Jason McPhate

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis

We present new high-speed, multi-observatory, multi-instrument photometry of the eclipsing polar UZ For in order to measure precise mid-eclipse times with the aim of detecting any orbital period variations. When combined with published eclipse times and archival data spanning ∼27 years, we detect departures from a linear and quadratic trend of ∼60 s. The departures are strongly suggestive of two cyclic variations of 16(3) and 5.25(25) years. The two favoured mechanisms to drive the periodicities are either two giant extrasolar planets as companions to the binary [with minimum masses of 6.3(1.5) and 7.7(1.2)MJup) or a magnetic cycle mechanism (e.g. Applegate’s mechanism) of the secondary star. Applegate’s mechanism would require the entire radiant energy output of the secondary and would therefore seem to be the least likely of the two, barring any further refinements in the effect of magnetic fields (e.g. those of Lanza et al.). The two-planet model can provide realistic solutions but it does not quite capture all of the eclipse times measurements. A highly eccentric orbit for the outer planet would fit the data nicely, but we find that such a solution would be unstable. It is also possible that the periodicities are driven by some combination of both mechanisms. Further observations of this system are encouraged.

Gallium Nitride Photocathode Development for Imaging Detectors

Recent progress in Gallium Nitride (GaN, AlGaN, InGaN) photocathodes show great promise for future detector applications in Astrophysical instruments. Efforts with opaque GaN photocathodes have yielded quantum efficiencies up to 70% at 120 nm and cutoffs at ~380 nm, with low out of band response, and high stability. Previous work with semitransparent GaN photocathodes produced relatively low quantum efficiencies in transmission mode (4%). We now have preliminary data showing that quantum efficiency improvements of a factor of 5 can be achieved. We have also performed two dimensional photon counting imaging with 25mm diameter semitransparent GaN photocathodes in close proximity to a microchannel plate stack and a cross delay line readout. The imaging performance achieves spatial resolution of ~50μm with low intrinsic background (below 1 event sec-1 cm-2) and reasonable image uniformity. GaN photocathodes with significant quantum efficiency have been fabricated on ceramic MCP substrates. In addition GaN has been deposited at low temperature onto quartz substrates, also achieving substantial quantum efficiency.

High Resolution Photon Counting With MCP-Timepix Quad Parallel Readout Operating at

The unique capability of microchannel plates (MCPs) to convert a single photon/ electron/ ion/ neutron into a charge of 104 -107 electrons localized within 4-12 μm from the event position is widely used in event counting imaging detectors. The high spatial and timing resolution of MCP detectors have been demonstrated with different readout techniques. A compromise between the spatial and temporal resolution, the global/local counting rate and active area must always be made for each detector application. In this paper we present a 28 × 28 mm2 MCP detector with 2 × 2 Timepix ASICs for readout, capable of ~ 10 μm spatial resolution at event rates up to ~ 3 MHz, and in excess of 200 MHz with ~ 55 μm pixels. This detector has a unique capability to detect multiple simultaneous events, up to several thousand with ~ 10 μm resolution and > 25000 for the 55 μm mode. The latter is enabled by the new fast readout electronics capable of readout speeds of ~ 1200 frames/sec. Despite its limitations (relatively small active area, readout dead time of 300 μs) the MCP-Timepix detector can be very attractive for applications where high spatial resolution needs to be preserved for nearly simultaneous events, e.g., time of flight measurements with pulsed sources. The low noise of the Timepix readout enables operation at gains as low as 104 -105, which should extend the lifetime of the MCP detectors operating at high counting rates.

> 1~{\rm KHz}

We demonstrate that the G140L segment B channel of the Cosmic Origins Spectrograph (COS) recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ∼ 10 cm in the bandpass between the Lyman edge at 912 Å and Lyman β, rising to a peak in excess of 1000 cm longward of 1130 Å. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale-length of the hardness variation in the metagalactic ionizing background over the redshift range 2 < z . 2.8, measure the ratio of CO to H2 in dense interstellar environments with AV > 3, or harness the high temperature diagnostic power of the O VI λλ 1032, 1038 doublet can now be carried out with unprecedented sensitivity. Subject headings: instrumentation: spectrographs, ultraviolet: generalWe demonstrate that the G140L Segment B channel of the Cosmic Origins Spectrograph recently installed on the Hubble Space Telescope (HST) has an effective area consistent with ~10 cm2 in the bandpass between the Lyman edge at 912 ? and Lyman ?, rising to a peak in excess of 1000 cm2 longward of 1130 ?. This is a new wavelength regime for HST and will allow opportunities for unique science investigations. In particular, investigations seeking to quantify the escape fraction of Lyman continuum photons from galaxies at low redshift, determine the scale length of the hardness variation in the metagalactic ionizing background over the redshift range 2 3 can now be carried out with unprecedented sensitivity.

Frame Rates

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ~55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ~30 mA energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mA level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ~few hundreds μistrain for 300s of data acquisition time.

FAR-ULTRAVIOLET SENSITIVITY OF THE COSMIC ORIGINS SPECTROGRAPH

Abstract The Cosmic Origins Spectrograph (COS) is a high throughput spectrometer that will be placed on the Hubble Space Telescope (HST) during the last servicing mission in the year 2003. COS will be the most sensitive UV spectrograph ever flown aboard HST and will investigate such fundamental issues as the ionization and baryon content of the intergalactic medium and the origin of large-scale structure of the Universe. The driving design goal for COS is to maximize throughput at a moderate spectral resolution of >20,000 using optics with very few reflections and detectors with high quantum efficiency in two bandpass channels: FUV (1150– 1775 A ) and NUV (1750– 3200 A ). The COS FUV detector, a windowless microchannel plate (MCP) detector, consists of two segments each 85 mm ×10 mm concatenated end to end with a 9 mm gap between them. The design is based on the Far Ultraviolet Spectroscopic Explorer detectors with identical format and front surface radius of curvature that matches the grating focal plane of the spectrograph. However, enhancements have been made in the design and fabrication of the MCPs, the photocathode, the delay line anode and the readout electronics. We discuss these design enhancements and their significance.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

The unique ability to record photon X,Y,T high fidelity information has advantages for high speed recording devices for some important time dependent applications. For microchannel plate sensors our most commonly used readout configuration is the cross delay line anode. We have achieved resolutions of < 25 μm in tests over 65 mm x 65 mm (>2.5k x 2.5k resolution elements) with excellent linearity for random photon rates of > 500 kHz, while time tagging events using the MCP output signal to better than 100 ps. Open face and sealed tube microchannel plate cross delay line detectors of this kind have been built and used for observation of flare stars, orbital satellites and space debris with the GALEX satellite, time resolved imaging of the Crab Pulsar with a telescope as small as 1m, biological fluorescence imaging and synchrotron diagnostics. To achieve better efficiency, higher counting rate and extended lifetime we are now developing cross strip anode readouts. These have already demonstrated 5μm resolution at <10x lower gain than the cross delay line schemes, and high speed electronics for the cross strip are currently in development.

The FUV detector for the cosmic origins spectrograph on the Hubble Space Telescope

Neutron transmission radiography can be strongly enhanced by adding spectroscopic data spatially correlated with the attenuation coefficient. This can now be achieved at pulsed neutron sources, utilizing a neutron detector with high spatial and temporal resolution. The energy of transmitted neutrons can be recovered from their time-of-flight, simultaneously with the acquisition of the transmission radiographic image by a pixelated detector. From this, the positions of Bragg edges can be obtained for each pixel of the radiographic image. The combination of both spectroscopic and transmission information enables high spatial resolution studies to be carried out on material composition, phase transitions, texture variations, as well as strain analysis, as long as the resolution and statistics are favorable. This paper presents initial results from proof-of-principle experiments on energy-resolved neutron transmission radiography, using a neutron counting detector consisting of neutron-sensitive microchannel plates (MCPs) and a Medipix2 electronic readout. These experiments demonstrate that the position of Bragg edges are measurable with a few mAring resolution in each 55-mum pixel of the detector, corresponding to DeltaE/E~0.1%. However, the limited intensity of most current neutron sources requires a compromise between the energy resolution and the area over which it was integrated. Still, the latter limitation can be overcome by combining energy information for several neighboring pixels, while transmission radiography can still be done at the limit of the detector spatial resolution.

High performance microchannel plate imaging photon counters for spaceborne sensing

Objectives for the next generation of UV microchannel plate astronomical detectors include development of efficient photocathodes, including gallium nitride (GaN), and diamond, and optimization of silicon based MCPs. Goals include the development of GaN photocathodes in sealed tube and open detectors with >50% DQE in the UV (>110nm), with tunable cutoffs around 400nm. Activated diamond photocathodes with >40% DQE @ >110nm, cutoffs >200nm, and their application to Si MCPs are also of interest. GaN photocathodes have been developed with efficiencies >60% and cutoffs of ~380nm. Diamond photocathodes with ~40% efficiency at 40nm have been achieved, and Cs activation shows promise for high efficiencies (>20% at 180nm). Silicon based MCPs have qualities that make them preferable to glass MCPs (very low intrinsic background, low fixed pattern noise). Large 8cm Si MCPs have been fabricated with large open area ratio, good lifetest data has been obtained, and techniques for coating high temperature/robust photocathode layers have been explored. We also report on a novel MCP imaging readout scheme, the Cross Strip (XS). This anode uses charge division, and centroiding, of microchannel plate charge signals detected on two orthogonal layers of sense strips to encode event X-Y positions, time tags and signal amplitudes. The XS anode is fabricated as a multilayer ceramic/metal structure that can be implemented in a footprint that is not much larger than the active area, and may accommodate formats up to 10cm x 10cm. To date the XS scheme has been tested with a 32 mm x 32 mm prototype anode and customized electronics. This has demonstrated excellent resolution (<7μm FWHM, ~5k x 5k resolution elements (limited by the MCP pore size)) using low MCP gain (~4 x 105), with anode & electronics resolution of ~3μm FWHM.