Niels J. Westergaard

Graded d-spacing multilayer telescope for high-energy x-ray astronomy

A high energy telescope design is presented which combines grazing incidence geometry with Bragg reflection in a graded d-spacing multilayer coating to obtain significant sensitivity up to --6O keV. The concept utilizes total reflection and first order Bragg reflection in a graded d-spacing multilayer structure in a way that higher energies are reflected from the deepest layers in the stack. The specific design presented in this paper is based on Ni/C and Mo/C structures with dspacings ranging from 25A to 100 A. X-ray reflectivity data obtained with Cu Kc1 (8. 05 keV) are presented from the first graded d-spacing structures of this kind.

Cosmic ray propagation in a closed galaxy

A simple model of cosmic ray propagation is proposed from which the major experimental results can be derived: The model reproduces the observed nuclear abundances and accounts for the observed changes of nuclear composition with energy, the high degree of isotropy of cosmic ray flux at all energies, and the high degree of its constancy throughout the history of the Solar System. It is consistent with the observed size distribution of extensive airshowers, the intensity and energy distribution of the electron component, and the diffuse emission of γ-rays and radio waves.The model is characterized by the two basic assumptions: (1) that cosmic rays have been injected at an unchanging rate by sources located in the galactic spiral arms and (2) that a large-scale magnetic field retains all particles in our galaxy, where they interact with interstellar gas, so that all complex nuclei are finally fragmented and their energy dissipated in meson production and electro-magnetic interactions.

X-ray calibration of the SODART flight telescopes

The on- and off-axis imaging properties and effective area of the two SODART flight telescopes have been measured using the expanded beam x-ray facility at the Daresbury synchrotron. From on-axis measurements the encircled power distribution and the point spread function at three energies 6.627 keV, 8.837 keV and 11.046 keV have been measured using a one-dimensional position sensitive detector. We found that the point spread function can be presented well by a function which is a sum of a Gaussian divided by the radius and two exponential terms where the first has a 1/e value close to 2 arcmin and the other a 1/e value of ca. 15 arcmin. The data have been used to calculate the half power diameter (HPD) for three different SODART focal plane detectors, the high energy proportional counter (HEPC) with a field of view (FOV) of 65 arcmin, the low energy proportional counter (LEPC) with a FOV of 33 arcmin and the 19 element solid state detector array (SIXA) with a FOV of 18 arcmin. We found that the HPD decreases with increasing energy due to poorer figure of the outermost mirrors. The HPD falls in the range from 2.4 to 3.8 arcmin depending on energy and FOV. Data have also been obtained on the on- and off-axis effective area at all three energies and compared to that obtained from a raytracing of an ideal telescope configuration. We found that the measured on-axis effective area integrated over a FOV of 105 arcmin is ca. 65% of the area predicted from an ideal geometry. Finally the one- dimensional detector data has been used to obtain the radial dependence of the on-axis HPD and the on-axis effective area and the data from the two-dimensional position sensitive detector has been used to obtain the azimuthal dependence of the on-axis HPD and the on-axis effective area.

Electrons in a closed galaxy model of cosmic rays

We consider the consistency of positrons and electrons with a propagation model in which the cosmic rays are stopped by nuclear collisions or energy losses before they can escape from the Galaxy (the closed-galaxy model). The fact that we find no inconsistency between the predictions and the data implies that the protons which produce the positrons by nuclear reactions could have their origin in a large number of distant sources, as opposed to the heavier nuclei which in this model come from a more limited set of sources. The closed-galaxy model predicts steep electron and positron spectra at high energies. None of these are inconsistent with present measurements; but future measurements of the spectrum of high-energy positrons could provide a definite test for the model. The closed-galaxy model also predicts that the interstellar electron intensity below a few GeV is larger than that implied by other models. The consequence of this result is that electron brems-strahlung is responsible for about 50% of the galactic gamma-ray emission at photon energies greater than 100 MeV.

Doubly curved imaging Bragg crystal spectrometer for x-ray astronomy

An x-ray spectrometer which is sensitive in the 0.5–7-keV energy range and is intended for use onboard astronomical satellites has been studied. The Bragg reflected rays from a doubly bent crystal positioned downstream of the focal plane of a grazing-incidence concentrator are focused along the axis of a position sensitive detector. A high throughput concentrator with densely nested thin-foil conical mirrors approximating a Wolter-1 system can be used without loss of spectral resolution. Other advantages include simultaneous (scanning free) coverage of the spectral range of interest, nearly stigmatic imaging, and crystal-limited resolution.

Studies of multilayers and thin-foil X-ray mirrors using a soft X-ray diffractometer

A versatile x-ray diffractometer is described in detail. Two applications to the study of x-ray optical elements are presented. The first is a Bragg reflection study of state-of-the-art multilayers deposited both on conventional Si-wafer substrates and on superpolished substrates such as fused quartz and electroless nickel. These data are compared to data previously obtained at FeKα. The second study is a reflectivity and scattering study of various thin-foil x-ray reflectors proposed for up-coming x-ray satellite missions. All the data have been obtained at MgKα = 1.2536 keV.

A versatile three/four crystal X-ray diffractometer for X-ray optical elements: Performance and applications

Abstract A versatile X-ray diffractometer for the study of X-ray optical elements such as grazing incidence mirrors, crystals and X-ray gratings has been built and put into operation at the Danish Space Research Institute. The diffractrometer is built on a 1.5 m long granite bench with the X-ray source located at one end of the bench where it can be rotated around a fixed vertical axis. The beam defining elements are perfect crystals of Si, Ge or quartz. With these it is possible to define a highly collimated beam of a few arcsec fwhm in the scattering plane. Examples of measurements on various X-ray optical elements are presented. The deconvolution of the experimental data is discussed in general terms and as an example the results of a calculation of the diffractometer resolution function for a four crystal setup are presented.

The X-ray imager on AXO

DSRI has initiated a development program of CZT X-ray and gamma-ray detectors employing strip readout techniques. A dramatic improvement of the energy response was found operating the detectors as the so-called drift detectors. For the electronic readout, modern ASIC chips were investigated. Modular design and the low-power electronics will make large area detectors using the drift strip method feasible. The performance of a prototype CZT system will be presented and discussed. One such detector system has been proposed for future space missions: the X-Ray Imager (XRI) on the Atmospheric X-ray Observatory (AXO), which is a mission proposed to the Danish Small Satellite Program and is dedicated to observations of X-ray generating processes in the Earth’s atmosphere. Of special interest will be simultaneous optical and X-ray observations of sprites that are #ashes appearing directly above an active thunderstorm system. Additional objective is a detailed mapping of the auroral X-ray and optical emission. XRI comprises a coded mask and a 20]40 cm2 CZT detector array covering an energy range from 5 to 200 keV. ( 2001 Elsevier Science B.V. All rights reserved.

Development of large-area CZT detector systems

DSRI has initiated a development program of CZT x-ray and gamma ray detectors employing strip readout techniques. A dramatic improvement of the energy response was found operating the detectors as so-called drift detectors. For the electronic readout, modern ASIC chips were investigated. Modular design and the low power electronics will make large area detectors using the drift strip method feasible. The performance of a prototype CZT system will be presented and discussed.

Optical measurements of the XSPECT mirrors and the assembled mirror modules for the SODART x-ray telescope on the Spectrum-X-Gamma satellite

The Danish Space Research Institute is supplying high- throughput x-ray mirror modules (MM) for the SODART telescopes to be flown on the Russian Spectrum-X-Gamma mission. We have finished the process of building two flight units and one spare unit. We have measured 5500 flight quality single mirrors using laser scanning. Edge deformations have been found to be a persistent phenomenon giving rise to reduced effective area and alignment accuracy. The degree of foil straightness is a function of the radius of curvature. Detailed optical measurements of the MMs is a control of the mechanical alignment achieved by repeated adjustments between the stages of assembly. The results of these investigations are discussed and compared with x-ray calibration measurements. A combination of the optical results and x ray results is fed into a raytracing code resulting in a reliable prediction of the imaging capabilities of the SODART telescopes.