低能电子加速器的感生放射性

THE RADIOACTIVITY INDUCED IN LOW ENERGY ELECTRONIC ACCELERATOR

运行在１０ＭｅＶ能量以上的放射治疗和工业辐照用的电子加速器，均可能产生感生放射性，其强度取决于加速电子能量、束功率和被照材料的类型．电子能量在阈能或近似于３５ＭｅＶ之间，中子基本上产生于“巨光核共振”．医用加速器的Ｘ射线靶，固定和可变准直器，Ｘ线过滤器，电子散射箔等是产生感生放射性的主要部件，辐照加速器中束流偏转扫描装置辐照容器等也是感生放射性的主要来源．

The radioactivity may be indUced in electronic accelerators working abovethe energy of 10 MeV used for radiation therapy and industry irradiation. its intensity depends on the energy, beam power of the accelerator and material irradiated. The energiesbetween the thresholds and near 35 MeV, neutrons are primarily produced from 'Giantphotonuclear resonance#.The physical mechanism can be described as one in which theelectric field of the photon transfers its energy to the nuclei by inducing an oscillation inwhich the protons as a group move oppositely to the neutrons as a group.The yield of secondary particles per incident electron Y(E,) of energy E, is ob-tained by an intergration over the photon energy K:Saturation activity:This is the amount of activity to be found at the instant of accelerator turn off if it hasbeen operating steadily for a period of long time compared with the half--life of the pro-duced nuclide(s).The exact formula for exposure rate is:Where t, is the bund up or irradiation time, t. is the decay time following turn offand is the half-life of the nuclide.X-ray target of medical accelerators, fixed and moveable collimators, X-rays filter,electron scattering foil are importent components which produce radioactivity. The beamdeflection, scanning system, and irradiation vessels are also very importent parts ofradioactivity in industry accelerators.The experiment was performed by using an electron linear accelerator at the Depart-ment of Physics of Naming University. Radioactivity may be induced in aluminium, ironand copper that are irradiated by the 13 MeV eletron beams. Beam power is 0.7kw, beamutilization factor is equal to 0.5.For the 13 MeV electron irradiation of aluminium andiron, the irradiation time tbwas 1200 seconds and 3200 seconds; the exposure rates x were 0.2(10-5Sv/ h) and0.5(10-5Sv) respectively.For 13MeV electorn irradiation of copper, the td was 266 seconds and x is equal to2.5(10 -5Sv / h). The experiment measurement results were in agreement with those of calculation. Ti isevident that aluminium. concrete and lead are preferred metrials which have lesser extentradioactivity. Copper and iron are moderately susceptible to activation, and stainless steel,Tungstem, Tantalum and Nickel are highly susceptible to activation. It is notability, because induced radioactivity is often the major cause of radiation exposure to personnel.