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conantee
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« on: December 31, 2009, 08:32:51 AM »

17. Suppose that _{A}^{Z} X decays by natural radioactivity in two stages to  _{A-4}^{Z-1} Y. The two stages would most likely be which of the following?
                               First Stage                                         Second Stage
(A) \beta ^- emission with an antineutrino                \alpha emission
(B) \beta ^- emission                                               \alpha emission with a neutrino
(C) \beta ^- emission                                               \gamma emission
(D) Emission of a deuteron                                       Emission of two neutrons
(E) \alpha emission                                                  \gamma emission
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conantee
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« Reply #1 on: December 31, 2009, 09:14:43 AM »

In order to do the problem, one needs to understand the natural radioactive decays. The usual decays are following:
1. alpha decay: alpha particle is helium's nucleus (_{2}^{4} He^{2+}) and the reaction is
                                                         _{Z}^{A} X \to _{Z-2}^{A-4} Y + _{2}^{4} He^{2+}
2. beta negative becay: beta particle is actually electron from weak interaction and the reaction is
                                                         _{Z}^{A} X \to _{Z+1}^{A} Y + e^- +\bar{\nu _e} +\gamma
,where \bar{\nu _e} is anti-neutrino.
3. gamma decay: this process does not involve with the nuclear numbers. It justs the transition from excited state to ground state.
                                                         _{Z}^{A} X^* \to _{Z}^{A} X +\gamma

Using this information, we can track down mass number and atomic number of X in each choice. One can see that (C) is wrong since the mass number (A) does not change in the process, and (E) is wrong since the atomic number (Z) changes by 2, not 1 in the process.

Now, we have only (A), (B) and (D).
To make a guess, alpha emission with a neutrino, and emission of two neutrons are unlikely process since they do not even appear in regular texts.
Let's guess (A) for the answer.


The correct answer is (A), and 32 of 100 people did it correctly.
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