Fission fragment angular anisotropy in neutron-induced fission of U235 measured with a time projection chamber

Abstract

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of 235 U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

Correlations Between Fission Fragment and Neutron Anisotropies in Neutron-Induced Fission

“…Of course, the picture is more compli-cated than this since the opening of the second-chance fission channel leads to a possibility of fission from either the A+1 or A compound nucleus. A recent measurement of the anisotropy of 239 Pu confirmed that the changes in anisotropy track the shape of the fission cross section [16], in agreement with many previous measurements that observed the same trends, e.g. [15,22,23].…”

“…The enhancement with respect to the beam axis can be as much as 30% to 60%. There have, in particular, been several measurements of the anisotropy of actinides, including 235 U [12][13][14][15][16], 238 U [12,15,[17][18][19][20][21], and 239 Pu [15,22,23].…”

Anisotropy in fission fragment and prompt neutron angular distributions

“…Angular anisotropies of fission fragments as a function of incident neutron energy have been measured for several fissile targets [17][18][19][20][21][22][23][24][25][26][27] and have been shown to change as the incident energy increases. It was postulated [8] that this preferential direction of fragment emission comes from the transition states in the compound nucleus that are populated at the outer saddle point.…”

Angular distribution of fragments in neutron-induced nuclear fission at energies 1−200 MeV: Data, theoretical models and relevant problems

“…To calculate the coefficients A Q (17), ( 16) one needs to know the spin-tensors of orientation (11) and the anisotropy parameters (14) for all levels of all fissioning nuclei formed in the reaction. As for the spin orientation of the initial compound nuclei, it can be found in the framework of the optical model (see, e.g., [14]; a general expression for the corresponding spin-tensors is given in [29]).…”

“…Measurements were made with the use of the pulsed spallation neutron source and neutron TOF spectrometer GNEIS based on the 1 GeV proton synchrocyclotron in the Petersburg Nuclear Physics Institute [7,8]. Similar investigations were performed also by collaborations n TOF and NEFFTE for target nuclei 232 Th [9], 235 U, 238 U [10] and 235 U [11], respectively. These studies are of significance because almost all earlier obtained data on fission fragment angular distributions refer to the energy region below 20 MeV; only in the work [12] the angular anisotropy of fragments in neutron induced fission of 232 Th and 238 U was measured up to the energy 100 MeV.…”