Tim Vidmar

Calculation of true coincidence summing corrections for extended sources with EFFTRAN

A deterministic code was developed for the calculation of true coincidence summing correction factors and has been incorporated into the EFFTRAN tool. The approach is aimed at the analysis of extended samples measured on p-type HPGe detectors in environmental gamma-ray spectrometry and was verified against the results of a state-of-the-art full Monte Carlo code. The two sets of results matched on average within 1%. Our code requires no measurements in addition to a standard full-energy-peak calibration, has a very short run time and takes into account the spatial variation of the efficiency across the sample volume. The EFFTRAN code is free software, available from the authors on request.

MONTE-CARLO CALCULATION OF THE SPATIAL DEPENDENCE OF THE COAXIAL HPGE DETECTOR EFFICIENCY FOR POINT SOURCES

Abstract It is shown that fine geometric modelling of the HPGe detector structure using the GEANT system leads to a good match between the calculated and measured spatial dependence of efficiency for point sources in the region of photon energies from 60 keV to 1115 keV.

A theoretical description of diffusion and migration of 137Cs in soil.

Careful measurements of activity concentrations of 137Cs in soil samples taken layer by layer in autumn of 1999 in Slovenia are confronted with a prediction based on the diffusion-convection equation with a boundary condition which--unlike the boundary conditions applied in the literature so far--conserves the deposited activity over time, except for the natural decay. It is shown that it is essential to consider the deposits from atmospheric nuclear weapons tests and the Chernobyl accident to arrive at a good fit to the measured data. The corresponding Greens function as well as the diffusion constant and migration speed based on the analysis are given.

A semi-empirical model of the efficiency curve for extended sources in gamma-ray spectrometry

Abstract A semi-empirical formula for the full energy peak efficiency of extended sources in the range from 4 to 3000 keV is presented, taking into account self-absorption in the sample. The formula is based on physically sound premises and features a low number of free parameters, which exhibit physically viable values. Along with very good fits it furnishes, this makes the model suitable for extrapolation of the efficiency values towards the energies where measurements are not available. The model we use is based on the assumption of independence of the intrinsic peak-to-total ratio on the emission point of the gamma-ray. For extended sources, it features a term which takes into account the attenuation of gamma-rays in the sample. The proposed model was tested against a number of experimental efficiency curves measured with point and extended sources on n- and p-type HPGe detectors, as well as against data sets obtained from Monte Carlo calculations using the GEANT and MCNP codes.

Precise modeling of a coaxial HPGe detector

Measured spatial dependence of the full energy peak efficiency for point sources in the region of photon energies from 60 to 1115 keV was analyzed using the GEANT system. A systematic iteration procedure was developed for the fine geometric modeling of the HPGe detector structure. It revealed the details of the bulletized crystal edge, the surface dead layers structure, the core shape and its surrounding inactive layer.

Efficiency transfer between extended sources.

An application of the efficiency transfer method of efficiency determination in gamma-ray spectrometry is described, with both the standard and the measured sample being extended sources. The approach was successfully tested against experimental data.

Monte Carlo calculation of entire in situ gamma-ray spectra.

We present a method for the synthesis of entire in situ gamma-ray spectra based on Monte Carlo calculations and measured data that characterize the detector properties. The method can serve for the determination of the effective depth of 137Cs in soil based on the information contained in the low-energy part of an in situ spectrum. Effective depth is defined as the depth of a plane distribution of 137Cs beneath the surface that reproduces the fluence energy and angular distribution at 1 m above the ground of gamma rays belonging to the real 137Cs distributions. We managed to reproduce the measured in situ spectra with our method and to demonstrate that the method allows the determination of the effective depth of 137Cs with a precision of 10(-2) m. The method requires minimal experimental characterization of the detector and is not sensitive to the details of the detector model and the soil composition and density employed in the Monte Carlo calculations.

Close-geometry efficiency calibration in gamma-ray spectrometry using radio-nuclides with a two-step cascade decay

Abstract When efficiency calibration is performed in gamma-ray spectrometry with point sources in close geometry, radio-nuclides emitting photons of a single energy are usually utilized in order to avoid problems arising from true coincidence summing. Radio-nuclides emitting gamma-rays in a simple two-step cascade are therefore not considered suitable for such measurements. It is, namely, not possible to determine the full-energy-peak and total efficiencies for the gamma-rays such radio-nuclides emit from the system of equations which determine the number of counts registered in individual peaks in their spectra. A method was developed to overcome this difficulty by making use of additional constraints, based upon sound physical grounds, which can be imposed on these equations to render the combined system solvable. The accuracy of the method was successfully tested with point sources of 60 Co , 46 Sc and 94 Nb . The method provides six additional energies in the range between 700 and 1400 keV for which full-energy-peak and total efficiencies can be determined, which is important in view of the fact that only seven single-energy emitters are generally available for close-geometry calibration. We applied the method to several distances of the point source from the detector and studied the influence of angular correlations on the determination of the efficiencies. The effect is significant for total efficiencies and larger distances of the source from the detector, which has not been noted before.

A recursive deterministic algorithm for treatment of true coincidence summing effects in gamma-ray spectrometry.

Different algorithms exist that can be applied to the calculation of the effects of true coincidence summing in gamma-ray spectrometry. Some of these, however, are not capable of reproducing the count rates in all the pure sum peaks that a spectrum may contain. A recursive, easy-to-implement deterministic algorithm has been developed that overcomes this shortcoming.

Extended relative method of activity determination.

An extension of the classical relative method of activity determination in gamma-ray spectrometry to standards that differ from the measured sample in size, density, composition and radionuclide contents is described. The approach is based on Monte Carlo simulations and was successfully tested against experimental data.