Doppler-tuned x-ray spectrometer

Abstract

A novel spectrometer designed especially for a fast foil‐excited beam source has been studied and tested. The spectrometer makes use of a large Doppler shift to bring the emission spectrum into coincidence with known features of the absorption spectrum of a suitable filter material. Tuning is accomplished by varying the angle between the beam and the detector, with the filter material between the source foil and the detector. Knowledge of the beam velocity, angle, and the absorption spectrum of the filter is sufficient to determine the emission spectrum from the beam. The resolving power easily exceeds 10³ and probably can exceed 10⁵ with some effort, making this spectrometer competitive with conventional crystal diffraction devices. A prototype has been constructed, and tested at the Berkeley HILAC. In the initial experiments, the 2¹P₁ − 1¹S₀ line of heliumlike argon at 3.3 keV, produced by passing a 412 MeV argon beam through a thin carbon foil, was brought into coincidence with the silver L_III absorption edge near 3.4 keV. The expected sudden jump in the count rate when the detector was moved past 58° was observed. This device can be used either for wide range, low resolution scans, or for narrow range, high resolution work, and its range of applicability can be easily extended to higher and lower energies.