Study of Radiation Damage to Luminosity Counters during Run 2a

1. Study of Radiation Damage to Luminosity Counters during Run 2a Brendan Casey, Kayle DeVaughan, Yuji Enari, Richard Partridge, Sahal Yacoob April 21, 2006

2. Expectations • The possibility of scintillator radiation damage was explored when the Run II LM was designed • Charged particle flux: dN/dh ~ 4 gives ~1 MRad/r2/fb-1 • For inner edge of luminosity scintillator (3.3 cm) gives 90 kRad/fb-1 • Actual dose will be larger: • Charged particles from photon conversions • Low pT “curlers” • Secondary particles from charged particle interactions • MC study for LM TDR estimated dose at inner edge of the scintillator to be ~300 kRad/fb-1 Richard Partridge

3. Expectations • Radiation damage in plastic scintillator studied by Bross and Pla-Dalmau • Dominant effect is decreased light transmission in substrate • Dopants largely unaffected by radiation • Significant uncertainties in the impact of dose rate, annealing process, Richard Partridge

4. Expectations • TDR (1997): “At these radiation levels, we expect there to be some darkening of the scintillator closest to the beam pipe that may require the scintillator to be replaced every few fb-1 to avoid a loss in efficiency. We anticipate that replacing the scintillator wedges will be a relatively simple and inexpensive operation that can be performed during a typical summer shutdown.” Richard Partridge

5. Radiation Damage Studies • Visual Inspection • Light Transmission Measurements • Radioactive Source Rate Measurements • Cosmic Ray Pulse Height Measurements Richard Partridge

6. Visual Inspection • This is the show and tell part of the meeting • We have three scintillator samples • An old scintillator that was in the luminosity monitor and was radiated • An old scintillator that was a spare and has never been in the beam • A new scintillator that is ~ 6 months old • Can you identify which sample is which? Richard Partridge

7. Visual Inspection • On close examination, there is some browning of the old scintillator in the region nearest the beam pipe • Based on this observation, we decided to replace the LM scintillator Richard Partridge

8. Light Transmission Measurements • We used an HP spectrophotometer in Lab 6 to measure light transmission through both new and old scintillators • Typical Spectrum for new scintillator is shown below Richard Partridge

9. Fitting the Transmission Curve • Goal: measure attenuation in light due to radiation damage • First step: understand new counters with no radiation damage • The transmission measurements are sensitive to both bulk attenuation and surface condition, which vary point to point • We find that the following function gives a reasonably good fit for the undamaged counters • l0 = 407.7 nm and Dl = 3.2 nm parameterize the wavelength shifter absorption and are fixed for all fits • T0 and K parameterize the surface effects and are determined for each transmission measurement Richard Partridge

10. Outer inner Transparency Measurements • Have measured transmission at 7 locations • Measurements made on two new scintillators and two old scintillators • Determine surface transmission parameters by fitting the following wavelength ranges: • 500 – 570 nm • 590 – 650 nm • 660 – 700 nm • These ranges exclude the shorter wavelengths where we expect to see radiation damage and observed absorption lines Richard Partridge

15. Transmission Measurements • Measurements establish decreased transmission at shorter wavelengths for scintillator that has been irradiated • Largest effect is seen in postions 1 and 2, which are closest to the beam • To quantify the magnitude of the absorption, we compare the measured transmission with our fit to the transmission curve Richard Partridge

20. Transmission Conclusions • The old scintillator shows increased absorption due to radiation damage • The increase in absorption is largest for the measurements made at small radius, where the radiation dose is expected to be highest • The peak of the scintillator light output is at 430 nm, where we see ~5% absorption in the 1.6 cm counter thickness • The typical path length for charged particles hitting at the inner edge of the scintillator is 9 cm • Thus, reduced light transmission in the irradiated scintillator should reduce the measured light output at small radii by a factor of: Richard Partridge

21. Source Rate Measurements • The counting rate from a radioactive source was measured for each counter • PMT signal was required to fire a a fixed threshold discriminator • HV on PMT was adjusted to equalize gain * QE • Count rate was measured with a scaler • Measurements were taken with both the old and new scintillator for each PMT • Measurements were taken at 7 different source positions • Challenge: Converting count rates to light yield • Spectrum of energy deposit is very non-linear • Assume that reduced light yield is equivalent to reduced PMT gain • Fit counting rate vs PMT HV plots • Use PMT gain vs HV to relate counting rate to light yield (G~V7.1) Richard Partridge

22. Counting Rate vs HV Fits • Measured for a reference counter Richard Partridge

23. Relative Light Yield Richard Partridge

24. Average Light Yield Richard Partridge

25. Source Measurement Summary • Have modeled the very non-linear relation between counting rate and light yield by using variation in counting rate with PMT gain • See substantial counter-to-counter variations • On average, old counters have 74-79% light yield of new counters at small radius • Consistent with expected value of ~75% from transparency measurements • Decrease in light yield near PMT is 8-10% • Would have expected ~5% from transparency measurements Richard Partridge

26. Cosmic Ray Measurements • Three old counters and three new counters were brought to Brown and studied using our cosmic ray test stand • Test stand uses high precision TOF counters to determine position, arrival time of cosmic rays hitting the luminosity counter • Position accuracy ~ 1 cm • Accumulate usable distributions of pulse height vs position in ~ 8 hour run Richard Partridge

27. So, Which is the Old Counter?? Richard Partridge

28. Average Cosmics in Position Bins Richard Partridge

29. Old / New Ratio Richard Partridge

30. Cosmic Summary • Data consistent with a 10-15% drop in light yield in region nearest beam pipe relative to PMT region Richard Partridge

31. Can we Compensate? • Yes! Richard Partridge

32. Conclusions • Modest loss of light yield in region closest to beam pipe • Consistent with expectations from 10 years ago • Generally consistent results from Transparency, Source, and Cosmic measurements • Plan: • Evaluate as luminosity increases • Current detector likely sufficient for Base luminosity goal • Probably would need one more scintillator replacement if Design luminosity goal is obtained • Such a scintillator replacement is a modest undertaking, likely can be performed during a scheduled shutdown • Will compensate for loss of light yield by increasing HV as necessary Richard Partridge