Contribution Of Bremsstrahlung To Buildup Factor Of Shielding Materials:A MCNP Study

In the shielding design,it is very important to select a reliable buildup factor.Because it can ensure safety,save shielding materials,and optimize radiation protection.At present,the standard reference value of the buildup factor is only related to the contribution of Compton scattered photons.In fact,the bremsstrahlung of secondary electrons,which are produced by the physical processes such as photo-electricity and positive and negative electron effects of ?-rays,also make an important contribution to the buildup factor.Since the vary of ?-ray energy,the change of shielding thickness,or the vary of shielding material all affect the contribution of secondary electron bremsstrahlung,studying the contribution of bremsstrahlung to buildup factor under different c onditions has practical significance for shielding and shielding applications.The Monte Carlo program MCNP has a cross-section database of various particles,so it can realistically simulate the actual physical processes in the particle transport process,and can use the Phys card to perform bremsstrahlung radiation selection in the physical process.Therefore,the MCNP program is used in this paper to calculate:(1)the wide-beam ?-ray exposure and the narrow-beam ?-ray exposure of different energy gamma ray(0.5,1,1.5,2,3,4,5,6,8,10 MeV)with or without bremsstrahlung,when the shielding thickness of Pb is one free path;(2)the wide-beam ?-ray exposure and the narrow-beam ?-ray exposure of different shielding thickness(?d=1 ? 2 ? 4 ? 7 ? 10)with or without bremsstrahlung,when the shielding material and ?-ray energy are constant;(3)the wide-beam ?-ray exposure and the narrow-beam ?-ray exposure of different shielding materials(Al?Fe?Cu?Pb)with or without bremsstrahlung,When the shielding thickness and ?-ray energy are constant.And then we obtain the buildup factor with or without bremsstrahlung under different conditions.In addition,we compare buildup factor of calculation and buildup factor of theory.By analyzing the relationship between the contribution of bremsstrahlung to buildup factor and the gamma ray energy,the shielding thickness or the atomic number Z of shielding material,the following conclusions can be obtained:(1)If the thickness of Pb is one free path,the bremsstrahlung of low energy(< 2 Me V)?-rays has little effect on buildup factor,which is less than 2%;the bremsstrahlung of high-energy(> 6 Me V)?-rays has a significant effect on buildup factor,which is greater than 10%.(2)With the increasing of ?-ray energy,shielding thickness or shielding materials atomic number Z,the contribution of the bremsstrahlung to buildup factor is getting lager.(3)Considering the calculation of If the buildup factor with bremsstrahlung of shielding materials is needed to calculated in engineering applications,MCNP simulation can be used to obtained a reliable reference value of buildup factor.