In the environmental radioactivity analyzing methods using gamma-ray spectrometry, the natural activities of radionuclides were required to be higher than the minimum detectable activity (MDA). To reduce MDA, one of the popular methods is to improve the ability of reducing the background radiation of the gamma-ray spectrometry. In this work, we designed the shielding configuration with 5 cm lead and 2 mm copper (thickness of walls and top). The MDAs of gamma-ray spectrometer were 2.6–4.24 times times for 40K (1460.8 keV), 232Th (208Tl- 2614.5 keV) and 238U (214Pb- 352 keV; 214Bi- 609.3 keV, 214Bi- 1764.5 keV). In the other hand, MDA for 238U with this shielding configuration is smaller than the activity of 238U inside surface soils in Vietnam. These results showed that the gamma spectrometer with NaI(Tl) detector and this shielding configuration was suitable for measurements activity of 238U in the environmental samples.

In this work, the Compton cross section of some materials such as aluminum, iron, copper, C45 steels and CT3 steels was calculated by Mathematica and Fortran 95 codes. Besides, the MCNP5 code and experiment using the gamma transmission techniqne to determine linear attenuation coefficients, were used to calculate the Compton cross section. The maximum deviation of about 6 % showed that our codes was good agreement with NIST data, MCNP code and experiment.