1. Daisuke Kaneko, ICEPP, Univ. of Tokyo on behalf of MEG collaboration
Performance of UV-sensitive MPPC for liquid xenon detector in MEG experiment
Daisuke Kaneko, ICEPP, Univ. of Tokyo
on behalf of MEG collaboration

2. μ+→e++γ MEG experiment
MEG is searching for charged lepton flavor violating decay
Highly suppressed in the standard model. (ℬ∼ 10 −50 )
But, sizable decay rate (ℬ∼ 10 −12 ⋯ 10 −14 ) is predicted in theories beyond the standard model.
We published the latest result with data.　　 𝓑<𝟓.𝟕× 𝟏𝟎 −𝟏𝟑 (90% CL), the most stringent experimental upper limit Phys. Rev. Lett. 110, (2013)
Data taking finished in 2013 August.
μ+→e++γ
2013 Nuclear Science Symposium and Medical Imaging Conference

3. MEG upgrade MEG is planning a major upgrade.
Our proposal was approved by Paul Scherrer Institute (Switzerland) in 2013 January. (arXiv: )
Upgrade items
Increase beam intensity
Active and thinner target
Unified volume, stereo angle wire drift chamber
Pixelated, SiPM read out timing counter
Radiative μ decay counter to remove accidental background
SiPM readout liquid xenon γ detector
Sensitivity goal is 𝟓× 𝟏𝟎 −𝟏𝟒 , 10 times higher than current MEG.
2013 Nuclear Science Symposium and Medical Imaging Conference

4. Upgrade of liquid xenon detector
Present
Improved granularity by smaller photo-sensor.
MPPC
PMT
γ-ray
Upgraded
CG image
12mm
PMT
46mm
γ-ray
Resolution was limited by sensor size, especially for shallow events.
2013 Nuclear Science Symposium and Medical Imaging Conference

5. Performance : Energy resolution
depth ＜ 2cm
40 % of events
depth ≧ 2cm
60 % of events
Upgraded
Upgraded
σup
2.4% ↓
1.1%
σup
1.7% ↓
1.0%
Present
Present
Response to 52.8MeV γ (MEG signal)
2013 Nuclear Science Symposium and Medical Imaging Conference

6. Performance : Position resolution
Red : Present
Blue : Upgraded
Depth from inner face [cm]
Position resolution in σ [mm]
2013 Nuclear Science Symposium and Medical Imaging Conference

7. Cross-sectional image of MPPC
UV sensitive MPPC
Development is performed in collaboration with Hamamatsu Photonics.
Requirements
Sensitivity to liquid xenon scintillation (λ = 175 nm)
Large active area (12×12 mm2)
Single photon counting capability
Moderate trailing time constant (τ ＜ 50 ns)
In order to improve sensitivity
Remove protection layer
Matched refractive index
　 to liquid xenon
Anti reflection coating
Cross-sectional image of MPPC
2013 Nuclear Science Symposium and Medical Imaging Conference

8. Achieved performance ↑Charge spectrum with 12×12㎟ MPPC 12×12㎟ MPPC
↑17% of PDE (crosstalk & after pulse removed) is already achieved. Even higher PDE is expected after new technologies of Hamamatsu will be included.
3×3㎟ MPPC
2013 Nuclear Science Symposium and Medical Imaging Conference

9. Series connection of MPPC
Remaining issue was long tail in waveform (τ ～ 200ns) due to large capacitance of large-area sensor.
In order to shorten long decay time, reduce sensor capacitance by subdividing active sensor region and connect them in series.
6mm
12mm
Signal is expected to be sharpened due to smaller capacitance. However there is anxiety that S/N becomes worse.
2013 Nuclear Science Symposium and Medical Imaging Conference

10. How many segments to divide
We simulated the series connection using four independent small MPPCs (6×6㎟ each).
C is capacitance when all segments are connected in parallel.
4 segments (C/16)
4 × 6mm × 6mm
2 segments (C/4)
2 × 12mm × 6mm
2013 Nuclear Science Symposium and Medical Imaging Conference

11. 2 types of series connection
There are two options for series connection of segments; "simple" and "hybrid“.
Serial – signal
Parallel - bias
Simple series connection
Hybrid connection
amplifier
○ Less parts
○ Automatic over
voltage adjust
× Higher voltage
× Different potential between each sector
× Extra parts
× Gain uniformity
is required
○ Same voltage as
single MPPC
○ All sectors have common potential
bias
Both connection types work in LXe, but hybrid is more advantageous.
2013 Nuclear Science Symposium and Medical Imaging Conference

12. Set up for liquid xenon test
6mm
MPPC
LED
10 kΩ
10 nF
2013 Nuclear Science Symposium and Medical Imaging Conference

13. Waveform of 1 photo electron
Signal tail is successfully reduced down to 30-50ns !
The tail for 4 segmentation is as short as that for 3×3㎟ single sensor as expected.
1 p.e. signal can be resolved
4segments
V →
Vov = 3.0V
2segments
Vov = 3.0V
0 p.e.
1 p.e.
2 p.e.
charge →
Parallel
Vov =1.5V
×10 amp
t →
Baseline RMS have no significant difference, ～2mV
2013 Nuclear Science Symposium and Medical Imaging Conference

14. Decay time and connection type
Calculated decay constant by fitting waveform by event.
　4 segments 25ns
　2 segments 46ns ↕
　Parallel ns
50ns of target time constant is achieved in both cases.
2 segments
4 segments
2013 Nuclear Science Symposium and Medical Imaging Conference

15. Gain (multiplication factor)
Gain is 4～10×105
Gain is lower when capacitance is smaller, while pulse amplitude is almost the same.
Lower than that of parallel connection, but 1 p.e. peak is resolved with proper voltage.
2 segments
4 segments
2013 Nuclear Science Symposium and Medical Imaging Conference

16. Effect of long cable
In MEG LXe detector, long coaxial cable of about 10m exists between sensor and readout electronics.
2 segments
4 segments
Cable Length [m]
Gain does not change, while decay time slightly increases (1ns/m).
Constant is 56ns at 11m of 2 segments, but still acceptable.
2013 Nuclear Science Symposium and Medical Imaging Conference

17. Summary of serial connection
Gain
3V)
Peak separation
Noise rms [mV]
Decay
const [ns]
Rise
4 segments
4×105 ○
2.0 – 2.5
25 ～ 33
2 – 3
2 segments
8×105 ◎
46 ～ 56
4 - 5
* 1 segment
1.2×106
3～5
～200
～10
* estimation from previous measurement
Gain is larger and peak is clearer when number of segment is few.
This makes charge calibration easier and relative gain error smaller.
On the other hand, signal is sharper with finer segmentation. Timing resolution is expected to be better, and pile-up will be reduced.
4 segments is favorable, because gain error is small enough, and timing resolution should be as good as possible. In addition, if single p.e. can not be resolved, it is easier to switch 2 segments.
2013 Nuclear Science Symposium and Medical Imaging Conference

18. Prototype detector γ
We are planning to make prototype of MPPC read-out detector
576 MPPCs on incident face.
Beam test will be performed in 2014
CG image of prototype detector
2013 Nuclear Science Symposium and Medical Imaging Conference

19. Summary
We are developing UV-sensitive MPPC for upgrade of liquid xenon detector in collaboration with Hamamatsu Photonics.
>17% PDE was already obtained by 12×12㎟ and 1p.e. countable MPPC prototype.
In order to solve problem of long waveform due to the carge capacitance of large-area MPPC, we proposed series connection.
Serial connection was tested in LXe environment.
30 – 50 ns of decay constant was achieved
Single p.e. can be resolved
Prospects
Prototype detector with ～600MPPCs is being prepared.
Detector construction will start in 2014 for next MEG experiment from 2016.
2013 Nuclear Science Symposium and Medical Imaging Conference

20. 2013 Nuclear Science Symposium and Medical Imaging Conference
Fin.
2013 Nuclear Science Symposium and Medical Imaging Conference

21. PCB based feedthrough Slope (Charge/Voltage)
= ± 1.2 [fF] LEMO
= ± 2.0 [fF] MMCX
Charges of 1 p.e. event are compared between usual LEMO-connector and PCB feedthroughs.
About for charge, there is no difference.
Another properties are being analyzed.
2013 Nuclear Science Symposium and Medical Imaging Conference

22. Serial connection test at room temp.
Set up for room temp. test
Observed Waveform
LED light, averaged
LED
MPPC
mask
4 serial 30ns
2serial,2parallel 50 ns
4parallel 200ns
2013 Nuclear Science Symposium and Medical Imaging Conference

23. Reminder 1 etc. How to reduce long tail ?
Application to LXe detector
Sensitivity to VUV (λ=175nm)
1 p.e. resolve with 12x12㎟
0 p.e.
1 p.e.
2 p.e.
Long tail
(τ～200ns)
Worse S/N
Increase pileup
How to reduce long tail ?
Target is about 50ns (～Scintillation decay)
etc.
2013 Nuclear Science Symposium and Medical Imaging Conference

24. Reminder 2 How to reduce long tail of 12×12㎟ MPPC ?
Reduce quench resistance as low as possible.
Reduce input resistance of amplifier
Divide and connect in series active sensor area
Tail is not be reduced than expected with lower quench
Waveform is distorted with long coaxial cable
Rs = 33Ω, 10m cable
200ns
Most realistic plan
The effect was confirmed at room temperature test.
2013 Nuclear Science Symposium and Medical Imaging Conference

25. Raw-Waveform 25μ Low Rq 50μ Low Rq 25μ Mid Rq 50μ Mid Rq 25μ High Rq
2013 Nuclear Science Symposium and Medical Imaging Conference

26. pre amplifier
2013 Nuclear Science Symposium and Medical Imaging Conference

27. Result : Waveform and Quench-R
Fitting waveforms with a double-exponential function works well.
・The tail time constant do not depend on Rq so much
・25µm pitch MPPC is not so different from 50µm.
1 Rising & 1 Trailing component, τr ,τt
MPPC Type
Quench
R [kΩ]
Trail time constant
τt [ns]
Rise time constant
τr [ns] 50
Over voltage [V]
1.0
1.2
1.5
R1 Low
349
246
255
276
19.6 21
24.3
R2 Mid
606
277
288
314 19
19.4
20.7
R3 High
8867 -
783
16.5 25
Over voltage [V]　
2.0
2.5
3.0
719
214
1170
218
20.1
21433
538
23.5
2013 Nuclear Science Symposium and Medical Imaging Conference

28. … Vb Rp Rs Rq Cs Cd Cause of long waveform
AMP side Vb
Waveform can not be shortened only by reducing the quench resistance and cell capacitance.
bias voltage Rp
protect
resistance
MPPC side
amplifier
quench
resistance Rs Rq …
shunt
resistance Cs Cd
stray
capacitance
Rs,Rp×Cs term is dominant against Rq×Cd term under small Rq condition?
diode
(sensor)
2013 Nuclear Science Symposium and Medical Imaging Conference

29. How to shorten waveform
Smaller Rs
→　Effective, but only in limited situation.
↓Data taken with large-area MPPC at room temperature↓
Rs = 50Ω
short cable
Rs = 33Ω
short cable
Rs = 33Ω
long cable
τt= 192 ns
τt= 138 ns
Tail is reduced with small Rs, but the waveform is distorted with a long read-out cable because of the impedance mismatch.
Smaller Rp
→　Not effective
2013 Nuclear Science Symposium and Medical Imaging Conference

30. 2013 Nuclear Science Symposium and Medical Imaging Conference

31. Width of 1 p.e. peak
2013 Nuclear Science Symposium and Medical Imaging Conference

32. Leading edge and connection type
2 segments
4 segments
Leading time constant is also shorter in smaller capacitance.
Leading time constant seems to depend on o.v. , fitting for small waveform was difficult. ↓
To use pico sec pulse laser, Hamamatsu PLP nm.
Available soon.
2013 Nuclear Science Symposium and Medical Imaging Conference

33. Detailed PDE calculation of G-type MPPC
Voltage
1.4
1.7
Errors
Gain
3.1%
1.6%
Alpha peak
0.26%
0.21%
Correction
3.2%
1.7%
Model syst.
14%
22%
total
15.3%
22.2%
In 2013Feb-1 test UM-R2
voltage
1.4
1.7
PDE
+/-
15.5
2.5
16.9
3.7
2013 Nuclear Science Symposium and Medical Imaging Conference