基于7Li(p,γ)8Be共振核反应伽马源的光核嬗变实验原理验证研究

Proof-of-principle Study of Photon-nuclear Transmutation Based on 7Li(p,γ)8Be Reaction

光核嬗变可作为长寿命核废料嬗变的一种补充手段.随着强流质子加速器技术的发展,共振核反应γ源的流强可达到较高强度.7Li(p,γ)8Be共振核反应产生的148MeV与176MeV伽马射线恰好位于巨共振区内,是研究光核嬗变的较好选择.本工作基于中国原子能科学研究院2×17MV串列加速器,利用7Li(p,γ)8Be共振核反应产生的高能伽马射线对197Au进行了光核嬗变实验研究.在质子流强081μA的条件下,当金靶厚度为2mm时,反推得到4π方向上的总嬗变率为(58683±2298)s-1.该实验结果为将来利用共振核反应伽马源进行光核嬗变实验研究奠定了技术基础.

With the development of economy,there is an increasing need of nuclear energy,which means that nuclear waste transmutation is becoming more and more important.Usually,there are two ways to preform nuclear transmutation,one of which is neutron transmutation.Neutron transmutation is the main way to process most of nuclear waste,including almost all transuranic elements and several kinds of fission products.However,the reaction cross sections of some fission products are small for neutron transmutation.So,it is worthwhile to develop an auxiliary way.Photon nuclear transmutation could be a reasonable way to process nuclear waste when the intensity of high energy gamma beam reaches a certain level.There are several different methods to produce high energy gamma rays,like Bremsstrahlung,in-flight positron annihilation,laser Compton backscattering(LCS)and(p,γ)resonant reactions.High energy gamma rays from Bremsstrahlung have very low production rate.The methods of LCS and in-flight positron annihilation need large-scale electron accelerators.Gamma rays from(p,γ)resonant reactions are based on tandem accelerators,which is only working on hundreds of kilovolts voltage.It makes(p,γ)resonant gamma source a very attractive way to be used in nuclear waste transmutation,since it has a relatively low cost.With the technology development of high intensity proton beam accelerator,the beam intensity can reach more than 100 mA.Under such condition,the gamma beam intensities can be improved greatly.Then,(p,γ)resonant gamma source becomes a promising way to process nuclear wastes.There are some different reactions can be used to produce high energy gamma rays,7Li(p,γ)8Be resonant reaction is the best choice among them.The gamma ray energies from this reaction,14.8 and 17.6 MeV,just locate in the giant resonance region.The experiment was done by using the 2×1.7 MV tandem acceleration at China Institute of Atomic Energy.Gamma rays emitted from 7Li(p,γ)8Be reaction were detected by HPGe detector.In order to get maximized gamma ray yield,the LiF target was positioned 45°to the proton beam direction.The 197 Au target was parallel placed with the LiF target while it was irradiated by high energy gamma rays.It has been irradiated for 3.85 h.After irradiation,the 197 Au target was placed in the low background chamber.A Compton suppressed HPGe spectrometer was used to measure the decay gamma rays from 196 Au.The transmutation rate from this experiment is finally obtained to be(586.83±22.98)s-1 under the proton beam intensity at 0.81μA for an 18.5 mm diameter and 2 mm thick gold target.The experimental results lay a technical foundation for the experimental study of photon nuclear transmutation by using(p,γ)resonant gamma source.