Application of analytical, experimental, semiempirical and Monte Carlo methods for efficiency calibration of HPGe detectors in environmental samples gamma spectrometry

Квалитет гама спектрометријских мерења у општем сучају директно зависи од познавања ефикасности детекције за посебне случајеве геометрије мерења и карактеристика мереног узорка. С обзиром на то да узорци који се мере могу бити веома различити по хемијском саставу и геометрији, калибрација није доступна у свакој ситуацији. Поред тога, код мерења у контактној геометрији, што је најчешћи случај приликом мерења узорака из животне средине, феномен коинцидентног сумирања пертурбује спектар и мења површине испод пикова у спектру што резултује нетачним резултатима мерења. Тада се јавља потреба за развијањем различитих метода за калибрацију и специфичних референтних материјала за сваки појединачни случај. Методе калибрације могу подразумевати мерење сертификованих извора или секундарних референтних материјала, одређивање параметара потребних за израчунавање ефикасности или симулирање спектралног одговора инструмента. У овом раду представљено je и примењено неколико различитих приступа калибрацији ефикасности мерног система. То су: - Експериментални метод, који подразумева мерење секундарних референтних материјала који су по геометрији и хемијском саставу најприближнији узорцима који се мере. Овај метод даје најтачније резултате али истовремено захтева дуго време припреме секундарних референтних материјала и дуго време мерења. Све ово може бити извор низа мерних несигурности које су последица управо те припреме. -Аналитички метод, који подразумева успостављање аналитичке зависности између ефикасности и геометријских карактеристика мереног узорка и детектора. Овај метод је апроксимативан с обзиром на то да су интеграли који описују зависност за цилиндричне геометрије врло компликовани и њихово решавање је могуће једино нумеричким путем. Уведене апроксимације доприносе мерној несигурности али у оквирима потреба мерења узорака из животне средине могу бити прихватљиве. -Полуемиемпиријски метод (представљен у овом раду програмом за трансфер ефикасности, EFFTRAN) користи зависност ефикасности детектора од ефективног просторног угла под којим се узорак налази у односу на детектор и на основу тога рачуна трансфер ефикасности од референтне калибрационе криве до криве за реалну мерну геометрију и корекцију на коинцидентна сумирања. -Монте-Карло метод (представљен у овом раду програмским пакетима PHOTON и GEANT4) који се користи за симулацију спектралног одговора детектора на основу познавања процеса који доводе до депоновања енергије у медијуму детектора и случајног избора бројева. Сваки од наведених метода је примењен за калибрацију HPGe детектора који се користе у Лабораторији за заштиту од зрачења и заштиту животне средине Института за нуклеарне науке Винча. Као резултат добијене су ефикасности детектора, са придруженим мерним несигурностима. С обзиром на то да се мерење секундарних референтних материјала сматра најтачнијим методом калибрације, ефикасности добијене другим методама су упоређене са експерименталним резултатима да би се утврдило слагање или неслагање у оквиру мерне несигурности. Поређењем је установљено да се ефикасности добијене аналитичком методом разликују у односу на експериментално добијене ефикасности у опсегу од 1 до 14%, при чему су вредности одређиване за 3 геометрије које су најчешће у употреби у мерењу узорака из животне средине. Резултати добијени трансфером ефикасности, коришћењем калибрације за тачкасти извор као референтне калибрације, одступају од експерименталних од мање од 1% до више од 20%. Највећа одступања су уочена код матрикса веће густине и то на крајевима опсега испитиваних енергија. Остали резултати се налазе у оквиру мерне несигурности. Ефикасности су одређиване и Монте Карло симулацијом помоћу два различита програма. Резултати добијени применом програмског пакета GEANT4 показују одступање од експерименталних вредности која су реда величине неколико процената и та одступања су последица дефинисања геометрије детектора. Симулација програмом PHOTON је дала задовољавајуће резултате са одступањима која се крећу од мање од 1% до приближно 20% у односу на експерименталне резултате, али уз изостављање најнижих енергија, за које су одступања била неприхватљиво велика. Додатни критеријум за поређење различитих метода калибрације била је тачност која је проверена мерењем узорака из животне средине познате активности. Ови узорци су мерени у оквиру различитих међулабораторијских интеркомпарација или су као секундарни референтни материјал набављени од сертификоване лабораторије. Након мерења, активности радионуклида у узорцима добијени су коришћењем ефикасности добијених различитим методама. Поређењем овако добијених активности са референтним вредностима датим за те узорке, извршено је објективно поређење свих метода и установљена њихова еквиваленција по резултатима. Tакође, утврђене су границе применљивости аналитичког модела и програмског пакета PHOTON. Дате су и препоруке у вези са комбиновањем различитих приступа за будуће калибрације. У овој дисертацији први пут је примењено више принципијелно различитих метода на калибрацију детектора за мерење узорака из животне средине у цилиндричној геометрији.The quality of the gamma spectrometry measurements is in general directly dependent on determining of the efficiency for the specific measurement geometry and measured sample characteristics. Due to the fact that measured samples can be very versatile in terms of geometry and chemical composition, the efficiency calibration is not readily available. That is when the need for developing different efficiency calibration methods and specific reference materials becomes obvious. Methods of efficiency calibration may consist of measurement of certified sources and secondary reference materials, determination of the parameters needed for efficiency calculation or simulation of the detector system response. Besides, the measurement in contact geometry, such is the case in measurement of environmental samples, implies the perturbation of the spectrum via coincidence summing effect, which leads to incorrect results of the measurement. Several methods for efficiency calibration of the measurement system are presented and utilized in this thesis. These are: - Experimental method, which implies the measurement of the secondary reference materials that are similar to the realistic samples in terms of geometry and composition. This method produces high accuracy results but at the same time requires long time for production of the secondary reference materials and long measurement time. All this can be the source of large measurement uncertainty. -Analytical method, which implies the existence of analytical dependence between the efficiency and geometrical characteristics of the measured sample and detector. This method is approximate, due to the fact that the integrals describing the efficiency dependence for cylindrical geometry are very complex and their solution is only possible by using a numerical integration. The approximations introduced in the calculation contribute to the uncertainty of the results, but can be acceptable in case of environmental samples. -Semi empirical method (represented in this thesis by EFFTRAN – efficiency transfer software) utilizes the dependence of the efficiency on the solid angle between the sample and detector and calculates the transfer of the efficiency from the reference calibration curve and coincidence summing corrections, accordingly. -Monte Carlo method (represented in this thesis by software package GEANT4 and PHOTON) utilizes Monte Carlo simulation of the detector spectral response based on known processes that lead to photon energy deposition in the medium and random number generation. Each of mentioned methods is applied for efficiency calibration of the HPGe detectors readily used in the Laboratory for Radiation and Environment Protection of the Institute for Nuclear Sciences Vinča. As the result, detector efficiencies and associated uncertainties are obtained. Since the measurement of the secondary reference materials is considered to produce the most accurate results, efficiencies obtained using other methods are compared to the experimental results in order to establish the accordance of the results within the uncertainty limits. The comparison determined that the efficiencies obtained using analytical method differ from the experimental results within the range of 1 to 14%, for the 3 measurement geometries that were analyzed and that are most frequently in use for environmental samples measurement. Results obtained by the efficiency transfer, using point source calibration as the reference, differ from the experimental results that ranges from less than 1% to more than 20%. The largest discrepancies are for high density matrices and at the edges of the investigated energy span. Other results obtained by this method are within the measurement uncertainty limits. Efficiency calibration by Monte Carlo simulation was performed using two different software. Results obtained by using GEANT4 software package show discrepancies from the experimental results that are of order of magnitude of several percent, which is the consequence of detector geometry definition. Simulation performed by PHOTON software produced satisfactory results with the discrepancies that ranged from less than 1% up to 20%, with the remark that the lowest energies were omitted due to unacceptably large discrepancy. Additional criterion for comparison of different efficiency calibration methods was testing of the accuracy by measurement of the realistic environmental samples. These samples were measured within different interlaboratory intercomparisons, or were acquired from certified laboratory as a secondary reference material. After measurement, the radionuclide activities in the samples were calculated using efficiencies obtained by different methods. By comparing the results with the activity target values provided by the intercomparison organizer, an objective accuracy comparison was performed and the equivalence of the results was determined. Also, the limits of the aplicability of anaytical method and PHOTON simulation were established. Recommendations regarding future efficiency calibrations are stated. This thesis applies, for the first time, several principielly different approaches to efficiency calibration of the HPGe detectors for environmental samples measurement in cylindrical geometry

Quality control of HPGe detectors for gamma spectrometry of environmental samples

Quality assurance and quality control (QA/QC) is a series of procedures aimed at verifying the validity of the measurement results and are defined in the Standard ISO 17025/17. QC should be planned, described in the quality control documentation, performed in a systematic manner, recorded and reviewed. All proposed measures for QC/QA are implemented in the Radiation and Environment Protection Department of Institute for Nuclear Sciences Vinča, Belgrade. One of these procedures prescribes a regular quality control of the instruments used for the measurement. In this paper, the quality control of three High Purity Germanium (HPGe) detectors, produced by Canberra, are presented. The accuracy and reproducibility of gamma spectrometry systems are verified on a weekly basis. Total background count rate is used to verify that the detector and shield has not been contaminated. Measurement of the total activity of calibration source is used to check the efficiency calibration and general operating parameters of the system such as peak shape and peak drift. These values are observed and verified if they are within the acceptance limits. For that purpose, 60Co and 133Ba point sources, produced by Czech Metrology Institute and traceable to Bureau International des Poids et Mesures (BIPM), are used. The acceptance limits for each controled parameter are defined according to the statistical analysis of the previous QC period. The acceptance limits are set to be ±2σ and ±3σ of the mean value taken over a previous year. Results of the QC measurement falling between ±2σ are considered to be satisfactory, the ones between ±2σ and ±3σ are warning and those exceeding ±3σ indicate that a problem with the measurement system has occurred. Analysing the QC data acquired for 2021, we can conclude that major part of parameters were within the limits of acceptance. Occasional discrepancies were minor and were addressed immediately. Discrepancies of the peak shape were corrected simply by additional cooling of the instrument, peak position by performing an energy calibration and background was corrected by ventilating the laboratory.Twentieth Young Researchers’ Conference - Materials Science and Engineering: Program and the Book of Abstracts; November 30 – December 2, 2022, Belgrade, Serbi

Interlaboratory comparison material homogeneity testing

The homogeneity of fertilizer samples for interlaboratory gamma-ray spectrometry comparison was tested by determination of the total. count rate and the count rates for two U-238. lines, one K-40 line and one common U-235 and Ra-226 line. Homogeneity testing was accomplished by determination of the minimum, maximum, mean and standard deviation for each parameter and comparison of their standard deviations with predefined tolerances, by Cochrans test, and by a one-way ANOVA. The standard deviations were all less than these tolerances. All samples passed Cochrans test and the one-way ANOVA test for homogeneity

Ground Level Air Beryllium-7 and Ozone in Belgrade

Three sets of data covering the 2004-2007 period are examined: two beryllium-7 series and ozone measured in ground level air. The measuring sites are at three different locations in Belgrade, Serbia. The temporal evolution of beryllium-7 and ozone is presented, as well as their mutual correlations. Beryllium-7 data for Belgrade agree well with the results for other locations in the region. The correlation between two beryllium-7 data sets is 0.57. The results for ozone indicate that Belgrade is not a common continental site, as the maximum in ozone distribution is reached in springtime. The overall correlation between beryllium-7 and ozone is good, but varies over different seasons. A large correlation (0.67) is noted between beryllium-7 measured at the site in Vinca, Serbia, and the monthly maximum ozone in autumn. An analysis which assumes the transport of air masses from the stratosphere, along which the only process changing the air mass composition is radioactive decay of beryllium-7, does not conclusively confirm the high correlation between beryllium-7 and ozone in autumn

Seasonal variations of naturally occurring radionuclides and 137cs in the leaves of deciduous tree species at sites of background radioactivity levels

Activity concentration of natural radionuclides and 137Cs were studied in leaves of the deciduous trees. In the spring and autumn season, leaves were collected in the area of normal background radiation levels represented by city parks in a multi-year period (2002-2012). Measurements by means of gamma-ray spectrometry showed 226Ra and 210Pb seasonal accumulation in leaves, while 238U and 235U could be detected only in autumn. Difference between seasons was not found significant for 40K and 137Cs. The study of radionuclides transfer factors was conducted by analyzing its relationships with basic soil properties at the beginning and the end of the vegetation period. © 2019, Editura Academiei Romane. All rights reserved

Activity concentration of 210Pb in aerosol in Serbia in the period of 2014-2020 and the comparison between results from urban and rural sampling stations

Aerosol sampling and monitoring of naturally occurring and artificial radionuclides is readily performed within the frame of Environmental radioactivity monitoring in the Republic of Serbia. It provides information important for the preservation of the public health and the environment from the harmful effects of ionizing radiation. This type of monitoring has been performed in Institute of Nuclear Sciences Vinča for more than 30 years now. The radionuclides that are measured are: naturally occurring 210Pb, artificial radionuclide 137Cs and cosmogenic radionuclide 7 Be. Particle reactive radionuclides such as 210Pb and 7 Be have been used as atmospheric tracers for studying environmental processes such as cloud scavenging and precipitation [1], aerosol transit and residence times in the troposphere [2], and aerosol deposition velocities [3]. The fission product 137Cs is an indicator of anthropogenic pollution caused by nuclear weapon atmospheric tests and nuclear power plant accidents. Seasonal variation in 137Cs in air is an indicator of the stratosphere-troposphere exchange processes [4]. In most aerosol samples in later years, the activity concentration of 137Cs is below the minimal detectable activity and therefore it was not taken into the consideration in this paper. The activity concentration of 7 Be is dependent on the movements of large air masses and vertical mixing in the atmosphere and also was not discussed in this paper. In this paper, the activity concentration of 210Pb in aerosol samples, obtained over the period from 2014. to 2020., was discussed. Sampling is performed using constant air flow pumps and Whatman filter papers on 6 measuring stations distributed in various parts of the country. Three stations are in urban areas of cities of Belgrade, Vranje and Zaječar and other three are in rural areas (Palić and Zlatibor) or removed from the urban part of the city (Meteorological station in Vinča). The monthly composite sample from each sampling station was ashed at the temperature below 400o C. After ashing, the residue aerosol is measured on HPGe detectors using a standard gamma spectroscopy method [5]. The results for 210Pb are of the order of magnitude of 10-4 to 10-3 Bq/m3 . The activity concentration in all investigated samples shows temporal dependence with the maxima in the winter months and minima observed during the spring and summer. Also, the comparison between the results obtained from the rural and urban locations was performed in order to investigate the influence of the air pollution present in the urban areas on the activity concentration of 210Pb. The values, obtained at stations located in rural parts, ranged from (0.056 – 3.3) mBq/m3 , while for stations located in urban areas the range was from (0.125 – 2.9) mBq/m3 . It can be seen that from all the data that the activity concentration of 210Pb was lower in rural areas. Two outliers were observed at Palic station, located in the plane and along the path of the prevailing south – east wind, possibly bringing aerosols from Deliblatska pescara (Deliblato Sands) [6].IV International Conference on Atmospheric Dust : October, 4-7, 2021, Torre Cintola Conference Centre | MONOPOLI | Italy

Radionuclide content in samples of berries

After the Chernobyl accident, the presence of artificial radionuclides in foodstuff, especially in berries (blueberries, cranberries etc.), was heightened. Legislative, presented in Rulebook on Radioactivity Control of Goods During the Import, Export and Transit (Official Gazette RS, 86/19 and 90/19), establishes the mandatory gamma spectrometry measurement of berries, while the limits of radionuclide content in these products is defined in the Rulebook on Limits of radionuclide content in drinking water, foodstuff, feedstuff, medicines, items of general use, building materials and other goods placed on the market (Official Gazette RS, 36/18). At the Radiation and Environment Protection Department of the Vinča Institute of Nuclear Sciences, measurements of the radionuclide activity in foodstuff are readily performed. In this paper, the results of the investigation of berries in the period of 2014 to 2021 are presented along with the amount of this produce that can be ingested by the various age groups without exceeding the annual effective dose limit of 0.1 mSv

Seasonal variations of naturally occurring radionuclides and 137cs in the leaves of deciduous tree species at sites of background radioactivity levels

Activity concentration of natural radionuclides and 137Cs were studied in leaves of the deciduous trees. In the spring and autumn season, leaves were collected in the area of normal background radiation levels represented by city parks in a multi-year period (2002-2012). Measurements by means of gamma-ray spectrometry showed 226Ra and 210Pb seasonal accumulation in leaves, while 238U and 235U could be detected only in autumn. Difference between seasons was not found significant for 40K and 137Cs. The study of radionuclides transfer factors was conducted by analyzing its relationships with basic soil properties at the beginning and the end of the vegetation period. © 2019, Editura Academiei Romane. All rights reserved

Quality control of gas flow proportional counter for beta spectrometric determination of 90Sr

In the Radiation and Environmental Protection Department, Vinča Institute of Nuclear Sciences, the validated radiochemical analytical method for 90Sr determination in environmental samples is used. The Laboratory for Radiation Measurements within mentioned Department is accredited according to ISO/IEC 17025. The activity of 90Sr is determined indirectly, by beta counting of its daughter product 90Y, and after established radioactive equilibrium during 18 days from the moment of 90Y radiochemical separation. Beta spectrometry by gas flow proportional counter Thermo Eberline FHT 770T is used for detection of 90Sr activity in investigated samples. In order to assure the reliability of the measurement results in accordance with Standard ISO/IEC 17025:2017, quality assurance and quality control (QA/QC) corresponding procedures need to be applied. For this purpose, activities ought to be planned in a systematic manner as described in the quality control documentation. The efficiency calibration of the detection system needs to be performed in a proper manner. The calibration of the detector for beta counting is performed using the Sr-90 certified radioactive standard point source (9031-OL-335/11, produced by Czech Metrology Institute) which is traceable to Bureau International des Poids et Mesures (BIPM). As lowlevel measurements require strict QA/QC requests, the accuracy and reproducibility of measurement systems must be verified periodically. A regular internal quality control of the gas flow proportional counter is performed once a week. Quality control activities include the background measurement and checking of efficiency using adequate radioactive standard source. The measurement values are recorded and verified if they are within the acceptance limits. The obtained results together with acceptance limits during the period of 3 months are graphically presented in control charts. The acceptance limits are set to be ±2σ and ±3σ in relation to the calculated mean value and standard deviation. The results of QC verification within ±2σ are considered to be satisfactory, those within ±3σ are warning, while those that exceeding ±3σ indicate that there is a problem with the measurement system. Consequently, the analysis of causes and design, as well as the application of corrective measures is needed.Twenty-First Young Researchers’ Conference - Materials Science and Engineering: Program and the Book of Abstracts; November 29 – December 1, 2023, Belgrade, Serbi

A Multi-Year Study of Radioactivity in Surface Air and Its Relation to Climate Variables in Belgrade, Serbia

Activities of Be-7 and Pb-210 were monitored in surface air in Belgrade, Serbia, from 2004 to 2012. The measurements were taken from two locations, in an open field of a city suburb and in the central city area. The activities were determined on HPGe detectors by standard gamma spectrometry. The Be-7 activity shows a pronounced seasonal pattern, with the maximum in spring-summer and minimum in winter, while the Pb-210 activity exhibits two maxima, in autumn and late winter. The mean monthly concentrations measured at both sites are below 9 mBq/m(3) and 1.3 mBq/m(3) for Be-7 and Pb-210, respectively. The obtained correlation of the Be-7 activity with the number of sun-spots is not statistically significant. Relations of the radionuclides activities with climate variables (precipitation, temperature, relative humidity, cloud cover, sunshine hours, and atmospheric pressure) are also investigated, but the only significant correlations are found for the Be-7 activity with temperature and sunshine hours, and the Pb-210 activity with atmospheric pressure. The maximum Be-7 and Pb-210 activities corresponding to binned total monthly precipitation data imply different modes of the radionuclide scavenging from the atmosphere. During dry periods, accumulation of the radionuclides in the atmosphere leads to their increased activities, but no correlation was found between the activities and the number of consecutive dry days