Angular correlation between the fission fragment intrinsic spins

Abstract

The generation of fission fragment (FF) spins and of their relative orbital angular momentum has been debated for more than six decades, and no consensus has been achieved so far. The interpretation of recent experimental results of Wilson et al., Nature (London) 590, 566 (2021) has been challenged by several recent theoretical studies, which are not in agreement with one another. According to the interpretation by Wilson et al., Nature (London) 590, 566 (2021), the FF spins emerge very long after scission occurrs. Randrup and Vogt Phys. Rev. Lett. 127, 062502 (2021), while agreeing that the FF spins are uncorrelated, conclude on the basis of a phenomenological model that these spins are uncorrelated already before scission. Bulgac et al., Phys. Rev. Lett. 128, 022501 (2022) in a fully microscopic study demonstrate that the primordial FF spin final values are defined before the emission of prompt neutrons and statistical gammas and are strongly correlated with a relative angle between spins close to $2\ensuremath{\pi}/3$, a result in full agreement with the present independent analysis. The prompt neutrons and statistical gammas carry a significant amount of angular momentum according to the study of Stetcu et al., Phys. Rev. Lett. 127, 222502 (2021), which can lead to a decorrelation of the FF bandhead spins of the yrast lines measured by Wilson et al., Nature (London) 590, 566 (2021), and which also provides arguments why these measured spins are so different from the primordial FF spins evaluated by Bulgac et al., Phys. Rev. Lett. 128, 022501 (2022). Here, I show that the unexpected character of the angular correlation between the primordial FF intrinsic spins, recently evaluated by Bulgac et al., Phys. Rev. Lett. 128, 022501 (2022), which favors FF intrinsic spins pointing predominantly in opposite directions, can be understood by using simple general phase space arguments. The observation by Wilson et al., Nature (London) 590, 566 (2021) that the FF spins are uncorrelated follows from both the results of the microscopic calculation of Bulgac et al., Phys. Rev. Lett. 128, 022501 (2022) and the present analysis of the full correlated probability distribution of the FF spins together with the relative orbital angular momentum of the primordial FFs. These arguments may apply also to heavy-ion collisions and, since there is no use of specifics of the particle interactions, the present results might apply to atomic and molecular systems as well.