Reaction Mechanism of ^4He Photodisintegration and Charge Symmetry

~4He在π阈值以下能区的光核反应对于研究少核子系统的基本性质和核电荷对称性是非常有用的。本工作旨在通过精确测量反应事件，研究~4He在30-100 MeV能区发生光解的反应机理。为此，我们开发了一种新的实验方法。在这种方法中，我们使用东北大学核科学实验室(LNS)的标记光子束和充满氦和CD_4混合气体的时间投影室(TPC)。光子束入射到TPC中的~4He原子核上，带电粒子被TPC探测到。由于TPC是一种漂移室，因此需要一个外部触发信号来测量…的轨迹反应中释放出更多的带电粒子。但LNS标记的光子束是连续的光束，不能用作外部触发源。因此，我们在TPC中安装了一个由塑料闪烁体制成的触发计数器(TR)。在时间分辨率为1.1 ns(1σ)的情况下，TR可以提供能量为1 MeV的粒子的提示信号。为了研究测量系统的性能，我们在LNS标记光子装置上进行了测试实验，在42300s的测量时间内积累了大约40万个事件的原始数据。通过在离线分析中要求TPC和光子标签计数器(TAG)之间的一致性，有效地减少了背景事件，并使2万个事件存活下来。然后我们检查了剩余事件中粒子的轨迹，我们可以选择反气体的主要成分^4He、D和^&lt；12&gt；C的光反应事件的候选者。实验结果表明，我们的实验方法具有良好的性能。更详细的分析目前正在进行中。较少

The photonuclear reactions of ^4He in the energy region below the pion threshold are quite useful to investigate fundamental properties of few-nucleon systems and the nuclear charge symmetry. The present work was aimed to investigate the reaction mechanism of the photodisintegration of ^4He occurring in the energy region from 30MeV to 100MeV by measuring the reaction events precisely. For this purpose, we developed a new experimental method. In this method we used the tagged photon beam at Laboratory of Nuclear Science (LNS) of Tohoku University and a time projection chamber (TPC) filled with a gas mixture of He and CD_4. The photon beam impinged upon ^4He nuclei contained in TPC, and the charged particles were detected by TPC. So TPC served as an active target, and that provided extremely high detection efficiency of -100% and very large acceptance of almost 4π of the solid angle.Since TPC is a kind of drift chambers, an external trigger signal was needed to measure the tracks of the … More charged particles emitted from the reactions. But the LNS tagged photon beam is a continuous beam, it cannot be used as a source of the external trigger. So we installed a trigger counter (TR) made of plastic scintillators in TPC. TR could provide a prompt signal of the particles with the energies down to 1MeV, and the time resolution was 1.1ns(1σ). To study the performance of the measurement system, we made a test experiment at the LNS tagged photon facility We accumulated the raw data of about 400000events during the measuring time of 42300s. By requiring the coincidence between TPC and the photon tagging counter (TAG) in the offline analysis, the background events were efficiently reduced, and 20000events survived. Then we checked the tracks of the particles in the remaining event, and we could select the candidates of the photoreaction events of ^4He, D and ^<12>C, which were main compositions of the counter gas. The present result shows the good performance of our experimental method. More detailed analysis is now in progress. Less