Scintillator strip KLM detector for Super Belle

1. Scintillator strip KLM detector for Super Belle P.Pakhlov for ITEP group

2. Motivation for a new KLM design • 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

3. Scintillator KLM set up • 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 • Geometrical acceptance > 99% y-strip plane Iron plate x-strip plane Mirror 3M (above groove & at fiber end) Optical glue increase the light yield ~ 1.2-1.4) Aluminiumframe WLS: Kurarai Y11 1.2 mm GAPD Strips: polystyrene with 1.5% PTP & 0.01% POPOP Diffusion reflector (TiO2)

4. h Al R 50 Depletion Region 2 m Substrate Ubias Si+ resistor Al conductor GAPD characteristics: general • Matrix of independent pixels arranged on a common substrate • Each pixel operates in a self-quenching 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) Discharge is quenched by current limiting with polysilicon resistor in each pixel I<10A Short Geiger discharge development < 500 ps Pixel recovery time ~ CpixelRpixel=100-500ns

5. 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

6. 20 40 15 30 one pixel gain (exp. data) e% 5 10 20 One pixel gain M, 10 Efficiency of light registration ~565nm APD 5 10 operating voltage 0 0 QE,% PMT 0 1 2 3 4 5 6 GAPD , V Overvoltage U=U-U D breakdown Wavelength, nm GAPD: efficiency and HV • Working point Vbias=Vbreakdown + V; V  50-60 V (experimental series with 20-120V) ; V  3V above breakdown voltage • Photon Detection Efficiency is a product of • Quantum efficiency>80% (like other Si photodetectors) • Geometrical efficiency = sensitive are/total area ~30-50% • Probability to initiate Geiger discharge ~ 60% • Finite recovery time  dead time depends on noise rate and photon occupancies • Each pixel works as a Geiger counter with charge Q=VC, C~50fmF; Q~350 fmC ~106e – comparable to vacuum phototubes

7. GAPD production Around 1990 the GAPD were invented in Russia. V.Golovin (CMTA), Z.Sadygov (JINR), and B.Dolgoshein (MEPHI- PULSAR) 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 is very useful to get good GAPD quality and lower the price (~20$). All producers has an experience of “moderate“ mass production ~ 10000 pieces JINR R8 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)

8. Electronics • Single photon produce a signal of several milivolts on a 50 Ohm load. A simple amplifier is needed. • Each GAPD has individual optimal HV (spread ~ 5V). HV to be set by microcontoller (from a db) • 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 • Such a scheme has been realized for the test module (100 channels) using CAMAC/NIM modules: home made ITEP HV control and NIM discriminators.

9. Efficiency and GAPD noise 1m × 40mm ×10 mm strip imperfection of the trigger 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

10. 1m 1m Test module at KEKB tunnel • 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. • Key issues of the 2007 fall test run • 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

11. 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 Independent method: neutron dose has been measured at ECL via observed increase of the APD dark current: ΔI∼5nA Endcap GAPD endcap Barrel GAPD barrel  ECL APD 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

12. Radiation damage measurements Dark current increases linearly with flux Φ as in other Si devices: ΔI = αΦ 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~1011n/cm2 but the efficiency degradates ITEP Synchrotron Protons E=200MeV Estimated flux in 5 years SuperB Radiation hardness of GAPD is sufficient for SuperBelle, but we do not have a large safety margin for more ambitious luminosity plan Radiation damage by 1 MeV neutron is similar to 200 MeV protons

13. before after ADC 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 • 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.

14. Veto: ADC<180 Hit: ADC>1000 Veto: ADC<180 MIP detection MIP • Standalone MIPs can be triggered: not obligatory from the interaction, most are from bg – large theta) No LED calibration Use MIP as a reference Hit map display (typical events) • The MIP efficiency with noisy conditions vs threshold is similar to those obtained with no beam bg data • The MIP efficiency with noisy conditions vs threshold is similar to those obtained with no beam bg data

15. Backgrounds in the KEKB tunnel Hit map: neutron & gamma’s • The major backgrounds (~65%) seen as a single hit in all channels. It’s due to gamma & neutrons. • EM Showers ~ 5% produce many hits up to 20 Hit map: showers • 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

16. Stored sc-KLM data • 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

17. Summary • Scintillator KLM design is OK for SuperB: • 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