Fundamental study on uranium enrichment by nuclear excitation in laser plasma

对激光产生的铀等离子体中<^(235)U>的核激发进行了实验和理论研究。这一方案可能适用于铀浓缩。用聚焦的<CO_2>激光束照射金属天然铀靶。<^(235)U>的26 min同分异构体由NEET（核激发电子跃迁）产生，并通过检测<^(235)U>核同分异构体水平的内部转换电子来证实。根据被激发的原子核数，估计了激光等离子体中<^(235)U>的核激发概率。实验得到的数值与理论期望值相当吻合。通过改变激光束对铀靶的聚焦条件，测量了等离子体中激发率对同分异构体水平的温度依赖性，并与理论预测的依赖性进行了比较。当电子温度低于50eV时，未观察到异构体的生成。400 ~ 500ev的电子温度有利于等离子体<^(235)U>的核激发。核激发效率随等离子体电子密度的增加而线性增加，这一点从1 <微> m （Nd:YAG激光器）和10 <微> m （CO_2>激光器）辐照的激发率比较中得到证实。这表明球团内爆可以提高<^(235)U>异构体的生产率。

Nuclear excitation of <^(235)U> in a laser produced uranium plasma has been investigated experimentally and theoretically. This scheme may be applicable to uranium enrichment.A metallic natural uranium target was irradiated by a focused <CO_2> laser beam. The 26 min isomers of <^(235)U> were produced by NEET (Nuclear Excitation by Electron Transition), which were confirmed by detecting the internal conversion electrons from the isomeric level of <^(235)U> nucleus.The probability of the nuclear excitation of <^(235)U> in the laser produced plasma was estimated from the number of the excited nuclei. The value experimentally obtained was fairly in good agreement with the theoretically expected value.The temperature dependence of the excitation rate to the isomeric level in the plasma was measured by varying the focusing condition of the laser beam to the uranium target, and was compared with the theoretically predicted dependence. When the electron temperature is lower than 50eV, the production of the isomer was not observed. The electron temperature of 400-500eV is favorable for the nuclear excitation of <^(235)U> in the plasma.The efficiency of nuclear excitation increased linearly with the electron density of the plasma, which was confirmed from the comparison of the excitation rates by 1 <micro> m (Nd:YAG laser) and 10 <micro> m ( <CO_2> laser) irradiations. This shows the availability of the pellet implosion to improve the production rate of <^(235)U> isomers.