The Heavy Ion Beam is a Limited Edition weapon made available on May 4, 2012--the same day that most of the Avengers move-themed uniforms were released. Unlike the new uniforms, this weapon is available for 2 days only.
This weapon deals moderate damage and causes Radiation Exposure, but only targets one enemy. Nonetheless, it's a very nice weapon due to the Quick Action property and cool-down of only 1 round.
Radiation Exposure is a very nasty debuff that is difficult to cure. Another weapon that has this debuff is the Cosmic Ray Bomb, one of the two rewards from the premium mission in Chapter 7.
The Heavy Ion Beam is similar to the S.A. Pincer in that both are ranged guns with the Quick Action property. However, the Heavy Ion Beam deals more total damage, deals energy damage, hits 4 instead of 1 time, has a cool-down of 1 round instead of 2 rounds, and causes Radiation Exposure.
In the image, the cost of the Heavy Ion Beam is shown to be 12 Gold, which is a good deal. Some players have reported that this weapon is being offered to them at higher costs such as 32 Gold or 64 Gold.
The screenshot was taken when my Agent was level 84.