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
upgrade!
(CG)
inner face
2inch PMT× 216
12x12mm2 MPPC× 4092
3
1m
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
Hamamatsu S10943-4186(X)
Successfully developed a new
type of MPPC!
- PDE for VUV light > 20%
- Large area
fall time < 50ns with series
connection
- 50μm pitch pixel
- Four chips
- metal quench resister
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
4
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. 2450 are already tested.
Setup for the mass test
Temperature:
20 deg
~40 MPPCs (~160 chips)
are measured at once.
relays to change the readout
chips
We can test ~250 MPPCs / day
I-V curve measurement
We measure I-V curve of all chips to identify bad chips.
We check:
I
① Breakdown voltage (Vbd)
compare to the data sheet
② Current at Vover = 5.0V (Imeas)
③Any strange shape in the I-V curve?
chip0123
Vbd
(from linear + quadratic fit)
5V
chip0
1
3
2
Imeas (from spline fit)
7
Results
Measured values were compared to the data sheet.
Operation voltage
from data sheet
(gain=2.0x106 @ 25℃)
Measured
breakdown voltage
Vhama – Vbd
Measured current at
V = Vbd+5.0V
Current at Vhama
from data sheet
Imeas – Ihama
8
one outlier
(next page)
large current chips
x ~10
Fraction of outlier chips is very small.
Chips with strange I-V curves
I-V curves for some of the chips were strange.
I
I
Examples of bad chips
strange bump
9
current offset
2 chips
8 chips
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.
PDE (Photon Detection Efficiency)
vs.
Incident angle
10
In our previous measurement …
In the mass test for prototype MPPCs (~600 pcs), we noticed that the
PDE is different for the MPPCs placed at different position.
PDE vs. incident angle
in LXe
α
α sources
241Am
θ
MPPCs
detected photons
PDE = incident photons
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).
11
Additional measurements
We plan to do two different type of tests.
Large chamber
Small chamber
Liquid Xe
gas Xe
12
α
MPPC
(4 chips)
• Quick setup, only 4 chips
• Confirm angular dependence
↑ Today’s topic
α
~20 MPPCs
on moving
stage
α
70cm
• Movable stage, 20~30 chips.
• Check the angular dependence and
MPPC by MPPC variation of PDE
Small chamber setup
● Use 1 MPPC (= 4 chips)
2 chips are placed at 30 deg,
other 2 chips are placed at 50 deg.
● The effect of reflection from the wall
is considered carefully.
 chips are set very close to α
Direct light >> Reflected light
100
<10
α
6mm
27°
chip by chip
readout
49°
● Relative position of the α and MPPC
must be measured precisely.
13
Alignment check
We used 3D laser scan to measure the position of the sensors and α.
(+ reconstruction from photo image)
Scanned data
FARO laser
scanner
14
α
z
y
x
MPPC
position accuracy:
50~70μm
Basic properties check
1p.e. charge
Basic properties such as gain, crosstalk and afterpulse are measured
by using LEDs.
Crosstalk+afterpulse prob. vs. Vover
Gain vs. Vover
0.3
this difference
comes from
different
amplifier that
we used.
15
chip 0
1
2
3
0.2
4
6
8
Over voltage [V]
Results are consistent with previous measurements with sample MPPC.
(This is the first time to use the a final MPPC in LXe.)
PDE at different angle
PDE vs. Vover
PDE reduction
chip1,2
chip0,3
angle ~
30deg
angle ~
50deg
PDE reduction
expected from
prev. meas.
16
parameters for
calculation:
α energy = 4.78MeV
W = 19.6eV
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
Summary
• Mass test of final MPPCs
– Final MPPCs are being produced in Hamamatsu.
– 2920 pcs are already delivered. 2450 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
Liquid Xe
γ-ray detector
stopping rate x ~2
γ
resolution &
efficiency x 2
e+
μ+
e+ drift chamber
& timing counter
Gradient magnetic field
 Sensitivity will improve by factor 10
beam
19
More investigations for bad chips
In total, we found ~20 bad chips out of 9800.
For some of the bad chips, we did more detailed tests:
Waveform check by oscilloscope, basic property measurement with LED etc.
• Dark noise rate for the large current
chips seemed to be high.
• Waveform of each pulse looked OK.
• Gain and Vbd were not different from
the normal chips.
• Nothing strange was found in a visual
inspection with microscope.
bad chip
(large current)
normal chip
Some of the bad MPPCs are sent back
to Hamamatsu for more investigation.
20
Transmittance factor (LXe-Si)
21
Small chamber
Incident angle will be different for different pixels in the chips.
 How different are they?
α
Angle at center of the chips:
27 deg, 49 deg
distributio
n of angle
30 deg
chips
22
50 deg
chips
This distribution must be taken into account when
we consider this measurement with Kebab.
Alignment check
adhesive (dropped here
Data point distribution by mistake)
Position of the α source along the wire is
reconstructed by comparing photo image
and FARO scan data.
Projection along x axis
FARO scan data:
adhesive, wire holder ring
23
compare
Photo image:
adhesive, wire holder ring,
alpha source
accuracy of the position
measurement is 50~70 μm.
adhesive
𝑦
𝑧
α source
𝑥 on wire
0.5mm