Correlations in Intermediate Energy Two-Proton Removal Reactions

Abstract

We report final-state-exclusive measurements of the light charged fragments in coincidence with 26 Ne residual nuclei following the direct two-proton removal from a neutron-rich 28 Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triplecoincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially-correlated pair-removal mechanism. The fraction of such correlated events, 56(12) %, is consistent with the fraction of the calculated cross section, 64 %, arising from spin S = 0 two-proton configurations in the entrance-channel (shell-model) 28 Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams. Access to nuclear reactions that can probe the states of pairs of nucleons in an atomic nucleus is a long-standing ambition. Specifically, an ability to probe the spinstructure of nucleon pairs and, for example, to identify and quantify two-nucleon correlations in the [S, T ]=[0, 1] spin-isospin channel, is required. Intermediate-energy reactions that remove two nucleons (2N) suddenly from a fast projectile, in collisions with a light target nucleus, have been shown [1-4] to provide a sensitivity to the 2N configurations near the projectile surface. Experimentally, the importance of nucleon removal reactions derives from their high detection efficiency (forward focusing of the reaction residues) and use of thick reaction targets, increasing the effective luminosity of the relatively low intensity exotic beams. Prior to the present work these 2N removal cross section measurements have been inclusive with respect to the final states of both the target and the removed nucleons but often exclusive with respect to the bound final-states of the forward-traveling projectilelike residues, allowing spectroscopic studies based on the different residue final-state yields and the shapes of their momentum distributions [5][6][7][8][9][10].The present work sacrifices, temporarily, this betterunderstood residue final-state sensitivity to investigate new information that might be forthcoming from more exclusive measurements of the final states of the removed nucleons. The 9 Be( 28 Mg, 26 Ne) reaction was used at an intermediate energy of 93 MeV/u. The chosen reaction was studied previously in a 26 Ne-γ coincidence measurement [10] and was used to confirm the predicted relative populations of the four bound 26 Ne final states [1]. The present data set has also been used [11] to confirm that the measured contributions to the inclusive 2N-removal cross section from each of the possible elastic and inelastic removal mechanisms [1] were consistent with calculations that use eikonal reaction dynamics and sd-shellmodel structure inputs for the 28 Mg to 26 Ne(J π ) 2p overlap functions. There it was shown that only 8(2) % of the inclusive 9 Be( 28 Mg, 26 Ne) reaction cross s...

Sequential Nature of (p,3p) Two-Proton Knockout from Neutron-Rich Nuclei

“…The typical time between the two protonproton collisions is 10 −24 s, meaning that the wave function of the projectile does not rearrange between collisions, making ðp; 3pÞ a candidate for a rescattering probe, currently unexplored in nuclear physics but similar to successful ones in hadronic [61,62] and molecular [63,64] physics. Previous works have shown a relative importance of correlated pairs of around 50% in twonucleon knockout at lower energies (93 MeV=nucleon) with heavier targets [14]. We have not found such a strong contribution of correlated pairs, possibly due a larger mean free path of the protons in the nucleus.…”

“…In particular, one-nucleon knockout reactions at intermediate energies, above ∼50 MeV/ nucleon, have been extensively used for the spectroscopy of unstable isotopes, both with heavy-ion [10,11] and proton targets [12,13]. Given the success of one-nucleon knockout reactions, two-nucleon knockout has naturally garnered much interest, as it allows us to explore further nuclear properties, such as nucleon-nucleon correlations [14][15][16][17][18]. Two-proton knockout from neutron-rich nuclei has been used to populate very exotic species, in a process assumed to be direct, due to the inhibited proton evaporation in these nuclei [19][20][21][22][23].…”

Different mechanism of two-proton emission from proton-rich nuclei 23Al and 22Mg

“…As mentioned above, diproton emission is of interest. In this case, a strong correlation of p-p relative momentum around 20 MeV/c will emerge together with a small opening angle between the two protons in the rest frame of the three decay products as demonstrated in the experimental studies of 17,18 Ne [12][13][14].…”

“…In addition, it is a good way for investigating the astro-nuclear (2p,γ), and (γ,2p) processes which are closely related to the waiting point nuclei [6][7][8]. Although some experimental investigations on the 2p emitter have been done [9][10][11][12][13][14][15][16][17], the two-proton decay mechanism is still not well understood and further experimental and theoretical studies are required. * ygma@sinap.ac.cn † dqfang@sinap.ac.cn…”

Microscopic two-nucleon overlaps and knockout reactions from12C

“…The reaction observables used are, currently, inclusive with respect to the final states of both the removed nucleons and the struck light target nucleus. More exclusive measurements, e.g., of the light charged fragments in the final state, may in the future provide additional probes of the projectile structure [4].…”