Varghese Anto

• Doctor of Philosophy
• PostDoc Position at The University of Texas at Arlington

This talk will present the investigation into the enhancement of the piezoelectric potential in a metal/ZnO thin film under surface plasmon resonance (SPR) conditions for the development of novel piezo-plasmonic based UV photon detectors. Metal thin films are used in numerous high-tech applications, such as electronic devices, surface plasmon reson...

We propose a novel detection scheme that utilizes the piezo-pyroelectric properties of ZnO thin films. The aim is to realize a zero external bias vacuum ultraviolet (VUV) detector that can be used in liquid noble time projection chambers. We present initial results from our measurements that test our hypothesis that stress-induced in ZnO thin films...

Magnetic bottle Time-of-Flight (ToF) spectrometers can measure the energy spectra of all electrons emitted into a 2π sr solid angle simultaneously, greatly reducing data collection time. When the detection of the annihilation gamma (γ) and the detection of the electron (e) are used as timing signals for ToF spectrometers, the e-γ time difference sp...

Magnetic bottle Time-of-Flight (ToF) spectrometers can measure the energy spectra of all electrons emitted into a 2$\pi$ sr solid angle simultaneously, greatly reducing data collection time. When the detection of the annihilation gamma ($\gamma$) and the detection of the electron (e) are used as timing signals for ToF spectrometers, the e-$\gamma$...

We demonstrate the ability to obtain the direction of the gamma rays using a standard coaxial high purity germanium (HPGe) detector using the direction-sensitive information embedded in the shape of the pre-amplified HPGe signals. We deduced the complex relationship between the shape of the signal and the direction from which the gamma-ray enters t...

We present a spectroscopic method which utilizes virtual photons to selectively measure the electronic structure of the topmost atomic layer. These virtual photons are created when incident positrons transition from vacuum states to bound surface states on the sample surface and can transfer sufficient energy to excite electrons into the vacuum. Th...

We report the results of the measurements of three pieces of the new Photonis miniPLANACON microchannel-plate photomultipliers (MCP–PMTs) intended for use in the demanding environment of the Large Hadron Collider (LHC) beamline as a part of the AFP Time-of-Flight detector. These photomultipliers were modified in cooperation with the manufacturer by...

In this work, we studied and optimized the performance of electrostatic quadruple focusing lens system. Combining the power of three simulation software, we explored the lens hyper-parameter space to maximize the Figure of Merit Q that is proportional to the magnitude of the product of the horizontal and vertical demagnifications and inversely prop...

We have applied positron annihilation induced Auger-gamma coincidence spectroscopy to identify important mechanisms responsible for the emission of low energy electrons following the sudden creation of holes in bilayer graphene on copper substrate. The novel surface spectroscopic method measures the energy of the Doppler shifted annihilation gamma...

In this manuscript, we demonstrate a novel technique to probe the electronic structure of the top-most atomic layer of a solid using the virtual photon exchange between a positron and a valence band electron. This process, termed Auger-mediated positron sticking, is initiated by the coupling of the energy and momentum associated with the trapping o...

We report here on a set of experiments that focus on measuring the timing performance of the Photonis miniPlanacon XPM85212/A1-S microchannel plate photomultiplier tubes along with the detector response at high event rates. The detector has a single photoelectron timing resolution of 30ps at low rates (10kHz). We show that both the gain and the tim...

Oxygen, the third most abundant element in the universe, plays a key role in the chemistry of condensed matter and biological systems. Here, we report evidence for a hitherto unexplored Auger transition in oxides, where a valence band electron fills a vacancy in the 2s state of oxygen, transferring sufficient energy to allow electron emission. We u...

We describe here an efficient machine-learning based approach for the optimization of parameters used for extracting the arrival time of waveforms, in particular those generated by the detection of 511 keV annihilation γ-rays by a 60 cm³ coaxial high purity germanium detector (HPGe). The method utilizes a type of artificial neural network (ANN) cal...

https://arxiv.org/abs/2007.13938 We present a novel method for the analysis of the Doppler broadened spectrum of gamma photons emitted following the annihilation of surface trapped positrons with the electrons of bilayer graphene supported on polycrystalline Cu substrate. The method relies on the measurement of the energy of the Doppler shifted an...

https://arxiv.org/abs/2004.00008 We describe here an efficient machine-learning based approach for the optimization of parameters used for extracting the arrival time of waveforms, in particular those generated by the detection of 511 keV annihilation gamma-rays by a 60 cm3 coaxial high purity germanium detector (HPGe). The method utilizes a type o...

In this paper, we present evidence of low-energy electron emission resulting from the LVV Auger decay of oxygen 2s holes. Low-energy Auger electron emission is difficult to observe principally because of the large, primary beam-induced secondary electron beam. We have overcome this background limitation by using positron-electron annihilation to in...

In this study, we describe an advanced multi-functional, variable-energy positron beam system capable of measuring the energies of multiple “positron-induced” electrons in coincidence with the Doppler-shifted gamma photon resulting from the annihilation of the correlated positron. The measurements were carried out using the unique characteristics o...

We present preliminary results that were collected using a new positron beam system at the University of Texas at Arlington. The beam allows for the coincident measurement of the energy of positron annihilation gamma photon and time-coincident positron-induced electrons. This system enables the parallel collection of a 2- dimensional array of coinc...

Here we describe an advanced multi functional, variable-energy positron beam system capable of measuring the energies of multiple positron-induced electrons in coincidence with the Doppler-shifted gamma photon resulting from the annihilation of the correlated positron. The measurements were carried out using the unique characteristics of the digita...

In this paper, we present and analyze measurements of the positron induced electron spectra (PIES) from highly oriented pyrolytic graphite (HOPG). The spectra were obtained using a time of flight spectrometer attached to a variable energy positron beam. In the first measurements presented, the system was configured to obtain high resolution data fr...

Positronium (Ps) formation on the surface of clean polycrystalline copper (Cu), highly oriented pyrolytic graphite (HOPG) and multi layer graphene (MLG) grown on a polycrystalline copper substrate has been investigated as a function of incident positron kinetic energy (1.5eV to 1keV). Measurements on Cu indicate that as the kinetic energy of the in...

Preliminary results are presented from a new positron beam system currently under development at the University of Texas at Arlington for the coincident energy measurement of Doppler-shifted annihilation quanta and positron-induced Auger electrons. We report data based on an analysis of the pulses resulting from the detection of positron induced se...

We present Doppler broadened gamma spectra, obtained using the newly developed advanced positron beam at the University of Texas at Arlington, from a sample consisting of 6 to 8 layers of graphene (MLG) on polycrystalline Cu. The kinetic energy of the positron beam was varied form 2 eV to 20 keV allowing for a depth resolved measurement. The ratio...

In this paper, we present the simulations of an electrostatic quadrupole quadruplet (EQQ) lens system that will be used in a new sub-micron multi-ion beam for radiobiological studies. The new sub-micron beam will be used to carry out fine controlled irradiation of sub-cellular matrix with a variety of ions (H⁺, He²⁺, C⁶⁺) with energies up to 3 MeV/...

Here we describe the methods employed for the analysis of digitized data produced by electron time-of-flight and gamma spectrometers that are part of the recently completed advanced positron beam system at the University of Texas at Arlington. The digitization of the data acquisition methods enabled the coincident measurement of the energies of the...

In this paper, we present and analyze measurements of the positron induced electron spectra (PIES) from highly oriented pyrolytic graphite (HOPG). The spectra were obtained using a time of flight spectrometer attached to a variable energy positron beam. In the first measurements presented, the system was configured to obtain high resolution data fr...

We present Doppler broadened gamma spectra, obtained using the newly developed advanced positron beam at the University of Texas at Arlington, from a sample consisting of 6 to 8 layers of graphene (MLG) on polycrystalline Cu. The kinetic energy of the positron beam was varied form 2 eV to 20 keV allowing for a depth resolved measurement. The ratio...

Positronium (Ps) formation on the surface of clean polycrystalline copper (Cu), highly oriented pyrolytic graphite (HOPG) and multi layer graphene (MLG) grown on a polycrystalline copper substrate has been investigated as a function of incident positron kinetic energy (1.5eV to 1keV). Measurments on Cu indicate that as the kinetic energy of the inc...

Preliminary results are presented from a new positron beam system currently under development at the University of Texas at Arlington for the coincident energy measurement of Doppler-shifted annihilation quanta and positron-induced Auger electrons. We report data based on an analysis of the pulses resulting from the detection of positron induced se...

We report the first measurement of the full energy spectrum (down to 0 eV) of electrons emitted from the surface of Ag(100) as a result of the N3VV Auger transition. A comparison with a Monte-Carlo model was used to obtain an estimate of the contribution of multi-electron Auger processes to the low energy part of the Auger spectrum. The experimenta...

Auger processes involving the filling of holes in the valence band are thought to make important contributions to the low-energy photoelectron and secondary electron spectrum from many solids. However, measurements of the energy spectrum and the efficiency with which electrons are emitted in this process remain elusive due to a large unrelated back-...

Low energy (~2 eV) positrons were deposited onto the surface of highly oriented pyrolytic graphite (HOPG) using a positron beam equipped with a time of flight (TOF) spectrometer. The energy of the electrons emitted as a result of various secondary processes due to positron annihilation was measured using the University of Texas at Arlington's (UTA)...

We report the first measurements on 6-8 layers graphene using a low energy positron beam employing a high efficiency rare gas moderator. Doppler broadening of the annihilation gamma was measured from graphene over layers at positron kinetic energies as low as 2 eV. The efficient trapping of positrons on the surface state of graphene at these low en...

We report here the energy spectra of positron-induced electrons emitted from single- and multilayer graphene films. One of UT Arlington's two Time of Flight positron beam systems was used to deposit very low energy (KE < 1.5 eV) positrons at the surface of graphene samples consisting of a) 1 layer of graphene on polycrystalline Cu and b) 6-8 layers...

In this paper, we present results of numerical modelling of the University of Texas at Arlington's time of flight positron annihilation induced Auger electron spectrometer (UTA TOF-PAES) using SIMION® 8.1 Ion and Electron Optics Simulator. The time of flight (TOF) spectrometer measures the energy of electrons emitted from the surface of a sample as...

An advanced variable energy positron beam (~2 eV to 20 keV) has been designed, tested and utilized for coincidence Doppler broadening (CDB) measurements at the University of Texas at Arlington (UTA). A high efficiency solidified rare gas (Neon) moderator was used for the generation of a slow positron beam. The gamma rays produced as a result of the...

We report the first direct observation of electron emission into the vacuum as a result of a VVV Auger transition resulting from the relaxation of a deep hole in the valence band. A beam of low energy (<1.25eV) positrons was used to deposit positrons onto the surface samples consisting of single layer graphene, multi-layer graphene and graphite. Th...

Evolution of chemical states and defect structure in titanium during low energy nitrogen ion implanta-tion by Plasma Immersion Ion Implantation (PIII) process is studied. The underlying process of chemical state evolution is investigated using secondary ion mass spectrometry and X-ray photoelectron spectroscopy. The implantation induced defect stru...

We describe a novel spectrometer designed for positron annihilation induced Auger electron spectroscopy employing a time-of-flightspectrometer. The spectrometer’s new configuration enables us to implant monoenergetic positrons with kinetic energies as low as 1.5 eV on the sample while simultaneously allowing for the detection of electrons emitted f...

The effect of thermal stress and the role of defects in residual stress relaxation have been investigated in a nanocrystalline iron thin film coated on Si (1 0 0) substrate using glancing incidence x-ray diffraction (GIXRD) and depth resolved positron beam studies. The film has been annealed isothermally at a low homologous temperature of 400 K (~0...

This report deals with studies concerning vacancy related defects created in silicon due to implantation of 200 keV per atom aluminium and its molecular ions up to a plurality of 4. The depth profiles of vacancy defects in samples in their as implanted condition are carried out by Doppler broadening spectroscopy using low energy positron beams. In...

The scaling behavior of hardness with ligament diameter and vacancy defect concentration in nanoporous Au (np-Au) has been investigated using a combination of Vickers Hardness, Scanning electron microscopy, and positron lifetime measurements. It is shown that for np-Au, the hardness scales with the ligament diameter with an exponent of −0.3, that i...

Positron lifetime experiments, coupled with scanning electron microscopy studies, have been used to follow the coarsening of the ligaments of nanoporous Au (np-Au), prepared by electrochemical dealloying of Ag75Au25. Positron lifetime measurements in the as-prepared np-Au indicate two lifetime components, identified with annihilation at vacancies w...

We report here the design of ultra high vacuum (UHV) compatible pulsed positron beam lifetime system, which combines the principles of a conventional slow positron beam and RF based pulsing scheme. The mechanical design and construction of the UHV system to house the beam has been completed and it has been tested for a vacuum of {approx} 10{sup -10...

Highly Oriented Pyrolytic Graphite samples are irradiated with 200 keV Carbon ions to fluences of 10{sup 14} and 10{sup 15} C{sup +} ions/cm{sup 2}. Depth resolved Doppler lineshape S-parameter exhibited large increase in peak damage regions of the sample, indicating the existence of irradiation induced vacancy defects. The depth profile of the def...

Titanium modified austenitic steel has been subjected to nitrogen implantation to different fluences. There is a considerable deviation of the experimental depth profile of nitrogen from the theoretical profile obtained using Monte-Carlo codes. Available literature ascribes such redistribution to sputtering; by following the implanted nitrogen and...

A pulsed positron lifetime system has been designed using RF pulsing scheme. The mechanical design of the ultra high vacuum(UHV) beam line has been made to incorporate the basic requirements of a magnetically guided slow positron beam as well as that of an RF pulsing system consisting of a reflection type grid based chopper, a double gap pre-bunche...

Depth-resolved positron annihilation studies have been carried out on nitrogen-implanted Si to investigate defect evolution and thermal stability. Si(1 0 0) wafers have been implanted with 60 keV N+ ions to a fluence of 1 × 1014 and 1 × 1015 ions/cm2. From positron diffusion analysis of defect-sensitive S-parameter profiles, it is found that the hi...

Positron lifetime studies have been carried out on CaSO4:Dy phosphors with and without Teflon binder. The effect of Co-60 gamma irradiation exposure in these samples has been investigated. Unexposed CaSO4:Dy embedded Teflon discs exhibited four lifetime components viz., 228, 755, 3865 and 77 ps with intensities of 62%, 18%, 14% and 6%, respectively...

Positron annihilation, microstructural and hardness studies were carried out in a 9Cr reduced activation ferrtic - martensitic steel (9%Cr-1%W-0.1%C) subjected to isochronal annealing in the temperature range of 300 to 1273 K. The measured lifetime in the ‘as-received’ (normalized and tempered) condition was 147 ps, which exhibited considerable red...