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What makes a material fissile? 
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#1
Dec2109, 05:32 AM

PF Gold
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What are the reasons that fissile materials are fissile? What makes them fissile?



#2
Dec2109, 10:04 AM

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#3
Dec2209, 07:31 AM

PF Gold
P: 113

So the more unstable the nucleas, higher the chances that it will sustain fission?(I said fissile not fissionable)
And what do you mean by internal resonances? 


#4
Dec2209, 09:35 AM

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What makes a material fissile?
Fissile nuclei include those of U233, U235, Pu239, Pu241, Am242, Cm243 and other heavier nuclei (these generally tend to be odd A isotopes, and Am242 is an exception). These have the ability to fission with low energy neutrons, but there is also the probability that they will absorb a neutron and not fission. Example  http://hyperphysics.phyastr.gsu.edu...235chn.html#c1 Whatever happens, when fissile nuclei absorb a low energy neutron, it sets ups oscillations within the nucleus that causes two charge distributions to form such that they repel one another. These become the fission product nuclei. Two or three neutrons are released. 


#5
Dec2209, 11:05 AM

PF Gold
P: 113

Thank you. That really helped.



#6
Dec2209, 11:13 AM

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In answer to the more general question, this is mainly a classical physics thing. The liquid drop model of the nucleus says that the nucleus has a certain energy as a function of deformation. There are various ways of parametrizing nuclear deformation, but say we define some deformation parameter x. x=0 is a sphere, x=.4 or something might be an American football. For large values of x (say ~1 for most parametrizations) you get shapes that want to form a neck  in the sense that of all shapes with a specific amount of elongation, the one that minimizes the energy is one with a neck. Once the neck forms, you're basically on your way to fission. The main terms in the liquid drop model that determine the shape of the curve of E versus x are the surface energy and the Coulomb energy. The latter scales with Z^2, so it's much more important for heavy nuclei. For nuclei with Z greater than about 120 or something, there is no stable minimum at all, and that's why we don't ever expect to be able to form such nuclei, even artificially. The main quantummechanical ingredient you need is that there is a barrier in the potential energy curve, and the nucleus has to tunnel through this barrier in order to fission. This is why you get spontaneous fission. If you look at heavy nuclei in general, some tend to decay by spontaneous fission, some by alpha decay, and some do both, with some branching ratio. This difference is mainly quantummechanical in origin. The N and Z of the parent nucleus influence the detailed shape of the E(x) curve. In addition, the N's and Z's of the daughter nuclei determine their binding energies, and therefore influence the amount of phase space available for the decay. 


#7
Dec2209, 12:09 PM

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In fact, in the case of U235, roughly 5/6 (85%) absorptions of thermal neutrons cause fission, while ~1 in 6 (16%) result in the formation of U236, which spontaneously emits a gammaray. Similary, Pu239 can either fission, or form Pu240, which spontaneously emits a gammaray. Fissionable and fertile nuclides can fission, but require higher energy neutrons. U235, U236 and U238 all undergo alpha decay as well as spontaneous fission, but only U235 is fissile. The SF rates are 7.0E9 %, 9.4E8 % and 5.5E5 % of decays, respectively. Ref: http://www.nndc.bnl.gov/chart/reCenter.jsp?z=92&n=143 (hit zoom 1 to see detail) When I used the term resonance, I was meaning nuclear oscillations, which are not to be confused with neutron (absorption) resonances. The emission of extra neutrons ( 2 or 3) is what makes possible a fission chain reaction, provided that at least one of the two or three neutrons survives to cause a new fission. 


#8
Dec2209, 08:58 PM

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I wrote:



#9
Dec2309, 04:32 AM

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#10
Dec2309, 07:29 AM

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I was referring to oscillations in shape, or some distortion that allows two fission products form as opposed to gamma emission. When U235 absorbs a neutron, the resulting U236* has an ~85% change of fissioning, and a ~15% of gamma emission. Pu 239, 240 and 241 have spontaneous fission rates of 3E10%, 5.7E6 %, and 2.5E3%, respectively, but 240 is not fissile, whereas 239 and 241 are fissile. Evenso, there is a probability that Pu239 will form Pu240, which decays by gamma emission rather than fission following thermal neutron capture. http://www.nndc.bnl.gov/chart/reCenter.jsp?z=94&n=146 (use zoom 1) Fissile, fissionable and fertile isotopes do have potential for spontaneous fission, but the probability varies over orders of magnitude. 


#11
Dec2309, 10:44 AM

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#12
Dec2309, 08:00 PM

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Both U235 and U236 can also experience spontaneous fission, which are very low probability compared to alpha decay. See also a past discussion why is U235 used for chain reaction but not U238 http://www.physicsforums.com/showthread.php?t=115453 


#13
Dec2309, 09:44 PM

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I wrote:



#14
Dec2309, 09:48 PM

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What does one consider a 'resonance' or 'resonance effect'? 


#15
Dec2309, 10:18 PM

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I think resonance, in a reaction of this kind, would refer to a case where the incoming projectile (a neutron, say) has just the right energy to excite some specific, discrete state in the compound nucleus. 


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