E. Picatoste, D. Gascon PMT afterpulsing and SPD trigger 1 Looking to the problem from the instrumentation point of view LHCb Calorimeter Meeting – CERN – April 21th 2010

Afterpulsing in PMTs (literature) Definition: PMT 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. – Therefore afterpulse probability AP is the probability to find an afterpulse for each incoming photon, that is to say the rate for single photoelectron ilumination – AP depends on the size of the afterpulse AP depends on the high voltage (photocathode and first dynodes tension) 2

Afterpulsing in PMTs AP rate depends on the level of the afterpulse 3

Afterpulsing in PMTs (literature) AP distributions for different gain100K 4

Afterpulsing in PMTs (literature) – Two types of afterpulsing time distribution: Exponential decay Ionized rest gases Peaks Ions travelling from fixed positions Surfaces ions in dynodes 5

Afterpulsing in PMTs (literature) 6 AP for 3 phe negligible after 2 us for different PMTs

Afterpulsing in PMTs (literature) AP depends on the HV – Very difficult to predict, depends on PMT – AP seems to be ion feedback to photocathode and firsts dynodes – Electric field in photocathode and first dynode region seems to be critical HV Divider Size of the PM 7

What about our PMT? Measurements at CERN are performed at too high HV (890 V) BUT tests at CERN see almost no AP with only clear fiber and PMT !!! – Even exceeding nominal PM HV !!!!!!!! Hamamatsu says that the AP of 800 V is 4 % for > 4 phe Our nominal HV is below 600 V Our AP threshold would be around 15 phe The AP in nominal conditions should be 1 to 3 orders of magnitude smaller It should correspond to something 15 phe 8

What about our PMT? Test ongoing at BCN Preliminary results VFE output Th < 0.5 MIP 600 V > 1 hour acquisition with infinite persistence LED pulse of > 1 MIP Ch3 is VFE output Ch2 is LED trigger About 3000 waveform/s No AP after 1 us AP < 1e-7 after 1 us (40 Bx) 9

What about our PMT? We could perform a more precise estimation of AP up to 30 cycles Using VFE test DAQ LED pulse of about 1 MIP (cycle 0) AP for after pulse > 0.5 MIP Cumulative AP (sum from cycle 2 to cycle 600 V: < 0.01 % !!! 10 Average of 8 ch

What about our PMT? Making PMT life very hard: EXTREME CONDITIONS LED pulse >> 10 MIP 800 V > 1 hour acquisition with infinite persistence Ch3 is VFE output Ch2 is LED trigger About 3000 waveform/s Almost no AP after 2 us 11

After pulse and SPD trigger We can do some estimations on the relationship between AP and tails in SPD trigger based on some assumptions The AP 1MIP probability of an afterpulse larger than 0.5 MIP generated by a MIP signal is AP*nphe MIP – AP is the probability of an AP generated by a single photoelectron – nphe MIP =30 Trigger is generated by the coincidence of n SPD hits We consider that afterpulses are generated by events with a typical multiplicity m We assume afterpulses duration is almost 1us (d=30 clock cycles) – AP typically follows an exponential decay distribution – Time constant:  – Cm,n are the possible combinantions of m elements in groups of n 12

After pulse and SPD trigger For m=100,  =6 cycles and SPD>2 13

After pulse and SPD trigger For m=100,  =6 cycles Increasing coincidence 14

After pulse and SPD trigger For SPD>2,  =12 cycles Varying multiplicity 15

Conclusions Trying to find if SPD trigger tails is consistent with possible AP ? AP < 0.01 % is too low to explain tail in SPD trigger Possible correlations to be studied – AP is supposed to be ion feedback – Several channels could be fired by a single ion feedback in a MaPMT??? Anyway SPD>2 tail duration (d) does not fit with AP duration – For beam-beam interactions d is about 400 cycles (10 us) – PMT AP duration is < 1 us !!!!!! 16

Conclusions There are PM afterpulses but … Main component of tail in SPD trigger does not seem to come from neither PM afterpulses nor electronics oscillation Consistent with Miriam’s analysis Is there something outside? 17

Conclusions Other candidates in the instrumentation? Jacques reminded that we have seen a long decay time constant in cosmic ray test It’s true Scintillator afterglow ? Anyway probability seems too low… 18 Average time waveform of the signal produced by cosmic rays