**2.2 Gamma spectroscopy analysis**

The collected sealed samples were left for a minimum of 4 weeks to allow secular equilibrium prior to the counting of the samples for radioactivity concentration. The counting for radioactivity was carried out using a 7*:*6*cm* 7*:*6*cm* NaI(Tl) detector for 10 h (Model Bircom, USA) housed in a 10-cm-thick lead shield to reduce background gamma radiation. The detector with energy resolution (FWHM) of 7.5% at 662 keV was coupled to a set of electronics which consist of pre-amplifier, main amplifier, analog-to-digital converter (ADC), and a Canberra Multichannel Analyzing (MCA) computer system. The integrated spectroscopy system was used for the power supply and the data acquisition of the energy spectra and utilized SAMPO S100 software package from Canberra (MAESTRO window USA). The energy calibration of the detector was performed between the gamma energy range of 83 keV and 1875 keV using International Atomic Energy Agency standard point sources (109Cd, 57Co, 137Cs, 54Mn, and 22Na), the energy range of the radionuclide to be identified. To simulate the milk samples, 100 g of IAEA-375 reference sample was used. The radioactivity concentrations of 226Ra/238U were determined from the photopeaks of 609.32 keV (214Bi), 1120.20 keV (214Bi), and 352.6 keV (214Pb) and that of 232Th from 969.3 keV (228Ac) and 583.78 keV (208Tl), while the radioactivity of 40K was evaluated from 1460.3 keV photopeak following the decay of 40K. The background spectrum measured under the same settings for

both the standard and sample measurement was used to correct the computed sample activity concentration in agreement with Arogunjo et al. [13].
