Quality assessment of in-pile guides by energy resolved neutron imaging

The energy resolved pinhole-imaging method can be applied for surveying the condition of neutron beam extraction systems without the need to remove them. The only condition is the access to a position where direct view of the source is available. Generally this is possible at some stages of the guide system installation or during replacement of first out of pile guide sections. Some facilities have removable sections downstream of the main shutters where the proposed measurement can be performed. The measurement is based on position sensitive neutron detection in time of flight regime. In case of continuous sources, a chopper is required, while in case of pulsed sources the time frame is provided by the source period. The proposed method can be considered an in situ reflectometry experiment. To each pixel of a reflection region corresponds a pixel of the direct view. The ratio of the neutron counts in the image pixel with respect to the corresponding direct pixel gives the reflectivity of the involved mirror plate at the angle which can be determined from the experiment geometry and at the wavelength which results from the flight length and time. The measurement gives an average reflectivity over the mirror surfaces viewed through the given pinhole position and finally the transfer function of the investigated guide sections. The accuracy of the results is affected by the alignment accuracy of the guide sections because the reflection angles are determined with respect to the theoretical positions of the mirror surfaces. As an example, the results obtained at a 100 mm high and 25 mm wide cold neutron beamline at the Budapest Neutron Centre are presented.