For 10 points, name this Greek letter, which follows alpha and beta.
A crystal can absorb radiation of this type without recoil in the Mossbauer effect. This letter symbolizes a quantity that multiplies time intervals in time dilation and is the reciprocal of "the square root of one minus beta squared" in special relativity. This letter denotes an extension of the (*) factorial function to any real number. This letter denotes an electromagnetic wavelength shorter than x-rays. For 10 points, name this Greek letter, which follows alpha and beta.
A crystal can absorb radiation of this type without recoil in the Mossbauer effect.
{gamma} [prompt on {electromagnetic}; prompt on {em}]