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Impact of particles on the Planck HFI detectors: Ground-based measurements and physical interpretation
The Planck High Frequency Instrument (HFI) surveyed the sky continuously from
August 2009 to January 2012. Its noise and sensitivity performance were
excellent, but the rate of cosmic ray impacts on the HFI detectors was
unexpectedly high. Furthermore, collisions of cosmic rays with the focal plane
produced transient signals in the data (glitches) with a wide range of
characteristics. A study of cosmic ray impacts on the HFI detector modules has
been undertaken to categorize and characterize the glitches, to correct the HFI
time-ordered data, and understand the residual effects on Planck maps and data
products. This paper presents an evaluation of the physical origins of glitches
observed by the HFI detectors. In order to better understand the glitches
observed by HFI in flight, several ground-based experiments were conducted with
flight-spare HFI bolometer modules. The experiments were conducted between 2010
and 2013 with HFI test bolometers in different configurations using varying
particles and impact energies. The bolometer modules were exposed to 23 MeV
protons from the Orsay IPN TANDEM accelerator, and to Am and Cm
-particle and Fe radioactive X-ray sources. The calibration data
from the HFI ground-based preflight tests were used to further characterize the
glitches and compare glitch rates with statistical expectations under
laboratory conditions. Test results provide strong evidence that the dominant
family of glitches observed in flight are due to cosmic ray absorption by the
silicon die substrate on which the HFI detectors reside. Glitch energy is
propagated to the thermistor by ballistic phonons, while there is also a
thermal diffusion contribution. The implications of these results for future
satellite missions, especially those in the far-infrared to sub-millimetre and
millimetre regions of the electromagnetic spectrum, are discussed.Comment: 11 pages, 13 figure
Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene
Atomic resolution high angle annular dark field imaging of suspended,
single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al and Au atoms
had been deposited was carried out in an aberration corrected scanning
transmission electron microscope. In combination with electron energy loss
spectroscopy, employed to identify individual impurity atoms, it was shown that
nano-scale holes were etched into graphene, initiated at sites where single
atoms of all the metal species except for gold come into close contact with the
graphene. The e-beam scanning process is instrumental in promoting metal atoms
from clusters formed during the original metal deposition process onto the
clean graphene surface, where they initiate the hole-forming process. Our
observations are discussed in the light of calculations in the literature,
predicting a much lowered vacancy formation in graphene when metal ad-atoms are
present. The requirement and importance of oxygen atoms in this process,
although not predicted by such previous calculations, is also discussed,
following our observations of hole formation in pristine graphene in the
presence of Si-impurity atoms, supported by new calculations which predict a
dramatic decrease of the vacancy formation energy, when SiOx molecules are
present.Comment: final version accepted in ACS Nano + supplementary info. 22+6 pages,
4+5 figure
Novel Boron-10-based detectors for Neutron Scattering Science
Nowadays neutron scattering science is increasing its instrumental power.
Most of the neutron sources in the world are pushing the development of their
technologies to be more performing. The neutron scattering development is also
pushed by the European Spallation Source (ESS) in Sweden, a neutron facility
which has just started construction. Concerning small area detectors (1m^2),
the 3He technology, which is today cutting edge, is reaching fundamental limits
in its development. Counting rate capability, spatial resolution and
cost-effectiveness, are only a few examples of the features that must be
improved to fulfill the new requirements. On the other hand, 3He technology
could still satisfy the detector requirements for large area applications
(50m^2), however, because of the present 3He shortage that the world is
experiencing, this is not practical anymore. The recent detector advances (the
Multi-Grid and the Multi-Blade prototypes) developed in the framework of the
collaboration between the Institut Laue-Langevin (ILL) and ESS are presented in
this manuscript. In particular two novel 10B-based detectors are described; one
for large area applications (the Multi-Grid prototype) and one for application
in neutron refectometry (small area applications, the Multi-Blade prototype)
Transverse Beam Profiles
The performance and safe operation of a particle accelerator is closely
connected to the transverse emittance of the beams it produces. For this reason
many techniques have been developed over the years for monitoring the
transverse distribution of particles along accelerator chains or over machine
cycles. The definition of beam profiles is explained and the different
techniques available for the detection of the particle distributions are
explored. Examples of concrete applications of these techniques are given.Comment: 37 pages, 53 figure
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