Nuclear level density and $\gamma$-ray strength functions of $^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the ($^3$He,$^3$He$^\prime\gamma$) and ($^3$He,$\alpha \gamma$) reactions. The level densities of $^{121,122}$Sn display step-like structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for $E_\gamma \gtrsim 5.2 $ MeV. This enhancement is compatible with pygmy resonances centered at $\approx 8.4(1)$ and $\approx 8.6(2)$ MeV, respectively, and with integrated strengths corresponding to $\approx1.8^{+1}_{-5}%$ of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in $^{116-119}$Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in $^{116-122}$Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths
The reduced transition probabilities, B E2; 0 gs ! 2 1 , have been measured in the radioactive isotopes 108;106 Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B E2; 0 gs ! 2 1 0:222 19 e 2 b 2 for 108 Sn and B E2; 0 gs ! 2 1 0:195 39 e 2 b 2 for 106 Sn were determined relative to a stable 58 Ni target. The resulting B E2 values are 30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N Z 50 shell closure. DOI: 10.1103/PhysRevLett.101.012502 PACS numbers: 23.20.Js, 21.60.Cs, 25.70.De, 27.60.+j Precision measurements in unstable nuclei together with recently developed models of the nucleon-nucleon interaction, stemming from many-body techniques and QCD, show promise to improve our understanding of the finer aspects of the dynamics of the atomic nucleus. One approach to this question is to measure reduced transition probabilities -B E2; 0 gs ! 2 1 -for specific nuclei in the vicinity of a shell closure and to compare these results with calculations based on such models. In particular, one of the pressing questions in nuclear physics today is whether the shell closures, that are well established close to stability, remain so also for isotopes with a more extreme proton-toneutron ratio. Intuitive models, such as the generalized seniority scheme [1], predict that these B E2 values follow a parabolic trend, that peaks at midshell, for a sequence of isotopes between two shell closures. In the following we address the 100 Sn shell closure and consequently present results from measurements in the sequence of neutron-deficient even-mass Sn isotopes. This approach has been made possible by newly developed facilities that produce high-quality radioactive ion beams. Recent measurements in 110;108 Sn [2 -4] consistently deviate from the broken-pair model as given by the generalized seniority scheme and from current large-scale shell-model calculations [2]. Parallel work [4], using intermediate energy Coulomb excitation, suggests a constant trend of the reduced transition probabilities extending to 106 Sn. In this Letter we report results from the first measurements of 108;106 Sn using subbarrier Coulomb excitation. This is the only experiment so far for 106 Sn that has permitted for complete control of the scattering process and thus explicitly fulfills the conditions for safe Coulomb excitation. Our result still deviates significantly from theoretical predictions but indicates a decreasing trend of the B E2 with a decreasing number of valence particles outside of the 100 Sn core. Note that with this Letter three different isotopes have been used for normalization as 112 Sn [2] and 197 Au [4] have been used previously. All three experiments yield similar PRL 101, 012502 (2008)
Article:Bree, N., Wrzosek-Lipska, K., Petts, A. et al. (67 more authors) (2014) Shape coexistence in the neutron-deficient even-even 182-188Hg isotopes studied via Coulomb excitation.
We have analyzed primary γ-ray spectra of the odd-odd 238 Np nucleus extracted from 237 Np(d, pγ) 238 Np coincidence data measured at the Oslo Cyclotron Laboratory. The primary γ spectra cover an excitation-energy region of 0 ≤ E i ≤ 5.4 MeV, and allowed us to perform a detailed study of the γ-ray strength as function of excitation energy. Hence, we could test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the γ strength. In this work, using the available highquality 238 Np data, we show that the γ-ray strength function is to a very large extent independent on the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis has been experimentally verified for γ transitions between states in the quasi-continuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined.PACS numbers: 24.30. Gd, 21.10.Ma, 25.40.Hs Sixty years ago, David M. Brink proposed in his PhD thesis [1] that the photoabsorption cross section of the giant electric dipole resonance (GDR) is independent of the detailed structure of the initial state. In his thesis, he expressed his hypothesis as follows: "If it were possible to perform the photo effect on an excited state, the cross section for absorption of a photon of energy E would still have an energy dependence given by (15)", where equation (15) refers to a Lorentzian shape of the photoabsorption cross section. Brink's original idea, the Brink hypothesis, was first intended for the photoabsorption process on the GDR, but has been further generalized, applying the principle of detailed balance, to include absorption and emission of γ rays between resonant states [2,3]. In addition to assuming independence of excitation energy, there is no explicit dependence of initial and final spins except the obvious dipole selection rules, implying that all levels exhibit the same dipole strength regardless of their initial spin quantum number. We will refer to this as the generalized Brink-Axel (gBA) hypothesis. A review of the history of the hypothesis was given by Brink in Ref. [4].The gBA hypothesis has implications for almost any situation where nuclei are brought to an excited state above ≈ 2∆, where ∆ ≈ 1 MeV is the pair-gap parameter. Here, the nucleus will typically de-excite via γ-ray emission and/or by emission of particles. In this context, it is usual to translate the γ-ray cross section σ (E γ ) into γ-ray strength function (γSF) by f (E γ ) = (3π 2h2 c 2 ) −1 σ (E γ )/E γ .To describe and model the electric dipole part of the γ-decay channel, the gBA hypothesis is frequently used, applying in particular the assumption of spin independence [5]. For example, a rather standard approach to calculating E1 strength is to apply some variant of the quasi-particle random-phase approximation (QRPA) to obtai...
The γ-ray strength function of 56Fe has been measured from proton-γ coincidences for excitation energies up to ≈11 MeV. The low-energy enhancement in the γ-ray strength function, which was first discovered in the (3He,αγ)56Fe reaction, is confirmed with the (p,p'γ)56Fe experiment reported here. Angular distributions of the γ rays give for the first time evidence that the enhancement is dominated by dipole transitions.
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