Astronomy 10 Quiz 4, fall semester 2006Cuesta College, San Luis Obispo, CA
Astronomy 10 learning goal Q4.2
Consider a sample with differing amounts of unstable isotopes, embedded gaseous decay products, and inert material (which is not involved in the radioactive decay process), schematically shown at right above. This sample is then melted completely, and then cooled and solidified. Which one of the following choices (A)-(E) schematically shown below best corresponds to the sample after it has resolidified? Clearly circle your answer.
Correct answer (highlight to unhide): (B)
The solidification age of a sample (how long ago has it been since it was last molten) is determined by the ratio of decay products to its unstable isotopes. A larger ratio of decay products to unstable isotopes corresponds to a sample with a very old solifidification age. Because these decay products are typically gaseous, when a sample is heated up to a molten state, the decay products bubble out, and thus the ratio of decay products to unstable isotopes is "zeroed out," corresponding to a zero-age solification age as it cools back down and resolidifies.
The plurality of (A) responses seems to indicate that students are thinking that heating up and melting a sample "resets" the decay products back into the original unstable isotopes.
Student responses:
(A): 11 students
(B): 7 students
(C): 5 students
(D): 4 students
(E): 6 students
Success level: 21%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.86
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