10 research outputs found
FACT -- The G-APD revolution in Cherenkov astronomy
Since two years, the FACT telescope is operating on the Canary Island of La
Palma. Apart from its purpose to serve as a monitoring facility for the
brightest TeV blazars, it was built as a major step to establish solid state
photon counters as detectors in Cherenkov astronomy. The camera of the First
G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes
(G-APD), equipped with solid light guides to increase the effective light
collection area of each sensor. Since no sense-line is available, a special
challenge is to keep the applied voltage stable although the current drawn by
the G-APD depends on the flux of night-sky background photons significantly
varying with ambient light conditions. Methods have been developed to keep the
temperature and voltage dependent response of the G-APDs stable during
operation. As a cross-check, dark count spectra with high statistics have been
taken under different environmental conditions. In this presentation, the
project, the developed methods and the experience from two years of operation
of the first G-APD based camera in Cherenkov astronomy under changing
environmental conditions will be presented.Comment: Proceedings of the Nuclear Science Symposium and Medical Imaging
Conference (IEEE-NSS/MIC), 201
Epigenetic Transitions and Knotted Solitons in Stretched Chromatin
The spreading and regulation of epigenetic marks on chromosomes is crucial to
establish and maintain cellular identity. Nonetheless, the dynamical mechanism
leading to the establishment and maintenance of a given, cell-line specific,
epigenetic pattern is still poorly understood. In this work we propose, and
investigate in silico, a possible experimental strategy to illuminate the
interplay between 3D chromatin structure and epigenetic dynamics. We consider a
set-up where a reconstituted chromatin fibre is stretched at its two ends
(e.g., by laser tweezers), while epigenetic enzymes (writers) and
chromatin-binding proteins (readers) are flooded into the system. We show that,
by tuning the stretching force and the binding affinity of the readers for
chromatin, the fibre undergoes a sharp transition between a stretched,
epigenetically disordered, state and a crumpled, epigenetically coherent, one.
We further investigate the case in which a knot is tied along the chromatin
fibre, and find that the knotted segment enhances local epigenetic order,
giving rise to "epigenetic solitons" which travel and diffuse along chromatin.
Our results point to an intriguing coupling between 3D chromatin topology and
epigenetic dynamics, which may be investigated via single molecule experiments.Comment: Accepted version; Supplementary movies can be found at
http://www2.ph.ed.ac.uk/~dmichiel/KnottedSolitons.html and
https://www.youtube.com/watch?v=Osghh9nEhe