He-4 third-sound pulse collisions - art. no. 054524

We investigate the behavior of pulses of He-4 third sound that propagate an d collide on a glass substrate. Third sound is created by a thermal source and detected by evaporated metal superconducting transition-edge thermomete rs. Two thermal sources located symmetrically on either side of a detector allow third-sound pulses to be created, propagate through each other, and b e detected. By varying the delay of the pulse launch between the two source s, the collision can be placed at an arbitrary position on the substrate. W e present detailed observations of the evolution of the amplitudes of the t hird-sound pulses in collision. We also report measurements on the effect o f the pulse collision on the propagation time of Hight of the third-sound p ulses. Experimental results for the time of flight change induced by the co llisions are compared with quantitative expectations based on the two fluid model and a stronger effect is observed than expected.