Speaker
Description
In gamma-ray spectrometry the analysis of low-energy gamma-emitting nuclides like 210Pb, 129I needs special attention because the gamma attenuation in the sample depends on the specific element composition of the sample material. Hence, a material- or sample-specific self-attenuation correction is necessary. For that purpose, attenuation correction procedures have been developed in the past that rely on transmission measurements. The Cutshall (Cutshall, 1983) correction method is probably the best-known and most used but it suffers from measurement bias. Moreover, attenuation correction procedures relying on efficiency transfer require the mass attenuation coefficients to be known and hence need a different approach. Practice shows that attenuation coefficients at low gamma-ray energy of unknown sample materials are not easily obtained by transmission measurements with a parallel beam of gamma-rays and they generally suffer different experimental limitations. We propose non-collimated transmission measurements using gamma-rays of an 241Am and 133Ba source to determine attenuation coefficients at the different energies emitted by the source. The measurement principle relies on setting up a set of calibration curves for the attenuation data for each of the gamma-ray energies of the source. These calibration curves are specific for the detector and the sample geometry used. They are relating the count rate of the transmitted gamma-rays through the known sample relative to the count rate measured for the water sample to the attenuation coefficients for this material relative to that of water. For all materials used to set up these calibration curves, and for all considered energies, the mass attenuation coefficients are taken from the XCOM database (Berger, 2010). Measurements were done on an extended range HPGe detector, and the energies used range from 21.16 keV until 302.9 keV. Mass attenuation coefficients ranged from 0.7 to 33 cm²/g at 21.16 keV. The counting geometry used to set up the calibration curves for the transmission uses a source positioned on top of a cylindrical container (h 23 mm x70 mm) resulting in a fan beam of gamma-rays towards the detector. The apparent mass density of the materials used for the calibration is used to compute the linear attenuation coefficient. By measuring, the count rates for transmission relative to water of an unknown sample at the different source energies and by accounting for the apparent density of the unknown sample material, the mass attenuation coefficients relative to that of water can be determined directly from the calibration curves. The attenuation data obtained in this way allow interpolation between the data points to obtain the attenuation also at other energies. The procedure is easily tested by comparing measured attenuation data for materials of known composition with their attenuation data read from XCOM. For routine analyses with this method, the measured attenuation data are transcribed to a sample-specific material file to be used by the efficiency transfer software. We use EFFTRAN (Vidmar, 2005) for efficiency transfer. The paper will outline the set-up of the different calibration curves used to determine the attenuation data at low gamma-ray energy and will illustrate how the procedure can be used in combination with efficiency transfer by EFFTRAN. Uncertainty propagation from the transmission measurement and the calibration curves towards activity determination in samples with unknown composition will be outlined. The fact that this method does not yield information on absorption edges will also be discussed in view of its general applicability to the analysis of low-energy gamma emitters in environmental and NORM samples. References Berger, M. H. (2010). XCOM: Photon Cross Section Database. NIST STandard Reference Database, na. Retrieved from http://www.nist.gov/pml/data/xcom/ Cutshall, N. (1983). Direct analysis of 210Pb in sediment samples: Self-absorption corrections. Nucl.Instrum.Methods, 206, 309-312. Vidmar, T. (2005). EFFTRAN A Monte Carlo efficiency tranfer code for gamma-ray spectrometry. Nucl. instrum. Methods, 550, 603-608.
