Detector for neutron time-of-flight spectrometry with improved response to low energy neutrons

NUCLEAR INSTRUMENTS

AND M E T H O D S 57

(1967) 2 3 7 - 2 4 4 ; ~L~ N O R T H - H O L L A N D P U B L I S H I N G

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DETECTOR FOR NEUTRON TIME-OF-FLIGHT SPECTROMETRY WITH IMPROVED RESPONSE TO LOW ENERGY NEUTRONS* L. P. WISHARTt, R. PLATTNER + and L. CRANBERG

University of Virginia, Charlottesville, Virghda, U.S.A. Received 24 July 1967 Design and operating characteristics are described of a neutron detector based on coincident detection of proton-recoil scintillations. Large area, compact construction, and efficient operation at neutron energies in the range of tens of keV are major features.

A light output curve for proton energies down to 50 keV is reported, together with absolute photoelectric yields at several proton energies.

1. Introduction Detection of neutrons by means of proton-recoil scintillations has proved to be widely useful, particularly where fast signals are required for time measurement. Despite the many, even unique advantages enjoyed by this method, its usefulness has been limited largely to the energy range above 300 keV. Below 300 keV neutron energy, photomultiplier tube noise is a serious problem for detection of neutrons by protonrecoil. Two methods of suppressing it have been used in the past: 1. cooling the photomultiplier, and 2. viewing the scintillator with more than one photomultiplier and demanding coincidence between them. Because of the considerable technical complication involved in method 1, we decided to produce an improved version of a method 2 detector. Several groups have previously reported the use of

such detectors 1-4). Our detector uses as scintillator a -~" thick disc of NE 102 plastic 5) of 5" dia., to combine reasonable counting efficiency with good timing resolution. To view a scintillator of similar dimensions with good light collection efficiency, Adams et al. 4) used a tapered light guide of volume similar to the volume of the scintillator, and 2 photomultipliers of 2" dia. in coincidence, mounted at an angle of 20 '~ relative to each other. This arrangement has the disadvantages of a rather cumbersome shape and increased background from scattering and scintillations in the light guide. We therefore tried a scheme using four 2"

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* This work was supported in part by the National Science Foundation. + R. Plattner, Oak Ridge Technical Enterprises, Oak Ridge, Tennessee. t L. P. Wishart, U.S.A.

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photomultipliers in direct contact with the scintillator, eliminating the light guide completely (fig. 1). The photomultipliers are RCA type 8575 recently developed to provide exceptionally low noise and high photocathode efficiency.

observing the four anode pulses produced with a light pulser separately, on a sampling oscilloscope triggered by the pulser. The maximum relative delay necessary between any two tubes was 2 ns. Slow signals taken from the 9th dynodes of the two pairs of PM's diagonally opposite to each other are added immediately so that each of these two pairs effectively acts like one PM with a figure-8-shaped photocathode. Coincidence is required between the two pairs. The signals of all four PM's are also summed and fed to a multichannel analyzer to record the pulse height distributions. The single channel analyzers shown in the diagram were used as integral dis-

2. Electronic arrangement A block diagram of the electronics is shown in fig. 2. Fast signals are taken from the anodes of all four tubes and added in an impedance matching circuit as shown in the figure. Delay cables are inserted to compensate for differences in the transit times of the photomultipliers. The correct delay times were obtained by

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criminators only, and were set in such a way, that for the lowest signals accepted by the discriminator in the sum channel, either of the pairs of P M ' s had to receive at least one fourth of the total light produced in the scintillator. The coincidence resolution time was 125 ns.

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3. Operational characteristics Fig. 3 shows in the upper portion the background counts for 1 min without the coincidence requirement and in the lower part with coincidence switched on. Note that the vertical scale is logarithmic. The energy scale is such that the threshold corresponds to 1/18 of the peak of the 60 keV 24~Am g a m m a ray. The upper end of the single tube noise spectrum (ch. 64) corresponds to 10 keV gamma-ray energy (or approximately 100 keV neutron energy, as will become clear from the subsequent analysis, viz. fig. 7). The following numbers demonstrate the efficiency of single tube noise suppression: Count rate per second without coincidence 10000, with coincidence 45. These 45 cps are presumed to be due to background radiation impinging onto the scintilIator leading to true coincidence events. The chance coincidence rate measured with a fixed delay in one of the coinciding channels is

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less than one per second, so that the single tube noise is suppressed by a factor of more than 104. Fig. 4 shows the pulse-height spectrum obtained from the 60 keV g a m m a ray of Z4~Am. The earliest report of such a well-defined maximum in the pulse-height distribution from a plastic scintillator illuminated with low energy g a m m a rays appears to be that of Thomson6). The occurrence of such a maximum is not altogether obvious in view of the expectation that g a m m a rays will react with greatest probability with the low-Z materials of the scintillator via the Compton process, and indeed, it has been suggested 4) that the maximum is due to multiple Compton collisions. For scintillator volumes such as are under consideration here, however, this interpretation is by no means obviously correct. Thus, for 60 keV gamma-rays, the average energy loss in C o m p t o n collision is only 10%, O/ while the mean free path for with a maximum of 20/o, Compton collision is approximately 5 cm. A simpler and more convincing interpretation of the maximum is suggested by the fact that the cross section for photoabsorption in carbon is approximately three per cent of the Compton cross section for 60 keV photons. Estimates of relative probabilities of various event-

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sequences based on the above numbers suggest that the maximum observed is due predominantly to prompt photoabsorption events in carbon, and to such photoabsorption following one or two C o m p t o n scatterings, rather than to multiple C o m p t o n events. The peak-to-valley ratio of the 241Am spectrum is a sensitive indicator of the light collection efficiency of the detector. Our value of2.5:1 compares satisfactorily with the reported values of 3 : 1 for a detector using a 600

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single-photomultiplier°), and 2:1 for a coincidence type detector4). Fig. 5 shows the measured efficiency of our detector for neutrons between 50 and 360 keV. To obtain the curve we used the 7Li(p,n)VBe reaction as source of monoenergetic neutrons with proton energies varying from threshold v) (1.8807 MeV) to 2.5 MeV. The target was an evaporated layer of metallic lithium of 2.6 keV thickness for 2 MeV protons. Neutron energies were

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These interpretations of the spectra provide a basis for estimates of maximum proton recoil energy associated with the various pulse-height spectra. Thus, at higher energies, where the spectrum can be regarded in first approximation as due to single n-p events, a point half-way down the shoulder is assumed to correspond to a full-energy recoil. At the lowest energies, where the spectrum is regarded in first approximation as a total absorption spectrum, the pulse-height corresponding to the maximum of the curve is assumed to correspond to the maximum proton recoil energy. Fig. 7 shows our estimate of the response of NE 102 plastic scintillator to low energy protons on the basis of the foregoing interpretation of our pulse-height spectra. Also shown are a straight line drawn through the origin and a point corresponding to the peak of the pulse-height spectrum for the 60 keV gamma rays from 24~ Am, and the curves of Gettner and Selove ~~) and of Czirr, Nygren and Zafiratos ~z) for low-energy protons. Our data are in excellent agreement with those of Gettner and Selove down to their lowest energy of 140 keV, and in disagreement with those of Czirr et al. Fig. 8 shows the time spectrum produced by neutrons and gamma rays originating from a LiF target of 10 keV thickness bombarded with I. 895 MeV protons. The gamma line is due to the J~F(p,c~7)~60 reaction. The width of 4.4 ns fwhm is reduced approximately to 1.5 ns when a narrow side-channel window is used. The width of the neutron peak can be attributed to the target thickness, which gives a smear in neutron energy

calculated from the proton energies with the help of the tabulation of Langsdorf8). The proton energies Ep in MeV were determined from the N M R frequency measuring the field of the analyzing magnet. For every point on the efficiency curve we measured the response of our detector for a given number of protons impinging on the target, and immediately afterwards replaced the detector with a "long counter" of the type described by Hanson and McKibben 9) for comparison, assuming its efficiency was constant in our energy range. The average absolute efficiency of the "long counter" was determined with a Pu-Be source of known strength. Also plotted in fig. 5 is a theoretical curve based on single scattering from protons in the scintillator and zero bias~°). The pulse height spectra produced by the monoenergetic neutrons were also recorded. It is interesting to note the gradual change from the well-known flat distribution characteristic of single neutron-proton scattering (fig. 6a) to the peaked distribution (fig. 6c) as neutron energy decreases. This change can be interpreted in terms of a gradual increase of the probability for multiple n-p scattering events as neutron energy decreases. The mean free path of a 100 keV neutron in the plastic scintillator is 0.88 cm, which is about half of its thickness. It seems plausible, therefore, that the once-scattered neutron is scattered again, particularly since its mean free path decreases sharply with its energy, and that the spectrum takes on the shape of a full energy peak. co z 6O o r~"

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Let us assume our interpretation is correct that the pulse-height distributions for low-energy neutrons are, in first approximation, total absorption peaks. Let us also assume that only a small number of photoelectrons is being detected in any single event at low neutron energies. It then follows that the detailed shapes of the pulse-height distributions may be strongly influenced, even dominated, by the statistical fluctuations to be expected when the average number of photoelectrons is small. On the foregoing set of assumptions it seemed reasonable to try to fit the pulse-height distributions with Poisson functions corresponding to an average number, m, to be determined from the fitting process. (fig. 9). This number is presumably to be identified with the average number of photoelectrons released in a neutron interaction process. Table 1 summarizes the results of the fitting process for three neutron energies. Also shown in table 1 are numbers for the energy of an electron which produces the same light output in the scintillator as the neutron. These data are taken from the straight line in fig. 9, which represents the light output in the plastic scintillator for electrons, as previously determined (fig. 7). The final column in table 1 gives the quotient of the numbers in the other two columns in terms of numbers of photoelectrons per keV of electron energy - a number which measures the energy efficiency of the

detection system in absolute terms. Values for this quantity have been determined previously by a number of methods, and it is pertinent to compare them with ours. Meyerott e t a l . 13) measured the response of Pilot B scintillator ~4) to 1-10 keV photons using a R C A 6810A photomultiplier. They found a non-linear behavior with photon energy, with P = 0.3 at 5 keV and P = 0.45 at 10 keV, approaching approximately P = 0 . 5 for higher energies, using a method of analysis similar to ours. Pilot B and NE 102 appear to be very similar in all their properties. Only one correction has to be applied in comparing their results with ours, namely, for the 10% higher luminous efficiency of the 8575 cathode. Their results are therefore in good agreement with ours. Porter et ai.15) measured the response of NaI(TI) as well as of anthracene and Pilot B to low energy electrons. Using monoenergetic electrons with energies between 5 and 1000 keV, they obtain pulse height spectra with TABLE 1

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Fig. 9. Poisson-fits to three proton-recoil spectra (m = mean number in Poisson distribution). characteristic peaks. They relate their width to the statistics of the photoemission process in the phototube in the well known fashion16), which shows that the width is inversely proportional to the square root of R, the number of photoelectrons released. Thus they find P = 3 . 7 photoelectrons/keV for 12.4 keV electrons impinging onto NaI(TI). They also find some nonlinearity of the electron response in all scintillators below 1000 keV, so that the relative light efficiency NaI/Pilot B is, for example, 7:1 at 10 keV, 4:1 at 500 keV. Using the first figure, we get P = 0.5 photoelectrons/keV for Pilot B at 12.4 keV. Tentatively extrapolating Porter's data, this value would decrease for small electron energy, but probably still lies well within a factor of 2 of ours. Lynch 17) obtained values of P by measuring the timing uncertainty in the appearance of the Q,h photoelectron (Q is any fixed number) from a gammaexcited NaI(T1) crystal. Q depends sensitively on the total number R of the photoelectrons produced. Using 31.6 keV gammas and a 8575 tube he gets P = 8 . 7 photoelectrons/keV. Using Porter's value for relative light efficiency of Ne 102/NaI(TI) = 1 : 5 at 30 keV his results give P = 1.7 photoelectrons/keV. Finally we made a few observations with lower slow-

channel bias of the detector. Fig. l0 shows a spectrum of the 5.8 keV g a m m a line from 5SFe, taken with a sidechannel bias of 1/60 of 241Am. Because of the strong attenuation of the 55Fe line by the front cover of the detector, a hole of z"l It dia. was drilled at its center and the source placed in front of it. The left side of fig. 10 shows the spectra with source in place and without source, for equal times. To the right is the difference between the two. The position of the 55Fe peak is at 1/18 of the z a l A m peak, which indicates non linear response of our scintillator for very low energy g a m m a w Z < T U

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244

L . e . WISHARX et al.

rays, in qualitative agreement with findings by Meyerott ~3) and PorterlS), and indicates that our bias of 1/60 of the 60 keV 24~Am peak corresponds to approximately 2 keV g a m m a energy. If we adjust the electron response curve of fig. 7 to fit this result q u a n t i tatively, assuming linearity above 10 keV g a m m a ray energy, the values for P in the three cases of table 1 are changed to 0.5, 0.4, and 0.4 respectively, which improves the agreement with Meyerott ~4) a n d Porter 15).

5. Conclusions If we assume that at 47 keV n e u t r o n energy the mean n u m b e r of photoelectrons is a b o u t 3, we can expect to be able to use this detector with good efficiency for the detection of n e u t r o n s whose energies extend to well below this value. Thus, n e u t r o n s which m a y produce on average only 1 photoelectron, will produce the 2 or more photoelectrons which are necessary to trigger a coincidence 26% of the time. We may therefore expect an appreciable efficiency for the detection of n e u t r o n s down to 20 keV or less. This means that proton-recoil detection now overlaps very substantially the energy range in which detectors based on the t°B(n,c~,) reaction have had a n almost exclusive d o m a i n . The light-output curve and absolute photo-efficiencies reported in this paper represent a c o n t r i b u t i o n in a field in which there have been substantial discrepancies a n d give agreement in each case with sets of previously indicated values. T h a n k s are due to Mr. A. Z i n k for his skill and

assistance in the mechanical aspects of the work a n d to Mr. D. A. Hills for his help with the Van de G r a a f f accelerator.

References 1) L. Cranberg, R. K. Beauchamp and J. S. Levin, Rev. Sci. Instr. 28 (1957) 89. 2) R. Batchelor and J. H. Towle, Proc. Phys. Soc. A73 (1959) 193. 3) R. Batchelor, W. B. Gilboy, A. D. Purnell and J. H. Towle, Nucl. Instr. and Meth. 8 (1960) 146. 4) j. M. Adams, E. Barnard, A. T. G. Ferguson, W. R. McMurray and 1. J. Van Heerden, N ucl. Instr. and Meth. 34 (1965) 21. ~) Nuclear Enterprises Ltd., Winnipeg, Canada. ") D. B. Thomson, Phys. Rev. 129 (1963) 1649. 7) j. B. Marion, Rev. Mod. Phys. 33 (1961) 139. 8) A. S. Langsdorf, J. E. Monahan and W. A. Reardon, A tabulation of neutron energies from monoenergetic protons on lithium, ANL 5219 USAEC (1954). 0) A. O. Hanson and J. L. McKibben, Phys. Rev. 72 (1947) 673. 10) j. B. Marion and J. L, Fowler, Fast neutron physics 1 (Interscience, New York, 1963) ch. IIB. H) M. Gettner and W. Selove, Rev. Sci. Instr. 31 (1960) 450. le) j. B. Czirr, D. R. Nygren and C. D. Zafiratos, Nucl. Instr. and Meth. 31 (1964) 226. 13) A. J. Meyerott, P. C. Fisher and D. T. Roethig, Rev. Sci. Instr. 35 669 (1964). ~4) Pilot Chemical Co., Waltham, Mass. 15) F. T. Porter, M. S. Freedman, F. Wagner Jr., I. S. Sherman, Nucl. Instr. and Meth. 39 (1966) 35. t6) j. B. Birks, The theory and practice of scintillation counting (New York, 1964) p. 149. 17) F. J. Lynch, IEEE Trans. Nucl. Sci. NS-13, no. 3 (Tenth Scintillation and Semiconductor Counter Symposium, Washington, D.C., March, 1966) 140. Note added in proof: With a bias of 1/60 241Am, the detector has been shown by B. S. Chesnun to be capable of detecting neutrons of 14 keV energy.