Study of the NaI(Tl) efficiency by Monte Carlo method

The Sodium Iodide detector NaI(Tl) presents a great efficiency for γ-photon and the X-ray detection. The purpose of the present work is to study this efficiency versus different parameters that are related to the detection phenomenon. This has been achieved by using both analytical method and Monte Carlo simulation which give the same results. We have found that the intrinsic efficiency has a minimum at d/R≈0.7 (d is the source-NaI(Tl) distance and R is the NaI(Tl) radius). In order to explain this minima we have used the Dirac theory for the mean chord image of photons in the NaI(Tl) crystal and we have determined asymptotic limits of the efficiency corresponding to the following asymptotic limits of the mean chord: image=L (L: NaI(Tl) length) for large d/R and image=4/3R for small d/R for point source. For distributed source (disk) the minimum is less pronounced and the asymptotic limit of the intrinsic efficiency is less than for the punctual source for small d/R while they are the same for large d/R. The total efficiency falls down at the d/R ratio corresponding to the minimum of the γ-photons mean chord in the NaI(Tl) crystal so for a good photon detection we have shown that the source–detector distance must respect the inequality: d/R<0.01.