Soil radioactivity-depth profiles from regularized inversion of borehole gamma spectrometry data.

An in situ borehole gamma logging method using a LaBr3 gamma detector has been developed to characterize a137Cs contaminated site. The activity-depth distribution of 137Cs was derived by inversion of the in situ measurement data using two different least squares methods, (i) Least squares optimization (LSO) and (ii) Tikhonov regularization. The regularization parameter (λ) of the Tikhonov regularization method was estimated using three different methods i.e. the L-curve, Generalized Cross Validation (GCV) and a prior information based method (PIBM). The considered inversion method variants were first validated for a137Cs contaminated pipe, and in most of the cases, the calculated activity of 137Cs was found to be within the acceptable range. The calculated 137Cs activity-depth profiles from in situ measurements were also in good agreement with the ones obtained from soil sample analysis, with an R2 ranging from 0.76 to 0.82. The GCV method for estimating λ appeared to perform better than the two other methods in terms of R2 and root mean squared error (RMSE). The L-curve method resulted in higher RMSE than the other Tikhonov regularization methods. Instability was observed in the activity concentration depth profile obtained from the LSO method. Therefore, we recommend the Tikhonov regularization with GCV for estimating λ for estimating the activity concentration-depth profile. The site studied showed 137Cs activity concentrations above the exemption limit down to depths of 0.50-0.90 m.

Optimization and validation of a LaBr3(Ce) detector model for use in Monte Carlo simulations.

A reliable detector model is needed for Monte Carlo efficiency calibration. A LaBr3(Ce) detector model was optimized and verified using different radioactive sources (241Am,133Ba,137Cs,60Co and152Eu) and geometries (point, extended and surface). PENELOPE and MCNP were used for Monte Carlo simulations. A good agreement was observed between simulated and experimental full energy peak efficiencies (FEPE) as their mean relative difference was 2.84% ± 1.93% and 2.79% ± 1.99% for PENELOPE and MCNP simulation, respectively. The differences between simulated FEPEs of two Monte Carlo codes were negligible except for low energies (< 100 keV).

Negative correlation between the number of sunspots and the occurrence of 7Be and 22Na in the surface air and their contribution to radiation doses.

This article presents yearly mean concentrations of cosmogenic radionuclides 7Be and 22Na occurring in dry and wet depositions (fallout) and aerosols. Time dependencies negatively correlated with the yearly mean number of sunspots. Activity concentrations of 7Be and 22Na in aerosols in the surface air had a correlation of near-unity. 7Be in aerosols exhibited a smoother time dependence than 22Na, implying that the production of 22Na is more sensitive to the solar activity than the production of 7Be. The effect of the measured doses on the general population through internal and external exposure to radiation from cosmogenic radionuclides was small.

InSiCal - A tool for calculating calibration factors and activity concentrations in in situ gamma spectrometry.

In situ gamma spectrometry is a widely applied analysis technique for the determination of radioactivity levels in soil. Compared to traditional laboratory analysis of soil samples, in situ techniques offer a quick and low-cost way of obtaining accurate results from on-site measurements. However, although the technique is well-known, the dependence of in situ gamma spectrometry on complex and time-consuming calibration procedures as well as in-depth knowledge of the geometric distribution of the source in the ground deters many potential users from employing it in their routine work. Aiming to alleviate this issue, a software tool named InSiCal (In Situ gamma spectrometry Calculator) has been developed to make in situ gamma spectrometry more accessible to both experts and non-experts in the field. This is done by simplifying and streamlining both calibration and activity calculation through a simple and intuitive graphical user interface. Testing in real field conditions show that InSiCal is capable of yielding results which are in very good agreement with soil sample analyses, and that the results may be obtained using different detector types (HPGe, NaI, LaBr and CZT). Overall, InSiCal, provides results which are comparable in accuracy to laboratory measurements, indicating that it fulfills its purpose successfully.

The first experimental test of the MEFFTRAN software on HPGe detector calibration for environmental samples.

The gamma spectrometry of environmental samples poses a series of specific problems for the practitioner. The determination of full energy peak efficiencies is, in the case of environmental samples, one of the greatest challenges. In this paper, the calibration of two HPGe detectors for environmental samples in Marinelli beakers was performed using the newly developed MEFFTRAN software. The results obtained with MEFFTRAN were compared to the experimental efficiencies measured using the calibration sample produced at the Laboratory for Radiation and Environment Protection, Institute for Nuclear Sciences Vinca, Belgrade, in order to confirm the validity of the calculation. It is shown that calculated and experimental efficiencies are in good agreement with the discrepancies from -3.9% to +1.5%. The results were also validated by measuring secondary reference materials provided by the IAEA within several interlaboratory proficiency tests. The activity concentration of the radionuclides in the secondary reference materials was determined using the efficiency obtained by MEFFTRAN and compared with the target value given by the IAEA. All the results in materials provided by the IAEA were acceptable except one, proving that MEFFTRAN produces satisfactory results.

Half-life measurements of lutetium-176 using underground HPGe-detectors.

The half-life of (176)Lu was determined by measuring the (176)Lu activity in metallic lutetium foils. Three different HPGe-detectors located 225 m underground were employed for the study. Measurements using the sum-peak method were performed and resulted in an average massic activity of (52.61±0.36) Bq g(-1). The foils were of natural isotopic abundance so using the massic activity and the value of the natural isotopic abundance of (2.59±0.01)%, a half-life of (3.722±0.029)×10(10)a could be calculated.

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.

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.

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.

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.

Intrinsic error of the point-source and the LS-curve approximations in treatment of true coincidence summing.

Apart from an experimental determination of the relevant correction factors or a full Monte Carlo simulation, the most popular computational approach to the problem of true coincidence summing effects is the point-source approximation. An improvement of this method is based on the so-called linear-to-square or "Third curve" of gamma-ray spectrometry. Both of these computational methods carry with them an intrinsic error, the magnitude of which we investigate for the case of environmental measurements with p-type HPGe detectors.

Crystal rounding and the efficiency transfer method in gamma-ray spectrometry.

The effect of failing to properly model the rounding of the crystal edges in the application of the efficiency transfer method to the measurement of environmental samples is investigated for coaxial and thick planar (BeGe) HPGe detectors. It is found that for lower energies proper modelling of the crystal rounding is necessary to achieve the accuracy of the computed efficiency usually required in this type of measurements.

A method for calculation of true coincidence summing correction factors for extended sources.

In gamma-ray spectrometry, true coincidence summing correction factors for an extended sample can be calculated from full-energy-peak and total efficiencies as if the sample were a point source, if the so-called linear-to-square- (LS) curve, introduced by Blaauw and Gelsema, is known and properly applied. A method is described for obtaining the efficiencies and the corresponding LS-curve for an arbitrary cylindrical sample from calibration measurements in a reference geometry. The approach is aimed at the analysis of samples measured on p-type HPGe detectors in environmental gamma-ray spectrometry and was successfully verified against experimental data.

On the invariability of the total-to-peak ratio in gamma-ray spectrometry.

It is demonstrated that the assumption on the independence of the total-to-peak ratio of the source position in gamma-ray spectrometry holds for coaxial detectors with sufficient accuracy to make it applicable to the determination of the full energy peak efficiencies in environmental measurements via the so-called efficiency transfer method. The calculations show that for planar detectors the assumption breaks down at higher gamma-ray energies and that for such detectors the use of the efficiency transfer method is only warranted for gamma-ray energies up to 200 keV.