Gain and Breakdown Voltage Measurements CLICdp: ECAL Lab Meeting (CERN) Magdalena Munker March 2, 215
Setup for study of Scintillator tiles with SiPM Readout Setup in cooled dark room ( temperature about 2 C): elektron source (strontium 9) elektron gun B ca..2 T Electron source strontium 9 Sr: Sr Y (e 2 + νe) + e 1 + νe Selectable electron energy Opening: ( X, Y) = (1.2 mm, 1.2 mm) opening of electron gun scintillator tile SiPM Scintillators covered by reflecting foil: Trigger fibres: 1 mm x 1 mm x 2 mm Scintillator tile: 15 mm x 15 mm x 1 mm scintillating fibre = movable direction SiPM scintillating fibre SiPM SiPMs from Hamamatsu: area of 1 mm, 4 pixel (5 µm) direct coupling to the scintillator This talk: About characterization of the SiPM which is used to readout signal in scintillator tile ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 2
Setup for Characterization of SiPM ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. Same setup, but: Replace scintillator tile by optical fibre, connected to LED pulser Do not use trigger and electron gun Simplified schematic of the setup for the characterization of the SiPM: V(bias) LED pulser optical fibre SiPM pulse generator trigger signal PCB SiPM readout data readout
Observable to Measure Signal in SiPM Amplitude [ mv ] Observable to measure signal in SiPM = charge in SiPM: Q = pulses dt F amplification R picoscope (R picoscope = 5 Ω, F amplification = 9) Pedestal correction: Temperature correction: Q(MPV) [pc] 7.5 8 dq / dt = ( -.562 ±.1 ) pc / C 7 Pedestal Time [.8 ns ] Signal 6.5 18 19 2 21 22 Temperature [ C] Pedestal and temperature correction of the charge: ) Q corrected = (Q signal Nsignal 1 Qpedestal Npedestal 1 + α r (T T) α r = dq dt 1 Q(T ) ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 4
Measured Charge Spectrum ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 5 Gain = ( 978 ± 8 ) x 1 Temperature [ C] = 19; V(bias) [ V ] = 71.5 # Trigger 6 4 2.5 1 1.5 Q [pc] ( Pedestal & temperature corrections applied ) Peaks from different number of photoelectrons clearly visibly Underlying noise spectrum from afterpulses?
Gain Calculation Check linear dependence of Q(MPV) Q(µ) for different photoelectron peaks to validate calibration of charge to number of photoelectrons ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 6 # Trigger 5 4 2 Temperature [ C ] = 19; V(bias) [ V ] = 71.5 Gain = ( 99 ± 2 ) x 1 Gain calculation: g i,j = Q i Q j /e {i, j} = peaks in charge distribution with i j = 1 e = elementary charge 1.5 1 1.5 2 Q [pc] weighted mean: Gain = i,j w i,j g i,j W First peak corresponds to pedestal Fit Gauss function to pedestal peak Q(µ) Other peaks correspond to signal of 1 photoelectron Fit Landau Gauss functions to each other peak Q(MPV) weights of uncertainties: w i,j = 1 σ 2 g i,j, W = i,j w i,j
Cross Check of the Gain Calculation ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 7 Identify peaks in charge spectrum with different numbers of photoelectrons # p.e. Calculate for each # p.e. difference in charge to pedestal peak Q Q [ pc ].6 V(bias) = 71.5 V.4.2 1 2 4 # p.e. Can calibrate measured charge to # p.e. with the measured gain: # p.e. = Q/(Gain e) (e = electron charge)
Uncertainties of the Gain Calculation ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 8 Motivation: Breakdown voltage calculation by linear fit of gain values for different bias voltages Composition of uncertainties in gain measurement: 1.) Propagate uncertainties of Q(MPV) and Q(µ) ( in the order of 1 % ) 2.) Uncertainties from the temperature correction: Temperature [ C ] = 19; V(bias) [ V ] = 71.5 6 Gain = ( 989 ± 2 ) x 1 4 2 Temperature correction (+) 5 4 2 1 Temperature [ C ] = 19; V(bias) [ V ] = 71.5 Gain = ( 14 ± 2 ) x 1 Temperature correction (-).5 1 1.5 2.5 1 1.5 2 Additional uncertainty of.75 % from the temperature correction
Breakdown Voltage Calculation ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 9 Motivation: Currently used bias voltage of V(bias) = 71.5 V is optimal value for T = 25 C Temperature in the dark room is in the range of T = (18 22) C Calculate breakdown voltage to check if V(bias) = 71.5 V is optimal Measurement of breakdown voltage corrected to T = 19 C: Repeat gain calculation for different bias voltages Gain ( V(bias) V(bd) ) ( V bd = breakdown voltage ) Linear fit of Gain vs. bias voltage Gain / 1 15 V(bd) = ( 7. ±.2 ) V 1 5 Χ2 / ndof = 1.6 Determine V bd from intersect of linear fit function with x-axis 7 7.5 71 71.5 72 V(bias) [ V ]
Temperature Dependence of the Breakdown Voltage Expectations: Higher temperature leads to higher thermal vibrations of silicon lattice Smaller path length of the electron Higher breakdown voltage Correct measurement of breakdown voltage for different temperatures: Gain / 1 15 1 V(bd) = ( 7. ±.2 ) V Χ2 / ndof = 2 Gain / 1 14 12 1 8 6 V(bd) = ( 7.1 ±.2 ) V Χ2 / ndof = 1. 5 Corrected to T = 18.6 C 7 7.5 71 71.5 72 V(bias) [ V ] 4 2 Corrected to T = 21. C 7 7.5 71 71.5 72 V(bias) [ V ] Value of breakdown voltage is stable in considered temperature range of the measurements Is V(bias) = 71.5 V the optimal value of the bias voltage? ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 1
Breakdown Voltage / Interpretation of Results ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 11 Is V(bias) = 71.5 V the optimal value of the bias voltage? : Bias voltage should be safely above breakdown voltage But the signal to noise ratio might get worse with higher bias voltage Measure signal in scintillator tile for different bias voltages: Use the initial setup ( see page 2 ) Measure charge in center of the scintillator tile for different bias voltages of the SiPM at the scintillator tile / Q(MPV) σ Q(MPV).8.6.4.2 7 7.5 71 71.5 72 V Bias [ V ] Used bias voltage of V(bias) = 71.5 V seems to be in a good range
Summary and Outlook ECAL Lab Meeting M. Munker Gain and Breakdown Voltage Measurement for the used SiPM p. 12 Measurement of gain: Conversion of charge into number of photoelectrons Comparison to other studies Measurement of breakdown voltage: Confirmation that used bias voltage is in a good range To do: Characterisation of new SiPMs Application of new SiPMs for uniformity studies