Primary Electrons Research Articles

Design of a mechatronic system for accurate measurement of secondary electron emission

Accurate measurement of Secondary Electron Emission (SEE) in scientific experiments provides better insight and understanding of field emission and multipacting properties in accelerator cavities. SEE properties of Niobium (Nb) are of significant importance to the performance and operation of accelerator cavities. Additionally, accurate Secondary Electron Yield (SEY) measurements from primary electrons generated from samples measured under different incident angles represent a technical challenge. For that purpose, the sample positioning system presented in this article is a design that allows for precise sample orientation and handling during single pump-down of the vacuum system in the experimental setup. Measurement techniques supported by a mechatronic system provide the means for acquisition of large quantities of data and at the same time reduce the experimental errors related to sample positioning. The authors describe a sample manipulation system and method for controlling the position of samples used for the measurement of SEY.

Analyzing the performance of pseudo-random single photon counting ranging Lidar.

The detection principle and performance of the pseudo-random single photon counting ranging (PSPCR) Lidar system are investigated. The detection probability and single photon detection efficiency (SPDE) of the macro code for the PSPCR Lidar system are derived based on statistical theory. The effects of the echo primary electrons number and the dead time on the detection probability and SPDE are analyzed. The detection probability increases with the increase of the primary electron number and tends toward saturation. The change of dead time length has little effect on the detection probability of the macro code, especially when the number of echo primary electrons is large. However, the length of dead time is inversely proportional to the number of detected codes. The longer the dead time, the fewer the number of detected codes, and the worse the ranging performance. The signal-to-noise ratio (SNR) of the PSPCR Lidar is analyzed based on the cross-correlation function. The Monte Carlo simulation results show that the PSPCR Lidar has a satisfactory SNR even in a high noise level. As the number of signal primary electrons increases, the SNR gradually increases and tends to be saturated. As the noise increases, the SNR gradually decreases, and the greater the noise, the more severe the SNR decreases. At the same time, based on the assumption that the power of the single code in Gaussian distribution and the time resolution of the photon counting module are less than the code width, the theoretical formula of the range error is deduced. The effects of the echo signal primary electron number and code width on the range error are analyzed. The results show that the fewer the primary electron numbers or the narrower the code width, the smaller the range error of the PSPCR Lidar system. The range error of the PSPCR Lidar system is verified by Monte Carlo simulation. The simulation results are in good agreement with the theoretical analysis.

Angular, temperature, and impurity effects on secondary electron emission from Ni(110)

The secondary electron emission from a temperature-controlled Ni(110) sample was examined for 50–1500 eV electrons impacting at 0°–35°, 50°, and 78°. Measurements showed a non-cosine dependence on an electron incidence angle: the yield has a maximum at 0°, minima at ±12°, and increases at larger angles up to 35°. This trend in angular dependence is characteristic of single crystal materials and is due to increased secondary electron generation when primary electrons are directed along a close-packed direction. For example, compared to polycrystalline nickel, the yield for Ni(110) from primary electrons at 0° (i.e., along the [110] direction) is up to 36% larger. Additionally, secondary electron yields are highly sensitive to incident electron energy (most notably between 0 and 500 eV) and to the presence of adsorbed carbon monoxide [with an up to 25% decrease compared to clean Ni(110)]. However, yields are independent of sample temperature between 300 and 600 K and of exposure to deuterium ions leading to formation of subsurface hydrogen. These results reaffirm the unique secondary electron emission properties of single crystals materials and highlight the importance of crystal orientation. Results are important for plasma-enhanced chemistry applications that utilize Ni(110) catalysts, since larger secondary electron emission may facilitate reactions of adsorbed species.

Characteristics and Performance of the CALorimetric Electron Telescope (CALET) Calorimeter for Gamma-Ray Observations

Abstract The CALorimetric Electron Telescope primary detector (CALET-CAL) is a 30 radiation-length-deep hybrid calorimeter designed for the accurate measurement of high-energy cosmic rays. It is capable of triggering on and giving near complete containment of electromagnetic showers from primary electrons and gamma rays from 1 GeV to over 10 TeV. The first 24 months of on-orbit scientific data (2015 November 01–2017 October 31) provide valuable characterization of the performance of the calorimeter based on analyses of the gamma-ray data set in general and bright point sources in particular. We describe the gamma-ray analysis, the expected performance of the calorimeter based on Monte Carlo simulations, the agreement of the flight data with the simulated results, and the outlook for long-term gamma-ray observations with the CAL.

Thermal Design of a 100 kW Electron to Gamma Converter at TRIUMF

The electron target station of the TRIUMF-ARIEL Facility will employ an electron “driver” beam to irradiate Isotope Separator On-Line (ISOL) targets for the production of radioactive isotopes via photofission [1]. 35 MeV electrons will be converted to gamma spectrum Bremsstrahlung photons via an electron to gamma (e-γ) converter located upstream of the ISOL target. The e-γ concept uses a composite metal with two layers: One high-Z material to stop and convert electrons to photons, and one low-Z material to provide structural support, thermal dissipation, and maximal transparency to the produced gamma photons as well as to provide attenuation of remaining primary electrons.. Several material combinations and bonding processes are currently being evaluated and tested using TRIUMF’s e-linac. Water-cooling and thermal design are being optimized for 100 kW electron beam power operation and have thus far been experimentally validated up to 10 kW driver beam power equivalent. The latest test results and future prospects are summarized.

Secondary electron yield of Cu (1 1 0) with adsorbed nitrogen: a 3D Monte Carlo simulation

We developed an adsorption model to examine secondary electron (SE) emission from a surface adsorbed by N2 molecules. In this model, the scattering processes of electrons and N2 molecules are simulated by Monte Carlo method. The traces of both primary electrons (PEs) and SEs are tracked by combining the electron scattering in the material and adsorbed layer. The SE yield (SEY) is then calculated from the final states of electrons that can emit out from the adsorbed layers. Moreover, the effects of the work function variation induced by adsorption are also taken into account in the model. The SE emission properties of the surface under different pressures and surface coverage are examined by our adsorption model. We find that the SEY reaches its minimum for an intermediate coverage of 1 ~ 2 × 1015/cm2 for the Cu substrate. Further investigations on returning and outgoing ratios and energy distributions of SEs indicate that an adsorbed layer can generate new electrons but weaken the electron energy, especially at a high PE energy. The competing mechanism of ‘SE generation event’ and ‘energy losing event’ controls the variation of SEY. Accordingly, our model could serve as a powerful tool and our results can provide comprehensive insight into the SE emission of the surface covered by gas molecules, thereby helping us to find a rationale for the behaviour of SEs under various surface conditions.

The influence of internal and external electric fields on the transport of energetic electrons in nanostructures

Sandwich like metal–insulator–metal (MIM) nanostructures consisting of a 50 nm silver film and a 30 nm aluminum film separated by a few nanometer aluminum oxide layer were irradiated with a focused e-beam (diameter 0.5 mm) at kinetic impact energies in the range of 100 eV to 1000 eV. To distinguish between internal transport of hot charge carriers across the buried insulator (tunnel junction) and parasitic electron transport mediated by externally emitted electrons re-entering the sample, an additional “dome” electrode was implemented which was biased to positive or negative potential in order to establish an external accelerating or retarding field above the nanostructure's surface. Different device currents induced by the primary electron irradiation were measured either by metering the irradiated or non-irradiated electrode, respectively. The dependence of the detected device currents on impact parameters such as the irradiated position on the MIM surface, the kinetic energy and impact angle of the primary electrons was studied. These experiments were accomplished while changing the internal electric field by an internal bias voltage between the top and the bottom electrode of the MIM and while changing the external electric field by applying a voltage to the dome electrode. The measured currents are interpreted in terms of external and internal emission yields. It is shown that the external electric field allows a clear discrimination between true internal electron transport and external electron transport leaving the MIM nanostructure on one site and re-entering at another site. The results demonstrate that “internal” currents measured without an external dome electrode may be strongly influenced or even falsified by such cross-absorption effects.

Role of Auger electrons in X‐ray excitation by electron beam

We present an algorithm that allows an estimation of the number of ionizations of atoms with low atomic number by Auger electrons produced in the matrix during excitation of X‐rays by an electron beam. The dependence of Auger ionization on the atomic number of the matrix has a maximum resulting from the increasing energy of Auger electrons and the decrease of their number as a function of the atomic number. Dependence of the contribution of Auger electrons to the analytical signal on the energy of primary electrons is calculated. Estimates of this contribution to X‐ray emissions by boron, carbon, nitrogen, and fluorine in different matrices are given.

A Photo-hadronic Model of the Large-scale Jets of 3C 273 and PKS 1136–135

Abstract X-ray-bright knots of kiloparsec-scale jets of several radio-loud quasars are an actively discussed issue. Among various models for explaining observations of these phenomena, synchrotron radiation from the electron population that differs from radio to IR-emitting electrons, is promising. However, the origin of this electron population has been debated. Recently, we proposed that this electron population is produced by proton–photon collisions (mainly, the Bethe–Heitler process), and we applied this model to PKS 0637–752. We found that this model works if the proton power is an order of magnitude larger than the Eddington power. In this paper we apply this model to the X-ray emission in the knots of 3C 273 and PKS 1136–135. The target photons for electron–positron pair production are supplied by synchrotron radiation at the radio–IR by primary electrons and by the active galactic nucleus (AGN) core, as well as cosmic microwave background (CMB) radiation. The effects of the AGN photons are included for the first time in the hadronic model. Though the observed X-ray flux is obtained with the contribution of the AGN photons, the required proton power turns out to be highly super-Eddington. However, we find that our model works for a nearly Eddington proton power, if the photon density of the AGN is enhanced. This can occur if the AGN photons are more beamed toward the X-ray knots than toward our line of sight and the AGN photon frequency is shifted by the Doppler effect.

Spatial resolution optimization in a THGEM-based UV photon detector

THick Gas Electron Multiplier (THGEM) is considered in many UV photon detector applications. It has the capability of detecting single photon and imaging with high sensitivity. Operating parameters such as choice of gas mixture, pressure, drift field, drift gap, multiplication voltage, induction field and induction gap play an important role in deciding the spatial resolution of the detector. Detailed simulation study enables to optimize the above-mentioned parameters for a given THGEM-based imaging detector and hence to achieve improved performance for the same. Simulation, using ANSYS and Garfield++, starts with the release of primary electrons at random coordinates on the photocathode plane. They are tracked as they pass through the drift gap and THGEM hole till the electron cloud reaches anode plane. Distribution of electron cloud on the anode plane along X and Y axis is plotted in histogram and fitted with Gaussian function to determine spatial resolution. Ar/CO2 (70:30) mixture, which shows higher ETE and lower transverse diffusion, is chosen for this simulation study. Transverse diffusion has a major impact on both ETE and the spatial resolution. Lower transverse diffusion coefficient is always desired for having better resolution as well as for ETE. It is found from the simulation study that higher gas pressure, lower drift field and induction field, smaller drift and induction gap can provide optimum detection efficiency with the best spatial resolution. The simulation method proposed here can also be extended to X-ray imaging detectors.

Backscattered electron spectra from graphite

The backscattered electron spectra from graphite sample were studied both experimentally and theoretically at impact energies between 500 and 5000 eV. The angle of the incident electron beam was 50° and the detection angle was 0° with respect to the surface normal, respectively. Monte Carlo (MC) simulations were performed based on the Classical Transport Theory (CTT) model to mimic the experimental spectra. In our simulations, both elastic and inelastic scattering of primary electrons and secondary electron emission from graphite are taken into account. There is found satisfactory agreement between measured and calculated electron spectra.

Characteristics of the Plasma Generated by Primary Electrons at Lowered Pressure

Plasma characteristics in the gap at lowered pressure in the presence of primary electrons are analyzed for the hydrodynamic model in the cold ion approximation. It is demonstrated that at a certain critical ionization level, the plasma state changes abruptly. The potential drop in the plasma can be not only more, but also less than ln(2)kTe/e, and hence the velocity of ions exiting from the plasma can be more or less than the Bohm velocity. For low ionization intensity and formation of the plasma with a high fraction of primary electrons, the velocity of secondary electron exiting from the plasma becomes equal to their thermal velocity, and their distribution differs significantly from the Boltzmann distribution.

Estimation of the Heating of a Semiconductor Target Surface by a Low-Energy Electron Beam

The problem of heat distribution in semiconductor materials irradiated with sharply focused lowenergy electron beams in the absence of heat exchange between the target and external medium is considered by methods of mathematical modeling. The model is based on solving a multidimensional steady-state heatconduction equation using Green’s function. The source function is a model applicable to a wide class of solids and a wide range of energies of primary electrons, based on separate description of the contributions of absorbed and backscattered electrons to the energy dissipated in the target. Using the features of such an approach, a nonmonotonic dependence of the temperature of maximum heating of the target on the energy of primary electrons is explained. The results of calculations using the considered model of heating the target with an electron probe are presented for various semiconductor materials of electronic engineering.

Particle modeling of radial electron dynamics in a controlled discharge of a Hall thruster

An improved radial particle-in-cell model of an annular Hall effect thruster discharge with secondary-electron emission from the walls and a radial magnetic field is presented. New algorithms are implemented: first, to adjust the mean neutral density to the desired mean plasma density; second, to avoid the refreshing of axially accelerated particles; and third, to correctly weigh low-density populations (such as secondary electrons). The high-energy tails of the velocity distribution functions of primary and secondary electrons from each wall are largely depleted, leading to temperature anisotropies for each species. The secondary-electron populations are found to be partially recollected by the walls and partially transferred to the primary population. The replenishment ratio of the primary high-energy tail is determined based on the sheath potential fall. Significant asymmetries at the inner and outer walls are found for the collected currents, the mean impact energy, and the wall and sheath potentials. Radial profiles in the plasma bulk are asymmetric too, due to a combination of the geometric expansion, the magnetic mirror effect, and the centrifugal force (emanating from the E × B drift). The temperature anisotropy and non-uniformity, and the centrifugal force modify the classical Boltzmann relation on electrons along the magnetic lines.

Two discharge modes of a repetitive nanosecond pulsed helium glow discharge under sub-atmospheric pressure in the repetition frequency range of 20 to 600 kHz

Two discharge modes, α and γ, of a repetitive nanosecond pulsed helium glow discharge at a gas pressure of 10 kPa in the repetition frequency range from 20 to 600 kHz are reported for the first time. The pulsed glow discharge is produced in a pair of parallel plate metal electrodes without insertion of dielectrics. The α mode discharge is volumetrically produced in the electrode gap at a low-repetition frequency, whereas the γ mode discharge is localized at the cathode surface at a high-repetition frequency. At high-repetition frequency, the time interval between voltage pulses is shorter than the lifetime of the afterglow produced by the preceding discharge. Then, the γ mode discharge is maintained by a large number of secondary electrons emitted from the cathode exposed to high-density ions and metastable helium atoms in the afterglow. In the α mode discharge with a low-repetition frequency operation, primary electrons due to gas ionization dominate the ionization process. Thus, a large discharge voltage is needed for the excitation of the α mode discharge. It is established that the bifurcation of α-γ discharge mode, accompanied by a decrease in the discharge voltage, occurs at the high-repetition frequency of ∼120 kHz.

A Combined Model of Charging of the Surface and Bulk of a Dielectric Target by Electrons with the Energies 10–30 keV

A physico-mathematical model of the processes of radiation-induced charging of dielectric materials with open surfaces, irradiated with monoenergetic electrons in the energy range 10–30 keV, is described. The model takes into account the relationship between the processes of surface and bulk charging for the given conditions of the experimental design, which accounts for the effect of anomalously long charging of dielectrics after the incident energy of primary electrons during charging is reduced to below the second critical energy for the secondary electronic emission coefficient. The initial fast phase of charging a high-resistivity dielectric material (Al2O3) is investigated. It is shown that as the incident electron energy is approaching the second critical energy during charging, the secondary electronic emission is partially suppressed due to negative charging of the open surface of the dielectric and formation of a near-surface inversion electrical field retarding the electronic emission yield.

Evaluation of the Use of New Matrix Corrections in Quantitative X-Ray Spectral Analysis

New possibilities for the quantitative description of matrix corrections for the absorption of characteristic X-ray radiation and backscattering of primary beam electrons are considered when solving the direct problem of quantitative X-ray spectral microanalysis. It is shown that the proposed procedure for calculating the given matrix corrections, together with the correctly chosen correction for the stopping power, makes it possible to calculate the quantitative content of elements for a large array of binary compositions from B to U.

Characteristics of an elongated plasma column produced by magnetically coupled hollow cathode plasma source

An elongated plasma column in the presence of an axial magnetic field has been formed using a cylindrical hollow cathode (HC) and a constricted anode (CA). The plasma characteristics of the central line have been found to vary with the magnetic field strength and the axial distance from the source. It is believed that the primary electrons constituting the discharge current are steered by the axial magnetic field to undertake ionizing collisions along the plasma column. The current carrying electrons from the HC reach the anode by cross-field diffusion towards the central line. The above observation has been substantiated using a phenomenological model which links the observed characteristics of the source with the plasma column. The experimental results are found to be in qualitative agreement with the model.

Propagation of modulated electron and X-ray beams through matter and interactions with radio-frequency structures

The generation and evolution of modulated particle beams and their interactions with resonant radiofrequency (RF) structures are of fundamental interest for both particle accelerator and vacuum electronic systems. When the constraint of propagation in a vacuum is removed, the evolution of such beams can be greatly affected by interactions with matter including scattering, absorption, generation of atmospheric plasma, and the production of multiple generations of secondary particles. Here, we study the propagation of 21 MeV and 25 MeV electron beams produced in S-band and L-band linear accelerators, and their interaction with resonant RF structures, under a number of combinations of geometry, including transmission through both air and metal. Both resonant and nonresonant interactions were observed, with the resonant interactions indicating that the RF modulation on the electron beam is at least partially preserved as the beam propagates through air and metal. When significant thicknesses of metal are placed upstream of a resonant structure, preventing any primary beam electrons from reaching the structure, RF signals could still be induced in the structures. This indicated that the RF modulation present on the electron beam was also impressed onto the x-rays generated when the primary electrons were stopped in the metal, and that this RF modulation was also present on the secondary electrons generated when the x-rays struck the resonant structures. The nature of these interactions and their sensitivities to changes in system configurations will be discussed.

Hadronic model for the non-thermal radiation from the binary system AR Scorpii

Abstract AR Scorpii is a close binary system containing a rotation powered white dwarf and a low-mass M type companion star. This system shows non-thermal emission extending up to the X-ray energy range. We consider hybrid (lepto-hadronic) and pure hadronic models for the high energy non-thermal processes in this binary system. Relativistic electrons and hadrons are assumed to be accelerated in a strongly magnetised, turbulent region formed in collision of a rotating white dwarf magnetosphere and a magnetosphere/dense atmosphere of the M-dwarf star. We propose that the non-thermal X-ray emission is produced either by the primary electrons or the secondary e± pairs from decay of charged pions created in collisions of hadrons with the companion star atmosphere. We show that the accompanying γ-ray emission from decay of neutral pions, which are produced by these same protons, is expected to be on the detectability level of the present and/or the future satellite and Cherenkov telescopes. The γ-ray observations of the binary system AR Sco should allow us to constrain the efficiency of hadron and electron acceleration and also the details of the radiation processes.