Low Energy Nuclear Physics at Princeton - The Borexino Solar Neutrino Experiment

0077423年，汉斯·贝特（Hans Bethe）第一次解释了太阳内部的能量产生，解释了我们在地球生命周期中观察到的能量通量。 这个理论是基于太阳核心的热核反应过程。 在贝特的开创性工作中，讨论了两种能量产生机制：所谓的碳-氮-氧（CNO）和质子-质子（pp）循环。 今天，后一个过程被认为是负责生产超过98%的太阳能量。 pp循环由一系列核反应组成，将四个氢原子转化为一个氦原子，每次转化释放26.7 Mev的能量。 能量以带电粒子动能、伽马射线和中微子的形式释放。 由于它们之间的相互作用很弱，中微子离开太阳时的相互作用非常少，“太阳中微子问题”早在35年前就出现了，当时雷·戴维斯和合作者的开创性“氯”实验表明，地球上实际测量到的中微子数量只有“标准太阳模型”预测数量的1/3左右。 从那时起，其他实验已经进行，所有发现的中微子赤字相比，预测。 目前的实验证据表明，这种缺陷不是由于我们对太阳的理解存在缺陷，而是由基本中微子过程引起的，该过程将太阳中产生的正常电子型中微子转换为当前探测器无法检测到的中微子。 如果中微子的质量不为零，中微子转换或“中微子振荡”就会发生。 但中微子质量不为零与标准粒子物理学理论相悖。 因此，“太阳中微子问题”已成为基本粒子物理学的一个重要问题。 Borexino是一种新型的太阳中微子探测器，它利用300吨级的液体闪烁体探测低能太阳中微子。 探测器将位于意大利的格兰萨索地下实验室。 它的主要目标是探测能量为0.86 MeV的单能Be-7中微子。 根据目前对太阳中微子数据的分析，预计Be-7中微子将表现出最大的中微子振荡效应。 Borexino将试图观察中微子振荡的直接证据，并证实中微子质量的假设。 要搜索的信号包括非常大的年度和昼夜变化的计数率，这是独特的中微子振荡引起的。

0077423CalapriceIn 1938, Hans Bethe provided the first explanation of the energy production inside the Sun that was able to account for the energy flux we have observed over the lifetime of the Earth. This theory is based on thermonuclear reaction processes occurring in the Sun's core. In Bethe's seminal work, two energy production mechanisms were discussed: the so-called carbon-nitrogen-oxygen (CNO) and the proton-proton (pp) cycles. Today, it is the latter process that is thought to be responsible for the production of more than 98% of the energy flowing from the Sun. The pp cycle consists of a series of nuclear reactions that convert four hydrogen atoms to a helium atom, releasing an energy of 26.7 Mev for each conversion. The energy is released in the form of charged particle kinetic energies, gamma rays, and neutrinos. Because of their weak interaction, the neutrinos leave the sun with very few interactions.The "solar neutrino problem" came in existence more than 35 years ago, when the pioneering "Chlorine" experiment of Ray Davis and collaborators showed that the actual number of neutrinos measured on Earth was only about 1/3 of the number predicted by the "standard solar model". Since then, other experiments have been performed and all find a deficit of observed neutrinos, compared to the prediction. The present body of experimental evidence indicates that the deficit is not due to a flaw in our understanding of the Sun, but rather, is caused by fundamental neutrino processes that convert the normal electron-type neutrinos produced in the sun to neutrinos not readily detected in the current detectors. The neutrino conversions or "neutrino oscillations" can occur if the neutrino has a non-zero mass. But a non-zero neutrino mass is contrary to standard particle physics theory. Thus, the "solar neutrino problem" has become an issue of fundamental importance to elementary particle physics. If the neutrino does have mass, as suggested by the current experiments, new fundamental physics theories will be required.Borexino is a new solar neutrino detector designed to detect low energy solar neutrinos by use of a large 300 ton liquid scintillator. The detector will be located in the Gran Sasso underground laboratory in Italy. It's principle objective is to detect the mono-energetic Be-7 neutrino at an energy of 0.86 MeV. According to current analysis of solar neutrino data, the Be-7 neutrino is expected to exhibit maximal neutrino oscillation effects. Borexino will attempt to observe direct evidence of neutrino oscillations and confirm the present hypothesis of neutrino mass. The signals to be searched for include very large annual and day/night variations in count rate that are uniquely caused by neutrino oscillations.