Kurtis L. Dietz

First Images from HERO: A Hard-X-Ray Focusing Telescope

We are developing a balloon-borne hard X-ray telescope that utilizes grazing-incidence optics. Termed HERO, for High-Energy Replicated Optics, the instrument will provide unprecedented sensitivity in the hard X-ray region and will achieve millicrab-level sensitivity in a typical 3 hr balloon-flight observation and 50 μcrab sensitivity on ultralong-duration flights. A recent proof-of-concept flight, featuring a small number of mirror shells, captured the first focused hard X-ray images of galactic X-ray sources. Full details of the payload, its expected future performance, and its recent measurements are provided.

HERO: high-energy replicated optics for a hard-x-ray balloon payload

We are developing high-energy grazing-incidence optics for a balloon-borne hard-x-ray telescope. When completed the instrument, termed HERO for High Energy Replicated Optics, will have 200 cm2 effective collecting area at 40 keV and <EQ 30 arcsec angular resolution. The payload will offer unprecedented sensitivity in the hard-x-ray region, with milliCrab level sensitivity on a one-day balloon flight and 100 microCrab on an ultra-long-duration flight. While the full science payload is scheduled for flight in 2002, an engineering/proving flight is currently awaiting launch. This flight, consisting of just two mirror modules, each containing three nested shells above a pair of gas scintillation proportional counter focal plane detectors, is intended to test a newly designed gondola pointing and aspect system and to examine the stability of optical bench designs. This paper provides an overview of the HERO program.

A large-area microstrip-gas-counter for X-ray astronomy

We have developed a large-area coded-mask telescope for hard-X-ray astronomy. The heart of the instrument is an imaging microstrip-gas-counter of active area 30 cm × 30 cm and filled with 2 × 105 Pa of xenon + 2% isobutylene. Fabricated on a single sheet of borosilicate glass, 1 mm thick, the microstrip features fine anodes (10 μm) and interleaved cathodes from which the position sensing is derived. Rear pickup electrodes provide the second coordinate. Full details of the instrument and its performance are presented. A first flight, from a high-altitude balloon, is scheduled for the Spring of 1997.

Shielding simulations for the Advanced X-Ray Astrophysics Facility (AXAF)

The Advanced X ray Astrophysics Facility (AXAF) contains two focal-plane science instruments -- the microchannel-plate high-resolution camera (HRC) and the AXAF CCD imaging spectrometer (ACIS). Each of these instruments provides two low-internal- background detectors, one for imaging and the other for reading out AXAFs objective transmission gratings. Taking maximum advantage of the low internal background of these instruments requires shielding against external electromagnetic and particle radiation. To optimize the shielding within weight constraints, we performed extensive numerical simulations of the photon transport and of the particle transport and activation within the AXAF. We discuss the simulations and report the results of the shielding study. With the adopted shielding design, the particle-induced background and the x-ray-induced background are each comparable to or less than the anticipated internal background of AXAFs imaging detectors (HRC and ACIS). Thus, the predicted total non-imaged background is very low -- less than 0.2 counts arcsec-2 day-1 for the HRC and even lower for the ACIS half-arcsec angular resolution, this low background has negligible effect upon point-source detection, for reasonable observation durations.

Background in X-ray astronomy proportional counters

The authors report the results of an investigation into the nature of background events in proportional counters sensitive to X-ray photons having energy >

HERO: program status and first images from a balloon-borne focusing hard x-ray telescope

HERO is a balloon payload featuring shallow-graze angle replicated optics for hard-x-ray imaging. When completed, the instrument will offer unprecedented sensitivity in the hard-x-ray region, giving thousands of sources to choose from for detailed study on long flights. A recent proof-of-concept flight captured the first hard-x-ray focused images of the Crab Nebula, Cygnus X-1 and GRS 1915+105. Full details of the HERO program are presented, including the design and performance of the optics, the detectors and the gondola. Results from the recent proving flight are discussed together with expected future performance when the full science payload is completed.

Neutron induced background in the MIXE X-ray detector at balloon altitudes

The MIXE X-ray astronomy detector has experienced high backgrounds on balloon flights although well shielded against atmospheric gamma-rays and charged particles. Monte Carlo simulations show that this can be due to neutron-induced backgrounds in the payload arising from cosmic-ray proton interactions and atmospheric albedo neutrons.

Marshall Space Flight Center imaging x-ray experiment (MIXE)

The MIXE includes a state-of-the-art imaging proportional counter which utilizes fluorescence gating in order to achieve high sensitivity in the bandwidth from the xenon k-edge (35 keV) to 100 keV. The current detector system includes a parallel amplification stage to allow for improved energy resolution without sacrificing spatial resolution. Another novel feature is the molybdenum/stainless steel pressure vessel, which is an important factor in achieving low background rates. In this paper we compare the results of Monte Carlo simulations with laboratory experiments and present data obtained during a recent balloon flight from Palestine, Texas.

Performance of a multistep fluorescence-gated proportional counter for hard X-ray astronomy

The results from the first flight of our proportional counter in an imaging telescope led us to rebuild the detector in anticipation of a Fall 1992 flight. We have used a Penning gas mixture (xenon + 1% isobutylene) and introduced a preamplification region to improve the energy resolution. We have rebuilt the pressure vessel making novel use of molybdenum as the housing material in order to reduce the residual instrument background, particularly in the fluorescence-gated mode for which the detector design has been optimized. We have also increased the sensitive gas depth from 9 cm to 14 cm to further increase the sensitivity to both fluorescent pairs and conventional singles. Our calibrations have shown that the overall energy resolution of the detector has been enhanced by a factor of 2, and we predict that the sensitivity at float will increase by a factor of 3 in the 50 - 70 keV energy band.

Background in a balloon-borne fluorescence-gated proportional counter

The results of an analysis of the background in a fluorescence-gated proportional counter operating over the energy range 3-150 keV are presented. It is found that the dominant background component is that produced by high energy qamma-rays that penetrate the shields and undergo multiple scattering in the detector body, resulting in photoelectric absorption in the detector gas. A careful choice of materials and thickness can move the peak of this emission outside of the detector sensitive range, thereby dramatically reducing the residual background.