Secondary Photons Research Articles

Small accelerators and their applications in the CANAM research infrastructure at the NPI CAS

The Nuclear Physics Institute (NPI) of the Czech Academy of Sciences (CAS) operates, among other facilities, a Tandetron linear accelerator, TR-24 and U-120M cyclotrons, and an MT25 microtron. A new accelerator mass spectrometry (AMS) instrument, MILEA, has recently been acquired. Except for the MT25, all facilities are synergic parts of the Centre of Accelerators and Nuclear Analytical Methods Research Infrastructure (CANAM RI) ( http://canam.ujf.cas.cz ). This paper demonstrates the instrumental, scientific and application capabilities of the devices for many fundamental nuclear physics experiments and a wide spectrum of applications. The Tandetron Laboratory has a full arsenal of ion-beam analytical methods, ion-beam nano- and microstructuring techniques, as well as tools for the doping and synthesis of new progressive materials using energetic ion beams. It uniquely utilises ion beams for 3D elemental mapping; it studies internal morphology using ion-microprobe methods applicable in many scientific branches. The TR-24 and U-120M cyclotrons provide the primary beams of accelerated ions as well as the generated secondary fast neutrons, including the necessary instrumentation and the target technology, and enable fundamental experiments in nuclear physics, astrophysics, dosimetry, etc. The microtron MT25 is a source of relativistic electrons (the primary electron beam), the secondary photon beam (bremsstrahlung) and neutrons from photonuclear reactions which are used, e.g. for analysis and preservation of cultural heritage. The MILEA AMS system offers a highly sensitive isotopic ratio measurement of very long-lived radionuclides at levels up to 10–15.

PeV–EeV Neutrinos from Gamma-Ray Blazars due to Ultrahigh-energy Cosmic-Ray Propagation

Abstract Blazars are potential sources of cosmic-ray acceleration up to ultrahigh energies (E ≳ 1018 eV). For an efficient cosmic-ray injection from blazars, pγ collisions with extragalactic background light (EBL) and cosmic microwave background (CMB) can produce neutrino spectra with peaks near to PeV and EeV energies, respectively. In this work, we analyze the contribution of these neutrinos to the diffuse background measured by the IceCube neutrino observatory. The fraction of neutrino luminosity originating from individual redshift ranges is calculated using the distribution of BL Lacs and FSRQs provided in the Fermi-LAT 4LAC catalog. Furthermore, we use a luminosity-dependent density evolution to find the neutrino flux of unresolved blazars. The results obtained in our model indicate that as much as ≈10% of the flux upper bound at a few PeV energies can arise from cosmic-ray interactions with EBL. The same interactions will also produce secondary electrons and photons, initiating electromagnetic cascades. The resultant photon spectrum is limited by the isotropic diffuse γ-ray flux measured between 100 MeV and 820 GeV. The latter, together with the observed cosmic-ray flux at E > 1016.5 eV, can constrain the baryonic loading factor, depending on the maximum cosmic-ray acceleration energy.

Comparison between PHITS and GEANT4 Simulations of the Heavy Ion Beams at the BEVALAC at LBNL and the Booster Accelerator at BNL

Heavy charged particles have been discussed for clinical use due to their superior dose-depth distribution compared to conventional radiation such as X-rays. In addition, high-charge and energy (HZE) ions in galactic cosmic rays (GCR) present important health risks for crewed space missions to the Earth's moon or Mars. Experiments at heavy ion accelerators are used in radiobiology studies; however, numerical simulations of track segment or Bragg peak irradiations are complicated by the details of the beam-line and dosimetry systems. The goal of the present work is in support of biophysics modeling of historical radiobiology data at Lawrence Berkeley National Laboratory (LBNL) and more recent results from the Brookhaven National Lab (BNL) facility (NASA Space Radiation Lab (NSRL)). In this work, the Spread-Out Bragg Peak (SOBP) of 4He, 12C, and 20Ne particles, and a Bragg curve of 56Fe ion have been simulated numerically in the geometries of LBNL and BNL using the Monte-Carlo based PHITS and GEANT4 simulation toolkits. The dose contributions of primary particles and secondary particles, including neutrons and photons, in the target material are computed and discussed as well. Comparisons suggest more contributions of secondaries in GEANT4 simulations compared to PHITS simulations, and less statistical fluctuation and better prediction of neutrons in PHITS simulations. Neutrons and gamma-rays are estimated to make minor contributions to absorbed doses for these beams.

Out-of-field doses for scanning proton radiotherapy of shallowly located paediatric tumours—a comparison of range shifter and 3D printed compensator

The lowest possible energy of proton scanning beam in cyclotron proton therapy facilities is typically between 60 and 100 MeV. Treatment of superficial lesions requires a pre-absorber to deliver doses to shallower volumes. In most of the cases a range shifter (RS) is used, but as an alternative solution, a patient-specific 3D printed proton beam compensator (BC) can be applied. A BC enables further reduction of the air gap and consequently reduction of beam scattering. Such pre-absorbers are additional sources of secondary radiation. The aim of this work was the comparison of RS and BC with respect to out-of-field doses for a simulated treatment of superficial paediatric brain tumours. EURADOS WG9 performed comparative measurements of scattered radiation in the Proteus C-235 IBA facility (Cyclotron Centre Bronowice at the Institute of Nuclear Physics, CCB IFJ PAN, Kraków, Poland) using two anthropomorphic phantoms—5 and 10 yr old—for a superficial target in the brain. Both active detectors located inside the therapy room, and passive detectors placed inside the phantoms were used. Measurements were supplemented by Monte Carlo simulation of the radiation transport. For the applied 3D printed pre-absorbers, out-of-field doses from both secondary photons and neutrons were lower than for RS. Measurements with active environmental dosimeters at five positions inside the therapy room indicated that the RS/BC ratio of the out-of-field dose was also higher than one, with a maximum of 1.7. Photon dose inside phantoms leads to higher out-of-field doses for RS than BC to almost all organs with the highest RS/BC ratio 12.5 and 13.2 for breasts for 5 and 10 yr old phantoms, respectively. For organs closest to the isocentre such as the thyroid, neutron doses were lower for BC than RS due to neutrons moderation in the target volume, but for more distant organs like bladder—conversely—lower doses for RS than BC were observed. The use of 3D printed BC as the pre-absorber placed in the near vicinity of patient in the treatment of superficial tumours does not result in the increase of secondary radiation compared to the treatment with RS, placed far from the patient.

Generation of secondary radiation on the surface of endoprostheses used for breast reconstruction during mastectomy, after radiation therapy

Objective: Experimental estimate of the secondary radiation on the surface of implants and expanders, used for breast reconstruction in case of mastectomy, after making radiation therapy.Methods: Special equipment was constructed by radiation study department. It consists of linear accelerator as a source of ionizing radiation and a gamma-ray spectrometer with the detector on the basis of ultrapure germanium. The objects of research were the implants and expanders of the different producers.Results: The maximum amount of the secondary photons on the implants and expanders was generated on the energy level lower than 700 KEV. The level of photons’ generation between 600–300 KEV was higher on the surface of the implants than on the surface of expander filled with saline.Conclusion: The photons of the secondary radiation with the energy level from 100 to 700 KEV are generated under the influence of ionizing radiation on the implants and tissue expander filled with saline. It can be one of the causes of capsular contractures development after mastectomy.

Mathematical algorithm model of absolute dose in radiotherapy

A algorithm of obtaining absolute dose at each tissue depth only by the mathematical calculation of formula is reported. The algorithm is based on integrating the energy flux of the electron generated by X-ray in the range of irradiation field, and the energy spectrum of ray and the secondary scattered ray are considered in this process. In this algorithm, the water phantom in the irradiation field is divided into several thin layers, and the energy flux of the electrons generated by interaction between the ray and thin layer reaching the calculation point is calculated. Finally, the absolute dose of the calculation point can be obtained by accumulating the energy flux contribution of all thin layers. For the X-ray with continuous energy spectrum, the expected mass attenuation coefficient is calculated for obtaining the photon flux at each depth in this process. The absolute dose calculated by this algorithm is verified by Monte Carlo simulation, and the difference between the algorithm and simulation is compensated for by a dose function about multiple scattering photons, and the function shows fast descent and then slow ascent. It is found that the ratio of the dose caused by backscatter to the surface dose, and the relationship among forward scatter, backward scatter and primary ray, and the relationship between the dose and the depth of the secondary scattered rays show a trend of first rising and then declining, and the depth of the peak value deviates from the position of the thin layer. Three-dimensional energy spectra of the secondary photon and the secondary electron are also compared with each other, and the spectrum is a function of particle flux about particle energy and particle direction. From the perspective of Compton effect, the physical meanings of different positions in the three-dimensional energy spectrum of the two particles are explained. It is found that the difference between algorithm percentage depth dose and simulation percentage depth dose is similar to the difference between small irradiation field percentage depth dose and big irradiation field percentage depth dose from simulation, and it is verified that the difference between algorithm and simulation comes from the increase of scattered rays. Finally, the algorithm is applied to the dose calculation of non-uniform phantom, which can accurately reflect the dose distribution characteristics and have less error.

Extension and benchmarking of the OpenMC code for accelerator-based neutron source applications

OpenMC is a community developed Monte Carlo (MC) code for neutron and photon transport calculations. In this work, the OpenMC code has been extended and benchmarked for accelerator-based neutron source applications, such as the IFMIF-DONES (International Fusion Material Irradiation Facility- DEMO Oriented NEutron Source) with neutron energies up to 55 MeV. A spherical computational benchmark and the TIARA high energy neutron shielding experiment has been employed for the benchmarking including also comparisons to the results of the well-validated MC code MCNP as reference. The neutron fluxes computed for spherical benchmarks of selected elements show good agreement between OpenMC and MCNP. A problem has been identified and fixed in the secondary photon transport of the OpenMC code. For the TIARA experiment on iron and concrete shielding, the results of OpenMC are in good agreement with MCNP, and the discrepancies between the experiment results and OpenMC simulated results are considered acceptable. In summary, these benchmarks provide good evidence of the suitability of OpenMC for accelerator-based neutron source applications in the energy range up to 55 MeV, with possibility of further extensions depending on the used nuclear data.

Exploring the Chemistry Induced by Energetic Processing of the H2-bearing, CO-rich Apolar Ice Layer

Abstract Interstellar ice mantles on the surfaces of dust grains are thought to have a bilayered structure, with a H2O-rich polar layer, covered by a CO-rich apolar layer that probably harbors H2 and other volatiles such as N2. In this work, we explore the chemistry induced by 2 keV electrons and Lyα photons in H2:CO:15N2 ice analogs of the CO-rich layer when exposed to similar fluences to those expected from the cosmic-ray-induced secondary electrons and UV photons during the typical lifetime of dense clouds. Six products were identified upon 2 keV electron irradiation: CO2, C2O (and other carbon chain oxides), CH4, H2CO, H2C2O, and H15NCO. The total product abundances corresponded to 5%−10% of the initial CO molecules exposed to electron irradiation. Lyα photon irradiation delivered one to two orders of magnitude lower yields with a similar product branching ratio, which may be due to the low UV-photon absorption cross section of the ice sample at this wavelength. Formation of additional N-bearing species, namely C2 15N2 and 15NH3, was only observed in the absence of H2 and CO molecules, respectively, suggesting that reactants derived from H2 and CO molecules preferentially react with each other instead of with 15N2 and its dissociation products. In summary, ice chemistry induced by energetic processing of the CO-rich apolar ice layer provides alternative formation pathways for several species detected in the interstellar medium, including some related to the complex organic molecule chemistry. Further quantification of these pathways will help astrochemical models constrain their relative contribution to the interstellar budget of, especially, the organic species H2CO and HNCO.

Carbon isotopic fractionation in molecular clouds

Context. Carbon fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and low-temperature isotopic exchange reactions. Aims. Recent chemical models predict that isotopic exchange reactions may lead to a depletion of 13C in nitrile-bearing species, with 12C/13C ratios two times higher than the elemental abundance ratio of 68 in the local interstellar medium. Since the carbon isotopic ratio is commonly used to evaluate the 14N/15N ratios with the double-isotope method, it is important to study carbon fractionation in detail to avoid incorrect assumptions. Methods. In this work, we implemented a gas-grain chemical model with new isotopic exchange reactions and investigated their introduction in the context of dense and cold molecular gas. In particular, we investigated the 12C/13C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2 × 103 to 2 × 107 cm−3, respectively. Results. We suggest a possible 13C exchange through the 13C + C3 → 12C +13CC2 reaction, which does not result in dilution, but rather in 13C enhancement, for molecules that are formed starting from atomic carbon. This effect is efficient in a range of time between the formation of CO and its freeze-out on grains. Furthermore, the parameter-space exploration shows, on average, that the 12C/13C ratios of nitriles are predicted to be a factor 0.8–1.9 different from the local 12C/13C of 68 for high-mass star-forming regions. This result also affects the 14N/15N ratio: a value of 330 obtained with the double-isotope method is predicted to vary in the range 260–630, up to 1150, depending on the physical conditions. Finally, we studied the 12C/13C ratios of nitriles by varying the cosmic-ray ionisation rate, ζ: the 12C/13C ratios increase with ζ because of secondary photons and cosmic-ray reactions.

Runaway electron diagnostics using silicon strip detector

We present a proof-of-principle measurement of runaway electrons in a small tokamak using a silicon strip detector. The detector was placed inside the diagnostic port of the tokamak vessel and detected the runaway electron signal directly. The measured signal was compared to the signal provided by other tokamak diagnostics, especially the hard X-ray scintillation detector, which detects secondary photons created by interaction of accelerated electrons with tokamak walls (indirect detection of runaway electrons). The preliminary results show that when not saturated, direct detection with a segmented silicon strip detector provides promising new diagnostic information including spatial and temporal distribution of the runaway electron beam, and the measurement results are in good agreement with hard X-ray measurements with a scintillation detector.

Analysis and evaluation of secondary photons originated in jaws as contamination particles at the phantom surface

Analysis and evaluation of secondary photons originated in jaws as contamination particles at the phantom surface

Development of a Gd2O2S:Tb Phosphor Screen with an Anti-reflective Layer Fabricated Using Poly Chloro-para-xylylene to Improve Optical Properties in Radiography

In this study, we developed a Gd2O2S:Tb screen with a parylene-C-based antireflective (AR) layer and evaluated its optical and X-ray response properties; in particular, we focused on developing an AR coating layer to improve transmission efficiency. The phosphor screen with the parylene-Cbased AR layer can be fabricated via screen printing and chemical vapor deposition. In particular, experimental results obtained using the AR layer showed improvements of 1.47% in luminescence intensity, as well as a lower full width at half maximum of 9.40, compared to the case without using the AR layer. We observed that the roughness of the AR layer was higher than that of the phosphor screen; the image resolution improved owing to the narrow distribution of the secondary photons resulting from forward scattering attributable mainly to Mie scattering. These results indicate that parylene-C is suitable for developing AR coating materials to improve exposure efficiency while maintaining resolution in radiography systems. Therefore, we believe that the number of photons would be further increased if the AR layer were utilized in commercial products.

Proposal for a Procedure for Measuring the Transverse Dimensions of a Beam of Relativistic Electrons with a Small Longitudinal Size

The possibility of implementing a previously proposed procedure for determining the beam dimensions at a target is analyzed; it includes the measurement of two-dimensional angular distributions of the coherent radiation of fast electrons for two distances between a crystal, where radiation is generated, and a coordinate detector. The use of two mechanisms of parametric X-ray radiation and diffracted transition radiation is considered. The limits of the method sensitivity and the influence of the departure of secondary electrons and photons on them are discussed.

Constraints on the intergalactic magnetic field from γ -ray observations of GRB 190114C

Very high energy photons from cosmological gamma-ray bursts (GRBs) are expected to interact with extragalactic background light (EBL) and produce electron-positron pairs when they propagate through intergalactic medium (IGM). These relativistic pairs will then up-scatter cosmic microwave background (CMB) photons and emit secondary GeV emission. Meanwhile, the motion of these pairs are deflected by intergalactic magnetic field (IGMF), so the secondary GeV photons arrive later than the primary emission. It has been suggested that the properties of the secondary GeV emission can be used to constrain IGMF. Recently, TeV gamma-ray emission has been detected, for the first time, from a GRB (GRB 190114C) by the MAGIC telescope and its steep ${\rm \gamma-ray}$ spectrum shows a clear evidence of absorption by EBL. We then constrain the IGMF with the GeV flux limit obtained from the $Fermi$-LAT observations. We find a limit of $>10^{-19.5}$ G for the coherence length of $\lambda \leq 1$ Mpc. Although this limit is weaker than that obtained by using blazars, it represents the first limit from ${\rm \gamma-ray}$ observations of GRBs, which provides an independent constraint on IGMF. We also find that, for transient ${\rm \gamma-ray}$ sources, one can choose a favorable time window to search for the echo emission at a particular energy.

Method of spatial size measurement of relativistic electrons beams with small bunch length

We investigate the practical implementation of a previously proposed method for determining the transverse dimensions of an electron beam on a target by measuring the two-dimensional angular distributions of diffracted transition radiation of relativistic electrons for two distances between the crystal where the radiation is generated and the coordinate detector for femtosecond bunches. We show that determining electron microbunches with small longitudinal sizes requires an increased photon energy, achieved by decreasing the observation angle. The sensitivity limits of the method and the influence of the emission of secondary electrons and photons from the detector pixels are discussed.

Eye lens dosimetry for workers at Fukushima Daiichi Nuclear Power Plant—1: Laboratory study on the dosemeter position and the shielding effect of full face mask respirators

We conducted an eye lens dosimetry study on workers, engaged in recovery and decommissioning operations at Fukushima Daiichi Nuclear Power Station. To investigate the optimal positioning of eye lens dosemeters and the shielding effect of full face mask respirators, photon irradiation experiments to various eye lens dosemeters attached to a head phantom were performed at the secondary photon calibration fields of the Japan Atomic Energy Agency. As a result, no marked difference was observed in readings of the dosemeters attached to the different positions on the head phantom. Moreover, two types of full face mask respirators provided an insignificant shielding effect from photons of 83–662 keV. Comparative measurements between AP and ROT geometries suggested that head-mounted dosemeters would generally produce a small angular variation in readings specifically in multidirectional fields as compared to the torso-mounted dosemeters.

X-ray interaction characteristic functions in semiconductor detectors

Secondary photons produced by x-ray interactions, such as scattered and fluorescent x-rays, can degrade the performance of semiconductor imaging detectors. In this article, we define characteristic functions describing the spatial-frequency-dependent absorbed energy and counting spectra for energy-integrating and single-photon counting detectors, respectively. We describe the characteristic function as a sum of contributions due to several x-ray interaction mechanisms. To obtain the characteristic function of a detector, we performed list-mode analysis on the Monte Carlo simulation results obtained for an impulsive x-ray excitation, hence discriminating x-ray interactions depositing energies over space. We apply this analysis to cadmium telluride detectors for wide ranges of energy and detector designs. The shape of characteristic functions is mostly governed by fluorescent x-ray interactions rather than scattered photons. The effects of x-ray energy, detector pitch, and threshold energy on the characteristic functions of single-photon counting are discussed.

Photon transport physics in Serpent 2 Monte Carlo code

This paper presents the photon physics models used in Serpent 2 Monte Carlo particle transport code. The four main photon interactions – photoelectric effect, Rayleigh scattering, Compton scattering and electron–positron pair production – are included in Serpent, accompanied by three secondary photon production mechanisms (electron–positron annihilation, atomic relaxation and bremsstrahlung emitted by electrons and positrons). Models, algorithms and data used for these interaction mechanisms are described, including new efficient sampling algorithms for photoelectron direction and electron–positron pair production.

Monte Carlo simulation technology based on characteristic γ-ray spectrum analysis

<sec>Monte Carlo method is an ideal way to simulate criticality, shielding and nuclear detection. JMCT is a multipurpose 3D Mont Carlo (MC) neutron-photon-electron and coupled neutron /photon /electron transport code which is developed by IAPCM. The program is developed based on the combinatorial geometry parallel infrastructure JCOGIN and has the most functions of general Monte Carlo particle transport code, including the various variance reduction techniques. In addition, some new algorithms, such as Doppler broadening on-the-fly (OTF), uniform tally density (UTD), consistent adjoint driven importance sampling (CADIS), fast criticality search of boron concentration (FCSBC), the domain decomposition (DD), the two-level parallel computation of MPI and OpenMP, etc. have been developed, where the number of geometry zones, materials, tallies, depletion zones, memories and period of random number are big enough to simulate various extremely complicated problems. Also the JMCT is hybrid the discrete ordinate SN program JSNT to generate source biasing factors and weight window parameters for deep-penetration shielding problems. The input is based on the CAD modeling, and the result is a visualized output. The JMCT can provide technology support for radiation shielding design, reactor physics and criticality safe analysis. Especially, the JMCT is coupled depletion and thermal-hydraulic code for simulating the reactor feedback effect, including depletion, thermal feedback. In recent years, new function of γ-ray spectrum analysis has been developed. </sec><sec>In this paper, the working principles of timing measure are introduced. The advanced calibration count is developed for distinguishing between inelastic γ-ray and capture γ-ray based on time bin tally. On the other hand, when neutron collides with nuclide, the secondary photon is labeled into the primary line photon and primary continuous photon, where energy of primary line photon does not change with the incident neutron energy, such as carbon spectral-line at 4.43 MeV and oxygen spectral-line at 6.13 MeV. The element components of detected object can be determined by the primary line photon. On the other hand, expect value estimator (EVE) is used to produce the secondary photons. The advantage of EVE does not leak any event even with a small probability which is important for detecting the hide exploder. However the shortage of the EVE results in producing a great number of photons with small weight. If all of these small weight photons are simulated one by one, a great amount of computation time and memory will be consumed. For avoiding this case, a new algorithm is design by coupling EVE and DE (direct estimator). The all of secondary photons from EVE only make the direct tally take a little computing time, then end the photon history and return to the DE production photon model (one photon production at most). Final, the total tally is a summation of EVE direct tally and DE scattering tally. The use of new algorithm to realize the analysis of γ-ray spectrum will increase only a little computing time. The numerical tests are done by using own Monte Carlo code JMCT. The correctness and validity of the algorithm are shown preliminarily.</sec>

Characteristic X-ray yields and cross sections of thick targets of Al, Ti, Zr, W and Au induced by keV-electron impact

In this paper, pure thick Al (Z = 13), Ti (Z = 22), Zr (Z = 40), W (Z = 74) and Au (Z = 79) targets are bombarded by electrons in an energy range of 5–27 keV, and the experimental thick-target characteristic X-ray yields of K-shell and L-shell, the X-ray production cross sections and the ionization cross sections of inner shells are presented. The present experimental setup and data processing are improved, specifically, a deflection magnet is installed in front of the X-ray detector to prevent the backscattered electron from entering into the X-ray detector, and the bremsstrahlung background spectra calculated from PENELOPE Monte Carlo simulations are used to deduce the net peak areas. The X-ray detector used in this experiment is the XR-100SDD manufactured by Amptek Inc. with a 25 mm<sup>2</sup> C2 ultra-thin window which can detect the low-energy x-rays down to boron Kα line (0.183 keV). Standard sources (<sup>55</sup>Fe, <sup>57</sup>Co, <sup>137</sup>Cs and <sup>241</sup>Am) with an activity accuracy range of 1%–3% (<i>k</i> = 2), supplied by the Physikalisch-Technische Bundesanstalt, Germany (PTB), are used to perform the detector’s efficiency calibration, and in a low-energy range (< 3.3 keV) the efficiency calibration is accomplished by measuring characteristic X-ray spectra produced by 20 keV electron impacting various thickness solid targets (i.e. by the characteristic peak method). The uncertainty of the detector’s efficiency calibration obtained in this paper is ~1.6%. The experimental thick-target characteristic X-ray yield data with an uncertainty of 1.7%–6.2% are compared with the PENELOPE Monte Carlo simulations, in which the inner-shell ionization cross sections are based on the distorted-wave Born approximation (DWBA) calculations, and they are in good agreement with a difference of less than or ~10%. According to the measured thick-target characteristic x-ray yields, the K-shell ionization cross sections for Al, Ti and Zr and the L-shell X-ray production cross sections for Zr, W and Au are also obtained with an uncertainty of 5%–8% (except for Al due to large K-shell fluorescence yield uncertainty), the difference between the experimental and theoretical data is also less than or ~10%. Moreover, by comparing the thick-target characteristic X-ray yields obtained from the analytical model and the PENELOPE Monte Carlo simulations at the electrons’ incident angles of 45° and 90°, it is found that the degree of agreement between the results from the analytical model and the Monte Carlo simulations at the incident angle of 90° is better than at the incident angle of 45°. Moreover, the contributions of the secondary electrons and bremsstrahlung photons to the characteristic X-ray yield are also given based on the PENELOPE Monte Carlo simulations. As for the elements studied in this paper, for the low ionization threshold energy, the contribution of the secondary electrons is ~2%, and however, for the high ionization threshold energy, the contribution is ~10%–20%. These contributions depend weakly on the energy of the incident electrons and show that these contributions are closely correlated with atomic number.