AUSTRALIAN FRONTIERS OF SCIENCE, 2008

The Shine Dome, Canberra, 21-22 February

Session 3: Discussion

Question: Nanda, does your research on quantum effects in fusion have any application for commercial fusion? Is there something that you should be telling the people building the ITER reactor in France?

Mahananda Dasgupta: No, because when we go to commercial fusion or power generation, it basically involves very light nuclei, basically protons, and that part is, we think, very well understood. The challenge there, of course, is controlling it and containing it.

The people who are interested in the results are astrophysicists, because they extrapolate down to very low energies where fusion occurs purely by quantum tunnelling. The kinds of effects we are seeing imply a reduction in the probability of tunnelling, and so they might have to rethink their models. So it is going back to element formation, if you like. But no, not in power generation.

Question: When you are building these superheavy elements, is it possible one day that you will build one which will be stable – that is, theoretically could there be the possibility of attaining stability again?

Mahananda Dasgupta: That is a very nice question. The implication is that as we are going towards the Island of Stability, so-called, the lifetimes are on the increase. Maybe we will go to days, maybe months. I do not think they will go to millions of years, because then we would have found them on Earth. You can find articles which say that if you find superheavy elements in nature, then we can explore them for energy generation – which I do not quite understand, but never mind!

I think we won't get to the stage where they live for years, but they will be shorter-lived. But, having said that, I need to say that the parameters for all these calculations which are predicting these superheavy elements have been tweaked, based on the nuclei which are known. Beyond that, we do not know. So we do not think so, given our experience, but we can always be pleasantly surprised.

Question: I almost had the same question, but I think your answer pretty much told me what I need to know. I think you were saying, though, that if these elements do exist, then a supernova would have generated them already. There is no way you can do something in a lab that a supernova couldn't do. Is that right?

Mahananda Dasgupta: No, what I am saying is that if they were generated in a supernova – indeed, there have been searches for superheavy elements in meteorites, for example – because they are short-lived they would have died away. In the lab we can make them and they live for seconds, let's say, or milliseconds, and that allows people to do the chemistry. That's another interesting aspect: the chemistry of one atom. An interesting concept.

So we can make them in the lab, and the advantage of making them in the lab is that we can study them, whereas when they were made in a supernova, then being short-lived they are not accessible for further investigations.

David Jamieson: I would like to inject a question from the chair, if I may. I would like to ask Toshi on the previous question about making atoms. I think you mentioned that one of the assumptions of your methods was that the cosmic production rate of the rare elements was constant, and so you could measure the accumulation as a measure of the age. But how do you know that the atoms are being made at a constant rate?

Toshiyuki Fujioka: To be honest, it is a bit of an approximation. Actually, that production rate depends on the flux of the cosmic rays, and cosmic ray flux does fluctuate through time. This fluctuation depends on what kind of time scale you are looking at. So if you are looking at just, maybe, 100 years or 70 years, maybe fluctuation is just a few per cent or less. But if you are looking at, say 1,000,000 years, the fluctuation of cosmic ray flux may be significant and should be considered for age estimation. But at the moment, at this stage, we don't have much knowledge about cosmic ray fluctuation over about 800,000 years. Up to 800,000 years, there is some calibration of how the cosmic ray flux has fluctuated. Then, considering that fluctuation we can also calculate the uncertainties of production rate.

Question: Does the variation in cosmic ray flux dominate your errors, or there is another term that dominates the errors in your dating?

Toshiyuki Fujioka: Well, I will tell you that the error from the fluctuation of cosmic ray flux is the most significant one. But, everyone in this field knows that production rates needs to be further investigated in terms of time and spatial variations, so often we exclude these uncertainties from the final age estimate at this stage. This is quite enough if you look at relative sequences of the events of your interest. Also, it may not matter if the time-scale is short, say less than 500 ka, because as I said, we have some calibration up to 800 ka now. The uncertainties of cosmic-ray flux variation and thus production rate will definitely be a significant concern in the future of the method, when we start talking absolute ages with very small analytical uncertainties, as we do now in radiocarbon dating, and longer time-scales, say >1 Ma.

David Jamieson: What Toshi is really saying is that we need the astronomers to understand the local environment better, to help us with all these things.

Question: Nanda, early in your talk you gave a plot of the form of the potential as a function of distance from the nucleus, which was the addition of the electrostatic force and the strong nuclear force. My question is: what is the smallest size scales on which the 1/r2 form of the electrostatic force has been verified, and with what accuracy can we really tell if it is a straight, sharp cut-off in the effect of the strong nuclear force?

Mahananda Dasgupta: People have been looking at deviations from the 1/r2 law, and I couldn't tell you right now the reference for it. It is at much smaller scales than we are talking about. We are still talking about 10 fermi or something like that. We have found that if we consider uniformly charged spheres when penetrating inside, then it is sufficient. Coulomb's law and its electroweak generalisation has been tested for separations down to 10-18 m in e+e- leptonic interactions at high energy colliders.

The main problem comes in the nuclear force, in the sense that the Coulomb force at that level we assume to be – and we know it to be – well known. The nuclear force is, on the other hand, the biggest uncertainty. That is what is stopping us from doing fundamental calculations, if you like, and that is what actually still stops us from knowing what the barrier height is exactly until we do the measurement.

Once we have done the measurement, then we know the barrier height, and in some respects it doesn't matter what the exact components were because I know the barrier height.

Well inside the barrier, we do not know about the Coulomb force or the nuclear force, but in a way it doesn't matter, because everything has come together and it has gone to an irreversible process well inside, and we are not seeing any evidence of that well outside. But the touching part is what I am talking about.

Question: Nanda, I was wondering how your data allows you to differentiate between quantum superposition and simply having a population where you have some nuclei in the ground state and some in the excited states.

Mahananda Dasgupta: That is what initially people thought, that if we say, 'Oh, this nucleus is in an excited state,' or, 'a ground state,' and we do the calculations then, it will give us the right result. Then one of the very nice pieces of evidence was that if the nucleus is excited such that it costs energy – for example, it has gone to an excited state – then the kinetic energy is, effectively, lowered, classically speaking. So the fusion cross-section should come down. But actually that is not what happens: it goes up. And that is what told us that it is not like, 'The nucleus is in this state, or that state, or that state,' but only if you consider superposition will you get that increased fusion cross-section. That was the critical piece of evidence, if you like.

Question: I still have a question about the potential profile that you have. You have a weak nuclear force and you have Coulombic interactions. It is very similar to the Lennard-Jones potential that you have in the colloidal domain. My question is this: is it these nuclear forces that you find with the [inaudible] attractions that in colloids science been treated as phenomenological quantities?

Mahananda Dasgupta: Yes. The nuclear force is treated as a phenomenological quantity, and that comes back to the question I was answering earlier. Because it is phenomenological, we do not know a priori. So yes, it is phenomenological.

It is interesting that you are talking about this in connection with metals, because there have been theorists who were working in nuclear physics, and researchers working in nuclear physics, who apply the same sort of physics in metallic clusters. Maybe it is that relationship, I don't know.