A fast, bidimensional, position-sensitive detection system for heavy ions

NUCLEAR INSTRUMENTS AND METHODS

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A FAST, BIDIMENSIONAL, POSITION-SENSITIVE DETECTION SYSTEM FOR HEAVY IONS* A. BRESKINi', I. TSERRUYA and N. ZWANG

Department of Nuclear Physics, Weizmann Institute Of Science, Rehovot, Israel Received 30 June 1977 A fast heavy ion detection system, composed of a bidimensional drift chamber and a parallel plate avalanche counter is described. Time resolution of 180 ps (fwhm) and position resolution better than 0.55 mm (fwhm) in both dimensions have been measured with 30 MeV oxygen ions at 18 torr of isobutane.

1. Introduction The increasing interest in heavy ion nuclear reactions has stimulated many efforts to develop new detection techniques enabling a complete identification of the products resulting from the interactions. Generally a high efficiency for data collection is required, besides the requirements of good accuracy in the product identification and in the energy measurements. A survey of several heavy ion detection techniques may be found in ref. 1. An accurate and highly efficient heavy-ion identification system is under development at the Weizmann Institute of Science2). It is based on the kinematical coincidence method (for two-body reactions) where the time of flight and position of the products are measured. In the present work we describe a fast position-sensitive detector which will be used as stop and position detector in the above system. It consists of a bidimensional drift chamber, DC, (providing x and y signals) and a parallel plate avalanche counter, PPAC, (providing the time signal) built together as a compact unit. This detection system is inexpensive, easy to build and to operate. It offers high detection efficiency, high counting rate capabilities 3'4) and combines the excellent position resolution of a DC 5) with the excellent time resolution of a PPAC6"7). The detector has been operated at the EN tandem accelerator of the Weizmann Institute of Science, with 30 MeV oxygen ions and we report here on some of its characteristics such as time, position and AE resolution. Section 2 gives a description of the detection system and the electronics. The operation of the * Supported in part by the Israel Commission for Basic Research. * Hattie H. Heineman Research Fellow.

detectors is described in section 3. The results are shown and discussed in section 4.

2. The detection system The detector is composed of two independent parts: a drift chamber which provides a bidimensional position of the incident particle, and a parallel plate avalanche counter which provides a fast timing signal. The two counters are mounted in a vessel, as shown in fig. 1, and operate at the same gas pressure. The entrance window is made of 2.5 #m Hostaphan foil evaporated with 20/~g/cm 2 of aluminium, to avoid upcharging. It is SUPported by a set of 50/zm tungsten wires, stretched on an extra frame at an angle of 45 ° and spaced by 20 ram. At the present set up, the distance between the two counters was kept to about 40 mm, but they may be set at a closer distance if necessary. Fig. 2 shows a photograph of the detector assembly, the DC being removed out of the vessel. 2.1. THE DRIFTCHAMBER The DC, which is similar in design to the one described in ref. 8, has a sensitive area of 100×50 mm 2. It consists on a 100 mm long drift space with a proportional counter at the end. A uniform electric field is created by applying a constant potential gradient to the two rows of parallel electrodes, made of 50/zm tungsten'wires spaced by 2 ram. The sense wire (20/~m) is centered inside a square brass tube having a 5 mm wide entrance slit which ensures a collection of charges only from the central, homogeneous, part of the drift space. The rare wall of the tube is replaced by a delay-line which provides the orthogonal coordinate. The delay-line is made of a 70/zm insulated copper wire wound around a square (10× 10 mm 2) brass tube, covered with 0.9 mm thick epoxy resin

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plates. It has a characteristic impedance of 2.2 kl2, an ohmic resistance of 200/2, a delay of 5.3 n s / m m and an attenuation factor of 10% after 50 mm, and for input signals having a rise time of 12 ns. The position along the drift space is given by the time difference between the PPAC signal and the moment the electrons reach the sense wire. The orthogonal position is obtained by extracting the delayed induced signals from the two edges of the delay-line. The sense wire of the DC provides also a slow signal, proportional to the charge of the particle. 2.2. THE PPAC The PPAC has the same sensitive area as the DC. The two electrodes are made of 6/~m aluminized Mylar foils, glued to brass frames. A gap of 1.6 mm between the electrodes is maintained by means of an epoxy resin printed board which provides also the electrical contacts to the electrodes, all around their frame. The gas is supplied through an opening made in one of the frames and in the spacer, and is evacuated through a similar opening in the opposite frame (see fig. I), thus providing an efficient circulation of the gas between the electrodes.

The cathode,,close to the DC, is grounded and the timing signals are extracted from the anode, which is at a positive potential, through a 500 pF capacitor. 2.3. ELECTRONICS A block diagram of the electronics is shown in fig. 3. Good timing conditions were achieved for all counters by processing the time signals with fast, low noise preamplifers 9't°) or amplifiersg), timing filter amplifiers (TFA, Ortec 454) and snap-off timing discriminators (Elscint, STD-Nl). Time-to-amplitude converters (TAC) were used to provide time and position information which have been analysed by an on-line computer or a multichannel analyser. A charge-sensitive preamplifier (Ortec 121) has been used to provide the charge information from the DC. 3. Operation The detector has been operated at the EN tandem accelerator of the Weizmann Institute, with 30 MeV oxygen ions scattered from a Au target. It has been placed inside a large scattering chamber at a distance of about 230 mm from the target. The positions of the collimators and that of the de-

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tector itself could be varied from outside. The gas has been supplied by a differential pumping system including a monostat to keep the pressure constant within - 1%. Several gases have been tried. We have found isobutane to be the most convenient for the simultaneous operation of both counters the DC and the PPAC. The detector has been operated at a pressure range of 3-25 torr. It has been checked that there was no influence of one detector upon the other. 4. Results and discussion 4.1. TIME RESOLUTION The time resolution was measured between the PPAC and a surface barrier detector mounted behind it (see fig. 1)• The PPAC was operated with isobutane at a pressure of l0 torr. With a potential of 530 V between the two el~,ctrodes, the signal to noise ratio was - 1 0 0 and the risetime at the preamplifier output was 2.7 ns. The time spectrum is shown in fig. 4 with a time resolution of 240 ps (fwhm) for the total system. The contribution from the solid state detector was estimated to be 120 ps and the electronic contribution determined with a pulser was 100 ps, thus yielding an intrinsic time resolution of 180 ps (fwhm) for the PPAC. Very similar results were obtained at pressures 1 nsec

ranging from 3 to 25 torr and with other gases like ethylene and n-heptane. 4.2. POSITION RESOLUTION The detectors have been operated with isobutane at 18 tort. The sense wire was at a potential of + 1500 V and the drift potential was - l l 0 0 V• The PPAC was operated at 850 V. 4•2• l• X-resolution As pointed out previously the x-position is determined by measuring the time difference between the signals of the PPAC and the sense wire, which is practically equal to the drift time of the electrons, produced by the ionisation of the gas at the DC. The drift velocity of electrons in isobutane is shown in fig. 5. It is seen that the operating conditions of the DC are at the beginning of the region of constant drift velocity, 09= 5.5 c m / # s . A slit of 0.1 m m width and 3 cm height was mounted in front of the DC at a distance of 2.6 cm from the central plane of the drift space. The upper part of fig. 6 shows the distribution of the x-position signals as a function of x where x is the drift distance, it is seen that the distributions have a gaussian shape, without distorsions or tails• Both the peak position and the width are independent of the height of the slit indicating the independence of the x response on y. The experimental fwhm zlexp is indicated on the figure and is given by

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introduced by the electronics, may be neglected. A ~ was calculated using the tables of ref. 11 and found to be A~ca=0.15mm. &m was calculated geometrically ,'l~m= 0.25 ram. The intrinsic x-resolution deduced from eq. (1) is shown in the lower part of fig. 6. It is seen that the position resolution deteriorates by ~40% from one end to the other of the detector. This is due to the diffusion of electrons which increases with x, resulting in an increasing risetime (from 15 to 50 ns) and decreasing amplitude of the signals at the sense wire. 4.2.2. y-Resolution The y-position is determined by measuring the time difference between the signals at the two edges of the delay-line. A collimator composed of two thin slits of 0.1 mm width and separted by 20 mm in the vertical direction has been placed in front of the detector. The geometrical image of the slits, in the central plane of the drift space, has the following characteristics: vertical separation

25 mm, width 0.25 mm. The distribution of the y-position signals is shown in fig. 7 for two different incident positions along the drift space at x = 26 and 88 ram. The experimental y-resolution is quoted in the figure with an accuracy of 0.05 ram. It is seen that the y-resolution is almost independent of both y and x. The diffusion of electrons along t h e drift space affects the risetime and the pulse height of the induced signal on the delay-line, but not the y-resolution, since the ysignal is in fact a measure of the centroid of the induced signal. The experimental fwhm is equal to - 0 . 4 5 mm. Using eq. (1) one finds an intrinsic y-resolution of 0.35 mm. 4.3. LINEARITY TO demonstrate the linearity of the position response in both dimensions a mask of radial slots has been placed in front of the detector. Its image on the detector is shown in fig. 8. As pointed out previously the x-response is independent of y and

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a perfect x-linearity was obtained. In the y direction small deviations of less than 2% were observed due to distortions of the electric field at the edges of the sensitive area of the counters. The edge effects may be minimized by adjusting the field shape by means of a wire, at a correct potential, placed at the end of the drift space.

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4.4. ,JE RESOLUTION The charge integrated signal from the sense wire is proportional to the energy loss of the incident ion in the sensitive region across the drift space thus providing information on the atomic 6E -spectrum

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number of the ion. Fig. 9 shows the alE-spectrum obtained at the same operation conditions as described in section 4.2. A dE-resolution (fwhm) of 12% was obtained. It is independent of the position, beside some small influences at the end of the detector due to edge effects. Note that the energy loss of the incident ~60 ions in the effective drift space is rather small (E~800 keV). A better dE-resolution may be obtained at higher pressuresS). Note that a higher pressure is not likely to affect the time resolution of the PPAC as shown recentlyT).

5. Conclusion The combination of a DC and a PPAC provides an accurate, fast position-sensitive detection system. (Time resolution= 180 ps and position resolution better than 0.55 mm in both dimensions.) The detector behaves linearly in both dimensions, thus making very easy its calibration. The detector is simple to construct inexpensive and easy to operate. The results reported here are constant over a wide range of parameters (gas, pressure and potentials) and are not very sensitive to electronic adjustments. The detector has a high intrinsic efficiency (transparency of 95%, due to the potential wires of the DC) and can cover a large solid angle. It can also be operated at very high counting rates (higher than 10 4 part/s). All these properties make this detection system very attractive and particu-

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larly suitable for application in heavy ion identification system. The authors would like to thank Prof. U. Smilansky for his interest in this work. The wire stretching machine, used for the construction of the drift chamber, has been designed by Mr. L. Dumps from CERN and built with great skill by Mr. Y. Asher. We would like to express our gratitude to them here. We are grateful to Messrs. R. Anati and 1. Cohen for their technical support during the construction of the detector. References ]) F. S. Goulding and B. G. Harvey, Ann. Rev. Nu¢l. Sci. 25 (1975) 167. 2) 1. Tserruya, A. Breskin, N. Trautner, U. Smilansky and N. Zwang, to be published. 3) A. Breskin, G. Charpak, F. Sauli, M. Atkinson and G. Schultz, Nucl. Instr. and Meth. 124 (1975) 189. 4) G. Gaukler, H. Schmidt-B~3cking,.R. Schuch, R. Schule, H. J. Specht and I. Tserruya, Nucl. Instr. and Meth. 141 (1977) 115. s) A. Breskin and N. Trautner, Nucl. Instr. and Meth. 134 (1976) 35. 6) H. Stelzer, Nucl. Instr. and Meth. 133 (1976) 409. 7) A. Breskin and N. Zwang, Nucl. Instr. and Meth., in press. 8) A. Breskin, G. Charpak and F. Sauli, Nu¢l. Instr. and Meth. 125 (1975) 321. 9) I. S. Sherman, R. G. Roddick and A. J. Metz, IEEE Trans. Nucl. Sci. NS-15, no. 3 (1965) 500. 10) M. Birk, A. Breskin and N. Traumer, Nu¢l. Instr. and Meth. 137 (1976) 393. ]l) p. Sigmund and K. B. Winterbon, Nu¢l. Instr. and Meth. 119 (1974) .541.