Page 247 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 231

premixed with acid and scintillator solution before counting; the use of that system is
described in w

8.9. Absolute disintegration rates

The determination of absolute disintegration rates is of great importance in all areas of
nuclear chemistry, tracer work, age calculation, etc. Numerous methods have been
employed, many using techniques described above, as track counting, liquid scintillation
measurements, 47r proportional counters, etc. If the nuclei decay through/~-7 emission, the
absolute rate may be obtained by two detectors placed close to each side of a thin sample,
one detector/~-sensitive and the other "y-sensitive.
When only a single detector in a conventional counting set-up is available (e.g. detector
arrangement in Fig. 8.4), absolute counting rates can be obtained for unknown samples by
comparison with known standards.
When standards are not available it is possible to obtain an approximate estimation of the
absolute disintegration rate from a knowledge of the various factors that influence the
counting efficiency. The detection efficiency ff is defined as a ratio between the count rate
and the absolute disintegration rate (4.45). This detection efficiency, which was discussed
briefly in w is the product of all the factors which influence the measured count rate and
may be expressed as

(8.16)
= ~det~res~'geom~back~self~abs
where •det = counting efficiency of detector,
= resolving time correction (see w
!~geom = geometry factor (see w
= backscattering factor,
~back
~self = self-absorption factor (~bsample = ~bbackl~self ), and
= absorption factor (see w

The efficiency of the detector is a measure of the number of counts registered compared
to the number of particles that enter the sensitive volume of the detector. This efficiency
is approximately 100% for a- and high-energy/3-particles in most detectors, but often
substantially lower for -y-rays. Inasmuch as it is quite difficult to apply simple geometric
considerations to the solid angle subtended by a detector for a source which is not
concentrated at a point, usually the factors ~det and l~geo m are determined experimentally
by using a very thin standard source of approximately the same area as the unknown. The
factor ~bgeo m can be calculated for circular samples and detector windows, see w
It was noted in w that/~-rays undergo large angle deflections. As a result,/~-particles
from the sample which may start in a direction away from detector can be deflected by
several scattering events back into the detector. Such backscattering is dependent upon the
atomic number of the material upon which the sample is supported (cf. Fig. 6.14). ffback
increases with backing material thickness up to a saturation thickness beyond which it is
constant. Counting is usually done with either an essentially weightless backing (~kback =
1) or with a backing sufficiently thick as to have saturation. The saturation thickness