Question 3 of 8: Airborne Radiometrics and a Gamma-Ray Spectrum

Question 3: Airborne Radiometrics and a Gamma-Ray Spectrum (7 marks)

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.

(a) Three types of natural radioactivity. Alpha decay (emission of a helium nucleus, $^4_2\text{He}^{2+}$), beta decay (emission of an electron or positron as a neutron converts to a proton or vice-versa), and gamma decay (emission of a high-energy photon as an excited daughter nucleus relaxes to its ground state, usually immediately following an alpha or beta event). Only gamma rays are measured in airborne radiometric surveying because alpha particles are stopped by a few centimetres of air (or even the outer few microns of the rock itself) and beta particles by at most a few metres of air, so neither can reach an aircraft flying tens of metres above ground; gamma rays, by contrast, are electromagnetic radiation that can travel hundreds of metres through air (attenuated but detectable) before being absorbed, making them the only one of the three that is remotely sensible at survey altitude.

(b)(i) Cause of the peaks, and their labels. Each peak is a photopeak: a full-energy gamma-ray line emitted at a fixed, characteristic energy by a specific radionuclide in the natural decay chain, superimposed on a smoothly falling Compton-scattering continuum (gamma rays that lost only part of their energy to electron scattering before reaching the detector, rather than depositing it all at once). Because each parent isotope's daughters emit gammas at reproducible energies, the peak energy directly identifies which decay series produced it. The three peaks used for airborne gauging (and used in the redrawn spectrum below) are:

• Potassium (K): the 1.46 MeV gamma from the direct decay of ⁴⁰K itself (a primordial isotope, not a daughter in a chain) — the largest, sharpest peak in the spectrum.
• Uranium (U): the 1.76 MeV gamma from ²¹⁴Bi, a daughter well down the ²³⁸U decay chain (used as a PROXY for U because ²³⁸U itself emits no useful gamma; this assumes secular equilibrium in the chain).
• Thorium (Th): the 2.62 MeV gamma from ²⁰⁸Tl, a daughter in the ²³²Th chain (again a proxy, assuming equilibrium) — the highest-energy, lowest-count-rate peak, consistent with the falling continuum.

[Figure not reproduced: Gamma-ray spectrum for the granitic gneiss outcrop, redrawn from the exam figure (log count-rate vs. energy), with the K/U/Th photopeaks identified by their standard windows. See the official exam paper.]

(b)(ii) Correction needed for the potassium count. The raw potassium-window count rate must be corrected for (1) height/distance attenuation — converted to a standard survey altitude via an altitude-attenuation calibration, since gamma flux falls off strongly (roughly exponentially) with the air path between source and detector; (2) Compton stripping — subtracting the fraction of counts in the K window that are actually degraded (scattered) photons from the higher-energy U and Th peaks, using stripping ratios determined on calibration pads; (3) background subtraction — cosmic-ray and aircraft/detector-material background measured at altitude over water or on a shielded pad; and (4) conversion of the corrected, stripped count rate to a %K by mass using a sensitivity (counts per second per %K) determined from a calibration pad of known potassium content. Only after all four corrections is the number a meaningful %K estimate rather than a raw count rate.