Low-lying dipole strength in theN=28shell-closure nucleus52Cr

Abstract

The low-lying dipole strength of the N = 28 closed-shell nucleus 52 Cr was studied with nuclear resonance fluorescence up to 9.9 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC. Twenty-eight spin-1 states were observed between 5.0 and 9.5 MeV excitation energy, 14 of which for the first time. Both electric dipole excitations (E1, around 8 MeV) and magnetic dipole excitations (M1, around 9 MeV) were detected. Microscopic calculations within the quasiparticle-phonon nuclear model were performed and show good agreement with experimental results. The structure of E1 and M1 excitations, respectively, is discussed.

Investigation ofJ=1states and theirγ-decay behavior inCr52

“…Enders et al [44] erroneously assigned a transition at this γ-ray energy to a ground-state decay. Three γ-decay branching ratios have been identified in previous bremsstrahlung experiments [34]. For the J = 1 states at E x = 7014.7 keV (π = −) and E x = 8014.6 keV (π = +) they are in very good agreement with the results of this work, see tables II and I, respectively.…”

“…3 for backward angles. Such a broad peak is also observed by Pai et al [34] in previous NRF measurements with unpolarized bremsstrahlung. At a γ-ray beam energy of 7.5 MeV a strong E1 tran- 52 Cr.…”

“…At E x = 8179.1 keV, the measured γ-decay branching ratio is higher by a factor of ≈ 2 in Ref. [34]. The intensity of the γ-decay to the 2 + 1 state might be too large in the previous bremsstrahlung experiment because of contributions from the single escape peak of the ground-state decay of the 1 − state at E x = 7252.3 keV.…”

“…To further investigate the magnetic dipole strength distribution, complementary experiments with photons, that selectively excite J = 1 states, are very useful. Bremsstrahlung experiments with continuos photon flux have been performed for 40,44,48 Ca, 50,52 Cr, 56 Fe and 58,60 Ni [23][24][25][26][27][32][33][34][35]. Additionally, for some of those nuclei, almost monoenergetic γ-ray beams from Laser-Compton backscattering (LCB) have been used [25][26][27]35].…”

“…Most reliable systematic comparisons in the Cr-chain can be achieved by the same experimental approaches for each isotope. 52 Cr has already been investigated in bremsstrahlung experiments, but mainly ground-state transitions have been observed [34]. In bremsstrahlung experiments nonresonant scattered background is increasing towards lower energies in the spectrum which makes the observation of low-energy tran- sitions quite difficult.…”