271 research outputs found
High Efficiency Positron Accumulation for High-Precision Measurements
Positrons are accumulated within a Penning trap designed to make more precise
measurements of the positron and electron magnetic moments. The retractable
radioactive source used is weak enough to require no license for handling
radioactive material and the radiation dosage one meter from the source gives
an exposure several times smaller than the average radiation dose on the
earth's surface. The 100 mK trap is mechanically aligned with the 4.2 K
superconducting solenoid that produces a 6 tesla magnetic trapping field with a
direct mechanical coupling.Comment: 7 pages, 9 figure
Design and Assembly of a Large-aperture Nb3Sn Cos-theta Dipole Coil with Stress Management in Dipole Mirror Configuration
The stress-management cos-theta (SMCT) coil is a new concept which has been
proposed and is being developed at Fermilab in the framework of US Magnet
Development Program (US-MDP) for high-field and/or large-aperture accelerator
magnets based on low-temperature and high-temperature superconductors. The SMCT
structure is used to reduce large coil deformations under the Lorentz forces
and, thus, the excessively large strains and stresses in the coil. A
large-aperture Nb3Sn SMCT dipole coil has been developed and fabricated at
Fermilab to demonstrate and test the SMCT concept including coil design,
fabrication technology and performance. The first SMCT coil has been assembled
with 60-mm aperture Nb3Sn coil inside a dipole mirror configuration and will be
tested separately and in series with the insert coil. This paper summarizes the
large-aperture SMCT coil design and parameters and reports the coil fabrication
steps and its assembly in dipole mirror configuration
Telomere disruption results in non-random formation of de novo dicentric chromosomes involving acrocentric human chromosomes
Copyright: Β© 2010 Stimpson et al.Genome rearrangement often produces chromosomes with two centromeres (dicentrics) that are inherently unstable because of bridge formation and breakage during cell division. However, mammalian dicentrics, and particularly those in humans, can be quite stable, usually because one centromere is functionally silenced. Molecular mechanisms of centromere inactivation are poorly understood since there are few systems to experimentally create dicentric human chromosomes. Here, we describe a human cell culture model that enriches for de novo dicentrics. We demonstrate that transient disruption of human telomere structure non-randomly produces dicentric fusions involving acrocentric chromosomes. The induced dicentrics vary in structure near fusion breakpoints and like naturally-occurring dicentrics, exhibit various inter-centromeric distances. Many functional dicentrics persist for months after formation. Even those with distantly spaced centromeres remain functionally dicentric for 20 cell generations. Other dicentrics within the population reflect centromere inactivation. In some cases, centromere inactivation occurs by an apparently epigenetic mechanism. In other dicentrics, the size of the alpha-satellite DNA array associated with CENP-A is reduced compared to the same array before dicentric formation. Extrachromosomal fragments that contained CENP-A often appear in the same cells as dicentrics. Some of these fragments are derived from the same alpha-satellite DNA array as inactivated centromeres. Our results indicate that dicentric human chromosomes undergo alternative fates after formation. Many retain two active centromeres and are stable through multiple cell divisions. Others undergo centromere inactivation. This event occurs within a broad temporal window and can involve deletion of chromatin that marks the locus as a site for CENP-A maintenance/replenishment.This work was supported by the Tumorzentrum Heidelberg/Mannheim grant (D.10026941)and by March of Dimes Research Foundation grant #1-FY06-377 and NIH R01 GM069514
First observation of cyclotron radiation from MeV-scale following nuclear beta decay
We present an apparatus for detection of cyclotron radiation that allows a
frequency-based beta energy determination in the 5 keV to 5 MeV range,
characteristic of nuclear beta decays. The cyclotron frequency of the radiating
beta particles in a magnetic field is used to determine the beta energy
precisely. Our work establishes the foundation to apply the cyclotron radiation
emission spectroscopy (CRES) technique, developed by the Project 8
collaboration, far beyond the 18-keV tritium endpoint region. We report initial
measurements of beta^-s from 6He and beta^+s from 19Ne decays to demonstrate
the broadband response of our detection system and assess potential systematic
uncertainties for beta spectroscopy over the full (MeV) energy range. This work
is an important benchmark for the practical application of the CRES technique
to a variety of nuclei, in particular, opening its reach to searches for
evidence of new physics beyond the TeV scale via precision beta-decay
measurements
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