PMT AFTERPULSE STUDIES

1
PMT AFTERPULSE STUDIES

• By
• Ugur Akgun
• The University of Iowa

2
Outline

• What is PMT Afterpulse and its Rate?
• The University of Iowa Afterpulse Tests
• Hamamatsu Afterpulse Tests
• TB2004 Analysis for PMT Afterpulses
• Conclusion

3
Definitions and Facts

• Definition Afterpulses are spurious pulses that
  appear in the wake of true pulses.
• FACT Every true pulse may be followed by one or
  more afterpulses.
• FACT The afterpulse size(charge) does not vary
  with respect to the incoming pulse height.
• FACT For bigger incoming light more afterpulses
  appear, So total charge ratio stays the same.
• i.e. Finding the ratio of afterpulse to any main
  pulse does not mean anything, because IT IS
  DIFFERENT for every incoming light intensity.
• i.e. Finding the number of afterpulses for any
  incoming pulse does not mean anything, either.
  Because IT IS DIFFERENT for every light
  intensity, too.
• Only meaningful AFTERPULSE RATE definition is
• RATE (SQp,i / SQa,i) x 100

4
The References

• The ratio does not change with the number of true
  pulses, or the amount of charge they contain.
  When the charge transferred by each true pulse is
  very small (i.e. SPE level), that transferred by
  each afterpulse may be as large or even larger.
  However, as proportionally fewer true pulses are
  then followed by afterpulses, the charge ratio
  remains the same (Phillips, Photomultiplier
  Tubes Principles and Applications).
• For further information please read
• Hamamatsu and Philips PMT books.
• B.H. Candy Rev. Sci. Instr. 56, 183 (1985)
• G.A. Morton et al. IEEE Tran. Nucl.
  Sci.NS-14No.1, 443, (1967)
• R. Staubert et al. NIM 84, 297 (1970)
• S.J. Hall et al. NIM 112, 545 (1973)

5
The University of Iowa Afterpulse Tests

• The tests are performed on 3 different PMT Types
  (R7525, R6427, R1398).
• We Tested 83 R7525 (HF PMTs).
• Blue LED (with 420 nm peak) is used as light
  source. The light intensity set to be 2TeV.
• The LED driver is running with 100 kHz, providing
  the gate signal as well.
• The gate signal is generated by LeCroy 222 Dual
  Gate Generator.
• The measurements are taken by LeCroy 2249 ADC.
• Every measurement contains 10,000 ADC counts.
• We DID NOT use any amplifier to eliminate the
  noise factor.

6
The University of Iowa Afterpulse Tests
7
Iowa Results
R7525

• Every PMT we tested shows afterpulses.
• There is no magic PMT that gives no afterpulses.

R6427
R1398
8
Iowa Results
Afterpulses appear to localize at 3 different
regions. These locations shift 10 with different
PMT high voltages.
9
Iowa Results
Delay of the so called afterpulse regions with
respect to the main signal.
10
Iowa Results
Afterpulse Rates of these 3 regions for 83 R7525
PMTs
11
Iowa Results
PMT afterpulse rates stay around 1 for different
High Voltage values.
12
Iowa Results
Single Photoelectron level No charge
accumulation over pedestal level !
13
Iowa Results
Signal and its afterpulse
14
Iowa Conclusion

• There are afterpulses for R7525 as every other
  PMT.
• The afterpulses appear to localize at 3 distinct
  time delay regions.
• The charge accumulation rate due to these
  afterpulses are less than 1.
• It does not vary with respect to the batch
  number.
• It does not vary with respect to the incoming
  light intensity or frequency.
• It does not vary with PMT high voltages.

15
Hamamatsu Tests
Charge Mode

• Hamamatsu performed afterpulse tests in Charge
  Mode and Counting Mode.
• They used 400 GeV light for charge mode, 80 GeV
  for counting mode.

Counting Mode
16
Hamamatsu Charge Mode Setup
Charge coming from the afterpulses are integrated
on a 9500 ns range!!
17
Hamamatsu Counting Mode Setup
Number of afterpulses coming 150ns after the main
pulse are counted
18
Hamamatsu Results
19
Hamamatsu Results
Afterpulse rates for different PMT gains.
20
Hamamatsu Results
Afterpulse Rates for different light intensities.
21
Hamamatsu Conclusion

• R7525 Afterpulses are not different than any
  other PMT Hamamatsu preoduces.
• The afterpulse tests done in 2001 give the same
  result with 2004 tests.
• Afterpulse rate does not change with PMT gain
  (HV) or incoming light intensity.
• In the charge mode the integrated afterpulses on
  9500ns range produce
• 2-3 rate.

22
TB2004 PMT Afterpulse Analysis
20 Time Slice Data, and its afterpulses.
23
PMT Afterpulses at 60TS Data
The 3 peaks can easily be seen.
24
PMT afterpulse rates for three afterpulse regions
at 1150V.
Signal width is 50ns, but we integrated 200ns
region for third afterpulse rate. That is why it
is higher, Same time slice gives lt1, always.
25
PMT afterpulse rates for three afterpulse regions
at 1350V.
There is one problematic tower (tower 20). There
might be problem with readout system.
26
Integrated PMT Afterpulse Rate
The total charge accumulated by the afterpulses
on 1200ns range.
27
TB2004 Analysis Conclusion

• The afterpulses could be seen with 20TS data, but
  60TS data helped to see the whole structure.
• The 3 afterpulse regions give around 1 rate.
• Whole (1200ns) spectrum gives 3-4 rate.
• In 25 ns time slice (LHC Runs) we always get LESS
  THAN 1 AFTERPULSE RATE.