In addition to providing the initial gamma-ray burst trigger and location, the Swift Burst Alert Telescope (BAT) will also perform an all-sky hard x-ray survey based on serendipitous pointings resulting from the study of gamma-ray bursts. BAT was designed with a very wide field-of-view (FOV) so that it can observe roughly 1/7 of the sky at any time. Since gamma-ray bursts are uniformly distributed over the sky, the final BAT survey coverage is expected to be nearly uniform. BAT's large effective area and long sky exposures will produce a 15 - 150 keV survey with up to 30 times better sensitivity than any previous hard x-ray survey (e.g. HEAO A4). Since the sensitivity of deep exposures in this energy range is systematics limited, the ultimate survey sensitivity depends on the relative sizes of the statistical and systematic errors in the data. Many careful calibration experiments were performed at NASA/Goddard Space Flight Center to better understand the BAT instrument's response to 15-150 keV gamma-rays incident from any direction within the FOV. Using radioactive sources of gamma-rays with known locations and energies, the Swift team can identify potential systematic errors in the telescope's performance and estimate the actual Swift hard x-ray survey sensitivity in flight. These calibration results will be discussed and a preliminary parameterization of the BAT instrument response will be presented. While the details of the individual BAT CZT detector response will be presented elsewhere in these proceedings, this talk will focus on the translation of the calibration experimental data into overall hard x-ray survey sensitivity.
The Burst Alert Telescope (BAT) aboard the Swift Gamma-ray Burst Explorer (scheduled for launch in January 2004) is a coded aperture telescope that includes an array of 32,768 CZT planar detectors, each 4 mm x 4 mm x 2 mm thick. The mobility-lifetime products for holes and electrons are used to characterize the charge transport properties of each detector and were measured during pre-flight calibration tests in early 2003. These values were found to vary over an order of magnitude from detector to detector. In addition to the charge transport models, other more empirical models are used to characterize the remaining components of the response of the BAT array to 15 - 150 keV gamma radiation from any direction in BAT's field-of-view (FOV). The full angular and spectral response of the array as a whole can be parameterized and the spectral response of the instrument over the range of angles and energies can thus be generated in an overall instrument response model. Here we present a preliminary instrument response model and discuss the general characteristics of the detector array. The software to generate the response matrix for BAT is currently under development.
The properties of 32k CdZnTe detectors have been studied in the
pre-flight calibration of Burst Alert Telescope (BAT) on-board the
Swift Gamma-ray Burst Explorer (scheduled for launch in January 2004).
After corrections of the linearity and the gain, the energy resolution
of summed spectrum is 7.0 keV (FWHM) at 122~keV. In order to construct
response matrices for the BAT instrument, we extracted
mobility-lifetime (μτ) products for electrons and holes in the
CdZnTe. Based on a new method applied to 57Co spectra taken at different bias voltages, μτ for electrons ranges from
5.0x10-4 to 1.0x10-2cm2V-1, while μτ for holes ranges from 1.0x10-5 to
1.7x10-4cm2V-1. We show that the distortion of the spectrum and the peak efficiency of the BAT instrument are well reproduced by the μτ database constructed in the calibration.
The observation of EUV spectra of multiply charged ions excited by an ultrafast capillary discharge suggest the possibility of developing an intense compact source of soft x- ray and EUV radiation for practical applications (EUV lithography, microscopy, etc.). The discharge is driven by a compact water transmission line having a very fast current pulse (12 ns FHWM, rise time approximately 1 ns). Spectra of OV-OVI ions from a polyacetal capillary and ArVII-ArVIII ions from an Ar filled capillary were observed and investigated in a spectral range of 100 - 300 Angstrom using a 1-m grazing incidence spectrograph. The resulting capillary discharge plasmas have an electron temperature of approximately 25 - 35 eV, which could be increased to approximately 100 eV using a more powerful transmission line.