LHAASO-WCDA: Design &
Performance
Zhiguo Yao
IHEP, Beijing
February 17-19, 2011
Outline
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Technique & physics goals
Detector configuration
Trigger setting
Charge calibration
Water system
Performance & competition
Requirements on electronics
Summary
Technique & physics goals
• Technique:
– 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:
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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;
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• Energy range:
– 100 GeV – 10 TeV.
Detector unit

e

4m
1m
5m
5m
• Top: electromagnetic detectors (ED);
• Bottom: muon detector (MD);
• 4 EDs share same area of 1 MD.
10m
10m
Detector configuration: 4 sub-arrays
• ¼ array:
– 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
• 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:
– Tier 2 – master 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.
• 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:
15cm
– Put down a shading cover
above the PMT;
– Measure charge distribution
of cosmic muons;
– Find and fit the second peak
of the distribution.
Pond water
Ultrafiltration
0.22 m
Fine filtration
1 m
Fine filtration
5 m
Carbon filter
Multi-media
filter
Water system
UV 254 nm
UV 185 nm
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;
– Significance threshold: 5 s.d.
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.