We have developed a polarimetry of ultrashort pulse $\gamma$ rays based on the fact that $\gamma$ rays penetrating in the forward direction through a magnetized iron carry information on the helicity of the original $\gamma$ rays. Polarized, short-pulse $\gamma$ rays of $(1.1\pm 0.2)\times {10}^6/\mathrm{\text{bunch}}$ with a time duration of 31 ps and a maximum energy of 55.9 MeV were produced via Compton scattering of a circularly polarized laser beam of 532 nm off an electron beam of 1.28 GeV. The first demonstration of asymmetry measurements of short-pulse $\gamma$ rays was conducted using longitudinally magnetized iron of 15 cm length. It is found that the $\gamma$-ray intensity is in good agreement with the simulated value of $1.0\times {10}^6$. Varying the degree of laser polarization, the asymmetry for 100% laser polarization was derived to be $(1.29\pm 0.12)\%$, which is also consistent with the expected value of 1.3%.

• Received 2 May 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.164801