Application of GUM Supplement 1 to uncertainty of Monte Carlo computed efficiency in gamma-ray spectrometry

Uncertainty and covariance matrix of the coincidence-summing correction factors due to decay-data uncertainties

The uncertainty component of high efficiency gamma-ray spectrometry measurements due to the uncertainty of the decay parameters is evaluated. It is based on the analysis of a large set of random decay schemes, constructed using the values and uncertainties of the decay parameters taken from data libraries. The distributions of the coincidence-summing correction factors FC, of the factors p⋅FC (p = photon emission intensity), and of the correlations between the distribution of pairs of these factors are constructed and analysed.

Effects of the uncertainty contributions on the methods used for measurement uncertainty evaluation

Adequate measurement uncertainty evaluation is crucial for supporting basic measurement purposes. However, the most prevalent approach, the uncertainty propagation, may not be validly applicable under certain conditions which require the use of less restricted alternative method (MCM-based propagation of distributions). We demonstrate the effects of major conditions, e.g. non-linear measurement model, non-Gaussian input quantities, on the reliability of uncertainty evaluation methods, highlighting the importance of the less frequently examined impact of uncertainty component contributions to the standard uncertainty of measurand.

Calculation of decision threshold and detection limit in radiometric measurements using a Monte Carlo Method

Calculation of the decision threshold and detection limit of a measurement, or measurement method, are crucial in order to decide if an analyte is present or not and with what confidence it can be quantified. That decision is important in view of possible actions if something would be detected. In this work, a method for calculating these limits using a Monte Carlo method is presented. In the Monte Carlo method any a priori distribution (e.g. normal distribution, rectangular distribution, triangular distribution) of an input quantity can be selected. Differences between the Monte Carlo calculated characteristic limits and the ones calculated according to ISO 11929:2010 is presented. Moreover, suggestions how to calculate the detection limit when it can not be calculated according to the ISO 11929:2010 are given.

Uncertainty evaluation in gamma spectrometric measurements: Uncertainty propagation versus Monte Carlo simulation

Calculation and reporting of combined measurement uncertainties are important in decision making processes, and a more proper uncertainty estimation can reduce the risk and/or the cost associated with decisions for example after radiological incidents and in free release measurements of radioactive waste. However, sound decisions demand a sound uncertainty estimation. In this work we present the possible consequences when uncertainty propagation is applied to gamma-ray spectrometry measurements involving assumed probability density functions for an efficiency transfer having different metrological quality by comparison with Monte Carlo simulations.

SUMCOR: Cascade summing correction for volumetric sources applying MCNP6

The main features of code SUMCOR developed for cascade summing correction for volumetric sources are described. MCNP6 is used to track histories starting from individual points inside the volumetric source, for each set of cascade transitions from the radionuclide. Total and FEP efficiencies are calculated for all gamma-rays and X-rays involved in the cascade. Cascade summing correction is based on the matrix formalism developed by Semkow et al. (1990). Results are presented applying the experimental data sent to the participants of two intercomparisons organized by the ICRM-GSWG and coordinated by Dr. Marie-Cristine Lépy from the Laboratoire National Henri Becquerel (LNE-LNHB), CEA, in 2008 and 2010, respectively and compared to the other participants in the intercomparisons.

On homogeneity approximation in calibration of gamma-spectrometry assessment of bulk samples

The effect of the intrinsic inhomogeneity of a sample on the efficiency for the measurement of the sample by gamma-ray spectrometry is studied. The difference between the efficiency for the inhomogeneous sample and the efficiency for a sample made from the equivalent homogeneous matrix is evaluated.

Absolute peak-efficiency calibration of a well-type germanium detector using multiple gamma-emitting nuclides with the "Solver" add-in in Excel™

When calibrating absolute peak efficiency and total efficiency functions for high efficient germanium detectors, single gamma-emitting nuclides are used in general to avoid coincidence summing effects. As another approach, Blaauw developed calibration technique using multiple gamma-emitting nuclides with least-square technique (Blaauw, 1993; Blaauw and Gelsema, 2003). In the present study, we tried to calibrate efficiency functions of a well-type germanium detector based on Blaauw's approach using a home-made iteration calculation algorithm with the "Solver" add-in in Excel™.

Efficiency computation for gamma-ray spectrometry assessment of samples with intrinsic inhomogeneity

The full energy peak efficiency for inhomogeneous samples, comprising a number of components with different activity and matrix is evaluated by Monte Carlo simulation. The distribution of the values of the efficiency due to the effects of inhomogeneity is constructed. In the particular case when one of the components is a highly active and highly attenuating medium, then the efficiency and its uncertainty at low energies is sensitive to the properties of this component, including the dimensions of the grains.

Uncertainties in calculated correction factors for true coincidence-summing (TCS)

The aim of this work was to estimate the uncertainties in Monte Carlo calculated correction factors for true coincidence summing (TCS). In this work TCS-factors and their uncertainties were calculated for ¹³⁴Cs and then the corrected activities compared to empirical data. The study was carried out using a close-end coaxial p-type detector (Ø80mm×54.5mm, 80% relative efficiency) and a cylindrical glass fiber sample (Ø60mm×14mm). It was shown that the uncertainty in the calculated correction factor for the primary gamma ray was below 0.5%, which means it will not contribute significantly to the combined uncertainty in an activity measurement for e.g. environmental monitoring.

A practical evaluation of measurement uncertainty in gamma-ray efficiency curve determination through an analytical approach

An example of uncertainty evaluation of an efficiency curve is reported. In the applied method, the efficiency curve is determined through the weighted least square method based on measured efficiencies employing a mixed-radionuclide standard source. The variance-covariance matrix of the efficiencies is determined by evaluating the uncertainties of the counting, the calibrated radioactivities, the correction factors for true coincidence summing, and the inadequacy of the fitting function. The proposed method can be a basis to develop a more general procedure.