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FIG. 1. 
Neutron-detecting spark counter 
FIG. 2. Multiple-wire anode for spark counter 
BF, Neutron Spark Counter 
A BF:-filled spark counter is sensitive to neutrons yet insensitive to gammas. 
Cadmium shielding plus paraffin moderation make it useful for fast-neutrons too 
By MARTIN JAY SWETNICK* and NICHOLAS G. ANTON 
Anton Electronic Laboratories, Brooklyn, New York 
WITH A ROSENBLUM spark counter (1). 
operated in a BF; atmosphere we have 
achieved a slow-neutron detection eff- 
ciency of 0.6%, and the system is com- 
pletely insensitive to Co® radiation at 
500 r/hr. External circuitry required 
is limited to a high-voltage supply and 
a scaler. 
Sensitivity 
Detection efficiency is proportional 
to the probability of slow-neutron cap- 
ture in B! and the probability of alpha 
particles or lithium recoils from the 
B!°(n,a)Li’? reaction reaching the sensi- 
tive corona region of the electrode as- 
sembly (2). Sensitivity is not a function 
of the total volume of BF; gas. The 
sensitive volume of the spark counter 
* PRESENT ADDRESS: Physics Department 
of the University of Maryland, College 
Park, Maryland. 
Slow-Neutron Response 
Applied No cadmium Cadmium 
voltage (cpm) (cpm) 
2,000 10 0 
2,050 30 0 
2,100 62 0 
2,150 80 0 
2,200 115 4 
2,250 200 8 
2,300 560 75 
36 
extends only to the alpha-particle 
range. Our sensitivity is decreased by 
shielding of additional gas since the 
anode-wall distance is greater than the 
alpha range. 
The slow-neutron-sensitive spark 
counter can be converted into a fast- 
neutron-sensitive spark counter by en- 
casing the detector in a paraffin shield 
to moderate fast neutrons and covering 
the exposed surface of the paraffin with 
a 2-mm cadmium sheet to absorb inci- 
dent slow neutrons. 
Counter Construction 
Our counter is enclosed in an air- 
tight stainless-steel cylinder filled with 
BF; to 45 cm Hg (Figs. 1 and2). The 
boron in the gas is 96% enriched in 
B'°, The electrode assembly consists 
of a° smooth-surfaced stainless-steel 
cathode plate 2 in. wide and 8 in. long 
and an anode array of five parallel 
0.005-in. stainless-steel wires. The 
wires are in a plane 1 mm from the 
cathode surface and parallel to it. 
Between power supply and counter 
we use a two-resistor series combina- 
tion to attenuate output pulses from 
the ~—200 volts at the anode to 
~—50 volts. A 50-uuf condenser 
couples the pulse to a scaler. 
Slow-Neutron Response 
The detector was exposed to a con- 
stant flux of slow neutrons from a 
paraffin-encased 10-mc radium-bery]- 
lium neutron source to determine its 
response as a function of applied volt- 
age. Runs were made at 50-volt inter- 
vals between 2,000 and 2,300 volts with 
and without cadmium shields about the 
détector. Although the pulse heights 
were observed to increase with applied 
voltage, all pulses had identical shapes 
and magnitudes at any one operating 
voltage. The results of the measure- 
ments are presented in the table. In 
the absence of neutrons, counting rate 
was zero over the entire voltage range 
we studied. 
Gamma Response 
With slow neutron source removed, 
gamma radiation response of the detec- 
tor was checked by placing a 1 me Co®® 
needle source in contact with the outer 
wall of the counter cylinder. The in- 
tensity of the gamma radiation at the 
sensitive region of the electrode assem- 
bly was computed to be >500 r/hr. 
The counter was found to be completely 
insensitive to this intense gamma 
radiation. i 
* * * 
This work was supported in part by the 
Bureau of Ships and the AEC, 
BIBLIOGRAPHY 
1. W. Y. Chang. S. Rosenblum, Phys. Rev. 67, 
222 (1945) 
2. N. K. Saha, Narendra Nath, Nucueontcs 16, 
No. 6, 94 (1955) 
