以太魯閣-4 電波天線陣列探尋極高能宇宙射線

Abstract

能量超過 10^18 eV 的超高能宇宙射線的研究是當前物理學中的一個活躍研究領域，物理學家希望揭示這些高能粒子的來源及其加速機制。該領域的主要挑戰在於超高能宇宙射線的稀有性——每平方公里每年僅有約一次事件發生。因此，實現大規模探測面積和長時間觀測變得尤為重要。一種有效的超高能宇宙射線觀測方法是探測由其引發的廣延大氣簇射所發射的無線電脈衝。無線電輻射的主要機制是地磁效應。當廣延大氣簇射的帶電粒子在地球磁場中傳播時，洛倫茲力會使其偏轉，從而產生橫向電流。這個隨時間變化的橫向電流會發射出地磁輻射，其偏振方向與洛倫茲力方向一致。這種地磁輻射在幾兆赫到幾百兆赫的頻率範圍內具有相干性，以相對論性前向束射的形式在廣延大氣簇射前進方向發出，形成可以被探測到的短暫脈衝（持續時間為數納秒）。 除了超高能宇宙射線，廣延大氣簇射還可以由超高能微中子在地球表面以下通過帶電流相互作用產生的濤輕子衰變引發。超高能微中子可能通過宇宙射線與宇宙微波背景輻射的相互作用（即 GZK 效應）產生。已觀測到在 4X10^17 eV 以上的超高能宇宙射線通量抑制，這支持了 GZK 效應的存在。與會被星際磁場偏轉的超高能宇宙射線不同，宇宙對超高能微中子幾乎是透明的，因此探測超高能微中子是間接發現超高能宇宙射線源的一個有效策略。 臺灣天文粒子地磁同步輻射電波觀測站（簡稱太魯閣，TAROGE）是一個位於臺灣東部沿海高山上的天線陣列，面向海洋，專為探測超高能宇宙射線引發的近地平線廣延大氣簇射和地表掠過的超高能濤微中子而設計。TAROGE 具有高有效觀測時間、低單位成本和良好擴展性等優點。從 2014 年到 2019 年，已有四個 TAROGE 站點在臺灣部署，每個站點的儀器都有所改進，以提高探測效率。最新部署的站點 TAROGE-4 配備了一種基於表面聲波濾波器的多頻帶觸發系統。該新型觸發系統能夠有效區分地磁同步輻射脈衝信號與郊區人類活動背景噪聲。 本文首先總結了 TAROGE-4 站點的概念和儀器設計。接著描述了用於校準接收系統響應以實現精確時間測量的分析步驟和差分響應反卷積方法。在降噪和響應反卷積的基礎上，本文實現了對廣延大氣簇射方向和入射電場的可靠重建。基於地磁輻射特性和環境噪聲，本文提出了事件選擇標準。包括觸發模擬以獲取探測效率。此外，還提出了蒙特卡羅探測模擬以驗證觸發和分析效率。最終目標是表徵探測事件的初級粒子並測量其通量。

The study of Ultra-High Energy Cosmic Rays (UHECRs) with energies greater than 10^18 eV is an active research area where physicists hope to find the sources and the acceleration mechanisms of such high energy particles. The primary challenge in this field is their rarity, approximately one event per square kilometer annually, it becomes essential to implement expansive detection area and lengthy observational periods. One effective way to observe UHECRs is by detecting radio pulses emitted from Extensive Air Shower (EAS) induced by them. The main mechanism of radio emission is geomagnetic effect. When charged particles of EAS propagating in the earth’s magnetic field, the Lorentz force will deflect them and thus induces a transverse current. The time-varying transverse current will emit geomagnetic radiation which polarization is aligning with the Lorentz force. This geomagnetic radiation is coherent at frequencies of a few to hundreds of MHz, generating short transient pulses (a few nanoseconds) relativistically beamed in the EAS forward direction at a detectable level. In addition to UHECRs, EAS can also be induced by the decay of tau leptons created through charged-current interaction of UHE (Ultra High Energy) neutrinos under the Earth’s surface. UHE neutrinos could be produced through the interactions between cosmic rays and cosmic microwave background radiation (CMB), which is the so-called GZK effect. The suppression of the UHECR flux above 4X10^17 eV has been observed, which supports the existence of the GZK effect. Unlike UHECRs will be deflected by intergalactic magnetic fields, the universe is almost transparent to UHE neutrinos, thus detecting UHE neutrinos is a good indirect strategy to discover UHECR sources. Taiwan Astroparticle Radiowave Observatory for Geosynchrotron Emissions (TAROGE) is an antenna array atop the high mountains on the eastern coast of Taiwan, pointing to the ocean. It is designed for the detection of near-horizon EAS induced by UHECRs and Earth-skimming UHE tau neutrinos with advantages in high effective live time, low unit cost and scalability. Four TAROGE stations in Taiwan have been deployed from 2014-2019, each station has improvements in instruments to increase detection efficiency. The latest deployed station TAROGE-4 has been equipped with a new trigger system by using Surface Acoustic Wave (SAW) filters based multi-bands coincidence technique. This new trigger system provides an effective discriminating power for impulsive geo-synchrotron signals against suburban anthropogenic backgrounds. In this paper, The concept and the instrument of TAROGE-4 station are first summarized. Then the analysis steps and differnetial response deconvolution method to calibrate receiver response to get precise measurement of timing are described. Then the noise reduction and the deconvolution of the response are applied to obtain reliable reconstruction of EAS direction and incident electric field. Event selection criteria are proposed, based on the characteristics of geomagnetic emission and environmental noise. Trigger simulated are included to obtain the detedction efficiency. Monte-Carlo detection simulation are also proposed to verify trigger and analysis efficiency. The final goal is to characterize primary particles of detected events and measure their fluxes.