Question

The site in question has pegmatite which includes uranium-bearing microlite. The age of the pegmatite is 1.3 billion years. I've measured the pegamtite at 4.2 mR/hr (see photo) The chart below show the decay series for U-238. The first number is 4.468 * 10^9 years = 4,468,000,000. What can be said about the composition of radioactive element(s) and the nature/strength of the radioactivity itself? (Edit: Can I assume it started with U-238? How much U-238 remains? What other of the decayed elements are there? What is the exposure danger to a human?) Nuclide Half-Life Radiation *

U-238 4.468*10^9 years alpha
Th-234 24.1 days beta
Pa-234m 1.17 minutes beta
U-234 244,500 years alpha
Th-230 77,000 years alpha
Ra-226 1,600 years alpha
Rn-222 3.8235 days alpha
Po-218 3.05 minutes alpha
Pb-214 26.8 minutes beta
Bi-214 19.9 minutes beta
Po-214 63.7 microseconds alpha
Pb-210 22.26 years beta
Bi-210 5.013 days beta
Po-210 138.378 days alpha
Pb-206 stable -

Answer 1

The initial radioactive element in the 1.3 billion-year-old pegmatite was U-238, with less than half remaining due to decay. Daughter nuclides, including Ra-226, Rn-222, and Po-210, contribute to the measured radioactivity of 4.2 mR/hr, indicating potential health risks from prolonged exposure to the site's radiation.

Step-by-step explanation:

When we are dealing with a pegmatite that has a measured age of 1.3 billion years and contains uranium-bearing microlite, it's reasonable to start with the assumption that the original radioactive element was U-238. Given that the half-life of U-238 is 4.5 × 10⁹ years, and the pegmatite is 1.3 billion years old, we can calculate that approximately less than half of the original U-238 remains, because the pegmatite has not yet reached its first half-life.

Regarding the exposure danger to humans, the measured radioactivity of 4.2 mR/hr indicates that the site has a significant level of radiation, which can pose health risks with prolonged exposure. Radiation safety protocols must be utilized to minimize exposure, such as limiting time spent near the site, using shielding, and employing personal protective equipment.