Belle
Barrel PID system (TOP counter)

Read out

Belle II experiment
Belle ll experiment is the B-factory
experiment and designed to collect 50 ab–1e+ e− colidion data at the SuperKEKB
which is a luminosity frontier accelerator.
Its aims are precision measurement of thestandard model and searches for newphysics. The particle identification (PID)
performance largely influences the physics
sensitivity.

π/K

The Time Of Propagation (TOP) counter is aring image Cherenkov detector. We measure
the time of propagation of the photons andseparate π and K meson.

HVprism～200 psTime ofPropagation

Micro Channel Plate Photomultiplier Tube (MCP-PMT)
MCP-PMT is the photon detector with the best time resolution. The time
resolution for a single photon is less than 50 ps. It is necessary for excellent
PID performance of the TOP counter.

～7 m～7.5 m

MCP

13.1 mm

After TOP counter installation, we found a problem that PMT rotates in B-field. First section is the study to fix against the magnetic force.
Second section is the status of TOP after installation. And the last section, we introduce the study for improvement MCP-PMT lifetime.

The measures for PMT rotation problem
wavelength cut filter

Destructive testing

MCP-PMTspacer
front boadGapIn B-field, MCP-PMT receives magnetic force(about 1 kgf),
then it rotates in module and comes off the filter.
The cause is space between PMT and front board.
We consider to put spacer in that space.
There is a difference in thickness between each PMT,
so we also prepare various size of shim.

DM0031 is thinner than DM0042 and uneven.
This is caused by silicone potting on the PMT bottom.

Recording the load when PMTpeel off from filter.
Fix PMT with vise and add
stress to only module frame.

DDM0031DM0042

Raw signal& sampling points

Hight~ 50 ps resolution is also
required for electronics

We compare laser hit efficiency at B=0 T with 1.5 T. The hit
efficiency decrease at some PMTs due to optical contact breaking.

sampling

Read out Asic do wave form sampling
every ~400 ps (different size bin by bin ! ).

Laser onLaser
resource

Hight

Time resolution of read out electronics

PMT rotation causes loss of photon !!

Clockmodule

We must measure 256 bin size
Time (ns)Out put file

1. Input 2 calibration pulses with constant time difference (ΔT) to TOP electronics
2. Get input sample number vs ΔT (bin) plot and fit 256 bin size
3. If we get constant ΔT (ns) value for all sample number, the set of bin size is correct

Calibration pulse(t = 0)

Ratio map of hit efficiency (□ mark : lower than 1σ from mean ratio)

20

ΔT

Collect many data(sample vs ΔT)

Optimize bin size

Sample number
1 PMTSample number

PMT peel off

We estimate that the bad effect of PMT rotation toward PID
performance down is less than 3% .
(These will be repaired during the term between superKEKB
Phase 2 and Phase 3.)

Sample number (bin)ΔT (ns)Photoelectron signal

DAQ start

ΔT (bin)Undesired
air gap(~100 μm)BC

Fix PMT with vise

In the shrot term, PMT with spacer did not peel off from filter around 1 kgf force.
We are planning a long term test.

Damage to PMT optical contact

Laser

ARed means PMT already peel off at before test, so recording the load when peeling off is proceeded.
Green means PMT already peel off at fixing with vise.
Blue means PMT is peeling off after making module.

EPDM gum spacer (left, 1.5 mm)with shim(0.5 mm)
Glass epoxy spacer (right, 1.6 mm)

PMT

Add stress to frameTime resolution～600 psTime resolution～50 psNext : We measure time resolution of laser signal (including PMT time resolution)
That is the time resolution for Belle II TOP physics run.

The measurement of after pulse for improvement MCP-PMT lifetime

The weighted mean after
Delay time distribution of after pulse by TDC
pulse rate of each type PMTAfter pulse rate[%]After pulse rate[%]<Motivation>• The background of Belle ll may increase more than the current estimation.
Lifetime define
Red : Rate around 10 ns+
Black : NormalH
Green:Ratearound40nsQE⇒MCP-PMT is required to have longer lifetime.
= 0.80
Red : ALDBlue : Total rate
• The lifetime is defined by Quantum Efficiency (QE) drop of photocathode.
Initial QE
Green : Life-extended ALD⇒If QE drop happen, the PID performance of TOP counter also decreases.
Blue : Candidate of ion
Themechanism⇒Want to understand the mechanism of QE drop to improve the lifetime.
+He+ , CO 2+ , O+ofemittingionHO• The photocathode is reacted with and/or damaged by neutral gas and ion from MCP.
22
⇒In this study, measurement of ion’s effect.
e−<Approach>
MCP• When the ion interacts, it will emit electron.
Delay time[ns]Gas :• This electron can be observed as second pulse.
<Summary>⇒This is called after pulse.
•IneachPMT,thedifferentionarrives
atthe⇒The time difference between after pulse and main pulse relates to mass
photocathode.
of ion : 𝑡 ∝ 𝑚𝑖𝑜𝑛++⇒Normal:H,He
Measure three type of sample PMTs which have different lifetime.
ALD stands for Atomic Layer Deposition.
2+ ++ALD coating : H2 O , CO2 , O
Normal : 1 year
It is intended to suppress residual gas• The total after pulse rate is no correlation of PMT type.
ALD : 8 years
Lifetime of each PMT in Belle llemitting.
• ALD and life-extended ALD don’t have clear difference.
Life-extended ALD : 15 years