307 research outputs found
Structure of unbound neutron-rich He studied using single-neutron transfer
The 8He(d,p) reaction was studied in inverse kinematics at 15.4A MeV using
the MUST2 Si-CsI array in order to shed light on the level structure of 9He.
The well known 16O(d,p)17O reaction, performed here in reverse kinematics, was
used as a test to validate the experimental methods. The 9He missing mass
spectrum was deduced from the kinetic energies and emission angles of the
recoiling protons. Several structures were observed above the neutron-emission
threshold and the angular distributions were used to deduce the multipolarity
of the transitions. This work confirms that the ground state of 9He is located
very close to the neutron threshold of 8He and supports the occurrence of
parity inversion in 9He.Comment: Exp\'erience GANIL/SPIRAL1/MUST
Search for low lying dipole strength in the neutron rich nucleus Ne
Coulomb excitation of the exotic neutron-rich nucleus Ne on a
Pb target was measured at 58 A.MeV in order to search for low-lying E1
strength above the neutron emission threshold. Data were also taken on an
Al target to estimate the nuclear contribution. The radioactive beam
was produced by fragmentation of a 95 A.MeV Ar beam delivered by the
RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged
fragment hodoscope and a neutron wall. Using the invariant mass method in the
Ne+n channel, we observe a sizable amount of E1 strength between 6 and
10 MeV. The reconstructed Ne angular distribution confirms its E1
nature. A reduced dipole transition probability of B(E1)=0.490.16
is deduced. For the first time, the decay pattern of low-lying
strength in a neutron-rich nucleus is obtained. The results are discussed in
terms of a pygmy resonance centered around 9 MeV
Experimental investigation of ground-state properties of <sup>7</sup>H with transfer reactions
The properties of nuclei with extreme neutronâtoâproton ratios, far from those naturally occurring on Earth, are key to understand nuclear forces and how nucleons hold together to form nuclei. 7H, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutronâtoâproton ratio known so far. However, its sheer existence and properties are still a challenge for experimental efforts and theoretical models. Here we report experimental evidences on the formation of 7H as a resonance, detected with independent observables, and the first measurement of the structure of its ground state. The resonance is found at âŒ0.7 MeV above the 3H+4n mass, with a narrow width of âŒ0.2 MeV and a 1/2+ spin and parity. These data are consistent with a 7H as a 3H core surrounded by an extended four-neutron halo, with a unique four-neutron decay and a relatively long half-life thanks to neutron pairing; a prime example of new phenomena occurring in what would be the most pure-neutron nuclear matter we can access in the laboratory.</p
Linear Responses in Time-dependent Hartree-Fock-Bogoliubov Method with Gogny Interaction
A numerical method to integrate the time-dependent Hartree-Fock Bogoliubov
(TDHFB) equations with Gogny interaction is proposed. The feasibility of the
TDHFB code is illustrated by the conservation of the energy, particle numbers,
and center-of-mass in the small amplitude vibrations of oxygen 20. The TDHFB
code is applied to the isoscalar quadrupole and/or isovector dipole vibrations
in the linear (small amplitude) region in oxygen isotopes (masses A = 18,20,22
and 24), titanium isotopes (A = 44,50,52 and 54), neon isotope (A = 26), and
magnesium isotopes (A = 24 and 34). The isoscalar quadrupole and isovector
dipole strength functions are calculated from the expectation values of the
isoscalar quadrupole and isovector dipole moments.Comment: 10 pages, 13 figure
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Gamma Ray Multiplicity of 252Cf Spontaneous Fission using LiBerACE
We are studying the gamma ray and neutron multiplicity of various fission processes, beginning with the spontaneous fission of {sup 252}Cf, for a variety of basic and applied science purposes. The Livermore-Berkeley Array for Collaborative Experiments (LiBerACE) consists of six high-purity germanium Clover detectors (HPGe) each enclosed by an array of 16 bismuth-germanate (BGO) detectors. These detectors were arranged in a cubic pattern around a 1 {micro}Ci {sup 252}Cf source to attempt to cover as much solid angle of gamma ray emission as possible with a high level of segmentation. The single-gamma detector response function is determined at several energies by tagging in a HPGe detector on the photopeak of one of two gamma rays in two-gamma ray calibration sources and observing the multiplicity of the remainder of the array. Summing these single-gamma responses in groups yields the response function of the array to higher multiplicity events, which are convolved with multiplicity distributions from theoretical models and compared to the measured results to test the models validity
The N = 16 spherical shell closure in 24O
The unbound excited states of the neutron drip-line isotope 24O have been
investigated via the 24O(p,p')23O+n reaction in inverse kinematics at a beam
energy of 62 MeV/nucleon. The decay energy spectrum of 24O* was reconstructed
from the momenta of 23O and the neutron. The spin-parity of the first excited
state, observed at Ex = 4.65 +/- 0.14 MeV, was determined to be Jpi = 2+ from
the angular distribution of the cross section. Higher lying states were also
observed. The quadrupole transition parameter beta2 of the 2+ state was
deduced, for the first time, to be 0.15 +/- 0.04. The relatively high
excitation energy and small beta2 value are indicative of the N = 16 shell
closure in 24O.Comment: to be submitted to Physical Review Letter
Statistical \u3cem\u3eÎł\u3c/em\u3e Rays in the Analysis of Surrogate Nuclear Reactions
The surrogate nuclear reaction method is being applied in many efforts to indirectly determine neutron-induced reaction cross sections on short-lived isotopes. This technique aims to extract accurate (n,Îł) cross sections from measured decay properties of the compound nucleus of interest (created using a different reaction). The advantages and limitations of a method that identifies the Îł-ray decay channel by detecting any high-energy (âstatisticalâ) Îł ray emitted during the relaxation of the compound nucleus were investigated. Data collected using the Silicon Telescope Array for Reaction Studies and Livermore-Berkeley Array for Collaborative Experiments silicon and germanium detector arrays were used to study the decay of excited gadolinium nuclei following inelastic proton scattering. In many cases, this method of identifying the Îł-ray decay channel can simplify the experimental data collection and greatly improve the detection efficiency for Îł-ray cascades. The results show sensitivity to angular-momentum differences between the surrogate reaction and the desired (n,Îł) reaction similar to an analysis performed using low-lying discrete transitions even when ratios of cross sections are considered
Absolute and Relative Surrogate Measurements of the \u3csup\u3e236\u3c/sup\u3eU(\u3cem\u3en,f\u3c/em\u3e) Cross Section as a Probe of Angular Momentum Effects
Using both the absolute and relative surrogate techniques, the 236U(n,f) cross section was deduced over an equivalent neutron energy range of 0 to 20 MeV. A 42 MeV 3He beam from the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory was used to perform a (3He,α) pickup reaction on targets of 235U (JÏ=7/2â) and 238U (JÏ = 0+) and the fission decay probabilities were determined. The 235U(3He,αf) and 238U(3He,αf) were surrogates for 233U(n,f) and 236U(n,f), respectively. The cross sections extracted using the surrogate method were compared to directly measured cross sections. The sensitivity of these cross sections to the JÏ -population distributions was explored
Surrogate Ratio Method in the Actinide Region Using the (\u3cem\u3eα,α\u27f\u3c/em\u3e) Reaction
In the Surrogate Method, the measured decay probability of a compound nucleus formed via a direct reaction is used to extract the cross section for a reaction with a different entrance channel that proceeds through the same compound nucleus. An extension of the Surrogate Method, the Surrogate Ratio Method (SRM), uses a ratio of measured decay probabilities to infer an unknown cross section relative to a known one. To test the SRM we compare the direct-reaction-induced fission probability ratio of 234U(α, αâ f ) to 236U(α, αâf ) with the ratio of cross sections of 233U(n, f ) to 235U(n, f ). These ratios were found to be in agreement over an equivalent neutron energy range of 0.4â18 MeV
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