342 research outputs found
Gold fragmentation induced by stopped antiprotons
A natural gold target was irradiated with the antiproton beam from the Low
Energy Antiproton Ring at CERN. Antiprotons of 200 MeV/c momentum were stopped
in a thick target, products of their annihilations on Au nuclei were detected
using the off-line gamma-ray spectroscopy method. In total, yields for 114
residual nuclei were determined, providing a data set to deduce the complete
mass and charge distribution of all products with A > 20 from a fitting
procedure. The contribution of evaporation and fission decay modes to the total
reaction cross section as well as the mean mass loss were estimated. The
fission probability for Au absorbing antiprotons at rest was determined to be
equal to (3.8+-0.5)%, in good agreement with an estimation derived using other
techniques. The mass-charge yield distribution was compared with the results
obtained for proton and pion induced gold fragmentation. On the average, the
energy released in pbar annihilation is similar to that introduced by ~ 1 GeV
protons. However, compared to proton bombardment products, the yield
distribution of antiproton absorption residues in the N-Z plane is clearly
distinct. The data for antiprotons exhibit also a substantial influence of
odd-even and shell effects.Comment: 14 pages, 9 figures, Revtex 4, to be published in Physical Review
Information on antiprotonic atoms and the nuclear periphery from the PS209 experiment
In the PS209 experiments at CERN two kinds of measurements were performed:
the in-beam measurement of X-rays from antiprotonic atoms and the
radiochemical, off-line determination of the yield of annihilation products
with mass number A_t -1 (less by 1 than the target mass). Both methods give
observables which allows to study the peripheral matter density composition and
distribution.Comment: LaTeX (espcrc1 style), 6 pages, 3 EPS figures, 1 table, Proceedings
of the Sixth Biennal Conference on Low-Energy Antiproton Physics LEAP 2000,
Venice, Ital
Muscle Activity during Maximal Isometric Forearm Rotation Using a Power Grip
This study aimed to provide quantitative activation data for muscles of the forearm during pronation and supination while using a power grip. Electromyographic data was collected from 15 forearm muscles in 11 subjects while they performed maximal isometric pronating and supinating efforts in nine positions of forearm rotation. Biceps brachii was the only muscle with substantial activation in only one effort direction. It was significantly more active when supinating (µ = 52.1%, SD = 17.5%) than pronating (µ = 5.1%, SD = 4.8%, p \u3c .001). All other muscles showed considerable muscle activity during both pronation and supination. Brachioradialis, flexor carpi radialis, palmaris longus, pronator quadratus and pronator teres were significantly more active when pronating the forearm. Abductor pollicis longus and biceps brachii were significantly more active when supinating. This data highlights the importance of including muscles additional to the primary forearm rotators in a biomechanical analysis of forearm rotation. Doing so will further our understanding of forearm function and lead to the improved treatment of forearm fractures, trauma-induced muscle dysfunction and joint replacements
Neutron density distributions from antiprotonic 208Pb and 209Bi atoms
The X-ray cascade from antiprotonic atoms was studied for 208Pb and 209Bi.
Widths and shifts of the levels due to the strong interaction were determined.
Using modern antiproton-nucleus optical potentials the neutron densities in the
nuclear periphery were deduced. Assuming two parameter Fermi distributions
(2pF) describing the proton and neutron densities the neutron rms radii were
deduced for both nuclei. The difference of neutron and proton rms radii /\r_np
equal to 0.16 +-(0.02)_{stat} +- (0.04)_{syst} fm for 208Pb and 0.14 +-
(0.04)_{stat} +- (0.04)_{syst} fm for 209Bi were determined and the assigned
systematic errors are discussed. The /\r_np values and the deduced shapes of
the neutron distributions are compared with mean field model calculations.Comment: 22 pages, 8 tables, 15 figure
Optimal search strategies for hidden targets
What is the fastest way of finding a randomly hidden target? This question of
general relevance is of vital importance for foraging animals. Experimental
observations reveal that the search behaviour of foragers is generally
intermittent: active search phases randomly alternate with phases of fast
ballistic motion. In this letter, we study the efficiency of this type of two
states search strategies, by calculating analytically the mean first passage
time at the target. We model the perception mecanism involved in the active
search phase by a diffusive process. In this framework, we show that the search
strategy is optimal when the average duration of "motion phases" varies like
the power either 3/5 or 2/3 of the average duration of "search phases",
depending on the regime. This scaling accounts for experimental data over a
wide range of species, which suggests that the kinetics of search trajectories
is a determining factor optimized by foragers and that the perception activity
is adequately described by a diffusion process.Comment: 4 pages, 5 figures. to appear in Phys. Rev. Let
Tracking the phase-transition energy in disassembly of hot nuclei
In efforts to determine phase transitions in the disintegration of highly
excited heavy nuclei, a popular practice is to parametrise the yields of
isotopes as a function of temperature in the form
, where 's are the measured yields
and and are fitted to the yields. Here would be
interpreted as the phase transition temperature. For finite systems such as
those obtained in nuclear collisions, this parametrisation is only approximate
and hence allows for extraction of in more than one way. In this work we
look in detail at how values of differ, depending on methods of
extraction. It should be mentioned that for finite systems, this approximate
parametrisation works not only at the critical point, but also for first order
phase transitions (at least in some models). Thus the approximate fit is no
guarantee that one is seeing a critical phenomenon. A different but more
conventional search for the nuclear phase transition would look for a maximum
in the specific heat as a function of temperature . In this case is
interpreted as the phase transition temperature. Ideally and would
coincide. We invesigate this possibility, both in theory and from the ISiS
data, performing both canonical () and microcanonical ()
calculations. Although more than one value of can be extracted from the
approximate parmetrisation, the work here points to the best value from among
the choices. Several interesting results, seen in theoretical calculations, are
borne out in experiment.Comment: Revtex, 10 pages including 8 figures and 2 table
Thermal excitation of heavy nuclei with 5-15 GeV/c antiproton, proton and pion beams
Excitation-energy distributions have been derived from measurements of
5.0-14.6 GeV/c antiproton, proton and pion reactions with Au target
nuclei, using the ISiS 4 detector array. The maximum probability for
producing high excitation-energy events is found for the antiproton beam
relative to other hadrons, He and beams from LEAR. For protons
and pions, the excitation-energy distributions are nearly independent of hadron
type and beam momentum above about 8 GeV/c. The excitation energy enhancement
for beams and the saturation effect are qualitatively consistent with
intranuclear cascade code predictions. For all systems studied, maximum cluster
sizes are observed for residues with E*/A 6 MeV.Comment: 14 pages including 5 figures and 1 table. Accepted in Physics Letter
B. also available at http://nuchem.iucf.indiana.edu
The liquid to vapor phase transition in excited nuclei
For many years it has been speculated that excited nuclei would undergo a
liquid to vapor phase transition. For even longer, it has been known that
clusterization in a vapor carries direct information on the liquid- vapor
equilibrium according to Fisher's droplet model. Now the thermal component of
the 8 GeV/c pion + 197Au multifragmentation data of the ISiS Collaboration is
shown to follow the scaling predicted by Fisher's model, thus providing the
strongest evidence yet of the liquid to vapor phase transition.Comment: four pages, four figures, first two in color (corrected typo in Ref.
[26], corrected error in Fig. 4
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