722 research outputs found
Reduction of light output of plastic scintillator tiles during irradiation at cold temperatures and in low-oxygen environments
The advent of the silicon photomultiplier has allowed the development of
highly segmented calorimeters using plastic scintillator as the active media,
with photodetectors embedded in the calorimeter, in dimples in the plastic. To
reduce the photodetector's dark current and radiation damage, the high
granularity calorimeter designed for the high luminosity upgrade of the CMS
detector at CERN's Large Hadron Collider will be operated at a temperature of
about -30C. Due to flammability considerations, a low oxygen
environment is being considered. However, the radiation damage to the plastic
scintillator during irradiation in this operating environment needs to be
considered. In this paper, we present measurements of the relative decrease of
light output during irradiation of small plastic scintillator tiles read out by
silicon photomultipliers. The irradiations were performed using a
source both to produce the tiles' light and as a source of
ionizing irradiation at dose rates of 0.3, 1.3, and Gy/hr, temperatures
of -30, -15, -5, and 0C, and with several different oxygen
concentrations in the surrounding atmosphere. The effect of the material used
to wrap the tile was also studied. Substantial temporary damage, which annealed
when the sample was warmed, was seen during the low-temperature irradiations,
regardless of the oxygen concentration and wrapping material. The relative
light loss was largest with 3M Enhanced Specular
Reflector Film wrapping and smallest with no wrapping, although due to the
substantially higher light yield with wrapping, the final light output is
largest with wrapping. The light loss was less at warmer temperatures. Damage
with oxygen was similar to that in standard atmosphere. Evidence of a
plateau in the radical density was seen for the 0C data
Effects of oxygen on the optical properties of phenyl-based scintillators during irradiation and recovery
Plastic scintillators are a versatile and inexpensive option for particle
detection, which is why the largest particle physics experiments, CMS and
ATLAS, use them extensively in their calorimeters. One of their challenging
aspects, however, is their relatively low radiation hardness, which might be
inadequate for very high luminosity future projects like the FCC-hh. In this
study, results on the effects of ionizing radiation on the optical properties
of plastic scintillator samples are presented. The samples are made from two
different matrix materials, polystyrene and polyvinyltoluene, and have been
irradiated at dose rates ranging from Gy/h up to kGy/h at room
temperature. An internal boundary that separates two regions of different
indices of refraction is visible in the samples depending on the dose rate, and
it is compatible with the expected oxygen penetration depth during irradiation.
The dose rate dependence of the oxygen penetration depth for the two matrix
materials suggests that the oxygen penetration coefficient differs for PS and
PVT. The values of the refractive index for the internal regions are elevated
compared to those of the outer regions, which are compatible with the indices
of unirradiated samples.Comment: Replaced with published version. Added journal DOI. 30 pages, 15
figures. Published in Nuclear Instruments and Methods in Physics Research
Section
Titan's interaction with the supersonic solar wind
After 9 years in the Saturn system, the Cassini spacecraft finally observed Titan in the supersonic and super-Alfvénic solar wind. These unique observations reveal that Titan?s interaction with the solar wind is in many ways similar to unmagnetized planets Mars and Venus and active comets in spite of the differences in the properties of the solar plasma in the outer solar system. In particular, Cassini detected a collisionless, supercritical bow shock and a well-defined induced magnetosphere filled with mass-loaded interplanetary magnetic field lines, which drape around Titan?s ionosphere. Although the flyby altitude may not allow the detection of an ionopause, Cassini reports enhancements of plasma density compatible with plasma clouds or streamers in the flanks of its induced magnetosphere or due to an expansion of the induced magnetosphere. Because of the upstream conditions, these observations may be also relevant to other bodies in the outer solar system such as Pluto, where kinetic processes are expected to dominate.Fil: Bertucci, Cesar. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; ArgentinaFil: Hamilton, D. C.. University of Maryland; Estados UnidosFil: Kurth, W. S.. University of Iowa; Estados UnidosFil: Hospodarsky, G.. University of Iowa; Estados UnidosFil: Mitchell, D.. University Johns Hopkins; Estados UnidosFil: Sergis, N.. Academy of Athens; GreciaFil: Edberg, N. J. T.. Swedish Institute of Space Physics,; SueciaFil: Dougherty, M. K.. Imperial College London; Reino Unid
Space Weather in the Saturn-Titan System
New evidence based on Cassini magnetic field and plasma data has revealed that the discovery of Titan outside Saturn’s magnetosphere during the T96 flyby on 2013 December 1 was the result of the impact of two consecutive interplanetary coronal mass ejections (ICMEs) that left the Sun in 2013 early November and interacted with the moon and the planet. We study the dynamic evolution of Saturn's magnetopause and bow shock, which evidences a magnetospheric compression from late November 28 to December 4 (at least), under prevailing solar wind dynamic pressures of 0.16-0.3 nPa. During this interval, transient disturbances associated with the two ICMEs are observed, allowing for the identification of their magnetic structures. By analyzing the magnetic field direction, and the pressure balance in Titan’s induced magnetosphere, we show that Cassini finds Saturn’s moon embedded in the second ICME after being swept by its interplanetary shock and amid a shower of solar energetic particles that may have caused dramatic changes in the moon’s lower ionosphere. Analyzing a list of Saturn's bow shock crossings during 2004-2016, we find that the magnetospheric compression needed for Titan to be in the supersonic solar wind can be generally associated with the presence of an ICME or a corotating interaction region. This leads to the conclusion that Titan would rarely face the pristine solar wind, but would rather interact with transient solar structures under extreme space weather conditions
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Evaluating Local Ionization Balance in the Nightside Martian Upper Atmosphere during MAVEN Deep Dip Campaigns
Combining the Mars Atmosphere and Volatile Evolution (MAVEN) measurements of atmospheric neutral and ion densities, electron temperature, and energetic electron intensity, we perform the first quantitative evaluation of local ionization balance in the nightside Martian upper atmosphere, a condition with the electron impact ionization (EI) of CO2 exactly balanced by the dissociative recombination (DR) of ambient ions. The data accumulated during two MAVEN Deep Dip (DD) campaigns are included: DD6 on the deep nightside with a periapsis solar zenith angle (SZA) of 165 degrees, and DD3 close to the dawn terminator with a periapsis SZA of 110 degrees. With the electron temperatures at low altitudes corrected for an instrumental effect pertaining to the MAVEN Langmuir Probe and Waves, a statistical agreement between the EI and DR rates is suggested by the data below 140 km during DD6 and below 180 km during DD3, implying that electron precipitation is responsible for the nightside Martian ionosphere under these circumstances and extra sources are not required. In contrast, a substantial enhancement in EI over DR is observed at higher altitudes during both campaigns, which we interpret as a signature of plasma escape down the tail.Strategic Priority Research Program of the Chinese Academy of Sciences [XDA17010201]; National Science Foundation of China [41525015, 41774186, 41525016]; National Aeronautics and Space Administration; Swedish National Space Agency [135/13, 166/14]; Swedish Research Council [621-2013-4191]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
Molecular dynamics simulation of polymer helix formation using rigid-link methods
Molecular dynamics simulations are used to study structure formation in
simple model polymer chains that are subject to excluded volume and torsional
interactions. The changing conformations exhibited by chains of different
lengths under gradual cooling are followed until each reaches a state from
which no further change is possible. The interactions are chosen so that the
true ground state is a helix, and a high proportion of simulation runs succeed
in reaching this state; the fraction that manage to form defect-free helices is
a function of both chain length and cooling rate. In order to demonstrate
behavior analogous to the formation of protein tertiary structure, additional
attractive interactions are introduced into the model, leading to the
appearance of aligned, antiparallel helix pairs. The simulations employ a
computational approach that deals directly with the internal coordinates in a
recursive manner; this representation is able to maintain constant bond lengths
and angles without the necessity of treating them as an algebraic constraint
problem supplementary to the equations of motion.Comment: 15 pages, 14 figure
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