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

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

3. 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.

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

5. 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 = 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.

6. 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 = m2;
= 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.

7. 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).

8. 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

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

10. 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.

11. 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;

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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