II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Neutron intruder states and collectivity beyond N = 50 towards 78 Ni
L. Plagnol,∗1 F. Flavigny,∗1 for the RIBF196 and HiCARI collaborations

∗1

34

33
10

3

10

232

Z

Recent spectroscopy studies in the vicinity of 78 Ni1,2)
hint to the presence of low-lying deformed conﬁgurations. These intruder conﬁgurations are predicted tooriginate from the multiparticle-multihole excitations3)of nucleons above the N = 50 and Z = 28 shellgaps becoming competitive in energy owing to neutronproton correlations enhancing quadrupole collectivity.
The quantiﬁcation of collective eﬀects above N = 50 is
crucial since it directly impacts binding energies, dripline limits,4) and consequently r-process nucleosynthesis calculations. Because these states involve manyparticle excitations, their theoretical description is very
challenging: thus, identifying them experimentally is ofprime interest to constrain models.
We report here on the analysis status of the RIBF196
experiment to identify and characterize 2p-1h intruderstates above N = 50 in 83 Ge and 81 Zn for the ﬁrst timeusing a neutron knockout reaction from N = 52 isotopes. This selective reaction enables neutron removalfrom the g9/2 orbital populating preferentially 9/2+states with a signiﬁcant intruder ν(g9/2 )−1 (s1/2 d5/2 )+2
conﬁguration. Simultaneously, low-lying states in 82 Zn
were populated using proton removal from 83 Ga, andtheir lifetimes will be extracted using line-shape analysis to better characterize the onset of deformation developing in Zn above N = 50.5)
To attain these scientiﬁc objectives, a secondary
cocktail beam containing 84 Ge, 83 Ga, and 82 Zn was produced and puriﬁed in the BigRIPS spectrometer to induce knockout reactions on a secondary 6-mm thick Be
target. De-excitation γ-rays of the reaction products
83Ge, 82 Zn, and 81 Zn were detected using the HiCARI
germanium array6) surrounding the secondary reaction
target. Event-by-event particle identiﬁcation for beam
nuclei and reaction products was achieved in the BigRIPS and ZeroDegree spectrometers by reconstructing their atomic number (Z) and their mass-to-charge
ratio (A/Q) using the TOF-Bρ-ΔE method combinedwith a two-fold Bρ method.
We performed higher-order optical corrections of theA/Q values to compensate for remaining dependencies
on spectrometer variables such as as positions and angles at the focal planes. This additional correction isequivalent to a higher-order determination of opticaltransfer matrices and improves the identiﬁcation. It
was performed using a multi-dimensional ﬁtting procedure, considering cross-terms. An optically correctedparticle identiﬁcation diagram (PID) is shown in Fig. 1.
The A/Q resolution achieved through this opticalcorrection procedure is of 7.10−2 % calculated on theidentiﬁcation of 84 Ge in BigRIPS with an ameliorationof approximately 14% compared with the uncorrected

3130

10

29
2.56

2.58

2.6

2.62

2.64

2.66

A/Q

2.68

2.7

2.72

2.74

1Fig. 1. Particle identiﬁcation of the reaction products in theZeroDegree spectrometer. The black, red, and purple
ellipsoids indicate 83 Ge, 82 Zn, and 81 Zn, respectively:
see text for details.

A/Q resolution, and 9.10−2 % on 83 Ge in ZeroDegree
spectrometer with an amelioration of more than 50%.
In addition, we cleaned the PID by removing unwanted
events such as delta rays in PPAC detectors, pileup
events in plastic detectors and ionisation chambers, andchange in charge states. The most impacting selectionis the pileup events removal cutting away approximately
10% of events on the total PID.
The HiCARI detectors have been aligned in time, calibrated in energy (38 crystals and 376 segments) using
sources of 60 Co, 152 Eu, and 133 Ba, and the array eﬃciency was determined (2.9% at 1 MeV with a 0.5 mm
Pb shield). The non-linearities of the ADCs were quantiﬁed from the residuals of the energy calibrated spectra
and exhibit a saw-tooth pattern with variations up to 2–3 keV.7) Their impact on Doppler corrected spectra will
be quantiﬁed using simulations. Finally, a preliminary
Doppler-corrected gamma spectrum has been produced
using photogrammetry positions of the clusters and aﬁxed velocity at the target, but we aim at improving
this result using event-by-event Doppler correction using the measured velocity distributions in the BigRIPS
and ZeroDegree spectrometers.
References
1) R. Taniuchi et al., Nature 569, 53 (2019).
2) X. F. Yang et al., Phys. Rev. Lett. 116, 219901 (2016).
3) K. Heyde, J. L. Wood, Rev. Mod. Phys. 83, 1467 (2011).
4) J. Erler et al., Nature 486, 509 (2012).
5) C. Shand et al., Phys. Lett. B 773, 492 (2017).
6) K. Wimmer et al., RIKEN Accel. Prog. Rep. 54, S27(2020).
7) K. Dolde et al., Nucl. Instrum. Methods Phys. Res. A848, 127 (2017).

LPC Caen, CNRS/IN2P3- 19 -1