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A large high altitude air shower observatory (LHAASO) located at Daocheng in Sichuan province, China, with an altitude up to 4410 m above the sea level, takes the function of hybrid technology to detect cosmic rays. It is composed of three sub-arrays: a 1.3 km2 ground-based particle detector array (KM2A) for γ-ray astronomy and cosmic ray physics, a 78000 m2 water Cherenkov detector array (WCDA) for γ-ray astronomy, and 18 wide field-of-view air Cherenkov/fluorescence telescopes array (WFCTA) for cosmic ray physics. As the major array of LHAASO, KM2A is composed of 5195 electromagnetic particle detectors (EDs, each with 1 m2) and 1188 muon detectors (MDs, each with 36 m2). In the ground-based experiments, there are two common independent data acquisition systems, corresponding to the shower and scaler operation modes. Up to now, the KM2A array operates only in shower mode with the primary energy threshold of about 10 TeV. In the scaler mode, it is not necessary for too many detectors to be hit at the same time. The energy threshold of the experiment can be greatly lowered. In order to learn more about the scaler mode in LHAASO-KM2A, we adopt the CORSIKA 7.5700 to study the cascade processes of extensive air showers in the atmosphere, and employ the G4KM2A (based on Geant4) to simulate the detector responses. The KM2A-ED array is divided into dozens of clusters. For one cluster (composed of 64 EDs), the event rates of showers having a number of fired EDs ≥ 1, 2, 3, 4 (in a time coincidence of 100 ns) are recorded. The average rates of the four multiplicities are ~88 kHz, ~1400 Hz, ~220 Hz, and ~110 Hz, respectively. The particle multiplicities m ≥ 3 are almost completely due to cosmic ray secondary particles. The corresponding primary energies and effective areas are also given in this paper. According to our simulations, the energy threshold of the scaler mode can be lowered to 100 GeV, and the effective areas reach up to ~100 m2. The simulation results in this work are helpful in the online triggering with the scaler mode, and provide information for the subsequent data analysis in LHAASO-KM2A.
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Keywords:
- cosmic rays /
- scaler mode /
- Monte Carlo simulations
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图 1 KM2A-ED阵列(左)和scaler模式中cluster的ED布局图(右)
Figure 1. Layout diagram of KM2A-ED array (left) and the cluster in scaler mode (right).
图 2 经过1个ED探测器响应后宇宙线计数率分布的模拟结果
Figure 2. Event rate distribution for one ED.
图 3 Scaler模式中多重数 m ≥ 1, 2, 3和4的计数率分布
Figure 3. Event rate distribution with m ≥ 1, 2, 3 and 4 in scaler mode.
图 4 Scaler 模式中不同多重数时有效面积随原初能量的分布 (a)质子; (b)氦
Figure 4. The Aeff as a function of the primary energy in scaler mode: (a) Proton; (b) Helium.
图 5 Scaler模式中不同多重数时有效面积随天顶角的分布 (a)质子; (b)氦
Figure 5. The Aeff as a function of the zenith angle in scaler mode: (a) Proton; (b) Helium.
图 6 Scaler 模式中不同多重数时探测到的原初质子能量分布
Figure 6. Energy distribution for primary Proton in scaler mode.
图 7 Scaler 模式中不同多重数时探测到的原初氦能量分布
Figure 7. Energy distribution for primary Helium in scaler mode.
表 1 Scaler模式中不同多重数时的平均计数率和宇宙线贡献率
Table 1. Average rates and the contribution of cosmic rays in scaler mode.
多重数(m) ≥ 1 ≥ 2 ≥ 3 ≥ 4 平均计数率 88 kHz 1400 Hz 220 Hz 110 Hz 宇宙线贡献率 42.3% 62.3% 96.9% 99.7% 
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表 2 Scaler模式中探测原初质子和氦的平均有效面积
Table 2. Average effective area for primary Proton and Helium in scaler mode.
多重数(m) Proton: $ \langle $Aeff$ \rangle $ /m2 Helium: $ \langle $Aeff$ \rangle $ /m2 ≥ 1 129.69 73.07 ≥ 2 2.38 0.91 ≥ 3 0.67 0.49 ≥ 4 0.34 0.27
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表 3 Scaler模式中探测原初质子和氦的平均能量
Table 3. Average energy of primary Proton and Helium in scaler mode.
多重数(m) Proton $ \langle $E$ \rangle $/GeV Helium $ \langle $E$ \rangle $/GeV ≥ 1 93.8 236.0 ≥ 2 1.4 × 103 5.4 × 103 ≥ 3 6.3 × 103 1.4 × 104 ≥ 4 9.5 × 103 1.9 × 104
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[1] 张潇, 陈阳 2019 现代物理知识 31 9
Zhang X, Chen Y 2019 Modern Physics 31 9
[2] 周勋秀, 胡红波, 黄庆 2009 58 5879
Google Scholar
Zhou X X, Hu H B, Huang Q 2009 Acta Phys. Sin. 58 5879
Google Scholar
[3] Bartoli B, Bernardini P, Bi X J 2017 Astrophys. J. 842 31
Google Scholar
[4] Abeysekara A U, Alfaro R, Alvarez C 2015 Astrophys. J. 800 78
Google Scholar
[5] Aharonian F, An Q, Axikegu 2021 Chin. Phys. C 45 025002
Google Scholar
[6] Aielli G, Bacci C, Barone F 2009 Astrophys. J. 699 1281
Google Scholar
[7] Di Girolamo T 2016 Nucl. Part. Phys. Proc. 279-281 79
[8] Bartoli B, Bernardini P, Bi X J 2015 Astrophys. J. 806 20
Google Scholar
[9] Bartoli B, Bernardini P, Bi X J 2015 Phys. Rev. D 91 112017
Google Scholar
[10] Bartoli B, Bernardini P, Bi X J 2015 Phys. Rev. D 92 092005
Google Scholar
[11] Vernetto S 2000 Astropart. Phys. 13 75
[12] Aielli G, Bacci C, Barone F 2008 Astropart. Phys. 30 85
Google Scholar
[13] Di Girolamo T, Vallania P, Vigorito C 2011 Astrophys. Space Sci. 7 239
Google Scholar
[14] Bartoli B, Bernardini P, Bi X J 2014 Astrophys. J. 794 82
Google Scholar
[15] Dasso S, Asorey H 2012 Adv. Space Res. 49 1563
Google Scholar
[16] Bartoli B, Bernardini P, Bi X J 2018 Phys. Rev. D 97 042001
Google Scholar
[17] 周勋秀, 王新建, 黄代绘, 贾焕玉, 吴超勇 2015 64 149202
Google Scholar
Zhou X X, Wang X J, Huang D H, Jia H Y, Wu C Y 2015 Acta Phys. Sin. 64 149202
Google Scholar
[18] 徐斌, 别业广, 邹丹 2012 空间科学学报 32 501
Google Scholar
Xu B, Bie Y G, Zou D 2012 Chin. J. Space Sci. 32 501
Google Scholar
[19] Heck D, Knapp J, Capdevielle J N, Schatz G, Thouw T, https://www.ikp.kit.edu/corsika/70.php [2021-04-08]
[20] 陈松战, 赵静, 刘烨, 何会海, 侯超, 李秀荣, 张忠泉, 李骢, 刘佳, 李哲, 王玲玉 2017 核电子学与探测技术 37 1101
Chen S Z, Zhao J, Liu Y, He H H, Hou C, Li X R, Zhang Z Q, Li C, Liu J, Li Z, Wang L Y 2017 Nuclear Electronics & Detection Technology 37 1101
[21] 卢晓旭, 顾旻皓, 朱科军, 李飞 2020 核技术 43 80
Lu X X, Gu M H, Zhu K J, Li F 2020 Nuclear Techniques 43 80
[22] He H H 2018 Radiation Detection Technology and Methods 2 1
Google Scholar
[23] 曹臻, 陈明君, 陈松战 2019 天文学报 60 19
Cao Z, Chen M J, Chen S Z 2019 Acta Astronomica Sinica 60 19
[24] Cao Z, Chen M J, Chen S Z 2019 Chin. Astron. Astrophys. 43 457
Google Scholar
[25] Jin C, Chen S Z, He H H 2020 Chin. Phys. C 44 065002
Google Scholar
[26] Zhang Z Q, Hou C, Cao Z 2017 Nucl. Instrum. Methods A 845 429
Google Scholar
[27] Allison J, Amako K, Apostolakis J 2016 Nucl. Instrum. Methods A 835 186
Google Scholar
[28] Aguilar M, Aisa D, Alpat B 2015 Phys. Rev. Lett. 114 171103
Google Scholar
[29] Aguilar M, Aisa D, Alpat B 2015 Phys. Rev. Lett. 115 211101
[30] Aharonian F, An Q, Axikegu 2021 Nucl. Instrum. Methods A 1001 165193
Google Scholar
[31] Zhou X X, Gao L L, Zhang Y, Guo Y Q, Zhu Q Q, Jia H Y, Huang D H 2016 Chin. Phys. C 40 075001
Google Scholar
[32] Vallania P, Girolamo T Di, Vigorito C 2006 Proceedings of the 20th European Cosmic Ray Symposium, Lisbon, Portugal, September 5–8, 2006 p5
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