Gas Scintillation Proportional Counter - GSPC

We have developed high-pressure Gas Scintillation Proportional Counters (GSPC) for the focus of our balloon-borne hard-x-ray telescope. The device has a total active diameter of 50 mm, of which the central 20 mm only is used, and is filled with xenon + 4% helium at a total pressure of 106 Pa giving a quantum efficiency of greater than 85% up to 60 keV. The detector entrance is sealed with a beryllium window, 3-mm thick, which provides useful transmission down to 6 keV, well below the atmospheric cut-off at balloon float altitudes. Scintillation light exits the detector via a UV transmitting window in its base and is registered by a Hamamatsu position-sensitive crossed-grid-readout photomultiplier tube.

Further information (published in SPIE, PDF format) on the initial testing and preliminary measurements of light yield, energy resolution and spatial resolution show that a spatial resolution of 0.5 mm FWHM or better should be achievable up to 60 keV, and this is well matched to the angular resolution and plate scale of the mirror system. The energy resolution has been measured to be around 5% at 22 keV. A paper in PDF format on in-flight cosmic-ray background simulations, based on an older detector configuration, is also available. An even earlier proportional counter design, called MIXE, used a mechanical collimator instead of focusing optics.

Below are photos of an earlier version of the GSPC (left) and the photomultiplier tubes (right).