LHAASO-WCDA Design & Performance Zhiguo Yao for the LHAASO Collaboration IHEP, Beijing 2011/08/17

LHAASO-WCDA Water Cherenkov Detector Array of Large High Altitude Air Shower Observatory

Physics Motivation

All Sky Survey: Multi-Wavelength

TeV Sky Survey  >0.20 Crab unit northern sky: MILAGRO; ARGO-YBJ.  Only patches along the galactic plane (including the Cygnus region):  H.E.S.S.;  VERITAS;  MAGIC.  We eager to have a more sensitive ALL SKY MAP ! MILAGRO ARGO-YBJ

AGN Flares: IACT or Ground Array?  IACT could do following- up observations after receiving alerts from other wavelength: But, more fails than successes; And the sample is biased; And < 1/2 of the flares occur in their duty circle.  Ground particle detector array with improved sensitivity could do a better job! Cen A

Project Overview Charged Particle Array  Detector Array Water C Array Water C Array Wide FOV C-Telescope Array & Core Detector Array

Physics Goals of LHAASO-WCDA  Technique: Ground particle detector array at the high altitude  LHAASO; 100 GeV – 10 TeV: water Cherenkov technique  WCDA.  Main goals of WCDA: Sky survey for VHE extragalactic sources, and their flares; Long time monitoring variable sources; High energy emissions from GRBs; Cosmic ray physics, such as anisotropy; Solar flares & IMF; Dark matter; …

Technical Design Details

Detector Cell  Originally designed for HAWC;  Water are partitioned into 5  5 m 2 cells by black curtains;  Water depth 4.5 m;  A 8”PMT placed at the bottom, looking upward.

Detector Configuration: 4 Sub-arrays  ¼ array: Octagonal pond inscribed a rectangle of 150  150 m 2 ; Side length: (  2-1)  150 m = 62 m; Area: 2(  2-1)  150  150 m 2 = m 2 ; 25-4 = 21 clusters; 4 groups per cluster; 9 EDs per group; EDs are partitioned by curtains; Total ED PMTs: 720 (8”).

Water Purifying & Recirculation  Tiny holes are punched along the pipes;  Water is injected at the bottom of the pond, and drawn out on the top;  Recirculation speed: 1 pond water per month.  UV lamps in 185 nm are very essential, as it can destroy the dissolved organic carbon (TOC)!

Trigger  Level 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.  Level 2 – station 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 station 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).  We are also working on another approach: triggerless system.

Time Calibration  LED array + double-fiber system: Long & short fibers; 1 pair of LED arrays per cluster; Two fibers can cross-check themselves; Exchange long fibers for two nearest PMTs from two neighboring clusters for cross calibration; A cluster exchanges fibers with at least two neighboring clusters; LED pulses can be generated in the master station and sent from the trigger / synchronizing cables. See poster: ID-0771

Charge Calibration  Make use of the SPE measurement: Obtain the gain.  Make use of cosmic muons: Put down a shading pad above the PMT; Measure charge distribution of cosmic muons; Find and fit the second peak of the distribution; 20 minutes (e.g., every 10 days) running with shading pad can reach a precision of 2%!  ee  15cm See poster: ID-1123

WCDA Sensitivity  For known stable sources.  For flares.

Optimizations  Cell size;  Number, type, size of PMTs;  Layout of the array;  Out-triggers;  Altitudes (there are several viable candidate locations in Tibet);  …  These studies will use experimental data (from prototype or engineering array) as input.

Some Technical Parameters

Characteristics: On 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 100 ns.  It is not a big issue with modern electronics.

Characteristics: On Charge Measurement  PE distribution (gammas from CRAB): 1 PE: 43%, 2 PE: 18%; nPE>2000: 2  (7  10 -4, E>5 TeV).  Sensitivity: No difference between nPE max =50000 and nPE max =2000 ; Slight difference between nPE min =1 and nPE min =2.  For R5912, with the dynode + anode (or two dynodes) readout, we have achieved the dynamic range of PEs with nonlinearity < 5%!

Characteristics: On DAQ & Data Storage  ¼ array: Trigger rate: 16 kHz; Data rate: 94 Mbps; Data volume: 1 TB/day.

R&D Status: Prototypes

2 layers of 1 m  1 m Scintillators 1 layer of 1 m  1 m Scintillator 5 m 7 m WCDA Prototype Setup in the very beginning

Results: Water & Rate See NIM A644 (2011) 11-17

Results: Second Peak Hamamatsu R5912 EMI 9350KB Hamamatsu R5912EMI 9350KB Hamamatsu R5912 See NIM A644 (2011) 11-17

R&D Status: Engineering Array

Engineering Array of WCDA

Engineering Array Later: permaflex coating will be used to replace PE plastics. We have succeeded in potting / sealing 11 PMTs in a very cheap way (<200 yuan!) See posters: ID-0261, 0732

First Test Results  9 PMTs: CH1, CH2: with charge calibration covers (shading pad); CH4: not immersed into the water;  Water depth 30 cm above the photo cathodes.

Summary

Summary & Outlook  Water Cherenkov detectors have good performance in surveying the whole sky for extragalactic sources, complementary to next generation IACTs;  The experiment union (LHAASO-WCDA & HAWC) in East and West doubled the observation time on any sources in northern hemisphere, to realize the best monitoring of their emissions.  LHAASO-WCDA is conceptually designed;  R&D is still in the very beginning stage but progressing smoothly.  The LHAASO project has a big chance to be financially supported in next 2 years, and we wish it can be successfully built by 2016;  The first version of technical design report (TDR) of LHAASO is expected to be released by the middle of next year.

Scientific Problem: Gamma Rays SNRs Cold Dark Matter Pulsars GRBs Test of the speed of light invariance cosmological g-Ray Horizon AGNs Origin of CRs Microquasars Possible Source of UHECRs

VHE Gamma Astronomy: Techniques  IACT: HESS, VERITAS, MAGIC, … Better angular resolution; Fair background rejection; Low duty cycle; Narrow FOV.  More focused on deep observation.  Ground particle detector array: AS , Milagro, ARGO-YBJ, …  Reasonable angular resolution;  Ordinary background rejection;  Full duty cycle;  Wide FOV.  More oriented on all sky survey and flares detection

WCDA: Configuration Optimizations

WCDA Big Pond + MD  For known stable sources.  For flares.