Nuclear Reaction Defies Expectations

All our workhas been focused on naturally occurring fission reactions and living with theconsequences.  Here we have an empiricalresult that questions the present theoretical regime and we need to ask what isnext?

We havelearned what we have learned by hurling neutrons mostly at speeds sufficient toovercome the electrostatic potential of the target.  Now we have an unusual alternative outcomethat is unpredicted in our modeling.

There couldbe a whole range of very low probability events in play that could completelyreshape our knowledge of the detail.  Oneshould not think that what we have is anything more than a good approximationto the empirical data that is likely to run foul of the facts as has justhappened.

Cold fusion,by the way, is a strong hint.

Theelectrostatic fields are not necessarily continuous or mathematically convenientand many good questions have never been asked let alone answered in thelab.  I thought cold fusion was anapparatus able to ask and answer some of those questions.  Other similar apparatus need to befabricated.

Wouldn’t itbe lovely to be able to move a low speed neutron along an axis to directcontact with an elemental nucleus at a specified location?  If we ever pull that off, then perhaps weknow something that can be trusted about the nucleus.

Nuclear reaction defies expectations

Dec 10, 2010 

A novel kind of fission reaction observed at theCERN particle physics laboratory in Genevahas exposed serious weaknesses in our current understanding of the nucleus. Thefission of mercury-180 was expected to be a "symmetric" reaction thatwould result in two equal fragments but instead produced two nuclei with quitedifferent masses, an "asymmetric" reaction that poses a significantchallenge to theorists.

Nuclear fission involves the splitting of a heavynucleus into two lighter nuclei. According to the liquid-drop model, whichdescribes the nucleus in terms of its macroscopic quantities of surface tensionand electrostatic repulsion, fission should be symmetric. Some fissionreactions are, however, asymmetric, including many of those of uranium and itsneighbouring actinide elements. These instead can be understood by also usingthe shell model, in which unequal fragments can be preferentially created ifone or both of these fragments contains a "magic" number of protonsand/or neutrons. For example, one of the fragments produced in many of thefission reactions involving actinides is tin-132, which is a"doubly-magic" nucleus, containing 50 protons and82 neutrons.

The latest work, carried out by a collaboration ofphysicists using CERN's ISOLDE radioactive beam facility,investigated the interplay between the macroscopic and microscopic componentsof nuclear fission. It used what is known as beta-delayed fission, a two-stepprocess in which a parent nucleus beta decays and then the daughter nucleusundergoes fission if it is created in a highly excited state. This kind ofreaction allows scientists to study fission reactions in relatively exoticnuclei and was first studied at the Flerov Laboratory in Dubna, Russia,about 20 years ago, although the Dubna measurements did not reveal themasses of the fragments produced.

Firing protons at uranium

The experiment at ISOLDE involvedfiring a proton beam at a uranium target and then using laser beams and amagnetic field to filter out ions of thallium-180 from among the wide varietyof nuclei produced in the proton collisions. These ions then became implantedin a carbon foil, where they underwent beta decay and some of the resultingatoms of mercury-180 then fissioned. Silicon detectors placed in front of andbehind the foil allowed the energies of the fission products to be measured.

The researchers were expecting the fissionreaction to be symmetric, with the mercury-180 splitting into two nuclei ofzirconium-90, a result thought to be particularly favoured because these nucleiwould contain a magic number of neutrons (50) and a "semi-magic"number of protons (40). What they found, however, was quite different. Theenergy of the fission products recorded in the silicon detectors did not peakat one particular value, which would be the case if only one kind of nuclei wasbeing produced in the reactions, but instead showed two distinct peaks centredaround the nuclei ruthenium-100 and krypton-80.

Collaboration spokesperson Andrei Andreyev of the University ofLeuven, Belgium, (and currently at the University of West of Scotland) saysthat this asymmetric fission was unexpected because the observed fragments donot contain any magic or semi-magic shells. His colleague, theorist Peter Möller ofthe Los Alamos National Laboratory in the US had in fact devised a model ofthe nucleus that predicted that mercury-180 would undergo asymmetric fission.But he wasn't able to explain why that is, having plotted a three-dimensionalpotential energy surface for the fission of mercury-180 and then identified aminimum in that surface, but he couldn't identify which of the three variableswere responsible for that minimum.

'Beautiful experimental achievement'

Phil Walker of the University of Surreyin the UK, who is not a member of the collaboration, describes the research asa "beautiful experimental achievement" that has "an impressivetheoretical outcome". He says that the result will be mainly of interestto academics but believes that it might just have practical implications."Much of our energy generation depends on nuclear fission," he pointsout, "and if we want to make reactors safer and cheaper we need to be ableto trust the basic theory of the fission process. I would say that the theoryhas been found to be sadly lacking, and it needs to be fixed."

Andreyev agrees. "I hope that as a result ofour paper theorists will start to think about this problem and tell us what ishappening," he says. "For the moment we don't know."

The research appears in Physical Review Letters.

Aboutthe author

EdwinCartlidge is a science writer based in Rome

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