Calibration of a 5x5 NaI(Tl) for Prompt In-Situ Gamma-ray Spectrometry System

Energy calibration of the detector was performed by weighing the standard materials, RGU, RGK, and RGTh, from the International Atomic Energy Agency (IAEA) in petri dishes. Total masses of 144.00 g, 183.90 g, and 130.02 g of RGU, RGK, and RGTh, respectively, were used. The standard materials in the petri dishes were arranged to depict the inhomogeneous distribution of the radionuclide in the environment. The detector was placed directly above the arrangement at a height of 140 mm. This was allowed because as much as possible only the natural radioactivity, as a result of the low gamma emitting radionuclides present in the soil, was intended to be captured. The Window Analysis Method (WAM) was applied as spectrum analysis method. In this method, only the region of interest of the spectrum is considered [10]. The assessment of the concentration of Potassium was therefore done by studying the single peak emitted by ⁴⁰K at the energy of 1,460 keV, Uranium, ²³⁸U was at the energy of 1,765 keV from ²¹⁴Bi, while Thorium, ²³²Th, was through the gamma rays of ²⁰⁸Tl with energy equal to 2,614 keV. After a preset time of 300 s [13], a spectrum was captured, and the channels of the various photopeaks corresponding to the gamma energies were identified. Figure 1 shows the spectrum as captured during the experiment. The photopeaks of interest were 295 keV, 1,120 keV, and 1,765 keV. The calibration procedure continued with the selection of the calibration option of the software and inserting the gamma energies of each of the peaks of interest against their channel numbers. A relation of the gamma energy versus the channel number was thus established by the software. The regions of interest (ROI) were carefully determined around these photopeaks in order to obtain count rates due to each radio-nuclide under its reference peak. These count rates were converted to soil activity concentration of the radionuclides using the conversion factors.

Figure 1:

The pulse height distribution that was obtained earlier from the detector and MCA only informs us about the distribution of energy depositions in the active volume of the detector. In order to do a radionuclide-specific quantification of the activity, efficiency calibration linking the number of recorded counts in the detector to the ground deposition activity level is required [14]. According to [15], the number of counts per second, N_f, obtained under a photopeak due to a particular gamma energy, E, is related to the soil radioactivity concentration, A of the radionuclide producing the peak by Equation 1 [16]. (1) NfA=NfNoNo∅∅A {{{N_f}} \over A} = {{{N_f}} \over {{N_o}}}{{{N_o}} \over \emptyset}{\emptyset \over A}

In Equation 1, NfA {{{N_f}} \over A} is the photopeak count rate at the gamma energy per unit activity concentration of the radionuclide in the soil (cpsBqkg−1) \left({{{cps} \over {Bqk{g^{- 1}}}}} \right) , NfNo {{{N_f}} \over {{N_o}}} is the angular correction factor, which accounts for the nonuniformity of the detector response to gamma rays incident at varying angles, No∅ {{{N_o}} \over \emptyset} is the on-axis response of the detector (normal to detector face) given in the count rate per uncollided flux of parallel gamma rays (cpsγm−2s−1) \left({{{cps} \over {\gamma {m^{- 2}}{s^{- 1}}}}} \right) , ∅A {\emptyset \over A} represents the total uncollided flux per unit source activity concentration (γm−2s−1Bqkg−1) \left({{{\gamma {m^{- 2}}{s^{- 1}}} \over {Bqk{g^{- 1}}}}} \right) , and ∅ is the gamma r y unscattered flux on the detector (γm⁻²s⁻¹).

The factor No∅ {{{N_o}} \over \emptyset} represents the response of the detector to photons at normal incidence. Its value was determined by placing the uranium standard material (RGU-1) obtained from the International Atomic Energy Agency (IAEA) at a distance of 140 mm from the detector face. After a preset counting time of 300 s, a spectrum was obtained. Figure 1 is the spectrum showing the response of the detector to the associated radionuclide with the photopeaks of interest, with 1,460 kev for ⁴⁰K, 1,764 kev for ²²⁶Ra, and 2,615 kev for ²³²Th. A region of interest was carefully marked around the photopeaks so that the count rate, N_o, was obtained. The flux ∅ was determined by using Equation 2 [16]. (2) ∅=A×γ4πR2 \emptyset = {{A \times \gamma} \over {4\pi {R^2}}}

Here γ is the gamma yield at a particular gamma energy, R is source to detector distance (m), and A is radiation source activity (Bqkg⁻¹).

NfNo {{{N_f}} \over {{N_o}}} was determined from count rate under photopeaks obtained by moving the standard source from 0⁰–50⁰ in steps of 10⁰. If Nf(E,θ)No {{{N_f}\left({E,\theta} \right)} \over {{N_o}}} is the ratio of the detector response to gamma ray of energy E, at angle, θ, with respect to the response at θ= 0⁰, then NfNo {{{N_f}} \over {{N_o}}} can be determined using Equation 3 [15]. (3) NfNo=1∅∫0π/2∅(θ)Nf(E,θ)Nodθ {{{N_f}} \over {{N_o}}} = {1 \over \emptyset}\mathop \smallint \nolimits_0^{\pi /2} \emptyset \left(\theta \right){{{N_f}\left({E,\theta} \right)} \over {{N_o}}}d\theta

The determination of NfNo {{{N_f}} \over {{N_o}}} can be achieved by numerically integrating Equation 3 (Jibiri and Farai, 2005). The integration was performed in this work by using the experimental values of Nf(E,θ)No {{{N_f}\left({E,\theta} \right)} \over {{N_o}}} and the value of ∅. By substituting the values of No∅ {{{N_o}} \over \emptyset} , NfNo {{{N_f}} \over {{N_o}}} , and ∅A {\emptyset \over A} in Equation 1, the desired conversion factor NfA {{{N_f}} \over A} is obtained.

A body free from the presence of any radionuclide was simulated. This was achieved by putting distilled water into a fairly large bathtub to attenuate the natural background gamma rays. The in-situ gamma spectrometer was mounted on the water body, and counting was done for a preset time of 300 s. A spectrum was captured, and the count rate under the photopeaks of ⁴⁰K, ²²⁶Ra, and ²³²Th were determined by carefully taking the region of interest, ROI, around the photopeaks.