The evolution of nuclear detector instrumentation

NUCLEAR INSTRUMENTS AND METHODS 162 (1979) 699-718 ; ® NORTH-HOLLAND PUBLISHING CC . X111. Nuclear detector data handling

THE EVOLUTION OF NUCLEAR DETECTOR INSTRUMENTATION K. KANDIAH UK Atomic Energy Authority, AERE, Harwell, Didcot, England

1 . Introduction

according to the logical functions following on front the detector do not coincide with the physical The development of radar, computers, communi- subdivisions into equipment units. The final section cations and space systems generate the advanced deals with overall systems and some of the engitechnology essential for all instrumentation. Pulse neering aspects. and digital techniques are essential ingredients in these areas as well as in nuclear instruments . The relatively small funds available for the development 2 . Nuclear instrumentation of nuclear instruments cannot justify the development of the special components and technologies This review deals with equipment that receives which are required to carry out the numerous diffisignals from radiation detectors, processes the sigcult and complex functions . The cavelopment of nals and presents the output in a predetermined nuclear instruments has therefore depended heavily format . The input signal is invariably a small packet on the imaginative use of every new component or of charge or a current and the chosen output he technology as they become available and the posi- modem equipment can be C.R.T. display, graph, tion held by the nuclear electronics community is photographic record, tabulated printout a evidence of the success of their efforts. data for further processing or control of other equipment . In the early 1940s the output consisted In a number of instances the adaptation of a technology to a nuclear instrument has led to furthof a meter deflection or numbers in an electronreer exploitation in other fields. The ever increasing chanical register . There are numerous reasons for the sophisticademands for higher xc.tacy, speed, and flexibility as well as the expansion of the nature and range of tion of the instruments used in nuclear measurethe measurements have tested not only the rements . In the first place the charge packets from the detector range from only a few hundred elecsources of the designer but the performance and availability of components and systems. In this trons as in low energy X-ray detection to sonic tents review it is hoped to highlight the advances mainly of millions of electrons, i.e . about a few pC in the of an innovative nature although the engineering case of the most energetic particles. These packets achievements will not be neglected . Comparisons arrive randomly in time with magnitudes in a merge determined by the nature of the incident radiation with other fields will be made where appropriate and contributions mace to the general technical and the detector response. It is necessary to measure one or more of the following parameters Ric community will be mentioned . each of these packets of charge : The published books dealing with nuclear instrua) magnitude of the charge which is usually ments hardly do justice to the many innovations in this broad subject . This review will inevitably be proportional to the energy of the radiation highly selective and limited by the author's experabsorbed in the detector, ience and ability to search the literature . Some b) the time of the pulse of charge in relation to trace the originator of an another pulse in the satin or in another deteceffort has been made to tor, idea as well as its successful application . It is hoped that the selected references will be adequate for c) the shape of the current pulse from the tor which constitutes the response to the those who find the sitar descriptions inadequate . individual particle, The reader who is familiar with the subject may d) relation between magnitudes as well as times omit section 2 where an attempt is made to introof arrival of the charge pulses firim two a dude others to the range of topes covered in the more detectors, later sections . The sub-divisions of the subject x111 . NUCLEAR DETECTOR DATA HANDLING

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feeding a meter. For reasonable volumes of the chamber the currents to be measured range from < 10 - ' S A up to nearly 10 -4 A. In the 1940s electrometers that had been developed for other applications wefe put to use. The early valve electrometer circuits were extremely difficult to use and particularly tedious when currents of <10-11 A were to be measured . The vibrating reed electrometer was therefore developed in the late 1940s although the design principles were known for some time previously. This class of vibrating reed electrometers became standard laboratory instruments for measurements of currents down to about 10-'b A and are still widely used . For currents > 10 -" A it is now possible to use MOST amplifiers using matched transistors of special design. An important requirement in radiation measuring instruments for radiological protection and for reactor control is the display of a wide range of currents on a single scale. A range of do amplifiers with a logarithmic meter indication was developed in the early 1950s. These instruments utilised the property that the voltage across a thermionic diode is proportional-to the logarithm of the current through the diode. It was realised that by feeding the ion chamber current directly into the grid of an electrometer valve the anode current was proportional to the logarithm of the ion chamber'current. Accurate logarithmic response requires more complicated circuits and an important contribution was the scheme used by Cox et al .') in which a pentode electrometer received the chamber current into. the first grid and the amplifier was arranged to . maintain a constant anode current by feeding back a voltage to the second grid. This voltage was then accurately proportional to the logarithm of the current in the first grid and was less dependant on heater voltage and temperature than previous arrangements. The instrument covered a cur-gent range of 5x 10 -" A to 5x10-1A with one type of electrometer valve and 10 - " A to 10 -' A with another type. The relationship between the collector 'current of a bipolar transistor and the emitter base voltage is also logarithmic and modern logarithmic amplifiers use such transistors in a feedback circuit with 3. Amplification temperature compensating elements to cover cur3.1 . DC AMPLIFIERS rents from less than 10 - ' 2 A up to about 10 -° A. The simplest radiation measurement instrument The basic circuit of such a system is shown in consists of an ionisation chamber and_a do amplifier fig. 1.

e) the position of entry of the radiation into the detector. The accuracy of measurement of the magnitudes of the charge packets is often required to be better than 0.1 % and in many cases somewhat better than 0.01% with long term stabilities of the same order. The relative time of occurrence of an event is measûred in the rangeof a fraction of a nanosecond to milliseconds or more . The final data will consist of the statistical distribution of one or more parameters averaged over a large number of events. We are therefore concerned with tow noise amplifiers which can raise the signal levels to those required by the processing circuits. The amplifier has to be highly linear and stable with moderately high speed for the amplitude measurements and very ~ fast for the time measurements. It is often not possible to meet all the requirements in a single amplifier and parallel amplifying paths are then used. The processing circuits select events according to amplitudes or time delay. The simple amplitude discriminators select events above a threshold level and coincidence circuits select events within a small time interval . After such sorting the mean rate of events in each category can be converted to a meter deflection through a counting rate meter. Alternatively the number of events in a predetermined time is recorded in a counting register or scaer. The term 'scaer' dates from the 1930s when electronic circuits were used to scale down the event rate to a value low enough to be recorded by electromechanical registers. A complete amplitude distribution is recorded in a pulse height analyser. These sort pulses according to their amplitudes in contiguous equal intervals called channels . The number of channels in a typical analyser ranges from 100 to many 1000es . Similarly time sorters record the time delay distribution. Each channel in such a distribution records the number of events in that group for the period of the measurement. The amplitude distribution may represent an energy spectrum and the time delay may represent a velocity distribution which in turn could be related to, say, the energy distribution of neutrons .

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Fig. 1. Logarithmic amplifier using bipolar transistor feedback.

Another interesting system for do measurements which gives a nearly logarithmic function, is that used in a scintillation instrument using a photomultiplier. If the voltage across the dynode divider resistor chain is controlled in such a way as to maintain constant anode current as the radiation intensity varies this control voltage is approximately proportional to the logarithm of the radiation intensity. This system is useful only over about three decades and the accuracy of the logarithmic function is much worse than that using diodes described earlier. 3.2. PULSE PREAMPLIFIERS The preamplifier, which is mounted as close as possible to the detector to reduce stray capacitance and maintain good signal-to-noise ratio, converts the charge signal from the detector into a voltage or current output of sufficiently high level to transmit along a cable which may be from 1 m-jo 100 m long . Work on preamplifiers has been active since the introduction of the Frisch grid ionisation chamber in the 19409. Thermionic valves were then used,in the input stage and, the type of valve which gave the best performance varied, according to the requirements, from some which were primarily designed for very high frequency applications to others meant for audio frequency applications. The noise! limitations of these amplifiers were beginning to be understood in 1947 and were not fully explained until the publication o0 the detailed

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analysis by Gillespie'). This proved that triode valves in the input stage connected in a cascode circuit under conditions which had the appropriate ratio of arithmetic grid current to mutual conductance gave the lowest noise. This analysis is still the most general treatment of the problems of noise in preamplifiers for use with capacitance sources. There was little change in the design of preamplifiers for about two decades from the mid 1940s except that there was a greater recognition of the need for choice of valves for the input stage and to apply negative feedback to maintain the stability of the amplifier parameters . It was realised in this period that the valves produced for telephone repeaters had the important characteristics of low grid current and long term stability of characteristics . Such valves gave consistent low noise performance in alpha particle spectrometers. In these amplifiers the statistical fluctuations of the charge produced in the chamber by the alpha particle were of the same order as the contribution of the amplifier noise to the line width. Typical noise.contribution of the amplifier was between 250e and 300e r.m .s . referred to the ion chamber for pulse shaping time constants of some tens of us: Using other valves optimised for 2 ,us shaping time constant the noise of the amplifier would be about 600e r.m .s . The ballistic deficit in signal pulse amplitude owing to the charge collection time in the chamber not being negligible compared to 2 us introduces further broadening of the line with this time constant . The preamplifiers used with the Frisch grid ionisation chambers use the voltage sensitive configuration, i .e . the charge from the detector was integrated in the total capacitance at the detector which included the amplifier and strays. There was negative feedback to the cathode of the input stage in order to improve linearity and gain stability. With the advent of semiconductor detectors there was considerable doubt about the constancy of the detector capacitance . Variation of this capacitance would alter the calibration of pulse amplitude at the output of the preamplifier against incident energy of the particle in the detector. Around 1960 there was an important change in tke design of the circuits of the preamplifier. Thelfeedback to stabilise the gain and improve lineàrityy ycnsisted of a capacitance from an appropriate point in the amplifier to the input as proposed by Cottini et ai .') in 1956 . The basic XII1 . NUCLEAR DETECTOR DATA HANDLING

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advances in the use of Si(i) detectors for energy dispersive X-ray spectrometry . Many prûblems had to be solved and perhaps the most important was described by Radekas) in 1968 . Lossy dielectrics at the gate of a FET connected to a very high impedance signal source can contribute -.significant amounts of noise . A number of workers discovered that the dielectric in the header on which FETs are normally mounted and around the high value resistances used as the do path for the gate bias were major sources of noise in Fig. 2 . Principle of charge-sensitive amplifier. Si(Li) X-ray detector systems. Optoelectronic feedback as proposed by , Kandiahb) in 1966 and circuit is shown in fig . 2. The value of this capa- the elegant,"method of Goulding et al .') who used citance, is usually small compared to the detector the gate-drain diode of the FET as the path for capacitance but because of the large forward gain the current from the negatively biassed detector it helps to maintain a constant charge sensitivity combined with low-loss insulators for mounting in spite of changes of detector capacitance . This the FET virtually eliminated this source of noise . charge sensitive configuration is used universally , The residual sources of noise in the modern Xin preamplifiers for nuclear detectors and its ray detector systems are the thermal noise_of the virtues are beginning to be appreciated in such FET channel, generation-recombipation noise fields as photovoltaic infrared detectors and due to defects near the channel land excess charge coupled devices . With improved valves currenvnoise having an (f") frequency-power the amplifier noise with a shaping time constant law relationship. Radekas) studied the effect of of 2 ,us was about 350e r .m.s. in 1960 . temperature on the noise contribution of the FET The use df field effect transistors (FETs) for in these systems but it has turnedJout that the low noise amplification took a long period of problem of FET noise is more complicated than development not only in nuclear electronics but that indicated in the earlier papers. Active work in most applications . The results of early measure- in this area is being continued ano useful coniTiments reported by Radeka°) were encouraging butions to the general unders anding ôf the although' the noise was not 'lower than that o~ sources of noise in .FETs are being made'-), thermionic valves but they demonstrated that n-`, Largely as a result of these ac ivities FETs o channel devices were, superior to p-channel de- improved' pérformance are be oming availabl vices . These early ETs suffered from poor and being used with infra-red d tectors and othef geometry and a high density of defects in the low noise systems . Optoelectr nic generation aft silicon . There has been a steady improvement in FET gate bias for low noise/systems has also, the quality of FETs in the past 15 years. By the, been adopted in some of these application's . early 1970s the noise contribution of the FET to These advances in amplifie techniques co~a large Ge detector system was- about 120e r .m.s. bined with techniques of cooling detectors atd at 4 us shaping time cdristant and the lire width ô1` FETs have resuked in dramatic improvements n the 1 .33 MeV gamma. ray was about 2 ReV fwhm. energy resolutions in the p`*st decade and sôrpe A large range of n-channel FETs is now available .results will be given later . and it is possible to match detector capacitances in the range 1 pF to 30 pF for minimum noise. The gate leakage currents of these - FETs are 3.3. FURTHER AMPLIFICATION negligible compared to the detector leakage . - 'The amplification necessary td raise the leHel Puise rise times of about 20 ns are readily of the signal to that refiuired by the amplitude obtained and the timing accuracy is limited discriminators, analysef5 and time sorters co d mainly by the variations in charge collection time not all be achieved in Fhe preamplifier. From t e of Ge detectors - 2 ns accuracy of time being 1940s to the present day,;.with minor exceptions possible in some cases . which will be dealt with under pulse processing, In the past ten years there have been dramatic it has been standard practice to use a main

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amplifier which also contained the pulse shaping to match the photomultiplier anode impedance to circuits . The pattern was set by the'design of the the discriminators . The analytical treatment of first general purpose AI amplifier described-by noise and 'resolution in connection with ionisaJordan and Bell'°) in 1947. This design was typi-, tion chambers was not generally considered to be cal of many that wire produced in various labo- ' relevant to the scintillation counter . There was a ratories in the world. Negative feedback over tendency to use a rather empirical approach to three stages (the last being a cathode-follower) the -circuits following the photomultiplier in the was commo4l , sod as it had been since the days early 1950s. Work on the main amplifiers for use of the Man rattan Project as recorded by Elmore with I ionisation chambers continued and there and , s") and pulse shaping by integrating was an intensification in,the efforts to improve and ifferentiating circuits or delay lines was the; counting rate capabilities of these systems in usually provided . Since there was little recorded an ;attempt to match the scintillation counter . information or. the benefits of pulse shaping in Most of this work was concerned with pulse the 1)40s it was left to the experimenter to ,shaping and will be discussed in a later section. choose the shaping time constants: that best Tttermionic valve amplifiers have many di suited his requirements. In some laboratories the vantages compared to the modern transistor amconditions necessary for good signal -ti)-noise ra- . plifier. The power consumption of a transistor tio and the effect of pulse shaping on spectral amplifier is so small that very high gain amplif distortion due to pile-up' were beginning to be iers with good stability and reliability can be appreciated in the late 1940s. In such laboratories , readily achieved. The absence of the heater supthe amplifiers had a flexible range of shaping ply makes it possible to use longer pulse shaping circuits in order to achieve the desired optimum. time constants with transistors without running However this was not universally adopted so that into serious problems of pick-up at the mains -there were considerable differences in the prac- frequency or its multiples. Microphony is also tice until well,'after the publication of the analy- less of a problem with transistor amplifiers. The sis by Gilles~e2) of the significance of the pulse gain-bandwidth product of an amplifying device shape. It is :::i--'. sad -comment . oil the lèvel of is proportional to, the ratio of its mutual conduccommunication within the nuclear instrupenta- tance to the sum of its input and output capacition community, and between laboratories, that tances. The transistor has a gain bandwidth some publications reviewing detectors in the product typically one order of magnitude higher 1950s gave the impression that the use of pulse than that of valves. When pulse rise times of a shaping was an arbitrary and poorly understood few ns are required it is necessary to use distritechnique. buted amplification' 2) whey: using valves. In this The main amplifier charact ristics are largely system the valves have their grids connected to determined by the requirements of the amplitude taps on a transmission line and the anodes to discriminators or analysers . Sincè`.most of these taps on another transmission line thus obtaining circuits in the 1940s and 1950s required pulse an additive effect from each stage rather than the amplitudes up to 100 V the power consumption 'multiplicative effect of a conventional amplifier of the amplifiers was large. A large number of cascade. A design produced by Orman") in 1957 controls defining the pulse shape, the gain and, had a gain of 10 using 100d? lines and a pulse more recently, base line restoring circuits gave rise tinie of 2.5 ns but it used nine valves with a the experimenter many . parameters on which he total power consumption of 40 W. A typical had to make decisions in order to optimise the transistor amplifier such as that described by system. This is therefore a major point of contact Manfredi and Rimini'°) in 1967 can obtain a simibetween him and the designers of nuclear instru- lar gain and pulse rise time when terminated ments. with 50 d2 with only two transistors in a circuit The advent of the scintilation counter not only using resistive loads and negative feedback with opened up gamma ray spectrometry but the a power consumption of about 200 mW and a possibility of high counting rates. Although it size less than I valve. It is however important to could not replace ionisation chambers for high remember that the valve amplifier can deliver a resolution alpha particle spectrometry there was much larger output voltage as required in C.R .T. no need for a pulse amplifier apart from the need deflection systems . XIII . NUCLEAR DETECTOR DATA HANDLING

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Although a large number of compact, highgain, low-power amplifiers in monolithic integrated circuit form have become available in recent years their use in nuclear spectrometry is somewhat limited. This is due to the fact that concurrent with these developments there has been important progress in the performance of semiconductor radiation detectors. The requirements of low noise, high gain bandwidth and gain stability are even more stringent than before the advent of integrated circuits. Furthermore it is necessary to have a degree of flexibility in the performance of the amplifier which permits switching of amplifier gain without a change in the transient response of the amplifier. Monolithic integrated circuits, without exception, show major changes in transient response when the feedback factor is changed. As a result amplifiers for high resolution spectrometry continue to use discrete transistor circuits or hybrid integrated circuits but in the less demanding applications it is possible to use monolithic integrated circuits . It is interesting to note that many transistor amplifiers deliver less than 10 V maximum amplitude compared to the 50-100 V of the thermionic valve amplifier. Yet the percentage resolution of modern spectrometer systems is perhaps two orders better than that achieved in the 1940s. This is due to the improved stability, speed and discrimination of the amplitude discriminators and tuning circuits using transistors . 4. Pulse shaping

4.1 . SHAPING FOR AMPLITUDE MEASUREMENT This is an important subject but somewhat complex owing to numerous basic and practical limitations that have to be taken into account and to the divergences of the requirements of different detectors and of the objectives of experimenters . Ideally it is necessary to characterise the radiation falling on the detector according to its intensity and energy distribution with very good energy resolution and negligible counting losses . The basic limitations of detector systems are due to the finite energy required to produce an ion pair and the statistics of this process, the finite charge collection time and electronic noise in the preamplifier. The shaping circuits aim to minimise the effects of these limitations and to be tolerant to the statistical nature of the rate of occurrence of the events. In the early 1940s pulse

shaping consisted essentially of a differentiating time constant - a CR circuit. It was realised a little later that limiting the high frequency response of the amplifier would reduce noise drastically and this was achieved by means of an integrating RC circuit. The optimum value of this CR-RC circuit combination for shaping the pulse was analysed by Gillespie2) who showed that in the presence of shot noise and grid current noise in the valve the best signal-to-noise ratio was obtained when the CR and RC time constants were equal. The optimism value of this time constant is proportional to Co/x/(gmld where Co is the total capacitance at . thé input, gm is the mutual conductance and I` is the grid current. The use of delay lines in preference to the CR differentiator - delay line clipping as it is often referred to yields shorter resolving times, i.e. there is less interaction between amplitude measurements of closely spaced pairs of pulses. In the 1950s and 1960s methods of pulse shaping received considerable attention stimulated by the development of the scintillation counter and semiconductor detector. Pulse pile-up which refers to the interaction between pulses which occur close to each other in time, variations of mean level at the output of the pulse amplifier referred to as baseline fluctuations and overloading of the amplifier on very large pulses resulting in the distortion of the amplitude of subsequent pulses were some of the practical problems which were studied. Most of these arise from the need to have ac coupling between the amplifying stages and result in shifts and broadening of the lines in an energy spectrum. There were many and varied solutions to these problems and a good summary of the situation in 1965 will be found in the series of articles by Fairstein and Hahn"). A pulse shaping technique developed in that period and still widely used

Fig . 3. Active pulse shaping with RC network.

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generates an output pulse which is almost gausSian in shape. An elegant method of achieving this is to use operational amplifiers with frequen. cy dependant feedback with only a few components as in fig. 3 which can simulate the effect of the multiple integration necessary to generate a Gaussian shape. Gaussian pulse shaping has the merits of good signal to noise ratio and reasonably short pulse duration compared to the simple CR-RC filter. One of the important requirements of all filters is the ability to produce good spectral lines at the higher counting rates and even the gaussian pulse shaper is not adequate for some applications in this respect. The experimenter is therefore usually offered the choice of shorter time constants or double differentiation which can impove the relative performance at the higher counting rates for a small sacrifice in the noise which determines the width of the low energy lines . The solution to the problem of base-line fluctuation consisted of baseline restoring circuits of the type proposed by Robinson 16) with subsequent improvements . Since these circuits introduced some non-linearities and slightly enhanced noise it became standard practice to provide a choice of circuits in each system in order to help the experimenter to reach his objectives . An important development which improved the performance of baseline restorers was the introduction of pole-zero cancellation by Blankenship and Nowlin") in 1965 . This method of cancelling the effect of the ac coupling led to more difficult adjustments in amplifiers but a

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worthwhile improvement in line shapes. Most of the pulse shaping circuits in nuclear instrumentation systems available at present use various combinations of the above and represent a level of sophistication in analogue signal processing which is hardly equalled in any other area of instrumentation . It is not likely however that these important developments will find many applications elsewhere, with the possible exception of radar, since most of the problems which have been solved in these ways have arisen because of the randomness in time of the detected pulses . A landmark in the development of radiation detectors was the introduction of GOLD detectors in 1960 . With the valve preamplifiers available at that time the noise was already about 500e r.m .s. even with the large detectors . Not long after the introduction of FET input preamplifiers in the mid 1960s the percentage resolution on the 1.33 MeV gamma ray line was better than 0.2% fwhm . Another important characteristic was that the charge collection time in germane ium detectors was not longer than that of the inorganic scintillators. In principle germanium detectors could therefore offer much better resolution than scintillation counters and similar counting rates . The performance of the pulse shaping circuits available in the mid 1960s did not match the potentialities of high performance Ge(Li) detectors . Kandiah") proposed the use of electronically switched time constants and gated integrators activated by each pulse from the

RECOGNITION AND LOGIC

Fig. 4. Principle of time-variant filter using switched time constants and gated integrator. X111 .

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detector in order to obtain high resolution which was much less dependent on the counting rate than the pulse shaping circuits use previously . These signal processing methods could achieve energy resolutions and counting efficiencies which were limited almost entirely, except" at very low energies, by the charge collection characteristics of the detector and the effects of the noise in the preamplifier. The first complete instrument embodying these principles was described") at the International Symposium on Nuclear electronics at Versailles in 1968, and the block diagram of the basic processing method is shown in fig. 4. With these signal processing methods using time-variant filters the experimenter had to make smaller sacrifices") in energy resolution and accepted signal rates in order to attain his objectives . In spite of the large number of papers on time-variant filters at the International Symposium in 1968 there are only a few instruments embodying these principles available at present. The general performance of such systems can be seen in fig. 5 which shows two spectra obtained with the same detector at a very low and high counting rates . The degradation at high counting rates is entirely due to undepleted regions of the detector. 4.2. PULSE SHAPE DISCRIMINATION AND POSITION SENSING The exact shape of the current pulse from a detector can give an indication of the type of incident particle . Owen21 ) and Brooks") showed that in some scintillators the fluorescence decay has a fast and a slow component and the ratios of these depends on the type of incident particle. The method used by Owen to distinguish between neutron generated events and photon generated events was to shape the output pulses from the photomultiplier through a short and a long differentiating time constant . The ratio of the heights of these pulses are different for neutrons and photons . The method used by Brooks belongs to a class which is called the charge comparison method and later variants such as the time for zero-crossing of a differentiated pulse were also used . The optimum pulse shaping method has been considered by Gatti and De Martini"). The disadvantages of these timeinvariant filters for processing long pulses have been recognised for many years. A time-variant filter which eliminates many of the problems encountered in the earlier pulse shape discriminators was

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described by Whiten") in 1975. It uses do coupling and automatic stabilisation of the zero of the system which is important when discrimination at lower energies is required . The instrument was easy to operate and had stable characteristics. Useful separation of neutrons from photons for neutron energies down to 50 KeV using the NE213 phosphor was obtained. The use of a charge collecting electrode with distributed resistance in a detector can provide information on the location of the point of charge collection . This idea has been applied to proportional counters and semiconductor detectors and the principle of position sensing is that of studying the shapes and relative times of the . current pulses at the two ends of the resistive electrode . The low noise amplifiers at the two ends cannot be direct coupled and there are major problems in using the charge sensitive configuration for these amplifiers . Consequently one of the techniques for position sensing uses a voltage sensitive configuration fo! these amplifiers. The position information is obtained by measuring the time delay between zerocrossing times of the two pulses") after dilTerentiation . The alternative method uses the charge sensitive configuration fdr the amplifiers and determines the position of the ionisation by measuring the ratio of the charges flowing into the two ends of the collecting electrode. A general noise analysis for these measurements is complex owing to the need to consider the correlation of the noise in the two channels . In the usual design the detector resistance is made much larger than the equivalent noise resistance of the FETs in the amplifiers. Since the resistance of the electrode is therefore of the order of many kO the spatial resolution is limited by the noise to about 1 % for a total collected charge of the order of lobe. 5. Pulse amplitude discriminating and timing 5.1 . AMPLITUDE DISCRIMINATORS We have so far considered analogue circuits but a vital element for counting systems is the threshold circuit which can discriminate between classes of events according to amplitude and/or time. Prior to the use of the Schmitt trigger circuitz') in the late 1930s the classification of nuclear pulse amplitudes was mostly carried out by visual observation of records of individual pulses . Simple diode discrimiX111 . NUCLEAR DETECTOR DATA HANDLING

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nators continue to be used for reasons of speed or a stability of better than 200#V. It is seen how the simplicity but a very large number of modern thre- signal levels of 100 V in the 1940s have shrunk to shold detecting circuits are based on the Schmitt about 5 V today with an improved fractional accu circuit which has been a common element in racy of amplitude measurement. nuclear counting systems for more than three decades. The threshold stability of the Schmitt circuit using thermionic valves is of the order of â 5.2 . TIMING Amplitude discriminators must precede all timing few 100 mV but a more important parameter which sets a lower limit to the pulse amplitudes that can circuits and timing accuracy is therefore critically be handled is the backlash of the Schmitt which is dependant on the speed of the amplitude discrimiusually many volts. These factors determine the nator. The Schmitt trigger circuit using valves or range of pulse amplitudes required at the output transistors will switch in about 100 ns. Higher of the pulse amplifier feeding these discriminators speeds can only be obtained with large input ampliand therefore their power consumption. One devel- tudes or at some risk in the certainty of the output opment in the 1950s which made it possible to have pulse amplitudes. The fast discriminator of Moody good threshold stability was the sensitive trigger et al. 19) made it possible to obtain 10 ns timing circuit using valves whose threshold depended on accuracies with pulse amplitudes from 1 V to 10 V. the incremental resistance of a diode passing a fixed This circuit used a diode to perform the discriminacurrent") and these principles are used in modern tion function followed by a trigger circuit which trigger circuits using semiconductors. utilised the fact that positive feedback can be The advent of the transistor reduced the power obtained from the dynode to the grid of a secondaconsumption of electronic equipment but Schmitt ry emission pentode valve. It may be noted that discriminators using transistors in the early days this development took place very shortly after still had a large backlash and the threshold stability secondary emission valves became available. Alwas not much better than 10 mV . Therefore there though the stability of the amplitude discrimination was some point in continuing to use discriminators was only about 0.1 V these were adequate for the relying on incremental properties of diodesz8). With many applications of scintillation counters . The use the advent of integrated circuits the ability to obtain of- the secondary emission valve flourished in the closely matched transistors has made it possible to 1950s partly owing to the high speed but also have threshold stabilities of a fraction of 1 mV . because of other interesting properties of the However it is important to bear in mind that direct dynode as will be seen later. coupled trigger circuits will inevitably have a backThe next important event was the invention in lash and in the simplest transistor circuit this will 1958 of the tunnel diode'°) which is a two-terminal device with a negative resistance characteristic over be some hundreds of mV. In order to ov9rcome this problem it is usual to combine a sensitive trig- part of the range. The important feature was that ger circuit with a latch arrangement which holds the device does not suffer charge storage characterthe circuit in the triggered state until the input istics in this range unlike most other semiconductor signal has fallen below the threshold. Proper use of devices. Nuclear instrument designers took advanthese arrangements requires some design skill and tage of these properties and, as soon as devices many instruments still suffer from the defects of became available, designed many variations of trigincomplete output pulses under some conditions ger circuits which acted as sensitive amplitude leading to various distortions in the recorded data. discriminators with timing accuracies of about However if these problems are adequately dealt 2 ns . with it is possible to use modern comparators in Since the tunnel diode, like the secondary emisintegrated circuit form for pulse amplitude discrim- sion pentode contained its own latch it was necesination of high accuracy and in many cases to sary to reset it after the input pulse goes below the combine it with high speed. Integrated circuit threshold as mentioned earlier. A typical arrangecomparators capable of switching in 100ns typically ment for this operation was that used by Sugarman have an absolute error in their threshold voltage of et al . 31 ) as shown in fig. 6 in which the tunnel diode no more than -1 mV with a stability of about 20#V . is reset after the time delay determined by the line . A fast comparator with a switching time of 5 ns This particular development like many others in may only have an absolute error of about 3 mV and nuclear instrumentation where early advantage is

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Fig. 6. Tunnel diode discriminator with delay line reset .

taken of new technologies or components had its share of problems. It turned out that many of the early tunnel diodes had poor reliability and manufacturers witdrew them from their lists . An important landmark in timing accuracy is the development of the zero-crossing or constant fraction timing discriminator. These utilise the principle that the relative shapes of the pulses from some detectors, especially scintillation counters, are very nearly independent of their actual amplitudes over a large range. It was suggested by Weinzierl 3z) that this could be used to obtain better timing accuracy than the method used up to that time of using the output of a normal amplitude discriminator which shows a time-walk at the output as a function of input amplitude for a pulse with finite rise time. Fairstein33) and Gruhle34) used the fact that the time at which a double differentiated pulse crosses the zero line is independant of the amplitude. A number of variants of this are now in use but the essential elements are a linear pulse shaping method which gives a bipolar pulse and a discriminator which triggers when the pulse crosses the baseline. It will be obvious that the logic design of such a circuit is complicated by the presence of, noise, and statistical fluctuations of the charge collected by the detector in the case of the scintillation counter, rise time variations and the fact that the circuit will have a backlash. A circuit which used a tunnel diode in a zero-crossing discriminatorwas described by Orman35) in 1963 and represents the state of the art of such systems with a time walk of a fraction

of a ns with plastic scintillators. Accuracies of better than 200ps have been reported for the constant fraction shaping in which the original pulse is subtracted from a delayed and attenuated version of itself and the zero-crossing time is detected by a fast trigger circuit . A problem arises in the case of ionisation detecfors owing to the variation of charge collection time. In the systems before the introduction of the semiconductor detector no attempt was made to correct for these. variations in order to obtain timing accuracies better than the charge collection time. In the past ten years there has been a great deal of interest in this field since this is a major aspect in which the semiconductor detector is inferior to the scintillation counter. Various methods of picking off a fast pulse as early as possible in the amplifying chain have been combined with pulse shaping techniques basically using the principles of delaying, inverting and subtracting a portion of the main pulse. A survey of many of these methods has been given by Cho and Chase"). Timing accuracies of a few ns have been achieved with coaxial detectors in which the maximum charge collection time could exceed 200 ns and with planar detectors, using systems which rejected slower events ;) the timing accuracy could be better than 1 ns. 5.3 . COINCIDENCE CIRCUITS The principles of the original Rossi coincidence circuit form the basis of the AND gate which is one of the most important parts of all modern computer XIII . NUCLEAR DETECTOR DATA HANDLING

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r OUT DIAS Fig. 7. Bell and Petch coincidence circuit .

logic circuits . In the early days the definition of the input pulse duration to the coincidence circuit was not very high owing to the slow charge collection times in ionisation chambers and the long dead times of Geiger counters. Soon after the introduction of the scintillation counter the interest in short resolving times led to the intensive study of pulse shaping for coincidence counting. The circuits described by Bell et al . 38) formed the basis of a family of fast coincidence circuits in which the resolving time was determined by a delay line. These circuits of the basic form shown in fig. 7 required a negative pulse with a fast edge to cut-off the valves . When used in conjunction with the fast discrimina-. tors using secondary emission pentodes29) it was possible to obtain resolving times as short as 10 ns in the early 1950s. With improved scintillatoos and circuits and the introduction of transistors which require much smaller signals to cut off the current, resolving times have gradually been reduced to a fraction of 1 ns with modern transistors . Many parameters such as scintillator response, photomultiplier current pulse shapes and transistor speeds will make it difficult to obtain much shorter resolving times with simple circuits. Another classic arrangement introduced by Bell and Petch39) forms the other main ingredient of all coincidence systems in which event selection depends not only on the time of detection but also on other characteristics of the detected event such as energy, particle type or position . It may be noted that after pulse shaping and timing, using the methods mentioned earlier, it is now possible to perform all the functions of coincidence, anticoincidence and the more complex functions of the fast-slow systems derived from the

arrangement of Bell and Petch39) by using standard logic gates in the ranges offered by many integrated circuit manufacturers . They take up little space and low power for resolving times greater than a few tens of ns . Faster circuits offering the possibility of a few ns resolving time are also available but they consume a little more power. 6. Counting rate meters and scalers 6 .1.

COUNTING RATE METERS

In portable instruments and monitoring systems the counting rate meter has occupied a prominent position for 40 years. The principle of the diode pump and a practical rate meter were first described by Elmore and Sands°°) in which feedback was used over the integrating capacitor but not over the resistor at the output of the pump. It could therefore be used with reasonable accuracy only over a limited range of output voltage. The linear and logarithmic ratemeters described by Cooke-Yarborough4' . 42 ) form the basis of most ratemeters used to the present day. In the linear ratemetersl) the major improvement of applying feedback to the resistor as well as the capacitor at the output of the pump improved the accuracy and stability of the system. In the logarithmic version42) advantage is taken of the saturation characteristics of a diode pump when fed with pulses of a finite height and the correct transfer function is obtained by using a number of parallel pump circuits into a common integrator. The recent development of a versatile range of digital integrated circuits of high speed has opened up the possibility of simple practical ratemeters using entirely digital operation on the pulses at the

NUCLEAR DETECTOR INSTRUMENTATION

output of a discriminator as proposed by Vincent interest in the use of cold cathode tubes in these and Rowles"). A versatile instrument of this type laboratories . with digital readout and with the functions such as Multielectrode cold-cathode tubes called dekathe integrating time constant of an analogue rate- trons became available in 19505°) and a large meter with preselectable standard deviation was number of scaling units using these were availdescribed by White°°). It covers a counting rate able") in the early 1950s. The virtues of these range from 10 per second to 106 per second full saalers were that they were simple, economic, scale with standard deviations selectable between compact, provided self-indication and worked basi7% and 0.22% and it is interesting to note that the cally in a decimal system . In many cases a single time constant at 10 pps for a standard deviation of cold cathode trigger tube generated the drive wave0.22% is 104 s - a figure almost unobtainable in any form required by the dekatron and there were only analog ratemeter. These principles can also be used about six other components per decade . It was in portable instruments by using the low power possible to build a scale of 1000 in a small rack integrated circuits used in modem calculators and mounting unit without the need for forced cooling. digital watches . An interesting phase in the delop- The only disadvantage was that the resolving time ment of portable instruments was the use of cold was about 400,us and if a prescaler using thercathode trigger tubes in the late 1940s. Although mionic valves was used to overcome this limitation, they did not provide the accuracy required in some the power consumption, cost, size and reliability of monitoring systems they were simple, reliable and the whole scaling unit was dominated by the charconsumed very little power. An early instrument by acteristics of the prescaler. It was no wonder that Franklin and Loosemore 43) used cold cathode tubes two families of scaling units, with and without in a ratemeter circuit and as an oscillator of prescalers were made for about a decade in the constant amplitude to generate the high voltage to 1950s. In the late 1950s a disturbing type of unrethe Geiger counter through a Cockcroft-Walton liability, which made the scaling tube stick in a multiplier. Another interesting development of that position where it had been static for some hours, period was a variable voltage stabiliser46) for low was discovered . The development of transistor scalcurrents using a cold cathode trigger tube . ers which offered many advantages caused the decline in the use of the dekatron but transistors were used to generate the drive pulses for the deka6.2 . SCALING tron during the late 1950s. In spite of the small size Modern counting circuits owe a great deal to the and low power consumption of the modern intedevelopment of scaling circuits for nuclear instru- grated circuit BCD counter it is difcul° to visualise ments. Binary counters and binary-coded-decimal a modern counterpart as aesthetically "efficient" as (BCD) counters were used during World War II the dekatron. It is conceivable that a ctsarge coupled although the earlier versions of the BCD counters device (CCD) with liquid crystal display (LCD) using valves were not universally accepted as being could be designed and made to simulate the dekareliable even up to the 1950s45 .48). This was no tron combining high speed and small size with all . doubt related to the failure rate of thermionic the other virtues of the dekatron but in these days valves. Not without justification therefore some of automatic data accumuu Jon and analysis there laboratories embarked on a programme of develop- is no need for such a devise. ment of new counting devices and systems . Some, The most commonly used counting systems are such as ring circuits using magnetic core memories, the binary, ring and the various ways of achieving a did not stand the test of simplicity, reliability and binary coded decimal system . In some situations user convenience. One important development in Gray code counters have an advantage since the the late 1940s was the development of ring saalers ambiguities in the code during the transition from using cold cathode tubes for use as resigsters in one one to the next are less serious and codes of this of the earliest multichannel pulse analysers 4'). type are sometimes used in analog-to-digital conThese were very simple using one trigger tube and verters. An interesting ring-of-five circuit was defive small components per element in the ring and veloped52) during the early stages of the use of consumed a negligible amount of power. Since they transistors. It used a ring of three and a scalewere self-indicating and found to be at least as of-two combined to obtain only five codes instead reliable as thermionic valves there was a growing of the six normally possible with this configuration. XI11 . NUCLEAR DETECTOR DATA HANDLING

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opened sequantially to the detector pulses by means of clock pulses for channel widths greater than about 1 us. With modern transistor circuits it should be possible to use these techniques for channel widths down to about 50 ns but below this the effective channel width profile which is related to the overlap between adjacent channels becomes unacceptable . In principle such multigate spectrometers can record multiple events following each start pulse to the timing clock. However in practice the characteristics of the pulse shaping circuits between the detector and the time spectrometer have finite time resolution and usually limit this multistop function by imposing a dead time of a few us in most cases. Consequently time delay spectrometers with multiple gates are only used for the larger channel widths such as in slow neutron time-offlight spectrometry . In the case of short time channels it is common practice to use some encoder which can generate only one delay address per start event in the most economic way. The simplest method is to feed the 7. Multichannel spectrometers start and stop pulses into a register which is countIn the past 30 years the work done in this field ing a clock which defines the channel width. With including the evolution of multiparameter systems modern transistor circuits it is possible to use clocks and the use of computers both interactive and off- up to a few hundred MHz implying channel widths line is so vast that even a book would hardly do down to a few ns. Before the availability of such justice to the many important developments espe- high speed circuits it was necessary to use an cially if an attempt is made to maintain the histori- encoding method using delay lines and coincidence cal aspects. The author need therefore not apologise gates for channel widths of many tens of ns. These for the necessary limitations of the choice of chronotronsss) were developed in the late 1950s and subjects . Since computer usage is dealt with in more refined systems called vernier chronotrons another paper in this issue only developments lead- have been developed more recently for very narrow ing to that stage will be mentioned . It is also time channels. In the vernier chronotrons it is not assumed that the reader is familiar with the'general necessary to have the multiple fast coincidence subject and may wish to consult the book by circuits of the simpler chronotrons. The method Chase"') which serves as a good introduction and a first described by Cottini and Gattin) used two record of early developments. oscillators at two frequencies which are well defined but slightly different from each other. These are fed 7.1 . TIME DELAY ENCODING to a mixer and the time delay when the output of These spectrometers have two main parts - an the mixer goes through zero is an expanded version encoder and a storage system together with ancilla- of the initial time delay. Another method later ry parts performing control functions on the condi- introduced by Lefevre and Russell") uses two recirtions of the measurement and the display and culating trans of pulses which are started by the output of the data. It is assumed that the input input pulses and whose periods are accurately pulses have been suitably shaped to define the defined by two delay lines whose time delays are detection times and the encoder transforms the slightly different from each other as in fig. 8. The analogue time delay into a digital code which can number of pulses before coincidence takes place at be used to select an address for the storage of the the gate is a measure of the initial time delay but accumulated counts. In the early days of time delay expanded . Channel widths of I ns can be obtained spectrometers it was possible to obtain the coding by using these techniques. by means of a number of electronic gates which are Another important form of encoding is to converti

Modern integrated counting circuits use a combination of the standard JK bistable element to obtain the appropriate code . It is not difficult to obtain any desired code, the problem only being one of containing the entire circuit on one chip. Synchronous and asynchronous counters some of which are reversible are available in BCD codes with counting speeds in the tens of MHz. The contents of these can be displayed in compact LED displays and the modern scale of 10` with displays takes up less space than a scale of 10 before the dekatron era. However it may be interesting to note that a sealer with a counting speed of 300MHz in the Harwell 2000 series was used in one of the early studies into the statistical properties of light fields53) in 1965. In this series of measurements extensive use was made of amplifiers, discriminators and sealers from the nudéar instrumentation field for studying the statistics of the single photoelectron generated pulse distributions at the output of a photomultiplier .

NUCLEAR DETECTOR INSTRUMENTATION

71 3

Fig. 8. Vemier chronotron.

the time delay between the start and stop pulses to a proportional pulse amplitude . This amplitude is then measured with a pulse amplitude analyser. Most of the circuits described use a current flowing into a capacitor for the time interval as the conversion technique. The merits of the various methods cannot be discussed here but it could be stated that using test pulse generators some of the recent systems claim a resolution of about 10 ps fwhm . 7.2. PULSE AMPLITUDE ENCODING

The simplest method of measuring an amplitude uses a number of amplitude discriminators with their threshold voltages set at progressively higher levels. The pulses are fed to all the discriminators in parallel and anticoincidence circuits are connected between adjacent discriminators. The anticoincidence circuits then feed counting registers. Such stacked discriminator systems were used in the 1940s to make multichannel analysers with up to 10 channels49- 58 .59). Obtaining a large number of channels is beset with a number of problems which were partly connected with the accuracy and stability of the discriminators and the sizes and power consumption of the counting registers used in that period. This basic system still offers the only satisfactory way of obtaining a high speed encoder. In contrast to those early developments it may be noted that recently an integrated circuit encoder of this type with 64 discriminators (6 bits) on one chip with a maximum coding rate in excess of 25 MHz has been made. In the above systems it is necessary to strobe the discriminators at the peak of the pulse in order to obtain a correct measure. A major breakthrough in analog-to-digital conversion techniques was the invention of the ramp-

type of converter by Wilkinson6°) in 1950. A linear ramp is made to run from zero to the pulse amplitude and the time taken by the ramp is measured in a digital form . In the above method it is necessary to store the peak pulse amplitude for the duration of the conversion and this is carried out in a pulse stretcher. An alternative method is to allow a capacitor which has been charged to the peak amplitude of the pulse to run down linearly to zero voltage so that this class of converter is sometimes called the run-down type. The majority of analogto-digital converters (ADCs) available today use these methods or some variants of these. The main work in the past three decades on converters of this type for nuclear measurements has been on improving the accuracy and speed of the peak stretchers and the circuits for detecting the end of the run-down period. The ramp conversion technique is also used in the majority of modem digital voltmeters. In the early converters the pulse-stretcher used a diode to charge a capacitor leading to non-linearities which increased as the input pulse width decreased . Attempts to improve this by reducing the storage capacitance led to problems of uncertainty of the ramp rate owing to leakages and hum pick-up from valve heater supplies. The second major improvement in ADCs was the use of negative feedback to charge a capacitor of moderately large value to the peak of the input pulse by using a differential amplifier to sense the difference between the pulse level and the capacitor voltage and to feed an appropriate current to the diode. This technique greatly improved the linearity of the charging and in the circuit used byChasebl) in which the dynode of a secondary emission pentode was used to charge the capacitor the leakage problem was also greatly XII1 . NUCLEAR DETECTOR DATA HANDLING

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o~-

try is concerned with the pulse amplitude distribution on random events. Their sliding scale technique has been effectively applied in a number of ADCs and conversion times less than S us for 8192 channels has been achieved in contrast to the 82 us which would be required in a ramp type ADC using 100 MHz. These developments are particularly significant when sophisticated signal processing methods using time variant filters have been shown to give excellent spectra with Ge(Li) detectors with a total dead time of less than 4 us enabling data output rates in excess of 10 5 events per second. The pace of development of ADCs is very encouraging in that owing to the general use of digital processing in many fields there is growing interest in improvements to ADCs in terms of speed, accuracy, size and power consumption. The last of the major family of ADCs developed especially for pulse amplitude analysis was that described by Arirel and Kurz") which obtained a conversion time of about 25 ns for 256 channels. A conversion time of about 1 us for 8192 channels has recently been obtained in an experimental high speed converter for radar applications. A large number of integrated (monolithic and hybrid) ADCs have become âvailable in the past few years. Most of these are of the successive approximation type and will require correction using the sliding-scale principle63) for nuclear pulse spectrometry. Conversion to 4096 channels in 4 as and 8192 channels in 10 us can be obtained with these compact ADCs. As mentioned earlier conversion times of less than 100 ns for at least 256 channels will be offered soon in such integrated circuits.

INPUT

Fig. 9. Principle of accurate ADC.

reduced. There still remained appreciable errors due to the relative drifts of thedifferential amplifier used for charging the capacitor and of the circuit which detects the null between the ramp and capacitor voltages . A converter of high accuracy in which the same difference amplifier was used first to charge the capacitor in a negative feedback arrangement and then used as a sensitive null detector to detect the null point with the ramp was described by Kandiah 61 ) in 1962. This circuit also used â further interesting property of the secondary emission pentode viz that the direction of the dynode current could be changed by altering the anode potential. The general principles of such an ADC are shown in the block diagram of fig. 9. A positive poing input pulse charges the capacitor to the peak voltage. After the peak is passed a constant current discharges the capacitor and a clock is gated on until the discharge is complete and during this latter period the input pulse is gated off. Modern ADCs using balanced high speed transistor pairs for the difference amplifier can convert up to 8192 channels using 100MHz clocks with differential linearities of better than 1% over most of the input amplitude range of about 5V. In some applications the speed of the ramp type ADCs using balanced high speed transistor pairs mation method which is used extensively in industrial measurements can offer higher speed. A major disadvantage is that these ADCs will have major channel width non-uniformities near the transition points defined by the more significant bits. These non-linearities are not acceptable in nuclear spectrometry and Cottini et al .63) devised a means of alleviating the effects of these non-linearities which takes advantage of the fact that nuclear spectrome-

7.3.

DATA STORAGE

It has already been stated that individual electromechanical registers sometimes preceded by sealers were used for storing the counts in each channel in the 1940s. This was soon found to be inadequate when the Wilkinson type of ADCs offered the possibility of 100 or more channels since the storage circuits alone would have taken more than one 6-ft rack (some laboratories did tolerate more than one rack for their analyser) . Consequently there has been considerable interest in storage systems as well as pulse stretchers and ADCs since 1950. The first major development was the Hutchinson-Scarrott analyser 65) which used an ultrasonic delay line as the memory soon after such memories were developed for use in the early computers. In this

NUCLEAR DETECTOR INSTRUMENTATION

sytem the start of the ramp for the Wilkinson type of converter and an oscilloscope display of the data in the line were synchronised with the master clock of the recirculating store . It was an impressive development since simple switching of the arrangement of the store could change the conversion factor (number of channels) for a corresponding sacrifice in the number of counts stored in the channel. Many variants of this basic design were in use throughout the world for about a decade. The main attraction was the compactness of the complete analyser for such a larger number of channels . Some of these analysers showed signs of poor reliability owing to problems of stability of the storage system which led to the development of a 100channel pulse analyser which used the unique properties of the dekatron") to provide the facilities of matrix selection and information storage in a 10 x 10 array of such tubes. In the mid 1950s analysers continued to keep pace with the major developments of storage systems for computers. Analysers with capacities ranging from 100channels to 256 channels were being developed in many laboratories and one design which became popular was that described by Schumann and McMahonb') which used magnetic core stores and a feedback pulse stretcher providing 256 channels. Since the encoding problem was not a major obstacle in time delay spectrometers, instruments with larger numbers of channels") were also developed during the mid 1950s. An interesting parallel development to core stores was magnetic tape storage for computer data . In an attempt to overcome the bulk and cost of large digital stores Cavanagh and Boyce") used a tape recorder to record individual pulse amplitudes, after stretching . These pulses were then played back, analysed and stored off line. This was intended to open up the possibility of multi-parameter systems of low cost . A more successful development was the use of digital magnetic tape storage of data on-line with analysis off-line in a computer") to obtain data from 31 detectors in a neutron scattering experiment . The information included time-offlight data with 9-bit accuracy having an overall 16 k channel capacity . A requirement of such systems is the temporary storage of a number of words in a buffer store so that the random bursts from the detectors could be fed to the magnetic tape as a smoother flow of data. Other solutions to this class of problems were being found and one that is typical of the use of

magnetic drums is the 4096 channel two parameter analyser described by Chase' 1 ) . In many multiparameter experiments short resolving times are essential in the situations where good statistical accuracy is required in the weaker peaks, especially if due attention is given to the cost of running large accelerators or other facilities for the experiment . An early attempt to provide large capacity magnetic core stores forthis class of work wasthe 20k channel system described by Goodman et al . 72 ). At the Harwell Symposium where this was reported there was a report of a digital magnetic tape storage system with 65k channel capacity for neutron timeof-flight and associated gama ray spectra"). Modern memories are so small in physical dimensions that it is possible to have 8k channels with 22 °counts per channel in a bench top instrument including the ADC and other ancillaries. The semiconductor industry forecasts that memories of this capacity may become economic in integrated circuit form in a few years. Indeed there are reports that an attempt is currently being made to provide this capacity on one chip although the store access times may not come below 100gs. In the light of these developments and the uncertain nature of other delopments such as magnetic bubble memories it is not surprising that there is a temporary slowing down of the rate of adaptation of new storage systems into nuclear pulse analysers. 8. Instrumentation systems In this section it is proposed to deal with a number of developments which are not closely identified with the functional sub-divisions discussed earlier. One of the important subjects is that of constructional practice. Rack mounting of equipment for laboratory use has been dictated by the availability of the hardware designed for other applications. They have not always been convenient. In order to accommodate the total equipment for a fast time-of-flight analyser with 30channels the author had to resort to a special 4-ft wide rack in 1944. Such an instrument would take up one or two printed circuit cards using modern integrated circuits. Owing to the variety and complexity of the tasks the development of nuclear instruments has been associated with considerable efforts in the design of the mechanical features of the assemblies . It is not a trivial matter to assemble an ionisation chamber and electrometer with a minimum current sensitivity of 10 -' 3 A in a small hand held instruXI11 . NUCLEAR DETECTOR DATA HANDLING

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ment. Nor is it easy to take signals from detectors near a reactor to instruments many tens of meters away . There has been a wealth of development of special insulators, capacitors, cables, shielding and it is not possible to isolate any of these here. One of the tangible results of instrument design in the nuclear field is the early use of modular systems and the painstaking documentation to enable such systems to be used in many laboratories. The history of these developments is not easy to trace but the use of the plug-in scale-of-two described by Elmore and Sands") is probably the earliest example where advantage was taken of the repetitive nature of parts of the equipment in order to simplify construction and ease problems of maintenance. Major developments in the various national laboratories which led to many important ranges of modular equipment started late in the 1950s although the benefits of the modular system were discussed59) in earlier papers.The disadvantages of these early systems were that they used extensive inte!connections between modules using sockets on the front panels which did not improve the accessibility or the appearance of complex assemblies. More significant however was that they were standardised on the basis of consensus on a national basis and therefore inhibited the commercial exploitation of these modules on an international basis. The first major modular system that gained some acceptance on an international basis was the NIM system which like previous modular systems aimed largely at equipment for experimental use. With increasing use of digital systems, some of which were closely associated with on-line computers, the design of a modular sytem primarily aimed at the rationalisation of these assemblies began in the late 1960s. As a result we now have the CAMAC system which not only finds its place in the nuclear laboratories but in many others where there is a need for comprehensive equipment which can be easily interfaced to computers. These combine high performance with neat front panel assemblies which are therefore also acceptable for routine control equipment in industrial environments. The measure of rationalisation that has been achieved is a tribute to the ingenuity and patience of those responsible . Many of the early designs of multichannel analysers were designed to have a console for easy access to the many controls and to provide room for the displays of the counting registers. Although the modern bench top versions are much more compact they contain more controls. The reasons

for these are partly technical although some of them are purely historical . The various switches for controlling the input and output configurations of the store including the display are gradually being simplified partly by the availability of cheap large stores but mainly by using microprocessors with simple control keys or buttons. Most analysers still use analogue signal processing methods and ADCs which require the optimisation of a number of apparently independant parameters with the consequent need for switches to be set by the user . The use of the proven time variant titters and optoelecironic charge neutralisation methods has brought about a reduction in the number and, partly as a result of the removal of human intervention, significant improvement in performance. Further improvements in this direction can be envisaged if advantage is taken of the more sophisticated semiconductor devices that are becoming available. The significance of the large size of the CRT and its supplies is well demonstrated in a recent design of a miniature 64 channel analyser'°) which uses a matrix of LEDs for its display which is only 21 x 13 x6 cm' and weights 1 .3 kg including batteries. The small size and particularly the low power consumption of modern equipment are such that limitations imposed by the performance of power supplies have been virtually eliminated since it is a relatively simple matter to filter these supplies. Thus it has been possible to make noise measurements') ranging from 0.4 nV/%/Hz up to 50 nV/ JHz over time constants from 1 us to 1000 us to an accuracy and long term stability of better than 1% . Similarly the stability of the line width and its independance from pick-up or line frequency or its harmonics is demonstrated by the ability to obtaine noise line widths less than 40 eV fwhm, with Si(Li) X-ray detectors for pulse shaping times from 200 to 1000us and with cryostats which are not insulated from the probe assemblies . A number of instances of the impact of nuclear instrumentation in general have already been cited. Some well known instruments of wide application were derived from multichannel analysers which employ the technique of storage of complex data and presentation of distribution functions . The ideas for the use of associative memories were being developed for multiparameter analysers alongside similar developments in computers. Smoothing of data such as for MSssbauer experiments or recording noisy waveforms are well known examples of lasting contributions. An inter-

NUCLEAR DETECTOR INSTRUMENTATION

esting through short-lived development was the use by Maeder's) of a CRT output on to photographic film to obtain an integrated spectrum in a truly analogue storage system . A more recent example of the expanding use of nuclear instrumentation technology is the application of energy dispersive X-ray spectrometers' 6) to environmental applications pioneered by Goulding and his co-workers . The importance of one of the activities of instrument designers cannot be overestimated. Whenever an attempt is made to improve an instrument there is a need to prove that the improvement is what would be predicted on the basis of the design . Furthermore in the use of any instrument there comes a time when a failure or suspected malfunction is of an unexpected nature. These situations require reliable and sophisticated test equipment and many such instruments have been developed in the past 40 years but the author can only pick interesting instances within his orbit of experience . The earliest and most useful piece of test equipment is the sliding pulse generator which is mentioned by Elmore and Sands"). A modern version of it was used to test the 8192-channel l ,us ADC mentioned earlier to the delight of the designer who was unaware of the existence of such a simple tester. In the course of the study of high speed pulse circuits in the late 1950s it was clear that the sampling oscilloscope then available in the laboratory, of the form described by McQueen in hisclassic paper") in 1952 was too bulky and lacked the reliability for the work in hand. This led to the development of an adaptor unit using transistors which could convert a standard low frequency laboratory oscilloscope into a sampling oscilloscope. This adaptor, using avalanche transistors and transistor mixers to obtain an effective bandwidth of 300Mc/s, was described by Chaplin1e) and formed the basis of the compact sampling oscilloscopes of later years. One item of test equipment of particular significance to nuclear instruments which has also been used in many otherapplications is the random pulse generator described by White"). This generates standard pulses with a true Poisson distribution in time at mean rates from 10 pulses per second to 6x 10' pulses per second only subject to the errors of the finite pulse length of the output. 9. Conclusions This small insight into the process of innovation in nuclear instrumentation shows how the community has made contributions to the main pro-

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grammes of research and development which it supports. Some of those involved in the earlier developments are now making contributions to other fields based on their experience in nuclear instrumentation . This may be partly the reason for the significantly fewer references quoted here covering the past decade but may also imply a lack of interest by the author in these developments . I suspect that in any case the stimulus provided by the wealth of developments of new solid state devices and technologies associated with communications, computers, defence and radar will as ever generate many new trends in nuclear instrumentation . References

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