Johann spectrometer

A schematic of a typical Johann spectrometer on a tokamak is shown in Fig. 8.1. Typically, the detectors limit the resolution, as long as gas detectors are used. 

Johann-type crystal spectrometers record an energy interval according to the size of the source simultaneously, if a position-sensitive X-ray detector of corresponding extension is available. The use of spherically-bent crystals allows partial vertical focusing, which increases the count rate by a factor of almost two for Bragg angles around 55°. 

Estimates of shifts of spectra in curved crystal geometries are often calculated for an ideal detector located on the Rowland circle. However, the detection surface is usually fiat and therefore cannot lie on the Rowland circle. Detectors located on a fixed length detector arm will additionally travel off the Rowland circle as the Bragg angle is scanned unless the crystal curvature is simultaneously scanned (which raises problems of stress hysteresis). Conventional shifts calculated for detection on the Rowland circle do not agree with shifts at a flat extended detector and this systematic error can be 100-200 ppm for any Johann curved crystal spectrometer. We have incorporated fiat surface detectors located off the Rowland circle into the general theory [18,17]. 

Fig. 2. Johann geometry crystal spectrometer coupled to the Oxford EBIT. Not to...

Our experimental setup consists of a Johann-type X-ray spectrometer [15] coupled to the Oxford electron beam ion trap [16], as shown in Fig.2. 

The use of a Johann-type curved crystal spectrometer and an electron beam ion trap allow us to obtain X-ray specta of the Lyman-a lines of medium Z... 

Fig. 8.1. Overview of X-ray spectrometer (Johann type) installed at a tokamak...

Fig. 7 The central component of a high resolution emission spectrometer is a Bragg crystal that spectrally analyzes the fluorescence from the sample and reflects it onto a photon detector. This particular example utilises a spherically bent Johann type crystal in a one-to-one focusing Rowland geometry in connection with a solid state detector. The solid angle of collection can be increased by increasing the number of analyzer crystals, all aligned to intersect at the two focal points. ...

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