146 research outputs found
Super-MeV Compton Imaging and 3D Gamma-Ray Imaging Using Pixelated CdZnTe
The dissertation presents work in gamma-ray imaging in the MeV range, 3D Compton imaging, and time encoded imaging. The first thrust in high energy gamma-ray imaging begins with analyzing the artifacts produced. These factors include the increase in pair-production events, incorrect event sequencing, and charge sharing due to the larger electron clouds. They all result in shift-variant artifacts that degrade the signal-to-noise ratio as well as create artifacts that might be mistaken for a hot spot. The degradation from artifacts is discussed and possible mitigation techniques are presented to allow for recovery of the Compton image.
One of the presented mitigation techniques proposes a new sequencing algorithm for 3-or-more interaction events, called FIL-MSD. Missequencing presents one of the more dominant artifacts and by fixing the first interaction to be the largest deposited energy, the sequencing efficiency has increased by 20% in simulated data. Experimental results show an almost twofold increase in the signal to noise ratio (SNR) for simple backprojection images of a 22Na (1.7 MeV) source.
The image resolution using filtered backprojection (FBP) was improved on by developing an analytical point spread function model for high energy 3-interaction events. Previous models did not account for missequencing effects in the model. Adding these effects into the model improved the resolution of the image, but at a cost of increased artifact production. In addition, the Wiener filter was formalized for spherical harmonics, which could be used for any number of interaction given an appropriate point spread function model.
Next, demonstration of a 3D Compton imaging system is accomplished via sensor fusion of a foot-mounted odometer and a CdZnTe detector. A comparison between 3D Compton imaging and inverse-square image-reconstruction algorithms for certain measurement conditions is presented. The experiments demonstrate the advantage of 3D Compton imaging over traditional localization techniques in those scenarios. Improvements in time encoded imaging (TEI) were also made with advancements in the reconstruction algorithms and was done so in three thrusts: use of subpixel sensing, depth of interaction correction, and 3D imaging of extended sources. Complex 3D objects was accomplished via the use of magnification-parallax effects which allowed for the estimation of a source in distance away from the detector. Both the 3D Compton imaging and TEI techniques were explored at the Idaho National Laboratory.PHDNuclear ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155099/1/shyd_1.pd
Cone carving for surface reconstruction
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The Angular Momentum of the Circumgalactic Medium in the TNG100 Simulation
We present an analysis of the angular momentum content of the circumgalactic
medium (CGM) using TNG100, one of the flagship runs of the IllustrisTNG
project. We focus on Milky Way-mass halos () at
but also analyze other masses and redshifts up to . We find that the
CGM angular momentum properties are strongly correlated with the stellar
angular momentum of the corresponding galaxy: the CGM surrounding high-angular
momentum galaxies has a systematically higher angular momentum and is better
aligned to the rotational axis of the galaxy itself than the CGM surrounding
low-angular momentum galaxies. Both the hot and cold phases of the CGM show
this dichotomy, though it is stronger for colder gas. The CGM of high-angular
momentum galaxies is characterized by a large wedge of cold gas with rotational
velocities at least of the halo's virial velocity, extending out to
of the virial radius, and by biconical polar regions dominated by
radial velocities suggestive of galactic fountains; both of these features are
absent from the CGM of low-angular momentum galaxies. These conclusions are
general to halo masses and for ,
but they do not apply for more massive halos or at the highest redshift
studied. By comparing simulations run with alterations to the fiducial feedback
model, we identify the better alignment of the CGM to high-angular momentum
galaxies as a feedback-independent effect and the galactic winds as a dominant
influence on the CGM's angular momentum.Comment: Accepted to ApJ. 16 pages, 12 figure
Inpainting hydrodynamical maps with deep learning
From 1,000 hydrodynamic simulations of the CAMELS project, each with a
different value of the cosmological and astrophysical parameters, we generate
15,000 gas temperature maps. We use a state-of-the-art deep convolutional
neural network to recover missing data from those maps. We mimic the missing
data by applying regular and irregular binary masks that cover either or
of the area of each map. We quantify the reliability of our results
using two summary statistics: 1) the distance between the probability density
functions (pdf), estimated using the Kolmogorov-Smirnov (KS) test, and 2) the
2D power spectrum. We find an excellent agreement between the model prediction
and the unmasked maps when using the power spectrum: better than for
Mpc for any irregular mask. For regular masks, we observe a systematic
offset of when covering of the maps while the results become
unreliable when of the data is missing. The observed KS-test p-values
favor the null hypothesis that the reconstructed and the ground-truth maps are
drawn from the same underlying distribution when irregular masks are used. For
regular-shaped masks on the other hand, we find a strong evidence that the two
distributions do not match each other. Finally, we use the model, trained on
gas temperature maps, to perform inpainting on maps from completely different
fields such as gas mass, gas pressure, and electron density and also for gas
temperature maps from simulations run with other codes. We find that visually,
our model is able to reconstruct the missing pixels from the maps of those
fields with great accuracy, although its performance using summary statistics
depends strongly on the considered field.Comment: 14 pages, 6 figures, Submitted to AP
Modeling Galactic Conformity with the Color-Halo Age Relation in the Illustris Simulation
Comparisons between observational surveys and galaxy formation models find
that the mass of dark matter haloes can largely explain galaxies' stellar mass.
However, it remains uncertain whether additional environmental variables,
generally referred to as assembly bias, are necessary to explain other galaxy
properties. We use the Illustris Simulation to investigate the role of assembly
bias in producing galactic conformity by considering 18,000 galaxies with
> . We find a significant signal of
galactic conformity: out to distances of about 10 Mpc, the mean red fraction of
galaxies around redder galaxies is higher than around bluer galaxies at fixed
stellar mass. Dark matter haloes exhibit an analogous conformity signal, in
which the fraction of haloes formed at earlier times (old haloes) is higher
around old haloes than around younger ones at fixed halo mass. A plausible
interpretation of galactic conformity can be given as a combination of the halo
conformity signal with the galaxy color-halo age relation: at fixed stellar
mass, particularly toward the low-mass end, Illustris' galaxy colors correlate
with halo age, with the reddest galaxies (often satellites) being
preferentially found in the oldest haloes. In fact, we can explain the galactic
conformity effect with a simple semi-empirical model, by assigning stellar mass
based on halo mass (abundance matching) and by assigning galaxy color based on
halo age (age matching). We investigate other interpretations for the galactic
conformity, particularly its dependence on the isolation criterion and on the
central-satellite information. Regarding comparison to observations, we
conclude that the adopted selection/isolation criteria, projection effects, and
stacking techniques can have a significant impact on the measured amplitude of
the conformity signal.Comment: 15 pages, 8 figures; accepted for publication in MNRAS (minor
revisions to match accepted version
Radiation Damage of Pixelated CdZnTe Due to High-Energy Protons
Pixelated CdZnTe detectors are a promising imaging-spectrometer for gamma-ray
astrophysics due to their combination of relatively high energy resolution with
room temperature operation negating the need for cryogenic cooling. This
reduces the size, weight, and power requirements for telescope-based radiation
detectors. Nevertheless, operating CdZnTe in orbit will expose it to the harsh
radiation environment of space. This work, therefore, studies the effects of
protons on pixelated
CdZnTe and quantifies proton-induced radiation damage of fluences up to . In addition, we studied the effects of
irradiation on two separate instruments: one was biased and operational during
irradiation while the other remained unbiased. Following final irradiation, the
centroid and nominal resolution of the detectors
were degraded to and $653.8 \
\mathrm{keV}, 1.75 \% \ (\mathrm{FWHM})60^{\circ}\mathrm{C}$ annealing
The CAMELS multifield data set: Learning the universe’s fundamental parameters with artificial intelligence
We present the Cosmology and Astrophysics with Machine Learning Simulations (CAMELS) Multifield Data set
(CMD), a collection of hundreds of thousands of 2D maps and 3D grids containing many different properties of
cosmic gas, dark matter, and stars from more than 2000 distinct simulated universes at several cosmic times. The
2D maps and 3D grids represent cosmic regions that span ∼100 million light-years and have been generated from
thousands of state-of-the-art hydrodynamic and gravity-only N-body simulations from the CAMELS project.
Designed to train machine-learning models, CMD is the largest data set of its kind containing more than 70 TB of
data. In this paper we describe CMD in detail and outline a few of its applications. We focus our attention on one
such task, parameter inference, formulating the problems we face as a challenge to the community
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