PMT AFTERPULSE STUDIES

By

Ugur Akgun

The University of Iowa

Outline

• What is PMT Afterpulse and its Rate?

• The University of Iowa Afterpulse Tests

• Hamamatsu Afterpulse Tests

• TB2004 Analysis for PMT Afterpulses

• Conclusion

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 = (ΣQp,i / ΣQa,i) x 100

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)

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.

The University of Iowa Afterpulse Tests

Iowa Results • Every PMT we tested

shows afterpulses.• There is no magic PMT

that gives no afterpulses.

R7525

R1398

R6427

Iowa Results

Afterpulses appear to localize at 3 different regions. These locations shift %10with different PMT high voltages.

Iowa Results

Delay of the so called afterpulse regions with respect to the main signal.

Iowa Results

Afterpulse Rates of these 3 regions for 83 R7525 PMTs

Iowa Results

PMT afterpulse rates stay around 1% for different High Voltage values.

Iowa Results

Single Photoelectron level; No charge accumulation over pedestal level !

Iowa Results

Signal and its afterpulse

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.

Hamamatsu Tests

• Hamamatsu performed afterpulse tests in Charge Mode and Counting Mode.

• They used ~400 GeV light for charge mode, ~80 GeV for counting mode.

Charge Mode

Counting Mode

Hamamatsu Charge Mode Setup

Charge coming from the afterpulses are integrated on a 9500 ns range!!

Hamamatsu Counting Mode Setup

Number of afterpulses coming 150ns after the main pulse are counted

Hamamatsu Results

Hamamatsu Results

Afterpulse rates for different PMT gains.

Hamamatsu Results

Afterpulse Rates for different light intensities.

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.

TB2004 PMT Afterpulse Analysis

20 Time Slice Data, and its afterpulses.

PMT Afterpulses at 60TS Data

The 3 peaks can easily be seen.

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 <1%, always.

PMT afterpulse rates for three afterpulse regions at 1350V.

There is one problematic tower (tower 20). There might be problem with readout system.

Integrated PMT Afterpulse Rate

The total charge accumulated by the afterpulses on 1200ns range.

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.