An approach to neutron spectrometry based on triton spectroscopy in the 6Li(n,α)3H reaction

Nuclear Tracks, Vol. 12, Nos I-6, pp. 603-606, 1986. Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain.

0191-278X/86 $3.00+.00 Pergamon Journals Ltd.

AN APPROACH TO NEUTRON SPECTROI~TRY BASED ON TRITON SPECTROSCOPY IN THE 6Li(n,a)3H REACTION

Judith M Brock, A Peter Fews and Denis L Henshaw H H Wills Physics Laboratory University of Bristol Tyndall Avenue Bristol BS8 1TL, England.

ABSTRACT The feasability of neutron energx determination in the range 10-100keV from measurement of the t r i t o n energy in the 6Li(n,a)3H reaction is explored. A theoretical analysis predicts the energy spectrum of r e c o i l t r i t o n s recorded in CR-39 and produced by a neutron beam incident on a Li-doped radiator. Experimental measurements of t r i t o n track length distributions from unmoderated and moderated 252Cf neutron sources and a 24 keV neutron beam i l l u s t r a t e the principle of the technique and that i t shows promise for further developement. INTRODUCTION A passive neutron spectrometer in the energy range 100 keV to 15 MeV has been developed in this laboratory based on energy and direction measurements of recoil protons recorded in CR39 (Fews and co-workers 1985). The low energy l i m i t of t h i s method of detection using conventional track etching is approximately 100 keV and is governned by the range of the recoil proton. At 100 keV this is only ~l~m. Also below 100 keV the range-energy and hence the energy versus track etch rate r e l a t i o n becomes i l l - d e f i n e d . We have examined the p o s s i b i l i t y of e x p l o i t i n g the favourable energy t r a n s f e r to the t r i t o n in the 6Li(n.a)3H reaction and the corresponding t r i t o n range enhancement to the design of a CR-39 neutron spectrometer in the energy range 10- 100 keV. The t r i t o n range as a function of incident neutron energy as shown in Fig. 1. I t is seen that the range enhancement is appoximately 1~m/10 keV. High resolution measurement of the t r i t o n energy in CR-39 suggests that this can be exploited as a possible means of determining neutron energy in the range 10-100 keV and may be used to supplement measurements on direct recoil protons at higher energies. This is investigated in this paper.

THEORY Glass doped with 6% 6Li acts as a radiator in contact with CR-39. The Tritons recorded in CR-39 obey the track etch rate versus range and etch induction time versus range r e l a t i o n s h i p s given in Figs (2) and (3) r e s p e c t i v e l y . The data for a l p h a - p a r t i c l e s and protons are also shown for comparison. In Fig. 4 a photomicrograph is shown of a t r i t o n and a-particle cluster recorded in CR-39 from an adjacent p e l l e t of LiF and exposed to thermal neutrons. The tritons and a-particles are c l e a r l y distinquished being the large over-etched cones and the t r i t o n tracks the smaller non-etched-out cones. In the present application tritons must be distinquished both from their associated Q-particles and recoil protons from any high energy component of the neutron source. A theoretical analysis has been developed which may be used to predict the energy spectrum of t r i t o n s recorded in the p l a s t i c from a given source. The 6Li(n,a)3H has an energy release, Q=4.785 MeV. The t r i t o n energy Et is given by:

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JUDITH M. BROCK et al.

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J where ~ is the scatterinq angle in the laboratory frame. The t r i t o n range-energy relation may , ~t 3/% whereA=11.35 from which the range in CR-39 for a t r i t o n emmited at be expressed Rt = a.L height h above the plastic surface is:

L,_ I f 9 is the scattering angle in the centre of mass frame then the neutron scattering cross sections in the two frames are related by:

ANALYSIS

The above r e l a t i o n s have been incorporated i n t o a computer programme to simulate the predicted t r i t o n range distribution recorded in CR-39 from incident neutrons of given energy. The predicted d i s t r i b u t i o n s f o r neutrons from an unmoderated 252Cf source and a source moderated by 30 cm of p a r a f f i n wax are shown in Figs (5). The corresponding experimental measurements are shown in Figs (6) and (7). The two d i s t r i b u t i o n s are well distinguished. The measurements were obtained using three etch times, 3, 3.75 and 4.5 h at 98°C. This is necessary because for a given etch time there exists only a comparatively narrow window for high resolution track measurements, that is, from tracks that are just etched to or beyond the end of t h e i r range (Fews and Henshaw 1982). The lower range l i m i t is governed by the limitations of measurents from heavily overetched tracks, while the upper l i m i t is governed by the resolution obtained from measurements on non-etched-out tracks and the corresponding accuracy of VT determination. The etch times employed provide upper range thresholds of approximately 66, 81 and 95 ~m from which the composite observed range d i s t r i b u t i o n s were obtained. For these etch times t r i t o n s are r e a d i l y distinquished from the associated ~p a r t i c l e s . Tritons were distinquished from knock-on protons by t h e i r measured track etch rate-range value p l o t t e d alongside the curves in Fig. 2. As an example of the t r i t o n distribution obtained from a single neutron energy, a 24 kev neutron beam under development f o r neutron capture therapy at AEREHarwell UK has been used. The beam is characterised by low contamination, -3% from higher energy neutrons. The predicted t r i t o n range distribution from this beam is shown in Fig. (8) and the corresponding measured values in Fig. (9). DISCUSSION I t is seen that the measured t r i t o n range distributions from the three neutron sources are well distinquished. The absence of measured particles at low range is due to the limitations of the mesaurements that can be made on over-etched tracks as discussed above. Also i t is c l e a r l y necessary to employ several d i f f e r e n t etch times to obtain windows of optimum resolution. In future work i t w i l l be possible to use automated image analysis to measure the t r i t o n tracks so that greatly improved statistics can be obtained. The authors conclude that the technique shows promise for further development as a passive

NEUTRON SPECTROMETRY BASED ON TRITON SPECTROSCOPY

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spectrometer for neutron energy determinations both in the energy range 10 - 100 keV and to supplement measurents on direct recoil protons at higher neutron energies. ACK~EDGEI, IENTS The authors are grateful to Dr G Constantine at AERE Harwell for arranging the exposures to the 24keV beam from the DENIS f a c i l i t y and to Dr A Spowart of SNS Engineering Services, Argyll who provided the lithium-doped glass. REFERENCES A P Fews, T Portwood, D L Henshaw, T Wturner and A Worley. Proc 5th Symposium on Neutron Dosimetry, Munich/Neuherberg Sept 1984. CEC publication EUR 9762 vol 1; pp 479-499. A P Fews and D L Henshaw, Nucl Instr Meth 197 (1982) 517-529.

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