Timing of pulses from surface barrier detectors

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TIMING OF PULSES F R O M S U R F A C E BARRIER D E T E C T O R S W.-D. E M M E R I C H , A. H O F M A N N and R. STOCK*

Phys~alisches lnstimt der Universit~t Erlangen-N~rnberg, Er~ngen, G~many Receded 22 November 1968 Both the crossover m ~ h o d and the ~ a ~ n g edge m ~ h o d are ava~able for o b t ~ n g time m a r ~ n g ~ g n ~ s for c o ~ o d e n c e experiment. For s~id state detectors the two m ~ h o d s are discussed. If ~ is necessary to a v o ~ count~g ~ e s , w ~ c h • ~ o ~ particle spectra, the ~ a d ~ g edge m ~ h o d was found to

work wall for coincidence resolving times 2 r ~ 30 ns (fwhm = 3.4 n~, the crossover m ~ h o ~ howeveL o ~ y for 2 ~ ~ 90 ns. T ~ s r e s d t ~ due to v a f i ~ n s ~ the rise time of the detector p~se~

Introduction In the ~udy of nuclear reactions- espedM~ by panicle discrimination with counter tdescopesarrangements are o~en needed which allow the derivation of a lo~c ~gnM to mark the time of occurrence of an event in the detector. For this purpose, two different methods are suitabl~ the crossover m~hod and the ~ading edge m~hod (fig. 1). In the crossover method the d ~ e ~ o r pulse is doubly differentia~d; the time of baseline crossover is identified by suitabL drcuR~ sen~ng zero amplitude a~er an inifiM threshold trigger. In the Lading edge m~hod the rise of the pulse above a present threshold trigge~ the time fignM. For this purpose, fast time marking discfiminato~ are necessary. Both m~hods have been known ~nce a long time. The in~ruments needed are commerciM~ avM~bl~ The purpose of this paper is to show that the Lading edge method is superior to the crossover m~hod in timing pulses from s~id state dete~o~ for a wide dynamic range of pulse h~ght~

used for identifying and analyzing the deuterons and the other reaction products in the final channd with an online two-dimen~onal computer program2). The ~gnals which are needed for controlling the coincidence were first obtained by the crossover method. This method should be superior to the leading edge method because the time of the baseline crossover of the bipolar pu~e is theoretically independent of the pu~e amplitude and therefore independent of the energy of the detected particles. Varying the pulse height by a factor of 10 produces a walk in the crossover time of only +_2 ns in both fast circuits.

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2.

Methods of time derivations This problem arose during the investigation of the reactions ~) 27Al(d, d')27A1* and 25Mg(d, d') 25Mg*, at the tandem accelerator of the University of Edangen-Nfirnberg. A solid state detector telescope was

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Fig. 1. The two m ~ h o d s of time d e d v ~ n : and cro~over timing.

2.1. CROSSOVERTIMING In order to ~udy the time d~tfibufion of the fast ~gnal~ obtained from the reaction products of 27A1+ d by the crossover method, a TPC was started by the time signal of the dE-detector and ~opped by the delayed time ~gnal of the E-detector (fig. 2). The full width at half maximum (fwhm) of this di~ribution and its shape give information about the time unce~ainty of ~gnals from coincident events in both detectors and therefore indicate how to set the resolving time of a coincidence unit. In fig. 3 two measured TPCspectra are shown. They are shined by 30 ns. The fwhm is 12.5 ns. Th~ shows that a coincidence resolving time of 30 ns leads to a counting loss of 17%. Thus, di~ortions of particle spectra are possibl~ because different types of particles are related to different regions in the TPC-spectrum. [One is able to dui~inguish between types of particLs by observing the rise times of the p u l s e s 3 ~ . This result is due to variations in the rise time of the E-detector pulses. The pulse rise ttme is a function of the colLcting time of the charge carriers produced in the detector wh~h in turn depends on the d~tanee to be transversed by the charge carr~rs. The pulse rise

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~ 3. ~me dhtfibufion of the ~st fign~s ~ing ~ r tim~g m ~ d with a time to p~se h~ght conve~er (TPC). time therefore depends on the range of the pa~icM in the detector~. A dependence of the rise time on location and incident a n # e was excluded by two smM1 diaphragm~ one in front of the telescope and another one, with the same ape~ure between the two detector. A posfible influence of the ionization denfity of different particles due to recombination, dectric fi~d, etc. is probably smML Ufing the crossover m~hod, variations in the pulse rise time cause a wMk in the time of the baseline crossover s~nM. This wMk is of the same order of magnitude as the variation in the pulse rise time. This causes a deterioration in the permissibM resolfing time of the coincidence unit. Satisfactory results from the crossover method can only be expected in experiments where the d e ~ c t o n supply pulses of Mmo~ constant rise time. 2.2.

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In order to reduce the dependence of the time marking ~gnal on the pulse rise t i m e - in principle this is imposfible in the crossover method - t h e leading edge method was used. When the time fignal is picked off the lower part of a fast pulse rise, variations in the rise time should be of less influence then in the crossover method. HoweveL it is important to avoid a dependence of the time signal on the pulse height and therefore on the energy by employing appropriate measures. This teelmique has been successfully used in the ~udy of the scattering experiments 27Al(d, d') 27A1" and 25Mg(d, dO 2SMg*. Fa~, time marking threshold amplifiers * were used (amplification factor 100, rise time 2 n~. They are driven by the fast analog output of a charge sen~five preamplifier * (25 mV]MeV, rise f Ffieseke und HoepfneL Eflangen.

T I M I N G OF P U L S E S FROM S U R F A C E B A R R I E R D E T E C T O R S

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time 20 n~. With a detector pulse rise time of 100 ns (measured at the fast output of the charge senfifive preamplifie0, results in a time walk at the output of the threshold amplifier of ± 1.7 ns by a variation in the energy by a factor of 10. Some improvement can be achieved through the "constant fraction of pu~e height trigger" ~). In fig. 4 a basic block diagram of the arrangement used in the experiment is shown. Aside from the time marking dectronics, everything remains unchanged in regard to the crossover set up. The deuteron bombarding energy was 10.1 MeV, i.e. at laboratory angle of 70 ° in respect to the incident deuteron beam and with a 27A14arget, the maximum energy of the ~-particles was 14.60 MeV, that of the protons 14.77 MeV and that of the elastically scattered deuterons 9.15 MeV. Apart from the strong elastic scattering, the cross sections for all reactions are of the same order of magnitude. That mean~ that all pulse heights (except elastic scattering) occur with comparable yield.

For this pulse bright d~tribufion and with a threshold of the discfiminatovamp~fier of 3 mV (& 0.12 MeV), a T P C - s p e c ~ u m (fig. 5) with a fwhm of only 3.4 ns was measured. Thus, with a coinodence resolfing time of 30 ns there are pract~ally no counting losses ( < I % ) and therefore no d~tortion of energy spectra. Compared with the crossover method used at first a distinct improvement is noted under comparable conditions. The same result was obtMned after the telescope detecto~ were replaced by other surface barrier detectors of fimflar dimenfion~ A typical spec~um of deuterons, scaRered on 27A1 at 0ub = 70 ° is shown in fig. 6. The coincidence r e s o l i n g time was 30 ns. The pulses from the dEand E-detectors, sdected by fa~ coincidence were added by a summing amplifieL The total energy resolution is 37 keV. Instead of the time marking discriminator, pulse

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transformers, so-called time pickoff uniU (TPO's)* are Mso used. In this casG the current pulse produced by a charged panicle in the semiconductor detector induces a voyage pulse in the secondary winding of the pulse transformer. This voAage pulse is then ~ansformed into a fast time ~gnal by the subsequent amplifieL discriminator and pulse former. Such TPO's were used in earlier experiments with 14 MeV neutronsT). Here two difficulties arose: These pulse transformers respond only to relativdy high pulses ( ~ 5 MeV). Since solid state detectors with large counting surfaces and deep depletion layers had to be used for these ~ u d ~ the pulse rise times obtained were ~ow. Further difficult~s are caused by thor high rf-sensV fivity. Interchanging the TPO and the preamplifier made but fi~M differenc~ In the experiments discussed in this pape~ TPO's were not used.

S u ~ Ap#fing the leading e d ~ method and u~ng ~st time mar~ng threshed a m # ~ , it is posfiNc to obtMn r e s t i n g times of 30 ns in coinddence experiments ~ t h sur~cc barrier d ~ e ~ o ~ ~ o m cou~ losses (< 1%). Thcre~rc no ~ o r t i o n s of the p a ~ c ~ a resulting from c o u ~ Msses occuL Becau~ of the v a f i ~ o n s in the p ~ rise time of s~id state d ~ c ~ o ~ , such resets arc ~ n c r ~ y not avMMMc ~ t h t ~ crossover m~ho& The di~rcnce in the ~ r ~ r m a n ~ of the two m ~ h o ~ can be seen in fig. 7. There the ratio of the coincident events w ~ arc re#~cred in t ~ time in~rvM z to the totM number of coinddcnt events is #ottcd versus ~. It is ob~ous that ~ r a

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counting loss of e.g. 10% the crossover method allows only a coincidence resolving time of 50 ns whereas 15 ns can be achieved with the leading edge method.

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* O~ec, Oak Ridge.

The authors express thor thanks to P r o C t o r R. Fldschmann for helpful discusfions and his kind in~re~ in this work. The hdp of G. Philipp and K. Thomas ~ Mso appredated.

References ~ ~ ~ ~

W.-D. Emmerich, D ~ s e ~ a t ~ n , to be puM~hed. P. ElzeL Diplomarbeit (Eflangen, 1969). J. C. Leg~ Nud. InstL and M ~ h . 36 (1965) 343. C. A. & Ammeflaan, R. F. R u m p h o ~ t and L. A. Ch. Koert~ Nud. InstL and M e t k 22 (1963) 189. ~ D. A. Gedcke and W. & McDonMd, Nud. In~L and Meth. 58 (196~ 253 and references ther6n. ~ M . P . Endt and C. van der Leun, Nuclear Phys~s A 105 (196~ 1. ~ W.-D. Emmerich and A. H o f m a n ~ Z. Phyfik 201 (196~ 241.