Timing properties of parallel plate avalanche counters with light particles

NUCLEAR

INSTRUMENTS

AND METHODS

144 ( 1 9 7 7 )

609-611

'. ©

NORTII-HOLLAND

PUBLISItlNG

CO.

TIMING PROPERTIES OF PARALLEL PLATE AVALANCHE COUNTERS WITH LIGHT PARTICLES* A. B R E S K I N t and N. Z W A N G

Department o/ Nuclear Physics. Weizmann Institute ol Science. Rehovot. Israel Received 30 March 1977 W e have operated a 15 cm 2 parallel plate avalanche c o u n t e r (PPAC) with protons and ~-particles. A time resolution o f 140 ps (fwhm) has been m e a s u r e d with 5.5 MeV :z-particles at 30 torr of isobutane.

We report on some characteristics of parallel plate avalanche counters (PPAC) operated in a range of pressures from 10 to 60 torr, with low energy protons (0.8-2.3 MeV) and 5.5 MeV ~z-particles. PPAC are known to be an excellent tool for fast timing measurements with heavily ionizing particles. Since their discovery1,2), they have mainly been employed for the detection of ~z-particles3) and fission fragments<5), offering high time resolution (better than 1 ns), high counting rate capability and good detection efficiency. This technique has recently been applied to heavy ion physics. Large area detectors have been built for timeof-flight measurements with heavy ions6). Time resolutions of 160 and 320 ps (fwhm) have been achieved with an 160 beam and 5.5 MeV ~z-particles, using isobutylene at a pressure of 8-15 torr. A rather poor time response of such counters to light particles has been related to the small amount of energy deposited in their narrow gap (1-3 mm), resulting in difficult timing conditions. In a recent work v) it has been found that low pressure MWPC present a very stable operation when filled with isobutane. Due to the good photon quenching properties of this gas, the amplification could be pushed to higher values than in other gases tested. We have found the operation of PPAC with isobutane an attractive attempt. We have built two test counters of 40 x 40 mm 2 (fig. 1). The electrodes have been made of 2.5/~m Hostaphan foils evaporated with 10/,zg/cm 2 of aluminimum. The foils were glued to thin frames, made of epoxy resin plates, currently used for printed boards. A gap of 1.6 mm between the elec* Supported in part by the Israel C o m m i s s i o n for Basic Research. 1" Hattie H. H e i n e m a n Research Fellow.

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trades was maintained by means of spacers made of the same material. Contact to the anodes has been applied all around their frame, by means of a copper layer on the spacers. i

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610

A.

BRESKIN

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The counters have been placed in a small vessel, separated from the vacuum by means of a 2.5 ~m Hostaphan foil. Amplification curves (fig. 2) as a function of the gas pressure have been obtained with 5.5 MeV c~particles, showing that the amplification factor does not vary much but that the pulse height increases with gas density. The pulse height resolution improves significantly with increasing pressures, as shown in fig. 3. The fact that the reduced electric field, E/p, drops only by a factor of two, when going from 10 to 60 tort, while the

N.

ZWANG

pulse height grows almost by a factor of 10 seems to indicate that better timing may be achieved at higher pressures due to better slope-to-noise conditions. For the timing measurements, pulses from the two counters were sent through fast amplifiers 8) (Zm = 50 ,(2) directly to snap-off timing discriminators (Elscint STD-N-2). A signal-to-noise ratio better than 100 has been measured with u-particles at the higher pressure range. The rise time of the pulses, measured at the output of the fast amplifier, varied between 2.5 and 3.Sns (p= 10-60 torr). The time resolution was measured by observing the delay between pulses from the two counters by means of a time-to-amplitude converter. All the pulses were analysed and no further corrections have been applied to take into account the walk introduced by the spread in the pulse height. A collimator, with an opening of 5 mm diameter, has been placed in front of the first counter to avoid a spread due to the propagation time of the signals along the anode foil. Our results are shown in fig. 4. Best results have been obtained with ~z-particles at a pressure of 30-60 torr (fig. 5). The time spread of 200 ns (fwhm) corresponds to coincidences between the two counters. The time resolution of each counter is better than 140 ps (fwhm). The resolution with protons is slightly worse due to signal-to-noise problems and to the larger influence of the walk introduced by the spread in pulse height. Some improvements can be achieved by means of computer corrections to the walk9). We have developed a simple and accurate method for position sensing of PPAC ]0). The position of the particle provides a correction to the propagation time 500

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PARALLEL

PLATE

AVALANCHE

COUNTERS

611

Further development in this domain is pursued at present in our laboratory. We would like to thank Prof. G. Goldring for his interest in this work. We are grateful to Eng. B. Feldman, Messrs. L. Sapir and A. Kariti for their technical support. We are very much obliged to Mr. A. Faibis for his collaboration during the experiments with the Van de Graaff accelerator. References

ig. 5. Time distribution measured with ~z-particles at a presare of 55torr. h v ( + ) = 1 5 0 0 V , fwhm is 200ps (for coincience between the two counters). Delay between peaks is 1 ns.

long the anode and enables us to achieve good tiaing performances even with large area counters. We have shown that when operating PPAC at Ligher gas pressures than usual, one can achieve xcellent timing properties even with light partiles. The large improvement in pulse height res,lution may be helpful for particle identification.

I) j. W. Keuffel, Rev. Sci. Instr. 20 (1949) 202. 2) R. W. Pidd and L. Madansky, Phys. Rev. 75 (1949) 627. 3) F. W. Bfisser, J. Christiansen, H. P. Hermsen, F. Niebergall and G. SiShngen, Z. Physik 187 (1965) 243. 4) j. Christiansen, G. Hempel, H. Ingwersen, W. Klinger, G. Schatz and W. Witthuhn, Nucl. Phys. A239 (1975) 253. 5) G. Hempel, F. Hopkins and G. Schatz, Nucl. lnstr, and Meth. 131 (1975) 445. 6) H. Stetzer, Nucl. Instr. and-Meth. 133 (1976) 409. 7) A. Breskin, to be published in Nucl. Instr. and Meth. 8) I. S. Sherman, R. G. Roddick and A. J. Metz, IEEE Trans. Nucl. Sci. NS-15, no. 3 (1965) 500. 9) N. E. Sanderson, B. R. Fulton and J. B. A. England, Nucl. Instr. and Meth. 137 (1976) 399. 10) A. Breskin and N. Zwang, to be published.