Reactions of Polarized Gamma Rays on Polarized HD Targets

这项NSF项目试图利用偏振的伽马射线和偏振的核子来测量核子结构。偏振伽马射线是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的偏振激光后向散射产生的。在后向散射中，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。极化的核子来自强磁场中产生冻结的HD靶。极化的H、D或两者都可以产生适合于核物理实验的目标，在H的情况下松弛时间为两周，而对于D的松弛时间更长。这些双极化实验允许测量以前没有测量过的介子光致产生的幅度。这对于完整地描述伽马-核子相互作用是很重要的。伽马和核子的吸收截面的差异是平行和反平行的，这对核子的自旋结构很敏感。我们将能够测试Gerasimov-Drell-Hain求和规则的主要部分，并直接确定核子的自旋极化率。由于检验求和法则所需的能量范围很大，没有一个单独的阐述者可以完成这项任务。然而，由于LEGS情况下的伽马能量范围从光产生阈值到超出三角共振，所以主要的敏感区域被覆盖。

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.