Journal of Multidisciplinary Engineering Science and Technology (JMEST) ISSN: 3159-0040 Vol. 2 Issue 6, June - 2015 www.jmest.org JMESTN42350859 1562 Investigation Of The Effects Of Variation Of Neutron Source-Detector Distance On The Emitted Neutron Dose Equivalent Igwesi, D. I. Physics and Industrial Physics Department, Faculty of Physical Sciences, Nnamdi Azikiwe University, P. M. B. 5025 Awka, Anambra State, Nigeria. igwesidavid@yahoo.com Abstract—Monte Carlo radiation transport simulations were performed to investigate the effect of variation on the neutron source-detector distances on emitted neutron dose equivalents using pure polythene and borated Polythene as shielding materials. The measurements were taken at three different neutron source-detector distances of 100 cm, 80 cm and 60 cm by varying the position of the neutron source inside the water basin. The experimental set up was modelled using MCNP4 code with a He 3 proportionate counter serving as the detector and different thicknesses of pure polythene and borated polythene as shields. The results from the MCNP simulations were used to calculate the neutron dose equivalents. The calculated results showed that the neutron dose equivalent decreases exponentially as the thickness of both shielding materials increases. The variations observed on the neutron dose equivalents for the three neutron source-detector distances indicated that decreasing the neutron source-detector distance increases the neutron dose equivalent which by implication established that the neutron source- detector distance of 100 cm was obtained as the optimum shielding configuration for any occupational workers among the three distances considered. Furthermore, 6.24 cm of pure polythene and 5.73 cm of 5% borated polythene were obtained as the thickness of the shielding materials that can attenuate 50% of the neutron dose equivalent. Therefore, the results obtained showed that the effectiveness of any shielding material for high neutron source, such as 241 Am/Be depends on the neutron source-detector distance and the density of the shielding materials. Keywords— 241 Am/Be Neutron Source, Polythene, Borated Polythene, Occupational Workers, MCNP4 Code, Half Valued Layer. 1. INTRODUCTION Nuclear science and technology finds applications in many fields such as scientific research, agriculture, industry and medicine and it offers many advantages but not without some difficulties. Since the radiations involved are ionizing that have damaging effects on human health and environment, it is important therefore to evaluate the risks involved and possibly quantify the level of exposure to such ionizing radiations by radiation workers and subsequently develop technological configuration that guarantees the safety of radiation workers. Radiation shielding involves placing a shielding material(s) between the sources of ionizing radiations (such as 241 Am/Be) and the worker or environment. These ionizing radiations which include alpha particles, beta particles, gamma rays, X-rays, neutrons etc, interact differently with shielding materials. Thus, the efficiency and effectiveness of the shielding varies with the types and energy of the radiation to be shielded as well as the shielding materials. The best materials for protection against radiation produced by neutron source are mixture of hydrogenous materials (polythene, water and many plastics), heavy elements, and neutron absorbing elements (such as boron, chromium), because they reduce both the intensity of gamma rays and neutrons. Indeed, hydrogenous materials slow down fast and intermediate neutrons energy via inelastic scattering, and they become thermal neutrons which can easily be absorbed by neutron absorbing materials that have a very high neutron absorption cross-section. One material useful for high energy neutron is polythene. Polythene is a good neutron shielding material [1], [2], [3] and more effective when the thickness increases [4]. However, the shielding effect of polythene can be improved by the addition of boron [5], [6], [7], [8], [9], which is a good neutron absorber. Thus, borated polythene materials shield against neutron source better than the pure polythene materials Unlike other forms of radiations, neutron’s shielding introduces some complication because of the wide range of energies needed to be considered. Therefore, fast neutrons are first moderated before being captured. The moderation is achieved by the use of a non-radioactive material of low atomic number, and subsequently, appropriate materials are used to shield the thermalized neutron. Neutrons are associated with significant health issues as they are highly penetrating and can induce secondary deep body ionizing radiation doses. Large amount of neutron exposure biologically affect cells.