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LHAASO Simulation: Performance of the Water CherenkovDetector Array

Zhiguo Yao∗, Min Zha∗, Zhen Cao∗, Huihai He∗ for the LHAASO Project∗Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, CAS, Beijing, 100049, P.R. China

Abstract. The Water Cherenkov has been provedto be an efficient technique in detecting air showersinduced by cosmic rays. Aiming at the sub-TeVgamma astronomy and as an important componentof the LHAASO project, a water Cherenkov detectorarray (WCDA) is planned to be built in YBJ Tibet,China. As proposed, the whole array is 90000 m2 indimension and consists of 3600 water detector cells.

In this presentation, the whole detector setup isintroduced, and a detailed simulation to a singlewater cell is demonstrated. And next the performanceof the LHAASO-WCDA, such as the trigger rate, theeffective area and the sensitivity is discussed.

Keywords: gamma astronomy, water Cherenkov,EAS, IACT

I. INTRODUCTION

High energy gamma-rays are a powerful and cur-rently unique probe to the most violent non-thermalastrophysics phenomenon under the extreme conditionsbeyond the Earth. The window of the very high energy(VHE, >100 GeV) gamma rays has been opened sincethe advent of the new technique in detection of TeVgamma radiations from the Crab Nebula in 1989 [1].Since then the VHE gamma astronomy is developedsteadily and until now more than 80 VHE sources havebeen discovered, and some important information aboutthe ultra-relativistic universe have been obtained.

Two general detection techniques are widely used inthe field of the VHE gamma astronomy: imaging airCherenkov telescope (IACT) and ground-based exten-sive air shower (EAS) array. The former is composed ofa huge reflection mirror and several hundreds of PMTpixels at the focus plane to detect Cherenkov lightsgenerated along the route of the air shower cascade,allowing a very good angular resolution (< 0.1◦) anda strong background rejecting power (> 99%), butsuffering for a short duty circle (10%) and a smallfield of view (∼ 5◦). The latter consists of severalhundreds of particle detectors to sample the tail ofthe EAS particles from which the shower front canbe reconstructed, indicating a modest angular resolu-tion (∼ 0.3◦) and a not so good background rejection(∼ 90%), but exhibiting a high duty cycle (∼ 100%) anda wide field of view (∼ 1.5 sr). Major instruments inthe IACT category include HESS, MAGIC and Veritas;Typical experiments of EAS array are Tibet ASγ (plasticscintillator), Milagro (water Cherenkov) and ARGO-YBJ (RPC carpet). Undoubtedly, IACT technique hasachieved a big success during past 15 years. But as a

complement to IACTs, the EAS array enables a fullsky survey alternatively, providing hot spots for IACTto implement a deep observation; Furthermore, the EASarray has a strong potential in detecting transient flaringsignals, monitoring variable emissions, and exploringgalactic extended sources which can not be well ma-nipulated by IACTs.

The water Cherenkov technique has been employedby many ongoing big experiments such as Super-Kamiokande, Milagro, Pierre Auger, ICECUBE, An-taras, as well as some projected experiments for instanceKM3Net and HAWC. The pioneered EAS array experi-ment on gamma astronomy as Milagro has demonstrateda good performance of water Cherenkov in detecting airshowers, and especially a better background rejectionpower than other shower detectors due to its particularcalorimeter nature. For the purpose of a competitivegamma astronomy instrument, the next generation ofwater Cherenkov array must be big enough to contain thesub-core of hadronic showers, and a location at higheraltitude is very essential in order to record more showersecondaries at the low energy band.

Aiming at sub-TeV gamma astronomy and as animportant component of the LHAASO project, followingthe pioneer practice of EAS array experiments andfulfilling the merit of high observation level, a waterCherenkov detector array (LHAASO-WCDA) is plannedto be built in Yang-Ba-Jing, Tibet, China. The wholearray is projected to be 90000 m2 in dimension.

In this paper, a full Monte Carlo simulation is carriedout to study the WCDA performance, such as the back-ground rejection power, the effective area, the triggerrate, the angular resolution. And after taking account allabove factors, the sensitivity of the array is given.

II. DETECTOR SETUP

The proposed LHAASO-WCDA will locate close toYang-Ba-Jing Cosmic Ray Observatory Tibet, China atan altitude of 4300 m a.s.l., corresponding to a verticalatmospheric depth of 606 g/cm2. The whole array is90, 000 m2 in dimension and split into 4 sub-arrays(ponds) with size of 150 × 150 m2 each, shown as theblue circles in figure 1.

For every sub-array, a single layer of photomultipliertube (PMT) with 5 m spacing are anchored along agrid of 30 × 30 under 4 meter depth in the water,facing upwards for Cherenkov lights yielded by chargedparticles traversing in the water. The type of PMTstentatively chosen is 20 cm Hamamatsu R5912, and

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Fig. 1: The proposed LHAASO project.

altogether 900 PMTs will be deployed for every sub-array. An opaque curtain is stretched between the PMTsintended to optically isolate each sensor. Every isolatedspace with a PMT inside is called a cell, which is thesmallest module of the array.

III. MONTE CARLO SIMULATION

A. Simulation for a Cell Detector

A GEANT4 [2] based simulation is employed to studythe detection efficiency of a cell detector. Particles suchas muons, gammas and electrons at different energiesare injected into the cell detector, tracing their propa-gation and interaction in water, registering the numberof photon-electrons (PE) yielded at the PMT cathode -after taking into account the absorption of water and thequantum efficiency of PMT. Figure 2 shows the averagenumber of detected PEs as a function of the radialdistance for a gamma vertically injected at differentwater depth. It reveals that an EM component can throwout Cherenkov lights uniformly at the PMT plane withina radial distance of 2.5 m after a water depth about 4 m.That is why a 4 m depth of water and a 5 m spacing ofPMT is chosen in the detector configuration. Simulationsshow that in average 20 PEs per GeV is produced by EMparticles, and 20-600 PEs for passing through muonswhich strongly depends on the radial distance betweenthe PMT and the muon track.

B. Simulation for a Sub-array

The air showers of both gamma emissions and back-ground cosmic rays along the Crab transit are simu-lated in the frame work of CORSIKA6735 [3] programwith the hadronic interaction model QGSJET II [4]and GHEISHA. The Crab gamma flux measured byHEGRA [5] and the chemical flux (Z=1, 2, . . . , 26)of background cosmic rays compiled in [6] are usedto generate the primary particles. In order to ensureenough statistics in high energy region and to im-prove the computing efficiency, events are generatedin 6 energy intervals: Emin–20–50–100 GeV–1–10–100 TeV, where Emin = max(10, 1.1 · A) GeV fornuclei with atomic number A and simply 10 GeV for

radial distance from the center of the tank(m)0 0.5 1 1.5 2 2.5 3 3.5

Np

e

-210

-110

1

10

210

H=1.2m

H=2.2m

H=3.2m

H=4.2m

H=5.0m

Fig. 2: The average number of detected PEs as a functionof the radial distance. 100 MeV gammas are injectedvertically at various water depth.

Fig. 3: The incoming direction of generated cosmic raysfor an arbitrary day. The line in the middle is the Craborbit.

gammas. The sampled event ratio for these intervals are0.05:0.1:0.5:1.0:0.5:1.0. A proper weighting factor forevery event is calculated and stored into the Corsika databank for scaling back the spectrum in the analysis. Thekinematic cut for hadrons, muons, electrons, photonsand pions are 50, 50, 0.3, 0.3, 0.3 MeV respectively,very close to the energy threshold of the Cherenkovproduction in water.

As is mentioned, the Crab transit is followed up fromhorizon to horizon in a step of 60 seconds for both thegamma emission and the background cosmic rays. Theonly difference is that gamma is always emitting froma point, but background cosmic rays are sampled in acone with a radius 7◦ which is perpendicular to the sightof view, see figure 3.

When each simulated gamma shower reaches theobservation level, the shower core location is randomlyprojected within an rectangular area with the dimensionchanging from 600 × 600 m2 to 1500 × 1500 m2 in5 levels, corresponding to 5 primary energy ranges.For background cosmic rays, the core projection areais 1800 × 1800 m2 and 2100 × 2100 m2 below andabove 10 TeV respectively, where the big projectionarea is intended to take the geomagnetic deflections intoaccount. A shower is reused for 10 times with different

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core locations, for the sake of saving CPU time on theair shower simulation.

The detector response of the water Cherenkov arrayis simulated based on a GEANT4 programme developedby the Milagro collaboration. The air shower secondariesat ground level is shot into the top of the water, theeffects of the detector structure, passage of particles inthe water, Cherenkov lights production and propagation,PMT geometry and quantum efficiency are fully takinginto account. Finally the number of PEs and their arrivaltime on each PMT are registered. In this simulation,the cosmic noise for each cell detector and the PMTdark noise has not been taken into account, and onlythe multiplicity (≥20) of fired PMTs is treated as thetrigger pattern.

The triggered events are reconstructed with the Mi-lagro reconstruction package Milinda. After conicallyfitting the arrival times of signals on PMTs and theirpositions, by help of an algorithm based on the centerof gravity, the shower direction and the core position canbe found.

The performance of the water Cherenkov array de-pends much on the spectrum of the primary gammas.The Crab spectrum (slope: −2.62) is actually simulated,but circumstances for other spectra (such as −2.12,−3.12, −3.62, −4.12) can be easily acquired by ap-plying an extra weighting factor to each event.

IV. DISCRIMINATION OF GAMMA SHOWERS

A hadronic primary tends to produce a sub-core atthe ground level made up of muons or a cluster of EMparticles. The sub-core usually creates a very bright PMThit outside the region of the main core. The Milagro/ HAWC collaboration developed a parameter knownas compactness [7] to locate the sub-core, defined asC = nPMT/cxPE, i.e., 1/nPE of the brightest PMToutside the shower core (R >30 m), weighted by thePMT multiplicity nPMT. Statistically the compactnessdistribution of gammas and background hadrons aredifferent, a simple cut to the distribution can reject mostof the hadronic showers while dominant gamma showersremained. Figure 4 shows an example for the case ofnPMT≥200.

V. RESULTS AND DISCUSSIONS

A. Trigger Rate and Effective Area

Figure 5 shows the trigger rate of background cosmicrays as a function of the PMT multiplicity at differentcompactness cuts. The maximum trigger rate for a sub-array amounts to 15 kHZ at the PMT multiplicity of 20,which is modest for the capacity of modern electronics.In this calculation, the cosmic and dark noise has notbeen counted, however a practical trigger design of 2levels will efficiently eliminate the influence of thesenoises. Furthermore, if an on-line parallel computing canbe implemented for a quick reconstruction of showers toset a compactness cut, as the 3rd level of trigger, the ratecan be greatly depressed to a desirable level.

nPMT/cxPE

0 5 10 15 20 25 30

En

trie

s (n

orm

aliz

ed)

1

10

210

310

410

510Q = 9.41C = 11.00

R = 17.20%, 0.03%

Fig. 4: The compactness distribution for gamma andhadronic showers.

nPMT

20 30 40 100 200 300 1000

nP

MT

) [k

Hz]

≥R

ate

(

-410

-310

-210

-110

1

10

nPMT/cxPE>0: 15.73 kHznPMT/cxPE>1: 7.50 kHznPMT/cxPE>2: 3.56 kHznPMT/cxPE>3: 2.14 kHznPMT/cxPE>4: 1.40 kHz

Fig. 5: The trigger rate of a sub-array.

[TeV]0E0.02 0.1 0.2 1 2 3 4 10 20 100

]2E

ffec

tive

Are

a [m

1

10

210

310

410

510

20≥nPMT 30≥nPMT 50≥nPMT100≥nPMT200≥nPMT500≥nPMT

Fig. 6: The effective area of a sub-array for gammas.

The effective area of primary gammas as the functionof the energy is computed as well, via simulating thegamma emissions from the Crab transit with Zenith< 30◦, after applied the background rejection power(compactness >5) and the angular resolution restriction(space angle < 1◦), see figure 6. Thanks to the highaltitude of the location, the detector still has ∼ 100 m2

acceptance at the energy of 100 GeV.

B. Angular and Energy Resolution

Statistically speaking, the number of fired PMTs(nPMT) as well as the total number of PEs (nPE) of thearray has a certain relationship with the shower energy.When a particular nPMT cut is applied, the median en-

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[ TeV ]medianE = E0.04 0.1 0.2 1 2 3 4 10 20 100

[ d

egre

e ]

αδ

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 Slope = -2.12Slope = -2.62Slope = -3.12Slope = -3.62Slope = -4.12

Fig. 7: Angular resolution as a function of gammamedian energies. The curves for several other presumedspectra are drawn as well in coping with alternativegamma emissions.

nPE

log

(E/G

eV)

2.5

3

3.5

4

4.5

5

210 310 410 510

NucleiGamma

Fig. 8: Logarithmic energy as a function of total numberof PEs.

ergy of the primaries can be obtained. Figure 7 shows theangular resolution of the array to the primary gammasas a function of median energies. The angular resolutionat the minimum nPMT cut (nPMT≥20, correspondingto a median energy of 510 GeV) is as good as 0.5◦,and turns better rapidly to < 0.2◦ at the median energy10 TeV (nPMT≥570).

The primary energy can be estimated with the mea-surement of the total number of PEs of the sub-array.Figure 8 gives the mean logarithmic energies at a seriesof PE ranges, where the error bar at each bin shows thespread (RMS) of the distribution at this particular PErange. Obviously the water Cherenkov array is not goodat the energy measurement especially for low energies.But when assuming reasonably a power law spectrumof an observed source, the estimation to the flux and theslope will still be in a satisfactory precision.

C. Sensitivity

In order to obtain a better sensitivity of the sub-arrayto a gamma source, the compactness cut and the angularbin size should be optimized. For a particular nPMTrange, the procedure of the optimization is explained asthe following: Firstly the angular bin size is optimizedfor gammas with an initial cut of compactness > 5;

[ TeV ]medianE = E0.04 0.1 0.2 1 2 3 4 10 20 100

]-1 s

-2 (

>E)

[ T

eV c

mΦ•

E

-1310

-1210

-1110

Slope = -2.12Slope = -2.62Slope = -3.12Slope = -3.62Slope = -4.12

Fig. 9: The sensitivity of a sub-array to sources orbitingin the Crab transit. The thick dashed lines from top tobottom are corresponding to 1, 0.1, 0.01 Crab fluxesrespectively.

Then the compactness cut is optimized for those eventswithin the optimized angular bin size; Finally, applyingboth cuts the sensitivity for this particular nPMT rangeis calculated. Figure 9 gives the flux sensitivity as afunction of the median energy. The curves for otherspectra are drawn as well.

In above optimization procedure, a premise to beconformed to is that the number of detected gammasat the obtained sensitivity must be greater than 30. Itguarantees a physical rather than a too optimized resultobtained.

VI. CONCLUSION

The combined observation of four separate sub-arraysof LHAASO-WCDA would be capable of detecting theCrab Nebula with 5σ within 3.5 hours and surveying theall sky at a level of 2% Crab in one year’s operation,thanks to its powerful background rejection and thehigh altitude location. The sensitivity of an alternativeconfiguration with for example a big pond of 90000 m2

would be able to achieve a better sensitivity which isstill under study.

VII. ACKNOWLEDGEMENTS

The author would like to thank Milagro Collaboration,and particular Gus Sinnis and Andrew Smith for offeringus their software programme and beneficial instructions.This work is partly supported by Knowledge Innovationfund (H85451D0U2) of IHEP, Beijing.

REFERENCES

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