Pre-calibration of PM gain by photo-statistics A = k * G * N(p.e.) * e / s(adc) N(p.e.) = is the number of photoelectrons; N(p.e.) = f *[A(LED)-A(PED)]²/[σ(LED)²-σ(PED)²]; e is electron charge; s(adc) is ADC sensitivity; s(adc) = 0.0195 pC per ADC count; f is Fano factor; f ~ 1.2; k is clipping coefficient; k = 1/3; A(LED) is LED peak position; σ(LED) is LED peak r.m.s; A(PED) is pedestal peak position; σ(PED) is pedestal peak r.m.s.; G = 304688 * (σ(LED)² - σ(PED)²) / (A(LED) – A(PED)) Irina Machikhiliyan, 08.07.2009

Gain by photostatistics-II Contribution from the photostatistics should dominate other possible sources. Natural strategy is to decrease the intensity of the LED light as much as possible increasing simultaneously the gain of phototubes; To exclude dependence on time-alignment quality: 5TAE events are collected in the form of timing scan of many steps: For each step #i (i=1,100): for each cell: find maximal value of LED response over 5 TAE A(i); Over all steps: for each cell find points with optimal timing over A(i). Calculate gain in this point; Several maxima give additional benefit allowing to estimate the stability of the PM response in time. A(i), ADC cnt Step i Irina Machikhiliyan, 08.07.2009

STEP #1 Settings: PM gain: 250K (Hamamatsu reg. curves are used); LED intensity: 0V; DAQ: timing scan, 5TAE events, 100 points / 2400 events per step (300 entries in LED peak per step); Many ECAL LEDs produce residual light at zero intensity setting. This light is seen by phototubes if their gain is high enough. Signals from some LEDs are so large that PM responses saturate ADC. Response on LED Irina Machikhiliyan, 08.07.2009

STEP #1(cont) Measured value of the gain with respect to the prediction (HAMAMATSU reg curves): R.M.S. is 18% HV is in kV Bend at HV > 1.45kV The same effect is seen if compare with test-benches prediction The assumption: MV setting 90V is not enough to reach desired HV values Irina Machikhiliyan, 08.07.2009

STEP #2 Settings: Medium voltage is set to 97V; PM gain: tuned, for most of the cells: 250K; LED intensity: tuned, from 0V up to 1.3V; DAQ: timing scan, 5TAE events, 100 points / 5000 events per step (625 entries in LED peak per step); Gain settings: from 80K up to 250K Response on LED Much better, but further tuning is needed, sometimes on the level of individual cells Irina Machikhiliyan, 08.07.2009

STEP #2(cont) LAL reg. curves Hamamatsu reg. curves Ratio G(meas)/G(LAL) depends on HV in the similar way we saw during lab-studies (see talk from 24 Nov 2008) With MV=97V: no bend! Irina Machikhiliyan, 08.07.2009

LAL: systematics 1.23; r.m.s.: 14.4% fit: 12.1% STEP #2(cont) Measured value of the gain with respect to predictions LAL: systematics 1.23; r.m.s.: 14.4% fit: 12.1% IHEP: systematics 1.24; r.m.s.: 14.4% fit: 12.5% Irina Machikhiliyan, 08.07.2009

Measured value of the gain with respect to the prediction STEP #2(cont) Measured value of the gain with respect to the prediction ITEP: for some PMs measurements seems to be not correct. Further (in this presentation): ITEP data will be excluded from consideration HAM: systematics 1.44; r.m.s.: 14.0% fit: 11.1% Irina Machikhiliyan, 08.07.2009

Relative width of LED peak STEP #2(cont) Relative width of LED peak G(HAM) ≤ 100K 100K< G(HAM) ≤ 200K G(HAM) > 200K Irina Machikhiliyan, 08.07.2009

Absolute width of LED peak STEP #2(cont) Absolute width of LED peak G(HAM) ≤ 100K 100K< G(HAM) ≤ 200K G(HAM) > 200K Absolute width of LED peak is higher than 9 ADC counts: pedestal r.m.s. (1.2÷1.3) can be neglected Irina Machikhiliyan, 08.07.2009

Many thanks to Jacques and Pavel for very fruitful discussions STEP #2(cont) Results seems to be promising. Another iteration has been done during last weekend, data processing is now in progress. Next request on LED data will be issued in one-two weeks; During access periods: the cells with measured gains which differ significantly from expected values were checked. As a result, several PMs were found which position was mixed with respect to DB. Also several signal connectors of bad quality were repaired; Our summer student Viveca is here to participate in the data analysis; Many thanks to Jacques and Pavel for very fruitful discussions Irina Machikhiliyan, 08.07.2009

Lab measurements: factor f By I. Machikhiliyan & P. Shatalov Test setup: PM Vctrl 3.2÷3.5V (1.35÷1.48kV) R/O crate with FEB and CROC_v2 was borrowed from Jacques and Frederic (many thx!) CAT DAQ. Timing scan to find the optimal point. Few photoelectrons mode: Additional amplifier: gain g(amp)=18.18 Very low LED Normal mode: No additional amplifier higher LED (position Qn, width sn) Fit function to obtain one photoelectron response (position Q1, width s1): E.H. Bellamy et al “Absolute calibration and monitoring of spectrometric channel using a photomultiplier” NIM A339 (1994) 468-476 PM gain: G(1 p.e.) = Q1 * s / (g(amp)*k*q) = 20111.4*Q1, where s – ADC sensitivity, k=1/3 – clipping factor, q – charge of the electron G(normal mode) = 365625 * sn^2 / (Qn-256) ( here f=1.2 is excluded ) New value of f: f = G(normal mode) / G(1 p.e.) Irina Machikhiliyan, 08.07.2009

HV=3.3V; 49K of events (1st 1K is rejected) Example: PM LA4612 (1) HV=3.3V; 49K of events (1st 1K is rejected) g(1 p.e.) = 851114 G(normal mode)=1193176 f=1.4 Irina Machikhiliyan, 08.07.2009

HV=3.3V; 9K of events (1st 1K is rejected), different LED intensity-I Example: PM LA4612 (2) HV=3.3V; 9K of events (1st 1K is rejected), different LED intensity-I HV=3.3V; 9K of events (1st 1K is rejected), different LED intensity-II Irina Machikhiliyan, 08.07.2009

Lab measurements (cont) In general we checked 4 phototubes at different HV/LED LV settings. Calculated value of f varies between 1.4 and 1.6 Ratios from step #2 when f=1.2 is excluded from the formula for G(meas): New value of f gives better agreement of G(meas) with test-benches data; Can be pure coincidence… The goal is to achieve acceptable level of inter-calibration. Absolute normalization can be extracted confidently from physical signals only… Which value of f should be used? Irina Machikhiliyan, 08.07.2009

Spare slides Irina Machikhiliyan, 13.06.2009

PM LA4612 (3) TADC=23 TADC=0 TADC=2 TADC=4 TADC=5 TADC=6 Irina Machikhiliyan, 13.06.2009

PM LA4612 (4) TADC=8 TADC=10 TADC=12 Irina Machikhiliyan, 13.06.2009