# In-beam γ-ray spectroscopy with fast-neutron probes at NFS

H Sengar, E Clément, D Ackermann, O Aktas, L de Arruda, G de France, P Dessagne, G Henning, I Jangid, D Kalaydjieva, et al.

► To cite this version:

H Sengar, E Clément, D Ackermann, O Aktas, L de Arruda, et al. In-beam γ-ray spectroscopy with fast-neutron probes at NFS. Nuclear Physics A, 2025, 1061, pp.123135. 10.1016/j.nuclphysa.2025.123135. hal-05066644

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# In-beam γ-ray spectroscopy with fast-neutron probes at NFS

H. Sengar<sup>a, ID</sup>,*, E. Clément<sup>a</sup>, D. Ackermann<sup>a</sup>, O. Aktas<sup>a</sup>, L. de Arruda<sup>a</sup>, G. de France<sup>a</sup>, P. Dessagne<sup>e</sup>, G. Henning<sup>e</sup>, I. Jangid<sup>a</sup>, D. Kalaydjieva<sup>b</sup>, M. Kerveno<sup>e</sup>, N. Kumar<sup>e</sup>, X. Ledoux<sup>a</sup>, A. Lemasson<sup>a</sup>, I. Matea<sup>d</sup>, P. Miriot-jaubert<sup>c</sup>, A. Navin<sup>a</sup>, J. Piot<sup>a</sup>, D. Ramos<sup>a</sup>, T. Roger<sup>a</sup>, P. Sharma<sup>a</sup>, T. Tanaka<sup>a</sup>

## ARTICLE INFO

### Keywords:

Fast-neutron probes

Neutrons For Science (NFS)

(n,xn) reactions

In-beam γ-ray spectroscopy

HPGe

## ABSTRACT

Using the high flux of fast neutron beam provided at the Neutrons For Science (NFS) facility of GANIL-SPIRAL2, high resolution γ-ray spectroscopy of nuclei produced in the (n,xn) reaction is performed with the EXOGAM HPGe array. The nuclei produced by the (n,2n) and (n,3n) channels of <sup>58</sup>Ni are re-examined through prompt γ-ray spectroscopy in coincidence with fast neutrons to provide a comprehensive and possibly new description of its level schemes and excitation functions.

## 1. Introduction

Understanding and predicting the evolution of nuclear structure has long been a central focus of nuclear physics. Established experimental techniques such as charged particle induced reactions and fusion evaporation reactions have been used extensively for this purpose. However, the possibilities expand by fast-neutron probes as they provide a complementary path. With this goal in mind, an experiment was performed at the NFS facility [1,2] to probe nuclear structures using fast neutrons for the first time. Fast neutrons can penetrate deep into the nucleus without being deflected or slowed down by the electromagnetic field, allowing direct interaction through the strong nuclear force.

The (n,xn) reactions, while studied for cross-sectional data evaluation, have rarely been employed in nuclear structure studies. Consequently, the understanding of the associated reaction mechanism remains limited. The high energy thresholds and relatively low cross sections of these reactions make them challenging to study. However, the unprecedented neutron flux up to 40 MeV at the NFS facility allows the possibility of a high statistics measurement to perform γ-ray spectroscopy on nuclei produced through these exotic channels in the perspective of nuclear structure physics. As an initial test case, we chose to look at the well-studied Ni-shell. With <sup>nat</sup>Ni target, via the (n,3n) channel, the doubly magic <sup>56</sup>Ni was investigated for the first time using a pure neutron probe. Alongside, Co and Fe isotopes that are produced from (n,p/d/t/α) reactions will also be studied. The nuclei near <sup>56</sup>Ni are of particular interest because they are well-suited for various microscopic theoretical approaches, allowing us to explore the interplay between

<footnote><sup>a</sup> Grand Accélérateur National d'Ions Lourds, CEA/DRF - CNRS/IN2P3, Caen, France</footnote>

<footnote><sup>b</sup> University of Guelph, Canada</footnote>

<footnote><sup>c</sup> IRFU, CEA Saclay, Université Paris-Saclay, France</footnote>

<footnote><sup>d</sup> IJCLab, Université Paris-Saclay, France</footnote>

<footnote><sup>e</sup> Université de Strasbourg, CNRS, IPHC/DRS UMR 7178, 23 Rue du Loess, F-67037 Strasbourg, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: hemantika.sengar@ganil.fr (H. Sengar).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Prompt γ spectrum, gated on 2700 keV, showing the evidence of $^{56}$Ni along with other contributions.</figcaption></figure>

single-particle and collective excitations. For all the isotopes produced, the primary objective is to investigate what Yrast surface is populated by these reactions and to understand how they differ from particle transfer and heavy-ion reactions. New spectroscopic information to be collected will also be relevant for nuclear reaction tools (such as TALYS [3]) and nuclear data evaluation libraries.

## 2. Experimental setup

The neutron beam at NFS was produced by d + Be reaction, significantly higher average flux of ~10⁸ neutrons/cm² up to 40 MeV, compared to n_TOF, GELINA or WNR. In the Time of Flight (TOF) hall, at ~8 metres from the converter, a 1 mm thick $^{nat}$Ni target is placed, around which 12 EXOGAM detectors [4] were arranged. Using the Co calibration source, the absolute efficiency is determined to be (5.70 ± 0.28)% at 1332 keV. To avoid being overwhelmed by inelastic scattering events, for data taking across the full neutron energy range provided by NFS, we implemented strict neutron TOF validations (175 and 200 ns) to collect data coincident with fast neutrons. Despite the limited timing resolution of HPGe detectors, we were still able to observe a clear distinction between the fast neutron region and the γ-flash (photons produced due to reactions on the Be converter).

During an effective beam time of ~8 days with the Ni target, about 10¹⁰ γγ coincidences have been sorted after the addback procedure. Additional ~3 days of data was taken with Pb target. This is the first time EXOGAM experienced such high doses of fast neutrons, hence the corresponding resolution damage was studied extensively. All clovers were warmed, re-cooled, annealed at ~100 °C for a week and remeasured before the 2024 beam time. The Gaussian shape and the resolution of the peaks was successfully recovered.

## 3. Results and discussion

Evidence of the $^{58}$Ni(n,3n)$^{56}$Ni reaction channel was observed as shown in Fig. 1, with additional contributions leaking into the gate. Notably, $^{57}$Co accounts for ~60% of the 1224 keV yield, and the remaining 40% must come from $^{56}$Ni. Furthermore, $^{56}$Ni was also seen in the γγγ analysis, reinforcing the presence of this isotope.

Statistical model calculations show that the (n,2n) channel is two orders of magnitude higher cross-section compared to the (n,3n) channel [5], hence we focused our studies on $^{57}$Ni. The observed transitions for this isotope were compared with previous studies. From an initial analysis, none of the states populated exclusively by particle transfer reactions were observed in the present data-set. We see that the (n,2n) channel behaves similarly to heavy-ion fusion evaporation reactions, but is limited to low angular momentum. Despite these similarities, new transitions, previously not seen in heavy-ion fusion evaporation experiments, are observed in the (n,2n) channel and are to be integrated into the level scheme.

In the present experiment, 4 (8) clover detectors are placed at ~135° (90°) with respect to the neutron beam axis. Based on the directional co-relation ratio ($R_{DCO}$) calculations [6], shown in Fig. 2, the fast neutrons seem to align the nuclei like in the case of fusion evaporation of heavy-ions. Thanks to EXOGAM's ability to act as a polarimeter [7], we are also sensitive to asymmetry measurements, as used to validate the positive parity of the 3701 keV state based on $g_{2/2}$ orbital as measured by Rudolph et al. [8,9], which were different from the earlier spin and parity assignments from pick-up reactions [10] (Fig. 3).

In addition to collecting spectroscopic information, we were also able to measure excitation functions based on the event-by-event measurement of the TOF between the HPGe CFD and RF of the LINAC. Specifically, we compared the excitation function for the $5/2^-$ → $3/2^-$ transition in the (n,2n) reaction with predictions from TENDL data [5] and observed significantly higher values. This discrepancy highlights the need for further refinement in theoretical models to accurately represent the experimental data.

## 4. Conclusions and prospects

The experiment was successfully conducted, demonstrating that high-resolution γ-ray spectrometry is feasible even for fast neutron-induced reactions. Preliminary results suggest that the (n,2n) channel resembles heavy-ion fusion evaporation, though limited to low angular momentum. The present work indicates that the reactions induced by fast neutrons produce aligned nuclei. We
<figure><img src="image_5.png" /><figcaption>Fig. 2. Measured DCO ratios for known dipole and quadrupole transitions using gates on stretched quadrupoles.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Asymmetry ratio as a function of the DCO ratio for the transition from the 3701 keV state in $^{57}$Ni.</figcaption></figure>

also potentially observed new γ transitions that could be integrated into the level scheme, with the possibility of assigning spin and parity. Further studies are planned, including neutron-rich Co isotopes from $^{60}$Ni and data collected with a Pb target.

Present work is an initial effort of a bigger goal to investigate the interplay between direct and compound nuclear reaction mechanisms. Methods to approach this complex problem are being explored.

### CRediT authorship contribution statement

**H. Sengar:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Shifts in experimental data taking: all authors.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

We acknowledge the GANIL facility and the NFS team for providing the neutron beam, and the EXOGAM collaboration for their support.

### Data availability

Data will be made available on request.

### References

[1] X. Ledoux, et al., EPJ Web Conf. 146 (2017) 03003.

[2] X. Ledoux, et al., EPJ A 57 (8) (2021) 257.

[3] A. Koning, et al., EPJ A 59 (6) (2023) 131.

[4] F. Azaiez, et al., Nucl. Phys. A 654 (1) (1999) 1003c–1008c.

[5] A.J. Koning, et al., Nucl. Data Sheets 155 (2019) 1–55.

[6] K.S. Krane, et al., At. Data Nucl. Data Tables 11 (5) (1973) 351–406.

[7] Y. Zheng, et al., Phys. Rev. C 87 (4) (Apr. 2013) 044328.

[8] D. Rudolph, et al., EPJ A 4 (2) (1999) 115–145.

[9] D. Rudolph, et al., EPJ A 6 (4) (1999) 377–380.

[10] M.R. Bhat, et al., Nucl. Data Sheets 85 (1998) 415.