20160615

Physics quiz question: electron capture decay of iodine-125

Physics 205B Quiz 7, spring semester 2016
Cuesta College, San Luis Obispo, CA

I(125,53) is used as a radiation source placed surgically implanted next to tumors for treatment of eye cancer[*]. I(125,53) decays via electron capture (with a half life of 59.43 days) to become:
(A) Te(123, 52).
(B) Te(125, 52).
(C) Xe(125, 52).
(D) Xe(127, 52).

[*] cancer.ucsd.edu/care-centers/radiation-oncology/procedures/Pages/eye-plaques.aspx.

Correct answer (highlight to unhide): (B)

Electron capture takes in an electron to covert a proton into a neutron, which also leaves the nucleon number the same (a total of 125 protons and neutrons), but decreases the atomic number by one (from 53 protons to 52 protons).

(Response (C) corresponds to the daughter product of I(125,53) undergoing β decay.)

Sections 30882, 30883
Exam code: quiz07eYE5
(A) : 1 student
(B) : 32 students
(C) : 7 students
(D) : 0 students

Success level: 80%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.13

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