357 research outputs found
Final Scientific/Technical Report Development of Large-Area Photo-Detectors
This proposal requested ADR funds for two years to make measurements and detector proto-types in the context of planning a program in conjunction with Argonne National Laboratory to develop very large-area planar photodetectors. The proposed detectors have integrated transmission-line readout and sampling electronics able to achieve timing and position resolutions in the range of 1-50 psec and 1-10 mm, respectively. The capability for very precise time measurements is inherent in the design, and provides a �third� coordinate, orthogonal to the two in the plane, for the point of origin of photons or charged particles, allowing �tomographic� reconstruction in 3-dimensions inside a volume
Low-Dose TOF-PET Based on Surface Electron Production in Dielectric Laminar MCPs
We present simulations of whole-body low-dose time-of-flight positron
emission tomography (TOF-PET) based on the direct surface production [1] by 511
keV gamma rays of energetic electrons via the Photo-electric and Compton
Effects, eliminating the scintillator and photodetector sub-systems in PET
scanners. In Ref. [1] we described Microchannel Plates (MCP) constructed from
thin dielectric laminae containing heavy nuclei such as lead or tungsten
(LMCP). The laminae surfaces are micro-patterned to form channels,
which can then be functionalized to support secondary electron emission in the
manner of conventional MCPs.
We have simulated direct conversion using modifications to the TOPAS
Geant4-based tool kit. A 20 20 2.54 cm LMCP, composed of
150-micron thick lead-glass laminae, is predicted to have a %
conversion efficiency to a primary electron that penetrates an interior wall of
a pore. The subsequent secondary electron shower is largely confined to one
pore and can provide high space and time resolutions.
In whole-body PET scanners the technique eliminates the scintillator and
photodetector subsystems. The consequent absence of a photocathode allows
assembly of large arrays at atmospheric pressure and less stringent vacuum
requirements, including use of pumped and cycled systems.
TOPAS simulations of the Derenzo and XCAT-brain phantoms are presented with
dose reductions of factors of 100 and 1000 from a literature benchmark. New
applications of PET at a significantly lower radiation dose include routine
screening for early detection of pathologies, the use in diagnostics in
previously unserved patient populations such as children, and a larger
installed facility base in rural and under-served populations, where simpler
gamma detectors and lower radiation doses may enable small low-cost portable
PET scanners.Comment: Version 9d incorporates reviewer comments during revision process. 22
pages, 18 figure
A 96-Channel FPGA-based Time-to-Digital Converter
We describe an FPGA-based, 96-channel, time-to-digital converter (TDC)
intended for use with the Central Outer Tracker (COT) in the CDF Experiment at
the Fermilab Tevatron. The COT system is digitized and read out by 315 TDC
cards, each serving 96 wires of the chamber. The TDC is physically configured
as a 9U VME card. The functionality is almost entirely programmed in firmware
in two Altera Stratix FPGA's. The special capabilities of this device are the
availability of 840 MHz LVDS inputs, multiple phase-locked clock modules, and
abundant memory. The TDC system operates with an input resolution of 1.2 ns.
Each input can accept up to 7 hits per collision. The time-to-digital
conversion is done by first sampling each of the 96 inputs in 1.2-ns bins and
filling a circular memory; the memory addresses of logical transitions (edges)
in the input data are then translated into the time of arrival and width of the
COT pulses. Memory pipelines with a depth of 5.5 s allow deadtime-less
operation in the first-level trigger. The TDC VME interface allows a 64-bit
Chain Block Transfer of multiple boards in a crate with transfer-rates up to 47
Mbytes/sec. The TDC also contains a separately-programmed data path that
produces prompt trigger data every Tevatron crossing. The full TDC design and
multi-card test results are described. The physical simplicity ensures
low-maintenance; the functionality being in firmware allows reprogramming for
other applications.Comment: 32 pages, 13 figure
The Boundary Conditions of the Heliosphere: Photoionization Models Constrained by Interstellar and In Situ Data
The boundary conditions of the heliosphere are set by the ionization, density
and composition of inflowing interstellar matter. Constraining the properties
of the Local Interstellar Cloud (LIC) at the heliosphere requires radiative
transfer ionization models. We model the background interstellar radiation
field using observed stellar FUV and EUV emission and the diffuse soft X-ray
background. We also model the emission from the boundary between the LIC and
the hot Local Bubble (LB) plasma, assuming that the cloud is evaporating
because of thermal conduction. We create a grid of models covering a plausible
range of LIC and LB properties, and use the modeled radiation field as input to
radiative transfer/thermal equilibrium calculations using the Cloudy code. Data
from in situ observations of He^O, pickup ions and anomalous cosmic rays in the
heliosphere, and absorption line measurements towards epsilon CMa were used to
constrain the input parameters. A restricted range of assumed LIC HI column
densities and LB plasma temperatures produce models that match all the
observational constraints. The relative weakness of the constraints on N(HI)
and T_h contrast with the narrow limits predicted for the H^O and electron
density in the LIC at the Sun, n(H^0) = 0.19 - 0.20 cm^-3, and n(e) = 0.07 +/-
0.01 cm^-3. Derived abundances are mostly typical for low density gas, with
sub-solar Mg, Si and Fe, possibly subsolar O and N, and S about solar; however
C is supersolar. The interstellar gas at the Sun is warm, low density, and
partially ionized, with n(H) = 0.23 - 0.27 cm^-3, T = 6300 K, X(H^+) ~ 0.2, and
X(He^+) ~ 0.4. These results appear to be robust since acceptable models are
found for substantially different input radiation fields. Our results favor low
values for the reference solar abundances for the LIC composition.Comment: 14 pages, 4 figures, submitted to Astronomy & Astrophysics together
with papers from the International Space Sciences Institute workshop on
Interstellar Hydrogen in the Heliospher
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A proposal to study hard hadron - hadron collisions
We propose a novel technique to study hadron-hadron collisions in which a significant fraction of the available energy is carried off in directions transverse to the beam. The technique is independent of the details of the final state so is ideally suited to searching for the jet-like particle clusters predicted by the parton model. The detector consists of a small calorimeter and several planes of 1 meter square proportional chambers
The "Solar Model Problem" Solved by the Abundance of Neon in Stars of the Local Cosmos
The interior structure of the Sun can be studied with great accuracy using
observations of its oscillations, similar to seismology of the Earth. Precise
agreement between helioseismological measurements and predictions of
theoretical solar models has been a triumph of modern astrophysics (Bahcall et
al. 2005). However, a recent downward revision by 25-35% of the solar
abundances of light elements such as C, N, O and Ne (Asplund et al. 2004) has
broken this accordance: models adopting the new abundances incorrectly predict
the depth of the convection zone, the depth profiles of sound speed and
density, and the helium abundance (Basu Antia 2004, Bahcall et al. 2005). The
discrepancies are far beyond the uncertainties in either the data or the model
predictions (Bahcall et al. 2005b). Here we report on neon abundances relative
to oxygen measured in a sample of nearby solar-like stars from their X-ray
spectra. They are all very similar and substantially larger than the recently
revised solar value. The neon abundance in the Sun is quite poorly determined.
If the Ne/O abundance in these stars is adopted for the Sun the models are
brought back into agreement with helioseismology measurements (Antia Basu 2005,
Bahcall et al. 2005c).Comment: 13 pages, 3 Figure
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A search for direct muon production in the forward direction
We propose to search for direct muon production in the forward direction from 300 GeV protons incident on a heavy nuclear target. By using the first stage of the Ml beam as a source of a diffracted proton beam and the second and third stages of Ml as a spectrometer, one can make a measurement of the direct muon to pion ratio at values of x between 0.3 and 0.75. If this ratio is on the order of 10{sup -4}, the event rates are on the order of 800 direct muons/hr. at an x of .5. The modifications to the Ml beam are minor
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