Catania Terrestrial Magnetic Field Effects on Large Photomultipliers

Catania Terrestrial Magnetic Field Effects on Large Photomultipliers E. Leonora, S. Aiello INFN, sez. Catania on behalf of the KM 3 Ne. T consortium supported by the EU in FP 6 under Contract no. 011937 and FP 7 under Grant no. 212525

Optical Modules in KM 3 Ne. T Catania One of the proposed designs: - a single large area photon sensor (8”-10” PMT) - housed in a glass sphere (13”) resistant to high hydrostatic pressures - surrounded of a mu-metal cage as magnetic shield - optical and mechanical link by optical gel Picture of an assembled OM (10” PMT) Schematic 3 D view (KM 3 Ne. T Technical Design Report km 3 net. org/KM 3 Ne. T-TDR. pdf) Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 2

Aims of the work Catania The influence of the Earth’s magnetic field on three large area PMTs was studied The aims were twofold: - to evaluate effects of the Earth’s magnetic field on PMT performance - to decide whether the use of magnetic shielding is necessary The three tested PMTs were produced by Hamamatsu R 7081 Hamamatsu (10” STD): 10 inch. photocathode Standard bialkali photocathode (QE ≈ 25% @ 400 nm) box and line with 10 stages R 5912 Hamamatsu (8” STD): 8 inch. photocathode Standard bialkali photocathode (QE ≈ 25% @ 400 nm) box and line with 10 stages Sketch of 8 in. PMT. (courtesy of Hamamatsu K. K. ) R 5912 HQE Hamamatsu (8” HQE): 8 inch. photocathode Super bialkali photocathode (QE ≈ 32% @ 400 nm) box and line with 10 stages Sketch of 10 in. PMT. (courtesy of Hamamatsu K. K. ) Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 3

Measurement facility Catania • A dark box (1 x 0. 5 m) able to rotate 360° with respect to vertical axis (1° step) and to change its vertical inclination (10°step) was made. • No magnetic materials were used in its construction Picture of the box in Horizontal position Picture of the box in vertical position Picture of the box in tilted position Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 4

Measurement setup Catania • A pulsed laser source (410 nm, 60 ps width, 10 KHz) attenuated in spe condition (Picoquant PDL 800 -B) • Light pulses conducted by means of multimode optical fibres • An optical diffuser was used to produce homogeneous illumination over the photocathode (Thorlabs) • Charge measurements made by NIM QDC 7422 Silena • Time measurements made by NIM 7072 T FAST • A second PMT was used as monitor of the light source …thanks to O. Kalekin and P. Keller for their contribution ! Erlangen 12 -14 October 2011 Sketch of the apparatus emanuele. leonora@ct. infn. it 5

Positions measured Catania Every one of the three PMTs was measured in 3 inclinations: - vertical downwards ( Tilt = 0° ) ; 50° downwards ( Tilt = 50° ); horizontal ( Tilt = 90° ) Tilt : 50° Vertical position Tilt : 0° Horizontal position Tilt : 90° Z Vertical axes Z 90° North 180° 270° 0° Horizontal angles Z Vertical axes Tilt 90° North 180° 270° 0° Horizontal angles North 180° 0° Horizontal angles 270° NORTH 0° Top view For each inclination, the PMT under test was rotated 360° around its vertical axis in 30° steps Dy 1 Dy 2 90° 270° Each PMT started its rotation from the same position with respect to the box and to the Earth’s magnetic field PMT downwards 180° Starting position for each PMT

Measurements and conditions For each PMT and for each position the following parameters were measured: - Detection Efficiency - Gain - P/V ratio - Charge Resolution (sigma) - TT (relative) - TTS (FWHM) PMTs were powered with an ISEG PMT active base PHQ 7081 -i-2 m PMTs were at same Gain condition = 1. 5 E 7 All measurements were made on PMTs un-shielded and repeated with a mu-metal magnetic shield Catania

Magnetic shield Catania Characteristics of the cage of mu-metal wire (ITEP, Moscow): • a hemispherical part ( 30 cm diameter, 14 cm height) • a flat part (30 cm diameter ) with a hole in its centre ( 12 cm diam. ) • wire of 1 mm of diameter • pitch of 68 x 68 mm • shadow on the photocathode ≈ 5% • shielding effect measured ≈ a factor 4 Picture of the parts of the cage Erlangen 12 -14 October 2011 Picture of the PMT under test surrounded by the cage emanuele. leonora@ct. infn. it 8

Measurements of the Earth’s magnetic field Catania Magnitude and uniformity of the Earth’s magnetic field were measured in the place of the test Map of the local measurements of Earth’s magnetic field (step 0. 6 m) …thanks to P. Vernin for his contribution and tools The signed point was selected as position of the Test BOX: - B ≈ 40 micro. Tesla - good uniformity over 1 meter area Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 9

0° 0° Detection efficiency Catania 50° • In naked 8” PMTs the impact of the magnetic field was smaller than naked 10” 90° • The shield reduced considerably the variations for the 10” PMT • The increased QE in the HQE 8” PMT compensates the smaller detection area respect to the 10” NAKED Erlangen 12 -14 October 2011 SHIELDED emanuele. leonora@ct. infn. it 10

0° 0° Gain Catania 50° • Greatest gain variation was less than 10% for both the naked 8” • Considerable variations in the 10” PMT naked, up to 29% 90° • The shield reduces variations in both the 8” PMTs, with variations less than 4. 4 % • The shield reduces strongly the variations in the 10” PMT, with variation less than 7% Erlangen 12 -14 October 2011 NAKED emanuele. leonora@ct. infn. it SHIELDED 11

0° 0° Peak to Valley ratio Catania 50° • Considerable variations for all the un-shielded PMTs 90° • Great reductions in variation by using the magnetic shield • Small improvements in average values with the magnetic shield Erlangen 12 -14 October 2011 NAKED emanuele. leonora@ct. infn. it SHIELDED 12

0° 0° Charge Resolution (sigma) Catania 50° • Large effects of magnetic field for the unshielded 10” PMT 90° • The mu-metal cage greatly reduced the variation in 10” PMT • No large effects due to the magnetic field for 8” PMTs. • The mu-metal cage reduced variations for both 8” PMTs Erlangen 12 -14 October 2011 NAKED emanuele. leonora@ct. infn. it SHIELDED 13

0° 0° TT Catania 50° • No significant variations due to magnetic field for all the PMTs 90° Erlangen 12 -14 October 2011 NAKED 90° • No considerable effects of the mu-metal cage • Difference in average value due to the different size between 8” and 10” PMT emanuele. leonora@ct. infn. it SHIELDED 14

0° TTS Catania 50° • Variations over the 10% in all the un-shielded PMTs 90° • Considerable reduction in variations with the magnetic shield • No significant improvement in average values with magnetic shield Erlangen 12 -14 October 2011 NAKED emanuele. leonora@ct. infn. it SHIELDED 15

Measurements of fraction of spurious pulses Catania The fraction of the number of spurious pulses on the number of main pulses was measured Measurements conditions: - Laser source at 10 KHz in spe condition - PMTs vertically downward - Horizontal angles: [ 0°, 180° ] (range with max detection variation) - All measurements were done on PMTs naked and repeated with the mu-metal cage Spurious pulses definition Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 16

Fraction of delayed and pre- pulses Catania Fraction [%] prepulse delayed naked shielded 8'' STD 8'' HQE 10'' STD 8'' STD 8'' HQE 10'' STD Min 0. 01 0. 00 Ave 0. 02 0. 03 0. 01 0. 02 0. 01 Max 0. 03 0. 04 0. 01 0. 04 0. 02 0. 03 Min 5. 36 4. 77 5. 36 5. 70 4. 92 5. 90 Ave 5. 99 4. 87 6. 47 5. 97 5. 02 6. 23 Max 6. 60 4. 98 7. 80 6. 08 5. 04 6. 46 Erlangen 12 -14 October 2011 • No significant magnetic effects on fraction of pre-pulses, and nor improvements with the shielding • No significant effects in delayed pulses for 8” PMTs • Considerable variation in delayed pulse for the 10”, reduced by mu-metal cage emanuele. leonora@ct. infn. it 17

Fraction of After Pulses type 1 and type 2 Catania • no significant variations in fraction of type 1 and type 2 After pulse • no significant improvements with the mu-metal cage • standard PMTs had similar fractions of type 1 and 2 After pulse • HQE PMT had fractions of type 1 and type 2 After pulse greater than standard PMTs Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 18

Conclusion Catania • The impact of the magnetic field was found to be smaller on the 8” PMTs than on the 10” PMT. • The magnetic shielding reduced strongly the variations in 10” PMT and even improved performance. Also for 8” PMTs, but with lower effects • The increased Quantum Efficiency in the HQE 8” PMT almost compensates smaller detection surface compared to the 10” PMT • No significant magnetic effects were measured on Transit Time, and on fraction of after pulses type 1 and 2 • Significant variation in delayed pulsing was measured only in naked 10” PMT, reduced by the magnetic shield • HQE photocathode produced greater type 1 and type 2 after-pulsing than the standard bialkali photocathode. Erlangen 12 -14 October 2011 emanuele. leonora@ct. infn. it 19