Gamma spectroscopy for environmental radiation measurements L Visca

Gamma spectroscopy for environmental radiation measurements L. Visca 1, 2, R. Cirio 1, 2, A. Solano 1, 2 1. Universita’ degli Studi di Torino, Dipartimento di Fisica Sperimentale 2. INFN, Sezione di Torino XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Summary Overview about natural background radiation Description of a portable scintillation detector system Na. I(Tl) Calibration of a scintillation detection system Analysis of gamma spectra from different samples Activity assessment from measured spectra XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Natural radiation exposures All living organisms are continually exposed to ionizing radiation, which has always existed naturally. The sources of that exposure are cosmic rays that come from outer space and from the surface of the Sun, terrestrial radionuclides that occur in the Earth’s crust, in building materials and in air, water and foods and in the human body itself. Some of the exposures are fairly constant and uniform for all individuals everywhere, for example, the dose from ingestion of potassium-40 in foods. Other exposures vary widely depending on location. Exposures can also vary as a result of human activities for example building materials of houses and the design and ventilation systems strongly influence indoor levels of the radioactive gas radon and its decay products, which contribute significantly to doses through inhalation. XI ICFA School From the Report of the United Nations Scientific Committee on the Effects of Atomic San Carlos de Bariloche January 2010 (2000) Radiation to the 11 -22 General Assembly

Gamma rays from terrestrial sources Naturally occurring radionuclides of terrestrial origin are present in all media in the environment, including the human body itself. Irradiation of the human body from external sources is mainly by gamma radiation from radionuclides in the 238 U and 232 Th series and from 40 K. Indoor exposure to gamma rays, mainly determined by the materials of construction, is inherently greater than outdoor exposure in earth materials have been used; the source geometry changes from half-space to a more surrounding configuration indoors. When the duration of occupancy is taken into account, indoor exposure becomes even more significant. From Annex B of UNSCEAR Report 2000 XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Instruments and methods Scintibloc is a scintillator cristal Na. I[Tl] directly coupled to Photo. Multiplier. Tube (PMT). Nano. SPEC is a Na. I(Tl) detector base. It includes HV module, spectroscopic amplifier and Multi. Channel. Analyser (MCA). It can be connect to a PC using a serial interface win. TMCA 32 is the software required to set-up the detector and acquire the gamma ray spectrum from the samples.

Instruments and methods Nuclide. Navigator Gamma. Vision (GV) is the software used to for the edit your analysis nuclide. database.

Win. TMCA 32: Initialization Before starting the set-up, set the switch of the Nano. Spec to: Int 1. Select the acquisition mode (Acquisition_Mode) on: PHA (pulse height analysis). In this way every value is assigned to one channel and the channel content is incremented by one if a corresponding signal is counted. 2. Define the Spectrum length (Spectrum_Length): this command changes the number of channels and sub spectra of the actual spectrum. You can select values from the list in the LENGTH field of the input (i. e. 1024, 2048, 4096…) or enter an optimal value (in our case, 2048). 3. Setup the hardware (Hardware_Setup) : a) High Voltage: type the desired voltage value (in our case 450 V; WARNING: do not exceed 500 V) in the text field, then confirm the input by pressing the ENTER button. b) ULD…: i. ULD (upper level discriminator): defines the upper level of the signal acquisition. The settings 0 to 255 don’t belong to a channel directly, but 0 relates to the lowest channel and 255 to the top channel. (set ULD to 255) San Carlos de Bariloche 11 -22 January 2010

Win. TMCA 32: Initialization ii. LLD (lower level discriminator): discriminates the lowest level of signal acquisition. The settings 0 to 255 are not directly related to to the channels. In order not to discriminate a channel, Set LLD to 0. iii. Conversion Gain: defines the number of channels for spectra acquisition, thus the relation of channels and voltage. The value varies from 0 to 255. Set Conv. Gain to 0 (this corresponds to a spectrum with 2048 channels). iv. Noise: defines which of the signals registered are events and which are noise. Only events which give a higher voltage value than the noise value will be registered. In principle Noise works like LLD but events beyond the Noise-level are not considered for the dead time and base line calculations. Set Noise to 0. XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Win. TMCA 32: Initialization c) Gain: Sets up the coarse gain for the acquisition hardware. Set the coarse gain to 20 XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Win. TMCA 32: Spectrum acquisition Select window Real time Starts spectrum acquisition Status: STP =acquisition is stopped ACQ=acquisition is active Stops spectrum acquisition Erases the actual spectrum Defines spectrum length (2048) Opens the hardware setup menu Integrates the marked area within the actual spectrum Opens a dialog to select spectra files Save the spectrum Show the preset menu XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Energy calibration Ask the tutor how to convert win. TCA file to Gamma. Vision file. The energy calibration of a spectrum has to be realized with calibration sources. As example we could used a Marinelli calibration source, containing a mixture of nuclides. Nuclide Gamma-ray energy (ke. V) Activity @ 1 february 2000 (Bq) Emission rate (s-1) Caesium-137 662 3. 18 E+03 2. 70 E+03 Cobalt-60 1173 3. 49 E+03 3. 48 E+03 Cobalt-60 1333 3. 49 E+03 XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Energy calibration procedure 1. Select from GV main menu ROI mark. 2. Define the ROI using the marker. 3. With the marker inside the ROI, use the Peak Info button, to determine the Centroid of the peak. 4. Select Calibrate, Energy calibration and insert the energy of the calibration radionuclide. 5. Repeat the procedure for all the available peaks. 6. Remember to save your calibration Calibrate Save calibration. XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Efficiency calibration The efficiency calibration of Na. I detector has to be realized with calibration sources. As example we could used the Marinelli calibration source constituted by radionuclidic mixture. 1. With the marker in your ROI Select Calibrate, Efficency calibration 2. You can use GV to calculate the efficency from your source data. 3. Repeat the procedure for all the available peaks. 4. Chose the knee and quadratic interpolation mode above the knee. 5. Remember to save your calibration Calibrate Save calibration. XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Sample analysis - Acquire the spectrum from your sample using the same set up used for the calibration. - Open the GV spectrum. - Load the energy calibration. - Define the relevant ROIs. - For each ROI collect centroid and FWHM using the Peak Info button. XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Nuclide search: Sample analysis -Load Nuclide Navigator -Open the Master Lybrary (File Open source library master) -Use the search command to search by photopeak energy the nuclide (half life and secondary peak emission can help you !) XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Sample analysis Edit your nuclide database: - Nuclide Navigator Select target library choose a name for your library in gammavision format - Using the Library manager view edit your library using drag and drop for the radionuclide you’ve found in your spectrum. XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Sample analysis Load your library in your Gamma Vision analysis: Library Select File Your Library The peak info window shows the activity [Bq]calculated from the net count rate, the efficiency calibration and the branching ratio of the nuclides listed in your library. XI ICFA School San Carlos de Bariloche 11 -22 January 2010

Conclusions Give a brief report of your experiment comparing the activity form your sample with the ones of a background spectrum. In which way you can assess the gamma absorbed dose rate? The mass of the detector is 0. 378 kg. XI ICFA School San Carlos de Bariloche 11 -22 January 2010