Shape distortion of ¹²⁸I ß ⁻spectrum observed by a self-activated CsI(Tl) scintillator for high-sensitivity neutron measurements

The factors causing the distortion of the ¹²⁸I ß⁻ spectrum detected by a self-activated CsI(Tl) scintillator were studied to verify the correctness of the spectral shape and the appropriateness of the discrimination setting for ß⁻-particle counting by the scintillator. These criteria are essential for the correct evaluation of radioactivity generated in a scintillator volume by the self-activation method, which was recently proposed by our group. A pulse height defect caused by the partial escape of ß⁻ particles from the surface of the scintillator crystal shifts the ß⁻ spectrum toward the lower-energy region when smaller CsI(Tl) scintillators are used (the ß⁻-escape effect). For larger CsI(Tl) scintillators, an increase in pulse height caused by the summing of 0.443 MeV prompt γ-rays from the excited state of the ¹²⁸I daughter nuclide (¹²⁸Xe) affects the shape of the ß⁻ spectrum considerably, resulting in a shift toward the higher-energy region (the γ-summing effect). The extent of the contributions of these two effects was examined by a Monte Carlo simulation of various cubical CsI(Tl) crystals of different sizes. It was found that the distortions caused by those two effects effectively cancel each other out for a medium-size cubical CsI(Tl) crystal with a side length of approximately 3 cm. This finding is very useful for the practical applications of the self-activation method. In addition to the factors mentioned above, the efficiency of scintillation light collection by the photodetectors also affects the shape distortion of the ß⁻ spectrum slightly through spectral line broadening due to the degradation of the energy resolution. This effect was estimated using a simple model with different discrimination settings for ß⁻ pulse counting.