Betsy Ancker‐Johnson

Restricted Diffusion in Biophysical Systems: Experiment

The pulsed-gradient spin echo nuclear magnetic resonance (PGSENMR) technique was used to measure restricted diffusion of water in three types of animal tissue: human blood plasma and red cells; rat and rabbit heart; rat and rabbit liver. Characteristic lengths (L) for restriction of diffusion are estimated from dependence on the measuring time. Limitations on the range of observable restrictive lengths (1.5-15 mum) are discussed.The decrease in diffusivity due to 1 mum alumina powder (volume fraction = 0.18) in glycerin/water mixtures agrees with the Wang theory assuming spherical particles and no hydration. The characteristic length (L approximately 4 mum) is larger than the particle size (1 mum) or separation (1.8 mum). Comparison of the diffusivities in tissues at short diffusion times with the Wang theory indicates some bound or trapped water.For packed red blood cells, a restriction (L approximately 2.3 mum) was attributed tothe red cell membrane. A permeability p approximately 0.014 cm/s may be estimated from the decrease in diffusivity. Average values of diffusivity ratio in heart were: 0.36 +/- 0.02 for rat; and 0.26 +/- 0.03 for rabbit; and in liver: 0.25 +/- 0.01 for rat; 0.25 +/- .04 for 10-day old rabbit; and 0.195 +/- 0.03 for 2-yr old rabbit. A restriction (L approximately 2.7 mum) in rat liver probably results from the mitochondria.

Microwave Emission from Nonequilibrium Plasmas in InSb Subject to Magnetic Fields

This paper draws together a large amount of experimental information obtained by others as well as the author. Out of the maze of conflicting results one set of consistent characteristics of the microwave emission and its associated phenomena results. The reconciliation is obtained by means of a model for the spatial distribution of plasmas during emission. It shows why some samples emit preferentially for B ∥ E, others for B⊥E, and still others for intermediate orientation. Indeed, under the proper sets of conditions the same sample can be made to emit for all these various orientations in consonance with the model. The model also clarifies the various observed threshold dependencies. Why megahertz resistance oscillations of two different natures attend emission under some conditions and why under others such oscillations are totally absent is explained. Thus three characteristics of the emission and its associated phenomena can be consistently predicted, namely, the polar dependencies, the sometimes att...

MICROWAVE EMISSION FROM MAGNETIC‐FIELD‐FREE ELECTRON‐HOLE PLASMAS

Emission is observed only under special plasma conductance conditions. These have been obtained by applying voltages in the form of decreasing steps. Some plasma produced during the initial step remains to produce a higher plasma density during subsequent steps than can be obtained at those lower voltages if they are applied during the initial step. By this means a variety of usually unobtainable conductance conditons have been produced and an approximate threshold curve for the emission has been obtained in terms of current and field strength.

Restricted diffusion in biophysical systems: Theory☆

Abstract Theoretical results are presented on measurements of restricted diffusion in biophysical systems by the pulsed gradient spin echo nuclear magnetic resonance (PGSENMR) technique. A Fokker-Planck equation is developed to describe restricted diffusion, and it is shown that only two basic types of penetrable diffusion barriers exist, those in which the diffusing particles are partially excluded from the barrier region because of an increased free energy, and those in which the diffusing particles are not excluded but experience increased viscosity in the region. The Fokker-Planck equation is used to obtain expressions for the spin echo amplitude in PGSENMR experiments, and it is shown that for restricted diffusion the average diffusion coefficient measured in these experiments over short intervals is larger than that measured over long intervals. The possibility of distinguishing between the two types of barriers is considered. The experimental parameters required for intracellular restricted diffusion measurements are discussed, and it is shown that the interpretation of PGSENMR results in animal tissues should include the possibility of penetrable barriers rather than just the impenetrable barriers of previous PGSENMR calculations.

MULTIMODE OSCILLATION AND AMPLIFICATION IN PINCHED ELECTRON‐HOLE PLASMAS

An azimuthally symmetric perturbation in the density distribution of a pinched electron‐hole plasma produces spontaneous pinch oscillations which grow in time and space. Applied waves with this symmetry are amplified as they propagate down the pinch while the plasma is biased just below the current threshold for spontaneous oscillation. Magnetic fields applied parallel to the current cause helical perturbations to grow. Waves with helical symmetry grow markedly only when the plasma is just below its pinch threshold. Phase‐discriminating wave amplification is achieved when the plasma is properly biased.

Dynamic and Steady‐State Injection of Electron‐Hole Plasma in p‐Type InSb

Potential measurements as a function of time and space show in detail the passage of an injected electron‐hole plasma front, and the eventual establishment of a nonequilibrium steady state in a long bar of p‐type InSb at 77°K. The front is preceded by a depletion layer which vanishes as plasma reaches the anode. Thereafter the current, with the voltage held constant, grows exponentially until just before the steady‐state plasma density is reached. These results are compared with a theory by Dean and by Ancker‐Johnson, Robbins, and Chang describing plasma injection into a semiconductor with deep traps. The measured front arrival time as a function of constant applied voltage agrees satisfactorily with Deans prediction. Four observations are at variance with his theory: the time constants of the exponential current growth are density‐dependent instead of being independent as predicted; the current at the front arrival is not a function of voltage as his theory states; the electric field behind the front is...

TRANSIENT HIGH‐DENSITY INJECTION IN A SEMICONDUCTOR WITH TRAPS

The properties of high‐density injection into p‐InSb at 77 °K are compared with theory. An extension of Deans theory is in satisfactory agreement with the extensive observations which include density‐dependent growth times.

Magnetoconductance of nonequilibrium plasmas in InSb

Abstract The conductance of injected and impact-ionized plasmas in p -InSb at 77°K has been measured as a function of amplitude and direction of magnetic field relative to the electric field strength. Very large anisotropy in magnetoconductance occurs as the relative orientation between electric and magnetic fields is varied, and in the vicinity of the maxima large amplitude conductance oscillations develop with a frequency in the tens of MHz range. The plasma magnetoconductance for given magnitudes of applied fields abruptly drops to zero at a precisely defined orientation θ between the fields. The MHz oscillations have two characteristic waveforms, either rather regular for θ corresponding to the conductivity maxima or very irregular (spike-shaped) for θ corresponding to the nearly discontinuous changes in magnetoconductance. An enhancement in the conductance occurs for alignments within a few degrees of parallel and antiparallel for sufficiently small and large magnetic fields, an enhancement compared to the conductance in the absence of a magnetic field. The magnetoconductivity anisotropy is essentially independent of crystal orientation; it is strongly affected by sample surface conditions. All these properties are explained qualitatively by the well-known helical instability and by a theory for the role of a transverse component of magnetic field on the plasma distribution. Impact-ionized plasmas produce current-field strength characteristics with negative resistance regions both in the presence and absence of magnetic fields. These effects are also qualitatively explained.

The spectrum of microwave emission from InSb

Conflicting descriptions in the literature about the frequency dependence of microwave emission from InSb are shown to be the result of faulty experimental techniques. The amplitude as a function of frequency is apparently not periodic nor is frequency bunching confirmed. Several authors have claimed that the amplitude decreases with increasing frequency and this is confirmed. Coherent emission has been reported and some substantiation of this claim is given.

Impact ionization in p-type InSb

Abstract Impact ionization in p -InSb is shown to be initiated by injected electrons. At sufficiently high average electric field strength. 1 kV/cm, impact ionization begins next to the cathode. An impact ionization wavefront forms there and propagates along the sample with a velocity ν ii =3·2×10 8 cm/sec at 77 K. At moderate field strengths an impact ionization wavefront is initiated at some plane between the cathode and anode. Before its initiation in such a case, injected electrons exclusively propagate in a well defined front at 10 7 cm/sec, then when the field ahead of the injected front reaches ∼490 V/cm, impact ionization is initiated and the impact ionization wavefront propagates to the anode with ν ii . After the impact ionization wavefront reaches the anode, carrier density grows exponentially to a steady-state magnitude. The growth rate dependence on field strength is best expressed by g(E) φ exp (0·8×10 6 / E 2 ). Thus the energy dependence of the electrons during impact ionization in p -InSb at 77 K appears to be Maxwellian.