p-11B: is it Fusion or Fission?

[BSFusion]

I’m curious, why is p-11B classified as aneutronic *fusion* instead of aneutronic *fission*?

The way I understand it, nuclear fission is a process that splits a nucleus into two or more smaller pieces, whrereas fusion joins nuclei together to form heavier nuclii. With that in mind, the p-11B reaction, in which two nuclii come together to produce three small alpha particles, should be classified as fission.

Please cast a vote.

[zapkitty]

BSFusion wrote: Please cast a vote.

The energy released in the pB11 process comes from fusion: the fusion of a hydrogen nucleus (a proton) with a boron-11 nucleus into a stable and quite non-radioactive carbon 12 nucleus.

But the C12 nucleus can’t contain the energy that resulted from its creation and it flies apart… but the [em]cause[/em] of breakup is not an unstable heavy nucleus decaying, as happens in fission.

The cause is instead an external energy input into the nucleus… even though that input came from the event that actually created the nucleus.

And thus even though it results in the C12 breaking apart and ending up with 3 helium nuclei (alpha particles)… the energy released in the pB11 process comes strictly from fusion.

[BSFusion]

The energy released in the pB11 process comes from fusion: the fusion of a hydrogen nucleus (a proton) with a boron-11 nucleus into a stable and quite non-radioactive carbon 12 nucleus.

Humm… it instantly decays – this stable nucleus??

OK, but isn’t it also true that energy is released according to Einstein’s E=mc2, with the final products (three alphas) containing less mass than the original reactants (p+B11), regardless of whether or not there was an intermediate C12* formed?

But the C12 nucleus can’t contain the energy that resulted from its creation and it flies apart… but the [em]cause[/em] of breakup is not an unstable heavy nucleus decaying, as happens in fission.

So, are you saying, fission can only be thought of as a process of spontaneous radioactive decay, not induced through neutron bombardment?

… the energy released in the pB11 process comes strictly from fusion.

A less strict interpretation would be that energy comes from the change in mass between product nuclii and reactant nuclii.
Maybe a third option, “aneutronic spallation,” should be added to the poll. What do you think?
Anyway, this is only an opinion poll, nothing serious, but please remember to cast a vote.

Thanks

[Ivy Matt]

It’s a nuclear reaction. 😛

[asymmetric_implosion]

Zap: Your definition seems pretty arbitrary. U-236 is stable in a small fraction of events when a gamma is emitted. If I understand the reaction correctly, C-12 does the same thing in a small fraction of events from p+B-11. The C-12 does not break apart due strictly to energetics. Conservation of momentum is the reason. Can’t conserve momentum in the reaction without a minimum of two particles ejected. If we could avoid two particles, D+D-> He-4 would work and our cold fusion friends would be very happy. 🙂

The definition I’ve run across (Mayo, Intro to Nuclear Concepts for Engineers) is about producing a heavier particle than the initial pieces with a smaller partner (Again momentum conservation). By this definition, p+B-11 is fission. The two products are an alpha and Be-8 which quickly decays by two alpha emission. Fission does not require neutrons. Proton fission is a known quantity. It is terribly inefficient but it does exist with high Z elements. One can choose the banner to carry by shaping the definition for their purpose. I have no problem with fission so I don’t mind calling it fission. I know many on this site don’t share my feelings so they unite under the fusion banner. I find it funny that the unknown of fusion energy is more appealing to some than the known of fission. Fission is far from perfect but we know the problems. Fusion energy is still a mystery and the unknown unknowns are still many.

In the end it doesn’t matter; a nuclear reaction without neutrons is still sweet.

[mchargue]

So far as I know, fusion is defined by: (the mass of the reaction products) < (the mass of the mass of the reactants) The difference is released as energy.

[asymmetric_implosion]

mchargue wrote: So far as I know, fusion is defined by: (the mass of the reaction products) < (the mass of the mass of the reactants) The difference is released as energy.

Total mass is always lost in an exothermic nuclear reaction. Fission meets the same conditions. Fission and fusion are subclasses of exothermic reactions. My opinion is the names came about to satisfy someone with the desire to classify things until only a few were in each category. The holy grail of nuclear energy is an exothermic nuclear reaction that does not produce neutrons but can produce a controllable chain reaction. p+11B meets the neutron condition. We shall see if it can meet the chain reaction condition.

When I was referring to mass, I should have said atomic number (Z). In my experience, fusion usually means one product has a higher Z than either one of the reactants. Fission leads to products that are both lighter (smaller Z) than the heaviest reactant.

[dennisp]

Yes, carbon 12 is stable. It’s the same isotope as most of the carbon in your body. You wouldn’t say a brick is unstable because you blows up when you pack it with C4 and detonate it.

Fusing a proton with Boron-11 is equivalent to detonating C4 in a brick. It’s not the breaking of the brick that causes the energy release. The energy release causes the breakage.

Energy releases when you fuse nuclei lighter than iron, or when you fission nuclei heavier than iron.

Therefore I’ll vote “fusion.”

[Lerner]

I thik to the vast majority of nuclear physicists, fission is a process produced by the interaction of neutrons–neutral particles–and nuclei, while fusion is a process produced by the interaction of charged nuceli with each other. The names, like many names in physics and elsewhere, have historical origns, but do not show thier actual meaning on the surface. The techncial meaning of “chaos” is another example, since motions that a physicist would term “chaotic” are not random or thermal as a non-technical meaning of chaos implies.
Since pB11 is produced buy the interactions of charged nuclei, everyone would call it fusion, even though the final nuclei are smaller than one of the initial nuclei.

[asymmetric_implosion]

Dennisp: I don’t agree with you analogy. Carbon-12 has many possible exit channels to release its energy such as re-emitting the proton, emitting a gamma or falling apart. The brick does not have the same option. There are reasons the carbon-12 is not stable and energy plays a role as I said above. The ability to conserve momentum is the reason. Energy and momentum are linked quantities. If you look at the fission of U-235 by neutrons it produces the largely stable U-236 after absorbing the neutron. The time of stability is 12C->4He+8Be; 8Be->2*4He. p+11B is a bit unique in that it takes advantage of the stability of He-4 for it’s location on the periodic table. He-4 has a binding energy per nucleon of 7 MeV/nucleon. Fe-56 has binding energy per nucleon of 8.7 MeV/nucleon (near top of the charts). 11B is 6.9 MeV/nucleon. The general trend is binding energy per nucleon increases with Z until you reach Fe. After Fe, binding energy per nucleon decreases leading to less stable nuclei. p+11B moves toward stability by reducing product atomic number rather than increasing it. Why work harder when you can work smarter? Why gain atomic number per product when losing atomic number per product is more favorable?

Look at the reactions below. Which ones are the most alike?

D+T->5He->4He+n
D+D->4He->p+T
n+235U->236U->Xn+products (Too many to list, X~2.3)
p+11B->12C->4He+8Be->3*4He

[dennisp]

I was going to ask you about your previous post mentioning momentum…if there’s a simple explanation, why does that force the nucleus to split? Why couldn’t the proton just stick to the boron and give it a push, like a bullet hitting a block of clay?

I understand there will be energy that has to go somewhere…the clay heats, but the carbon could, as you say, emit a gamma.

(As I’m sure you can tell, my physics training only extends to a couple semesters.)

[asymmetric_implosion]

dennisp wrote: I was going to ask you about your previous post mentioning momentum…if there’s a simple explanation, why does that force the nucleus to split? Why couldn’t the proton just stick to the boron and give it a push, like a bullet hitting a block of clay?

I understand there will be energy that has to go somewhere…the clay heats, but the carbon could, as you say, emit a gamma.

(As I’m sure you can tell, my physics training only extends to a couple semesters.)

While the gamma can carry momentum to balance the reaction, imagine the state of the 12C nucleus after absorbing the proton. The five protons and six neutrons were dancing around in their quantum mechanical happy place. Balance between the electromagnetic force and strong nuclear force is achieved. Suddenly, some jerk from out town shows up. He has both strong nuclear attributes (attractive over short distances) and electromagnetic attributes (repulsive over long distances) and he’s not afraid to show it. By the time the proton gets close enough for the strong nuclear force to gab onto everyone the electromagnetic force is pushing on the other protons. The neutrons have to stay near the protons to maintain balance so they start to move as well. Like a desperate person falling, the out-of-towner grabs what he can. For some reason, grabbing another proton and two neutrons is the most favorable. The remaining nucleons suddenly find their peace disrupted. Factions grow up in the 8Be nucleus and talks break down. It is far easier to divorce than stay together. Every now and then, the protons course is such that it can grab onto everyone or enough neighbors to stick. At that point, a photon is released to conserve momentum and energy.

If you imagine the case of a neutron going into the nucleus, it has only strong nuclear properties. It can cozy up to the other nucleons. Trying to find its place in the group it fights the other nucleons. Depending on the instability in the nucleus to start with, it can lead to some particles expelled (host of neutron reactions), an invitation to stay (neutron comes and photon goes) or the Jerry Springer show breaks out (fission). Protons and photons can do the same thing but the energy requirements tend to be much larger. Photons of sufficient energy are rare. Protons are charged particles and they are repelled by other charged particles in the nucleus.

Protons reacting with nucleons are throwing a ping-pong ball into the wind. You cannot say the ping-pong ball collided with the wind in the ball hitting the wall sense. Instead, you say the ping-pong ball was continuously deflected. Take a golf ball and throw it into the wind. You won’t notice much change relatively speaking.

[Ivy Matt]

On a historical note, the term “splitting the atom” originally referred to some of the better-known aneutronic nuclear reactions. “Fusion” originally referred to the D+D nuclear reaction.

[BSFusion2]

The following footnote, which deals with the p + Li7 → 2α + energy reaction, was taken from Ken Ford’s new (2015) book, titled “Building the H-Bomb. His book can be downloaded from http://www.worldscientific.com.

*This reaction can be viewed as a kind of fission process. The combination of a proton and a Li7 nucleus creates, momentarily, a beryllium-8 nucleus (containing four protons and four neutrons), which then splits into two alpha particles, each with two protons and two neutrons. This fission process is an exception to the rule explained later in this chapter that fission releases energy for heavy nuclei, whereas fusion releases energy for light nuclei. That rule applies only for stable or long-lived nuclei. The highly unstable, and very short-lived Be8 nucleus does release energy when it undergoes fission.

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