Investigations of energy dependence of saturation thickness of multiply backscattered gamma photons in elements and alloys - an inverse matrix approach

Abstract

Abstract. In Compton scattering experiments employing thick targets one observes that the numbers of multiply backscattered photons increases with increase in target thickness and then saturate at a particular target thickness called the saturation thickness. The energy of each of gamma ray photons continues to decrease as the number of scatterings, the photon undergoes, increases in the sample having finite dimensions. The present experiment is an independent study of energy and intensity distributions of 279-, 320-, 511-, 662 keV, and 1.12 MeV gamma rays multiply backscattered from targets of different atomic numbers and alloys of various thicknesses, and are carried out in a backscattering geometry. The backscattered photons are detected by a NaI(Tl) scintillation detector. The detector response unscrambling, converting the observed pulse-height distribution to a true photon energy spectrum, is obtained with the help of a 12x12 inverse response matrix. The present experimental results confirm that for thick targets, there is significant contribution of multiply backscattered radiations emerging from the targets, having energy equal to that of singly scattered Compton process. The measured saturation thickness (in units of mean free path) for multiply backscattering of gamma photons is found to be decreasing with increase in energy of incident gamma photons.

Evaluation of Multiple Backscattering and Saturation Thickness of Gamma Rays

“…On the other hand, number of multiply scattered events decreases with increasing energy of incident photons because the penetration of gamma ray photons increases with increase in incident energy. This behaviour is in agreement with experimental results obtained from the works of Sabharwal et al [1,3,6,14]. The values of the saturation thicknesses, being compared with experiment and MCNP [2], are presented in Table I.…”

“…The backscattering of gamma rays from the surface of a material is of fundamental importance in radiation shielding, radiation absorption and non-destructive testing of finite samples of industrial, medical and agricultural interest [1]. When gamma rays are allowed to interact with thick targets, they undergo multiple scattering incidents within the dimensions of the target before they escape from it [2].…”

“…These low energy photons get registered in the spectrum along with the singly scattered events. Therefore, the energy spectrum of such photons is broad and never completely separates from the singly scattered distribution [1,3].…”

“…The backscattering of gamma rays from various materials has been the subject of experimental investigation by many studies [1][2][3][4][5][6][7][8][9][10]. Because of the large number of experimental parameters that must be followed by Monte Carlo simulations, these become both unwieldy and time consuming, in addition to the unavoidable increase in statistical errors that follow this type of correction procedure [5].…”

Backscattering factor for some personal dosimeters and impact on Hp(10)

“…The BSF of a phantom for a dosimeter is an important parameter for accurate dose estimation in terms of operational quantity, Hp(10). Since the number of multiple backscattered events increases with decrease in photon energy and is saturated with an increase in target thickness (Sabharwal et al, 2009a(Sabharwal et al, , 2009bAkar et al, 2012), backscattered photons from a phantom are expected to be optimum at 662 keV calibration photon energy.…”