Abstract Present paper studies the γ -ray spectrum observed using NaI (Tl) scintillation detector over a year. The paper reports the presence of a distinct diurnal pattern in the total number of γ -ray counts detected by the NaI detector. The counts start decreasing after sunrise and show gradual recovery after sunset. The amplitude of this variation is quite significant ( > 10%) at the observation site Tirunelveli, South India. However, further investigation based on different energy ranges reveals that the mentioned diurnal pattern is actually present only in the energies related to the terrestrial background radioactivity. The study demonstrates that the pattern is associated with the radioactivity of isotopes of radon (222Rn and 220Rn) and their daughter radionuclides. The amplitude of the diurnal variation is found to have seasonal dependence, with the lowest amplitude during June–October ( ∼ 2%), and highest in April–May months ( ∼ 14%). The dependence of the amplitude of the diurnal variation on various atmospheric parameters, namely temperature, relative humidity, and the vertical wind is examined. The observed diurnal pattern is attributed to the generation of the inversion layer. The distribution of concentration of radon and thoron progenies in an air column changes with the vertical mixing and atmospheric boundary layer (ABL), resulting in the diurnal variation of γ -rays. The observed lower amplitudes in the months of June–October may be due to the presence of downward winds restricting the radon movement to a thin layer, giving smaller amplitudes of the diurnal variation in total γ -ray flux resulting in seasonal variation.

中文翻译：

---

使用 NaI (Tl) 探测器在伽马射线光谱中观察到的昼夜变化的原因

摘要 本文研究了使用 NaI (Tl) 闪烁探测器一年多来观测到的 γ 射线光谱。该论文报告了 NaI 探测器探测到的 γ 射线计数总数中存在明显的昼夜模式。日出后计数开始下降，日落后逐渐恢复。在印度南部蒂鲁内尔维利的观测点，这种变化的幅度非常显着（> 10%）。然而，基于不同能量范围的进一步调查表明，上述昼夜模式实际上仅存在于与地球背景放射性相关的能量中。研究表明，这种模式与氡（222Rn 和 220Rn）同位素及其子放射性核素的放射性有关。发现昼夜变化的幅度具有季节性依赖性，6-10月的幅度最低（~2%），4-5月的幅度最高（~14%）。检查了昼夜变化幅度对各种大气参数（即温度、相对湿度和垂直风）的依赖性。观察到的昼夜模式归因于逆温层的产生。气柱中氡和钍子的浓度分布随着垂直混合和大气边界层（ABL）而变化，导致γ射线的日变化。在 6 月至 10 月观察到的较低振幅可能是由于向下风的存在将氡运动限制在薄层中，