For the purpose of future visualization of the flow field in superfluid helium-4, clusters of the triplet state excimer 4He2 * are generated along the micro-scale recoil tracks of the neutron-absorption reaction n + 3He → 3T + p. This reaction is induced by neutron irradiation of the 3He fraction contained in natural isotopic abundance liquid helium with neutron beams either from the Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility (JPARC)/Materials and Life Science Experimental Facility or from the Kyoto University Institute for Integrated Radiation and Nuclear Science. These 4He2 * clusters are expected to be ideal tracers of the normal-fluid component in superfluid helium with several advantageous properties. Evidence of the excimer generation is inferred by detection of laser induced fluorescence emitted from the 4He2 * clusters excited by a purpose-built short pulse gain-switched titanium:sapphire (Ti:sa) laser operating at a wavelength of 905 nm. The setup and performance characteristics of the laser system including the Ti:sa and two continuous wave re-pumping lasers are described. Detection at the fluorescence wavelength of 640 nm is performed by using optical bandpass filtered photomultiplier tubes (PMT). Electrical noise in the PMT acquisition traces could successfully be suppressed by post-processing with a simple algorithm. Despite other laser-related backgrounds, the excimer was clearly identified by its fluorescence decay characteristics. Production of the excimer was found to be proportional to the neutron flux, adjusted via insertion of different collimators into the neutron beam. These observations suggest that the apparatus we constructed does function in the expected manner and, therefore, has the potential for groundbreaking turbulence research with superfluid helium.

中文翻译：

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通过中子 3He 吸收反应在超流氦 4 中创建和成像 4He2* 准分子簇示踪剂的实验装置

为了将来对超流氦 4 中的流场进行可视化，沿着中子吸收反应 n + 3He → 3T + p 的微尺度反冲轨迹生成了三重态准分子 4He2 * 的簇。该反应是由日本质子加速器研究中心、材料和生命科学实验设施 (JPARC)/材料和生命科学实验设施或京都的中子束对天然同位素丰度液氦中所含的 3He 馏分进行中子辐照引起的大学综合辐射与核科学研究所。这些 4He2 * 簇有望成为超流氦中正常流体成分的理想示踪剂，具有多种优势。准分子生成的证据是通过检测从 4He2 * 簇发出的激光诱导荧光来推断的，这些荧光是由在 905 nm 波长下工作的专用短脉冲增益开关钛蓝宝石 (Ti:sa) 激光器激发的。描述了包括 Ti:sa 和两个连续波再泵浦激光器的激光系统的设置和性能特征。使用光学带通滤波光电倍增管 (PMT) 进行 640 nm 荧光波长的检测。通过使用简单算法进行后处理，可以成功抑制 PMT 采集轨迹中的电噪声。尽管存在其他与激光相关的背景，但通过其荧光衰减特性可以清楚地识别准分子。发现准分子的产生与中子通量成正比，通过将不同的准直器插入中子束进行调整。这些观察结果表明，我们构建的装置确实以预期的方式运行，因此具有使用超流氦进行开创性湍流研究的潜力。