IceCube DOM Calibration

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IceCube DOM Calibration

• Jim Braun

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Motivation
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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
XML Files
Database
Calibrator.java
Analysis
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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

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ATWD Calibration

• Bootstrap process!

Muons
Mainboard LED
V 5 x bias_dac / 4096
V 0.0001220 x (0.4 x disc_dac 0.1 x bias_dac)
f 20MHz
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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

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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

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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

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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

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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 Hagars ch1ch2 charge discrepancy

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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

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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

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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

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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 (lt1 Hz)
• Slow when discriminator rate is high
• Use LED if necessary

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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 _at_ 20MHz,
  ATWD frequency is given by 20MHz bins
• Not quite linear
• newer version will sample closer to 850 DAC value

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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

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Gain Calibration
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Gain Calibration
Repeat from 1200V to 1900V in 100V intervals
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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
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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

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DOM-Cal XML Files

• Hopefully, users wont 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

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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/

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DOM-Cal Results

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

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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

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DOM-Cal Results

• The bad news
• Probably wont 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!

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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