611 research outputs found
Electrode level Monte Carlo model of radiation damage effects on astronomical CCDs
Current optical space telescopes rely upon silicon Charge Coupled Devices
(CCDs) to detect and image the incoming photons. The performance of a CCD
detector depends on its ability to transfer electrons through the silicon
efficiently, so that the signal from every pixel may be read out through a
single amplifier. This process of electron transfer is highly susceptible to
the effects of solar proton damage (or non-ionizing radiation damage). This is
because charged particles passing through the CCD displace silicon atoms,
introducing energy levels into the semi-conductor bandgap which act as
localized electron traps. The reduction in Charge Transfer Efficiency (CTE)
leads to signal loss and image smearing. The European Space Agency's
astrometric Gaia mission will make extensive use of CCDs to create the most
complete and accurate stereoscopic map to date of the Milky Way. In the context
of the Gaia mission CTE is referred to with the complementary quantity Charge
Transfer Inefficiency (CTI = 1-CTE). CTI is an extremely important issue that
threatens Gaia's performances. We present here a detailed Monte Carlo model
which has been developed to simulate the operation of a damaged CCD at the
pixel electrode level. This model implements a new approach to both the charge
density distribution within a pixel and the charge capture and release
probabilities, which allows the reproduction of CTI effects on a variety of
measurements for a large signal level range in particular for signals of the
order of a few electrons. A running version of the model as well as a brief
documentation and a few examples are readily available at
http://www.strw.leidenuniv.nl/~prodhomme/cemga.php as part of the CEMGA java
package (CTI Effects Models for Gaia).Comment: Accepted by MNRAS on 13 February 2011. 15 pages, 7 figures and 5
table
Modelling electron distributions within ESA's Gaia satellite CCD pixels to mitigate radiation damage
The Gaia satellite is a high-precision astrometry, photometry and
spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012.
Its primary science drivers are the composition, formation and evolution of the
Galaxy. Gaia will achieve its unprecedented positional accuracy requirements
with detailed calibration and correction for radiation damage. At L2, protons
cause displacement damage in the silicon of CCDs. The resulting traps capture
and emit electrons from passing charge packets in the CCD pixel, distorting the
image PSF and biasing its centroid. Microscopic models of Gaia's CCDs are being
developed to simulate this effect. The key to calculating the probability of an
electron being captured by a trap is the 3D electron density within each CCD
pixel. However, this has not been physically modelled for the Gaia CCD pixels.
In Seabroke, Holland & Cropper (2008), the first paper of this series, we
motivated the need for such specialised 3D device modelling and outlined how
its future results will fit into Gaia's overall radiation calibration strategy.
In this paper, the second of the series, we present our first results using
Silvaco's physics-based, engineering software: the ATLAS device simulation
framework. Inputting a doping profile, pixel geometry and materials into ATLAS
and comparing the results to other simulations reveals that ATLAS has a free
parameter, fixed oxide charge, that needs to be calibrated. ATLAS is
successfully benchmarked against other simulations and measurements of a test
device, identifying how to use it to model Gaia pixels and highlighting the
effect of different doping approximations.Comment: 12 pages, 6 figures, appearing in Proc. of SPIE Optics and Photonics
Conference (Focal Plane Arrays for Space telescopes IV), 2-6 August 2009, San
Diego, US
Challenges in photon-starved space astronomy in a harsh radiation environment using CCDs
The Charge Coupled Device (CCD) has a long heritage for imaging and spectroscopy in many space astronomy missions. However, the harsh radiation environment experienced in orbit creates defects in the silicon that capture the signal being transferred through the CCD. This radiation damage has a detrimental impact on the detector performance and requires carefully planned mitigation strategies. The ESA Gaia mission uses 106 CCDs, now orbiting around the second Lagrange point as part of the largest focal-plane ever launched. Following readout, signal electrons will be affected by the traps generated in the devices from the radiation environment and this degradation will be corrected for using a charge distortion model. ESA’s Euclid mission will contain a focal plane of 36 CCDs in the VIS instrument. Moving further forwards, the World Space Observatory (WSO) UV spectrographs and the WFIRST-AFTA coronagraph intend to look at very faint sources in which mitigating the impact of traps on the transfer of single electron signals will be of great interest. Following the development of novel experimental and analysis techniques, one is now able to study the impact of radiation on the detector to new levels of detail. Through a combination of TCAD simulations, defect studies and device testing, we are now probing the interaction of single electrons with individual radiation-induced traps to analyse the impact of radiation in photon-starved applications. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only
Mapping radiation-induced defects in CCDs through space and time
The Charge Coupled Device (CCD) has long been the detector of choice for many space-based applications. The CCD converts the signal X-rays or visible light into electrons (n-channel devices) or holes (p-channel devices) which are stored in the pixel structure during integration until the subsequent transfer of the charge packets through the device to be read out. The transfer of this signal charge is, however, not a perfect process.
Throughout the lifetime of a space-based mission the detector will be bombarded by high-energy particles and gamma rays. As time progresses, the radiation will damage the detectors, causing the Charge Transfer Efficiency (CTE) to decrease due to the creation of defects or “traps” in the silicon lattice of the detector. The defects create additional energy levels between the valence and conduction band in the silicon of the detector. Electrons or holes (for n-channel or p-channel devices respectively) that pass over the defect sites may be trapped. The trapped electrons or holes will later be emitted from the traps, subject to an emission-time constant related to the energy level of the associated defect. The capture and emission of charge from the signal leads to a characteristic trailing or “smearing” of images that must be corrected to enable the science goals of a mission to be met.
Over the past few years, great strides have been taken in the development of the pocket-pumping (or strictly-speaking “trap pumping”) technique. This technique not only allows individual defects (or traps) within the device to be located to the sub-pixel level, but it enables the investigation of the trap parameters such as the emission time constant to new levels of accuracy. Recent publications have shown the power of this technique in characterising a variety of different defects in both n- and p-channel devices and the potential for use in correction techniques, however, we are now exploring not only the trap locations and properties but the life cycle of these traps through time after irradiation. In orbit, most devices will be operating cold to suppress dark current and the devices are therefore cold whilst undergoing damage from the radiation environment. The mobility of defects varies as a function of temperature such that the mix of defects present following a cryogenic irradiation may vary significantly from that found following a room temperature irradiation or after annealing. It is therefore essential to study the trap formation and migration in orbit-like conditions and over longer timescales.
In this paper we present a selection of the latest methods and results in the trap pumping of n- and p-channel devices and demonstrate how this technique now allows us to map radiation-induced defects in CCDs through both space and time. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only
Modelling Gaia CCD pixels with Silvaco 3D engineering software
Gaia will only achieve its unprecedented measurement accuracy requirements
with detailed calibration and correction for radiation damage. We present our
Silvaco 3D engineering software model of the Gaia CCD pixel and two of its
applications for Gaia: (1) physically interpreting supplementary buried channel
(SBC) capacity measurements (pocket-pumping and first pixel response) in terms
of e2v manufacturing doping alignment tolerances; and (2) deriving electron
densities within a charge packet as a function of the number of constituent
electrons and 3D position within the charge packet as input to microscopic
models being developed to simulate radiation damage.Comment: 4 pages, 3 figures, contributed poster, appearing in proceedings of
the ELSA conference: Gaia, at the frontiers of astrometry, 7-11 June 2010,
S\`evres, Pari
Radial Distribution of Stellar Motions in Gaia DR2
By taking advantage of the superb measurements of position and velocity for
an unprecedented large number of stars provided in Gaia DR2, we have generated
the first maps of the rotation velocity, , and vertical velocity,
, distributions as a function of the Galactocentric radius, , across a radial range of ~kpc. In the
map, we have identified many diagonal ridge features, which are compared with
the location of the spiral arms and the expected outer Lindblad resonance of
the Galactic bar. We have detected also radial wave-like oscillations of the
peak of the vertical velocity distribution.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Lette
Risk of death and cardiovascular outcomes with thiazolidinediones: a study with the general practice research database and secondary care data.
OBJECTIVE: To describe the likely extent of confounding in evaluating the risks of cardiovascular (CV) events and mortality in patients using diabetes medication. METHODS: The General Practice Research Database was used to identify inception cohorts of insulin and different oral antidiabetics. An analysis of bias and incidence of mortality, acute coronary syndrome, stroke and heart failure were analysed in GPRD, Hospital Episode Statistics and death certificates. RESULTS: 206,940 patients were identified. The bias analysis showed that past thiazolidinedione users had a lower mortality risk compared to past metformin users. There were no differences between past users of rosiglitazone and pioglitazone (adjusted RR of 1.04; 95% CI 0.93-1.18). Current rosiglitazone users had an increased risk of death (adjusted RR 1.20; 95% CI 1.08-1.34) and of hospitalisation for heart failure (adjusted RR of 1.73; 95% CI 1.19-2.51) compared to current pioglitazone users. Risk of mortality was increased two-fold shortly after starting rosiglitazone. Excess risk of death over 3 years with rosiglitazone was 0.3 per 100 in those aged 50-64 years, 2.0 aged 65-74, 3.0 aged 75-84, and 7.0 aged 85+. The cause of death with rosiglitazone was more likely to be due to a disease of the circulatory system. CONCLUSIONS: Higher risks for death (overall and due to cardiovascular disease) and heart failure were found for rosiglitazone compared to pioglitazone. These excess risks were largest in patients aged 65 years or older. The European regulatory decision to suspend rosiglitazone is supported by this study
Local Stellar Kinematics from RAVE data - VII. Metallicity Gradients from Red Clump Stars
We investigate the Milky Way Galaxy's radial and vertical metallicity
gradients using a sample of 47,406 red clump stars from the RAVE DR4. This
sample is more than twice the size of the largest sample in the literature
investigating radial and vertical metallicity gradients. The absolute magnitude
of Groenewegen (2008) is used to determine distances to our sample stars. The
resulting distances agree with the RAVE DR4 distances Binney et al. (2014) of
the same stars. Our photometric method also provides distances to 6185 stars
that are not assigned a distance in RAVE DR4. The metallicity gradients are
calculated with their current orbital positions ( and ) and with
their orbital properties (mean Galactocentric distance, and ),
as a function of the distance to the Galactic plane:
d[Fe/H]/d- dex/kpc for kpc and
d[Fe/H]/d- dex/kpc for kpc. This
reaffirms the radial metallicity gradient in the thin disc but highlights that
gradients are sensitive to the selection effects caused by the difference
between and . The radial gradient is flat in the distance
interval 0.5-1 kpc from the plane and then becomes positive greater than 1 kpc
from the plane. The radial metallicity gradients are also eccentricity
dependent. We showed that d[Fe/H]/d-, -,
- and - dex/kpc for , ,
and sub-samples, respectively, in the distance
interval kpc. Similar trend is found for vertical
metallicity gradients. Both the radial and vertical metallicity gradients are
found to become shallower as the eccentricity of the sample increases. These
findings can be used to constrain different formation scenarios of the thick
and thin discs.Comment: 18 pages, including 16 figures and 6 tables, accepted for publication
in PAS
Silvaco ATLAS model of ESA's Gaia satellite e2v CCD91-72 pixels
The Gaia satellite is a high-precision astrometry, photometry and spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012. Its primary science drivers are the composition, formation and evolution of the Galaxy. Gaia will achieve its unprecedented accuracy requirements with detailed calibration and correction for CCD radiation damage and CCD geometric distortion. In this paper, the third of the series, we present our 3D Silvaco ATLAS model of the Gaia e2v CCD91-72 pixel. We publish e2v's design model predictions for the capacities of one of Gaia's pixel features, the supplementary buried channel (SBC), for the first time. Kohley et al. (2009) measured the SBC capacities of a Gaia CCD to be an order of magnitude smaller than e2v's design. We have found the SBC doping widths that yield these measured SBC capacities. The widths are systematically 2 ?m offset to the nominal widths. These offsets appear to be uncalibrated systematic offsets in e2v photolithography, which could either be due to systematic stitch alignment offsets or lateral ABD shield doping diffusion. The range of SBC capacities were used to derive the worst-case random stitch error between two pixel features within a stitch block to be ±0.25 ?m, which cannot explain the systematic offsets. It is beyond the scope of our pixel model to provide the manufacturing reason for the range of SBC capacities, so it does not allow us to predict how representative the tested CCD is. This open question has implications for Gaia's radiation damage and geometric calibration models
Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite
The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors
which will suffer from increased charge transfer inefficiency (CTI) as a result
of radiation damage. To aid the mitigation at low signal levels, the CCD design
includes Supplementary Buried Channels (SBCs, otherwise known as `notches')
within each CCD column. We present the largest published sample of Gaia CCD SBC
Full Well Capacity (FWC) laboratory measurements and simulations based on 13
devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in
the upper half of each CCD that are systematically smaller by two orders of
magnitude (<50 electrons) compared to those manufactured pre-2004 (thousands of
electrons). Gaia's faint star (13 < G < 20 mag) astrometric performance
predictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs as
inputs to their simulations. However, all the CCDs already integrated onto the
satellite for the 2013 launch are post-2004. SBC FWC measurements are not
available for one of our five post-2004 CCDs but the fact it meets Gaia's image
location requirements suggests it has SBC FWCs similar to pre-2004. It is too
late to measure the SBC FWCs onboard the satellite and it is not possible to
theoretically predict them. Gaia's faint star astrometric performance
predictions depend on knowledge of the onboard SBC FWCs but as these are
currently unavailable, it is not known how representative of the whole focal
plane the current predictions are. Therefore, we suggest Gaia's initial
in-orbit calibrations should include measurement of the onboard SBC FWCs. We
present a potential method to do this. Faint star astrometric performance
predictions based on onboard SBC FWCs at the start of the mission would allow
satellite operating conditions or CTI software mitigation to be further
optimised to improve the scientific return of Gaia.Comment: Accepted for publication in MNRAS, 16 pages, 19 figure
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