Single-ion dosemeter based on floating gate memories.

Floating Gate (FG) nonvolatile memories are based on a tiny polysilicon layer (the FG) which can be permanently charged with electrons or holes, thus changing the threshold voltage of a MOSFET. Every time a FG is hit by a high energy ion, it experiences a charge loss, depending on the ion linear energy transfer (LET) and on the transistor geometrical and electrical characteristics. This paper discusses the opportunities to use this devices as single an ion dosemeter with sub-micrometer spatial resolution and capable of distinguish the impinging ion LET.

Sensitivity and dynamic range of FGMOS dosemeters.

UVPROM memory devices employing FGMOS transistors as memory cells make excellent dosemeters for applications involving ionising radiation. With proper preparation and programming, these devices can be used in remote-sensing applications in high-radiation environments with no power required during exposure.

Uncertainties in radiation effect predictions for the natural radiation environments of space.

Future manned missions beyond low earth orbit require accurate predictions of the risk to astronauts and to critical systems from exposure to ionizing radiation. For low-level exposures, the hazards are dominated by rare single-event phenomena where individual cosmic-ray particles or spallation reactions result in potentially catastrophic changes in critical components. Examples might be a biological lesion leading to cancer in an astronaut or a memory upset leading to an undesired rocket firing. The risks of such events appears to depend on the amount of energy deposited within critical sensitive volumes of biological cells and microelectronic components. The critical environmental information needed to estimate the risks posed by the natural space environments, including solar flares, is the number of times more than a threshold amount of energy for an event will be deposited in the critical microvolumes. These predictions are complicated by uncertainties in the natural environments, particularly the composition of flares, and by the effects of shielding. Microdosimetric data for large numbers of orbits are needed to improve the environmental models and to test the transport codes used to predict event rates.

Radiation pattern of fluorescence from molecules embedded in small particles: general case.

A method is presented for calculating and analyzing the angular distribution of fluorescent emission from randomly oriented anisotropic molecules embedded in small dielectric particles with the nonzero reorientation angle between absorption and emission moments suggested by physical considerations now taken into account. Calculations performed on the basis of this method are compared with some of the available experimental data for fluorescent dye molecules embedded in microspheres, and good quantitative agreement is found. It is shown how fitting the computed results to experimental data determines an effective reorientation angle between absorption and emission transition moments. A more definitive test to which the model could be subjected is described.

Light scattering and fluorescence by small particles having internal structure.

We consider two related, yet distinct queries: 1. How does the internal morphology of a small particle affect the elastic light scattering signals? We have devised an algorithm, presently accurate for particles comparable only to small biological spheres (diameter less than 1 micron), which suggests that light scattering is sensitive to internal morphology only in the backward directions. Accordingly, observations should be obtained in these directions when probing for internal morphology. 2. How are fluorescent signals affected when the active molecules are variously distributed within small particles? One cannot assume that the fluorescent signals are simply proportional to the number of active molecules contained in the particle because there may also be a dependence upon the geometrical and optical properties of the particle and upon the particular spatial distribution of these molecules within the particle. Indeed, even the measured emission spectrum may be affected by such morphological features. Here, too, these calculations are mainly restricted to small particles (diameter less than 1 micron) in which the fluorescent molecules are isotropic and immobile. Under these conditions the effects are quite dramatic. These effects should be considered in quantitative procedures which utilize fluorescence for determining the concentration of specific molecules in small particles such as biological cells. They may provide a clue for discriminating among cells which differ morphologically or in which the spatial distribution of the fluorescent moiety differs. These effects may be minimized by utilizing a light source which is polarized perpendicularly to the scattering plane.

Visual phenomena induced by relativistic carbon ions with and without Cerenkov radiation.

Exposing the human eye to individual carbon ions (6C+) moving at relativistic speeds results in visual phenomena that include point flashes, streaks, and larger diffuse flashes. The diffuse flashes have previously been observed by astronauts in space but not in laboratory experiments with particles of high atomic number and energy. They are observed only when the nucleus moves fast enough to generate Cerenkov radiation.

Role of nuclear stars in the light flashes observed on Skylab 4.

The astronauts on Skylab 4 observed bursts of intense visual light-flash activity when their spacecraft passed through the portion of the earth's inner trapped radiation belt known as the South Atlantic Anomaly (SAA). Two experimental sessions were carried out on board Skylab 4 under the auspices of Pinsky et al. who compare the flash rates with the measured flux of Z > or = 1 particles that would pass through the astronaut's eyes. They concluded that the flash rates, which became as great as 20/min, were anomalously high. We explored a number of alternative explanations for the anomalous flash rates that would be consistent with the accepted SAA flux values and the laboratory data on particle induced visual sensations and found that when one includes the effect of nuclear interactions in and near the retina which result in star formation (the emission of slow protons, neutrons and alpha particles form the nucleus in an evaporation-like process) the apparent anomaly is removed.

Comparison of the light-flash phenomena observed in space and in laboratory experiments.

Astronauts on Apollo and Skylab missions have reported observing a variety of visual phenomena when their eyes were closed and adapted to darkness. These observations were studied under controlled conditions during a number of sessions on board Apollo and Skylab spacecraft and the data available to date on these so-called light flashes are in the form of descriptions of the phenomena and frequency of occurrence. Similar visual phenomena have been demonstrated in a number of laboratories by exposing the eyes of human subjects to beams of neutrons, alpha particles, pions and protons. More than one physical mechanism is involved in the laboratory and space phenomena. No direct comparison of the laboratory and space observations has been made by observers who have experienced both. However, the range of visual phenomena observed in the laboratory is consistent with the Apollo and Skylab observations. Measured detection efficiencies can be used to estimate the frequencies with which various phenomena would be observed if that subject was exposed to cosmic rays in space.

Light flashes observed on skylab 4: the role of nuclear stars.

The astronauts on Skylab 4 observed bursts of intense visual light flash activity when their spacecraft passed through the South Atlantic Anomaly. Flash rates as high as 20 per minute have in the past been considered unexpectedly high. When the effect of nuclear interactions in and near the retina is included, the apparent anomaly is removed.

Role of Cerenkov radiation in the eye-flashes observed by Apollo astronauts.

Visual phenomena in the form of colorless flashes of light were observed by astronauts in deep space when their eyes were closed and adapted to darkness. We describe in this paper laboratory experiments and calculations which indicate that many of these flashes are the result of visible light generated within the astronauts' eyeball in the form of Cerenkov radiation when a relativistic HZE particle traverses it. The sensitivity to Cerenkov radiation measured for three subjects exposed to pulses of pions and muons and the visual phenomena observed were found to be consistent with the reports of flashes observed at rates as high as 2 per minute on Apollo missions 11 through 17.

Muon-induced visual sensations.

The visual phenomena induced by the passage of a pulse of extremely relativistic muons through the vitreous humor have been studied at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. The visual phenomena include flashes that range from small crescents of light in the peripheral field of view to large clouds of light that fill the entire field of view as well as bright flashes with dark centers. Three subjects have been exposed to date. Arguments are given to show that the physical mechanism behind these flashes is Cerenkov radiation. Standard psychophysical techniques are used to determine the threshold for muoninduced visual sensations for one subject. Comparison is made with his pion treshold measured under the same condition.

Visual sensations induced by Cerenkov radiation.

Pulses of relativistic singly charged particles entering the eyeball induce a variety of visual phenomena by means of Cerenkov radiation generated during their passage through the vitreous. These phenomena are similar in appearance to many of the visual sensations experienced by Apollo astronauts exposed to the cosmic rays in deep space.

Visual sensations induced by relativistic nitrogen nuclei.

The ability of the human eye to detect nitrogen nuclei that enter the retina at speeds just above the Cerenkov threshold has been confirmed in an experiment at the Princeton Particle Accelerator. A system for beam transport and subject alignment delivered individual nitrogen nuclei onto a spot 3 millimeters in diameter on the retina at a visual angle of 7 degrees on the temporal side of the fovea. The beam particles entered the retina within 25 degrees of normal and induced visual sensations that had the appearance of streaks for three out of four subjects.

Acceleration of argon ions to 1.17x1010 electron volts.

Argon ions were accelerated to 1.17x10(10) electron volts in the Princeton Particle Accelerator. The synchrotron was tuned by use of a neon beam with a charge-to-mass ratio equal to that of the argon ions. The fully accelerated argon ions were detected by the observation of etched tracks in cellulose nitrate sheets and also by the use of scintillation counters. Predictions of the range and of the characteristics of argon tracks in plastics were confirmed.