Radioactive isotope of tin confirmed to have doubly magic nucleus

May 28, 2010 by Lin Edwards report

( -- Scientists in the U.S. and U.K. have demonstrated that the short-lived radioactive and neutron-rich isotope tin-132 has a doubly magic nucleus.

Protons and neutrons in nuclei occupy orbital shells in a similar way to in . Nuclear magic numbers were first identified in 1948 by Maria Goeppert-Mayer, who shared in the 1963 Nobel prize for physics for her work on developing the nuclear shell model. Nuclear magic numbers are the numbers of nucleons (protons or neutrons) that form full (closed) outer shells in an , like the magic numbers of electrons in helium, neon and other noble (super-stable) gases. The magic numbers for nuclei are 2, 8, 20, 28, 50, 82, and 126, while for electrons the magic numbers are 2, 8, 18, 32, and 50.

Magic proton or neutron numbers give the nucleus greater stability and stronger binding, and are therefore usually more common than nuclei with unfilled orbital shells. In doubly magic nuclei both proton and neutron shells are filled, leading to even stronger binding and stability. The outer shells of doubly magic nuclei are rigidly spherical.

Other confirmed doubly magic nuclei include helium-4, oxygen-16, lead-208, calcium-49, and nickel-48, which are abundant and stable, and nickel-56, which was discovered in 1998 and is less stable than the others, having a half-life of just 5.9 days. Tin-132 is even more unstable with a half-life of only four seconds, which has made confirmation of its doubly magic nature difficult. It has 50 protons and 82 neutrons, and is the first confirmed doubly magic isotope that is both neutron-rich and radioactive.

Kate Jones, from the Department of Physics and Astronomy at the University of Tennessee in Knoxville, Tennessee, and her colleagues used a cyclotron at the Oak Ridge National Laboratory dating from the 1950s to carry out the experiments. Jones said they were excited to have managed the feat, when most people thought it would not be possible until newer facilities were available.

One method used to identify doubly magic nuclei has been to strip neutrons from the isotope and examine the properties of the stripped-off neutrons to infer the structure of the nucleus. To do this a film is made of the isotope in question, and this is then hit with a beam of deuterium (an isotope of hydrogen having one proton and one neutron).

Tin-132’s extremely short half-life made this process impossible, so Jones and her team reversed it, using the isotope as the firing beam instead of as the film being hit. They accelerated a beam of tin-132 to about 10 percent of the speed of light and fired it at a target of deuterated polyethylene. As the tin-132 collides with the target some of the neutrons are stripped from the deuterium molecules to form tin-133, leaving the deuterium proton to fall back to the target.

The researchers then analyzed the energy and angular distribution of the particles and were able to confirm the stripped neutron fell into a separate orbital shell above the closed inner shells of tin-132, which means the nucleus must be robustly spherical as expected for a doubly magical nucleus.

The research findings will help physicists studying what is called the R-process, which is thought to be the process by which over half the elements heavier than iron are created through a series of neutron captures on seed nuclei such as nickel-56, probably in neutron star mergers or in collapsing supernovae.

The research paper is published in the journal Nature.

Explore further: Research group figures out a way to film a laser in normal air bouncing off mirrors (w/ Video)

More information: The magic nature of 132Sn explored through the single-particle states of 133Sn, Nature 465, 454-457 (27 May 2010) doi:10.1038/nature09048

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5 / 5 (2) May 28, 2010
Most certainly calcium-49 is not doubly-magic as the unpaired fermion is allowed ample room for single particle excitations. It is calcium-48 of course.

It's true that the experimental confirmation of the double magicity of tin-132 was still lacking, but there have been several theoretical studies claiming this since 30 years at least.
not rated yet May 28, 2010
If Tin-32 has a half-life of 4 seconds how was a beam of it produced? It seems to me they'd have to make it by firing yet another element at another target, producing a mix of Tin-32 and other byproducts. Seems messy, unless they filtered out the other stuff somehow.
not rated yet May 28, 2010
So what are the mechanical properties of these types of materials? any real differences to none magic elements?
not rated yet May 29, 2010
I'm SO glad LOST is over... As I was reading the magic number that are listed in the above story I was almost ready to say 4, 8, 15... oh skip it, it's way to embarrassing. :)
not rated yet May 30, 2010
If Tin-32 has a half-life of 4 seconds how was a beam of it produced? It seems to me they'd have to make it by firing yet another element at another target, producing a mix of Tin-32 and other byproducts. Seems messy, unless they filtered out the other stuff somehow.

Do a search of how a muon reaches the ground from the upper levels of our atmosphere. That will give you a good idea.
not rated yet May 31, 2010
What if Maria was wrong?
An alternate nuclear arrangement would be elements about lithium to calcium filled shells each with four protons and four neutrons. Eg O16 2 shells, Ca40 5 shells. Outer shells always filled. Innermost shells take the variations in the void at the nuclear center. For elements above calcium 40 to about barium a filled shell of 8 protons (at axes pointing outward at cube corners) and 12 neutrons (at axes pointing inward at cube edges). Sometimes the innermost neutron on a neutron axis decays to a proton, but it can stay in place.
Elements above calcium a filled shell is protons at cube corner axes and cube face center axes. Neutron axes point inward at cube edge and on the diagonal between a cube corner and a face center. The outer shell generally filled. Inner shells partially filled.
Nucleii not built on magic numbers at all. these are artefacts of wishfull thinking.

I'm not a physicist. Just me thinking out loud.
1 / 5 (1) Jun 02, 2010
OK here is a dumb question.

"...In doubly magic nuclei both proton and neutron shells are filled, leading to even stronger binding and stability. "
So how is Sn-132 "even more unstable with a half-life of only four seconds"?

Sounds like a bit of a contradiction.

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