Advances in Soft X-Ray Spectroscopy Using Charge - Devices

Details Advances in Soft X-Ray Spectroscopy Using Charge - Devices Advances in Soft X-Ray Spectroscopy Using Charge - Devices

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........17k&link_type=abstract

Thesis (PH.D.)--THE PENNSYLVANIA STATE UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-09, Section: B,

Astronomy and Astrophysics

Astronomy

6

Thin Gates, Floating Gate Output Amplifiers

Scientific paper

We present new results in the development of charge -coupled devices for X-ray astronomy. A charge-coupled device with two novel features (floating gate output amplifiers and thin gates) was built and tested for use as a non-dispersive X-ray spectrometer. The floating gate output amplifier has allowed us to achieve a readnoise of 0.9 electrons rms with 16 readouts per pixel. The soft X-ray quantum efficiency is enhanced compared with other front-side illuminated devices by using a novel thin gate structure. The combination of sub-electron noise and good soft X-ray quantum efficiency has enabled us to detect photons below the Si L edge (E < 100 eV) in photon counting mode. These low energy events are well separated from the readnoise floor. The measured energy resolution is 16.3 eV (FWHM) at 66 eV (Al L from rm Al_2O_3 ), 33.8 eV at C K_α (E = 277 eV) and 120 eV at Mn K_ α, (E = 5.9 keV). The soft X-ray quantum efficiency of this detector was measured using direct continuum synchrotron radiation. The spectrum of the synchrotron radiation can be accurately computed, so the observed pulse height spectrum is a direct measure of the detector response. A key function in modeling the efficiency is the energy dependence of the fraction of detected single pixel events. We find that a model including the effects of charge drift and diffusion plus two different depletion depths (corresponding to the collection and barrier phases under the gates) is required. With this model the absolute QE can be determined to within ~5% over the 500 eV to 4500 eV bandpass. This low noise, thin gate design is compared with the ACIS Backside Illuminated CCD for three different astrophysical observations below 1200 eV. The thin gate CCD has better energy resolution but lower quantum efficiency. We find that the thin gate CCD is generally superior to the ACIS backside illuminated CCD for spectroscopy between ~400 eV and 1000 eV, and below 100 eV (if both detectors could be operated in single photon counting mode in this energy range). Even if the CCD can only be operated in integrating mode below 100 eV, the thin gate CCD provides a viable alternative to backside illuminated CCDs or microchannel plates for EUV astronomy.

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