374 research outputs found
Correlations in Nuclear Arrhenius-Type Plots
Arrhenius-type plots for multifragmentation process, defined as the
transverse energy dependence of the single-fragment emission-probability,
-ln(p_{b}) vs 1/sqrt(E_{t}), have been studied by examining the relationship of
the parameters p_{b} and E_{t} to the intermediate-mass fragment multiplicity
. The linearity of these plots reflects the correlation of the fragment
multiplicity with the transverse energy. These plots may not provide thermal
scaling information about fragment production as previously suggested.Comment: 12 pages, Latex, 3 Postscript figures include
A statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy
Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV
has been studied with multidetector systems covering nearly 4-pi in solid
angle. The correlations of both the intermediate mass fragment and light
charged particle multiplicities with the transverse energy are explored. A
comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The
experimental trends are compared to statistical model predictions.Comment: 7 pages, submitted to Phys. Rev.
Lattice gas model for fragmentation: From Argon on Scandium to Gold on Gold
The recent fragmentation data for central collisions of Gold on Gold are even
qualitatively different from those for central collisions of Argon on Scandium.
The latter can be fitted with a lattice gas model calculation. Effort is made
to understand why the model fails for Gold on Gold. The calculation suggests
that the large Coulomb interaction which is operative for the larger system is
responsible for this discrepancy. This is demonstrated by mapping the lattice
gas model to a molecular dynamics calculation for disassembly. This mapping is
quite faithful for Argon on Scandium but deviates strongly for Gold on Gold.
The molecular dynamics calculation for disassembly reproduces the
characteristics of the fragmentation data for both Gold on Gold and Argon on
Scandium.Comment: 13 pages, Revtex, 8 figures in ps files, submitted to Phys. Rev.
Size Matters: Origin of Binomial Scaling in Nuclear Fragmentation Experiments
The relationship between measured transverse energy, total charge recovered
in the detector, and size of the emitting system is investigated. Using only
very simple assumptions, we are able to reproduce the observed binomial
emission probabilities and their dependences on the transverse energy.Comment: 14 pages, including 4 figure
The three-dimensional Ising model: A paradigm of liquid-vapor coexistence in nuclear multifragmentation
Clusters in the three-dimensional Ising model rigorously obey reducibility
and thermal scaling up to the critical temperature. The barriers extracted from
Arrhenius plots depend on the cluster size as where
is a critical exponent relating the cluster size to the cluster
surface. All the Arrhenius plots collapse into a single Fisher-like scaling
function indicating liquid-vapor-like phase coexistence and the univariant
equilibrium between percolating clusters and finite clusters. The compelling
similarity with nuclear multifragmentation is discussed.Comment: (4 pages, 4 figures
To the Continuum and Beyond: Structure of U Nuclei
An experiment was performed at the 88-inch cyclotron at LBNL to investigate the structure of uranium isotopes and concurrently test the so-called surrogate ratio method. A 28 MeV proton beam was used to bombard 236U and 238U targets and the outgoing light ions were detected using the STARS silicon telescope allowing isotopic assignments and the excitation energy of the compound nucleus to be measured. A fission detector was placed at backward angles to give particle-fission coincidences, while the six clover germanium detectors of the LIBERACE array were used for particle-Îł coincidences. The (p,d) reaction channels on 236U and 238U targets were used as a surrogate to measure the Ï(234U(n,f))/Ï(236U(n,f)) cross section ratio. The results give reasonable agreement with literature values over an equivalent neutron energy range between 0 MeV and 6 MeV. Structure results in 235U include a new (3/2â) level at 1035 keV, that is tentatively assigned as the 3/2â[501] Nilsson state. The analogue 3/2â[501] state in 237U may be associated with a previously observed level at 1201 keV, whose spin/parity is restricted to JÏ = 3/2â on the basis of newly observed decays to the ground band
Charge correlations and dynamical instabilities in the multifragment emission process
A new, sensitive method allows one to search for the enhancement of events
with nearly equal-sized fragments as predicted by theoretical calculations
based on volume or surface instabilities. Simulations have been performed to
investigate the sensitivity of the procedure. Experimentally, charge
correlations of intermediate mass fragments emitted from heavy ion reactions at
intermediate energies have been studied. No evidence for a preferred breakup
into equal-sized fragments has been found.Comment: 12 pages, TeX type, psfig, submitted to Phys. Rev. Lett, also
available at http://csa5.lbl.gov/moretto/ps/zcor_pp.p
Utilizing (\u3cem\u3ep,d\u3c/em\u3e) and (\u3cem\u3ep,t\u3c/em\u3e) Reactions to Obtain (\u3cem\u3en,f\u3c/em\u3e) Cross Sections in Uranium Nuclei Via the Surrogate-Ratio Method
The surrogate ratio method has been tested for (p,d) and (p,t) reactions on uranium nuclei. 236U and 238U targets were bombarded with 28-MeV protons and the light ion recoils and fission fragments were detected using the Silicon Telescope Array for Reaction Studies detector array at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. The (p,df) reaction channels on 236U and 238U targets were used as a surrogate to determine the Ï[236U(n,f)]/Ï[234U(n,f)] cross-section ratio. The (p,tf) reaction channels were also measured with the same targets as a surrogate for the Ï[235U(n,f)]/Ï[(233U(n,f)] ratio. For the (p,df) and (p,tf) surrogate measurements, there is good agreement with accepted (n,f) values over equivalent neutron energy ranges of En=0â7 MeV and En=0â5.5 MeV, respectively. An internal surrogate ratio method comparing the (p,d) and (p,t) reaction channels on a single target is also discussed. The Ï[234U(n,f)]/Ï[233U(n,f)] and Ï[236U(n,f)]/Ï[235U(n,f)] cross-section ratios are extracted using this method for the 236U and 238U targets, respectively. The resulting fission cross-section ratios show relatively good agreement with accepted values up to EnâŒ5 MeV
Effect of a Single Amino Acid Change in MHC Class I Molecules on the Rate of Progression to AIDS
Background From studies of genetic polymorphisms and the rate of progression from human immunodeficiency virus type 1 (HIV-1) infection to the acquired immunodeficiency syndrome (AIDS), it appears that the strongest susceptibility is conferred by the major-histocompatibility-complex (MHC) class I type HLA-B*35,Cw*04 allele. However, cytotoxic T-lymphocyte responses have been observed against HIV-1 epitopes presented by HLA-B*3501, the most common HLA-B*35 subtype. We examined subtypes of HLA-B*35 in five cohorts and analyzed the relation of structural differences between HLA-B*35 subtypes to the risk of progression to AIDS. Methods Genotyping of HLA class I loci was performed for 850 patients who seroconverted and had known dates of HIV-1 infection. Survival analyses with respect to the rate of progression to AIDS were performed to identify the effects of closely related HLAB* 35 subtypes with different peptide-binding specificities.
Results HLA-B*35 subtypes were divided into two groups according to peptide-binding specificity: the HLA-B*35-PY group, which consists primarily of HLAB* 3501 and binds epitopes with proline in position 2 and tyrosine in position 9; and the more broadly reactive HLA-B*35-Px group, which also binds epitopes with proline in position 2 but can bind several different amino acids (not including tyrosine) in position 9. The influence of HLA-B*35 in accelerating progression to AIDS was completely attributable to HLAB* 35-Px alleles, some of which differ from HLA-B*35- PY alleles by only one amino acid residue.
Conclusions This analysis shows that, in patients with HIV-1 infection, a single amino acid change in HLA molecules has a substantial effect on the rate of progression to AIDS. The different consequences of HLA-B*35-PY and HLA-B*35-Px in terms of disease progression highlight the importance of the epitope specificities of closely related class I molecules in the immune defense against HIV-1
Liquid-gas phase transition in nuclear multifragmentation
The equation of state of nuclear matter suggests that at suitable beam
energies the disassembling hot system formed in heavy ion collisions will pass
through a liquid-gas coexistence region. Searching for the signatures of the
phase transition has been a very important focal point of experimental
endeavours in heavy ion collisions, in the last fifteen years. Simultaneously
theoretical models have been developed to provide information about the
equation of state and reaction mechanisms consistent with the experimental
observables. This article is a review of this endeavour.Comment: 63 pages, 27 figures, submitted to Adv. Nucl. Phys. Some typos
corrected, minor text change
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