The "nation's attic" opens an exhibit today of items you won't find in anyone else's attic. They're atom smashers, machines so big some of them have to be shown in pieces and so expensive they had to be donated before they could be exhibited.

They all can be found for at least the next five years on the first floor of the Smithsonian Institution's Museum of History and Technology, alongside the Hall of Textiles and just west of the Foucault Pendulum.

The idea for the exhibit is not a new one. The Smithsonian began collecting pieces of atom smashers as long as 20 years ago in anticipation of the day it would want to document a piece of the Nuclear Age.

Atom smasher is the catch-all word used to described these giant machines Physicists call them "particle accelerators" because they accelerate charged atomic particles like the electron and proton to such high energies that the particles crumble like crackers into subatomic particles with such names as meson, muon and omega.

In the less than 50 years that atom smashers have existed, the physicists who built them have used them to discover more than 200 of these subatomic particle. The bigger they made their machines that the smaller the particles that they discovered became. The machines became the tools of physics, just as telescopes are the tools of astronomers and microscopes the tool of biologists.

"The hope has always been," said Pual Forman, the Smithsonian physcist who put the exhibit together, "that out of all these tiny subparticles would come a more complete understanding of the makeup of the atom."

The understanding is nowwhere near complete but it's getting there. A monument to that is the fact that most of the Nobel Prizes in physics in the last 20 years have been won for work with atom smashers. On exhibit at the Smithsonian are the Nobel diplomas of physicist Samuel C.C. Ting and Burton Richter, who won the prize a year aog for their work with atom smashers.

Also on exhibit are some of the chines, thathelped win some earlier Nobels. The bubble chamber that won the Nobel Prize for Donald Glaser at Brookhaven is here; the electron synchrotron that won a Nobel for Edwin McMillan at Berkeley is here.

The Berkeley synchrotron is the biggest machine on display at the Smithsonian. Built almost 30 years ago, it achieved particle energies of 330 million volts. There are only pieces of the machines built since them a steel plate from the magnet of the Brookhaven Cosmotron that broke 3 billion volts. A 10-foot reconstructed section of the Standford University accelerator that at 2 miles is one of the worlds longest.

The longest and the biggest is also the newest, the accelerator run outside Chicago by the Fermi National Accelerator Laboratory. It's built in a tunnel that runs through 4 miles of Illinois countryside and produces energies of 500 billion volts. Visitors to the exhibit walk through a 35-foot section of this tunnel, reconstructed with painstaking care by Smithsonian personnel.

Serious as it all sounds, the exhibit is not without a sense of humor. It's proivded by six satirical songs, played in the background and written by a physicts at Fermilab named Arthur Roberts. He's been writing these songs since 1939 and apparently he's still writing them. The music may not be as funny as Tom Lehrer's but then the subject is more serious.