I disagree. Most stories about the dead will almost always name the cause of death. Besides, it's no secret that atomic bombs put out lots of contamination in the form of radiation. The fact is, no one will probably ever know if being in that area after the fact, was the cause of his death, many years later.
Highly unlikely that the radiation contributed to his death in any way. The highest levels of radiation produced by atomic bombs tend to dissipate fairly quickly (short half life or are simply dispersed by win and rain) after detonation. Being near the detonation or around ground zero very shortly thereafter can cause serious radiation exposure. However, weeks or months later the radiation has already started to dissipate.
In December 1961, the United States detonated an atomic bomb in a salt dome about 40km south of Carlsbad, New Mexico. Six months later, crews drilled down and entered the cavity created by the detonation. While radiation still existed, the intense radiation created by the blast had long dissipated.
"It's no secret", huh? Well, FYI, the nuclear detonations at Hiroshima and Nagasaki were air bursts above 5,000 Ft, AGL. There was very little nuclear contamination in the debris at these sites. The initial Gamma Ray exposure is what killed many people after a few days or weeks. The destruction that you see in these photos of Hiroshima and Nagasaki are from the blast of the explosion, not anything related to "atomic" other than that was the process used to create a destructive pressure wave. The same way that conventional explosives "kill people and break things".
Unless McGovern was at Hiroshima on the day the bomb was dropped, his exposure to nuclear radiation was next to nil.
When McGovern photographed the destruction on the ground, there was very little nuclear contamination. It's the "Dirty Bombs" that one needs to worry about. And anthrax scares me more.