PD 07 @ Kobe Univ. 06/29/2007 General performance of the IceCube detector and the calibration results I am Mina Inaba from Chiba university. I will talk about “General performance of the IceCube detector and the calibration results”. Mina Inaba1 S.Yoshida1 , K.Mase1, C.Rott2 ,Y.Hasegawa1 1.Dept. of Physics, Chiba University 2. Pennsylvania State University for the IceCube Collaboration

Outline Introduction of the IceCube experiment General performance of the IceCube detector PMT/DOM Production Procedure Calibration of PMT Calibration of DOM Comparison between PMT and DOM Detector status in the ice Summary and future work This is the outline of my talk. Firstly, I’d like to introduce the IceCube experiment and its detector. Next, I show some general performances of our detector. Then, after I introduce the production procedure , I will give a main part of my talk about photomultiplier tubes including charge response behavior ,photocathode uniformity mapping and absolute measurement. And I will also talk about calibration of the module so-called “DOM”. Finally, I will discuss how we can implement the measured results into the IceCube simulation.

The IceCube experiment South pole ~1 km IceTop IceCube IceCube AMANDA-II Cherenkov light detector 60 PMTs x >70 strings = >4200 PMTs Neutrino energy of 107(SNs)-1020eV is detectable. 22 strings are deployed up to now, and taking data as the biggest neutrino detector. 1450m IceCube is a very high energy cosmic neutrino Cherenkov detector at the South Pole. This is the schematic view of IceCube. The total size of the detector will be about a cubic-kilometer. It will consist of more than 70 strings, and 60 modules are attached to each string. So, we will deploy more than 4200 photomultiplier tubes. 22 strings have been deployed so far, it corresponds to about one-third of the full volume and taking data as the biggest neutrino detector. This is a Monte Carlo simulation of 80 strings. Color means the arrival time and the size of the circle means the magnitude of the energy. The energy of this event is more than 10 to the 9th GeV. We aim at such high energy event with full strings. 〜1 km 2450m 1km3 1 km

Digital Optical Module (DOM) Module of the IceCube Digital Optical Module (DOM) PMT HV Base Board Typical digitized waveform Glass Pressure Sphere Flasher board 35.6cm 10inch R7081-02 PMT Hamamatsu Optical gel Magnetic Shield Cage This is the module of IceCube. Digital Optical Module , so-called “DOM” contains a 10inch diameter photomultiplier tube supported by coupling gel, a signal processing electronics board ,an LED flasher board and PMT base with high-voltage supplier. Our photomultiplier tubes are made by Hamamatsu. This is the typical digitized waveform. DOM has two digitizers in itself. One is an ATWD, and it has three different gain channels for wider dynamic range. Another one is FADC for long duration pulses. Onboard capture of PMT waveforms - ATWD : 300MHz 14 bits (~400nsec) 3 different gains (x16, x2, x0.25) - FADC : 40MHz 10 bits (~6.4μsec)

General performances of the PMT Type R7081-02　(bialkali) Dynode 10 stages Quantum efficiency 25% @420nm Dynamic range 1000p.e. /30nsec @107 gain Time resolution 2.2nsec Noise rate 300Hz @ -40℃ ~ -20℃ Operation gain 1x107 @1300V QE Noise rate Dynamic range I show here some general performances of the IceCube detector. IceCube PMT is the standard bialkali material , it has 10 dynode stages . A peak quantum efficiency is approximately 28% at 420nm. IceCube PMT has a linear response up to approximately one thousand photoelectrons at the gain of 10 to 7th. Time resolution is about 2.2nsec. Noise rate is 300Hz under -40 ~ -20 degree, the same temperature in the ice. The nominal operation gain is 10 to the 7th at 1300V, Charge [pC] QExCE 25% Dark count 300Hz 103 p.e. Wavelength [nm] temp

Production Procedure PMT DOM Hamamatsu Chiba Univ. Wisconsin Univ. HV board metal cage Flasher board Main board 10 inch PMT Optical gel Hamamatsu Chiba Univ. Wisconsin Univ. Chiba Univ. South Pole Normal check: Linearity of the PMT gain dark noise rate charge resolution 2D sensitivity scan Golden PMTs: normal check + Absolute calibration Golden DOMs: 2D sensitivity scan Absolute calibration This is the production procedure of PMT and DOM. All PMTs are made by Hamamatsu and at Chiba University we check the basic performance such as linearity of the gain ,dark noise rate, charge response and 2 dimensional uniformity scan. In addition to the normal check , some of them are measured absolute efficiency. Then, such PMTs are sent to Wisconsin University to assemble into DOM . After that, DOMs which contains absolutely calibrated PMT are sent back to Chiba, and we calibrate the DOMs absolutely. Finally ,all DOMs are sent to the South Pole. assembles DOMs check all DOMs (under low temperature) Linearity of the DOM gain dark noise rate component check cold reboot communication test

Set up of SPE measurement Diffuser attached to the UV LED In the Freezer box Freezer -32℃ This is the set up for single photo electron measurement. To imitate the same condition at the South Pole, we made measurements in a freezer box which can cool down to -32 degree. An ultraviolet LED are used as the light source and a diffuser is attached on it . We controlled the LED by a pulse generator and took data by CAMAC. 375nm UV LED 2kHz, ~0.01 photo-electron/shot

The parameterization of SPE CR 2 ( ) The exponential term Comes from an incomplete multiplication at the first dynode We derived a function to represent the single photo electron distribution. This function is implemented in the IceCube simulation. In general, the charge of SPE pulse follows a gaussinan distribution, but as you see, here is a shoulder. This is due to an incomplete multiplication at the first dynode. So we fitted the SPE histogram with 1 exponential and 1 gaussian terms. Fitted the spe charge response with 1 exponential + 1 gaussian terms.

The averaged charge response One measurement Peak of gaussian ~ 1 p.e. Charge resolution 29% Average over 118 PMTs This is the averaged charge response of the IceCube PMT. Red line represents the average. For comparison, I plot some PMT ‘s charge response by dotted lines. The averaged function is derived from measurements of 118 PMTs over 3 years. The peak value of gaussian component correspond to 1p.e. However, the mean value of the function is 15% less than 1 p.e because of the exponential term. This parameter is used by the detector Monte Carlo simulation. The charge resolution of a single photo electron is 29%. Mean 15% less

Absolute calibration of PMT Using Rayleigh scattering 5x10-11 337nm N2 laser Energy meter Rayleigh scattering 4x109 Cathode hit photon# ~0.2 Photoelectron# QE= Cathode hit photon# Next, I would like to talk about absolute calibration of the PMT. The absolute calibration of PMT is very important because IceCube uses the observed number of photons to estimate energy of an event derived from a neutrino. This figure shows our calibration system. We used 337nm nitrogen laser for the light source. It is too strong to hit the laser directly to the PMT so to dump the light intensity we had photon scattered in the chamber filled with nitrogen gas. Only photons with scatter angles of ~90 °will reach the IceCube PMT photocathode to provide an SPE signal. Cross section of the Rayleigh scattering is well understood so we can estimate the photon number at the cathode surface by energy meter and numerical calculation. By dividing measured photo-electrons by the photon numbers, we measured a quantum efficiency of a PMT. x Energy meter Numerical calculation Rayleigh scattering cross section is well understood

Results of absolute calibration Systematic error Photoelectron# : 1% Light yield : 4% Initial photon fluctuation : 4 % Pressure : 1% Photon energy probe : 5% Total ~8% This is the results of the absolute calibration. X-axis is the QE measured by Hamamatsu and Y-axis is QE*CE measured by us. Hamamatus measurement is based on DC light source while Chiba uses a pulse. Measured absolute efficiency distributed from 18% to 23% . Our system has a 8% systematic error and the results have a good correlation with Hamamatsu data within the error. Our data have a good correlation with Hamamatsu QE measured by Hamamatsu(340nm)%

Averaged PMT 2D CEMap(~94PMTs) PMT 2D Uniformity Scan Efficiency depends on the cathode surface. Averaged PMT 2D CEMap(~94PMTs) PMT LED motor The IceCube PMT ‘s photocathode surface is very large, so we have checked the uniformity. We have systematically measured the variation of efficiency for each photocathode position, using a two dimensional scan system. This is the picture of the system. An UV LED can move along the curved PMT surface. This plot shows averaged PMT efficiency map over 94 PMTs , where the X-Y coordinates represent a projection of distance on a PMT surface. One can see that the efficiency falls down at the edge and the effect of first dynode direction. The direction of the first dynode

2D CE combined with QExCE We can draw Absolute 2D QExCE Map ! CE Map normalized to QE Efficiency[%] 2D Collection efficiency Map Absolute efficiency This shows the measured detection efficiency as a function of distance on a PMT surface from the cathode center. Blue points are the absolute efficiency and the histogram shows the relative efficiency . It shows that the absolute efficiency measurements follow closely the shape measured by a 2D relative efficiency scan. By matching the relative efficiency with the absolute one which we only took a few points, we can know the absolute efficiency for all the PMT surface. Length on cathode [m]

2D QE x CE Map of PMT @337nm 20 15.0% 16.7% 18.1% 15.5% 17.9% 16.0% Systematic error ～8% TA2259 TA2349 TA1895 TA2026 20 -0.15m 0.15m 15.0% 16.7% 18.1% 15.5% TA2086 TA2146 TA2182 TA2374 These are the PMT two dimensional Map combined with absolute efficiency. This value is the QE at the PMT center. You see the PMT characteristic. PMT to PMT variation of the collection efficiency at a given spot on the photocathode can be as great as 40%. However, in fact, the spread of characteristic is an order of 15 %. 17.9% 16.0% 18.7% 17.8%

Variance of the Collection efficiency By. Y.Hasegawa 90° 0° Variation of the collection efficiency ~ 15% QExCE[%] QExCE[%] 90° 0° Length on cathode [cm] Length on cathode [cm]

Reminder …what is a DOM ? PMT Gel Glass PMT Absolute Calibration Digital Optical Module = DOM PMT Absolute Calibration Glass + Gel measurement PMT Gel Glass DOM Absolute Calibration

Calibration of DOM QE×CE Absolute calibration 4πCE scanning Reference PMT Absolutely calibrated X-stage LED DOM This is the absolute calibration system of DOM. We used 4 wavelength LEDs as the light source. We used a ND filter to split the light for the both the reference PMT and the one which we measure. The transmission and the reflectivity of the ND filter and the gain of the reference PMT are calibrated within 5% precision. This is the 2D collection efficiency scanning system for the DOM. Basic mechanism is the same as PMT scanning system but this can scan 4 pi direction. 365nm wavelength LED is used for the light source. LED Slit 1mm NDfilter Reflectivity : 14.5%±0.73 Transmission : 50.7%±2.54 Systematic error ~7% @ room

2D QE x CE Map of DOM @365nm 14.0% 15.3% 15.4% 16.3% 17.5% 16.6% 18.0% By. Y.Hasegawa @365nm Systematic error ～7% TA2259 TA2349 TA1895 TA2026 18 -0.3m 0.3m 14.0% 15.3% 15.4% 16.3% TA2086 TA2146 TA2182 TA2374 These are the 2 dimensional collection efficiency Map combined with absolute efficiency of DOM. Unlike the case of PMT 2D collection efficiency, we don’t see the effect of 1st dynode direction. The information is lost by a glass and gel, we think. 17.5% 16.6% 18.0% 16.0%

Comparison between PMT and DOM PMT + Glass/Gel DOM Calibration Finally I will discuss how our measured results are implemented into the IceCube simulation. As I said earlier , we rely on simulation to predict detector response . The photon propagation in the glass and gel has been independently simulated by the GEANT4, based on the measurement of the transmittance of a glass and a gel. Detector simulation reads that table generated by the GEANT simulation and simulate PMT response based on the PMT measurements. Since we have calibrated absolutely both for PMT and DOM, so we can calibrate the simulation. This figure show the photon penetration in glass and gel as a function of wavelength. This may include large uncertainty because of our incomplete knowledge of the characteristic of the gel. As you see, there is a sharp cut-off at around 350nm. This indicates that the transparency can be changed dramatically at short wavelength by even a tiny shift of the penetration curve. So, we evaluate values of the wavelength shift in the glass/gel transmittance such that simulation describes the DOM calibration data better. Wavelength [nm] PMT Calibration Transparency of glass/gel

Comparison between PMT and DOM Absolute calibration of DOM 365nm 16.1% 470nm 17.5% LED DOM 520nm 10.7% NDfilter 337nm 8.21% PMT + Glass/Gel This plots shows the comparison between DOM measurements and the PMT measurement+glass-gel simulation. The histogram is the expected efficiency which is provided by the simulation explained in the previous slide. The red points are the value of DOM measurements. The difference between simulation and data was 12% at 337nm at first, and then We found that a shift of -3 nm of the penetration curve of a glass and a gel make the difference 0.5%. You can see that data and simulation agrees well over the wavelengths. 572nm 4.99% Good agreements over the wavelengths.

Detector status IceCube is now working smoothly!! Temperature HV -30℃ -10℃ string Shallow ← depth 　→　 deep Variation ~7% Mean :1285V IceCube is now working smoothly!! gain rate These are the real information sent from the South Pole. There are a lot of infomations. For example, This is the distribution of the temperature. This shows that going deeper in the ice, getting higher the temperature. This is the distribution of the high voltage, it varies within 7%. This is the distribution of the gain, it within varies 2.5%. This is the launch rate. IceCube is now taking data smoothly. Variation ~2.5% Mean : 1.0 x 107 700Hz 4.5Hz UTC Time

Summary General performances of the IceCube PMT were shown. Averaged charge response and 2D CE map over 100 PMTs are derived. We have implemented the fundamental characteristics of our detector into the detector simulation. We measured absolute efficiency of our PMTs. Our measurement has a good correlation with Hamamatsu. We compared between PMT+ Glass/Gel and DOM. It showed good agreement over the wavelength. IceCube is now taking data smoothly. コントラストが悪い。 あと、埋まったDOMは氷中でもちゃんと期待したように動いているとか書いたら？でも、それを示す絵をあんまり出してなかったか。 モニタリングの話でいえるかな？ あと、ほんとは物理をやるのに基本的な性能がちゃんとでているとか言えたらかっこいいけどね。 例えば、角度分解の１度を達成するのにどれぐらいの時間分解能が必要でそれに必要な時間分解能を 実際に満たしている事を確かめたとか。 PMTペーパーに少し書いてあった気がするけど。 ほんとはこの辺がサマリーに来るべきだよな。 それで、それを千葉でいっぱい測定して確かめて、シミュレーションに組み込んだと。

Absolute Calibration in the ice Future work Absolute Calibration in the ice For the future work, we will calibrate the photon propagation in the ice using the absolutely calibrated instruments. Absolutely calibrated source so called standard candle and absolutely calibrated DOM are both deployed in the ice. DOM receive photons which emitted from the standard candle like this. From this measurement, we can analyze how many photons propagate in the ice. This study will reduce the systematic uncertainty in the ice and improve our estimation of the target particle’s energy. Golden DOM Absolute Calibrated detector Standard Candle Calibrated N2 laser Cone reflected light 130.04m

Thank you !

Backup Slides

Obtained Parameters Decay time (qtau/q0) Exponential/SPE (P_ex) Charge resolution (sq0/q0) 0.51±0.016 0.30±0.01 0.29±0.01 Summarized by Y. Hasegawa このスライドはなし。バックアップにまわす。 These are the obtained parameters. X-axis is the PMT gain. This shows the distribution of the index of the exponential term of fitting function which corresponds to the decay time. This one corresponds to the ration of gaussian term or exponential term. This one corresponds to the charge resolution. As you can see , many PMTs failed to fit at low gain. We did SPE measurement about 118 PMTs over 3 years. The present data is rich enough to picture average behavior. Many PMTs failed to fit at low gain Total 118PMTs (FY2004~2006) statistics is enough to picture average behavior !!

QE measurement at -32℃ QE measured at -32℃　was 5~10% lower than measured at 24℃ Keno_Inuzaka(TA1056) Daikaku_Inumura(TA1176) Sosuke_Inukawa(TA1181) 16.2% 13.5% 14.3% 15.4% 17.8% 16.7% 11.5% 10.5% 18.0% 17.0% 19.8% 17.6% 9.89% 9.66% 12.2% 11.3% 17.3% 14.4% 5.08% 4.65% 4.58% 4.05% 5.28% 4.29% Red : measured at 24℃ Blue : measured at -32℃

After pulses 2 μsec 8μsec show around 600nsec Peak 13SPE(600nsec) Rate Hz After pulses ここに測定について 載せる予定です Chirisからの返事待ち After pulses are common feature of the PMTs. This is the original peak, and afterpulses peaks occur around 600nsec , 2 micro sec and 8 micro sec after the main response peak. This single after pulse have typical charge of 13 SPE. Original pulse The various peaks are believed to correspond to ions of different mass

Charge Response @different gains @1x107 @5x107 @1x108 Here are the results of single photo-electron measurements for each gain. As you see, it’s hard to separate one photo-electron peak from the pedestal at 1x107 Gain. SPE pedestal