High resolution spectrometry with nuclear radiation detectors

NUCLEAR

INSTRUMENTS

AND

METHODS

95

(197I) 289-300; ©

NORTH-HOLLAND

PUBLISHING

CO.

H I G H R E S O L U T I O N S P E C T R O M E T R Y W I T H NUCLEAR R A D I A T I O N DETECTORS K. KANDIAH

Electronics & Applied physics Division, A.E.R.E. Harwell, England

Received 4 March 1971 A high resolution spectrometer system which contains optoelectronic charge restoration at the detector, direct coupled signal amplification, active signal filters and a versatile analogue-todigital converter is briefly described. The performance of this system with semiconductor gamma ray and X-ray detectors, scintillation counters and proportional counters over a very wide

range of counting rates is described. Owing to the unique characteristics of this spectrometer the performance with every detector tested is believed to be substantially better than that hitherto reported for any other spectrometer. It is shown that the overall performance is limited solely by noise at the input and by detector characteristics even with the best available detector.

1. lwtroduetion The energy spectrum obtained with a radiation detector is often distorted by the signal amplifying and processing equipment. In some instances the distortion is priimarily due to the detector itself. Most of the current views on these problems are the natural result of the commonly used methods of amplification and signal processing which are derived from the basic ideas introduced by Lewis (1948) and others. Improved pulse shaping for the best signal to noise ratio was discussed by Den Hartog and Muller 1) and a pulse shaping system with good performance at moderate counting rates was described by MaederP). The above methods assumed that the problem of unidirectional charge flow in the detector can be solved by using a high value resistor for neutralising this charge and for the definition of the operating point of the input stage of the amplifier. Since the amplifiers used ac coupling many parts of the system contributed to their limitations. Kandiah 3) proposed charge neutralisation at the detector with an opto-electronic

device followed by direct coupled amplification and a new signal processor which gives nearly ideal performance with every type of detector. Most aspects of this system were presented and discussed at the Conference on Semiconductor Nuclear Particle Detectors and Circuits held in Gatlingburg in 19677). A practical form of equipment along these lines, with some preliminary results, was described by Kandiah et al.4). The maximum advantages of these proposals are obtained when they are adopted as a whole although limited improvements can be obtained in some situations with only opto-electronic charge neutralisation or by using the new signal processor alone. This paper presents some results obtained with the equipment previously described by the author. A variety of detectors has been used and some obvious effects closely associated with the detectors are discussed. It is demonstrated that, in most circumstances, the shortcomings, especially at high counting rates, are due to the detector and source geometry problems, apart from the predictable deterioration of the spec-

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1

290

K. K A N D I A H

trum due to finite levels of noise or background. The results presented here have been chosen primarily to illustrate these points and do not necessarily represent the best that can be obtained in a particular situation. For example the Si X-ray detector system reported in this paper gave a resolution (fwhm) of 120 eV and 170 eV on noise and Fe X-rays respectively during these

measurements. The same detector now gives a resolution of 80 eV on noise and 145 eV on Mn X-rays with a better FET.

2. Brief description of the system A fuller description can be found elsewhere4). A simplified block diagram is shown in fig. 1 and a

Fig. 2. Photograph of complete spectrometer assembly.

HIGH RESOLUTION

SPECTROMETRY

WITH NUCLEAR RADIATION

photograph of the complete equipment with a detector and cryostat assembly is shown in fig. 2. The equipment includes the following: 1. A preamplifier with opto-electronic charge neutralisation at the detector controlled by the signal processor. The charge neutralisation takes place after each signal has been processed or when an overload exists. This method does not show the count rate limitation or enhanced noise of the simple rate dependent dc feedback system adopted recently by Goulding et al. 2) recognised in a more recent paper by Landis et al.8). 2. Elimination of distortion caused by events in the detector overlapping one that is being processed. It will be shown later that the effectiveness of these safeguards is seriously limited in some cases by detector characteris~fics. One important feature of the system is that no event is accepted for processing unless it was preceded by a silent period, called the quiescent time, during which no event was recognised. 3. Signal processing using active filters in which the quiescent time following a recognised event is variable, independently of the processing time. The dead time is the sum of the processing and quiescent times. The memory of the processed signal level, after the dead time, is virtually zero. The dead time on rejected events can be made less than that necessary on accepted events. The dead time even for an accepted event is much less than in most known systems for a given signal to noise ratio6). 4. Facilities for inhibiting the operation of the system for a defined interval of time. The recovery to normal perfbrmance after being inhibited or after an overload signal is extremely rapid and is virtually independent of the overload magnitude or gate duration. This feature is intended to give optimum performance with pulsed sources which produce high levels of unwanted radiation. 5. A medium speed, high performance analogue-todigital converter, which has been fully described elsewhereS), with automatic spectrum stabilisation. Simple digital control of all ADC parameters is obtained either manually by means of the control box shown in fig. 2 or automatically by a computer that may be used with the system. 6. A simple design of a single channel analyser. This is a consequence of the signal processing method used which produces a rectangular pulse proportional to energy. Six such single channel analysers are contained in the equipment shown in fig. 2.

3. Signal processing All processes necessary to ensure that each output

DETECTORS

291

signal is unambiguously proportional to the energy of the associated particle are included in this broad heading. The major processes are recognition of the event, processing the signal for optimum signal-tonoise ratio, neutralisation of the charge at the detector and protection against pile up. 3.1. RECOGNITION AND PROTECTION The signals are recognised by means of a threshold discriminator following double delay line shaping. The delays of the lines are made independent of the processing time necessary for optimisation of signal-tonoise ratio. There are two limitations of such a simple recognition system. The first is that some events falling within the duration of the shaped pulse will be missed and thus lead to pile-up distortion of the spectrum. This situation can be improved if a zero-crossing discriminator is included and events are accepted only if the zero crossing time falls within a defined interval following the triggering of the threshold discriminator. This method also has disadvantages since smaller pulses falling in the period corresponding to the second half of the delay line shaped pulse will fail to be recognized. Simple considerations might lead to the use of short delay lines in order to recognise closely spaced events. Since this leads to increased noise in this recognition channel it results in a failure to recognise some small amplitude signals which occur during negative excursions of the noise (signal being assumed positive). Reduction of the threshold level of the discriminator in an attempt to improve this will lead to a high rate of triggering due to noise. The most difficult problem arises from the variable charge collection time in the detector. Two types of problems exist. In the first place there are small variations of collection time, in say a coaxial Ge-Li detector, depending on the position of the charge generated by the particle in the depleted region of the detector. In order to maintain high detection efficiency it therefore becomes necessary to tolerate a large variation in the delay time for the output of the zerocrossing discriminator. In this situation two events which produce fast rising pulses but occurring within the expected long collection time cannot be separately identified. Secondly, some of the particles will produce charge in the undepleted region of the detector. These events will cause charge flow into the depleted region with a characteristic rise time similar to the carrier lifetime of the material. Since this is usually long, these events may not exceed the discriminator threshold after shaping with a (relatively short) delay line. This

292

K. K A N D I A H

charge will then contaminate later events which are recognised and processed. 3.2. SIGNAL FILTFRING AND CHARGE NEUTRALISATION

One of the problems of active filters used here is that the output noise does not retain the simple stationary characteristic of the output of a passive filter. The definition of signal to noise becomes complex and the shape of a spectral line will change with counting rate when it is very high. The major advantage of an active filter is that the distortions associated with base line fluctuations in a conventional passive filter can be completely eliminated if direct coupling is used in all circuits other than those used for signal filtering. In order to do this effectively, charge neutralisation of any event at the detector must be independent of the magnitudes and rate of preceding events in the detector. The opto-electronic elements in the system used by the author are brought into conduction only during well defined intervals and are non-conducting at all other times and direct coupling is employed throughout. 4. Results

In the following sections some spectra obtained with a range of detectors under a variety of conditions are presented. The main aspects to be noted are: 1. energy calibration, linearity and dynamic range; 2. line widths as a function of energy at low and high counting rates; 3. distortions at high counting rates and the rate of occurrence of spurious peaks; 4. effects of rapidly changing rates and the gating of unwanted events. The counting rates quoted are the total event rates at the detector. The analysis rates are lower than this as a result of the processing time of accepted events, the quiescent time and the dead time of the ADC. The quiescent time employed in most cases was about twice the processing time. In some cases the effect of variations in the quiescent time were investigated and these are quoted where necessary. It is important to quote the signal processing time and the quiescent time since the usefulness of a system is not only dependent on the maximum counting rate that can be tolerated at the input but on the analysis rate when the input rate is high.

line shaping with a delay time suited to the decay characteristics of the scintillator. Since the current pulse, determined purely by the scintillator characteristics, is used directly there is no need for charge neutralisation at the detector. The energy resolution of the system and the behaviour at high input counting rates will depend purely on the relationship of the imposed quiescent time to the scintillator decay time. A long quiescent time will maintain good resolution even at high input counting rates with the appropriate reduction in the analysis rate of the system. Fig. 3 shows 6°Co gamma ray photo peaks at low and high counting rates obtained with a 2 inch Nal scintillator. The overall processor dead time used for these measurements was 3/~s. The lower curve A shows the spectrum obtained at any rate up to about l0 s counts/s whereas the upper curve B is the spectrum at 4 x 105 counts/s. The deterioration in peak/valley ratio at the high rate can be accounted for almost completely by the overlapping of charge from previous events. The recording rate in the multichannel analyser was limited by the A D C dead time. This limitation in analysis rate does not apply to the single channel analysers built into the system which can record the full output rate of the processor. This scintillation counter system will not be affected by rapid variations of counting rate or by fast gating against a high intensity of unwanted radiation provided that the photomultiplier does not show fatigue effects.

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Fig. 3. ~°Co gamma ray photopeaks with 2 in. NaI scintillator. A - Counting rates < 10S/sec; B - counting rate of 4 x 105/sec.

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4.2. GAMMA RAY SPECTRA WITH A Ge-Li DETECTOR

the system had been used for a number of different types of measurements in the meanwhile the virtue of automatic spectrum stabilisation is demonstrated by the consistency of the energy calibration. The spectra were plotted without applying any corrections. The

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DETECTORS

295

presence of some background lines due to 4°K and sources of °°Co and xaVcs in an adjacent room is seen on the low rate spectrum A. The energies can be read off directly to an accuracy of better than 500 eV at any point in the spectrum. The high rate spectrum B in fig. 4 shows a tailing off on the higher energy side of each line. This can be seen in greater detail in fig. 5 and fig. 6 which show the photo-peaks of the 6°Co spectrum taken with two different detectors and pre-amplifiers. It will be noticed that the lines break away from the continuum quite abruptly on the low energy side but have a tail on the higher energy side at high counting rates. It is also noticed that for a given count on the nominated peak the Compton rate is apparently higher. It is assumed that the broadening at the higher energy side of the lines and the apparent increased relative level of the continuum are due to the presence of very slow pulses caused by particle interactions in the undepleted regions of the detector. Since these are undetected by the threshold discriminator which used 200 ns pulse shaping, they contaminated recognised events and increased their pulse amplitudes. Evidence of this is

Fig. 7. Typical w a v e f o r m s at o u t p u t of opto-electronic head amplifier due to g a m m a rays in 40 c m 3 G e detector.

296

K. KANI)IAH

seen in the four photographs in fig. 7 of typical pulses at the output of the preamplifier. The first frame shows pulses which are all good. Slow pulses of varying magnitudes are seen at A, B, C, D and E in the other frames. The slow pulses at A and B would not contribute to any error in the measurements of the later recognised pulses which eventually caused a restoration of charge at the detector. Event C would have caused a small increase in the amplitudes of 3 recorded events. The rarer event D probably occurred just inside but very close to the edge of the depleted region of the detector with some of the primary charge being generated just outside the depleted region as shown appearing later at E. Thus the recorded amplitude would be smaller than that corresponding to the total charge generated by that event. Apart from the distortions in the spectrum at or near the main lines caused by very slow pulses, an important effect in some applications is the failure to recognize closely spaced events. As explained earlier, sophistication in such recognition systems gives limited improvements when the detector has a large variation in its collection time due to finite mobility of the charge carriers and has poor geometry. This problem is clearly more serious in a coaxial detector than in a planar one. The effect will be seen as increased counts at much higher energies where no single event can cause that amount of charge. Fig. 8 shows the effect in a coaxial detector in which the collection time for single events varied from under 100 ns to over 250 ns thus necessitating the use of 200 ns delay lines and integrating times in the recognition circuits. The counting rates in the 2A and 2B peaks are seen to be equal while that in the A + B peak is higher owing to the presence of

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true coincidences. Since the signal processing method has been designed to be insensitive to charge collection time within reasonable limits these sum peaks also have the appropriate line widths without further broadening. 4.3. PROPORTIONALCOUNTER It is inevitable that owing to the long collection time of the heavy ions the energy resolution of a proportional counter will be strongly dependent on the imposed quiescent time at high counting rates. The performance of a thin walled counter on Mn X-rays is summarized in table 1. The processing time was kept constant at 2 l~s for these measurements and the ADC dead time was 5 Its. These figures demonstrate the effect of quiescent time and show that a proportional counter can be used at high rates. However, the considerably better performance of Si or Ge detectors will make them preferable unless there are special problems connected with counter geometry, efficiency or relative sensitivity at different energies. TABLE I Resolution of proportional counter on Mn X-rays.

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for the amplifier have very good energy resolution. Detector window thickness can be small enough to give good transmission for X-rays of energies down to about 2 keV. These detectors have short charge collection time because of their planar geometry and high field. The depleted region of the detector can be wide enough to accommodate X-ray energies up to about 40 keV.

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RADIATION

297

DETECTORS

The opto-electronic charge restoration system is ideally suited to this application since it will not have the noise associated with the finite value of the feedback resistor nor need it have excess noise due to lossy dielectrics of the type first noted by Radekal°). G o o d photodiodes do not contribute additional noise charge referred to the input except that due to the increase in the total input capacitance. Since some silicon X-ray detectors have a capacitance less than 1 pF and conveniently have a negative bias applied to the H.T. electrode (collecting electrode being connected to the gate of the n-channel FET input stage of the amplifier) Goulding et al. 2) proposed the use of the FET for providing the return path for the detector mean current without adding further capacitance. This mean current was generated with a long time constant in the feedback circuit. Simple considerations will show that at high counting rates this will lead to spectral line broadening due to base line fluctuations if the time constant is short or overloading of the amplifier if the time constant is long. Further, there will be shot noise associated with this mean current. Some X-ray spectra obtained with a 12 m m 2, 2 m m thick Si detector with active opto-electronic charge restoration will now be discussed. Fig. 9 shows the Mn X-ray spectrum on the right with a linear count scale. On the left is seen the line due to noise. It is worth noting that the total counts on the peak of the Mn line is about 106. Our runs aimed at an accurate measurement of the line width typically had 105 counts on the peak. In all cases the threshold setting of the recognition circuits was such that the noise line was also recorded at a reasonable counting rate. In-

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spection of the shape of this line, which should ideally be Gaussian, will reveal the presence of any effects due to delayed charge flowing to the input associated with previous events in the detector or other causes. The width of this line also gives the electronic noise line width of the system. In all the major experiments the spectra were plotted with a logarithmic counts scale since this will reveal many aspects of the system which would not be apparent when a linear scale is used. To demonstrate this the whole of the Mn spectrum is shown on a log scale in fig. 10 covering energies up to 13 keV. Long delay lines (1 ~s) were used in the pulse recognition unit for these measurements in order to demonstrate the presence of sum peaks which are seen at 11.8, 12.3 and 12.8 keV. By measuring the intensity of the sum peaks in relation to the main peaks it is possible to deduce the time interval during which the recognition circuits fail to identify double pulses. In all the measurements that are reported below, the delay lines were 200 ns

long and the relative intensity of the sum peaks would be correspondingly lower. The low intensity fluorescence line of the Si and associated escape peaks in the detector are clearly seen in fig. 10. In order to demonstrate the ability to accept high input rates a number of fluorescence spectra were taken with an X-ray machine as the source. This provides a reliable method of varying the counting rate over a wide range without some of the usual source geometry problems. Fig. 11 shows the fluorescence spectrum of Fe at two counting rates with a processing time of 44/~s and a quiescent time of 70/~s. A larger fraction of the events will be accepted for analysis if shorter processing times are used but this will lead to a widening of the lines both at low and high counting rates. An example of the use of 2/~s processing time and 8 l~s quiescent time is shown in fig. 12. A summary of the behaviour of this detector with different processing and quiescent times is given in table 2. The deterioration at high rates is believed to be largely due to finite detector

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299

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TABLE 2 Line widths of Fe K~ X-rays.

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1.6 8.4 40 8.4 340 340 170 175 206 306 370 338 120 124 160 265 340 265 44 44 44 2 2 2 70 70 70 8 8 16 67 33 13 88 12 0.3

w i n d o w thickness a n d the presence o f u n d e p l e t e d regions in the detector. A d e m o n s t r a t i o n o f the clean spectra o b t a i n e d with a Si d e t e c t o r is given in fig. 13. I n the lower p a r t o f this figure two sections o f the N p s p e c t r u m are p l o t t e d on an e x p a n d e d scale in o r d e r to emphasise the ease with which lines which d o n o t have a G a u s s i a n shape are identified. The use o f such lines for calculations o f F a n o factor can lead to considerable error. The squares o f the line widths as a function o f energy over the range

1.74 keV to 34 keV are p l o t t e d in fig. 14. The F a n o factor for this d e t e c t o r is estimated to be 0.130. It is to be noted that the points for the K , lines o f M n and F e and the 13.9 keV line o f the Np, which consist o f unresolved pairs, lie a b o v e the m e a n line for F = 0.13. 5. Conclusions It is c o n c l u d e d that, with the signal processing m e t h o d s used in the system r e p o r t e d here, the perf o r m a n c e o f a s p e c t r o m e t e r is largely g o v e r n e d by the m a g n i t u d e s o f the noise o f the input stage and other fluctuations present at the input. In a large n u m b e r o f cases there are errors due to intrinsic detector characteristic which can be b r o a d l y t e r m e d slow charge collection. W i t h the current p e r f o r m a n c e o f these two c o m p o n e n t s the limitations o f the rest o f the spectrometer can be m a d e negligible. The p r o p o r t i o n a l counter used for the m e a s u r e m e n t s r e p o r t e d here was supplied by Dr. J. Leake and the Si

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Fig. 13. The complete spectrum of Np X-rays at moderate counting rates with Si detector. X - r a y d e t e c t o r by Mr. G. Knill. T h e a u t h o r is very grateful also to Messrs. A. Stirling, D. L. T r o t m a n a n d G. W h i t e for c o n s i d e r a b l e assistance d u r i n g the measurements.

x 10 4 I0

8

References

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6

=O.13

5

4 3 2

I O

5

i

I

IO 15 ENERGY

i

I

F

20

25

30

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Fig. 14. P l o t o f r e s o l u t i o n v e r s u s e n e r g y f o r Si d e t e c t o r .

1) H. den Hartog and F. A. Muller, Physica 13 (1947) 571. e) F. S. Goulding, J. T. Walton and D. F. Malone, Nucl. Instr. and Meth. 71 (1969) 273. 3) K. Kandiah, Radiation measurements in nuclear power (Institute of Physics and the Physical Society, London, 1966). 4) K. Kandiah, A. Stirling, D. L. Trotman and G. White, Intern. Symp. Nuclear electronics (Versailles, 1968). '~) K. Kandiah, A. Stirling and D. L. Trotman, lspra Nuclear electronics Syrup. (Euratom, 1969). ~) K. Kandiah, Semiconductor nuclear particle detectors and circuits, Nat. Acad. Sci. Publ. 15943 (1969) p. 544. 7) K. Kandiah and A. Stirling, ibid., p. 495. s) D. A. Landis, F. S. Goulding and J. M. Jaklvic, Nucl. Instr. and Meth. 87 (1970) 211. u) D. Maeder, Helv. Phys. Acta 21, no. 3 4 (1948) 174. 10) V. Radeka, Intern. Symp. Nuclear electronics (Versailles, 1968).