Background and material gamma spectra of air sampler filters using neutron activation analysis

I’m using a high-purity Germanium detector to investigate the concentrations of tracer metals in a calm, indoor environment. Glass fiber filters were used for their efficient capture of 1 micron particulates, which is the form of the tracer powder Europium oxide. Though measurements of the tracer quantities are difficult to see and uncertain, the background gamma spectrum can be analyzed.

Naturally occurring radionuclides present in gamma spectrum

Figure 1 show an example of the background spectrum of a neutron-irradiated filter in a coin envelope, over which gamma peaks from activated Europium isotopes will appear when detected. Notable features include the asymptotic edge from Compton scattering of gammas, a wide peak resulting from the backscatter of electrons in the detector and shielding, and a series of prominent environmental radionuclides; these include members of the Uranium-238 decay chain (Bismuth-214, Lead-212) and Thorium-232 decay chain (Actinium-228, Bismuth-212, Lead-212, Thallium-210), as well as naturally radioactive Potassium (K-40) and Sodium (NA-24).

Figure 1: Background gamma spectrum of neutron activated glass fiber filter.

Barium appears in gamma spectrum of neutron-activated of glass fiber filters

Glass fiber filters are made from threaded borosilicate fibrils, and sometimes carry trace quuantities of barium and zinc from the manufacturing process. In Figure 1, prounounced levels of barium-139 (ground and metastable) can be seen. Should a follow-up pure background spectroscopic assessment omit these peaks, they could indicate presence of Barium within the filters. Alternatively if the peaks remain, it may instead result from interference of Caesium-137 test sources present though stowed in the laboratory room.