Entrance collimators

In both neutron and y-ray detection, the shielding of the detector is extremely important. Especially in the neutron detection measurements, the long target-to-detector distance (2-4 in) which is required to obtain velocity resolution via the neutron TOF technique means many more neutrons are produced than are actually scattered from the sample and then detected. These extraneous neutrons create a disastrous background unless the detector is adequately shielded. We have accomplished this with a large cylindrical shield which contains a lead cavity surrounded by Li2C03 loaded paraffin. The entrance collimator has steel and lead liners the main detector shield weighs about 2000 kg (see B in Fig. 1). 

The unsymmetrical, or Straumanis, method of film loading is shown in Fig. 6-5(c) and Fig. 3-12. Two holes are punched in the film so that it may be slipped over both the entrance collimator and the beam stop. Since it is possible to determine from measurements on the film where the incident beam entered the film circle and where the transmitted beam left it, no knife-edges are required to make the film-shrinkage correction. The point X (20 = 180°), where the incident beam entered, is halfway between the measured positions of lines 4,4 similarly, the point Y (20 = 0°), where the transmitted beam left, is halfway between lines 1,1. The difference between the positions of X and Y gives W, and 0 is found by proportion ... 

Imaging on the entrance collimator. A lens is placed immediately in front of the entrance slit and should image the relevant part of the radiation source on the entrance collimator (Fig. 16A). This has the advantage that the entrance slit is homogeneously illuminated, however, stray-radiation may easily occur inside the spectrometer. The distance between the source and the entrance slit (a) is given by the magnification required, as ... 

Fig. 16. Illumination of the optical spectrometer with lenses. (A) Imaging on the entrance collimator, (B) illumination with intermediate image, (C) imaging on the entrance slit.

Image on the entrance slit. With the aid of one lens this is also possible. Here the structure of the source appears on the entrance collimator and on the detector. This allows spatially-resolved line intensity measurements to be made when a detector with two-dimenional resolution such as a photographic emulsion or an array detector (see Section 2.2.2) are employed. This type of imaging is often used for diagnostic studies. 

A dispersive spectral apparatus contains an entrance collimator, a dispersive element and an exit collimator (for an example discussion, see Refs. [48, 49]). 

With an entrance collimator a quasi parallel beam is produced from the radiation coming through the entrance aperture, which has a width se and a heigth he. The entrance collimator has a focal length and a width W. The diffraction slit width (so) and the diffraction slit heigth (h0) are the half-widths given by ... 

The number of photons entering the monochromator is determined by the space angle of observation and in its turn determines the flux at the entrance collimator Oen) as ... 

X is the width (or diameter) of the source, W is the width of the entrance collimator and its focal length. The/number of the lens than is given by ... 

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