Page 233 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 217

NaI(TI) 4x4"_ ~.~

Q~

0
X
^
'~-'--- .- : __.A_.:- : ........
o
o
Oo HPGe 2 2Z
0
i ! i i i i )
0 500 1000 1500
E(keV)
FIG. 8.15. Energy spectrum of ~~ obtained with scintillation and HPGe detectors.

eqn. FWHM = (n 2 + kEv)~h, where FWHM is the full ~ width at half maximum, n the
noise line width, k = 2, and E "),-energy, all in eV units. This resolution is far superior to
that of the scintillation detector, as is seen from Figure 8.15. On the other hand, the
detection efficiency is usually smaller. However, an efficiency higher than 100% of that
obtained with a 3" x 3" NaI(TI) scintillation detector for 1.33 MeV "y-rays (which is the
reference for Ge-detector efficiency) has been obtained in the best designs, but typical
values are 10 - 30 %.

8.4.3. Intrinsic detectors

One reason for drifting lithium into silicon and germanium is the necessity to compensate
some p-type (acceptor) impurities normally l~resent in pure materials. It is now possible to
increase the purity of germanium to 1:l0 b (compared to earlier 1" 1011) Which makes
lithium-drifting unnecessary. In addition, these intrinsic germanium detectors make
uninterrupted cooling less important; it is not required when storing the crystals, but should
be used when measuring in order to improve resolution and prevent crystal overheating.
Depending on the type of dominating impurity, HPGe crystals can be of either n- or p-type.
HPGe-detectors of p-type have usually a lower energy cutoff, beginning at about 100 keV,
caused by the detector capsule wall and insensitive entrance layer. Commercially available
p-type detectors have relative efficiencies of 10 - 100 %. On the other hand n-type HPGe-
detectors can be made with a very thin entrance layer and, when fitted with a Be-window,
have a low energy cutoff beginning at about 6 keV. Relative efficiencies for n-type HPGe
detectors are in general somewhat less than for p-type detectors, i.e. 10 - 80%. Intrinsic
germanium detectors are to a large extent replacing the lithium-drifted germanium detectors
because of their greater handling simplicity at no higher cost or loss of resolution.