Page 225 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 209

90% At" + 10% CH 4 ("P-gas") and 96 % He + 4 % i-C4Hlo ("Q-gas"). Many other pure
gases or combinations are possible, but molecules which readily attach electrons must be
avoided. Figure 8.8(b) shows a proportional counter tube using pure CH~ and the tube
characteristics (i.e. count rate versus voltage) for 239pu cr and for "roSe B-particles
entering through the thin aluminized mylar window.
The gas mixture and electrodes may be separated by a thin window from the radioactive
sample, or the counter may be operated windowless. For windowless operation, after
insertion of the sample the chamber must be flushed with gas to eliminate all the oxygen
and water vapor as these molecules absorb electrons readily to form negative ions and, by
so doing reduce the pulse size. For cr and ~-particles whose ranges do not exceeA the
dimensions of the chamber, windowless counters are often referred to as 27r counters since
a solid angle of 2r is subtended above the sample. In these cases, with proper care, the
measured count rate is very close to 50 % of the true disintegration rate. Such windowless
proportional counters are very useful for measuring low energy radiation such as the ~-
emissions of 14C and 3H, and for absolute counting. Figure 8.10 shows a 47r proportional
counter in which the sample is inserted in the middle between the two half-domes
(cathodes).
Proportional counters can be used also for neutron detection by using a gas containing
nuclei that capture neutrons and in the nuclear reaction produce charged particles, e.g. 10B
or 3He. In BF3-counter the gas is BF 3, usually enriched in 10B. With neutrons the reaction

10B + n~7Li + 4He (Q = 2.78 MeV)

occurs (cf. Ch. 14). The ionization of the two products produces a heavy pulse, which is
easy to discriminate against an intense ~-background. Some properties of a BF 3 neutron
detector are given in Table 8.2. From Figure 14.4 it is clear that the BF 3 counter has a
higher efficiency for thermal than for fast neutrons.

UPPER UPPER GAS
A IUI/'~I'~I tA/IDI= CONNECTION
SC
RING
9 .'h"~ i

SLIDE

HIGH VOLTAGE
CONNECTORS

D r

FIG. 8.10. A 4a" proportional counter for measuring absolute decay rates. (Acc. to
O'Kelley.)