Reactions of Polarized Gamma Rays on Polarized HD Targets

这个NSF项目旨在利用极化伽马射线和极化核子来测量核子结构。在布鲁克海文国家实验室的国家同步加速器光源的存储环中循环的高能电子产生的反向散射偏振激光产生了偏振伽马射线。在反向散射中，极化是不变的。通过探测散射电子，伽马射线能量被“标记”，所有能量的测量都可以同时进行。极化核子来自于在高磁场中产生冷冻HD靶。偏振H， D或两者都可以在适合核物理实验的尺寸目标中产生，并且在H的情况下弛豫时间为两周，在D的情况下弛豫时间更长。这些双偏振实验允许测量以前没有测量过的介子光产生振幅。这对于完整地描述伽马-核子相互作用是很重要的。平行和反平行自旋的吸收截面的差异对核子的自旋结构很敏感。我们将能够测试格拉西莫夫-德雷尔-赫恩求和规则的主要部分，并直接确定核子自旋极化率。由于测试和规则所需的能量范围很大，没有一个实验室可以完成这项任务。然而，由于LEGS情况下的伽马能量范围从光产生阈值到超出delta共振，因此覆盖了主要的灵敏度区域。

This NSF project seeks to make measurements of nucleon structureusing polarized gamma rays and polarized nucleons. The polarizedgamma rays are created at LEGS by back scattering polarized laserlight from the high energy electrons circulating in the storage ringof the National Synchrotron Light Source at Brookhaven NationalLaboratory. In back scattering the polarization is unchanged. Bydetecting the scattered electron, the gamma ray energy is "tagged",and measurements at all energies can be made simultaneously. Thepolarized nucleons come from producing targets of frozen HD in ahigh magnetic field. Polarized H, D or both can be produced intargets of a size suitable for nuclear physics experiments and withrelaxation times of two weeks in the case of H and longer for D. These double polarization experiments allow measurements ofpion-photo-production amplitudes that have not been previouslymeasured. Such are important to completely describe thegamma-nucleon interaction. The difference in the absorption crosssections for gamma and nucleon spins parallel and anti-parallel aresensitive to the spin structure of the nucleon. We will be able totest the major part of the Gerasimov-Drell-Hearn Sum Rule and makedirect determinations of nucleon spin polarizabilities. Due to thelarge energy range required to test the sum rule no singlelaboratory can accomplish the task. However, as the gamma energiesin the LEGS case range from photo-production threshold to beyond thedelta resonance, the major region of sensitivity is covered.