Development of (p,p′γ) detection capabilities at iThemba LABS through the study of low-lying E1 strength in 58Ni

This work is a feasibility study designed to test and develop the (p,p′γ) detection capabilities at iThemba LABS when the K600 magnetic spectrometer at 0° is coupled to an array of high purity germanium and cerium-doped lanthanum bromide detectors. This is done through an investigation into the low-lying E1 strength of 58Ni with a proton beam of E p = 80 MeV. The coincidence matrix of the energies of the excited states and the subsequent γ decays is presented and decays to the first excited state and ground state were identified. The results of this study are compared with previous (α, α′γ) studies.


Introduction
The detection of medium-energy hadron scattering at 0 • presents an advantage of selectivity to excitations with low spin transfer [1].This selectivity can be further improved for the study of low-lying states in nuclei by coupling particle spectrometers with γ-ray detectors.In two earlier experiments, the K600 magnetic spectrometer at iThemba LABS was successfully coupled with γ-ray detectors for α-γ coincident measurements.In this feasibility study, the (p,p ′ γ) measurement capabilities at 0 • are tested and exploited for the characterisation of the decay path of low-lying E1 excitations in 58 Ni.Coincident measurements allow for the determination of branching ratios, the assignment of multipolarities and the investigation of the isospin character in nuclei of interest [2].
The low-lying E1 strength being investigated here is commonly referred to as the Pygmy Dipole Resonance (PDR) and it lies below the Giant Dipole Resonance (GDR).In the naive picture it is characterised by the out-of-phase oscillation of an N=Z core against the neutron skin in nuclei and it is attributed to a concentration of J π = 1 − states located below the neutron separation energy in nuclei [3,4].The PDR is linked to the symmetry term of the nuclear binding energy and the nuclear equation of state [5,6].The neutron capture rate in astrophysical processes is affected by the presence of increased dipole strength close to the neutron separation threshold [7].

Experimental setup
An 80-MeV proton beam from the Separator Sector Cyclotron (SSC) was used to excite a 5 mg/cm 2 thick 58 Ni target.The spectrometer was operated in the high-dispersion mode.Twelve high-resolution high purity germanium (HPGe) detectors and five high-efficiency cerium-doped lanthanum bromide (LaBr 3 :Ce) detectors composed the BaGeL (Ball of Germanium and LaBr) array.The HPGe detectors were placed at distances ranging from 17-19.5 cm away from the target at backward angles, while four of the five LaBr 3 :Ce detectors were placed at 20 cm and the other one at 13.2 cm.One Multi-Wire Drift Chamber (MWDC) was used at the focal plane of the spectrometer for position measurements, while the coincidence signal of the two scintillator detectors, just downstream of the MWDC, was used to provide event trigger signals for the Maximum Integrated Data Acquisition System (MIDAS) [8].The proton-γ measurement was K600-trigger based such that when there was an event at the focal plane that triggered the acquisition system, the γ-ray event in the BaGeL detectors that fell within an appropriately time delayed window was recorded and represented the coincidence data.

Data analysis, results discussions and outlook
The coincidence events are identified through the γ-time spectrum.This spectrum represents the time difference between the trigger timing of the spectrometer and the time the γ-rays are detected.The true p-γ coincidence events come from the same beam burst and are found on the prompt peak of the γ-time.To discriminate these true coincidences from random events, the coincidence matrix is gated on the prompt peak.Matrices gated on the prompt peak and a random peak are shown in Fig. 1.The random coincidence matrix shows that indeed no coincidence events of interest are found outside the prompt peak and the background is uniformly distributed across the excitation energy.Nonetheless, a detailed investigation has to be conducted to improve the γ-detector shielding since it is suspected that the random gamma rays could come from scattered protons off the collimator.The typical time resolution of the HPGe and the LaBr 3 :Ce are 37 ns and 4 ns, respectively.Below the particle threshold, the low-lying E1 excitations mostly decay to the ground state and states with very low spin relative to the ground state [2].Therefore, being able to isolate the decays to these states is crucial for studies of the PDR.In the coincidence matrix shown in Fig. 1  resolution are included.On the diagonal bands, the locus that can be observed at E x = 3.27 MeV and 4.76 MeV are thinner along the y-axis than they are along x -axis.This is because the energy resolution of the γ-ray spectrum (of the HPGe detectors) is 28 keV compared to the 48 keV of the corresponding excitation-energy spectrum.The γ-ray efficiency of the 12 HPGe detectors was ∼0.63% at E γ =1.8 MeV.It should be noted that the γ-ray decay data from the LaBr 3 :Ce are still under analysis at this point; however, the obtained energy resolution of the preliminary photo-peaks is currently ∼ 54 keV.The excitation-energy spectrum shown in Fig. 2 was also gated on the prompt peak of the γ-time to discriminate against beam halo and random coincidences.Two states, at 8.24 ± 0.004 MeV and 8.51 ± 0.002 MeV were assigned J π = 1 − with reference to NuDat of the NNDC [9] and could possibly be associated with the PDR.The first coincidence measurements to study low-energy E1 strength in 58 Ni were done by T.D. Poelhekken et al. [10] at Groningen through an (α,α'γ) reaction.An extension of that work was conducted by D. Savran et al. [2].In these two studies, a prominent J π = 1 − 1 at 6.027 MeV was observed.This peak could be hidden or not excited in this present work.However, it is suspected that the structure in Fig. 2 that is marked with a vertical line could be the 6.027 MeV state.Two states at 5.906 MeV and 5.942 MeV corresponding to J π =2 + (11) and J π =0 +

Figure 1 :
Figure 1: Two-dimensional coincidence matrices of the γ-ray decay in the HPGe detectors against the excitation energy where the matrix is gated on the prompt peak of the γ-time (left panel) and the matrix is gated on a random peak (right panel).
(left), the upper and lower diagonal bands represent decay to the 0 + 1 (ground state) and 2 + 1 (first-excited state), respectively.The diagonals are produced by imposing the conditions |E x − E γ | < 200 keV and |E x − (E γ + 1454.21)|< 200 keV for the upper and lower band, respectively, where 1454.21keV is the energy of the first excited state of 58 Ni.The diagonal lines were opened 400 keV wide in order to ensure that all the events that are within the detection 28th International Nuclear Physics Conference (INPC 2022) Journal of Physics: Conference Series 2586 (2023) 012069

Figure 2 :
Figure 2: The excitation-energy spectrum of 58 Ni showing the two identified states with J π = 1 − just above the proton threshhold.

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respectively, were observed in D. Savran et al. and this work.The comparison with this recent study in terms of tracking different transitions is, however, limited due to the low statistics of the (p,p'γ) γ-decay spectrum.The (p,p'γ) experiment was considered a commission run and only 11.5 hours of continuous beam time was provided.For further analysis, a longer beam time was requested.It is anticipated that developing this detection system can be helpful for experiments where the interest is in resolving high density nuclear states.