# Fast-timing spectroscopy with the *ν*-ball2 spectrometer

S. Pascu, C. Hiver, Zs. Podolyák, P.H. Regan, M. von Tresckow, J.N. Wilson

► To cite this version:

S. Pascu, C. Hiver, Zs. Podolyák, P.H. Regan, M. von Tresckow, et al. Fast-timing spectroscopy with the *ν*-ball2 spectrometer. 17th International Symposium on Capture Gamma Ray Spectroscopy and Related Topics (CGS17), Jul 2023, Grenoble, France. pp.01010, 10.1051/epjconf/202532901010 . hal-05381735

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# Fast-timing spectroscopy with the v-ball2 spectrometer

S. Pascu¹,²,* , C. Hiver³, Zs. Podolyák¹, P.H. Regan¹,⁴, M. von Tresckow⁵, and J.N. Wilson³

¹School of Mathematics and Physics, University of Surrey, Guildford GU2 7XH, United Kingdom

²National Institute for Physics and Nuclear Engineering, R-77125 Bucharest-Magurele, Romania

³Université Paris-Saclay, CNRS/IN2P3, IJC Laboratory, Orsay, France

⁴National Physical Laboratory, Teddington TW11 0LW, United Kingdom

⁵Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstrasse 9, 64289, Darmstadt, Germany

**Abstract.** The fast-timing method has been used in a high-statistics experiment at ALTO@IJCLab to perform lifetime measurements of excited states in exotic nuclei. These observables and their corresponding transition probabilities are key fingerprints for the structure of atomic nuclei, as they are sensitive to small components of the wave function. Neutron-rich nuclei have been populated in a two-step fission-induced experiment. In the first stage, the neutron beam was produced by the LICORNE source in an inverse kinematics reaction with a pulsed $ ^7 $Li beam delivered by the Tandem accelerator, incident on an H$_2$ gas cell. The resulting neutron beam with an average energy of about 2 MeV was used in a second step to induce fissions on the $ ^{238} $U secondary target. The hybrid high-efficiency v-ball2 array comprising 24 clover detectors and 20 UK FATIMA LaBr$_3$(Ce) scintillators was used to detect the emitted gamma rays. Improvements are discussed in comparison to the previous v-ball1 campaign and preliminary results of the current experiment are presented.

## 1 Introduction

The atomic nucleus is a quantum system whose properties are determined by the delicate interplay between single-particle and collective effects. In the vicinity of the valley of stability, where an extensive set of information has been gathered in the last decades, most modern theoretical calculations are starting to agree with each other and to give a reasonable reproduction of the data. However, the structure of exotic nuclei is far from being understood, with new emerging phenomena appearing as we move away from the stable nuclei. This happens in part because very exotic nuclei cannot be easily produced, and consequently, the experimental spectroscopic information is very sparse, sometimes only a few properties of the ground state being known.

One type of information that is missing, especially in neutron-rich nuclei, is the transition probabilities between the low-lying states. These observables can be deduced from dedicated half-lives measurements that cover a broad range, typically from nanoseconds down to the picoseconds region. As such, there are two obstacles that need to be overcome, the production mechanism of neutron-rich nuclei and an appropriate technique for lifetime measurements that covers a broad range. One method to populate these nuclei relies on the fission of heavy nuclei, traditionally done with spontaneous fission [1, 2] or from fission induced by thermal neutrons [3, 4]. A third option is to employ fission induced by fast neutrons, which will populate nuclei with a higher N/Z ratio as compared to the two

previous methods. This last technique was chosen for the present study, while the fast-timing method was selected for lifetime measurements.

The aim of the present contribution is to report on the improvements made on the v-ball γ-ray spectrometer available at ALTO that allowed population of neutron-rich nuclei with high yields, as well as to present the first preliminary results.

## 2 Experimental setup

Neutron-rich nuclei were populated in a two-step reaction. The first step consisted of generating the fast neutrons with an average energy of 1.7 MeV using the LICORNE neutron source and the p($ ^7 $Li,n)($ ^7 $Be inverse reaction [5, 6]. A pulsed beam (with a 400 ns period) of $ ^7 $Li was accelerated by the 14 MV Tandem accelerator at ALTO to an energy of 17.6 MeV and was impinged on a H$_2$ gas cell kept at 1.6 atm. One of the main advantages of using this reaction is that in inverse kinematics, the neutrons will be emitted at forward angles in a narrow cone, without interacting with the neighboring γ-ray detectors, of which especially HPGe detectors are known to exhibit a considerable worsening of their resolution when exposed to a high neutron flux. In a second step, the neutrons were used to induce fission reactions on a conical-shaped $ ^{238} $U target with a mass of 71 g. The γ rays emitted by the decaying fission products were recorded with the v-ball2 array comprising 24 Compton-shielded clover HPGe detectors from the GAMMAPOOL collaboration and 20 UK FATIMA 1.5"x2" cylindrical LaBr$_3$(Ce) scintillators [7]. Two

<footnote>*e-mail: s.pascu@surrey.ac.uk</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Comparison between the arrangement of the detectors in the *v*-ball1 (left) and *v*-ball2 (right) setups. The efficiency of the LaBr₃(Ce) array of detectors could be improved considerably by moving ten scintillators at backward angles, where they are completely outside of the neutron cone (see text for details).</figcaption></figure>

NE213 liquid scintillators were used for neutron detection, placed at 2.7 m from the reaction point. One of them was connected to an analog MPD4 Mesytec neutron-γ discrimination module and was further connected to a counter for monitoring the beam intensity. The second detector was connected to the DAQ where a digital neutron-γ discrimination and time-of-flight measurements were performed.

The signals from the preamplifiers of the HPGe detectors, as well as the anode signals of the scintillators, were processed by FASTER [8], a fully digital acquisition system developed by the LPC Caen that allows high data transfer rates. 125 MHz/14-bit MOSAHR daughterboards were used for the HPGe detectors, while the scintillator signals were processed with 500 MHz/12-bit CARAS digitizers. Energy and efficiency calibrations were performed with standard ¹⁵²Eu and ¹³³Ba sources, as well as with a natural Th source for high-energy γ rays. Time and alignment calibrations were performed with a ⁶⁰Co source to

correct for the random time differences introduced when initializing various FASTER cards.

### 3 *v*-ball1 versus *v*-ball2

The *v*-ball1 array consisted of 24 clover HPGe detectors placed at 75° and 105°, ten coaxial Phase1 HPGe detectors at 134°, while the 20 LaBr₃(Ce) scintillators were placed at forward angles, at 46°. Due to the opening cone of the neutron angular distribution of about 20°, the LaBr₃(Ce) detectors were pushed back with respect to the other experiments in the campaign that did not use a neutron beam, considerably reducing the detection efficiency. Therefore, for the *v*-ball2 setup, the arrangement of the detectors was modified, the ten coaxial Phase1 HPGe detectors were dropped, while the LaBr₃(Ce) scintillators were positioned at forward and backward angles (46° and 134°). In this way, the efficiency of the scintillators could be increased by a factor of three compared to the *v*-ball1 array. A com-

<figure><img src="image_3.png" /><figcaption>**Figure 2.** HPGe spectra from the *v*-ball1 (top) and *v*-ball2 (bottom) campaigns. Total projection of a symmetric 2D matrix (left) and gated spectra of ¹⁵²Ce (right), as well as its relevant part of the level scheme. The decrease of the contaminant transition at 351 keV of ²¹Ne is marked on the figure (left).</figcaption></figure>
<table><caption>**Table 1.** Summary of the modifications made for the v-ball2 setup in comparison with the corresponding v-ball1 array.</caption><thead><tr><th>v-ball1</th><th>v-ball2</th><th>Improvement</th></tr></thead><tbody><tr><td>Pb stopper and collimator</td><td>Au stopper and W collimator</td><td>Cleaner spectra</td></tr><tr><td>Up to 10 nA pulsed <sup>7</sup>Li beam</td><td>Up to 100 nA pulsed <sup>7</sup>Li beam</td><td>x10</td></tr><tr><td>H<sub>2</sub> gas cell pressure: 1.3 atm</td><td>H<sub>2</sub> gas cell pressure: 1.6 atm</td><td>x1.25</td></tr><tr><td>Detector geometry: 24 clovers+10 coax HPGe<br>+ 20 forward LaBr<sub>3</sub>(Ce) detectors</td><td>Detector geometry: 24 clovers<br>+20 forward-backward LaBr<sub>3</sub>(Ce) detectors</td><td>Increased efficiency for<br>LaBr<sub>3</sub>(Ce) detectors</td></tr><tr><td>Efficiency(LaBr<sub>3</sub>(Ce)) @ 1 MeV: 0.7%</td><td>Efficiency(LaBr<sub>3</sub>(Ce)) @ 1 MeV: 2.1%</td><td>x3</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Figure 3.** LaBr<sub>3</sub>(Ce) spectra from the v-ball1 (top) and v-ball2 (bottom) campaigns for <sup>140</sup>Xe, gated by the 2<sup>+</sup> → gs transition in the HPGe. Main transitions in <sup>140</sup>Xe are marked on the figure and the relevant level scheme is given on the right-hand side.</figcaption></figure>

parison between the two arrangements of the detectors is shown in Fig. 1, with v-ball1 array on the left and the v-ball2 configuration on the right.

In addition, the use of a new sputtering source allowed an increase in the beam intensity by a factor of up to 10, reaching a maximum current of 100 nA on the target. Finally, the pressure inside the H<sub>2</sub> gas cell could be increased by 25% using a tantalum foil with a 4 µm thickness.

Another issue noted in the previous campaign was the presence of strong peaks in the γ-ray spectra coming from light nuclei (e.g., <sup>18,19</sup>F, <sup>21</sup>Ne, etc.) produced in fusion evaporation reactions of the <sup>7</sup>Li beam incident on the oxide layer of the surrounding lead elements. For the v-ball1 experiment, a lead collimator placed outside LICORNE and a lead stopper placed inside the gas cell were used, resulting in a large quantity of oxide available. Therefore, in the subsequent v-ball2 setup, the lead collimator was replaced with tungsten, while the lead stopper was replaced with Au, resulting in much cleaner spectra. The quality of the spectra can be seen in the left panels of Fig. 2, where a total projection of a 2D matrix is given for v-ball1 (upper panel) and v-ball2 (lower panel). A clear decrease of the contaminant transition at 351 keV of <sup>21</sup>Ne (marked on the figure) can be observed in the v-ball2 spectrum.

A summary of the improvements made for the v-ball2 setup is presented in Table 1. The increase in statistics for HPGe detectors was assessed from gated spectra on the nuclei produced with high yield in the fission reaction

(e.g., <sup>140</sup>Xe). This comparison revealed an increase of a factor of eight in the v-ball2 campaign as compared to the v-ball1 experiment. This reflects mainly the considerable increase in the beam intensity that could be achieved. On the right side of Fig. 2 we present the effect these improvements had on <sup>152</sup>Ce, a nucleus at the edge of the mass distribution, produced with a very low yield. The gate is set on the 2<sup>+</sup> → gs transition in <sup>84</sup>Se, the fission partner of <sup>152</sup>Ce. In the upper panel corresponding to v-ball1 setup, the 4<sup>+</sup> → 2<sup>+</sup> and 6<sup>+</sup> → 4<sup>+</sup> γ rays are barely visible, while in the lower panel corresponding to the v-ball2 configuration, the peaks are considerably more prominent. The experimental data for the two level schemes were taken from Refs. [9, 10].

The comparison of the two campaigns for the LaBr<sub>3</sub>(Ce) detectors is given in Fig. 3 in the case of <sup>140</sup>Xe. An HPGe-LaBr<sub>3</sub>(Ce) matrix projection is shown with a gate on the HPGe axis on the 2<sup>+</sup> → gs transition. The 458 keV peak of the 4<sup>+</sup> → 2<sup>+</sup> γ ray is integrated in both cases, and a factor of about 25 enhancement in the case of v-ball2 configuration is found. This will translate into a factor of 72 increase in the number of HPGe-LaBr<sub>3</sub>(Ce)-LaBr<sub>3</sub>(Ce) coincidences.

## 4 Fast-timing analysis

Since the fission process produces a variety of nuclei in the medium-mass region, several physics cases have been proposed, each one addressing a specific topic. Most of them were concerned with the determination of lifetimes using the fast-timing method, while some of them addressed the building of the level scheme, determination of statistical properties or constraining the isomeric fission yield ratios. The analysis of the data is ongoing, so in this section only a generic lifetime determination will be presented that was used to check the feasibility of the technique when dealing with dense γ ray spectra as the ones produced by the deexcitation of all fission products.

The fast-timing method uses the fast response of the LaBr<sub>3</sub>(Ce) scintillators to measure the time difference between two gamma rays, one populating and the other one deexciting the level of interest. This method was introduced some decades ago for measuring lifetimes using the beta decay of neutron-rich isotopes [11, 12] and was further adapted for in-beam studies in Refs. [13, 14]. In this case, triple-γ coincidences are measured and an additional gate on the HPGe detectors, which have superior energy resolution, is used to clean the LaBr<sub>3</sub>(Ce) spectra.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Time spectrum of the first $4^+$ state in $^{134}$Te measured as the time difference between the $6_1^+ \rightarrow 4_1^+$ and $4_1^+ \rightarrow 2_1^+ \gamma$ rays, gated by the 1279 keV $2_1^+ \rightarrow \text{gs}$ transition. The relevant level scheme of $^{134}$Te is shown on the right.</figcaption></figure>

Energy calibration was made at the beginning and the end of the experiment using standard calibration sources, as described in Section 2. Due to the beam intensity fluctuations and the small drifts of the high-voltage power supply, a random gain shift can be seen in the spectra. As a result, the registered peaks are broader than their intrinsic resolution. However, this effect happens over a period of hours and allows a run-by-run gain correction, with one run typically lasting for about 1-2 hours. An automatic procedure of this correction was implemented for a broad range of energies [15].

Since the fast-timing method uses time differences of pairs of LaBr₃(Ce) detectors, the timing information has to be treated correctly. Therefore, the dependence of the time response on the energy has to be known. The so-called prompt response distribution (PRD) is determined by placing the ¹⁵²Eu source in the middle of the array and recording coincidence events. One of the detectors is chosen as a reference and an energy gate is placed on a γ transition. Then, an average energy-time difference matrix is constructed with respect to the reference detector and time differences between all the coincident γ rays are deduced. The calibrated PRD is used to extract the lifetimes using the centroid shift difference, suited for situations when the lifetime to be measured is smaller than the time resolution of the array. A careful PRD calibration was performed for the v-ball2 array using this procedure [16].

To check the performances of the v-ball2 array, the lifetime of the $4_1^+$ state in $^{134}$Te was determined. This lifetime is known from previous measurements [17] using the fast-timing method. The relevant decay scheme is presented in Fig. 4. To obtain the time spectrum, a gate is set in the HPGe detectors on the 1279 keV $2_1^+ \rightarrow \text{gs}$ transition, while the time difference is measured between the $6_1^+ \rightarrow 4_1^+$ and $4_1^+ \rightarrow 2_1^+ \gamma$ rays. All coincident events within a 400 ns time gate were considered. A preliminary result with about 30% of the statistics available in the experiment was used. An exponential plus polynomial fit is performed for the tail of the time distribution and results in a value of $T_{1/2}=1.1$ (3) ns, in reasonable agreement with the 1.36(11) ns in the literature [17]. Note that for this

preliminary analysis no background was subtracted from the HPGe or LaBr₃(Ce) gates. Future work will also take advantage of the $6^+$ isomer in the yrast band that offers a possibility to determine the half-life of the $4_1^+$ by selecting delayed coincidences which will help clean the spectra.

## 5 Conclusions

A new fast-neutron-induced fission experiment has been performed at ALTO using the v-ball2 array optimised for fast-timing measurements. Several improvements with respect to the previous campaign were made, most notable by placing the LaBr₃(Ce) detectors at forward and backward angles in a close configuration, which increased the efficiency by a factor of three. In addition, the beam intensity could be improved, as well as reducing the unwanted background from light nuclei produced in fusion-evaporation reactions. This allowed the population of exotic nuclei produced with low yields in the fission reaction with reasonable intensity. Examples of the spectra collected and the use of the fast-timing method have been given. The quality and the high statistics of the fission data obtained with the v-ball2 array represent a new benchmark for these types of experiments.

## 6 Acknowledgements

We appreciate the excellent work of the ALTO team to deliver a high-intensity beam throughout the experiment. This work was supported by the UK STFC grant No. ST/P005314.

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