37 research outputs found
Positron Induced Electron Emission from Graphite
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 from the annihilation induced KVV
Auger transition of carbon. In the second set of data presented, PIES spectra
were obtained for 3 different positron beam energies (1.25 eV, 3.5 eV and 4.5
eV). The resulting time of flight (ToF)-PIES exhibit contributions arising from
either positron annihilation induced Auger processes (PAES), Auger mediated
positron sticking (AMPS), or secondary electron emission. Our analysis
indicates that for incident positron energies 3.5 eV and less, the ToF-PIES can
be accounted for considering only two mechanisms: positron annihilation induced
Auger processes or positron sticking.Comment: Presented at 18th International Conference on Positron Annihilation,
August 19-24, 2018 | Orlando, USA. The following article has been accepted by
AIP Conference Proceedings. After it is published, it will be found at this
https://aip.scitation.org/journal/ap
Using Remotely Piloted Aircraft and Onboard Processing to Optimize and Expand Data Collection
Remotely piloted aircraft (RPA) have the potential to revolutionize local to regional data collection for geophysicists as platform and payload size decrease while aircraft capabilities increase. In particular, data from RPAs combine high-resolution imagery available from low flight elevations with comprehensive areal coverage, unattainable from ground investigations and difficult to acquire from manned aircraft due to budgetary and logistical costs. Low flight elevations are particularly important for detecting signals that decay exponentially with distance, such as electromagnetic fields. Onboard data processing coupled with high-bandwidth telemetry open up opportunities for real-time and near real-time data processing, producing more efficient flight plans through the use of payload-directed flight, machine learning and autonomous systems. Such applications not only strive to enhance data collection, but also enable novel sensing modalities and temporal resolution. NASAs Airborne Science Program has been refining the capabilities and applications of RPA in support of satellite calibration and data product validation for several decades. In this paper, we describe current platforms, payloads, and onboard data systems available to the research community. Case studies include Fluid Lensing for littoral zone 3D mapping, structure from motion for terrestrial 3D multispectral imaging, and airborne magnetometry on medium and small RPAs
Positronium formation in graphene and graphite
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 incident positrons increases from 1.5eV to
900eV, the fraction of positrons that form Ps () decreases from ~0.5 to
~0.3. However, in HOPG and MLG, instead of a monotonic decrease of
with positron kinetic energy, a sharp peak is observed at ~ 5eV and above
~200eV,remains nearly constant in HOPG and MLG. We propose that in HOPG and
MLG, at low incident positron energies the Ps formation is dominated either by
a surface Plasmon assisted electron pick up process or by an energy dependent
back scattering process. Both these processes can explain the peak observed and
the present data can help to augment the understanding of Ps formation from
layered materials.Comment: Presented at 18th International Conference on Positron Annihilation,
August 19-24, 2018 | Orlando, USA. The following article has been accepted by
AIP Conference Proceedings. After it is published, it will be found at
https://aip.scitation.org/journal/ap
Coincident Measurement of the Energy Spectra of Doppler-Shifted Annihilation Gamma Quanta and Positron-Induced Secondary Electrons
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 secondary electrons by a micro-channel
plate detector in coincidence with the pulses resulting from the detection of
associated annihilation gamma rays in a NaI(Tl) gamma detector.Comment: Presented at 18th International Conference on Positron Annihilation,
August 19-24, 2018 | Orlando, USA. The following article has been accepted by
AIP Conference Proceedings. After it is published, it will be found at
https://aip.scitation.org/journal/ap
Application of Doppler Broadened Gamma Spectroscopy to Study the Surface of Graphene
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 curves formed by dividing the
measured Doppler broadened gamma spectra obtained at low positron kinetic
energies (~2eV) to the gamma spectra obtained at 20 keV were compared to ratio
curves found by dividing the calculated spectra of bulk graphite to bulk Cu.
The ratio curves obtained from the measured results show qualitative agreement
with those obtained from the calculated spectra. In particular, both sets of
curves indicate a much reduced intensity at high momentum. The agreement
between the measured and calculated curves is consistent with the hypothesis
that the 2eV spectra correspond to the Doppler broadened spectra from the thin
overlayer of Graphene (which we anticipate should be similar to the spectra
obtained from bulk graphite) and that the spectra taken at 20 keV corresponds
to bulk Cu due to the fact that most of the positrons implanted at this energy
annihilate in the Cu substrate. The results taken at 2 eV provide evidence that
it is possible to obtain chemically sensitive information from the top atomic
layers of surfaces (both internal and external) from an analysis of the high
momentum tail of the Doppler broadened gamma spectra obtained from the
annihilation of positrons at the surface.Comment: Presented at 18th International Conference on Positron Annihilation,
August 19-24, 2018 | Orlando, USA. The following article has been accepted by
AIP Conference Proceedings. After it is published, it will be found at
https://aip.scitation.org/journal/ap
Monte Carlo analysis of the contributions of long-lived positronium to the spectra of positron-impact-induced secondary electrons measured using an annihilation-gamma-triggered time-of-flight spectrometer
Magnetic bottle Time-of-Flight (ToF) spectrometers can measure the energy
spectra of all electrons emitted into a 2 sr solid angle simultaneously,
greatly reducing data collection time. When the detection of the annihilation
gamma () and the detection of the electron (e) are used as timing
signals for ToF spectrometers, the e- time difference spectra
(e- TDS) are reflective of the positron-induced electron energy
distributions provided the times between the impact of the positrons and the
emission of the annihilation gammas are short compared to the flight times of
the electrons. This is typically the case since positrons have short lifetime
in solids ( 100 - 500 ps) compared to the flight times of the secondary
electrons ( ns to ns). However, if the positron leaves the surface
as a positronium atom (a bound electron-positron state), the annihilation gamma
photons can be appreciably delayed due to the longer ortho-positronium (o-Ps)
lifetime. This can result in an e- TDS having an exponential tail with
a decay constant related to the o-Ps lifetime. Here, we present an analysis of
the e- TDS using a Monte Carlo model which estimates the spectral
contributions resulting from o-Ps annihilations. By removing the contributions
from the delayed gamma signal, the energy spectrum of Positron Impact-Induced
Secondary electrons (PIISE) can be isolated. Furthermore, our analysis allows
an estimation of the intensity of the exponential tail in the e- TDS
providing a method to measure the fraction of positrons that form Ps at solid
surfaces without relying on assumed 100% Ps emitting surfaces for calibration
Weathering the Storm: Unmanned Aircraft Systems in the Maritime, Atmospheric and Polar Environments
Remotely piloted aircraft (RPA) have the potential to revolutionize local to regional data collection for geophysicists as platform and payload size decrease while aircraft capabilities increase. In particular, data from RPAs combine high-resolution imagery available from low flight elevations with comprehensive areal coverage, unattainable from ground investigations and difficult to acquire from manned aircraft due to budgetary and logistical costs. Low flight elevations are particularly important for detecting signals that decay exponentially with distance, such as electromagnetic fields. Onboard data processing coupled with high-bandwidth telemetry open up opportunities for real-time and near real-time data processing, producing more efficient flight plans through the use of payload-directed flight, machine learning and autonomous systems. Such applications not only strive to enhance data collection, but also enable novel sensing modalities and temporal resolution. NASAs Airborne Science Program has been refining the capabilities and applications of RPA in support of satellite calibration and data product validation for several decades. In this paper, we describe current platforms, payloads, and onboard data systems available to the research community. Case studies include Fluid Lensing for littoral zone 3D mapping, structure from motion for terrestrial 3D multispectral imaging, and airborne magnetometry on medium and small RPAs
Subclinical ketosis: Prevalence and risk factor analysis in early lactation dairy cattle of Thrissur district
Present study was conducted in Kerala Veterinary and Animal Sciences University farms,
various organized and unorganised farms and individual households in Thrissur district during the
period of May 2021 to April 2022 with the objectives of estimation of prevalence of subclinical ketosis
(SCK) by screening dairy cows on 14th and 28th day post-partum. Multiparous cows with a blood
beta-hydroxy butyrate (BHB) value ≥ 1.2 mmol/L without any overt clinical signs were considered
as positive for SCK. One hundred and fourteen animals from second to seventh lactation were
screened for SCK. Prevalence of SCK was 23.7 per cent on 14th day and 14.04 per cent on 28th
day with a total prevalence of 27.2 per cent. Occurrence of SCK was highest in animals in second
lactation (54.8 per cent) with more occurrence in crossbred Holstein Friesian cows. Maximum
number of positive cases were observed in dairy cows of age between four to six years (54.8 per
cent). Body condition score of diseased animals were higher than normal animals.
Prevalence was highest in animals calved between months of July to September and 58.1
per cent of affected animals had male calf. Concurrent abnormalities during transition period were
also assessed based on clinical examination and farm records