Values Of Decay Constants Research Articles

Mechanism for the decay of Ar(3P2) atoms in pure argon. Reconciliation of low pressure and high pressure values for the decay constant

A new set of decay constant values for Ar(3P2) in pure argon at 298 K determined by means of single frequency cw laser probe spectrophotometry in the 100–700 Torr region is reported. An excimer forming mechanism initiated by a reversible termolecular step and carried forward by a sequence of reversible collisional relaxation stages with provision for radiative decay at each stage is proposed to account for the kinetic behavior of Paschen-1s atoms. It is demonstrated that this mechanism is quantitatively consistent with measured collisional decay constant values at pressures in the 0–22 and 100–700 Torr regions and that adherence of such values to form, k3P2=AP2+BP, is fortuitous and that the values of A and B have no mechanistic significance. It is similarly indicated that the nature of the probable decay mechanism precludes the possibility of recovering excited atom precursor decay constant values by analysis of excimer emission and absorption data without detailed numerical modeling.

A users' guide to fission track dating calibration

Misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations. Problems of registration geometry, inhomogeneous uranium distribution, accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods, and impose strict limitations upon even their routine application. Complexities of unique, absolute thermal neutron dosimetry are illustrated by the calibration and use of the NBS pre-irradiated glass SRM 962. The 20% disparity in measured λ f decay constant values cannot be isolated from neutron fluence (φ) measurement and calibration. Frequently a calibration ratio (φ/λ f) is evaluated against an age standard and then split into its component parts. To illustrate this interdependence of λ f and φ, zircon fission track ages, in agreement with independent K Ar ages, are obtained by calculating the same track count data with each of the preferred values of λ f (λ f = 7.03 × 10 −17yr −1and8.46 × 10 −17yr −1) together with appropriate, selected neutron dosimetry schemes. An alternative approach is presented, formally relating unknown ages of samples to known ages of standards, either by direct comparison of standard and sample track densities, or by the repeated calibration of a glass against age standards. Practical recommendations are given for the reporting and critical assessment of fission track data.

New Leaky Surface Waves in Anisotropic Metal-Diffused Optical Waveguides

Propagation of guided waves in anisotropic metal-diffused optical waveguide is investigated. Two-dimensional guide-mode dispersion curves are computed and classified for a metal-diffused waveguide with arbitrary optic-axis orientation in various diffusion depth. It is found that a new leaky surface wave exists in the region where the refractive index is above the cutoff value, not below it. Typical values of decay constant are about 5 dB/cm for the wave propagating along X axis on 128/spl deg/ rotated Y-cut LiNbO/sub 3/, and 35 dB/cm for the wave propagating along the direction making an angle of 70/spl deg/ to X axis on Y-cut plane LiNbO/sub 3/. We were able to observe the leaky surface waves experimentally.

Thermal-Neutron Transport Processes in H2O-D2O Mixtures in the Temperature Range 253 to 21 K: The Possible Use of These Moderators as Cold-Neutron Sources

The results of a detailed and systematic study of time-dependent, space-dependent, and steady-state neutron spectra in different assemblies of various H2O-D2O mixtures in the temperature range of 253 to 21 K are reported.By the matrix diagonalization method, the multigroup Boltzmann diffusion equation was solved to obtain asymptotic and transient spectra in assemblies at 253 K, with buckling values ranging from 0 to 0.4 cm−2. Mixtures with D2O content of 20, 50, and 80 wt% are considered. The calculated values of the fundamental mode decay constant and the diffusion parameters are compared with the experimental values reported by Salaita and Robeson.The multigroup time-independent source-free Boltzmann equation was also diagonalized to obtain spatial eigenvalues and eigenfunctions. By using these eigenfunctions, neutron spectra and the effective diffusion length L(x), at different distances from the source plane, were calculated at 253 K. The asymptotic values of L(x) compare fairly well with those calculated by using the measured values of D0 and in mixtures with a D2O content of 20, 50, and 80%.For the steady-state problem, the multigroup inhomogeneous Boltzmann diffusion equation was solved by the matrix inversion method for assemblies at 77 and 21 K with buckling values ranging from 0 to 0.1113 cm-2. Mixtures with D20 content of 0, 5, 10, 20, ... 90, 95, and 100% are considered. At 21 K, the relative usefulness of different assemblies as cold-neutron sources is discussed.For some selected assembly sizes, we determined the optimum H2O-D2O mixtures that would give maximum cold-neutron flux. The optimum mixtures for buckling values of 0.1113, 0.0158, and 0.001 cm-2 are those with nearly 30, 70, and 90% D2O, respectively; the corresponding gain factors for graphite-filtered neutrons are 1.85, 4.17, and 13.57, respectively. We find that in mixtures, as in the cases of H2O and D2O ice, cooling an assembly below 21 K does not decrease the effective temperature of the neutron distribution below that obtained at 21 K.

Comparability of geochronological measurements of coexistent (coeval) minerals, made with different values of decay constants of K40, Rb87, and other radioactive elements1

Comparability of geochronological measurements of coexistent (coeval) minerals, made with different values of decay constants of K⁴⁰, Rb⁸⁷, and other radioactive elements¹

VLF radiation from subsequent return strokes in multiple stroke lightning

Two papers have been published presenting experimental results on comparison of frequency-amplitude spectrum of first and subsequent return stroke fields in multiple stroke lightning. One result supports the predicted frequency shift to low frequencies for subsequent return strokes, but the other result contradicts this predicted shift. A corrected theory here shows that the low frequency shift is a function of the values of decay constant of current pulse and height of ionized current column and in particular does not exist for some values.

Measurements of Neutron Thermalization Parameters in Light Water by Pulsed Source and Non 1/vAbsorber Method

Measurements of thermalization parameters in light water have been made by the pulsed neutron source and non 1/v absorber method. In an effectively infinite system of poisoned water, an asymptotic decay constant was measured by detecting capture γ-rays emitted from the whole volume of this system. The thermalization parameters were derived from values of decay constants measured over a wide range of non 1v absorber concentration. Measured values of these parameters were compared with values calculated from Nelkin Egelstaff-Schofield and McMurry-Russell kernels.

Measurements of Neutron Thermalization Parameters in Light Water by Pulsed Source and Non 1/ν Absorber Method

Measurements of thermalization parameters in light water have been made by the pulsed neutron source and non 1/v absorber method. In an effectively infinite system of poisoned water, an asymptotic decay constant was measured by detecting capture γ-rays emitted from the whole volume of this system. The thermalization parameters were derived from values of decay constants measured over a wide range of non 1v absorber concentration. Measured values of these parameters were compared with values calculated from Nelkin Egelstaff-Schofield and McMurry-Russell kernels.

Energy distribution of thermal neutrons in semi-infinite and finite blocks of beryllium oxide assembly

Energy distributions of thermal neutrons in semi-infinite and finite blocks of beryllium oxide assembly have been studied by solving the Boltzmann equation in the diffusion approximation by numerical iteration. In the steady-state problem a constant source of thermal neutrons in contact with the free face of the semi-infinite block has been assumed. Different absorptions of the medium have been considered. For pure beryllium oxide moderator, the value of diffusion length obtained is in good agreement with the experimental results. The decay rates of neutrons in assemblies of different sizes are calculated by using two different scattering kernels. Placzek kernel is found to give higher values of decay constant A. The value of the diffusion constant D agrees well with the experimental results but the diffusion cooling constant C comes out to be about twice the experimentally observed value.

The radioactive age of four Western Australian pegmatites by the potassium and rubidium methods

Age determinations using the K-A and Rb-Sr decay schemes have been made on mica and feldspar minerals from several Western Australian pegmatitic intrusions. The results suggest that Rb-Sr and K-A ages determined on micas generally yield consistent ages, although a case of low mica K-A age is reported. The K-A ages of the feldspars measured appear to be considerably lower than their corresponding Rb-Sr ages, but again an exception is noted. It is concluded, using particular values of decay-constants and K branching ratio, that the four pegmatites dated are contemporaneous, with an age of some 2800 million years.