Radiation damage to 3He proportional counter tubes

NUCLEAR

INSTRUMENTS

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R A D I A T I O N D A M A G E TO SHe P R O P O R T I O N A L C O U N T E R TUBES* A L B E R T E. E V A N S , H O W A R D

O. M E N L O V E , R O D D Y

B. W A L T O N a n d D A R R Y L

B. S M I T H

University o f California, Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, U.S.A. Received 28 J a n u a r y 1976 N e u t r o n d a m a g e to 3He proportional-counter tubes is inhibited by coating the cathodes o f the tubes with activated charcoal. This coating also improves pulse-height resolution.

Counting of delayed neutrons from fission has been found to be a valuable method for the nondestructive assay of many types of nuclear materials. In one recently developed technique for the fissile assay of nuclear material inventory-verification samples1), the samples are interrogated with 300500 keV neutrons in pulses 35 ms long every 100 ms, and delayed neutrons are counted in the interval between pulses by a highly efficient array of 39 3He proportional counter tubes imbedded in polyethylene. These counters, size 2 . 5 c m diameter by 50.8cm active length and containing 4 atm of 3He, are subjected during irradiation to a partially thermalized neutron flux of between 107 and 108 neutrons/cm 2 s. It was found that this flux causes deterioration of the pulse output of the tubes to such a degree that the resolution of tubes becomes very poor after a few hundred hours of operation. Accordingly, a testing * W o r k supported by the U.S. Energy Research a n d Development Administration.

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program was initiated to determine the cause of tube failure and find a remedy. Tubes meeting the above specifications were obtained from four different manufacturers and irradiated with neutrons from a 252Cf source moderated with polyethylene. Fission chamber measurements and Monte Carlo calculations established the neutron flux at the irradiation position to be approximately 9 x 1 0 6 neutrons/cm 2 s, 60% of which were thermal neutrons. Progressive deterioration of the tubes is clearly evident from the data shown in fig. 1. Curve 1 is the thermal neutron response of a tube which had been subjected to an irradiation of 2 . 5 x l 0 1 3 n / c m 2 in 753 h, without high voltage applied to the anode. In this case, the pulse-height distribution is little, if any, changed from that obtained with a new tube. The fullwidth at half maximum of the thermal peak indicates an energy resolution of 6.4%. Curve 2 is the thermalneutron pulse-height distribution from an identical tube which had been irradiated with 9 x 1012 fast neutrons/cm 2, with 1500V applied to the anode of the tube. The average gain of the tube has dropped

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Fig. I. Progressive radiation d a m a g e to 3He proportional counter tubes. (1) T u b e irradiated without applied a n o d e voltage; (2) tube irradiated with 9 x 1012 fast n e u t r o n s / c m 2 (Cd cover during irradiation); (3) tube irradiated with 2.6 x 1013 n e u t r o n s / c m z, 60% thermal.

Fig. 2. C o m p a r i s o n o f thermal peaks f r o m two activatedcharcoal-lined 3He proportional counter tubes. ( + ) : new tube, unirradiated. O p e n circles: tube irradiated with 3.5×1013 n e u t r o n s / c m 2. T h e non-zero abscissa should be noted.

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Fig. 3. Pulse-height distribution obtained by bombarding an activated-charcoal-lined 3He proportional-counter tube with 600 keV neutrons. by 16% and the thermal p e a k has b r o a d e n e d to a resolution of 25%. Curve 3 (open circles) shows the complete deterioration o f a tube which has been irradiated with 2.6 x 10 ~3 n e u t r o n s / c m 2 with o p e r a t i n g voltage applied. The d a m a g e has been f o u n d to be p e r m a n e n t a n d reasonably reproducible from tube to tube and from m a n u f a c t u r e r to manufacturer. Since the application o f high voltage is a prerequisite for the occurrence of radiation d a m a g e in the tubes, it was assumed that the p h e n o m e n o n is due to the buildup o f gaseous poisons (e.g., electronegative gases) in the tubes. It was suggested that a layer o f activated charcoal on the tube c a t h o d e would serve to a b s o r b such poisons and thus inhibit radiation damage. BF 3 tubes i n c o r p o r a t i n g this feature have long been available2). Accordingly, test models o f aluminum-walled, activated-charcoal-lined 3He tubes were p u r c h a s e d from the Reuter Stokes C o r p o r a t i o n . in fig. 2 the pulse-height distribution for thermal

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neutrons incident on a new tube o f this test g r o u p is c o m p a r e d w i t h that f r o m an identical tube which h a d been irradiated with 3.5× 10 ~3 n e u t r o n s / c m 2 over a p e r i o d of 1000 h. The gas gain o f the irradiated tube has d r o p p e d by 10% ; however, there has been no d e t e r i o r a t i o n o f the shape o f the thermal peak. The test indicates that it should be possible to get m a n y years o f service f r o m tubes o f this type when they are used in a pulsed neutron field for the a p p l i c a t i o n described above. In a d d i t i o n to inhibiting r a d i a t i o n damage, the activated charcoal liner a p p e a r s to offer an additional i m p r o v e m e n t in the p e r f o r m a n c e o f 3He p r o p o r t i o n a l counter tubes. W h e n used with low-noise chargesensitive preamplifiers and spectroscopy-type main amplifiers, the tubes consistently exhibited resolution better than 3% (23 keV) for thermal neutrons. Fig. 3 is a pulse-height distribution o b t a i n e d by b o m b a r d i n g one o f these tubes with monoenergetic 6 0 0 k e V neutrons. The resolution o f the 600 keV p e a k is 45.6 keV, which is only a factor o f two b r o a d e r than that o b t a i n a b l e with the spectrometer tube o f Shalev 3.4). It w o u l d a p p e a r that the charcoal lining may be an economical short-cut to the manufacture o f inexpensive 3He spectrometers. The a u t h o r s are grateful to E. L. A d a m s for t a k i n g the d a t a used in p r e p a r i n g this note.

References t) A. E. Evans, IEEE Trans. Nucl. Sci. NS-21, no. 1 (1974) 628. 2) A.J. Stokes, T.J. Meal and J. E. Meyers, Jr., 1EEE Trans. Nucl. Sci. NS-13, no. 1 0966) 630. a) A. E. Evans and L. V. East, Los Alamos Scientific Laboratory Progress Report LA-5291-PR (1973) p. 17. 4) j. Cuttler, S. Shalev and U. Dagan, Trans. Am. Nucl. Soc. 12, no. l (1969) 63.