The happy-looking fellow posing outside the Quonset hut in his cute little shorts is Harold M. Agnew, physicist and future director of Los Alamos National Laboratory.
In his hand, he is holding a cheerful little box which looks not entirely unlike a miniature Weighted Companion Cube. The value of that box (in inflation-adjusted 2013 money) is approximately five billion dollars.
The box contains the plutonium core for the Fat Man atomic bomb, which killed somewhere between 60,000 and 80,000 people shortly after the photograph was taken.
The death toll was actually much lower than that of the earlier Hiroshima bombing, for several reasons. Kokura was the prime target for the second mission, however cloud cover over the city on Aug 9 prevented the Bockscar from being able to sight the target. After making several orbits, Bockscar diverted to its secondary target, Nagasaki, which was similarly obscured. Running low on fuel, crew began to configure for a radar approach, when a last-minute break in the clouds allowed bombardier Frederick Ashworth to make a visual sighting, and the weapon was released.
Due to all of the anomalies which had occurred over the course of the mission, the bomb was released nearly two miles off the designated ground-zero, with the result that its blast was largely confined within the Urakami Valley, which shielded the majority of the city proper from its prompt effects.
A few random images:
This second shot is actually quite interesting mostly because of what it is not.
It is not a photograph of the Fat Man, nor of any American-built weapon.
This is, in fact, a Soviet-made RDS-1, which was first detonated in August, 1949. While not an exact replica, the RDS-1 turned out to bear an exceedingly strong resemblance to the Fat Man both in appearance and function. Amazingly, both Theodore Hall and Klaus Fuchs escaped the hangman's noose for their pivotal roles in the espionage project which revealed to the Soviets both theoretical data as well as design specifications produced during the Manhattan Project, of which they were both members.
This is a Fat-Man type bomb, on display at Wright-Patterson AFB Museum in Dayton, OH. In the background is the Bockscar B-29 which dropped the Fat Man bomb on Nagasaki. (Not a replica, the actual for-real airplane.)
I have actually touched this bomb, and the airplane next to it.