Session B4: Nuclear Physics I

Chiral basis for particle-rotor model for odd-odd triaxial nuclei

In the last decade nuclear chirality resulting from an orthogonal coupling of angular momentum vectors in triaxial nuclei has been a subject of numerous experimental and theoretical studies. Three perpendicular angular momenta can form two systems of the opposite handedness, the right-handed and the left-handed system; the time-reversal operator, which reverses orientation of each of the components, relates these two systems. In the simplest case of odd-odd nuclei, two mutually orthogonal angular momenta are provided by the high-spin valence proton and neutron which are of particle and hole character. The third angular momentum component is provided by the collective core rotation and aligns along the axis of the largest moment of inertia; this is the intermediate axis for irrotational flow-like moments of inertia for a triaxial body. This simple picture leads to prediction of distinct observables manifesting chirality in rotational structures, most notably to the doubling of states. All these effects can be demonstrated using particle-rotor model for triaxial nuclei, and are especially transparent when a newly developed chiral basis is used in calculations. The model, the basis, numerical results, and comparison to the data will be presented and discussed.   

The nucleon-nucleon interaction up to sixth order in the chiral expansion

We have calculated the nucleon-nucleon potential up to sixth order (N5LO) of chiral perturbation theory[1,2]. Previous calculations extended only up to N3LO (fourth order)[3] and typically showed a surplus of attraction, particularly, when the $\pi$-$N$ LECs from $\pi$-$N$ analysis were applied consistently. Furthermore, the contributions at N2LO and N3LO are both fairly sizeable, thus, raising concerns about the convergence of the chiral expansion. We show that the N4LO contribution is repulsive and, essentially, cancels the excessive attraction of N3LO. The N5LO contribution turns out to be considerably smaller than the N4LO one, hence establishing the desired trend of convergence. The predictions at N5LO are in excellent agreement with the empirical phase shifts of peripheral partial waves. \\ 1. D. R. Entem, N. Kaiser, R. Machleidt, and Y. Nosyk, Phys. Rev. C 91, 014002 (2015). \\ 2. D. R. Entem, N. Kaiser, R. Machleidt, and Y. Nosyk, Phys. Rev. C 92, 064001 (2015). \\ 3. R. Machleidt and D. R. Entem, Phys. Reports 503, 1 (2011).   

Doppler-shift lifetime measurements in $^{84}$Kr using the TIGRESS Integrated Plunger

Intense re-accelerated beams delivered by the ISAC-II facility at TRIUMF, Canada's national laboratory for particle and nuclear physics, permit access to nuclear structure information for a wide range of radionuclides via in-beam gamma-ray spectroscopy with TIGRESS, a high-efficiency and Compton-suppressed segmented HPGe array. To take advantage of this opportunity, the TIGRESS Integrated Plunger (TIP) has been constructed at Simon Fraser University [1]. The TIP infrastructure supports Doppler-shift lifetime measurements via the Recoil Distance Method (RDM) using a 24-element TIP CsI(Tl) wall for charged-particle identification. A commissioning experiment aimed towards a high-precision ($<10$\%) measurement of the $B(E2,2^+_1\rightarrow 0^+_1)$ reduced transition probability in $^{84}$Kr was performed in October 2015 using inelastic scattering near the Coulomb barrier coupled with an RDM lifetime measurement of the stable $^{84}$Kr beam. A Geant4-based code for TIP is being developed as a tool to aid the analysis and for the optimization of future experiments. The device, experimental approach, analysis, and preliminary results will be presented and discussed. This work is presented on behalf of the TIP and TIGRESS collaborations. [1] P. Voss et al., NIMA 746 (2014) 87.   

Study of $^{22}$Ne and $^{28}$Mg excited states using fusion-evaporation and Doppler shift measurements

Electromagnetic transition rate measurements serve as a fundamental probe of nuclear structure and provide a stringent test for theoretical models. Doppler shift lifetime measurements offer an opportunity to directly access information about electromagnetic transition rates and discriminate between model calculations. The TIGRESS Integrated Plunger device (TIP), constructed at SFU, supports Doppler shift lifetime measurements via gamma-ray spectroscopy with the TIGRESS segmented Ge array as part of the experimental program at the ISAC-II facility of TRIUMF. A recent study using TIP employs the fusion-evaporation reaction of $^{18}$O + $^{12}$C at beam energies of 56 and 48 MeV, with reaction channel selection provided via coincident charged particle detection using ancillary CsI(Tl) detectors. Transitions were identified belonging to the 2 alpha particle and 2 proton evaporation channels from the compound system $^{30}$Si, corresponding to $^{22}$Ne and $^{28}$Mg respectively. Lineshapes, from which lifetimes can be determined by comparison to simulated data, have been observed for these transitions. The experimental approach, analysis procedure, and preliminary comparison of lineshapes to simulations using the GEANT4 toolkit will be discussed.   

Measurements of Mixing Ratios in $^{\mathrm{116}}$Sn.

The $\beta $ decay of $^{\mathrm{116m1,g}}$In to $^{\mathrm{116}}$Sn, conducted at TRIUMF-ISAC utilizing the 8$\pi $ array of 20 HPGe detectors augmented with 5 Si(Li) detectors, produced a high statistics data set from which E2/M1 mixing ratios from $\gamma \gamma $ angular correlations were obtained. Several new mixing ratios were measured for the first time for transitions among the I$^{\mathrm{\pi }} \quad =$ 4$^{\mathrm{+}}$ states within $^{\mathrm{116}}$Sn. Additionally, a re-measurement of mixing ratios between the 2$_{\mathrm{2}}^{\mathrm{+}}$ to 2$_{\mathrm{1}}^{\mathrm{+}}$ and 2$_{\mathrm{3}}^{\mathrm{+}}$ to 2$_{\mathrm{1}}^{\mathrm{+}}$ states shows consistency with mixing ratio systematics in neighboring even-even tin isotopes. Previous analyses of $^{\mathrm{116}}$Sn have rationalized the nature and mixing of the 4$^{\mathrm{+}}$ states on the basis of shared phonon strength [1]. The results presented here demonstrate that generalized seniority, without the inclusion of vibrational components, may better describe these states. This has implications for ongoing theoretical and experimental research on $^{\mathrm{116}}$Sn and neighboring tin isotopes. [1] S. Raman \textit{et al.}, Phys Rev C 43 521 (1991)