宇宙线直接探测进展概述

宇宙线从发现起至今已超过百年。在20世纪上半叶,大型粒子加速器技术成熟以前,对宇宙线的研究引领着基本粒子物理的发展,从宇宙线研究中取得的多项成果斩获诺贝尔奖。21世纪,宇宙线因其与极端高能的物理规律和暗物质等新物理现象联系密切而绽放出新的活力,宇宙线起源、加速、传播等相关的天文学及物理学问题也备受关注。简述了近年来在空间直接观测宇宙线实验方面取得的进展,以及其对理解宇宙线物理问题的推动。最后概述了中国在相关领域的研究历程和现状。     

宇宙线的超新星遗迹起源

宇宙线的起源是高能天体物理的核心问题之一.一直以来,超新星爆发被认为是能谱膝区以下宇宙线的主要来源.多波段观测表明,超新星遗迹有能力加速带电粒子至亚PeV (10~(15)eV)能量.扩散激波加速被认为是最有效的天体高能粒子加速机制之一,而超新星遗迹的大尺度激波正好为这一机制提供平台.近年来,一系列较高精度的地面和空间实验极大地推动了对宇宙线以及超新星遗迹的研究.新的观测事实挑战着传统的扩散激波加速模型以及其在银河系宇宙线超新星遗迹起源学说上的应用,深化了人们对宇宙高能现象的认识.结合超新星遗迹辐射能谱的时间演化特性,构建的时间依赖的超新星遗迹粒子加速模型,不仅能够解释200 GV附近宇宙线的能谱反常,还自然地形成能谱膝区,甚至可以将超新星遗迹粒子加速对宇宙线能谱的贡献延伸至踝区.该模型预期超新星遗迹中粒子的输运行为表现为湍流扩散,这需要未来的观测以及与粒子输运相关的等离子体数值模拟工作来进一步验证.     

DAMPE的氦核能谱以及高能天体物理研究

<正>宇宙线的研究自19世纪末开始,人们对其已进行了深入广泛的百年探索.近些年人们发现宇宙线在几千亿电子伏特(几百GeV)处存在明显的能谱变硬特征,这一发现跟此前预测的单幂律能谱存在明显的冲突,可能对理解宇宙线加速、传播等物理过程有重要意义.近些年空间宇宙线直接探测实验也触及TeV能段,     

基于暗物质粒子探测卫星的宇宙线直接探测研究

高能宇宙线的起源、加速和传播是重大的前沿科学问题,回答该问题需要对宇宙线的能谱、各向异性以及各类高能天体电磁辐射进行精确观测.通过空间粒子探测器对宇宙线各成分能谱的直接测量是研究宇宙线物理问题的重要手段.中国于2015年底发射并持续运行至今的暗物质粒子探测卫星以其大接受度、高能量分辨率等特点,在宇宙线直接探测方面取得了系列重要成果,揭示出质子、氦核、硼碳和硼氧比例等宇宙线能谱的新结构,为理解宇宙线起源等科学问题提供了新的依据.介绍了暗物质粒子探测卫星的仪器设置、运行状况、科学成果及其物理意义.     

CCD图像中宇宙线μ子甄选技术

电荷耦合器件(Charge Coupled Device, CCD)图像中的宇宙线是来自外太空的高能粒子,它穿过大气层,与大气层中的粒子发生相互作用形成次级粒子,最后落在CCD上。μ子是大气层宇宙线的主要成分,为了研究这些μ子的性质和变化规律,首先需要对图像中的μ子进行甄选。介绍一种快速有效地从图像中甄选μ子的方法,采用拉普拉斯边缘检测法从图像中提取宇宙线候选像素列表,去除其中的坏像素和噪声,然后使用凝聚层次聚类算法将宇宙线像素聚类成宇宙线事件,对宇宙线事件进行特征提取和分类,从中甄选宇宙线μ子。最后对甄选结果进行总结分析。     

暗物质粒子探测卫星的能量重建和宇宙线质子能谱的分析

宇宙线的观测研究和暗物质粒子的间接探测是高能天体物理领域两个重大研究课题. 自1912年V. Hess发现宇宙线开始, 人类对宇宙线的观测历史已经超过了一个世纪, 传统理论模型预言``膝''区以下能段的宇宙线能谱应服从单一幂率分布, 而近些年的空间和高空气球实验表明10 GeV--100 TeV的宇宙线质子能谱可能存在偏离单一幂律谱分布的重要结构, 这对研究银河系内宇宙线的起源、传播和加速机制具有重要意义. 另一方面, 得益于宇宙线和伽马射线观测精度的提高和观测能段的拓宽, 暗物质粒子的间接探测在国际上受到越来越多的关注, 暗物质粒子可能会发生湮灭或衰变产生稳定的普通高能粒子, 包括正负电子对、正反质子对、伽马射线和中微子等, 进而在宇宙线或伽马射线留下可探测的信号.     

高能中微子和极高能宇宙线起源天体的理论研究

极高能宇宙线一般指来自地外的能量高于1018电子伏特(eV)的高能质子与原子核,其起源的研究一直是高能天体物理和粒子天体物理领域的热点问题.近年随着一些大型探测器(如Pierre Auger天文台)的运行,极高能宇宙线的研究取得很大进展.然而由于极高能宇宙线事例相对较少及其在从源到地球传播过程中的复杂性(如与宇宙微波背景辐射以及磁场的作用),需要通过观测这些宇宙线在强子反应中产生的次级粒子(如中微子)来获得其起源的额外信息.最近,位于南极的IceCube中微子天文台探测到了54个能量分布在60TeV{3PeV内的中微子事例,开启了高能中微子天文学的新时代.在本文中,我们研究了高能中微子、极高能宇宙线的天体物理起源以及它们之间可能的联系.     

宇宙线“膝区”物理简介及国内观测研究进展

宇宙线发现100年来极大地推动了粒子物理学、天体物理学的发展,然而其起源、加速和传播的问题依然是宇宙线研究的三个根本问题。宇宙线“膝区”物理的研究是解决宇宙线这些基本问题的途径之一。简要回顾了宇宙线研究的历史和能谱特点;重点阐述了宇宙线“膝区”的实验观测数据以及重要实验组测量的差异和争论的焦点;归纳讨论了解释宇宙线“膝区”成因的四种主要理论模型,并结合最新实验数据说明了第四种成因(新物理过程)的可能性不大;叙述了国内研究者对宇宙线“膝区”物理研究的贡献;最后对宇宙线“膝区”物理研究的前景进行了展望。     

捕捉宇宙线的中国身影

夜空中除了繁星,还隐藏着一个巨大的秘密。无数神秘的粒子正以接近光的速度在广袠的宇宙中飞驰,无时不在,无处不在。这些神秘的粒子就是宇宙线。自发现它们之曰起,科学家对宇宙线的研究一直推动着天体物理和空间物理的发展,丰富着人类对宇宙中天体演化现象的了解和认识。宇宙线给人们带来了太阳系以外的物质样本,携带着其产生地"源"天体及其经过的空间环境,乃至天体演化及宇宙早期的奧秘,是人类探索宇宙的重要途径。     

太阳磁场和宇宙线实验规律初探

本文总结了近20年来太阳磁场、太阳行星际磁场的观测结果,较详细地结出了银河宇宙线在空间分布和随太阳活动变化的时间特性,描绘出一幅比较完整的太阳磁场与宇宙线关系的图像。 对于太阳磁场的研究,得到了一般的结论。给出了宇宙线粒子在太阳系中受太阳磁场调制的四种物理因素。     

Primary cosmic-ray nucleon spectrum in the energy range 40–400 TeV reconstructed from the secondary gamma-ray spectrum in the stratosphere

We present the spectrum of all primary cosmic-ray nucleons in the energy range 40–400 TeV reconstructed from the spectra of gamma rays recorded along with charged particles at an altitude of 30 km in the stratosphere in the RUNJOB balloon experiment. Gamma rays are produced by the interaction of primary cosmic-ray particles with atomic nuclei of the residual atmosphere and are a component whose spectrum follows the nucleon spectrum at the atmospheric boundary in the approximation of quasi-scaling models. Comparison of the nucleon spectrum with the proton spectrum indicates that the contribution of nucleons from nuclei increases toward the 100-TeV energy region compared to the 1-TeV region. This result confirms the conclusions reached in the JACEE, SOKOL, ATIC-2, and KASCADE experiments: the fraction of helium nuclei and, possibly, heavier nuclei in the primary cosmic-ray flux increases with energy.     

Propagation in 3D spiral-arm cosmic-ray source distribution models and secondary particle production using Picard

We study the impact of possible spiral-arm distributions of Galactic cosmic-ray sources on the flux of various cosmic-ray nuclei throughout our Galaxy. We investigate model cosmic-ray spectra at the nominal position of the sun and at different positions within the Galaxy. The modelling is performed using the recently introduced numerical cosmic ray propagation code Picard. Assuming non-axisymmetric cosmic-ray source distributions yields new insights on the behaviour of primary versus secondary nuclei.We find that primary cosmic rays are more strongly confined to the vicinity of the sources, while the distribution of secondary cosmic rays is much more homogeneous compared to the primaries. This leads to stronger spatial variation in secondary to primary ratios when compared to axisymmetric source distribution models. A good fit to the cosmic-ray data at Earth can be accomplished in different spiral-arm models, although leading to decisively different spatial distributions of the cosmic-ray flux. These lead to different cosmic ray anisotropies, where even reproducing the data becomes possible. Consequently, we advocate directions to seek best fit propagation parameters that take into account the higher complexity introduced by the spiral-arm structure on the cosmic-ray distribution. We specifically investigate whether the flux at Earth is representative for a large fraction of the Galaxy. The variance among possible spiral-arm models allows us to quantify the spatial variation of the cosmic-ray flux within the Galaxy in presence of non-axisymmetric source distributions.     

Relative abundances of 10≤Z≤26 nuclei in medium energy cosmic rays

Abundances of primary cosmic-ray nuclei from neon to iron, in the energy interval 200 to 650 MeV n^–1, have been determined using a balloon exposed cellulose nitrate plastic detector. These abundances have been extrapolated back to the source regions using various models of cosmic ray interstellar propagation. Some aspects of cosmic-ray propagation and origin have been discussed in the light of results of the present experiment.     

Energy losses of galactic cosmic rays in the interplanetary medium

Using realistic models of cosmic-ray propagation in interplanetary space we present, for electrons, protons and helium nuclei of a given energy near Earth, calculations of their distribution in energy before entering the solar cavity and their mean energy loss. Interplanetary conditions appropriate for the epochs 1965 and 1969 have been used. Cosmic-ray energies in the range of 20 MeV/nucleon to 1000 MeV/nucleon have been considered.     

Observations on the abundance of nitrogen in the primary cosmic radiation

New measurements of the intensity and spectrum of cosmic ray nitrogen nuclei made by instruments flown on balloons and on the Pioneer-8 space probe are reported. The nitrogen spectrum is found to be identical with that of the other medium nuclei, carbon and oxygen, over the range of measurement from 100 MeV/nuc to > 22 GeV/nuc. The ratio of N to all M nuclei is found to be =0.125, constant to within 10% over this energy range. This ratio is extrapolated to the cosmic-ray source using the most recently obtained abundances of oxygen and heavier nuclei and fragmentation parameters for the production of nitrogen from these nuclei. Taking an average material path length of 4 g/cm² of hydrogen constant with energy, as required to make the abundance of L nuclei 0 at the cosmic-ray source, the resulting N/M source ratio is 0.03. In other words, to the same degree that the so-called L nuclei are absent in the cosmic-ray sources, N nuclei are also absent. This nitrogen abundance is therefore different from the estimated solar atmospheric abundance of 0.10 for the N/M ratio which is believed to represent the integrated effects of nucleo-synthesis in the galaxy at the time of the formation of the sun. Nevertheless under certain conditions in the CNO bi-cycle that operates for the production of nitrogen in stellar objects a negligible production of nitrogen might be expected. It is suggested that these conditions exist in the cosmic-ray sources. The C/O ratio of 0.9 deduced for cosmic-ray sources is compatible with the observed low nitrogen abundance arising in this CNO bi-cycle.NRC-NASA Resident Research Associate at Goddard Space Flight Center.     

The nature of protons ejected from the nuclei of extragalactic radio sources

We have estimated the proton injection flux from the nuclei of some typical extragalactic radio sources (EGRS). To do so, we have used measured values of radio luminosities from these sources and have assumed the proton-proton collision model as a source of relativistic electrons which give rise to radio emission. The estimated values of the proton flux is in fairly good agreement with theoretical estimates of cosmic-ray fluxes within the same range of energy. This lends support to the fact that the nuclei of EGRSs might be the site for the generation of primary cosmic rays.     

The age distribution of galactic cosmic rays

The distribution of ages for secondary and interacting cosmic-ray nuclei is derived for a class of steady-state, bounded models of diffusion in the galaxy. Results are presented in detail for the model in which diffusion is in one dimension, sources are uniformly distributed throughout the scattering region, and particles are observed near the central plane of the galaxy. The leakage-lifetime approximation is shown to be accurate as long as the ratio of the cosmic-ray diffusive lifetime to the mean interaction or decay lifetime is less than about ten.     

Production of neutrinos and secondary electrons in cosmic sources

We study the individual contribution to secondary lepton production in hadronic interactions of cosmic rays (CRs) including resonances and heavier secondaries. For this purpose we use the same methodology discussed earlier [C.-Y. Huang, S.-E. Park, M. Pohl, C.D. Daniels, Astropart. Phys. 27 (2007) 429], namely the Monte-Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles with leptons as the final decay states, that result from inelastic collisions of cosmic-ray protons and Helium nuclei with the interstellar medium of standard composition. By combining the simulation results with parametric models for secondary particle (with resonances included) for incident cosmic-ray energies below a few GeV, where DPMJET appears unreliable, we thus derive production matrices for all stable secondary particles in cosmic-ray interactions with energies up to about 10 PeV.

We apply the production matrices to calculate the radio synchrotron radiation of secondary electrons in a young shell-type SNR, RX J1713.7-3946, which is a measure of the age, the spectral index of hadronic cosmic rays, and most importantly the magnetic field strength. We find that the multi-mG fields recently invoked to explain the X-ray flux variations are unlikely to extend over a large fraction of the radio-emitting region, otherwise the spectrum of hadronic cosmic rays in the energy window 0.1–100 GeV must be unusually hard.

We also use the production matrices to calculate the muon event rate in an IceCube-like detector that are induced by muon neutrinos from high-energy γ-ray sources such as RX J1713.7-3946, Vela Jr. and MGRO J2019+37. At muon energies of a few TeV, or in other word, about 10 TeV neutrino energy, an accumulation of data over about 5–10 years would allow testing the hadronic origin of TeV γ-rays.     

Variations of cosmic-ray energy in interplanetary space

A consistent theory of energy exchange between high-energy charged cosmic-ray particles and the random inhomogeneities of a magnetic field frozen in the moving solar wind plasma is developed. It is shown that the mode of the particle energy variations at a given law of plasma velocity variation in space is determined by the specific form of the particle distribution function. The equation for the density of cosmic-ray energy is obtained. Consideration is given to the generation of a charged particle energy spectrum in the course of multiple scatterings by the random inhomogeneities of the magnetic field.     

Evidence for the deficiency of short cosmic ray pathlengths and a physically realistic explanation — The no-near-sources model

From our recent observations of the charge and energy spectra of cosmic-ray nuclei we have constructed secondary-to-primary charge ratios at the two ends of the charge spectrum. These ratios are found to be inconsistent with thead hoc leaky-box model of cosmic-ray propagation which leads to an exponential pathlength distribution. Models for which the pathlength distribution function is deficient in short pathlengths provide a more consistent picture. Several of these models are investigated, bothad hoc and physical. The physical model considered here is one for which detailed galactic propagation parameters and boundary conditions are used and for which there exists no near sources of cosmic rays over a time interval corresponding to a few times the cosmic-ray age.