5,278 research outputs found
Study of X-ray Radiation Damage in Silicon Sensors
The European X-ray Free Electron Laser (XFEL) will deliver 30,000 fully
coherent, high brilliance X-ray pulses per second each with a duration below
100 fs. This will allow the recording of diffraction patterns of single complex
molecules and the study of ultra-fast processes. Silicon pixel sensors will be
used to record the diffraction images. In 3 years of operation the sensors will
be exposed to doses of up to 1 GGy of 12 keV X-rays. At this X-ray energy no
bulk damage in silicon is expected. However fixed oxide charges in the
insulating layer covering the silicon and interface traps at the Si-SiO2
interface will be introduced by the irradiation and build up over time.
We have investigated the microscopic defects in test structures and the
macroscopic electrical properties of segmented detectors as a function of the
X-ray dose. From the test structures we determine the oxide charge density and
the densities of interface traps as a function of dose. We find that both
saturate (and even decrease) for doses between 10 and 100 MGy. For segmented
sensors the defects introduced by the X-rays increase the full depletion
voltage, the surface leakage current and the inter-pixel capacitance. We
observe that an electron accumulation layer forms at the Si-SiO2 interface. Its
width increases with dose and decreases with applied bias voltage. Using TCAD
simulations with the dose dependent parameters obtained from the test
structures, we are able to reproduce the observed results. This allows us to
optimize the sensor design for the XFEL requirements
Design, development and evaluation of Stanford/Ames EVA prehensors
Space Station operations and maintenance are expected to make unprecedented demands on astronaut EVA. With Space Station expected to operate with an 8 to 10 psi atmosphere (4 psi for Shuttle operations), the effectivness of pressurized gloves is called into doubt at the same time that EVA activity levels are to be increased. To address the need for more frequent and complex EVA missions and also to extend the dexterity, duration, and safety of EVA astronauts, NASA Ames and Stanford University have an ongoing cooperative agreement to explore and compare alternatives. This is the final Stanford/Ames report on manually powered Prehensors, each of which consists of a shroud forming a pressure enclosure around the astronaut's hand, and a linkage system to transfer the motions and forces of the hand to mechanical digits attached to the shroud. All prehensors are intended for attachment to a standard wrist coupling, as found on the AX-5 hard suit prototype, so that realistic tests can be performed under normal and reduced gravity as simulated by water flotation
Neutron irradiation effect on SiPMs up to = 5 10 cm
Silicon Photo-Multipliers (SiPM) are becoming the photo-detector of choice
for increasingly more particle detection applications, from fundamental physics
to medical and societal applications. One major consideration for their use at
high-luminosity colliders is the radiation damage induced by hadrons, which
leads to a dramatic increase of the dark count rate. KETEK SiPMs have been
exposed to various fluences of reactor neutrons up to =
510 cm (1 MeV equivalent neutrons). Results from the I-V,
and C-V measurements for temperatures between 30C and 30C
are presented. We propose a new method to quantify the effect of radiation
damage on the SiPM performance. Using the measured dark current the single
pixel occupation probability as a function of temperature and excess voltage is
determined. From the pixel occupation probability the operating conditions for
given requirements can be optimized. The method is qualitatively verified using
current measurements with the SiPM illuminated by blue LED light
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Inequality in mortality decreased among the young while increasing for older adults, 1990–2010
Many recent studies point to increasing inequality in mortality in the U.S. over the past twenty years. These studies often use mortality rates in middle and old age. Here we study inequality in mortality for all age groups in 1990, 2000, and 2010. Our analysis is based on groups of counties ranked by their poverty levels. Consistent with previous studies, we find increasing inequality in mortality at older ages. For children and young adults below age 20, however, we find strong mortality improvements that are most pronounced in poorer counties, implying a strong decrease in mortality inequality. These younger cohorts will form the future adult U.S. population, so this research suggests that inequality in old age mortality is likely to decline in future
Design, development and evaluation of Stanford/Ames Extra-Vehicular Activity (EVA) prehensors
A summary is given of progress to date on work proposed in 1983 and continued in 1985, including design iterations on three different types of manually powered prehensors, construction of functional mockups of each and culminating in detailed drawings and specifications for suit-compatible sealed units for testing under realistic conditions
Optimization of the Radiation Hardness of Silicon Pixel Sensors for High X-ray Doses using TCAD Simulations
The European X-ray Free Electron Laser (XFEL) will deliver 27000 fully
coherent, high brilliance X-ray pulses per second each with a duration below
100 fs. This will allow the recording of diffraction patterns of single
molecules and the study of ultra-fast processes. One of the detector systems
under development for the XFEL is the Adaptive Gain Integrating Pixel Detector
(AGIPD), which consists of a pixel array with readout ASICs bump-bonded to a
silicon sensor with pixels of 200 {\mu}m \times 200 {\mu}m. The particular
requirements for the detector are a high dynamic range (0, 1 up to 10E5 12 keV
photons/XFEL-pulse), a fast read-out and radiation tolerance up to doses of 1
GGy of 12 keV X-rays for 3 years of operation. At this X-ray energy no bulk
damage in silicon is expected. However fixed oxide charges in the SiO2 layer
and interface traps at the Si-SiO2 interface will build up. As function of the
12 keV X-ray dose the microscopic defects in test structures and the macro-
scopic electrical properties of segmented sensors have been investigated. From
the test structures the oxide charge density, the density of interface traps
and their properties as function of dose have been determined. It is found that
both saturate (and even decrease) for doses above a few MGy. For segmented
sensors surface damage introduced by the X-rays increases the full depletion
voltage, the surface leakage current and the inter-pixel capacitance. In
addition an electron accumulation layer forms at the Si-SiO2 interface which
increases with dose and decreases with applied voltage. Using TCAD simulations
with the dose dependent damage parameters obtained from the test struc- tures
the results of the measurements can be reproduced. This allows the optimization
of the sensor design for the XFEL requirements
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