MEG II 実験のための陽電子タイミングカウンター実機建

設

Construction of Positron Timing Counter for MEG II experiment

西村美紀（東大）

他　 MEGII コラボレーション

日本物理学会 2015 年 秋季大会

大阪市立大学（杉本キャンパス）

2Contents

• Introduction• Physics Motivation• MEG II Experiment• Positron Timing Counter

• Status• Counter Mass Production• Scintillator• SiPM• Counter property

• Summary

Physics Motivation• Search for cLFV (charged lepton flavor

violation), μ+ e+γ decay• Forbidden in the SM• Sizable branching ratio is expected by many

bSMs

• Most stringent upper limit of the branching ratio is set by the MEG experiment;

90% C.L. (Phys. Rev. Lett. 110(2013) 201801)

Already started exploring bSM region!

3

Upgrade MEG experiment (MEG II)Sensitivity

One example

4What should we measure? 4

We should measure Timing ・ Position ・ Momentum precisely.

Accidental BGDominant

Physics BG

Signal Background

(Michel decay)

The resolutions of every detector improved twice.

55

Drift Chamber

Positron Timing Counter

Liquid Xenon Gamma-ray Detector

Superconducting Magnet

MuonPositron

Gamma-ray

Radiative Decay Counter

MEG II

66Overview of Timing Counter

4 cm

or 5 cm

12 cm

Thickness 5 mm

6 SiPMs in series at the both endsAdvanSiD (Italy) 3x3 mm2, 50x50 um2 pixels

Fast Plastic ScintillatorBC422, rise time 0.35 nsattenuation length 8 cmWith reflector (3M film)

PCB

Cable （ RG178 ）Non-magnetic

PCB

Back planeLong PCB ~80 cmMulti layer, coaxial like

Optical Fiber for laser lightInter-counter time offset is calibrated by laser.

256 x 2 (up and down stream)

counters

Next talkM. Nakao

77Multiple Hit

Since TC consists of many small counters, the positrons hits many counters.

Number of hit counters (MC)

*In magnetic field

The time resolution is improved by multiple hits.

Averaging timing information, final TC resolution scales as . （ We already proved the principle in beam tests. ）

Expectation of Overall Resolution 8

Resolution vs. # of hit counters

σ <30 ps

Res

olut

ion

(pse

c)

~5 ps

~30 ps

Multiple-scattering

Inter-counter jitter from electronics chain

* Path length is corrected by drift chamber track information

99Up to the Present

Single Counter R&DStudy basic property

Optimize the counter design

Construction

Multi Counter R&DProve multiple hit scheme

Check rate dependence

Software development• Reconstruction• Simulation

Calibration• Laser• Muon Normal

Decay

Engineering run

Beam test @PSI

Single Counter Test

Next talkM. NakaoNext next talk

K. Yoshida

9

Engineering run

Schedule

• We will construct positron timing counter with SiPMs in this year.• Engineering run with half counters of TC down stream will start in

this November to confirm the final performance in the MEG II environment.

10

Aug.

Downstream Construction Upstream

Engineering runand physics run

Oct. Dec. Feb. Apr. Jun.2016

128 counters

11Counter Mass Production• Goal for this year: 128 counters by middle of October.

SiPM Array

Scintillator

Single SiPM

Counter production

Counter Test

• Scintillator• Saint-Gobain BC422• 600 plates

• 360: 120x40x5 mm3

• 240: 120x50x5 mm3

• SiPM• AdvanSiD, ASD-NUV3S-P High-Gain

(MEG), 3x3 mm2 ,50x50μm2 pixels• 3248 SiPMs (correspond to 541 arrays)

• SiPM array• 6 SiPMs in series connection

12Scintillator• Test with a SiPM array on one side of

scintillator. Check the light yield of plates by monitoring current of SiPMs.

• 3/4 scintillators are worse than we ordered. We will replace it.• The company already started new

production for 3/4.

Reso

lutio

n (p

s)

Current of SiPMs (μA)5.5 6 6.5 7 7.5 8

60

65

70

75

80

85

905 cm result

Different colors correspond to different batch.

The same as prototype

The same as prototype

Pico ammeter

Measure the current

13SiPM Single IV curve• Break down voltages are stable. (RMS 0.03 V)• However IV curve behavior is not the same.

We see two type of IV curve. One brows up faster. Type-2 seems to have larger dark current. (45.5 % is the type-2)

Type -1

Type -2

14SiPM dark current and production

• Some group have lower dark current, but that in the other group spread large range.

• We use all of them even though some SiPMs have high dark current.• We selected similar dark current 6 SiPMs for one array.

15SiPM array

6 SiPMs are connected in series (= array). We got 541 arrays. 16 arrays cannot be used.• 12 arrays: Window become yellow because soldering temperature is high.• 1 array: One SiPM window is broken.• 1 array: We didn’t see any signal.• 2 array: IV curve is strange.yellowed

normal

16SiPM array selection

We rejected 24 arrays by comparing IV curve with single SiPM IV.• 21 arrays: current is

inconsistent with single SiPM measurement.• 3 arrays: break down

voltage is inconsistent with single SiPM measurement.

Current of arrays (uA) @ 3.0 OV

Expe

cted

cur

rent

from

sin

gle

SiPM

(uA)

@ 3

.0 O

V

for 4 cm

0-1 1BD Difference (V)

BD Difference b/w single and array meas.

10 14 18622

6

10

14

18

17SiPM array

4 cm5 cm

Prototype 0.225(4 cm)

• Relative PDE with scintillator light• PDE of final SiPMs are worse than one of

prototype.

• Check IV curve • They also have two type of IV behavior.• 2 arrays are strange.

18Optimal voltage

• If IV curve blows up fast, optimal OV range becomes narrow because of dark current.

Final counter with Type -1 SiPMs

Final counter with Type -2 SiPMs

Bias scan with type-1 and type-2 counter

19Counter Mass ProductionMake combination with similar property (= dark current and relative PDE) arrays.With Optical cement (BC600), we attached SiPM arrays to scintillator. We made 138 counters.

Remove void in vacuum

20Counter Mass TestCheck the resolution for every final counter. (measure at 3 points)• Under the way; now we finished 62 counters.

Source Sr90(<2.28MeV, β-ray)

Digitizer（ 1.6GHz sampling

speed ）

Reference Counter5x5x5 mm (BC422), 1 Hamamatsu SiPM

Trigger, Collimator ShapingPole Zero cancellation

Test Counter

Amplifier

@30 degree

21Counter Mass Test

• Final counter resolutions are 70-80 ps.

• With prototype we expected 64 ps resolution.

Þ Overall resolution becomes bad 15 %

Still TC overall resolution is ~30 ps.

• This difference come from low PDE. • SiPM production • Soldering process

under investigation.

w/ prototype SiPMs

*Not with all scintillator

Final counters

The difference is 10 ps.

22Support structure

• For this year run we have only produced part of the full TC DS structure (50% )• 16 backplanes (final layout)

has been produced for this year run.

23Summary

• Positron timing counter based on segmented scintillation detector with multiple-SiPM readout has been developed for MEG II.

• The construction of TC is on-going.• Scintillators and SiPMs tests were curried out.

• Light yield of 3/4 scintillator seems small, but they will be replaced.• SiPMs PDE are smaller than the prototype one.

• All counters for engineering run were produced.• Counter resolution is 70-80 ps.• Though the resolution of the final single counters are worse than

one of a prototype counter, overall resolution of ~30 ps is still expected.

Back up

24

25

26Reminder (5 cm)

0.11 0.12 0.13 0.14 0.15 0.16 0.1765

70

75

80

85

90

95

Signal amplitude in quick test (V)

Reso

lutio

n (p

s)@ 3.1 OV

27