Development of the Hadron Absorber for the 2nd Phase Operation of the T2K Neutrino Oscillation Experiment

开展连接东海至神冈295公里的下一代长基线中微子振荡实验T2K是建造J-PARC的主要动机之一。它使用50GeV质子同步加速器作为中微子超束源，使用Super-Kamiokande作为远中微子探测器。它可以以前所未有的精度探索中微子的质量和混合。特别是首次观测到微小的v-μ-to-v_e现象信号。一条中微子束线正在东海园区建设中，束调试计划于2009年4月进行。该项目的目的是开发一个可以接受1 mW束流损失的强子吸收系统。对于未来的T2K计划来说，重点是发现轻子区的CP破缺是至关重要的。束流堆场位于衰变体积的下游，强子吸收体是束流堆场的核心，它与铁屏蔽体安装在一起。强子吸收体的设计如下：它由14个核心模块和一个支撑它们的框架组成。它的大小为3.1米x 5.4米x 3.2米，总重量为70吨。堆芯模块由7个大的挤压石墨块组成，紧固在带有冷却水通道的铝铸板上。为了保证冷却面和负荷面的平整度为0.1 mm，7块石墨块一次加工在一起。采用多个弹簧垫圈的设计来紧固石墨和铝，以最大限度地减少梁操作时由于温升而减少的连接力，即两者之间的热对流。经过详细的设计，我们已经成功地生产出了第一个核心模块。

To carry out T2K, a next-generation long base-line neutrino oscillation experiment connecting 295 km from Tokai to Kamioka, is one of the main motivations to construct J-PARC. It employs a 50GeV proton synchrotron as a neutrino super-beam source, and Super-Kamiokande as a far neutrino detector. It can explore mass and mixing of neutrinos with unprecedented precision. It especially aims to observe tiny vμ-to-v_e appearance signal for the first time. A neutrino beam-line is under construction at the Tokai campus, and beam commissioning is scheduled in April, 2009. The purpose Of the project is to develop a hadron absorber system which can accept 1MW beam loss. It is vitally important for the future project of T2K to focus on the discovery of the CP violation in the lepton sector. The beam dump is located at the downstream of the decay volume, where a hadron absorber, the core of the beam dump, is installed together with iron shields. The design of the hadron absorber is as follows : It is composed of 14 core modules and a frame supporting them. It has a size of 3.1m x 5.4m x 3.2m, and total weight of 70t. A core module is composed of 7 large extruded graphite blocks, fastened to an aluminum cast plate with cooling water paths in it. To keep flatness of 0.1mm for the cooling surface and for the loading surface, 7 graphite blocks are machined together at once. A design with multiple spring washers is adopted to fasten graphite and aluminum, to minimize the reduction of joint force, i.e. heat convection between them, by temperature rise at beam operation. Following to the detailed design, we have produced the 1st core module successfully.