As Requested, IAM someone who has witnessed an atomic bomb test...I've seen 18 or 20 upclose and personal...my job was to fly thru the mushroom clouds and collect air samples
(my son is the redditor for this post, but I will answer your questions)
My participation in operation Dominic as a B57 pilot began in January 1962. At the time I was flying the F89 interceptor for the 103FIS (PANG) at the Philadelphia international Airport. When a request came down from Wing Headquarters for volunteers to go on a classified mission to a small island in the South Pacific, three other pilots, and I signed up. Our first stop was Louisville KY, where we checked out in the B57. Two weeks later we were on our way to the 1211th test squadron in Albuquerque NM. Where we flew several missions learns how to do air sampling.
Then off to Hickam Air Base, Hawaii where we practiced until we learned that we would be going to Christmas Island to perform air sampling missions during the atomic bomb testing. We then flew our B57s to Christmas Island. Once the bomb testing started we took turns flying through the mushroom cloud immediately after the detonation. Most detonations took place about 20miles downwind of the island. A transport ship was moored off the island should the winds suddenly turn around during a test. . (It never did)
A typical mission went like this. Each pilot would have a specific take-off time. It could be five, ten or fifteen minutes before detonation or immediately after detonation time. We would climb to a designated altitude and toward the mushroom cloud. (If we took off before detonation we would make sure we were heading in the opposite direction at zero time). Another B57 pilot with an engineer in the back seat would join up with us for a few minutes to give us an exact heading to hold while we flew through the cloud. We would also be given an emergency exit heading should the cloud become too hot. We would know this by the reading of the radiation detection instruments, which were installed in the back seat. My navigator would read these gage numbers over the air as we flew through the cloud. I would be responsible for opening the air sampling valves on the empty tip tanks. If the gages did not max out I would hold the heading until I came out the other side of the cloud, I would immediately head back to the airstrip, land, and taxi to the decontamination area.
After shutting down the engines, I would raise the canopy. This allowed the decon specialists, who were dressed in white protective gear and wearing big gloves, to drive a forklift with a raised wooden platform on its tongs to the edge of the cockpit. An airman on the platform would first lift the navigator, then me out of the cockpit. This procedure prevented us from touching the outside of the airplane. The only protection we wore was a lead vest over our thin summer flight suit. Instead of the usual heavy flight boots, we wore light athletic sneakers. The reason for this was that after we were taken to the decontamination building we discarded all of our clothing into a large empty oil drum. I guess these were then washed and used again. We were then directed to the shower area where we used some strong hard soap to wash off any external radiation we might have accumulated. After drying off we were checked with a Geiger counter and if the numbers were too high we returned to the showers until we got the numbers down to a safe? Number. Normally two showers would suffice, but I heard the record for one crew was seven showers. Short hair was a must, as hair would trap the radiation.
To measure how much radiation each crewmember accumulated, we would wear a dosimeter attached to a string around out neck and would also swallow a radiation detection pill. It was about one inch long and shaped like a football. It was hinged in the center to allow a dosimeter in its center to be read after retrieval. The method of retrieving it was not something we looked forward to.
Meticulous records were kept and if a crewmember had high accumulations of radiation he/she would not be allowed to continue the air sampling missions. I never accumulated more than ten Roentgens. Some years later the Atomic Energy Dept sent me documentation of my radiation exposure during operation Dominic. Included was a list of possible health hazards associated with exposure to radiation received while flying through atomic bomb clouds. Fortunately, to date, I have not experienced any those symptoms.