1. Design and optimization of Electromagnetic particle Detectors (EDs) in LHAASO-KM2A Xiangdong Sheng, Jia Liu, Jing Zhao on behalf of the LHAASO collaboration Institute of High Energy Physics, CAS August 17, 2011

2. 2 Outline Introduction Design and optimization of ED prototypes Design of the size of Scintillator tile with fiber readout Long-term stability of the candidate scintillators Selection of the PMTs Suitable module power supplies ED’s assembly Conclusion

3. 33 1.Introduction KM2A aims at, Discovery of the Galactic CR sources Survey of the northern sky, exploring the origin of galactic cosmic rays Cosmic ray physics Electromagnetic particle detector (ED) Muon detector (MD) 5137 EDs and 1209 MDs,  Compose one square Kilometer extensive air shower array (KM2A) (Φ1020m);  Obtain the highest sensitivity (1%ICrab) at the highest Gamma ray energy. (Shuwang Cui, ICRC2011 Poster ID: 776)

4. 4 The requirements of performances of ED (in present design, as a kind of scintillator detector) are related, 1) To realize the KM2A sensitivity,  Detection area: 1m×1m;  Time resolution: < 2ns;  Detection efficiency: > 95%;  Dynamical range: 1 ～ 4000 particles/m 2 ; 25%@1 particle, 5%@4000 particles. 2)To the long-term operation in YBJ environment (4300m, 0.6 atm., -25ºC to 40ºC)  Good Scintillator with long-term stability;  Stable gains of PMTs, monitored in any time;  Suitable power supply;  Appropriate ED structure, water-repellent and easy to maintain; 2. Design and optimization of ED prototypes

5. 5 Key points,  Suitable photo-electrons obtained in photocathode, 20p.e. (1 particle)~80000p.e. (4000 particles)  Suitable scintillation light transmission paths and low fluctuation; Time resolution < 2ns Candidate scintillators, KEDI type, ST-401 type (~40% Anthracene) and ET9903kB type PMTs, R11102 type ones are applied for the tests, as well as BCF92 WLS fiber of Φ1.5mm. (1) Design of the size of Scintillator tile with fiber readout The following processes, scintillation light generation, its transmission and collection and the conversion at PMT’s photocathode, are considered. Scintillator samples (Tiles),  length: 25cm;  width: 5cm (KEDI), 15cm (KEDI), 25cm(KEDI, ST-401);  thickness: 1.0cm (KEDI), 1.5cm (KEDI), 2cm (ST-401)  holes-making: 5, 8 (only for KEDI) (25cm×Φ1.5mm each)  grooves: 8 (ST-401), 25cm×1.6mm×1.8mm( or 2.5mm )

6. 6  Length : 25cm suitable for time resolution;  Width : slowly variation (exceeding 15cm) Np.e. (increase); Time resolution (worse);  Depth: < 2cm  Scintillator covered with a layer of Tyvek sheet;  ~ 90% : Fiber coated an aluminium layer;  > 40% : Fiber-scintillator coupling (BC600 to air)  Deeper grooves has a little superiority ~ 20% : coupling with air; ~ 10% : coupling with BC600;  >30% : Fiber-PMT’s window coupling with silicon oil than with air In case of incoming single particle, Experimental results show the scintillation light collection efficiency affected by different factors. Time resolution : ~ 1.5ns A GEANT4-based simulation package have been developed, which results are consistent with the testing ones, which benefits the design and optimization greatly, combining with the tiles testing results.

7. 7  Scintillator size: 25cm× 25cm× 2cm; 8 grooves (25cm×1.6mm×1.8mm each)  8 WLS fibers (150cm long and Φ1.5mm)  Fiber-Scintillator coupling with air  One ET9903kB PMT applied ~ 21 p.e.  Scitillator size: 25cm× 25cm× 1.5cm; 8 holes (25cm×Φ1.5mm each)  8 WLS fibers (150cm long and Φ1.5mm)  Fiber-Scintillator coupling with air  One ET9903kB type PMT is equipped ~ 16 p.e. Two present optimization sizes of scintillator tiles

8. 8 (2) Long-term stability of the candidate scintillators Two accelerated ageing experiments are done to obtain the critical times (when the light output has decreased by 20%) of the candidate scintillators. (See Tianfu Zhou’s poster, ID 267)  High and low temperature aging experiment In simulation environment box: iterative variations: -30 ℃～ 40 ℃ ; Record the iterative times and assume one time means 2 days. ST-401 scintillator : ~ 9 years; KEDI scintillator : ~ 15 years.

9. 9 (3) Selection of the PMTs HAMAMATSU R11102 type and Electron Tubes (ET) 9903kB type are suitable PMTs. (See Bin Zhou, Chao Hou, ICRC2011 poster ID 261)  Linearity : ~ 3.6 orders of magnitude (<5% non-linearity)  Uniformity : (better than 80%);  Operating gain : ~ 10 5.6 ;  Low gain variation affected by geomagnetic field : (<10%); ET 9903kB type PMT

10. 10 (4) Suitable module power supplies To keep the PMTs’ operating gain variation within ±5% at YBJ site, it needs each module power supply with temperature coefficient less than 0.01%/ºC and ripple index less than 0.01%. Ration variation Vs. the temperatures Three kinds of candidate module power supplies have been validated to be eligible. (See Shaoru Zhang, ICRC2011 poster ID: 642)

11. 11  Each ED consist of 4×4 scintillator tiles, while 128 WLS fibers in total will guide the scintillation light to one PMT’s photocathode. Here, each PMT is equips with a module power supply;  Scintillator Tiles, one PMT and a set of power supply are packed in one aluminium/plastic box (one black box). On the top of the box, a layer of LEAD convertor with 5mm thickness are used to increase the detection efficiency, which is of benefit to improve angular resolution and position resolution of KM2A. (5) ED’s assembly Based on the present designs, some ED prototypes will be assembled by the end of this year with two kinds of scintillator tiles, respectively. Now the corresponding works are doing, such as the tests of scintillator tiles, PMTs and power supplies.

12. 12 3. Conclusion  Good progresses on the design and optimization of ED have been made. Here, some characteristics or performances of Scintillators, PMTs and power supplies have been studied in detail.  Based on the present designs, some new ED prototypes will be assembled this year.  It still needs more efforts to obtain the final optimization of ED.

13. 13 Thank you!