[论文解读] Extreme Universe Space Observatory on a Super Pressure Balloon 1 calibration: from the laboratory to the desert

Jenni Adams, Shadab Alam+64 more|arXiv (Cornell University)|Nov 18, 2020

Astrophysics and Cosmic Phenomena参考文献 20被引用 5

一句话总结

本文介绍了极端宇宙空间观测站超压气球1号（EUSO-SPB1）的飞行前校准，该观测站为搭载于平流层高空气球的紫外望远镜，旨在通过荧光光探测超大能量宇宙射线。通过实验室和现场测试（包括基于激光的触发效率、角分辨率和均匀场响应测量），该仪器实现了10%的绝对光探测效率、11.1°的视场角以及亚度级角分辨率，验证了其在临近空间高度开展宇宙射线探测的准备就绪状态。

ABSTRACT

The Extreme Universe Space Observatory on a Super Pressure Balloon 1 (EUSO-SPB1) instrument was launched out of Wanaka, New Zealand, by NASA in April, 2017 as a mission of opportunity. The detector was developed as part of the Joint Experimental Missions for the Extreme Universe Space Observatory (JEM-EUSO) program toward a space-based ultra-high energy cosmic ray (UHECR) telescope with the main objective to make the first observation of UHECRs via the fluorescence technique from suborbital space. The EUSO-SPB1 instrument is a refractive telescope consisting of two 1m$^2$ Fresnel lenses with a high-speed UV camera at the focal plane. The camera has 2304 individual pixels capable of single photoelectron counting with a time resolution of 2.5$\mu$s. A detailed performance study including calibration was done on ground. We separately evaluated the properties of the Photo Detector Module (PDM) and the optical system in the laboratory. An end-to-end test of the instrument was performed during a field campaign in the West Desert in Utah, USA at the Telescope Array (TA) site in September 2016. The campaign lasted for 8 nights. In this article we present the results of the preflight laboratory and field tests. Based on the tests performed in the field, it was determined that EUSO-SPB1 has a field of view of 11.1$^\circ$ and an absolute photo-detection efficiency of 10%. We also measured the light flux necessary to obtain a 50% trigger efficiency using laser beams. These measurements were crucial for us to perform an accurate post flight event rate calculation to validate our cosmic ray search. Laser beams were also used to estimated the reconstruction angular resolution. Finally, we performed a flat field measurement in flight configuration at the launch site prior to the launch providing a uniformity of the focal surface better than 6%.

研究动机与目标

• 通过受控的实验室和现场测试，表征EUSO-SPB1探测器在飞行前的性能表现。
• 验证仪器通过荧光光探测并重建超高能宇宙射线（UHECRs）引起的广延空气簇射（EAS）轨迹的能力。
• 测量关键仪器参数，如光探测效率、视场角（FoV）、触发效率和角分辨率。
• 确保在各组件（PDM与光学系统）及整体配置中，校准的准确性与一致性。
• 通过建立基准性能指标，实现飞行后事件率计算与源重建的准确性。

提出的方法

• 在巴黎APC实验室使用积分球和校准的378 nm LED光源，对光电探测模块（PDM）进行了相对与绝对光度校准。
• 利用准直激光束与校准光电二极管，测量双透镜与三透镜配置的光学透过率，以确定光通量。
• 在美国犹他州的望远镜阵列（Telescope Array）站点开展了为期8晚的现场试验，使用脉冲紫外激光模拟EAS引发的荧光信号。
• 测量触发效率随激光脉冲能量的变化，确定50%探测阈值为0.9 mJ，验证仪器灵敏度。
• 在新西兰南地（Wanaka）发射场进行均匀场响应测量，评估像素间灵敏度均匀性（<6%偏差）。
• 通过以不同角度（13°–88°）相对于光轴发射激光，利用几何簇射模型进行时间拟合，重建簇射到达方向并评估角分辨率。

实验结果

研究问题

• RQ1 EUSO-SPB1仪器的绝对光探测效率是多少？其与各组件实验室测量结果相比如何？
• RQ2 根据现场测试结果，EUSO-SPB1仪器的有效视场角（FoV）是多少？
• RQ3 仪器在何种激光脉冲能量下实现50%的触发效率？该值与超高能宇宙射线簇射预期信号水平有何关联？
• RQ4 在真实现场条件下，仪器的重建角分辨率如何？其随簇射入射角的变化趋势如何？
• RQ5 探测面各像素的灵敏度是否均匀？非均匀性对事件重建有何影响？

主要发现

• EUSO-SPB1仪器的绝对光探测效率测定为10% ± 1%，与PDM和光学系统实验室测量结果的综合值一致。
• 根据犹他州望远镜阵列站点的现场测量，仪器视场角确定为11.1° ± 0.2°。
• 在355 nm波长下，激光脉冲能量为0.9 mJ时实现50%触发效率，该能量水平对应于33 km高度、45°天顶角下3.5 EeV广延空气簇射的预期信号。
• 最坏情况（13°入射角）下重建角分辨率优于2.2°，在68°入射角时提升至0.4°，表明对倾斜簇射具有高精度方向重建能力。
• 在发射场进行的均匀场测量确认，探测面像素灵敏度均匀性优于6%，支持可靠事件重建。
• 仪器表现出约0.9°的系统性偏低角重建偏差，可能源于激光倾斜或望远镜指向校准的微小错位。