LHAASO-WCDA: Design & Performance Zhiguo Yao IHEP, Beijing February 17-19, 2011

Outline Technique & physics goals Detector configuration Trigger setting Charge calibration Water system Performance & competition Requirements on electronics Summary

Technique & physics goals Water Cherenkov: surface detector for air shower particles; Wide field of view (2/3) and full duty cycle (>90%); Sky survey: 8/3. Main goals: Sky survey for extragalactic sources, especially AGN flares; Sky survey for galactic sources; Long time monitoring known sources; Cosmic ray physics, such as anisotropy; Solar flares & IMF; GRBs; Dark matter; … Energy range: 100 GeV – 10 TeV.

Detector unit  e  4m 1m 5m 10m Top: electromagnetic detectors (ED); Bottom: muon detector (MD); 4 EDs share same area of 1 MD.

Detector configuration: 4 sub-arrays Octant inscribed a rectangle of 150150 m2; Side length: (2-1)150 m = 62 m; Area: 2(2-1)150150 m2 = 18640 m2; 25-4 = 21 clusters; 4 groups per cluster; 9 EDs per group; 9 MDs per cluster; Total ED PMTs: 720 (8’’); Total MD PMTs: 1852 = 370 (8’’) or 185 (10’’); Total PMTs: 1090 or 905.

Detector configuration: a big array Octant inscribed a rectangle of 300300 m2; Side length: (2-1)300 m = 124 m; Area: 2(2-1)300300 m2 = 74558 m2; 100-12 = 88 clusters; 4 groups per cluster; 9 EDs per group; 9 MDs per cluster; Total ED PMTs: 2916 (8’’); Total MD PMTs: 7202 = 1440 (8’’) or 720 (10’’); Total PMTs: 4356 or 3636.

Trigger setting Two tiers of trigger: Tier 1 – group trigger: When a PMT is fired, the slave station produces a signal of 100 ns. For a group, the signals are summed. At any rising edge of the pipe line clock, if the sum is equal to or greater than 3, and the sum is falling after a rising change, send a digital signal containing the sum (hit multiplicity) and group ID to the master station. Tier 2 – master trigger: When the master station receives a group signal, extend it to 700 ns. At any rising edge of the pipe line clock, if there are group triggers satisfying one of the following condition, produce a master trigger: a: 1(M9); b: 1(M7) + 1(M3); c: 1(M6) + 1(M4); d: 2(M5); e: 1(M5) + 2(M3); f: 3(M4) + 1(M3); g: 2(M4) + 3(M3); h: 6(M3).

Charge calibration   e  Make use of the LED+fiber system: Tune the luminosity of LED to be very weak; Measure the SPE of PMT; Obtain the gain. Make use of cosmic muons: Put down a shading cover above the PMT; Measure charge distribution of cosmic muons; Find and fit the second peak of the distribution.   e  15cm

Water system Multi-media filter Carbon filter Pond water UV 254 nm Ultrafiltration 0.22 m UV 254 nm Pond water UV 185 nm Fine filtration 1 m Fine filtration 5 m

Sensitivity Limitations in this calculation (to be solved soon): A rectangular pond is still used instead of an octant; Noises added but with charge of 1 PE; Cosmic muon noises have not been added; Detected muon numbers (outside the core) is used instead of detected PEs.

Competition: observation of known stable sources IACT: Total 800 hours observation time per year; At most 200 hours per year for a source; A reasonable number: 50 hours per year for a source. Significance threshold: 5 s.d. LHAASO-WCDA: A big array of 300300 m2 ; 8000 hours observation time per year; Source transit effect is already included in the calculation;

Competition: sky survey for unknown stable sources IACT: Field of view: < 3, about 1/800 of wide FOV detector; Half of the live-time (5 years) is for survey; Scan each sky region for 2.5 hours; Significance threshold: 3 s.d. LHAASO-WCDA: Field of view: > 2; Significance threshold: 6 s.d.

Competition: sky survey for flaring sources IACT: No way if no alarm is received; Even with an alarm, only ¼ chance to observe it. LHAASO-WCDA: Suppose the duration of the flare is 3 days; Significance threshold: 7 s.d.

Requirement: time measurement Arrival time of PEs to a PMT: Pulse width (0-90% PEs) for 90% PMTs: <13 ns; Same thing for large zenith showers: < 18 ns. Sensitivity: With / without 1 ns jitter: no big difference. Multiple hits: About 50 kHz counting rate of noise; Early arrived noise of a fired PMT may cause recording a wrong time; Electronics shall be able to record multiple hits if they are separated by 25 ns.

Requirement: charge measurement PE distribution (gammas from CRAB): 1 PE: 43%, 2 PE: 18%; nPE>2000: 210-4 (710-4 , E>5 TeV). Sensitivity: No difference between nPEmax=50000 and nPEmax=2000; Slight difference between nPEmin=1 and nPEmin=2.

Requirement: DAQ & data storage ¼ array: Trigger rate: 16 kHz; Data rate: 94 Mbps; Data volume: 1 TB/day. Estimation to a big array: Trigger rate: 70 kHz; Data rate: 500 Mbps; Data volume: 5.4 TB/day.

Summary LHAASO-WCDA is conceptually designed; It has good performance in detecting AGN flares, challenging next generation IACT; Progress on implements of these designs, please see Mingjun Chen’s talk: Status of R&D of LHAASO-WCDA, and also next several talks on electronics. Physics design report will be given in this year, hopefully.

Time calibration LED + double-fiber system: Long & short fibers; 1 unit per cluster; Exchange short fibers for two nearest PMTs from two neighboring clusters for cross calibration; A cluster exchanges fibers with at least two neighboring clusters; LED pulse is generated in the master station and sent from the trigger cable. 18