20120612

Physics final exam question: carbon-14 decay process

Physics 205B Final Exam, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 29.4, Problem 29.26

Discuss why radioactive (14,6)C will not undergo β+ decay, and must instead undergo β decay. Explain your reasoning using the properties of nuclides and radioactive decay modes.

Solution and grading rubric:
  • p:
    Correct. Discusses β+ decay as converting protons into neutrons, and since carbon-14 has more neutrons than protons, this would not lead to a more stable nucleus, thus β decay (which converts a neutron into a proton) would result in a more stable nucleus.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some attempt at finding the number of neutrons and protons in carbon-14.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.

Grading distribution:
Section 30882
Exam code: finalsM0k
p: 20 students
r: 0 students
t: 0 students
v: 1 student
x: 2 students
y: 0 students
z: 1 student

A sample "p" response (from student 0524):
Another sample "p" response (from student 1413):

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