Electron thermalization length in solid para-hydrogen at low-temperature

We report the first ever measurements of the thermalization length of low-energy electrons injected into solid para-hydrogen at a temperature $T\approx 2.8\,$K. The use of the pulsed Townsend photoinjection technique has allowed us to investigate the behavior of quasi-free electrons rather than of massive, slow negative charges as reported in all previous literature. We have found an average thermalization length $\langle z_0\rangle\approx 260\,$\AA\ which is 3 to 5 times longer than that in liquid helium at the same temperature.