The vertical drift chamber as a high resolution focal plane detector for heavy ions

Nuclear Instruments and Methods in Physics Research 224 (1984) 421-431 North-Holland, Amsterdam

THE VERTICAL DRIFT CHAMBER AS A HIGH RESOLUTION HEAVY IONS

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FOCAL PLANE DETECTOR FOR

T.P. S J O R E E N , J.L.C. F O R D , Jr. *, J.L. B L A N K E N S H I P , R.L. A U B L E , F.E. B E R T R A N D , E.E. G R O S S , D.C. H E N S L E Y a n d D. S C H U L L * Oak Ridge National Laboratory **, Oak Ridge, Tennessee 37830, USA

M.V. H Y N E S Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

Received 16 January 1984

A vertical drift chamber in the focal plane of a magnetic spectrometer has been tested with 12C(129 MeV), 14N(176 MeV) and 160 (140 MeV) ions. An experimental position resolution of < 0.5 mm was measured, corresponding to an intrinsic detector resolution of about 0.1 mm (fwhm). The angle of the ion trajectory with respect to the focal plane was measured to within - 1 0 mrad, which corresponds to an intrinsic detector resolution of about 7 mrad. The angle measuring capability of the counter permits the measurement of angular distributions across the spectrometer opening and the correction of the counter data for magnet aberrations. Details of the electronics and gas handling systems and the detector performance with heavy ions are presented.

1. Introduction Magnetic spectrometers are among the most useful instruments for nuclear research due to the unmatched energy resolution with which charged particle spectra can be measured. In addition, these instruments provide excellent particle identification and background suppression, features particularly important in heavy ion research where a large number of different reaction products are frequently concentrated at forward angles. The advantages for heavy ion research offered by a magnetic spectrometer require, however, a sophisticated focal plane counter capable of high position resolution. Furthermore, parameters such as the energy, energy loss and time-of-flight of the detected particle are usually required for particle identification. The Holifield Heavy Ion Research Facility ( H H I R F ) at Oak Ridge possesses two magnetic spectrometers an Elbek [1] modified by the addition of a quadrupole to increase the solid angle [2] and an Enge split-pole [3]. While both of these instruments, designed primarily for light-ion research, have distinct limitations for heavy-ion work, they have many valuable features as well. One such feature is their broad energy range - an asset in * Deceased. * Visiting scientist from GSI, Darmstadt, Fed. Rep. Germany. ** Operated by Union Carbide Corporation under contract W-7405-eng-26 with the US Department of Energy. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

heavy ion reaction studies where reaction products can be emitted over a wide range of energies and charge states. However, this broad range is bought at the expense of a low dispersion for both these magnets, which corresponds to an energy shift on the order of 1 k e V / m m - MeV for the detected particles, or an energy difference of 100 k e V / m m if 100 MeV particles are observed. Since the coupled Oak Ridge tandem and cyclotron accelerators can produce beams with energies up to 25 M e V / a m u , a focal plane detector capable of submillimeter position resolution is clearly required, since otherwise the counter will limit the energy resolution with which spectra can be measured. The importance of having a focal plane counter capable of such high intrinsic position resolution was demonstrated by scattering 400 MeV 160 ions from the H H I R F coupled tandem-cyclotron accelerators from a 120 p g / c m 2 thick 2°8pb target mounted on a 40 p g / c m 2 12C backing [4]. The scattered particles were detected with a 100 # m deep position sensitive solid state counter mounted at the focal plane of the Elbek broad range spectrometer (BRS). The dispersion in the focal plane at the elastic peak was 280 k e V / m m and the overall resolution observed was E / A E - 3 5 0 0 . These figures imply that the resolution of the focal plane counter should be 1 / 4 mm or less in order to take full advantage of the high resolution capabilities of the spectrometer and accelerator near the maximum energy of 25 M e V / a m u available from the facility. Although solid

422

72 P. Sjoreen et al / Vertical drift chamber as focal plane detector

state counters are capable of such position resolution, their limited size ( - 1 cm high and typically < 5 cm in length) is too small for most focal plane applications. A detector which could possibly meet our dual requirements of 1) submillimeter position resolution and 2) an active focal plane length of > 35 cm (10% range in momentum) was the vertical drift chamber (VDC) [5]. This detector also had the capability of measuring trajectory angles. However, previously such counters had only been used to detect highly penetrating particles such as electrons, pions and protons, and had been operated at gas pressures of 1 atm - a pressure too high for the _< 25 M e V / a m u heavy ions at HHIRF. In order to determine the performance of the VDC as a heavy-ion detector, we designed, constructed and tested a detector similar to the original design [5], but suitable for use at gas pressures _< 200 Torr while in a high vacuum chamber. The principle of operation of this counter, its design and construction and its operating characteristics are given in detail in an accompanying paper [6]. The performance of the detector with heavy ion beams from the ORIC cyclotron at HHIRF and the overall system consisting of the spectrometer, detector, gas handling apparatus and electronics will be described here.

2. Counter operation and construction The counter was designed to match the modified Elbek spectrometer. The solid angle (with quadrupole lens) of this magnet varies from about 3 msr at the high-energy end of the focal plane to 10 msr at the low-energy end, with the average value for the solid angle being approximately 5 msr. Without the quadrupole lens the magnet does not have vertical focusing. The value of the vertical magnification with the lens varies from - 4 at the low- energy end of the focal plane, to - 8 at the high-energy end. The desirable vertical height of the focal plane counter is then 5 cm in order to intercept the full image at the focal plane when operating without vertical focusing and to make the effective solid angle insensitive to the vertical position of the beam spot on the target when the quadrupole is on. The mass-energy product of this spectrometer (_< 240), together with the broad range and high resolution capabilities with even a modest length vertical drift chamber, provided a good match to the performance of the H H I R F accelerators. The VDC is a multiwire proportional structure used as a drift chamber. As schematically shown in fig. 1, the counter consists of an anode wire plane symmetrically spaced between two high-voltage, aluminized-mylar, cathode planes. The wire plane consists of active sense wires, each separated from its neighbor by two guard wires. All wires are evenly spaced. The larger-diameter,

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T.P. Sjoreen et al. / Vertical drift chamber as focal plane detector

cerning the principles on which the counter operates, see refs. [5] and [6]. The actual design and operating characteristics of the detector are detailed in ref. [6]. The V D C was designed as a transmission counter so that the detectors providing timing and particle identification information can follow it. In the tests reported here the V D C was backed by a 5.08 cm high, 60 cm long and 1.27 cm thick NE102 plastic scintillator viewed from both ends by photomultiplier tubes, which provide the start signal for measuring the drift times within the VDC. Each sense wire of the V D C is connected to an amplifier with a gain of about 140 followed by a leading edge timing discriminator circuit. The timing discriminator modules plug into a c o m m o n power supply located outside the vacuum system. The multiple cables between these modules and the counter are cable ribbons [7], with each ribbon containing 25 fifty ohm coaxial lines similar to R G 174U. The ribbon cables from the electronics and from the counter plug into the two ends of three separate microstrip line circuit boards epoxied into the high vacuum flange. These microstrip

423

transmission lines transmit the separate signal and ground of each of the 25 coaxial cables through the vacuum wall of the spectrometer camera box. The ribbon cables and strip lines provide a means by which up to 75 coaxial cables can penetrate a high vacuum bulkhead with a relatively small flange and allow easy, error free disassembly and reconnection of a large number of coaxial cables. Fig. 2 shows a photograph of the VDC, backed by the scintillator, mounted in the camera box of the spectrometer. The ribbon cables are attached to the multiplicity of lemo connectors at the upper and lower edges of the counter, which are connected to the sense and guard wires of the counter, respectively. The ribbon cables connected to the ends of the strip line outside the vacuum system lead to the 51 discriminator modules mounted in the box seen in the upper right hand portion of the photograph (see sect. 3). The detector was operated with isobutane as the counter gas. It is important for best results that the gas remain uncontaminated (requiring a fairly fast flow rate) and that the pressure remain constant for stable

Fig. 2. Photograph of the vertical drift chamber backed by the plastic scintillator mounted in the camera box of the Elbek spectrometer.

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gas gain and drift times. The electronic control and regulation system [8] constructed for this purpose is shown schematically in fig. 3. An automatic valve regulated according to the pressure measured by a transducer [9] maintained the absolute counter pressure within I Torr.

3. Eiectronks Although the V D C is a multiwire detector, the associated electronics are simplified by the fact that tapped delay lines are used to encode the trajectory information. As shown schematically in fig. 4, each sense wire is connected to an amplifier and a leading-edge discriminator circuit to provide a proper timing signal. The output of each of the discriminators is encoded onto 1

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of 5 delay lines. Hence. cells 1.6. 11, - •. are connected to delay line 1; cells 2, 7, 12, • -. to delay line 2; cells 3. 8, 13, . - - to delay line 3; cells 4, 9, 14, . - . to delay line 4 and cells 5, 10, 15, • • • to delay line 5. The arrival times o f the signals from the two ends of each delay line are measured by a T D C relative to a reference start signal obtained from the scintillation counter in the initial tests of the chamber. The circuit diagram for the timing discriminators designed for this application is shown in fig. 5. This circuit has low noise ( < 50 #V), and the discriminators were set at about 1 mV to reduce the number of spurious signals from the counter and electronic system. A NIM-logic compatible negative pulse is produced by the discriminator circuit with a width, risetime and fulltime of about 20, 1 and 1 ns, respectively. The time jitter is about 0.3 ns for a 3 mV input pulse, with a 3 ns risetime. The delay lines consist of 30.5 cm lengths of RG-174 cable. With these cables the delay between each timing discriminator board is 1.76 ns. The 51 dis-

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T.P. Sjoreen et al. ,/ Vertical drift chamber as focal plane detector

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criminators mounted in the common power supply box visible in fig. 2 are shown in the photograph of fig. 6. The scintillator and 10 delay line signals were processed by a CAMAC based data acquisition system located at the spectrometer in order to avoid degradation of the fast timing pulses. The basic electronic system is presented in the schematic of fig. 4. The updating discriminators served to fan the signals out to the time-to-digital converter (TDC) and the gated latch (CB) which provided a bit pattern. The results from the CAMAC system were then transmitted to the main data acquisition computer for further processing and storage and to the counting room for display.

4. Detector performance The performance of the VDC as a heavy ion detector was tested with 129-MeV a2C, 176-MeV 14N and 140MeV ~60 ions from the Oak Ridge isochronous cyclotron and the modified Elbek spectrometer. For these tests, the VDC sense-wire plane was positioned so that it coincided with the focal plane which has very little curvature. The proper focal plane position was determined by minimizing the resolution of the elastic peak as a function of the detector position. Measurements were made with isobutane gas at pressures of 200, 100 and 50 Torr. The targets consisted of 100/~g/cm 2 of 2°spb on a 2 0 / ~ g / c m 2 12C backing, a layer of 140 p,g/cm 2 of 14aNd between 40 p,g/cm 2 12C foils and a 100 p,g/cm 2 19VAufoil. In order to calculate correctly and accurately the position and angle of a particle trajectory through the

VDC, it is necessary to have both an unambiguous cell number identification and a constant drift xelocity in each cell. These two features of the VDC and its associated electronics were tested by uniformly illuminating the counter. This was achieved by ramping the current of the dipole magnet of the Elbek spectrometer to sweep elastically scattered particles back and forth across the detector. An example of the cell identification spectra obtained with this procedure is shown in fig. 7. Each of the five rows of peaks in this figure corresponds to a particular delay line and each peak corresponds to a particular cell (or sense wire). These spectra were obtained by taking the time difference between the two ends of each delay line [5,6]. Each peak has a width of typically 250 ps fwhm. The peak to valley ratios are typically 10 4 indicating the extremely low ambiguity in cell identification. The widths of the peaks in delay line # 1 are somewhat broadened, probably because the TDCs for this delay line were not calibrated as accurately as those for the other delay lines. Fig. 8 is a typical drift time distribution for an individual cell measured with the uniform illumination procedure by summing the drift times from the two ends of a delay line. This distribution is characteristic of that expected for a vertical drift chamber [5,6]. The fiat region between 25 and 240 ns is indicative of a uniform drift velocity (0.05 m m / n s at 4.4 kV bias and 200 Torr pressure) for particle trajectories which traverse the cell between 1.7 and 12.7 mm from the sense wire, while the peak within 25 ns arises from nonuniformities in the field near the wire. Although it is possible to correct for the peak in the drift time distribution [5], we have excluded these drift times in the present analysis. The geometry of the detector and spectrometer insures that any trajectory which passes near a sense wire will also traverse five adjacent cells. Therefore, we ignored the shortest drift times and analyzed such events with time information from the remaining four cells. Consequently about 97% of the events have been analyzed with information from four cells. The remaining 3% were three cell events. Measurements such as in figs. 7 and 8 provide fairly rapid checks that both the counter and electronics are operating satisfactorily over the entire active length of the VDC and so are routinely performed at the beginning of each experimental run. The intersection of a particle trajectory with the wire plane of the VDC and its angle with respect to this plane were determined by transforming the cell number and drift time values for each of the four cells involved into x - y position coordinates and performing a least squares fit to a straight line. Measurements of the position and angle resolution using the VDC as a focal plane detector for heavy ions demonstrate the excellent characteristics of this counter. Examples of the energy

T.P. Sjoreen et al. / Vertical drift chamber as focal plane detector

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(i.e. position) resolution measurements are shown in figs. 9, 10 and 11. These figures also illustrate the usefulness of this counter for heavy ion research. Fig. 9 is an inelastic spectrum at 17 o (lab) obtained by scattering 176 MeV 14N ions from 2°spb. The width of the elastic peak in this spectrum is 110 keV fwhm or 0.88 ram. About 10 keV of the line width as well as much of the tail arises from aberrations resulting from the large angular acceptance (AO = 70 mrad) of the spectrometer for this run. Energy loss straggling and incident beam energy spread account for another 80 keV. Most of the remaining line width is expected to be due to beam

wander. The contributions to the line width from multiple scattering in the gas volume, the windows and the high voltage planes are negligible in all these measurements. A better example of the VDC's capability is shown in the inelastic spectrum of fig. 10 obtained from the scattering of 12C from 14aNd. For this run the spectrometer entrance aperture was set at + 5.0 mrad and contributions to the line width from magnet aberrations were negligible. The beam energy spread and energy loss straggling are estimated to be 75 keV. Thus the remaining contributions to the line width, assuming that

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they add in quadrature, can be no more than 31 keV or 0.4 mm. Again we believe that most of the remainder is due to beam wander. To eliminate the effects of beam wander, several short elastic scattering runs were also made. An example of the energy resolution achieved during these runs is the elastic scattering of 14N by 197Aushown in fig. 11.

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T.P. Sjoreen et al. / Vertical drift chamber as focal plane detector

The width of the elastic peak is 69 keV fwhm (0.55 mm), or an energy resolution A E / E of - 1/2500. The target and beam contributions to this width are estimated to be - 6 7 keV, which implies that all other remaining factors to the line width are consistent with a contribution of - 16 keV, or about 0.13 mm. Similar results were obtained from other short runs using different beams, different targets and different gas pressures. Based upon these measurements, we estimate that the position resolution of the VDC is about 0.1 mm. The angle measuring capability of the VDC is shown in fig. 12, where the counts obtained by elastically scattering 14N from 2°8pb are displayed as a function of the spectrometer entrance angle a. For this run the entrance angle slits of the spectrometer were set at the maximum opening ( + 35 mrad). Because the angular magnification of the spectrometer is about 3.5, the 70 mrad (4 °) angular range in a shown in fig. 12 corresponds to a 14 ° spread at the focal plane where the particle trajectories traverse the VDC at angles between 30 and 44 ° with respect to the wire plane. In order to estimate the angular resolution of the VDC, a mask consisting of four equally spaced 1 mm wide wire strips was positioned during this run between the target and the spectrometer entrance slits. The relative positions and widths of the mask wires in the angular spectrum are indicated by the cross hatched areas in fig. 12. The scattering angle subtended by the widths of these wires was 3 mrad. As indicated by the valleys appearing in fig. 12, the wire mask is well

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429

resolved in the entrance angle spectrum. We estimate that the angular resolution with which the scattering angle can be measured is about 3 mrad. This figure then corresponds to a - 1 0 . 5 mrad resolution at the focal plane due to the angular magnification of the spectrometer. The values of - 0.1 mm for the position resolution and - 1 0 . 5 mrad for the angular resolution compare favorably with expectations based upon the cell precision measurements discussed in the accompanying paper (Hynes et al. [6]), which indicate that the position resolution should be about 0.12 mm and the angular resolution about 7 mrad. The values for the predicted [6] and measured position resolution are in good agreement. However, the measured angular resolution is about 50% greater than the predicted value [6]. This difference is probably largely due to the finite size of the beam spot on target ( - 1 mm in width). The ability to measure trajectory angles accurately will enable us to operate the modified Elbek spectrometer with larger acceptance angles than previously. Earlier measurements without angle measuring capability were made with a restricted range of a so that magnet aberrations would not degrade the focal plane resolution. The effect of these aberrations is shown in fig. 13c, which is an uncorrected, distribution of elastically scattered 14N particles along the focal plane, when the a acceptance slits were set at _+35 mrad. The tail on the left side of the main peak and the satellite peak on the right are due to magnet aberrations. (The narrow valleys in each peak are the detector's response to the 50 lam field shaping wires and the broad valley between the two peaks is an artifact of the wire mask described earlier.) That the poor resolution in fig. 13c is due to those events with large values of lat is indicated in the correlation between focal plane position Xfp and a shown in fig. 13a. The solid line drawn in fig. 13a is the calculated central image position computed by a geometrical ray tracing program [10]. These calculations do not include the sources of finite resolution from the beam and target, which dominate the spread in the line shape seen in the data. Previously, in order to avoid limiting the experimental resolution, it was necessary to reduce the angular opening of the spectrometer, to clip the wings observed in fig. 13a. This, of course, also reduced the solid angle of the instrument. However, the VDC makes it possible to correct empirically for the t~ dependence in the focal plane position. The results of the correction are shown in fig. 13b. That the position resolution is recovered over the entire range of a is made apparent in the corrected distribution shown in fig. 13d.

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5. Conclusion The vertical drift chamber is an excellent focal plane counter for energetic heavy ions. Measurements with light heavy ions such as 12C and 14N at energies above 10 M e V / a m u yield values for the position resolution of

about 0.6 mm. Removing the estimated contributions due to the energy spread in the incident b e a m and energy loss and straggling in the target indicates that the intrinsic resolution of the counter is close to 0.1 mm. This value is comparable to the intrinsic position accuracy expected from the present detector configuration.

T.P. Sjoreen et al. / Vertical drift chamber as focal plane detector

Therefore the VDC provides position resolution fully compatible with magnets such as the split-pole and Elbek spectrometers. The limitations on the position resolution obtained with heavy ions will be controlled by factors such as the finite energy resolution of the beam from the accelerator, the size and angular convergence of the beam on the target and energy loss as well as the energy and angular straggling in the target. With the 25 M e V / a m u heavy ion beams available from the coupled Oak Ridge tandem-cyclotron system, it should be possible to perform good resolution experiments with projectile masses up to about A = 50. The vertical drift chamber also measures the angle with which the trajectory crosses the detector. This information determines the angle at which the particle enters the spectrometer. Therefore, large solid angles can be used to accumulate data, while at the same time, fine structure in the angular distributions can still be measured across the spectrometer entrance aperture and corrections can be made for magnet aberrations. It is necessary to measure additional parameters of the detected particle in order to identify its mass, charge state and atomic number. Because the VDC is a transmission counter, it can be followed by additional counters for this purpose. The high position and angular resolution of the VDC make it particularly suitable for relatively simple spectrometers having small dispersion, but a very broad range of momentum. This broad range can then be

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utilized without loss of resolution as the VDC is increased in length (but at the expense of more anode wires and electronics). A large momentum bite can be significant in heavy ion studies because of the presence of multiple charge states and it is also desirable for simultaneously measuring many different reaction products focused at widely different positions along the focal plane. References [1] J. Borggreen, B. Elbek and L. Perch Nielsen, Nucl. Instr. and Meth. 24 (1963) 1. [2] E.E. Gross, Nucl. Instr. and Meth. 121 (1974) 297. [3] J.E. Spencer and H.A. Enge, Nucl. Instr. and Meth. 49 (1967) 181. [4] Holifield heavy ion research facility newsletter, No. 18, ed., R.L. Robinson, (April 1981) Oak Ridge National Laboratory. [5] W. Bertozzi, M.V. Hynes, C.P. Sargent, C. Creswell, P.C. Dunn, A. Hirsch, M. Leitch, B. Norum, F.N. Rad and T. Sasanuma, Nucl. Instr. and Meth. 141 (1977) 457. [6] M.V. Hynes, J.L.C. Ford, Jr., T.P. Sjoreen, J.L. Blankenship and F.E. Bertrand, Nucl. Instr. and Meth. 224 (1984) 89. [7] Obtained from AMP, Inc., Harrisburg, PA 17105. [8] Designed by C.A. Reed, Oak Ridge National Laboratory. [9] Obtained from MKS Instruments, Inc., Burlington, Massachusetts, USA. [10] Performed by J.B. Ball, Oak Ridge National Laboratory.