Status of LHAASO updates from ARGO-YBJ

Zhen Cao19B Yuquan St., Shijingshan, Beijing 100049, China

For LHAASO Collaboration

a r t i c l e i n f o

Available online 14 December 2013

Keywords:VHE gamma ray astronomyAir shower detector arrayWater Cherenkov techniqueWide FOV imaging air Cherenkov telescopesBurst detector arrayCosmic rays

a b s t r a c t

The Large High Altitude Air Shower Observatory (LHAASO) is a multipurpose project with a complexdetector array for high energy gamma ray and cosmic ray detection. The array of 1 km2 is composedof five types of detectors to measure shower arrival direction, total number of secondary particles,muon content, Cherenkov image and high energy gamma rays near shower core, respectively. Themain scientific goals are (1) searching for galactic cosmic ray origins by extensive spectroscopyinvestigations of gamma ray sources above 30 TeV; (2) all sky survey for gamma ray sources atenergies higher than 300 GeV; (3) energy spectrum and composition measurements of cosmic raysover a wide range covering knees with fixed energy scale and known fluxes for all species at the lowenergy end. In this paper, the progress on relevant detector developments is reported, includingconstructions of prototype detectors at Tibet site and coincidence operation with the ARGO-YBJresistive plat chamber full coverage array at 4300 m a.s.l. The energy spectrum of cosmic rayhydrogen and Helium nuclei up to 0.8 PeV is reported as the first piece of physics measurements bythe LHAASO experiment.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Large High Altitude Air Shower Observatory (LHAASO) isdesigned for three major scientific goals [1–3]. (1) Searching forhigh energy cosmic ray origins by extensive spectroscopyinvestigations of gamma ray sources above 30 TeV. More than140 gamma ray sources have been discovered in this so-calledVery High Energy (VHE) range. Observing gamma rays withgood statistics and measuring their energy spectrum up to1 PeV with high energy resolution for galactic sources is apromising approach to collect important evidences for theorigins of the photons, either from cosmic ray pevatrons orwell known electron sources. Besides, the high energy sky hasrevealed a stunning richness of new phenomena and puzzlingdetails in the observation of the existing sources, (2) deepsurveying over the whole sky for more sources with highsensitivity and clock-round monitoring for transient phenom-ena of the VHE sources. They are very important as an essentialpart of the multi-wavelength investigation in order to under-stand the evolution of galaxies (such as AGN) and particleacceleration procedures and radiation mechanisms in thegamma ray sources. With strong complementary to the Cher-enkov telescopes, the ground-based particle detector arraysplay irreplaceable roles in the gamma ray astronomy due totheir large acceptance in terms of high duty cycle of 495% as

well as large field of view of the whole hemisphere. Particularly,the proposed project will be at least one order of magnitudemore sensitive than the Cherenkov Telescope Array (CTA) above10 TeV and (3) measuring energy spectra above 1 PeV for individualcosmic ray species. This is the ultimate way to understand the originof knees. Major difficulty is to distinguish different primary cosmicray composition in the air shower observations. A detector array likeLHAASO at an altitude of 4400 m could naturally be used for thispurpose because air showers around few PeV just reach theirmaximum as they touch down the ground, thus the effects due toshower fluctuations can be minimized. In order to gain photon/hadron discrimination power, the proposed LHAASO array has beenequipped with the large muon detector array. The high statisticmeasurements on muon content will make a significant contributionto the separation between primary species. In addition, the highaltitude of the site enables a threshold energy to be lower than100 TeV in the spectrum measurements. It is important because ofthe overlap with the balloon or space borne experiments such asCREAM. The comparison between the direct measurement andground based experiments will provide a natural calibration in bothenergy scale and flux normalization for each species. The scales willbe propagated up to higher energies in the experiments withLHAASO. This will bridge the space borne direct measurements ofcosmic rays and ground-based ultrahigh energy cosmic ray experi-ments which are troubled by the issues.

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Nuclear Instruments and Methods in Physics Research A 742 (2014) 95–98

2. Base-line design of the detector array

In order to fulfill all the goals mentioned above, a large scalecomplex of many kinds of detectors is needed. Fig. 1 is a sketchmap of LHAASO detector array which is composed of three majorcomponents. The simulated sensitivity curve of LHAASO projectfor the gamma ray observation is shown in Fig. 2. The sensitivecurves for other projects are also shown in the same figure forcomparison.

Among known sources discovered in the all sky survey, many ofthem will be investigated for their emission mechanism. This canbe done by measuring the energy spectra of gamma rays up to afew hundred TeV. To search for galactic cosmic ray origins amongthem, the focus is the high energy ends of the spectra where oneexpects to see differences between origins of gamma rays, eitherthrough inverse Compton scattering of high energy electrons orfrom decays of neutral pions which are produced in interactionsbetween accelerated high energy nuclei, such as proton, andambient material near the sources. For this purpose, a particledetector array with an effective area of 1 km2 (KM2A) is proposed,including a muon detector array using water Cherenkov techniquewith 40 000 m2 active area. This allows a background-free mea-surement of gamma ray spectra above 50 TeV without any con-tamination by simply selecting muon-poor air showers. 5635scintillator detectors (1 m2 each) are arranged in a triangle gridwith a spacing of 15 m, while the spacing is set as 30 m between1221 muon detectors.

To survey gamma ray sources, a Water Cherenkov Detector Array(WCDA) with a total active area of 90 000 m2 is proposed [4], markedby the four squares in Fig. 1. It is sensitive to gamma ray showersabove few hundred GeV. The sensitivity to a source like the CrabNebula is about 0.7% of the crab unit, Icrab, namely the significancereaches to 5s in one year observation, shown in Fig. 2.

WCDA consists of 4 water ponds, each of which has a size of150�150 m2. The depth of the pond is about 4.5 m. Each pond issubdivided into 30�30¼900 cells sized 5�5 m2 each, partitionedby black plastic curtains to prevent the penetration of light yielded inneighboring cells. An 8 in. hemispheric PMT resides at the bottom ofeach cell, looking upward to collect Cherenkov lights produced bycharged particles in the water pond, recording the arrival time andthe charge of pulses. The later is in proportional to the product of thenumber of particles in the shower and their energies.

To measure energy spectra for individual species of cosmic rayparticles, one needs a reliable primary particle identificationalgorithm for observed air showers. A multi-parameter measurementis the most plausible approach. In general, the shower maximumposition, the muon content and the high energy component near theshower core are three independent parameters that can be used fordeducing signatures of showers induced by different nuclei [5,6].Thus, two more detector arrays are proposed in the LHAASO project.They are designed to measure the shower maximum location andhigh energy components near cores, respectively. They are WideField of view Cherenkov Telescope Array (WFCTA) composed of 24telescopes and the high threshold Shower Core Detector Array(SCDA) with an effective area of 5000 m2, shown as a rectangleand two rows of small squares near WCDA ponds at the center of thearray in Fig. 1, respectively.

To extend the spectrum measurements to higher energies withcalibrated energy scale and composition, we have to re-arrange thedetector arrays for larger effective area and larger exposure.A simple re-arrangement of the WFCTA is necessary to form a hybridexperiment together with the 1 km2 KM2A as a whole instead ofwith SCDA which is only 5000 m2. The energy coverage can beextended to 10 PeV regime. In order to connect with other experi-ments, such as TA and Auger, at altitudes around 1600 m a.s.l., aneven larger effective area is required. The wide FOV telescopes will bere-arranged and modified to measure shower fluorescence light andmonitor the space above the ground array from a distance of 4–5 km.This modification and reconfiguration is nearly cost-free. The detec-tor configuration is shown in Fig. 1, in which the main detector arrayis composed of 16 telescopes covering elevations from 31 to 591 andtwo other detector arrays, covering elevations from 31 to 311, toobserve showers from perpendicular directions. Showers above100 PeV will be detected stereoscopically to maintain a high resolu-tion of shower maximum position. Combining with the muoncontent measured by KM2A, the telescopes will achieve the spec-trum and the composition measurements around the second knee.

3. R/D and engineering arrays

The Electromagnetic particle Detector (ED) in LHAASO-KM2Aconsists of 4�4 plastic scintillation tiles (25 cm�25 cm�2 cmeach) packed in a steel box with an area of 1 m�1.2 m.

Fig. 1. Left panel is LHAASO detector layout, and on the right-hand side the layout of the fluorescence detector array and the LHAASO array.

E = Emedian [ TeV ]0.1 1 10 100 1000

E•Φ

(>E)

[ Te

V cm

-2s-1

]

10-14

10-13

10-12

10-11

10 crab1 crab

0.1 crab

0.01 crab

0.001 crab

0.0001 crab

Fig. 2. The sensitivity of LHAASO-WCDA þ LHAASO-KM2A. The curves for otherexperiments and projects are drawn for comparison. The observation times are1 year and 50 h for wide field-of-view detectors and IACT respectively.

Z. Cao / Nuclear Instruments and Methods in Physics Research A 742 (2014) 95–9896

Four wavelength-shifting fibers (BCF92) of 1.5 mm in diameter and150 cm long in each are embedded in eight dips on the tile tocollect the scintillation light as charged particles pass through.A photomultiplier tube touched to all ends of the 128 fibers fromthe 16 titles collects the scintillating photons.

To verify the design and exam the performance of the detec-tors, an array with a size of 1% of the full scale LHAASO-KM2A,namely 42 detector units, has been built and tested at ARGO-YBJsite in Tibet since March 2010. The prototype detectors wereuniformly distributed on top of the central carpet of the ARGO-YBJ array [7] with the spacing of 15 m as proposed for LHAASO-KM2A covering an area around 75 m�75 m. The detectorarray [8] has been in operation since October 2010, the triggerrate is about 48 Hz with a criterion of at least 5 detectors beingtriggered. Around 1.5 GB data per day is collected and transferredto IHEP. Among them more than 95% events are matched withARGO-YBJ experiment within a time window of7500 ns. Themoon shadow in cosmic rays is successfully observed with asignificance of 5.3s, as shown in Fig. 3. Both significance andlocation of the shadow are within the statistic errors as expected.

The Muon Detector (MD) in LHAASO-KM2A is a water-Cherenkov detector in a concrete tank covered by 2.5 m of dirt.Each MD is 3.6 m in radius�1.2 m in height and 2.5 m underneaththe surface, i.e. about 12 radiation length before shower particlesreach to the water surface. Most of electromagnetic particles in theair showers cannot survive from the absorption layer. Each tank isequipped with an 8 in. PMT watching into a highly reflecting linerbag fully filled with pure water. A prototype detector is running atthe ARGO-YBJ site. Some preliminary results, such as the charge ofmuon signal pulses are quite stable within 2% over last threemonths and the detector simulation reproduces muon arrival timedistribution quite well as shown in Fig. 4.

After the single unit prototype of the water Cherenkov detectorconcluded on a good agreement between the measured and thedetailed simulation of the detector [9], an engineering array of

3�3 cells of LHAASO-WCDA [10] was constructed at the north-west corner of the ARGO-YBJ experiment hall in Tibet. It is about1% of one of the four full scale water ponds in LHAASO. For nine8 in. PMTs, all front-end-electronics, DAQ, water recycling andpurification system, slow control system, optical calibration sys-tem [11], etc., are fully equipped as a full detector array. In thecoincident operation with the ARGO-YBJ RPC carpet together, thewater Cherenkov detector performance is tested, including thearrival direction reconstruction accuracy, trigger efficiency of airshowers and zenith/azimuth angle distributions and so on. Thedistribution of core positions of all showers triggered on bothdetectors is shown in Fig. 5. The core locations are reconstructedby using ARGO-YBJ carpet data.

Fig. 6 is the picture of the prototype Shower Core Detector Array(SCDA) which is currently operated together with AS-gamma, one ofthe two major experiments at the Tibet site. Relevant analysis isunder going mainly for cosmic ray composition and energyspectrum.

4. The combined spectrum of hydrogen and heliumby prototype WFCTA and ARGO-YBJ

The prototype telescope of the Wide Field of view CherenkovTelescope Array (WFCTA) in LHAASO is mainly composed of a

Fig. 3. The significance map of the moon shadow observed by LHAASO-KM2A 1%array using two years data.

Fig. 4. The arrival time comparison between data and MC simulation.

Fig. 5. The distribution of shower core position from the coincident events fromLHAASO-WCDA and ARGO-YBJ experiments.

Fig. 6. A picture of the prototype shower core detector.

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4.7 m2 spherical aluminized reflective light collector and animaging camera, a set of 256 hexagonal PMTs arranged in a16�16 array located at the focal plane of the reflector, thus forma field of view (FOV) of 141�161 in total. Each single PMT as anindependent pixel has a FOV of approximately 11. The whole system,including electronics, DAQ, slow control and monitoring system, isinstalled in a compact shipping container with a dimension of2.5 m�2.3 m�3 m. The telescope as a whole is mounted on astandard dump truck frame with a hydraulic lift that allows thepointing direction to cover the elevation angle from 01 to 671. Theparticular portable design of the telescope enables a flexibility ofswitching between the configurations of the telescope array depend-ing on different physics targets. The detailed testing results andperformance of the telescope can be found elsewhere [12].

Two prototype telescopes for LHAASO-WFCTA were deployedto the ARGO-YBJ experiment site in Tibet in 2007. Millions ofcosmic ray events that simultaneously trigger the telescopes andthe ARGO-YBJ RPC carpet array have been collected. Many

interesting works have been carried out, such as monitoring thelocal weather using background starlight, etc. With its indepen-dent measurement about the shower longitudinal development,the two telescopes are used to select proton and Helium showerstogether with the RPC carpet that measures the lateral distributionof shower particles in very small regions near the cores, namelyless than 2 m. The hybrid measurement of the energy spectrum ofHydrogen and Helium nuclei [13,14] is shown in Fig. 7. It shows aclear extension of the spectrum from the similar measurements byARGO-YBJ and CREAM at slightly lower energies. Recently anenergy calibration and atmospheric monitoring facility usingnitrogen laser was deployed on the site [15].

References

[1] Z. Cao, LHAASO Project, in: 31st ICRC, 2009.[2] H.H. He, LHAASO Project: detector design and prototype, in: 31st ICRC, 2009.[3] M. Zha, Nuclear Instruments and Methods in Physics Research A 692 (2012)

77.[4] Z.G. Yao, et al., Design and performance of LHAASO-WCDA experiment, in:

32nd ICRC, 2011.[5] J.L. Liu, et al., The performance of shower maximum reconstruction by WFCTA

Telescope, in: 32nd ICRC, 2011.[6] M. Amenomori, et al., Test of the hadronic interaction models at ground few

10 TeV with Tibet EAS core data, in: 32nd ICRC, 2011.[7] G. Aielli, et al., Nuclear Instruments and Methods in Physics Research A 562

(2006) 92.[8] J. Liu, et al., Performances of the LHAASO-KM2A engineering array, in: 32nd

ICRC, 2011.[9] Q. An, et al., Nuclear Instruments and Methods in Physics Research A 644

(2011) 11.[10] M.J. Chen, et al., R and D of LHAASO-WCDA, in: 32nd ICRC, 2011.[11] B. Gao, et al., An optical calibration system for engineer array of LHAASO-

WCDA, in: 32nd ICRC, 2011.[12] S.S. Zhang, et al., Nuclear Instruments and Methods in Physics Research A 629

(2011) 57.[13] L.L. Ma, et al. The monitoring of weather and atmospheric condition of

LHAASO site, in: 32nd ICRC, 2011.[14] S.S. Zhang, Hybrid measurement of CR energy spectrum and composition

r200 TeV by using ARGO-YBJ and WFCTA, in: 32nd ICRC, 2011.[15] Y. Zhang, et al., Energy calibration for WFCTA using Nitrogen laser, in: 32nd

ICRC, 2011.

Fig. 7. The energy spectrum of protons and Helium nuclei using the hybridmeasurement with LHAASO-WFCTA prototype and ARGO-YBJ RPC carpet array.

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