Transcript ppt

Scintillator strip KLM
detector for Super Belle
P.Pakhlov
for ITEP group
Motivation for a new KLM design
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The present RPC design for KLM demonstrated nice
performance at Belle
However with the present luminosity the efficiency
degradation is observed due to high neutron background
and large RPC dead time
The paraffin shield helps to reduce neutron background
just slightly in the outermost superlayers.
The background rate
in the innermost
superlayers are
only ~2 times
smaller and cann’t
be shielded
With 20 times
higher occupancy
the efficiency
becomes
unacceptably low
(<50%)
For SuperB new KLM design in endcup is required
Scintillator KLM set up
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Two independent (x and y) layers in one superlayer made of
orthogonal strips with WLS read out
Photodetector = avalanche photodiod in Geiger mode (GAPD)
~120 strips in one 90º sector (max L=280cm, w=25mm)
~30000 read out channels
y-strip
Geometrical acceptance > 99%
plane
Iron plate
Mirror 3M (above
groove & at fiber end)
Optical glue increase the
light yield ~ 1.2-1.4)
WLS: Kurarai Y11 1.2 mm
Diffusion reflector (TiO2)
x-strip
plane
Aluminium frame
GAPD
Strips: polystyrene with 1.5% PTP & 0.01% POPOP
GAPD characteristics: general
h
Al
Depletion
Region
2 m
R 50
Substrate
Ubias
Matrix of independent pixels
arranged on a common substrate
Each pixel operates in a selfquenching Geiger mode
Each pixel produces a standard
response independent on number
of incident photons (arrived
within quenching time): logical
signal 0/1
GAPD at whole integrates over
all pixels: GAPD response =
number of fired pixels
Dynamic range ~ number of
pixels (200-2000)
Si+ resistor
Al conductor
Short Geiger
discharge
development
< 500 ps
Discharge is quenched by
current limiting with
polysilicon resistor in each
pixel I<10A
Pixel recovery time ~
CpixelRpixel=100-500ns
GAPD signals
connect pixels in parallel
via an individual limiting
resistor.
Oscilograph view & ADC spectra of
Hamamtsu GAPD eluminated with LED.
Photo of Geiger discharge in
one pixel and cross-talk
GAPD: efficiency and HV
 Working point
Vbias=Vbreakdown + V; V 
50-60 V (experimental
Quantum efficiency>80% (like
series with 20-120V) ; V
other Si photodetectors)
 3V above breakdown
Geometrical efficiency =
voltage
sensitive are/total area
one pixel gain (exp. data)
Finite recovery time  dead
time depends on noise rate
and photon occupancies
APD
20
40
15
30
10
20
5
One pixel gain M, 10
~30-50%
Probability to initiate
Geiger discharge ~ 60%
~565nm
5
QE,%
0
PMT
GAPD
Wavelength, nm
Efficiency of light registration e%
Photon Detection Efficiency
is a product of
10
operating voltage
0
1
2
3
4
,
Overvoltage  U=U-U breakdown
5
V
0
6
 Each pixel works as a
Geiger counter with charge
Q=VC, C~50fmF; Q~350 fmC
~106e – comparable to vacuum
phototubes
GAPD production
Around 1990 the GAPD were invented in
Russia. V.Golovin (CMTA), Z.Sadygov
(JINR), and B.Dolgoshein (MEPHIPULSAR) have been the key persons in
the development of GAPDs.
Now produced by many companies:
 CMTA Moscow, Russia
 JINR Dubna, Russia
 PULSAR Moscow, Russia
 HAMAMATSU Hamamatsu City, Japan
 And many others in Switzerland, Italy,
Island…
Strong competition between producers isJINR R8
very useful to get good GAPD quality
and lower the price (~20$).
All producers has an experience of
“moderate“ mass production ~ 10000
pieces
We work with CMTA (Moscow) where the producer is eager to
optimize the GAPD for our purposes (the spectral
efficiency to Y11 fiber / the GAPD shape)
Electronics
Single photon
produce a signal of
several milivolts on
a 50 Ohm load. A
simple amplifier is
needed.
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Each GAPD has individual
optimal HV (spread ~ 5V).
HV to be set by
microcontoller (from a db)
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Such a scheme has been
realized for the test module
(100 channels) using
CAMAC/NIM modules: home made
ITEP HV control and NIM
discriminators.
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Each GAPD has
individual gain
thus individual
thresholds
required
Slow control/control run to
be developed:
Measure dark currents,
temperature etc
 Self calibration using GAPD
noise
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Efficiency and GAPD noise
1m × 40mm × 10 mm
strip
Use cosmic (strip
integrated) trigger
LED spectra is used to
calibrate GAPD
Random trigger is used to
measure noise
Discriminator threshold
at 99% MIP efficiency
(6.5 p.e.) results in
GAPD noise 100 Hz only!
<10% variation of light
yield across the strip;
~20% smaller light yield
from the far end of the
strip
imperfection of
the trigger
Test module at KEKB tunnel
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100 1m-strips arranged in 4 layers
Initially supposed to be installed
in the iron gap instead of the not
working outermost RPC layer.
However dismantling of RPC turns
out to be a hard job. Finally
installed in the KEKB tunnel almost
without any shield (2mm lead).
Tested during 40 days of 2007 run.
Tests to be continued in 2008.
1m
Key issues of the 2007 fall test run
1m
 Study radiation ageing of GAPD:
1 day dose at the KEKB tunnel equivalent to 7 days
dose at the prospective position at SuperB.
 Measure background rate for MC simulation (QED
backgrounds to be shielded by iron plates / neutron
background at the KEKB tunnel and at the prospective
position are similar)
 Test compatibility with Belle DAQ: try to store test
module hits on data tapes
 Check MIP registration efficiency in a noisy
conditions
Estimate of neutron dose at SuperB
Now(₤=1.4×1034) ~1mSv/week  15mSv/week at SuperB(₤=2×1034)
 3Sv/5 years  conservatively  9×109 n/cm2/5years
Luxel budges (J type)
measure fast neutron dose
Barrel
GAPD
Independent method: neutron
dose has been measured at ECL
endcap via observed increase of the
APD dark current: ΔI∼5nA
GAPD barrel
Endcap
ECL APD
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Conservative: 5×108 n/cm2/500fb-1
conservatively assuming dose ~ 1/r
 1010 n/cm2/5years
Both methods are conservative and give consistent estimates 1010 n/cm2/5years
Neutron dose at barrel part can be twice higher
Radiation damage measurements
Dark current increases linearly with
flux Φ as in other Si devices:
ITEP Synchrotron
Protons E=200MeV
ΔI = α Φ Veff Gain, where
α = 6 x 10-17 A/cm, Veff ~ 0.004 mm3
determined from observed ΔI
Since initial GAPD resolution of
~0.15 p.e. is much better than in
other Si detectors it suffers sooner:
After Φ~1010 n/cm2 individual p.e.
signals are smeared out, while MIP
efficiency is not affected
MIP signal are seen even after Φ~1011
Estimated flux in
5 years SuperB
Radiation damage by 1
MeV neutron is similar
to 200 MeV protons
n/cm2 but the efficiency
degradates
Radiation hardness of GAPD is
sufficient for SuperBelle, but we
do not have a large safety margin
for more ambitious luminosity plan
Radiation damage measurements at KEKB tunnel
The test module has been
exposed to neutron
radiation in KEKB tunnel
during 40 days. The
measured neutron dose is
0.3 Sv, corresponding to
half year of Super KEKB
operation
before
after
ADC
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Increase of dark currents after 40 days in KEKB tunnel
Iafter – Ibefore ~ 0.1 A (within the accuracy of the
measurement)
More accurate estimate of GAPD degradation is done using
ADC spectra: the 1 p.e. noise increased by 10% only
after 40 days in KEKB tunnel for the GAPDs irradiated
with the highest dose 0.3Sv.
MIP detection
Veto: ADC<180
MIP
Hit: ADC>1000
Veto: ADC<180
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No LED calibration
Use MIP as a reference
Standalone MIPs can
be triggered: not
obligatory from the
interaction, most are
from bg – large
theta)
Hit map
display
(typical events)
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The MIP efficiency with noisy
conditions vs threshold is
similar to those obtained with
no beam bg data
Backgrounds in the KEKB tunnel
Hit map: neutron
& gamma’s
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The major backgrounds (~65%) seen as a
single hit in all channels. It’s due to
gamma & neutrons.
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EM Showers ~ 5%
produce many hits up
to 20
Hit map: showers
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Single tracks (presumably
electrons) ~ 30% (2--6 hits
in the test module)
QED backgrounds should be suppressed in the prospective
position by iron plates, neutron backgrounds is not
significantly shielded. We need to separate neutron/gamma’s
to estimate the future bg rate
Stored sc-KLM data
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Sc-KLM hits are stored in
the data tapes: the raw
hit rate is ~10 times
higher than RPC hits
Muons from ee → are seen
with proper time
Proper time hits show the
position of the test
module in the tunnel
Muon tag
required
Muon
vetoed
The distribution of the muons hits (x%y)
extrapolated from CDC to z = ztest module
with the proper time sc-KLM module hits
Summary
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Scintillator KLM design is OK for SuperB:
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the efficiency of MIP detection can be kept at high level
(>99% geometrical; thresholds: compromise between efficiency
and neutron bg rate)
KL reconstruction: rough estimates were done for LoI; full
MC simulation to be done by TDR using the information from
the test module
Radiation hardness of GAPD is sufficient for SuperBelle
for endcap and barrel parts, but we do not have a large
safety margin.
The test with a real prototype showed a good performance
of the proposed design; further optimization is to be
done before TDR: compromise between physical
properties/cost
The tests to be continued in 2008 to see further GAPD degradation
Many thanks
to the Belle KLM group for the help in tests
and
D. Epifanov for providing us
the ECL neutron flux measurements