Explorations across the neutron dripline – EXPAND

The investigations will be undertaken using kinematically complete measurements of the beam velocity reaction products – charged fragment and neutrons – arising from the in-flight decay of the system of interest. The detection of the charged fragment will be undertaken using the SAMURAI superconducting dipole and associated detectors. The EXPAND project focuses on developing the neutron detection and aims to provide a world class array to match the beams provided by the RIBF.

Specifically it is planned to transform the existing neutron array (NEBULA) through the doubling of the number of scintillator walls and increasing their individual thicknesses by 30% – forming the “NEBULA-Plus” array – to enable 3 and 4 neutron detection capabilities, as well as augmenting dramatically the single and two-neutron detection efficiencies. As such, during the EXPAND project, 28O (Z=8, N=20), the only remaining unexplored doubly-magic nucleus yet to be observed will be investigated for the first time and its structure and multi-neutron correlations probed. In parallel, the “heavy” candidate two-neutron halo nuclei 29,31F and the associated unbound systems 28,30F will be explored. In addition, the search for multi-neutron correlations in the form of resonances in the 4n system will be pursued in combination with the study of the most neutron-asymmetric nucleus known, 7H.

The last 12 months of the project have produced the following principal results:

• Extension, including their successful bench marking, of the two independent and realistic simulation packages for the SAMURAI+NEBULA setup to include the NeuLAND demonstrator.

• The validation and refinement of the neutron-cross talk rejection schemes for the hybrid setup including the NeuLAND demonstrator.

• Analysis of the data acquired for the commissioning of the NeuLAND demonstrator and the validation of its performances.

• The running of the experiment to search for the first 2+ level of 22C.

• Discovery of 24N and uncovering of evidence for 25N.

• Finalsiatioon of the NEBULA-Plus design.

• Testing of prototype modules from the different suppliers.

• Installaion and commission of NEBULA-Plus at the RIBF-RIKEN.

“Nucleus 26O: A Barely Unbound System Beyong the Dripline”
Y Kondo et al.,
Phys. Rev. Lett 116 (2016) 102503

“Interaction Cross Section Study of the Two-Neutron Halo Nucleus 22C”
Y Togano et al.,
Phys. Lett B761 (2016) 412

“Single-Neutron Knockout from 20C and the Structure of 19C”
J Hwang et al.,
Phys. Lett. B769 (2017) 503

“Progress Report on NEBULA-Plus”
NA Orr
SAMURAI International Workshop, Wako shi, Fukuoka, Japan, 7-8 Sept 2015

“Structure in the Region of N=16 and the Status of the NEBULA-Plus Upgrade”
NA Orr
SAMURAI International Workshop, University of Kyushu, Fukuoka, Japan, 5-6 Sept 2016

“Formation and Detection of Neutron Clusters”
FM Marqués
ECT* Workshop – Physics Beyond the Neutron Dripline: Exploring the Continuum
17 - 21 Oct 2016

“Experimental Constraints on the Formation and Detection on Neutron Clusters”
FM Marqués
ESNT Workshop – Dynamics of Highly Unstable Exotic Light Nuclei and Few-Body Systems
30 Jan - 3 Feb 2017

“Neutral Nuclei: Probes and Persepctives”
FM Marqués
ECT* Workshop – Walk on the Neutron-Rich Side
10 - 13 April 2017

One of the central themes in modern day nuclear physics is the exploration of the structure of nuclei far from stability – the so-called “exotic” nuclei. Of particular interest is the light (A<40) neutron dripline region, which not only provides an ideal testing ground for many theoretical models of nuclear structure but also exhibits a number of unique phenomena, such as neutron haloes. Experimentally it has become possible in recent years to begin to access light unbound systems lying beyond the neutron dripline in this region and, as such, probe the evolution of structure into the continuum.

The overall physics objectives of the EXPAND project are to undertake, using the unique opportunities offered by the recently commissioned RIBF facility at RIKEN (Saitama, Japan), the study of the structure and intrinsic correlations present in light neutron-rich nuclei lying at and beyond the neutron dripline and to search for exotic new phenomena, including resonance structures in multi-neutron systems such as the tetra-neutron. The choice of the RIBF as the venue for this work is a compelling one: high intensity primary beams (most notably 48Ca) coupled with the BigRIPS fragment separator allow for the production of the most intense secondary beams of light neutron dripline nuclei in the world – typically three or four orders of magnitude higher than any other facility. Moreover, in many cases, the RIBF is the only facility capable of producing these beams. In addition, the beam energies (~250 MeV/nucleon) are ideally matched, both experimentally and theoretically, to the reactions of interest – nucleon “knockout”, breakup and inelastic scattering.

The investigations will be undertaken using kinematically complete measurements of the beam velocity reaction products – charged fragment and neutrons – arising from the in-flight decay of the system of interest. The detection of the charged fragment will be undertaken using the SAMURAI superconducting dipole and associated detectors. The present proposal focuses on developing the neutron detection and aims to provide us with a world class array to match the beams provided by the RIBF. Specifically it is planned to transform the existing neutron array (NEBULA) through the doubling of the number of scintillator walls – forming the “NEBULA-Plus” array – to enable 3 and 4 neutron detection capabilities, as well as augmenting dramatically the single and two-neutron detection efficiencies. As such, within the four year timeframe of the EXPAND project, 28O (Z=8, N=20), the only remaining unexplored doubly-magic nucleus yet to be observed will be investigated for the first time and its structure and multi-neutron correlations probed. In parallel, the “heavy” candidate two-neutron halo nuclei 29,31F and the associated unbound systems 28,30F will be explored. In addition, the search for multi-neutron correlations in the form of resonances in the 4n system will be pursued in combination with the study of the most neutron-asymmetric nucleus known, 7H and the four-neutron dissociation of 8He. These lines of research will continue well beyond the period of the grant whereby the investigation of heavier systems will be pursued. It is expected that a state-of-the-art physics programme with the NEBULA-Plus array will be maintained for at least 15 years.

In summary, the EXPAND project will allow the highest intensity beams of light neutron dripline nuclei to be coupled with unique neutron detection capabilities, thus permitting the exploration of nuclear structure in the most exotic neutron-rich systems. The project will position us as leaders in the field for at least the next decade and provide an essential base, both technical and scientific, for extending these studies with the future European 3rd generation radioactive beam facility EURISOL.