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

T2K是连接东海和神冈295公里的下一代长基线中微子振荡实验，是建设J-PARC的主要动机之一。它采用50GeV质子同步加速器作为中微子超级束源，超级神冈探测器作为远端中微子探测器。它可以以前所未有的精度探索中微子的质量和混合。特别是首次观测微小的vμ-to-v_e外观信号。东海校区正在建设一条中微子束线，并计划于2009年4月进行束流调试。本课题的目的是研制一种能承受1MW束流损耗的强子吸收系统。关注轻子扇区CP违逆的发现对T2K的未来项目至关重要。束流场位于衰变体的下游，束流场的核心是强子吸收体和铁屏蔽体。强子吸收体的设计如下：它由14个核心模块和支撑它们的框架组成。尺寸3.1米× 5.4米× 3.2米，总重70吨。核心模块由7个大型挤压石墨块组成，固定在带有冷却水通道的铝铸板上。为保证冷却面和装料面平整度为0.1mm， 7块石墨块同时加工。采用多弹簧垫圈的设计将石墨与铝紧固，最大限度地减少了在横梁运行时温度升高对石墨与铝的结合力即热对流的减少。在详细设计之后，我们成功制作了第一个核心模块。

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.