Sterilization of foods with low-energy electrons (“soft-electrons”)

Electrons with an energy of 300 keV or lower were defined as “Soft-electrons”, which showed several advantages over conventional irradiation with gamma-rays or high-energy electrons in decontamination of grains and spices. Energies of electrons necessary to reduce microbial loads to levels lower than 10 CFU/g were 60 keV for brown rice, 75 keV for wheat, 100 keV for white pepper, coriander and basil, 130 keV for buckwheat, 160 keV for rough rice, and 210 keV for black pepper. Electrons with such energies did not significantly influence the quality.     

Electron-beam-induced conduction in fluorinated ethylene propylene copolymer and its acrylonitrile graft copolymer

Electron-beam-induced conductivity (EBIC) studies were carried out on fluorinated ethylene propylene copolymer (FEP) and its acrylonitrile graft copolymer (FEP-g-AN). The current transport parallel to the surface under the influence of an electron beam of medium energy (6–30 keV) is reported. The gain in EBIC is calculated and its characteristic variation as a function of beam energy is studied. Minima in the gain versus energy curves have been obtained around 21 keV for FEP, 12 and 18 keV for FEP-g-AN, and 12 keV for polyacrylonitrile (PAN). This behavior is explained by assuming the existence of electron trapping bands close to the surface, at characteristic distances below the surface of the polymeric film corresponding to the range of incident electrons. Gains of the order of 5, 12, and 30 have been obtained respectively for FEP, FEP-g-AN, and PAN for the maximum energy of incident electrons used.     

Experimental results on 2–30 keV bremsstrahlung from thick and thin targets

The recent experimental investigations on electron bremsstrahlung produced from impact of 2–30 keV electrons with thick solid and thin gaseous targets are reviewed. The theoretical models describing the energy and angular distributions of bremsstrahlung photons are discussed with their brief outlines and formulations to explain the experimental data. The results on thick target bremsstrahlung (TTB) spectra produced by keV electrons have suggested that there is a need to develop a comprehensive theory for accounting the solid state effects. It is further noted that the prediction of the modified KKD formula gives a reasonable agreement with the TTB data, whereas a semi-empirical formula gives a better fit to the data for thick targets. The available experimental data for dependence of double differential cross-sections of emitted photons on impact energy and their emission angles for gaseous atoms and molecules exhibit a good agreement with the theoretical calculations of Kissel et al., [1983. Shape functions for atomic-field bremsstrahlung from electrons of kinetic energy 1–500 keV on selected neutral atoms 1<Z<92. Atom. Data Nucl. Data Tables 28, 381–460].     

Differential track structure of electrons in liquid water

The differential spectrum of energy loss events produced in liquid water by electrons has been calculated using a differential cross-section incorporating exchange and binding. The average energy loss in a single event is found to vary from about 28 to about 68 eV for 100 eV for 100 eV to 1 MeV electrons, respectively. All energy loss events above a certain level (set either to 1 or to 5 keV) are assumed to produce branch tracks, which are further degraded using a Monte Carlo technique. The total track averaged energy loss per event for a 1 MeV electron is found to be about 57 eV.     

The study of sensitivity and resolution of microautoradiography using Auger electrons

By using low energy beta emitter to microautoradiography, it is expected that high resolution microautoradiogram can be made. In this report, the authors pay attention to the use of monochromatic and low energy electron on the metallic samples, and used⁵¹Cr that emits Auger electrons with the energy of 4.5keV and⁶⁵Zn that emits Auger electrons with energy of 7keV in the study of the resolution of Auger electron microautoradiograph. From the results of the study, it was clearly confirmed that the resolution of the microautoradiograph using⁵¹Cr was better than 0.5 m and the resolution using⁶⁵Zn was better than 1 m.In order to make high resolution microautoradiograph, it is essentially important to know the sensitivity of the nuclear emulsion to the low energy electrons in advance, for securing the reasonable exposure to the nuclear emulsion. In this regard, the investigation on the sensitivities of those emulsions to the Auger electrons with the energy of 5.4keV from⁵⁵Fe and 7keV from⁶⁵Zn was made using Ilford and Fuji nuclear emulsions.     

Measurements and calculations of absorbed dose in electron beam radiation processing

An electron beam current densimeter has been described and used for dose measurement in EB radiation processing. An improved bipartition model of electron transport is applied to calculate the reflection coefficients and energy deposition distributions produced by 0.2–3 MeV electrons in semi-infinite media of Al, C, MAR and nylon, and the energy deposition produced by 2 MeV electrons in the two-layer medium consisting of copper and polystyrene. In addition, the depth dose distribution of 300 keV electrons in Ti-air-nylon composite-layer has been evaluated. The calculation results are in good agreement with the measurement data.     

Doppler effect in fluorine K-Auger line produced in electron-induced core ionization of SF6

An experimental evidence is reported on the observation of the Doppler effect in fluorine K-Auger line emitted from a core-ionized SF6 molecule under an impact of 16 keV electrons. The emitting source of the Auger line is found to acquire a kinetic energy of 4.7+/-0.3 keV. We propose that such large energy is released from the Coulomb repulsion taking place between F+ and SF5+ fragment ions under influence of an intense focusing field of the incident electrons. In the presence of the Coulomb field of these ions, the Auger line obtains a polarization P = 76%+/-7%.     

Electron reflectivity from clean and oxidized steel surface

This paper aims to elucidate the effect of an air-formed native oxide covering mild steel surface on the contrast in the scanning electron microscopy (SEM) images obtained with the landing energy from 5 keV down to 0 eV. Part of the mild steel surface was in-situ cleaned by Ar⁺ ion sputtering process in order to remove native oxide from the surface. It enabled us to observe the oxide-free and the naturally oxidized area on the mild steel surface simultaneously in the SEM micrographs. Presence of the native oxide starts to play a role in the SEM images acquired at landing energy below roughly 3 keV. Contrast between differently oriented grains situated inside the area covered by the native oxide starts to be negligible with landing energy decreasing below 3 keV, up to some ultra-low values where the contrast increases again. Total reflectivity contrast between the clean and the oxidized area increases exponentially with landing energy decreasing below 3 keV. The reflectivity-versus-energy curves of the cleaned and the naturally oxidized mild steel surface are markedly different. The reflectivity of the electrons is correlated with the density of states (DOS), as is demonstrated at very low landing energies. Sensitivity of the very low-energy electrons to the electronic structure was verified by comparison of the experimental data with the simulations of reflectivities, band structure, and DOS. The theoretical predictions are based on the density-functional theory calculations and they have been performed in energy range corresponding to specular reflectivities of the Fe-BCC (001) orientation. We have also observed that close to the mirror condition, that is, near-zero landing energies, the primary electrons become sensitive to the surface potential differences caused by the work function differences of clean and native oxide-covered steel surfaces.     

Asymmetry of bremsstrahlung from transversely polarized electrons

By using an electron-photon coincidence method the photon emission asymmetry in the elementary process of bremsstrahlung from transversely polarized electrons was measured. For an electron beam of 300 keV incident on a gold target the asymmetry for spin up and down of the primary beam was measured for fixed direc-tions of the outgoing electrons and photons as a function of the energy split. Asymmetries up to 30% were found. The measurements are a proper test for theories going beyond the first Born approximation.     

Computer simulation of charge recombination in model tracks of high-energy electrons in nonpolar liquids; kinetics and escape

The evolution in time of the recombination of ions in model tracks and spurs produced by high-energy electrons was calculated by computer simulation of the diffusion and drift of the ions in each other's Coulomb field. Tracks of high-energy electrons are subdivided into tracks of secondary electrons that can be considered as independent one from another. Energy losses smaller than 50 or 100 keV are assumed to give rise to correlated groups of charges. The diffusion and recombination in the groups is directly simulated. For electrons with an initial energy in excess of 50 or 100 keV calculations are performed by summing the contributions of energy losses below this value. The nonhomogeneous kinetics of the charge recombination has been studied for both the short-time and long-time domains. The survival probability as a function of time, W(t), has been calculated for the charged species in electron tracks with different initial energy of the electron, for gaussian and exponential distributions of the initial distance between the positive and the negative species. The behaviour of W(t) and comparisons with the available short-time experimental data do not provide any clear distinction between the two distributions. The behaviour of W(t) at long times was also investigated in detail. The region of applicability of the theoretical limiting behaviour t^−0.5 was checked and found to be very small. The experimentally observed behaviour t^−0.6 was critically examined. Results are obtained for the probability of ion escape from recombination in the track as a function of the initial energy of the electron. The experimentally observed decrease of the yield of escape with decreasing energy of the electron for two liquids is adequately explained.     

Ionizing radiation-stimulated diffusion and desorption of hydrogen from metals

The processes of hydrogen diffusion from a sample depth activated by electrons with an energy of tens of keV are studied. The difference from the known models of electron-stimulated desorption, which consider as a rule electron energies from 0.5 to several keV, is noted. The proposed model is shown to correspond to at least two established experimental facts: the nonlinear dependence of hydrogen isotope desorption on the electron beam current density affecting the sample and the dependence of hydrogen desorption on the irradiation time of the sample.     

Energy loss of scattered ions at glancing-angle incidence on the crystal surface

Experimental findings of glancing-angle scattering of energetic ions are reviewed with particular emphasis on the energy loss of ion during scattering. The position-dependent stopping power of the surface which is derived from the angular dependence of observed energy loss is explained in terms of single and distance electron excitations of surface electrons. For the ions with velocities less than the Fermi velocity of target valence electrons, it is shown that the stopping power of a surface is determined only by the elastic collisions of the ion with valence electrons. From the analysis of the energy losses of 12–30 keV He ions reflected from the (001) surface of SnTe, a method is proposed to derive the electron density distribution averaged over the plane parallel to the surface from this position-dependent stopping power.     

Thin layer spectroelectrochemical (RVC-OTTLE) studies of pertechnetate reduction in acidic media

A Peltier-cooled silicon drift detector was successfully applied for conversion electron spectrometry. The energy resolution of the detector for 45 keV electrons was 0.50 keV (FWHM). The approximate thickness of the dead layer was determined to be 140 ± 20 nm Si equivalent. The relative efficiency of the detector was verified to be approximately constant in the energy range of 17–75 keV. This is concordant with the high transparency of the thin dead layer and the sufficient thickness of the detector (450 μm) to stop the electrons. The detector is suitable for use in plutonium analysis of chemically prepared samples. Moreover, it was demonstrated that conversion electron spectrometry is better than alpha spectrometry in preserving its capability to determine the ²⁴⁰Pu/²³⁹Pu isotopic ratio as a function of sample thickness. The investigated measurement technique can be considered a promising new tool in safeguards, complementary to existing methods.     

A comparison of the microbicidal effectiveness of gamma rays and high and low energy electron radiations

The radiation response of spores of Bacillus pumilus were examined for irradiation with cobalt 60 photons, 10 MeV electrons and low energy electrons at 100 and 80 keV. The responses were found to be the same for all types of radiation within the measurement uncertainties and were also in agreement with a previously published value.     

Signal-to-background in EPMA: Measurement and Monte Carlo calculation

By the method of Monte Carlo calculation, the dependence of the signal-to-background ratio of detected X-rays on the energy of electrons as well as on the thickness of the sample was calculated. The range of energy was 40 ÷ 120 keV, the range of thickness was approximately 8 ÷ 80 g/cm² (40 ÷ 400 nm at density = 2 g/cm³). The results were compared with measurements in electron microscope on thin resin standard for biological microanalysis. The measured dependence of signal-to-background ratio on the energy of electrons has the maximum at 80 keV, the calculated one changes at increased thickness from a monotonic form to one with a maximum at a particular thickness. The absolute values (Hall correction procedure was used for measured values) differs mainly at the highest energy used (120 keV); the difference is probably caused by unproper correction of measured value of background at this energy. Simultaneously, the source distribution of emitted X-ray photons is calculated. Its knowledge gives the possibility to estimate simply the interaction volume diameter and, by this way, to determine the spatial resolution of electron probe X-ray microanalysis.     

The application of long wavelength radiation in X-ray analysis

The authors used long wavelength X-ray radiation excited by low energy electrons for analytical purposes. Low energy electron excitation was achieved with an open window tube. The authors used a type that was developed in Philips laboratories. The electron source was a glow discharge from which the electrons ( 15 keV) are extracted and directed towards the anode. A photon spectrum of “Bremsstrahlung” and characteristic peaks is generated at a gold anode. In addition about half of the impinging electrons is reflected and may, thanks to the open window, be used to irradiate a specimen in spite of a partial loss of energy due to the collisions.The authors mention the following attractive characteristics: the tube is simple to operate and may easily be exchanged for a closed tube; no high vacuum is needed, a feature which, however, at the same time inhibits the determination of traces of O, N, C; about 7 cm² of the specimen, which may be an insulator, is irradiated. A discussion of applications illustrates the usefulness of the tube.     

Low energy tail spectra of characteristic X-rays detected by a Si(Li) X-ray detector

Characteristic X-ray spectra are inherently accompanied by the tail spectra in the low energy side when X-rays are detected with a semiconductor X-ray detector. The tail spectra of Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr and Mn KX-rays have been observed using two different Si(Li) X-ray detectors. An annular source of 55Fe has been used to excite the Mg (1.25 keV) to Ti (4.5 keV) KX-rays, and 54Mn and 55Fe sources have been used for the detection of Cr (5.4 keV) and Mn (5.9 keV) KX-rays, respectively. Observed intensity ratios of the tail area Nt to the Gaussian X-ray peak area NP have exhibited to change remarkably at the Si-K adsorption edge energy 1.84 ke V. When X-ray spectra detected with different Si(Li) detectors are compared at some specific characteristic X-ray, different values of Nt/Np intensity ratios as well as different line shapes of tail spectra have been observed. Using a simple model, the thickness of Si layer which generates the tail spectrum has been estimated, i.e., the thicknesses are about 0.05 micron for one detector and 0.09 micron for the other detector. The generation of the tail spectrum is known to be partially due to the escape effect of photoelectrons or Auger electrons from the intrinsic region.(ABSTRACT TRUNCATED AT 250 WORDS)     

New developments in theory of fast electron scattering in solids: Applications to microbeam analysis

The study of fast electron interaction with solids in the energy range from 100 eV to several tens of keV is prompted by quickly developing microbeam analysis techniques such as electron probe microanalysis, scanning electron microscopy, electron energy loss spectroscopy and so on. It turned out that for random solids the electron transport problem might be solved on the basis of the generalized radiative field similarity principle. The latter states that the exact differential elastic cross section in the kinetic equation may be replaced by an approximate one provided the conditions of radiative field similarity are fulfilled. Application of the generalized similarity principle to electron scattering in solids has revealed many interesting features of electron transport. Easy to use and effective formulae have been obtained for the angular and energy distribution of electrons leaving a target, total yields of characteristic photons and slow electrons escaping from a sample bombarded by fast primaries, escape probability of Auger electrons as a function of depth etc. The analytical results have been compared with Monte Carlo calculations and experiments in a broad range of electron energies and scattering properties of solids and good agreement has been observed.     

Influence of organic ions on DNA damage induced by 1 eV to 60 keV electrons

We report the results of a study on the influence of organic salts on the induction of single strand breaks (SSBs) and double strand breaks (DSBs) in DNA by electrons of 1 eV to 60 keV. Plasmid DNA films are prepared with two different concentrations of organic salts, by varying the amount of the TE buffer (Tris-HCl and EDTA) in the films with ratio of 1:1 and 6:1 Tris ions to DNA nucleotide. The films are bombarded with electrons of 1, 10, 100, and 60?000 eV under vacuum. The damage to the 3197 base-pair plasmid is analyzed ex vacuo by agarose gel electrophoresis. The highest yields are reached at 100 eV and the lowest ones at 60 keV. The ratios of SSB to DSB are surprisingly low at 10 eV (～4.3) at both salt concentrations, and comparable to the ratios measured with 100 eV electrons. At all characteristic electron energies, the yields of SSB and DSB are found to be higher for the DNA having the lowest salt concentration. However, the organic salts are more efficient at protecting DNA against the damage induced by 1 and 10 eV electrons. DNA damage and protection by organic ions are discussed in terms of mechanisms operative at each electron energy. It is suggested that these ions create additional electric fields within the groove of DNA, which modify the resonance parameter of 1 and 10 eV electrons, namely, by reducing the electron capture cross-section of basic DNA units and the lifetime of corresponding transient anions. An interstrand electron transfer mechanism is proposed to explain the low ratios for the yields of SSB to those of DSB produced by 10 eV electrons.     

Applicability of the condensed-random-walk Monte Carlo method at low energies in high-Z materials

The predictions of several Monte Carlo codes were compared with each other and with experimental results pertaining to the penetration of through gold foils of electrons incident with energies from 128 to 8 keV. The main purpose was to demonstrate that reflection and transmission coefficients, for number and energy, can be estimated reliably with a simple Monte Carlo code based on the condensed-random-walk and continuous-slowing-down approximations.