Exclusive measurements of nuclear breakup reactions of 17

We have studied one-proton-removal reactions of about 500MeV/u Ne beams on a carbon target at the RB/LAND setup at GSI by detecting beam-like Op and determining their relative-energy distribution. We exclusively selected the removal ae-mail: f.wamers@gsi.de DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2014 , / 03094 (2014) 201 66 epjconf EPJ Web of Conferences 4 6 6030 94 This is an Open Access article distributed under the terms of Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. the Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20146603094 of a Ne halo proton, and the Glauber-model analysis of the F momentum distribution resulted in an s contribution in the Ne ground state of about 40%.


Introduction
The proton-dripline nucleus 17 Ne has raised considerable interest in nuclear physics in the past two decades.While this nucleus also has been studied in the context of nuclear astrophysics [1] and in the search for two-proton radioactivity [2], the focus of the present work has its origin in nuclear-structure physics.The goal is to clarify the 17 Ne ground-state structure and its long proclaimed candidacy as a 2proton halo nucleus [3].Like the famous 2n-halo nuclei 11 Li or 6 He, 17 Ne is a Borromean three-body system.Its binary subsystems (p-p and 15 O-p) are unbound, and it can be described as an 15 O core with two valence protons, predominantly in (1s 1/2 ) 2 or (0d 5/2 ) 2 configurations.The spin and parity of 17 Ne are J π = 1 2 − , its half-life for β + -decay to 17 F is T 1/2 = 109.2ms, its proton-separation energies are S 1p =1479 keV and S 2p =943 keV, and it does not have any bound excited states.While earlier experiments had already revealed certain indications for the halo characteristics of 17 Ne (e.g., an anomalous beta decay branching ratio [4], and large matter radius [5]), the more recent experimental and theoretical studies have focused on the s 2 -weight in the s 2 /d 2 configuration mixture of the valence proton pair in the 17 Ne ground state.Yet, there is no final conclusion: the three-body model of Grigorenko et al. [6], which is the benchmark for our study, predicts a value of 48 %.Kanungo et al. [7] have measured narrow longitudinal-momentum distributions of the 15 O core in connection with a large cross section in 2p-removal reactions from 17 Ne beams on a Be target.They determined an s 2 -weight of 70 % and concluded to observe a 2-proton halo.However, this experiment was not exclusive to the removal of halo protons, but also includes that of the core protons.Further reported s 2 -weights are 40 % [8], "dominant" [9], and 15 % [10].Our aim is to provide a conclusive result.

Experimental Technique and Setup
We report on an experiment performed at GSI in 2007.Secondary 17 Ne beams were produced in fragmentation reactions of (630 MeV/u) primary 20 Ne beams impinging on a thick (6.3 g/cm 2 ) beryllium target at the entrance of the fragment separator (FRS).They were delivered to the experimental setup, impinging on a secondary (370 mg/cm 2 ) carbon target at an energy of about 500 MeV/u and an intensity of about 10 4 ions/s.In the one-proton knockout reactions induced there, individual protons were instantly removed from the 17 Ne projectiles, leading to unbound 16 F decaying in-flight to 15 O+p.The 16 F fragments were spectators, thus in the c.m. system carried only the recoil to the intrinsic momentum of the removed proton: p(p) = − p( 16 F).Correspondingly, their momentum distribution reflects the removed protons' intrinsic momentum.Since the width and shape of these momentum distributions are sensitive to the angular-momentum value of the removed proton, a configuration mixture in the studied nucleus (such as the mentioned s 2 /d 2 mixture in 17 Ne) can be decomposed by a fit with momentum distributions calculated in the Eikonal approximation using single-particle wave funtions.At GSI, such reactions were studied using the R 3 B/LAND complete-kinematics reaction setup.The 17 Ne ions were identified and tracked onto the carbon reaction target, which was surrounded by a box of Si strip detectors and a thick 4π NaI shell segmented into 162 detectors for measuring light recoil particles and gamma rays from excited fragments. 15O fragments and beam-like protons travelling under forward angles were detected in two silicon strip detectors closely behind the target, separated in the ALADIN dipole magnet, and identified and tracked by two fibre detectors and a ToF wall under 16.7 degrees ( 15 O fragments), and by two drift chambers and a ToF wall under 31 degrees (protons).

03094-p.2 3 Analysis and Preliminary Results
For each one-proton-knockout event, the 17 Ne projectile and the 15 O+p ejectiles were identified, tracked, and their four-momenta were reconstructed.Using the invariant-mass technique, the 15 Op relative kinetic energy distribution has been determined and is shown in figure 1(a), next to the 16 F level scheme (b), and a reaction scheme (c).In figure 1 (a), the experimental data (black markers)

Figure 1 :
Figure 1: (a) Measured relative-energy distribution of 15 O-p (black markers), and locations of the 16 F resonances states (coloured arrows).(b) Level scheme of 16 F resonances in terms of the 15 O-p relative energy[11].The lowest 0 − state is the 16 F ground state.(c) Proton-knockout processes from 17 Ne core and halo states, as depicted in[6].

Figure 2 :
Figure2: Momentum distributions of the 16 F ( 15 O+p) system after one-proton knockout from the halo of 17 Ne.A Glauber-model calculation for proton knockout from single-particle states (green dash: swave, blue dots: d-wave) in 17 Ne is used to fit (red full) the experimental data (black markers).