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Figure 1.
Precipitating electrons produce X-rays through the bremsstrahlung process.
These X-rays can be observed by balloons, but do not penetrate all the way
to the ground.
Typical auroral X-rays that reach balloon altitudes have energies
between about 20 and 120 keV.
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Figure 2.
The lead pinhole collimator focuses X-rays like a camera. (A different
collimator uses multiple
pinholes in a coded-aperture mask to allow more X-rays to be detected.)
The sodium iodide scintillator emits visible photons when hit by an X-ray.
Photomultiplier tubes (PMTs) detect photons and send signals to the
electronics.
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Figure 3.
The distribution of light among the PMTs tells the location of the X-ray.
The total of the four PMT responses tells the energy of the X-ray.
The spatial resolution is about 10 km for aurora at 100 km altitude.
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