Low-lying dipole strength of the open-shell nucleus94Mo

Abstract

The low-lying dipole strength of the open-shell nucleus 94 Mo was studied via the nuclear resonance fluorescence technique up to 8.7 MeV excitation energy at the bremsstrahlung facility at the Superconducting Darmstadt Electron Linear Accelerator (S-DALINAC), and with Compton backscattered photons at the High Intensity γ-ray Source (HIγ S) facility. In total, 83 excited states were identified. Exploiting polarized quasi-monoenergetic photons at HIγ S, parity quantum numbers were assigned to 41 states excited by dipole transitions. The electric dipole-strength distribution was determined up to 8.7 MeV and compared to microscopic calculations within the quasiparticle phonon model. Calculations and experimental data are in good agreement for the fragmentation, as well as for the integrated strength. The average decay pattern of the excited states was investigated exploiting the HIγ S measurements at five energy settings. Mean branching ratios to the ground state and first excited 2 + 1 state were extracted from the measurements with quasi-monoenergetic photons and compared to γ-cascade simulations within the statistical model. The experimentally deduced mean branching ratios exhibit a resonance-like maximum at 6.4 MeV which cannot be reproduced within the statistical model. This indicates a nonstatistical structure in the energy range between 5.5 and 7.5 MeV.

Direct determination of ground-state transition widths of low-lying dipole states in 140 Ce with the self-absorption technique

“…3 Interestingly, the previous literature [20,22,23] reported that b 0 as a function of excitation energy was difficult to be described within the statistical model. A maximum of b 0 in the energy domain of the PDR has, e.g., been reported for 94 Mo [23] and for 130 Te [22], whereas b 0 calculated within the statistical model steadily decreases as a function of increasing energy.…”

“…3 Interestingly, the previous literature [20,22,23] reported that b 0 as a function of excitation energy was difficult to be described within the statistical model. A maximum of b 0 in the energy domain of the PDR has, e.g., been reported for 94 Mo [23] and for 130 Te [22], whereas b 0 calculated within the statistical model steadily decreases as a function of increasing energy. This deviation may occur because the statistical model can either not describe the decay behavior of the strongly excited states that are concentrated in the excitation-energy domain of the PDR where also the maximum of b 0 is observed or because the statistical model cannot describe the concentration of strongly excited states in this excitation-energy range.…”

“…Recently, NRF measurements with quasi-monoenergetic photons at the High Intensity γ -Ray Source (HIγ S) [17,18] at Triangle Universities National Laboratory (TUNL) in Durham, NC, USA gave experimental insight in mean decay properties of dipole excited states in the energy region of the PDR [12,[19][20][21][22][23]. All of these measurements demonstrate that the decay via intermediate states cannot be neglected, however, the data do not answer the question how individual states behave.…”

“…All of these measurements demonstrate that the decay via intermediate states cannot be neglected, however, the data do not answer the question how individual states behave. Recent studies also revealed that the mean branching ratio b 0 to the ground state cannot completely be described within the statistical model [20,22,23], demonstrating the need to study 0 directly. One opportunity to study 0 is provided by inelastic proton scattering at forward angles.…”

Contemporary research with nuclear resonance fluorescence at the S-DALINAC

“…Furthermore, the method of relative self absorption (RSA) [8,9] allows for the model-independent, direct determination of the ground-state transition width, the total level width, and, consequently, also of the branching ratio to the ground state of individual excited states. On one hand, this technique is well suited to study the decay pattern of lowlying dipole excited states such as those attributed to the PDR, which recently draw a lot of attention [10,11,12]. On the other hand, it provides the opportunity to improve the accuracy of, e.g., calibration standards that are used in NRF measurements [13].…”