1. LAWCA for Air Shower Detection at High Altitude IHEP, Beijing Zhiguo Yao VCI, 11-15/02/2013

2. LAWCA - Large Area Water Cherenkov Array

3. Physics Goals  VHE gamma sky survey (100 GeV-30 TeV): Extragalactic sources & flares; VHE emission from Gamma Ray Bursts; Galactic sources; Diffused Gamma rays.  Cosmic Ray physics (1 TeV-10 PeV): Anisotropy of VHE cosmic rays; Cosmic electrons / positrons; Cosmic ray spectrum; Hadronic interaction models.  Miscellaneous: Gamma rays from dark matter; Sun storm & IMF.

4. VHE  -astronomy: Two Techniques  IACTs: H.E.S.S., VERITAS, MAGIC, … good angular resolution (~0.1  ); fair background rejection power; short duty cycle (~10%); narrow FOV (<5  ); Low energy threshold (~100 GeV);  Mainly focused on deep observation.  Ground particle array: AS , ARGO-YBJ, Milagro, …  not-so-good angular resolution (~0.5  );  poor background rejection power (but much elaborated in water Cherenkov);  full duty cycle (>95% ， ~10  IACT);  Wide FOV （ >2/3  ， ~150  IACT);  High energy threshold  improved by construction at high altitude (~1 TeV);  Good at sky survey, extended sources and flares.

5. Instrumentation History 2007 1980Whipple 0.2 Crab Crab detected! VERITAS 0.008 Crab ARGO-YBJ 0.6 Crab Tibet-AS  1.5 Crab Milagro 0.9 Crab 2012 2015 HAWC 0.06 Crab LAWCA 0.06 Crab 2004 H.E.S.S. 0.008 Crab MAGIC 0.02 Crab 2009 2001 1989 CTA 0.001 Crab LHAASO-WCDA 0.01 Crab Usually IACT is 10  better in sensitivity. 143 Sources observed 6 Sources 2017? HEGRA, CANGAROO, CAT … 0.04 Crab 10 years delay!

6. Water Cherenkov for Air Showers  Developed by Milagro, Auger, IceTop, etc.  to detect shower secondary particles: electrons/positions; muons; gammas: ~10x more, a benefit of water Cherenkov.  What are actually measured: energy flux in the water.  VHE: Two kinds of layouts: pool / tank.

7. “Sub-core” of Hadronic Showers Proton Gamma  Brightest “sub-core”: signal of the brightest PMT outside the shower core region (e.g., 45 m); mainly caused by muon (mean PE = 20, fluctuating with a long tail).  “Compactness” ( invented by Milagro):  nPMT/cxPE; proton: small; gamma: big.  “Compactness” can be employed to reject cosmic ray background efficiently

8. Detector Layout of LAWCA  An L-shape water pool: 4300 m a.s.l. North-East of ARGO-YBJ hall; 23,000 m 2 ; 4.5 m depth; 916 cells, with an 8” PMT in each cell; Cells are partitioned with black curtains. Original idea is credited to Milagro/HAWC.

9. Angular Resolution & Background Rejection  Good angular resolution: Optimized bin size: 0.85  @ 1 TeV; 0.45  @ 5 TeV.  Fair background rejection power: Q-factor: 3 @ 1 TeV; 14 @ 5 TeV.

10. Effective Area & Sensitivity  Effective area: 500 m 2 @ 100 GeV; 30,000 m 2 @ 1 TeV; 60,000 m 2 @ 5 TeV.  Sensitivity per year: 0.1 CRAB @ 1 TeV; 0.06 CRAB @ 5 TeV 。 ~10x better than ARGO-YBJ. 4 个 ¼ 阵列

11. Sensitivity to Flares  Minimum requirements: 30 events; 5 s.d.  Mainly limited by statistics. DurationSensitivity (Crab) 1 year0.06 30 days0.2 10 days0.4 3 days1 1 day2 3 hours4 2 hours5 1 hour10 3 days’ flare

12.  Requirements: water-proof: loss <1/1000 volume/day; light-proof: luminous flux (300-650 nm) <100k photons/m 2 /s; tolerance to snow, rain, wind, dust, earth-quake; anti-icing; clean water compatible; light roof and top materials. Engineering of Water Pool

13. Water Purifying & Circulation  Purifying: Absorption length >30 m @ 400 nm;  Water in pool: Absorption length >20 m @ 400 nm; Uniformity: >85%.  Circulation speed: 30 days per pool volume.

14. LAWCA PMT8”/9” Input polarityPositive Single rate50 kHz Charge dynamic range 1-4000 p.e. Charge resolution 50% @ 1 p.e. 5% @ 4000 p.e. Discriminator threshold 0.25 p.e. Time range0-2000 ns Time resolution 0.5 ns Multi-hit separation 100 ns Channels3600 Cable length30 m PMT / Electronics Specifications  Single counting rate is very high: robust DAQ system;  Single PE, large dynamic range: low noise, dynodes readout;  Time resolution: essential for shower direction measurement.

15. Trigger Scheme  Cluster-based;  Neighboring clusters are half-overlapped;  Pattern: Multiplicity during 250 ns of any cluster  12; Noise trigger <1 kHz.  Besides a hardware solution, a software-based trigger mechanism is also proposed. Noise trigger

16. Trigger Rate & Data Volume  Trigger rate: ~17 kHz.  Data volume after trigger: 240 Mbps = 1 PB/year.  DAQ data volume (input, soft trigger): 4.6 Gbps = 18 PB/year. Huge amount of data: an online- reconstruction solution is under investigation. Trigger rate Data volume

17. PMT Readout  Tapered voltage divider circuit;  A specialized decoupling circuit to reduce the effect of charge piled-up;  Two dynode outputs set for SPE resolution and dynamic range;  Dynamic range 1-4000 PE can be achieved with a linearity level 5%.

18. Electronics DY10 DY8  Charge: analog shaping, digital peak detecting;  Timing: pulse front discrimination;  9 PMTs share 1 FEE board;  FEEs are synchronized with central station via White Rabbit protocol;  Hit signals are transferred to the central DAQ system via TCP/IP network, shared with WR;  DAQ: based on Atlas TDAQ software framework (soft trigger compatible).

19. Charge Calibration: Low Range  Method: single rate ~50 kHz; SPE signal dominated; Including PMT Gain + cable + pre-amp + electronics low range;  Precision: 2% per 30 seconds; Real time (hardware trigger): 2% per 30 minutes. Temperature effect: PMT + cable Variation over a month Fitted with a convolution of power law  Poisson  Gaussian + SPE noise

20. Charge Calibration: High Range  Method: muon peak ~10 Hz; muons hitting the photo-cathode; PMT gain + QE + CE + cable + pre-amp + electronics high range.  Precision: 2% per 30 minutes; Real time (hardware trigger): 2% per day. Gaussian fitting after a power law of charge is multiplied. Variation over a month Temperature effect: PMT + cable

21. Time Calibration  Cluster-based, cross- calibrated: 2 fibers per PMT (naming: short & long); 2 LEDs per cluster, lightened in turn; 2-4 fibers are crossed among neighboring clusters; Frequency of LED pulsing: 5-10 Hz.  Requirements: Time offset measurement: ~0.1 ns.

22. Time Calibration: Test Results Two fibers on a PMT Two fibers on 2 PMTs Short fibers of 2 PMTs:  = 0.07 ns. Long fibers of 2 PMTs:  = 0.12 ns. Distribution of mean offset, 3 months. Mean value: 10 minutes @ 5 Hz. Distribution of mean offset, 3 months. Mean value: 10 minutes @ 5 Hz. unit: 1/5.6 ns Distribution of single measurements, 5 minutes @ 1 kHz, different thresholds.

23. Prototype Detector (2009-2010) Single rate: 16 kHz  30-50 kHz (4300 m a.s.l.) Single rate: 16 kHz  30-50 kHz (4300 m a.s.l.)  -peak is first observed. 2 layers of 1 m  1 m Scintillators 1 layer of 1 m  1 m Scintillator 5 m 7 m

24. Engineering Array (2010-now) 9 cells, effective area 225 m 2, 1% scale of LAWCA.

25. Installation 2011/03: dry run 2011/07: wet run >10 TB test and physics data obtained so far. 2011/03: dry run 2011/07: wet run >10 TB test and physics data obtained so far.

26. Event Reconstruction and Coincidence with ARGO-YBJ

27. Support & Schedule  Provisional support from IHEP-Beijing is available: ~2 M$; Land preparation is going to start in 04/2013; Preparation for production has started, including PMTs, electronics, detector installation facilities, DAQ, data storage, …  Full support from NSFC is to be decided in 06/2013: ~10 M$; Pool construction will then start soon and is to be completed in 10/2013; Detector installation is to be completed in 07/2014; Physics run may start in 10/2014.

28. Summary  A new VHE air shower detection instrument “LAWCA” is proposed to be built at YBJ in 2 years.  Similar to HAWC, it employs water Cherenkov techniques, aimed mainly at a full sky survey for new gamma ray sources;  The detector has been designed and partially tested with the prototype and the engineering array.