Uffe C C Korsbech

Search for lost or orphan radioactive sources based on NaI gamma spectrometry

Within recent decades many radioactive sources have been lost, stolen, or abandoned, and some have caused contamination or irradiation of people. Therefore reliable methods for source recovery are needed. The use of car borne NaI(Tl) detectors is discussed. Standard processing of spectra in general can disclose strong and medium level signals from manmade nuclides. But methods for detecting low level signals from weak, distant or shielded sources can be improved. New methods for source detection and identification based on noise adjusted singular value decomposition and on area specific stripping of spectra are presented.

A new technique for processing airborne gamma ray spectrometry data for mapping low level contaminations

A new technique for processing airborne gamma ray spectrometry data has been developed. It is based on the noise adjusted singular value decomposition method introduced by Hovgaard in 1997. The new technique opens for mapping of very low contamination levels. It is tested with data from Latvia where the remaining contamination from the 1986 Chernobyl accident together with fallout from the atmospheric nuclear weapon tests includes 137Cs at levels often well below 1 kBq/m2 equivalent surface contamination. The limiting factors for obtaining reliable results are radon in the air, spectrum stability and accurate altitude measurements.

Core density scanning, degree of induration and dynamic elastic moduli of Palaeogene limestone in the Copenhagen area

Abstract Density scanning by gamma transmission measurements on whole cores is here demonstrated to be valuable both as a sampling guide in the core laboratory, and for assessing the degree of induration of Palaeogene limestones in Denmark. Fracture intensity is also indicated. Moduli of elasticity calculated from acoustic velocities are systematically related to density (and thus porosity), so we have attempted to relate the degree of induration to the dynamic moduli of elasticity. The dynamic moduli of elasticity fall, as is commonly observed, above those measured from uniaxial compression tests. P-wave as well as S-wave velocities are anisotropic in the entire porosity interval (12%–44%). This is reflected in a significant anisotropy of the dynamic shear modulus at high densities, whereas the anisotropy for lower densities and for the dynamic Youngs modulus and dynamic bulk modulus remain below the level of significance. The anisotropy probably reflects minor horizontal fractures formed during exhumation of the limestone and unloading of the cores. The density scanning tool thus proves a useful method for determination of geophysical and geotechnical parameters.

Atmospheric dispersion of argon-41 from a nuclear research reactor: measurement and modelling of plume geometry and gamma radiation field

An atmospheric dispersion experiment was conducted using a visible tracer along with the routine releases of 41Ar from the BR1 research reactor in Mol, Belgium. Simultaneous measurements of plume geometry and radiation field from 41Ar decay were performed as well as measurements of the 41Ar source term and the meteorological parameters. Good overall agreement is found between measurement data and model results using the mesoscale atmospheric dispersion and dose rate model RIMPUFF.

Radiation damage effects in CZT drift strip detectors

At DSRI, in collaboration with the cyclotron facility at Copenhagen University Hospital, we have performed a study of radiation effects exposing a 2.7 mm thick CZT drift strip detector to 30 MeV protons. The detector characteristics were evaluated after exposure to a number of dose loads in the range from 2*108 to 60*108 p+/cm2. Even for the highest dose loads, which had a dramatic effect on the spectroscopic performance, we were able to recover the detectors after an appropriate annealing procedure. The radiation damage was studied as function of depth inside the detector material. A numerical model that emulates the physical processes of the charge transport in the CZT detector was used to derive the charge trapping parameter , μτe (the product of charge mobility and trapping time) as function of dose. The analysis showed that the electron trapping increased proportional with the proton dose. The radiation contribution to the electron trapping was found to obey the following relation: (μτe)-1rad =(2.5±0.2)*10-7*Φ (V/cm2) with the proton fluence, Φ in p+/cm2. The trapping depth dependence, however, did not agree well the damage profile calculated using the standard Monte Carlo simulations, TRIM for the proton induced radiation effects. The present results suggest that proton induced nuclear reactions contribute significantly to the radiation damage. Further work will elaborate on these effects. The detector energy resolution was investigated as function of proton dose. It was found that the observed degradation is well explained by the decrease of μτe when the fluctuations of the electron path length are taken into account. The proton irradiation produced In meta stable isotopes in the CZT material. Their decay and production yield as function of depth were analyzed.