Experimental study of the knockout reaction mechanism usingO14at 60 MeV/nucleon

Abstract

Background: For the deeply-bound one-nucleon removal at intermediate energies using a 9 Be or 12 C target, a strong reduction of cross section was observed relative to the prediction of eikonal theoretical model. The large disagreement has not been explained and the systematic trend is inconsistent with results from transfer reactions. The recently observed asymmetric parallel momentum distribution of the knockout residue indicates the significant dissipative core-target interaction in the knockout reaction with a composite target, implying new reaction mechanisms beyond the eikonal reaction descriptions. Purpose: To investigate the reaction mechanism for deeply-bound nucleon removal at intermediate energies. Method: The neutron removal from14 O using a 12 C target at 60 MeV/nucleon was performed. Nucleon knockout cross sections were measured. The unbound excited states of 13 O were reconstructed using invariant mass method with the residues and the associated decay protons measured in coincidence. The measured cross sections are compared with an Intra-Nuclear Cascade (INC) prediction. Results: The measured cross section of ( 14 O, 11 C) is 60(9) mb, which is 3.5 times larger than that of ( 14 O, 13 O) channel. This 2pn-removal cross section is consistent with INC prediction, which is 66 mb mainly contributed by the non-direct reaction processes. On the other hand, the upper limit of the cross section for one-neutron removal from 14 O followed by proton evaporation is 4.6(20) mb, integrated up to 6 MeV above the proton separation energy of 13 O. The calculated total cross section for such reaction processes by INC model is 2.5 mb, which is within the measured upper limit. Conclusions: The data provide the first constraint on the role of core excitation and evaporation processes in the deeply-bound nucleon removal from asymmetric nuclei. The experiment results suggest that non-direct reaction processes, which are not considered in the eikonal model, play an important role in the deeply-bound nucleon removal from asymmetric nuclei at intermediate energies.

Improvement of one-nucleon removal and total reaction cross sections in the Liège intranuclear-cascade model using Hartree-Fock-Bogoliubov calculations

“…The HFBRAD calculations also allow us to constrain the fuzziness parameter f , which is taken to be f = 0.5 for protons and f = 0.3 for neutrons. The same calculations were performed for other nuclei (not shown), resulting in the same values obtained for 208 Pb and 25 Mg. Whereas those values could be slightly different from one target to another, in Sec.…”

“…2 25 Mg, respectively. This comparison confirms that the excitation energy associated with the ejection of one nucleon is underestimated by the standard INCL (solid line), as pointed out previously in Ref.…”

“…Recently, some works have pointed that this kind of model describes reasonably inclusive measurements of one-nucleon-removal reactions on light nuclei, such as carbon or oxygen, at projectile energies below 120 MeV [23][24][25] where, basically, one-nucleon removal reactions must fulfill two conditions simultaneously: Only one nucleon must be knocked out in the intranuclear-cascade process and the residual excitation energy of the remnant must be below the neutron and proton separation energies. However, these models fail in the description of inclusive measurements of one-nucleon-removal cross sections of medium and heavy nuclei [26,27].…”

Binding-energy independence of reduced spectroscopic strengths derived from (p,2p) and (p,pn) reactions with nitrogen and oxygen isotopes

“…These studies have been later extended to more asymmetric systems, using heavy-ion knockout reaction experiments at medium energies up to 100 MeV in which a fast-moving projectile nucleus collides with a stable composite nucleus (such as 9 Be or 12 C) losing a nucleon. The analysis of these reactions with the eikonal reaction theory [7], assuming spectroscopic factors from shell-model calculations shows also a sizable quenching but, most notably, with a strong isospin dependence, which manifests as a dependence on the difference between separation energies ∆S = S p(n) − S n(p) , for proton (neutron) removal.…”

Breakup reactions and their ambiguities

“…However, in non-direct reaction processes, such as multiple scattering inside of the projectile, the excitation and decay of the spectator core could influence the deeply bound nucleon-removal cross section with a composite target [63]. In particular, recent theoretical and experimental works suggest that there may exist core excitation contributions that could significantly reduce the deeply bound nucleon-removal cross sections [63,64]. In fact, intranuclear cascade calculations, which include these effects but no microscopic information on the detailed nuclear structure, support the important role of evaporation in the cross sections for deeply bound nucleon removal from very asymmetric nuclei, i.e.…”