GE•SPE•COR

Introduction

GESPECOR software has been developed initially for the computation of the self-attenuation corrections and of the coincidence-summing corrections useful in high resolution gamma ray spectroscopy. Later the computation of the full energy peak efficiency and of the total efficiency was added (see the Updates in this page).

Among the procedures available for solving the same problems, GESPECOR is unique with respect to its precision, flexibility and user friendliness.

Coaxial and well-type HPGe detectors described in full detail and cylindrical and Marinelli beaker samples with any composition and density are properly addressed; the tedious task of deciphering the decay scheme data required for coincidence summing corrections (including all higher order contributions) is replaced by an automatic procedure based on a nuclide library containing the necessary data for about 225 from the most frequently encountered nuclides.

Self-attenuation corrections are required if the efficiency calibration curve established with a standard calibration source is used for the analysis of a spectrum measured for a sample which has a composition and a density that is different with respect to the matrix of the calibration source (usually water equivalent). The correction factors are higher for large volume, high Z and high density samples, for low energy photons.

Coincidence summing effects are important in high efficiency measurement conditions, such as well-type detector measurements or measurements with high volume detectors and sources placed close to the detector. The origin of these effects is the fact that in the case when several photons interact with the detector in a very short time one after the other the detector is unable to give separate signals for each of the photons; a single summed signal is produced instead. Nulides like Mn-56, Co-60, Cs-134, Ba-137, Eu-152, Eu-154 and many others (in fact most of the nuclides) decay through gamma cascades, i.e. through successive emissions of photons following each after the other in a very short time. When more than one photon from a cascade interacts with the detector coincidence summing effects are produced. The magnitude of the effects depends on the decay scheme of the nuclide, on the detector efficiency, on the measurement geometry (including the sample, the detector, the shield), on the sample composition and density. Due to coincidence summing effects, the apparent efficiency at the energy of a photon from a cascade differs from the efficiency at the same energy obtained from the calibration curve measured with coincidence-free nuclides.

Coincidence summing corrections are therefore required in order to apply the usual efficiency calibration curve for the analysis of nuclides which possess a decay scheme with cascading transitions.

It is difficult to estimate the self-attenuation corrections and the coincidence-summing corrections on a purely experimental basis. In the GESPECOR software the Monte Carlo simulation method is applied. This is a sophisticated computational method, including all the relevant experimental details and a proper description of all the important physical processes. In spite of the complex calculational procedures a friendly user interface is provided which makes the use of GESPECOR very easy.

This WINDOWS interface provides the means to input the necessary detector and geometry data; graphical representations may be used to avoid mistakes regarding the significance of various parameters. Any sample matrix may be easily accomodated, if the composition is known, or if some standard composition may be assumed. The methods used in the GESPECOR package are theoretically sound and the results have been thoroughly tested by comparisons with experimental measurements carried out at Physikalisch-Technische Bundesanstalt (Braunschweig, Germany).

Roughly speaking, for commonly used measuring geometries and samples, the uncertainty of the self-attenuation computations is lower than (usually much lower than) 5% at energies between 50 and 100 keV and lower than 1% at energies above 100 keV. Even in such cases when the self-attenuation is responsible to a reduction of the efficiency by 10 to 20 times the computations reproduced very well the measured values, in the limit of experimental errors.

In the case of measurements with well-type detectors where the effects are particularly high, the uncertainty of the computed coincidence summing correction factors is lower than 5% for nuclides with simple decay schemes and lower than l0% to 15% for nuclides with complex decay schemes and with substantial higher order summing. In the case of measurements with coaxial HPGe detectors, the corresponding values of the uncertainty are about two times lower.

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