Gamma Spectrometric Efficiency Measurement Uncertainty of 137Cs in the Vegetation Sample

Celal Bayar University Journal of Science Volume 15, Issue 1, 2019, p 51-55 Doi: 10.18466/cbayarfbe.455494 Y. Agus Gamma Spectrometric Efficiency Measurement Uncertainty of 137Cs in the Vegetation Sample Yusuf Ağuş* TAEK, Department of Radiation and Accelerator Technologies, Kahramankazan /Ankara, Turkey * Received: 28 August 2018 Accepted: 22 January 2019 DOI: 10.18466/cbayarfbe.455494 Abstract In this study, gamma spectrometric efficiency measurement uncertainty of 137Cs in vegetation sample is investigated. Each uncertainty component is evaluated and quantified. For this purpose, n-type 70% efficiency HPGe detector was used. The combined standard uncertainty of 137Cs efficiency was found to be 2.43%. Keywords: Efficiency Uncertainty, Gamma Spectrometry, Cs-137, Vegetation. The efficiency measurement uncertainty is an essential part of the determination of the radioactivity concentration measurement results. Nuclear analytical laboratories (NAL’s) must take into account efficiency measurement uncertainty while preparing radioactivity concentration uncertainty budget. Additionally, measurement uncertainty evaluation is a part of the ISO/IEC 17025 technical requirements of the NAL’s and quantifying uncertainty in nuclear analytical measurements. Therefore, more attention is given to the evaluation of the measurement uncertainty and the preparation of uncertainty budget [8-11]. 1. Introduction Artificial radionuclides occur as a result of the nuclear power plant accidents and nuclear tests. One of these artificial radionuclides is 137Cs. Due to the relatively high half-life (T1/2=30.05 years), 137Cs remains in ecosystem many years after the nuclear tests and accidents [1]. After the Chernobyl (1986) and Fukushima Daiichi (2011) nuclear power plant accidents, artificial radionuclides released and contamination has occurred. Contamination takes place as deposition of the radionuclides in the air. Deposited radionuclides in the soil pass into the vegetation. Radionuclides in the vegetation pass into the human by the consumption of the vegetation within the food chain [2-6]. Therefore, determination and the evaluation of the contamination level and radioactivity concentrations for natural and artificial radionuclides are important for ensuring public health and food sector workers safety. In this study, components of the FEP efficiency measurement uncertainty of the 137Cs in vegetation sample were evaluated and quantified using gamma spectrometric method. Contribution of the uncertainty components to the combined standard uncertainty evaluated and expanded uncertainty obtained multiplying by coverage factor k. Gamma spectrometric analysis with the High Purity Germanium (HPGe) semiconductor detectors is a powerful radioanalytical analysis technique. Determination of the radioactivity concentration of the gamma-emitting radionuclide in the sample is the main goal of the gamma spectrometric analysis. Additionally, full energy peak (FEP) efficiency and photon emission intensities (f) can be obtained with this technique [7]. The measurement of radioactivity concentration via gamma spectrometric analysis should be performed with high accuracy and precision. Therefore, FEP efficiency calibration must be done accurately and precisely before the radioactivity measurements. FEP efficiency calibration depends on the source-detector geometry, sample matrix and on the photon energy. Sample and standard must be counted exactly same geometry and in the same type of sample container for the wellestablished efficiency calibration. 2. Materials and Methods 2.1. Sample Preparation and Experiment Radioactive vegetation reference standard was used to obtain efficiency value at 661.6 keV 137Cs photon energy. Vegetation standard had 9871.67 Bq.kg-1 137Cs radioactivity concentration at reference date. Standard were put into cylindrical plastic analysis container (6x5 cm diameter x height dimension). Then, analysis container sealed and weighed. Efficiency measurement was performed by using HPGe gamma spectrometer. Specifications of the gamma spectrometric system are given in Table 1. 241Am, 137Cs and 60Co standard point sources were used for energy calibration. Before the measurements, an empty analysis container counted for the determination of the background effects. 51 Celal Bayar University Journal of Science Volume 15, Issue 1, 2019, p 51-55 Doi: 10.18466/cbayarfbe.455494 Y. Agus where µ and x are the linear attenuation coefficient and thickness of the volume sample[8]. Table 1. Specifications of the gamma spectrometric system. Detector Type n-type %Rel. Eff. (1.33MeV) 70% Crystal diameter [mm] 73.2 Crystal length [mm] 65.0 Peak/Compton 60:1 1332.5 keV 60Co 2.3 keV Resolution 122.0 keV 57Co 1.1 keV  C4 is random summing correction factor. C4  e 2R (2.5) where  is the resolution time of the measurement system(s). R is the mean count rate [9].  2.2. Efficiency Determination C5 is coincidence correction factor [9]. There is no true coincidence factor for 137Cs. Efficiency is calculated by the following formula [9]  N A    t s  m  C1  C 2  C 3  C 4  C 5 (2.1) where,  N is corrected net peak area of the corresponding photo peak. It is calculated as N  N  t s  N . Ns s b tb     and Nb are the net peak area in the sample spectrum and in the background spectrum respectively. ts and tb live time of the sample measurement and of the background measurement in seconds. A is radioactivity concentration of the 137Cs in the sample. m is dried mass of the sample (kgs).  is emission probability of the 137Cs photo peak energy. C1 is correction factor for nuclear decay during the sampling to measurement C1  e  ln(2 )t T1 / 2 Figure 1. Gamma-ray spectrum of the radioactive vegetation reference standard. Repeatability (precision) is the closeness of the result of the consecutive measurements. During the measurement process, experimental condition remains unchanged. In the repeatability measurements analyst, detection system, laboratory conditions are the same and successive measurements carrying out in short period of time. Repeatability (precision) is the random error and expressed as the standard deviation(s) or the percent relative standard deviation (RSD%). Reproducibility is closeness of results of measurements carried out under changed conditions. In the reproducibility measurements detection system, analyst, laboratory condition and measurement interval can change. Reproducibility of the efficiency measurement was determined by using different analyst’s results. (2.2) where t is the elapsed time from the time of sampling to the beginning of measurement(s), T 1/2 is the half-life(s) [9] 3. Results and Discussions Efficiency value was calculated by using equation 2.1. Activity concentration of the reference standard material was 9871.67 Bq.kg-1. Measurement live time, background measurement live time, net photopeak area in sample spectrum, net (...truncated)