Masquerading as a modern day Man in the Iron Mask, William (Bill) Jameson wore a mask that emitted ion rays that when in the presence of Encephalonic waves (such as those his brother Ed's brain emitted) would black out those nearby and rob them. Chronos would pluck the Man in the Ion Mask from time to battle Ryan Choi. Synopsis from his appearance in Atom #25: With Jean running late for a masquerade party, she calls in to her fiancee Ray Palmer to tell him that she will be there for sure. When Ray asks her what she'll be dressed up as, she tells him that he has to try and find her. Sure enough, while attending the party, Ray manages to correctly guesses which party goer is Jean. When she calls his bluff Ray reveals that he knew her because she had ticket number 155, and when working the door he saw through her disguise when she handed him her own ticket.
As the party continues, everyone is impressed by a party goer who has dressed up as a 20th-century Man in the Iron Mask. When the man wins first prize, he announces to all the party goers that he is looking to collect a different prize all together. Suddenly, the mask begins to glow brightly causing everyone to black out. When the party goers regain their facilities that find that they have all been robbed. When the police arrive, they ask for Ray's help in trying to solve the case. Ray goes to his lab and upon examining evidence he learns that ion energy was the cause of everyone at the party to black out.
Left in private, Ray exposes his Atom costume to an anti-ion ray so that when he runs into the "Man in the Ion Mask" again, he will be immune to the masks effects. Going out to buy Jean a birthday present Ray pops into a local jewelry store. Coincidentally this is the same store that the Man in the Ion Mask intends to rob. This time however, Ray is ready for him and quickly changes into the Atom and engages in battle with the masked criminal. Despite getting the upper hand, the Atom fails to stop the crook when the Man in the Ion Mask traps the Atom in a display case. As the Atom breaks free of the trap, the Man in the Ion Mask manages to escape.
While puzzling how the Man in the Ion Mask's powers work, Ray deduces that perhaps the white dwarf star components of his costume could be the source. However later when the Man in the Ion Mask robs the Ivy Town Bank without Ray being present ruins that theory. Looking over the list of witnesses to the previous two crimes he notices one thing in common: That a man named Ed Jameson is always present. Paying Jameson a visit at the Atom, he learns that while he wasn't at the third crime scene, he was eating at a restaurant next door. Realizing that Ed is the key to discovering the Ion Mask's powers, he decides to follow Ed around to see where the Man in the Ion Mask will strike next.
Sure enough, the Man in the Ion Mask appears at a horse betting track and attempts to rob the box office. The Atom springs into action, grabbing a horseshoe and banging it over the head of the Man in the Ion Mask, disorientating him long enough for the Atom to pull his sports coat over his arms and yank off his mask. The Atom easily knocks the man out, and Ed is shocked to find that it's his brother Bill. Bill revives and tells them that he was developing an Ion ray that didn't work until he realized that it would only operate when in proximity to Encephalonic waves, that just happen to be emitting from Ed's mind. As the police are taking him away, he explains that the ion device works by effecting the cerebellum of the brain causing the black outs.