Investigation of J=1 states and their γ -decay behavior in Cr52

Abstract

Background: In the A ≈ 50 mass region M1 spin-flip transitions are prominent around 9 MeV. An accumulation of 1 − states between 5 and 8 MeV generating additional E1 strength, also denoted as Pygmy Dipole Resonance (PDR), has been established in many nuclei with neutron excess within the last decade. Purpose: The γ-decay behavior of J = 1 states has been investigated in an NRF experiment. M1 excitations have been compared to shell model calculations. Methods: J = 1 states were excited by quasi-monoenergetic, linearly polarized γ-ray beams generated by Laser-Compton backscattering at the HIγS facility, Durham, NC, USA. Depopulating γ-rays were detected with the multi-detector array γ 3 . Results: For eleven beam-energy settings the γ-decay behavior of dipole states was analyzed by a state-to-state analysis and average γ-decay branching ratios have been investigated. 34 parity quantum numbers were assigned to J = 1 states. Conclusions: Six 1 − states and two 1 + states have been investigated in NRF experiments for the first time. The M1 strength distribution is in good agreement with shell-model calculations.

Electric and magnetic dipole strength in Fe54

“…The summed strength B(M1) = 1.76μ 2 N of the calculated values greater than 0.005 μ 2 N between E γ = 6.8 and 10.6 MeV exceeds the upper limit of the experimental value of B(M1) = 1.59(4)μ 2 N by about 8%. This relation resembles the one found for the isotone 50 Cr [16]. Possible reasons for the difference are that some of the states with unknown parity in Table I may be 1 + states that are not taken into account in this comparison or may be unobserved branching transitions.…”

“…Furthermore, unobserved transitions in the quasicontinuum contribute to the summed strengths as well. Nevertheless, these summed strengths are of magnitudes similar to the ones found for 50 Cr [13] and for the N = 28 isotone 52 Cr [15,16]. The distribution of M1 strength is compared with predictions of shell-model calculations in the following.…”

“…This relation seems to change when going to lighter nuclides, such as the ones in the iron-nickel region. In 50 Cr [13,14], 52 Cr [15,16], 54 Cr [14], 56 Fe [17], and 58,60 Ni [18], several strong isolated M1 excitations have been observed. A detailed investigation of these finding requires the study of further nuclei with varing properties, such as nuclides at shell closures and within open shells.…”

Valence-shell dependence of the pygmy dipole resonance: E1 strength difference in Cr50,54

“…The results for 52 Cr have already been reported in Ref. [36][37][38]. In this publication, data for 50 Cr and 54 Cr are presented.…”

Monte Carlo Simulation of γ − γ Correlation Functions

“…In each case, we could extract the dimensions of the target and the detector from the corresponding reports, and then we precisely simulated the experimental setups. In a recent study, Wilhelmy et al [26] assigned 35 parity quantum numbers of 52 Cr to either M1 or E1 transition. The asymmetry ratios A Exp (π/2) averaged 0.878(313) overall assignments.…”