X-ray calorimetry

A story on: Charge exchange x-ray emission: Astrophysical observations and potential diagnostics (AIP Conference Proceedings 1525 (1), 49-54)

Background

Some comets grow a tail as they approach the sun. Originating from the interaction of (a) solar wind: bursts of charged particles or ions flowing radially outwards from the sun, and (b) the comet’s thin atmosphere if any at all.

A clue to the composition of a comet’s atomsphere may be in its emission spectrum, since the neutral gases of which it might be composed of should be able to capture an electron from the incident solar wind. During this interaction, a recaptured electron may bounce and scatter among different atomic states while emitting photons in random directions. Based on the energy of the emitted light and its relative intensity (i.e. its spectrum), it is possible to study a comet’s atmosphere.

Experiment

Unfortunately, comets and similar bodies rarely visit our solar neighborhood (at human timescales anyways), so collecting high-quality data to study their atmosphere is hard. Nevertheless, it is possible to recreate the conditions found in the solar wind and comet interaction.

This AIP conference proceedings reports a study on the fundamental electron recapture process done by three different neutral gases which may be present in comets: Helium (He), deuterium (diatomic Hydrogen), and Kripton (Kr). To this end, we emulate the solar-wind in the lab by accelerating highly-charged Carbon ions (C6+) which are made to collide onto a buffer gas located right below a high resolution spectrometer. The emitted spectrum is a proxy for the multiple atomic Lymann transitions happening during the charge exchange process as such process is followed by an electronic decay event with soft X-rays (energies of a few keV).

The spectrometer in question was a calorimeter. Calorimetric detection is typically used to increase the photon energy resolution at the cost of slow detection rates (limited by the thermal load of the absorbed radiation on the active detection element). In this case, opting for a higher resolution allowed a precise determination of the relative strength between different emission peaks, which in turn allows for a calibration of the existing and future observed spectra from astrophysical observations.