Radioactivity of Rock from the Val Verde Tunnel

Citation

Clarke, Robert Alma (1937) Radioactivity of Rock from the Val Verde Tunnel. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pj2s-xj13. https://resolver.caltech.edu/CaltechTHESIS:11232024-021848045

Abstract

l. Suitable apparatus for the determination of the radioactivity of course-grained low activity samples was developed, and measurements of the alpha particle activity of 73 representative samples from in and around the Val Verde tunnel were made.

2. These measurements show:

a. Active substances are concentrated in grains distributed in such a way that for samples less than 175 – 210 mg mg the graininess is important. For 25-35 mg samples taken from a 65 gram mass of rock ground to 60 mass and well mixed the average deviation from sample to sample is 18% while for 175 - 210 mg samples it is 4%.

b. Sampling around a point indicates that the majority of carefully chosen samples taken within a few feet of each other have fairly constant radioactivity.

c. The radioactivity increases in the western end of the tunnel and shows a strong secondary correlation both with the percentage and acidity of the plagioclase feldspar in the rock.

d. Measurements on 21 non-country rocks give a striking confirmation of the already observed variation of radioactivity with acidity among different rock types. Measurements also indicate that the inclusions came from the gabbro which definitely gives the order of intrusion of the successive magmas into the schist, and suggests that this may be a powerful method of studying the origin of inclusions.

3. Radium determinations were made on 6 samples of tonalite and combination of these with counter measurements gives:

a. An average radium content of 0.47 x 10^-12 grams/gram of rock.

b. A surprisingly constant Th/U ratio of average value 3.69.

c. An average content of .53 x 10^-6 grams of thorium per gram of rock.

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