Paul Oxley

The EBEX experiment

EBEX is a balloon-borne polarimeter designed to measure the intensity and polarization of the cosmic microwave background radiation. The measurements would probe the inflationary epoch that took place shortly after the big bang and would significantly improve constraints on the values of several cosmological parameters. EBEX is unique in its broad frequency coverage and in its ability to provide critical information about the level of polarized Galactic foregrounds which will be necessary for all future CMB polarization experiments. EBEX consists of a 1.5 m Dragone-type telescope that provides a resolution of less than 8 arcminutes over four focal planes each of 4 degree diffraction limited field of view at frequencies up to 450 GHz. The experiment is designed to accommodate 330 transition edge bolometric detectors per focal plane, for a total of up to 1320 detectors. EBEX will operate with frequency bands centered at 150, 250, 350, and 450 GHz. Polarimetry is achieved with a rotating achromatic half-wave plate. EBEX is currently in the design and construction phase, and first light is scheduled for 2008.

First positron cooling of antiprotons.

Positrons are used to cool antiprotons for the first time. The oppositely charged positrons and antiprotons are first simultaneously accumulated in separate Penning trap volumes, and then are spatially merged in a nested Penning trap. The antiprotons cool until they reach a low relative velocity with respect to the cold positrons, the situation expected to be optimal for the production of cold antihydrogen.  2001 Published by Elsevier Science B.V.

Stacking of cold antiprotons

Abstract The stacking of cold antiprotons is currently the only way to accumulate the large numbers of the cold antiprotons that are needed for low energy experiments. Both the largest possible number and the lowest possible temperature are desired, especially for the production and study of cold antihydrogen. The antiprotons accumulated in our particle trap have an energy 10 10 times lower than the energy of those delivered by CERNs Antiprotons Decelerator (AD). The number accumulated (more than 0.4 million in this demonstration) is linear in the number of accepted high energy antiproton pulses (32 in this demonstration). Accumulation efficiencies and losses are measured and discussed.

Millimeter-wave achromatic half-wave plate

We have constructed an achromatic half-wave plate (AHWP) suitable for the millimeter wavelength band. The AHWP was made from a stack of three sapphire a-cut birefringent plates with the opticalaxes of the middle plate rotated by 50.5 deg with respect to the aligned axes of the other plates. The measured modulation efficiency of the AHWP at 110 GHz was 96 +/- 1.5%. In contrast, the modulation efficiency of a single sapphire plate of the same thickness was 43 +/- 4%. Both results are in close agreement with theoretical predictions. The modulation efficiency of the AHWP was constant as a function of incidence angles between 0 and 15 deg. We discuss design parameters of an AHWP in the context of astrophysical broadband polarimetry at the millimeter wavelength band.

Studies of magnetic dipole transitions in highly charged argon and barium using an electron beam ion trap

Using the Oxford electron beam ion trap (EBIT), we have studied a number of magnetic-dipole fine-structure transitions in highly charged argon and barium, which lie in the visible and near-UV region of the spectrum. Our wavelength measurements, with uncertainties of between 7 and 20 ppm, are the most accurate yet reported using an EBIT as a spectroscopic source of ions, and provide a useful test of atomic structure calculations for many-electron systems. The argon transitions studied are also of astrophysical interest. Finally, we present the first studies of the polarization of a visible transition from a highly charged ion trapped in an EBIT.

Apparatus for Magnetization and Efficient Demagnetization of Soft Magnetic Materials

This paper describes an electrical circuit that can be used to automatically magnetize and ac-demagnetize moderately soft magnetic materials and with minor modifications could be used to demagnetize harder magnetic materials and magnetic geological samples. The circuit is straightforward to replicate, easy to use, and low in cost. Independent control of the demagnetizing current frequency, amplitude, and duration is available. The paper describes the circuit operation in detail and shows that it can demagnetize a link-shaped specimen of 430FR stainless steel with 100% efficiency. Measurements of the demagnetization efficiency of the specimen with different ac-demagnetization frequencies are interpreted using eddy-current theory. The experimental results agree closely with the theoretical predictions.

Laser spectroscopy of the 1s22s2p 3P2-3P1 transition in beryllium-like argon using the Oxford EBIT

A study of the 1s22s2p 3P2-3P1 fine-structure transition in beryllium-like argon is planned on the Oxford electron beam ion trap (EBIT), using laser-induced resonance. This transition wavelength was measured previously as 594.373(4) nm, which is accessible using a dye laser. The ions are produced and excited in the EBIT and are confined during laser irradiation using the magnetic trapping mode. The 3P2 level population is depleted by the laser and by subsequently monitoring the emitted fluorescence a laser resonance signal can be obtained.

Finite element solution of Laplace's equation for ion-atom chambers

The finite element method is used to solve Laplace’s equation for ion-atom chambers. We first consider a simplified model chamber for which an analytical solution can be obtained; the model chamber serves as a test case to verify the accuracy and convergence of the finite element method. We apply the finite element method to an experimental chamber consisting of five equipotential rings in a grounded cylindrical shell. We determine the strength and homogeneity of the electric field in the region of the chamber where the atoms undergo laser excitation into a Rydberg state.

Observations of cold antihydrogen

ATRAP se þ cooling of p in a nested Penning trap has led to reports of cold H produced during such cooling by the ATHENA and ATRAP collaborations. To observe H, ATHENA uses coincident annihilation detection and ATRAP uses field ionization followed by p storage. Advantages of ATRAPs field ionization method include the complete absence of any background events, and the first way to measure which H states are produced. ATRAP enhances the H production rate by driving many cycles of e þ cooling in the nested trap, with more H counted in an hour than the sum of all the other antimatter atoms ever reported. The number of H counted per incident high energy p is also higher than ever observed. The first measured distribution of H states is made using a pre-ionizing electric field between separated production and detection regions. The high rate and the high Rydberg states suggest that the H is formed via threebody recombination, as expected. 2003 Elsevier B.V. All rights reserved.

Laser spectroscopy of the 2s lamb shift in hydrogen-like silicon using an electron beam ion trap

We describe progress towards a precision measurement of the 2s Lamb shift in hydrogen-like silicon. This will use the Oxford electron beam ion trap (EBIT) as a source of highly charged ions for laser spectroscopy. We have successfully trapped silicon ions in the EBIT, and are determining the optimum operating conditions required to maximize the number of Si13+ ions. The laser system required for the experiment is currently under development; the first stage of this work involves locking a frequency-stabilized laser operating at 734 nm to a high-finesse enhancement cavity.