Abstract
Positron-lifetime spectra in KCl and NaF have been measured in the temperature range 9-353 K and 8-400 K, respectively. A large change in the spectra has been observed in the temperature range in which the angular-correlation data showed a transition from free to localized positronium. These experiments strongly support the hypothesis that at low-temperature Ps is in a free Bloch-like state while at high temperature it is localized, perhaps in a dilated interstitial region of the crystal.

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