Onboard detection of intrinsic Ly. alpha. radiation from a neutral particle beam
- Montana State Univ., Bozeman, MT (USA). Dept. of Physics
- Los Alamos National Lab., NM (USA)
We consider photometers onboard a hydrogen neutral particle beam (NPB) space platform which monitor the instrinsic radiation from excited atoms in the NPB in flight. The radiation of choice is the Lyman {alpha} (Ly{alpha}) line 1216 A, emitted when the beam's {approx equal} 7% fraction of H (2S) atoms is motionally quenched in the earth's magnetic field. At nominal 20-MeV NPB energy, the Ly{alpha} radiation persists at 1% of its initial intensity out to 100 m along the beam, and is red-shifted to 1494 A when viewed form behind the exciting NPB pulse. A photosensitive detector with a {approximately} 5{degree} field of view, placed adjacent to the NPB exit port and viewing the NPB pulse along its limb, shows marked changes in detected Ly{alpha} intensity when the NPB axis shifts direction. If the NPB pulse is nominally 50 MA times 100 {mu}s, and if the detector is a 1-cm{sup 2} array of 25 {mu}m times 25 {mu}m photosensitive pixels located in the focal plane of an 8-cm diam. f/1 LiF lens, then pixels at the brightest part of the beam image are illuminated by up to 2300 Ly{alpha} photons per NPB pulse. The pixel quantum efficiency, optics transmission losses, and a geometric correction for viewing angle reduce the maximum count rate to {approximately} 200 photoelectrons per pixel per pulse under realistic operating conditions, and at a limb-viewing angle (i.e., angle between beam and detector axes) of {approx equal} 6 mrad. At smaller viewing angles the pixels count rate declines rapidly, but rapidly, but becomes sensitive to small angular shifts in the NPB axis direction. In the limit of shot-noise on the pixel count, and at optimum viewing angle ({approx equal} 0.54 mrad), we find that a single pixel can sense beam-axis shifts of {approx equal} {plus minus}50 {mu}rad. 8 refs., 7 figs., 2 tabs.
- Research Organization:
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- DOE/DP
- DOE Contract Number:
- W-7405-ENG-36
- OSTI ID:
- 6856684
- Report Number(s):
- LA-11776-MS; ON: DE90010341
- Country of Publication:
- United States
- Language:
- English
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