Experimental measurement of Rayleigh to Compton cross-section ratio for 279 keV gamma photons M.P. Singh 1 , Amandeep Sharma 2 , Bhajan Singh 1 , B.S. Sandhu 1,* 1 Department of Physics, Punjabi University Patiala-147002 2 Department of Math. Stat. & Physics, Punjab Agricultural University Ludhiana-141004 *E-mail: balvir@ pbi.ac.in Abstract The present work involves measurement of Rayleigh to Compton scattering cross-section ratio of elements in the range 13 ≤ Z ≤ 79 for 279 keV incident gamma photons. An HPGe gamma detector, placed at 50 to the incident beam, detects gamma photons scattered from the target under investigation. The intensities of Rayleigh and Compton scattered peaks observed in the recorded spectra, and corrected for photo-peak efficiency of gamma detector and absorption of photons in the target and air column present between the target and detector, along with the other required parameters provides differential cross-sections ratio for Rayleigh to Compton scattering. The measured values of cross-section ratio are found to agree with existing theories for low Z elements, but deviate for high Z elements. 1. Introduction Rayleigh (coherent) scattering results in scattering of gamma photon without any change in energy, and is predominant at low incident photon energies, small scattering angles and high atomic number elements. The atomic Compton (incoherent) scattering results in degradation of gamma photon energy, and can be calculated with the incoherent scattering function modification to Klein-Nishina formula for Compton scattering. Rayleigh to Compton scattering ratio, R, has a power relation to Z in the region of elemental interest and this power dependence is based upon the ratio F 2 /S, with F(q,Z) being the form factor and S(q,Z) is the incoherent scattering function. Hubbell et al. [1] have provided theoretical model for the calculation of Rayleigh to Compton cross-section ratio from the parameters F(q, Z) and S(q, Z). Various theories have been developed to calculate atomic form factor based on non-relativistic form factor [1], relativistic form factor [2], modified relativistic form factor [3] and S-matrix theory [4]. According to these theories, the atomic form factor, F(q, Z), is the Fourier transform of atomic charge distribution and can be evaluated by different wave functions. Icelli and Erzeneoglu [5] have measured the ratio of differential cross-sections for coherent and Compton scattering of 59.54 keV at scattering angles of 55 and 115 for Fe, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Sn, Ta, Au and Pb targets using Ge(Li) detector. More recently, we [6] have determined coherent to incoherent scattering cross section ratio of elements for 145 keV gamma photons. An accurate determination of ratios of coherent to incoherent scattering for different elements is important because of their wide use in the fields of atomic and radiation physics, and non- destructive elemental analysis of materials. There have been various investigations on coherent (Rayleigh) to incoherent (Compton) scattering ratio method. This intensity ratio method has been quite successful in various fundamental and medical applications of gamma radiations. Our group has successfully used this ratio technique for non- destructive evaluation of scientific and biological samples, measurements of mandibular bone density and pulmonary edema etc. [7]. The knowledge of coherent to incoherent scattering cross-section ratio is useful in the calculations of radiation attenuation, reactor shielding, industrial applications and also in the field of medical sciences in a number of ways. In the present experiment, Rayleigh to Compton scattering cross-section ratio values are measured for elements in the range, 13 ≤ Z ≤ 79, for 279 keV incident gamma photons. The measured results are compared with various existing theories available in literature (corresponding to 9.510 o A -1 photon momentum transfer). 2. Experimental set-up and Method of Measurements The principle of present measurements is to observe the intensities of Rayleigh and Compton scattered gamma photons at a particular scattering angle. The measurements are performed using 203 Hg (279 keV) radioactive source strength of activity 0.925 GBq, which is placed at the end of a cylindrical cavity of depth 20 mm and diameter 11 mm fabricated in a lead cube having each side 160 mm (Fig. 1). An aluminium hollow sleeve of internal diameter 12 mm, fitted in the centre of a rectangular block of lead having dimensions 80 mm  80 mm  15 mm, is placed coaxially adjacent to the cavity to obtain a narrow beam of photons. The distance of thin target under study from the source collimator (radius 3 mm) is kept as 100 mm. The source- target assembly is aligned in such a way that the 52 www.ijert.org International Journal Of Engineering Research and Technology (IJERT) AMRP-2013 Conference Proceedings