Efficiency of Nal Tl Detectors

The efficiency increases with crystal size. The user should be aware, however, that when the detector volume increases, the background counting rate increases too. In fact, the background is roughly proportional to the crystal volume, while the efficiency increases with size at a slower than linear rate. Thus, there may be a practical upper limit to a useful detector size for a given experiment. 

Calculated absolute total efficiencies of a Nal crystal are given in Fig. 12.15 for several source-detector distances. They have been obtained by integrating Eq. 8.20, which is repeated here (refer to Figs. 8.21 and 12.12 for notation)  

The inherent approximation of Eq. 8.20 is that it considers detected every photon that interacted at least once inside the detector. 

In Fig. 12.15, note that the efficiency decreases with energy up to about 5 MeV. Beyond that point, it starts increasing because of the increase in the pair 

13) changes with energy for a source located 0.10 m from detectors of different sizes. 

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Scintillation detectors with Nal(Tl) crystals may also be used for y spectrometry. Because Nal(Tl) crystals can be made in larger size than Ge crystals and because the atomic number of I is larger than that of Ge, the internal counting efficiency of Nal(Tl) detectors for y rays is higher than that of Ge crystals, as already discussed in section 7.5 (Fig. 7.16). On the other hand, the energy resolution is appreciably lower (5 to 7% for y energies of the order of 100 keV). Scintillation detectors are operated in a way similar to that used with Ge detectors, but without cooling. 

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