EAS Changed With Zenith Angle
Researchers: Liu Yifan, Yuan Jiaqi, Chen Huiyang, Ma Haipeng,Zhang Huairen, Zuo Hongkang.
Instructor: Ma Shuncun.
The High School Attached To Hunan Normal University
Cosmic rays usually consist of high-energy particles from space. On the way high-energy cosmic rays shoot from space to earth, they need to pass through the atmosphere and it is the time that they interact with atomic nuclei in the atmosphere, knocking out various secondary particles. These secondary particles will interact with atomic nuclei in the atmosphere again during the flight, producing more secondary particles, which are sprinkled from the air to the earth like a rainstorm, and conducting cascade reactions continue to occur in this way. This process is also known as "air shower". More specific, these secondary particles produced by cosmic rays repeatedly act to produce more secondary particles until the average energy is equal to a certain critical value, and the number of secondary particles reaches the maximum value, which is called shower maximum. After that, the particles gradually decay or are affected by the atmosphere. Absorption, so that the number of secondary particles gradually decreases.
High-energy cosmic rays can form "showers" with large area, which produce a large number of secondary charged particles reaching the ground almost simultaneously. By measuring these charged particles arriving at the same time, the "shower" cases can be obtained. Scientists usually study on cosmic rays by indirectly detecting these "showers" reaching the surface of the earth. In general, the higher the energy of cosmic rays, the larger the shower area that reaches the surface. In practical experimental setup, detector array are often used for detection.
In this article, we analyzed the data detected by the detector array of Dongzhimen Middle School in Beijing, China(39.933°N latitude, 116.417°E longitude, 46.4 meters above sea level). The detector array is composed of 9 scintillation detectors, separated by 10 meters, set in a 3×3 matrix pattern, and the sensitive area of each detector is 0.5 square meters.
Ⅱ.Data processing and analysis
We analyzed the direction of each EAS event within 5 days since 0:00 Beijing time on October 4, 2021, and obtained the number of events Ni with the uniform range of the zenith angle from 0 to 90° when i=9. Its standard error σi=Ni0.5. For the zenith angle range A to B, it covers a solid angle Ω=2π(cosA-cosB), and the effective area of the array is 400m2×COSθi (θi is the median of the zenith angle range i). In this way, we get the average EAS intensity Ii=Ni÷(3×Ωi×COSθi) within the unit solid angle range of each zenith angle within a day. The standard error is σli=σi÷(3×Ωi×COSθi). The following is the table after measurement and data processing (Table 1):
We expect the curve decrease monotonically at in the region (0°, 90°), however from data analysis the intensity of EAS takes the minimum value around (75°, 85°) and get slightly increase in the region (85°, 90°). The reason may relate to accidental coincidence fake events taken account into detection due to relatively large coincident time window.
Ⅲ.Research discussion and result
From the data analysis, we can easily tell that the number of events within the corresponding zenith angle range always reaching the maximum value around X ∈ (20°, 25°) as shown in Figure 1. We believe that: on the one hand, as the zenith angle increases, the solid angle Ω, equals to 2π(cosA-cosB), is increasing, resulting in an increase in the number of events within the corresponding zenith angle; on the other hand, as the zenith angle increases, the effective detection area (0.5m2*cosX) of the detector decreases. Under the combined influence of these two factors, number of events within the corresponding zenith angle will achieve the maximum value at a certain angle position.
In the analysis of the curve of Intensity of EAS, we found that the curve varies steeply with the zenith angle at the beginning and the latter part is relatively flat as shown in Figure 2. From this result, we believe that: according to the principle of extended atmospheric shower, as the zenith angle increases , the particles need to pass through a thicker atmosphere and the primary particles need more energy to reach our detection threshold after being dispersed. Besides, the intensity of the isotropic primary cosmic ray is approximately proportional to E-2, so the EAS intensity is a descending function of the zenith angle.
Second to that, we found that the curve of Intensity of EAS changing with the zenith angle tends to take the minimum value around (75°, 85°) as shown in Figure 2. We believe that the slightly increase of intensity in the region (85°, 90°) may relate to accidental coincidence fake events taken account into detection due to relatively large coincident time window.
Last but not the least, in general, within EAS (Extensive Atmospheric Shower), the number of cosmic rays after area correction decreases as the zenith angle increases (ignoring accidental coincidence fake events)