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

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

Limitations of the generalized coupled two-level model during the multiphoton absorption in different gas mixtures

Generalized coupled two-level model is applied in different gas mixtures and investigated for high fluence regime. Functional dependences of mean number of absorbed photons per molecule LT n GT (total) on buffer-gas pressure (P-buff) are presented, used to confirm or predict some possible physical and chemical processes, like enhanced absorption and/or dissociation. Limitations of proposed models are analyzed depending on both gas pressure and laser fluence. Results are compared with other previously obtained by the same experimental technique, but for different absorbing molecule.International School and Conference on Optics and Optical Materials, Sep 03-07, 2007, Belgrade, Serbi

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

Calculation of the highly excited SF6 vibrational state distributions and dissociation yields in different gas mixtures

Influence of the buffer gas on the multiphoton absorption and dissociation in different mixtures was investigated. Simple method based on the empirical and theoretical vibrational energy distribution is applied for high fluence regime. Collisional effects of buffer gas (Ar) are introduced to enhance the absorption and relaxation of irradiated molecules (SF6 and C2H4). Functional dependences of mean number of absorbed photons per molecule ( LT n GT (total)) on the Molecular excitation level are presented, enabling us to confirm or predict the level of excitation, number of molecules directly involved in the absorption process and dissociated during the laser pulse.International School and Conference on Optics and Optical Materials, Sep 03-07, 2007, Belgrade, Serbi

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

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

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

Transfer Factors for ,,The Soil-Cereals System in the Region of Pcinja, Serbia

The aim of the paper was to estimate the values of transfer factors for natural radionuclides (K-40, Ra-226, Th-232, U-235, and U-238) and Cs-137 from soil to plants (cereals: wheat, corn and barley) as important parameters for the agricultures in the selection of the location and the sort of cereals to be planted on. The results presented in this paper refer to the ,,soil -cereals system in the region of Pcinja, Serbia. Total of 9 samples of soil and 7 samples of cereals were measured in the Department of Radiation and Environmental Protection, Irnica Institute of Nuclear Sciences, using three high -purity germanium detectors for gamma spectrometry measurements. In all the samples, transfer factors for Ra-226 are significantly lower than for K-40, but they are all in good agreement with the literature data. On the three investigated locations, the calculated values of transfer factors for K-40 were in the range of 0.144 to 0.392, while in the case of Ra-226, the transfer factors ranged from 0.008 to 0.074. Only one value (0.051) was obtained for transfer factor of Th-232. Specific activities of Cs-137, as well as uranium isotopes, in all the investigated cereal samples, were below minimal detectable activity concentrations. Also, the absorbed dose rate and the annual absorbed dose from the natural radionuclides in the soil, were calculated. The absorbed dose rate ranged from 49-86 nSv/h, while the annual absorbed dose ranged from 0.061-0.105 mSv. The measurements presented in this manuscript are the first to be conducted in the region of Pcinja, thus providing the results that can be used as a baseline for future measurements and monitoring

Correlation Between Beryllium-7 in Atmospheric Deposit and Ground Level Air in Serbia for 2014 Year

Activity density of beryllium-7 in atmospheric deposit and in ground level air at five monitoring stations (MS Nis, Vranje, Zajecar, Zlatibor, Palic) in Serbia were determined during the period January-December 2014. Activity of cosmogenic radionuclide beryllium-7 was determined on HPG detectors (Canberra, relative efficienty 20%.) by gama spectrometry method. Activity density of beryllium-7 in aerosols were in range 0.5 - 9.8 mBq/m(3) and in deposits were in range 1.8 - 233 Bg/m(2). Based on the obtained results correlation coefficient between aerosols and deposits was calculated and its value ranged from 0.15 to 0.59.3rd International Conference on Radiation and Applications in Various Fields of Research (RAD), Jun 08-12, 2015, Budva, Montenegr