MEG II 実験 液体キセノンガンマ線検出器に用いる 光検出器 MPPC の 実装に向けた最終試験 家城 佳 他 MEG II collaboration + 九大の方々

Introduction 2

LXe detector upgrade 3 2inch PMT × 21612x12mm 2 MPPC × m inner face upgrade! (CG) Granularity will improve  γ energy resolution & position resolution expected to improve by a factor of 2! Requirements for the MPPC - sensitive to VUV scintillation light - large area (reduce the number of readout ch)

MPPC for MEG 4 Successfully developed a new type of MPPC! - PDE for VUV light > 20% - Large area fall time < 50ns with series connection - 50μm pitch pixel Hamamatsu S (X) - metal quench resister - Four chips Performance tests done so far: - Small subsample (prototype & sample of final model) tests in LXe - Mass test of prototype MPPCs (600 pcs) in room temperature and in LXe This talk: ① Mass test of final MPPCs (4200 pcs) ② PDE incident angle dependence check

Mass test of final MPPCs 5

Mass test Production of MPPCs is ongoing in Hamamatsu. (Production will finish in the end of October. ~2 month delay due to the quality control issue.) 2920 out of 4200 MPPCs are delivered are already tested. Temperature: 20 deg ~40 MPPCs (~160 chips) are measured at once. relays to change the readout chips Setup for the mass test We can test ~250 MPPCs / day

I-V curve measurement 7 We measure I-V curve of all chips to identify bad chips. We check: Any strange shape in the I-V curve? V bd (from linear + quadratic fit) compare to the data sheet chip0123 chip V I meas (from spline fit) I Breakdown voltage (V bd ) Current at V over = 5.0V (I meas ) ① ② ③

Results 8 V hama – V bd I meas – I hama Measured values were compared to the data sheet. Operation voltage from data sheet Measured breakdown voltage Measured current at V = V bd +5.0V Current at V hama from data sheet one outlier (next page) large current chips x ~10 Fraction of outlier chips is very small.

Chips with strange I-V curves 9 current offset strange bump Examples of bad chips 2 chips 8 chips I-V curves for some of the chips were strange. Some of the strange or large current chips were checked in more detail. Visual check, gain, waveform …  nothing strange. Some of them are sent back to Hamamatsu to do more checks. Anyways, the fraction of these suspicious chips is small. I I

PDE (Photon Detection Efficiency) vs. Incident angle 10

In our previous measurement … 11 PDE vs. incident angle α θ PDE = incident photons detected photons α sources 241 Am in LXe In the mass test for prototype MPPCs (~600 pcs), we noticed that the PDE is different for the MPPCs placed at different position. MPPCs Angular dependence? (much larger than what we expect from attenuation or reflection.) Also, the absolute value is smaller than our previous measurement (PDE~0.25).

Additional measurements 12 Large chamber gas Xe ~20 MPPCs on moving stage α α α Liquid Xe Small chamber MPPC (4 chips) We plan to do two different type of tests. Quick setup, only 4 chips Confirm angular dependence Movable stage, 20~30 chips. Check the angular dependence and MPPC by MPPC variation of PDE ↑ Today’s topic 10cm 70cm

Small chamber setup 13 α ● The effect of reflection from the wall is considered carefully.  chips are set very close to α Direct light >> Reflected light 100 <10 ● Relative position of the α and MPPC must be measured precisely. 6mm 27°49° chip by chip readout ● Use 1 MPPC (= 4 chips) 2 chips are placed at 30 deg, other 2 chips are placed at 50 deg.

Alignment check 14 (+ reconstruction from photo image) Scanned data x z y α We used 3D laser scan to measure the position of the sensors and α. position accuracy: 50~70μm FARO laser scanner MPPC

Basic properties check 15 Over voltage [V] 46 1p.e. charge chip Basic properties such as gain, crosstalk and afterpulse are measured by using LEDs. Gain vs. V over Crosstalk+afterpulse prob. vs. V over Results are consistent with previous measurements with sample MPPC. (This is the first time to use the a final MPPC in LXe.) this difference comes from different amplifier that we used. 8

PDE at different angle 16 PDE vs. V over angle ~ 30deg angle ~ 50deg PDE angular effect seems to be roughly consistent to what we expected. However, there is a large uncertainty in the correction factor for saturation of pixels.  detailed measurement in large chamber PDE reduction parameters for calculation: α energy = 4.78MeV W = 19.6eV PDE reduction expected from prev. meas. Preliminary chip1,2 chip0,3

Summary Mass test of final MPPCs – Final MPPCs are being produced in Hamamatsu. – 2920 pcs are already delivered are already tested. – Three types of bad chips were found (~20/9800 chips). They are being checked in detail. The fraction of bad chips is small. PDE vs. incident angle – In our previous test, PDE was measured to be smaller at larger angle. – This effect was reproduced in the new measurement. – Detailed test will be done in large chamber. – Effect of this to MEG II is studied in simulation  Next talk 17

Backup 18

MEG  MEG II 19 μ + beam e+e+ γ Liquid Xe γ-ray detector Liquid Xe γ-ray detector e + drift chamber & timing counter e + drift chamber & timing counter Gradient magnetic field  Sensitivity will improve by factor 10 stopping rate x ~2 resolution & efficiency x 2

More investigations for bad chips 20 bad chip (large current) normal chip Dark noise rate for the large current chips seemed to be high. Waveform of each pulse looked OK. Gain and V bd were not different from the normal chips. Nothing strange was found in a visual inspection with microscope. In total, we found ~20 bad chips out of For some of the bad chips, we did more detailed tests: Waveform check by oscilloscope, basic property measurement with LED etc. Some of the bad MPPCs are sent back to Hamamatsu for more investigation.

Transmittance factor (LXe-Si) 21

Small chamber 22 α Angle at center of the chips: 27 deg, 49 deg Incident angle will be different for different pixels in the chips.  How different are they? distributio n of angle 30 deg chips 50 deg chips This distribution must be taken into account when we consider this measurement with Kebab.

Alignment check 23 Data point distribution Projection along x axis α source on wire adhesive 0.5mm Position of the α source along the wire is reconstructed by comparing photo image and FARO scan data. FARO scan data: adhesive, wire holder ring Photo image: adhesive, wire holder ring, alpha source adhesive (dropped here by mistake) compare accuracy of the position measurement is 50~70 μm.