So we had this test in nuclears physics today, and part of one of the quesitons was the following:

When $\rm U^{235}_{92}$ nuclides are bombarded with one neutron, they can decompose to krypton $\rm Kr_{36}$, barium $\rm Ba^{141}_{56}$ and 3 more unknown particles. Given the above data, find the 3 unknown particles, write the reaction formula, and state the mass number of the krypton.

Here is what I did:

We know that the 3 particles are neutrons because of the charge conservation principle.

Thus the reaction formula is the following: $$\rm U^{235}_{92} + n^1_0\to Kr_{36}+Ba^{141}_{56}+3n^1_0$$

Now, due to the nucleon conservation law, we know that:

$$A_{\rm Kr}=235+1-141-3=92$$ And that finishes the question. All fine here, and the answer is correct.

However, assuming that we don't know the real number of nucleons in the krypton, we can think that the 3 unknown particles are photons. The reaction then will be:

$$\rm U^{235}_{92} + n^1_0\to Kr_{36}+Ba^{141}_{56}+3\gamma^0_0$$

Everything seems legit - the charge is conserved, and then the mass number of the krypton will be:

$$A_{\rm Kr}=235+1-141=95$$

Where is the problem in the second reaction? How can I know which one of them is correct in the next test? :)

Note that energy values are not given in the question, so we can not rely on them.


I forgot to mention that the 3 unknown particles are from the same type (i.e 3 photons, 3 neutrons 3 protons and so on)

  • 1
    $\begingroup$ I can't tell from your description what the author of your test had in mind here. According to the nuclear wallet cards, krypton-95 and krypton-92 are both produced in uranium fission. Since you took the exam today, the best thing for you to do is to follow up with your instructor next time. $\endgroup$ – rob Jun 30 '15 at 3:11

The test was not written by my teacher, so he had no answer and he did not notice it either. But after a while, we thought that photons are not realy elementry particles (or classical ones), like neutrons or protons. And the question states 3 more unknown particles. So maybe photons dont apply to that condition.

But this is not a real answer, since the quesiton of the particle nature of photons is not suited for high school students like me :). So I'm geussing the one who wrote the test didn't think about it at all.


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