Probability of gamma-emitting branch of p+11B...

93143 · Post by **93143** » Sun Dec 06, 2009 8:53 pm

Wikipedia's aneutronic fusion page states that about one in ten thousand p+11B fusions will result in gamma radiation rather than alpha. Energies mentioned are 4, 12, and 16 MeV.

Does anyone have more details on this?

Most importantly, does anyone know whether the gamma emission depends on collision energy? For instance, could a well-focused annealing wiffleball operating at the 50 kV resonance peak with a narrow quasi-beamlike bimodal velocity distribution have a substantially reduced proportion of gamma-branch fusions? Or do we need a foot of lead between the core and any nearby workers no matter what?

Art Carlson · Post by **Art Carlson** » Sun Dec 06, 2009 9:17 pm

Nuclear physics is not my strong suit, but I believe the first step in the p-B11 fusion process is always the formation of C12*. When it's got several MeV of energy to get rid of, it won't care whether it has a tenth of an MeV more or less from the energy of the reactants. The branching ratio is probably something basic like the square of the fine structure constant. I think even if you could tune the ion energies precisely you would still have to live with the gammas. The good news is that, as far as I know, it shouldn't be too hard to do.

93143 · Post by **93143** » Sun Dec 06, 2009 9:36 pm

Thanks for the quick reply!

Art Carlson wrote:I believe the first step in the p-B11 fusion process is always the formation of C12*.

That much I knew...

I think even if you could tune the ion energies precisely you would still have to live with the gammas.

Dammit.

The good news is that, as far as I know, it shouldn't be too hard to do.

Not for a ground-based power station, no...

chrismb · Post by **chrismb** » Sun Dec 06, 2009 9:44 pm

I have previously pondered on this potential myself, but no such reaction appears on NNDC database, which tends to support my doubt that such a reaction occurs/is as common as this. I would presume it is an erroneous wiki-edit by someone that has made a guess at it, until such time as I see a reference otherwise. I don't understand why it suggests you'd get different gamma strengths out of the same reaction. Doesn't seem to add up.

The isotopic mass differences are 11B+p=11.0093054u + 1.00782503207u and 12C is 12u, so the difference is 0.01713u (at 1u=931.494 MeV/c^2) which would be your 16MeV. I can't really see a 12C holding together with that much energy as the emission of an alpha from a 12C is endothermic at 7.5MeV, so I don't see why a 12C would stay together with 16MeV in it.

Art Carlson · Post by **Art Carlson** » Sun Dec 06, 2009 10:42 pm

chrismb wrote:I have previously pondered on this potential myself, but no such reaction appears on NNDC database, which tends to support my doubt that such a reaction occurs/is as common as this. I would presume it is an erroneous wiki-edit by someone that has made a guess at it, until such time as I see a reference otherwise. I don't understand why it suggests you'd get different gamma strengths out of the same reaction. Doesn't seem to add up.

The isotopic mass differences are 11B+p=11.0093054u + 1.00782503207u and 12C is 12u, so the difference is 0.01713u (at 1u=931.494 MeV/c^2) which would be your 16MeV. I can't really see a 12C holding together with that much energy as the emission of an alpha from a 12C is endothermic at 7.5MeV, so I don't see why a 12C would stay together with 16MeV in it.

Looking at the history of the wiki article it appears I put that statement in myself when I created the article. I'm pretty sure it came from

W. Kernbichler, R. Feldbacher, M. Heindler. "Parametric Analysis of p-11B as Advanced Reactor Fuel" in Plasma Physics and Controlled Nuclear Fusion Research (Proc. 10th Int. Conf., London, 1984) IAEA-CN-44/I-I-6. Vol. 3 (IAEA, Vienna, 1987).

My comments on this paper as documented in http://en.wikipedia.org/wiki/Talk:Nucle ... B11_fusion are as follows:

This paper is somewhat hard to understand but is probably the latest and best reference for the power balance of p-B11. Includes effects of non-Maxwellian ions and nuclear elastic scattering (NES) recoils. Concerning Q: "However, there is little reason to believe that the fuel has the potential to reach reasonable Q-values, regardless of the quality of the confinement. ... Even for the 'reference' parameter set - which defines the upper bound of the fuel performance with respect to confinement and impurity parameters - the maximum value of Q was found to be as low as 1.89 at Ti=320keV; moderate synchrotron losses shift the maximum to Q_p=1.68 at 250 keV, as compared to the breakeven requirement of Q_be = 3.6." To understand exactly what they are saying requires some close study of their paper, but we call attention to their pessimistic tone and to the fact that their definition of Q_p is the normal definition plus one. Concerning radioactivity: "The neutron production occurs mainly through 11B(fast-alpha,n)14N." Also mentioned are the 11B(p,gamma)12C, 11B(fast-alpha,p)14C, and 11B(p,n)11C reactions.

I suspect there is an excited level at 4 MeV (or possibly 12 MeV) so that the excited nucleus sometimes decays in two steps. Does anyone have time to dig out the paper?

93143 · Post by **93143** » Sun Dec 06, 2009 10:58 pm

I can't find it on Google Scholar, and I haven't remembered to get a proxy set up through the university library, so I'll have to wait until I can get to the lab. I'm feeling a bit ill, so that may or may not be tomorrow...

rcain · Post by **rcain** » Mon Dec 07, 2009 2:58 pm

Art Carlson wrote:...W. Kernbichler, R. Feldbacher, M. Heindler. "Parametric Analysis of p-11B as Advanced Reactor Fuel"

Hi Art, theres much mention there of impracticaly low Q values for pB11. Can I ask, what are the theoretical reasons behind this?

Art Carlson · Post by **Art Carlson** » Mon Dec 07, 2009 5:13 pm

rcain wrote:
Art Carlson wrote:...W. Kernbichler, R. Feldbacher, M. Heindler. "Parametric Analysis of p-11B as Advanced Reactor Fuel"
Hi Art, theres much mention there of impracticaly low Q values for pB11. Can I ask, what are the theoretical reasons behind this?

I haven't looked at this paper for many years. Maybe I'll get a chance later this week. Considering the comment about "regardless of the quality of the confinement" and the mention of synchrotron losses making things worse, they can really only be talking about the classic problem with p-B11 - bremsstrahlung. In this forum it should be of interest that they included effects of non-Maxwellian ions, although I don't remember in just what way.

93143 · Post by **93143** » Mon Dec 07, 2009 6:26 pm

Well, I can't find that paper online. The university library has a hard copy, but it's downtown and could be a pain to get. Does anyone have a link?

TallDave · Post by **TallDave** » Thu Dec 10, 2009 7:20 pm

I searched around and couldn't find it either.

If some kind soul should scan and post, I will award them 100 internets.

Axil · Post by **Axil** » Thu Dec 10, 2009 8:26 pm

Reference:

http://www-ferp.ucsd.edu/PUBLIC/SNOWMASS/Energy-A.pdf

The p11B fuel cycle avoids exotic isotopes, so that no breeding of fuel is required and the potential fuel supply is essentially unlimited. It is also nearly aneutronic, thus addressing materials damage and much of the materials activation concern. However, there are residual activation issues associated with high energy y-rays produced via the reaction 11B(p,y)12C, and with neutron production from the reactions 11B(y,n)14N and 11B(p,n)11C; and safety concerns associated with possible equilibrium inventories of
MCi/GW of 11C. More fundamentally, there is the problem that the p-11B fusion reactivity appears to be too low to compete with bremsstrahlung radiation losses, so that ignition (or even high fusion gain) cannot be achieved with this fuel.

MSimon · Post by **MSimon** » Thu Dec 10, 2009 9:05 pm

Axil wrote:Reference:

http://www-ferp.ucsd.edu/PUBLIC/SNOWMASS/Energy-A.pdf

The p11B fuel cycle avoids exotic isotopes, so that no breeding of fuel is required and the potential fuel supply is essentially unlimited. It is also nearly aneutronic, thus addressing materials damage and much of the materials activation concern. However, there are residual activation issues associated with high energy y-rays produced via the reaction 11B(p,y)12C, and with neutron production from the reactions 11B(y,n)14N and 11B(p,n)11C; and safety concerns associated with possible equilibrium inventories of
MCi/GW of 11C. More fundamentally, there is the problem that the p-11B fusion reactivity appears to be too low to compete with bremsstrahlung radiation losses, so that ignition (or even high fusion gain) cannot be achieved with this fuel.

Is that true at all drive voltages? There is a nice resonance peak at 50 to 65KV drive (depending on well depth) that should change the loss/gain ratio.

I have a few ideas on how to control such a contraption. Pilot frequencies and phase detectors. I'm keeping my hand in by designing a HF transceiver in my spare time. It is pretty slick.

So the question is: how much difference does drive make on Brems?

rcain · Post by **rcain** » Thu Dec 10, 2009 9:35 pm

MSimon wrote:...There is a nice resonance peak at 50 to 65KV drive (depending on well depth) that should change the loss/gain ratio.
...
So the question is: how much difference does drive make on Brems?

just found some info on it here - http://fds.oup.com/www.oup.co.uk/pdf/0-19-856264-0.pdf (p13) - seems relevant:

For this {'Advanced Fuels'}group of reactions the Gamow energy is higher than for the previous group, leading to smaller cross-sections at relatively low energy. At high energy the cross sections are intermediate between that of DD and that of DT.
The proton–boron reaction
p + 11B → 3α + 8.6 MeV, 1.43
is particularly interesting, because it does not involve any radioactive
fuel, and only releases charged particles. Its cross section exhibits
a very narrow resonance at = 148 keV, where the S factor peaks
at 3500 MeV·barn and a broader resonance at = 580 keV, where
S ≈ 380 MeV·barn

... still trying to hunt down some stuff on Brem.