Calibration and Characterization of
Photomultiplier tubes ofthe IceCube Neutrino Detector
Hiroko Miyamoto
Dept. of Physics
CHIBA University

Introduction
The IceCube neutrino observatory will consist of an array of 4800 Digital Optical Modules (DOMs) located in the deep ice at the South Pole, and also an IceTop air shower array of
320 DOMs in 160 ice tanks located on the surface. A 10 inch PMT is housed in each DOM for the detection of Cherenkov light. This poster describes the methods of calibration andcharacterization of the IceCube PMTs in the laboratory, which are germane to improving the detector resolution and reducing systematic uncertainties. Two dimensional scans on theentire photocathode to map out photon conversion efficiency have been carried out for 60 PMTs. The quantum/collection efficiency has been calibrated in an absolute manner using
the Rayleigh scattered light from our newly built chamber filled with nitrogen gas. The charge response of the PMTs at temperatures below freezing has been extensibely studied andfound to be well represented by the analytical model. All these results have been combined and implemented in the detector Monte Carlo simulation.

PMT Calibration

Digital Optical Module
PMT R7081-02
penetrator

IceCube detector uses the HAMAMATSU 10 inch R7081-02 photomultiplier tube with 10 dynodes
inside. It has a spherical surface of the photocathode to enlarge its collection area.
We must learn at least about these three parameters of the PMT below in the event reconstruction.

HV board
flasher board

pressure sphere

DOM
main board

Signal = QE × CE × Gain・ records timestamps
・ digitizes waveforms
・ transmits to surface atrequest via digitalcommunications• important design
requirement
PMT
Noise rate < ~1.5 kHz
mu metal cage SN monitoring within
our Galaxy
delayboard

Charge response of the PMT
The most simplest formula to represent the signal part of ADC spectrum(single p.e.) can be easily supposed to be a gaussian function. However,our data shows the existence of another component: exponential-likecomponent appears together with the main gaussian feature anddominates especially at lower ADC range. This behavior probably
arises from the instability of first dynode gain.
We thus sum up both components and assumed it to be a model functionof our data. Reduced fitting chi-square between our data and model
were below 1.0, which justify our model assumption.

optical gel

2D uniformity and 2D Gain scanning measurement
C E /G a in U n ifo r m it y
2D scan
1 0 8 G a in
5E07Gain

PMTUV LED

7 5 n se c2D uniformity scanning

Two figures in the panel below show the relative
efficiency in two azimuthal angle slices (135deg and
157.5deg) for 4 tubes, SF0004, SF0016, SF0050 andSF0030. The efficiencies plotted here are normalized sothat the average efficiency over entire photocathode is
100 %. You can see the efficiency varies photon
locations to locations by +- 20 %. The difference between
different tubes is also visible. We have implemented
these effects to the IceCube detector MC.

R - g u id e

The collection efficiency depends on where a photoelectron is emitted on the photo-cathod surface. The
figure below shows our scan results of 8 IceCube PMTs.
TA1052

TA1056

TA1059TA1062

TA1069TA1167

TA1176

R o t a t io n - b e d
M o to r2D Gain scanning

TA1181The plots of the right panel shows the gain variancealong the three slices with different rotation angles on
the photocathode. The vertical axis represents therelative gain normalized as 100% in the overall
average. The actual (absolute) gain in this data taking
is approximately 4.4E7 (at room temperature). The
spikes in the center is caused by an interpolation of
the data points and you should neglect it.
Although there are slight pmt to pmt variances, but thetrend is quite same, in contrast to the case of thecollection efficiency.

TA0451

TA0532

TA0538Three figures below are of the gain map for threedifferent tubes. You can see they share the samegeneral behavior.

Absolute Efficiency (CE× QE) Measurement
Data Analysis
I. Estimate the photoelectron per laser shot. Supposing Poisson distribution of the charge response,
photoelectron number can be described by the number of pedestal events and the all the event as bellow.

photoelectron # = -ln(Nped / Nall)
Signal Subtraction
Pedestal

Charge histogram

Measurement Setup
We use N2 laser (Laser Science VSL-337BD-S), the wavelength of the shot is 337.1 nm.
Each parameters are monitored as bellow.
Absolute energy of the laser shot : Si energy probe (Laser Probe Inc. RjP-465)
Pressure of inside the chamber : Pressure meterTemperature of inside the chamber : Platinum resistance thermometer

SignalCharge histogram + normalized pedestal

II.Estimate incidental photon #.

cathode hit photon # ~ initial photon# x aperture x Cross Section

Results

Energy meter

TA1052

Numerical calculation

CenterTA1059

Typically,~ 4E9 x 5E-11 ~ 0.2

QE(absolute efficiency)= photoelectron # / cathode hit photon #TA1167

EdgeTA1176

Distribution of Initial Photon #Photon # as a function of time

Projection of the right panel

Horizontal axis is the length on cathode. 0 corresponds to the center of the photocathode, while the edge does
about 15 cm off from the center. The light spot size is approximately 1.0 cm. You can see the efficiency
doesn't change so much around center and the asymmetry of efficiency distribution. The detection efficiency
in the central area of the photocathode is measured to be 18-20%. There's almost no efficiency outside area atmore than 15 cm from the center.
The data of these four PMTs look like similar distribution, but trend of individual tubes starts to appear.

~40min.
Non-Gaussian tail●Remove the events out of the two sigma.

Verification
Check air condition(Rayleigh dominant)Time dependence(stability check)by forward and backward monitor PMT

Error budget-- Statistics --•photoelectron # : 6 %Gaussian-- Systematics --Combine the Absolute Efficiency Data and the 2D Uniformity Data
The data points in the left panels show the determined detection
efficiency as a function of the distance from the cathode center.
The histogram shows the two dimensionally scanned relativecollection efficiency described above, after applying theappropriate normalization.
The locally measured absolute efficiency data points agree very
well with this histogram. Consequently, this allows us to estimate
the those averaged over the entire photocathode.

Pressure dependence(no offset)•photoelecton # : 0.4 %•Light Yield ( aperture ) : 4 %•Initial photon fluctuation : 0.02 %•Pressure : 1 %•Photon energy probe : 5 %Total Error Budget : 10 %