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Calibration of the LHAASO-KM2A
using extensive air showers
Lv Hongkui
Institute of High Energy Physics, Chinese Academy of Sciences
TIPP2017, Beijing, China
Outline

Introduction

Requirement of KM2A-EDs

A automatic calibration method using EASs
-- Principle
-- Experimental calibration && cross - check
-- Calibration results && precision

Conclusion
One square kilometer
array (KM2A) :
5242 electromagnetic
particle detectors (EDs)
1171 muon detecters
Wide Field Cherenkov
telescope Array
Water Cherenkov
Detector Array
78,000 m2
Coverage area: 1.3 km2
Altitude: 4400 m a.s.l.
LHAASO-KM2A

As a main component of LHAASO, the 1km2 array (KM2A) has several physics
goals such as:

Origin of cosmic rays ;

UHE gamma sky survey ;

Energy spectrum measurement ;

…
Performance of the KM2A
Value
Energy range
10 TeV~100 PeV
Sensitivity
1%Crab @ 50 TeV
Angular resolution
Background rejection
~0.5o
@ 30 TeV
Backgroung free
above 50 TeV
MD
ED
Electromagnetic particle detectors (EDs)

5242 EDs is designed to measure the particles (𝑒 ± , 𝜇 ± , 𝛾) density and arrival times of
the particles within the EAS;

From which the primary energy and direction can be reconstructed.
Lead (0.5 cm)
Scintillation
tiles (2 cm)
Specifications of ED
Value
Effective area
1 m2
Number of
scintillation tiles
4
Size of each
scintillation tile
100×25×2.5 cm3
Number of WLS fibers
32 / tile
Requirement of
electromagnetic particle detectors (EDs)


High precision time synchronization: the direction of the primary cosmic ray is
directly related to the particle arrival times recorded. Timing calibration is critical to
guarantee the optimal angular resolution.
Requirement
Value
Precision of
time synchronization
among all EDs
0.5 ns
Time resolution
2 ns
Dynamic range
1 – 104 particles/m2
Detection efficiency
> 95 %
Wide dynamic range: the particle density varies significantly due to the large variation
in the primary energy (10 TeV - 100 PeV) of cosmic rays and different distances of
detectors from the shower core.
Calibration of
electromagnetic particle detectors (EDs)

Challenge: The huge number of EDs extremely demand on a fast, high precision and
real-time calibration procedure;

Exceed the capability of traditional methods;

A automatic, self-calibration method, which use the charged particles within extensive air
showers (EAS) as calibration beam, is explored;

Focusing on its applicability to the up-coming LHAASO-KM2A.
Outline

Introduction

Requirement of KM2A-EDs

A automatic calibration method using EASs
-- Principle
-- Experimental calibration && cross - check
-- Calibration results && precision

Conclusion
A automatic timing calibration method

Timing calibration: The 5242 EDs timing must be synchronized to within sub-nanoseconds.

Thus, detector time offsets must be determined and corrected very fast (on hour timescale).

Calibration principle: The secondary particles of the EAS front approximately preserve a
conical shape, providing a common standard timing signal to calibrate the EDs.
Fiber
PMT
𝛿𝑡𝑖
Length:1m

Expected hit time of ED (ID:i): 𝑡𝑖 𝑡ℎ = 𝑎𝑡ℎ
𝑥𝑖
𝑐
+ 𝑏𝑡ℎ
𝑦𝑖
𝑐
+ α𝑟𝑖 + 𝑡0
Electronics
Monte Carlo studies

Measured time offsets VS. Preset time offsets: Good agreement ;

Precision: 𝑅𝑀𝑆(𝑡𝑚𝑒𝑎𝑠𝑢𝑟𝑒 − 𝑡𝑝𝑟𝑒𝑠𝑒𝑡 ) ~ 0.46 ns.

Calibration speed: at least 2*106 EAS events were needed.  0.5 hour of event collecting;

High precision && Very fast && Without manpower;
Experimental calibration && cross - check

KM2A prototype array (at Yangbajing, Tibet, 4300 m a.s.l)
• Composed of 40 EDs (~1 % scale of KM2A);
• Began to operate in October 2014 and has been
running steadily for more than two years
.
• Two independent calibration methods have been
applied in the KM2A prototype array to provide an
estimation of uncertainties.
ED
array
ARGO
112.5 m
52 m
 40 EDs distributed on 52m×112.5m
Experimental calibration && cross - check

Muon telescope system: backgroung muons pass through 3EDs simultaneously, thus
the time differences between these EDs can be determined;

Provides an independent calibration method to cross-check the results obtained from
the software-level method using EAS.
particle
3EDs

High precision ~ 0.1 ns but very slow ~ 10 min/ED, limited by muon event rate;
Calibration results && precision

Muon telescope method VS. EAS method: Good agreement;

Precision: 𝑅𝑀𝑆(𝑡𝑚𝑢𝑜𝑛 − 𝑡𝐸𝐴𝑆 ) ~ 0.42 ns.

Agree with the simulation result very well;
Calibration results && precision

The precision of EAS methods can meet the requirements of KM2A EDs.
Muon Telescope
Accuracy：~ 0.1 ns
High accuracy, too much time and manpower
Timing calibration
EAS method
Accuracy: ~ 0.42 ns
High speed without much manpower
Conclusion
• The presence of cosmic ray and the EAS they produced in every field observed, makes
them potentially very useful for calibration purposes.
• Calibration of ED timing through the use of EAS events has been shown to be a robust,
automatic approach enabling an independent calibration of many hardware
technologies.
• The self-calibration method can be used to determine the detector time offsets
constants on hours timescale at a precision level of sub-nanosecond.
• The calibration procedure meet the requirements of KM2A-EDs and will be applied in
up-coming LHAASO-KM2A experiment.
Thank you!