Physics quiz question: alpha decay product of bismuth-209

Physics 205B Quiz 7, spring semester 2017
Cuesta College, San Luis Obispo, CA

Bi(209,83) has the longest known half-life of any isotope that undergoes alpha decay, with a half-life of 1.9×10¹⁹ years[*].

Bi(209,83) undergoes alpha decay to become:
(A) Tl(205, 81) .
(B) Tl(207, 81) .
(C) Bi(209, 82) .
(D) Po(209, 84) .

[*] Pierre de Marcillac, Noël Coron, Gérard Dambier, Jacques Leblanc and Jean-Pierre Moalic, "Experimental Detection of α-Particles from the Radioactive Decay of Natural Bismuth," Nature, vol. 422 (April 24, 2003), 
pp. 876-878 nature.com/nature/journal/v422/n6934/full/nature01541.html.

Correct answer (highlight to unhide): (A)

Bi(209,83) is a nuclide with 83 protons, and 209 nucleons, and thus has 209 – 83 = 126 neutrons. When it undergoes alpha decay, it emits a helium-4 nucleus (He(4,2)) that contains two neutrons and two protons, such that the resulting nucleus has 81 protons, and 124 neutrons, and thus has a total of 81 + 124 = 205 nucleons, and so the result is Tl(205,81).

Sections 30882, 30883
Exam code: quiz07N4ci
(A) : 10 students
(B) : 11 students
(C) : 3 students
(D) : 2 students

Success level: 38%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.71