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PrincePhoenix
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What are the reasons that fissile materials are fissile? What makes them fissile?
The particular nucleus is relatively highly unstable and internal resonances are such that two mass distributions form such that the coulomb force/pressure overcomes the nuclear force binding them.PrincePhoenix said:What are the reasons that fissile materials are fissile? What makes them fissile?
It's a particular type of instability. Some unstable nuclei decay by beta emission or alpha emission, while others decay by positron emission.PrincePhoenix said: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?
PrincePhoenix said:What are the reasons that fissile materials are fissile? What makes them fissile?
Actually, no. I was trying to explain why some isotopes are fissile (readily fission), while others are fissionable or fertile.bcrowell said:I think Astronuc's answer involving resonances is about something more specialized, which is the question of why some substances are able to sustain a nuclear chain reaction. The OP's question was more general: why are some materials fissile?
I think Astronuc's answer involving resonances is about something more specialized, which is the question of why some substances are able to sustain a nuclear chain reaction. The OP's question was more general: why are some materials fissile?
Astronuc said:Actually, no. I was trying to explain why some isotopes are fissile (readily fission), while others are fissionable or fertile.
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When I used the term resonance, I was meaning nuclear oscillations, which are not to be confused with neutron (absorption) resonances.
Yes neutron induced fission was intended. I'm sorry I didn't make myself clear. We haven't been taught about spontaneous fission yet.bcrowell said:I wrote:
Are you talking about (1) spontaneous fission, or (2) neutron-induced fission? I read the initial post as being a question about #1, although it's possible that #2 was intended.
The term fissile infers that an isotope is readily fissioned with low energy (thermal) neutrons - neutron induced fission. Fissile also means that a particular nucleus forms an excited nucleus that is closer to the threshold for fission as opposed to just decaying by gamma emission.bcrowell said:I wrote:
Are you talking about (1) spontaneous fission, or (2) neutron-induced fission? I read the initial post as being a question about #1, although it's possible that #2 was intended. In order to get #2, you have to have #1 first, although the branching ratio, as you point out, may be very small. When you refer to "nuclear oscillations," are you talking about shape oscillations? If so, then I'm pretty sure resonances are not at all relevant to spontaneous fission. If you're talking about resonances of shape oscillations, maybe you could clarify what you're referring to; are you talking about a potential with two different minima, as in the fission isomers? When 235U absorbs a neutron, the compound nucleus is above the fission barrier ( http://www.nature.com/nature/journal/v409/n6822/full/409785a0.html ), so I don't see how a resonance of shape oscillations could be relevant there. Figures 4 and 5 of the Nature paper give some examples of potential energy landscapes.
Astronuc said:I was referring to oscillations in shape, or some distortion that allows two fission products form as opposed to gamma emission.
bcrowell said:Could you respond to the other questions in my post about resonance?
But I'm not satsified with that discussion. I'd like to find the papers on nuclear structure, particularly U-235 and U-236. Once U-235 absorbs a thermal neutron and forms an excited nucleus, it may fission (asymmetrically) or it may decay by gamma emission. The fission or gamma emission is determined by the nuclear shape - going from an oblate spheroid (by the models in the Nature paper). The gamma emission allows the nucleus to fall into a more stable shape or energy state - whereas if the nucleus assumes a more elongated necked shape - it will fission. Fission of U-235 and Pu-239 are almost always asymmetric, but there is a small probability of symmetric fissioning - one the order of 0.01% of fissions. Ref: S. Katcoff, "Fission-Product Yields from Neutron Induced Fission," Nucleonics 18, 11, 201 (Nov 1960).7. The liquid drop model can be used to provide a qualitative Nuclear Fission: explanation of fission. Consider a large nucleus that has at least 200 nucleons. All nucleons are being acted upon to some degree by their nearest neighbors because all are within the potential well. That is, all are within range of the nuclear force. The degree of attraction is not the same between all pairs of nucleons because of the saturation effect. The nucleons are all moving and forces average out so, over time, nucleons in a similar location (interior or surface) experience the same net force. For those in the interior, individual forces cancel with the net force being zero. For those on the surface, there are no nucleons on the “outside” to balance those in the “inside.” So, the surface nuclei are subject to a net force that pulls them inward. On average, the shape of the nucleus is therefore spherical. However, that is only the average. The nucleus, like a drop of liquid, oscillates, and forms an ellipsoid. If the oscillations grow, the ellipsoid splits in two. This splitting in two is the fission process. The probability of fission increases if energy is added from some exterior source. If the added energy exceeds a certain “critical” value, the likelihood of fission is very great. For nuclides below thorium, the critical energy is quite large (i.e., for Pb-208, its 20 MeV). For nuclides above Thorium, it is much lower, 4-6 MeV.
Could you respond to the other questions in my post about resonance?
Astronuc said:An explanation is here -http://ocw.mit.edu/NR/rdonlyres/Nuclear-Engineering/22-05Fall-2006/272838A6-E400-4297-BCF0-7E604236F310/0/lecture05.pdf
I'm using the term 'resonance' to refer to 'oscillations' with respect to "The nucleus, like a drop of liquid, oscillates, and forms an ellipsoid. If the oscillations grow, the ellipsoid splits in two. This splitting in two is the fission process." This is what I learned 30 years ago. I believe it was an effort to explain U-235 + n => U236* => fission or U236 + γ (no fission). Ostensibly the shape U236 after γ-emission is quite different than U236* which fissions.bcrowell said:The word "resonance" never occurs in that lecture. As far as I can tell at this point, you're just mistaken about resonances involving shape oscillations having anything to do with the general phenomena of spontaneous and neutron-induced fission. There may be such an effect in certain special cases, as in the isotopes that have a pronounced second minimum in the potential (isotopes that display fission isomers). As I pointed out in post # 8, the compound nucleus in 235U+n is always above the height of the barrier, so I don't see how you can get any resonance effect there. My field is low-energy nuclear structure, not nuclear reactions or nuclear engineering, so it's possible that I'm wrong about this. If so, then I would be interested to learn more. But so far it seems to me that you are incorrect on this point.
Astronuc said:I'm using the term 'resonance' to refer to 'oscillations' with respect to "The nucleus, like a drop of liquid, oscillates, and forms an ellipsoid. If the oscillations grow, the ellipsoid splits in two. This splitting in two is the fission process." This is what I learned 30 years ago. I believe it was an effort to explain U-235 + n => U236* => fission or U236 + γ (no fission). [...]
What does one consider a 'resonance' or 'resonance effect'?
I'm not clear on what you mean by this. The 236U* has an excitation energy that's higher than the barrier to fission. That means that it can't be modeled in terms of fluctuations about some equilibrium shape, and I don't think it makes any sense to talk about its shape. Re the shape of the 236U after gamma emission, are you thinking of a superdeformed fission isomer? I don't know whether 236U has a superdeformed second minimum or not. Some heavy nuclei do; they were the first known examples of superdeformation.Astronuc said:Ostensibly the shape U236 after γ-emission is quite different than U236* which fissions.
Fissile refers to a material that is capable of sustaining a nuclear chain reaction.
A material is considered fissile if it contains isotopes with a high neutron absorption cross section and can be easily split by neutrons, releasing energy.
The most commonly known fissile elements are uranium-233, uranium-235, and plutonium-239.
Fissile materials are capable of sustaining a nuclear chain reaction on their own, while fissionable materials require a neutron moderator to sustain the reaction.
No, there are naturally occurring fissile materials such as uranium-235, but most fissile materials used in nuclear reactors are man-made by irradiating non-fissile materials in a reactor.