Image-forming detectors to observe fine spatial distributions of auroral X-rays

Observations of fine auroral X-ray images at balloon altitude are important for studies of acceleration and precipitation mechanisms of energetic particles in the magnetosphere. Balloon-borne auroral X-ray image-forming detectors with some angular resolution are proposed to observe spatial distributions of auroral X-ray sources, as fine as possible and also as a function of time. Two systems of image-forming detectors are being developed. One system consists of two sets of Nal (Tl) scintillation detectors with very thin scintillators. The first set has a collimator with multi-pinholes, giving an angular resolution of 5°. The second has a collimator with a single larger pinhole, and it eliminates the redundant images unavoidable in the multi-pinhole type of detector. The incident position of an X-ray on the thin scintillator is determined by using the signals from four photomultiplier tubes attached to the scintillator. The other system is constructed with the Nal (Tl) scintillator viewed by a microchannel plate photomultiplier tube (MCP PMT) through a Fresnel lens. The pinhole aperture of the collimator is selected according to the auroral X-ray intensity, giving an angular resolution of 2°, 5°or 10°. It is expected that auroral X-ray images can be observed with an angular resolution of 5°and with a time resolution of 1s, for the moderate activity of an auroral X-ray burst