Dependence of NaI(Tl) detector intrinsic efficiency on source-detector distance, energy and off-axis distance: Their implications for radioactivity measurements

Abstract

In this work the dependence of intrinsic efficiency of a NaI(Tl) detector of radius 3.82 cm and height 7.62 cm on source-detector distance (d), source-off-axis distance (d0) and γ-photon energy have been investigated using analytical and Monte Carlo methods. The results showed that, for a given off-axis distance, there exists a value of the ratio of source-detector distance (d) to detector radius (R) where intrinsic efficiency is minimum. This d/R value at which minimum efficiency occurs approaches zero as offaxis distance increases and it is almost constant with increase in energy. In the region where d/R < 0.01, a criteria given by Jehouani et al [1] for good photon detection, intrinsic efficiency decreases with increasing off-axis distance. The implications of the results for radioactivity measurement and radiation protection are discussed. Chacteristics of intrinsic efficiency in the regions d/R < 0.01 and d/R > 10 are also compared.

Evaluation of the total and intrinsic efficiencies of a 3 in x 3 in NaI(Tl) crystal by using the hybrid Monte Carlo method

“…When d/R increases, the intrinsic efficiency decreases, and after passing through minimum, it increases again. Similar results have been reported by Jehouani [8] and Ogundare [6].…”

A method to calculate the gamma ray detection efficiency of a cylindrical NaI (Tl) crystal

“…Although both geometric and intrinsic efficiencies are the function of source's situation and detector's shape, the intrinsic efficiency can be also improved by changing the internal material and construction of detector [20]. Incident photon interacts with detector by three phenomena: photoelectric effect, Compton scattering, and pair production, which depending on the energy of photon and detector's material, one of them is dominant [21,22]. The total interaction probability of 662 keV-gamma ray was used for the NaI (Tl) detector.…”