Observed Decay Time Research Articles

Convective Motions Due to Trapped Particles of Plasmas Confined in an l = 3 Stellarator Field

Confinement of the plasmas in a stellarator of l = 3 is studied. The distribution of density and potential over the plasma cross section shows the existence of convective cells due to the trapped particles at the three corners of the magnetic surfaces, where the magnitude of the field is a minimum along the lines of force. The observed decay time can be explained in an order-of-magnitude discussion by the observed convective motions and the low-frequency fluctuations observed in the boundary region.

The decay phase of solar flare events

A study of the properties of the cosmic radiation of energy ∼- 10 MeV generated by solar flares is reported. Data from four Pioneer spacecraft in interplanetary orbits, and separated by ∼ 180° in heliocentric longitude are employed. Attention is restricted to the properties evident at times in excess of 1 day after the occurrence of the parent flare. The anisotropic character of the radiation; the gradients in heliocentric longitude; the decay time constants; and the energy spectra of the radiation are all studied in detail. It is found that the equilibrium anisotropy assumes a direction ∼- 45° E of the satellite-Sun line at very late times. It is suggested that the anisotropy at such times is parallel to E × B. This observation confirms that convection is the determining process in the escape of the solar cosmic rays from the solar system. It indicates that a positive radial gradient of solar cosmic radiation density has builtup at orbit of Earth some 4 days after a flare. This results in an effective convective velocity of approximately 1/2 the solar wind velocity. Direct measurements indicate the presence of strong gradients in heliocentric longitude even at very late times (≳ 4 days). These gradients are essentially invariant with respect to time, e-folding angles of n ∼- 30° have been observed at ∼- 10 MeV. The presence of these gradients has a major effect on the temporal variation of the cosmic ray flux during the decay phase of the flare effect. Thus, the observed decay time constant is either increased or decreased relative to the ‘convective’ value depending on the position of the observer relative to the centroid of the cosmic ray population injected by the flare. The effect of the gradient becomes more pronounced at lower energies, and may even exceed the convective removal rate. The observed decay time constant, the characteristics of the anisotropy, and the gradient in longitude are shown to be inter-related as demanded by theory. It is shown that the exponent of the cosmic ray spectrum is dependent on the location of the observer relative to the centroid of the cosmic ray population injected by the parent flare. At a given point in the frame of reference of the cosmic ray population, the spectral exponent is invariant with time.

The proton relaxation of CH3OD in the presence of Co(II) ions, as studied by spin-echo techniques

By measurement of the proton relaxation times T 1 and T 2 of CH3OD containing Co++ ions in dilute solution and the dependence of the observed decay time (T 2 †) on t ep the separation between pulses in a Carr-Purcell sequence the coordination number is shown to be six. Since this result has previously been obtained by an independent method, the use of the dependence of T 2 † on t ep to determine coordination numbers is justified for solutions of paramagnetic ions. The validity of analytical expressions derived previously has been checked by numerical methods, and the analytical expression is shown to be valid even in the presence of H 0 and H 1 inhomogeneity and inaccurate pulses of finite duration, provided that the Meiboom-Gill method is used. In an Appendix insight derived from the results of the numerical solutions is used to give a brief derivation of the dependence of T 2 † on H 0, exchange rate and t ep.

Zero-Field Level-Crossing Effects in a Cascade Process Induced by Highly Saturated Xenon Laser Field

Intensity change of spontaneous emission from the lower laser level induced by a linearly polarized laser transition has been studied as a function of magnetic field. By using a zero-field interference effect between magnetic sublevels, the decay times of the upper and lower laser levels can be measured. The intensity change of spontaneous emission has been investigated experimentally in a 3.51 µm (5 d 33 →6 p 22 ) and a 5.57 µm (5 d 34 →6 p 23 ) xenon laser as a function of polarization directions of the laser radiation and the spontaneous emission, the parameters being the gas pressure and the discharge current. The observed intensity change has two overlapping components of different widths, their ratio being about 1:20. It is found that the narrow component is caused by fourth-order perturbation of the laser field, the coherence among sublevels of the upper laser state being transferred to the lower state. The observed decay time of the lower laser level is 3.5×10 -8 s for the 6 p 22 level and 2.5×10 -8 s for the 6 p 23 state at a pressure of 13 mTorr. The observed decay time of the upper level is very sensitive to the discharge conditions and takes a value of (3∼8)×10 -7 s for the 5 d 34 level.

Recombination Processes in Insulators and Semiconductors

The discrete states in the forbidden zone are divided into ground states and shallow trapping states. The major recombination traffic passes through the ground states. The shallow trapping states cause the observed decay time of free carrier concentrations to exceed the lifetime of a free carrier in the conduction (or valence) band. At low rates of excitation (free carrier concentrations less than ground state concentrations) the electron lifetime and hole lifetime are independent and, in general, significantly different. At high rates of excitation the free electron and hole lifetimes are equal. For an insulator having one class of ground states (a class being defined by the capture cross sections for electrons and holes) the high-light lifetime is bracketed by the two low-light lifetimes.The behavior of a model having one class of ground states can be described relatively simply and quantitatively. The behavior of a model having more than one class of ground states becomes sufficiently complex that only special cases can be treated easily. More than one class of ground states, however, is required to account for infrared quenching, and the ability of added ground states to reduce the rate of recombination. These phenomena involve a redistribution of electrons and holes amongst the classes of ground states. Such redistributions can give some meaning to the phrases: filling of traps or saturation of centers.The recombination behavior of a semiconductor is significantly different from that of an insulator. For example, superlinearity can occur in a semiconductor having only one class of ground states. Also, the photocurrents in a semiconductor can be intrinsically more noisy than the photocurrents in an insulator.

Isotope Effect in the Imprisonment of Resonance Radiation

On theoretical grounds it is expected that the decay time of imprisoned resonance radiation in the vapor of a single even isotope of mercury should be some six times larger than that observed with the natural samples of mixed isotopic constitution. To investigate this effect experimentally, decay measurements were carried out with samples of ${\mathrm{Hg}}^{198}$ (3 percent ${\mathrm{Hg}}^{199}$ and 0.1 percent ${\mathrm{Hg}}^{200}$). For vapor densities $N\ensuremath{\lesssim}\frac{{10}^{14}}{\mathrm{cc}}$, the predicted effect was verified. For $N>\frac{{10}^{14}}{\mathrm{cc}}$, the observed decay time drops below that predicted theoretically for pure ${\mathrm{Hg}}^{198}$. This secondary effect is here attributed to the transfer of excitation from ${\mathrm{Hg}}^{198}$ to ${\mathrm{Hg}}^{199}$ and ${\mathrm{Hg}}^{200}$ by collisions of the second kind. Comparison of experimental data with an appropriately generalized theory permits estimation of the cross section for such collisions. The value so obtained is ${10}^{\ensuremath{-}13}$ ${\mathrm{cm}}^{2}$, some twenty times the gas-kinetic cross section, and in order-of-magnitude agreement with theoretical expectations.