Northern Colorado Grotto

A lot of caves are wet and muddy, but others are dry and dusty. Horsethief Cave, along the Wyoming/Montana border, is one of the latter, at least in places. Horsethief is also quite "hot"...er, I mean radioactive. After returning from a trip to the cave, I thought I'd take a closer look at some of the dirt I harvested from my coveralls.

A routine analysis of the dirt was unable to detect any increased radioactivity relative to the background radiation in the laboratory, but I wasn't very surprised by that fact as the major source of radiation is from Radon gas, which has a very short half-life of about 3 days. Nevertheless I loaded a small sample of the dirt into a JEOL JSM-5610 scanning electron microscope (SEM) and took a closer look at the dirt. I discovered that the dirt is comprised exclusively of small irregularly shaped particles with sharp edges and features that measure on average between about 50 and 100 microns. In order to determine the elemental composition of the cave dirt, x-rays were detected and analyzed using an attached energy-dispersive x-ray detector/analyzer manufactured by Princeton Gamma-tech.

Energy-dispersive x-ray spectroscopy (EDXS) collects the x-rays generated by the incident electron beam integral to the SEM. X-rays are generated when electrons are "captured" by individual atoms. As a result of being captured or bound by the atom, the electrons release a portion of their energy in the form of an x-ray. The x-rays have specific energies which are determined by the quantum state in which the electrons are bound. Based on the energy of the x-rays, the elemental composition of the material being probed can be determined.

Analysis of the EDX results indicates the following elemental composition in atomic percentages:

• Ca - 46.598
• Si - 25.338
• Fe - 11.770
• Mg - 7.655
• Al - 3.597
• S - 3.276
• K - 1.457
• Mn - 0.268
• Pb - 0.040

There was no indication of uranium or any of the decay products between uranium-238 and lead-206; however, a substantial amount of lead is present. It is not possible to determine from the evidence the percentage of lead that results from radioactive decay.  Because radon is gas, and denser than air, it is entirely possible that the heavier radiactive materials are present in other regions of the cave and that the radon has migrated to the cave floor where it later decays to lead.  See below for a complete breakdown of the decay process from uranium-238 through radon to lead-206.  Uranium-238 is the most plentiful isotope of uranium.

• ²³⁸U → ²³⁴Th + α (t_1/2 = 4.5 x 10⁹ years)
• ²³⁴Th → ²³⁴Pa + β (t_1/2 = 24.5 days)
• ²³⁴Pa → ²³⁴U + β (t_1/2 = 1.14 minutes)
• ²³⁴U → ²³⁰Th + α (t_1/2 = 2.33 x 10⁵ years)
• ²³⁰Th → ²²⁶Ra + α (t_1/2 = 8.3 x 10⁴ years)
• ²²⁶Ra → ²²²Rn + α (t_1/2 = 1.59 x 10³ years)
• ²²²Rn → ²¹⁸Po + α (t_1/2 = 3.825 days)
• ²¹⁸Po → ²¹⁴Pb + α (t_1/2 = 3.05 minutes)
• ²¹⁴Pb → ²¹⁴Bi + β (t_1/2 = 26.8 minutes)
• ²¹⁴Bi → ²¹⁴Po + β (t_1/2 = 19.7 minutes)
• ²¹⁴Po → ²¹⁰Pb + α (t_1/2 = 1.5 x 10^-4 seconds)
• ²¹⁰Pb → ²¹⁰Bi + β (t_1/2 = 22 years)
• ²¹⁰Bi → ²¹⁰Po + β (t_1/2 = 5 days)
• ²¹⁰Po → ²⁰⁶Pb + α (t_1/2 = 140 days)
• ²⁰⁶Pb (stable)