D. Arnold

GESPECOR: A versatile tool in gamma-ray spectrometry

GESPECOR is a Monte Carlo based software developed for the computation of efficiency, of matrix effects and of coincidence summing effects in gamma-ray spectrometry. GESPECOR can be applied to coaxial and well-type HPGe or to Ge(Li) detectors and to various types of sources, including point, cylindrical, and spherical sources or Marinelli beakers. In this paper the structure of GESPECOR is presented and the procedures applied are described. The uncertainty of the results computed by GESPECOR is carefully analyzed. The analysis shows that GESPECOR is able to provide results with a well defined uncertainty, in a user friendly WINDOWS environment.

Intercomparison of efficiency transfer software for gamma-ray spectrometry

The EUROMET project 428 examines efficiency transfer results for Ge gamma-ray spectrometers when the efficiency is known for a reference point source geometry. For this, different methods are used, such as Monte Carlo simulation or semi-empirical computation. The exercise compares the application of these methods to the same selected experimental cases to determine the usage limitations versus the requested accuracy. For carefully examining these results and trying to derive information for improving the computation codes, this study was limited to a few simple cases. The first part concerns the simplest case of geometry transfer, i.e., using point sources for 3 source-to-detector distances: 2, 5 and 20 cm; the second part deals with transfer from point source geometry to cylindrical geometry with three different matrices. The general conclusion is that the deviations between the computed results and the measured efficiencies are mostly within 10%. The quality of the results is rather inhomogeneous and shows that these codes cannot be used directly for metrological purposes. However, most of them are operational for routine measurements when efficiency uncertainties of 5-10% can be sufficient.

Accurate computation of coincidence summing corrections in low level gamma-ray spectrometry

The GESPECOR (Germanium Spectrometry Correction factors) software, previously developed for computing the self-attenuation and coincidence summing corrections, was applied to the computation of the coincidence summing correction factors for a well-type and two coaxial HPGe detectors. Cylindrical samples as well as Marinelli beaker samples were considered. The computed values are in good agreement with carefully measured values. A detailed study of the uncertainties assigned to the results was carried out. The analysis shows that the procedures used in GESPECOR are reliable and provide results with a well defined accuracy.

The PTB underground laboratory for dosimetry and spectrometry

In 1991, the Physikalisch-Technische Bundesanstalt established an underground laboratory for dosimetry and spectrometry (UDO) at the Asse salt mine, near Braunschweig. Due to the depth of 925 m below ground (equivalent to about 2100 m of water), the cosmic ray muon intensity in this facility is reduced by more than 5 orders of magnitude. In addition, the low specific activity of the pure rock salt and a low concentration of radon lead to an extremely low ambient dose equivalent rate of less than 1 nSv/h. The UDO facility is therefore well suited for dosimetry at very low dose rates, as well as for Ultra-Low-Background (ULB) gamma-ray spectrometry. In 1998, a coaxial low-background HPGe-detector (88% relative efficiency, FWHM 2.0 keV at 1.33 MeV) with an extended shielding (20 cm low-activity lead, 1 cm electrolytic copper, N2-flushing) was installed at UDO; the count rate per mass of germanium, integrated over the energy range from 40 to 2750 keV, was measured to be 0.012 s(-1) kg(-1). Results from test measurements and first applications are reported. The design of a ULB gamma-detector system, presently under construction, is described.

An intercomparison of Monte Carlo codes used in gamma-ray spectrometry

In an intercomparison exercise, the Monte Carlo codes most commonly used in gamma-ray spectrometry today were compared with each other in order to gauge the differences between them in terms of typical applications. No reference was made to experimental data; instead, the aim was to confront the codes with each other, as they were applied to the calculation of full-energy-peak and total efficiencies. Surprising differences between the results of different codes were revealed.

Transfer of the efficiency calibration of Germanium gamma-ray detectors using the GESPECOR software

The GESPECOR software was extended to incorporate procedures for the computation of the efficiency transfer factor for cases of practical interest: (a) sources with identical geometry, but different matrices, (b) sources with similar (but not identical) geometry; and (c) transfer from a point source to a volume source. Fast and accurate algorithms. based on correlated sampling, were implemented for solving the first two cases. A procedure to take into account the imperfect charge collection in the detector was implemented.

Self-attenuation and coincidence-summing corrections calculated by Monte Carlo simulations for gamma-spectrometric measurements with well-type germanium detectors

Abstract A Monte Carlo simulation package for the computation of the full-energy peak efficiency, self-attenuation correction factors and coincidence-summing corrections has been developed to assist in the calibration of well-type germanium detector measurements. Due to the almost 4π solid angle for this measurement geometry, particularly high coincidence-summing effects occur in the case of multi-photon emitting nuclides. Besides pair coincidences, the correction terms required to describe higher order coincidences have been included in the computation of the coincidence-summing effects. A general algorithm for self-attenuation calculations, working accurately as well in the case of high attenuating media, has been implemented. The computed results are in good agreement with the experimental data.

A new low-level γ-ray spectrometry system for environmental radioactivity at the underground laboratory Felsenkeller

A low-level gamma-ray spectrometry system, based on an HPGe-detector with 92% relative efficiency recently installed in the underground laboratory Felsenkeller at 110 m water equivalent (w.e.) depth, is described. The integral background count rate normalised to the Ge-crystal mass in the energy range from 40 keV until 2.7 MeV of 0.034 s(-1)kg(-1) has been achieved by careful material selection of the detector construction material, a graded shielding construction and effective radon suppression. The detector is highly suitable for the effective surveillance of water for human consumption with decision thresholds for (226)Ra and (228)Ra in the order of some mBq L(-1).

On the Monte Carlo simulation of HPGe gamma-spectrometry systems

Typical applications of Monte Carlo simulations in low-level gamma-ray spectrometry are presented. The current state of coincidence summing computations is briefly reviewed. Several problems concerning direct computation of the efficiency by Monte Carlo simulations are discussed.

Intercomparison of methods for coincidence summing corrections in gamma-ray spectrometry

A comparison of the coincidence summing correction methods is presented. Since there are several ways for computing these corrections, each method has advantages and drawbacks that could be compared. This part of the comparison was restricted to point sources. The same experimental spectra, decay scheme and photon emission intensities were used by all the participants. The results were expressed as coincidence summing correction factors for several energies of (152)Eu and (134)Cs, and three source-to-detector distances. They are presented and discussed.