Page 245 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 229

10"2

32% Relative Efficiency

>. - r GEM and GMX
(.1 I
z 10 "=
uJ I
r I GEM
It_
u.
ILl I
~ _ I
GMX
.J I
0 10-4 ._
I
! i all 1
I I ill i i Ill I I I
lO-S
10-~ 10-2 10 -t 100 10 t
ENERGY (MeV)
FIG. 8.20. Typical efficiency curves for HPGe detectors. GEM and GMX are detectors of
n- and p-type with Be-window respectively. (Acc. to ORTEC.)

8.8. Special counting systems

For low intensity measurements, in which a level of radioactivity comparable to the
normal background radiation is to be measured, special electronic circuits incorporating two
detectors are used. The detectors are coupled so that a signal registers only when both
detectors are activated at the same time (coincidence circuit) or, alternatively, when only
one but not the other is activated at the same time (anticoincidence circuit). With such
arrangements the normal/~, "y background of a detector may be decreased by more than a
factor of 100. In the most advanced coincidence techniques both detectors are energy
sensitive as well, providing information on the type of radiation being measured. For
example, B-T coincidence measurements are used for absolute determination of radioactivity
for samples in which B-decay is immediately followed by -y-emission.
Detectors which are direction sensitive have been developed primarily for use in medical
diagnosis with radioactive isotopes. The simplest version involves a scintillation detector
surrounded by a lead shield with a small hole (collimator) through which radiation reaches
the detector. More complex systems with many detector/collimator pairs are in common use
(T-cameras); of. w Scanning instruments (Fig. 9.13) have been developed which permit
the measurement of radioactivity as a function of several coordinates as illustrated in Figure
9.14. Such instruments make it easy to detect the accumulation of a radioactive tracer in
a particular organ of the body.
Whole-body counters were originally developed for investigation of poisoning by
radioactive substances such as radium. They are now used diagnostically and consist of a
large scintillation, or semiconductor, detector with the whole system, including the patient,
placed in a heavily shielded room. The sensitivity is sufficient to measure natural
radioactivity in the human body from such nuclides as 4~