Xcov 20 Deadtime Measurement

Deadtime Measurement Number 1

  1. . Using the Texas photometer as a basis, choose two apertures (one smaller, one larger) and arrange to illuminate them both with a light source that shows stability of counting rate for several minutes. I found an LED that had been on for long enough to come to temperature equilibrium works fine, if it is powered by a regulated power supply.
  2. . Take two readings of the counting rate, one through each aperture, the higher chosen so that the dead time should be significant at that counting rate -- between 500,000 cps and 2,000,000 cps worked for me.
  3. . Now decrease the illumination intensity, so the counting rate drops about a factor of 10 or more. After the LED is stable again, take two more readings, through each of the same apertures.
  4. . There are four data points (the readings) and four unknowns:
  5. I worked out the equations for this, but I don't have them in front of me ... I can probably find them somewhere. But the exercise is not very hard, and is useful to know about.
I recommend, as an exercise to the student, that you work out the simultaneous equations and see if you get something you can understand ... and believe in. They might even be the same as the ones I worked out many years ago ... To first order, if the dead time is negligible at the lower counting rate, then the observed ratio calibrates the aperture area ratio, which should be the same at the higher rate. Any discrepancy can be blamed on the dead time. The proper way to do it, though, is to solve the simultaneous equations for the dead time explicitly. I got the 20 ns dead time figure for the current system that way. Repeated measurements gave the same dead time to within 10 percent.
ADVANTAGES: Takes little time - Can be done any time
DISADVANTAGES: Less accurate - Sensitive to measurement errors - Requires constant artificial light source

Deadtime Measurement Number 2

  1. On a nice clear evening, wait until the Sun is about 8 degrees below horizon.
  2. Find an aperture which gives you a count rate of about 1,000,000 cps at a blank piece of sky. Use a filter, if useful. ALWAYS start with the smallest aperture when trying to approach this count rate.
  3. Start recording data. Switch back and forth between this aperture and a second one with at most a quarter of the other's diameter; use 2x10 seconds of integration in each aperture. Continue switching until sky counts drop below about 3000 cps in the larger aperture. Then do longer integrations in both apertures to get good statistics for measuring the aperture size ratio.
  4. Take a number of dark integrations, then quit this run.
  5. Check that there is no star in the larger aperture.
  6. To determine the deadtime, first subtract the mean dark count from the data. Separate the curves for the different apertures.
  7. Determine the aperture area ratio from the count rate ratio in the last (flat) part of the curves when you used longer integration times to get better photon statistics. Multiply the curve with the smaller count rates by this ratio.
  8. Bring the steep parts of the curves to agreement by using trial deadtimes, choose the best value. Small adjustments to the aperture area ratio may be necessary. Or plot a graph of ratio of counts in big to (counts in small interpolated to time of counts in big) versus counts in big. This will be a straight line whose negative slope will be (1-alpha)tau where alpha is the intercept = ratio of apertures and tau is the dead time in seconds (convert counts per 10 sec to counts per sec before plotting).
  9. Obviously, you can also do this in morning twilight in reverse order.
ADVANTAGES: More accurate - Incorrect measurements are easily recognized
DISADVANTAGES: Might take away precious observing time - Requires clear skies
Send comments to wetmaster. Last updated on 6 Nov 2000.