ITHACA, N.Y. -- Boyce D. McDaniel, the Cornell University physicist and Manhattan Project scientist who gave the atomic bomb its final check before the first test at Trinity site in July 1945, died of a heart attack May 8 in Ithaca, N.Y. He was 84.

McDaniel's faculty career at Cornell spanned 56 years. But his professional start was sudden and dramatic. In 1943, as a newly fledged Ph.D., McDaniel was hired, at $250 a month working 10- to 15-hour days at a secret facility in Los Alamos, N.M., to conduct nuclear physics research on a device nicknamed "the gadget." The device was the atomic bomb, and McDaniel had been hired as a protégé of Robert Bacher, one of several Cornell physicists assigned to the Manhattan Project. The young McDaniel would play a critical role on physicist Robert Wilson's cyclotron research team, which helped identify the amount of the isotope uranium-235 (U-235) needed to create the atomic fission to detonate the world's first nuclear weapon.

McDaniel had earned his doctorate at Cornell in the early years of World War II, researching the absorption rates of neutrons in the element indium. While his thesis was not considered classified information by the U.S. government, McDaniel and Bacher understood its implications for weapons research. They marked each page "secret" and locked two copies away in the university's library.

Physicists on the Manhattan Project worked simultaneously on different materials to power the bomb. McDaniel's group, however, needed to know if the U-235 chain reaction occurred fast enough to create an explosive device. By the summer of 1945, enough U-235 (an isotope, comprising only .7 percent of the atoms of naturally occurring uranium) and sufficient plutonium-239 had been processed.

On July 15, 1945, the plutonium bomb was ready for its test at Trinity site on the U.S. Army's Alamogordo Bombing Range at the Jornada del Muerto (Journey of Death) desert. The test was scheduled for the next day at 4 a.m.

One of McDaniel's tasks was to check the bomb's radiation levels every few hours. The gadget had been hoisted atop a 100-foot metal tower, and as McDaniel climbed the tower at 1 a.m., a strong thunderstorm developed over the site. McDaniel later wrote, "During the evening a misting rain had been falling, and by the time I started up [the tower], there were thunderstorms playing around the site with frequent flashes of lightning followed by rolling thunder. With considerable fear and trepidation, I made the trip to the top and returned safely, heaving a sigh of relief."

McDaniel would be the last man to check and even touch the bomb at the Trinity site before it was detonated at 5:29:45 a.m., Mountain War Time, July 16, 1945. The young McDaniel was surrounded his friends, mentors and colleagues, physicists Bacher, Isadore Isaac Rabi and Enrico Fermi. McDaniel recalled them "staring intently into the darkness. Then came the last-minute countdown with the switch to automatic time out. Finally, the brilliant flash of an ever-growing sphere was followed by the billowing flame of an orange ball rising above the plain."

In 1946, McDaniel joined the Cornell faculty as an assistant professor and became a full professor in 1955. With Cornell physicist Robert Walker he invented the pair spectrometer, an important tool used to measure gamma ray energies.

He was a leader in establishing the Cornell Laboratory of Nuclear Studies (LNS), and had a major role in designing and building the 300 megavolt (MeV) electron synchrotron, one of the first such accelerators in the world. He and Wilson built three more electron synchrotrons of successively higher energies, each of which enabled physicists to study phenomena in a new energy range. "Each of these accelerators was a masterpiece of technology, built rapidly and economically by a small team of physicists," said Peter C. Stein, Cornell professor of physics. "Mac played a leading role in the construction of all of these accelerators, and brilliantly completed the construction of the last of these accelerators, the 10 GeV [gigavolt] synchrotron."

McDaniel became director of the LNS in 1967, and remained in that position until he became an emeritus professor in 1985. He pioneered the technique of tagged gamma rays, and performed important measurements, including a long series of work in K-meson and lambda-meson photo production and measurements of the neutron electromagnetic form factors.

In 1972, McDaniel took a year's leave from Cornell to become acting head of the accelerator section at the Enrico Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. "This was a very difficult time for Fermilab and the entire particle physics community," Wilson, Fermilab's first director, who died in 2000, wrote. "Though the accelerator had operated at a near-design energy, component failure was frequent and operation intermittent. Mac threw himself into the fray with his usual enthusiasm." Because of McDaniel's leadership, the Fermilab accelerator was working as it should by the end of the year.Back at Cornell in 1974, McDaniel proposed upgrading the existing 10 GeV synchrotron into an 8 GeV electron-positron storage ring. "This radical but risky proposal, if it worked, would reduce the cost and construction time by a large factor," said Stein. "This would be just enough to make its funding possible. Mac convinced the National Science Foundation to support the project, and threw himself heart and soul into the job of making it work. That it worked at all was miraculous, but not even Mac dared hope for the rich treasure trove of science that it would uncover." The storage ring, known as CESR, became the world's leading source of information about the b-quark, one of the fundamental building blocks of matter.

Boyce Dawkins McDaniel was born on June 11, 1917, in Brevard, N.C. He graduated from Chesterville High School, Chesterville, Ohio, in 1933. He earned his bachelor's degree from Ohio Wesleyan University in 1938, his master's degree from the Case School of Applied Science (now Case Western Reserve University) in 1940 and his doctoral degree in physics from Cornell in 1943.

McDaniel was a Fulbright Research Fellow in 1953 at the Australian National University, Canberra, and a Fulbright Research Fellow and a Guggenheim Fellow at the University of Rome and the National Laboratory, Frascati, Italy, in 1959.

He was a member of the National Academy of Sciences, a trustee of the Associated Universities, a member of the governing board of Brookhaven National Laboratory and of the Department of Energy High Energy Advisory Panel, a trustee of the Universities Research Association, a governing board member of Fermilab and chair of the Superconducting Supercollider Board of Overseers.

McDaniel is survived by Jane, his wife of 61 years; a son, James of Victoria, British Columbia, and a daughter, Gail, of New York City.