Determination of the shielding factors for gamma-ray spectrometers

• A model is described to assess shielding factors for gamma-ray spectrometers.
• The background due to the detector, shield and ambient radiation must be known.
• The sample attenuation, its dimensions and distance from the crystal are considered.
• Shielding factors for gamma-rays from the 232Th and 226Ra decay chains are assessed.
• For a water sample with a mass of 0.25 kg, shielding factors above 0.88 are obtained.

A method for determining the shielding factors for gamma-ray spectrometers is described. The shielding factors are expressed by decomposing the peaked background of the spectrometer into contributions of the detector, spectrometer shield and ambient radiation to the spectrometer background. The dimensions of the sample and its mass-attenuation coefficient are taken into account using a simple model. For six spectrometers, with contributions to the background quantified, the shielding factors were determined for the background based on the thorium decay series and the radon daughters. For a water sample with a diameter of 9 cm and a thickness of 4 cm and the nuclides of the thorium decay series that are in the spectrometer shields, the values of the shielding factors lie in the interval 0.95–1.00. For a spectrometer exhibiting the diffusion of radon into the shielding material, the values of the shielding factors for the same sample for gamma-rays from the radon daughters lie in the interval 0.88–1.00.