Study of the Pygmy Dipole Resonance in ( p , p ’ γ ) and ( d , p γ ) experiments with SONIC @ HORUS

Last year, the new silicon-detector array SONIC with up to 8 silicon-detector positions was installed inside the existing γ-ray spectrometer HORUS consisting of 14 HPGe detectors. The combined setup SONIC@HORUS allows for a coincident detection of γ-rays and light charged particles in the exit channel of inelastic scattering and transfer reactions. As a first physics case, the Pygmy Dipole Resonance (PDR) in 92Mo has been investigated in a (p,p’γ) experiment at Ep = 10.5MeV. Since a specific excitation energy can be chosen offline in the coincidence data, the sensitivity to weak decay branchings of PDR states is increased. Additionally, a second reaction mechanism for the excitation of PDR states has been tested with the new setup. In a 119Sn(d,pγ) transfer reaction at Ed = 8.5MeV, PDR states in 120Sn could be excited. Since this one-neutron transfer reaction is sensitive to the neutron single-particle structure, it could reveal new information on the microscopic structure of the PDR.


Introduction
Low-lying electric dipole strength in neutron-rich nuclei, commonly denoted by Pygmy Dipole Resonance (PDR), has recently attracted a lot of experimental and theoretical interest due to its implications for the neutron skin [1], the symmetry energy in the equation of state [2], and isotopic abundances from astrophysical network calculations [3].To complement to previous experimental approaches, presented in a recent review [4], a new experimental setup was commissioned at the 10 MV Tandem accelerator at the Institute for Nuclear Physics in Cologne.This setup consists of the HPGe-detector array HORUS for high-resolution γray spectroscopy and the silicon-detector array SONIC for light charged-particle identification and spectroscopy and will be described in more detail in section 2. By applying the particle-γ coincidence technique, this setup enables the investigation of the PDR in inelastic particle-scattering and light-ion transfer reactions to learn more about its underlying structure.

Experimental setup 2.1 γ-ray detection: HORUS
The HORUS array is an HPGe-detector array consisting of 14 HPGe detectors, 6 of which are equipped with BGO shields for active Compton suppression.Under typical experimental conditions, it has a photo-peak efficiency of the order of 2% at 1.3 MeV and a resolution of 2 keV at 1.3 MeV.Due to the granularity of the setup, it has already a e-mail: pickstone@ikp.uni-koeln.debeen used very successfully for γ-γ coincidence experiments and to extract multipolarities from angular correlations of the emitted γ-rays.

Particle detection: SONIC
The new particle-detector array SONIC consists of passivated implanted planar silicon (PIPS) detectors inside the target chamber on up to 8 positions.To allow for particle identification, each position can be equipped by a ΔE-E telescope of two detectors of 300 μm and 1500 μm thickness.The active area of one detector is 150 mm 2 , leading to a maximum solid angle coverage of the whole array of 4%.The energy resolution of the PIPS detectors is 15 keV using a 241 Am standard source, but is worsened in beam to around 70 keV due to straggling effects in the target and the solid angle coverage of the detectors.

Particle-γ coincidences: SONIC@HORUS
The great improvement for nuclear physics experiments comes from the combination of the two devices (SONIC@HORUS).Since the excitation energy E X can be determined from the ejectile energy, a two-dimensional matrix can be constructed with the excitation energy and the γ-ray energy as axes, see Fig. 1.In this matrix, certain structures appear, which can be studied selectively: Horizontal columns and vertical bars correspond to a certain E X and E γ , respectively.Most importantly, transitions to any level at E level are appear as diagonals and can be selected by requiring the condition E X = E level + E γ .To take advantage of the excellent energy resolution of the HPGedetectors, the events fulfilling this condition (gate) are then Table 1.Observed decay pattern of J = 1 states of 92 Mo in the PDR energy region (up to E X =7 MeV).
projected onto the γ-ray axis for further analysis.Due to this procedure, several advantages can be achieved: First of all, the peak-to-background ratio is greatly improved compared to the ungated spectra.Most importantly, the level scheme and branching ratios can be extracted rather easily with the additional information on E X .However, the calculation of branching ratios also relies on the correct determination of detector efficiency and angular correlations, which will not be covered in this article.

Inelastic proton scattering: 92 Mo(p,p'γ)
Last year, a proton scattering experiment on 92 Mo was performed at the 10 MV Tandem accelerator in Cologne to investigate the PDR in this nucleus.A 10.5 MeV proton beam impinged on a 0.6 mg/cm 2 self-supporting target of 94% enriched 92 Mo.The average beam current was 20 nA and the experiment lasted 5 days.The goal of the experiment was to investigate the decay branching of previously observed J = 1 states [5] below the particle threshold by taking advantage of the p-γ-coincidence technique as explained above.For many states, a sizeable decay branching to excited states was observed.A summary of the decay pattern up to 7 MeV is given in Table 1.For all these transitions, branching ratios will be determined.However, the analysis is still ongoing and some aspects of the setup (e.g.efficiency and angular correlations) still have to be accounted for before branching ratios can be determined.Afterwards, they will be compared with theoretical calculations within the Quasiparticle Phonon Model [6].Recently, a transfer experiment has been performed with the new setup to investigate the PDR in a pioneering (d,pγ) experiment.By using this reaction on 119 Sn, the singleparticle structure of the PDR in 120 Sn was investigated, since this reaction represents a neutron stripping.After applying a gate on the outgoing proton channel in the ΔE-E plot and on ground state decays, the preliminary spectrum as seen in Fig. 2 was obtained.A clear excitation of states in the PDR region was observed as well as peaks from the individual states previously assigned to the PDR in an NRF experiment [7].The analysis of this experiment is still ongoing and the excitation and de-excitation of states in the PDR region will be investigated.

Figure 1 .
Figure 1.p-γ coincidence matrix of the 92 Mo(p,p'γ) experiment.Due to the good excitation energy resolution and the very good γ-ray energy resolution, all transitions to low-lying states can be completely resolved.

Figure 2 .
Figure 2. γ-ray spectrum of the 119 Sn(d,pγ) 120 Sn experiment obtained by requiring protons as ejectiles and gating on E X ≈ E γ , i.e. ground-state transitions, as explained in 2.3.Low-lying transitions and strong dipole transitions are marked with spin and parity of the initial state.