The use of the lithium-6 semiconductor sandwich spectrometer for the measurement of fast-neutron spectra

NUCLEAR INSTRUMENTS AND METHODS IO5 (1972) 397-411; © NORTH-HOLLAND PUBLISHING CO.

T H E USE OF T H E L I T H I U M - 6 S E M I C O N D U C T O R S A N D W I C H S P E C T R O M E T E R F O R T H E M E A S U R E M E N T OF F A S T - N E U T R O N S P E C T R A I.C. RICKARD Imperial College of Science and Technology, Exhibition Road, London, S. Ire". 7, England

Received 31 July 1972 In order to meet the stringent target accuracies for neutron spectrum measurements in fast reactor assemblies, the existing experimental methods must be refined and the sources of experimental error identified and, where possible, eliminated. The construction and properties of the lithium-6 semiconductor detector are discussed. The technique may be used to measure neutron spectra over the energy range (10 keV-6 MeV). Major sources of experimental error have been isolated, enabling improvements to be incorporated into the existing electronics system. The life-span of the detectors is severely limited by fast neutron radiation damage of the silicon diodes. The 6Li cross-

section has been remeasured to an accuracy of ~4% (one standard deviation). The method of background subtraction has been tested and found to be adequate. Fast neutron spectra have been measured in three ZEBRA assemblies in the energy range (10 keV-6 MeV) and their accuracies assessed. The experimental results are compared with results from competing experimental techniques and with the theoretical predictions made using current methods and data. It is concluded that the results provide a sufficient overlap with existing techniques to give an accurate measurement of the fast-neutron spectrum above 1 MeV.

P A R T I. T H E T E S T I N G A N D P R O V I N G O F T H E S P E C T R O M E T E R SYSTEM

1. Introduction Stringent target accuracies have been set for the measurement of fast-neutron spectra on the zero energy fast reactor ZEBRAS). The attainment of these target accuracies necessitates a systematic and searching study of all possible sources of experimental error. The lithium-6 semiconductor sandwich spectrometer has been used for the measurement of fast-neutron spectra in various laboratories, including ZEBRA2-5). Although several fast-neutron spectra had been measured with these devices, it was felt that no serious study had been made of the overall accuracy of the experimental method. In addition, doubt existed as to the validity of the background subtraction method. Consequently, a study was made of the electronics and the experimental method in an attempt to isolate the principal sources of error. A study was also made of the short lifetimes of the detectors. The accuracy of the technique is ultimately limited by uncertainty in the 6Li(n, ~) cross-section. For this reason a measurement of this cross-section was undertaken over the neutron energy range (150keV-3.9 MeV). Full details of this measurement will be reported elsewherer), but the results are shown in fig. 1 compared with previous measurements of the cross-section. The results were normalised to the recent measurements of Coates et al. 7) over the energy range (300 keV-600keV). It is interesting to note that the results of Gabbard DECE~R 1972

et al. s) which form the basis of the Pendlebury compilation 9) are in excellent agreement with the present results if renormalised to the more recent measurements of Coates et al. at 600 keV. The probable error on the new measurement is + 4 % (one standard deviation). The triton analysis technique has shown the possibility of extending spectrum measurements to energies of 10 keV and belowt7'~s). To prove this technique, a number of spectra were measured with a range of different spectrometers to test the reproducibility of the resultS. These measurements were compared with results from other experimental techniques to obtain an estimate of the experimental error. It is possible, using the triton analysis technique, to obtain a complete normalised spectrum measurement from one spectrometer over the energy range 10 keV-6 MeV. 2. Description of spectrometer

and electronics system The spectrometer is similar in construction to designs first suggested by Love and Murray 2) and Lee and Awcock3). The spectrometer employs two charged particle detectors, silicon surface barrier diodes, whose active faces are placed parallel to each other and are encased under vacuum. A layer of lithium-6 fluoride is deposited on the surface of one of these diodes. Fig. 2 shows an outline of the diode's construction.

397

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Neutrons are detected by means of the reaction In+tLi

similar methods of achieving the background measurement have been suggested, viz: 1) Using an essentially identical spectrometer which has no sensitive layer of 6LiF3). The principal disadvantage of this technique is the assumption of identical responses to silicon-neutron events from the two detectors. It is necessary to lay rigorous standards on the reproducibility of the spectrometer design (see appendix). 2) The alternate approach would be to redesign the spectrometer to incorporate a removable film. There are two main objections to this approach: (a) Any system of mechanically removing the film from between the diodes would be liable to damage the thin film. The film has to be kept thin to avoid the formation of double response peaks. (b) If the spectrometer were to be made completely

~ 4He+13T+Q,

Q = 4.787 MeV. The reaction products travel in opposing directions and are detected in the two diodes. Suitable manipulation of the pulses and a knowledge of the lithium-6 (n,e) cross-section leads to a measurement of the incident neutron spectrum. It is possible for protons and alpha particles from the (n, e) and (n, p) reactions in silicon to pass through the depletion layers of both diodes to form a time coincident background distribution. This background can be as high as 40% of the total count over certain energy ranges, and it is important to be able to obtain an accurate measurement of this contribution. Two IO'O

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demountable, there is evidence to suggest that frequent gassing and degassing of the detectors would shorten the diode lifetime and damage the film. For the above reasons, the background measurements were obtained using separate background detectors. As mentioned above, the validity of this approach has been questioned, and consequently, a first priority in spectrum measurements was to obtain consistent results from pairs of spectrometers having different background fractions. A complementary method of background subtraction is achieved by means of a two-dimensional accumulattion of data, by obtaining the distribution of (E~ + Et) VS E t (E, = alpha energy; E t = triton energy). Since the values of (Et)r,,x and (E,)ml, for a given neutron energy can be calculated, any counts outside of this range must be background counts. This allows an accurate check on the shape and normalisation of the background distribution. Extensive tests of the spectrometer system were carried out in the thermal column of the neutron

source reactor NESTOR19). From these straightforward thermal column measurements an understanding of the basic limitations of the spectrometer electronics system was achieved. Three distinct improvements were made to the detector electronics: 1) The incorporation of an improved pre-amplifier system to reduce the length of the detector leads to a minimum. 2) A fast pulse timing system to operate at coincidence resolving times of 100 ns and below. 3) A form of pulse pile-up rejection and base-line restoration to improve the resolution in the presence of high y-fluxes. Since measurements were required at the centre of the test region of the ZEBRA reactor, the only feasible manner in which the connecting leads between detector and pre-amplifier could be reduced was to design a pre-amplifier to fit into the ZEBRA fuel sheath immediately above the detector. Consequently, a fully transistorised pre-amplifier was designed which limits the necessary cable-length to about three inches. Using this configuration, a detector with a lithium

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fluoride coating of 80/~g/cm2 gives a fwhm for the triton peak of about 45 keV, compared with the 70 keV obtainable previously. To obtain fast timing, two discriminators were used. One of these, the high level discriminator, determines which pulses are to be counted, i.e. the level is determined by the minimum energy from the 6Li(n, ct) reaction. The second discriminator, the low level discriminator, is set just above the electronics noise level and provides a precise timing pulse (i.e. minimum discriminator time-walk). 50 ns delay-line differentiation on the signal to this discriminator ensures a precise leading edge (see fig. 3). The coincidence unit will be triggered if, and only if, AND gate (a) has already been triggered by the highlevel discriminator. The trigger from the low-level discriminator is delayed 20 ns to ensure that it is this signal which opens the AND gate and hence provides the necessary precise timing. This technique allows a coincidence resolving time of 20 ns to be employed. The effect of pulse pile-up from y-ray pulses (due to Compton interactions in the silicon) and between pulses from the 6Li(n, ct)T reaction has been reduced by a simple pile-up rejection circuit. The logic can be understood by considering the case of a signal capable of triggering the high-level discriminator which has not been preceded (for at least 1/is) by another pulse. The trigger from the discriminator opens the exclusive OR gate (b), providing a trigger for the A N D gate (a). 20 ns later, the timing pulse from the low-level discriminator opens the AND gate (a) to trigger the coincidence unit. Simultaneously, it triggers the Time Extender unit to close gate (b) for the following 1/~s. Any further pulses arriving within 1 #s of the first pulse will be rejected, but the Time Extender will cause the gate (b) to be closed for a further 1 #s. The logic described will detect pile-up from pulses up to 1 ps before the useful analogue pulse. Pulses arriving during the 1 kts in which the linear gate/integrator is open are detected by feeding the gate-on pulse from the linear gate/integrator to an AND gate (c) which can be triggered by the low-level discriminator. In the event of a second pulse arriving, an anticoincidence pulse is fed to the pulse-height analyser and the signal is rejected. 3. The efficacy of the electronics system Thermal column tests have shown that the system is capable of operating adequately with resolving times down to 20 ns. The pile-up of pulses from neutron events was reduced to negligible proportions. The case of resolution degradation is not so straightforward. The effect was studied in ZEBRA Core-9 using the

anti-pile device and the results obtained are shown in table 1. TABLE 1

The contribution of ),-rays to the detector resolution at different reactor power levels. Reactor power 0 5x102 1 x 103 2 x 108 4 x 108

),-flux 0 (assumed) 8R/h (-4-2) 16R]h (4-4) 32 R/h (4- 8) 65 R/h (4-16)

),-contribution 0 (assumed) 40keV 70 keV 120 keV 360 keV

The figures are not significantly better than those obtained during earlier measurements. In particular there exists a high energy tail to the distribution extending (in the worst cases) as high as 1 MeV in energy from the peak of the distribution. Although this tail constitutes only some 0.1-1% of the amplitude of the distribution peak, it is possible for this to contribute 20-50% to the measured spectrum at 1 MeV, depending on the power level and spectrum shape. The degree of y-ray pile-up was therefore investigated at each reactor power level. It was measured from the distortion of a test-pulse input. The spectrum was then corrected by resolution unfolding2°). If practicable, a second measurement was made with the power level reduced by a factor of three (and pile-up reduced by approximately a factor of nine). The results indicate that useful measurements can be achieved in gamma fluxes of 20 R/h and less. From this approach, the error in the spectrum can be deduced and accurate corrections applied. 4. The effect of neutron radiation damage The surface-barrier detectors have had a history of short life spans and have been notoriously difficult to manufacture. To investigate the causes of the limited detector life span, the reverse biased leakage-currents were measured regularly. The long term variation of the leakage-currents is plotted in fig. 4. Detector no. 070-324 remained unirradiated with fast neutrons over the period of interest ( ~ 8 weeks), and therefore provides a useful yardstick with which to judge any deterioration in the other detectors. The leakage currents remained effectively constant over the period of measurement. The behaviour of the irradiated diodes was not so straightforward. It was found that irradiation in

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ZEBRA with a fast neutron dose rate of approximately 3 x 1011 fast neutrons/h for periods up to 4 h produced large increases in the magnitude of the leakage current. The detectors recovered slightly after being removed from the reactor for a few hours. The leakage currents were measured at regular intervals during an irradiation and showed a monotonic increase with time (see fig. 5). That this increase was not due to any temperature drift (the in-core detector was cooled by a forced air current) can be seen from the fact that the leakage currents remained effectively constant before the reactor power was raised, and tended to drop slightly after the reactor was shut down. This increase in leakage-current was accompanied by a loss of resolution. In the example shown in fig. 5 the fwhm's* of the thermal peaks before and after the irradiation were 237 and 270 keV respectively. This change is * F w h m = full width at half maximum.

troublesome, but can probably be tolerated. More serious is the fact that another detector (SRD7A/N/13) exhibited electrical breakdown and became very noisy during a second period of irradiation in the reactor. The total integrated neutron dose was approximately 1.5x 1012 fast neutrons/cm 2. Dearnaley 21) has summarised the possible mechanisms for bulk radiation damage in silicon and the consequent changes in electrical properties. He gives a table of typical allowable exposures for such detectors and quotes a figure of 1012-1013 fast neutrons/cm 2 for the significant deterioration of surface barrier detectors. This has been confirmed by the present observations. The measurements have shown that the most probable cause of SRD7 diode failure is radiation damage during operation, and that the maximum allowable exposure to fast neutrons is around 1.5 x 1012 fast neutrons/cm 2. This is the equivalent of between two and four spectrum measurements of high statistical accuracy per detector.

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The limitations of the experimental technique were investigated and from these investigations it was possible to recommend improvements to the experinaental technique. As described in section 2, the electronics system has been improved to optimise the resolution and minimise pile-up effects. The finite lifetime of the silicon diodes has been shown to be a severe limitation. Although a similar detector based on the 3He(n,p) reaction would have a greater neutron sensitivity and hence a longer effective lifetime, experience has shown that such a detector pro-

duces overwhelming problems from the 7-background pulses. The larger Q-value of the 6Li reaction reduces such problems to manageable proportions. To minimise the effect of neutron damage in the diodes, the maximum 6LiF thickness should be used. For this reason a minimum of two experiments should be performed. The first, at a low reactor power level and with a 6LiF coating of ,~80/~g/cm 2 to cover the low energy neutron spectrum. The second experiment at a higher power level and a 6LiF coating of ~ 2 4 0 / l g / c m 2 should be designed to accrue sufficient counts to cover the high energy portion of the spectrum up to 10 MeV.

P A R T II. T H E M E A S U R E M E N T O F F A S T - N E U T R O N S P E C T R A I N Z E B R A ASSEMBLIES 8D, 8G A N D 8H 1. Introduction

The lithium-6 semiconductor sandwich detector was utilised for spectrum measurements in Z E B R A assem-

blies 8D, 8G and 8H. The results are compared with the proton-recoil measurements taken in assemblies 8D and 8G and with time-of-flight measurements obtained

404

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RICKARD

from assembly 8D. From the comparison a measure of the experimental error has been obtained. Spectra have been measured using both the sum and triton analysis techniques and the results have been compared with multi-group calculations over the energy range from 10 keV up to about 6 MeV. The Zebra 8 series experiments 23) were designed to provide integral results sensitive to key items of the nuclear data used in fast power reactor calculations. Core 8D had a unit cell containing plutonium, natural uranium, carbon and sodium. Core 8G had a composition and spectrum similar to that of a sodium cooled oxide fuelled power reactor with a cell containing PuO2/UO2, UO2 and sodium. Core 8G was a simple 235U-238U system with a spectrum sensitive to the inelastic scattering of 23Su.

2. The experimental method When possible, several detectors were used to span the energy range of interest, and measurements were made at a number of different reactor power levels. The latter was important in determining the degree of spectrum distortion which can be attributed to ~-pulse pile-up, and hence in determining an estimate of the experimental error. The detector and pre-amplifier were suspended from the top-cap of an empty fuel can placed in the centre of the reactor test region. The applied detector bias voltage was corrected for the voltage drop across the 2 MY2 bias resistors. The leakage currents of both diodes were monitored during the course of an experiment. For cases of severe neutron radiation damage, it was necessary to adjust the bias voltage during the irradiation. Two similar versions of the lithium-6 semiconductor sandwich detector (SRD7) were used in these experiments. The SRD7A version, built at A.E.R.E. Harwell, employs diodes with an active area of 1 cm 2 and a diode separation'S'of 1 mm. The alternative version, manufactured by 20th Century Electronics Ltd., has an active area of either 1 cm 2 or 2 cm 2, and a diode separation of 3.5 mm. A range of coating thicknesses from 40 pg/cm 2 to 240 #g/cm 2 was employed. The energy calibration is obtained from the detector response to thermal neutrons. The thermal neutron reaction produces a 2.73 MeV triton and a 2.05 MeV at-particle. The two pulses are added to give a 4.78 MeV analogue pulse. It is these three energies which allow a detector calibration. The background counter has a small Am-c~ source deposited on the face of one of the two diodes. The 5.47 MeV ~-particle energy can be

used as the reference energy for the background calibration. In deriving an exact energy calibration, an allowance must be made for energy losses in the gold and lithium layers. The possible errors on the assumed thicknesses of gold and lithium lead to uncertainties in the calculated energies of the order of _ 1.5%. The background distribution was measured using a similar spectrometer but with no lithium-6 coating. The shape of the background distribution is very sensitive to the depletion depth and the diode spacing. The magnitude of the background is sensitive to the thickness of the diodes. Two multi-channel analysers are necessary for a complete spectrum measurement since both the sum spectrum and the coincident triton spectrum are recorded simultaneously. The Argus 500 on-line computer was used as a one-parameter 512 channel analyser to record the sum spectrum and a TMC 400 channel analyser recorded the triton distribution. 3. The method of analysis 3.1. SUM TECHNIQUE The basic neutron spectrum is given by the relationship: q~(E) dE = N(E) dE

G(E) a(E)'

(1)

where N(E)dE is the number of neutron counts corresponding to an energy of E (MeV), G(E)dE is the geometrical efficiency of the detector22), a(E) is the 6Li(n, at) cross-section. This derivation is valid for neutron energies above 1.5 MeV. Below 1.5 MeV the finite resolution of the detector must be taken into account. N (E) = re, ~b(E) a (E) R (E' - E) dE',

(2)

where R is the response function of the detector. This equation is solved by forming the corresponding approximate matrix equation:

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This equation is solved by the iterative unfolding process described by Gold2°). If measurements are produced at two different power levels, a good estimate of the degree of pile-up can be obtained by comparing the low energy distributions, as mentioned in Part I. The thermal column counts distribution was found to give a poor description of the actual detector res-

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3.2. TRITON ANALYSIS The distribution of tritons obtained from the 6Li (n,~)T reaction can be written in the form:

ponse under reactor conditions. Unfolding the counts spectrum with this distribution did not reduce the discrepancies between the results obtained at high and low reactor power levels. This can be explained by observing the change in shape of the pulse-height distribution from a pulse generator as the reactor power is raised. Not only does the function broaden, but it also develops a high energy tail which can seriously distort the results below 2 MeV. Thus, the resolution chosen for the unfolding process should be the thermal column response function modified in the manner suggested by the response to a pulse generator. No accurate estimate of the size of this tail was obtained for the cores 8D and 8G but a careful measurement was made in assembly 8H. By modifying the response function in the manner described above, it was found that the unfolding process produced a much improved agreement between measurements made at different power levels. This emphasises the importance of recording the response to a pulse generator for each power level at which a measurement is to be made.

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IO2 TRITON ANALYSIS SUM TECHNIQUE

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than 600 keV becomes larger and more difficult to calculate. Apart from the uncertainties inherent in the unfolding process the major limitation on accuracy lies in the calculation of the response matrix. The calculation depends upon a knowledge of the geometrical efficiency as a function of energy, the resolution of the counter and the differential cross-section of lithium-6. The effect of uncertainties in the first two quantities is known to be small but the influence of the latter has not been determined.

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In analysing the results, only the distribution above a triton energy of 2.73 MeV is considered, since this will contain all the essential information and will have the added advantage of a negligible alpha particle background. Eq. (7) is solved by the Gold unfolding technique using a 5 0 x 5 0 calculated response matrix. The program used for analysing the results is basically the same as that written by Dr. M. G. Silk of A.E. R.E. Harwell 1°) after modification by Dr. P. J. Clements and the author. The matrix represents neutron. energies from 1 keV to 600 keV. Above 400 keV the results have tended to be oscillatory in nature and have been disregarded. Also, above 400 keV the effect on the triton spectrum of neutron events of energy greater

4. The experimental results The results of measurements in ZEBRA cores 8D, 8G and 8H are shown in fig. 6, 7 and 8. The results are compared with calculations made using the 2000 group collision probability code MURAL. The spectra were measured using both the sum and the triton analysis techniques which were normalised absolutely one to the other. There appears to be no gross discrepancies between theory and experiment in the range 10 keV-6 MeV. The experimental spectrum tends to fall below the theoretical spectrum in the

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TABLE2 Typical errors involved in the s u m t e c hni que i n ¼-lethargy intervals. Energy range (MeV)

Statistical

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Cross-section

4.72 - 6.06 3.68 - 4 . 7 2 a 2.87 - 3 . 6 8 2.23 - 2.87 a 1.74 - 2.23 a 1.35 - 1 . 7 4 1.05 - 1 . 3 5 0.821-1.05 0.639 - 0.821 0.498 - 0.639 0.388 - 0.498 0.302 - 0.388 0.235 - 0.302 0.183 - 0.235 0.143 - 0.183 0.111 - 0.143

4-2.2% 4-1.8% 4-1.4% 4-1.4% 4- 1.4% 4-1.0% 4-0.8% 4-0.7% 4-0.6% 4-0.5% 4-0.4% 4-0.3% :t:0.3% 4-0.3% 4-0.3% 4-0.2%

4-2.0% 4-3.0% 4-2.0% 4-1.2% 4-1.2% 4-0.7% 4-0.4% 4-0.2% 4-0.1% :t:0.1% 4-0.1% 4-0.05% 4-0.05% 4-0.05% 4-0.05% 4-0.05%

~10% 4- 8% 4- 4 % 4- 4 % 4- 4 % 4- 4 % 4- 4 % 4- 4 % 4- 3% 4- 2 % 4- 2 % ± 2% 4- 2 % 4- 2 % 4- 2 % 4- 2 %

Unfolding

-4- 1.0% 4- 1.0% 4- 1.0% 4- 1.0% -4- 2.0% 4- 2.0% 4- 3.0% 4-12.0% q- 10.0% 4-10.0% 4-15.0% 4-15.0% 4-15.0% -4- 15.0% 4-15.0% 4-20.0%

C a l i b r a t i o nb e rror

Total

4- 0.5% 4- 0.5% 4- 0.5% 4- 1.0% 4- 1.0% 4- 1.0% 4- 2.0% 4- 2.0% 4- 3.0% 4-10.0% 4-15.0% 4-15.0% 4-15.0% 4-20.0% 4-18.0% 4-15.0%

4-10% 4- 9 % 4- 4.8% 4- 4.5% 4- 4.7% 4- 4.7% 4- 5.4% 4-12.8% q- 10.9% 4-14.3% 4-21% 4-21% 4-21% 4-25% 4-24% 4-25%

a In these energy ranges art a d d i t i o n a l error o f 4- 5% m u s t be a d d e d i n the core 8D m e a s u r e m e n t s for the u n c e r t a i n t y in the pulse pile-up correction. b C a l c u l a t e d for a 15 keV shift in the p o s i t i o n o f the zero channe l a n d a -4- 1.5% u n c e r t a i n t y i n the ke V / c ha nne l .

408

I. C. R I C K A R D

range 150-300 keV but this is probably due to the inability of the detector to resolve the 250 keV resonance in the lithium-6 cross-section which is considerably narrower than the detector resolution. A separate measurement was made of the detector efficiency and this has been used to calculate the spectra. Calculations were also made with the cross-sections recommended by Pendlebury9). This analysis gave results which fell well below theory in the energy range 300-700 keV. Since proportional counter and time-offlight measurements are in reasonable agreement with theory, this can be taken as evidence that the present efficiency measurement gives an improved representation of the 6Li cross-section. Triton analysis provides a good estimate of the flux levels between 10 keV and 400 keV but has not properly resolved any of the resonances in this region. +o2

The reason for this failure is probably due to inadequacies in the unfolding technique. It is interesting to note how well triton analysis predicts the higher flux gradient at low energies in Core 8H compared to 8D and 8G. Reproducible results were obtained from different detectors having different background fractions. Fig. 9 shows 2 sets of results obtained from Core 8H. The two sets were normalised to the same integrated fission count. This was further evidence on the reliability of the method of background subtraction. Similar results have been obtained from the other spectrum measurements. 5. Sources of experimental error 5.1. CALIBRATIONERRORS The detector calibration requires a knowledge of the

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thickness of the lithium fluoride and gold layers. This leads to an uncertainty in the energy calibration of about + 1.5% in the value of (En+ Q).

SANDWICH

TABLE 3 Efficacy o f unfolding technique for 8H s p e c t r u m results.

U p p e r energy

5.2. STATISTICAL ERRORS In all but the highest energy ¼-lethargy groups these are less than + 5 %, and below 1 MeV they are usually negligible. The errors quoted in table 2 include an estimate of the contribution of statistical errors from the background fraction. 5.3. BACKGROUNDSUBTRACTIONERRORS Tests on the sensitivity of the results to a shift in the background calibration reveal a maximum sensitivity of about + 10% in the upper energy groups to a shift in the background calibration of 10%. In fact, the maximum discrepancy between the calibration of the foreground and background will be less than + 3%. Consequently, the maximum error attributable to the background has been placed at + 3%. Tests on the reproducibility of spectra obtained from different detectors indicate that the error from background subtraction is of the same magnitude or less than the corresponding statistical errors for each ~-lethargy group. 5.4. ERRORS ARISING FROM )~-PULSE PILE-UP As mentioned earlier, the formation of a high energy tail on the detector response function can lead to distortion of the measured spectrum. The magnitude of this tail is roughly proportional to the square of the reactor power level. Thus, a good estimate of its effect on the measured spectrum can be derived by observing the change in spectrum shape from an increase in reactor power level. A further check on the magnitude of this error is obtained by unfolding the spectrum using a response function derived from the convolution of the thermal response function and the experimentally determined response to mono-energetic pulses. This has been successfully attempted for core 8H measurements. Table 3 shows the predicted correction from the difference between measurements obtained at two power levels, and the actual correction obtained from resolution unfolding. From the above calculations it is estimated that in the present experiment the error from this source over the energy range (600 keV-1.5 MeV) is approximately :__+_10% and from (1.5-2.0 MeV) it is approximately :+3%. This error can be minimised by reducing the reactor power level. Above 2 MeV the results remain effectively unchanged after resolution unfolding, and

409

SPECTROMETER

1.35 1.05 821 639 498 388 302

Predicted correction

Correction p r o d u c e d by unfolding

-- 4.4% -28.5% -17.8% - 15.0% + 5.1% +13.9%

-- 3.5% - 15% -13.1% - 15.8% -16.0% +15.5%

MeV MeV keV keV keV keV keV

any changes can be traced to oscillations produced by poor experimental statistics. These errors can be reduced by further lowering the reactor power levels used for spectrum measurements. The effect appears to become significant for v-ray fluxes above 15 R/h and it is suggested that a measurement at a v-ray flux below 10 R/h should produce pile-up errors of less than 5%. 5.5. ERRORS ARISING FROM THE UNFOLDING TECHNIQUE

These can arise from both inaccuracies in the assumed detector response function and from errors caused by poor convergence of the solution. The major uncertainty in the formation of the triton analysis unfolding matrix derives from uncertainties in the shape of the differential cross-section. The magnitude of these errors is not known, but by comparing the results obtained by the triton technique with those obtained from the proportional counter method (whose errors are thought to be less than _+5%), X 2 fitting tests indicate that the error in ¼-lethargy groups is of the order of _ 15%. Whether or not these errors can be fully explained by inaccuracies in the unfolding matrix has not been determined but it is known that the treatment of low energy events by the present response matrix is inadequate. 5.6. SPECTRUM PERTURBATION The major perturbations to the spectrum caused by the presence of the detector material are due to neutron reactions in the material immediately surrounding the 6LiF layer. The principal reactions of interest are elastic scattering in the silicon and inelastic scattering in the stainless steel diode supports. Monte Carlo calculations indicate that this effect will have no significant effect on the measurement of reactor spectra

410

L C. RICKARD

but in the measurement of mono-energetic neutron spectra, the inelastically scattered neutrons will significaptly perturb the count-rate because of the rapidly increasing nature of the 6Li cross-section at low neutron energies. 5.7.

TABLE 4 Normal±sat±on o f the lithium-6 m e a s u r e m e n t s to proton-recoil m e a s u r e m e n t s over the energy range 400 k e V - 1 . 0 5 M e V (core 8D). I

U p p e r energy

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388 keV 498 keV 639 keV 821 keV 1.05 M e V

29.6 27.6 25.7 20.4 15.1

31.7 28.1 24.3 18.8 15.5

SRD7/PRC

ERRORS RESULTING FROM THE DETECTOR EFFICIENCY CORRECTION

It was estimated that with the new measurement of the detector efficiency, the lithium-6 cross-section is known relatively to ± 2 % over the energy range 300-600 keV; to ± 4 % from 600keV to 4 MeV; to ± 10% from 4 MeV to 7 MeV. These errors have been applied directly to the spectrum errors. Bunching of the experimental results will tend to slightly reduce the effects of local inconsistencies in the data. 6. Comparison of results with proton-recoil and time-of-flight techniques Time-of-flight and proton recoil measurements have been produced for core 8D 22'2a) and proton-recoil measurements only in core 8G. In core 8D a comparison between the triton analysis results and the proton recoil measurements indicates that the former are consistent with a standard error of ± 15 % in ¼-lethargy intervals. As can be seen from table 4, if the results are compared over the energy range (400keV-1.05MeV), agreement within experimental error is obtained between the proton-recoil and lithium-6 results. This indicates that it is now possible to produce sufficient overlap with proton-recoil and time-of-flight techniques to normalise the measurements of the spectrum above 1 MeV. The triton analysis is a good guide in obtaining an unambiguous normal±sat±on over the energy range I0 keV-4 MeV. This enables a further check on the high energy data by comparing the result of normal±sing to theory over the complete energy range with that of normal±sing to the proton-recoil measurements over the range 400 keV-1.05 MeV. 7. An assessment of the technique The purpose of the research was to fully assess the sources of experimental error associated with the lithium-6 spectrometer and to seek methods of improving the experimental technique. The technique is now capable of approaching the target experimental accuracies in the energy range (1.0-4 MeV) and will be capable of reaching these target accuracies at higher energies after an improvement of the basic lithium-6 cross-section data at these energies. The triton analysis technique has been shown to

0.93 0.98 1.06 1.08 0.97

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n S R D 7 = 6Li detector results. b P R C = proton-recoil results.

provide a useful method of normal±sing the high energy sum technique results to theory and to measurements from competing techniques. The main limitations in the technique arise from deficiencies in the unfolding technique and in the knowledge of the lithium-6 differential cross-section data. There is no reason, in principle, why the results should not have been extended to 1 keV and below. Preliminary efforts in this direction have been encouraging. The detectors have been shown to have a lifetime of only two spectrum measurements. Since the diodes are difficult and expensive to fabricate, this is a serious disadvantage. If the technique is to be applied further, a large and regular supply of detectors must be assured so that sufficient measurements can be made of each spectrum as a check on the experimental errors. The life-time can be increased by using spectrometers with a thick ( > 2 0 0 p g / c m 2) LiF layer. This will produce only a small worsening of resolution except in 7-fluxes of less than 10 Rad/h. In the absence of competing techniques, the lithium-6 semiconductor sandwich detector is capable of providing an estimate of the fast neutron reactor spectrum from 10keV to 6 MeV. From 10keV to 600keV the accuracy is approximately + 1 5 % in broad lethargy intervals. Between 600 keV and 2 MeV the principal source of error is thought to be due to pulse pile-up and lies between 5 and 10% (see table 2). Above 2 MeV the accuracy is limited by the uncertainties in the lithium-6 (n, ct) cross-section which are thought to be ~ ± 4 % and ± 10% respectively below and above 4 MeV. When measurements are available from competing methods, the technique has been shown capable of providing sufficient overlap in the energy range above 400 keV to give an unambiguous description of the high energy portion of the spectrum.

LITHIUM-6 SEMICONDUCTOR SANDWICH

Appendix Specification for manufacture of 6Li sandwich spectrometer. To ensure accurate background subtraction, the neutron and background spectrometers must be closely matched ~during manufacture. The following specifications were chosen to allow an accuracy on the background measurement of +_5% i.e. to reduce the error on the neutron spectrum to less than +_2%. 1) The diodes for both detectors should be manufactured from the same silicon crystal. 2) Diode leakage currents should be less than 1 #A on receipt. 3) The thickness of the diodes should not vary by more than 2%. 4) The active area of the diodes should be 100 mrn 2 +_3%. 5) The spacing between the diodes should be 1 mm +5%. Since no significant error due to background subtraction has been noted, it must be concluded that these specifications are sufficient to ensure an accurate background subtraction. The author wishes to express his thanks to Prof. P. J. Grant and Dr. C. B. Besant of the Nuclear Power Section, Imperial College, and to Dr. C. G. Campbell, Dr. J. E. Sanders and Dr. D. Jakeman of the Fast Reactor Physics Division, A.E.E. Winfrith, for their expert guidance. Sincere thanks are also due to many people at A.E.R.E. Harwell and A.E.E. Winfrith-in particular, to Mr. W. Abson and Mr. Awcock for provision of spectrometers, to Dr. P. J. Clements for his assistance and advice in the measurement of detector efficiency, to Mr. A. M. Broomfield for helpful discussions on spectrum measurements, to Mr. J. Marshall for assistance in computer programming and to Mr. R. G. P. Batt for the design of the electronic equipment.

SPECTROMETER

411

The author gratefully acknowledges the support of both Imperial College and the United Kingdom Atomic Energy Authority under the joint auspices of whom this work has been carried out.

References 1) A. M. Broomfield, W. J. Paterson and J. E. Sanders, AEEWM997 (1970). s) T. A. Love, R. B. Murray, J. J. Manning and H. A. Todd, Nuclear electronics 1 (IAEA, Vienna, 1962) p. 415. 3) M. E. Lee and M. L. Awcock, Neutron dosimetry I (IAEA, Vienna, 1963) p. 441. 4) S. K. I. Patterson and M. J. Stevenson, Proc. CEGB Conf. Radiation measurements in nuclear power (Sept. 1966). 5) H. Bluhm and D. Stegemann, Nucl. Instr. and Meth. 70 (1969) 141. 6) p. j. Clements and 2. C. Rickard, A E R E R7075 (1972). 7) M. S. Coates, G. J. Hunt and C. A. Uttley, A E R E Harwell Progress Report AERE PR/NP 18 ( 1970/1 ). s) F. Gabbard, R. H. Davis and T. W. Bonner, Phys. Rev. 114, no. 1 0959). 9) E. D. Pendlebury, AWREO-60/64 (1964). lo) M. G. Sowerby, B. H. Patrick, C. A. Uttley and K. M. Diment, AERE-R6316 (1970). 11) E. Fort, IAEA Conf. Nuclear data for reactors (Helsinki, 1970) Paper CN-26/72. 12) S. Schwartz, L. G. Stromberg and A. Bergstrom, Nucl. Phys. 63 (1965) 593. la) R. V. Babcock, AFSWC TR 61-67 (1961). 14) C. B. Murray and H. W. Schmitt, Phys. Rev. 115 (1959) 6. 15) F. C. Ribe, Phys. Rev. 103 0956). 16) j. B. Weddell and J. H. Roberts, Phys. Rev. 95 0954) 117. 17) C. Beets, S. De Leeuw and G. De Leeuw-Gierts, Proc. CEGB Conf. Radiation measurement in nuclear power (Sept. 1966). is) M. G. Silk, AERE-M2009 (1968). 19) I. C. Rickard, Ph.D. Thesis (Imperial College of Science and Technology, London University, 1971). 20) R. Gold, ANL 6984 (1964). 21) G. Dearnaley, Nucleonics 22 (1964) 78. 22) A. M. Broomfield and M. D. Carter, AEEW-M9005 (1969). sa) A. M. Broomfield, C. F. George, G. Ingram, D. Jakeman and J. E. Sanders, Proc. BNES Conf. Physics of fast reactor operation and design (June 1969).