Radiations have found many uses in biomedical research, in the diagnosis and treatment of hundreds of types of diseases, in industrial applications, in chemical and agricultural research, food irradiation and much more due to wide properties of Radiations. Radiations interact with matter to some extent while remaining radiations are transmitted. The elemental composition, density of the irradiated material, types of radiations and its energy tells us about the interaction and transmission of radiations. Radiations do not require the direct contact between matter and can be used with any type of medium regardless of its nature, i.e., the material can be in the form of a gas, liquid or solid, it can be a conductor or insulator of heat or electricity; it can be ferromagnetic or non-magnetic, ceramic or metal, porous or impermeable, sealed or open, etc. Review of the work carried out by different people on gamma attenuation studies in different media and at different moisture levels have been given in this section.
One of the oldest methods for measuring moisture gradient is bandsaw slicing technique (McMillen 1955). The effect of relative humidity variation on tensile and compressive stresses, perpendicular to the grain, during dry of 2 inches red oak at 110o F is reported. This technique is not entirely accurate because of kerf and moisture loses caused by heat generated during high speed cutting.
Sastry and Jnanananda (1958) estimated attenuation coefficients in seven different alloys and in Perspex for 1.17 and 1.33 MeV gamma radiation. They obtained the effective atomic number of two tungsten steels by interpolation. Their results indicated the necessity of different effective atomic numbers to describe the various types of gamma ray interactions in a mixture of elements.
Rothwell (1974) studied breast height sapwood water content fluctuations of lodgepole pine (Pinus contorta var. latifolia Engelum). Also, gamma radiation attenuation was tested as a method for in situ sapwood water content measurement. Cesium 137 sources of 0.661MeV peak energy is used for the gamma radiation attenuation in lodgepole pine trees.
Bradley et al (1991) have determined linear attenuation coefficients of a range of tropical hardwoods at a photon energy of 59.5 keV. They demonstrated a good linear dependence of attenuation on mass density. At 60Co energies, they observed that seasoned Rhizophora spp with a mass density of 1040 kg m-3, provides percentage depth doses that are in agreement to within 2% with those tabulated for water.
Malan and Marais (1991) determined the wood density in about 35 wood species by studying the direct gamma ray densitometry; representing a wider range of densities using 60 keV collimated gamma radiation from a 241Am radioisotope. Mass attenuation coefficients of the wood samples varied from 0.170 to 0.210 cm2/gm mainly due to differences in moisture, extractive and ash contents, differences in chemical composition of the wood and the presence of varying amounts of heavy metals. On the average, the mass attenuation coefficients for air dry and oven dry wood were 0.185 cm2/gm and 0.18 cm2/gm respectively. This study indicated that the linear attenuation coefficient of wood correlates well with its density.
Bhandal et al (1992) measured the total photon mass attenuation coefficients of 7 fatty acids in the energy range 356 to 1116 keV. The experimental results are analysed in terms of the total cross-section, effective atomic numbers, and electron densities.
Arora et al (2005) studied gamma attenuation studies in selected wood samples at varying moisture levels. In this, the transmission studies in five different varieties of wood i.e., Oak, Poplar, Shisham (heart and sap) and Babul at different moisture levels in between oven dry and completely wet states is done at 662 keV from 137Cs. Correlation coefficient which lies in between -0.97 to -0.99 in all the cases studied. The parameter which is basically responsible for the absorption of gamma rays in materials is the effective atomic number. In this paper it was concluded that as moisture content increases attenuation coefficient decreases.
Khazaei (2008) studied water absorption characteristics of three different wood varieties (Afra, Ojamelesh, and Roosi). The mean values of water absorption at initial stages of moisture sorption for Afra, Ojamalesh, and Roosi genotypes were equal to 13.44, 6.05 and 5.44 (kg/m2 s1/2), respectively. The corresponding mean values of this parameter for the entire soaking process were equal to 6.8, 4.6 and 3.9 (kg/m2 s1/2), respectively.
Raje and Chaudhari (2009) determined the mass attenuation coefficients of various soil samples using gamma energy at 0.662 MeV. The results are presented in graphical form between a number of particles of radiation counted without absorber (I0) per number of particles of radiation counted with an absorber (I) and the thickness of absorber. This shows that the mass attenuation coef?cient decreases exponentially with increasing density and con?rms the interaction of gamma radiations with various soil samples of various components.
Pawar (2011) measures mass and linear attenuation coefficients of gamma rays of Al for 514, 662 and 1280 keV photons. Three standard gamma sources Sr 90(0.514), Cs137 (0.662) and Na22 (1.280) MeV are used. Mass (µ/?) and linear attenuation coefficients (µ) of Al have been measured using the well-type scintillation spectrometer. The measured attenuation coefficients and mass of the element are useful for dosimetry and radiation shielding purpose.
Alallak and Sarhan (2012) studied various factors affecting the gamma ray transmission in the range of 661.6 keV-1332.5 keV. Attenuation coefficients, cross-sections, electron densities and effective atomic number affect the gamma ray transmissions which were studied for brass, Fe, Al, PVC, Perspex. The inverse relations between energy and attenuation coefficients, as well as cross-sections, were highlighted. Brass appears as a good attenuator from the analysis.
Udagani (2013) estimated the dependence of mass and linear attenuation coefficients of gamma rays on the concentration of manganese (II) chloride (MnCl2). GSpec gamma spectroscopy system was used which consists 2″x2″ NaI (Tl) detector with a Multichannel analyzer (MCA). In this, 137Cs radioactive source emitting 0.662 MeV gamma rays has been used. Results show that the linear attenuation coefficient varies linearly with the concentration of the MnCl2 solution. The linear variation of attenuation coefficient with the concentration of chemical solution will be a useful phenomenon for estimating the concentration of the chemical solution.
Rajashekhar and Kumar (2014) investigated gamma radiation shielding characteristics of wood. NaI (Tl) scintillation detector is used. Gamma radiation shielding characteristics such as linear attenuation coefficient, mass attenuation coefficient, half –value layer, etc. of eight type of wood materials were measured using gamma energy range from 0.511 MeV-1.322MeV. In this paper, it was concluded that density of wood increases attenuation coefficient also increases and attenuation coefficient decreases with increasing energy.
Rajashekhar et al (2015) compare the gamma ray attenuation coefficient of different wood materials. NaI (Tl) scintillation detector is used for measuring linear and mass attenuation coefficients connected to 8K Multi Channel Analyser at gamma ray energies of 662, 1173 and 1332 keV. Sources that are used are for this are 130Cs and 60Co respectively. It is found that mass attenuation coefficient decreases as photon energy increases. In all of the wood materials, high attenuation wood is considered as a very good absorber and a good quality material for radiation shielding.
Thukral and Kumar (2015) studied the moisture content of selected wood samples and its variation with mass attenuation coefficient using 137Cs gamma source. The study describes the use of gamma radiation techniques to determine the mass attenuation coefficients of Eucalyptus species and Pinus roxburghii wood samples at various moisture levels. By keeping the samples in an electric oven for fixed durations the moisture content of the samples was varied. The mass attenuation coefficients were found to be increasing with decreasing moisture content.
Onwoke et al (2017) aim to determine the parameters which estimate the attenuation of woods. In this study, a total of five types of woods, namely, African Balsam, Gmelina, Tectona Grandis, Ironwood, and Mahogany are taken into consideration. To remove water, the wood samples were oven-dried with a temperature range of 90 – 120oC and the attenuation coefficient determining parameters estimated. In this research, the linear attenuation coefficient of the wood species was found to depend on the energy of incident photons, the density, and nature of the wood species.