109 research outputs found
Direct reaction measurements with a 132Sn radioactive ion beam
The (d,p) neutron transfer and (d,d) elastic scattering reactions were
measured in inverse kinematics using a radioactive ion beam of 132Sn at 630
MeV. The elastic scattering data were taken in a region where Rutherford
scattering dominated the reaction, and nuclear effects account for less than 8%
of the cross section. The magnitude of the nuclear effects was found to be
independent of the optical potential used, allowing the transfer data to be
normalized in a reliable manner. The neutron-transfer reaction populated a
previously unmeasured state at 1363 keV, which is most likely the
single-particle 3p1/2 state expected above the N=82 shell closure. The data
were analyzed using finite range adiabatic wave calculations and the results
compared with the previous analysis using the distorted wave Born
approximation. Angular distributions for the ground and first excited states
are consistent with the previous tentative spin and parity assignments.
Spectroscopic factors extracted from the differential cross sections are
similar to those found for the one neutron states beyond the benchmark
doubly-magic nucleus 208Pb.Comment: 22 pages, 7 figure
FY2010 Annual Report for the Actinide Isomer Detection Project
This project seeks to identify a new signature for actinide element detection in active interrogation. This technique works by exciting and identifying long-lived nuclear excited states (isomers) in the actinide isotopes and/or primary fission products. Observation of isomers in the fission products will provide a signature for fissile material. For the actinide isomers, the decay time and energy of the isomeric state is unique to a particular isotope, providing an unambiguous signature for Special Nuclear Materials (SNM). Future work will include a follow-up measurement scheduled for December 2010 at LBNL. Lessons learned from the July 2010 measurements will be incorporated into these new measurements. Analysis of both the July and December experiments will be completed in a few months. A research paper to be submitted to a peer-reviewed journal will be drafted if the conclusions from the measurements warrant publication
Neutron time-of-flight measurements of charged-particle energy loss in inertial confinement fusion plasmas
Neutron spectra from secondary ^{3}H(d,n)Ī± reactions produced by an implosion of a deuterium-gas capsule at the National Ignition Facility have been measured with order-of-magnitude improvements in statistics and resolution over past experiments. These new data and their sensitivity to the energy loss of fast tritons emitted from thermal ^{2}H(d,p)^{3}H reactions enable the first statistically significant investigation of charged-particle stopping via the emitted neutron spectrum. Radiation-hydrodynamic simulations, constrained to match a number of observables from the implosion, were used to predict the neutron spectra while employing two different energy loss models. This analysis represents the first test of stopping models under inertial confinement fusion conditions, covering plasma temperatures of k_{B}Tā1-4āākeV and particle densities of nā(12-2)Ć10^{24}āācm^{-3}. Under these conditions, we find significant deviations of our data from a theory employing classical collisions whereas the theory including quantum diffraction agrees with our data
The magic nature of 132Sn explored through the single-particle states of 133Sn
Atomic nuclei have a shell structure where nuclei with 'magic numbers' of
neutrons and protons are analogous to the noble gases in atomic physics. Only
ten nuclei with the standard magic numbers of both neutrons and protons have so
far been observed. The nuclear shell model is founded on the precept that
neutrons and protons can move as independent particles in orbitals with
discrete quantum numbers, subject to a mean field generated by all the other
nucleons. Knowledge of the properties of single-particle states outside nuclear
shell closures in exotic nuclei is important for a fundamental understanding of
nuclear structure and nucleosynthesis (for example the r-process, which is
responsible for the production of about half of the heavy elements). However,
as a result of their short lifetimes, there is a paucity of knowledge about the
nature of single-particle states outside exotic doubly magic nuclei. Here we
measure the single-particle character of the levels in 133Sn that lie outside
the double shell closure present at the short-lived nucleus 132Sn. We use an
inverse kinematics technique that involves the transfer of a single nucleon to
the nucleus. The purity of the measured single-particle states clearly
illustrates the magic nature of 132Sn.Comment: 19 pages, 5 figures and 4 table
Measurement of the Entry-Spin Distribution Imparted to the High Excitation Continuum Region of Gadolinium Nuclei Via (\u3cem\u3ep,d\u3c/em\u3e) and (\u3cem\u3ep,t\u3c/em\u3e) Reactions
Over the last several years, the surrogate reaction technique has been successfully employed to extract (n,f) and (n,Ī³) cross sections in the actinide region to a precision of ā¼5% and ā¼20%, respectively. However, attempts to apply the technique in the rare earth region have shown large (factors of 2ā3) discrepancies between the directly measured (n,Ī³) and extracted surrogate cross sections. One possible origin of this discrepancy lies in differences between the initial spin-parity population distribution in the neutron induced and surrogate reactions. To address this issue, the angular momentum transfer to the high excitation energy quasicontinuum region in Gd nuclei has been investigated. The (p,d) and (p,t) reactions on 154,158Gd at a beam energy of 25 MeV were utilized. Assuming a single dominant angular momentum transfer component, the measured angular distribution for the (p,d) reactions is well reproduced by distorted-wave Born approximation (DWBA) calculations for ĪL=4 ā transfer, whereas the (p,t) reactions are better characterized by ĪL=5 ā. A linear combination of DWBA calculations, weighted according to a distribution of L transfers (peaking around ĪL=4ā5 ā), is in excellent agreement with the experimental angular distributions
Remnants of Spherical Shell Structures in Deformed Nuclei: The Impact of an \u3cem\u3eN\u3c/em\u3e = 64 Neutron Subshell Closure on the Structure of \u3cem\u3eN\u3c/em\u3e ā 90 Gadolinium Nuclei
Odd-mass gadolinium isotopes around N = 90 were populated by the (p,d) reaction, utilizing 25-MeV protons, resulting in population of low-spin quasineutron states at energies near and below the Fermi surface. Systematics of the single quasineutron levels populated are presented. A large excitation energy gap is observed between levels originating from the 2d3/2, 1h11/2, and 3s1/2 spherical parents (above the N = 64 gap), and the 2d5/2 (below the gap), indicating that the spherical shell model level spacing is maintained at least to moderate deformations
Surrogate Measurement of the \u3csup\u3e238\u3c/sup\u3ePu(\u3cem\u3en,f\u3c/em\u3e\u3c/em\u3e) Cross Section
The neutron-induced fission cross section of 238Pu was determined using the surrogate ratio method. The (n,f) cross section over an equivalent neutron energy range 5ā20 MeV was deduced from inelastic Ī±-induced fission reactions on 239Pu, with 235U(Ī±,Ī±ā²f) and 236U(Ī±,Ī±ā²f) used as references. These reference reactions reflect 234U(n,f) and 235U(n,f) yields, respectively. The deduced 238Pu(n,f) cross section agrees well with standard data libraries up to ~10 MeV, although larger values are seen at higher energies. The difference at higher energies is less than 20%
Spectroscopy of \u3csup\u3e88\u3c/sup\u3eY by the (\u3cem\u3ep,dĪ³\u3c/em\u3e) Reaction
Low-spin, high-excitation energy states in 88Y have been studied using the 89Y(p,dĪ³) reaction. For this experiment a 25 MeV proton beam was incident upon a monoisotopic 89Y target. A silicon telescope array was used to detect deuterons, and coincident Ī³ rays were detected using a germanium clover array. Most of the known low-excitation-energy low-spin states populated strongly via the (p,d) reaction mechanism are confirmed. Two states are seen for the first time and seven new transitions, including one which bypasses the two low-lying isomeric states, are observed
Relative \u3csup\u3e235\u3c/sup\u3eU(\u3cem\u3en,Ī³\u3c/em\u3e) and (\u3cem\u3en,f\u3c/em\u3e) Cross Sections From \u3csup\u3e235\u3c/sup\u3eU(\u3cem\u3ed,pĪ³\u3c/em\u3e) and (\u3cem\u3ed,pf\u3c/em\u3e)
The internal surrogate ratio method allows for the determination of an unknown cross section, such as (n,Ī³), relative to a better-known cross section, such as (n,f), by measuring the relative exit-channel probabilities of a surrogate reaction that proceeds through the same compound nucleus. The validity of the internal surrogate ratio method is tested by comparing the relative Ī³ and fission exit-channel probabilities of a 236Uā compound nucleus, formed in the 235U(d,p) reaction, to the known 235U(n,Ī³) and (n,f) cross sections. A model-independent method for measuring the Ī³-channel yield is presented and used
Ī³-ray decay from neutron-bound and unbound states in Mo 95 and a novel technique for spin determination
The emission of Ī³ rays from neutron-bound and neutron-unbound states in Mo95, populated in the Mo94(d,p) reaction, has been investigated. Charged particles and Ī³ radiation were detected with arrays of annular silicon and Clover-type high-purity Germanium detectors, respectively. Utilizing p-Ī³ and p-Ī³-Ī³ coincidences, the Mo95 level scheme was greatly enhanced with 102 new transitions and 43 new states. It agrees well with shell model calculations for excitation energies below ā2 MeV. From p-Ī³ coincidence data, a new method for the determination of spins of discrete levels is proposed. The method exploits the suppression of high-angular momentum neutron emission from levels with high spins populated in the (d,p) reaction above the neutron separation energy. Spins for almost all Mo95 levels below 2 MeV (and for a few levels above) have been determined with this method
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