1,517 research outputs found
Scintillation detectors constructed with an optimized 2x2 silicon photomultiplier array
Silicon photomultipliers (SiPMs) are a good alternative to photomultiplier
tubes (PMTs) because their gain and quantum efficiency are comparable to PMTs.
However, the largest single-chip SiPM is still less than 1~cm. In order to
use SiPMs with scintillators that have reasonable sensitivity, it is necessary
to use multiple SiPMs. In this work, scintillation detectors are constructed
and tested with a custom 2x2 SiPM array. The layout of the SiPMs and the
geometry of the scintillator were determined by performing Geant4 simulations.
Cubic NaI, CsI, and CLYC with 18~mm sides have been tested. The output of the
scintillation detectors are stabilized over the temperature range between --20
and 50~C by matching the gain of the SiPMs in the array. The energy
resolution for these detectors has been measured as a function of temperature.
Furthermore, neutron detection for the CLYC detector was studied in the same
temperature range. Using pulse-shape discrimination, neutrons can be cleanly
identified without contribution from -photons. As a result, these
detectors are suitable for deploying in spectroscopic personal radiation
detectors (SPRD).Comment: IEEE Nuclear Science Symposium Conference Record (2016
Diversity in density profiles of self-interacting dark matter satellite halos
We present results from N-body simulations of self-interacting dark matter
(SIDM) subhalos, which could host ultra-faint dwarf spheroidal galaxies, inside
a Milky-Way-like main halo. We find that high-concentration subhalos are driven
to gravothermal core collapse, while low-concentration subhalos develop large
(kpc-sized) low-density cores, with both effects depending sensitively on the
satellite's orbit and the self-interaction cross section over mass .
The overall effect for is to increase the
range of inner densities, potentially explaining the observed diversity of
Milky Way satellites, which include compact systems like Draco and Segue 1 that
are dense in dark matter, and less dense, diffuse systems like Sextans and
Crater II. We discuss possible ways of distinguishing SIDM models from
collisionless dark matter models using the inferred dark matter densities and
stellar sizes of the dwarf spheroidal galaxies.Comment: 9+4 pages, 4+4 figures. Comments are welcom
On the -Intensity Correlation in Gamma-Ray Bursts: Subphotospheric Heating with Varying Entropy
The emission mechanism during the prompt phase in gamma-ray bursts (GRBs) can
be investigated through correlations between spectral properties. Here, we
revisit the correlation relating the instantaneous flux, , and the photon
index below the spectral break, , in individual emission pulses, by
studying the 38 most prominent pulses in the Fermi/GBM GRB catalogue. First, we
search for signatures of the bias in the determination of due to the
limited spectral coverage (window effect) expected in the synchrotron case. The
absence of such a characteristic signature argues against the simplest
synchrotron models. We instead find that the observed correlation between
and can, in general, be described by the relation , for which the median . We suggest that this
correlation is a manifestation of subphotospheric heating in a flow with a
varying entropy. Around the peak of the light curve, a large entropy causes the
photosphere to approach the saturation radius, leading to an intense emission
with a narrow spectrum. As the entropy decreases the photosphere secedes from
the saturation radius, and weaker emission with a broader spectrum is expected.
This simple scenario naturally leads to a correlated variation of the intensity
and spectral shape, covering the observed range.Comment: 15 pages, 10 figures, 1 table, accepted for publication in MNRA
Magneto-Josephson effects and Majorana bound states in quantum wires
A prominent signature of Majorana bound states is the exotic Josephson
effects they produce, the classic example being a fractional Josephson current
with 4\pi periodicity in the phase difference across the junction. Recent work
established that topological insulator edges support a novel `magneto-Josephson
effect', whereby a dissipationless current exhibits 4\pi-periodic dependence
also on the relative orientation of the Zeeman fields in the two banks of the
junction. Here, we explore the magneto-Josephson effect in junctions based on
spin-orbit coupled quantum wires. In contrast to the topological insulator
case, the periodicities of the magneto-Josephson effect no longer follow from
an exact superconductor-magnetism duality of the Hamiltonian. We employ
numerical calculations as well as analytical arguments to identify the domain
configurations that display exotic Josephson physics for quantum-wire
junctions, and elucidate the characteristic differences with the corresponding
setups for topological insulators edges. To provide guidance to experiments, we
also estimate the magnitude of the magneto-Josephson effects in realistic
parameter regimes, and compare the Majorana-related contribution to the
coexisting 2\pi-periodic effects emerging from non-Majorana states.Comment: 8+ pages, 8 figures, minor changes, figures added; published versio
Search for Relativistic Curvature Effects in Gamma-Ray Burst Pulses
We analyze the time profiles of individual gamma-ray burst (GRB) pulses, that
are longer than 2 s, by modelling them with analytical functions that are based
empirical descriptions of GRB spectral evolution. These analytical profiles are
independent of the emission mechanism and can be used to model both the rise
and decay profiles Using this method, we have studied a sample of 77 individual
GRB pulses, allowing us to examine the fluence, pulse width, asymmetry, and
rise and decay power-law distributions. We find that the rise phase is best
modelled with a power law of average index and that the
average decay phase has an index o.f . We also find that the
ratio between the rise and decay times (the pulse asymmetry) exhibited by the
GRB pulse shape has an average value of 0.47 which varies little from pulse to
pulse and is independent of pulse duration or intensity. We compare these
parameters with those predicted to occur if individual pulse shapes are created
purely by relativistic curvature effects in the context of the fireball model,
a process that makes specific predictions about the shape of GRB pulses. The
decay index distribution obtained from our sample shows that the average GRB
pulse fades faster than the value predicted by curvature effects, with only 39%
of our sample being consistent with the curvature model. We discuss several
refinements of the relativistic curvature scenario that could naturally account
for these observed deviations
Modality Cycles with Masked Conditional Diffusion for Unsupervised Anomaly Segmentation in MRI
Unsupervised anomaly segmentation aims to detect patterns that are distinct
from any patterns processed during training, commonly called abnormal or
out-of-distribution patterns, without providing any associated manual
segmentations. Since anomalies during deployment can lead to model failure,
detecting the anomaly can enhance the reliability of models, which is valuable
in high-risk domains like medical imaging. This paper introduces Masked
Modality Cycles with Conditional Diffusion (MMCCD), a method that enables
segmentation of anomalies across diverse patterns in multimodal MRI. The method
is based on two fundamental ideas. First, we propose the use of cyclic modality
translation as a mechanism for enabling abnormality detection.
Image-translation models learn tissue-specific modality mappings, which are
characteristic of tissue physiology. Thus, these learned mappings fail to
translate tissues or image patterns that have never been encountered during
training, and the error enables their segmentation. Furthermore, we combine
image translation with a masked conditional diffusion model, which attempts to
`imagine' what tissue exists under a masked area, further exposing unknown
patterns as the generative model fails to recreate them. We evaluate our method
on a proxy task by training on healthy-looking slices of BraTS2021
multi-modality MRIs and testing on slices with tumors. We show that our method
compares favorably to previous unsupervised approaches based on image
reconstruction and denoising with autoencoders and diffusion models.Comment: Accepted in Multiscale Multimodal Medical Imaging workshop in MICCAI
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