Development of gas wire chambers for in-beamcharged particle detector in the KOTO experiment
K.Nakagiri (Kyoto University), for the KOTO collaboration

Introduction to the KOTO experiment
Goal of the KOTO experiment0= Discovery of KL ! ⇡ ⌫ ⌫¯ d¯0s

•direct CP-violating rare dicay•loop-induced decay➡ BSM particles can contribute•suppressed decay in the SM (2x10-11)•small theoretical uncertainty (2%)

Principle of the experiment

VtsVtd⇤Z0+nothing

Calorimeterd¯dW ( ˜)t (ũ)KL ! ⇡ ⌫ ⌫¯ = 2νveto detectorγ⌫⌫¯KL

Feynman diagram

hermetic vetoback ground event
veto detector

Calorimeter

γ(π0)

KLγ

Calorimeter

π+(π0)π-γ

BHCVGood Probe for New Physics !

BHCV

ν

Beam Hole Charged Veto(BHCV)BHCV is …
•in-beam charged particle veto counter (efficiency > 99.5% is required)➡ cover downstream in-beam area
•exposed to a high flux of photons and neutrons (0.6GHz/20x20cm @300kW)
high counting rate
➡ generate accidental veto signals• BHCV in 2013(old BHCV): 3-mm-thick plastic scintillator + PMT➡ a significant acceptance loss is expected
for the planned increase of the beam intensity

2

Design of new BHCV(a) 1.8$mm

use thin gap chamber

upgrade of BHCV is required !
beam test for a prototype chamber

φ50$um$Au/W$wire$$$$$$$$$(5$cm)

Chamber (HV 2.7kV)

1.4$mmtrigger scinti.

low mass
high rate tolerance

500MeV/c e-5cm

Feature 1 : thin cathode film
low mass ➡ reduce photon/neutron hitscoinTDC

start

ADC gate

160 (+2) wires
16 channel readout

HV supply・read out board

Feature 2 : fast gas
short e- drift time ➡ short time jitter➡ veto time window can be shortened➡ the probability of detecting
an accidental veto signal is reduced➡ acceptance loss is reduced

300 mmADCTDCTDC distribution

ChamberADC
ChamberADC

4500

4000

4000

3500

3500

4000

4000

4000

4000

3500

350099.9% efficiency

3000

3000

3000

3000

2500

2500

2500

2500

2000

2000

2000

1500

1500

1500

1500

1000

1000

1000

1000

500

500

500

500

➡enough efficiency to2500+ - 2000
0suppress
K→ππ
π
BG
L
2000

1500

1500

1000

1000

500

500

00

(HV:2.7kV)

40 0060 20
80200100 120
4060 140
80 160100 180
120 200
140 160 180 2000010 20 3000401050 2060 3070 4080 5090 60
100 70 80 90 0100
[ns]
[ns]

Charge output[pC] output [pC]
Charge1inefficiency v.s. HVinefficiency

10-1

10-2

3500
3000

(HV:2.7kV)
50

0
100
0

time jitter v.s. HVinefficiency v.s. HV• Efficiency(ε=1-η) becomes better( ηtot = 3η3 + 3η2(1-η) < η )ADCTDC

time jitter
3500~33ns 3000

2000

install 3 modules in line along beam axis
➡ 2-out-of-3 logic

threshold

4500

2500

Al frame

ChamberTDC ChamberTDCTime jitter[ns]use graphite-coated polyimide film
as a cathode plane

ADC distribution

300 mm

4035302520-310• time jitter becomes short by defining
detect time as the fastest hit timein 3 modules➡ CF4 + n-Pentane gas mixture
quench gas

1510-4

2500

2600

2700

2800

2900

30003100

103200HV [V]

2500 2600 2700 2800 2900 3000 3100 3200 3300
HV[V]rate tolerance was also checked➡ No gain drop at 60kHz/cm2(maximum rate expected in KOTO)

Performance of new BHCV
Efficiency measurement

new BHCV was installed
30cm
ADC counts

30cm

HV:2.7kV600100ns0.7

4500.675mV

4000.50.43 modules are glued andplaced in line along
beam axis

0.3

3000.22500.1100200160

newBHCV

300400wave form

5000time [ns]MIP peak ~75mV in peak height

11/4 MIP threshold

-26

140

BHPV※

high pμ±,π±oldBHCV
trigger Scinti.
trigger Scinti.
22 x 22 x 0.5 cm320 x 20 x 0.5 cm3calorimeterrequired hits of old BHCV and2 trigger scintillators

8060p40

20000.5il
re1nimyra1.52.5

3[keV]

newBHCVPeak~99.6% efficiency (each module)✳BHPV is a photon counter made of aerogel and lead
➡ 99.9% w/ 2-out-of-3 logic(total radiation length ~ 6.2 X0)➡select high momentum charged particles

Conclusion
BHCV was upgraded with thin gap chamber
99.9% efficiency was achieved
counting rate was reduced to 1/3 of that of old BHCV
➡ acceptance loss: 6% → 4% @24kW beam power (23% → 15% @100kW)

-22

-20

-18-16-14-12-10rate reduction(HV:2.7kV)

2-24

considering TOF distribution,veto timing window is ~ 30ns
(2 times wider than old BHCV)

120

100

nimyr
ap ~24ns

Energy distribution

10.8

500

2000ilreh_Mod0MaxEne_wAllCut
0.9350

beam

102

10

Data[2][0]:2*Iteration$ {Entry$==10}550

Timing distribution

h_minimumtime(24kW)

oldBHCV

newBHCVrate

4.4 MHz

1.4 MHz:( threshold)
1/4 MIP peakcounting rate of new BHCV was~1/3 of that of old BHCV

15050