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

1938年，汉斯·贝特提出了太阳内部能量产生的第一个解释，这个解释能够解释我们在地球生命周期中观察到的能量流。这个理论是基于发生在太阳核心的热核反应过程。在Bethe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，后者被认为是产生98%以上来自太阳的能量的原因。pp循环由一系列的核反应组成，这些核反应将四个氢原子转化为一个氦原子，每次转化释放26.7兆电子伏的能量。能量以带电粒子动能、伽马射线和中微子的形式释放出来。由于它们的相互作用很弱，中微子离开太阳时几乎没有相互作用。“太阳中微子问题”早在35年前就存在了，当时Ray Davis及其合作者的开创性“氯”实验表明，在地球上测量到的中微子的实际数量仅为“标准太阳模型”预测的中微子数量的1/3左右。从那以后，进行了其他实验，与预测相比，都发现了观测到的中微子的缺陷。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有缺陷，而是由基本的中微子过程引起的，这种过程将太阳中产生的正常电子型中微子转化为目前探测器不易检测到的中微子。如果中微子具有非零质量，就会发生中微子转换或“中微子振荡”。但非零中微子的质量与标准粒子物理理论相悖。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。如果中微子确实有质量，正如目前的实验所表明的那样，那么就需要新的基础物理理论。Borexino是一种新的太阳中微子探测器，设计用于探测低能太阳中微子，使用一个300吨的大型液体闪烁体。该探测器将位于意大利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，Be-7中微子预计会表现出最大的中微子振荡效应。Borexino将尝试观察中微子振荡的直接证据，并证实目前关于中微子质量的假设。要搜索的信号包括非常大的年度和昼夜计数率变化，这些变化是由中微子振荡引起的。

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.