Simple logarithmic pulse height analysers

NUCLEAR

INSTRUMENTS

AND

METHODS

16 (1962)

39--43;

NORTH-HOLLAND

PUBLISHING

CO.

SIMPLE LOGARITHMIC PULSE HEIGHT ANALYSERS J. c. B A R T O N a n d A, C R I S P I N

Northern l'olylechnie, London R e c e i v e d 6 March 1962 A t y p e of m u l t i e h a n n e l a n a l y s e r is described in which t h e pusle h e i g h t is encoded b y a p p l y i n g it to a r i n g i n g circuit a n d c o u n t i n g t h e n u m b e r of cycles before t h e oscillation decays to a fixed level. ~A.'ith a 10 ~.'~ochannel w i d t h t h e d y n a m i c r a n g e of t h e 64

channel i n s t r u m e n t described is 400: 1. Possible t y p e s of r e a d o u t a r e discussed a n d v a r i o u s solutions s u g g e s t e d t h a t a r e c o m p a t i b l e w i t h t h e simplicity, reliability a n d s t a b i l i t y of t h e r e m a i n d e r of t h e a n a l y s e r .

1. Introduction There are now available commercially m a n y excellent multi-channel pulse height analysers. Unfortunately they cost several thousand pounds each so that few laboratories can afford to use them treely. But the potentialities of scintillation counters, in particular, are greatly reduced if their application is limited by the use of single channel analysers. We therefore regard it as important to devise multi-channel analysers which are sufficiently simple and cheap that their use can be regarded as normal rather than exceptional, even in experiments involving more than one counter. Only one previous paper 1) describes the sort of instrument which seems to be required; this was a 25 channel analyser using valves and relays. The present paper describes a transistor analyser which can be used for 2n channels and has been adapted for various types of read-out. The first requirement on an instrument of this sort is that it should be reliable and stable, even if this means some sacrifice of performance. Similarly, in order to use the analyser with confidence, it is essential that its calibration should be capable of being checked in a simple manner. A further requirement is that the dynamic range, the ratio of largest to smallest pulses which can be handled, should be large. We have found that this specification can be most easily met by using a logarithmic rather than a linear response. This use of a channel width proportional to the pulse height does at first seem less convenient, but after some experience x~ith the

technique tile disadvantages seem to us relatively unimportant. The only major difference is that the ordinate of the resulting differential pulse height distribution is E.N(E) instead of N(E) itself. On the other hand the fact that it is possible to measure directly the ratio of the energies of two pulses in a distribution is often valuable. Also, changing the photomultiplier supply voltage should only shift the whole distribution without altering its shape; this provides a useful simple check of the linearity of the last stage of the photomultiplier and enables readings taken at different supply voltages to be combined easily. The analyser consists of three sub-units, the logarithmic encoder, the pulse train counter and the read-out. 2. Logarithmic Encoder The logarithmic response is obtained very simply by using the decay of oscillations in a tuned circuit. The charge pulse from the photomultiplier is used directly to charge the condenser of this circuit. Thus the voltage across it subsequently is given by:

V=?-.exp ql ( - ~~/ot~ /.

cos 2rqot

where the symbols have their usual meaning. If this voltage is applied to a simple integral discriminator circuit, set at a voltage V', then the discriminator will be triggered n times (once each 1) G. F. Von Dardel, J. Sci. I n s t r . 32 (1955) 302. 39

40

J. c. B A R T O N A N D A. CI~.ISPIN

cycle) where ~¢ is the highest integer less than tile value found from l/' = ~ e-="IQ.

Thus the number of times the discriminator fires for an input charge pulse of magnitude qt is ,~ =

(2 ]r

log° (~,/cv').

The relative channel width is a constant and equal to 1 - e-'~iQ-:.- ~/Q. Thus the performance is truly logarithmic and depends on only three parameters. Of these C, which includes the photomuhiplier output capacity, should be quite constant and Q can easily be made sufficiently stable provided the tuned circuit is only lightly damped by the input impedance of the discriminator; so the circuit problem is reduced to designing a stable integral discriminator of sufficient sensitivity. The necessary triggering level can be calculated from the maximum available charge

circuits. However it is perfectly possible t o include an amplifier between the ringing circuit and the discriminator. Since the signal is essentially a sIowly modulated sine wave, this can be designed as a relatively narrow band amplifier and thus avoids the usual problems encountered in pulse amplifiers. On large signals the amplifier may be driven into saturation but this is unimportant, provided it recovers rapidly as the signal decays and operates in its linear region when the damped sine wave is just sufficient to fire the discriminator. The complete encoder unit is shown in fig. 1. The ringing frequency is about 300 ke/s ; this frequency was originally chosen as being the highest at which it is practicable to use simple alloy junction transistors, but it also ~dlows the analyser to be used with "slow" scintillators such as NaI(TI). The amplifier consists of two feed-back pair stages with a total gain of about 1000. The feedback used, of shunt-output series-input type, provides a high input (30 K O) and low output (1.2 k O ) i m p e d a n c e to each section ; this circuit is rather similar to one -6

oI 4J

330pF

330p[ K

IOOpF

--~UT +-

0C44

0<244

OC171

~5pV

&

l:ig. 1. Circuit d i a g r a m of logarithmic pulse height encoder.

pulse from the photo-multiplier and the chosen value of the relative channel width. In practice it seems necessary to use values of a few millivo!ts, which cannot easily be achieved with conventional

used by Chaplin et al.2). Although the power gain is less than if each circuit were matched to the next, 7) G. P;. B. Chaplin, C. J. N. Candy and A. J. Cole, Proc. I.I~2.E. 160 B (1959) 762.

S I M P L E L O G A R I T H M I C I ' U L S E I I E I G H T ANAI.YSI-'.RS

this technique prevents one circuit influencing another and hence increases the overall stability. The stability of both the amplifier and Schmitt type discriminator has been studied as a function of temperature and supply voltages. It was found that the variation with temperature was negligible between 0 and 50°(; and that the only critical voltage was the negative supply to the discriminator. Variation of the latter led to an approximately proportional change in the discrimination level, so that some form of voltage stabilization is necessary for this stage; a Zener diode in parallel has proved quite adequate. 3. Pulse Train Counter

The requirements here can e ~ i l y be satisfied by any of several types of scaler. For one instrument transistor core scalers 3) were used and for a portable equipment*) Eccles-Jordon tjy~pe transistor binaries of very low power consumption were developed. However, we have found that the least troublesome solution for general use is the employment of Combi-Element circuit sub-assemblies (Mullard Equipment Ltd.), These are only rated for operation at 100 kc/s but we have found that at 300 kc/s their margin of operation is still very wide. The slight extra cost is well justified by the saving

41

is triggered either by a signal from the last dynode of the photomultiplier or, in some experiments, by a master coincidence event. It remains open for about 200 ~sec so that'the full train of pulses passes to the scaler and sets up its number in binary form. The second and third one-shot multivibrators, each of which is fired by the training edge of the previous one, provide read-out and re-set pulses respectively. It has been found that the re-set pulse needs to be 20 t~sec long for reliable operation. 4. Read-out

Much of the cost and complexity of multichannel analysers is accounted for by the storage and read-out devices and wc do not know of any way to avoid this if the data is all to be processed in the most generally convenient manner. It has been our experience, however, that for any given experiment it is often possible to use much simpler read-out techniques by taking advantage of the particular circumstances. The various possibilities will be considered separately. 4.1. I¢.ECORDING ON MULTI-TRACK MAGNETIC TAPE

The read-out pulses from the six binary stages of the pulse counter are fed via single transistor ampli-

Inp~lt from

~ , s o r--T. ~ l

r---n I I

I

I

I

I

Fig. 2. Logical diagram of pulse height analyser.

of development time and the increased reliability achieved. Fig. 2 is a logical diagram of this part of the circuit. The pulses from the discriminator are gated by the first of three one-shot multiv~ibrators which

tiers to separate sections of the magnetic head. This method is very simple and consumes negligible power; it has been applied successfully in the s) j . C. Barton, Nucl. Instr. and Meth. 5 (1959) 332. I) j. C. Barton, Phil. Mag. 6 (1961) 1271.

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J. C. BARTON ANI) A. C R I S P I N

portable equipment mentioned above4). In this particular experiment the total number of counts was only a few thousand so it was practicable to analyse the tape by using a magnetic powder technique and examining it visually. It would of course be possible to devise an electronic play-back, perhaps reading out each channel in turn, but we have not yet built such an equipment. Magnetic heads with very large numbers of tracks are at present under development s) as this appears to be the simplest method of recording the digital output of sew'.ral multi-channel analysers simultaneously. 4.2. BINARY VISUAl. READ-OUT

The state of transistor binaries can be made visible by the use of sub-miniature indicator tubes type DM 1606). In an experiment at present in progress, in which other data are recorded photographically, it has proved convenient to use this technique. Again each event has to be examined individually but this is still simpler than, for example, having to measure the height of a pulse recorded directly on film. In this method the length of the read-out pulse is adjusted until the light output is adequate. 4.3. USE AS A SINGI.E C t t A N N E L ANAI,YSER

It may at first seem strange to suggest that this type of analyser should be used with only a single channel output. The reason for this suggestion is that commercial single channel analysers are not aiways adequately stable and require frequent realignment, particularly if used ~ t h a narrow " w i n d o w " setting. So that it m a y still be worth using the multi-channel encoder and pulse train counter instead of the customary amplifier and differential discriminator. However, this solution is obviously less satisfactory than reading out several channels simultaneously.

ten; this type of assembly can be made quick!y using sub-units avai able from the manufacturers (Ericson's Ltd.) To obtain the individual channel inputs the binary scaler outputs have to be "decoded" ; this is a standard operation in computer technique and can be accomplished using a diode decoding matrixS). Various simplifications are possible if only a fraction of the available channels is required. Thus if the first three and last three binaries are separately decoded into eight outputs each, then it is possible to arrange a manual switch so that it selects any one group of eight channels. Alternatively with a slight change in the "logic" of the circuitry it is possible to obtain a 16 channel output. In practice it seems that even an eight channel output is extremely useful. Although at times one regrets the inability to observe all channels simultaneously, there is often only one part of the spectrum which needs detailed study. This is usually (!) a part where the counting rates are low, so that the additional time necessary to obtain satisfactory statistics over the rest of the spectrum may well be relatively moderate. It is apparent that in at least some experiments the total time required will be much less than eight times as long as using a 64 channel read-out. On the other hand the advantages compared to a single channel read-out are very substantial. 5. Calibration It is essential to be able to calibrate the analyser when connected to the photomultiplier, since both the collector capacity and its "load" resistance are in parallel ~ t h the tuned circuit. The calibration signal can be introduced either through a small condenser or else across a small resistance placed between tile coil and earth. These components also will alter the Q of the circuit so they must be left connected.

4.4. USE OF AN F.IGHT-CIIANNEL READ-OUT

We have found that this is a satisfactory compromise for several experiments. In one instance, where the counting rates are low, each channel is simply a post-office register in a single transistor blocking-oscillator type circuitT). In another equipment each channel has four " D e k a t r o n " scales of

s) j. C. Barton, Proceedings of Nuclear Instrument Symposium, Rutherford Jubilee Conference, 1961 (lteywoods, London, 1962). s) p. A. Neeteson, Junction Transistors iv. Pulse Circuits (Pl~ilips' Technical Library, Eindhoven, 1959). ~) J. C. Barton J. Sci. Instr. 37 (1960) 303. s) M. V. Wilkes, Automatic Digital Computers (Methuen, London, 1956).

SI.MPLI~ L O G A R I T H M I C P U L S E n E I G H T A N A I . Y S E R S

Instead of using a standard pulse generator, which is expensive and of limited range, we have found it convenient to use a uniform size stepfunction fed through a cMibrated radio-frequency attenuator. The pulse is formed simply by driving an earthed emitter transistor from cut-off to saturation so that the amplitude of the step is very nearly that of the supply" voltage. The attenuator (Muirhead type D-239-A) has a range of 60 db in steps of 0.5 db and is accurate to + 0.2 db. This method of calibration is especially appropriate for a logarithmic analyser as the channel number should be directly proportional to tile negative of the attenuator setting.

6. Performance and Comments Several of these analyscrs have been constructed and lsed during the last two years. A typical 6oi 55

45 4O

35 3o

0

I 5

I ~0

t 15

I 20

A'Aenuator

I 25

I 30

Se:t:ng

I 35

~ 40

k 45

m dec!bels

43

responding to a dynamic range of about 400: 1, has proved most convenient but operation with narrower channel widths has also been used occasionally. The only voltage supplies are + 6 V and these can be derived conveniently from a potential divider across a single stabilised supply, as the current drawn is only ~ 60 m.,\. The cost of the analyser components and subunits, exch',sive of read-out, is less than g 100. The sub-units for the most expensive read-out system used, the eight-channel Dekatron one, cost another £ 300; all the other read-out systems described above are very much cheaper. For faster counting rates it is necessary to ensure that a second plflse does not arrive during the 200/1see for which the gate before the pulse train counter is open. It is possible to include a gate circuit between the photomultiplier and the ringing circuit but this is likely to upset the correct functioning of the latter. It therefore seems preferable to gate the photomultiplier directly by quenching the voltage between one pair of d}modes. This has been tried x~ith one analyser and has operated successfully. Recently Pizer 9) has suggested a means by which a ringing coil encoder can be adapted to give a linear response. This might t×: a useful modification if a logarithmic response is unacceptable but unfortunately no details are given concerning the stability of this technique. The main direction in which further development is necessary is in the reduction of the total time ( ~ 250/~sec) taken to process a pulse. It is already quite easy to build transistor binary counters which operate at speeds in excess of 10 Me/s; the remaining circuits should not present any greater problems so that an overall processing time of less than 10/~sec should be feasible for use with fast scintillators and ~erenkov counters.

Fig. 3. Calibration gra.ph for complete pulse height analyser.

calibration graph is shown in fig. 3 and has proved reproducible over periods of at least six months. For most of our work a 10°,o channel width, cor-

Acknowledgement We are indebted to Mr W. F. Carter for his help in constructing these analysers. ~) H. 1. Pizer, Rev. Sci. Instr. 32 (1961) 988.