A large light-mass component of cosmic rays at 1017-1017.5 electronvolts from radio observations
A large light-mass component of cosmic rays at 1017-1017.5 electronvolts from radio observations
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DOI：
10.1038/nature16976
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发表时间：
2016-03-03
期刊：
影响因子：
64.8
通讯作者：
Zensuss, J. A.
中科院分区：
文献类型：
--
作者：
Buitink, S.;Corstanje, A.;Zensuss, J. A.
宇宙射线是自然界中发现的最高能量粒子。测量能量为 10(17)-10(18) 电子伏的宇宙射线的质量成分对于了解它们是否具有银河源或河外源至关重要。还有人提出，天体物理中微子信号1来自能够产生这些能量的宇宙射线的加速器(2)。宇宙射线引发空气簇射（大气中的次级粒子级联），其质量可以通过测量簇射最大大气深度 3（Xmax；空气簇射包含最多颗粒时的深度）或到达地面的簇射颗粒的成分 (4) 来推断。电流测量 (5) 要么具有高不确定性，要么具有低占空比和高能量阈值。宇宙射线无线电探测 (6-8) 是一项快速发展的技术 (9)，用于确定 Xmax（参考文献 10、11），原则上占空比接近 100%。辐射是由地磁场中相对论性电子和正电子的分离以及簇射前沿中过量的负电荷产生的(6,12)。在这里，我们报告了由能量为 1017-1017.5 电子伏特的宇宙射线引发的空气簇射的 X-max 无线电测量，平均不确定度为 16 克/平方厘米。 Xmax 的高分辨率使我们能够确定宇宙射线的质谱：我们发现了一种混合成分，其轻质量分数（质子和氦核）约为 80%。除非与目前的预期相反，宇宙线的河外成分对低于 10(17.5) 电子伏特的总通量有很大贡献，否则我们的测量表明存在额外的银河成分，以解释我们在 10(17)-10(17.5) 电子伏特范围内测量的光成分。
Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 10(17)-10(18) electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal1 comes from accelerators capable of producing cosmic rays of these energies(2). Cosmic rays initiate air showers-cascades of secondary particles in the atmosphere-and their masses can be inferred from measurements of the atmospheric depth of the shower maximum3 (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground(4). Current measurements(5) have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays(6-8) is a rapidly developing technique(9) for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front(6,12). Here we report radio measurements of X-max with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 1017-1017.5 electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 10(17.5) electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 10(17)-10(17.5) electronvolt range.