A 1952 handbook from the US National Bureau of Standards affirms that "The numerical coefficient of the rep has been deliberately changed to 93, instead of the earlier 83, to agree with L. H. Gray 's 'energy-unit'." Gray's 'energy unit' was " one roentgen of hard gamma resulted in about 93 ergs per gram energy absorption in water". The lower range value of 83.8 ergs was the value in air corresponding to wet tissue. The rep was commonly used until the 1960s, but was gradually displaced by the rad starting in 1954 and later the gray starting in 1977.