51,068 research outputs found
Characteristics of Coulomb fission
Within an extended semiquantal theory we perform large-sized coupled-channel calculations involving 260 collective levels for Coulomb fission of 238U. Differential Coulomb fission cross sections are studied as a function of bombarding energy and impact parameter for several projectiles. In the Xe + U case, total cross sections are also given. We find a strong dependence on projectile charge number, PCF(180°)∼(Zp)6 in the region 50≤Zp≤92 for a fixed ratio E/ECoul, which might be helpful to separate Coulomb fission experimentally from sequential fission following transfer reactions. Since the cross sections are sensitive to the moment of inertia ⊖ at the saddle point, Coulomb fission can serve as a tool to investigate the dependence of ⊖ on elongation. The fragment angular distribution exhibits deviations from 1/sinθf which are pronounced at low incident energies. Our theory indicates that the recently measured Xe + U fission cross sections contain a major fraction of Coulomb-induced fission at E≤0.85 ECoul. NUCLEAR REACTIONS, FISSION Calculated Coulomb fission cross sections σ(Ep,θp) for 54Xe, 67Ho, 82Pb, 92U→92238U, fragment angular distribution, fission energy spectrum, mean spin value 〈Jf〉
Simulations of the Population of Centaurs I: The Bulk Statistics
Large-scale simulations of the Centaur population are carried out. The
evolution of 23328 particles based on the orbits of 32 well-known Centaurs is
followed for up to 3 Myr in the forward and backward direction under the
influence of the 4 massive planets. The objects exhibit a rich variety of
dynamical behaviour with half-lives ranging from 540 kyr (1996 AR20) to 32 Myr
(2000 FZ53). The mean half-life of the entire sample of Centaurs is 2.7 Myr.
The data are analyzed using a classification scheme based on the controlling
planets at perihelion and aphelion, previously given in Horner et al (2003).
Transfer probabilities are computed and show the main dynamical pathways of the
Centaur population. The total number of Centaurs with diameters larger than 1
km is estimated as roughly 44300, assuming an inward flux of one new
short-period comet every 200 yrs. The flux into the Centaur region from the
Edgeworth-Kuiper belt is estimated to be 1 new object every 125 yrs. Finally,
the flux from the Centaur region to Earth-crossing orbits is 1 new
Earth-crosser every 880 yrsComment: 15 pages, 2 figures, MNRAS in pres
Giant coherence in driven systems
We study the noise-induced currents and reliability or coherence of transport
in two different classes of rocking ratchets. For this, we consider the motion
of Brownian particles in the over damped limit in both adiabatic and
non-adiabatic regimes subjected to unbiased temporally symmetric and asymmetric
periodic driving force. In the case of a time symmetric driving, we find that
even in the presence of a spatially symmetric simple sinusoidal potential,
highly coherent transport occurs. These ratchet systems exhibit giant coherence
of transport in the regime of parameter space where unidirectional currents in
the deterministic case are observed. Outside this parameter range, i.e., when
current vanishes in the deterministic regime, coherence in transport is very
low. The transport coherence decreases as a function of temperature and is a
non-monotonic function of the amplitude of driving. The transport becomes
unreliable as we go from the adiabatic to the non-adiabatic domain of
operation.Comment: 15 pages, 9 figures, replaced by the version to appear in JSTA
Landau-Zener quantum tunneling in disordered nanomagnets
We study Landau-Zener macroscopic quantum transitions in ferromagnetic metal
nanoparticles containing on the order of 100 atoms. The model that we consider
is described by an effective giant-spin Hamiltonian, with a coupling to a
random transverse magnetic field mimicking the effect of quasiparticle
excitations and structural disorder on the gap structure of the spin collective
modes. We find different types of time evolutions depending on the interplay
between the disorder in the transverse field and the initial conditions of the
system. In the absence of disorder, if the system starts from a low-energy
state, there is one main coherent quantum tunneling event where the
initial-state amplitude is completely depleted in favor of a few discrete
states, with nearby spin quantum numbers; when starting from the highest
excited state, we observe complete inversion of the magnetization through a
peculiar ``backward cascade evolution''. In the random case, the
disorder-averaged transition probability for a low-energy initial state becomes
a smooth distribution, which is nevertheless still sharply peaked around one of
the transitions present in the disorder-free case. On the other hand, the
coherent backward cascade phenomenon turns into a damped cascade with
frustrated magnetic inversion.Comment: 21 pages, 7 figures, to be published in Phys.Rev.
Giant reversible barocaloric response of (MnNiSi)(1-x)(FeCoGe)(x) (x=0.39, 0.40, 0.41)
MnNiSi-based alloys and isostructural systems have traditionally demonstrated impressive magnetocaloric properties near room temperature associated with a highly tunable first-order magnetostructural transition that involves large latent heat. However, these materials are limited by a small field-sensitivity of the transition, preventing significant reversible effects usable for cooling applications. Instead, the concomitant large transition volume changes prompt a high pressure-sensitivity, and therefore, promise substantial barocaloric performances, but they have been sparsely studied in these materials. Here, we study the barocaloric response in a series of composition-related (MnNiSi)1-x(FeCoGe)x (x = 0.39, 0.40, 0.41) alloys that span continuously over a wide temperature range around ambient. We report on giant reversible effects of ~40 J K-1 kg-1 and up to ~4 K upon application of ~2 kbar and find a degradation of the first-order transition properties with pressure that limits the barocaloric effects at high pressures. Our results confirm the potential of this type of alloys for barocaloric applications, where multicaloric and composite possibilities, along with the high density and relatively high thermal conductivity, constructively add to the magnitude of the caloric effects.Peer ReviewedPostprint (published version
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