Detector spatial resolution

The improved neutron detector spatial resolution has been a recent advance, with user instruments first available at the end of 2006 and 2007 at NIST and PSI, respectively. The work can be classified as proof-of-principle,9,10 in situ measurement of the steady-state through-plane water content during fuel cell operation," 13 and dynamic through-plane mass transport measurements.14,15... 

Figure 4. SIT vidicon detected signalfrom a 250 x 750-nm portion of the image of a backlit 125-pm screen imaged with unity magnification at the focal plane. The SIT vidicon signal was obtained with the maximum attainable detector spatial resolution, with each picture element, or with pixel, 25 pm in diameter.

Other limitation for the spatial resolution can be found in the detector. A limited number of pixels in the camera array can be a reason for pure resolution in the case of a big field of view. For example, if field of view should be 10 by 10 nun with camera division 512x512 pixels the pixel size will be approximately 20 microns. To improve the relation of the field of view and the spatial resolution a mega-pixel sensor can be used. One more limitation for the spatial resolution is in mechanical movement (rotation) of the object, camera and source. In the case of a mechanical movement all displacements and rotations should be done with accuracy better than the spatial resolution in any tested place of the object. In the case of big-size assemblies and PCB s it is difficult to avoid vibrations, axle play and object non-planarity during testing. 

Detectors in EM can be categorized according to their different spatial resolution or in relation to the time it takes to actually see and process the signal (real-time/on-line capability). 

Future developments of this instrumentation include field emission electron sources at 200-300 kV that will allow better elemental detectability and better spatial resolution. Multiple X-ray detectors having large collection angles will also improve elemental detectability in X-ray microanalysis. The higher accelerating... 

This technique is invasive however, the particle can be designed to be neutrally buoyant so that it well represents the flow of the phase of interest. An array of detectors is positioned around the reactor vessel. Calibration must be performed by positioning the particle in the vessel at a number of known locations and recording each of the detector counts. During actual measurements, the y-ray emissions from the particle are monitored over many hours as it moves freely in the system maintained at steady state. Least-squares regression methods can be applied to evaluate the temporal position of the particle and thus velocity field [13, 14]. This technique offers modest spatial resolutions of 2-5 mm and sampling frequencies up to 25 Hz. 

In summary, the NLE technique offers a concepmally new approach to observe NFS. Existing limits for time resolution could be overcome by a microfocused synchrotron beam (as planned for PETRA III) and by detectors with high spatial resolution and background from SAXS could be suppressed by employing high-energy transitions and crystalline sapphire as rotor material. 

In practice, the user selects the spatial resolution combined with the readout time of a modern 2D detector. The lower resolutions are realized by binning12 of pixels on the detector. Typical ranges are 1024 x 1024 pixels with 0.7 s readout time and 2048 x 2048 pixels with 7 s readout time. A low resolution should only be selected if the high-resolution readout time is too long for the experiment. 

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