Talk:Neutron moderator

From Wikipedia, the free encyclopedia

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
???	This article has not yet received a rating on the assessment scale. [FAQ]
???	This article has not yet received an importance rating within physics.
Help with this template Please rate this article, and then leave comments to explain the ratings and/or to identify its strengths and weaknesses.

[edit] Why are fast neutrons LESS likely to cause fission?

From the article:

"Faster neutrons are much less likely to cause further fission....The newly-released fast neutrons, moving at roughly 10% of the speed of light, must be slowed down...if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction."

That seems counterintuitive. A nonexpert like me would think that a faster-moving object (neutron, whatever) would be MORE likely to cause its target to break apart (i.e., fission) on impact. Can someone explain this? 24.6.66.193 18:08, 7 May 2007 (UTC)

It is counterintuitive, but only because we tend to assume that a faster-moving object has as much chance of hitting a given target as a slow-moving object, and therefore the only difference is the greater energy delivered by the faster object. This is certainly true if you're talking about, say, a gun pointing at a stationary target, and two types of projectiles (slow and fast).

Now, however, put yourself in the target's shoes and think about which is easier to catch - a baseball at 95 mph, or a baseball lobbed at slow speed. The slower ball is easier to catch basicallyl because it spends more time in the vicinity of your hand, giving your brain more time to effect the correct muscle movements to catch the ball. (And even if the fast ball hits your hand it might just sail on through since it would hurt too much to hang on to it.)

Similarly, a slow neutron that spends more time in the vicinity of a nucleus is more likely to be caught by that nucleus. If it can do this and still have enough energy to induce the nucleus to fission, that's a win-win situation. This is the case with U-235. Whitlock 23:01, 7 May 2007 (UTC)

To my understanding, Enrico Fermi found that slower neutrons were far more effective in causing fission, because what happens is that the neucleus "catches" a nuetron that gets added to its neucleus. This stretches out the neucleus which weakens the force that keeps the neucleus together. Remember that protons repel each other due to sharing positive charges, but neucleur forces are greater than the forces that repel them. Now given an already unstable neucleus, by adding the additional nuetron weakens the neuclear forces enough that the forces that repel protons becomes greater and causes the breakdown of the nucleus. This is called fission. —Preceding unsigned comment added by 76.89.132.250 (talk) 11:15, 27 February 2008 (UTC)