Establishment of a Procedure to Calculate the Measurement Uncertainties in RadiationSurvey Meters Calibration

INTRODUCTION Error analysis works with the fact that any measurement procedure has imperfections that give rise to an error in the measurement result. Error is the result of a measurement minus the true value of the measurand. There are many good books on this subject (1,2). Since the true value cannot be determined, in practice a conventional true value is used. Based on these errors it is constructed a parameter associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurand and is called uncertainty (3). It’s quite important to know the uncertainty in a measurement, mainly in a calibration laboratory, because it is the clear expression of the capability and quality of theirs services (4). The Calibration Laboratory of IPEN (LCI/IPEN) calibrates more than one thousand gamma ray survey meters a year, beside other kinds of radiotherapy, diagnostic radiology and radiation protection instruments. In the gamma ray survey meter calibration procedure there are two values that have to get theirs uncertainties clearly known: measurements in the instrument under calibration and the true value. The uncertainty in the instrument under calibration is normally an A kind and is obtained by a series of measurements, at least ten, and then its standard deviation is multiplied by the t-factor to express the confidence level of the instrument (1). For the physical quantities exposure, absorbed dose or dose equivalent rates the true value is obtained through the indirect measurements performed with a reference ionization chamber. It is measured the integral of electrical current in a time interval, a charge, (Q±∆Q), formed in the active volume of the chamber when it is submitted to the radiation in a precisely distance (x±∆x) and instant (t±∆t). The measurement with the instrument under calibration (and its uncertainty) is compared with the true value (and its uncertainty) and if the normalization (57) is not obeyed adjustments are made in the instrument. The LCI has a reference (600cm) cylinder ionization chamber (Nuclear Enterprises Ltd. model 2511/3) traceable to the Brazilian Secondary Standard Dosimetry Laboratory (SSDL) which is traceable to the BIPM. Annually the beam dosimetry is performed using this ionization chamber and the results are used for the true values determination for calibration purposes. The uncertainties present in every direct or indirect measurement during the calibration procedure must be evaluated for purposes of laboratory quality control. All calculation steps in the propagation of errors are presented in this work starting from the ionization chamber charge measured with the reference instrument. Such propagation was made in space and time, considering even the pressure and temperature uncertainties. The propagation was necessary in space, because the ionization chamber measurements were performed at only one space position, that is (1,000±0,005)m. Actually we use a new procedure based in five measured positions and it will be published soon. The time propagation was essential due to the fact that the activity is a peculiar physical quantity, which changes with the time according to precise relations for a specific radionuclide. Nowadays the achievement of calibration laboratory quality systems requires the expression of all uncertainties and the procedure used to evaluate it. The FORTRAN program was used to calculate the true values, theirs uncertainties and an example of this procedure in the case of the calibration of a typical scale of a portable radiation survey meter is presented.