Pop-up detector calculations


Darren Dowell -- (626)395-6675 (office), -2600 (lab), 796-8806 (FAX)
Last modified Sunday, 13-Feb-2000 07:45 PDT
cdd@submm.caltech.edu

SHARC II Bolometer Specifcations -- January 17, 2000

The detector requirements for SHARC II were specified in papers submitted for publication in the proceedings of the Imaging at Radio Through Submillimeter Wavelengths conference in Tucson in June, 1999. The papers were submitted in July 1999 (Dowell et al.) and September 1999 (Moseley et al.). Moseley et al. describe the detectors in detail, while Dowell et al. only give a brief summary of SHARC II.

Moseley et al. (1999), PostScript, 8.3 Mbytes
Moseley et al. (1999), PDF, 0.3 Mbytes

Moseley et al. (1999), SHARC II tables only, PostScript, 0.6 Mbytes
Moseley et al. (1999), SHARC II tables only, PDF, 0.05 Mbytes

Dowell et al. (1999), PostScript, 0.06 Mbytes
Dowell et al. (1999), PDF, 0.06 Mbytes

Preliminary calculations for HAWC -- May 24, 1999

For the calculations, I am using the program described in a Postscript document that I sent out in February. The tables are missing, but the definitions of terms are in there. The program does not calculate the photon noise; the photon background only enters the calculations as a power level.

Numbers from Harvey for HAWC detectors:

  • R0 = 700 ohms
  • T0 = 30K
  • full-area thermometer (1 mm x 1 mm x 3000 angstroms)
  • G = 1.8x10^-9 W/K at 0.3 K, G has T^1 dependence
  • base temperature of 0.2 K
  • load resistors: 400 Mohms, at 4.2 K
  • FET noise: <= 8 nV/sqrt(Hz) at high frequencies
  • Q (background power) = 80-140 pW

    Given these parameters, I calculate the following electrical NEP at 10 Hz for an optimally-biased detector with Q = 80 pW: 1.1x10^-16 W s^1/2. Details.

    If the background power is instead 140 pW, the calculated NEP is: 1.3x10^-16 W s^1/2. Details. Note the different bias voltage.

    Suppose the temperature dependence of G is instead T^3, but the G at 0.3 K is still the same. The calculated NEP for Q = 80 pW is: 1.3x10^-16 W s^1/2. Details.

    Preliminary calculations for SHARC II -- May 24, 1999

    For a 5" x 5" pixel with 40% peak quantum efficiency, the expected background power for SHARC II is:
  • Q = 75 pW,
    the photon NEP (electrical) is 4.4x10^-16 W s^1/2 in good weather. The base temperature is:
  • TC = 0.32 K.
    For SHARC II, it is hoped that the bolometers will be quiet down to 0.1 Hz, so that frequency is used for the 1/f noise calculations.

    To coordinate with HAWC as much as possible, assume the following parameters are the same:

  • RL = 400 Mohms
  • full-area thermometer
    I will assume a T^3 temperature dependence of G.

    What if everything else were the same: R0 = 700 ohms, T0 = 30 K, G = 1.8x10^-9 W/K at 0.3 K?
    At 0.1 Hz, NEP = 6.0x10^-16 W s^1/2. Details. A huge loss in sensitivity results.

    Now reduce G0 by a factor of 7: NEP = 2.6x10^-16 W s^1/2. Details. Not too bad. Note small R of detector, which is good for AC-biased readout at ~50 Hz, where the FET noise will be low and the RC will be small, but not as good for DC-biased readout at 3 Hz.

    Now increase T0 to 40K and decrease G0 even more: NEP = 2.0x10^-16 W s^1/2. Details. There is now more latitude for noisier FETs.

    Finally, optimize R0 (higher than HAWC) and G0 for T0 = 40 K: NEP = 1.7x10^-16 W s^1/2. Details. The detector impedance is now at the target value of 10 megaohms.

    CONCLUSION: SHARC2 will at least need a much lower G0 than HAWC, by about a factor of 10, due primarily to the different operating temperature. Also, it is desirable to increase the detector impedance somewhat over HAWC.

    Go to SHARC II home page...