Translartion. Region: Russians Fedetion –
Source: Novosibirsk State University – Novosibirsk State University –
Scientists Faculty of Physics, Novosibirsk State University developed a method for measuring ultra-low concentrations of radioactive substances whose decay is accompanied by gamma radiation. Data is collected using a detector made of ultra-pure germanium, which is part of the equipment of the Interfaculty Laboratory of Atomic Physics and Spectrometry of NSU; a special hardware and software complex was created for data processing. The first project implemented using this method is research work on determining the level of radioactive substances (radon) in the soil of mines and coal opencasts in the Kemerovo Region.
To measure the radioactivity of soil samples for various nuclides, gamma-ray spectra were collected using a detector made of ultra-pure germanium. This is unique equipment that allows for very precise determination of the energy of gamma quanta emitted by radioactive substances. Germanium is a rare chemical element in the Earth’s lithosphere. Like silicon, it is a semiconductor and is used in microelectronics, but its scope of application is narrow. As a detector material, its efficiency of photon registration is higher than that of silicon, so it is used in detectors of not only X-rays, but also gamma radiation. Obtaining ultra-pure germanium is a complex and slow purification process using the zone melting method, which determines the high cost and complexity of equipment manufacturing.
There are devices that can register gamma radiation with even greater efficiency than a germanium detector, but only it can distinguish closely spaced gamma-quanta energies, and therefore gamma-quanta from different radionuclides. This is called high energy resolution; for a detector made of ultrapure germanium, it is approximately 0.01% in the energy range characteristic of gamma-quanta from atomic nuclei (units of megaelectron-volt). High resolution plays a decisive role in measuring ultra-low concentrations of radioactive substances, when it is necessary to separate background radiation and sample radiation and determine specific emitting radionuclides.
NSU scientists have developed a unique, highly sensitive method that allows determining ultra-low concentrations of radioactive substances in any samples – soil, ground, rocks, etc. The method has been tested and proven effective during the implementation of a project to determine the content of radioactive substances (in particular, radon) in the soil of mines and coal mines in the Kemerovo Region. Kemerovo State University employees approached NSU with this task in the spring of 2024. The KemSU study is aimed at determining the influence of soil types, artificial (for example, mining) and natural changes in soils and climate on the radioactive environment. In the future, this may make it possible to predict the radiation environment, for example, during housing construction.
— The main difficulty of the task was that the provided soil samples had a very low concentration of radioactive substances. Therefore, it was necessary to collect a lot of statistics for a reliable result, and statistics of both the sample itself and the background, the indicators of which were then “subtracted”. The work lasted almost half a year, we involved research associates of the educational Interfaculty Laboratory of Atomic Physics and Spectrometry of NSU, as well as students undergoing practical training as part of their studies, — says Elena Starostina, senior lecturer of the Physics Department of NSU.
The first stage involved collecting data directly on the detector. In total, colleagues from KemSU provided about 230 samples weighing from 100 to 250 grams, obtained from different places and from different depths – half a meter, one meter and one and a half meters. Data was collected daily from May to November 2024, and a background spectrum was also collected every week, without samples.
The experimental setup was as follows: a detector made of ultrapure germanium, cooled by a nitrogen cryostat, is surrounded by a lead tube with a wall thickness of about 10 mm. The tube suppresses the flow of background gamma quanta from the room by about three times. The tube rests on a table with an opening for the detector. Samples were placed directly on the detector.
— In the case of measuring ultra-low concentrations close to natural ones, the main difficulty is related to the fact that there is background radiation. It can be weakened with a lead screen, which is what we did, but it is impossible to completely eliminate it. Even with all the measures, the radiation of the samples was more than 7 times weaker than the background. In order to obtain a good contrast between the background and the actual study of the samples, it is necessary to collect the spectrum over a long period. The spectrum of each sample was collected in half-hour portions, for at least three hours, then half-hour spectra of good quality were selected so that the total statistics time was at least 2.5 hours. Once a week, multi-hour background spectra were collected, — Vyacheslav Kaminsky, senior lecturer, curator of the Interfaculty Laboratory of Atomic Physics and Spectrometry of NSU, shares the details of the experiment.
Another feature of the experiment is that the geometry of the measurements is such that only about 10% of the gamma quanta from the sample get into the detector. There are well-type detectors made of ultrapure germanium, which surround the sample from almost all sides, but they can only accommodate small samples. The detector made of ultrapure germanium at NSU allows working with samples of any size, and the developed technique in a sense compensates for the insufficient efficiency of gamma quanta registration.
The experimental data are presented as spectra with peaks from gamma lines and a continuous “substrate”. The peaks have a complex shape: they resemble a Gaussian curve with different widths on the left and right, they have a “tail” on the left, and the substrate on the left and right has a different level. The width of this “bell” in energy units characterizes the detector resolution: the narrower the peak, the finer the measurements that can be made. This peak shape is provided by both the processes of interaction of gamma quanta with the detector substance and the environment (for example, the Compton effect), and the processes of charge formation during the absorption of gamma quanta in the semiconductor and its collection.
After collecting the data, the researchers were faced with the task of determining the radiation of the samples, eliminating the background. The spectra were processed and the activity of the radionuclides was calculated.
— The method consisted in the fact that in the obtained data, in which the difference between the background and the sample was very small, a joint fitting of individual gamma lines was carried out for the spectra with the sample and the background. Each isotope that emits gamma quanta can have a dozen gamma lines, they are different, at different energies and with different intensities. First, good, intense lines were selected so that they were not very close to each other. According to the set of good, intense lines, each peak was fitted, it was done simultaneously for the background and for the background with the sample. Such a complex procedure is necessary in order to measure not only the amplitude of the peaks, but also to correctly estimate the measurement error. The resulting difference between the amplitudes for the sample with the inevitable background and only the background are the indicators of the sample itself, — says Vyacheslav Kaminsky.
Several programs written in Python were developed to collect and process the experimental data. The first one was for automatic spectral acquisition, which also recorded which operator placed the sample. Another one was for selecting, calibrating and summing the spectra. The third one was for calculating the activities of radionuclides. In addition, a separate program calculated the absolute efficiency of the detector. The scientists used classical statistical methods to determine the peak parameters, such as the least squares method, implemented in the MINUIT2 software library.
The study revealed that the samples contained only radioactive isotopes potassium-40, thorium-232 and uranium-238 and their decay products, which are common radionuclides found in soils, rocks and many building materials. The specific activity of the samples ranged from 0.1 to 2 becquerels per gram (decays per gram). These values are within safe limits, but the most active sample (with an error of about 7%) is equivalent to several bananas (see “banana equivalent”, bananas are active mainly due to the potassium-40 they contain). The least active sample is equivalent to half a banana with an error of more than 50%, which indicates a very high sensitivity of the method. At the moment, the KemSU research team has received the measurement results and is processing them.
Thus, the method developed by NSU scientists allows measuring very low levels of radiation, and linking it to specific radiating agents – radionuclides. This method will find application in monitoring the environmental situation, for drawing up maps of radioactive contamination after radiation accidents, etc.
The scientists plan to register a data processing program with Rospatent, certify and license the methodology, and in the long term, create a center for collective use that will conduct comprehensive work on chemical analysis of samples using spectral methods in the optical, X-ray, and gamma ranges.
The NSU Interfaculty Laboratory of Atomic Physics and Spectrometry (Atomic Workshop) is an educational laboratory where students become familiar with a range of atomic and nuclear phenomena, including atomic radiation, light absorption, visible radiation, visible light absorption, magnetic phenomena, nuclear magnetic resonance, electron paramagnetic resonance, electron diffraction, etc. The laboratory is equipped with special equipment, including a detector made of ultrapure germanium, which allows studying radiation from natural objects. Students from the Physics Department and the Natural Sciences Department study in the laboratory, and experimental research is also conducted as part of coursework.
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