The magnitude of the KS°~KL° mass difference Aw has been measured by monitoring the time dependence of the strangeness of neutral iTs produced in hydrogen and deuterium in the LRL 25-in. hydrogen bubble chamber. The particles originate as K° in Kr charge-exchange scatters at 0.85 to 1.15 BeV/c; the signature for an S--1 reaction is the production of a hyperon. Seventy-seven events were found, resulting in Ao> =0.50±0.15, measured in units of inverse Ks° lifetime. This and two other recent measurements using the same method are consistent with one another and with measurements of Aoo by other means.

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