Low Level - Low Energy (5-200 keV) Gamma Spectroscopy

The gamma spectroscopy analysis in the low energy region below 200 keV is conducted using a Low Energy Germanium Detector LEGe with the following specifications:

Active area 2000 mm²
Thickness 20 mm
0.5 mm Be window
Resolution: 340 eV at 5.9 keV, 530 eV at 122 keV.

The samples analysed are usually of environmental origin, such as soil, fly-ash, bottom-ash, building materials and water, with low activity, thus the use of large sample volumes is necessary.

When analysing bulk samples using low energy photons, such as 46.52 keV (Pb-210), 59.54 keV (Am-241) and 63.29 keV (Th-234), the countrate is highly affected by the intense self absorption of the low energy photons, which is material dependent.

Germanium Detector efficiency curve

The efficiency calibration of the detector in the energy region 0-2000 keV is obtained using standard mixed radionuclide solutions. The curve obtained is valid for the analysis of samples with the same geometry (cylindrical box 0.282 L) and, especially in the low energy region, the same material.

The difference in the absorbing properties between the calibration source and the sample requires the introduction of an efficiency correction factor. A method which is applicable for any cylindrical geometry was adapted for the determination of the above correction factor, using a newly developed experimental-numerical technique and a Fortran Program (Anagnostakis & Simopoulos '96 ).. This program using as input the detector geometry, the source-to-detector distance and the linear attenuation coefficients μ of both the calibration source and the material to be analysed, calculates the efficiency correction factor.

Linear atteniuation coefficient

The values of the linear attenuation coefficient μ needed for the above calculation are being experimentally estimated for each material to be analysed. According to the results obtained, in the case of surface soil, lignite and fly ash the values of μ ranges from 0.2 - 0.9 cm^-1, and leads to efficiency correction factors between the calibration source and analysed material in the range between 0.5 - 1.2, for photons with energies 46.52 and 59.54 keV. In the case of 185.99 keV photons, the efficiency correction factor is slightly lower than 1.0, even for the most absorbing of the materials analysed. For the analysis of the collected spectrum the in-house developed UNIX-based Computer Code SPUNAL is used.

Am-241 detection in soil sample

Among the isotopes identified in the soil samples analysed, was Am-241 (Simopoulos 89) . The samples were collected in the aftermath of the Chernobyl accident. Furthermore, Am-241 was identified in air-filters collected in Athens in May 1986, and the maximum air concentration was estimated to 8 mBq/m³ . The 63.29 photopeak in the air-filter sample is due to detector background.

Am-241 detection in air filter