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We report for the first time the discrimination of the core fragment knockout and valence nucleon knockout reaction mechanisms at medium energy range, by the use of the recoil proton tagging technique. Intense 8He beams at 82.3 MeV/u were supplied by the RIPS beam line at RIKEN, and impinged on both hydrogen and carbon targets. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The core fragment knockout mechanism is identified through the polar angle correlation and checked by various kinematics relations. This mechanism may be used to extract the cluster structure information of unstable nuclei. On the other hand, with the selection of the tagged valence nucleon knockout mechanism, a narrower peak of 7He ground state is obtained. The extracted neutron spectroscopic factor S n = 0.512 ( 18 ) is relatively smaller than the no-tagged one, and is in good agreement with the prediction of ab initio Greenʼs function Monte Carlo calculations.

International audience; The present work aims at exploring neutron-unbound states of 19C via the one-neutron knockout reaction. The invariant mass measurement in inverse kinematics was carried out with a carbon target and a 20C secondary beam at 290 MeV/nucleon. The preliminary relative energy spectrum and the parallel momentum distribution of the system of 18C + n were reconstructed from the measured momenta of the 18C fragment and decayed neutron. A new resonance was observed at Erel = 2.3 MeV, which corresponds to the unbound state of 19C at Ex = 2.9 MeV. The parallel momentum distribution for this resonance suggests a spin-parity assignment of 1/2−. Additionally, two known states were seen at Ex = 0.6 and 1.4 MeV, consistent with the 5/2+1 and 5/2+2 states, respectively.

The distribution of spectroscopic strength in nuclei can be extracted from direct-reaction cross section measurements, as one-nucleon knockout at intermediate energy or transfer at low energy. The study of deeply-bound nucleon removal from several sd-shell nuclei having a large difference of proton-neutron separation energies, DeltaS=Sp-Sn~20 MeV, exhibits experimental cross sections about four times smaller than theoretical predictions from state-of-the-art calculations. This trend is not observed from (d,p) transfer reactions with nuclei having smaller separation energy asymmetry, DeltaS~12 MeV.To investigate the origin of this reduction, we have performed two complementary experiments for the 14O case having a large energy asymmetry: (i) one-nucleon knockout from 14O (53 MeV/n) and 16C (75 MeV/n) on a 9Be target at the NSCL; (ii) one-nucleon transfer reaction using SPIRAL beam, 14O(d,t)13O and 14O(d,3He)13N at 18 MeV/n, and the MUST2 array. The analysis of the data presented in this document leads to a discrepancy between the spectroscopic factors extracted from these two experiment when a deeply-bound neutron is removed from 14O. In the neutron knockout from 14O, the cross section is strongly reduced compared to predictions based on an eikonal model and shell model spectroscopic factors. Moreover, several deviations from the eikonal prediction are observed on the shape of the parallel momentum distribution of the ejectile 13O. For the transfer 14O(d,t)13O, such a reduction is not observed and results are in agreement with stable nuclei values. These results call for new theoretical developments concerning the description of the reaction mechanism when a deeply-bound nucleon is removed from a nucleus., Les facteurs spectroscopiques nous renseignent sur l'occupation des couches nucléaires et peuvent être extraits par des réactions directes comme le transfert à basse énergie et le knockout aux énergies intermédiaires. L’étude récente de noyaux radioactifs de la couche sd ayant une large différence d'énergie de séparation, DeltaS=Sp-Sn~20 MeV, montre que les sections efficaces d’arrachage d’un nucléon très lié sont considérablement réduites par rapport aux prédictions théoriques. Cette tendance n’est pas observée pour des noyaux moins exotiques, jusqu’à DeltaS~12 MeV, par réaction de transfert (d,p).Pour comprendre l'origine de cette réduction, nous avons réalisé deux expériences complémentaires sur un noyau présentant une large différence d’énergie de séparation l’14O : (i) le knockout d'un nucléon au NSCL, 14O (53 MeV/n) et 16C (75 MeV/n), sur une cible de 9Be; (ii) le transfert d'un nucléon avec le faisceau d’14O à 18 MeV/n de SPIRAL, 14O(d,t)13O et 14O(d,3He)13N, étudié avec le dispositif MUST2. L'analyse des données présentée en détails dans ce manuscrit conduit à des facteurs spectroscopiques incompatibles entre ces deux expériences lorsqu'un neutron fortement lié est enlevé ou transféré de l'14O. Dans le cas du knockout, la section efficace mesurée est fortement réduite par rapport aux prédictions basées sur un modèle eikonal et le modèle en couches. La distribution en moment parallèle de l’13O mesurée après l’arrachage d’un neutron présente par ailleurs une forme qui n’est pas reproduite par le modèle eikonal. Dans le cas du transfert, une telle réduction n'est pas observée et le rapport entre expérience et théorie est compatible avec ce qui est obtenu pour les noyaux stables. Ces résultats suscitent de nouveaux développements théoriques pour la modélisation du mécanisme de réaction dans ces cas extrêmes où le nucléon enlevé est fortement lié.

Proton knockout reactions on carbon isotopes, p(9–16C,2p)8‐15B, at 250MeV/A were performed for systematic information on weakly to strongly bound 1p valence protons and deeply bound 1s protons. Various information such as energy gap between 1s and 1p orbits, widths of momentum distributions for 1s and 1p orbits, relative (p,2p) yields are obtained.