Dynamic Distortions in the HARP TPC: observations, measurements, modelling and corrections
A. Bagulya, A. Blondel, S. Borghi, G. Catanesi, P. Chimenti, U. Gastaldi, S. Giani, V. Grichine, V. Ivanchenko, D. Kolev, J. Panman, E. Radicioni, R. Tsenov, I. Tsukerman
The HARP experiment was designed to study hadron production in proton- nucleus collisions in the energy range of 1.5 GeV/c-15 GeV/c. The experiment was made of two spectrometers, a forward dipole spectrometer and a large-angle solenoid spectrometer. In the large-angle spectrometer the main tracking and particle identification is performed by a cylindrical Time Projection Chamber (TPC) which suffered a number of shortcomings later addressed in the analysis. In this paper we discuss the effects of time-dependent (dynamic) distortions of the position measurements in the TPC which are due to a build-up of ion charges in the chamber during the accelerator spill. These phenomena have been studied both theoretically and experimentally, and a correction procedure has been developed. First, the dynamics of the positive ion cloud and of the full electrostatics of the field-cage system have been modelled with a phenomenological approach and a general correction procedure has been developed and applied to all data settings. Then, the correction procedure has been benchmarked experimentally by means of recoil protons in elastic scattering reactions, where the track coordinates are precisely predictable from simple kinematical considerations. After application of the corrections for dynamic distortions the corrected data have a performance equal to data where the dynamic distortions are absent. We describe the theoretical model, the comparison with the measurements, the distortion correction method and the results obtained with experimental data.
Performance measurement of HARPO: a Time Projection Chamber as a gamma-ray telescope and polarimeter
我们分析了气体时间投影室（TPC）作为高性能的伽马射线望远镜和偏振仪在e $^+$ e $^-$对创建制度的性能。我们使用在已知偏振的伽马射线束收集的数据。TPC提供由气体体积中的每次转换引起的轨迹的两个正交投影$(x,z)$和$(y,z)$。我们使用一个简单的顶点查找程序，其中顶点和伪轨迹退出他们被确定。我们用蒙特卡罗模拟研究了单光子角分辨率的各种贡献，并将它们与实验数据进行了比较，发现它们非常吻合。由于探测器的非圆柱对称结构，对转换的方位角分布显示出偏差。这种偏差将在太空任务的长时间曝光中达到平均值，但对于这种双光束表征，我们通过双系统控制方案确保了其准确模拟，数据采集采用相对于光束偏振方向旋转几个角度的探测器，并采用非偏振光束进行系统控制。我们测量，第一次，在低能量对创建制度的线性偏振伽马射线束的偏振不对称性。这个亚GeV的能量范围对于宇宙源来说是至关重要的，因为它们的光谱是幂律的，随着能量的增加而迅速下降。这项工作可以为将极化伽马射线天文学扩展到MeV能量范围之外铺平道路。
