Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

The 92-keV resonance in the 25Mg(p,γ)26Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat(6)syst. The f0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γ rays.peerReviewe

Structure of resonances in the Gamow burning window for the Al 25 (p,γ) Si 26 reaction in novae

A γ-ray spectroscopy study of excited states in Si26 has been performed by using the Mg24(3He,n) reaction at a beam energy of 10 MeV. In particular, states have been studied above the proton threshold relevant for burning in the Al25(p,γ)Si26 reaction in novae. This reaction influences the amount of Al26 injected into the interstellar medium by novae, which contributes to the overall flux of cosmic γ-ray emission from Al26 observed in satellite missions. The present results point strongly to the existence of a 0+ state at an excitation energy of 5890 keV lying within the Gamow burning window, which raises questions about the existence and properties of another, higher-lying state reported in previous experimental work. The existence of two such states within this excitation energy region cannot be understood within the framework of sd-shell-model calculations. © 2015 American Physical Society

Xe 136 + Pb 208 reaction: A test of models of multinucleon transfer reactions

The yields of over 200 projectile-like fragments (PLFs) and target-like fragments (TLFs) from the interaction of (Ec.m.=450 MeV) Xe136 with a thick target of Pb208 were measured using Gammasphere and off-line γ-ray spectroscopy, giving a comprehensive picture of the production cross sections in this reaction. The measured yields were compared to predictions of the grazing model and the predictions of Zagrebaev and Greiner using a quantitative metric, the theory evaluation factor tef. The grazing model predictions are adequate for describing the yields of nuclei near the target or projectile but they grossly underestimate the yields of all other products. The predictions of Zagrebaev and Greiner correctly describe the magnitude and maxima of the observed TLF transfer cross sections for a wide range of transfers (ΔZ=-8 to ΔZ=+2). However, for ΔZ=+4, the observed position of the maximum in the distribution is four neutrons richer than the predicted maximum. The predicted yields of the neutron-rich N=126 nuclei exceed the measured values by two orders of magnitude. Correlations between TLF and PLF yields are discussed

Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

The 92-keV resonance in the 25Mg(p,γ)26Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat(6)syst. The f0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γ rays

Modeling multi-nucleon transfer in symmetric collisions of massive nuclei

Symmetric collisions of massive nuclei, such as 238U + 248Cm, have been proposed as ways to make new n-rich heavy nuclei via multi-nucleon transfer (MNT) reactions. We have measured the yields of several projectile-like and target-like fragments from the reaction of 1360 MeV 204Hg + 198Pt. We find that current models for this symmetric collision (GRAZING, DNS, ImQMD) significantly underestimate the yields of these transfer products, even for small transfers

γ -ray spectroscopy of Tl 209

States in Tl209 were populated using a multinucleon transfer reaction with a Xe136 beam impinging on a thick Pb208 target at E=785 MeV. The beam was pulsed at 825-ns intervals in order to perform isomer decay spectroscopy. The known Jπ=17/2+ isomer in Tl209 was located at 1228(4) keV and measured to have a half-life of T1/2=146(10) ns. A second isomer with Jπ=13/2+ was found to have T1/2=14(5) ns. The previously suggested low-energy X and Y transitions were found to have energies 57(2) and 47(2) keV respectively, while the measurement of conversion coefficients and a new decay path make the spin assignments below the isomers experimentally firm. Correlating the delayed γ transitions with the prompt beam flash allowed the decay of states above the isomer to be found. The longer-lived isomer represents full alignment of the simplest two-particle, one-hole configuration and illuminates the remarkably weak coupling of the proton hole to the Pb210 core

Shape coexistence and the role of axial asymmetry in 72Ge

The quadrupole collectivity of low-lying states and the anomalous behavior of the 02+ and 23+ levels in 72Ge are investigated via projectile multi-step Coulomb excitation with GRETINA and CHICO-2. A total of forty six E2 and M1 matrix elements connecting fourteen low-lying levels were determined using the least-squares search code, gosia. Evidence for triaxiality and shape coexistence, based on the model-independent shape invariants deduced from the Kumar-Cline sum rule, is presented. These are interpreted using a simple two-state mixing model as well as multi-state mixing calculations carried out within the framework of the triaxial rotor model. The results represent a significant milestone towards the understanding of the unusual structure of this nucleus

Triaxiality near the 110Ru ground state from Coulomb excitation

A multi-step Coulomb excitation measurement with the GRETINA and CHICO2 detector arrays was carried out with a 430-MeV beam of the neutron-rich 110Ru (t1/2=12 s) isotope produced at the CARIBU facility. This represents the first successful measurement following the post-acceleration of an unstable isotope of a refractory element. The reduced transition probabilities obtained for levels near the ground state provide strong evidence for a triaxial shape; a conclusion confirmed by comparisons with the results of beyond-mean-field and triaxial rotor model calculations

Identification of deformed intruder states in semi-magic Ni 70

The structure of semi-magic 2870Ni42 was investigated following complementary multinucleon-transfer and secondary fragmentation reactions. Changes to the higher-spin, presumed negative-parity states based on observed γ-ray coincidence relationships result in better agreement with shell-model calculations using effective interactions in the neutron f5/2pg9/2 model space. The second 2+ and (4+) states, however, can only be successfully described when proton excitations across the Z=28 shell gap are included. Monte Carlo shell-model calculations suggest that the latter two states are part of a prolate-deformed intruder sequence, establishing an instance of shape coexistence at low excitation energies similar to that observed recently in neighboring Ni68