118 research outputs found
The first direct measurement of ¹²C (¹²C,n) ²³Mg at stellar energies
Neutrons produced by the carbon fusion reaction ¹²C(¹²C,n)²³Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction ¹²C(¹²C,p)²³Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that ¹²C(¹²C,n)²³Mg is crucial to the production of Na and Al in Pop-III Pair Instability Supernovae. It also plays a non-negligible role in the production of weak s-process elements as well as in the production of the important galacti
In-beam spectroscopy of medium- and high-spin states in Ce
Medium and high-spin states in Ce were investigated using the
Cd(Ne, ) reaction and the Gammasphere array. The level
scheme was extended up to an excitation energy of MeV and spin 93/2
. Eleven bands of quadrupole transitions and two new dipole bands are
identified. The connections to low-lying states of the previously known,
high-spin triaxial bands were firmly established, thus fixing the excitation
energy and, in many cases, the spin parity of the levels. Based on comparisons
with cranked Nilsson-Strutinsky calculations and tilted axis cranking covariant
density functional theory, it is shown that all observed bands are
characterized by pronounced triaxiality. Competing multiquasiparticle
configurations are found to contribute to a rich variety of collective
phenomena in this nucleus.Comment: 20 pages, 11 figure
Probing the single-particle character of rotational states in F using a short-lived isomeric beam
A beam containing a substantial component of both the ,
ns isomeric state of F and its , 109.77-min ground
state has been utilized to study members of the ground-state rotational band in
F through the neutron transfer reaction , in inverse kinematics.
The resulting spectroscopic strengths confirm the single-particle nature of the
13/2 band-terminating state. The agreement between shell-model
calculations, using an interaction constructed within the shell, and our
experimental results reinforces the idea of a single-particle/collective
duality in the descriptions of the structure of atomic nuclei
Key Ne states identified affecting -ray emission from F in novae
Detection of nuclear-decay rays provides a sensitive thermometer of
nova nucleosynthesis. The most intense -ray flux is thought to be
annihilation radiation from the decay of F, which is destroyed
prior to decay by the F(,)O reaction. Estimates of
F production had been uncertain, however, because key near-threshold
levels in the compound nucleus, Ne, had yet to be identified. This
Letter reports the first measurement of the
F(He,)Ne reaction, in which the placement of two
long-sought 3/2 levels is suggested via triton--
coincidences. The precise determination of their resonance energies reduces the
upper limit of the rate by a factor of at nova temperatures and
reduces the average uncertainty on the nova detection probability by a factor
of 2.1.Comment: 6 pages, 4 figure
New -ray Transitions Observed in Ne with Implications for the O(,)Ne Reaction Rate
The O(,)Ne reaction is responsible for breakout
from the hot CNO cycle in Type I x-ray bursts. Understanding the properties of
resonances between and 5 MeV in Ne is crucial in the
calculation of this reaction rate. The spins and parities of these states are
well known, with the exception of the 4.14- and 4.20-MeV states, which have
adopted spin-parities of 9/2 and 7/2, respectively. Gamma-ray
transitions from these states were studied using triton--
coincidences from the F(He,)Ne reaction measured
with GODDESS (Gammasphere ORRUBA Dual Detectors for Experimental Structure
Studies) at Argonne National Laboratory. The observed transitions from the
4.14- and 4.20-MeV states provide strong evidence that the values are
actually 7/2 and 9/2, respectively. These assignments are consistent
with the values in the F mirror nucleus and in contrast to previously
accepted assignments
Fast-timing measurements in the ground-state band of 114Pd
Using a hybrid Gammasphere array coupled to 25 LaBr3(Ce) detectors, the lifetimes of the first three levels
of the yrast band in 114Pd, populated via 252Cf decay, have been measured. The measured lifetimes are Ï„2+ =
103(10) ps, Ï„4+ = 22(13) ps, and Ï„6+ 10 ps for the 2+
1 , 4+
1 , and 6+
1 levels, respectively. Palladium-114 was
predicted to be the most deformed isotope of its isotopic chain, and spectroscopic studies have suggested it
might also be a candidate nucleus for low-spin stable triaxiality. From the lifetimes measured in this work,
reduced transition probabilities B(E2; J → J − 2) are calculated and compared with interacting boson model,
projected shell model, and collective model calculations from the literature. The experimental ratio RB(E2) =
B(E2; 4+
1 → 2+
1 )/B(E2; 2+
1 → 0+
1 ) = 0.80(42) is measured for the first time in 114Pd and compared with the
known values RB(E2) in the palladium isotopic chain: the systematics suggest that, for N = 68, a transition from
γ -unstable to a more rigid γ -deformed nuclear shape occurs.This work was financially supported by the Science
and Technology Facility Council (STFC) Grants
No. ST/L005840/1, No. ST/L005743/1, and No.
ST/G000751/1. This work has also been partially supported
by the U.S. Department of Energy, Office of Science, Office of
Nuclear Physics under Contract No. DE-AC02-06CH11357
(ANL). E.R.G. would like to acknowledge the STFC for
funding via his Ph.D. studentship. D.J.H. acknowledges
the National Science Foundation, Grant No. PHY-1502092.
E.A.S. and O.Y. would like to acknowledge the project
DFNI-E02/6
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
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