This maser element was part of experiments conducted in the 1960s at MIT's Haystack radio and radar facility to confirm Einstein's prediction of the slowing of radiation by a strong gravitational field. The development of masers in the early 1960's greatly enhanced the sensitivity of radar receivers, making it possible to measure signals reflected from distant planets. In 1964 Irwin Shapiro, then at MIT's Lincoln laboratory, made careful measurements of the transit time of 7.84 GHz radar signals reflected from Mercury when that planet was about to disappear behind the solar limb. He found that the sun's gravitational field had added about 200 microseconds to the signal's round trip travel time. This confirmation of the prediction, now known as the "Shapiro time delay" was at the time the most accurate evidence for Einstein's theory. The receiver was donated to NASM by MIT and Irwin Shapiro.