Determination of narrow beam attenuation coefficients from a broad beam geometrical configuration for 320 keV photons

Narrow beam mass attenuation coefficients, μ/ρ, of 320 keV photons, emitted from the ⁵¹Cr radioisotope have been measured in various high Z materials such as Pb; BiPO₄; (CH₃COO)₂·Pb·3H₂O; Pb(NO₃)₂; Na₂WO₄·2H₂O, using a broad beam geometrical configuration, employing a 2′' × 1′' (5.1 cm × 2.54 cm) (0.051 m × 0.025 m) NaI (Tl) detector spectrometer system coupled to a 1 k multichannel analyser. The measured μ/ρ values in cm²/g for the above materials in the transmission range from 50% to 20% respectively are 0.34 ± 0.01 (0.35); 0.295 ± 0.002 (0.30); 0.23 ± 0.01 (0.24); 0.256 ± 0.003 (0.26); 0.255 ± 0.003 (0.256). The values given in parentheses are estimated theoretically from the mixture rule using tabulated values of Hubbell (1982). It is observed that for high Z compounds at low energies, the exponential law of attenuation will be valid even for a broad beam set up in the transmission range from 50% to 20%. The attenuation coefficients of elements derived from this method agree fairly well with the tabulated narrow beam attenuation coefficients indicating the validity of the mixture rule in this region of transmission.