# Experimental study of <sup>4</sup>n with <sup>8</sup>He(p,2p) reaction

S.W. Huang, Z.H. Yang, F.M. Marqués, N.L. Achouri, D.S. Ahn, T. Aumann, H. Baba, D. Beaumel, M. Böhmer, K. Boretzky, et al.

► To cite this version:

S.W. Huang, Z.H. Yang, F.M. Marqués, N.L. Achouri, D.S. Ahn, et al. Experimental study of <sup>4</sup>n with <sup>8</sup>He(p,2p) reaction. 27th International Nuclear Physics Conference (INPC2019), Jul 2019, Glasgow, United Kingdom. pp.012090, 10.1088/1742-6596/1643/1/012090. in2p3-03195738

HAL Id: in2p3-03195738

https://hal.in2p3.fr/in2p3-03195738

Submitted on 24 May 2024

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Experimental study of <sup>4</sup>n with <sup>8</sup>He(p,2p) reaction

S.W. Huang, Z.H. Yang, F.M. Marqués, N.L. Achouri, D.S. Ahn, T. Aumann, H. Baba, D. Beaumel, M. Böhmer, K. Boretzky, et al.

► To cite this version:

S.W. Huang, Z.H. Yang, F.M. Marqués, N.L. Achouri, D.S. Ahn, et al. Experimental study of <sup>4</sup>n with <sup>8</sup>He(p,2p) reaction. 27th International Nuclear Physics Conference (INPC2019), Jul 2019, Glasgow, United Kingdom. pp.012090, 10.1088/1742-6596/1643/1/012090. in2p3-03195738

HAL Id: in2p3-03195738

https://in2p3.hal.science/in2p3-03195738v1

Submitted on 24 May 2024

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
PAPER • OPEN ACCESS

Experimental study of $^4n$ with $^8He(p,2p)$ reaction

To cite this article: S. W. Huang et al 2020 J. Phys.: Conf. Ser. **1643** 012090

View the [article online](#) for updates and enhancements.

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# Experimental study of $^4n$ with $^8He(p,2p)$ reaction

To cite this article: S. W. Huang et al 2020 J. Phys.: Conf. Ser. **1643** 012090

View the [article online](#) for updates and enhancements.

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Experimental study of $^4n$ with $^8\text{He}(p,2p)$ reaction

S. W. Huang$^{1,2}$, Z. H. Yang$^{3,2,*}$, F. M. Marqués$^4$, N. L. Achouri$^4$, D. S. Ahn$^2$, T. Aumann$^{5,6}$, H. Baba$^2$, D. Beaumel$^7$, M. Böhmer$^8$, K. Boretzky$^{6,2}$, M. Caamaño$^9$, S. Chen$^{10}$, N. Chiga$^2$, M. L. Cortés$^2$, D. Cortina$^9$, P. Doornenbal$^2$, C. A. Douma$^{11}$, F. Dufter$^8$, J. Feng$^{1,2}$, B. Fernández-Domínguez$^9$, Z. Elekes$^{12,2}$, U. Forsberg$^{13,25}$, T. Fujino$^{14}$, N. Fukuda$^2$, I. Gašparić$^{15,2}$, Z. Ge$^2$, R. Gernhäuser$^8$, J. M. Gheller$^{16}$, J. Gibelin$^4$, A. Gillibert$^{16}$, B. M. Godoy$^4$, Z. Halász$^{12}$, T. Harada$^{17,2}$, M. N. Harakeh$^{6,11}$, A. Hirayama$^{18,2}$, N. Inabe$^2$, T. Isobe$^2$, J. Kahlbow$^{5,2}$, N. Kalantar-Nayestanaki$^{11}$, D. Kim$^{19}$, S. Kim$^{19}$, M. A. Knösel$^5$, T. Kobayashi$^{20}$, Y. Kondo$^{18}$, P. Koseoglou$^{5,6}$, Y. Kubota$^2$, I. Kuti$^{12}$, C. Lehr$^{5,2}$, P. J. Li$^{10}$, Y. Liu$^{1,2}$, Y. Maeda$^{21}$, S. Masuoka$^{22}$, M. Matsumoto$^{18,2}$, J. Mayer$^{23}$, H. Miki$^{18}$, M. Miwa$^{24,2}$, I. Murray$^2$, T. Nakamura$^{18}$, A. Obertelli$^5$, N. Orr$^4$, H. Otsu$^2$, V. Panin$^2$, S. Park$^{19}$, M. Parlog$^4$, S. Paschalis$^{5,13}$, M. Potlog$^{26}$, S. Reichert$^8$, A. Revel$^{27}$, D. Rossi$^5$, A. Saito$^{18}$, M. Sasano$^2$, H. Sato$^2$, H. Scheit$^5$, F. Schindler$^8$, T. Shimada$^{18}$, Y. Shimizu$^2$, S. Shimoura$^{22}$, I. Stefan$^7$, S. Storck$^5$, L. Stuhl$^{22}$, H. Suzuki$^2$, D. Symochko$^5$, H. Takeda$^2$, S. Takeuchi$^{18}$, J. Tanaka$^{5,6}$, Y. Togano$^{14,2}$, T. Tomai$^{18,2}$, H. T. Törnqvsit$^{5,2}$, J. Tscheuschner$^5$, T. Uesaka$^2$, V. Wagner$^5$, K. Wimmer$^{22}$, H. Yamada$^{18}$, B. Yang$^{1,2}$, L. Yang$^{22}$, Y. Yasuda$^{18,2}$, K. Yoneda$^2$, L. Zanetti$^{5,2}$, J. Zenihiro$^{28,2}$, T. Elidiano$^4$ and C. Lenain$^4$

<figure><img src="image_5.png" /></figure>

<footnote>$^1$ School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China</footnote>

<footnote>$^2$ RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan</footnote>

<footnote>$^3$ Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan</footnote>

<footnote>$^4$ LPC Caen, ENSICAEN, Université de Caen, CNRS/IN2P3, F-14050 CAEN Cedex, France</footnote>

<footnote>$^5$ Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany</footnote>

<footnote>$^6$ GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany</footnote>

<footnote>$^7$ Institut de Physique Nucléaire Orsay, IN2P3-CNRS, 91406 Orsay Cedex, France</footnote>

<footnote>$^8$ Technische Universität München, 85748 Garching, Germany</footnote>
Experimental study of $^4n$ with $^8\text{He}(p,2p)$ reaction

S. W. Huang$^{1,2}$, Z. H. Yang$^{3,2,*}$, F. M. Marqués$^4$, N. L. Achouri$^4$, D. S. Ahn$^2$, T. Aumann$^{5,6}$, H. Baba$^2$, D. Beaumel$^7$, M. Böhmer$^8$, K. Boretzky$^{6,2}$, M. Caamaño$^9$, S. Chen$^{10}$, N. Chiga$^2$, M. L. Cortés$^2$, D. Cortina$^9$, P. Doornenbal$^2$, C. A. Douma$^{11}$, F. Dufter$^8$, J. Feng$^{1,2}$, B. Fernández-Domínguez$^9$, Z. Elekes$^{12,2}$, U. Forsberg$^{13,25}$, T. Fujino$^{14}$, N. Fukuda$^2$, I. Gašparić$^{15,2}$, Z. Ge$^2$, R. Gernhäuser$^8$, J. M. Gheller$^{16}$, J. Gibelin$^4$, A. Gillibert$^{16}$, B. M. Godoy$^4$, Z. Halász$^{12}$, T. Harada$^{17,2}$, M. N. Harakeh$^{6,11}$, A. Hirayama$^{18,2}$, N. Inabe$^2$, T. Isobe$^2$, J. Kahlbow$^{5,2}$, N. Kalantar-Nayestanaki$^{11}$, D. Kim$^{19}$, S. Kim$^{19}$, M. A. Knösel$^5$, T. Kobayashi$^{20}$, Y. Kondo$^{18}$, P. Koseoglou$^{5,6}$, Y. Kubota$^2$, I. Kuti$^{12}$, C. Lehr$^{5,2}$, P. J. Li$^{10}$, Y. Liu$^{1,2}$, Y. Maeda$^{21}$, S. Masuoka$^{22}$, M. Matsumoto$^{18,2}$, J. Mayer$^{23}$, H. Miki$^{18}$, M. Miwa$^{24,2}$, I. Murray$^2$, T. Nakamura$^{18}$, A. Obertelli$^5$, N. Orr$^4$, H. Otsu$^2$, V. Panin$^2$, S. Park$^{19}$, M. Parlog$^4$, S. Paschalis$^{5,13}$, M. Potlog$^{26}$, S. Reichert$^8$, A. Revel$^{27}$, D. Rossi$^5$, A. Saito$^{18}$, M. Sasano$^2$, H. Sato$^2$, H. Scheit$^5$, F. Schindler$^8$, T. Shimada$^{18}$, Y. Shimizu$^2$, S. Shimoura$^{22}$, I. Stefan$^7$, S. Storck$^5$, L. Stuhl$^{22}$, H. Suzuki$^2$, D. Symochko$^5$, H. Takeda$^2$, S. Takeuchi$^{18}$, J. Tanaka$^{5,6}$, Y. Togano$^{14,2}$, T. Tomai$^{18,2}$, H. T. Törnqvsit$^{5,2}$, J. Tscheuschner$^5$, T. Uesaka$^2$, V. Wagner$^5$, K. Wimmer$^{22}$, H. Yamada$^{18}$, B. Yang$^{1,2}$, L. Yang$^{22}$, Y. Yasuda$^{18,2}$, K. Yoneda$^2$, L. Zanetti$^{5,2}$, J. Zenihiro$^{28,2}$, T. Elidiano$^4$ and C. Lenain$^4$

<figure><img src="image_5.png" /></figure>

<footnote>$^1$ School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China</footnote>

<footnote>$^2$ RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan</footnote>

<footnote>$^3$ Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan</footnote>

<footnote>$^4$ LPC Caen, ENSICAEN, Université de Caen, CNRS/IN2P3, F-14050 CAEN Cedex, France</footnote>

<footnote>$^5$ Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany</footnote>

<footnote>$^6$ GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany</footnote>

<footnote>$^7$ Institut de Physique Nucléaire Orsay, IN2P3-CNRS, 91406 Orsay Cedex, France</footnote>

<footnote>$^8$ Technische Universität München, 85748 Garching, Germany</footnote>
⁹ Departamento de Física de Partículas and IGFAE, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain

¹⁰ Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China

¹¹ KVI-CART, University of Groningen, Zernikelaan 25, 9747 AA Groningen, The Netherlands

¹² MTA ATOMKI, 4001 Debrecen, Hungary

¹³ Department of Physics, University of York, York YO10 5DD, United Kingdom

¹⁴ Department of Physics, Rikkyo University, Toshima, Tokyo 172-8501, Japan

¹⁵ Ruđer Bošković Institut (RBI), Zagreb, Croatia

¹⁶ CEA, Centre de Saclay, IRFU, F-91191 Gif-sur-Yvette, France

¹⁷ Toho University, Tokyo 143-8540, Japan

¹⁸ Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

¹⁹ Department of Physics, Ehwa Womans University, Seoul, Korea

²⁰ Department of Physics, Tohoku University, Miyagi 980-8578, Japan

²¹ Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192, Japan

²² Center for Nuclear Study, University of Tokyo, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

²³ Institut für Kernphysik, Universität zu Köln, Köln, Germany

²⁴ Department of Physics, Saitama University, Shimo-Okubo 255, Sakura-ku, Saitama-shi 338-8570, Japan

²⁵ Department of Physics, Lund University, 22100 Lund, Sweden

²⁶ Institute of Space Sciences, Magurele, Romania

²⁷ Grand Accélérateur National d'Ions Lourds (GANIL), CEA/DRF-CNRS/IN2P3, Bvd Henri Becquerel, 14076 Caen, France

²⁸ Kyoto University, Kyoto 606-8501, Japan

E-mail: *zaihong.yang@riken.jp

**Abstract.** The tetraneutron has attracted the attention of nuclear physicists during the past decades, but there is still no unambiguous confirmation of its existence or non-existence. A new experiment based on ⁸He(p,2p)⁷H{t+⁴n} reaction, with direct detection of the four neutrons, has been carried out at RIBF, which can hopefully help to draw a definite conclusion on the tetraneutron system.

# 1. Introduction

Many-neutron systems made of the chargeless neutrons, especially the tetraneutron($^{4}$n), have attracted a lot of attention of the nuclear physics community in recent years. Their existence, whether as bound or resonant states, is of fundamental importance in nuclear physics, serving as a sensitive probe to investigate the nuclear force free from the Coulomb interaction. Their properties are also crucial for a deeper understanding of neutron stars [1,2].
⁹ Departamento de Física de Partículas and IGFAE, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain

¹⁰ Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China

¹¹ KVI-CART, University of Groningen, Zernikelaan 25, 9747 AA Groningen, The Netherlands

¹² MTA ATOMKI, 4001 Debrecen, Hungary

¹³ Department of Physics, University of York, York YO10 5DD, United Kingdom

¹⁴ Department of Physics, Rikkyo University, Toshima, Tokyo 172-8501, Japan

¹⁵ Ruđer Bošković Institut (RBI), Zagreb, Croatia

¹⁶ CEA, Centre de Saclay, IRFU, F-91191 Gif-sur-Yvette, France

¹⁷ Toho University, Tokyo 143-8540, Japan

¹⁸ Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

¹⁹ Department of Physics, Ehwa Womans University, Seoul, Korea

²⁰ Department of Physics, Tohoku University, Miyagi 980-8578, Japan

²¹ Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192, Japan

²² Center for Nuclear Study, University of Tokyo, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

²³ Institut für Kernphysik, Universität zu Köln, Köln, Germany

²⁴ Department of Physics, Saitama University, Shimo-Okubo 255, Sakura-ku, Saitama-shi 338-8570, Japan

²⁵ Department of Physics, Lund University, 22100 Lund, Sweden

²⁶ Institute of Space Sciences, Magurele, Romania

²⁷ Grand Accélérateur National d'Ions Lourds (GANIL), CEA/DRF-CNRS/IN2P3, Bvd Henri Becquerel, 14076 Caen, France

²⁸ Kyoto University, Kyoto 606-8501, Japan

E-mail: *zaihong.yang@riken.jp

**Abstract.** The tetraneutron has attracted the attention of nuclear physicists during the past decades, but there is still no unambiguous confirmation of its existence or non-existence. A new experiment based on ⁸He(p,2p)⁷H{t+⁴n} reaction, with direct detection of the four neutrons, has been carried out at RIBF, which can hopefully help to draw a definite conclusion on the tetraneutron system.

# 1. Introduction

Many-neutron systems made of the chargeless neutrons, especially the tetraneutron($^{4}$n), have attracted a lot of attention of the nuclear physics community in recent years. Their existence, whether as bound or resonant states, is of fundamental importance in nuclear physics, serving as a sensitive probe to investigate the nuclear force free from the Coulomb interaction. Their properties are also crucial for a deeper understanding of neutron stars [1,2].
Many experimental trials have been made in search of the very exotic $^4n$ state in the past decades. However, all these attempts failed to draw a firm conclusion due to the extremely low statistics. In 2002, Marqués et al. [3] reported the possible existence of a bound or low-lying resonant $^4n$ state observed in the breakup reaction of $^{14}$Be → $^{10}$Be + $^4n$ channel. Another experiment using the $^4$He($^8$He, $^8$Be)$^4n$ reaction found the candidate resonant state with an energy $E_R = 0.83 \pm 0.65(\text{stat}) \pm 1.25(\text{syst})$ MeV above the $4n$ threshold and a width $\Gamma \le 2.6$ MeV [4].

Motivated by the experimental hints, many theoretical calculations were performed to study the tetraneutron system [5-11]. All of them agree that a bound state is ruled out based on standard nuclear forces but the existence of a tetraneutron as a low-lying resonant state is still under debate. It is supported by some theoretical models including Quantum Monte Carlo (QMC) [12] and No-Core Shell Model (NCSM) calculations [13] while some other ab-initio calculations exclude such a resonant $^4n$ state [8,9] since large (unrealistic) modifications of the three-body force would be necessary in order to reproduce the $^4n$ resonance reported in [4].

Here, we report a new experiment on $^4n$ by using the $^8$He($p,2p$)$^7$H{$t+^4n$} reaction at the RIKEN Radioactive Isotope Beam Factory (RIBF) facility.

## 2. Experimental Methods

The $^8$He($p,2p$)$^7$H{$t+^4n$} experiment was carried out in inverse kinematics at RIBF in 2017. The $^8$He secondary beam with an energy of 150 MeV/nucleon was produced through the projectile fragmentation reaction from the $^{18}$O primary beam bombarding on a $^9$Be primary target, and then purified and transported through the BigRIPS fragment separator [14]. The incident beam can be identified by TOF-$\Delta E$ method on an event-by-event basis. The trajectories of beam particles can be reconstructed from two multi-wire drift chambers (BDC1, BDC2) located upstream of the target. The $^8$He beam with an intensity of $10^5$ pps impinged onto the 150 mm-thick liquid hydrogen target MINOS [15] which can offer high luminosity and $^7$H was then produced by the ($p,2p$) reaction.

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Schematic view of the experimental setup.</figcaption></figure>

Figure 1 shows the schematic view of the experimental setup. The key ingredient of our experiment is the kinematically complete measurement of all the reaction products. The recoil
Many experimental trials have been made in search of the very exotic $^4n$ state in the past decades. However, all these attempts failed to draw a firm conclusion due to the extremely low statistics. In 2002, Marqués et al. [3] reported the possible existence of a bound or low-lying resonant $^4n$ state observed in the breakup reaction of $^{14}$Be → $^{10}$Be + $^4n$ channel. Another experiment using the $^4$He($^8$He, $^8$Be)$^4n$ reaction found the candidate resonant state with an energy $E_R = 0.83 \pm 0.65(\text{stat}) \pm 1.25(\text{syst})$ MeV above the $4n$ threshold and a width $\Gamma \le 2.6$ MeV [4].

Motivated by the experimental hints, many theoretical calculations were performed to study the tetraneutron system [5-11]. All of them agree that a bound state is ruled out based on standard nuclear forces but the existence of a tetraneutron as a low-lying resonant state is still under debate. It is supported by some theoretical models including Quantum Monte Carlo (QMC) [12] and No-Core Shell Model (NCSM) calculations [13] while some other ab-initio calculations exclude such a resonant $^4n$ state [8,9] since large (unrealistic) modifications of the three-body force would be necessary in order to reproduce the $^4n$ resonance reported in [4].

Here, we report a new experiment on $^4n$ by using the $^8$He($p,2p$)$^7$H${t+^4n}$ reaction at the RIKEN Radioactive Isotope Beam Factory (RIBF) facility.

## 2. Experimental Methods

The $^8$He($p,2p$)$^7$H${t+^4n}$ experiment was carried out in inverse kinematics at RIBF in 2017. The $^8$He secondary beam with an energy of 150 MeV/nucleon was produced through the projectile fragmentation reaction from the $^{18}$O primary beam bombarding on a $^9$Be primary target, and then purified and transported through the BigRIPS fragment separator [14]. The incident beam can be identified by TOF-$\Delta E$ method on an event-by-event basis. The trajectories of beam particles can be reconstructed from two multi-wire drift chambers (BDC1, BDC2) located upstream of the target. The $^8$He beam with an intensity of $10^5$ pps impinged onto the 150 mm-thick liquid hydrogen target MINOS [15] which can offer high luminosity and $^7$H was then produced by the ($p,2p$) reaction.

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Schematic view of the experimental setup.</figcaption></figure>

Figure 1 shows the schematic view of the experimental setup. The key ingredient of our experiment is the kinematically complete measurement of all the reaction products. The recoil
protons were tracked by the TPC surrounding the liquid hydrogen target and then detected in coincidence by an array of 36 NaI crystals [16], arranged in two symmetric rings. The energy resolution of the NaI scintillators was around 1% (FWHM) for 80 MeV protons. Energy calibration was performed by measuring the proton-proton elastic scattering at 175 MeV with the same setup. The trajectories of two protons are essential to reconstruct the reaction vertex in such experiments with a thick target.

Charged fragments were deflected in the SAMURAI [17] dipole magnet from the path of the neutrons. They passed through two drift chambers (FDC1, FDC2) located at the entrance and exit of the dipole magnet and finally detected by the HODO plastic scintillator array. The multiple neutron detection is crucial but extremely challenging in this kind of multi-neutron studies. The neutrons were detected by two plastic scintillator arrays, the NeuLAND demonstrator from GSI and the existing NEBULA array, placed downstream of the dipole magnet, which can together provide the highest 4n detection efficiency ($\epsilon_{4n} \sim 1\%$) at present. In addition, since we have access to the complete 7-body kinematics of the final state $(2p+t+4n)$, we can also obtain the invariant mass of $^7$H and $^4n$ by measuring only 3 of the 4 neutrons. The statistics can be enhanced markedly by this so-called “Missing-Invariant-Mass method” since the detection efficiency close to the threshold for 3n can be 10 times or more higher than that for 4n.

### 3. Preliminary results

As shown in figure 2(a), tritons and $^6$He can be separated clearly using the TOF-$\Delta E$ method. Figure 2(b) shows the polar-angle correlation for the two recoil protons in coincidence with triton fragments.

<figure><img src="image_7.png" /><figcaption>**Figure 2.** (a) PID of fragments identified by HODO. (b) $p-p$ polar-angle correlation.</figcaption></figure>

We first analyzed the $^6$He+$n$ channel, populated in the ($p,pn$) reaction, to validate the momentum analysis of fragments and neutrons. As shown in figure 3(a), the relative-energy spectrum of $^7$He reconstructed from $^6$He and one neutron exhibits a clear peak at around 0.4 MeV, in good agreement with previous works [18,19]. We also reconstructed the angular distribution of the polar angle $\psi$ defined as the angle between the $^7$He momentum $p_{^7\text{He}}$ and $^6$He-$n$ relative momentum $p_{^6\text{He}-n}$ [20]. As shown in figure 3(b), it is anisotropic but symmetric with respect to 90°, consistent with previous work [19,21].
protons were tracked by the TPC surrounding the liquid hydrogen target and then detected in coincidence by an array of 36 NaI crystals [16], arranged in two symmetric rings. The energy resolution of the NaI scintillators was around 1% (FWHM) for 80 MeV protons. Energy calibration was performed by measuring the proton-proton elastic scattering at 175 MeV with the same setup. The trajectories of two protons are essential to reconstruct the reaction vertex in such experiments with a thick target.

Charged fragments were deflected in the SAMURAI [17] dipole magnet from the path of the neutrons. They passed through two drift chambers (FDC1, FDC2) located at the entrance and exit of the dipole magnet and finally detected by the HODO plastic scintillator array. The multiple neutron detection is crucial but extremely challenging in this kind of multi-neutron studies. The neutrons were detected by two plastic scintillator arrays, the NeuLAND demonstrator from GSI and the existing NEBULA array, placed downstream of the dipole magnet, which can together provide the highest 4n detection efficiency ($\epsilon_{4n} \sim 1\%$) at present. In addition, since we have access to the complete 7-body kinematics of the final state $(2p+t+4n)$, we can also obtain the invariant mass of $^7$H and $^4n$ by measuring only 3 of the 4 neutrons. The statistics can be enhanced markedly by this so-called “Missing-Invariant-Mass method” since the detection efficiency close to the threshold for 3n can be 10 times or more higher than that for 4n.

### 3. Preliminary results

As shown in figure 2(a), tritons and $^6$He can be separated clearly using the TOF-$\Delta E$ method. Figure 2(b) shows the polar-angle correlation for the two recoil protons in coincidence with triton fragments.

<figure><img src="image_7.png" /><figcaption>**Figure 2.** (a) PID of fragments identified by HODO. (b) $p-p$ polar-angle correlation.</figcaption></figure>

We first analyzed the $^6$He+$n$ channel, populated in the ($p,pn$) reaction, to validate the momentum analysis of fragments and neutrons. As shown in figure 3(a), the relative-energy spectrum of $^7$He reconstructed from $^6$He and one neutron exhibits a clear peak at around 0.4 MeV, in good agreement with previous works [18,19]. We also reconstructed the angular distribution of the polar angle $\psi$ defined as the angle between the $^7$He momentum $p_{^7\text{He}}$ and $^6$He-$n$ relative momentum $p_{^6\text{He}-n}$ [20]. As shown in figure 3(b), it is anisotropic but symmetric with respect to 90°, consistent with previous work [19,21].
<figure><img src="image_8.png" /><figcaption>**Figure 3.** (a) Relative-energy spectrum of $^7$He. (b) Polar angular distribution for $^7$He decaying into $^6$He+$n$.</figcaption></figure>

The multi-neutron analysis is now in progress, for which rejection of crosstalk is essential. A crosstalk rejection algorithm based on the time-space separation and the energy deposition of the recorded hitting signals has been well established [22] and will be optimized in the current measurement according to the real experimental setup.

## Acknowledgements

We acknowledge the support of the RIBF accelerator staff and the BigRIPS team for providing the high-quality beam. Z. H. Yang acknowledges the financial support from the Foreign Postdoctoral Researcher program of RIKEN. T. Aumann acknowledges the support by DFG via SFB 1245. P. Koseoglou acknowledges the support from BMBF (NUSTAR.DA grant No.05P 15RDFN1).

## References

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[2] Brown B A 2000 *Phys. Rev. L* **85** 5296

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<figure><img src="image_8.png" /><figcaption>**Figure 3.** (a) Relative-energy spectrum of $^7$He. (b) Polar angular distribution for $^7$He decaying into $^6$He+$n$.</figcaption></figure>

The multi-neutron analysis is now in progress, for which rejection of crosstalk is essential. A crosstalk rejection algorithm based on the time-space separation and the energy deposition of the recorded hitting signals has been well established [22] and will be optimized in the current measurement according to the real experimental setup.

## Acknowledgements

We acknowledge the support of the RIBF accelerator staff and the BigRIPS team for providing the high-quality beam. Z. H. Yang acknowledges the financial support from the Foreign Postdoctoral Researcher program of RIKEN. T. Aumann acknowledges the support by DFG via SFB 1245. P. Koseoglou acknowledges the support from BMBF (NUSTAR.DA grant No.05P 15RDFN1).

## References

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[2] Brown B A 2000 *Phys. Rev. L* **85** 5296

[3] Marqués F M et al. 2002 *Phys. Rev. C* **65** 044006

[4] Kisamori K et al. 2016 *Phys. Rev. L* **116** 052501

[5] Bertulani C A and Zelevinsky V 2003 *J. Phys. G* **29** 2431

[6] Timofeyuk N K 2003 *J. Phys. G* **29** L9

[7] Sofianos S A, Rakityansky S A and Vermaak G P 1997 *J. Phys. G* **23** 1619

[8] Lazauskas R and Carbonell J 2005 *Phys. Rev. C* **72** 034003

[9] Hiyama E, Lazauskas R, Carbonell J and Kamimura M 2016 *Phys. Rev. C* **93** 044004

[10] Grigorenko L V, Timofeyuk N K and Zhukov M V 2004 *Eur. Phys. J. A* **19** 187

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[12] Pieper S C 2003 *Phys. Rev. L* **90** 252501

[13] Shirokov A M et al. 2016 *Phys. Rev. L* **117** 182502
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[22] Nakamura T and Kondo Y 2016 *Nucl. Instrum. Methods B* **376** 156-61