Use of a Mylar filter to eliminate vacuum ultraviolet pulse pileup in low-energy X-ray measurements

Abstract

We describe a method to reduce vacuum ultraviolet (VUV) pulse pileup (PPU) in X-ray pulse-height Silicon Drift Detector (SDD) signals. An Amptek FAST SDD, with C1 (Si3N4) window, measures bremsstrahlung emitted from PFRC-2 plasma to extract the electron temperature (Te) and density (ne). The C1 window has low transmissivity for photons with energy below 200 eV though will transmit some VUV and soft X-ray photons, which PFRC-2 plasmas abundantly emit. Multi-VUV-photon PPU contaminates the interpretation of x rays with energy > 100 eV, particularly in a low-energy exponential tail. The predicted low transmissivity of ∼1 μm thick Mylar [polyethylene terephthalate (PET)] to photons of energy <100 eV led to the selection of Mylar as the candidate filter to reduce VUV PPU. Experiments were conducted on an X-ray tube with a graphite target and on a quasi-Maxwellian tenuous plasma (ne ∼109 cm-3) with effective temperatures reaching 1500 eV. A Mylar filter thickness of 850 nm is consistent with the results. The Mylar-filter-equipped SDD was then used on the PFRC-2 plasma, showing a substantial reduction in the low-energy X-ray signal, supporting our hypothesis of the importance of VUV PPU. We describe the modeling and experiments performed to characterize the effect of the Mylar filter on SDD measurements.