Morris R. Jeppson, 87, one of two weaponeers who armed the atomic bomb that was dropped on Hiroshima in 1945, prompting the surrender of Japanese troops and the end of World War II, died March 30 at a hospital in Las Vegas. His family could not provide a specific cause of death but said he had been hospitalized for back pain and a severe headache.

Known as "Dick," Mr. Jeppson was a 23-year-old Army Air Forces second lieutenant when he boarded the Enola Gay, a B-29 bomber, for what would be his first and only combat mission.

It was the wee hours of the morning on Aug. 6, 1945, and "Little Boy," the bomb that would introduce the world to nuclear warfare, lay in the plane's belly in safe mode. It had to be armed in flight en route to its target to avoid accidental detonation during takeoff.

At 2:45 a.m., the Enola Gay and its cargo took off from Tinian Island in the Pacific and headed for Hiroshima, 1,800 miles and six hours away. Lt. Jeppson, the weapons test officer, and his boss, Navy Capt. William S. Parsons, climbed into the airplane's bomb bay to ready Little Boy for discharge.

Capt. Parsons was responsible for installing the charge that would be fired into the weapon's uranium core, setting off a nuclear explosion. Lt. Jeppson armed the bomb's electrical system, pulling out three green safety plugs and replacing them with red firing plugs.

Nobody but the two weaponeers knows for sure who was last to touch the warhead. Reference books and historical accounts differ.

But it's likely that Lt. Jeppson "put the last thing into the bomb that made it hot," said Dik Daso, curator of modern military aircraft at the Smithsonian's National Air and Space Museum.

The pair finished their work within 30 minutes after takeoff. Then they climbed into the pressurized cabin and waited another 5 1/2 hours before the plane jerked upward, signaling that Little Boy had been let go.

The flash came 43 seconds later, sending an enormous mushroom cloud into the sky and killing and wounding more than 100,000 people.

"No joy at that point," Mr. Jeppson told Time magazine in 2005. "But it was a job that was done."

The Enola Gay returned to Tinian, where her crew of 12 received a hero's welcome. Eight days later, after an atomic bomb was dropped on the city of Nagasaki, the Japanese agreed to surrender. A formal document of surrender was signed Sept. 2.

Mr. Jeppson received the Silver Star and went on to a civilian career in electronics and applied radiation. He maintained that he had no regrets about the bombing. He told reporters that his wife's car bore a bumper sticker that read "If there hadn't been a Pearl Harbor, there wouldn't have been a Hiroshima."

Morris Richard Jeppson was born in Logan, Utah, on June 23, 1922. He joined the Army Air Forces when he was 19, eager to become a pilot. Instead, unable to pass the required vision test, he was sent to electronics and radar training programs at Yale and Harvard universities and the Massachusetts Institute of Technology.

He was one of a small group sent to join the 509th Composite Group in Wendover, Utah. They trained for their atomic mission in secret over the Bonneville Salt Flats, a shimmering moonscape west of Salt Lake City.

The military's preparation for use of an atomic bomb was so secret that Lt. Jeppson had to remove the insignias from his uniform when he visited scientists at Los Alamos National Laboratory in New Mexico.

"The words 'atomic' or 'nuclear' were never even heard at Wendover," Mr. Jeppson told a Las Vegas newspaper in 2005. "The actual mission of the 509th was a secret that held until we were in the air and on the way to Hiroshima."

After the war, Mr. Jeppson received a bachelor's degree in physics at the University of California at Berkeley. He went on to work for the university's radiation laboratory and at the Lawrence Livermore National Laboratory before founding his own company, which built linear accelerators for research and medical applications.

In 1962, he started a second company, which produced industrial microwave ovens. He retired after selling that company in 1970. He lived in California until moving to Las Vegas about 20 years ago.

His first marriage, to Marge Jeppson, ended in divorce.

Survivors include his wife of 49 years, the former Molly Ann Hussey, of Las Vegas; two children from his first marriage, Carol English of Medford, Ore., and Nancy Hoskins of Colorado Springs; a daughter from his second marriage, Sally Jeppson of Gackle, N.D.; three stepchildren, Jane Ross of Midland, Ontario, Mike Sullivan of Pahrump, Nev., and John Sullivan of Lakeport, Calif.; a brother; 11 grandchildren; and 10 great-grandchildren.

Of the nine flight crew members and three scientists who flew to Hiroshima in the Enola Gay, only one now survives: Theodore Van Kirk, the aircraft's navigator.

In 2002, Mr. Jeppson opened a safe deposit box where he'd kept two of Little Boy's bomb plugs for more than half a century. One was a green-tipped safety plug; the other was a red-tipped spare. He sold them at auction for $167,500.

The U.S. Justice Department sued to keep the sale from going through, saying the design of the arming devices was classified.

Siding with Mr. Jeppson, a judge ruled that the plugs could be transferred to their buyer, a former rocket scientist who said the bomb had inspired him to become a physicist.

Morris R Jeppson