Calibration and performances of in-situ gamma ray spectrometer

Since 1896, when Henri Becquerel discovered that penetrating radiation was given off in the radioactive decay of uranium, the studies on radioactivity have been an interest of scientific world. With the spread of nuclear technologies applied to energy, health and industrial production, the theme of environmental radioactivity monitoring increasingly is becoming important to the policies of the health public protection both national and European level. Italy is required to comply with the recommendation of the European Commission of 8 June 2000 on the application of Article 36 of the Euratom Treaty concerning the monitoring of levels of radioactivity in the environment for the purpose of assessing the exposure of the population as a whole. In addition, the World Health Organization has identified the first group of carcinogens gas 222Rn, which is considered the second leading cause, after smoking, of lung tumors. In our environment there are various sources of radioactivity that can be natural or artificial origin. Gamma-ray spectrometry is a widely used and powerful method that can be employed both to identify and quantify radionuclides. The purpose of this work is calibration and performances of in situ a portable gamma ray spectrometer. In the first chapter I have given the necessary concepts for understanding the phenomenon of radioactivity. Qualitatively has been described the process of radioactive decay and its three types which can occur in nature. Three categories of environmental radionuclides, cosmogenic, primordial and man-made are discussed. We are exposed to environmental radiation from different sources. The origin of radioactivity in the environment can be divided into two main sources: (a) natural and (b) man-made sources. Mostly the naturally occurring radiation arises from terrestrial radioactive nuclides that are widely distributed in the earth’s crust and extra-terrestrial sources arising from cosmic ray. Also from human activities arise some other sources concerned with the use of radiation and radioactive materials from which releases of radionuclides into the environment may occur. In the second chapter is described the gamma radiation interacts with matter via three main processes: the photoelectric effect, Compton scattering and pair production The operation of a detector is based on the interaction of photons constituting the incident radiation with the material that constitutes the detector itself.. Thanks to these processes, all or part of the energy possessed by the radiation is transferred to the mass of the detector and then converted into an electrical signal. The basic notions related to the interaction of electromagnetic radiation with matter that we will provide in this chapter will therefore be useful to understand the mechanisms that are at the basis of the generation of a gamma spectrum. In addition, this chapter will briefly describe the two main types of gamma radiation detectors, i.e. the semiconductor detector and the scintillation, in particular the high-pure germanium detector (HPGe) and a sodium iodide detector activated by thallium NaI(Tl). In the third chapter is described the study area in which are performed the measurements of natural radioactivity. The area under consideration is the Ombrone basin located in southern Tuscany and Commune of Schio located in Region of Veneto. During the campaign were acquired in situ 338 spectra, including 80 with the ZaNaI_1.0L placed on the ground (Ombrone), 80 with the ZaNaI_1.0L placed on a tripod at 1m height (Ombrone), 89 with the ZaNaI_1.0L placed on the ground (Schio) and 89 spectra are acquired with a backpack placed on the shoulders of an operator (Schio). In each of the 80 sites which have been realized the measurements of radioactivity with the ZaNaI_1.0L instrument, also have been taken 5 different soil samples, for a total of 400 samples. The abundances of 40K, 238U and 232Th were obtained from the analysis of 338 spectra taken with the ZaNaI_1.0L and 400 spectra measured on soil samples in the laboratory with a high-pure germanium detector (MCA_Rad). Also it is described the procedures of ZaNaI_1.0L portable scintillation gamma-ray spectrometers for in-situ measurements. In the fourth chapter is described the procedure for the preparation of soil samples to be analyzed with the MCA_Rad system. The gamma-ray spectrometry system, called MCA_Rad introduces an innovative configuration of a laboratory high-resolution gamma-ray spectrometer featured with a complete automation measurement process, which can conduct measurements on each type of material (solid, liquid or gaseous) in less than 1 hour. The utilization of two coupled HPGe detectors permits to achieve good statistical accuracies in shorter time, which contributes in drastically reducing costs and man power involved. It is made a description of the characterization of absolute full-energy peak efficiency of such instrument reported here. In the fifth chapter are discussed the correlations between the abundances of 40K, 238U and 232Th measured with the ZaNaI_1.0L and those obtained from laboratory analysis on soil samples. The analysis was focused in particular on the study of four different types of correlation: correlation between in-situ acquisition on ground and laboratory measurements, correlation between in-situ acquisition on tripod and laboratory measurements, correlation between in-situ acquisition on ground and on tripod and correlation between in-situ acquisition on ground and on operator shoulder and the influence of vegetative cover during measurements in-situ.