Measurement of low-level radioactivity in environmental samples by gamma-ray spectrometry

A low-level high-purity Ge detection system has been set up at about 15 m above the sea level in a normal counting room where the exposure rate is about 60 nSv h⁻¹. A background reduction is achieved by material selection, cryostat configuration and a single lead shielding. The integrated count rate within the energy interval 70 to 3000 keV is (0.498 ± 0.001) s⁻¹. The background in this energy range is reduced by a factor of about 100. An efficiency calibration of the detector in the energy range 50 to 2000 keV and according to the Marinelli-beaker configuration, type IEC-450, filled up with different matrices and suitable radionuclides has been performed.Field sampling of phragmites communis and juncus communis and gamma-ray measurements performed on dry matrices after treatment of the fresh material, indicate low-level contents of the nuclides ¹³⁴Cs, ¹³⁷Cs, ⁶⁰Co, ²³²Th, ²²⁶Ra, ⁷Be and ⁴⁰K distributed in the plants. It can be concluded that both selected plants are suitable bioindicators to measure the distribution of some anthropogenic and natural radionuclides in the environment.     

Techniques and problems of low-level gamma-ray spectrometry

The origin of background events from internal and external sources is reviewed and methods of passive and active shielding are presented. It is shown that the shape of the background under a peak or multiplet can be evaluated directly from the experimental data without the use of a mathematical model shape. An algorithm for the computerized sub-division of a spectrum into regions is shown and problems associated with smoothing and automatic photopeak, or full energy-peak, detection methods are discussed. A time-dependent spectral analysis is taken as an example to demonstrate the use of averaging or trend analyses in quantitative spectrometry.     

Measurements of activation induced by environmental neutrons using ultra low-level gamma-ray spectrometry.

The flux of environmental neutrons is being studied by activation of metal discs of selected elements. Near the earth's surface the total neutron flux is in the order of 10(-2) cm(-2)s(-1), which gives induced activities of a few mBq in the discs. Initial results from this technique, involving activation at ground level for several materials (W, Au, Ta, In, Re, Sm, Dy and Mn) and ultra low-level gamma-ray spectrometry in an underground laboratory located at 500 m.w.e., are presented. Diffusion of environmental neutrons in water is also measured by activation of gold at different depths.     

Optimized background reduction in low-level gamma-ray spectrometry at a surface laboratory.

The background of a coaxial Ge detector placed at a surface laboratory has been reduced by means of a background reduction setup consisting of a passive shield of low-activity lead, a simple radon suppression system and an active shield with a plastic scintillation plate. In particular, we have devoted our efforts to in-depth optimization of each parameter associated with different anticoincidence setups and to their subsequent intercomparison. The overall performance of the active shield was improved by using the optimum time parameters for each setup. The final objective is to decrease the cosmic-ray background and, by this way, to reduce the detection limits of gamma-ray spectrometers at conventional laboratories, and consequently make them competitive for different measurements like (210)Pb dating.     

Intercomparison of efficiency transfer software for gamma-ray spectrometry.

The EUROMET project 428 examines efficiency transfer results for Ge gamma-ray spectrometers when the efficiency is known for a reference point source geometry. For this, different methods are used, such as Monte Carlo simulation or semi-empirical computation. The exercise compares the application of these methods to the same selected experimental cases to determine the usage limitations versus the requested accuracy. For carefully examining these results and trying to derive information for improving the computation codes, this study was limited to a few simple cases. The first part concerns the simplest case of geometry transfer, i.e., using point sources for 3 source-to-detector distances: 2, 5 and 20 cm; the second part deals with transfer from point source geometry to cylindrical geometry with three different matrices. The general conclusion is that the deviations between the computed results and the measured efficiencies are mostly within 10%. The quality of the results is rather inhomogeneous and shows that these codes cannot be used directly for metrological purposes. However, most of them are operational for routine measurements when efficiency uncertainties of 5-10% can be sufficient.     

Crystal rounding and the efficiency transfer method in gamma-ray spectrometry

The effect of failing to properly model the rounding of the crystal edges in the application of the efficiency transfer method to the measurement of environmental samples is investigated for coaxial and thick planar (BeGe) HPGe detectors. It is found that for lower energies proper modelling of the crystal rounding is necessary to achieve the accuracy of the computed efficiency usually required in this type of measurements.     

Improvements of a low-level gamma-ray spectrometry system at the underground laboratory “UDO”

The Physikalisch-Technische Bundesanstalt (PTB) operates various low-background γ-ray spectrometry systems at the underground laboratory for dosimetry and spectrometry “UDO” in the Asse salt mine. Experiences gained with these detector systems within 10 years of operation have led to technical changes and improvements of the most sensitive detector system at UDO. Key aspects are the precautions to suppress radon (and its progenies) and the performance of the detector system after exchanging the end cap. In addition, a brief summary of recent applications of this γ-ray detector system will be presented.     

中日γ能谱分析土壤样品中放射性核素的比对

目的 通过开展γ能谱测量分析比对活动,检验比对样品的采集、制备、测量和分析全过程,以促进放射性核素γ能谱测量分析技术的发展。方法 由中国疾病预防控制中心辐射防护与核安全医学所采集和制备环境土壤样品,比对各方分别对两个土壤样品中的²¹⁴Pb、²¹⁴Bi、²⁰⁸Tl、²²⁸Ac、⁴⁰K和¹³⁷Cs 6个核素进行测量和分析。根据3个实验室约定测量结果的评定标准,利用各实验室上报的活度浓度和总不确定度数据计算测量结果评价变量(En)值。结果 每两个实验室间计算的En值均小于1,3个实验室的结果均为可接受的结果。结论 各比对方的测量结果在一定范围内是一致的。本次比对既检验了比对方实验室的核素分析水平,又促进了本实验室样品制备中均匀性检验能力的提高。     

Development of a low-level background gamma-ray spectrometer by KRISS

A new low-level background and high-efficiency gamma-ray spectrometric system, to be used mainly for the activity certification of natural-matrix certified reference materials (CRMs) and environmental reference materials (RMs) that has been developed on the grounds of the Korea Research Institute of Standards and Science (KRISS). The spectrometer consists of a low-background high-purity germanium detector with a relative efficiency of 120% and various shielding devices to reduce radiation background. The cabinet-shaped device made of 10 ton of shielding materials encloses the germanium detector for protection against background from natural radioactivity and neutrons. Three plates of 50-mm-thick plastic scintillation detectors on top of the passive shielding cabinet suppress cosmogenic background by detecting high-energetic cosmic muons bombarding the germanium detector. The measured background rate of the spectrometer for the energy range 50–3000 keV was 1.72 s^–1.     

MCNP modelling of scintillation-detector gamma-ray spectra from natural radionuclides.

gamma-ray spectra of natural radionuclides are simulated for a BGO detector in a borehole geometry using the Monte Carlo code MCNP. All gamma-ray emissions of the decay of 40K and the series of 232Th and 238U are used to describe the source. A procedure is proposed which excludes the time-consuming electron tracking in less relevant areas of the geometry. The simulated gamma-ray spectra are benchmarked against laboratory data.     

Measurement of beta-plus emitters by gamma-ray spectrometry

The activity measurement of beta-plus emitters by gamma-ray spectrometry is studied. Experimental measurements are performed with ²²Na, ⁶⁵Zn and ⁶⁴Cu with sources included in a lead container. For these nuclides, the activity can be derived both from one photon emission peak and from the 511 keV annihilation peak, including annihilation in-flight correction and geometry correction computed by Monte Carlo simulation. The activity values obtained using the two types of peaks show satisfying agreement. The extension of the method to volume sources is discussed.     

The on-line characterization of a radium slurry by gamma-ray spectrometry.

We have developed an in-line monitor to directly measure the ²²⁶Ra concentration in a nuclear waste stream using quantitative gamma-ray spectrometry applied to the 186 keV emission. The waste stream is in the form of a slurry composed of the solid waste material mixed with water. The concentration measurement includes a self-attenuation correction factor determined from a transmission measurement using the 122 keV γ from ⁵⁷Co. Presented here is the model for the measurement system and results from some initial tests.     

A comparative study of the radon-induced background in low-level gamma-ray spectrometers

The radon-induced background of ten, high-resolution, germanium, gamma-ray spectrometers was analyzed. In the analysis the apparent activity was introduced, which is defined as the peak count rate normalized with respect to the emission probability and the detection probability. On the basis of its energy and time dependence, the contributions to the background count rates due to the radiation penetrating the shield and the radiation due to the contamination of the air with radon daughters were determined.     

On the invariability of the total-to-peak ratio in gamma-ray spectrometry.

It is demonstrated that the assumption on the independence of the total-to-peak ratio of the source position in gamma-ray spectrometry holds for coaxial detectors with sufficient accuracy to make it applicable to the determination of the full energy peak efficiencies in environmental measurements via the so-called efficiency transfer method. The calculations show that for planar detectors the assumption breaks down at higher gamma-ray energies and that for such detectors the use of the efficiency transfer method is only warranted for gamma-ray energies up to 200 keV.     

Determination of 210Pb concentration in the air at ground-level by gamma-ray spectrometry.

210Pb activity concentrations in the air of Palermo were determined by gamma-ray spectrometric analysis of 323 particulate samples collected in the period September 1995-December 1996. For each sample, the air filtered through a cellulose filter paper was 8600 m3 on average. The values of the daily activity concentration of 210Pb were ranging from 136 to 3390 microBq/m3.     

Minimum detectable activity in in situ gamma-ray spectrometry.

The algorithm used to calculate a minimum detectable activity (A(D)) of an environmental volume sample measured by gamma-ray spectrometry was modified and adapted for in situ measurements performed at the standard 1 m height above ground. AD values of target radionuclides were determined from the in situ spectra collected at two disparate sites. A linear relationship between the two sets of A(D)s was found and the ratio between these two sets is equal to the square root of the ratio of the two respective absorbed dose rates measured at the sites at 1 m height. Absorbed dose rates were calculated using the concentration of potassium, thorium and uranium in the soils at the sites and found to agree well with the measured values. A(D) values can be predicted easily in in situ gamma spectrometry applying a simple experimental procedure that is based on the linear relationship.     

3D GPS mapping of land contaminated with gamma-ray emitting radionuclides.

Two studies are given in this paper. Both studies use a combined gamma sensing and global positioning system to monitor land contaminated with gamma emitting radioisotopes. The first is the 3D profiling a large (100m x 200m x 15m) tip of the residue from radium extraction processing. The second is the dynamic monitoring of beaches to locate and recover small particles of radioactive material.     

Comparison of inductively coupled mass spectrometry and ultra low-level gamma-ray spectroscopy for ultra low background material selection

The main target of this study is the development and the validation of an internal procedure to measure the pg g⁻¹ levels, of naturally occurring radioisotopes to select ultra low background detector materials. As there are no ultra low-level certified reference materials commercially available, the performance of ICP-MS was compared with the well established ultra low-level γ-spectroscopy. This work shows also the reliability of very quick ICP-MS semi-quantitative analyses of an environmental matrix such as a geological sample treated according to conventional protocols.     

An anticoincidence-shielded gamma-ray spectrometer for analysis of low level environmental radionuclides.

We developed an ultralow-level background gamma-ray spectrometer, using active and passive shield devices at the same time. Cosmic-ray-induced background is suppressed by means of active shield devices consisting of plastic scintillating plates of 50mm thick and anti-coincidence electronic system. The observed background rate was 0.34 s(-1) (=0.12s(-1) per 100 cm(3) Ge volume) for energy regions between 50 and 3000 ke V. The detection efficiency curve for 10(3)ml Marinelli beaker samples is obtained over all the energy regions. The advantages of the method are demonstrated by measuring the activity of 137Cs in powdered milk sample prepared without taking any chemical procedure. The MDA for 137Cs is estimated to be (17+/-1.7)mBq at a confidence level of 95% and it is about a factor of 10 lower than the MDA obtained from the previous cryostat assembly with 10-cm thick lead shielding.     

Monte Carlo simulation of gamma-ray spectra from natural radionuclides recorded by a NaI detector in the marine environment.

The GEANT4 Monte Carlo code has been used to simulate gamma-ray spectra of natural radionuclides collected by a NaI scintillation detector immersed in seawater. The gamma-rays emitted from the decay of (40)K, and the series of (232)Th and (238)U, were used to describe the radioactive water source around the NaI crystal. The simulated gamma-ray spectra were compared with real data recorded in situ by a newly constructed NaI spectrometer and were found to be in good agreement. The NaI spectrometer was calibrated in the laboratory in a water tank, before its deployment in seawater. Activity concentrations were deduced from the gamma-ray spectra and discussed in comparison with results from the literature.