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We report a measurement of the diffractive structure function FjjD of the antiproton obtained from a study of dijet events produced in association with a leading antiproton in p̅p collisions at √s =630 GeV at the Fermilab Tevatron. The ratio of FjjD at √s =630 GeV to FjjD obtained from a similar measurement at √s =1800 GeV is compared with expectations from QCD factorization and other theoretical predictions. We also report a measurement of the ξ(x-Pomeron) and β(x of parton in Pomeron) dependence of FjjD at √s =1800 GeV. In the region 0.035 < ξ < 0.095, │t│< 1 GeV2, and β< 0.5, FjjD (β, ξ) is found to be of the form β -1.0±0.1 ξ -0.90±0.1, which obeys β-ξ factorization.