II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Characterization of a strongly Coulomb-excited state at an excitationenergy above 4 MeV in 136 Te
A. Jungclaus∗1 and P. Doornenbal∗2 for the NP1912-RIBF193 Collaboration
In April 2015, the experiment NP1306-RIBF98R1
was performed aiming for a methodical study of thesystematic uncertainties inherent to the analysis ofCoulomb excitation experiments at relativistic energies, using 136 Te as a high-statistics test case. Theanalysis of the obtained data is ﬁnished and the results are published.1–3) In that experiment, besides+
the known 607-keV, 2+
1 → 01 transition, additional γ
strength in the range 3.0–4.5 MeV was observed in theinelastic excitation of 136 Te on a gold target.4) The experimental spectra are nicely described assuming thattwo γ rays are detected, whereas the bump at highenergy is too broad to be described by one single transition. However, due to the limited in-beam energy
resolution of the DALI2 spectrometer, the individual
γ-ray energies can only be determined with rather large
uncertainties. The intensity ratio between the two lines
clearly depends on the γ-ray multiplicity of the event.
The line at higher energy has a higher yield when only
one γ ray is detected, while the one at lower energy
is more intense for events with γ-ray multiplicity two.
From this observation, it can be concluded that theγ ray with higher energy is emitted in cascades withlower multiplicity. Unfortunately, however, due to random coincidences with background γ rays, it is not possible to go beyond this qualitative statement. Based onthe available experimental information it is not possible to determine whether the two transitions populate+++136
the 4+Te.
1 and 21 or the 21 and 01 states ofThe large cross section measured for the excitationof a state above 4 MeV in a heavy nucleus such as 136 Te
in the inelastic scattering on a gold target is diﬃcult
to explain. Whatever the spin of the newly identiﬁed
state is, the corresponding transition probability is exceptionally large. Theoretical studies suggest that thenew excited state in 136 Te may be related to the existence of loosely-bound neutrons outside the 132 Sn core.
It is therefore of utmost interest to better characterize
this unusual state.
The availability of the HiCARI array at RIKEN in
2020 oﬀered the unique opportunity to further investigate this interesting case. The superb position resolution of the tracking detectors translates into a very
good in-beam energy resolution for high-energy γ rays.
Therefore, the inelastic excitation of 136 Te on a gold
target was measured again with the goal to determine
the energies of the two high-energy γ rays emitted inthe decay of the new state with suﬃcient precision inorder to establish to which low-lying levels of 136 Te the∗1∗2Fig. 1. HiCARI γ-ray spectrum of 136 Te populated via inelastic excitation on a gold target, considering either all
γ-ray multiplicities (black) or Mγ = 1 (red).

decay proceeds. In addition, also the inelastic excitation on a Be target was studied in order to allow for adetermination of the nuclear deformation length fromthe measured cross section. The aim of this part ofthe experiment was to obtain further information withrespect to the spin of the new state above 4 MeV in136Te.
The experiment NP1912-RIBF193 was performed
during the HiCARI campaign in November 2020. For
both the Au and Be targets the expected countingstatistics was accumulated and the taken data in theexperiment is currently under analysis. The selectionof the reaction channel was achieved via event-by-event
ion identiﬁcation in the BigRIPS and ZeroDegree spectrometers. A preliminary γ-ray spectrum of 136 Te in+
the region around the known 2+
1 → 01 transition, takenwith the HiCARI array following the inelastic excitation on a gold target, is shown in Fig. 1.
References

1) V. Vaquero et al., Phys. Rev. Lett. 118, 202502 (2017).
2) V. Vaquero et al., Phys. Lett. B 795, 356 (2019).
3) V. Vaquero et al., Phys. Rev. C 99, 034306 (2019).
4) V. Vaquero, PhD thesis, Universidád Autónoma deMadrid, 2018.

IEM-CSIC, Madrid
RIKEN Nishina Center- 10 -1