THE BERYLLIUM BOTTLENECK IN THE MAKING OF HEAVY ELEMENTS

Quoting from John Gribbin and Martin Rees's COSMIC COINCIDENCES. "The only place that heavier elements could have been manufactured, and could still be manufactured today, is inside stars. But how do stars do the trick?

THE BERYLLIUM BOTTLENECK

"Two alpha particles that collide with each other with the right energy (enough to overcome the electrical propulsion produced by the positively charged protons they each carry) will stick together to form a nucleus of beryllium-8. Unfortunately, however, beryllium-8 is the exception to the rule that nuclei containing whole numbers of alpha particles are stable. It is spectacularly unstable, and breaks apart into lighter particles within a lifetime of only 10^-17 seconds. So how can carbon, which requires the addition of another alpha particle to a beryllium-8 nucleus, ever be built up?

"Maybe, some theorists speculated, carbon-12 could be made directly inside stars, when THREE helium-4 nuclei just happened to collide with one another simultaneously. But a simple calculation soon showed that this is indeed as unlikely a prospect as it sounds. It might happen occasionally, but not often enough to produce all the carbon we see around us, the key element in the chemistry of living things.

"In 1952, Ed Salpeter, an American astrophysicist, suggested (more or less in desperation) that carbon-12 might be produced by a very rapid two-step process, with two alpha particles coliding to form a nucleus of beryllium-8, which was then in turn hit by a third alpha particle in the 10^-17 (10 to the negative seventeenth power) seconds before it had time to disintegrate. Since this did at least give 10^-17 seconds for the third particle to arrive, instead of requiring three to meet simultaneously, it was an improvement of the triple-collision idea. But since the arrival of a third particle might very effectively smash the unstable beryllium-8 nucleus to bits, it wasn't much of an improvement. Then Fred Hoyle, who had, back in 1946, written a classic paper expounding the idea that the chemical elements were made in stars, entered the story.

HOYLE'S ANTHROPIC INSIGHT

Hoyle (now Sir Fred) was based in Cambridge, England, but in the 1950s spent time in California, working with his friend, nuclear physicist Willy Fowler. Holyle puzzled over the problem of how heavy nuclei might be built up in stars (stellar nucleosynthesis) and became intrigued by the possibility that the energy levels of beryllium, helium, and carbon might be just right to encourage the two-step reaction Salpeter had proposed. It all hinged on a property known as resonance.

"Resonance works like this. When two nuclei collide and stick together, new nucleus that is formed carries the combined mass-energy of two nuclei, plus the combined energy of their motion, their kinetic energ (and minus a small amount of energy from the strong force, the binding energy that holds the new nucleus together). The new nucleus "wants" to occupy one of the steps on its own energy ladder, and if this combined energy from the incoming particles is not just right then the excess has to be eliminated, int he form of leftover kinetic energy, or as a particle ejected from the nucleus. This reduces the likelihood that the two colliding nuclei will stick together; in many cases, they simply bounce off each other and continue to lead their seperate lives. If everything meshes perfectly, however, the new nucleus will be created with exactly the energy that corresponds to one of its natural levels (it can then, of course, emit packets of energy and hop down the steps to the lowest level.) In that case, the interaction will proceed very effectively, and the conversion of lighter nuclei into a heavier form will be complete. The matching of energies to one of the levels appropriate for the new nucleus is the effect known as resonence, and it depends crucially on the strcutrue of the nuclei involved in the collisions.

"In 1954, Hoyle realized that the only way to make carbon inside stars is if there is a resonance involving helium-4, beryllium-8, and carbon-12. The mass-energy of each nucleus is fixed and cannot change; the kinetic energy that each nucleus has depends on the temperature inside a star, which Hoyle could calculate. Using that temperature calculation, Hoyle predicted that there must be a previously undetected energy level in the carbon-12 nucleus, at an energy that would resonate with the combined energies, including kinetic energy, of its constituent parts, under the conditions prevailing inside stars. He made a precise calculation of what that energy level must be, and he cajoled Willy Fowler's somewhat sketpical nuclear physics colleagues until they carried out experiements to test his predictions. To the astonishment of everyone except Hoyle, the measurements showed that carbon-12 has an energy level just 4 per cent above the calculated energy. This is so close that the kinetic energies of the colliding nuclei can readily supply the excess. This resonance greatly increases the chances of a helium-4 and a beryllium-8 nucleus sticking together, and ensures that enough alpha particles can be fused into carbon nuclei inside stars to account for our existence. "The remarkable nature of Hoyle's successful production cannot be overemphasized. Suppose, for example, that the energy levels in carbon had turned out to be just 4 percent lower than the combined energy of helium-4 and beryllium-8. There is no way that kinetic energy could SUBTRACT rather than add the difference, so the trick simply would not have worked. This is made clear when we look at the next putative step in steller nucleosynthesis, the production of oxygen-16 from a combination of carbon-12 and helium-4. When a carbon-12 and a helium-4 molecule meet, they would fuse into oxygen if there was an appropriate resonance. But the nearest oxygen-16 resonance has one percent LESS energy than helium-4 plus carbon-12. But that one percent is all it takes to ensure that this time resonance does not occur. Sure, oxygen-15 is manufactured in stars, but only in small quantities (at least, at this early stage of a star's life) compared with carbon. If that oxygen energy level were one percent lower, then virtually all the carbon made inside stars would be processed into oxygen and then (much of it) into heavier elements still. Carbon-based life-forms like ourselves would not exist. "Most anthropic insights are made with the benefit of hindsight. We look at the Universe, notice that it is close to flat, and say, 'Oh yes, of course, it must be that way, or we wouldn't be here to notice it.' But Hoyle's prediction is different, in a class of its own. It is a genuine scientific prediction, tested and confirmed by SUBSEQUENT experiments. Hoyle said, in effect, 'since we exist, then carbon must have an energy level at 7.6 MeV.' THEN the experiments were carried out and the energy level was measured. As far as we know, this is the only genuine anthropic principle prediction; all the rest are 'predictions' that MIGHT have beenmade in advance of the observatios, if anyone had had the genuis to make them, but that were never in fact made that way.

"Hoyle's remarkable insight led directly to a detailed understanding of the way in which all of the other elements are built up from hydrogen andhelium inside stars. He worked closely with Willy Fowler on this, and with the husband-and-wife team Geoffrey and Margaret Burbridge. Fowler (without Hoyle) later received a Nobel Prize for his part in the study of stellar nucleosynthesis.

"This combination of coincidences, just right for resonance in carbon-12, just wrong in oxygen-12, is indeed remarkable. There is no better evidence to support the argument that the Universe has been designed for our benefit--tailor-made for man. But there are alternative ways of viewing this coincidence, and others."

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