  By placing a ${}^{133}Cs$ $\gamma$-ray source embedded in a solid at the
center of a platinum (gold) cylinder, we try to change the width of the 81-keV
level. Our results show a narrowed energy level and, equivalently, a prolonged
lifetime. With a 0.5-mm-thick, 5-cm-long, 2-mm-diameter platinum cylinder, we
obtain a width narrower by $6.1 % $ at $4.2 \: K$.
