Level density for reaction cross section calculations

Transcription

1 Level density for reaction cross section calculations Edwards Accelerator Laboratory, Department Physics and Astronomy, Ohio University

2 Motivation The problem: Level density is the most uncertain Input in compound reaction cross section calculations What to do? Understand specific uncertainties of level densities and find the way of their reduction Level density inputs are based on experimental data on neutron separation spacing and discrete levels only The resonance spacing determines the density of resonances in the narrow spin interval only and mainly one parity. Reaction cross section calculations need level density for all spins and both parities (total level density).

3 Neutron resonance spacing Transformation is uncertain, spin cutoff parameter and parity ratio are not known experimentally Total level density =? Total level density Compound nuclear reaction cross section calculations

4 If we want to describe nuclear reactions we would be better off to get level density information directly from nuclear reactions but not from neutron resonances with its uncertain way of conversion. Level density systematics would need to be established on the basis of experimental data from nuclear reactions.

5 Models used in calculations are mostly based on 1. Fermi-gas formula (FG) 2. Gilbert Cameron formula (GC) U<~Bn U>~Bn

6 RIPL-3 R. Capote et al, Nuclear Data Sheets 110(2009) 3107 a= Does this regularity hold for total level densities needed for HF calculations?

7 Experiment Lithium beam energy is 15 MeV 6Li+54Fe 7Li+54Fe 6Li+56Fe 7Li+56Fe 6Li+58Fe 7Li+58Fe 59Ni+p 60Ni+p 61Ni+p 62Ni+p 63Ni+p 64Ni+p

8 Expected effect of shell corrections on parameter a for Ni isotopes δw 0 δw=0 a=

9 The total level density can be obtained studied experimentally from particle spectra of compound nuclear reactions The concept: d ( E) T ~ ( E) C in ( E') ( E i T f outi * ) de Make sure that the compound reaction mechanism dominates. 1. Select appropriate reactions (beam species, energies, targets). 2. Measure the outgoing particles at backward angles 3. Compare reactions with different targets and incoming species leading to the same final nuclei

10 Si Si Si Si Scheme of experimental set-up for charge-particle spectra measurements Edward s Accelerator Lab, Ohio University Si ΔE-E beam Target Si Si Si Si Si

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13 Lines are calculated assuming GC formula and shell corrections 0

14 Shell corrections =0

15 Lines are calculated assuming GC and shell corrections 0

16 Lines are calculated assuming shell corrections =0

17 Microscopic level density calculations Goriely, S. Hilaire and A.J. Koning, Phys. Rev. C78 (2008)

18 Conclusions Gilbert and Cameron composite formula reproduces experimental spectra much better compared to FG one. Level density parameter a obtained from proton spectra of compound nuclear reactions does not depend on shell corrections the way it is predicted from systematics based on neutron resonance spacing data We obtained this result for Ni isotopes only. Does this hold for other nuclei from different mass range? This is the problem which needs to be addressed in future experiments

19 The work has been performed at Edwards Accelerator Laboratory Physics and Astronomy Department Ohio University A.Voinov S.Grimes C.Brune T.Massey A.Schiller