Parity of Bound Dipole States inPb208

Abstract

The parities of eleven J= 1 levels in 208 Pb were determined by nuclear resonance fluorescence scattering of linearly polarized photons. A new 1 + level at E x = 5.846 MeV with r 0 Vr= 1.2 ± 0.4 eV was found. This level can probably be identified with the theoretically predicted isoscalar 1 + state in 208 Pb. All other bound dipole states below 7 MeV with r 0 2 /r> 1.5 eV have negative parity. The 1" assignment to the 4.842-MeV level is of special significance because of previous conflicting results about its parity.PACS numbers: 21.10. Hw, 25.20.+y, 27.80.+w For more than a decade the location of magnetic dipole (Ml) strength in 208 Pb has been a challenging problem for experimentalists and theorists. 1 This continued interest lies in the fact that from the energies of the Ml excitations and their particle-hole structure one expects information about the spin-dependent part of the effective nucleon-nucleon interaction in nuclei. In a simple shell-model picture, two 1 + states are expected in 208 Pb resulting from the one-particle, one-hole (lp-lh) configurations v(i 13 / 2 ml 9 iu/2) and ir(h 11 / 2 " 1 9 h Q / 2 ). Since these states are nearly degenerate in energy they are expected to mix strongly and most of the strength is moved to higher energy. 2 In one calculation 3 the upper 1 + state at 7.5 MeV is mainly isovector and carries most of the strength [B(Ml)i = 48fi 0 2 ] while the lower 1 + state at 5.4 MeV is mainly isoscalar and has little strength [J3(Ml)* = 1.2/x 0 2 ]. The distribution of the Ml strength between these two states depends critically on the coupling between the neutron lp-lh and the proton lp-lh configurations. The inclusion of 2p-2h configurations 4 ' 5 results in the fragmentation of the upper state into many components. However, the properties of the lower level are virtually unchanged because of its very weak coupling to 2p-2h states. 4 ' 6 The present experimentally established Ml strength gives a completely different picture. Original claims, prior to 1977, of finding a large concentration of strengths [B(Ml)i>50(d 0 2 ] as expected in the 7.0-8.3-MeV region have been reduced to a total definitive and probable Ml strength above 7 MeV of only ^8.5ju 0 2 each. 1 Therefore, new theoretical concepts have been considered either to shift the isovector state to higher energies 6 or to quench the intrinsic g s factor in the Ml operator. 7 The experimental results reported in this Let-ter clarify the situation of the Ml strength in the bound-state region of 208 Pb. The 4.842-MeV state which has a ground-state radiative width of 5 eV [J5(Ml)t = 11.4/i 0 2 ] was originally classified as 1 + on the basis of a linear polarization measurement using a Compton polarimeter. 8 This result was afterwards questioned and a 1" assignment was established. 9 A repetition of the linear polarization measurement, 10 now with an enriched 208 Pb target, found an inconsistency with either a pure El or Ml transition and it was concluded that there are two levels (1 + and 1") at 4.842 MeV less than 3 keV apart ...

Distribution ofM1Transitions inPb208

“…The results of the present experiment are in basic accord with previous claims that have been made for a 1 + state at 5.846 MeV 12. ' 25 The maximum M1 strength that we can attribute to a single state at this excitation is r<?…”

Particle-hole excitations in the208Pb mass region

“…This interpretation is not completely ruled out by the very recent experimental data presented in [32]. The present result does not reproduce the observed strength for the lowest lp-lh state [31], although the distribution of the Mt strength over the two 1 p-1 h states depends rather critically on the amplitudes of the two components [7~hla}2 h9/2) and [vi~ 2 i11/2) of the wave function, since the contributions of these two components interfere destructively for the lowest 1 p-1 h state.…”

Recent Progress on Nuclear Magnetic Dipole Excitations

“…The NRF experiments with polarized bremsstrah lung led to the discovery of the isoscalar M 1 transition in 208Pb (80). The isoscalar character of this state, which has also been excited in a 209 Bi(d, 3 He) reaction (8 1), was established later by inelastic electron elec tron and proton scattering experiments (82,83).…”