Determination of the fluxes and spectra of energetic particle precipitation into the Earth's atmosphere is of critical importance for radiation belt dynamics, magnetosphere‐ionosphere coupling, as well as atmospheric chemistry. To improve the assessments of precipitating electrons using X‐ray measurements requires deeper understanding of bremsstrahlung production, transport, and redistribution throughout the atmosphere. Here we use first‐principles Monte Carlo models to explore relativistic electron precipitation events from the perspective of bremsstrahlung X‐rays. The spatial distribution of X‐rays is quantified from the ground level up to satellite altitudes. We then simulate X‐ray images that would be captured using an ideal camera and calculate the energy spectra of X‐rays originating from monoenergetic beams of precipitating electrons. Moreover, we show how these impulse responses to monoenergetic beams can be used to reconstruct the precipitating source using an inversion technique. Modeling results show that space‐borne measurements of backscattered X‐rays provide a promising method to estimate precipitation spatial size, fluxes, and spectra.
- Award ID(s):
- 1917069
- PAR ID:
- 10325111
- Date Published:
- Journal Name:
- Physics of Plasmas
- Volume:
- 29
- Issue:
- 5
- ISSN:
- 1070-664X
- Page Range / eLocation ID:
- 053506
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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High-harmonic generation (HHG) is a unique tabletop light source with femtosecond-to-attosecond pulse duration and tailorable polarization and beam shape. Here, we use counter-rotating femtosecond laser pulses of 0.8
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The quality of electron beams produced from plasma-based accelerators, i.e., normalized brightness and energy spread, has made transformative progress in the past several decades in both simulation and experiment. Recently, full-scale particle-in-cell (PIC) simulations have shown that electron beams with unprecedented brightness (1020–1021 A=m2=rad2) and 0.1–1 MeVenergy spread can be produced through controlled injection in a slowly expanding bubble that arises when a particle beam or laser pulse propagates in density gradient, or when a particle beam self-focuses in uniform plasma or has a superluminal flying focus. However, in previous simulations of work on self-injection triggered by an evolving laser driver in a uniform plasma, the resulting beams did not exhibit comparable brightnesses and energy spreads. Here, we demonstrate through the use of large-scale high-fidelity PIC simulations that a slowly expanding bubble driven by a laser pulse in a uniform plasma can indeed produce self-injected electron beams with similar brightness and energy spreads as for an evolving bubble driven by an electron beam driver. We consider laser spot sizes roughly equal to the matched spot sizes in a uniform plasma and find that the evolution of the bubble occurs naturally through the evolution of the laser. The effects of the electron beam quality on the choice of physical as well as numerical parameters, e.g., grid sizes and field solvers used in the PIC simulations are presented. It is found that this original and simplest injection scheme can produce electron beams with beam quality exceeding that of the more recent concepts.more » « less
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Abstract Background Surface dose in megavoltage photon radiotherapy has a significant clinical impact on the skin‐sparing effect. In previously published works, it was established that the presence of medium atomic number (Z) absorbers, such as tin, decreases the surface dose. It was concluded that high‐Z absorbers, such as lead, increase the surface dose, relative to medium‐Z absorbers, due to the increased contributions from photoelectrons and electron‐positron pairs.
Purpose The purpose of this investigation is to revisit these conclusions in the context of photon beams from modern linacs.
Methods A metric estimating the relative intensity of charged particles emitted in the forward direction, , was proposed using cross‐sections for the photon interactions. The values were calculated for various absorbers using energy spectra of 6 and 10 MV photon beams from a Varian TrueBeam linac. Monte Carlo (MC) simulations were performed using TOPAS MC code to calculate the surface dose for various absorbers. Surface dose measurements were performed with 6 and 10 MV photon beams with tin and lead absorbers.
Results The values were found to decrease as a function of Z for both 6 and 10 MV photon beams indicating that the surface dose from electrons emitted in the forward direction consistently decreases with increasing Z. With the increasing Z of the absorbers, both experimental and MC‐calculated surface dose decreased without exhibiting a minimum at medium‐Z absorbers. The surface dose for lead and tin was determined to be within 1% of each other for both 6 and 10 MV photon beams using MC simulations and experimental measurements. Therefore, no statistically significant difference in surface dose was found between the tin and lead absorbers disproving the presence of any minima in the surface dose versus the Z curve as has been reported in the literature.
Conclusions Surface dose for modern photon beams can be reduced using both medium and high Z absorbers since a consistent decrease in surface dose was found with increasing absorber Z.
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We present a new model designed to simulate the process of energetic particle precipitation, a vital coupling mechanism from Earth's magnetosphere to its atmosphere. The atmospheric response, namely excess ionization in the upper and middle atmosphere, together with bremsstrahlung X-ray production, is calculated with kinetic particle simulations using the Geant4 Monte Carlo framework. Mono-energy and mono-pitch angle electron beams are simulated and combined using a Green's function approach to represent realistic electron spectra and pitch angle distributions. Results from this model include more accurate ionization profiles than previous analytical models, deeper photon penetration into the atmosphere than previous Monte Carlo model predictions, and predictions of backscatter fractions of loss cone electrons up to 40%. The model results are verified by comparison with previous precipitation modeling results, and validated using balloon X-ray measurements from the Balloon Array for RBSP Relativistic Electron Losses mission and backscattered electron energy and pitch angle measurements from the Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving CubeSat mission. The model results and solution techniques are developed into a Python package for public use.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0086579
