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QUESTION: When neutron bombards lithium-7, it becomes tritium and helium with the emission of neutron. Can this neutron initiate chain reaction, causing the fission of lithium and thus release energy? If it can be done, what is it critical mass? If it cannot be done, why?

ANSWER: You mean Lithium 6, not 7.  And that does not release another neutron, unless you mix it with Be.  (Berylium), but that would only cause a neutron-releasing reaction once in about 100,000 times.  It can't be used to create a critical mass.  And 6Li is the rare isotope, it's very expensive because most of it goes to the bomb-making industry.

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QUESTION: Then what about the bombardment of beryllium-9 by neutron, causing it to break into two helium nucleus and two neutrons? This process also releases energy, so can chain reaction happens when the neutrons from continuously breaks apart beryllium-9, releasing energy as a result?

Answer
That's normal, but the cross section (i.e. probability) and energy release is too low to create a chain reaction like you'd see in a weapon or a reactor.  Especially the probability of neutron capture without an n-gamma reaction and/or (especially) neutron escape.  You don't understand the massive probability of neutron escape without (or even with) a proper moderator structure.  That's just crazy, to even quote a "critical mass" without a deuterium scattering moderator.

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Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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