Direct reactions and spectroscopy with hydrogen targets: past 10 years at the RIBF and future prospects

31 Jul 2023, 08:00 → 4 Aug 2023, 20:40 Europe/London

Hilton York Hotel

Direct reactions and spectroscopy with hydrogen targets: past 10 years at the RIBF and future prospects

Employment of the MINOS liquid-hydrogen target and vertex-tracking system with the DALI2 scintillation-based gamma-ray spectrometer and other systems at the RIBF has led to abundant exciting physics results. Highlights include the first spectroscopy of doubly magic nuclei, investigation of multi-neutron decays, and shell evolution along isotopic chains. Through these studies with direct reactions, our understanding of nuclear reaction mechanisms and nuclear structure has been furthered. 

The advances simultaneously open new physics questions and pave the way for novel experimental setups to address them. Through combining particle- and gamma-spectroscopy with a broad range of nuclear reactions new frontiers will be established in our understanding in the structure and reaction mechanisms. In this symposium, we will commemorate the past ten years of studies on direct reactions and spectroscopy of exotic nuclei with liquid-hydrogen targets at the RIBF and define a vision for the future.

 

Key topics

• Shell migration at the neutron numbers N=32,34 around Ca
• The island of inversion at the neutron number N=40
• Shell structure around 78Ni isotope
• Collectivity from Zn to Zr
• Structure of halo nuclei
• Shell evolution towards 28O isotope
• Clustering and multi-neutron systems
• Theoretical and experimental studies in the reaction mechanism
• Development of new detection devices

 

Scientific Advisory Committee

• Pieter Doornenbal (RIKEN)
• Alexandre Obertelli (TU Darmstadt)
• Tomohiro Uesaka (RIKEN)

 

Organising committee

• Frank Browne (CERN)
• Sidong Chen (University of York)
• Martha Liliana Cortes (RIKEN)
• Julian Kahlbow (MIT and Tel-Aviv University)
• Yuki Kubota (RIKEN)
• Hongna Liu (Beijing Normal University)
• Clementine Santamaria (Morgan State University and MSU)
• Junki Tanaka (RIKEN)
• Ryo Taniuchi (University of York)
• Zaihong Yang (Peking University)
• Kazuki Yoshida (JAEA)

Sponsors

    

 

york_poster_June13.pdf

Monday, 31 July

08:30 Morning coffee + Registration

Welcome

1 Introduction

2 Greetings from University of York

Speaker: Michael Bentley (University of York)

3 Opening remark by JSPS

Speaker: Kobayashi Naoto (JSPS London)

4 Introduction by JSPS London office

Speaker: Miki Takahashi (JSPS London)

Keynote talks

5 Overview of the RIBF facility

Speaker: Hiroyoshi Sakurai (RIKEN)

10:35 Coffee break

6 Overview of the SEASTAR Project

In order to take full advantage of the RIBF's unprecedented secondary beame nergies, the "Shell Evolution and Search for Two-plus energies At RIBF" (SEASTAR) project was initiated in 2013. The project aims to systematically study shell
evolution in neutron-rich nuclei via in-beam gamma-ray spectroscopy and covers a wide range of nuclei from the neutron sub-shell closures at N=32,34 in Ca to the possible N=70 harmonic oscillator shell closure.

SEASTAR combined the NaI(Tl) based scintillator array DALI2+ with the liquid hydrogen and vertex tracker array MINOS. Three experimental campaigns were carried out in 2014, 2015, and 2017, the former two at ZeroDegree, the latter at
SAMURAI. In my presentation I will review the origins of SEASTAR, point out a few highlights, and venture a brief outlook towards possible future campaigns combining in-beam gamma-ray spectroscopy with quasi-free scattering.

Speaker: Pieter Doornenbal (RIKEN)

7 SAMURAI Overview

Speaker: Hideaki Otsu (RIKEN)

12:05 Lunch

Shell migration at the neutron numbers N=32,34 around Ca

8 Overview of shell evolution in the N=28-40 region and its analogy to IoI

Neutron-rich nuclei in the N=28-40 region provide a good testing ground of shell evolution. The conventional N=28 magic number is known to be disappear in S, Si, and Mg isotopes, and a new magic number N=34 had been predicted since 2001. One of the most important ingredients to cause those phenomena is the monopole interaction between a proton in the sd shell and a neturon in the pf shell. In this talk, I will show how such a shell evolution occurs on the basis of the coorperative and competitive effect of the central, tensor, and spin-orbit forces, in comparison to experimental results taken by the SEASTAR campaign. The shell evolution driven by those forces should lead to similar behaviors in other regions. I will focus on the similarity between calcium and oxygen region, especially the breaking of the doubly magic nature at 60Ca and 28O.

Speaker: Yutaka Utsuno (Japan Atomic Energy Agency)

9 Structure of Ca isotopes at N = 34 shell closure and above

The studies in the past decades revealed that the canonical magic numbers established for stable nuclei may not extend their universality to exotic nuclei, while new magic numbers emerge in some nuclei [1]. These new features often can be traced back to certain characteristic mechanisms of nuclear forces [1,2], for instance, the tensor force, which can vary the spin-orbit energy splitting and result in changes of shell structures. Discovering and interpreting these new features provide a fundamental test for the understanding of nuclear forces, and play a key role in the prediction of the dripline in the Segrè chart of nuclides. The calcium isotopes, with 20 protons (Z=20) forming the closed proton shell, exhibit a high sensitivity of the shell evolution according to the neutron number. In the neutron-rich side, signatures of new magic numbers or sub-shell closures have been found at N=32 and 34 [3,4], and interpreted as a consequence of the absence of a tensor attraction between valence protons and neutrons. For the N=34 sub-shell closure, the first experimental evidence was presented by the measured large E(2+1) in 54Ca [4]. It was then supported by the mass measurements of 55-57Ca isotopes [5]. Following these studies, we further investigated the nature of N=34 sub-shell closure by knockout reactions, and the structural evolution of calcium isotopes above the N=34 sub-shell closure.
The experiments were carried out at RIBF using the intense radioactive beams provided by the BigRIPS separator. A thick liquid hydrogen target of the MINOS device was used to induce the knockout reactions, and the DALI2+ high-efficiency array was arranged around the target for the detection of de-excitation γ rays of reaction products. The reaction products were identified by the SAMURAI spectrometer. The exclusive cross sections and momentum distributions of the 54Ca(p,pn)53Ca reaction channel were measured in the experiment, which provide access to the neutron occupation number of the 54Ca ground state and spin parity of the 53Ca final states [6]. Moving beyond the N=34 sub-shell closure, the first spectroscopy measurements of 56,58Ca were performed. The obtained results are confronted with state-of-the-art ab initio and shell-model calculations, permitting a sound prediction on the structure of 60Ca and the dripline of calcium isotopes [7]. In this talk, the physics interests of the calcium isotopes, the descriptions of the experiments and the discussions of the results shall be given in details.

[1] T. Otsuka et al., Rev. Mod. Phys. 92, 015002 (2020).
[2] O. Sorlin and M.-G. Porquet, Phys. Scr. T152, 014003 (2013).
[3] F. Wienholtz et al., Nature, 498, 346, (2013).
[4] D. Steppenbeck et al., Nature, 502, 207, (2013).
[5] S. Michimasa et al., Phys. Rev. Lett. 121, 022506 (2018).
[6] S. Chen et al., Phys. Rev. Lett. 123, 142501 (2019).
[7] S. Chen et al., Phys. Lett. B 843, 138025 (2023)

Speaker: S. Chen

10 Extended p3/2 Neutron Orbital and the N = 32 Shell Closure in 52Ca

The $^{52}$Ca(p,pn)$^{51}$Ca reaction was measured in inverse kinematics during the SEASTAR3 experimental campaign at the Radioactive Isotope Beam Factory (RIBF). The proton-induced quasi-free neutron knock-out reaction was performed at ∼230 MeV/nucleon using MINOS, a 150-mm long liquid hydrogen target and the MINOS TPC, combined with prompt $\gamma$ spectroscopy. Inclusive and exclusive cross sections to bound states of $^{51}$Ca were evaluated, as well as the momentum distribution corresponding to the removal of 1$f_{7/2}$ and 2$p_{3/2}$ neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation reaction framework, are consistent with a shell closure at the neutron number N = 32, found as strong as at N = 28 and N = 34 in Ca isotopes. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron 1$f_{7/2}$ and 2$p_{3/2}$ orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square radius of the 2$p_{3/2}$ orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

Speaker: Madalina Enciu (INSTITUT FÜR KERNPHYSIK DER TECHNISCHEN UNIVERSTITÄT DARMSTADT)

14:55 Coffee break

Shell migration at the neutron numbers N=32,34 around Ca

11 Global ab initio calculations for the structure of exotic nuclei

Breakthroughs in our treatment of the many-body problem and nuclear forces are rapidly transforming modern nuclear theory into a true first-principles discipline. This allows us to address some of the most exciting questions at the frontiers of nuclear structure and physics beyond the standard model.
In this talk I will briefly outline our many-body approach, the valence-space in-medium similarity renormalization group, and how recent advances now allow for global converged calculations of open-shell nuclei to the 208Pb region and beyond. In particular, I will focus on key topics in nuclear structure such as predictions of the proton and neutron driplines and evolution of magic numbers throughout the light and medium-mass regions, including new insights on the behavior of N=28,32,34 from the pf through the lower sd-shells as well as the existence of 28O including continuum degrees of freedom.

Speaker: Jason Holt (TRIUMF)

12 Spectroscopy of 52Ar and 54Ca with (p,2p) and (p,3p) reactions

Shell gaps represent the backbone of the nuclear structure and are a direct fingerprint of the in-medium many-body interactions. The nuclear shell structure is found to change, sometimes drastically, with the number of protons and neutrons, revealing how delicate the arrangement of interacting nucleons is. Recent experimental evidence favors a new doubly-magic nucleus 54Ca with a neutron subshell closure at N = 34, although the systematics of E(2+) and B(E2) in Ti and Cr isotopes do not show any evidence for the N = 34 magicity.

In order to study how the N = 34 subshell evolves below Z < 20 towards more neutron-rich systems, we measured the low-lying structure of 52Ar using the 53K(p,2p) one-proton removal reaction at ∼210 MeV/u at the RIBF facility. The 2+ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N > 20. This result is the first experimental signature of the persistence of the N = 34 subshell closure beyond 54Ca. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 2+ systematics of neutron-rich Ar isotopes and support a N = 34 subshell closure in 52Ar.

For the doubly magic nucleus 54Ca, several state-of-the-art nuclear structure calculations predict that it has a bond first excited 0+ state but with very different excitation energies. In particular, shell model calculations with the effective LNPS-U interaction predict significant intruder configurations in the first excited 0+ state in 54Ca, and suggest that its excitation energy can provide information on correlations of the gds orbitals lying above the N = 34 subshell closure, which will constrain the predictions for 60Ca (N = 40) and the dripline of the Ca isotopes. We therefore propose to search for the first excited 0+ state in 54Ca using 56Ti(p, 3p) reactions by means of missing-mass and in-beam γ spectroscopy, which is approved to by the NP-PAC committee at RIKEN. To summarize, in order to explore the shell evolution from N = 34 towards N = 40, we measured the first 2+ state in 52Ar and will search for the first excited 0+ state in 54Ca.

Speaker: Dr Hongna Liu (Beijing Normal Univerisity)

13 Detailed spectroscopy of the non-canonically doubly magic 54Ca

The nuclear magic numbers correspond to large energy gaps between successive nucleon orbits. In stable nuclei, these correspond to 2, 8, 20, 28, 50, 82, and 126, however, in exotic nuclei the 20 and 28 magic numbers are known to disappear, whilst 32 and 34 emerge as "non-canonical" magic numbers. The latter was first inferred through $\gamma$-ray spectroscopy of $^{54}$Ca at the RIBF, with subsequent mass measurements, and knockout reactions validating this picture.

Owing to the availability of higher beam intensities, and the experimental set-up of the SEASTAR2017 experiment, the spectroscopy of $^{54}$Ca could be extended to beyond the $S_{2n}$ level. Direct proton and neutron knockout reactions from $^{55}$Sc and $^{55}$Ca, respectively, on a liquid hydrogen target, a part of the MINOS device, were used to populate levels in $^{54}$Ca. The $\gamma$-ray decays of these states were recorded in the DALI2+ array and the neutron decays in the NeuLAND+NEBULA arrays. Comparison of the reaction products' momenta to DWIA calculations assigned the angular momentum of the nucleon that was removed to populate a given state.

During the talk, an overview of the basic conditions required for the emergence of the $N=34$ shell gap to occur shall be given, followed by the experimental evidence for the closure. Following this, the results of the spectroscopy described above shall be discussed in detail within the context of the shell model, specifically incorporating the GXPF1Br interaction, as well as pairing forces.

Speaker: Frank Browne (CERN)

Tuesday, 1 August

08:00 Morning coffee

Shell structure around 78Ni isotope

14 EXOTIC NUCLEI: THE $^{78}$Ni REGION (ZOOM)

In this contribution,
I will expose microscopic nuclear structure calculations for exotic nuclei far from stabilitity in the vicinity of $^{78}$Ni, in a key region needed for understanding nucleosynthesis paths of gold and some of the most heavy elements.
Our recent algebraic Nilsson SU3 self-consistent model[1] will be used to describe the intruder relative evolution in the vicinity of $^{78}$Ni. The spectroscopy of the exotic nucleus $^{78}$ performed at the RIKEN-RIF laboratory in Japan has been published in Nature [2]. The results support the doubly magic character N=50, Z=28, of the heaviest nickel isotope, that is spherical in its ground state. In addition they have detected the presence at very low energy (2.5 MeV) of another facet of the same nucleus which is radically different, characterized by its spheroidal shape. This atypical phenomenon of coexistence, more germane to molecular systems, was predicted by the Configuration-Interaction (LSSM) calculations of the Strasbourg-Madrid collaboration in 2016 [3].
The model predicts as well the vanishing of the magic closure at N=50 for the
more exotic isotones of Chromium and Iron which should be deformed in their ground states, leading to the idea of merging islands of collectivity from N=40 to N=50, as already observed from N=20 to N=28[4]. New cases of shape coexistence cases in the region will be discussed and interpreted with our newly developed DNO Shell Model approach employing beyond mean field techniques[5].
Finally we will briefly expose some of the latest developments for exotic nuclei far from stabilitity at the N=Z line[6] and new theoretical calculations for the very region of $^{80}$Zr.

[1] A. P. Zuker et al., Phys. Rev. C 92, 024320 (2015)
[2] R. Tanushui et al., Nature 569, 53-58 (2019)
[3] F. Nowacki, A. Poves, E. Caurier, B. Bounthong, Phys. Rev. Lett. 117, 272501 (2016)
[4] F. Nowacki, A. Obertelli, A. Poves, Prog. Part. Nuc. Phys. 120, 103866 (2021)
[5] D. D. Dao and F. Nowacki, Phys. Rev. C \textbf{105}, 054314 (2022).
[6] D. D. Dao, F. Nowacki, A. Poves in preparation

Speaker: Frédéric NOWACKI (Institut Pluridisciplinaire Hubert Curien)

15 Single-particle structure of neutron-rich copper

The neutron-rich copper isotopes are an ideal laboratory to investigate nuclear structure at intermediate mass as in principle they allow to follow the movement of a single proton around a core of magic nickel. In this talk I shall briefly present the experimental data gathered from $\beta$-decay, laser spectroscopy, Coulomb excitation and transfer reactions, after which I shall focus on the results obtained from $\gamma$-spectroscopy at RIBF.

Speaker: Serge Franchoo (IJC)

sf_york23.pdf

16 Nuclear Structure in neutron-rich Ni region

We have studied medium-mass nuclei of various regions by Monte Carlo shell model (MCSM) calculations. Our MCSM calculations for Ni isotopes reproduce well their properties such as levels and transitions. Shape coexistence of nuclei around 68Ni is predicted by the calculations and explained by considering change of shell structure. Neutron-rich nuclei around N=50 are calculated by using extended model space and shape coexistence of 78Ni is predicted.

Speaker: Yusuke Tsunoda (Center for Nuclear Study, the University of Tokyo)

10:30 Coffee break

Shell structure around 78Ni isotope

17 78Ni: Doubly magic nuclei at the onset of deformation

78Ni, which has 28 protons (Z = 28) and 50 neutrons (N = 50), 14 additional neutrons to the last stable nickel isotope 64Ni, is one of the most intriguing isotopes in the chart of nuclei. It is the most neutron-rich, exotic “doubly magic” nucleus that can be produced at present state-of-the-art facilities. The excited states of 78Ni have been investigated at the Radioactive Isotope Beam Factory, RIBF, by measuring their de-excitation γ rays after one and two-proton knock-out reactions from 79Cu and 80Zn beams. A 10 cm-thick liquid hydrogen target with a recoil proton tracking system, MINOS, and a surrounding NaI(Tl) based γ-ray detection array, DALI2 were employed. Eventually, 310 and 222 events with at least one detected γ-ray with more than 300 keV with the (p,2p) and the (p,3p) reactions, respectively, were obtained. As a result, at least two excited 2+ states and other higher-lying states were found. This casts a question about the nature of the shell closure in 78Ni and implies the possibility of shape coexistence.

The experimentally deduced level scheme was interpreted by comparison with several state-of-the-art theoretical predictions: Large-scale shell model, beyond mean-field, and ab initio calculations. Though the doubly magic nature was confirmed, at the same time, the shell gap is anticipated to vanish beyond 78Ni. In addition to the γ-ray analysis, particularly small inclusive cross sections of 79Cu(p,2p)78Ni and 80Zn(p,3p)78Ni were observed compared to neighbouring nuclei. To investigate this phenomenon, the cross sections to the excited states and the ground state deduced experimentally were compared with reaction theory based on a DWIA formalism.

In this presentation, the structural information of 78Ni will be discussed in detail with the obtained level scheme and the recent theoretical calculations.

[1] R. Taniuchi et al., Nature 569, 53-58 (2019).

Speaker: Ryo Taniuchi (Department of Physics, University of York)

18 Nuclear structure of 76Ni from the (p,2p) reaction

The nuclear structure of the 76Ni nucleus was investigated by (p,2p) reaction using a NaI(Tl) array to detect the deexciting prompt γ rays. A new transition with an energy of 2227 keV was identified by γγ and γγγ coincidences. According to these coincidence spectra, the observed transition connects a new state at 4147 keV and the previously known 4+1 state at 1920 keV. Two weaker transitions were also obtained at 2441 and 2838 keV, which could be tentatively placed to feed the known 2+1 state at 990 keV. Our shell-model calculations using the Lenzi, Nowacki, Poves, and Sieja interaction produced good candidates for the experimental proton hole states in the observed energy region, and the theoretical cross sections showed good agreement with the experimental values. Although we could not assign all the experimental states to the theoretical ones unambiguously, the results are consistent with a reasonably large Z=28 shell gap for nickel isotopes in accordance with previous studies.

Speaker: Zoltan Elekes (Atomki)

11:50 Conference Photo

12:05 Lunch

Island of inversion at N=40

Convener: Martha Liliana Cortes (RIKEN)

19 The development of deformation at N=40 (ZOOM)

Shell model calculations constitute a very powerful tool to predict with very good accuracy some experimental observables. However, when treating collective behavior, the large number of nucleons involved in the structure of the nuclear states and the size of the valence space may preclude these calculations because of the large matrices to be diagonalized.
The development of islands of inversion at the magic numbers, as for example that N=40 where large deformed nuclei have been found, can be interpreted in terms of the available single-particle levels around the Fermi surface (that favor quadrupole correlations) and the associated dynamical symmetries, variants of the SU(3) symmetry. Following the suggestion of these symmetries, a smart choice of the model space can be done, making these calculations doable.
After an introduction to the theoretical method, several results will be shown, also in comparison with the experimental data.

Speaker: Silvia Lenzi

20 First spectroscopy of $^{62}$Ti: Shell evolution towards $^{60}$Ca

Shell evolution for the N=40 isotones has recently attracted considerable attention. In a single-particle shell model N=40, which corresponds to the filling of the $fp$ neutron shells, is predicted to be a sub-shell closure. However, measurements of the first 2$^+$ states in $^{64}$Cr and $^{66}$Fe give evidence of a rapid weakening of the N=40 gap when removing protons from the $f_{7/2}$ shell. Conversely, the 2$^+$ energies of $^{58,60}$Ti show only a slight decrease towards N = 40. To further understand the shell evolution towards the supposedly doubly-magic $^{60}$Ca, we report on the measurement of the first excited 2$^+$ state of $^{62}$Ti.
Excited states in $^{62}$Ti were populated via the $^{63}$V(p,p2)$^{62}$Ti reaction and studied using $\gamma$-ray spectroscopy. The energies of the 2$^{+}\rightarrow 0^+$ and 4$^{+}\rightarrow 2^+$ transitions, observed here for the first time, indicate a deformed $^{62}$Ti ground state. These energies are increased compared to the neighboring Cr and Fe isotones, suggesting a small decrease of quadrupole collectivity. This result is well reproduced by large-scale shell-model calculations based on effective interactions, while ab initio and beyond mean-field calculations do not yet reproduce them. The shell-model calculations for $^{62}$Ti show a dominant configuration with four neutrons excited across the N=40 gap. Likewise, they indicate that the island of inversion extends down to Z=20, disfavoring a possible doubly magic character of $^{60}$Ca.

Speaker: Martha Liliana Cortes (RIKEN)

21 Restoration of the natural E(1/2+) -E(3/2+) energy splitting in odd-K isotopes towards N= 40 (ZOOM)

I will report on the first γ-ray spectroscopy of $^{51,53}$K produced from $^{52,54}$Ca(p,2p) reactions at ~250 MeV/nucleon at RIBF at RIKEN. The 1/2$_{1}^{+}$ →3/2$_{1}^{+}$ transitions in $^{51,53}$K were clearly observed [1], providing important information to understand the monopole drift effect of the proton 1d$_{3/2}$ and 2s$_{1/2}$ orbitals along the odd-K isotopic chain. Thanks to the MINOS setup based on reaction vertex tracking combined with a thick-hydrogen target [2], the final-state angular-momentum of $^{51,53}$K were determined unambiguously by comparing the shapes of the experimental exclusive parallel momentum distributions to distorted-wave impulse approximation calculations. 3/2+ ground states and 1/2+ first excited states in $^{51,53}$K were established quantifying the natural ordering of the 1d$_{3/2}$ and 2s$_{1/2}$ proton-hole states that are restored at N = 32 and 34. State-of-the-art ab initio calculations and shell-model calculations with improved phenomenological effective interactions reproduce the present data and predict consistently the increase of the E(1/2$_{1}^{+}$) - E(3/2$_{1}^{+}$) energy differences towards N = 40.

[1] Y. L. Sun et al., Phys. Lett. B 802, 135215 (2020).
[2] A. Obertelli et al., Eur. Phys. J. A 50, 8 (2014).

Speaker: Dr Yelei Sun (Technische Universität Darmstadt)

14:55 Coffee break

22 First spectroscopy of Cr-66, Fe-70 and Fe-72 in the N=40 Island of Inversion

We look back on the first ever SEASTAR campaign at RIBF, in 2014, using the DALI 2 γ-ray detector array and the MINOS device, and more especially focus on the measurement of the first $2^+$ and $4^+$ states of $^{66}$Cr and $^{70,72}$Fe via in-beam γ-ray spectroscopy after a (p,2p) knockout reaction. Experimental results and their interpretation within the shell model showed an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50, and paved the way to extend our knowledge in this neutron-rich region of the nuclear chart.

Speaker: Clementine Santamaria (Michigan State University)

23 First-time Investigation of Low-Lying Level Structures in V63, V59, and V61 Nuclei

In this presentation, we will discuss two publications that explore the low-lying level structures of V63, V59, and V61 nuclei. In the first study, the level structure of V63 was investigated using inelastic proton scattering and proton knock-out reactions in inverse kinematics. Comparing the observed γ-ray transitions with shell-model calculations, two excited states corresponding to the first 11/2− and 9/2− levels were established. Analysis of the (p,p′) excitation cross sections revealed large deformation parameters, placing V63 in the island of inversion below Ni68.
The second study focused on V59 and V61, where the low-lying level structures were explored using neutron knockout reactions and inelastic proton scattering. Several new transitions were identified for both isotopes and through comparison with shell-model calculations, three γ-rays in each isotope could be placed in the level scheme and assigned to the decay of the first 11/2− and 9/2− levels. The (p,p') excitation cross sections of V61 were analyzed, considering quadrupole plus hexadecapole deformations. However, due to the role of the hexadecapole deformation, the placement of V61 on the island of inversion remained ambiguous.

Speaker: Marcell Juhász (Institute for Nuclear Research)

24 Spectroscopy of 65,67Mn: Strong coupling in the N=40 “island of inversion”

Excited states in 63,65,67Mn were studied via in-beam γ-ray spectroscopy following knockout reactions from 68Fe. Similar level schemes, consisting of the 11/2−,9/2−,7/2− and 5/2− level sequence, connected by I→I−1 transitions, were established, the first time for 65,67Mn. Their level structures show features consistent with strongly-coupled rotational bands with K=5/2. State-of-the-art shell-model calculations with themodified LNPS effective interaction reproduce the observed levels remarkably well and suggest the dominance of 4-particle-4-hole neutron configurations for all the states. The data on the low-lying excited states of odd-mass 53−67Mn provide a textbook example of nuclear structure evolution from weak coupling through decoupling to strong coupling along a single isotopic chain on the n-rich side of the β stability line. These results help to deepen our understanding of the N=40 “island of inversion”.

Speaker: Xiaoyu Liu

Poster session with canapés

25 Characterizing GAGG Crystals for In Beam Gamma-Ray Spectroscopy at the RIBF

GAGG(Ce) is a novel scintillator that shows promise as a future material for gamma spectrometers. It has several benefits over traditional materials such as NaI(Tl) including its superior resolution (4.0±0.3%, Intrinsic FWHM at 662 keV), higher density, and it being non-hygroscopic [1][2]. A new scintillator-based array is planned to be deployed at the RIBF (Radioactive Isotope Beam Factory, Japan) for in beam gamma-ray spectroscopy. In this facility, the gamma-rays are emitted by fast-moving projectiles and are Lorentz-boosted up to energies of around 10 MeV necessitating large crystals for full gamma-ray absorption. At the same time, the effect of Doppler broadening in the reconstructed gamma-ray spectrum is driven by the size of the crystal and a high granularity of the array is required to correct for the angular dependence in the Doppler correction. Large, long, cuboidal shapes are being considered currently. It is therefore necessary to test and confirm that these larger crystals sizes maintain the material’s desirable properties such as light-yield uniformity, and good energy resolution.
In this study, we present the characterisation results for a 1”x3” cylindrical HR-GAGG crystal using various radioactive sources and a slit-collimator system. This has been done with different light collection methods, mainly using SiPMs (Silicon Photomultipliers) placed at different faces of the crystal. Additionally, the performance of the crystal has been tested using different wrapping materials, namely PTFE tape and ESR foil.
In particular, I plan to discuss the dependence of the signal on the gamma-ray interaction position within the volume of the crystal. This includes the observed variation in the mean gamma-ray energy, the energy resolution, and the pulse shape of the signal at different positions of interaction.
[1] B. Seitz, et. al., IEEE Transactions on Nuclear Science, vol. 63, no. 2, pp. 503-508, April 2016
[2] P. Sibczynski et. al., Nucl. Instrum. Methods Phys. Res., Sect. A, vol. 772, pp. 112-117, February 2015

Speaker: Mr William Marshall (University of York)

26 Development of GAGG Array at RIKEN for Next-generation Detection

The DALI2$^{+}$ array has well severed at RIBF, RIKEN for many years with fruitful research outcomes. However, the energy resolution of DALI2$^{+}$ array is far from adequate due to the intrinsic energy resolution of NaI crystal and the absence of interaction position reconstruction ability. Furthermore, the hygroscopic NaI crystal decays over time if the encapsulation is not good enough, which further affects the energy resolution and requires extra protection material that reduces detection efficiency. The next generation scintillator array needs to solve this problem by introducing crystal made of new scintillation material, ceramic GAGG $\mathrm{(Gd_{3}(Ga,Al)_{5}O_{12}(Ce))}$, coupled with segmented photomultiplier, to take the gamma-ray detection ability to another level.

Performance of several GAGG array geometry configuration was simulated, which shows promising overall improvement from DALI2$^{+}$ array, with halved energy resolution and doubled detection efficiency. By simulating the scintillation and light propagation process from the incident of gamma photon into GAGG crystal, correlation of interaction position and distribution of scintillation photons over detector surface were investigated. Preliminary result shows the reconstruction of first interaction can be achieved within about 2cm’s range from actual incident position, with the help of segmented photomultiplier. More details will be given in the poster presentation.

Speaker: Mr Ting Gao (University of Hong Kong)

27 Gamma-ray spectroscopy of the neutron-rich $\mathrm{^{55,57,59}}$Sc isotopes

Experimental data have shown that far from the valley of stability the nuclear shell structure evolves. New magic numbers can emerge and the traditional ones can disappear. In particular, two new magic numbers at N=32 and N=34 have been suggested in the vicinity of Z=20 based on $\gamma$-ray spectroscopy and mass measurements. In order to assess the impact of a single valence proton outside of the Z=20 shell on the shell-evolution mechanism in this region, it is necessary to study the neutron-rich Sc isotopes around, and even beyond, neutron number N=34. Investigation of exotic nuclei in this region was the goal of the third SEASTAR campaign at RIKEN-RIBF. Neutron-rich isotopes in the vicinity of $^{53}$K were produced by fragmentation of a primary $^{70}$Zn beam on a $^{9}$Be target. Known and new $\gamma$-ray transitions of the isotope $^{55}$Sc were observed and $\gamma$-rays from $^{57,59}$Sc were identified for the first time. Observed $\gamma$ spectra from $^{55,57,59}$Sc will be presented together with preliminary level schemes. They will be discussed in the framework of the tensor-driven shell evolution.

Supported by BMBF under Grant Nos. 05P19/21RDFN1.

Speaker: Radostina Zidarova (TU Darmstadt)

28 Identifying particle reaction channels using CALIFA’s QPID

The R3B setup at GSI employs many different detectors to study nuclear reactions. The CALorimeter for In Flight detection of gamma rays and high energy charged pArticles (CALIFA) is a highly segmented scintillation detector surrounding the target. It is formed from CsI(Tl) crystals, with a scintillation process formed from a fast and slow component with decay times of 700ns and 3.34s respectively. The generated pulse from an incident particle is analysed by recording the charge function in a first short and then delayed large window following the trigger signal [1]. Each component is dominant in the corresponding window and the contribution of light emitted is dependent on the type of particle incident, thus analysis of this feature allows for quick particle identification (QPID).
Analysis of a recent quasi-free scattering experiment will provide insight in CALIFA’s capabilities to distinguish different particle reaction channels and will help facilitate the employment of CALIFA in the future. Future experiments at R3B include measurements of p,pd QFS reaction cross sections for neutron rich carbon isotopes which will rely on CALIFA to correctly identify deuterons produced.

[1] Anna-Lena Hartig, “Evolution of CALIFA: From single detector modules to benchmark reactions”, 2021

Speaker: Matthew Whitehead (University of York)

29 Spectroscopy of 100Cd from one-proton removal

Low-lying states of 100Cd from one-proton removal reaction (101In, 100Cd+γ) have been studied using in-beam γ-ray spectroscopy at the Radioactive Isotope Beam Factory at RIKEN. A new method is proposed to separate peaks mixed together in the spectrum. Using this new method, two new γ transitions were identified and tentatively assigned as decays from two previously unknown states. The configurations of the two states are proposed by comparing with the shell model calculations and the systematic of Cd isotopes. The relative population intensities of the two lowest 4+ states were extracted, and the relative spectroscopy factors to these two states were deduced. The results suggest that the first 4+ state has neutron configuration and the second 4+ state is of proton configuration, in agreement with shell model predictions.

Speaker: Yingfeng Xu (Institute of Modern Physics, Chinese Academy of Sciences)

30 The first investigation of unbound states of 53,55Ca populated from neutron-knockout reactions

The calcium isotopes are an ideal test bench for studying the evolution of shell structure and magic numbers. Although many properties of surface nucleons in calcium have been successfully described by experiments and theories, it is still challenging to predict the shell structure for deeply bound nucleons. In this study, we report the first investigation of unbound states in 53Ca and 55Ca, populated from 54,56Ca (p,pn) reactions at a beam energy of around 216 MeV/nucleon. These states were analyzed in terms of their resonance properties, partial cross-sections, and momentum distributions. The momentum distributions are compared to calculations using the distorted wave impulse approximation (DWIA) reaction model, allowing orbit momentum l assignments for the observed states. The resonances at excitation energies of 5516(43)-keV in 53Ca and 5946(195)-keV in 55Ca, show clear signs of a significant l = 3 component, providing the first experimental evidence for the f7/2 single-particle strength of deeply bound hole-states in the neutron-rich Ca isotopes. The excitation energies and strengths of the observed states are compared to shell-model calculations using the effective GXPF1Bs interaction and ab initio calculations and found in good agreement with theoretical predictions. The vf7/2 single-particle strength was found to be very robust until N=36 where it's starting to show signs of fragmentation.

Speaker: Pengjie Li (Institute of Modern Physics, China)

Wednesday, 2 August

08:00 Morning coffee

Collectivity from Zn to Zr

Convener: Sidong Chen (University of York)

31 Multiple shape coexistence around Z = 30-48 studied with beyond-mean-field methods

Nuclei in the region of the nuclear chart between Z = 28 and 50 magic numbers show a collective behavior that can be attributed to the appearance of quadrupole shape mixing and/or coexistence. Advanced energy density functional (EDF) methods, including symmetry restorations and axial and triaxial shape mixing, are the perfect tools to study these phenomena from a microscopic point of view. In this contribution I will present recent systematic calculations performed with the Gogny EDF comparing with the available experimental data.
Furthermore, I will focus on the appearance of multiple shape coexistence in the nucleus 84Mo.

Speaker: Tomás R. Rodríguez (Universidad Complutense de Madrid)

32 Kiwis or frisbees? Onset of deformation above N=50

The SEASTAR campaigns allowed us to have a first look into the structural evolution of neutron rich Ge isotopes, up to N=56, through in-beam gamma spectroscopy. In addition, the availability of the EURICA array for the first campaigns allowed to efficiently do decay spectroscopy on most neutron rich isotopes, including Se up to N=60. The spectroscopy hinted at a rich picture for the evolution of quadrupole deformation in this region, ranging from the onset of collectivity next to the shell (from ⁸⁴Ge) to potentially triaxial structure in ⁸⁶Ge, and signatures for a prolate-oblate shape coexistence in then neutron-rich Se isotopes. The exploration into this region of the nuclear chart will be reviewed. It will be put into context with the structural evolution around N=56-60 in higher-Z isotopes like Zr, and will be related to more recent approaches taken at RIBF.

Speaker: Volker Werner (TU Darmstadt)

33 Shape coexistence in neutron-rich Kr and Se isotopes

Speaker: Sidong Chen (University of York)

10:20 Coffee break

Reaction mechanism

Convener: Kazuki Yoshida (Japan Atomic Energy Agency)

34 Current status and perspectives of nucleon and alpha knockout reaction

The proton-induced $\alpha$ knockout reaction, ($p$,$p\alpha$), is a powerful probe of the $\alpha$ formation in the nucleus. The author and the collaborators have shown that a modern theoretical calculation of the $\alpha$ amplitude combined with the ($p$,$p\alpha$) reaction calculation by the distorted wave impulse approximation can quantitatively reproduce the existing experimental data [1]. Although the $\alpha$ formation is known in the light mass nuclei as the $\alpha$ clustering and also in very heavy nuclei as the $\alpha$ decay, the universality of the $\alpha$ formation throughout the nuclear chart has been a question. Triggered by a theoretical prediction by S. Typel [2], $\alpha$ formation on the surface of medium mass nuclei, Tin isotopes $^{112,116,120,124}$Sn, has been experimentally confirmed [3]. Recently, the author and the collaborator have shown that the $\alpha$ reduced width, which is a key quantity for describing the $\alpha$ decay lifetime, can also be determined by the ($p$,$p\alpha$) reaction [4]. In this contribution, I will review the recent progress in the $\alpha$ formation phenomena studied by the above-mentioned ($p$,$p\alpha$) reaction, and give a perspective on the cluster knockout reactions and novel clustering phenomena.

[1] K. Yoshida, Y. Chiba, M. Kimura, Y. Taniguchi, Y. Kanada-En'yo, and K. Ogata, Phys. Rev. C. 100, 044601 (2019).
[2] S. Typel, Phys. Rev. C 89, 064321 (2014).
[3] Junki Tanaka et al., Science 371, 260 (2021).
[4] Kazuki Yoshida and Junki Tanaka. Phys. Rev. C 106, 014621 (2022).

Speaker: Kazuki Yoshida (Japan Atomic Energy Agency)

35 Theoretical model of the (p,pd) reaction for understanding deuterons inside nuclei

In the experiment performed 40 years ago at the University of Maryland, it was reported that the cross section of the $^{16}$O($p,pd$)$^{14}$N reaction [1] is almost half that of the $^{16}$O($p,2p$)$^{15}$N reaction [2]. This result may indicate that the existence probability of the deuteron in $^{16}$O is surprisingly high and that there are $pn$ correlations including the deuteron ``cluster.'' To describe this reaction, it is important to treat the fragility of the deuteron properly. The deuteron can be easily broken up by the incident proton in the elementary process. In addition, the knocked-out deuteron is expected to go through transition between the bound and breakup states by the final-state interactions (FSIs). Furthermore, the deuteron broken up in the elementary process can reform a deuteron by the FSIs. These processes are not included in the distorted wave impulse approximation (DWIA) framework [3], which is the standard reaction model for describing the knockout reactions as employed in the ($p,pd$) analysis of Ref. [1]. Therefore, even if measurement results of deuteron knockout reactions are systematically obtained, it is not possible to conclude clearly whether deuterons exist in nuclei or not by the DWIA analysis. Very recently, a project has been launched to measure the ($p,pd$), ($p,pt$), ($p,p^3$He), and ($p,p\alpha$) reactions on a variety of nuclei to reveal that the ground state of nuclei is a non-uniform state that contains various clusters. In this sense, there is a growing demand for a beyond-DWIA model that can quantitatively describe the ($p,pd$) reaction.

In this presentation, we are going to report the numerical results calculated with such a reaction model, CDCCIA, which we have been constructing [5]. In CDCCIA, the elementary processes of the ($p,pd$), i.e., the $p$-$d$ elastic scattering and the $d$($p,p$)$pn$ reaction, are described with an impulse picture employing a nucleon-nucleon effective interaction. In addition, the three-body scattering waves in the final state of the ($p,pd$) reaction are calculated with the continuum-discretized coupled-channels method (CDCC) [6-8]. We will shown that the deuteron reformation significantly changes the explicit cross section of the ($p,pd$) reaction through the interference between the elastic and breakup channels of deuteron. Our conclusion is that including these processes is important to quantitatively discuss the ($p,pd$) cross sections in view of the deuteron formation in nuclei.

1. C. Samanta et al., Phys. Rev. C 26, 1379 (1982).
2. C. Samanta et al., Phys. Rev. C 34, 1610 (1986).
3. T. Wakasa et al., Prog. Part. Nucl. Phys. 96, 32 (2017), and references therein.
4. T. Uesaka et al., Grants-in-Aid of Japan Society for the Promotion of Science, No. JP21H04975.
5. Y. Chazono et al., Phys. Rev. C 106, 064613 (2022).
6. M. Kamimura et al., Prog. Theor. Phys. Suppl. 89, 1 (1986).
7. N. Austern et al., Phys. Rep. 154, 125 (1987).
8. M. Yahiro et al., Prog. Theor. Exp. Phys. 2012, 01A206 (2012).

Speaker: Yoshiki Chazono (RIKEN Nishina Center)

36 Multiple mechanisms in proton-induced one nucleon removal from $^{14}$O at $\sim$100 MeV/nucleon

One nucleon removal reactions at intermediate energies has been a powerful tool for single-particle structure studies of exotic nuclei [1], but the reaction mechanism is not fully understood [2-5]. One debated phenomenon is the asymmetric parallel momentum distribution (PMD) of the residual nucleus occuring occasional in one nucleon removal induced from light ion-targets [4,6,7]. Recent theoretical calculation of ($p$,$pN$) reactions with $^{14}$O at 100 MeV/nucleon with the distorted-wave impulse approximation (DWIA) predicted also large asymmetric PMD for deeply-bound nucleon removal [4]. The low momentum tail is found to be due to the attractive potential between the residues and the outgoing nucleons and the steep falloff on the high momentum side is due to the energy and momentum conservation. Still, comparison with experimental data is necessary for validation and will be a basis for further spectroscopic factor studies. We have performed $^{14}$O($p$,$pN$)$^{13}$O and $^{14}$O($p$,$2p$)$^{13}$N reactions at $\sim$94 MeV/nucleon with a $\sim$2.4 mm thick solid hydrogen target at SAMURAI at RIKEN. Momentum of the residues were extracted from the SAMURAI spectrometer. An overview of the experiment and analysis will be given. We report the results for the cross section and PMDs, which exhibit that multiple reaction mechanisms occur. In addition to the ($p$,$pN$) knockout process, contributions of 50 % and 30 % from inelastic scattering ($p$,$p$') and transfer ($p$,$d$) for proton and neutron removal are observed, respectively. These processes should be considered in the analyses of one-nucleon removal cross section at intermediate energies for quantitative nuclear structure studies.

[1] T. Aumann $et\: al.$, Prog. Part. Nucl. Phys. 118, 103847 (2021).
[2] C. Louchart, A. Obertelli $et\: al.$, Phys. Rev. C 83, 011601(R) (2011).
[3] Y.L. Sun, A. Obertelli $et\: al.$, Phys. Rev. C 93, 044607 (2016).
[4] F. Flavigny $et\: al.$, Phys. Rev. Lett. 108, 252501 (2012).
[5] K. Ogata $et\: al.$, J. Phys. Rev. C 92, 034616 (2015).
[6] A. Gade $et\: al.$, Phys. Rev. C 71, 051301(R)(2005).
[7] K.L. Yurkewicz $et\: al.$, Phys. Rev. C 74, 024304 (2006).

Speaker: Dr Thomas Pohl (TU Darmstadt)

12:05 lunch

37 (TBC, (p,3p) talk)

Speaker: Alexandre Obertelli (TU Darmstadt)

38 Nucleon-nucleon correlations and the single-particle strength in atomic nuclei

Speaker: Marina Petri (University of York)

39 Nucleon knockout reactions within the intranuclear cascade model INCL

The main features of the intranuclear cascade model INCL for describing single-nucleon and multi-nucleon knockout cross sections will be presented in this talk. For the interpretation of experimental data on knockout cross sections, this model is coupled to the deexcitation code ABLA, which calculates the particle separation energies from the atomic mass evalutation AME2020. The results obtained for different data sets from RIKEN and GSI will be shown. Moreover, the new INCL improvements related to the implementation of short-range correlations will also be presented together with the results obtained for neutron and proton knockout cross sections measured in different worldwide facilities.

Speaker: Jose Luis Rodriguez Sanchez (University of Coruña)

40 Quasi-free knockout reaction to prove alpha clusters in nuclear surface

Speaker: Junki Tanaka (Riken, Nishina Center)

15:15 Coffee break

Development of new detection devices

41 Methodologies for Direct Reaction Studies with RI-beams ~ Present and Future ~

Speaker: T. Uesaka

42 NEBULA-Plus: the Upgrade of the NEBULA Fast Neutron Array

This presentation will briefly describe the NEBULA-Plus array, which is intended to complement the existing two-wall NEBULA fast neutron array of SAMURAI. After introducing the motivations for constructing NEBULA-Plus and the expected performances, the details of the setup and the installation and testing of the array will be presented. We will conclude with a few remarks on some of the possible experiments with NEBULA-Plus.

Speaker: Nigel Orr (LPC-Caen (CNRS/IN2P3))

NEBULA-Plus - York Symposium August 2023.ppt

Clustering and multi-neutron systems: (1)

Convener: Zaihong YANG (Peking University)

43 Tetra-neutron system populated by exothermic double-charge exchange reaction

Speaker: Susumu Shimoura (RIKEN Nishina Center)

York-shimoura.pdf

York-Shimoura-pptx.pdf

Thursday, 3 August

08:00 Morning coffee

Shell Evolution towards 28O

Convener: Takashi Nakamura (Tokyo Institute of Technology)

44 Ab-initio computations of exotic nuclei in and around the island of inversion

Neutron-rich nuclei at and beyond the magic neutron number 20 are interesting because of the breakdown of this shell closure, and the interplay between nuclear deformation and weak binding in the so-called island of inversion. Here I report on recent ab-initio computations of exotic nuclei in this region starting from chiral Hamiltonians. By breaking and restoring rotational symmetry ab-initio methods can now be used to address how rotational structure and shell-evolution evolves from the valley of beta-stability towards the dripline. Recent calculations indicate shape co-existence in $^{30}$Ne, $^{32}$Mg and $^{40}$Mg, predict a well deformed $^{34}$Ne, reveal systematic trends of charge radii, questions the existence of certain magic shell closures in neutron-rich nuclei, and confrontation with data also expose challenges for ab-initio methods.

New ways to make quantified predictions are now possible by the development of accurate emulators of ab-initio calculations. These emulators reduce the computational cost by many orders of magnitude. This allows us to perform global sensitivity analysis, and use novel statistical tools in making quantified predictions of nuclei. Using this approach, we predict with 98% probability that $^{28}$O is unbound and that it takes fine-tuned nuclear interactions to describe the subtle structure of neutron-rich oxygen isotopes within theoretical uncertainties. We also recently addressed the question: What drives deformation in exotic neon and magnesium isotopes from chiral interactions?

Speaker: Gaute Hagen (Oak Ridge National Laboratory)

45 Study of the heaviest fluorine isotopes

The nuclear shell model is one of the remarkable accomplishments in nuclear physics. Intriguingly, nuclear structure changes significantly in highly asymmetric nuclei. A particularly notable region in the chart of nuclei is the island of inversion around the neutron-rich neon to magnesium isotopes. I will discuss the structure of the neutron-rich fluorine isotopes, specifically 28F, 29F, and 30F, located at the southern shore of the island of inversion. The detailed spectroscopy reveals shell evolution and loss of magicity at neutron number N=20 in these isotopes as well. We performed the high-resolution invariant-mass spectroscopy of the neutron-unbound nuclei 28F, 30F, and of two-neutron decays in 29F for the first time at the SAMURAI setup.

Speaker: Julian Kahlbow (MIT & Tel Aviv University)

46 Four-neutron unbound nucleus 28O studied by the invariant mass method

Many experimental and theoretical studies have been devoted to studying doubly magic unstable nuclei as they are important cornerstones for understanding many-nucleon systems located far from the beta stability. The 28O nucleus is the last candidate of the doubly magic unstable nuclei that can be experimentally accessible but has never been observed so far. We have carried out the invariant mass experiment at RIBF. The 28O nucleus was produced by one-proton removal reaction from a secondary 29F beam. The decay products, 24O and four neutrons, are detected in coincidence by SAMURAI combined with the liquid hydrogen target system MINOS and large neutron detector array NeuLAND. Thanks to the high 29F beam intensity, high luminosity of the thick liquid hydrogen target provided by MINOS, and high neutron detection efficiency, we have successfully observed the 28O nucleus for the first time. I will present the result of the 28O observation and related studies.

Speaker: Yosuke Kondo (Tokyo Institute of Technology)

10:20 Coffee Break

Clustering and multi-neutron systems: (2)

Convener: Zaihong YANG (Peking University)

47 Exploring multi-neutron systems at SAMURAI at RIBF

I first characterize multi-neutron systems as pure-neutron nuclei and as neutron clusters at the surface of nuclei. Then, I show how to probe such a system using the proton-induced quasi-free scatterings in inverse kinematics for a rare-isotope beam. I introduce the ongoing development of experimental setups for such reactions at SAMURAI at RIBF: the STRASSE and the prototype PFAD, where, for the latter, I will show the results of the test experiment at HIMAC. I also present the planned experiment to search for multi-neutron clusters in the excited 10He and the 6n states at SAMURAI at RIBF. Finally, I will present future perspectives where I propose a new scheme of multi-neutron detection.

Speaker: Takashi Nakamura (Tokyo Institute of Technology)

48 Observing the four-neutron system in 8He(p,pa) reactions

The long-standing question regarding the existence of a bound four-neutron system, a so-called tetraneutron, was addressed in an experiment at SAMURAI using the MINOS LH2 target. A $^{8}$He beam was used to perform a (p,p$\alpha$) reaction in inverse kinematics at large center-of-mass angles, allowing for the prompt removal of the $\alpha$ and thus minimizing its influence on the 4n system. Results of this experiment will be presented, as well as plans for an upcoming experiment briefly explained.

Speaker: Dominic Rossi (TU Darmstadt / GSI)

49 Dineutron correlation in neutron drip-line nuclei

Speaker: Yuki Kubota (RIKEN)

12:10 Lunch

Clustering and multi-neutron systems: (3)

Convener: Zaihong YANG (Peking University)

50 3n Study at SHARAQ and New Multi-n Project at SAMURAI

Correlations in multi-neutron systems have been fascinating topics in recent nuclear physics. In this presentation, we will present two of new experimental approaches on multi-neutron systems recently performed and planned at RIBF.
First, we explain our experimental study on 3n system via the 3H(t,3He)3n reaction at 170 MeV/u using the SHARAQ spectrometer. We have developed a thick tritiate titanium target dedicated for this purpose. With this target we have successfully obtained the differential cross section of the 3H(t,3He)3n reaction at Ex<20MeV and Theta<4deg.
Second, we introduce our new project of multi-neutron experiment at SAMURAI. By means of the knockout reactions on helium isotopes, we will produce various multi-neutron systems with neutron numbers up to 6. The overview and outlook of this project will be explained in the presentation.

Speaker: Kenjiro Miki (Tohoku University)

51 Cluster structure of neutron-rich beryllium isotopes probed by (p,p$\alpha$) knockout reactions in inverse kinematics

The emergence of cluster structures within the nucleus is a fascinating phenomenon that requires a complete understanding of the nuclear structure and the fundamental nuclear interactions. So far $\alpha$-clustering has dominated cluster state studies among all other possible partitioning due to the large binding energy of the $\alpha$-particle and its inert character. The famous Ikeda diagram conveyed the idea that cluster states appear near the $\alpha$ threshold in stable N=Z nuclei [1]. However, clustering in the ground-state of exotic nuclei with large imbalance of proton and neutron number is still a question. Neutron-rich beryllium isotopes 10Be,12Be,14Be are the very appealing candidates of clustering studies as being built on the well-developed alpha-alpha rotor of 8Be (N=4, Z=4). It is predicted by calculations in antisymmetrized molecular dynamics model that alpha clustering in the ground-state develops from 10Be going to the dripline [2].
The SAMURAI-12 experiment performed at the Radioactive Isotope Beam Factory (RIBF) in RIKEN aims to investigate the cluster structure of neutron-rich beryllium isotopes using the cluster quasifree scattering reaction (p,p$\alpha$) in inverse kinematics. The reaction of interest was induced by radioactive 10,12,14Be beams at 150MeV/u impinging on a 2-mm-thick pure solid hydrogen target. Recoil protons were detected using the Recoil Proton Spectrometer (drift chamber, plastic scintillator, and NaI(Tl) rods) in a two-arm configuration, covering an angular range of 50°-70°. Two telescopes composed of silicon strip detectors and CsI(Tl) modules were placed at forward angles for detecting alpha clusters. The detection of helium residues was performed by using the SAMURAI spectrometer and its standard detectors. Experimental results concerning missing mass spectra and triple differential cross-sections will be presented. The latter will be compared to calculations using a microscopic description of the reactions of interest implemented in the distorted wave impulse approximation, allowing to probe the alpha cluster structures directly and quantitatively.

References:
[1] K. Ikeda, N. Takigawa, and H. Horiuchi, Prog. Theor, Phys. Suppl. Extra Number, 464 (1968).
[2] Y. Kanada-En'yo and H. Horiuchi, Phys. Rev. C 68, 014319 (2003).

Speaker: Pengjie Li (Institute of Modern Physics, China)

52 Structure and Two-Neutron Decay of 16Be

Investigating the properties of nuclei far from the line of beta-stability is one of the central themes of present day nuclear physics. Of particular interest as the neutron dripline is approached are the changes in shell structure and the correlations that may develop between the valence neutrons. In this presentation we will focus on the structure and neutron-neutron correlations in the heaviest known Be isotope, 16Be, which is unbound to two-neutron emission. Results will be presented from a study employing proton knockout from a secondary 17B beam and the SAMURAI spectrometer coupled to the MINOS active liquid hydrogen target system and the NEBULA fast neutron array. Comparison will be made with the results of realistic three-body calculations, which, importantly, incorporate the effects of the decay process. This will allow conclusions to be drawn regarding the spatial configurations of the levels observed in 16Be.

Speaker: Nigel Orr (LPC-Caen (CNRS/IN2P3))

53 Gamow shell model calculations of resonance states of drip line nuclei

The lightest nuclei, encapsulating some of the most fascinating systems in the nuclear landscape, have sparked considerable interest in the nuclear physics community. Their structures are characteristically intricate and complex, particularly evident in the $^{4,5}$He nuclei. These structures can be meticulously expounded with the implementation of realistic interactions in a no-core framework, illuminating the nuclei's internal dynamics. Light nuclei, accessible up to and even beyond drip-lines, provide perfect experimental and theoretical laboratories to probe the nucleon-nucleon interaction, contributing vital insights into the nature of nuclear forces and structures.

Recent years have witnessed significant strides in the Gamow shell model calculations of nuclear structures. These advancements provide the foundation for the dissection of complex nuclear phenomena, fostering a deeper understanding of the physical world at the nuclear level. This presentation focuses on the application of the Gamow shell model in studying the properties of weakly-bound and unbound nuclei. The Gamow shell model, endowed with resonance and continuum degrees of freedom, offers a potent tool for exploring the realm of weakly-bound systems and beyond.

Speaker: Jianguo Li (Institute of Modern Physics Chinese Academy of Sciences)

15:20 Coffee break

Clustering and multi-neutron systems: (4)

Convener: Zaihong YANG (Peking University)

54 TBD

Speaker: Zaihong YANG (Peking University)

55 Cluster formation around the neutron drip-line

The formation of clusters in finite and infinite nuclear systems is an issue of great interest in nuclear physics. Recently, experiments have revealed a negative correlation between alpha-cluster formation and neutron number in Sn isotopes, showing a trend opposite to that theoretically conjectured for Be and B isotopes.
Here, we investigate the cluster formation in Be and B isotopes to elucidate the clustering near the neutron drip line. The results indicate that the excess neutrons contribute to the formation of 6He and 8He clusters as well as alpha clusters.

Speaker: Masaaki Kimura (RIKEN)

19:00 Conference dinner

The entrance is only available from the Fossgate side. A drink reception will be open at 6 pm.
https://www.google.com/maps/place/53%C2%B057'29.4%22N+1%C2%B004'42.5%22W/@53.9581811,-1.0810329,17z/data=!3m1!4b1!4m4!3m3!8m2!3d53.958178!4d-1.078458?authuser=1&entry=ttu

Friday, 4 August

08:00 Morning coffee

Development of new detection devices

56 Development of the STRASSE system for quasi free scattering measurements at the RIBF

STRASSE (Silicon Tracker for RAdioactive nuclei Studies at SAMURAI Experiments) is a new detection system under construction for quasi-free scattering (QFS) measurements at 200-250 MeV/nucleon at the RIBF facility of the RIKEN Nishina Center. It consists of a charged-particle silicon tracker coupled with a dedicated thick liquid hydrogen target (up to 150-mm long) in a compact geometry to fit inside large scintillator or germanium arrays. Its design was optimized for two types of studies using QFS: missing-mass measurements and in-flight prompt γ-ray spectroscopy. In this presentation, we will report on i) the resolution requirements needed to go beyond the sensitivity of existing systems for these two types of measurements, (ii) the conceptual design of the system using detailed simulations of the setup and (iii) its complete technical implementation and challenges (iv) the current status of the project. The STRASSE system aims at a sub-mm reaction vertex resolution and is expected to reach a missing-mass resolution below 2 MeV in σ for (p,2p) reactions when combined with the CsI(Na) CATANA array.

Speaker: Dr Hongna Liu (Beijing Normal Univerisity)

57 CsI(Na) array CATANA to measure protons and gamma-rays

The CsI(Na) scintillator array CATANA has been enhanced as a dual-gain system with the capability to simultaneously measure proton and gamma-ray energies.

The simultaneous measurement of excitation energy of populated states and resultant de-excitation gamma-rays gives us an unique opportunity to investigate the detailed nuclear structures. To realize the simultaneous measurement for the (p,2p) reaction with STRASSE+CATANA setup, CATANA must measure the energies of two recoil protons and gamma-rays. Given that the typical energies of the protons and the gamma-rays are in the ranges of 100-250 MeV and 0.1-15 MeV, respectively, the system must have a wide dynamic range. Thus, new circuits and electronics have been introduced to CATANA to cover such a wide dynamic range, making it a dual-gain system.

In this presentation, we will present the specifics of the dual-gained CATANA and the results obtained by combining the dual-gained CATANA with a prototype proton tracker, PFAD.

Speaker: Yasuhiro Togano (RIKEN Nishina Center)

58 Development of the High-Resolution Neutron Detector HIME

High-resolution neutron detection plays a key role in spectroscopic measurements, in particular since in the past decades, the research focus shifted towards extremely neutron-rich nuclei. A prototype of a high-resolution neutron detector, called HIME, has been constructed at SAMURAI with an active area of $40 \times 40 \, \mathrm{cm}^{2}$ for precise measurements of two-neutron correlations in decays of states beyond the neutron dripline. However, its current size and active volume strongly limit the geometrical acceptance and the neutron-reconstruction efficiency.

In this talk, a status report will be given on the ongoing upgrade of the HIME prototype. The existing part of the detector is being extended to an active area of $100 \times 100 \, \mathrm{cm}^2$ and to a larger total depth, which will enhance significantly both acceptance and efficiency. In addition, it was equipped with the new TRB3 readout electronics, developed at GSI. The high granularity of the detector and the time precision of the new electronics guarantee high invariant-mass resolution. First data with the prototype and the new readout electronics were taken in 2022 using cosmic radiation, which showed promising performances.

Speaker: Marco Knösel (Technische Universität Darmstadt, Fachbereich Physik, Darmstadt, Germany)

10:20 Coffee break

Development of new detection devices

59 Recent upgrades of R3B detectors for the experiments with LH2 target

Speaker: V. Panin

60 New telescope for cluster-knockout reaction and cluster formation in neutron rich Ca isotopes

The Structure of nuclear and nuclear matter is explained as shell structure based on mean field theory. However new picture that the light cluster such as $d$, $t$, $^3\mathrm{He}$ and $^4\mathrm{He}$ form spontaneously in the low-density region (~1/10 of saturation density) is predicted . This phenomenon can be a clue to search mechanism which stabilizes nuclear via cluster formation and various phenomena of astrophysics such as neutrino response of low-density nuclear matter and pasta phase in the nuclear star.
But mechanism of such as clustering is not understood well. So, our group launched “ONOKORO project” to search mechanism of cluster formation by acquire systematical experimental data of clustering in clear. We use cluster-knockout reaction at 250 MeV/u energy which satisfies quasi-free condition to measure the ratio of cluster formation and information of cluster in nuclear such as momentum distribution and separation spectra . To extend the measurement to unstable nuclear, we plan measurements with inverse kinematics and develop new detector array named “TOGAXSI” which consisting of liquid hydrogen target, Si strip detector for measurement of scatter angle  and GAGG(Ce) calorimeter for measurement of energy.
In this presentation, we report development of TOGAXSI array and plan of the experiment with the neutron rich nuclei Ca isotopes at RIBF as the first measurement of neutron rich nuclei.

Speaker: Ryotaro Tsuji (Kyoto university / Riken, Nishina Center)

61 Future Programs at RIBF OEDO/SHARAQ: Utilizing Low Energy Heavy Ion Beams and Liq H2 Targets

This talk will outline future experimental programs planned at RIBF OEDO/SHARAQ. The unique capabilities of energy-degraded heavy ion beams at OEDO enable the execution of experiments involving reactions previously unachievable due to the beam energy constraints of RIBF. For instance, in-beam gamma-ray spectroscopy using fusion reactions has proven to be an invaluable tool for studying high spin physics, including exotic deformations of nuclei such as super deformation or parity-violating octupole deformation.
We will provide an overview of the future experimental setup, which includes the gamma-ray detector array and liquid hydrogen (Liq H2) targets used for fusion reactions. Although these programs are still in the early planning stages, we anticipate that our novel approaches will yield a wealth of new results on high-spin excited states across a wide range of exotic nuclei accessible at RIBF.
In this talk, we aim not only to showcase our upcoming experimental programs but also to stimulate discussion about the future of low energy heavy ion beam experiments at RIBF.

Speaker: Rin Yokoyama (Center for Nuclear Study, the University of Tokyo)

12:10 Lunch

Development of new detection devices

62 A New Scintillator-Based Gamma-Ray Spectrometer for the RIBF

Since advent of the RIBF, the NaI(Tl) based scintillation array DALI2+ has been the workhorse for in-beam gamma-ray spectroscopy experiments, now counting 100 peer-reviewed publications (2 Nature, 28 PRL, 23 PLB). Due to its modest energy resolution, caused by large opening angles and intrinsic energy resolution of NaI(Tl) scintillators, long absorption lengths of the scintillation material, as well as modest time resolution, the long-term potential is limited. Furthermore, limited available budget makes low cost alternatives to 4pi Ge tracking arrays with superior features, except energy resolution, desirable. Consequently, a new-generation scintillator array for in-beam gamma-ray experiments is being devised for the near future. Here, the scintillation materials GAGG and CeBr3 have been identified as the most promising choices. Key advantages for the former include its high density, low radiation length, and that it's neither hygroscopic nor self-emissive, while the latter offers a better intrinsic resolution and extremely fast
decay time.

It is envisioned that a hybrid array, composed of GAGG and CeBr3 crystals, will be employed at different experimental stations of the RIBF (F8, SAMURAI, SHARAQ), each having different performance requirements and constraints. Key experiments to be carried out in the future at the RIBF at intermediate energies involve inelastic scattering on high-Z targets to induce Coulomb excitation, as well as inelastic scattering and quasi-free scattering on liquid hydrogen.

In my presentation, I will provide an overview of the planned array, including how well its performance compares to other existing and planned gamma-ray spectrometer, and examples of possible future experiments.

Speaker: Pieter Doornenbal (RIKEN)

63 γ RIBF-UK: Scintillator-based high-resolution γ-ray spectrometer at RIBF

Speaker: M. Petri

Closing remarks

64 TBC

Speaker: Alexandre Obertelli (TU Darmstadt)

15:10 Coffee break