Effect of the Plasma Density Scale Length on the Direction of Fast Electrons in Relativistic Laser-Solid Interactions

The angular distribution of bremsstrahlung $\gamma$ rays produced by fast electrons accelerated in relativistic laser-solid interaction has been studied by photoneutron activation in copper. We show that the $\gamma$-ray beam moves from the target normal to the direction of the ${\mathbf{k}}_{\mathrm{laser}}$ vector as the scale length is increased. Similar behavior is found also in 2D particle-in-cell simulations.