IceCube DOM Calibration

1. IceCube DOM Calibration Jim Braun

2. Motivation

3. DOM-Cal – Big Picture • Application runs on DOM • Can calibrate all DOMs in parallel • Stores calibration data on DOM flash memory • Java client • Coordinates calibration • Reads calibration data from DOM • Produces DOM-Cal XML calibration files • Calibrator class reads XML file, applies calibration constants • All code in dom-cal project on glacier • Authors: John Kelley, Jim Braun, Kael Hanson DOM-Cal Client DOM hub Database XML Files Calibrator.java Analysis

4. Calibration Routine • ATWD Calibration • Reconstruct PMT voltage waveform from ATWD data • Requires: • Voltage calibration of ATWD • Measurement of amplifier gain for ch0, ch1, and ch2 • Frequency calibration of ATWD • Baseline measurement • PMT Gain Calibration • Find relationship between PMT gain and applied voltage • PMT Transit Time Calibration (NEW!) • Find PMT transit time as a function of applied voltage

5. ATWD Frequency Calibration ATWD Voltage Calibration Pulser Calibration Baseline Calibration Amplifier ch0 Calibration Amplifier ch1 Calibration Amplifier ch2 Calibration ATWD Calibration • Bootstrap process! Muons Mainboard LED f = 20MHz V = 0.0001220 x (0.4 x disc_dac – 0.1 x bias_dac) V = 5 x bias_dac / 4096

6. Pulser Calibration • Determine relationship between pulser DAC setting and peak voltage • Use known relationship between discriminator DAC and voltage • Set discriminator DAC, adjust pulser DAC until 50% of pulses cross discriminator threshold • At this point, the pulser peak voltage corresponds to known discriminator voltage. • Peak voltage distribution is very narrow

7. Pulser Calibration • Repeat for ~10 discriminator voltages • Relationship is very linear • Now know pulser peak voltage given DAC setting • Will use this relationship in amplifier calibration

8. ATWD Voltage Calibration • Determine relationship between ATWD value and signal voltage • Use known bias DAC voltage relationship • Bias is independent of amplifier gain • Set bias, record average ATWD value • For each bin (0-127) of each signal channel (0-2) of each ATWD (0-1) • O(100) samples • Apply linear fit to ATWD value vs. voltage data • Pedestal patterns are eliminated

9. ATWD Voltage Calibration • Now know voltage of any ATWD bin given a channel number, bin number, and value • Requires 768 linear fits! • ATWD response not entirely linear • Need to calculate and subtract small baseline offset for each channel during precision measurements

10. Baseline Calibration • New in DOM-Cal 5.0 • Measures average baseline offset for each ATWD channel • Need to measure baseline whenever the internal state of the DOM changes • Known to affect baseline: • Analog multiplexer • Mainboard LED power supply • PMT high voltage • Affects low gain channel the most • Main source of Hagar’s ch1:ch2 charge discrepancy

11. Baseline Calibration • We care most about affect of high voltage and residual baseline from imperfect ATWD calibration • Take baseline data both with HV off and HV at values spanning DOM operating points • Store all data points • Use baseline value closest to operational HV when calibrating TestDAQ data • For gain calibration, domcal chooses only the voltages where baseline calibration points exist

12. Amplifier Calibration • Calibrate high gain channel (0) with pulser • Pulser too weak to accurately calibrate lower gain channels • Set pulser peak to a known voltage, record peak voltage after amplification in ch0 • Use ATWD ch0 calibration data to find peak voltage • Maximize ATWD sampling speed to better localize peak • Record mean and error of O(250) ch0 peaks • Ratio of mean voltage and known pulser voltage yields amplification factor

13. Amplifier Calibration • Need source of high amplitude pulses to calibrate ch1 and ch2 amplifiers • Use PMT signals! – new in DOM-Cal 5.0 • Muons work well at surface • Mainboard LED needed in deep ice • LED power supply shifts baseline, need to recalibrate • For ch1: • Select pulses which have an ATWD peak value of 600-800 counts • Too few ch0 counts -- too much error in ch1 peak voltage • Too many ch0 counts -- ch0 nonlinearity becomes significant

14. Amplifier Calibration • For ch1: • Record ratio of ch1 peak voltage to ch0 peak voltage for O(250) iterations • We know ch0 gain, so ch1 gain is given by the product of ch0 gain and voltage ratio. • For ch2 • We now know ch1 gain, use previous procedure to find ch2 gain • Slow with muons (<1 Hz) • Slow when discriminator rate is high • Use LED if necessary

15. ATWD Frequency Calibration • Select mainboard oscillator in ATWD analog multiplexer channel (channel 3) • At various sampling speed DAC values, count number of bins between positive zero crossings in ATWD waveform • Average O(100) clock waveforms • Assuming oscillator operates in spec @ 20MHz, ATWD frequency is given by 20MHz * #bins • Not quite linear • newer version will sample closer to 850 DAC value

16. Gain Calibration • Capture PMT single photoelectron pulses in ATWD • Glass radioactivity emits enough light • Apply ATWD calibration to get PMT V(t) waveform • I(t) given by V(t)/50W • I(t) pulse integrated from –4 bins to +8 bins of pulse maximum (~-14ns - +28ns), yielding SPE charge • Repeat O(5000) times, histogram charge data, and apply nonlinear fit

17. Fit Exponential + Gaussian Single-Photoelectron Peak Noise Discriminator Edge Muons, etc. Charge (pC) Gain Calibration

18. 1400V 1500V 1700V 1600V Gain Calibration Repeat from 1200V to 1900V in 100V intervals

19. Gain (charge / e) Linear log - log fit yields operating point Voltage Gain Calibration Mean SPE charge vs. voltage is a power law The number of photoelectrons for any pulse at a given HV is now determined

20. Java Client • In dom-cal project on glacier • Main class: icecube.daq.domcal.DOMCal • Run with no args for usage instructions • Reads calibration data from DOM flash • Stores calibration data in local XML files • Stores data in domprodtest database • Can initiate calibration • Can run calibrations on many DOMs and DOM hubs in parallel • Most will never need to use the java client

21. DOM-Cal XML Files • Hopefully, users won’t need to know much about the XML files or database structure either! • Access provided through calibration applications • XML files are reasonably easy to read if needed • Contain: • DOM hardware ID (No name…..sorry Mark) • Temperature • Date • DAC settings and ADC readings • Calibration information • Linear fit data • Baseline data • Gain histogram data

22. DOM-Cal Calibrator • Icecube.daq.domcal.Calibrator java class provides access to calibration data. • I3DOMCalibration equivalent object in IceTray • Create a Calibrator for each DOM: • new Calibrator(XML_File); • Most important routine: • atwdCalibrateToPmtSig() • Takes an array of raw ATWD data and applies calibration to yield true voltage signal • Other methods to access raw data, described in javadoc • http://www.amanda.wisc.edu/jbraun/domcaldoc/

23. DOM-Cal Results • Currently running DOM-Cal v4.3 at pole • Baseline shift apparent in data from String 21 and from last year’s FAT runs • Analog multiplexer enabled in all string-21 runs – just recently disabled

24. DOM-Cal Results • DOM-Cal 5.0 results are encouraging • Baseline calibrated to zero for all channels • No analog multiplexer effects • Amplifier gains now calculated much more accurately • Pulse heights now agree between all three ATWD channels

25. DOM-Cal Results • The bad news • Probably won’t see much gain when analyzing String 21 data, even with DOM-Cal 5.0 • Local coincidence readout rate is a few Hz • Interval is long enough for baseline to drift • Hopefully there is a firmware fix!

26. DOM-Cal 6.0++ • Other features DOM-Cal may deal with: • PMT transit time – almost finished! • Signal droop • Evidence time constant can be easily measured (Chris W.) • Ch0 time offset, bandwidth limitation