Improvement in the calibration techniques of approved dosimetry services in Spanish nuclear power plants of whole body counters with NaI detectors.

Transcription

1 Improvement in the calibration techniques of approved dosimetry services in Spanish nuclear power plants of whole body counters with NaI detectors. E.Sollet (*), A.Felipe(**), JM.Perez (**), S. De Maria (**), P.Marchena(***), I. Villanueva (****) (*) Iberdrola, C.N. Cofrentes Paraje El Plano, S/N Cofrentes Valencia SPAIN (**)Iberdrola Ingeniería Consultoría, Avda. de Burgos 8 B Madrid 8036 SPAIN (***)TECNATOM, Ingeniería de Operación. Avda. Montes de Oca, 1. San Sebastián de los Reyes Madrid, SPAIN (****) Consejo de Seguridad Nuclear, Justo Dorado Madrid Scientific Thematic Area: 3 Dosimetry an Instrumentation Internal exposure Background From 1984 to nowadays the approved internal dosimetry services of nuclear power plants have used in the calibration procedures a phantom made of masonite (a type of wood with 1 gr/cm 3 ) This phantom is not provided with arms and has several holes in head, neck, lung, stomach, intestinal tract and legs where point radiactive sources are placed. Several point sources of different radionucleides are used with an activity in the range of 00 to 50 Bq. Calibration method requieres the individual measurement of each radionucleide to perform an empiric efficiency calibration curve. This curve is used to modify spectra from other radionucleides in the library to fit them to the curve. The calibration procedure has been carried out using long half-life radionucleides, so errors of calculating activity from radionucleides not included in the calibration method are increased with time and are out of control. 1. PHANTOM Once the research and development project will be finished the whole body counters will be calibrated with a antropometric phantom, so nowadays it has been bought two BOMAB (bottle manikin absrober) whole body calibration phantoms. These phantoms assemble ten cylindrical polyethylene bottles which represent torso, pelvis, head and neck, arms and legs for an adult. The calibration method will require the use of two BOMAB phantoms one of them will contain a known quantity of the required radionuclides in solution and the other one in form os a permanent source in a solid, tissue substitute matrix. The radionuclides included in the phantoms will be distributed in a way intended to represent the distribution in the human subject. Thyroid monitoring will requiere an special calibration phantom that reproduces the size and shape of organ and the attenuation of the overlying tissues, so the calibration will be made with a phantom designed by ANSI phantom made of one acrylic material cylinder with a 1-cm diameter and 1- cm high, the cylinder has one 5-cm diameter cavity 10-cm high, which has a 3-cm high slot where the calibration standard will be inserted. Next figure shows the ANSI thyroid phantom.

2 FIG. 1. ANSI thyroid phantom.. CALIBRATION TECHNIQUE. Calibrations and controls of primary calibration will be carried out. Primary calibration will be performed with a frequency between three and five years. Secondary calibrations or controls will be performed yearly..1 Primary calibration One calibration for each geometry type (fixed and scanning whole body, thorax counting used for radionuclides in lungs and thyroid monitoring) will be carried out. This primary calibration will be performed with the Bomab phantom filled with standardised radiactive water based solutions. This solution will contain a known quantity of some radionuclides as follows Co-57, Ba-133, Cs-134, Cs-137, Co-60, Y-89. The measurement time must be enough to acquire counts in the less-emission photo-peak. The calibration program will calculate the minimum time and will let the user to chose the photo-peaks used to calculate the calibration curves. These calibration curves will be use in the individual monitoring of exposed workers. Calibration method for Thyroid is similar to the calibration method proposed for whole body but instead using the Bomab phantom the ANSI Phantom wil be use. Calibration for peak analysis method will requiere the assessment of efficiency After calculating the calibration curves, a serie of measurements will be re-analyse using the new calibration curves. The quality control criteria will be +/- 5% for the accuracy.. Annual calibration controls. Yearly a calibration control will be performed in orden to assure the primary calibration is correct. In these controls a Bomab phantom containing a known quantity of radionuclides in form of permanent

3 source in a solid, tissue substitute matrix will be used. This phantom will contain Ba-133, Cs-137 and Co-60 This calibration control will be made in same conditions of geometry and counting time that primary calibration. Decay properties and distribution of radionuclides in phantom will take into account. Analysis results must fulfilled the established criteria. These criteria will be: an error less than +/- 0% for accuracy and an error less than +/- 10% for precision. Thyroid geometry will be checked with the suitable phantom using a radioactive source of Ba-133. Some approved dosimetry services are provided with Quick whole body counters, these devices are a vertical array of detector against which the subject stands for a short time. These equipments include INa detector and proportional counter detectors. This proportional counter detector will be checked periodically using point sources of, Cl-36 or Sr-90 The source positions must be properly established. A special bracket to hold the source shall be used..3 Exceptional calibration Exceptional calibration is called to any calibration out of schedule. It can be performed with any of two phantoms available depending on the needs of users..4 Daily controls A daily control of the calibration devices by means of adjustment refereed to the peak location versus energy calibration (channel number) A daily control procedure will be set up. This procedure must contain information about the point sources that will be used for performing these controls. Usually two point sources are used for this controls one Am-41 (59.6 Kev) placed inside of the whole body counter and other Co-60 (1173 y 133 Kev) Analysis of results obtained from these controls will lead to modify some parameters as the gain of the INa detectors until peak position lies into the range of allowance. The program will have the possibility of making an adjustment of the preamplifiers of the INa detectors, and it will be able to indicate the right position (v/kev) for each preamplifier. Users must check that the adjustment of the preamplifiers match up with values from the program, if not, user will modify the values with an error de +/- 1 channel. When the adjustments are finished, several measurements will be made with the daily point check source in order to assess its activity. The quality control criteria will allow errors less than +/- 0% for the accuracy..5 Calibration curves Three curves are generated in a calibration:

4 Energy / Channel. It correlates the spectrum channels with the emission energy of the isotopes, is a peak location versus energy calibration. On a simultaneous way, the system will calculate the resolution (FWHM) as a function of peak energy. The relationship between energy E and its corresponding channel x can be fit by means of a straight line or a function of -nd degree. The program calculates the coefficients Cx of the following equation E = C C x C3 x (1) FWHM / Energy. It determines the variation of the peak width with energy. In this case the relationship will be fitted to a first-degree function in the square root of the energy. FWHM = a + 1 a E () Efficiency / Energy. Efficiency calibration for peak-analysis methods requiere the assessment of the efficiency for several photon energies. Data are fitted to a polynomial curve of 4 th or 5 th degree, by means of least square method. ln ε = 3 4 ( ln E ) + f ( ln E) + f ( ln E) f ( E) 5 f f ln E + f ln (3) 3. PC PROGRAM ALEDIN ALEDIN has been developed to allow calibration, data acquisition, analysis of spectrums and calculation of radionuclide activity in human body A basic characteristic of the program is the reproducibility of any measurement or calibration. User can look for and recover any individual measurement or calibration process from the database of the program. The main points of the program are: - Measurements of environmental background, carrying out of calibrations, controls of calibrations and individual monitoring User is guided through the program in a straightforward way, from insertion of data: calibration curves and parameters, measurement geometry, personal data of the individual to be measured, etc going through data acquisition and following analysis in order to quantify the activity of radionuclide in human body. The program finishes by printing the results and final report Data analysis will be carried out using the method based in total areas over net spectrum. ALEDIN take four steps for locating the peaks in a spectrum, qualitative analysis, and for calculating the related activity, quantitative analysis: Calculating the net spectrum subtracting the environmental background. This is useful to remove environmental interference in some cases. Peak location in the region of interest of the spectrum using the first difference method complemented with data-smoothing techniques. The continuum under a peak is calculated using straight line or smooth-step functions. The peak shape is described as a function (Gaussian with or without tail) and data are fitted to such function.

5 Once isotope activity has been calculated, the program will call for calculating dose the program INDAC that will calculate dose according recommendations of ICRP 66. Results will be recorded in the database of INDAC, so the user could edit the measure and carry out new dose estimation by executing INDAC again.