MULTI-MODE FISSION OF ACTINIDES INDUCED BY THERMAL NEUTRON

尽管人们对热中子诱发铀、钚同位素的裂变机制进行了大量的研究，但在裂变过程中仍有许多问题尚未解决。近几年来，人们在前鳗系核素和重鳗系核素裂变实验中证实了双模裂变过程的存在。此外，理论方法可以在一定程度上解释氡系元素的裂变机制。我们研制了飞行时间仪器，以测量鳗系元素裂变的精确质量分布。飞行时间包括作为开始计数器的薄膜闪烁体和作为停止计数器的硅表面势垒探测器。首先，我们测量了252&gt；Cf自发裂变碎片的速度和动能，然后对该仪器的功能进行了检验。从裂变碎片中获得的激发中子的倍数与Wahl得到的值符合得很好。在此基础上，测量了京都大学研究堆研究所^&lt；235&gt；U+n<th>系统中裂变碎片的速度和动能，得到了裂变碎片的动能和质量分布。我们没有观察到多模裂变的存在。

Though the many investigations about the fission mechanisms of uranium and plutonium isotopes induced by thermal neutrons have been done, there are many problems undissolved in the fission. Recently, the existence of two-mode fission process was confirmed in the experiment of pre-actinide and heavy-actinide nuclides fission. Further, theoretical approaches can explain to a certain extent the fission mechanisms of actinides.We have developed the time-of-flight instrument in order to measure precise mass distribution of actinide fission. The time-of-flight consists of thin film scintilators as the start counter and silicon surface barrier detectors as the stop counter. First, we measured the velocities and the kinetic energies of the spontaneous fission fragments from ^<252>Cf and then the functions of this instrument were checked. The obtained multiplicities of the promptneutrons from the fission fragments were well agreed with the values obtained by Wahl. Further, we measured the velocities and the kinetic energies of the fission fragments in the ^<235>U+n_<th> system at The Reserch Reactor Institure, Kyoto University and obtained the kinetic energy and the mass distributions. We could not observe the existence of the multi-mode fission.