Number Of Low-energy Electrons Research Articles

Electron and Photon Energy Spectra Outside of 6 MV X-ray Small Radiotherapy Field Edges Produced by a Varian iX Linac

Due to the increase in the survival probability for patients treated with modern radiotherapy techniques to live enough for experimenting the late radiation effect, low dose outside the treatment volume becomes a concern. However, besides the absorbed dose, the beam quality outside the field edge should be taken into account. This work aimed at investigating the photon and electron fluence spectra outside the field edges for several small radiotherapy fields for determining the quality of the beams in order to better evaluate the secondary effect after modern radiotherapy treatments. Phase-space files of a 6 MV X-ray beam produced by a Varian iX linac for eight small fields of 0.7 × 0.7 cm2, 0.9 × 0.9 cm2, 1.8 × 1.8 cm2, 2.2 × 2.2 cm2, 2.7 × 2.7 cm2, 3.1 × 3.1 cm2, 3.6 × 3.6 cm2, and 4.5 × 4.5 cm2 and for the reference 10 × 10 cm2 field at SSD = 100 cm were generated using the BEAMnrc code. The photon and electron fluences in each field were calculated at 0.15, 1.35, and 9.85 cm water depth and several off-axis distances using FLURZnrc. The number of low-energy electrons between 1 and 10 keV at 2 cm outside the field edge increases by 60% compared to the central axis. Due to the relatively high linear energy transfer (LET) of these electrons, the results of this work should help to better evaluate the possible late effect of secondary radiation on healthy organs close to the tumor volume after radiotherapy treatment. We also observed high-energy electrons outside the field edge that are attributed to the leakage of the primary electron beam from the head of the linac. From a standpoint of radiological protection, these electrons should be taken into account when evaluating the dose delivered to the patient’s skin.

Production of secondary electrons and photons and energy absorption mechanisms in amorphous carbon irradiated by photons in the energy range of 0.03 to 17.4 keV

Energy absorption mechanisms in amorphous carbon under irradiation by 0.03–17.4 keV photons is studied by Monte Carlo simulation with accounting for the cascade decays of inner-shell vacancies and tracking all secondary electrons and photons including very-low-energy ones. Energy absorption by the atoms of the sample that underwent initial ionization by incident photons is only important at energies of several tens of eV (UV and XUV range). The principal mechanism of energy absorption on the whole incident photon energy interval is through inelastic processes caused by secondary electrons. At incident photon energies above C1s ionization threshold low-energy KLL Auger electrons produced by the cascade decay of C1s vacancy transfer energy to the medium in the vicinity of the site of initial photoionization. At energies far above the C1s-threshold, high-energy photoelectrons spread the absorbed energy over larger volumes and with smaller density. A great portion of the energy brought by incident photons, about 42 %, is transferred to the medium by low-energy secondary electrons and photons that are incapable of ionizing or exciting the atoms of the sample. The number of low-energy electrons is proportional to the energy of an incident photon, and at incident photon energies of several keV, one initial photoionization produces hundreds of secondary low-energy electrons.

Energy Spectra of Protons and Generated Secondary Electrons around the Bragg Peak in Materials of Interest in Proton Therapy.

The number and energy of secondary electrons generated around the trajectories of swift protons interacting with biological materials are highly relevant in proton therapy, due to the prominent role of low-energy electrons in the production of biodamage. For a given material, electron energy distributions are determined by the proton energy; and it is imperative that the distribution of proton energy at depths around the Bragg peak region be described as accurately as possible. With this objective, we simulated the energy distributions of proton beams of clinically relevant energies (50-300 MeV) at depths around the Bragg peak in liquid water and the water-equivalent polymer poly(methyl methacrylate) (PMMA). By using a simple model, this simulation has been conveniently extended to account for nuclear fragmentation reactions, providing depth-dose curves in excellent agreement with available experimental data. Special care has been taken to describe the electronic excitation spectrum of the target, taking into account its condensed phase nature. A predictive formula has been obtained for the mean value and the width of the proton energy distribution at the Bragg peak depth, quantities which are found to grow linearly with the initial energy of the beam, in good agreement with available data. To accurately characterize (in number and energy) the electrons generated around the proton paths, the energy distributions of the latter at each depth have been convoluted with the energy-dependent ionization inverse mean free paths. This results in a number of low-energy electrons around the Bragg peak larger than when only the proton beam average energy at the given depths is considered. The convoluted ionization inverse mean free path closely resembles the Bragg curve shape. The average energy of the secondary electrons is nearly constant (∼55 eV for liquid water and ∼43 eV for PMMA) in the plateau of the Bragg curve, independent of the proton incident energy and suddenly decaying once the Bragg peak is reached. These findings highlight the importance of a precise calculation of the proton beam energy distribution as a function of the target depth to reliably characterize the secondary electrons generated around the Bragg peak region.

The frequency dependence of the discharge properties in a capacitively coupled oxygen discharge

We use the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 to explore the evolution of the charged particle density profiles, electron heating mechanism, the electron energy probability function (EEPF), and the ion energy distribution in a single frequency capacitively coupled oxygen discharge, with driving frequency in the range 12–100 MHz. At a low driving frequency and low pressure (5 and 10 mTorr), a combination of stochastic (α-mode) and drift ambipolar (DA) heating in the bulk plasma (the electronegative core) is observed and the DA-mode dominates the time averaged electron heating. As the driving frequency or pressure are increased, the heating mode transitions into a pure α-mode, where electron heating in the sheath region dominates. At low pressure (5 and 10 mTorr), this transition coincides with a sharp decrease in electronegativity. At low pressure and low driving frequency, the EEPF is concave. As the driving frequency is increased, the number of low energy electrons increases and the relative number of higher energy electrons (>10 eV) increases. At high driving frequency, the EEPF develops a convex shape or becomes bi-Maxwellian.

Design and realisation of sensor system for detecting low-energy electron emission from metal microcracks

This paper investigates the low-energy electrons emitted from metal cracks. A mathematical model of low-energy electron emission from metals was established. A low-energy electron sensor system with a double grid air counter core was designed to measure the number of electrons emitted. The experiment was performed on 2024 Al. The results show that the detection system is effective for testing the number of low-energy electrons. When a crack expands due to applied force, the number of electrons detected increases with force, which shows that the double grid air counter sensor system can judge the development of cracks by measuring the number of low-energy electrons. The number of electrons detected increases with temperature, which verifies that the temperature influences low-energy electron emission.

The pressure dependence of the discharge properties in a capacitively coupled oxygen discharge

We use the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 to explore the evolution of the charged particle density profiles, electron heating mechanism, and the electron energy probability function (EEPF) in a capacitively coupled oxygen discharge with pressure in the pressure range of 10–500 mTorr. We find that at higher pressure (50–500 mTorr) the electron heating occurs mainly in the sheath region, and detachment by the metastable singlet molecule O2(a1Δg) has a significant influence on the electron heating process. At a low pressure (10 mTorr), Ohmic heating in the bulk plasma (the electronegative core) dominates, and detachment by O2(a1Δg) has only a small influence on the heating process. Thus at low pressure, the EEPF is convex and as the pressure is increased the number of low energy electrons increases and the number of higher energy electrons (>10 eV) decreases, and the EEPF develops a concave shape or becomes bi-Maxwellian.

Evolution of an electron energy distribution function in a weak dc magnetic field in solenoidal inductive plasma

We investigated the evolution of the electron energy distribution function (EEDF) in a solenoidal inductively coupled plasma surrounded by an axial dc magnetic field. The increase in the dc magnetic field caused the EEDF to evolve from a bi-Maxwellian to a Maxwellian distribution. At the discharge center, the number of low energy electrons was significantly reduced while the high energy electron population showed little change when a weak dc magnetic field was present. However, at the discharge radial boundary, the high energy electron population decreased significantly with the magnetic field while the change in low energy population was not prominent compared to the discharge boundary. These changes in EEDFs at the boundary and center of the discharge are due to the radial confinement and the restriction of radial transport of electrons by dc magnetic field.

A comparison of the measured responses of a tissue-equivalent proportional counter to high energy heavy (HZE) particles and those simulated using the Geant4 Monte Carlo code

Monte Carlo simulations of heavy ion interactions using the Geant4 toolkit were compared with measurements of energy deposition in a spherical tissue-equivalent proportional counter (TEPC). A spherical cavity with a physical diameter of 12.7 mm was filled with propane-based tissue-equivalent gas surrounded by a wall of A-150 tissue-equivalent plastic that was 2.54 mm to thick. Measurements and Monte Carlo simulations were used to record the energy deposition and the trajectory of the incident particle on an event-by-event basis for ions ranging in atomic number from 2 ( 4He) to 26 ( 56Fe) and in energy from 200 MeV/nucleon to 1000 MeV/nucleon. In the simulations, tracking of secondary electrons was terminated when the range of an electron was below a specified threshold. The effects of range cuts for electrons at 0.5 μm, 1 μm, 10 μm, and 100 μm were evaluated. To simulate an energy deposition influenced by large numbers of low energy electrons with large transverse momentum, it was necessary to track electrons down to range cuts of 10 μm or less. The Geant4 simulated data closely matched the measured data acquired using a TEPC for incident particles traversing the center of the detector as well as near the gas-wall interface. Values of frequency mean lineal energy and dose mean lineal energy were within 8% of the measured data. The production of secondary particles in the aluminum vacuum chamber had no effect on the response of the TEPC for 56Fe at 1000 MeV/nucleon. The results of this study confirm that Geant4 can simulate patterns of energy deposition for existing microdosimeters and is valuable for improving the design of a new generation of detectors used for space dosimetry and for characterizing particle beams used in hadron radiotherapy.

Electron acceleration by compressional ULF turbulence in the geomagnetosphere

In the quasi-linear theory, geomagnetospheric compressional ULF (ultra-low frequency) turbulence can enhance the energy of electrons via the mechanism of “transit-time acceleration”. The frequency range of typical compressional ULF turbulence is 2–15 mHz. The source electrons that are accelerated are background electrons in the vicinity of the geostationary orbit ( E < 30 keV) and substorm injected electrons (30–300 keV). When the wave-particle resonance takes place, the number of low energy electrons decreases and that of the relativistic ( E ≥ 1 MeV) electrons in the high-energy tail increases, implying that the electrons are effectively accelerated by the compressional turbulence. Besides, the more the substorm injection electrons, the more the relativistic electrons produced by acceleration. For the case with rate of substorm injection ε = 0.5, the number of relativistic electrons near the geostationary orbit can be obviously enhanced by this mechanism in 12 hours.

A Monte Carlo based development of a cavity theory for solid state detectors irradiated in electron beams.

Recent Monte Carlo simulations have shown that the assumption in the small cavity theory (and the extension of the small cavity theory by Spencer-Attix) that the cavity does not perturb the electron fluence is seriously flawed. For depths beyond dmax not only is there a significant difference between the energy spectra in the medium and in the solid cavity material but there is also a significant difference in the number of low-energy electrons which cannot travel across the solid cavity and hence deposit their dose in it (i.e. stopper electrons whose residual range is less than the cavity thickness). The number of these low-energy electrons that are not able to travel across the solid state cavity increases with depth and effective thickness of the detector. This also invalidates the assumption in the small cavity theory that most of the dose deposited in a small cavity is delivered by crossers. Based on Monte Carlo simulations, a new cavity theory for solid state detectors irradiated in electron beams has been proposed as: Dmed(p) = meanDdet(p) x s(med,det)S-A x gamma(p)c x S(T) where Dmed(p) is the dose to the medium at point p. MeanDdet(p) is the average detector dose to the same point, s(med,det)S-A is the Spencer-Attix mass collision stopping power ratio of the medium to the detector material, gamma(P)c is the electron fluence perturbation correction factor and S(T) is a stopper-to-crosser correction factor to correct for the dependence of the stopper-to-crosser ratio on depth and the effective cavity size. Monte Carlo simulations have been computed for all the terms in this equation. The new cavity theory has been tested against the Spencer-Attix cavity equation as the small cavity limiting case and also Monte Carlo simulations.

Angular and energy distributions of low energy electrons from backscattered-conversion electron Mössbauer spectroscopy

Angular and energy distributions of backscattered-low energy resonant and nonresonant electrons from iron films are studied both theoretically and experimentally. The results indicate that relatively few emerging resonant or nonresonant electrons are observed/predicted between 600 eV and 50 eV kinetic energy. A significant fraction of the total electron signal emerges, however, below 50 eV, which is shown to result from low energy Auger cascades, low energy shake-off electrons and secondary electrons attributed to higher energy events. The large number of low energy electrons permits relatively short acquisition times (ca. 1h) and enhanced surface sensitivities. They may be especially useful in future depth-deconvolution efforts.

Angular and energy distributions of low energy electrons in backscattered-Mössbauer spectroscopy

The angular and energy distributions of resonant and nonresonant electrons below 50 eV have been investigated during backscattered conversion electron Mössbauer spectroscopy (CEMS). Reasonable consistency is achieved between theoretical predictions and experimental studies. Relatively few resonant or nonresonant electrons are observed/predicted between 600 eV and 50 eV, while a significant fraction of the total electron signal emerges below 50 eV. The low energy signal is shown to result from low energy Auger cascades, low energy shake-off electrons and secondary electrons attributed to higher energy events. Despite the presence of secondary electrons, the signal arising from low energy Auger and shake-off electrons can be used to discriminate those resonant electrons (i.e., resonant spectra) arising from the topmost 1.0 nm of the absorber. The relatively large number of low energy electrons permits relatively short acquisition times (ca. 1 h) and enhanced surface sensitivities. They may be especially useful in future depth-deconvolution efforts.

Interactions between neutral dissociation and ionization continua in N2O

Autoionized Rydberg states converging to the à 2Σ+ and B̃ 2Π states of N2O+ are shown to be predissociated into neutral fragments. The decay to excited neutral fragments is observed by their fluorescence in the visible and ultraviolet regions of the spectrum. These decay channels are in competition with autoionization, which was shown in a previous study to yield unexpectedly large numbers of low energy electrons. On the basis of the striking similarity between the fluorescence excitation and the threshold photoelectron spectra, models are presented which connect dissociative channels to the production of threshold electrons. Ionization of dissociating neutral states and Rydberg states converging to the ground electronic state of N2O+(X̃ 2Π) are suggested as being responsible for the production of low energy electrons. The distribution of photoelectron energies resulting from such autoionization channels are calculated.

A health department hypertension education program: Mona Miles, North Central District Health Department, 1221 F Street, Lewiston, Idaho 83501; Deborah Pyke, Rosemary Shaber

Monte Carlo simulations of heavy ion interactions using the Geant4 toolkit were compared with measurements of energy deposition in a spherical tissue-equivalent proportional counter (TEPC). A spherical cavity with a physical diameter of 12.7 mm was filled with propane-based tissue-equivalent gas surrounded by a wall of A-150 tissue-equivalent plastic that was 2.54 mm to thick. Measurements and Monte Carlo simulations were used to record the energy deposition and the trajectory of the incident particle on an event-by-event basis for ions ranging in atomic number from 2 (4He) to 26 (56Fe) and in energy from 200 MeV/nucleon to 1000 MeV/nucleon. In the simulations, tracking of secondary electrons was terminated when the range of an electron was below a specified threshold. The effects of range cuts for electrons at 0.5 μm, 1 μm, 10 μm, and 100 μm were evaluated. To simulate an energy deposition influenced by large numbers of low energy electrons with large transverse momentum, it was necessary to track electrons down to range cuts of 10 μm or less. The Geant4 simulated data closely matched the measured data acquired using a TEPC for incident particles traversing the center of the detector as well as near the gas-wall interface. Values of frequency mean lineal energy and dose mean lineal energy were within 8% of the measured data. The production of secondary particles in the aluminum vacuum chamber had no effect on the response of the TEPC for 56Fe at 1000 MeV/nucleon. The results of this study confirm that Geant4 can simulate patterns of energy deposition for existing microdosimeters and is valuable for improving the design of a new generation of detectors used for space dosimetry and for characterizing particle beams used in hadron radiotherapy.

Measurement of the Emission of Low-energy Electrons in an Irradiated Vacuum Cavity Chamber

THE residual current in an ionization chamber, when the pressure is reduced to very low values, has been attributed by Greening1 to the transfer of slow electrons between the electrodes. He correlated several experimental studies2–9 in order to estimate the number of low-energy electrons emitted from unit area of the wall of a vacuum chamber under irradiation. For graphite the emission was about 5 × 105 electrons/cm2/rad (2.5 × 10−4 E.S.U./cm2/rad) and for elektron metal about 1.0 × 106 electrons/cm2/rad (5 × 10−4 E.S.U./cm2/rad), when irradiated by radium γ-rays. From some measurements at reduced pressure in ionization chambers Burlin10 also made an approximate estimate of the slow electron emission and obtained a value of 5 × 10−4 E.S.U./cm2/rad from graphite.