N U C L E A R I N S T R U M E N T S AND M E T H O D S 33 ([965) 283-288; © N O R T H - H O L L A N D P U B L I S H I N G CO. A NEUTRON TUBE WITH CONSTANT OUTPUT (101° n/see) FOR ACTIVATION ANALYSIS AND REACTOR APPLICATIONS J. E. BOUNDEN, P. D. LOMER and J. D. L. H. WOOD Services Electronics Research Laboratory, Baldock, Herts, England Received 22 September 1964 A compact source of fast neutrons capable of producing well over 1010 n/see has been developed. The tube, which uses the D-T reaction consists of an r.f. ion source from which a mixture of deuterons and tritons are accelerated onto a loaded erbium target. A life of at least 100 hour has been achieved. The design features which lead to this high output and long life are discussed. The application of the tube to reactor instrumentation and activation analysis is briefly described. 1. Introduction C o o n 5) with very thick t i t a n i u m targets having a halflife o f 600 p A h o u r / c m 2. A large i m p r o v e m e n t can be o b t a i n e d by using a mixture o f d e u t e r i u m and t r i t i u m in b o t h target a n d ion source. Then the percentages o f t r i t i u m a n d d e u t e r i u m in the target should remain u n c h a n g e d by ion b o m b a r d m e n t over very long periods. Experiments confirming this were carried o u t by C o o n 5) using the D(d,n) reaction ; the n e u t r o n o u t p u t r e m a i n e d c o n s t a n t t h r o u g h o u t e x p e r i m e n t s in which a total o f 3 C / c m 2 b o m b a r d e d the target. The use o f a mixture o f deuterium a n d t r i t i u m is p r e c l u d e d in a c o n t i n u o u s l y p u m p e d accelerator because o f the a t t e n d a n t health h a z a r d a n d the large a m o u n t o f t r i t i u m used. In a The present p a p e r describes a sealed-off fast n e u t r o n source which gives a c o n t i n u o u s yield o f 1010 n e u t r o n s / sec for over 100 hour. T h e source, which uses the D-T r e a c t i o n (D + T ~ n + He 4 + 17.6 MeV) is simple a n d c o m p a c t c o m p a r e d with an a c c e l e r a t o r a n d has the m a j o r a d v a n t a g e o f a c o n s t a n t o u t p u t over a long period. The stable n e u t r o n o u t p u t a n d reliability in use are p a r t i c u l a r a d v a n t a g e s in the a p p l i c a t i o n o f the source to i n d u s t r i a l p r o b l e m s r e q u i r i n g routine activation analysis, such as the d e t e r m i n a t i o n o f oxygen in steel, a n d to the p r o d u c t i o n o f short-lived isotopes. T h e n e u t r o n o u t p u t m a y also be m o d u l a t e d over a wide range o f pulse lengths a n d repetition frequencies, as required for m o r e s o p h i s t i c a t e d reactor experiments1). F o r these p u r p o s e s it m a y be assembled into a 6" d i a m e t e r c o n t a i n e r for insertion in the reactor. The main a d v a n c e offered by the present source over previous sealed-off sources 2-4) is an increase in neut r o n o u t p u t by a factor o f from 20 to 100 to b r i n g it within the levels o f interest for activation analysis. i i i i l i ~ i i i ,08 ~6 u o 2. Design factors T h e design c r i t e r i a for a neutron source using the D - T reaction are d e t e r m i n e d largely by the efficiency o f the reaction as a function o f the ion accelerating voltage and by the a s s o c i a t e d target p r o b l e m s o f life and p o w e r d i s s i p a t i o n . These c r i t e r i a are the same for a c o n t i n u o u s l y - p u m p e d accelerator or a sealed-off n e u t r o n tube except in c o n s i d e r a t i o n o f target life. In o r d e r to achieve n e u t r o n o u t p u t s o f I0 ' ° n/see, ion c u r r e n t s o f at least 200-300 ttA are needed. This represents a flow o f gas at a rate o f several cc at N T P per hour. Thus in a t i m e o f the o r d e r o f an hour, a 1 c m 2 t r i t i u m - l o a d e d target will be d i l u t e d with d e u t e r i u m from the ion source, sufficiently to halve the n e u t r o n o u t p u t . This has been confirmed by all users o f accelerators, the best results being those o b t a i n e d by I b E I0' g sa J w >. g z 3xiO~ O i 40 80 ION 120 BEAt,~ 160 ENERGY 200 I 240 I I (keV) Fig. 1. Calculated neutron yield from a thick erbium target as a function of the ion beam energy. Ion beam composition 50 ~ D! ~ 5 0 ~ Tl +. Target composition Er D I T 1. 283 J.E. BOUNDEN eta]. 284 diameter and an effective target area of s,, diameter has been designed for use with this. 3. Source configuration The neutron tube, fig. 3, operates at a uniform pressure ( ~ 1510 of a gas mixture of deuterium and tritium and consists essentially of a 1 mA ion source and a 100-150 kV accelerating gap. The gas is supplied from a heated titanium replenisher and the pressure measured by a pirani gauge. The ions are accelerated onto an erbium target and although most secondary electrons are suppressed, there are sufficient backstreaming electrons to require an electron "backstop" in the ion source to absorb the power dissipated by these electrons. In order to produce a high proportion of atomic rather than molecular ions of hydrogen isotopes and so obtain maximum efficiency, an r.f. ion source is used operating at 15 Mc/s. lons are extracted from the plasma through the central hole in the extractor by THREADED END . C O io~ , } 40 I I I I I I 80 120 160 ION BEAM ENERGY (keY) I I 200 1 Fig. 2. Neutron yield per joule of ion beam power dissipated at the target plotted vs ion energy for the conditions given in fig. 1. L L A R ~ END TUBE rARGET SUPPORT TUBE ..//TARGET TARGET COLLAR - - S H I E L D SUPPORT TUBE TARGETS/ - --TARGET sealed-off tube, however, this solution can be adopted without difficulty. Erbium has been selected as the target material in view of its high thermal stability6). The calculated yield from a thick erbium target containing a 50/50 D-T mixture, when bombarded by monatomic ions of the same mixture is plotted as a function of ion energy in fig. 1. In addilion to yield efficiency there is another consideration in determining the optimum acceleraling voltage, namely heat dissipation at the target. In fig. 2 the ratio of neutron yield to ion beam po ~¢er dissipated in the target is plotted against ion beam energy for the same conditions. The efficiency is a maximum at 210 keV but only falls off appreciably below 150 keV. Thus the accelerating voltage can be limited to 150 kV in ordcr to minimise the insulation problems without significant loss in efficiency. In prac:ice a neuCron source giving 10 ~° neutrons/sec will require an ion current of from 0.5 up to 1 mA at 150 kV. The size of target used is determined by the requirements of activation analysis in which the maximum flux is required through the sample being investigated. The tube to be described is based on a sample of 5 ir INSULATOR EXTRACTOR DISC ~ SHIELD SHIELD INSULATOR EXTRACTOR ION SOURCE ENVELOPE VITREOUS ENAMEL .EXTRACTOR S H I E L D SOURCE ELECTRON C H I M N E Y ~-~----------__ ION SOURCE END PLATE END T U B F METAL ENV E L O P L = ' ~ : PIRANI I GAUGE/j 2 W I R E ~ L E A D THROUGH IK~ GLASS COOLING COLLAR 'REPLENISHER 3 WIRE L E A D THROUGH k ~ I ~ i .,LO-K Fig. 3. Neutron generator type L. ! I" I A NEUTRON IA ¢ ,IOMA TUBE WITH IOK IT ~ ~1~ / " j f TARGET It ~ TARGETSHIELD , ~ ~ EXTRACTOR I PLASMA BACKSTOP l Fig. 4. H.T. circuit. applying a high negative potential to the target shield with respect to the extractor. A high proportion of atomic ions can only be achieved with an r.f. ion source if it is designed with inside surfaces having low coefficients for the recombination of neutral gas atoms impinging on them. This is in order that the dissociation of the gas by the discharge may lead to the build-up of a high concentration of neutral gas atoms, which when subsequently ionized, will result in a high yield of atomic ionsT). With the exception of aluminium, metals in general have high recombination coefficients, whereas glasses have low recombination co- CONSTANT OUTPUT 285 efficients, hence in the present design every effort has been made to keep the plasma away from metal surfaces. The extractor has therefore been screened with an extractor shield and also a coating of vitreous enamel has been fired onto its conical surface facing the plasma. Another reason for screening the metal surfaces is that they will be rapidly sputtered by ion bombardment from the plasma. The sputtered material would not only increase the atom recombination rate but would also act as an ion pump. The present ion source does not rely on probe and canal type extraction of the ionsS), instead the electron backstop and the extractor are at the same potential and the plasma is allowed to diffuse into the central hole in the extractor. The target shield and extractor geometry have been designed to eliminate vacuum sparking 9) and yet not enter the region of Paschen breakdown 1o) as previously discussed2). The extractor and the target shield fit fairly closely inside the glass shield insulator in order to shield glass to metal seals from voltage stresses and also to ensure that there are no long path lengths between electrodes along which Paschen breakdown would preferentially occur. A negative potential of about 400 V is applied to the target shield, which is insulated from the target (fig. 4). This target bias voltage serves to suppress secondary electrons liberated at the target by the impinging ions and some of the electrons from the ionization which is produced in the region between the target and the target shield by the ion beam. Electrons, which are drawn from this ionized region and accelerated back Fig. 5 (a) Neutron generator type L. 286 J.E. BOUNDEN et al. into the ion source are stopped at the end of the ion source by a molybdenum pressing, which is liquid cooled to absorb the power dissipated. The main components of the tube are constructed of Nilo K and Kodial glass. The tube is illustrated in fig. 5a. plasma increases. The yield also increases with the accelerating voltage, at the rate which would be expected from the theoretical yield voltage curves. Sources of the present type have produced yields of 10 l° DT neutrons/sec for periods of the order of 100 hour. The neutron output remains constant 4. Operating characteristics The neutron source is operated in a sealed container, an experimental version of which is shown in fig. 5b. The main purpose of the container is to surround the high voltage region of the tube with insulating oil, and also to immerse the ion source in a suitable fluid which is used to conduct away the heat dissipated from the ion source plasma. The container also prevents the escape of tritium should the tube envelope fracture. At the target end, the container has facilities for liquid cooling the target. Owing to the high voltage of the target this fluid must be a good electrical insulator and the halogenated hydrocarbon Arcton 113 fluid has been used. Normal operating conditions are as follows: TABLE I Pressure: Ion source power: Target voltage, VT: Tube current, 1A: Target current, IT Shield current, Is: Target bias voltage: Neutron output: 15 # Hg 500 W into r.f. oscillator 100-120 kV 1.6 mA 0.2 mA 1.8 mA 400 V 1010 neutrons/sec The r.f. power coupled into the discharge is about 200 W. The directions of the positive currents under normal operating conditions are as shown by the arrows in fig. 4. The target current I T corresponds to a negative current going to the target from the ion beam. This means that in fact the target is collecting more electrons produced by ionization of the gas inside the target shield than positive ions from the ion beam. The slow positive ions produced concurrently with the electrons by ionization inside the shield are collected by the shield and constitute most of the shield current I s . The ion beam which has a diameter of approximately ~5 t! at the target is not intercepted appreciably by tile target shield. The effect of the target bias voltage is shown in fig. 6. The decrease of the X-ray yield from the tube as the bias voltage is increased plainly demonstrates the suppression of high voltage backstreaming electrons. The neutron producing efficiency of the source increases as the power delivered to the oscillator is increased, as shown in fig. 7. This is due to an increase in the relative atomic ion yield as the power in the Fig. 5 (b) Experimental container for the neutron source. 287 A N E U T R O N TUBE WITH CONSTANT O U T P U T throughout the life of the tube and a typical 6.5 hour continuous run is shown in fig. 8. During activation analysis work rapid switching of the tube is also possible as illustrated in fig. 9. When filled solely with deuterium and operated under the conditions specified in table 1 the source will produce 108 D D neutrons/see. cies ranging from 0.0167 c/s to 5 kc/s, corresponding to pulse lengths ranging from 30 sec to 100/~s. Examples of the pulse shapes are shown in fig. 10. The fluctuations in pulse height more specially noticeable at the higher modulation frequencies, are due to statistical fluctuations in the detector. For modulation frequencies less than 1 c/s, restriking of the discharge at the beginning of the r.f. pulses is intermittent when a 100~ depth of modulation is used. This can be overcome by reducing the depth of modulation to 9 0 ~ , i.e. by maintaining a weak r.f. discharge in between the main r.f. pulses. Since the neutron yield is a sensitive function of the proportion of atomic ions formed by the ion source, the neutron yield risetime will be determined by the rate at which the gas in the ion source can be redissociated at the beginning of each pulse. This time is normally 1-2 ms ~~). Rise times of this order have been obtained, for modulation frequencies less than 0.5 c/s in the present case. As the modulation frequency is increased from 0.5 c/s, this effect will become steadily smaller since there is insufficient time between pulses for appreciable recombination. Thus at the higher modulation frequencies the rise time is limited to a few microseconds by the breakdown characteristics of the r.f. discharge. 5. Modulation of the neutron output 6. Applications For some applications a modulated neutron output is required. In particular for some nuclear reactor studies square wave modulation of the neutron output with risetimes less than one tenth of the neutron pulse length are required. By modulation of the r.f. power to the ion source, it has been possible to achieve this with the present neutron source for square wave modulation frequen- Compared with accelerators the present neutron source has the major advantage of providing a constant (~)IT X IS @ 400 i~) I T + Is • X-RAY / YIELD (ARBITRARY UNITS) XIRAY YIELD VT=E;OkV // PRESSURE 2 0 M I C R O N S R F O S C I L L A T O R POWER 300 20' WATTS ]S × ,¢ =:L ~i IT+Is kd ~r --EHAS V O L T A G E OF 300 400 IT TARGET RELATIVE -t- TO T A R G E T S H I E L D ( V O L T S ) -20 Fig. 6. Effect of the target bias voltage on Is,I T and the X-ray yield due to backstreaming electrons. o : o o f 3 o 6 4 5 OPERATING TIME (HOURS) Fig. 8. Illustration of output stability when operating for long periods. ..u c~ ). z I I00 I 200 POWER 1 300 SUPPLIED I 400 TO I 500 RE I 600 oSCrLLATOR I 700 I 800 (WATTS) Fig. 7. Relation between neutron source efficiency and power supplied to the r.f. oscillator. i F~I.'~tt~ 1 t ! ~11 1 !11 t t~l I 11 i ~ ii : / 1~ | o8 7 6 5 4 3 'r ~M F. (MINUTES) Fig. 9. Illustration of rapid switching of neutron output. i 288 J.E. BOUNDEN et al. uniform over a ~ inch disc and is efficiently used. T h e neutron o u t p u t r e m a i n s c o n s t a n t for long periods so t h a t the a c t i v a t i o n analysis technique is no longer confined to i m p u r i t i e s which p r o d u c e short-lived isotopes b u t can be e x t e n d e d to isotopes with h a l f lives o f m a n y hours. This provides an i m p o r t a n t and considerable extension o f the fast neutron activation analysis technique. t ~ I c/s - - r Ii Fig. 10. Neutron pulse shapes for square wave modulation. N = neutron yield/sec; t = time. stable o u t p u t over a p e r i o d o f 100 h o u r or more. T h e source is also c o m p a c t a n d easily p o r t a b l e being suitable for i n d u s t r i a l use. I n its sealed-off f o r m there is no t r i t i u m h a z a r d . T h e source has f o u n d two a p p l i c a t i o n s , one for r e a c t o r s a n d the o t h e r for a c t i v a t i o n analysis. The m a j o r a d v a n t a g e o f the source in r e a c t o r use is its size, which permits its insertion into a 6" d i a m e t e r hole, a n d the high o u t p u t , which m a y be m o d u l a t e d . This o u t p u t is a factor o f 100 greater t h a n t h a t p r o v i d e d by previous sealed-off n e u t r o n tubes for r e a c t o r a p p l i c a t i o n s . T h e a p p l i c a t i o n to activation analysis is o f wider significance a n d the main a d v a n t a g e in this case is the c o n t i n u o u s stable o u t p u t coupled again with the c o m pactness o f the source. This p e r m i t s the use o f relatively simple shielding facilities such as an eight foot diameter water t a n k , o r a hole in the g r o u n d c o n t a i n i n g the tube. The tube has been used for a c t i v a t i o n analysis with samples in the f o r m o f g5 t! dia, discs which can be inserted to within 3" o f the back face o f the target by means o f a p n e u m a t i c transfer system. Since n e u t r o n s are p r o d u c e d over a ~" d i a m e t e r section o f the centre o f the target, the n e u t r o n flux is fairly 7. Conclusions A c o m p a c t sealed-off n e u t r o n tube has been developed c a p a b l e o f p r o v i d i n g a c o n s t a n t stable o u t p u t of 101° neutrons/sec for over 100 hour. The neutron tube is being a p p l i e d to reactor i n s t r u m e n t a t i o n and also to activation analysis problems. The tube is expected to find p a r t i c u l a r a p p l i c a t i o n where routine analysis is required. Initial e x p e r i m e n t s on this tube were carried out by Dr. W. S. Whitlock. T h e c o n t r i b u t i o n s o f Mr. D. W. D o w n t o n in the testing a n d Mr. D. J. T a y l o r in the cons ruction o f these tubes are gratefully a c k n o w l e d g e d . This p a p e r is published by permission o f the M i n i s t r y o f Defence. References 1) T. E. Stern, A. Blaquiere and J. Valat, Reactor Sci. and Techn. 16 (1962) 499. 2) p. D. Lomer, J. D. L. H. Wood and R. C. Bottom[ey, Nucl. Instr. and Meth. 26 (1964) 7. 3) O. Reifenschweiler, Nucleonics 18 (1960) 69. 4) B. J. Carr, Nucleonics 18 (1960) 75. 5) j. H. Coon in Fast Neutron Physics, Part I (Interscience 1960) p. 677. 6) R. Redstone and M. C. Rowland, Nature 201 (1964) 1115. 7) C. C. Goodyear and A. von Engel, Proc. Phys. Soc. 79 (1962) 732. 8) C. D. Moak, H. Reese and W. M. Good, Nucleonics 9 (1951) 18. 9) W. D. Kilpatrick, UCRL Report No. 2321 (1953). 10) A. von Engel, Ionized Gases (Oxford, 1955), p. 172. l J) A. C. Riviere, private communication.