南極天壇陣列微中子觀測站的最佳排列方式

Abstract

南極天壇陣列微中子觀測站是觀測宇宙極高能微中子與冰層反應後產生的契倫可夫輻射的實驗裝置，該裝置設在南極冰層以下兩百公尺處。模擬能量約1017eV到1020eV的宇宙極高能微中子射入地球時，經過冰層與粒子反應發射出的契倫可夫輻射路徑，藉由冰層下的天線偵測到的訊號波形以及契倫可夫輻射的特性，再考量進真實情況的背景雜訊，重建出產生契倫可夫輻射的位置以及微中子入射進地球的方向，進而推斷宇宙極高能微中子的發射來源。在模擬過程中，測試各種可能的天壇陣列排列方式，改變測站間和天線間的距離，找出可偵測到最多微中子訊號的最佳排列方式。而最佳陣列是由49個測站組成的六角柱結構，每個測站裝設3串天線，每串有4個天線且依此偏振類型順序排列: 垂直-垂直-水平-垂直，最近的測站間距為3 km而天線間距為30 m，此為可得到最高效率和最大準確度的最佳排列方式。

Askaryan Radio Array (ARA) is an observatory designed to detect the radio frequency (RF) Cherenkov radiation generated by the shower induced by ultra high energy (UHE) cosmic neutrino whose energy lies between 10^17eV and 10^21eV. Tracing the UHE neutrinos is the best way to know the origin and the evolution of the cosmic accelerators, because neutrinos are undeflected by magnetic fields and unhindered by interactions with cosmic microwave background (CMB) when it traverses the universe from the source. ARA Observatory, to be located at the South Pole in Antarctica, takes abundant ice as the target. When UHE neutrinos propagate through the ice, they interact with the nucleons in the ice and generate the Cherenkov radiation via the Askaryan effect. In order to reconstruct more precisely the incident directions of the UHE neutrinos so as to identify their sources, it is desirable to explore different design geometries of the ARA array. We find the optimized configuration of ARA Observatory consists of 49 stations located on a hexagonal lattice with 12 antennas per station. The station spacing is 3 km, string spacing and antenna spacing is 30 m. It means the adjacent antenna spacing is 10 m which is the shortest distance in our simulation. There are three Vpol antennas and one Hpol antenna in a string with the sequence is Vpol-Vpol-Hpol-Vpol.