SAIP 2011

Low level counting using a NaI(Tl) detector

13 Jul 2011, 14:30

15m

Acro8

Oral Presentation Track B - Nuclear, Particle and Radiation Physics NPRP

Speaker

Mr Sive Noncolela (UWC)

Description

In this work a 7.5 cm x 7.5 cm NaI (Tl) detector was used to study activity concentrations of primordial radionuclides in soil and sand samples. The detector and the sample were placed inside a lead castle to reduce background measurements from the surroundings such as the wall and the floor. The samples were placed inside a 1L Marinelli beaker which surrounds the detector for better relative. Additional lead bricks were placed below the detector to further reduce the background by 10%. A Full Spectrum Analysis (FSA) method was used to extract the activity concentrations of 238U, 232Th and 40K from the measured data. The FSA method uses standard spectra corresponding to the radionuclides being investigated, which are multiplied by the activity concentrations and then added to fit the measured spectrum. Accurate concentrations are then extracted using a chi-squared (χ2) minimization procedure. Eight samples were measured using the NaI detector and analyzed using the FSA method. The samples were measured for about 24 hours for good statistics. The 238U activity concentration values varied from 10 ± 2 Bq/kg (iThemba soil, HS6) to 256 ± 10 Bq/kg (Kloof sample). The 232Th activity concentration values varied from 7 ± 1 Bq/kg (Anstip beach sand) to 45 ± 5 Bq/kg (Rawsonille soil #B31). The 40K activity concentration values varied from 45 ± 27 Bq/kg (iThemba soil, HS6) to 120 ± 9 Bq/kg (Rawsonille soil, #B28). The χ2 values also varied from sample to sample with the lowest being 0.13 (Anstip beach sand) and the highest being 6820 (iThemba soil, HS1). A high χ2 value usually represents incomplete gain drift corrections, improper set of fitting functions, proper inclusion of coincidence summing or the presence of anthropogenic radionuclei.