it was found that although the efficiency of electron collection was more than doubled, the size of the positive ion pulse on the Etot signal was only reduced by some 20% for equivalent charge collection on the PC wires. The time delay between the true ionization signal and the start of the interference varied considerably with the voltage applied to the body of the PC: for 500 V the delay was 0.3 ms but for 100 V it was 1.4 ms. No further attempts to capture all of the positive ions were made because the remaining interference could be filtered out by the main amplifier. With a shaPing time r of 4 #s the pulses had almost disappeared and at = 2 #s the effect of the positive ions was negligible. In general use, the counter has been run with 2 #s shaping times on the ionization signal amplifiers; this does not appear to significantly worsen the signal to noise ratio. A simpler overall design, without raised proportional counters, may have been possible if the proportional counters had shallow slots so that the field from the anode wire would be strong in the critical region just above the shielding grid. This would allow the ratio of the fields above and below this grid to be set to > 2 : 1, which was seen to improve the electron collection efficiency of the PCs and is thus particularly important for light-ion experiments. We conclude, however, that the positive ions are too numerous to be stoppe d entirely by shielding grids and that some filtering of the ionization signal is always necessary. Ophel [11] has come to a similar conclusion from experience with a small hybrid counter.
J.S. Winfield et al. / Focalplane detectorfor MDM-2 spectrometer 3.4. Capacitive coupling between PC body and A E plates Another problem caused by the large signals from the PCs is direct capacitive coupling to the ionization chamber electrodes. Careful routing of signal cables and the construction of anode dements with bevelled edges are standard precautions to reduce this effect in hybrid counters. Originally, the PCs were set on the anode plane with their cathodes held at ground potential and no evidence of capacitive pickup of PC signals was observed on the ionization signals. When the PCs were raised above the ground plane some fast, destructive interference was seen on the A E signals: the size of the A E signals was reduced as the PC wire voltage was raised until eventually the signal polarity appeared to be reversed. That this was a crosstalk problem rather than the drift of a positive ions was clear from the observation of delayed pulses some 300 #s later which were attributable to drifted ions. The crosstalk was eliminated by connecting 5 nF capacitors between the PC bodies and ground. As an additional precaution, the A E electrodes were shielded from the PC's by grounded copper plates.
3.5. Loss of total energy signal at sides of counter Tests in which an elastically scattered group of partides was systematically stepped across the detector showed that the energy signal began to drop in amplitude some 8 cm away from the centre. At 12 cm from the centre the peak height was about 5% lower than the value at the centre of the detector, and at 15 cm (i.e. close to the edge of the window) it had further fallen by another 5%, with a noticeable deterioration of resolution. Both effects were less pronounced when the spectrometer's horizontal aperture was reduced. Since it would appear from linearity tests of the proportional counters (see sect. 4.2) that the electrostatic field at the sides of the counter is quite uniform, we may conclude that the loss in energy signal arises because the cathode and Frisch grid do not form a complete Faraday cage. N o signals are taken from the potential defining wires around the sides of the counter and the positive ions left in the chamber after the electrons have passed through the grid will induce charge on the wires to an extent that varies with distance. The wires could be ac-coupled to the Etot signal to avoid the loss of induced charge, but this has not proved necessary for experiments so far. Alternatively, the present Eto t signal could be software-corrected if desired.
3.6. Signal reduction at localised regions of PSPC wires After long periods of use, some PSPC wires have shown a reduction in pulse height for signals corresponding to particular locations on the wire. The effect
301
may be conveniently monitored via a plot of the position parameter against the signal from one end of the counter (see fig. 2). The distortions in the position spectrum alone may be subtle and less easy to detect. It seems likely that this deterioration in the wire performance is a consequence of intense bombardment of the counter - by well-focussed elastically scattered beams, for example. Deposition of the products of isobutane cracking would cause a local increase in the effective diameter of the wire (this phenomenon has been noted in ref. [12]) and hence reduce the gas gain at that location.
4. Operation and performance 4.1. Energy and energy-loss signals Rather than capacitively coupling the cathode and Frisch grid to obtain a height-independent total energy signal, our usual practice is to sum the independent signals from the cathode and grid preamplifiers before the main amplification. This method makes the cathode signal available both for a measure of the height of the ion (see sect. 4.5) and as a timing signal (sect. 4.4). The energy-loss signals are taken from the plates on the anode plane. Tests with elastically scattered 35 and 63 MeV 12C particles stopped in the detector gave a best resolution of 0.9% for the Etot signal. The resolution of the A E signals is dominated by statistical fluctuations in the ionization. A typical result, for 43 MeV 9Be in 100 Torr of isobutane, is 7.0% measured resolution to be compared with 6.3% calculated contribution from energyloss straggling [13], including a factor of 1.6 for chargeexchange straggling [2]. An example of particle identification from an experimental run is shown in fig. 3. When the detector is used in light-ion mode, in which the particles of interest are not stopped in the counter, the Etot signal becomes a measurement of the energy loss. However, we have preferred in this case to use the summed signals from the two ends of the first proportional counter for particle identification. Then, any particles which are not stopped in the counter but which strike the sides before the veto counter do not cause anomalously small ionization signals (this is not a problem in heavy-ion mode since the particles of interest will give much greater Etot signals than any lighter particles which do not stop).
4.2. Position signals We chose charge division as the method of determing the position from the proportional counters. The timing method would probably be just as suitable for a 30 cm wire length, but would not have been as simple.
J.S. Winfield et al. / Focalplane detectorfor MDM-2 spectrometer
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Fig. 2. A plot of the signal from the right end of PC1 against the position parameter. The focal plane was illuminated by a-particles from the 12C(12C, aX) reaction. The arrows indicate two localized regions along the wire where a marked signal reduction occurs.
Rather than using an analog divider which may introduce nonlinearities, the division of the signal amplitude from one end of the counter by the summed amplitudes from the two ends is done in the data analysis software. This generally works well, but does give rise to a potential problem to do with the division of small integers. If the digitized signals from the ends of the wires are small relative to the number of channels in the position spectrum, a significant amount of accuracy will be lost. Then in a division operation of the form P
A L CL+R
(where L and R are the integer results of digital conversion, A is the number of channels in the A D C , and C is the compression factor used in the generation of the histogram of P), certain values of P will be suppressed and other values enhanced. For example, if
A = 512 and a 512 histogram of P is made ( C = 1), for L < R < 125 a result of P = 255 cannot be obtained whereas P = 256 can result from many combinations of L and R. This effect gives rise to sharp oscillations in a histogram of P (see fig. 4). The oscillations are particularly pronounced at one half, one third and two thirds along the spectrum. We find empirically that in order to avoid this problem, the size of the digitized signals L and R should be at least A / ( 2 C ) when particles are incident at the centre of the counter. Obviously, the situation is alleviated by compressing the histogram; the consequent loss of resolution could be avoided by the use of A D C s with more channels. We presently digitize the PSPC signals with 4096-channel ADCs. The resolution of the proportional counters has been investigated both on the bench with an a-particle source and with scattered beams. In the bench test, a doublecollimated alpha source was mounted inside the counter and the resolution of PC2 was investigated (this
J.S. WinfieM et al. / Focal plane detector for MDM-2 spectrometer
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Fig. 3. Plot of the energy-losssignal (AE) against the total energy signal (Etot) for ions analyzed by the spectrometer with a 48 MeV 11B beam incident on a 37C1 target. The spectrometer was set at 20 ° with a 7 msr aperture.
eliminated contribution to broadening from multiple scattering in the entrance foil). With 40 Torr of isobutane in the counter, the best resolution of 0.6 mm was observed with 1 kV bias applied to the anode wire. This was consistent with the geometrical broadening from the finite object slits and thus is an upper limit on the wire resolution. The position resolution observed during heavy-ion scattering experiments at the energies avalable from the Oxford 10 MV tandem Van de Graaff accelerator is almost always limited by target effects. On the other hand, the resolution we have seen with scattered protons or deuterons has been limited by the small signal to noise ratio from the proportional counters, a typical figure being 2 mm for 27 MeV deuterons which lost about 50 keV of energy below PC1. The best position resolution obtained, 0.6 mm, has been for the case of 27 MeV a-particles elastically scattered from a thin gold target ( - 2 btg/cm2) with a thin carbon backing. Again this is an upper limit on the resolution of the counter itself, since the beam spot size is esti-
mated to have contributed about 0.4 mm to the broadening and energy-loss straggling in the target also contributes. The linearity of the proportional counters has been investigated by stepping a collimated alpha source across the entrance window outside the detector. For both PC1 and PC2, the linearity was found to be good to within 0.9 mm across the 300 mm length seen through the window. Most of the deviations from perfect linearity arise at the extreme ends of the counters.
4.3. Timing signal Some experiments have required coincident detection of light particles in the focal plane with heavy-ion recoils detected in the spectrometer target chamber [14]. Thus, it has been necessary to derive a timing signal from the hybrid counter. The cathode and grid signals appear as soon as tlae electrons and positive ions drift apart in the ionization trail, and increase at a rate
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J.S. Winfield et aL / Focal plane detector for MDM-2 spectrometer I
I
I
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,
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Posif ion (channe[s) Fig. 4. A spectrum (taken with a PSPC similar to the ones in the Oxford detector) of the position parameter gated on a-particles. Because the gains of the PC and amplifiers were set for the detection of 12N ions, the signals for the a-particles were only about 1/40th of the ADC full scale. The spectrum is compressed by a factor of 4 and the insert shows a detail of the central region. The oscillations in the spectrum are caused by the division of small integers (see text). I
ResoLution (ns) : Energy toss (MeV) : Porficles :
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Time (l~S) Fig. 5. Composite spectrum of timing peaks observed when various particle types were detected at the focal plane of the spectrometer in coincidence with recoiling nuclei detected with a silicon surface-barrier counter in the target chamber. The timing signal at the focal plane is derived from the cathode of the gas counter. The timing resolution improves as the energy deposited in the focal plane detector increases.
J.S. Winfield et aL / Focalplane detectorfor MDM-2 spectrometer
characterized by the electron drift velocity. The starting time of these signals provides the only available timing references which are independent of the vertical height of the incident particle. Of the two, the cathode signal is the preferable choice since it has less capacitive and microphonic noise. The decoupled cathode signal was amplified with shaping time constants of rdiff = 10 #S and 'lint 1 #s in order to preserve the leading edge of the pulse, while filtering out some noise components. The rise time of the cathode signal is then of the order of microseconds. A discriminator on the cathode signal was used to start a time-to-amplitude converter (TAC) which was stopped by signals from a recoil coincidence detector (a silicon surface-barrier detector) mounted in the target chamber. Typical TAC peaks are shown in fig. 5. Note that the observed timing resolution (which is large relative to the contribution of - 1 6 ns from flight path differences in the spectrometer) is dependent on the energy deposited in the detector. Clearly, for the very light ions shown in fig. 5, the timing resolution is noise-limited. For heavy ions, the resolution is expected to be much improved: Erskine et al. [2] estimate a resolution of 5 ns with their small counter for 56 MeV 160 ions. =
4.4. Height signal
The vertical height of particles within the ionization chamber is conveniently measured by the drift time taken for electrons to reach the Frisch grid. A suitable arrangement is to start a TAC with the signal from a constant fraction discriminator set on the cathode signal and stop it with a signal from one of the front proportional counters. This method is now used with the Oxford counter because it is essentially independent of the energy deposited. Alternatively, the vertical position may be measured from the amount of charge induced\
2cm
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:
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i
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20
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on the cathode from the positive ions remaining in the ionization chamber after the electrons have been collected. This cathode signal, divided by the total energy (cathode + grid signals) to remove the energy dependence, gives a similar resolution to the timing method. Fig. 6 shows a spectrum derived with the latter method and with a monoenergetic scattered beam. A plate with three horizontal slots, all 2.5 mm high but of different lengths, covered the entrance to the counter. The observed 4 mm fwhm of the peaks is mostly accounted for by multiple scattering and the spread in vertical angle of the incident ions (which penetrated some 20 cm into the chamber). The height signal has been used to investigate the vertical image formed by the spectrometer [1] and as a diagnostic for the counter itself.
5. Summary A hybrid counter with an acceptance of 30 x 6 cm 2, covering half the focal plane of the Oxford MDM-2 spectrometer, has been built and tested. Two energy-loss, a total energy, a veto and two position signals are available and the counter is usually run with the cathode and Frisch grid decoupled in order that height and timing signals may be obtained. Several problems experienced with the original design have been corrected by relatively small modifications. Shaping of the electric field to maintain uniformity at the edges of the ionization region was found to be of crucial importance, particularly near the entrance window and first proportional counter. It was found advantageous to use a single (Frisch) grid rather than the conventional two grids in order to reduce capacitive noise. With additional small grids beneath the proportional counters, improved electron collection was achieved, although some interference on the ionization signals due to positive ions drifting down from the proportional counters still remained. The interference is filtered out at the main amplification stage, however, and has not been a problem during experiments. Overall, the counter's performance has been very satisfactory in spite of its large dimensions. It is capable of detecting both light and heavy ions, with typical heavy-ion results of 0.6 mm position resolution and 1% energy resolution.
Acknowledgements 75
125 175 Verfics[ position (chonnels}
Fig. 6. Vertical position spectrum obtained with a three-slit aperture placed in front of the counter entrance window. The positions and relative areas of the slits are indicated beneath the peaks.
We wish to thank L.K. Fifield and D. Sinclair who worked on the early stages of this project, W.D.M. Rae and R.K. Bhowmik for helpful suggestions, and the assistance of E.F. Garman and S.K.B. Hesmondhalgh during on-line testing. Financial support from the Sci-
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ence a n d Engineering Research Council ( U K ) is gratefully acknowledged. One of us (D.M.P.) is i n d e b t e d to the Carnegie Trust for the award of a fellowship.
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