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Count-Rate Performance for X-Ray Microcalorimeter Spectrometer

Count-Rate Performance for X-Ray Microcalorimeter Spectrometer . Richard Kelley NASA/GSFC. Counting Rate Issues. Most of the IXO science will be from what we think of as faint sources. But note that a 5 mCrab source corresponds to about 625 cps on the array!

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Count-Rate Performance for X-Ray Microcalorimeter Spectrometer

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  1. Count-Rate Performance for X-Ray Microcalorimeter Spectrometer Richard Kelley NASA/GSFC

  2. Counting Rate Issues Most of the IXO science will be from what we think of as faint sources. But note that a 5 mCrab source corresponds to about 625 cps on the array! Probably higher given latest effective area calculations. Further, we are still promoting IXO, so we need to be as inclusive as possible to the user community. X-ray binary sources as very strong (typically 0.1-1 Crab), but sometime several times higher, including x-ray busters. 1 Crab ~ 125,000 cps. So, we need to consider counting rates much higher than what we normally like to deal with for x-ray calorimeters

  3. Dealing with Pulse Pile-up: Event Grades (developed for XRS) • Grade events by relative arrival times: • Apply different pulse height algorithms according to grade. • Δt > T2 - “High-Res” • Full template • T1 < Δt < T2 – “Mid-Res” • Truncated template • Δt < T1, - “Low-Res” • Simple PHA (e.g., low-pass filter and single sample)

  4. Energy Resolution vs. Analysis Record Length We generally ignore the DC, or “zero”, frequency bin for the optimal filter. , where f0 = 1/T and T = record length Mid-res Hi-res Device model: α = 80 τ = C/G = 0.006 sec ΔE0= 2.5 eV τcrit = 1.5 x 10-4 sec

  5. Single-Pixel Hi-res Fraction vs. Rate

  6. Point Spread Function on array – spreading out the counts Gaussian PSF with HPD of 5 arcsec 20 m f/l  10.3 arcsec/mm 0.3 mm pixels  3 arcsec pixels 5” HPD a b c d

  7. Output rate vs. Input rate

  8. Event Grade Fractions vs. Input Rate

  9. Bright Source Diffuser – Slumped Micro Channel Plate (MCP) Inner array Outer array R. Willingale 2008

  10. Performance of Diffuser ~ 1180 pixels in “donut”

  11. Output Rates vs. Input Rate with Diffuser 40,000 cps 38% transmission of diffuser

  12. Comparison of Hi-Res Event Rate with and without Diffuser with BSDO without alone

  13. Summary & Conclusions Use digitized data for optimal filtering based on pulse inter-arrival times (“event grades”). Minimizes dead-time overall – all events are counted (up to rates higher than the Crab) Need to develop algorithm for low-res events (e.g., low-pass and sample peak, or weighted average near peak, etc.) For bright, Galactic sources, we should have a filter position with a MCP diffuser. Can observe sources with flux comparable to Crab with 10% of the events in Hi-Res + Mid-Res. Cross-over point is about 2000 cps input. Invest some effort to optimize the performance of the diffuser.

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