The National Institute of Standards and Technology (NIST) seeks a microwave resonator accessory add-on for its existing Bruker Biospin ELEXSYS-II E500 electron spin resonance spectrometer, intended to enable in situ quantitative measurements of atomic-scale defects in materials. The resonator must be a double rectangular cavity (TE104 mode) with an unloaded Q value of at least 7000, a nominal frequency of 9.7 GHz, and full compatibility with the existing spectrometer’s software and hardware. This is a combined sources sought notice and notice of intent to sole source to Bruker Biospin, as they are the only known provider capable of meeting the requirement without system compromise. No specific budget amount or performance location is mentioned in the description. Respondents must submit capability information as soon as possible, preferably before the notice’s closing date, and be registered in SAM to be considered for any future solicitation.