A new type of neutron spectrometer in the energy range 1–100 keV

N U C L E A R I N S T R U M E N T S AND METHODS

74 (I969)

256-260; © NORTH-HOLLAND

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A NEW TYPE OF N E U T R O N S P E C T R O M E T E R IN THE ENERGY RANGE 1-100 keV C. MARONI, F. RUSSO and E. VERONDINI

Istituto di Fisica dell' Universitd, Bologna, Italy and Istituto Nazionale di Fisiea Nucleare, Sezione di Bologna, Italy Received 3 June 1969 A neutron spectrometer using the 6Li(n,c03H reaction is described, in which the energy difference between the outgoing particles is recorded. A resolution of about 40 keV on the energy difference allows to obtain resolution as low as 1 keV on the neutron energies.

Several types of neutron spectrometers have been developed in recent years1-8). With the exception of the time-of-flight technique, the most widely used methods are rather simple in principle: the neutrons interact with a nucleus in a suitable target, causing the emission of one or more charged particles which can be detected. If only charged particles are produced in the reaction the neutron energy E, can be deduced from the sum of their energies minus the Q-value of the reaction. Using positive Q-value reactions and detecting the emitted particles in coincidence helps to discriminate against interfering background. 6Li(n,~z)3H (O 4.78 MeV) and 3He(n,p)3H (O = 0.764 MeV) are commonly used in commercial solidstate sandwich spectrometers*. The overall resolution response of such spectrometers to monoenergetic neutrons is more or less 100 keV for the 3He filled type and 300 keV for the LiF sandwich. Therefore their working range is limited to neutron energies =

* E.g. Ortec 525 System.

En > 500 keV. An improvement in resolution is possible (with corresponding reduction in efficiency) decreasing the thickness of the target until the energy loss fluctuations due to different paths inside, become lower than the intrinsic resolution of the detectors ( ~ 20 keV). The fwhm of the sum pulses spectrum is then not less than around 30 keV for incident monoenergetic neutrons. It is well known that important problems concerning the behaviour of fast reactors are connected with the knowledge of the low energy part (below 100 keV) of the neutron spectrum. As shown in fig. 1 for 6Li(n,~)3H reaction, the energy resolution of any "sum" spectrometer cannot allow a good precision in this energy range. In this paper a new principle of solid state spectrometer is described which may be employed in a collimated beam geometry, with keV resolution on the neutron energy. in this design, which is basically a solid-state

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6Li(n,~)3H sandwich, instead of the sum 2; of the eand 3H energies, their difference A is recorded when the particles are emitted at 0 ° and 180° with respect to the neutron direction of flight. In this kinematical situation the sensitivity of A to the neutron energy is much better than that of 27, as shown in fig. 2. Therefore, with the same efficiency and resolution on the detected particles, more precise information on the neutron energy E. can be obtained. Fig. 3 shows the spectrometer employed during measurements done in order to check the provisions and that are described below. A thin (about 350 A) 6LiF layer has been vacuum evaporated on one of the counters. If En ~ Q, the angle between e and 3H directions is very near 180°. In the assembly shown in fig. 3 two independent informations on the neutron energy spectrum can be simultaneously recorded, corresponding to the two different kinematical situations : * T h e n e u t r o n flux was p r o d u c e d by the A r g o n a u t R e a c t o r o f the L a b o r a t o r i o di Ingegneria N u c l e a r e (Montecuccolino, Bologna).

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0(~)= 180° a n d 0(3H) --- 0 °, (the angles are measured with respect to the direction of the incident neutron), with maximum angular spread of about 15°. These two situations can be distinguished for instance by checking the sign of the difference pulse. Fluctuations on the energy difference are mainly due to the following causes: 1. The final products energies are 0-dependent. 2. The energy loss of the a-particle is strongly dependent on the target depth at which the reaction takes place (this effect is negligible for 3H). 3. The 20 keV intrinsic energy resolution of the counters leads to a uncertainty on A of about 30 keV. The overall resolution on A is in our case about 40 keV, as shown in fig. 5, where the A spectrum for thermal neutrons is reported*. This value corresponds to a resolution on neutron energy of about 0.4 keV, which is approximately 2 orders of magnitude better than sum spectrometers resolutions.

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Fig. 3. Picture of the spectrometer and preamplifiers.

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Owing to the high Q-value, in the neutron energy range below 100 keV, the final products energies are nearly independent of E n, which therefore does not affect the resolution on A and for this reason the value quoted for thermal neutrons if valuable over the whole range of energies. Fig. 4 shows the calculated resolving power on En at different neutron energies. The blockdiagram of electronics is sketched in fig. 6. To obtain this good energy resolution, in the preliminary version of the spectrometer with only two counters, it was necessary to accept a rather poor

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Fig. 6. Block-diagram of the electronics (simplified). detection efficiency (about 5 x 1 0 - 7 for thermal neutrons). Nevertheless this figure may be improved more than an order of magnitude without loss in resolution simply by using larger area detectors and 6LiN targets (where the energy loss of c~-particles is lower). It is worthwhile to point out that the spectrometer performances reported above have been obtained by the use of standard components (detectors and preamplifiers). By using selected and cooled detectors 9) and very low-noise preamplifiers the resolution could be improved. Further developments of an instrument of this type are in the direction of hodoscopized sets of solid state counters.

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The authors are indebted to Prof. B. Ferretti for his suggestion about the method, to Dr. Placci for his initial work, to the SNAM group for reactor facilities, and to Mr. G. Busacchi for his technical help.

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References 1) T. A. Love and R. B. Murray, IRE Trans. Nucl. Sci. NS-8 (1961) 91. 2) E. F. Bennett, Rev. Sci. Instr. 33 (1962) 1153. ~) G. Dearnaley, A. T. G. Ferguson and G. C. Morrison, IRE Trans. Nucl. Sci. NS-9 (1962) 174.

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4) A. K. Furr and R. S. Runyon, Nucl. Instr. and Meth. 27 (1964) 292. 5) E. F. Bennett, Nucl. Sci. Eng. 27 (1967) 28. 0) M. Marseguerra and. C. M. Porceddu, C.N.E.N. Report RT/FI66 (1966) 36.

7) R. A. Rydin, 1EEE Trans. Nucl. Sei. NS-14 (1967) 377. 8) W. K. Brown, A. N. Ellis and D. D. Peterson, IEEE Trans. Nucl. Sci. NS-15 (1968) 404. a) C. F. Williamson and J. Alster, Nucl. Instr. and. Meth. 46 (1967) 341.