Indirect Ionization Research Articles

Electron-impact ionization for Ne[formula omitted] and Ne[formula omitted]

Ionization cross sections are studied for energy levels of the ground configurations of the Ne3+ and Ne4+ ions. The distorted wave (DW) approximation is used to analyze experimental data. The scaled DW cross sections are used to explain measurements for the Ne3+ ion. Study includes analysis of contributions from the direct and indirect ionization processes. Convergences of excitation-autoionization (EA) channels are estimated by analyzing excitations up to shells with the principal quantum number n⩽20. This study shows that the EA process provides from ∼10% to 20% for the energy levels of the ground configurations of the Ne3+ and Ne4+ ions.

Electron-impact double ionization cross sections and rate coefficients for Be-like ions: B+ to Ne6+

We report a detailed analysis of the electron-impact double ionization process for Be-like ions, spanning charge states from B+ to Ne6+. We considered both direct and indirect double ionization processes. For direct double ionization, we calculated the cross sections by considering three different processes: ionization-ionization (II), excitation-ionization-ionization (EII), and ionization-excitation-ionization (IEI). Our results show excellent agreement between the measured cross sections for Be-like-B and Be-like-O ions. The agreement is slightly weaker for Be-like-Ne ions when we take the potential of ionized ions in the distorted-wave method. For other ions for which experimental data are not available, our calculations should provide the missing data. We have summarized the trends of electron-impact double ionization cross sections with atomic number for isoelectronic sequences. Finally, we have provided the Maxwellian and non-Maxwellian rate coefficients for these Be-like ions. The data obtained should be useful for both equilibrium and non-equilibrium charge-state distributions in astrophysical research.

Excitation Dynamics of the 5p^6 Auger Spectra in Ba + e^– Collisions

The ionization of the 5p6 subshell in Ba atoms has been studied in an electron-impact energy range 23–105 eV by measuring the Auger spectra arising from the decay of the 5p5nln′l′ states. The energy dependences of the ionization cross-section have been obtained for nine states in the 5p55d2, 5p55d6s and 5p56s2 configurations. The analysis of the behavior of the cross-sections made it possible to determine the role of direct and indirect ionization processes involved in the 5p6 ionization of the Ba atom.

Electron-impact single ionization for the N[formula omitted] and N[formula omitted] ions

Electron-impact ionization cross sections are investigated for the ground levels of N2+ and N3+ ions. Additionally, the long-lived energy levels of the N2+2s2p2 and N3+2s2p configurations are analyzed. Study includes both direct and indirect ionization processes. Contribution of the excitation–autoionization (EA) channels to the ionization cross sections is approximately 15% to 23% for both ions. Convergence of the EA cross sections is analyzed by considering excitations up to shells with the principal quantum numbers n⩽20. The obtained results are compared to available experimental data for the N2+ and N3+ ions.

Electron-impact single ionization for the Fe2+ ion

Electron-impact ionization was analyzed for all 34 levels of the ground configuration of the Fe2+ ion. Both direct and indirect ionization processes are included in the study. For the direct process, the potential of the ionized ion was used to explain experimental data. For the indirect process, electron-impact excitation with subsequent autoionization was considered. The convergence of the cross sections of the indirect process was determined by considering the excitations up to shells with the principal quantum number n ⩽ 20. The main contribution to the indirect process was determined by the 3p → 3d excitations. The scaled distorted-wave approximation was applied to explain measurements for the Fe2+ ion. Scaling of the cross sections lowers values by ~35% at the peak for the ground level and ~50% for the highest level of the ground configuration of the Fe2+ ion. The experimental value for the single ionization threshold was used to provide better agreement with the measurements for the cross sections. This led to the cross sections for the levels of the ground configuration diminishing in size by 15–25%. The direct process contributes ~70% and ~50% to the total ionization from the lowest and highest levels of the ion, respectively.

Electron scattering with ethane adsorbed on rare gas multilayers: Hole transfer, coulomb decay, and ion dissociation.

Positive ion desorption following electron impact dissociative ionization of ethane adsorbed on Ar, Kr, and Xe multilayers has been studied as a function of incident electron energy from threshold to 100eV. Based on the dependence of ion yields on the identity of the rare gas, it is likely that the majority of ethane molecules undergo indirect ionization following hole transfer from the ionized underlying rare gas. This has also been corroborated by density of states calculations showing the energetic alignment of the outer valence states of ethane and the condensed rare gas ionization energies. Due to the near-resonant nature of charge transfer for single-hole states, the ethane molecular ion is excited to different final ionic states on different rare gases, which leads to differences in ion desorption yields and branching ratios. The quantitative yields increase with increasing ionization energy gap between the rare gas and ethane, in the order Ar > Kr > Xe. The large increase in yields from 25eV onwards for all rare gases is likely due to the formation and decay of two-hole states on neighboring rare gas and ethane molecules due to interatomic and intermolecular Coulomb decay (ICD) and not electron transfer mediated decay (ETMD). The ICD and ETMD pathways become accessible when the incoming electron has sufficient energy to excite the inner valence ns level of the rare gas to a Rydberg state or ionize it. The experimental findings are supported by calculations of thresholds, density of states for the final configurations of these processes, and coupling strengths for hole transfer between ethane and rare gases. The fragment ion branching ratios vary with energy from threshold to about 35eV, showing the fragmentation pattern changes with the mode of hole transfer and availability of excess energy. Sigma C-C bonds are more likely to break than C-H bonds in the mid-20eV range, and this effect is most pronounced for Xe, followed by Kr, and then Ar.

Secondary ionization of pyrimidine nucleobases and their microhydrated derivatives in helium nanodroplets.

Radiation damage in biological systems by ionizing radiation is predominantly caused by secondary processes such as charge and energy transfer leading to the breaking of bonds in DNA. Here, we study the fragmentation of cytosine (Cyt) and thymine (Thy) molecules, clusters and microhydrated derivatives induced by direct and indirect ionization initiated by extreme-ultraviolet (XUV) irradiation. Photofragmentation mass spectra and photoelectron spectra of free Cyt and Thy molecules are compared with mass and electron spectra of Cyt/Thy clusters and microhydrated Cyt/Thy molecules formed by aggregation in superfluid helium (He) nanodroplets. Penning ionization after resonant excitation of the He droplets is generally found to cause less fragmentation compared to direct photoionization and charge-transfer ionization after photoionization of the He droplets. When Cyt/Thy molecules and oligomers are complexed with water molecules, their fragmentation is efficiently suppressed. However, a similar suppression of fragmentation is observed when homogeneous Cyt/Thy clusters are formed in He nanodroplets, indicating a general trend. Penning ionization electron spectra (PIES) of Cyt/Thy are broad and nearly featureless but PIES of their microhydrated derivatives point at a sequential ionization process ending in unfragmented microsolvated Cyt/Thy cations.

Alpha-peptide receptor radionuclide therapy using actinium-225 labeled somatostatin receptor agonists and antagonists.

Peptide receptor radionuclide therapy (PRRT) has over the last two decades emerged as a very promising approach to treat neuroendocrine tumors (NETs) with rapidly expanding clinical applications. By chelating a radiometal to a somatostatin receptor (SSTR) ligand, radiation can be delivered to cancer cells with high precision. Unlike conventional external beam radiotherapy, PRRT utilizes primarily β or α radiation derived from nuclear decay, which causes damage to cancer cells in the immediate proximity by irreversible direct or indirect ionization of the cells' DNA, which induces apoptosis. In addition, to avoid damage to surrounding normal cells, PRRT privileges the use of radionuclides that have little penetrating and more energetic (and thus more ionizing) radiations. To date, the most frequently radioisotopes are β- emitters, particularly Yttrium-90 (90Y) and Lutetium-177 (177Lu), labeled SSTR agonists. Current development of SSTR-targeting is triggering the shift from using SSTR agonists to antagonists for PRRT. Furthermore, targeted α-particle therapy (TAT), has attracted special attention for the treatment of tumors and offers an improved therapeutic option for patients resistant to conventional treatments or even beta-irradiation treatment. Due to its short range and high linear energy transfer (LET), α-particles significantly damage the targeted cancer cells while causing minimal cytotoxicity toward surrounding normal tissue. Actinium-225 (225Ac) has been developed into potent targeting drug constructs including somatostatin-receptor-based radiopharmaceuticals and is in early clinical use against multiple neuroendocrine tumor types. In this article, we give a review of preclinical and clinical applications of 225Ac-PRRT in NETs, discuss the strengths and challenges of 225Ac complexes being used in PRRT; and envision the prospect of 225Ac-PRRT as a future alternative in the treatment of NETs.

Towards the ionizing radiation induced bond dissociation mechanism in oxygen, water, guanine and DNA fragmentation: a density functional theory simulation

The radiation-induced damages in bio-molecules are ubiquitous processes in radiotherapy and radio-biology, and critical to space projects. In this study, we present a precise quantification of the fragmentation mechanisms of deoxyribonucleic acid (DNA) and the molecules surrounding DNA such as oxygen and water under non-equilibrium conditions using the first-principle calculations based on density functional theory (DFT). Our results reveal the structural stability of DNA bases and backbone that withstand up to a combined threshold of charge and hydrogen abstraction owing to simultaneously direct and indirect ionization processes. We show the hydrogen contents of the molecules significantly control the stability in the presence of radiation. This study provides comprehensive information on the impact of the direct and indirect induced bond dissociations and DNA damage and introduces a systematic methodology for fine-tuning the input parameters necessary for the large-scale Monte Carlo simulations of radio-biological responses and mitigation of detrimental effects of ionizing radiation.

A new model for predicting proton SEE cross sections based on heavy ion data

Based on the physical mechanism of the proton single event effects (SEEs), a new analytical model is proposed to predict the proton SEE cross sections of electronic devices from the heavy ion data. The experimental data of five SRAMs from our experiments and the literature are used to test the new model, and the predicted results agree well with them. The existing models only apply to the devices with low LET threshold whose proton SEE cross sections are dominated by the p + Si reactions, and cannot predict the angular effects. Compared to them, our model applies to the devices with from low to high LET threshold whose proton SEE cross sections are dominated by the proton interactions with silicon and/or high-Z materials, and can also predict the angular effects. Moreover, for two SRAMs with low LET threshold, our model performs much better than two classic models, the BGR and microdosimetry models, in predicting their proton SEE cross sections at normal incidence. A new physical concept called the indirect ionization power of protons, which is very helpful for understanding the proton SEE mechanism, is put forward and used in our model. It is expected that the new model will be widely used in studying the proton SEE mechanism, predicting the proton SEE cross sections and on-orbit error rates of electronic devices in space.

Investigation of Neutron Shielding Behaviour of Unreinforced and Calcite Reinforced Concrete Samples

Neutron particles are different, due to their nature of interacting directly with the atomic nucleus and making indirect ionization. In this study calcite containing and non-calcite containing concrete samples neutron shielding capabilities was compared with each other. CaCO3 (calcite) added concrete and pure concrete was experimentally compared against an isotropic Am-Be neutron source. Naturally, calcite containing concrete was in heavy form in comparison with pure concrete, means that the aim of these experiment is comparing neutron shielding properties of heavy and light concrete. In both samples, thickness of the samples was started with 2 cm and reach to 10 cm with 2 cm increases. The effect of thickness and the effect of material type on neutron shielding was investigated. According to the test results, pure concrete samples shows better shielding characteristics.

Photoionization of Ne and Xe atoms induced by extreme ultraviolet photons

The interaction of extreme ultraviolet (XUV) photon with matter is a meaningful way to understand the electronic structure of microscopic particles. In this paper, the electron angular distributions of single ionization and double ionization of Ne and Xe atoms interacting with XUV photons are investigated by utilizing a reaction microscope. The <i>β</i>-asymmetric parameters of 2p electrons of Ne atom, and 5p, 5s electrons of Xe atom combined with the reported experimental data are compared with those from different theoretical models. The result shows that the electron correlation effect can be ignored in the ionization of 2p electron of Ne atom. While the ionization of 5p electron of Xe atom is strongly influenced by the electron correlation effect, but not by the relativistic effect. These two effects play an important role in ionizing the 5s shell of Xe atom. In addition, this study finds that both direct double ionization and indirect double ionization exist simultaneously during the ionization of Xe atom, and gives the photoelectron angular distributions and the <i>β</i>-asymmetric parameters of the first step and the second step of indirect double ionization.

Electron-impact single, double, and triple ionization of B+

Electron-impact single, double, and triple ionizations are investigated for the ${\mathrm{B}}^{+}$ ion. The direct and indirect ionization processes are studied for the single and double ionization. Good agreement with measurements for the single and double ionization is obtained when the scaled distorted-wave cross sections are used to study the collisional ionization and excitation processes. It is shown that ionization-excitation-autoionization and excitation-ionization-autoionization processes have to be introduced to the total double ionization to explain the measurements. The triple ionization is investigated by considering the direct double ionization with the subsequent autoionization.

Investigation of plasma mode transition and hysteresis in electron cyclotron resonance ion thrusters

Plasma mode-transition and hysteresis have been reported in several electron cyclotron resonance (ECR) plasma sources and have also been observed in ECR ion thrusters. From observation outside the thruster, the optical emission is significantly changed after plasma mode-transition, which indicates the electron heating, excitation and ionization processes are also changed. Therefore, to investigate these processes quantitatively, the ground-state neutral density and spontaneous emission by electron impact excitation are directly measured using two-photon absorption laser-induced fluorescence spectroscopy. This measurement system is applied to two types of thrusters. In one thruster, both mode-transition and hysteresis were observed, while in another, no hysteresis was observed due to the partial prevention of ECR heating. The experimental results indicate that the spontaneous emission sharply increases by the mode-transition and hysteresis. The tendency for this increase was relatively small with the partial prevention of ECR heating; however, the mode-transition was not deleted. Analysis of the excitation and ionization process revealed that the increase of indirect (stepwise) excitation and ionization from metastable particles can contributes to the sharp increase of the spontaneous emission. In addition, quantitative estimation of the collisional and ECR heating indicated that collisional heating cannot be neglected after the mode-transition, which indicates that collisional heating can contribute to the sharp increase due to the increase of electron temperature. The indirect ionization and collisional heating could thus be the main cause of the mode-transition and hysteresis.

Electron-impact ionization cross section calculations for lithium-like ions

The electron-impact ionization of lithium-like ions C3+, N4+, O5+, Ne7+, and Fe23+ is studied using a combination of two-potential distorted-wave and R-matrix methods with a relativistic correction. Total cross sections are computed for incident energies from 1 to 10 times of ionization energy and better agreements with the experimental results are obtained in comparison with the theoretical data available. It is found that the indirect ionization processes become significant for the incident energy larger than about four times of the ionization energy. Contributions from the exchange effects along the isoelectronic sequence are also discussed and found to be important. The present method can be used to obtain systematic ionization cross sections for highly charged ions across a wide incident energy range.

Electron-impact single and double ionization of Fe12+

The cross sections of electron-impact single and double ionization of Fe12+ are investigated using the distorted-wave (DW) method and two-step approximate formula based on the knockout (KO) mechanism. Both direct and indirect processes are studied in single and double electron-impact ionization. The better agreements between our theoretical results and experiment measurements are obtained contrasted to earlier theoretical works. Adequate indirect ionization channels with larger configurations space are used in our calculation, which is significant for understanding of the mechanism of single and double electron-impact ionization for highly charge ion Fe12+.

Electron-impact ionization of the Pb atom

Electron-impact ionization cross sections are calculated for the ground configuration of the Pb atom. Time-dependent close-coupling cross sections for the direct ionization of the 6s and 6p subshells leading to single ionization are calculated with and without a polarization potential. Configuration-average distorted-wave cross sections for the direct ionization of the 6s and 6p subshells leading to single ionization are also calculated with and without a polarization potential. We find the time-dependent close-coupling cross sections using a polarization potential to be in good agreement with convergent-close-coupling cross sections using a polarization potential. The total direct ionization cross sections are compared to two sets of experimental measurements. The differences between the direct ionization cross sections and the experimental measurements are mainly due to indirect ionization cross sections coming from the 6s^2 6p^2 rightarrow 6s 6p^3 excitation followed by autoionization.Graphic

Role of e-beam irradiation treatment on detemplation and structural hierarchy of ZSM-5 zeolite

In this work, detemplation of tetrapropylammonium ZSM-5 zeolites was performed by e-beam irradiation technique. It was found that exposure to high-energy e-beam irradiation induces the partial detemplation from the ZSM-5 zeolites at ambient condition and facilitates the supplementary detemplation. Direct radiolysis of water molecules coupling with indirect ionization and dissociation along the chain of extraframework-framework interactions have led to fragmentation of the organic compounds (dehydration and detemplation) and even the framework T-O-T bonds (amorphization). The completely detemplated zeolites with different Si/Al molar ratios and thus different hydrophilicity were found to have different hierarchical structures. A network of large-mouth funnel-shaped pores was formed in the Al-rich ZSM-5 zeolite with the Si/Al molar ratio of ca. 18, whereas channels with ink-bottle shaped pores were formed in the Si-rich ZSM-5 zeolite with the Si/Al molar ratio of ca. 30. • E-beam irradiation promotes partial detemplation from ZSM-5 zeolites. • Enhanced surface area and hierarchical structures achieved in detemplated ZSM-5. • High dose levels foster partial amorphization of Al-rich ZSM-5 with mechanisms. • Roles of e-beam irradiation to ZSM-5 structural changes are confirmed.

Radiation/Drug Interactions and the Combinational Effects of Tyrosine Kinase Inhibitors and Radiation Therapy in the Treatment of Cancer

Introduction: Progress in both the development of Tyrosine Kinase Inhibitors (TKIs) and modern radiation therapy (RT) have opened new horizons for the safe and effective treatments of cancer. Combination therapies of TKIs and radiation therapy have been evaluated with variable sequencing and dosing of both agents. Radiation therapy induces cellular damage to cancer and normal tissue DNA by the production of direct and indirect ionization leading to a cascade of biological events. These ultimately lead to potential loss of cellular reproductive capacity, cell death or impotency. Repair mechanisms of RT-induced cell damage require repair pathways dependent on TK. TKIs are key regulators of cellular function and signaling proteins that catalyze phosphorylation reactions of tyrosine molecules. The purpose of this study was to briefly review the basic biology of RT and TKIs and to review the modern literature pertaining to the toxic and therapeutic effects of their combination in the treatment of cancer based on preclinical and clinical data. Methods: A retrospective review of the basic biology of RT and TKIs with the aim of identifying their combined toxicity and benefit in the treatment of cancer was performed. A systematic search of the standard published radiotherapeutic, radiobiology, chemotherapy and radiotherapy texts, PubMed, Google Scholar, and Clinical Key using the search terms; TKI, RT and combination TKI and RT from 1985-2020 was employed. Data were abstracted from 222 entries from the literature published in English. Issues of toxicity and therapeutic efficacy were defined to evaluate the safe and effective use of combined modality therapy (CMT). Results: Few randomized studies were available for high-level recommendations. The combined treatment of cancer with TKIs and RT may have benefit for palliation, progression free survival, and potential survival under well-defined circumstances. Any benefit of combined therapy is accompanied by significant potential for enhanced radiation toxicity in addition to the baseline potential toxicities of both agents. Conclusion: The data reviewed suggest potential benefit from the CMT of TKIs and RT but at a significant risk of toxicity, which may include severe, hematological, cardiac, gastrointestinal, pulmonary and central nervous system toxicity. Further randomized prospective studies are necessary to define their safe and effective combined therapies. New TKI’s and radiotherapeutic modalities are constantly under development necessitating ongoing and often long term clinical evaluation to define benefits and risks.

Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions.

We report on the effects of electron collision and indirect ionization processes, occurring at photoexcitation and electron kinetic energies well below 30 eV, on the photoemission spectra of liquid water. We show that the nascent photoelectron spectrum and, hence, the inferred electron binding energy can only be accurately determined if electron energies are large enough that cross sections for quasi-elastic scattering processes, such as vibrational excitation, are negligible. Otherwise, quasi-elastic scattering leads to strong, down-to-few-meV kinetic energy scattering losses from the direct photoelectron features, which manifest in severely distorted intrinsic photoelectron peak shapes. The associated cross-over point from predominant (known) electronically inelastic to quasi-elastic scattering seems to arise at surprisingly large electron kinetic energies, of approximately 10-14 eV. Concomitantly, we present evidence for the onset of indirect, autoionization phenomena (occurring via superexcited states) within a few eV of the primary and secondary ionization thresholds. These processes are inferred to compete with the direct ionization channels and primarily produce low-energy photoelectrons at photon and electron impact excitation energies below ∼15 eV. Our results highlight that vibrational inelastic electron scattering processes and neutral photoexcitation and autoionization channels become increasingly important when photon and electron kinetic energies are decreased towards the ionization threshold. Correspondingly, we show that for neat water and aqueous solutions, great care must be taken when quantitatively analyzing photoelectron spectra measured too close to the ionization threshold. Such care is essential for the accurate determination of solvent and solute ionization energies as well as photoelectron branching ratios and peak magnitudes.