Monte Carlo Simulation of in situ Gamma-Spectra Recorded by Na I(Tl) Detector in the Marine Environment

To develop a NaI （T1） detector for in situ radioactivity monitoring in the marine environment and enhance the confidence of the probability of the gamma-spectrum analysis, Monte Carlo simulations using the Monte Carlo N-Particle （ MNCP ） code were performed to provide the response spectra of some interested radionuclides and the background spectra originating from the natural radionuclides in seawater recorded by a NaI （T1） detector. A newly developed 75 mm × 75 mm NaI （T1） detector was calibrated using four reference radioactive sources 137Cs, 60Co, 40K and 54Mn in the laboratory before the field measurements in seawater. A simulation model was established for the detector immersed in seawater. The simulated spectra were all broadened with Gaussian pulses to reflect the statistical fluctuations and electrical noise in the real measurement. The simulated spectra show that the single-energy photons into the detector are mostly scattering low-energy photons and the high background in the low energy region mainly originates from the Compton effect of the high energy y-rays of natural radionuclides in seawater. The simulated background spectrum was compared with the experimental one recorded in field measurement and they seem to be in good agreement. The simulation method and spectra can be used for the accurate analysis of the filed measurement results of low concentration radioactivity in seawater.

一台低本底反符合γ谱仪本底主要来源的确定

放射性核素４０Ｋ是我们已经研制成的ＨＰＧｅ低本底反符合γ谱仪本底的主要来源。本文介绍确定４０Ｋ来源的方法及其结果。结果表明，４０Ｋ本底主要来自由北京核仪器厂生产的ＳＴ１０５型ＮａＩ（Ｔｌ）环探测器所使用的６个光导玻璃窗，其钾含量为１５．８％，所含４０Ｋ总的放射性活度约为９１０Ｂｑ。

建筑材料放射性核素测量

详细叙述了采用NaI低本底多道γ谱仪测量建筑材料天然放射性核素镭、钍、钾放射性比活度的原理与方法。通过测量纯镭、纯钍、纯钾标准源,计算了谱仪对镭、钍、钾的响应系数。采用逐道最小二乘法分析建材样品中镭、钍、钾放射性比活度,运用以上方法对建材样品标准源进行测量,分析结果与标准数据之差在3%之内,远低于国标对测量不确定度的要求。

Study of radiation background at the north crossing point of the BEPC in collision mode

Understanding the radiation background at the north crossing point （NCP） in the tunnel of BEPCII is crucial for the performance safety of the High Purity Germanium （HPGe） detector, and in turn of great significance for long-term stable running of the energy measurement system. Therefore, as the first step, a NaI（T1） detector is constructed to continuously measure the radiation level of photons as background for future experiments. Furthermore, gamma and neutron dosimeters are utilized to explore the radiation distribution in the vicinity of the NCP where the HPGe detector will be located. Synthesizing all obtained information, the shielding for neutron irradiation is studied based on model-dependent theoretical analysis.

Measurement of radiation dose at the north interaction point of BEPC Ⅱ

The technique details for measuring radiation dose are expounded. The results of gamma and neutron radiation levels are presented and the corresponding radiation shielding is discussed based on the simplified estimation. In addition, the photon radiation level move as background for future experiments is measured by a NaI（T1） detector.