Collinear cluster tri-partition: Kinematics constraints and stability of   collinearity
  A new mode of nuclear fission has been proposed by the FOBOS collaboration, called Collinear Cluster Tri-partition (CCT), suggesting that three heavy fission fragments can be emitted perfectly collinearly in low-energy fission. It is surprising that CCT escaped observation for so long given the relatively high reported yield, of roughly 0.5% relative to binary fission. These claims call for an independent verification with a different experimental technique. Verification experiments based on direct observation of CCT fragments with fission fragment spectrometers require guidance with respect to the allowed kinetic energy range, which we present in this paper. We discuss corresponding model calculations which, if CCT is found in such verification experiments, could indicate how the breakups proceed. We also study the intrinsic stability of collinearity. Three different decay models are used, which span together the timescales of three-body fission. These models are used to calculate the possible kinetic energy ranges of CCT fragments in 235U(n,f) and 252Cf(sf). We use semi-classical trajectory calculations with a Monte-Carlo method to study the intrinsic stability of collinearity. CCT has a high net Q-value, but in a sequential decay, the intermediate steps are energetically and geometrically unfavorable or even forbidden. Moreover, perfect collinearity is extremely unstable, and broken by the slightest perturbation. According to our results, the central fragment would be very difficult to detect due to its low kinetic energy, raising the question of why previous experiments could not detect a missing-mass signature corresponding to CCT. We find that a realization of CCT would require an unphysical fine-tuning of the initial conditions. Our results enable independent experimental verification and encourage further critical theoretical studies of CCT. 
Patric Holmvall, Ulli K\"oster, Andreas Heinz, Thomas Nilsson
nucl-th