The nuclear time unit (NTU) is the length of time it takes for a ray of light, traveling in a vacuum, to span the diameter of a hydrogen atom.
According to most sources, the diameter of the hydrogen atom is nominally 1.06 x 10 -10 meters (m), or 0.106 nanometers (nm). The speed of light in free space is 3.00 x 10 8 meters per second (m/sec) to three significant digits. Therefore, an approximation of the NTU, in seconds to three significant digits, can be obtained as follows:
1 NTU = (1.06 x 10 -10 ) / (3.00 x 10 8 ) = 3.53 x 10 -19 sec