[en] The critical mass of the reactor RP-0 of the Peruvian Institute of Nuclear Energy is determined by using the Fermi model of diffusion and age in connection with an experimental subcritical neutron population. A desk computer HP9825 is used with a program in HP4. The observed discrepancy between the calculated and the measured mass is 5%

[en] Neutron moderation in an infinite homogeneous medium with constant cross sections, which is traditionally treated by Taylor series approximation methods, bears a close correspondence with the exact transcendental equation approach, as has been demonstrated by reducing the exact result to approximate forms. In this paper we give an alternative approach in which the convergence of the approximations to the exact result is derived by allowing the order of approximation to increase indefinitely. The approach is based on the fact that the usual approximations do not give a self consistent definition of the effective mass parameter. The present result can hence be considered as the slowing down counterpart of Davison's treatment of the one speed equation in the Psub(l) approximation. (auth.)

[en] The process of neutron slowing down in a nonabsorbing medium is reviewed, and the distinction between average energy or lethargy after n collisions and the average number of collisions to reach a given energy is emphasized. Fermi's remark about neutron slowing down in hydrogen is explained and rederived

[en] In the present work we have worked out a vector model for spontaneous emission based on the principle of moving vectors. This simplified approach reflects the work of Weisskopf and Wigner. Normalized probability of a transition to an upper level versus detuning has been plotted. Further we have shown that the areas which have been generated at phasor diagrams in different values of decay constants for moving vectors are directly proportional to time. This is a manifestation of Fermi's Golden rule. Fermi's Golden Rule in quantum mechanics is well known. But it is for the first time that we have worked out the concept of moving vectors using the transition probabilities and the concept of phasor diagrams to represent the Fermi's Golden rule. (author)

No abstract available

[en] This paper applies a recent semi-analytical treatment of the slowing-down and penetration of positrons in condensed matter to the slowing down of slow positrons in aluminium. The computations are based on the age equation (formally identical with the equation of heat conduction), of which a simplified derivation is given. The dominant energy loss mechanisms, scattering off conduction electrons and long wavelength acoustic phonons, have been treated using the Lindhard dielectric function and the deformation potential approximation, respectively

[en] The behaviour of neutron wave propagation is analyzed using energy, space and time-dependent diffusion theory with synthetic kernels in the slowing down region. A delta function source in energy and space which oscillates sinusoidally with frequency ω is used. Fermi age slowing down model is found to be valid only for very low frequencies. It is found that if a neutron wave experiment could be performed, information about the Greuling Georetzel parameters zeta and γ can be obtained by measuring the attenuation and phase at asymptotic distances from the source. (author)

[en] Complete test of publication follows. In this presentation, we will discuss the observation of the laser-assisted photoelectric effect on surfaces for the first time. This represents a high-field laser-dressing of the original manifestation of the photo-effect i.e. photoemission form a surface. To date, such a laser-assisted photoelectric effect (LAPE) has only been observed in isolated atoms, where it is used to measure femtosecond and attosecond duration x-ray pulses, as well for the study of ultrafast dynamics in atoms. In our work, a clean Pt(111) single crystal was used as the solid surface, since it exhibits a large density of states at the Fermi edge, with a characteristic peak-like structure in the photoelectron spectrum. In the presence of an intense laser field, sidebands appear in the Pt(111) photoemission spectrum. The modification of the photoelectron spectrum to theory. This effect promises to be useful to extend EUV pulse duration measurements to higher photon energies, as well as opening up femtosecond-to-attosecond time scale electron dynamics in solid and surface-adsorbate systems. We will also discuss the observation of intramolecular vibration dynamics using electrons rescattered during the process of high-order harmonic generation. We excite coherent vibrations in SF₆ using impulsive Raman scattering (ISRS) with a short laser pulse. A second, more-intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of ∼ 20-50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all the Raman-active modes of SF₆, with an asymmetric breathing model most visible. This is in contrast to conventional ISRS where only the symmetric breathing mode of the molecule is observed. Our results indicate that high harmonic generation is a more sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Since the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e. ∼ 1.5 A, small changes in the shape of the molecule lead to large changes in the high harmonic yield. This work therefore demonstrates a new spectroscopic technique for probing ultrafast internal dynamics in molecules, and in particular on the chemically-important ground state potential surface that is difficult to probe using other techniques. High harmonic generation from excited molecules may also be sensitive to Raman-active as well as infrared-active vibrational modes.

[en] In this paper, the first transcendental approximation as a very good Fermi formula improvement is presented. The first transcendental approximation exactness and application is discussed. (author)

[en] The problem of slowing-down density of neutrons for cylindrical shell of finite height with exact boundary conditions on all the surfaces in the age theory has been solved by operational method. (author)