836 research outputs found
Effects of High Charge Densities in Multi-GEM Detectors
A comprehensive study, supported by systematic measurements and numerical
computations, of the intrinsic limits of multi-GEM detectors when exposed to
very high particle fluxes or operated at very large gains is presented. The
observed variations of the gain, of the ion back-flow, and of the pulse height
spectra are explained in terms of the effects of the spatial distribution of
positive ions and their movement throughout the amplification structure. The
intrinsic dynamic character of the processes involved imposes the use of a
non-standard simulation tool for the interpretation of the measurements.
Computations done with a Finite Element Analysis software reproduce the
observed behaviour of the detector. The impact of this detailed description of
the detector in extreme conditions is multiple: it clarifies some detector
behaviours already observed, it helps in defining intrinsic limits of the GEM
technology, and it suggests ways to extend them.Comment: 5 pages, 6 figures, 2015 IEEE Nuclear Science Symposiu
Charge Transfer Properties Through Graphene for Applications in Gaseous Detectors
Graphene is a single layer of carbon atoms arranged in a honeycomb lattice
with remarkable mechanical and electrical properties. Regarded as the thinnest
and narrowest conductive mesh, it has drastically different transmission
behaviours when bombarded with electrons and ions in vacuum. This property, if
confirmed in gas, may be a definitive solution for the ion back-flow problem in
gaseous detectors. In order to ascertain this aspect, graphene layers of
dimensions of about 2x2cm, grown on a copper substrate, are transferred
onto a flat metal surface with holes, so that the graphene layer is freely
suspended. The graphene and the support are installed into a gaseous detector
equipped with a triple Gaseous Electron Multiplier (GEM), and the transparency
properties to electrons and ions are studied in gas as a function of the
electric fields. The techniques to produce the graphene samples are described,
and we report on preliminary tests of graphene-coated GEMs.Comment: 4pages, 3figures, 13th Pisa Meeting on Advanced Detector
Interpreting Attoclock Measurements of Tunnelling Times
Resolving in time the dynamics of light absorption by atoms and molecules,
and the electronic rearrangement this induces, is among the most challenging
goals of attosecond spectroscopy. The attoclock is an elegant approach to this
problem, which encodes ionization times in the strong-field regime. However,
the accurate reconstruction of these times from experimental data presents a
formidable theoretical challenge. Here, we solve this problem by combining
analytical theory with ab-initio numerical simulations. We apply our theory to
numerical attoclock experiments on the hydrogen atom to extract ionization time
delays and analyse their nature. Strong field ionization is often viewed as
optical tunnelling through the barrier created by the field and the core
potential. We show that, in the hydrogen atom, optical tunnelling is
instantaneous. By calibrating the attoclock using the hydrogen atom, our method
opens the way to identify possible delays associated with multielectron
dynamics during strong-field ionization.Comment: 33 pages, 10 figures, 3 appendixe
Demonstration of Gd-GEM detector design for neutron macromolecular crystallography applications
The European Spallation Source (ESS) in Lund, Sweden will become the world's
most powerful thermal neutron source. The Macromolecular Diffractometer (NMX)
at the ESS requires three 51.2 x 51.2~cm detectors with reasonable
detection efficiency, sub-mm spatial resolution, a narrow point spread function
(PSF) and good time resolution. This work presents measurements with the
improved version of the NMX detector prototype consisting of a Triple-GEM
detector with natural Gd converter and a low material budget readout. The
detector was successfully tested at the neutron reactor of the Budapest Neutron
Centre (BNC) and at the D16 instrument at the Institut Laue-Langevin (ILL) in
Grenoble. The measurements with Cadmium and Gadolinium masks in Budapest
demonstrate that the point spread function of the detector lacks long tails
that could impede the measurement of diffraction spot intensities. On the D16
instrument at ILL, diffraction spots from Triose phosphate isomerase w/
2-phosphoglycolate (PGA) inhibitor were measured both in the D16 Helium-3
detector and the Gd-GEM. The comparison between the two detectors show a
similar point spread function in both detectors, and the expected efficiency
ratio compared to the Helium-3 detector. Both measurements together thus give
good indications that the Gd-GEM detector fits the requirements for the NMX
instrument at ESS
The novel XYU-GEM to resolve ambiguities
Removing ambiguities within a single stage becomes crucial when one can not
use multiple detectors behind each other to resolve them which naturally is the
case for neutral radiation. An example would be RICH detectors. Commonly
pixilated readout is choosen for this purpose. However, this causes a
remarkable increase in quantity of channels and does not scale up well.
Therefore, the XYU-GEM was proposed as a three coordinate strip-readout which
is combined with a triple GEM detector. The readout complements a common XY
readout with an additional projection which is tilted by 45{\deg}. The
overdetermination due to three projections can be used to resovle ambiguities.
Following the detector design will be explained, first measurements discussed
to understand the response of the detector and a way to change the charge
sharing without changing the manufacturing parameters of the readout
Speciation, Luminescence, and Alkaline Fluorescence Quenching of 4-(2-methylbutyl)aminodipicolinic acid (H2MEBADPA)
4-(2-Methylbutyl)aminodipicolinic acid (H2MEBADPA) has been synthesized and fully characterized in terms of aqueous phase protonation constants (pKa\u27s) and photophysical measurements. The pKa\u27s were determined by spectrophotometric titrations, utilizing a fully sealed titration system. Photophysical measurements consisted of room temperature fluorescence and frozen solution phosphorescence as well as quantum yield determinations at various pH, which showed that only fully deprotonated MEBADPA2– is appreciably emissive. The fluorescence of MEBADPA2– has been determined to be quenched by hydroxide and methoxide anions, most likely through base-catalyzed excited-state tautomerism or proton transfer. This quenching phenomenon has been quantitatively explored through steady-state and time-resolved fluorescence measurements. Utilizing the determined pKas and quenching constants, the fluorescent intensity of MEBADPA2– has been successfully modeled as a function of pH
X-ray imaging with gaseous detectors using the VMM3a and the SRS
The integration of the VMM3a Application-Specific Integrated Circuit (ASIC) into RD51's Scalable Readout System (SRS) provides a versatile tool for the readout of Micro-Pattern Gaseous Detectors (MPGDs). With its self-triggered high-rate readout, its analogue part that allows to get information on the deposited energy in the detector, and its so-called neighbouring-logic that allows to recover information on the charge distribution, this new system has features of particular interest for digital X-ray imaging. In the present article, we want to emphasise the capabilities of VMM3a/SRS by presenting results of X-ray imaging studies. We will highlight the advantages on the energy and the spatial resolution provided by the neighbouring-logic. In the first part, we focus on spatial resolution studies. We show how segmented readout structures introduce a repeating pattern in the distribution of the reconstructed positions (using the centre-of-gravity method) and how this behaviour can be mitigated with the neighbouring-logic. As part of these studies, we explore as well an alternative position reconstruction algorithm. In the second part of the article, we present the energy resolution studies.Peer reviewe
Rate-capability of the VMM3a front-end in the RD51 Scalable Readout System
The VMM3a is an Application Specific Integrated Circuit (ASIC), specifically developed for the readout of gaseous detectors. Originally developed within the ATLAS New Small Wheel (NSW) upgrade, it has been successfully integrated into the Scalable Readout System (SRS) of the RD51 collaboration. This allows, to use the VMM3a also in small laboratory set-ups and mid-scale experiments, which make use of Micro-Pattern Gaseous Detectors (MPGDs). As part of the integration of the VMM3a into the SRS, the readout and data transfer scheme was optimised to reach a high rate-capability of the entire readout system and profit from the VMM3a’s high single-channel rate-capability of 3.6 Mhits∕s. The optimisation focused mainly on the handling of the data output stream of the VMM3a, but also on the development of a trigger-logic between the front-end cards and the DAQ computer. In this article, two firmware implementations of the non-ATLAS continuous readout mode are presented, as well as the implementation of the trigger-logic. Afterwards, a short overview on X-ray imaging results is presented, to illustrate the high rate-capability from an application point-of-view.Peer reviewe
Precise measurement of the W-boson mass with the CDF II detector
We have measured the W-boson mass MW using data corresponding to 2.2/fb of
integrated luminosity collected in proton-antiproton collisions at 1.96 TeV
with the CDF II detector at the Fermilab Tevatron collider. Samples consisting
of 470126 W->enu candidates and 624708 W->munu candidates yield the measurement
MW = 80387 +- 12 (stat) +- 15 (syst) = 80387 +- 19 MeV. This is the most
precise measurement of the W-boson mass to date and significantly exceeds the
precision of all previous measurements combined
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