Analogous Gamow-Teller Transitions Research Articles

Nonquenched Isoscalar Spin-M1 Excitations in sd-Shell Nuclei.

Differential cross sections of isoscalar and isovector spin-M1 (0(+)→1(+)) transitions are measured using high-energy-resolution proton inelastic scattering at E(p)=295 MeV on (24)Mg, (28)Si, (32)S, and (36)Ar at 0°-14°. The squared spin-M1 nuclear transition matrix elements are deduced from the measured differential cross sections by applying empirically determined unit cross sections based on the assumption of isospin symmetry. The ratios of the squared nuclear matrix elements accumulated up to E(x)=16 MeV compared to a shell-model prediction are 1.01(9) for isoscalar and 0.61(6) for isovector spin-M1 transitions, respectively. Thus, no quenching is observed for isoscalar spin-M1 transitions, while the matrix elements for isovector spin-M1 transitions are quenched by an amount comparable with the analogous Gamow-Teller transitions on those target nuclei.

Gamow–Teller transitions in exotic pf-shell nuclei relevant to supernova explosion

Gamow–Teller (GT) transitions starting from unstable pf-shell nuclei are of interest not only in nuclear physics, but also in astrophysics, e.g. in violent neutrino induced reactions at the core-collapse stage of type II supernovae. In the β-decay study of these pf-shell nuclei, half-lives can be measured rather accurately. On the other hand, in high-resolution (3He, t) charge-exchange reactions at 0°, individual GT transitions up to high excitations can be studied. Assuming the isospin symmetry for the strengths of Tz = ±1 → 0 analogous GT transitions, we present a unique ‘merged analysis’ for the determination of absolute B(GT) values.

New facet in the study of Gamow–Teller transitions

We introduce high-resolution (3He, t) reaction at 0° and at an intermediate beam energy as a new spectroscopic tool for studying Gamow–Teller (GT) excitations. Owing to the high energy resolution of the reaction (≈ 30 keV), individual transitions can be observed up to the region of GT giant resonance. In addition, the strengths of GT transitions can now be compared with the strengths of analogous GT transitions from β decays, M1 transitions from γ decays or (e, e′) reactions. The study on GT transitions with astrophysical interest, isospin symmetry of nuclei and other related topics are discussed.

Isospin symmetry ofTz=±3∕2→±1∕2Gamow-Teller transitions inA=41nuclei

Under the assumption that isospin $T$ is a good quantum number, isobaric analog states and various analogous transitions are expected in isobars with mass number $A$. The strengths of ${T}_{z}=\ifmmode\pm\else\textpm\fi{}3∕2\ensuremath{\rightarrow}\ifmmode\pm\else\textpm\fi{}1∕2$ analogous Gamow-Teller (GT) transitions and analogous $M1$ transitions within the $A=41$ isobar quartet are compared in detail. The ${T}_{z}=+3∕2\ensuremath{\rightarrow}+1∕2$ GT transitions from the ${J}^{\ensuremath{\pi}}={3∕2}^{+}$ ground state of $^{41}\mathrm{K}$ leading to excited ${J}^{\ensuremath{\pi}}={1∕2}^{+}$, ${3∕2}^{+}$, and ${5∕2}^{+}$ states in $^{41}\mathrm{Ca}$ were measured using the $(^{3}\mathrm{He},t)$ charge-exchange reaction. With a high energy resolution of $35\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, many fragmented states were observed, and the GT strength distribution was determined up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ excitation energy $({E}_{x})$. The main part of the strength was concentrated in the ${E}_{x}=4--6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ region. A shell-model calculation could reproduce the concentration, but not so well details of the strength distribution. The obtained distribution was further compared with two results of $^{41}\mathrm{Ti}$ $\ensuremath{\beta}$ decay studying the analogous ${T}_{z}=\ensuremath{-}3∕2\ensuremath{\rightarrow}\ensuremath{-}1∕2$ GT strengths. They reported contradicting distributions. One-to-one correspondences of analogous transitions and analog states were assigned up to ${E}_{x}=6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ in the comparison with one of these $^{41}\mathrm{Ti}$ $\ensuremath{\beta}$-decay results. Combining the spectroscopic information of the analog states in $^{41}\mathrm{Ca}$ and $^{41}\mathrm{Sc}$, the most probable ${J}^{\ensuremath{\pi}}$ values were deduced for each pair of analog states. It was found that ${5∕2}^{+}$ states carry the main part of the observed GT strength, while much less GT strength was carried by ${1∕2}^{+}$ and ${3∕2}^{+}$ states. The gross features of the GT strength distributions for each $J$ were similar for the isospin analogous ${T}_{z}=\ifmmode\pm\else\textpm\fi{}3∕2\ensuremath{\rightarrow}\ifmmode\pm\else\textpm\fi{}1∕2$ transitions, but the details were somewhat different. From the difference of the distributions, isospin-asymmetry matrix elements of $\ensuremath{\approx}8\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ were deduced. The Coulomb displacement energy, which is sensitive to the configuration of states, showed a sudden increase of about $50\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ at the excitation energy of $3.8\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The strengths of several $M1$ transitions to the IAS in $^{41}\mathrm{Ca}$ were compared with the strengths of analogous GT transitions. It was found that ratios of the $M1$ and GT transition strengths were similar, suggesting that the contributions of the $\ensuremath{\ell}\ensuremath{\tau}$ term in $M1$ transitions are small.