Question 3 of 10: Gamma-Ray Spectrometry — Radiation Types, the U-238 Decay Chain, and a Disequilibrium Discrepancy

Question 3: Gamma-Ray Spectrometry — Radiation Types, the U-238 Decay Chain, and a Disequilibrium Discrepancy (16.66% of paper)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Types of ionizing radiation. Radioactive decay of naturally occurring isotopes (principally the U-238, U-235 and Th-232 decay series, plus K-40) produces three types of ionizing radiation. Alpha particles (helium nuclei) are massive and highly ionizing but travel only a few centimetres in air and are stopped by a sheet of paper or a few tens of micrometres of rock — undetectable through any real rock surface. Beta particles (electrons) penetrate further, on the order of metres in air, but are still absorbed within a few millimetres of rock. Gamma rays (high-energy electromagnetic photons) are by far the most penetrating, travelling tens of centimetres through rock and up to hundreds of metres through air, which is what makes them the only one of the three that can be measured remotely — from a ground station, a vehicle, or an aircraft — over intact rock or soil. Gamma-ray spectrometry is therefore built entirely around detecting and energy-discriminating gamma photons; alpha and beta radiation are geologically real but geophysically invisible at any useful stand-off distance.

Uranium-238 decay. U-238 is not stable; it decays via a long chain of some 14 successive alpha and beta decays — through intermediate isotopes including Th-234, Pa-234, U-234, Th-230, Ra-226, Rn-222, Po-218, Pb-214, Bi-214, Po-214 and Pb-210 — ultimately terminating at the stable isotope Pb-206. Several of these intermediate daughters emit characteristic gamma rays as they decay, and it is these daughter-product gammas, not any radiation from U-238 itself (whose own decay emits only a very low-energy, weak gamma), that gamma-ray spectrometry actually detects.

Why bismuth-214 matters. Bi-214 is important because it emits a strong, high-energy, easily resolved gamma-ray photopeak at 1.76 MeV, which is used as the standard diagnostic "uranium channel" in every gamma-ray spectrometer. Because Bi-214 sits well down the U-238 chain (after Ra-226 and Rn-222), using its gamma emission as a proxy for uranium content is only valid under the assumption of secular equilibrium — that the whole decay chain from U-238 down to Bi-214 has had enough geological time (and no chemical disturbance) to reach a steady-state activity ratio, so that the measured Bi-214 gamma activity is proportional to the true U-238 abundance. The reading obtained on this assumption is conventionally reported as "equivalent uranium" (eU), to flag that it is inferred from a daughter product, not measured directly.

The 500 ppm eU vs. 15 ppm U discrepancy. This large mismatch (eU roughly 30× the assayed U) is a classic signature of radioactive disequilibrium, i.e. a breakdown of the secular-equilibrium assumption underlying eU. Uranium (U$^{6+}$, as the soluble uranyl ion) is far more mobile in oxidizing, near-surface groundwater than several of its mid-chain daughters (notably Ra-226, which is comparatively immobile and tends to be retained by adsorption/co-precipitation), so recent weathering or groundwater leaching of the outcrop can strip uranium out of the rock while the longer-lived daughter products — and hence the Bi-214 gamma signal, which reflects the daughter inventory built up over the preceding tens of thousands of years, not the present-day U content — remain behind at their pre-leaching level. The result is exactly this observation: a gamma-ray (eU) reading that is high, reflecting the historical uranium content recorded by the retained daughters, while a direct geochemical assay of the present rock shows the uranium itself has already been substantially removed. (The reverse disequilibrium — recently precipitated/re-concentrated uranium with daughters not yet grown in, giving low eU but high assayed U — is also possible and is the other standard explanation offered for this class of discrepancy, but does not fit here since eU is the higher of the two readings.)