A two-dimensional pulse-height analyser

NUCLEAR INSTRU~IENTS AND METHODS 6 (I960) 176-178; NORTH-HOLLAND PUBLISHING CO.

A r~WD-DIMENSIONAL PULSE-HEIGHT ANALYSER A. E. LITI-IERLAND and D. A. BROMLEY Chalk River Laboratovies, Onlario, Canada

Received 20 October 1959 A method is described whereby a 120-chamlel pulse-height analyser can be converted .into tt~ree forty-channel pulseheight analysers. This is accomplished by adding three

voltage pedestals of different height to the input pulses. The method has been successfully used in a study oi the N14(He3py)Ol~reaction.

1. Introduction

in a n y one of t h r e e voltage gates set on t h e s p e c t r u m from t h e o t h e r detector. Several two-dimensional analysers2, a) h a v e been described in t h e literature recently which are considerably more a d v a n c e d t h a n the one described in this communication. However, t h e pedestal insertion m e t h o d is simple to apply a n d h a s t h e a d v a n t a g e of visual presentation of t h e coincidence spectra b o t h during a n d after the experiment. The circuits to be described h a v e b e e n used successfu!ly 1) in t h e m e a s u r e m e n t of g a m m a - r a y spectra in coincidence w i t h protons from the reaction N14(Hea,p)O I6.

Many m e a s u r e m e n t s iv_ nuclear spectroscopy involve t h e recording of a pulse-height s p e c t r u m t h a t is in coincidence w i t h a p o r t i o n of a n o t h e r pulse-height spectrum. F o r example, in t h e recent s t u d y I) of the complex g a m m a - r a y decay schemes from states in 016 formed b y t h e reaction NI4(Hea,p)O I6 a 5-inch d i a m e t e r b y 4-inch long NaI(T1) crystal a n d a 1-inch diam e t e r b y ~ i n c h t h i c k CsI(T1) crystal were used in coincidence. M a n y 120-channel pulseheight spectra from one crystal were t a k e n in coincidence with a n a r r o w r a n g e of pulse heights in t h e second crystal. Since each coincidence s p e c t r u m took a p p r o x i m a t e l y one h o u r it is obvious t h a t a device for simultaneously recording several such pulse-height spectra would be of great v a l u e r Consequently additional circuits were devised which c o n v e r t e d t h e 120-channel analyser t h e n available into a two-dimensional analyser. W h e n c o m b i n e d w i t h t h e 120-channel analyser these circuits p e r m i t t e d , for example, the simultaneous recording a n d visual presentation of three separate 40-channel spectra in coincidence with three different and i n d e p e n d e n t voltage gates. This was accomplished b y adding voltage pedestals of a characteristic a m p l i t u d e to t h e linear i n p u t pulses from one d e t e c t o r w h e n t h e y occurred in coincidence w i t h a pulse Such a device is often referred to as an XYZ analyser~) or a coincidence sorter. In this note it will be referred to as a two-dimensional pulse-height anaIyser or more briefly a two-dimensional analyser.

2. Operation of the Circuits A block diagram of a typical experimental set-up is s h o w n in fig. 1. T h e pulses from two scintillation counters A a n d B are amplified b y linear amplifiers a n d analysed b y singlec h a n n e l analysers. Three single-channel analysers select Portions of t h e pulse s p e c t r u m from crystal A. The o u t p u t pulses from t h e singlec h a n n e l analysers are t h e n p u t in slow, 2v---2/~sec, coincidence with a pulse from a " F a s t Coincidence U n i t " , which requires t h a t t h e r e are pulses from t h e crystals A a n d B in fast, i) D. A. Bromley, H. E. Gove, J.'A. Kuehner, A. E Litherland and E. Almqvist, Phys. Rev. 114 (1959) 758. ~) L. Grodzins, A/Conf. 15/P/647 U.S.A. Second United Nations International Conference on the Peaceful Uses of Atomic Energy. 3) Multichanne! Pulse-Height Analysers, Edited by H. W. Koch and R. W. Johnstone, Nuclear Science Series, Report Number 20, National Academy of Sciences, Washington, D.C., U.S.A. (1957)

176

A TWO-DIMENSIONAL

PULSE-HEIGHT

2z = 40 nsec, coincidence4). The outputs from the three coincidence units are then applied to the three inputs, 1, 2 and 3, of the Pedestal Insertion Gate to be described below. It is

ANALYSER

177

pulses from crystal ]3 be limited to a certain range of pulse heights. This is accomplished quite simply as shown in the lower half of fig. 1. A wide voltage window is set by the single-

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Fig. 2- T h r e e 40-channel s p e c t r a d i s p l a y e d s i m u l t a n e o u s l y on a 120-channel k i c k s o r t e r . T h e s p e c t r a s h o w n are g a m m a r a y pulse s p e c t r a in coincidence w i t h p r o t o n g r o u p s f r o m t h e reaction NZ4(He3,pT)OZS. T h e energies in MeV of t h e p r i n c i p a l g a m m a - r a y s axe shown.

necessary for the successful operation of the Pedestal Insertion Gate that the spectrum of 4) R . E. Bell, R . G r a h a m a n d H . E . Perch, Can. J. P h y s .

30 (1952) 35. 5) 1~. L. Clarke a n d A. J . F e r g u s o u (unpublished).

channel anaIyser and the output pulses put in slow, 2z ~ 9 Fsec, coincidence with the pulses from the fast coincidence unit. The output pulses then open the Pedestal Free Gate 5) which passes the portion of the spectrum from

178

A.E.

LITHERLAND

crystal 13 selected by the single-channelanalyser. This pulse spectrum is then applied to input number 4 of the Pedestal Insertion Gate. A pulse at, for example, input 2 to the Pedestal Insertion Gate triggers one of three univibrators which produces a rectangular output of adjustable width. This pulse then cuts off one of three pentodes to produce a pedestal of the same width and of height determined by the quiescent current through the pentode. The currents through the three pentodes are adjusted to produce pedesta!s of three different heights corresponding to the three different inputs 1, 2 and ~. Addition of the pedestal to the appropriate pulse entering input 4 occurs in the common anode load of the pedestal forming pentode and a second linear-amplifier pentode stage which passes the pulse from input 4. The summed output is then mixed in a common cathode load of three cathode followers and applied to the 120-channel kicksorter. Pulses which enter the Pedestal Insertion Gate from crystal B which are not in coincidence with pulses at inputs 1, 2 and 3 are eliminated from the 120channel spectrum by using backbias. The pedestal heights are adjusted to divide the 120 channels into three groups of 40 channels. The range of pulses analysed from crystal ]3 is limited to 40 channels by means ot the single-channel analyser as described earlier. This ensures that pulses larger than the range of interest are not recorded in the next higher spectrum. The effective dead time of the system is that of the multichannel analyser which for the spectra shown in fig. 2 to be discussed below was approximately 4 milliseconds. An example of the use of the two-dimensional analyser described above is shown in fig. 2. I n this example three single-channel analysers were sel on the proton groups, P3 and P4, Ps, ]?9 and P10 from the reaction N14(He~.p)O16, corresponding to the formation of 018 in its third and fourth unresolved excited states, its

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£ . A. B R O M L E Y

fifth excited state and its ninth and tenth unresolved excited states. These excited states are at 6.92, 7.12, 8.87, 10.94 and ll.061~eV respectively. Gamma-radiation spectra in coincidence with these three proton channels, and therefore associated with the decay of the corresponding levels in 016, were displayed simultaneously on a 120-channel kicksolter. A discussion of these measurements has been publishedl). A pulse generator has been used to examine the linearity of the system with the results shown in fig. 2. I n the absence of any pulse arriving at the linear input 4 of fig. 1, as a result of the finite resolving time of the coincidence circuits used, the system produces three groups of pulses corresponding to the pedestals alone. These peaks are called separation markers in fig. 2 where the separation marke~ corresponding to the lowest pedestal has beeI1 back-biased just below the analyser threshold and the higher two markers appear in channels 40 and 80 respectively. This feature provides a convenient method of splitting the kicksorter display into its appropriate sections. Additional uses for the two-dimensional analyser described above suggest themselves readily. 1. Spectra taken with adjacent voltage gates, one set upon a particular peak and the other immediately above it, can be taken simultaneously and then subtracted to eliminate background contributions to the pulse spectrum in coincidence with the pulses corresponding to the peak. 2. The spectrum from a single detector can be displayed sequentially in three repetitions at any interval such that appreciable losses are not made by the kicksorter. This eliminates the time interval normally required for reading out of spectra until after three have been accumulate& 3. Spectra from three detectors can be recorded simultaneously.