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

Physics quiz question: Feynman diagram interpretation

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

This Feynman diagram depicts a(n) __________ process.
(A) β^–.
(B) β⁺.
(C) electron capture.
(D) (This process is not possible, as it is invalid.)

Correct answer (highlight to unhide): (D)

This Feynman diagram vertex is invalid, because it violates charge conservation. Tracing the proton line left-to-right shows that it converts into a neutron, and so the outgoing intermediate vector boson must be positively charged (W⁺) to conserve charge for that vertex. However, at the bottom vertex it is not possible for the neutrally charged neutrino (ν) to absorb that positively charged intermediate vector boson in order to convert to a negatively charged electron.

Sections 30882, 30883
Exam code: quiz07N4ci
(A) : 9 students
(B) : 12 students
(C) : 6 students
(D) : 0 students

Success level: 0%
Discrimination index (Aubrecht & Aubrecht, 1983): 0

Physics quiz question: interpreting β+ decay Feynman diagram

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

This (valid) Feynman diagram depicts the __________ process.
(A) α.
(B) β^–.
(C) β⁺.
(D) electron capture.
(E) γ.

Correct answer (highlight to unhide): (C)

This diagram shows β⁺ decay, where a proton transforms into a neutron, and the intermediate W⁺ particle decays into a neutrino and a positron, as shown by the left-to-right neutrino arrow, and a right-to-left electron arrow (corresponding to a positron--the "β⁺" particle--progressing left-to-right as time progresses).

Sections 30882, 30883
Exam code: quiz07eYE5
(A) : 2 students
(B) : 3 students
(C) : 30 students
(D) : 5 students
(E) : 0 students

Success level: 75%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.60

Physics quiz question: Feynman diagram vertex interpretation

Physics 205B Quiz 7, spring semester 2014
Cuesta College, San Luis Obispo, CA

This Feynman diagram vertex depicts a neutron:
(A) converting into a proton.
(B) converting into an antiproton.
(C) annihilating with a proton.
(D) annihilating with an antiproton.
(E) (this vertex is not possible, as it is invalid.)

Correct answer (highlight to unhide): (A)

This Feynman diagram vertex is valid, as it obeys the four rules of drawing Feynman diagrams, especially the first two:

1. You can draw two kinds of lines, a straight line with an arrow or a wiggly line [here, a dashed line]. You can draw these pointing in any direction.
2. You may only connect these lines if you have two lines with arrows meeting a single wiggly [dashed] line. Note that the orientation of the arrows is important! You must have exactly one arrow going into the vertex and exactly one arrow coming out.

   --Phillip "Flip" Tanedo, "Let's Draw Feynman Diagrams," Cornell University/USLHC Collaboration

Since the diagram is valid, tracing the straight line with arrows from left-to-right (the convention used in this class) shows a neutron converting into a proton (as it emits a negatively-charged π^– meson).

Sections 30882, 30883
Exam code: quiz07bC4n
(A) : 34 students
(B) : 0 students
(C) : 1 student
(D) : 1 student
(E) : 2 students

Success level: 89%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.20

Physics quiz question: interpreting β– decay Feynman diagram

Physics 205B Quiz 7, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 29.1

A Feynman diagram is shown at right. A __________ exits this interaction event.
(A) neutron.
(B) neutrino.
(C) positron.
(D) proton.
(E) (More than one of the above choices.)
(F) (None of the above choices.)

Correct answer (highlight to unhide): (D)

This diagram shows β^- decay, where a neutron transforms into a proton, and the intermediate W^- particle decays into an electron and an antineutrino, as shown by the left-to-right electron arrow, and a right-to-left neutrino arrow (corresponding to an antineutrino progressing left-to-right as time progresses). Thus of the above four choices, only a proton leaves this interaction event, as a neutron enters it, and an antineutrino and electron leave the interaction event (but neither of which are listed as choices).

Section 30882
Exam code: quiz07f3yN
(A) : 4 students
(B) : 6 students
(C) : 2 students
(D) : 9 students
(E) : 11 students
(F) : 0 students

Success level: 29%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.19