HALO at SNOLAB

氦和铅天文台（HALO）是一个长期、低成本、高寿命和低维护的专用超新星探测器，运行在SNOLAB。在世界上的超新星中微子探测器中，选择铅作为目标材料使HALO对电子中微子具有独特的敏感性，并将提供关于下一个银河系超新星的中微子通量和能量的补充信息。HALO还将参与超新星早期预警系统（SNEWS），以增加天文学界及时收到银河系超新星警报的几率。II型超新星是由大质量恒星核心的引力坍缩释放的能量提供能量的。最初非常热的原始中子星主要是通过释放各种类型的中微子来冷却的，这些中微子在几十秒内均匀分布。事实上，大约99%的引力势能是在~10MeV中微子的急剧爆发中释放出来的。从概念上讲，HALO是一个80吨重的铅，上面装有氦-3中子探测器。在HALO中，来自超新星的高能和强烈的中微子脉冲可以通过带电电流（CC）和中性电流（NC）弱相互作用激发，导致原子核进入一个或两个中子被射出的状态。随后HALO的氦-3中子探测器对这些中子的探测表明这是一颗银河系超新星。直接测量光度的时间演变，风味划分，和平均中微子能量是我们唯一的直接窗口到超新星动力学虽然引力波也预期。铅的核物理特性提供了对电子中微子通量的CC相互作用的主要敏感性；电子反中微子通量对CC相互作用完全不敏感；通过NC激励通道对所有口味都有轻微的敏感性。对超新星动力学的详细了解是大规模计算物理工作的前沿，当与观测数据相结合时，提供了提取基本中微子性质的可能性，并推进了我们对超新星机制，重元素核合成和其他天体物理问题的理解。超新星中微子通量和相互作用截面是如此之大，以至于质量在100吨到100千吨之间的探测器只对银河系中发生的超新星敏感。银河系超新星的速度估计为每世纪约3颗，这强调了用尽可能多的不同特征和灵敏度的探测器捕捉下一个这样的事件的重要性。从SN 1987A观测到的20个事件中可以学到很多东西。下一个银河超新星和当前一代的探测器应该有更多的可能，然而这些探测器中的大多数成本高，维护费用高，主要针对其他物理目标，并且由于长时间的校准运行，维修，重大修改等而受到明显的停机时间，因此可能相对较短。通过采用强大的中子探测技术，注重细节，并通过回收现有设备和材料，HALO实现了其成为长寿命，低成本和低维护的超新星探测器的目标。自2012年5月8日以来，HALO一直在一种模式下运行，其中充满了氦-3探测器，这些探测器几乎连续不断地读取数据。虽然还有一些重要的工作要做，但为了完成和完全投入使用HALO，该合作希望投入一些精力来测量ORNL散裂中子源的中微子-铅横截面的可能性，以及研究可能被证明适用于大规模铅基超新星探测器的技术。

The Helium and Lead Observatory (HALO) is a long-term, low cost, high livetime, and low maintenance dedicated supernova detector running at SNOLAB. Among the world’s supernova neutrino detectors the choice of lead as the target material makes HALO uniquely sensitive to electron neutrinos and will provide complementary information on neutrino fluxes and energies from the next galactic supernova. HALO will also participate in the SuperNova Early Warning System (SNEWS) increasing the odds that the astronomical community is promptly alerted to a galactic supernova. A type II supernova is powered by the energy liberated in the gravitational collapse of the core of a massive star. The initially very hot proto-neutron star cools primarily by the emission of neutrinos, of all flavours equally partitioned, in some tens of seconds. In fact, about 99% of the gravitational potential energy is released in a sharp burst of ~10MeV neutrinos. Conceptually, HALO is an 80 tonne mass of lead instrumented with Helium-3 neutron detectors. In HALO the energetic and intense pulse of neutrinos from a supernova can excite, via both charged current (CC) and neutral current (NC) weak interactions, the lead nuclei to states from which one or two neutrons are ejected. The subsequent detection of these neutrons in HALO’s Helium-3 neutron detectors signals a galactic supernova. The direct measurement of the time evolution of the luminosity, flavour partition, and average neutrino energy are our only direct window into supernova dynamics though a gravitational wave is also anticipated. The nuclear physics of lead provides for a dominant sensitivity to CC interactions of the electron neutrino flux; complete insensitivity to CC interactions of the electron anti-neutrino flux; and minor sensitivity to all flavours through NC excitation channels. A detailed understanding of supernova dynamics is at the forefront of large-scale computational physics efforts and, when combined with observational data, offers the possibility of extracting fundamental neutrino properties as well as advancing our understanding of the supernova mechanism, heavy element nucleosynthesis, and other astrophysical questions. Supernova neutrino fluxes and interaction cross-sections are such that detectors with masses between 100 tonnes and 100 kilotonnes are only sensitive to supernovae occurring in the Milky Way Galaxy. The rate of galactic supernovae is estimated at ~3 per century which underlines the importance of capturing the next such event with as many detectors of diverse characteristics and sensitivities as possible. A great deal was learnt from the 20 events observed from SN 1987A. Much more should be possible with the next galactic supernova and the current generation of detectors, however the majority of these detectors are high cost, high maintenance, primarily aimed at other physics objectives, and are subject to appreciable downtime due to extended calibration runs, repairs, major modifications, etc. and consequently may be relatively short-lived. By employing a robust technology for neutron detection, attention to detail, and by recycling available equipment and materials, HALO fulfills its objectives of a being a long lifetime, low cost and low maintenance supernova detector. Since May 8th 2012 HALO has been operating in a mode where it is fully populated with Helium-3 detectors with those detectors being read out nearly continuously. Although some important work remains, to complete and fully commission HALO, the collaboration wishes to invest some effort in the possibility of measuring neutrino-lead cross-sections at the ORNL Spallation Neutron Source, as well as to investigate technologies that may prove appropriate for larger-scale lead-based supernova detectors.