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Review of multichannel and multiparameter analyser systems
NUCLEAR
INSTRUMENTS
AND METHODS
(1966) 240-247; ©
43
NORTH-HOLLAND
PUBLISHING
CO.
REVIEW OF M U L T I C H A N N E L AND M U L T I P A R A M E T E R ANALYSER SYSTEMS H. GU1LLON D~partement d'tf.lectronique g~n~rale, Service d'Instrumentation Nucl~aire, CEA, Centre d'Etudes Nucl~aires de Saclay, France
During the recent years, pulse height analysers were improved to a high degree, as well for performances as for refinements. No major difficulties arise for the multiparametric analysers
realisation if modest capabilities are requested. Nevertheless development of more universal instruments raises important problems for compactness and for memory capacity.
I. M U L T I C H A N N E L PULSE H E I G H T A N A L Y S E R S 1. General The main characteristics of pulse height analysers are : a. pulse height resolution; b. speed of analysis; c. dynamic range. a. Today, 0.1% pulse height resolution in the upper region of the dynamic range is currently expected : this implies 1000 storage channels and sufficient stability for the content stored in the thousandth channel to be significant, within normally specified operational conditions (mains supply variations of + 10%, ambient temperature variations of about + 5°C, and counting rates of up to several times 104 pulses per second). b. Analysis of a single pulse should take about as long as storage access, which is about 20/as or less, with the usual ferrite core memories. c. The dynamic range should be at least 100. It is generally accepted that the maximum linear amplifier i" I
input l
output is 10 V: analysis should therefore be possible from at the worst 100 mV upwards. Substantial improvements in a., b. and c. above have been achieved in recent years. It is moreover also important for the P H A to be able to deal with input pulse amplitude alterations due to pile-ups and baseline shifts and for it to be flexible and easy to use. Most modern units are equipped with devices improving the latter two aspects of design. The schematic of an analyser (fig. 1) consists essentially of an analog-to-digital converter and a store. In most cases, a linear gate is connected at the input of the converter, if the latter does not have a built-in gating function. Delay is usually introduced at the input, to allow the necessary waiting time for the acceptance or rejection of each pulse. Logic circuits co-ordinate the various functions, which in the main are gate locking during operation of the ADC, storage control and gate I I I I
1 lo.oy I IGo
A. D,C.
I f
I
I
f
Ii
I One channel
I
I
J
Logic selector
I I I. . . . . . . . . .
I i l
i i
External gating Pulse
height
Pulse
encoder
height analyser
I
!_1 '1
Amplitude - to - time
Clock
coding
converter"
and scaler
I
I_
WILKI
NSON
TYPE.
A. D,C.
Fig. 1. Logical diagram of multichannel PHA's. 240
I '°z:'°7' i i I
,.o,.
REVIEW
OF
MULTICHANNEL
AND
MULTIPARAMETER
opening under external or internal control (such as the one-channel selector shown in dotted lines). More sophisticated systems can of course include several additional logical functions. Frequently, the complete analyser is contained in a single unit, although there is a general tendency to house the storage separately, using a pulse height encoder as an independent unit, or in the form of a plug-in sub-unit (the memory is thereby made directly available to the various sub-assemblies of a multiparameter analyser). 2. Analog-to-digital conversion methods The ADC is the most critical part of an analyser: the general performances of the system depend chiefly on this unit, linearity and stability being also influenced by the gate. The various ADC systems used therefore merit discussion. Several methods of converting pulse heights into digital form are available, but only three are commonly used in nuclear physics. These are: 1. The Wilkinson methodt), which converts the pulse amplitude into time, by charging a capacitor to a voltage proportional to the height of the pulse and counting the output pulses of a clock-pulse generator during constant current capacitor discharge. 2. The staircase methodZ), which is basically similar to the Wilkinson method, except that capacitor discharge is discontinuous, and it is the number of equal discharge "steps" that is used to represent the input pulse height. 3. The successive approximation method. Unlike the two foregoing "indirect" methods (numbers of pulses or steps proportional to input pulse height), this method directly states the amplitude of the measurand in a given weighted code. Using the pure binary code, for example, it first brackets the input pulse height in the upper or lower half of the measurement range by testing whether it is more or less than a reference at rangecenter, then brackets it again in the lower or upper half of the portion indicated, by comparison with a second, central reference, and so forth in a succession of discrete steps. The first approach towards application of this method in pulse height analysis seems to be attributable to Mac Mahon3). It is however the Wilkinson method that is almost universally adopted. The staircase method, capable of producing equally good results, has seen a few applications. It is attractive because it uses a direct relation of the charge supplied by the input pulse to a certain "unit" charge and is independent of the frequency stability of a clock-pulse
ANALYSER
SYSTEMS
241
generator. However, as we shall see later, it is too slow a method to be generally considered. The third method does in fact have the advantage of being the fastest if a large number of channels is used : its coding time is proportional to the log of the maximum number of channels, whereas conversion time, in the case of the fi~'st two methods, is directly proportional to the number of the channel in which the pulse is stored. But applications based on the approximation principle are liable to substantial differential faults. This liability prevented application of the method in pulse height analysis for a long time. It is inherent to the principle itself: unlike the first two methods, which use an unbroken process throughout analysis, comparison according to this method is made by application of successive calibrated voltages which must be related by accurately determined ratios. It can be easily demonstrated that the accuracy required for maximum differential linearity faults of 1% on 256 channels is practically unobtainable, and quite out of the question if the number of channels is larger. Further, it is necessary for the stretcher voltage to be absolutely constant during the analysis phase. These difficulties have however been surmounted : by Franz and Schulz4), through application of an incremental method already used with single-channel analysers 5) and by Gatti et al.6), through an ingenious method which makes it possible to obtain a substantially uniform average channel width. My personal opinion is that the Wilkinson method will remain the strongest for a long time yet, not only because of its excellent health since 1956, but for the following reasons: Subject to careful design-study, all three methods can provide excellent stability and linearity. The basis for comparison is thus speed and secondarily, convenience. A preliminary remark concerning speed of switching must first be made: pulse height analysis inescapably involves the accurate switching of known electrical quantities. Switching time, in what we shall call the analog type, is much longer than in digital switching, because of the necessity of waiting for virtually total disappearance of transient conditions, whereas in digital switching, the operation may generally be regarded as complete, once the quantity O/ disregarding subsequent has been switched to 90/o, ringing. The Wilkinson method employs two analog switching operations: constant current turn-on to the capacitor and its turn-off. The first only need be considered, as the second occurs after completion of analysis. The lia-
242
H. GUILLON Wilkinson method, above a certain number of channels, which depends on the ratio: (analog switching time)/(scaler resolving time) and must therefore be estimated on the basis of experimental data. At the moment, we can produce PHA's of the Wilkinson type using a 100 Mc/s time-base. This gives us an average 5t~s dead time for 1000 channels. To produce an encoder of the approximation type possessing the same speed, each basic operation could be allowed no more than 5/(2 log2 1024) = ~o = 0.25/Ls, which would certainly represent a remarkable performance. The approximation method therefore holds the advantage above 1000 channels. Parallel improvements in analog and digital switching times naturally will not reduce this limit. It may however be said that the limit of the Wilkinson method is not yet reached: the final limit would in fact be determined by amplitude-time conversion jitter, or the minimum resolving time of the scalers. Recent experience has shown us that the time-base frequency of a Wilkinson analyser can be quite reasonably increased to 200 Mc/s. In conclusion, we may affirm that in view of its technological complexity, the approximation method is a rational choice only in applications involving 2000 channels or more, which are not at all usual.
(a)
m l
l
lllllllllllllllllllll[l
(b)
(c)
Lp, 24 23 22 2t 2o
Fig. 2. Analog-to-digital conversion methods, a. Wilkinson; b. Staircase; c. Successive approximation.
bility of the first switching operation is non-linearity at the beginning of the voltage ramp (fig. 2a), though this can be eliminatedT). Owing to the smooth process employed, the theoretical speed of the method is limited solely by the resolving time of the scaler counting the clock-pulses, which uses faster, digital switching. In the staircase method, we must reckon with an average of 2" analog switching operations for n binary digits, since the staircase, which is furnished by a diode pump, involves two switchings per step (fig. 2b). Finally, the approximation method (fig. 2c) involves 2n analog switchings for n bits, as each bit involves two analog operations (test and possibly, incrementation). Thus, as stated earlier, the staircase method is prohibitively long and cannot be considered. The approximation method is faster than the
3. Wilkinson type pulse height analyzers Let us therefore concentrate on the Wilkinson analyser, and take a look at the operation of its various sub-units. 3.1. DESIGN There are various pulse height encoder schematics, mainly differing by the logical arrangement design. Essential parts are the pulse stretcher, the voltage ramp generator and the coding scaler. 3.1.1. Pulse stretcher
The pulse analysed must be stored in the capacitor at maximum height, the instant at which relative random fluctuations are minimum. The instant in question may be determined systematically, if a means of accurate event timing is available: it may for example be defined by the crossover of a double differentiated pulse. In such case, the pulse is sampled at an instant intrinsically according to its shape, and independent of its heightS). This method has doubtless the merit of maximum accuracy in the determination of low amplitudes, but also the stated demerit of requiring accurate timing according to pulse shape.
REVIEW OF MULTICHANNEL AND MULTIPARAMETER ANALYSER SYSTEMS
Stretcher
..~
\
i
D2
'I II
D1 M
J Switching
r
I ~ ----
constant TI current
circuit
I I I I
I I. . . . . .
J
Fig. 3. Discharge current switching diagram. A further method consists of detecting the passage through m a x i m u m by some form of differentiation; in particular, a stretcher with feedback path will indicate with good accuracy the instant at which the timederivative of the pulse reverses, by cutting the feedback path, §2.4 of ref. is). This method is more flexible than the former, but some distortion may be expected in the case of small pulses, whose rate of voltage variation is slow. However, it has the merit of eliminating certain errors due to bias drifts, when used also to sense the complete discharge of the storage capacitor 7, 9, 10). 3.1.2. l/oltage ramp generator circuit This circuit includes a constant current generator and usually a switching circuit accurately defining the beginning of the linear discharge ramp. It is wise to separate the switching function, so as not to disturb the current generator. We employ the diode switching circuit described by Alexander and Robinson 11), which is simple and efficient.
Fig. 4. Effect of switching transient on the differential linearity: replacement of the fast diode 1N4244 (below), by the snap-off diode SSA550 (above) as DI in fig. 3. The 100 Mc/s clock frequency settles the horizontal scale to 0.2/~s/div. The discharge current is the one corresponding to 128 channels on the left, and to 256 channels on the right.
243
More or less elaborate constant current circuits have been described 1°'11): for industrial manufacture, our encoder will use the circuit described in respect of the linear gate, fig. 6 of ref. 18). Concerning transients at the beginning of the voltage ramp, it is important to stress that not only transistor T a and diodes DI and D 2 (fig. 3) are involved, but also transistor TI, which supplies a constant current. These four components must therefore be fast. Fig. 4 shows the effect obtained by replacing the fast diode D 1 by a snap-off diode. As stated regarding biased amplifiers § 2.2 of ref. 18), a threshold can be obtained by applying a calibrated voltage step at the lower electrode of the storage capacitor before the beginning of the ramp: the residual charge in the capacitor at the end of the operation is then rapidly released by a special circuit. Alternatively, the voltage step can be applied during the fall of the voltage ramp, at an instant determined by the scaler, according to a method devised by Kandiah12). It is thus possible to create two regions of interest, which may have different channel widths, by changing the scaling factor at the instant of application of the step~°).
3.1.3. Scaler Special care must be exercised in the design and construction of the coding scaler in order to prevent cyclic differential non-linearities, which consists mainly in odd-even effects. In this connection, at high frequency clock-pulse rates, a peculiarly important requirement is that the last pulse of the address advance pulse train must be well formedS). 3.2. RESULTS The time-base frequency of our pulse height analyser was 20 Mc/s and has recently been increased to 100 Mc/s. Initial results are excellent: 1. differential linearity faults lie between +0.5% and - 0 . 5 % over most of the channels and initial non-
Fig. 5. Differential linearity with 1024 channels and 100 Mc/s clock. Vertical scales are respectively 100 counts/div, on the left and 1000 counts/div, on the right (average number of counts 11700 per channel).
244
H. GUILLON
linearity can moreover be eliminated by adding a pedestal (fig. 5). 2. the standard deviation of conversion time jitter is about 1 nanosecond, whatever the spectral region, for 256 channels (fig. 6). The only modification made in the amplitude-time converter has been replacement of the D1 diode type. It may be assumed that this has not affected stability, for which the following figures have already been given (including the gate): lower level shift: 0.5 mV/°C; conversion slope stability: 2.5 x 10-4/°C. Mention of two other 100 Mc/s analysers will indicate the state of progress in PHA designs: 1. the 256-channel Nanolyzer, with a resolving time of about 3 #s (thanks to its thin-film magnetic memory), became an industrially produced unit about a year ago; 2. the 256-channel analyser recently described by Emmera), which also uses a thin-film magnetic memory of 750 ns store dead time, has differential non-linearity
100"/.
computed for 0"= Ins
I0 ns
Fig. 6. Probability density in one channel located at 70% of the dynamic range when the PHA works with 256 channels. This curve is obtained by sampling during a differential linearity test. The triangular form is due to the not yet phasecorrelated clock.
of as little as +__1.3% on about 99% of its channels. This is the only performance published so far.
REVIEW
OF MULTICHANNEL
AND
MULTIPARAMETER
ANALYSER
SYSTEMS
245
II. M U L T I P A R A M E T E R A N A L Y S E R S 1. General We may first observe that whereas "multiparameter analysis" is a clear concept, the term "multiparameter analyser'" is not. Is there a single unit in existence truly worthy of being defined as such? It is in any case certain that no instrument possessing the universality of the PHA is as yet available. Multiparameter experiments actually preceded the creation of special ad hoc facilities: obtaining the energy spectrum from detector A, while that from B appears in a certain window, with variation in steps of the position of the window, may already be regarded as a biparameter experiment, in which the first parameter is explicitly measured, and the second implicitly contained in the experimental results. On such a basis, it is relatively easy to get together sufficient equipment for the storage of 8 or 16 spectra of respectively 128 or 64 channels in a memory with a capacity of 1024 locations. We will thereby have created a b/parameter analyser of undoubted usefulness, but very elementary, considering some ambitious projects. As the complexity of experiments grows, that is to say, as the number of parameters increases, we find ourselves facing a major difficulty: the average number of bits contained in a "descriptor" grows in proportion to the number of parameters; thus, while the volume of acquisition equipment does not increase very substantially, storage capacity must be increased exponentially. The cost of storage capacity thus rapidly becomes a limiting factor and various time-consuming, unwieldy expedients become necessary, such as the recording of addresses on punched or magnetic tape. In most cases, storage capacity economy is therefore of major importance. It can be achieved by various means: 1. precise delimitation of the experiment : 2. adaptation of resolution according to the behaviour of the region explored 1o. 13); 3. use of an associative memory. The general block diagram of a multiparameter analyser shows that if the basic function units remain such as they are today and independent, the system becomes quite monstrous in both size and number of connections. Concentration is therefore indispensable, and will be obtained through technological progress in microminiaturization, and the creation of new multiplefunction units. Unfortunately, we see as yet no outline of the
solution to these problems of concentration and storage capacity (as was the case ten years ago, in the development of pulse height and time analysers): if some inkling of a solution to these two problems were available, there would be less debate concerning the respective merits and demerits of "hardware" and "software". It may be guessed that the multiparameter analyser of the future will be a cross-breed of the two.
2. Organization Having these considerations in mind, we can now consider the three overall functions, which as in the case of the pulse height analyser are coding, logic and storage. They are of course in the present case of considerably greater complexity. Encoding has already been adequately discussed. Regarding logic, the function may be considered in two parts. 1. the logic to which the experimenter has access, or "input logic"; 2. the internal logic. The first part is much more than an assembly of Boolean logic circuits, since it carries out a task of analog data filtering. This function is essential, because the physical significance of the analyser output is as much dependent on its design, as it is on the experimental conditions around a target or a radioactive source. Its output is a signal permitting analysis of the event a) if it satisfies all selected criteria and b) if analysis under acceptable conditions is possible. The criteria involved are several: Coincidence: General coincidence of the various time-parameter pulses carries the strong probability that all detectors have responded to the same nuclear event. Window: Fast pulse height discrimination for each detector shows whether the detected energy is approximately equal to a selected level. By the same token, a time analyser will produce an output signal if the occurrence appears during an interval of interest. External parameters: Certain restrictions can be applied to a number of implicit parameters. Refinement: A sophistication of the experiment consists of checking input signals for the qualities permitting "clean" measurement: absence of pile-up, absence of overload, etc. The internal logic of the system co-ordinates the operation of the various units by determining the instants of writing in storage, and by holding up encoding while writing is in progress and so forth. There
246
H. GUILLON
is therefore no need for this logic to be accessible to the experimenter. At the moment, storage is almost always of the ferrite core type, or a tape storage for the recording o f addresses (or more precisely, of "descriptors"). In the latter case, a buffer m e m o r y m a y be necessary, not only to regulate and improve packing density, but also to avoid limitation of the system's resolving time by the tape14). Triggered intermittent tape-feed is also used for the first purposetS). In the case of "descriptor" address recording, stripping is selective. As a rule, the tape is seldom completely stripped, owing to the a m o u n t o f time this would require and because in certain regions the data is poor and m a y be ignored*, while in others, resolution need not be high. An important facility in search stations is therefore the "digital window", which makes it possible to select the events possessing a parameter included between two digital limits (or a number of parameters within a corresponding set of limits). Evidently, the more elaborate the equipment, the more sophisticated the range of restrictions applied. Results are stored in ferrite core memories with a capacity of at least 1024 channels. 3. Industrial
systems
With the considerations of the first section of this chapter in mind, we reach the following conclusions concerning the equipment that industry can offer at reasonable prices: 1. really "multiparameter" analysers of limited scope * In this connection, it is worth mentioning that from the bulk of experimental data, it is frequently found that an inadverted parallel experiment has simultaneously been conducted, and unexpected results obtained. Number of parameters
2 2 2,3,4...
but undoubted value, possessing storage capacities of about 10000 locations maximum. These are in fact available; 2. equipments for more sophisticated experiments, strictly "tailored" for the purpose, owing to imperative reasons of economy. They are supplied as separate functional units, which can be assembled at m i n i m u m cost for a given experiment. In our personal experience, such a multiparameter analysis assembly can however still ultimately cost as much as an average computer. This second type o f facility is not strictly speaking a self-contained multiparameter analyser; we tend to call it a "multiparameter analysis assembly". These assemblies can represent excellent solutions of current problems in large nuclear research centers: with adequate organization, the basic function units can in fact be used with m a x i m u m efficiency f r o m both the economic and operational standpoints. Equipment costs upto one million dollar for a major experiment could become perfectly acceptable, if the design of the equipment is " m o d u l a r " (as described above), so that it can be used again in various configurations for a n u m b e r of other or smaller experiments. With the help o f the following table we can now briefly c o m p a r e a few well-kown industrial models; one of them, deriving from the biparametric analyser type 2 x 3, ref. ~6, ~7), works with descriptor recording. [ wish to thank Mr. J. D. Fraser for his care in translating these papers and for his helpful remarks and suggestions. I am particulary pleased to acknowledge the valuable collaboration of Mr. V. G o u r s k y who designed the whole o f the original works mentioned here.
Memory capacity
Memory system
1 600 words 4 096 words 4 096 words
Magnetic cores Magnetic cores Magnetic cores
10 000 words 16 384 words Maximum word length : 28 bits (extensible to 52 bits)
Magnetic cores Magnetic c o r e s Descriptor recording on magnetic tape
References
General ref. on PHA: R. L. Chase, ref. 7) and Proc. Int. Symp. Nucl. Electr., Paris, 1963 (OECD, Paris, 1964) 275. General ref. on MPA" L. J. Lidofsky, Proc. Conf. Monterey (1963) 1.
Remarks 40 x 40 to 2 x 800 channels 64 x 64 channels - 5 t~s full memory cycle - possibility of associative memory Modular system Large choice of input plug-in units - Modular system - Display: 4096 channels
1) D. Wilkinson, Proc. Cambridge Phil. Soc. 46 (1950) 508. 2) H. Guillon, J. Phys. Radium 14 (1953) 128. 3) j. p. Mac Mahon, Proc. Int. Symp. Nucl. Electr. 1, Paris, 1958 (IAEA, Vienna, 1959) 291. 4) K. Franz and J. Schulz, Proc. Conf. Monterey (1963) 172.
REVIEW OF M U L T I C H A N N E L AND M U L T I P A R A M E T E R ANALYSER SYSTEMS 5) E. Gatti and F. Piva, Nuovo Cimento 10 (1953) 984. 6) C. Cottini, E. Gatti and V. Svelto, Proc. Int. Symp. Nucl. Electr., Paris, 1963 (OECD, Paris, 1964) 309. 7) R. L. Chase, Proc. Conf. Grossinger (1962) 79. 8) T. L. Emmer, IEEE Trans. Nucl. Sci. NS-12 (1965) 329. 9) A. F. Arbel, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) 3. 10) V. Goursky and H. Guillon, Proc. Int. Syrup. Nucl. Electr., Paris 1963 (OECD, Paris, 1964) 313. l l) T. K. Alexander and L. B. Robinson, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) 173.
247
12) K. Kandiah, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) I I. 13) j. Thenard and G. Victor, Nucl. Instr. and Meth. 33 (1964) 33. 14) y. Amram, H. Guillon and D. Tandardini, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) 91. is) F. H. Wells, J. G. Page and A. Lewis, Proc. Conf. Grossinger (1962) 69. 16) y. Aram, H. Guillon and J. Thenard, Proc. Conf. Nucl. Electr. 2, Belgrade 1961 .(IAEA, Vienna, 1962) 73, 85 and 101. XT) A. Boucherie, Proc. Int. Symp. Nucl. Electr. Paris, 1963 (OECD, Paris, 1964) 409. 18) H. Guillon, these proceedings, p. 230.
INSTRUMENTS
AND METHODS
(1966) 240-247; ©
43
NORTH-HOLLAND
PUBLISHING
CO.
REVIEW OF M U L T I C H A N N E L AND M U L T I P A R A M E T E R ANALYSER SYSTEMS H. GU1LLON D~partement d'tf.lectronique g~n~rale, Service d'Instrumentation Nucl~aire, CEA, Centre d'Etudes Nucl~aires de Saclay, France
During the recent years, pulse height analysers were improved to a high degree, as well for performances as for refinements. No major difficulties arise for the multiparametric analysers
realisation if modest capabilities are requested. Nevertheless development of more universal instruments raises important problems for compactness and for memory capacity.
I. M U L T I C H A N N E L PULSE H E I G H T A N A L Y S E R S 1. General The main characteristics of pulse height analysers are : a. pulse height resolution; b. speed of analysis; c. dynamic range. a. Today, 0.1% pulse height resolution in the upper region of the dynamic range is currently expected : this implies 1000 storage channels and sufficient stability for the content stored in the thousandth channel to be significant, within normally specified operational conditions (mains supply variations of + 10%, ambient temperature variations of about + 5°C, and counting rates of up to several times 104 pulses per second). b. Analysis of a single pulse should take about as long as storage access, which is about 20/as or less, with the usual ferrite core memories. c. The dynamic range should be at least 100. It is generally accepted that the maximum linear amplifier i" I
input l
output is 10 V: analysis should therefore be possible from at the worst 100 mV upwards. Substantial improvements in a., b. and c. above have been achieved in recent years. It is moreover also important for the P H A to be able to deal with input pulse amplitude alterations due to pile-ups and baseline shifts and for it to be flexible and easy to use. Most modern units are equipped with devices improving the latter two aspects of design. The schematic of an analyser (fig. 1) consists essentially of an analog-to-digital converter and a store. In most cases, a linear gate is connected at the input of the converter, if the latter does not have a built-in gating function. Delay is usually introduced at the input, to allow the necessary waiting time for the acceptance or rejection of each pulse. Logic circuits co-ordinate the various functions, which in the main are gate locking during operation of the ADC, storage control and gate I I I I
1 lo.oy I IGo
A. D,C.
I f
I
I
f
Ii
I One channel
I
I
J
Logic selector
I I I. . . . . . . . . .
I i l
i i
External gating Pulse
height
Pulse
encoder
height analyser
I
!_1 '1
Amplitude - to - time
Clock
coding
converter"
and scaler
I
I_
WILKI
NSON
TYPE.
A. D,C.
Fig. 1. Logical diagram of multichannel PHA's. 240
I '°z:'°7' i i I
,.o,.
REVIEW
OF
MULTICHANNEL
AND
MULTIPARAMETER
opening under external or internal control (such as the one-channel selector shown in dotted lines). More sophisticated systems can of course include several additional logical functions. Frequently, the complete analyser is contained in a single unit, although there is a general tendency to house the storage separately, using a pulse height encoder as an independent unit, or in the form of a plug-in sub-unit (the memory is thereby made directly available to the various sub-assemblies of a multiparameter analyser). 2. Analog-to-digital conversion methods The ADC is the most critical part of an analyser: the general performances of the system depend chiefly on this unit, linearity and stability being also influenced by the gate. The various ADC systems used therefore merit discussion. Several methods of converting pulse heights into digital form are available, but only three are commonly used in nuclear physics. These are: 1. The Wilkinson methodt), which converts the pulse amplitude into time, by charging a capacitor to a voltage proportional to the height of the pulse and counting the output pulses of a clock-pulse generator during constant current capacitor discharge. 2. The staircase methodZ), which is basically similar to the Wilkinson method, except that capacitor discharge is discontinuous, and it is the number of equal discharge "steps" that is used to represent the input pulse height. 3. The successive approximation method. Unlike the two foregoing "indirect" methods (numbers of pulses or steps proportional to input pulse height), this method directly states the amplitude of the measurand in a given weighted code. Using the pure binary code, for example, it first brackets the input pulse height in the upper or lower half of the measurement range by testing whether it is more or less than a reference at rangecenter, then brackets it again in the lower or upper half of the portion indicated, by comparison with a second, central reference, and so forth in a succession of discrete steps. The first approach towards application of this method in pulse height analysis seems to be attributable to Mac Mahon3). It is however the Wilkinson method that is almost universally adopted. The staircase method, capable of producing equally good results, has seen a few applications. It is attractive because it uses a direct relation of the charge supplied by the input pulse to a certain "unit" charge and is independent of the frequency stability of a clock-pulse
ANALYSER
SYSTEMS
241
generator. However, as we shall see later, it is too slow a method to be generally considered. The third method does in fact have the advantage of being the fastest if a large number of channels is used : its coding time is proportional to the log of the maximum number of channels, whereas conversion time, in the case of the fi~'st two methods, is directly proportional to the number of the channel in which the pulse is stored. But applications based on the approximation principle are liable to substantial differential faults. This liability prevented application of the method in pulse height analysis for a long time. It is inherent to the principle itself: unlike the first two methods, which use an unbroken process throughout analysis, comparison according to this method is made by application of successive calibrated voltages which must be related by accurately determined ratios. It can be easily demonstrated that the accuracy required for maximum differential linearity faults of 1% on 256 channels is practically unobtainable, and quite out of the question if the number of channels is larger. Further, it is necessary for the stretcher voltage to be absolutely constant during the analysis phase. These difficulties have however been surmounted : by Franz and Schulz4), through application of an incremental method already used with single-channel analysers 5) and by Gatti et al.6), through an ingenious method which makes it possible to obtain a substantially uniform average channel width. My personal opinion is that the Wilkinson method will remain the strongest for a long time yet, not only because of its excellent health since 1956, but for the following reasons: Subject to careful design-study, all three methods can provide excellent stability and linearity. The basis for comparison is thus speed and secondarily, convenience. A preliminary remark concerning speed of switching must first be made: pulse height analysis inescapably involves the accurate switching of known electrical quantities. Switching time, in what we shall call the analog type, is much longer than in digital switching, because of the necessity of waiting for virtually total disappearance of transient conditions, whereas in digital switching, the operation may generally be regarded as complete, once the quantity O/ disregarding subsequent has been switched to 90/o, ringing. The Wilkinson method employs two analog switching operations: constant current turn-on to the capacitor and its turn-off. The first only need be considered, as the second occurs after completion of analysis. The lia-
242
H. GUILLON Wilkinson method, above a certain number of channels, which depends on the ratio: (analog switching time)/(scaler resolving time) and must therefore be estimated on the basis of experimental data. At the moment, we can produce PHA's of the Wilkinson type using a 100 Mc/s time-base. This gives us an average 5t~s dead time for 1000 channels. To produce an encoder of the approximation type possessing the same speed, each basic operation could be allowed no more than 5/(2 log2 1024) = ~o = 0.25/Ls, which would certainly represent a remarkable performance. The approximation method therefore holds the advantage above 1000 channels. Parallel improvements in analog and digital switching times naturally will not reduce this limit. It may however be said that the limit of the Wilkinson method is not yet reached: the final limit would in fact be determined by amplitude-time conversion jitter, or the minimum resolving time of the scalers. Recent experience has shown us that the time-base frequency of a Wilkinson analyser can be quite reasonably increased to 200 Mc/s. In conclusion, we may affirm that in view of its technological complexity, the approximation method is a rational choice only in applications involving 2000 channels or more, which are not at all usual.
(a)
m l
l
lllllllllllllllllllll[l
(b)
(c)
Lp, 24 23 22 2t 2o
Fig. 2. Analog-to-digital conversion methods, a. Wilkinson; b. Staircase; c. Successive approximation.
bility of the first switching operation is non-linearity at the beginning of the voltage ramp (fig. 2a), though this can be eliminatedT). Owing to the smooth process employed, the theoretical speed of the method is limited solely by the resolving time of the scaler counting the clock-pulses, which uses faster, digital switching. In the staircase method, we must reckon with an average of 2" analog switching operations for n binary digits, since the staircase, which is furnished by a diode pump, involves two switchings per step (fig. 2b). Finally, the approximation method (fig. 2c) involves 2n analog switchings for n bits, as each bit involves two analog operations (test and possibly, incrementation). Thus, as stated earlier, the staircase method is prohibitively long and cannot be considered. The approximation method is faster than the
3. Wilkinson type pulse height analyzers Let us therefore concentrate on the Wilkinson analyser, and take a look at the operation of its various sub-units. 3.1. DESIGN There are various pulse height encoder schematics, mainly differing by the logical arrangement design. Essential parts are the pulse stretcher, the voltage ramp generator and the coding scaler. 3.1.1. Pulse stretcher
The pulse analysed must be stored in the capacitor at maximum height, the instant at which relative random fluctuations are minimum. The instant in question may be determined systematically, if a means of accurate event timing is available: it may for example be defined by the crossover of a double differentiated pulse. In such case, the pulse is sampled at an instant intrinsically according to its shape, and independent of its heightS). This method has doubtless the merit of maximum accuracy in the determination of low amplitudes, but also the stated demerit of requiring accurate timing according to pulse shape.
REVIEW OF MULTICHANNEL AND MULTIPARAMETER ANALYSER SYSTEMS
Stretcher
..~
\
i
D2
'I II
D1 M
J Switching
r
I ~ ----
constant TI current
circuit
I I I I
I I. . . . . .
J
Fig. 3. Discharge current switching diagram. A further method consists of detecting the passage through m a x i m u m by some form of differentiation; in particular, a stretcher with feedback path will indicate with good accuracy the instant at which the timederivative of the pulse reverses, by cutting the feedback path, §2.4 of ref. is). This method is more flexible than the former, but some distortion may be expected in the case of small pulses, whose rate of voltage variation is slow. However, it has the merit of eliminating certain errors due to bias drifts, when used also to sense the complete discharge of the storage capacitor 7, 9, 10). 3.1.2. l/oltage ramp generator circuit This circuit includes a constant current generator and usually a switching circuit accurately defining the beginning of the linear discharge ramp. It is wise to separate the switching function, so as not to disturb the current generator. We employ the diode switching circuit described by Alexander and Robinson 11), which is simple and efficient.
Fig. 4. Effect of switching transient on the differential linearity: replacement of the fast diode 1N4244 (below), by the snap-off diode SSA550 (above) as DI in fig. 3. The 100 Mc/s clock frequency settles the horizontal scale to 0.2/~s/div. The discharge current is the one corresponding to 128 channels on the left, and to 256 channels on the right.
243
More or less elaborate constant current circuits have been described 1°'11): for industrial manufacture, our encoder will use the circuit described in respect of the linear gate, fig. 6 of ref. 18). Concerning transients at the beginning of the voltage ramp, it is important to stress that not only transistor T a and diodes DI and D 2 (fig. 3) are involved, but also transistor TI, which supplies a constant current. These four components must therefore be fast. Fig. 4 shows the effect obtained by replacing the fast diode D 1 by a snap-off diode. As stated regarding biased amplifiers § 2.2 of ref. 18), a threshold can be obtained by applying a calibrated voltage step at the lower electrode of the storage capacitor before the beginning of the ramp: the residual charge in the capacitor at the end of the operation is then rapidly released by a special circuit. Alternatively, the voltage step can be applied during the fall of the voltage ramp, at an instant determined by the scaler, according to a method devised by Kandiah12). It is thus possible to create two regions of interest, which may have different channel widths, by changing the scaling factor at the instant of application of the step~°).
3.1.3. Scaler Special care must be exercised in the design and construction of the coding scaler in order to prevent cyclic differential non-linearities, which consists mainly in odd-even effects. In this connection, at high frequency clock-pulse rates, a peculiarly important requirement is that the last pulse of the address advance pulse train must be well formedS). 3.2. RESULTS The time-base frequency of our pulse height analyser was 20 Mc/s and has recently been increased to 100 Mc/s. Initial results are excellent: 1. differential linearity faults lie between +0.5% and - 0 . 5 % over most of the channels and initial non-
Fig. 5. Differential linearity with 1024 channels and 100 Mc/s clock. Vertical scales are respectively 100 counts/div, on the left and 1000 counts/div, on the right (average number of counts 11700 per channel).
244
H. GUILLON
linearity can moreover be eliminated by adding a pedestal (fig. 5). 2. the standard deviation of conversion time jitter is about 1 nanosecond, whatever the spectral region, for 256 channels (fig. 6). The only modification made in the amplitude-time converter has been replacement of the D1 diode type. It may be assumed that this has not affected stability, for which the following figures have already been given (including the gate): lower level shift: 0.5 mV/°C; conversion slope stability: 2.5 x 10-4/°C. Mention of two other 100 Mc/s analysers will indicate the state of progress in PHA designs: 1. the 256-channel Nanolyzer, with a resolving time of about 3 #s (thanks to its thin-film magnetic memory), became an industrially produced unit about a year ago; 2. the 256-channel analyser recently described by Emmera), which also uses a thin-film magnetic memory of 750 ns store dead time, has differential non-linearity
100"/.
computed for 0"= Ins
I0 ns
Fig. 6. Probability density in one channel located at 70% of the dynamic range when the PHA works with 256 channels. This curve is obtained by sampling during a differential linearity test. The triangular form is due to the not yet phasecorrelated clock.
of as little as +__1.3% on about 99% of its channels. This is the only performance published so far.
REVIEW
OF MULTICHANNEL
AND
MULTIPARAMETER
ANALYSER
SYSTEMS
245
II. M U L T I P A R A M E T E R A N A L Y S E R S 1. General We may first observe that whereas "multiparameter analysis" is a clear concept, the term "multiparameter analyser'" is not. Is there a single unit in existence truly worthy of being defined as such? It is in any case certain that no instrument possessing the universality of the PHA is as yet available. Multiparameter experiments actually preceded the creation of special ad hoc facilities: obtaining the energy spectrum from detector A, while that from B appears in a certain window, with variation in steps of the position of the window, may already be regarded as a biparameter experiment, in which the first parameter is explicitly measured, and the second implicitly contained in the experimental results. On such a basis, it is relatively easy to get together sufficient equipment for the storage of 8 or 16 spectra of respectively 128 or 64 channels in a memory with a capacity of 1024 locations. We will thereby have created a b/parameter analyser of undoubted usefulness, but very elementary, considering some ambitious projects. As the complexity of experiments grows, that is to say, as the number of parameters increases, we find ourselves facing a major difficulty: the average number of bits contained in a "descriptor" grows in proportion to the number of parameters; thus, while the volume of acquisition equipment does not increase very substantially, storage capacity must be increased exponentially. The cost of storage capacity thus rapidly becomes a limiting factor and various time-consuming, unwieldy expedients become necessary, such as the recording of addresses on punched or magnetic tape. In most cases, storage capacity economy is therefore of major importance. It can be achieved by various means: 1. precise delimitation of the experiment : 2. adaptation of resolution according to the behaviour of the region explored 1o. 13); 3. use of an associative memory. The general block diagram of a multiparameter analyser shows that if the basic function units remain such as they are today and independent, the system becomes quite monstrous in both size and number of connections. Concentration is therefore indispensable, and will be obtained through technological progress in microminiaturization, and the creation of new multiplefunction units. Unfortunately, we see as yet no outline of the
solution to these problems of concentration and storage capacity (as was the case ten years ago, in the development of pulse height and time analysers): if some inkling of a solution to these two problems were available, there would be less debate concerning the respective merits and demerits of "hardware" and "software". It may be guessed that the multiparameter analyser of the future will be a cross-breed of the two.
2. Organization Having these considerations in mind, we can now consider the three overall functions, which as in the case of the pulse height analyser are coding, logic and storage. They are of course in the present case of considerably greater complexity. Encoding has already been adequately discussed. Regarding logic, the function may be considered in two parts. 1. the logic to which the experimenter has access, or "input logic"; 2. the internal logic. The first part is much more than an assembly of Boolean logic circuits, since it carries out a task of analog data filtering. This function is essential, because the physical significance of the analyser output is as much dependent on its design, as it is on the experimental conditions around a target or a radioactive source. Its output is a signal permitting analysis of the event a) if it satisfies all selected criteria and b) if analysis under acceptable conditions is possible. The criteria involved are several: Coincidence: General coincidence of the various time-parameter pulses carries the strong probability that all detectors have responded to the same nuclear event. Window: Fast pulse height discrimination for each detector shows whether the detected energy is approximately equal to a selected level. By the same token, a time analyser will produce an output signal if the occurrence appears during an interval of interest. External parameters: Certain restrictions can be applied to a number of implicit parameters. Refinement: A sophistication of the experiment consists of checking input signals for the qualities permitting "clean" measurement: absence of pile-up, absence of overload, etc. The internal logic of the system co-ordinates the operation of the various units by determining the instants of writing in storage, and by holding up encoding while writing is in progress and so forth. There
246
H. GUILLON
is therefore no need for this logic to be accessible to the experimenter. At the moment, storage is almost always of the ferrite core type, or a tape storage for the recording o f addresses (or more precisely, of "descriptors"). In the latter case, a buffer m e m o r y m a y be necessary, not only to regulate and improve packing density, but also to avoid limitation of the system's resolving time by the tape14). Triggered intermittent tape-feed is also used for the first purposetS). In the case of "descriptor" address recording, stripping is selective. As a rule, the tape is seldom completely stripped, owing to the a m o u n t o f time this would require and because in certain regions the data is poor and m a y be ignored*, while in others, resolution need not be high. An important facility in search stations is therefore the "digital window", which makes it possible to select the events possessing a parameter included between two digital limits (or a number of parameters within a corresponding set of limits). Evidently, the more elaborate the equipment, the more sophisticated the range of restrictions applied. Results are stored in ferrite core memories with a capacity of at least 1024 channels. 3. Industrial
systems
With the considerations of the first section of this chapter in mind, we reach the following conclusions concerning the equipment that industry can offer at reasonable prices: 1. really "multiparameter" analysers of limited scope * In this connection, it is worth mentioning that from the bulk of experimental data, it is frequently found that an inadverted parallel experiment has simultaneously been conducted, and unexpected results obtained. Number of parameters
2 2 2,3,4...
but undoubted value, possessing storage capacities of about 10000 locations maximum. These are in fact available; 2. equipments for more sophisticated experiments, strictly "tailored" for the purpose, owing to imperative reasons of economy. They are supplied as separate functional units, which can be assembled at m i n i m u m cost for a given experiment. In our personal experience, such a multiparameter analysis assembly can however still ultimately cost as much as an average computer. This second type o f facility is not strictly speaking a self-contained multiparameter analyser; we tend to call it a "multiparameter analysis assembly". These assemblies can represent excellent solutions of current problems in large nuclear research centers: with adequate organization, the basic function units can in fact be used with m a x i m u m efficiency f r o m both the economic and operational standpoints. Equipment costs upto one million dollar for a major experiment could become perfectly acceptable, if the design of the equipment is " m o d u l a r " (as described above), so that it can be used again in various configurations for a n u m b e r of other or smaller experiments. With the help o f the following table we can now briefly c o m p a r e a few well-kown industrial models; one of them, deriving from the biparametric analyser type 2 x 3, ref. ~6, ~7), works with descriptor recording. [ wish to thank Mr. J. D. Fraser for his care in translating these papers and for his helpful remarks and suggestions. I am particulary pleased to acknowledge the valuable collaboration of Mr. V. G o u r s k y who designed the whole o f the original works mentioned here.
Memory capacity
Memory system
1 600 words 4 096 words 4 096 words
Magnetic cores Magnetic cores Magnetic cores
10 000 words 16 384 words Maximum word length : 28 bits (extensible to 52 bits)
Magnetic cores Magnetic c o r e s Descriptor recording on magnetic tape
References
General ref. on PHA: R. L. Chase, ref. 7) and Proc. Int. Symp. Nucl. Electr., Paris, 1963 (OECD, Paris, 1964) 275. General ref. on MPA" L. J. Lidofsky, Proc. Conf. Monterey (1963) 1.
Remarks 40 x 40 to 2 x 800 channels 64 x 64 channels - 5 t~s full memory cycle - possibility of associative memory Modular system Large choice of input plug-in units - Modular system - Display: 4096 channels
1) D. Wilkinson, Proc. Cambridge Phil. Soc. 46 (1950) 508. 2) H. Guillon, J. Phys. Radium 14 (1953) 128. 3) j. p. Mac Mahon, Proc. Int. Symp. Nucl. Electr. 1, Paris, 1958 (IAEA, Vienna, 1959) 291. 4) K. Franz and J. Schulz, Proc. Conf. Monterey (1963) 172.
REVIEW OF M U L T I C H A N N E L AND M U L T I P A R A M E T E R ANALYSER SYSTEMS 5) E. Gatti and F. Piva, Nuovo Cimento 10 (1953) 984. 6) C. Cottini, E. Gatti and V. Svelto, Proc. Int. Symp. Nucl. Electr., Paris, 1963 (OECD, Paris, 1964) 309. 7) R. L. Chase, Proc. Conf. Grossinger (1962) 79. 8) T. L. Emmer, IEEE Trans. Nucl. Sci. NS-12 (1965) 329. 9) A. F. Arbel, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) 3. 10) V. Goursky and H. Guillon, Proc. Int. Syrup. Nucl. Electr., Paris 1963 (OECD, Paris, 1964) 313. l l) T. K. Alexander and L. B. Robinson, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) 173.
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12) K. Kandiah, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) I I. 13) j. Thenard and G. Victor, Nucl. Instr. and Meth. 33 (1964) 33. 14) y. Amram, H. Guillon and D. Tandardini, Proc. Conf. Nucl. Electr. 2, Belgrade, 1961 (IAEA, Vienna, 1962) 91. is) F. H. Wells, J. G. Page and A. Lewis, Proc. Conf. Grossinger (1962) 69. 16) y. Aram, H. Guillon and J. Thenard, Proc. Conf. Nucl. Electr. 2, Belgrade 1961 .(IAEA, Vienna, 1962) 73, 85 and 101. XT) A. Boucherie, Proc. Int. Symp. Nucl. Electr. Paris, 1963 (OECD, Paris, 1964) 409. 18) H. Guillon, these proceedings, p. 230.