13,539 research outputs found
A solid state vertex detector for study of the photoproduction of heavy flavours
Imperial Users onl
X-ray time lags in AGN: inverse-Compton scattering and spherical corona model
We develop a physically motivated, spherical corona model to investigate the
frequency-dependent time lags in AGN. The model includes the effects of Compton
up-scattering between the disc UV photons and coronal electrons, and the
subsequent X-ray reverberation from the disc. The time lags are associated with
the time required for multiple scatterings to boost UV photons up to soft and
hard X-ray energies, and the light crossing time the photons take to reach the
observer. This model can reproduce not only low-frequency hard and
high-frequency soft lags, but also the clear bumps and wiggles in reverberation
profiles which should explain the wavy-residuals currently observed in some
AGN. Our model supports an anti-correlation between the optical depth and
coronal temperatures. In case of an optically thin corona, time delays due to
propagating fluctuations may be required to reproduce observed time lags. We
fit the model to the lag-frequency data of 1H0707-495, Ark 564, NGC 4051 and
IRAS 13224-3809 estimated using the minimal bias technique so that the observed
lags here are highest-possible quality. We find their corona size is ~7-15 r_g
having the constrained optical depth ~2-10. The coronal temperature is ~150-300
keV. Finally, we note that the reverberation wiggles may be signatures of
repeating scatters inside the corona that control the distribution of X-ray
sources.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Gamma at Transition in the Proposed PS2 Machine
The PS2 synchrotron is a normally conducting machine which is proposed as a replacement for the ageing PS to help meet the demand for brighter and more intense beams. In order to reprise the role of the PS, the new machine must be capable of a variety of beam manipulations. These impose constraints on the possible value of gamma at transition that can be considered
Non-invasive single-bunch matching and emittance monitor
On-line monitoring of beam quality for high brightness beams is only possible
using non-invasive instruments. For matching measurements, very few such
instruments are available. One candidate is a quadrupole pick-up. Therefore, a
new type of quadrupole pick-up has been developed for the 26 GeV Proton
Synchrotron (PS) at CERN, and a measurement system consisting of two such
pick-ups is now installed in this accelerator. Using the information from these
pick-ups, it is possible to determine both injection matching and emittance in
the horizontal and vertical planes, for each bunch separately. This paper
presents the measurement method and some of the results from the first year of
use, as well as comparisons with other measurement methods.Comment: 10 pages, 10 figures; added figure, minor textual additions; To be
resubmitted to Phys. Rev. ST-A
Understanding the measurement of forests with waveform lidar
The measurement of forests is essential for monitoring and predicting the role and response
of the land surface to global climate change. Globally consistent and frequent measurements
can only be made by satellites; unfortunately many current system’s measurements saturate at
moderate canopy densities and are not directly related to forest properties, requiring tenuous
empirical relationships that are insensitive to many of the Earth’s most important, Carbon rich forests.
Lidar (laser radar) is a relatively new technology that offers the potential to make direct
measurements of forest height, vertical density and, when ground based, explicit measurements
of structure. In addition measurements do not saturate until much higher forest densities.
In recent years there has been much interest in the measurement of forests by lidar, with
a number of airborne and terrestrial and one spaceborne lidar developed. Measuring a forest
leaf by leaf is impractical and very tedious, so more rapid ground based methods are needed
to collect data to validate satellite derived estimates. These rapid methods are themselves not
directly related to forest properties causing uncertainty in any validation of remotely sensed
estimates.
This thesis uses Monte Carlo ray tracing to simulate the measurement of forests by full
waveform lidars over explicit geometric forest models for both above and below canopy instruments. Existing methods for deriving forest properties from measurements are tested against
the known truth of these simulated forests, a process impossible in reality. Causes of disagreements are explored and new methods developed to attempt to overcome any shortcomings.
These new methods include dual wavelength lidar for correcting satellite based measurements
for topography and a voxel based method for more directly relating terrestrial lidar signals to
forest properties
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