Radioactive Decay: Also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting ionizing radiation. A material that spontaneously emits this kind of radiation — which includes the emission of alpha particles, beta particles, gamma rays and conversion electrons — is considered radioactive. Radioactive decay accounts for about 80% of the Earth's internal heat, powering the geodynamo and plate tectonics. The main heat-producing isotopes are potassium-40, uranium-238, uranium-235, and thorium-232. Radioactive primordial nuclides found in the Earth are residues from ancient supernova explosions which occurred before the formation of the solar system. They are the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. The naturally occurring short-lived radiogenic radionuclides found in rocks are the daughters of these radioactive primordial nuclides. The radioactive decay of these radionuclides in rocks within Earth's mantle and crust contribute significantly to Earth's internal heat budget. Radioactive decay is measured in becquerels (Bq) The becquerel is the SI derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.
Ionizing Radiation: Radiation that carries enough energy to liberate electrons from atoms or molecules, thereby ionizing them. Ionizing radiation is composed of energetic subatomic particles, ions or atoms moving at relativistic speeds, and electromagnetic waves on the high-energy end of the electromagnetic spectrum. Typical ionizing subatomic particles from radioactivity include alpha particles, beta particles and neutrons. Almost all products of radioactive decay are ionizing because the energy of radioactive decay is typically far higher than that required to ionize. Gamma rays, X-rays, and the higher ultraviolet part of the electromagnetic spectrum are ionizing, whereas the lower ultraviolet, visible light (including nearly all types of laser light), infrared, microwaves, and radio waves are considered non-ionizing radiation. Ionizing radiation is measured in sieverts (Sv) The sievert is the SI derived unit of ionizing radiation dose. One Sv represents the equivalent biological effect of the deposit of a joule of radiation energy in a kilogram of human tissue.
A Brief Radiological History of TI
The Army Corps of Engineers created Treasure Island for the 1939 Golden Gate International Exposition, encircling 400 acres of bay shoals with rock walls, draining them, filling the void with sand and soil, and naming it after the famous adventure novel.
After the exposition, the island was set to become a civilian airport — until the United States entered World War II. The Navy seized the land for the Treasure Island Naval Station and demolished the expo’s structures, leaving just the terminal and two hangars. For years, the base served its purpose until, in 1993, it landed on a decommission list. The military decamped, and dismantling & cleanup began.
For 46 years before the naval station was selected for closure, Treasure Island was home to nuclear war academies that used a variety of radionuclides in their training — including radium, plutonium and cesium-137.
Various operations involving radioactive material were distributed among facilities around Treasure Island. These efforts included academies that offered instruction in radiation detection, radiation instrument repair, ship decontamination, and atomic warfare damage control. Lessons involved handling samples of cesium-137 radioactive enough to severely burn unshielded people standing near them. Also stored on-site were sample lumps of radioactive tritium, plutonium, cadmium, cobalt, strontium and krypton.
The Navy built a full-scale mock-up of a 173-foot patrol craft on Treasure Island. To practice decontamination, sailors and other students would spray the mock-up, known as the USS Pandemonium, with short half-life radioactive material, then scrub it off. Inside the Pandemonium, 11 pieces of cesium-137 were routinely raised and lowered from lead enclosures so students could learn to use survey meters.
Radiological Impact Today
There are 6 radiologically impacted sites on Treasure island, with varying degrees of severity. Cleanup is still ongoing at several of the sites, however it must be stated that the levels of ionizing radiation that are currently present are very low, even around the sites that are fenced off and designated as radiologically controlled areas.
I spent several hours today wandering around the island taking measurements with my dosimeter. The average reading in most areas on the island was 0.07-0.08 μSv/hr. Readings were a bit higher near the radiologically impacted areas. Since ionizing radiation strength from a point source decreases with the square of the distance it travels, it's possible that higher doses would have been measured inside the controlled areas.
Dose measurements were taken at the coordinates listed for each waypoint and the maximum observed reading at each waypoint was recorded. The highest measured doses were found at WP1 and WP2 — both sites were known to have contained cesium-137, and remnants of it are likely present in the soil. Exposure to ionizing radiation from contaminated sites is possible in several ways:
- Walking on cesium-137 contaminated soil could result in external exposure to gamma radiation. Leaving the contaminated area would prevent additional exposure.
- Coming in contact with waste materials at contaminated sites could also result in external exposure to gamma radiation. Leaving the area would also end the exposure.
- If cesium-137 contaminated soil becomes air-borne as dust, breathing the dust would result in internal exposure. Because the radiation emitting material is then in the body, leaving the site would not end the exposure.
- Drinking cesium-137 contaminated water, would also place the cesium-137 inside the body, where it would expose living tissue to gamma and beta radiation.
You are not required to visit every waypoint in order to log this EarthCache as found, but I encourage you to do so — each site is unique and interesting in it's own way.
Logging Requirements — Send me a note with:
- The text 'Treasure Island ☢ Ionizing Radiation (GC5N6TJ)' on the first line.
- What is the qualitative difference between the units of measurement sievert and becquerel?
- Visit WP1 - Notice how close the homes are to this site. If you lived at the waypoint coordinates for one year, what would your total accumulated dose of ionizing radiation be for the year in mSv from this site? How does this compare to the EPA yearly limit on radiation exposure to a single member of the public? How does it compare to the normal yearly background dose?
- Visit WP2 - See if you can spot the former location of the USS Pandemonium. There is a fairly consistent prevailing wind on Treasure Island - what direction is it blowing from today? Speculate as to why, or why not, moving the site from the NW side of the island to the NE side was a good idea.
- Visit WP4 - You can see the entrance to the former underground radium storage bunker through the gate. Assuming a baseline dose of 0.08 μSv/hr on most of the island, if you took a nap on top of the bunker for 3 hours, what is the additional dose, in BED, that you would receive over baseline from being there that long?
- Given all of the above information, do you think it's dangerous to visit Treasure Island? How about living there?
No need to hear back from me after sending in your answers to the questions — just log the find and I will contact you if there are any problems. Have fun, eat a banana!