Mean free path

About: Mean free path is a research topic. Over the lifetime, 4412 publications have been published within this topic receiving 114418 citations.

Barium–borate–flyash glasses: As radiation shielding materials

Abstract: The attenuation coefficients of barium–borate–flyash glasses have been measured for γ-ray photon energies of 356, 662, 1173 and 1332 keV using narrow beam transmission geometry. The photon beam was highly collimated and overall scatter acceptance angle was less than 3°. Our results have an uncertainty of less than 3%. These coefficients were then used to obtain the values of mean free path (mfp), effective atomic number and electron density. Good agreements have been observed between experimental and theoretical values of these parameters. From the studies of the obtained results it is reported here that from the shielding point of view the barium–borate–flyash glasses are better shields to γ-radiations in comparison to the standard radiation shielding concretes and also to the ordinary barium–borate glasses.

Multiple Exciton Coherence Sizes in Photosynthetic Antenna Complexes viewed by Pump−Probe Spectroscopy

Abstract: The pump−probe signal from the light-harvesting antenna LH2 of purple bacteria is analyzed using a Green function expression derived by solving the nonlinear exciton-oscillator equations of motion (NEE). A microscopic definition of the exciton mean free path (Lf) and localization size (Lρ) is given in terms of the off-diagonal elements of the exciton Green function and density matrix, respectively. Using phonon-induced (homogeneous) and disorder-induced (inhomogeneous) line widths compatible with superradiane measurements, we find that at 4.2 K the localization size is Lρ = 15 and that the shift ΔΩ between the positive and negative peaks in the differential absorption is determined by a different effective size Lf/2 = 5.6 associated with the exciton mean free path. Our model further predicts the recently observed superradiance coherence size determined by Lρ.

Proximity effect under nonequilibrium conditions in double-barrier superconducting junctions

Abstract: A microscopic theory is developed for structures S-N-N′ (S is a superconductor, N, N′ are normal metals) with potential barriers at the interfaces. The length of the N layer is supposed to be shorter than the energy relaxation length but longer than the mean free path l , and the dirty case is considered ( l is shorter than the coherence length in the N-layer). The differential conductance is calculated for different values of the barrier resistances compared to the resistance of the middle layer. The conductance has an anomaly (a peak) at voltages V Δ due to an enhancement of the proximity effect at low energies (voltages). The anomaly is suppressed by pairbreaking mechanisms. It is shown that the magnetoconductance can be negative or positive depending on the ratio of the barrier resistances and on the voltage. The AC conductance (admittance) has a peak at a frequency ω Δ . At high voltages V ⪢ Δ a transition from an excess current at high barrier transparencies to a deficit current at low barrier transparencies is found in S-N-N′ (S) structures.

A theory of incoherent scattering of radio waves by a plasma: 3. Scattering in a partly ionized gas

Abstract: The theory of incoherent scattering in a plasma is extended to include the effect of ions and electrons colliding with neutral molecules, an effect that could be important in the ionosphere below perhaps 150 km for experiments at frequencies of the order of 50 Mc/s or less. We find, first of all, that the total scattered power is completely independent of collisions; the collisions only affect the shape of the spectrum. If, as is usually the case, the Debye length [6.9 (T/N)1/2 cm] is small compared to λ/4π. (λ is the radio wavelength), the spectrum begins to become narrower and peaked at the center when the mean free path of the ions becomes comparable to λ/4π. Any possible ion gyroresonant effects on the spectrum due to the presence of a magnetic field will be prevented if the collision frequency of the ions is greater than or equal to their gyrofrequency. If the Debye length should happen to be greater than λ/4π, the above comments would apply to the mean free path, collision frequency, and gyrofrequency of the electrons; however, such electron effects are not likely to be important in the ionosphere. It does seem that perhaps we might be able to measure ion collision rates by means of incoherent scattering. The principal difficulty would be in obtaining, from low altitudes, incoherent scattering signals uncontaminated by other types of scattering.

Interpretation of airborne oceanic lidar: effects of multiple scattering

Abstract: The effects of multiple scattering on the interpretation of the time dependence of elastic backscattering of laser pulses from the ocean (lidar) are investigated through solving the radiative transfer equation by Monte Carlo techniques. In particular, after removal of the geometric loss factors, it is found that the backscat-tered power is a decaying exponential function of time, over the time interval required for photons to travel four attenuation lengths through the water. The effective attenuation coefficient of this exponential decay is found to be strongly dependent on the parameters of the lidar system and on the optical properties of the water. The significant parameter is the ratio of the radius of the spot on the sea surface viewed by the lidar receiver optics to the mean free path of photons in the water. For values of this parameter near zero, the decay is determined by the beam attenuation coefficient, while for values greater than ~5-6, the decay is given by the attenuation coefficient for downwelling irradiance, often referred to as the diffuse attenuation coefficient. Between these two extremes the interpretation of the effective attenuation coefficient requires, essentially, complete knowledge of the inherent optical properties of the water: the beam attenuation coefficient and the volume scattering function.