HALO at SNOLAB

氦和铅天文台(HALO)是一个长期、低成本、长寿命和低维护的专用超新星探测器，在SNOLAB运行。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对电子中微子特别敏感，并将提供关于下一颗银河系超新星的中微子通量和能量的补充信息。光晕还将参与超新星早期预警系统(SNEWS)，这增加了天文学团体迅速得到银河系超新星警报的可能性。第二类超新星是由大质量恒星核心引力坍缩释放的能量提供动力的。最初非常热的原中子星主要是通过发射各种中微子来冷却的，各种中微子被平均分割，在几十秒内。事实上，大约99%的引力势能是在~10 MeV中微子的急剧爆发中释放出来的。从概念上讲，光晕是一种用氦-3中子探测器测量的80吨重的铅。在光晕中，来自超新星的高能而强烈的中微子脉冲可以通过带电电流(CC)和中性电流(NC)的弱相互作用激发铅原子核，从而使一个或两个中子被抛出。随后在Halo的氦-3中子探测器中检测到这些中子，就发出了银河系超新星的信号。直接测量光度、色泽分配和平均中微子能量的时间演化是我们了解超新星动力学的唯一直接窗口，尽管引力波也是预期的。铅的核物理提供了对电子中微子通量的CC相互作用的主要敏感性，对电子反中微子通量的CC相互作用的完全不敏感性，以及通过NC激发通道对所有味道的次要敏感性。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，与观测数据相结合，提供了提取基本中微子属性的可能性，并促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。超新星中微子的通量和相互作用截面使得质量在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。据估计，银河系超新星的速度约为每世纪3颗，这突显了用尽可能多的具有不同特性和灵敏度的探测器捕捉下一次这样的事件的重要性。从SN1987A观测到的20个事件中，我们学到了很多。下一颗银河系超新星和当前一代的探测器应该会有更多的可能性，然而，这些探测器中的大多数都是高成本、高维护、主要针对其他物理目标的探测器，并且由于延长的校准运行、维修、重大修改等而受到明显的停机时间的影响，因此可能相对较短。通过采用坚固的中子探测技术、对细节的关注以及对现有设备和材料的回收利用，Halo实现了其作为一个长寿命、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，光晕一直在一种模式下运行，在这种模式下，它完全填充了氦-3探测器，这些探测器几乎连续地被读出。虽然仍有一些重要的工作要做，但为了完成和充分启用Halo，该合作伙伴希望在测量ORNL散裂中子源的中微子-铅截面的可能性上投入一些努力，并调查可能被证明适合于更大规模的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.