Proposal for the after-pulse effect suppression  Observation of pulses and after-pulses  Shape measurement  Algorithm  Results  Efficiencies for after-pulse recognition for pulse detection  Conclusion

Lyon group, December Signals of (after-)pulses  Observations pulse: ns at the basis (scintillating effect and light transport mainly) after-pulse much shorter: less than 10 ns at its basis (tube effect only) proposal for AP identification on-line neutralization of the residual AP, suppression of after-triggers (AT) possible treatment of prepulses (direct impact on PMT cathods)

Lyon group, December Test bench in lab for shape recognition

 Suppress the pedestal (PED), choose a value for R, reduction factor  Algorithm triggered by each Discri signal (THR) - point the channel CM (TM) with the largest amplitude AM - search for T1 (  TM) and T2 (  TM) at A = AM / R, starting from CM T1 (T2) is the time with amplitude between a i A/R or with amplitude a i followed by a i+1 < a i and a i+2 < a i+1 - Compute the width W = T2 – T1 - Plot the correlation W vs AM Algorithm for P and AP recognition CM (TM) amplitude time AM T1T2 T1T2 W W AM/R = A CM (TM)

 Observation from 2300 HV data, 1 MIP and 11 MIPs (R = 4) for P: W larger for 11 MIPs than for 1 MIP for AP: W similar for 11 and 1 MIP  Distributions W(AP) independent on the number of MIPs (P amplitude) which gave rise to them a cut can be set to sign APs Correlation Width (at A/R) vs Amplitude (2300) AP (1+11)

 Observation from 2000 HV data, 1 MIP and 11 MIPs (R = 4) for P: W larger for 11 MIPs than for 1 MIP for AP: W (11 MIP) smaller than W from MIPs Correlation Width (at A/R) vs Amplitude (2000) AP (11)

 Observation from 2300 HV data, 1 MIP and 11 MIPs W increases with R for P: W larger for 11 MIPs than for 1 MIP at a given R for AP: W similar for 11 and 1 MIP W(P) – W(AP) increases with R  Observation from 2300/2000 HV data W(P) and W(AP) increase with HV a cut can be set to sign APs  W(P) vs HV similar to W(AP) vs HV (measured at R = 4) Correlation mean Width at A/R vs R no linearity

 For a given HV, W(P) depends on the number of MIPs, W(AP) das not  When the pulse charge (P) increases, the after-pulse (AP) number increases “after-pulses with long delay are caused by the positive ions which are generated by the ionisation of residual gases in the photomultiplier tube. These positive ions return to the photocathode and produce many photoelectrons which result in after-pulses.” (Hamamatsu)  When the after-pulse (AP) number increases, the after-charge (AC) remains constant the drift times for ions hitting the photocathode depend on the localization of their production. These times are thus different. Then each AP produces a charge associated to only one ion.  Common cut for selecting AP can thus be set whatever the pulse amplitude is efficiencies for cutting AP from 1 and 11 MIPs at a given HV should be the same After-pulse process 2000 V

Lyon group, December Efficiency for P/AP MIP detection (2300 V)  For a cut on the width at 15 ns:  ~ 95% for AP rejection  ~ 95% for P selection

Lyon group, December Efficiency for P/AP MIP detection (2000 V)  For a cut on the width at 14 ns:  ~ 95% for AP rejection  ~ 95% for P selection  The cut should be common for PMT working at the same gain

5/ /1800 2/ /16007/ /2300 Efficiency vs charge in pp collisions THR effect (168 mV) THR + Wcut effects

Lyon group, December Conclusion  The shape recognition of after-pulses can be done the production of parasite triggerings can be suppress ~95% of efficiency  The method can (has to) be developed in order to optimize the efficiency for rejecting the AP and detecting the MIP to minimize the time for on line cumputing  An upgrade of the electronics can be studied to introduce this algorithm to improve several present functions ….  For an upgrade ready in 2014 when LHC restarts decision needed as soon as possible for first step prototype board

Proposal  Upgrade of the Front End Electronics On-line correction of slewing effects. The time resolution is thus optimized and the time range for the selection of good events can be divided by a factor of about 5 as compared to the present situation. The after-pulse rate will be still reduced to a very significant level of the same order. Replacement of Beam-Beam and Beam-Gas time windows by an on-line selection of events from the corrected time distributions (see above). Present special runs to adjust the time windows are no more necessary. Exploitation of low level signals amplified by a factor 10. The amplified signal delivered by existing pre-amplifiers will be used in parallel with the direct signals in order to improve the detection efficiency at the MIP level. Replacement of the charge integration (QDC) of direct signal by amplitude measurements of direct and amplified signals with the use of 1 GHz flash ADC circuits. An on-line algorithm will identify pulses, after-pulses and noise. The residual after-pulse effects will be suppressed and the threshold level optimized to the benefit of the MIP detection efficiency. Moreover, the detection of pile-up within bunch clock intervals can be obtained. Present special runs to adjust integration gate time positions are no more necessary. Updating of firmware programs through Ethernet link. Accesses to the cavern are no more necessary. Zero suppression in recording data. Minimization of the event size can be achieved.