NEUTRON COLLIMATORS
Efficient neutron collimators can provide enhanced capabilities for neutron experimentation. There are two areas where neutron collimation can be beneficial: first, the probe neutron beam itself can be shaped by the collimators to reduce beam divergence leading to image blurring on the detector; and second, the collimators can be used for scatter rejection when they are placed between the object under study and the detector. Neutron collimators are presently used in neutron scattering experiments and are usually of the Soller slit type, comprising an array of absorbing films separated by neutron-transparent spacers or honeycomb-like packed structures.
The 10B- and/or Gd-loaded glass systems developed for radiography are now being applied as efficient and compact thermal neutron collimators. The ratio of the transmission channel length-to-width is one of the key parameters determining the efficiency of collimation. The larger this ratio, the higher the collimator performance within a given structure.
Collimator Modeling
Most neutron beams have some angular divergence, so that the predicted curves are necessarily convolved with the angular profile of the beam itself. Small angle reflections from the pore walls are not taken into account in these curves, although our experimental measurements indicate that such reflection does not have a large effect on the collimator performance - beyond moderate smoothing of the triangular shape of the rocking curve.
Our model has shown that a collimator doped with Gd atoms is more efficient than one doped with 10B. This is because the detection of low energy conversion electron and gamma ray emission from the 157Gd (η,γ) 158Gd reaction is difficult to discriminate from background events in the detector. These collimators are entirely passive, requiring no special activation processing (as with MCPs), to develop electronic characteristics.
The following figure shows the predicted rocking curves using cold neutrons for a natGd2O3 doped collimator with 8 micron pores on 11 micron centers and an L/D of 75:1.
The transmission as a function of the incidence angle is shown below for a much thicker collimator with thermal neutrons. Here the pore size is 10 µm pores on 12 µm centers and an L/D of 250:1. This system can demonstrate collimation within ± 0.25°.
The image degradation by the collimators has also been estimated when they are used for scatter rejection. The conclusion is that for a 10 micron pore collimator there will be no observable image degradation for any detectors with spatial resolution above 20 micron. As neutron detection reaches 20 micron levels we expect to reduce the periodic structure of the collimators to match the detector performance and perform scatter rejection with no associated image degradation.
Angular Selective Neutron Detection
Combining a thermal neutron collimator and the MCP-based neutron imaging detector into a single device can produce a thermal neutron detection system with high detection efficiency, spatial resolution, and angular sensitivity. Such a system is shown in the following figure.
The angular selectivity of such a device can be switched off and on by applying a retarding voltage to the front of the MCP detector. This accepts or repels the secondary electrons from being or to be amplified by the lower MCP stack. By applying a reverse bias to the front MCP and placing a neutron transparent electron collecting element in front of the first MCP, one can measure the ratio of "on-angle" and "off-angle" neutrons. All of this can be done by controlling the bias voltages applied without any mechanical changes. A very attractive feature is this system's compactness: less than 10 mm thick.
Neutron collimators are playing an important ancillary role in neutron scientific studies - through beam dispersion improvements with high L/D collimators, and through use of thinner collimators, rejection of off-axis neutrons scattered from the target. Alignment of the collimator will of course be critically important in attaining optimal performance for the entire imaging system.
Support for Nova's neutron collimator technology has primarily been supported by the US Department of Energy.
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