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Development of measurement services to calibratewavelength-dispersive detectors and spectrometers for low- to medium-energy x rays (10 to 300 keV): Bent-crystal spectrometers (and other x-ray diagnostics) are being developed and increasingly deployed to measure x-ray production in hot plasmas as diagnostics in fusion-energy research, solar and astrophysical studies, weapons effects and testing programs, and advanced medical-imaging applications. Quantitative results with these wavelength-dispersive detectors and spectrometers require absolute calibrations of the response or efficiency of each energy bin. There are currently no alternatives for such calibrations traceable to national standards.Such a service will be realized through a novel extension of NIST’s mature service to calibrate ionization chambers (which are not energy dispersive) in terms of air-kerma from low- and medium-energy x rays. By normalizing selected spectra obtained in the previous project to the directly realized air kerma, absolute fluence spectra can be determined. A scheme is being developed in which sets are chosen of overlapping spectra with peaks (where the uncertainties are expected to be smallest) that span the energy range at which to calibrate the wavelength-dispersive instrument, with the aim of minimizing the uncertainties across the energy range.  This will be the first service of its kind and is presently being requested from DOE laser facilities such as the National Ignition Facility (NIF).

Lawrence Hudson