A fission counter with high fissile material density for fast neutron fission counting

J. Nudar Bnaep. 1955.Vol. 2. pi. 38 to 40. F+a8monPIWELtd.. London

A FISSION COUNTER WITH HIGH FISSILE MATERIAL DENSITY FOR FAST NEUTRON FISSION COUNTING W. D. ALLXN and A. T. G. FERGUSON Atomic Energy Research Establishment, Harwell, Be&s. (Receicled9 June 1955) Ah&net-A multiplate counter is describedwhich contains up to 100 mg of &side material within one cubic inch with a plate capacity of about 100 pF. It is rezisonably robust, and has a transmission coetWent for fast neutrons of 93 per cent: and the quantity of fissile material can be easily varied. 1. INTROIjUCTION

OF THEmaking of fksion counters there is no end. Many varieties of simple parallel plate counters have been described in the literature. Spiral arrangements were described by ROSSIand STAUB(1949) and a spiral groove counter used by SALMON

FIN. I.-Diagrammatic sketch of counter.

(1954). Counters in the form of a nest of cylinders have been extensively used at A.E.R.E. (AVES,BARNES,and MACKENZIE,1954). A fission counter is required for the study of fission by fast neutrons. The sources of the latter. are the reactions Li(p, n) and T(p, n) in which the neutron energy and yield varies rapidly with angle. Practical considerations limit the total yield in the forward direction to an equivalent source strength of 108 neutrons per second. It is 38

A fission counter with high Guile material density for fast neutron &ion counting

39

necessary therefore to use a counter with the maximum of material in a minimum of volume. In order to avoid large and uncertain corrections it is necessary to keep the amount of scattering material in the counter to a minimum. 2.

THE

FISSION

COUNTER

At the outset of .this work, the Aves counter was the only one available. This counter, which was designed for pile monitoring, had several minor disadvantages: but eventually it was observed_ that the scattering of the pyrophyllite and nickel at the end was as large as 35 per cent. When this last point was established, it was clear that a counter of different design was called for. From a suggestion of Dr. M. J. POOLE, a counter was designed, of which a diagrammatic sketch is shown in Fig, 1. Plates, alternately earthed or at H.T., are connected to one or other of two sets of 1OBA rods; at any one plate, the three rods not connected to it are insulated by glass tubes of minimum thickness. In this way, an arbitrary number of plates can be built together. The earthed plates are of 0.005 in. platinum, coated on both sides with fissile material. The plates at H.T. are made from rings of 18 S.W.G. brass, with OXtO2in. silver sheet spun over them. The ring therefore acts as a corona &kid against breakdown. The vacuum wall of the counter is 0.010 in. steel. Thus, either in the axial or radial direction, the thickness of material is kept to a minimum: the only way we can see for further reduction is in the use of OXlO2in. platinum (which is not reliably flat) and in the use of 12 BA studding. With these materials, however, the counter would be rather flimsy to construct. 3.

QUANTITY

OF FISSILE

MATBRIAL

OF THE

AND

PERFORMANCE

COUNTER

The quantity of material which can be loaded into this counter depends on the maximum permissible fissile material thickness, the coated area, and the minimum distance between plates. It was found that, with a U-235 thickness of 14 mg/cms and an interplate distance of 5 mm, a flat plateau was obtained with a single plate coated to the inner edge of the studs. Four more plates were then added, and the area of coating extended to within 1.5 mm of thevacuum wall (this counter then contained 100 mg of material in a length 4 cm approximately). The plateau obtained with this counter were not so good, dropping by 24 per cent for 10 per cent change in volts bias in the operating region between 20 V and 30 V. This was regarded as acceptable, For plutonium, 11 mg were loaded in tliree plates ; the spacing between the plates was 1.25 cm, and the area of coating 3 cm 2. The plateau from this counter was better, falling only by 1.5 per cent for 10 per cent increase in bias, for bias levels between 20 V and 30 V. The U-233 plates were similar in specification to the U-235, and four of them were used: it was found, however, that the plateau of this was substantially worse than U-235. This was thought to be due to the considerable alpha build-up, and one plate was therefore removed: the resulting plateau was comparable with the U-235, and the final quantity of U-233, 40 mg approximately. Originally the counters were .made with 0 ring seals between the base and the vacuum wall. Noticeable deterioration in plateau was observed in seven to fourteen days. The rubber seal was therefore replaced with solder, and a metal tap interposed between the counter and the rubber diaphragm of the vacuum tap. Counters made in this way retained their characteristics for many weeks.

40

W. D. Amm

and A. T. G. FERGUSON

A scale drawing of the counter is shown in Fig. 2, and an X-ray of the U-233 counter is shown in Fig. 3. It should be mentioned that to date 11 mg of Pu is as much as other workers have been able to load into counters of carefully optimized design.

FIG. 2.-!kale

4.

drawing of counter.

ACKNOWLEDGMENTS

We are indebted to Dr. M. J. POOLEfor the suggestion of the method of support; and to Mr. F. HUDSWEL.~ for the prompt and competent supply of fission foils. REFERENCES Ross, B. B., and STAUB,H. S. (1949) Zonization ChambersandCounters,McGraw-Hill; New

SALMON, P. G. (1%) private Gommunicatio~. Av&$R., BARNES,D., and I$ACKENZIE, R. B. (1954) J. Nuclear Energy,Vol. 1, pp. 110-l 16.

York.