L. Rocks

Complex impedance measurements of calorimeters and bolometers : Correction for stray impedances

Impedance measurements provide a useful probe of the physics of bolometers and calorimeters. We describe a method for measuring the complex impedance of these devices. In previous work, stray impedances and readout electronics of the measurement apparatus have resulted in artifacts in the impedance data. The technique allows experimenters to find an independent Thevenin or Norton equivalent circuit for each frequency. This method allows experimenters to easily isolate the device impedance from the effects of parasitic impedances and frequency dependent gains in amplifiers.

Next generation of silicon-based x-ray microcalorimeters

After the design of the calorimeter array for the high-resolution x-ray spectrometer (XRS) on the original Astro-E was frozen, new fabrication techniques became available and our understanding of these devices continually increased. We are now able to complete the optimization of this technology and, potentially, to increase the capability of new XRS instrument for Astro-E2, our on-going sounding recket experiments, and possible further applications. The most significant improvement comes from greatly reducing the excess noise of the ion-implanted thermistors by increasing the thickness of the implanted region.

Observed limits on charge exchange contributions to the diffuse X-ray background

We present a high resolution spectrum of the diffuse X-ray background from 0.1 to 1 keV for a ~1 region of the sky centered at l=90, b=+60 using a 36-pixel array of microcalorimeters flown on a sounding rocket. With an energy resolution of 11 eV FWHM below 1 keV, the spectrums observed line ratios help separate charge exchange contributions originating within the heliosphere from thermal emission of hot gas in the interstellar medium. The X-ray sensitivity below 1 keV was reduced by about a factor of four from contamination that occurred early in the flight, limiting the significance of the results. The observed centroid of helium-like O VII is 568+2-3 eV at 90% confidence. Since the centroid expected for thermal emission is 568.4 eV while for charge exchange is 564.2 eV, thermal emission appears to dominate for this line complex, consistent with much of the high-latitude O VII emission originating in 2-3 x 10^6 K gas in the Galactic halo. On the other hand, the observed ratio of C VI Ly gamma to Ly alpha is 0.3+-0.2. The expected ratios are 0.04 for thermal emission and 0.24 for charge exchange, indicating that charge exchange must contribute strongly to this line and therefore potentially to the rest of the ROSAT R12 band usually associated with 10^6 K emission from the Local Hot Bubble. The limited statistics of this experiment and systematic uncertainties due to the contamination require only >32% thermal emission for O VII and >20% from charge exchange for C VI at the 90% confidence level. An experimental gold coating on the silicon substrate of the array greatly reduced extraneous signals induced on nearby pixels from cosmic rays passing through the substrate, reducing the triggered event rate by a factor of 15 from a previous flight of the instrument.

Microelectronic Fabrication of Transition Edge Sensors

Achieving optimum spectral energy resolution in conventional scanning electron microscopes (SEM) can be accomplished by using microcalorimeters. Improvements in device design are being studied, although many research groups have developed fabrication techniques that produce consistent results [1]. Here, we discuss the fabrication method that we are using to produce microcalorimeters using superconducting transition edge sensors (TES) as thermometers, and some of the roadblocks we are observing.