Sentence:  The impact rate on 1958 Alpha for 153 events was * * f for particles of mass greater than * * f ( Dubin , 1960 ) ; this <WSD>mass</WSD> threshold was derived from the detector calibration and an assumed impact velocity of * * f . 
Meaning of the word marked with <WSD> tags:  the property of a body that causes it to have weight in a gravitational field