200
PERKINS, THOMAS, AND NIELSEN
~ Nol(Tl} ANTICOINCIDENCE
SHIELD
.
[~~ SAMPLE LOCATION
h Nal(Tl) PRINCIPAL DETECTORS
(4 IN. THICK AND 6 IN.
IN DIAMETER)
PURE Nal CRYSTAL
LIGHT PIPE
PHOTOTUBES
Fig. 1—Detection system for multidimensional gamma-ray spectrometer.
detectors, the event is stored on the corresponding X or Y axis of the
memory. When two photons are emitted simultaneously, each interacting with a separate detector, the event is stored in an energyenergy plane at a point uniquely characteristic of the two energies.
The spectra of “Cu and Co illustrate (Figs. 2 and 3) the manner of
storage. The normal spectra of these radionuclides as seen by one of
the detectors are shown for comparison. With such a detection system,
the unique decay characteristic of many radionuclides is readily
resolvable. This permits a far greater sensitivity for detection and
direct measurement of many minor radioactive constituents. In addi-
tion, a much lower backgroundis obtained.
,
In Fig. 4 the background of a 4-in.-thick 6-in,-diameter singlecrystal detector without a light pipe or anticoincidence shielding is
compared with that of the two-crystal multidimensional detector sys-
tem in which the two detectors are separated by a 1-in.-thick lead