Speaker
Description
Information gained on neutron-rich N~126 nuclei is essential for the understanding of nuclear structure in heavy nuclei. Studies around doubly magic systems allow direct tests of the purity of shell model wave functions. From a longer-term perspective, experiments in this region pave the way toward the understanding of the nuclear-astrophysical r-process waiting point nuclei along the N = 126 shell closure.
The recent pioneering experiment with the Isolde Solenoidal Spectrometer (ISS) used a radioactive $^{206}$Hg beam impinging on a deuterium target. Excited states in $^{207}$Hg were populated via the (d,p) reaction and the properties (energies and spectroscopic factors) of the single-neutron states $g_{9/2}$, $_{d5/2}$, $s_{1/2}$, $d_{3/2}$ and $g_{7/2}$ have been determined [1]. We suggest a continuation of the study of the heavy neutron-rich region using $^{206}$Hg mercury beams, this time on a triton target.
Single-proton states will be studied in the semi-magic $^{205}$Au nucleus populated in (t,alpha) reaction. One excited state above the $d_{3/2}$ proton-hole ground-state is known in $^{205}$Au, which is the $h_{11/2}$ proton-hole isomer at 907(5) keV [2]. At ISS we aim to identify the $d_{5/2}$ and $g_{7/2}$ and proton-hole dominated states.
Two neutron states will be populated in $^{208}$Hg following (t,p) reactions. In addition to the neutron $g^2_{9/2}$ states known from an isomeric decay experiment [3], we expect to populate $i^2_{11/2}$ states in the 2-3.5 MeV region (but no $l$=0 transfer), and the yrast 3-. The latter is due the mixing between the collective octupole phonon and the neutron $g_{9/2} j_{15/2}$ 3- states. The observation of the 3- state will provide information on the evolution of octupole collectivity south-east of $^{208}$Pb.
The above predictions are based on equivalent (t,p) [4] and (t,alpha) [5] reactions performed on a $^{208}$Pb target at triton beam energies of 20 MeV and 17 MeV, respectively. The two reactions can be studied at ISS simultaneously, assuming the availability of Si detectors both downstream and upstream of the triton target. $^{205}$Au is expected to be populated with higher cross-section than $^{208}$Hg, and its associated particle spectrum will be cleaner. Presently, theoretical calculations are being performed by N. Timofeyuk, which will be used for detailed yield calculations.
[1] T.L. Tang et al., Phys.Rev.Lett. 124, 062502 (2020).
[2] Zs. Podolyak et al., Phys.Lett. B 672, 116 (2009).
[3] N. Al-Dahan et al., Phys. Rev. C 80, 061302 (2009).
[4] E.R. Flynn et al, Nucl. Phys. A 195, 97 (1972).
[5] E.R. Flynn et al, Nucl. Phys. A279, 394 (1977).
