Two neutrons at the same time: Discovery of dineutron decay

Apr 12, 2012

(Phys.org) -- Nuclear physicists recently witnessed an atomic nucleus do something that nobody had ever seen one do before – two neutrons at the same time.

Emitting them, that is.

The experiment revealed a brand new form of nuclear decay, the process by which unstable atoms release energy and transform into more stable forms. But instead of emitting known patterns of radiation, the nucleus ejected two correlated simultaneously – a dineutron. Though physicists had long theorized about the existence of this form of decay, this was the first experiment to see the dineutron event in action.

“We have for the first time unambiguously observed dineutron decay and clearly identified it in beryllium-16,” said Artemis Spyrou, professor of nuclear physics.

The newly discovered dineutron decay mode joins the 15 other known forms of atomic decay, including double proton emission, double beta decay and double positron emission. The results hold promise to strengthen scientists’ understanding of the strong force that holds nuclei together and the processes taking place within neutron stars.

The researchers caught the act red-handed. Beryllium-16 is an unbound, unstable isotope with 4 protons and 12 neutrons that decays in less than a trillionth of a second. To produce the extremely short-lived nucleus, the physicists smashed a beam of boron-17 into a solid target, occasionally knocking out a proton and forming the desired beryllium-16.

The neutrons emitted by the newly produced but instantly decaying nucleus flew straight into the Modular Neutron Array (MoNA) neutron detector, while the remaining beryllium-14 nucleus was deflected by a powerful magnet into a separate device to be measured. The resulting events clearly showed two neutrons travelling closely together – a dineutron – through the MoNA detector at the same time that a beryllium-14 was detected, giving direct evidence of the dineutron decay. In addition, the neutrons were sure to have been emitted simultaneously because it requires more energy to emit one at a time, making the dineutron decay the preferred mode.

Or, as Spyrou explained it, “You have to use energy to break off just one neutron, but the two neutrons just go.”

The results have been published in a recent issue of Physical Review Letters as an editors suggestion and have been highlighted as a focus article by Physics, an online journal by the American Physical Society spotlighting exceptional research.

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More information: PRL 108 (2012) 102501.
Physics Focus: physics.aps.org/articles/v5/30

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nuge
2.3 / 5 (6) Apr 13, 2012
This is interesting, but (correct me if I'm wrong) there doesn't seem to be anything fundamentally significant or important about it...it seems to be kind of like saying "if you have a very difficult to make and short-lasting unstable stack of marbles that would instantly fall over straight away anyway, in one particular exact way of arranging that stack, two marbles would fly off in the same direction when it fell apart". Repeatable experiment, kind of interesting, but you really have to wonder what useful information it really gives you about the universe. Maybe at best it helps you to fine-tune your understanding of the nature of the marbles, but the process itself isn't really that important.
gmurphy
5 / 5 (8) Apr 13, 2012
@nuge, in the words of Richard Feynman, whatever is not expressly forbidden is mandatory. This decay pattern was not disallowed by the laws of physics as we understand them therefore failure to observe it would indicate something amiss with our understanding.
slack
4.4 / 5 (7) Apr 13, 2012
Nuge,
you're right but in all sciences there's always room for doubt.
Proof of this kind simply adds weight to the theory that says that it is possible. Theory's fine, but there's nothing like data...
Terriva
not rated yet Apr 14, 2012
Before some time we discussed the case of the beryllium nuclei, which has a rod-like shape. The decay of one half of this nuclei would be therefore followed with decay of the another half at the same moment and it may not serve as a conclusive evidence of some specific interaction between the neutrons itself.
Terriva
not rated yet Apr 14, 2012
Beryllium-16 is an unbound, unstable isotope with 4 protons and 12 neutrons that decays in less than a trillionth of a second.
Actually it's life-time is about 10^-22 sec, which is not even enough for the dineutron to pass through the atom nuclei with speed of light. So that in this case it's rather correlated emission of neutron pairs, existing already inside the beryllium nuclei. The dineutron would violate Pauli exclusion principle, because it's spin would be zero. Which explains, why deuteron can and does exist, whereas the dineutron doesn't.

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