PTB News 1/2004 (111 kB) PTB News 1/2004, English edition Issue May 2004The extremely sensitive Lyman-alpha-photodetectors for NASA’s TWINS mission (Two Wide-angle Imaging Neutral-atom Spectrometers) were calibrated at PTB's radiometry laboratory at the Berlin synchrotron radiation facility BESSY in cooperation with the University of Bonn.
The TWINS Mission is to provide the first three-dimensional image of the plasma surrounding the Earth. Therefore different detector systems – payloads on two SMEX-satellites (Small Explorer) – are to picture the Earth's magnetosphere stereoscopically from two points in different orbits around the Earth. In order to create a model of the hydrogen geocorona, it is also necessary to register the Lyman-alpha intensity distribution of the terrestrial hydrogen exosphere.
For this purpose, two detector systems were developed by the Institute of Astrophysics and Extraterrestrial Research at the University of Bonn. The detectors are extremely sensitive in the wavelength region around 122 nm, so that the tests and calibrations at PTB's radiometry laboratory at BESSY had to be carried out in the picowatt radiant power range (10-12 Watt). This required PTB’s scale for spectral sensitivity in the UV and vacuum-UV spectral region to be expanded by five orders of magnitude to lower radiation power levels by exploiting the possibility of adjusting the beam current in the BESSY storage ring very precisely. Take-off for both satellites is scheduled for 2004/2005.