IsovectorM1 transitions inSi28and the role of meson exchange currents

Abstract

At the newly installed 180°Darmstadt electron scattering facility we have measured the isovector M 1 transition strengths in 28 Si in the excitation energy range from 10 to 18 MeV. Overall agreement with a shell-model calculation using the unified sd-shell interaction and effective g factors is obtained. Comparison with Gamow-Teller strength deduced from (p,n) data reveals the presence of meson exchange current contributions of the order of 25% to the summed M 1 transition strength.PACS number͑s͒: 21.10. Hw, 21.60.Cs, 25.30.Dh, 27.30.ϩt The role of non-nucleonic degrees of freedom, i.e., that of meson exchange currents ͑MEC͒ and isobars to the magnetic dipole (M 1) and Gamow-Teller ͑GT͒ transition strengths in nuclei, has been a subject of long-standing theoretical and experimental interest ͓1,2͔. It becomes a formidable task to quantitatively determine these contributions in complex nuclei. On the experimental side, the transition strengths are usually distributed over many levels and one has to ensure that almost all the transitions are observed. Theoretically, one has to be sure that all structural effects at the nucleonic level are taken into account. For sd-shell nuclei, the wave functions obtained with the Brown-Wildenthal unified sd-shell ͑USD͒ effective interaction ͓3͔ in the full 1s0d shell-model space, take into account all possible 0ប excitation strengths. They were able to deduce the effective g factors by fits to reliable data sets ͓4͔. These calculations, along with the data for M 1 strengths from reactions such as (e,eЈ) and the GT strengths from ( p, pЈ), (p,n), and (n, p) reactions provide a sensitive method of evaluating the MEC contributions, as has been shown recently ͓5͔.As was pointed out in Ref. ͓5͔, self-conjugate nuclei are special candidates for such tests of MEC's, as all these processes can be studied with the same target nucleus, and the pure isovector (⌬Tϭ1) nature of the M 1 transitions is well assured. For transitions from the ground state to the 1 ϩ ,Tϭ1 final states, one may write approximately ͓1͔where the numerical factor in Eq. ͑1͒ is 2.643 N 2 . The ratio of coupling constants (g A /g V ) is not included in the definition of B(GT). The spin matrix element M () and the isobar contribution M ⌬ appear in both expressions. Pion exchange is the main source of MEC contributions which are predicted ͓1͔ to be large for isovector M 1 currents, but should be strongly suppressed for axial-vector GT currents as a result of G parity conservation.The ratiomeasures the combined effects of orbital and MEC contributions, independent of the complexity of the nucleonic wave function and the ⌬-isobar component. In the absence of these contributions the ratio R should be unity. In sd-shell nuclei the orbital contributions can be predicted reliably from large scale shell-model calculations ͓3͔. Earlier, a comparison of M 1 and GT strengths was carried out in 24 Mg and gave clear evidence for the important role of MEC's in isovector M 1 transitions ͓5͔.Here we aim at a similar investigation of t...

Quenching of Isovector and Isoscalar Spin-M1 Excitation Strengths in N = Z Nuclei

“…Here, it should be noted that the isospin symmetry is reasonably assumed within the accuracy of the data in the present study although the meson exchange current contribution can be different between an IV M1 transition measured by electron scattering and the analogous GT transition by charge-exchange reaction (Richter et al, 1990;Lüttge et al, 1996).…”

Low-energy magnetic dipole response in 56Fe from high-resolution electron scattering

“…A quenching of the spin g-factor is observed in analogy to the renormalization of the axial-vector coupling constant (see, e.g., [2,3] and references therein). M1 transitions are also sensitive to non-nucleonic degrees of freedom in elementary nuclear excitations [4][5][6]. Furthermore, a collective orbital magnetic dipole mode-the scissors mode-has been identified [7] at low excitation energies whose strength and location are intimately related to rotational properties of the nucleus (see, e.g., [8][9][10][11] and references therein).…”

l-ForbiddenM1Transition inS32Reitz1, Neumann-Cosel2, Neumeyer3 et al. 1999Phys. Rev. Lett.Self Cite18150View full textAdd to dashboardCite

“…The difference between d S (GT) and d S (M1) is dominated by mesonic exchange currents (iv). The empirical values for d S obtained from a global fit of many sd-shell data [17] as well as specific transitions in 24 Mg [24] and 28 S [25,26] give a significant enhancement of d S (M1) over d S (GT) consistent with the microscopic calculations [2,7]. The predictive power of calculations using the USD interaction [21,22] and effective operators is demonstrated by an investigation of the B͑M1͒ strength distribution in 32 S also obtained from the experiments described above.…”