The performance damage mechanism of InP-based high electron mobility transistors (HEMTs) after proton irradiation has been investigated comprehensively through induced defects. The effects of the defect type, defect energy level with respect to conduction band E T, and defect concentration on the transfer and output characteristics of the device are discussed based on hydrodynamic model and Shockley–Read–Hall recombination model. The results indicate that only acceptor-like defects have a significant influence on device operation. Meanwhile, as defect energy level E T shifts away from conduction band, the drain current decreases gradually and finally reaches a saturation value with E T above 0.5 eV. This can be attributed to the fact that at sufficient deep level, acceptor-type defects could not be ionized any more. Additionally, the drain current and transconductance degrade more severely with larger acceptor concentration. These changes of the electrical characteristics with proton radiation could be accounted for by the electron density reduction in the channel region from induced acceptor-like defects.

Effect of defects properties on InP-based high electron mobility transistors

Cosmic rays offer an opportunity to search for dark matter, if the latter decays or annihilates into standard model particles. In the first part of this work, we address an excess that was claimed to be seen in the cosmic ray antiproton spectrum. Carefully evaluating the uncertainties of the analysis as well as their correlations, we show that the significance hardly exceeds 1σ. In the absence of an anomaly, we derive limits on the dark matter annihilation cross section that disfavor thermal WIMPs below roughly 500 GeV.
To go further, we question the general setup of typical homogeneous diffusion models.
We point out that the measured positron and electron+positron spectrum at TeV energies are consistent with the astrophysical background prediction, if energy losses are not relevant.
We propose alternative models of cosmic ray propagation that capture these properties, and discuss the consistency with various astrophysical measurements. It turns out that secondary electrons+positrons at TeV energy require short escape time ≤ 0.1 Myr. We show that this implies that the cosmic rays at these energies have seen an average density of interstellar matter of a few particles per cm3. This number is so far consistent with observations, but requires further investigation, e.g. through starlight extinction maps or γ-ray surveys.
In the last part we look at four explicit models of dark matter with a small coupling to the visible sector through the electromagnetic interaction, namely through a dark photon portal or through an electric or magnetic dipole moment. The small couplings prohibit thermalization with the standard model sector in the early universe, such that typical constraints for dark matter masses in the keV range are avoided. Instead, the dark matter may have been produced through annihilations and decay of standard model particles into the dark sector, so-called 'freeze-in' production. The parameters that reproduce the observed relic abundance can partially be constrained by stellar cooling arguments and upcoming direct detection experiments

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Various Aspects of Astroparticle Physics and the Implications for Dark Matter Searches

Total and partial charge-changing cross-sections of 600 A MeV Si14+ ions in a thick polyethylene target were measured by CR39 track etch detectors using an optical microscope DM 6000 M system installed with Leica QWin Plus software. The CR39 nuclear track detectors were used to identify the incident charged particles and their fragments using the cone-height distribution method. The charge resolution of CR39 detectors was achieved to be 0.17e in the present condition. The value of the total charge-changing cross-section was calculated to be σ
 tot
 = (766±17) mb. To calculate the partial charge-changing cross-sections for ΔZ = −11, −10, ..., −1, , number of events corresponding to each fragment were determined from multiple Gaussian fitting of cone-height distributions within 95.5% confidence level and the number of incident and survived beam ions were counted within 99.7% confidence level. Possible odd-even pattern in the values of the partial charge-changing cross-section is also presented. The measured charge pick-up cross-section for ΔZ = +1 is (7.2 ±1.7) mb.

Fragmentation cross-section of 600 A MeV Si14+ ions in thick polyethylene target

Realistic nuclear reaction cross-section models are an essential ingredient of reliable heavy-ion transport codes. Such codes are used for risk evaluation of manned space exploration missions as well as for ion-beam therapy dose calculations and treatment planning. Therefore, in this study, a collection of total nuclear reaction cross-section data has been generated within a GSI-ESA-NASA collaboration. The database includes the experimentally measured total nucleus-nucleus reaction cross-sections. The Tripathi, Kox, Shen, Kox-Shen, and Hybrid-Kurotama models are systematically compared with the collected data. Details about the implementation of the models are given. Literature gaps are pointed out and considerations are made about which models fit best the existing data for the most relevant systems to radiation protection in space and heavy-ion therapy.

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Total nuclear reaction cross-section database for radiation protection in space and heavy-ion therapy applications

Calibration of CR39 detectors with new system for Fe26+ ion beam and measurement of total charge changing cross-section in Al target

Simulation of depth-dose distributions for various ions in polyethylene medium

Total charge changing cross-section of 10 A GeV Fe26+ ion beam on polyethylene and CR39 targets was measured. Charge of the fragments of projectiles was detected using CR39 nuclear track detectors by a new technique of one-side etching using an automated optical microscope with an image analysing software. The calculated value of total charge changing cross-section is σtot = (2694 ± 142)mb and is in good agreement with the experimental values by other methods within error. The restricted energy loss (REL) at energy 10 A GeV for all the fragments was theoretically calculated by using Bethe–Bloch equation and then obtained a calibration curve of reduced etch-rate ratio (p) versus REL showing the response of CR39 track detectors to 10 A GeV Fe26+ beam. The curve was fitted by a polynomial showing the relation between p and REL.

Renu Gupta

Papers

Fragmentation cross-section of 600 A MeV Si14+ ions in thick polyethylene target

Simulation of depth-dose distributions for various ions in polyethylene medium

Response of CR39 track etch detector to 10 A GeV Fe26+ ion beam and total charge changing cross section measurement

Microwave dielectric properties and thermal conductivities of low-temperature sintered (Na1-K )2MoO4 (x ≤ 0.2) ceramics

Response of Cr39 detector to 5A GeV Si14+ ions and measurement of total charge changing cross-section