Analytical Chemistry

this problem. In addition, background currents result from the presence of impurity carriers (n-type or
p-type) within the crystal.

Intrinsic detectors employ semiconductor material of the highest purity to overcome the problem of
background currents and modern detectors are constructed on this basis. However, before the
technology for producing the highly pure material was routinely available, lithium drifted detectors
were in widespread use, and may still be encountered. Lithium drifted detectors were produced by first
doping an intrinsic crystal (Ge or Si) with p-type impurities such as B, AI, Ga or In. Lithium, a strongly
n-type element, was then 'drifted' into one end of the crystal. On refrigeration of the crystal a permanent
p–n junction was created within it. The electrical polarity produced led to n-type impurities migrating to
the p-type region and p-type impurities to the n-type region. Thus a depletion layer of highly pure
crystal was produced around the junction. Application of a reversed-bias high voltage to the crystal
increased the width of the depletion layer. Typical construction of semiconductor detectors is shown in
Figures 10.13 and 10.14.

Figure 10.13

Cross-sections through typical semiconductor radiation detectors. The same

geometry is used for both intrinsic and semiconductor types.

Figure 10.14

Construction of a 'dipstick' type semiconductor

detector system.