Abstract Experimental studies using a large-volume sodium iodide detector system show how the potassium, uranium, and thorium gamma-ray spectra vary with altitude above the ground. The shapes of the spectra were derived from measurements on large radioactive concrete calibration slabs using plywood sheets to simulate the absorption of the gamma radiation in the air. A mathematical analysis of the data showed that within the range covered by the experiment (0–112 m of air) the spectra of each of the three radioelements are made up essentially of two spectral components, whose proportions vary with the amount of absorbing material between the source and the detector. With this knowledge, it is shown how all the basic information relating to source–detector distance and source concentration may be extracted from the airborne gamma-ray spectrum.

Experimental potassium, uranium, and thorium gamma‐ray spectra covering a range of equivalent aircraft altitudes from 0 to 112 m were derived from measurements on large radioactive concrete calibration pads using plywood sheets to simulate the absorption effects of the air. A mathematical analysis of all three radioelement spectra showed they are composed of two basic spectral components which are added in different proportions dependent on the altitude of the aircraft above the ground. Above an energy of 0.662 MeV, the two‐component spectral model represented accurately the observed gamma‐ray spectrum from the ground. Below this particular energy, cesium-137 from atomic weapons fallout was found to contribute significantly to the airborne spectra. A theoretical analysis of the calculated spectrum from typical crustal material showed that by multispectral fitting above the cesium-137 energy, the thorium and uranium concentration errors could be reduced by approximately 25 percent compared to the standard ...

Noise-adjusted singular value decomposition (NASVD) is a spectral component analysis procedure for the removal of noise from gamma-ray spectra. The procedure transforms observed spectra into orthogonal spectral components. The lower-order components represent the signal in the original observed spectra, and the higher-order components represent uncorrelated noise. Noise is removed from the observed spectra by rejecting noise components and reconstructing the spectra from lower-order components. A synthetic dataset has been used to obtain new insights into the NASVD method. The dataset is based on an airborne survey over the Jemalong Plains area, NSW. The estimated ground concentrations of potassium (K), uranium (U) and thorium (Th) were spatially filtered and then used to synthesise airborne spectra using simulated Poisson noise. These spectra include a background component based on typical aircraft and cosmic background, and a simple model of the distribution of atmospheric radon. The application of NASVD smoothing to this dataset gives a much greater reduction in U concentration errors than that normally experienced. Careful investigation suggests that the reason for this is that the Jemalong Plains dataset exhibits high correlation between U and Th, and because the survey was flown on a day of low and near constant atmospheric radon concentration. If the dataset is modified by either adding spectra derived from an anomalous U/Th source, or by including atmospheric radon variations typical of most airborne surveys, then the large reductions in U concentration error are no longer achieved. Tests on the synthetic data suggest that the smaller the variation in spectral shape within the input signal, the greater the noise reduction. This is used to develop an improved implementation of the NASVD method that can be applied to large survey datasets. Instead of processing the survey data by flight, the entire survey database is sorted into clusters on the basis of similarity in spectral shape, and the NASVD method is applied to these clusters. This typically halves the K, U and Th fractional errors compared with those obtained when the data are processed according to flight. This increases the amount of geological information that can be extracted from enhanced images of the processed data.

Three hypotheses are tested for the pre-conditioning of airborne gamma-ray spectra to improve the accuracy of principal component-type (PC) spectral noise-reduction methods. First, I show that the distribution of the input variables (channel count rates) has little effect on the accuracy of the noise-reduction methods. Second, if there are insufficient spectra of a particular shape to form a statistically significant sample, then this shape will not be resolved by the noise-reduction methods, and will be removed as noise. However, by padding the data space with spectra exhibiting the full range of possible spectral shapes, an improvement in accuracy can be achieved. Third, the low signal-to-noise ratio in raw gamma-ray spectra limits the effectiveness of PC methods for removing the noise. If the signal-to-noise ratio in the input spectra is improved, the PC methods better remove the noise. Along-line summing of spectra improves the signal-to-noise ratio by exploiting the high correlation in signal between successive airborne gamma-ray spectra along each flight line. Summing spectra to optimum channels also improves the signal-to-noise ratio, but at the expense of spectral resolution. In both cases, spectral summing prior to the application of PC-type noise reduction results in a significant improvement in the accuracy of the noise-reduced spectra.

GR-820集优秀的稳谱技术、数字可视化设计和实时谱图显示于一身，提供了更为精确的伽马能谱测量。文中简要地介绍了GR-820的工作原理、性能和应用效果等几方面的内容，供读者了解当今航空伽马能谱仪的发展状况。

我国开展航空伽玛能谱测量虽然已经有20多年的历史,测量系统的标定却一直是借鉴国际原子能机构提供的航空伽玛能谱测量技术报告(323)作为测量规范,没有开展系统的质量评价研究.为了了解航空伽玛能谱测量的误差水平,2002年中国地质调查局开展了该项研究工作.

基于峰值采样保持的模拟能谱仪无法满足大尺寸多晶体高计数率的航空能谱测量。本系统采用了Y/U双通道数控增益放大器实现了谱漂的双16位分辨率调节范围,通过高速ADC与CPLD实现数字化能谱仪,在CPLD内部用VHDL语言编程实现基线恢复与峰高数据采集,保证了能量分辨率。结果表明:计数最大通过率大于100k/s,1 024道分辨率137Cs峰谱漂±1道,能量起始阈20 keV。实验证明本系统功耗低,性能可靠,体积小,可应用于航空能谱测量的场合。

航空数字化γ能谱测量系统以NaI(T1)晶体(10 inm×10 minx40 mm)和光电倍增管构成的闪烁计数器为丫射线探测器,采用4条下测晶体和1条上测晶体组合成航空γ能谱测量系统的探头,设计了温度传感器、电流反馈型前置放大器和低纹波高压供电电路,实现了γ光子与核信号的转换;采用Y/U双通道数控增益放大器、双回路自动稳谱电路和算法实现了谱漂的精细调节和自动稳谱,通过高速ADC与CPLD实现模拟核信号的数字化.该系统可接入NaI(T1)闪烁计数器γ探测器数20条,可探测能量范围0.02-10.0 MeV,采样周期0.5-1s,最大计数通过率>100 k/s,1024道谱漂<±1道.

航空伽玛能谱测量是直接寻找铀矿和进行区域地质调查的重要技术手段,在地质勘探、环境辐射评价、核应急和军控核查等方面具有重要作用。航空伽玛能谱仪探测K、U、Th核素衰变系列的伽玛射线窗口计数,要将测量计数率换算成地面放射性核素含量就需要对航空伽玛能谱测量系统进行标定,系统标定是航空伽玛能谱测量中最为复杂的程序,也是航空伽玛能谱测量的一项关键技术。论文系统介绍了目前中国国土资源航空物探遥感中心实施的航空伽玛能谱测量系统标定技术及相应的标定程序,这对理解航空伽玛能谱测量及资料应用具有实际意义。