FIG. 1. 
front of metal plate. 
justs separation 
Single-wire counter has wire stretched in 
Sliding handle in frame ad- 
pulse divider. 
characteristics 
FIG. 2. Circuit used for counting spark pulses has condenser 
Sum of Ri and R2 (=R) determines quiescent 
Operating Characteristics 
of the Spark Counter 
This highly specific instrument is sensitive only to alphas. 
With a boron plate it gives 1 count for 180 neutrons/cm* 
but < 1 count/hr in a gamma field of 1 roentgen/sec 
By N. K. SAHA and NARENDRA NATH 
Department of Physics, University.of Delhi, Delhi, India 
ALPHA PARTICLES have low penetrating 
power and ionize densely. To detect 
them, therefore, one should have an 
instrument with a thin window or no 
window at all, and this instrument 
should be sensitive to dense ionization 
and insensitive to the lighter ionization 
of beta and gamma rays. The Rosen- 
blum spark counter meets both of these 
criteria. It consists of a wire or a sys- 
tem of wires parallel to a metal plate 
and maintained at such a potential that 
the ionization produced by an alpha 
particle initiates a spark discharge be- 
tween the plate and the wire or wires. 
An efficient neutron detector is made 
by placing a thin layer of boron in front 
of the wires and detecting the alphas 
that result from the B!°(n,a) Li’ reaction. 
Following the original idea of Grein- 
acher (1), Chang and Rosenblum (2) 
designed a satisfactory alpha counter 
that uses the breakdown between a flat 
metal cathode and a fine wire anode 
stretched 1.5 mm in front of the cath- 
ode. Other workers have improved 
and modified the spark counter and 
studied its operation (3-7). A stand- 
ard form has not yet emerged, however, 
and the mechanism is still obscure in 
many respects. 
Conflicting reports have appeared re- 
garding some of the counter’s proper- 
ties. Connor has observed a decreasing 
counting rate with increasing applied 
voltage, while Payne has observed a 
nearly constant rate. Similar uncer- 
tainties exist as to whether the pulse 
size increases or decreases with applied 
voltage, whether a positive tempera- 
ture effect exists, etc. 
We have developed single-wire and 
FIG. 3. Counting characteristics (a, b, c) show hump where quiescent current (d) 
starts. 
With increasing source collimation slope decreases (a to b to c) as 
oblique rays fail to reach sensitive volume 
37 
