Saving the photons: mapping X-rays by position-tagged spectrometry

Since the early years of X-ray spectrometry in electron microscopes, mapping the locations of chemical elements has been important. The X-rays needed in large numbers for this are rare, owing to poor production efficiency compared with electron signals, and at risk of loss by many mechanisms such as missing the limited solid angle of the detector, absorption before reaching the detector and pulse pile-up conventional digital mapping hardware reduces the information contained in the X-ray spectrum at each pixel to the itegrated counts from a few regions of interest.The acquisition technique of position-tagged spectrometry eliminates the conflict between the desire to see full frame X-ray images quickly versus the analytical advantages of having complete spectra for each pixel. As the beam is scanned rapidly relative to traditional X-ray mapping, photons are counted in a virtual 3-D multichannel analyser on disk, preserving both spatial and spectral information. Along with the sophisticated post-processing allowed by storing an entire spectrum per pixel, a unique degree of dynamic interaction with the developing data is made possible by integrating many short scans instead of using a single long dwell time at each pixel.