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  • Astrophysics on the White Light Beamline

    A research team led by Dr. James E. Lawler of the University of Wisconsin-Madison has been using the SRC White Light beamline to develop a new experiment using ultra-high sensitivity absorption to determine accurate relative oscillator strengths of singly ionized iron (Fe+).

    Picture of Scott and Krista Krista and Scott at the end of the White Light Beamline

    Emission and absorption lines from iron group elements dominate many astrophysical spectra. Often the lines obscure more interesting elements which can be used for studying isotopic anomalies or the age of the universe. While the Fe+ database is adequate for measuring Fe abundance, it is not extensive enough or accurate enough to reliably "subtract out" the Fe lines from astrophysical spectra in order to study less abundant elements. Finding accurate oscillator strengths for Fe and other iron group elements remains a serious problem in modern astronomy. Lawler and his students hope to remedy this with their work at Aladdin.

    The experiment uses a hollow cathode discharge as a low pressure source of iron ions serving as the absorbing sample for light from the Aladdin continuum source. The light is separated with a 3m focal length echelle vacuum spectrometer and analyized with a CCD (charge coupled device) detector array which detects the emitted light. Modern CCD arrays provide enormous sensitivity advantages in absorption spectroscopy. A detector array, by monitoring all spectral channels simultaneously, makes an absorption experiment insensitive to low frequency noise or drift in the continuum source. The CCD array is superior to the sensitivity of a single-channel sequentially scanned absorption experiment which is often limited by such low frequency noise or drift in the continuum source. Another important limit on the sensitivity of an absorption experiment is Poisson statistical noise. A detector array provides a second major advantage by collecting good photon statistics simultaneously in all spectral channels. Since the glow discharge emits light at many of the same wavelengths at which it absorbs light, a digital subtraction technique is used to discriminate against line emission and detect only the transmitted radiation from the continuum source with absorption features from the glow discharge. The experiment extends high sensitivity absorption to near the limits of existing technology.

    Absorption Data

    Absorption spectrum of singly ionized vaporized iron. The high resolution Echelle monochromator on the beamline allows a relative absorption error less than 0.2%.

    The experimental apparatus uses the best available tools. The 3m echelle spectrometer achieves spectral resolving powers up to 350,000. The Aladdin electron storage ring provides a bright, spectrally smooth, broad bandwith continuum in the ultra-violet (UV) and vacuum ultraviolet (VUV) wavelength ranges. The hollow cathode discharge plasma has a rich population of both neutral and metastable atoms, providing a diverse collection of transitions that can be measured. All of these factors combined allow accurate measurements of relative oscillator strengths from a common lower level in the UV and VUV.

    Oscillator strengths for neutral iron have been measured accurately by Blackwell et. al. ( Mon. Not. R. Astr. Soc. 186, (1979) 633). By reproducing those results as well as measurements for well known lines in Fe+, the reliability of the experiment has been established. The experiment can now continue on to regions of the Fe+ spectrum that are not well understood.

    The design and implementation of this experiment has been thesis work for Scott Bergeson who will be graduating at the end of the summer and moving on to a job at NIST. He is turning the experiment over to Krista Mullman who will complete the measurements on Fe+ and extend the energy range into the VUV.

    Aladdin-Editor Brian Tonner