Fission fragment angular distributions in pre-actinide nuclei

Tathagata Banerjee, S. Nath, A. Jhingan, Gurpreet Kaur, R. Dubey, Abhishek Yadav, P. V. Laveen, A. Shamlath, M. Shareef, J. Gehlot, N. Saneesh, E. Prasad, P. Sugathan, and Santanu Pal
Phys. Rev. C 94, 044607 – Published 13 October 2016

Abstract

Background: Complete fusion of two nuclei leading to formation of a heavy compound nucleus (CN) is known to be hindered by various fission-like processes, in which the composite system reseparates after capture of the target and the projectile inside the potential barrier. As a consequence of these non-CN fission (NCNF) processes, fusion probability (PCN) starts deviating from unity. Despite substantial progress in understanding, the onset and the experimental signatures of NCNF and the degree of its influence on fusion have not yet been unambiguously identified.

Purpose: This work aims to investigate the presence of NCNF, if any, in pre-actinide nuclei by systematic study of fission angular anisotropies and fission cross sections (σfis) in a number of nuclear reactions carried out at and above the Coulomb barrier (VB).

Method: Fission fragment angular distributions were measured for six Si28-induced reactions involving isotopically enriched targets of Tm169,Yb176,Lu175,Hf180,Ta181, and W182 leading to probable formation of CN in the pre-actinide region, at a laboratory energy (Elab) range of 129–146 MeV. Measurements were performed with large angular coverage (θlab=41170) in which fission fragments (FFs) were detected by nine hybrid telescope (EΔE) detectors. Extracted fission angular anisotropies and σfis were compared with statistical model (SM) predictions.

Results: Barring two reactions involving targets with large non-zero ground state spin (J), viz., Lu17572+ and Ta18172+, experimental fission angular anisotropies were found to be higher in comparison with predictions of the statistical saddle point model (SSPM), at Ec.m. near VB. Comparison of present results with those from neighboring systems revealed that experimental anisotropies increasingly deviated from SSPM predictions as one moved from pre-actinide to actinide nuclei. For reactions involving targets with large nonzero J, this deviation was subdued. Comparison between measured σfis and predictions of SM indicated the presence of NCNF in at least four systems, when shell effects, both in the level density and the fission barrier, were included in the calculation.

Conclusions: Systematic SM analysis of measured fission angular anisotropies and σfis confirmed the onset of NCNF in pre-actinide nuclei. Discrepancies between results about the degree of its influence on complete fusion, as deduced from various experimental probes, remain challenges to be solved. Complete measurement of all signatures of NCNF for many systems and preferably a dynamical description of the collisions between projectile and target nuclei are warranted for a deeper understanding.

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  • Received 22 July 2016

DOI:https://doi.org/10.1103/PhysRevC.94.044607

©2016 American Physical Society

Physics Subject Headings (PhySH)

Nuclear Physics

Authors & Affiliations

Tathagata Banerjee1,*, S. Nath1, A. Jhingan1, Gurpreet Kaur2, R. Dubey1, Abhishek Yadav1, P. V. Laveen3, A. Shamlath3, M. Shareef3, J. Gehlot1, N. Saneesh1, E. Prasad3,†, P. Sugathan1, and Santanu Pal4,‡

  • 1Nuclear Physics Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, Post Box 10502, New Delhi 110067, India
  • 2Department of Physics, Panjab University, Chandigarh 160014, India
  • 3Department of Physics, School of Mathematical and Physical Sciences, Central University of Kerala, Kasaragod 671314, India
  • 4CS-6/1, Golf Green, Kolkata 700095, India

  • *he.tatha@gmail.com
  • Presently at Department of Nuclear Physics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 2601, Australia.
  • Formerly with Physics Group, Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata 700064, India.

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Vol. 94, Iss. 4 — October 2016

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