Number Of Secondary Electrons Research Articles

On the importance of excited state species in low pressure capacitively coupled plasma argon discharges

In the past three decades, first principles-based fully kinetic particle-in-cell Monte Carlo collision (PIC/MCC) simulations have been proven to be an important tool for the understanding of the physics of low pressure capacitive discharges. However, there is a long-standing issue that the plasma density determined by PIC/MCC simulations shows quantitative deviations from experimental measurements, even in argon discharges, indicating that certain physics may be missing in previous modeling of the low pressure radio frequency (rf) driven capacitive discharges. In this work, we report that the energetic electron-induced secondary electron emission (SEE) and excited state atoms play an important role in low pressure rf capacitive argon plasma discharges. The ion-induced secondary electrons are accelerated by the high sheath field to strike the opposite electrode and produce a considerable number of secondary electrons that lead to additional ionizing impacts and further increase of the plasma density. Importantly, the presence of excited state species even further enhances the plasma density via excited state neutral and resonant state photon-induced SEE on the electrode surface. The PIC/MCC simulation results show good agreement with the recent experimental measurements in the low pressure range (1–10 Pa) that is commonly used for etching in the semiconductor industry. At the highest pressure (20 Pa) and driving voltage amplitudes 250 and 350 V explored here, the plasma densities from PIC/MCC simulations considering excited state neutrals and resonant photon-induced SEE are quantitatively higher than observed in the experiments, requiring further investigation on high pressure discharges.

Secondary Electrons in Gold Nanoparticle Clusters and Their Role in Therapeutic Ratio: The Outcome of a Monte Carlo Simulation Study.

Gold nanoparticles (GNPs) are used in proton therapy radio-sensitizers to help increase the dose of radiation to targeted tumors by the emission of secondary electrons. Thus, this study aimed to investigate the link between secondary electron yields produced from a nanoshell of GNPs and dose absorption according to the distance from the center of the nanoparticles by using a Monte Carlo model. Microscopic evaluation was performed by modeling the interactions of secondary electrons in a phase-space file (PSF), where the number of emitted electrons was calculated within a spherical GNP of 15 nm along with the absorbed dose near it. Then, the Geant4-DNA physics list was used to facilitate the tracking of low-energy electrons down to an energy below 50 eV in water. The results show a remarkable change in the number of secondary electrons, which can be compared at concentrations less than and greater than 5 mg/mL, with increased secondary electron production exhibited around NPs within a distance of 10–100 nm from the surface of all nanospheres. It was found that there was a steep dose enhancement drop-off up to a factor of dose enhancement factor (DFE) ≤ 1 within a short distance of 100 nm from the surface of the GNPs, which revealed that the dose enhancement existed locally at nanometer distances from the GNPs. Overall, our results indicate that the physical interactions of protons with GNP clusters should not be considered as being directly responsible for the radio-sensitization effect, but should be regarded as playing a major role in NP properties and concentrations, which has a subsequent impact on local dose enhancement.

Development of a magnetic shield for 20-inch microchannel plate photomultiplier tubes

A newly developed 20-inch photomultiplier tube with good timing response and improved energy resolution uses a microchannel plate instead of traditional dynodes. Thin flexible sheets of high-permeability iron-based amorphous alloy foils encased in thin polyethylene terephthalate layers are used to shield the photomultiplier tube from the geomagnetic field. The work reported in this paper shows that the detection efficiency is increased by 87% on average with the help of this magnetic shield, and the number of secondary electrons is reduced by a factor of 2 for 20-inch MCP-PMT at LHAASO-WCDA.

The Multipactor Simulation and Thresholds Analysis for an Impedance Transformer by an Improved 3-D PIC Method Combined With Particle Merging

In this study, a secondary electron emission (SEE) module is updated to Alioth, a 3-D electromagnetic (EM) particle-in-cell (PIC) software for simulating the multipactor. To solve the problems of low computational efficiency and overburden caused by the considerable number of secondary electrons, a particle merging scheme based on the Russian roulette is introduced. The multipactor process of an impedance transformer is investigated from the generation of seed electrons to the saturation through the 3-D PIC method combined with the particle merging scheme. The time evolution characteristics of the space charge during the multipactor are calculated to analyze the multipactor threshold of the impedance transformer. Based on the evolution of the electrons and space charge, the changes of S₁₁, and the generation of the third harmonic, the multipactor threshold is researched thoroughly. The error of the threshold results between the simulation and experiment is 0.296 dB, which shows the good agreement of Alioth with the measures.

Methodological and Conceptual Progresses in Studies on the Latent Tracks in PADC.

Modified structure along latent tracks and track formation process have been investigated in poly (allyl diglycol carbonate), PADC, which is well recognized as a sensitive etched track detector. This knowledge is essential to develop novel detectors with improved track registration property. The track structures of protons and heavy ions (He, C, Ne, Ar, Fe, Kr and Xe) have been examined by means of FT-IR spectrometry, covering the stopping power region between 1.2 to 12,000 eV/nm. Through a set of experiments on low-LET radiations—such as gamma ray-, multi-step damage process by electron hits was confirmed in the radiation-sensitive parts of the PADC repeat-unit. From this result, we unveiled for the first-time the layered structure in tracks, in relation with the number of secondary electrons. We also proved that the etch pit was formed when at least two repeat-units were destroyed along the track radial direction. To evaluate the number of secondary electrons around the tracks, a series of numerical simulations were performed with Geant4-DNA. Therefore, we are proposing new physical criterions to describe the detection thresholds. Furthermore, we propose a present issue of the definition of detection threshold for semi-relativistic C ions. Additionally, as a possible chemical criterion, formation density of hydroxyl group is suggested to express the response of PADC.

Characteristics of non-Maxwellian magnetized sheath with secondary electron emission

In this paper, the effects of non-Maxwellian distribution of electrons on the characteristics of magnetized plasma sheath with secondary electron emission are investigated by using a magnetic fluid model of one-dimensional velocity and three-dimensional space. The velocity of electrons follows the non-extensive distribution, and the ions are magnetized in a magnetic field with a certain tilt angle relative to the wall. The effects of the non-extensive electron distribution parameter <i>q</i> and the magnetic field strength and angle on the Bohm criterion, the floating wall potential, the secondary electron number density at the sheath edge, the sheath thickness and the ion velocity are studied by establishing the self-consistent equations. When the electron velocity distribution deviates from the Maxwellian distribution, the results show that as the <i>q</i>-parameter increases, the value of the Bohm criterion decreases, the floating wall potential increases, the number of secondary electrons at the sheath increases, the sheath thickness decreases, the number density of ions and electrons decline faster, the number density of ions near the wall is higher, and the velocities of the ions in the three directions are all reduced. In addition, as the magnetic field strength increases, the sheath thickness decreases, and the number density of ions and electrons in the sheath area decrease rapidly; the larger the magnetic field angle, the more significant the influences of the parameter <i>q</i> on the wall potential and the sheath thickness are, while the velocity component of the ion in the <i>x</i>-direction decreases with the increase of the magnetic field angle, but in the case of super-extensive distribution (<i>q</i> < 1), the velocity change near the wall presents an opposite trend, the increase of magnetic field angle causes wall velocity to increase; when it is close to Maxwellian distribution (<i>q</i> → 1), the velocity near the wall does not depend on the change of the magnetic field angle and basically tends to be identical; in the case of sub-extensive distribution (<i>q</i> > 1), the velocity near the wall decreases with the magnetic field angle increasing.

Calculation Model of Self-Breakdown Voltage Distribution in Gas Switches

AMonte Carlo model is proposed to calculate the self-breakdown voltage distribution in gas switches in this study. The physical framework of the model has focused on the relationships between four random variables and development of self-breakdown. The self-breakdown process has been divided into four phases including waiting for a free electron, electron avalanche, generation of secondary electrons, and transformation from avalanche to breakdown. These random variables consist of the time and the position of the appearance of a free electron, the number of secondary electrons, and cathode protrusions. In addition, the influence of experimental conditions (such as gas pressure, gap structure, atmospheric ionization rate, and electrode morphology) on random variables has also been calculated. Then, the sampling methods of the random variables have been determined. Based on the above research, the calculation model which reflects the influence of experimental conditions on the self-breakdown stability has been established. Finally, some experiments have been designed to test the model. The calculation results conformed very well to the experimental ones.

MAXIMUM SECONDARY ELECTRON YIELDS FROM SEMICONDUCTORS AND INSULATORS

Based on the processes and characteristics of secondary electron emission and the formula for the yield due to primary electrons hitting on semiconductors and insulators, the universal formula for maximum yield [Formula: see text] due to primary electrons hitting on semiconductors and insulators was deduced, where [Formula: see text] is the maximum ratio of the number of secondary electrons produced by primary electrons to the number of primary electrons. On the basis of the formulae for primary range in different energy ranges of [Formula: see text], characteristics of secondary electron emission and the deduced universal formula for [Formula: see text], the formulae for [Formula: see text] in different energy ranges of [Formula: see text] were deduced, where [Formula: see text] is the primary incident energy at which secondary electron yields from semiconductors and insulators, [Formula: see text], are maximized to maximum secondary electron yields from semiconductors and insulators, [Formula: see text]; and [Formula: see text] is the maximum ratio of the number of total secondary electrons produced by primary electrons and backscattered electrons to the number of primary electrons. According to the deduced formulae for [Formula: see text], the relationship among [Formula: see text], [Formula: see text] and high-energy back-scattering coefficient [Formula: see text], the formulae for parameters of [Formula: see text] and the experimental data as well as the formulae for [Formula: see text] in different energy ranges of [Formula: see text] as a function of [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] were deduced, where [Formula: see text] and [Formula: see text] are the original electron affinity and the width of forbidden band, respectively. The scattering of [Formula: see text] was analyzed, and calculated [Formula: see text] values were compared with the values measured experimentally. It was concluded that the deduced formulae for [Formula: see text] were found to be universal for [Formula: see text].

Research on the development mechanism: from two-sided multipactor to one-sided multipactor

Multipactor discharge always causes disastrous damage to a vacuum window in high power microwave system, which actually becomes a limiting factor for the output power of vacuum device. To explore the multipactor phenomenon of complicated pill-box window, the mulitpactor in normal field between the metal boundary and the window disk is studied. Through Monte Carlo (MC) simulations, the susceptive curve is fitted and analyzed. The secondary electrons' avalanche behavior under the normal RF field is discussed. It is noticed that the one-sided multipactor is excited within a very limited Vrf-fD region when two-sided multipactor is excited initially. The development condition from two-sided multipactor to the one-sided multipactor is proposed. Through analyzing and MC simulation, the condition is achieved. When the normal RF electric field can satisfy the phase focus conditions of one-sided multipactor, the two-sided multipactor will develop into one-sided multpactor and then reach a saturation value. Meanwhile, the initial effect of electrostatic field on one-sided multipactor is also discussed. On condition that two-sided multipactor can be excited, the number of secondary electrons can increase up to a saturation value when Edc0 is lower than the minimal saturate value of Edc. When Edc0 is lager than the minimal saturate value of Edc and in the Edc/Erf threshold of one-sided resonant multipactor, the number of secondary electrons can also increase to a saturate value. However, When Edc0 is lager than the minimal saturate value of Edc but beyond the Edc/Erf threshold of one-sided resonant multipactor, secondary electrons will be suppressed.

SU-E-T-141: Effect of a Single Gold Nanoparticle with Different Sizes Inside a Small Water Phantom

Purpose: This study is to investigate the effects of the gold nanoparticles (GNP), a series of micrometre scale simulations have been constructed with Geant4 to track particles and simulate the effects of those particles as they pass through water phantom. Methods: The simulations were used to calculate the number of secondary electrons which are emitted from the particle tracks and the amount of energy which is deposited in the cell tissue. More electrons means that more water molecules can undergo hydrolysis and create potentially dangerous free radical molecules, therefore breaking up DNA and killing off cells or causing damaging mutations. Results: For the 20nm GNP, all three proton energies saw a small increase of electrons above the control, while the X-rays nearly tripled the number of electrons in the phantom. For the 50 nm GNP, the 3 and 2 MeV protons saw a small increase again, however the 1 MeV protons saw a decrease in electrons, the X-rays saw a large increase of nearly 4 times the number of electrons. For the 110nm GNP, all three proton energies saw a decrease in the total number of electrons in the phantom, while the X-rays saw an increase of 8 times as many electrons. Conclusion: From the range of GNP sizes used, it was found that the X-rays have a larger dose enhancement effect as the GNP size increases, the relation between electron emissions and GNP size was linear. This is because the majority of the dose from the X-rays is delivered to the cell tissue through the initial high energy secondary electrons, any dose lost from the Augerelectrons being trapped inside the GNP volume is small compared to the dose that escapes with the high energy electrons.

Accumulation and dissipation of positive charges induced on a PMMA build-up cap of an ionisation chamber by 60Co gamma-ray irradiation

The signal current from an ionisation chamber with a PMMA build-up cap decreases with irradiation time due to electric fields produced by positive charges induced on the cap. In the present study, it was confirmed that the signal current decreases faster for irradiation using narrower (60)Co gamma-ray beams. This is because the number of secondary electrons that are emitted from surrounding materials and penetrate the build-up cap is smaller in a narrower gamma-ray beam, so that fewer positive charges are neutralised. The ionisation chamber was first subjected to continuous gamma-ray irradiation for 24 h, following which it was irradiated with shorter periodic gamma-ray bursts while measuring the current signal. This allowed the coefficients of positive charge accumulation and dissipation to be determined. It was found that the dissipation coefficient has a large constant value during gamma-ray irradiation and decreases asymptotically to a small value after irradiation is stopped. From the coefficients, the minimum signal current was calculated, which is the value when accumulation and dissipation balance each other under continuous irradiation. The time required for the signal current to recover following irradiation was also calculated.

Formula for the Probability of Secondary Electrons Passing over the Surface Barrier into a Vacuum

Based on a simple classical model that primary electrons at high electron energy interact with the electrons of lattice by the Coulomb force, we deduce the energy of secondary electrons. In addition, the number of secondary electrons in the direction of velocity of primary electrons per unit path length, n, is obtained. According to the energy band of the insulator, n, the definition of the probability B of secondary electrons passing over the surface barrier of insulator into the vacuum and the assumption that lattice scattering is ignored, we deduce the expression of B related to the width of the forbidden band (Eg) and the electron affinity χ. As a whole, the B values calculated with the formula agree well with the experimental data. The calculated B values lie between zero and unity and are discussed theoretically. Finally, we conclude that the deduced formula and the theory that explains the relationships among B, χ and Eg are correct.

PIC-MCC Simulation for HPM Multipactor Discharge on Dielectric Surface in Vacuum

In order to understand the physical mechanism of multipactor discharge on dielectric window surface under high power microwave (HPM) excitation in vacuum, an electron movement simulation model based on the particle-in-cell (PIC) Monte Carlo (MC) is built in this paper. The influences of microwave electromagnetic field and electrostatic field from dielectric surface charging are simultaneously considered in this model. During the simulation, the emission velocity and angle distribution of secondary electrons from the dielectric surface are taken into account. The movement trajectories of electron clusters under complex field excitation are obtained. The influences of emergence angle and microwave electromagnetic parameters on the electron movement are analyzed. It is found that the emergence angle of electrons from the surface has significant effect on its movement, and both the impact energy and return time of electrons oscillate periodically with the phase of microwave field. The number of secondary electrons and induced electrostatic field from multipactoring are also investigated. The results reveal that both values oscillate periodically at twice the microwave frequency, which is due to the electron impact energy oscillating with microwave period. A schematic diagram is proposed to explain the periodical oscillation phenomena.

Universal Formula for Secondary Electron Yield of Metals at High Electron Energy and Incident Angle θ

On the basis of the main physical processes of secondary electron emission, the relationships among the incident energy (Wp0) of primary electrons, the number of secondary electrons (δPEθ) released per primary electron entering the metal at high electron energy and incident angle θ and incident angle (θ) are deduced. In addition, the relationship between the number of secondary electrons (δPE0) released per primary electron entering the metal at θ= 0° and Wp0 is determined. From the experimental results, the relationships among the ratio βθ (the subscript θ means the primary electron is incident at θ in this paper), the ratio at θ= 0° (β0) and θ are obtained. On the basis of relationships among δPEθ, δPE0, βθ, β0, backscattered coefficient ηθ, backscattered coefficient at θ= 0° (η0), secondary electron yield δθ, and secondary electron yield at θ= 0° (δ0), the universal formula for expressing δθ using δ0, ηθ, η0, and θ is deduced. The secondary electron yield calculated from this universal formula and the yields measured experimentally from aluminum and copper are compared. The results suggest that the proposed formula is universal for the estimation of secondary electron yields in the angle range of 0–80° and the energy range of 10–102 keV.

C60fragmentation in charge-changing collisions with slow Au+ions

We investigated ionization and fragmentation of C{sub 60} in the charge-changing collisions of 2.0 keV/amu Au{sup +} ions with C{sub 60} molecules. We performed simultaneous measurements of product ions, the number of secondary electrons, and a charge-selected outgoing Au projectile. The production of C{sub 60}{sup +} ions was predominant in the single-electron capture (1-capture) process. In contrast, multifragmentation of C{sub 60} was predominant in the single-electron loss (1-loss) process. The multifragmentation was enhanced in the 1-loss process compared with the 1-capture process when the same number of electrons was emitted from C{sub 60}. This enhancement was also observed when a fast Si ion traveled close to a carbon nucleus [T. Majima et al., Phys. Rev. A 74, 033201 (2006)]. The 1-loss process of slow Au{sup +} ions is considered to occur closer to a carbon nucleus than does the 1-capture process. Multifragmentation of C{sub 60} is caused by internal electronic excitation although it is probably assisted by nuclear stopping when, at least, the electronic excitation of C{sub 60} is small.

Enhancement of anode current in an HOPFED

In a HopFED, most electrons which get through the hop funnel and land on the anode are secondary electrons. The intensity of the anode current is largely determined by these electrons. In this paper, an extraction electrode is introduced to increase the number of secondary electrons. The influence of the extraction electrode voltage and the gate voltage on the anode current is also

Electronic stopping and velocity effect on multiple ionization and fragmentation of C60 in swift heavy ion impacts

Multipleionization and fragmentation of C60 induced by fast heavy ions have been studied by means of triple coincidence measurements of fragment ions, the number of secondary electrons and charge states of outgoing projectile ions. In order to investigate the roles of stopping power Se and of incident velocity v, we used projectile ions of 2.0MeV Li+, 0.4MeV O+ and 0.8MeV Si+, for which Se’s are equivalent (Li+ and O+) and v’s are also equivalent for O+ and Si+ ions. Mass spectra of fragment ions were found to be almost identical in the same v collisions (O+ and Si+) when the number of purely-ionized electrons from C60 is the same. By contrast, in the same Se collisions (Li+ and O+), mass spectra were completely different and the degree of multiple ionization was largely enhanced in O+ impacts. These results indicate evidently that partition of deposited energy Ed between ionization and internal excitation may be strongly dependent on the projectile velocity rather than the stopping power itself. We conclude that, in faster collisions, a larger part of the Ed is spent for electron emission, while in slower collisions Ed may be more converted into internal excitation.

Fragmentation processes ofC60in multiple electron loss and capture collisions of2−MeVSi2+

An experimental study of ionization and fragmentation of ${\mathrm{C}}_{60}$ has been performed for $2\text{\ensuremath{-}}\mathrm{MeV}$ ${\mathrm{Si}}^{2+}$ incident ions under charge-changing conditions. Time-of-flight (TOF) spectra of product ions and number distributions of secondary electrons $({n}_{e})$ were measured in coincidence with charge-selected outgoing projectiles. TOF spectra from transiently formed highly charged $\mathrm{C}_{60}{}^{r+}$ ions are obtained for individual charge states $r$, and ${n}_{e}$ distributions correlated with size-fixed $\mathrm{C}_{m}{}^{+}$ ions are also obtained for $ml15$. It is found that mass spectra and ${n}_{e}$ distributions are both significantly different between loss and capture collisions, whereas a remarkable similarity in the partial ${n}_{e}$ distributions is observed almost independently of the different charge-changing conditions when compared at given $\mathrm{C}_{m}{}^{+}$ ions size. We find also that a number of secondary electrons as high as 20 are emitted in the production of small fragment ions. Furthermore, small fragment ions are produced predominantly in close collisions even at very low charge states of $r\ensuremath{\simeq}3$ in comparison with distant collisions. This indicates that the energy partition rate between ionization and internal excitation might be considerably different in close and distant collisions.

Effects of secondary electrons emitted from surroundings on defect formation in silica glass under γ-ray irradiation

We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in silica glass under γ-ray irradiation. SiO2 (silica) glass plates and those sandwiched in a pair of various material disks (carbon, stainless steel or lead) were irradiated by γ-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra of the glass plates after the irradiation showed three absorption bands peaked around 2eV, 4eV and 5.8eV being assigned to color centers relate metal impurities (Al and Ge) and oxygen-deficient centers like E′ center, respectively. All three bands were found to grow with γ-ray irradiation dose and saturated at higher doses, and absorbance of the bands at the saturation for the sandwiched glass plates was higher than that for the bare glass plate. Moreover, the saturated absorbance was higher for the glass plate sandwiched with heavier materials. Employing Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process, enhanced energy deposition and numbers of secondary electrons and photons emitted from sandwiching material disks to a silica glass plate were calculated. The higher deposition energy correlates well to the higher saturated absorbance, indicating that the secondary electrons and photons emitted from the disks clearly enhanced the defect formation in the sandwiched silica glass plates. This suggests the existence of the dose effect above a critical does, i.e. the irradiation with higher dose will result in higher saturated absorbance.

Carbon buildup monitoring using RBS: Correlation with secondary electrons

The RBS technique is applied to solve the problem of on-line monitoring of the carbon deposited on a thin backed foil under ion bombardment. An iterative method is used to reliably extract quantities such as number of projectiles and target thickness in spite of beam energy changes and detector unstabilities. Experimental values for secondary electron yields are also deduced. Results are reported for the thickness variation of thin carbon foils bombarded with carbon ions of energies between 8.95 and 13 MeV. A linear correlation of this variation is found with both, the ion fluence at target and the number of secondary electrons emitted. The correlation exists even though a wide range of beam currents, beam energies and bombarding times was used during the experiment. The measured electron yields show evidence for a change in the emission process between the original foils and the deposited layer, possibly due to a texture change.