We have created a high-density gas of interacting positronium (Ps) atoms by irradiating a thin film of nanoporous silica with intense positron bursts and measured the Ps lifetime using a new single-shot technique. When the positrons were compressed to 3:3 10 10 cm ÿ2 , the apparent intensity of the orthopositronium lifetime component was found to decrease by 33%. We believe this is due to a combination of spin exchange quenching and Ps 2 molecule formation associated with colliding pairs of oppositely polarized triplet positronium atoms. Our data imply an effective cross section for this process of 2:9 10 ÿ14 cm ÿ2 .

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