Final Mass Distribution Research Articles

Intermittency: A test of nuclear fragmentation described in terms of sequential decay models

Nuclear fragmentation may show specific features such as intermittency which induces self-similarity at all scales in final mass or charge distributions. We test this property on specific rate-equation models which are aimed at describing sequential decay processes. We use analytical arguments which are corroborated by numerical calculations in order to show that the generic structure of these equations does not allow for the presence of intermittency.

Mass transhipment problems and ideal metrics

Given initial and final mass distributions P and Q on we study dual and explicit solutions of the mass transhipment problem: Minimize subject to for all Borel where m n is a signed measure with d.f. having derivative For the usual choice of the cost function, the solution of the transhipment problem determines an ideal metric of order kn + 1.

Sequential decay of excited nuclei and characteristic features of nuclear fragmentation

Sequential decay of excited nuclei is described as a succession of binary processes which lead to final mass and charge distributions. The production rates are obtained event-by-event by means of Monte Carlo simulation techniques. Moments of the distributions and other observables calculated for three different excited nuclei are compared with those obtained from percolation models. The results are discussed in the framework of the universality predictions of percolation theory.

Dynamical multifragmentation of highly excited nuclear systems

In the framework of a dynamical multifragmentation model we investigate the fragmentation of expanding nuclear systems. We find that the stable fragment mass multiplicity distribution differs (in some cases essentially) from the cluster distribution in the statistical equilibrium investigated so far. We especially investigate the pure power-law ansatz of the equilibrium distribution and find that in certain cases the fragmentation process leads to final mass distributions again in the form of a power-law, however, with an effective exponent.

Relaxation and tidal stripping in rich clusters of galaxies. I - Evolution of the mass distribution

A set of differential equations is derived which describes the evolution of a rich cluster of galaxies over relaxation time scales. Two physical processes are included. Exchange of orbital kinetic energy between galaxies of different masses, due to gravitational two-body encounters, is computed from the isotropic, orbit-averaged Fokker-Planck equation. Tidal stripping of galaxies with massive halos is approximated by the cross sections of Richstone. The fraction of cluster mass in the "background" is allowed to increase with time as the galaxies are stripped. The equations are integrated numerically, starting from a Schechter distribution of galaxy masses. Unless most of the virial mass is initially in the background, tidal stripping tends to obscure the effects of mass segregation by rapidly depleting the cluster core of massive galaxies. The final core mass distribution is qualitatively similar in both cases to that of a cD galaxy, in that it contains a central mass of low velocity dispersion surrounded by a high velocity dispersion envelope.

Mass distribution of heavy ion fission within a dynamical treatment

Dynamical calculations for the fluctuations of the collective variables involved in fission are obtained by a proper decomposition of the full density. The final mass distributions of the fission fragments appear to be independent on the angular momentum and on the friction strength.

Dependence of neutron yield on fragment mass for several low-energy fissioning systems

Neutron-yield functions for $^{245}\mathrm{Cm}$($n$,$f$), $^{252}\mathrm{Cf}$(sf), $^{254}\mathrm{Cf}$(sf), $^{254}\mathrm{Es}$($n$,$f$), and $^{256}\mathrm{Fm}$(sf) are derived with an iterative method in which successive approximations to the pre-neutron-emission (initial) mass distribution are compared with the post-neutron-emission (final) mass distribution. Initial mass distributions are deduced from kinetic energy measurements of the fragments. Final mass distributions are determined from radiochemical measurements of fission product mass yields. The $^{252}\mathrm{Cf}$(sf) neutron-yield function so derived agrees well with functions determined directly by neutron counting. All neutron-yield functions exhibit a saw-toothed character. The masses for which maximum neutron emission occurs and for which minimum neutron emission occurs are within \ifmmode\pm\else\textpm\fi{} 2 mass units of being complementary masses for the observed fissioning system, with the exception of the $^{245}\mathrm{Cm}$($n$,$f$) reaction. The effect of the neutron-yield function on various mass and energy correlations deduced from kinetic energy measurements is discussed.NUCLEAR REACTIONS, FISSION $^{254}\mathrm{Cm}$($n$,$f$), $^{252}\mathrm{Cf}$(sf), $^{254}\mathrm{Cf}$(sf), $^{254}\mathrm{Es}$($n$,$f$), $^{256}\mathrm{Fm}$(sf). Deduced neutron yields, fragment mass and energy distributions.

Distribution of mass, kinetic energy, and neutron yield in the spontaneous fission ofFm254

The mass distribution for spontaneous fission of /sup 254/Fm was determined for both pre- and post-neutron-emission fragments. The post-neutron-emission (final) mass distribution was deduced from radiochemical measurements of 24 mass yields. Starting with an assumed distribution of neutron yield versus fragment mass, the pre-neutron-emission (initial) mass distribution and neutron-yield distribution were derived from the kinetic-energy measurements of the fragments and the final-mass distribution by an iterative method. These distributions together with distributions of fragment kinetic energy, total kinetic energy of both fragments, and average kinetic energy and average total kinetic energy as functions of fragment mass are presented. The results are discussed in terms of a scission-point model of fission.

Dynamics of Plane Stellar Systems

AbstractThe evolution of initially balanced rotating disks of stars is investigated with a computer model for isolated disks of stars. An isolated, initially cold balanced disk is found to be violently unstable. Adding a sufficient amount of velocity dispersion will stabilize all small-scale disturbances. However, the disks are still unstable against slowly growing long wave-length modes and after about two rotations most disks tend to assume a bar-shaped structure. It is found that the final mass distribution over most of the disk can be closely approximated by an exponential variation, irrespective of the initial mass distribution. The gravitational two-stream instability is investigated by means of a modified computer model for infinite doubly periodic stellar systems.

Numerical Experiments with a Disk of Stars

view Abstract Citations (380) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Numerical Experiments with a Disk of Stars Hohl, Frank Abstract The evolution of an initially balanced rotating disk of stars with an initial velocity dispersion given by Toomre's local criterion is investigated by means of a computer model for isolated disks of stars. It is found that the disk is unstable against very large-scale modes. After about two rotations the central portion of the disk tends to assume a bar-shaped structure. A stable axisymmetric disk with a velocity dispersion much larger than that given by Toomre's criterion is generated. The final mass distribution for the disk gives a high-density central core and a disk population of stars that is closely approximated by an exponential variation. Publication: The Astrophysical Journal Pub Date: September 1971 DOI: 10.1086/151091 Bibcode: 1971ApJ...168..343H full text sources ADS |

Simulation of the evolution of stellar systems

Computer models for isolated disk galaxies and for infi nite doubly periodic stellar systems are used to study the self-consistent motion of large numbers of interacting stars as they move in the plane of the galactic disk. The doubly periodic model is used to investigate the develop ment of the gravitational instability for a plane stellar system. The results obtained show that the initial con densations which form are about two Jeans lengths in diameter. An isolated, initially cold, balanced disk is found to be violently unstable. Adding a sciffcient amount of ve locity dispersion will stabilize all small-scale disturb ances. Most disks investigated are found to be unstable against slowly growing long wavelength modes, and after about two rotations the disks tend to assume a bar- shaped structure. It is also found that the final mass distribution for most disks can be closely approximated by an exponential variation irrespective of the initial mass distribution.

Correlated mass and energy distributions of fission fragments from 235U by simultaneous velocity and energy measurements

The object of this work is the simultaneous determination of the mass and energy distributions both before (initial) and after (final) neutron emission of the fragments from thermal, neutron-induced fission of 235U. The method which is developed involves the measurement of the correlated energies and velocities of the fission fragments from 235U, by using a time-of-flight technique with solid state barrier layer detectors for the fragment detection. Fine structure in the initial mass yields, as well as in the final ones, is observed, with a characteristic periodicity of about 5 amu in both cases. The peaks in the structure of the final mass distribution are shifted by one to two mass units with respect to the peaks, which appear in the structure of the initial mass distribution. With about 2 % experimental mass uncertainty no additional structure is observed due to the dependence of the neutron emission on the mass.