College Park, MD (June 5, 2003)--In the movie The Hulk, intrepid Berkeley scientist Bruce Banner is zapped by a machine called Gammasphere. As a result, Banner transforms into a massive green monster at times of stress. Although Banner and his hulking alter ego are the latest fictional characters to emerge from comic book pages and make their way onto the big screen, Gammasphere is not merely a science fiction plot device. While scientists in real-world labs have never been turned into green giants by the machine, the real-life Gammasphere has provided valuable information about some other monstrosities.
"Gammasphere allows us to study the forces inside unstable atomic nuclei that don’t exist in nature," says Paul Fallon, a researcher at Lawrence Berkeley National Laboratory, where key scenes from The Hulk were filmed. Unlike the machine in the movie, the real Gammasphere doesn’t make the monsters...they have another machine for that. "We use a particle accelerator, either a cyclotron or a linear accelerator, to ram ions [electrically charged atoms] into a target," explains Fallon. "The ions combine with atoms in the target to create heavier atoms." This process is known as nuclear fusion. The resulting atoms are not monsters, of course, but like the Hulk they are large and unstable.
Generally, the new atoms are formed at high energy and are spinning, and rapidly disintegrate - emitting a spray of gamma rays along the way. The gamma rays are collected in the bristling and beautiful, spherical array of detectors that make up Gammasphere (see image here).
"By looking at the pattern of gamma rays," says Fallon, "we can infer the shape and the internal structure of the nucleus. The lifetime [before it decays] tells us something about deformations of the nucleus." The study is important in helping to explain why oxygen, carbon, iron and other common elements that comprise our world are stable.
Atoms are made up of negatively charged electrons in orbit around a nucleus. A nucleus contains positively charged protons and uncharged neutrons. Because the positive protons repel each other, a nucleus is under tremendous electrical stresses. Neutrons can help stabilize a nucleus because they bind to protons and each other through a short-range interaction called the strong force. The atoms that are fused by the accelerator and subsequently disintegrate inside Gammasphere, however, have too many protons to stay together for long. "The goal is to produce new and exotic nuclei," says Fallon. Creating unstable atoms at the extremes of stability lets us isolate different aspects of the problem that may not be evident in stable nuclei".
Although portions of the movie were filmed at the Lawrence Berkeley National Laboratory in California, where Gammasphere sometimes resides, the machine is presently in Illinois at the Argonne National Laboratory. "Gammasphere was built to be moved," says Fallon. "To date it has resided part of the time at Berkeley, and part of the time at Argonne, as part of a national research project." It takes about four months to disassemble, move, and reassemble the machine. The real Gammasphere itself never appears onscreen, but a faithful replica fills in. "As I understand it," says Fallon, "it's identical to the real thing, even down to the lab property stickers on the back."
One obvious difference between the real machine and the fictional version is the fact that the movie Gammasphere emits gamma rays that cause Banner to transform from human to Hulk every now and then. The real Gammasphere detects the weak gamma ray signals emitted by decaying atoms, and is harmless. "I think the producers were a little disappointed," chuckles Fallon, "when they found out that ours wasn’t dangerous."