Mean incident beam collimation

A typical Debye camera is shown in Fig. 6-1. It consists essentially of a cylindrical chamber with a light-tight cover, a collimator to admit and define the incident beam, a beam stop to confine and stop the transmitted beam, a means for holding the film tightly against the inside circumference of the camera, and a specimen holder that can be rotated. 

Another consideration is the beam cross section. Typical beam cross section at SSRL (SPEAR ) is on the order of several to tens of mm, while that on typical APS lines is in the tens of microns. For practical sample size, e.g., 2 cm, this means that the beam size cannot be larger than about 40 pm at a critical angle of 2 mrad. Any larger part of the beam will miss the sample (Fig. 6) and probably incur dramatic noise production. In practice this greatly reduces the utility of the GI experiment as so much incident beam is rejected by collimation (e.g., with SPEAR with a 2 mm vertical height beam, 98% of the beam is discarded). Thus the APS beams are ideal for samples of this size, and all of the output from an insertion device can in principle be directed on a GI sample of a few mm. 

The polychromatic beam from the cold source is monochronaated by a helical slot velocity selector which selects neutrons of +1- 10% about a mean wavelength determined by the rotation speed of the drum. Typically, a mean wavelength of -0.525 nm is used. The neutrons are then collimated by a series of movable guides. Guide sections are inserted into, or removed from, the beam depending on the incident beam divergence required. A position-sensitive neutron detector is located inside an evacuated tube and can be moved to any distance between 1.1 and 35 m from the sample. 

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