Simulated minimum detectable activity concentration (MDAC) for a real-time UAV airborne radioactivity monitoring system with HPGe and LaBr3 detectors

An automatic real-time UAV airborne radioactivity monitoring system with high-purity germanium (HPGe) and lanthanum bromide (LaBr₃) detectors (NH-UAV) was developed to precisely obtain small-scope nuclide information in major nuclear accidents. The specific minimum detectable activity concentration (MDAC) calculation method for NH-UAV in the atmospheric environment was deduced in this study for a priori evaluation and quantification of the suitability of NH-UAV in the Fukushima nuclear accident, where the MDAC values of this new equipment were calculated based on Monte Carlo simulation. The effects of radioactive source term size and activity concentration on the MDAC values were analyzed to assess the detection performance of NH-UAV in more realistic environments. Finally, the MDAC values were calculated at different shielding thicknesses of the HPGe detector to improve the detection capabilities of the HPGe detector, and the relationship between the MDAC and the acquisition time of the system was deduced. The MDAC calculation method and data results in this study may be used as a reference for in-situ radioactivity measurement of NH-UAV.     

Upgrade of beam energy measurement system at BEPC-Ⅱ

The beam energy measurement system is of great importance for both BEPC-Ⅱ accelerator and BES-III detector. The system is based on measuring the energies of Compton back-scattered photons. In order to meet the requirements of data taking and improve the measurement accuracy, the system has continued to be upgraded, which involves the updating of laser and optics subsystems, replacement of a view-port of the laser to the vacuum insertion subsystem, the use of an electric cooling system for a high purity germanium detector, and improvement of the data acquisition and processing subsystem. The upgrade system guarantees the smooth and efficient measurement of beam energy at BEPC-II and enables accurate offline energy values for further physics analysis at BES-Ⅲ.     

HPGe detector photopeak efficiency calculation including self-absorption and coincidence corrections for cylindrical sources using compact analytical expressions

Total and full-energy peak efficiencies, coincidence correction factors and the source self-absorption of a p-type coaxial HPGe detector for cylindrical sources have been calculated using direct analytical expressions. In the experiments gamma aqueous sources containing several radionuclides covering the energy range from 60 to 1836 keV were used. By comparison, the theoretical and experimental full-energy peak efficiency values are in good agreement.     

Measurement of transition quadrupole moments of high-spin states in the N=74 isotones Pr, Ce and La

The Doppler-Shift Attenuation Method has been used to extract transition quadrupole moments of high-spin bands in the N=74 isotones ¹³³Pr, ¹³²Ce and ¹³¹La, produced in the ³⁷Cl + ¹⁰⁰Mo reaction. The results appear to be configuration dependent and, for ¹³³Pr and ¹³²Ce, the involvement of Ω=1/2 νh_9/2 and νf_7/2 intruder orbitals appears to enhance the collectivity at high spin (I>25 ).     

Mass distribution in the bremsstrahlung-induced fission of Th,U and Pu

The yields of various fission products in the 10 MeV bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were determined using a recoil catcher and off-line γ-ray spectrometric techniques. From the yield data, mass yield distributions were obtained using charge distribution corrections. The higher yields of fission products around mass numbers 133–135, 138–140, 143–145 and their complementary products in the neutron and bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were interpreted based on nuclear structure effects. From the mass yield distribution, the peak-to-valley (P/V$P/V$) ratio was also obtained for the above fissioning systems. The present data, along with data from the literature on different bremsstrahlung- and mono-energetic neutron-induced fissions of ²³²Th and ²³⁸U are interpreted to examine the influence of excitation energy on the peak to valley ratio. For the same compound nucleus ²⁴⁰Pu^?, the data in the 10–30 MeV bremsstrahlung-induced fission of ²⁴⁰Pu were compared with similar data of thermal to 14 MeV neutron-induced fission of ²³⁹Pu and the spontaneous fission of ²⁴⁰Pu to examine the role of excitation energy due to bremsstrahlung radiation and mono-energetic neutrons.     

HPGe 探测器晶体尺寸及效率的研究

采用蒙特卡罗方法模拟计算光子的探测效率需对高纯锗( HPGe ) 探测器进行准确的建模。模拟计算研究了HPGe 晶体长度对探测效率的影响,并在空间不同的测量位置对放射源进行测量,将模拟计算与实验测量相结合,对探测器晶体的尺寸进行调整,获得了HPGe 探测器晶体的模拟计算的准确尺寸,并在122s1 332 keV 能量范围内对模拟结果进行了验证。结果表明,在此能量范围内HPGe 探测器的探测效率的模拟计算值与实验测量值的相对偏差大多数在5% 内,并建立点源探测效率与探测器轴向距离的关系。The accurate shape of HPGe detector is needed in order to calculate its detection efficiency with Monte Carlo methods. In our calculation, the influence of the HPGe crystal size on efficiency has been investigated; the final crystal sizes were determined by comparison with experiments and were validated by the experimental efficiency obtained at several source-to-detector positions. The results show that when the crystal dimensions determined are used to calculate the detection efficiency in the energy range 122s1 332 keV, the relative deviations between simulated and most experimental results were within 5%. In addition, the relationship between detection efficiency and axial distance to detector was established.     

Optimum condition of efficiency functions for HPGe γ-ray detectors in the 121-1408 keV energy range

The optimum condition of three commonly used functions in the Genie 2000 γ spectra analysis software have been studied in the 121-1408 keV energy range. The three functions are applied for fitting the full-energy peak efficiency of the HPGe gamma-ray detector. A detailed procedure to obtain the optimum condition is described. The HPGe detector is calibrated at 11 cm by three radioactive sources of point form （^152Eu, ^137Cs, ^60Co） providing 11 energy peaks. After data processing, results shows that the three functions used in the Genie 2000 gamma spectra analysis software fit best at orders 3-5. Lastly the standard radioactive source 133Ba is chosen to validate the results. Differences between the standard activity of 133Ba and the result obtained from the fitting functions are below 1.5%. Therefore the optimum orders of the three functions used in the Genie 2000 3, spectra analysis software are 3-5 with the 11 energy peaks.     

A formula used to subtract the effect of gamma-ray of the others to that of measured reaction in measurement of cross section of nuclear reaction and its application

According to the regulation of growing and decay of artificial radioactive nuclide, a formula used to subtract the effect of characteristic γ-ray of the others to that of measured reaction was deduced. And then the cross sections of ¹²⁰Te (n,2n)^119mTe reaction induced by neutrons around 14~MeV were measured by activation relative to the ⁹³Nb (n, 2n) ^92mNb. In the process of the cross sections measured to be calculated, it was subtracted that the effect of characteristic γ-ray of ¹²⁶Te (n, p) ¹²⁶Sb to that of measured ¹²⁰Te (n, 2n) ^119mTe reaction using the formula deduced. The experimental results were (689±37) and (750±41) mb at the neutron energies of (13.5±0.3) and (14.6±0.3)MeV, respectively. Measurements were carried out by γ-detection using a coaxial HPGe detector. As samples, spectroscopically pure tellurium powder has been used. The fast neutrons were produced by the T(d, n)⁴He reaction. The neutron energies in these measurements were determined by the method of cross-section ratios between ⁹⁰Zr (n, 2n) ^89m+gZr and ⁹³Nb (n, 2n) ^92mNb reactions.     

Method for developing HPGe detector model in Monte Carlo simulation codes

This paper describes a simulation model for a high-purity germanium (HPGe) detector by scanning the detector with a low-energy collimated source and calibrating the efficiency of the detector. The parameters of the initial simulation model, particularly the dead-layer thickness, were modified step by step according to the experimental results. The validity of codes, models, and methods was shown by implementing the model into the FLUKA and MCNPX codes and cross-checking against the experimental data.     

Working principles of the energy measurement system at BEPC Ⅱ

The proposed beam energy measurement system at BEPCⅡ is composed of three parts:the laser source and optics system,the laser-electron interaction system and the HPGe detector system.The working principles of each system are expounded together with the calculation for preliminary design.The normalizations of laser and electron beams are put forth and used for the evaluation of intensity of the backscattering photon.The simulation of HPGe detector is also performed for understanding the working properties.