Ann→ptransition has been observed for &agr;‐tin filaments irradiated with 0.675‐MeV electrons at 5 K. Resistance and thermoelectric emf measurements were used to establish that the electron dose needed to effect then→ptransition is roughly proportional to the impurity concentration forn‐type material. No transition was observed forp‐type material. The data are interpreted in terms of radiation‐induced trapping levels.