Energy transfer in slab dye lasers

A simple model of the energy transfer between a booster dye and the active dye in a flashlamp pumped planar waveguide dye laser is described.Analytical results are compared with experiments for several dyes in the 600–650 nm region.A twofold increase of laser output was found at a wavelength (630 nm) which is of particular interest for several applications.     

Possible explanation of fine structures in mass-energy distribution of fission fragments

Within an improved scission point model, experimental data on the relative yield, mean value and variance of the total kinetic-energy distribution of fission fragments are described. It is shown that for a fixed mass and charge fragmentation, the potential energy of the scission configuration has several minima as a function of the deformation parameters of the fragments. The scission at these minima leads to a relatively enhanced yield of the fragments with a certain TKE and creates fine structures in the TKE-mass distribution which are different from those produced by the odd-even effect.Received: 24 March 2004, Published online: 12 October 2004PACS: 24.75. + i General properties of fission - 21.60.Gx Cluster models     

Experimental evidence for muonicX-rays from fission fragments

In the spectrum of muonic X-rays of²³⁸U measured in coincidence with prompt fission events a structure has been found which is attributed to 2p1s transitions of the muon attached to heavy fragments. The intensityI _s of this structure relative to the strengthI _pf observed for prompt fission has been determined to beI _s/I _pf=(6.0± 2.1) %. Although the experimental significance for the appeareance of this phenomenon is weak, this is the first experimental indication for its occuxence.We are indebted to the following institutes or organizations for financial support: Bundesministerium für Forschung und Technologie der Bundesrepublik Deutschland contract number 06BN271 (CR,PD,HH,FR,WS), Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization of the Advancement of Pure Research (NWO) (JK,CTAMdL,AT), the Swiss National Foundation (SNF) (LAS,LS), and the Heinrich-Hertz-Foundation (HH) (FFK,BS).     

Effect of beta-electron capture to a bound state on delayed-neutron emission from fission fragments

The effect of ionization of the atomic electron shell on beta decay is investigated. The change in the beta-decay probability is due primarily to the appearance of the channel involving beta-electron capture to a bound state. It is shown that the effect may be significant at low beta-transition energies. The magnitude of the effect was studied in the case of the emission of one to three delayed neutrons following the beta decay of fission fragments. In the calculations, use was made of the beta-decay strength function with allowance for the population of isobaric resonances in daughter nuclei. The effect proved to be maximal for fragments where the numbers of protons and neutrons approach those in closed shells and subshells, thereby illustrating the role of the shell structure of the nucleus in fission.     

Prompt gamma radiation from fission fragments due to the Strutinsky-Denisov polarisation

The hard electric dipole radiation from fission fragments of ²³⁵U by thermal neutrons is predicted. The radiation arises due to the Strutinsky-Denisov-induced polarisation mechanism. The probability of the radiation is at the level of 0.0025 per fission, which is in agreement with experiment. The angular distribution exhibits left-right asymmetry with respect to the plane perpendicular to the neutron polarisation axis. That means that the emission of gamma quanta at the given angle depends on the neutron polarisation. The asymmetry is at the level of 10^-3. This effect is similar to that observed earlier for gamma quanta in binary and alphas in ternary fission. The study of this effect will give information about dissipation of the collective energy of the surface vibration in fragments with large amplitude, and gives a picture of the process of snapping back the nuclear surface.     

Nuclear charge distribution of heavy fission fragments from thermal-neutron-induced fission of235U

Fragments from thermal-neutron induced fission of²³⁵U have been separated by a mass spectrometer with respect to their masses and kinetic energies within 1 μsec. The separation principles are briefly described. For masses 130 to 139 amu the charge distributions have been determined by counting the number of beta tracks emitted from the individual mass selected fission fragments in a nuclear photographic emulsion. In another method, the average number of beta particles for each fragment mass is determined by use of a 4π-proportional counter. The mean nuclear charge as a function of mass is compared with other experimental results and theoretical curves. Contradictory to the radiochemical results, this experiment yields a dip in the mean nuclear charge versus mass curve at mass 132 amu corresponding to the doubly magic nucleus (N=82,Z=50)¹³²Sn. Recent theoretical calculations of Nörenberg are in agreement with this finding.     

Coherent electromagnetic radiation from the fragments of nuclear fission

The electromagnetic radiation from two vibrating fission fragments is classically treated. The rotational and translational motion of the fragments during emission and the interference of the radiation from the two individual fragments are taken into account. It is shown that the angular distribution of the radiation contains valuable information on the fission process and that the spectral distribution depends sensitively on the damping of the shape vibration.Dedicated to Professor P. Armbruster and P. Kienle on the occasion of their 60th birthday     

Dynamical approach to calculating angular distributions of fission fragments

A dynamical approach is proposed for calculating the angular distributions of fission fragments. The relaxation time for the degree of freedom associated with the projection of the total angular momentum of the nuclear system onto the symmetry axis and the coefficient of damping of the fission mode are the basic parameters of this approach. Experimental data on the anisotropy of the angular distributions of fission fragments and on the multiplicities of prescission neutrons are analyzed within the proposed model for ¹⁶O+²⁰⁸Pb (E _lab=110–148 MeV), ¹⁶O+²³²Th (120–160 MeV), ¹⁶O+²⁴⁸Cm (110–148 MeV), and ¹⁶O+²³⁸U (96–148 MeV). The relaxation time and the damping coefficient are estimated at τ_K=(5–6)×10^?21 s and β=4×10²¹ s^?1, respectively.     

Yield of fission fragments from the photofission of actinide nuclei

Yields of the ¹⁰¹Mo, ¹³⁵I, and ^135m Cs isotopes from the photofission of the actinide nuclei ²³²Th ²³⁸U, and ²³⁷Np are measured. These fission fragments have specific nuclear structure features or are of practical use. The measurements are performed in the bremsstrahlung from the microtron of the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research (JINR) at an electron energy of 22 MeV. The yields of the fragments are measured by the activation method using a γ radiation HPGe detector.     

Light collimation and focusing by a thin flat metallic slab

We present experimental and theoretical work showing that a flat metallic slab can collimate and focus light impinging on the slab from a point source. The effect is optimized when the radiation is around the bulk, not at the surface, plasma frequency. Also, the smaller the imaginary part of the permittivity is, the better the collimation. Experiments for Ag in the visible as well as calculations are presented. We also discuss the interesting case of Al, whose imaginary part of permittivity is very small at the plasma frequency in UV radiation. Generalization to other materials and radiations are also discussed.     

Identification of gamma transitions from He and Be ternary fission fragments

He and Be ternary fission processes of ²⁵²Cf have been studied in two experiments with the Gammasphere detector array with light charged particle detectors surrounding the source. From α-γ double gated spectra, neutron multiplicity distributions were determined for related α ternary fission pairs. In going from binary to α ternary SF for approximately the same mass splittings (A ≈ 104–146) the average neutron multiplicity decreases about 0.7 AMU. In the first light charged particle (LCP) γ-γ experiment, the ¹⁰Be spectrum was cutoff below 27 MeV and in the recent experiment, below 18 MeV. For high energy (E > 27 MeV) ¹⁰Be ternary fission, the data indicate that the largest yields go via the cold process (zero neutron evaporation). In the recent experiment with E cutoff of 18 MeV, the ¹⁰Be ternary SF was observed for zero to 4n emissions. It seems that in some cases like ¹³⁶Te, the On channel is the strongest and in the other cases like ¹⁰⁰Zr the 1n or 2n channel dominates. Clearly, there is a shift to lower average number of neutrons emitted for ¹⁰Be compared to α ternary SF. The ¹⁰⁴Zr and ¹³⁶Te cases where zero neutron emission occurs may be related to the fact that these nuclei are near the limits of the more neutron rich Zr and Te nuclei observed. The ¹³⁶Te is more spherical than the heavy partners in the other pairs and this may influence the 0n channel. Finally, the 0n channel may be more enhanced in the first data with the higher ¹⁰Be energy cutoff, leading to lower excitation energy. Also, we confirmed the 3368 keV peak with the FWHM of 60 keV emitted from the moving Be particles in the Doppler effect corrected spectrum.     

Mass spectrometric investigation of desorbed ions from a valine sample by252Cf fission fragments

Summary Spectra of positive and negative secondary ions desorbed from a valine sample by²⁵²Cf fission fragments were measured in a new time-of-flight mass spectrometer. The outstanding features of the obtained results, including fragmentation, cationization and clustering are discussed. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Energy transfer from a polymer host to a europium complex in light-emitting diodes

Energy transfer leading to electroluminescence of a lanthanide complex, Eu(Fod)₃MK (Fod=heptafluoro-dimethyl-octanedionate, MK=4, 4′-bis(N,N-dimethylaminobenzophenone) doped into PVK host is investigated. The optical properties of pure Eu(Fod)₃MK and its doping in PVK show that the PVK host can efficiently transfer energy to europium ions through a co-ligand of MK due to good spectrally overlap. Light-emitting diodes (LED) consisting of PVK doped with 1-5 wt% of Eu(Fod)₃MK also demonstrates this energy transfer. Organolanthanide complexes doped into a wide-gap conducting polymers would lead to efficient and pure red organic LEDs to solve the full colour conjugated polymer displays.     

Energy deposition by fission fragments in nuclear-pumped lasers: I. A general method of calculation

A general method of calculating the energy deposition by fission fragments in nuclear-pumped lasers is presented. A specific case of a cylindrical cell is considered.     

Energy transfer from paramagnetic ions to a lattice through rapidly relaxing centers

Spin-lattice relaxation is analyzed in a crystal containing two types of paramagnetic centers having approximately equal resonant frequencies but markedly different spin-phonon couplings. It is assumed that for the centers having the strong spin-phonon coupling this coupling is stronger than the spin-spin coupling with other paramagnetic centers. The Green's function method is used. The spin-lattice relaxation time for the centers coupled weakly with the lattice through rapidly relaxing centers is found as a function of the difference between the splittings of their spin levels, the strength of the spin-spin coupling between these centers, the concentrations of these centers, and the strength of the spin-phonon coupling over rapidly relaxing centers.Translated from Izvestiya VUZ. Fizika, No. 3, pp. 15–20, March, 1970.In conclusion the author thanks S. A. Al'tshuler and L. K. Aminov for useful discussions of these results.     

Generation of angular momentum of fission fragments in a cluster model

Based on the dinuclear system concept, the role of bending vibrations in creation of the angular momentum of primary fission fragments is investigated. For ²⁵²Cf spontaneous fission, the angular momenta of the fragments are calculated as a function of the neutron multiplicity and compared with available experimental data. Different cluster compositions of the ²⁵²Cf fission modes at the scission point are considered.     

Heat-transfer coefficient of a fine heated wire in a gas flow

Heat exchange between a linear heat source and a gas flow is studied theoretically. The coefficient of source-to-gas heat transfer is found as a function of the gas velocity in the limit of zero temperature drop. It is shown that the heat-transfer coefficient depends linearly on the gas flow velocity for Peclet numbers 0.1–1.0.