Physics news Update 789

Article Tools

Enlarge text

Shrink text
Print this page

Print

Subscribe
E-mail alert
RSS feed

Save and Share
Save to Digg

Digg this

Del.icio.us
Save to Furl

Furl

Number 789, August 22, 2006 by Phil Schewe, Ben Stein, and Davide Castelvecchi

Smallest Pyramids in the Universe

French physicists believe they can solve the mystery behind dozens of nuclear experiments conducted years ago. The experiments, conducted with a variety of detectors, energies, and colliding nuclear species, left puzzling results, so puzzling and hard to interpret that many of the experimenters turned their attention to the study of highly spinning nuclei, a quite fashionable subject at the time.
Now, Jerzy Dudek of the Université Louis Pasteur in Strasbourg, France, and his colleagues at Warsaw University and the Universidad Autonoma de Madrid claim that the old results can be explained by arguing that some nuclei, made in the tempestuous conditions of a sufficiently high-energy collision, can exist in the form of a tetrahedron or a octahedron.
Like a pyramid-shaped methane (CH₄) molecule held together by the electromagnetic force, a pyramidal nucleus would consist of protons and neutrons held together by the strong nuclear force. Such a nuclear molecule -- in effect the smallest pyramid in the universe -- would be only a few femtometers (10^-15 meter) on a side and millions of times smaller in volume than methane molecules.
Just as there are so-called "magic" nuclei with just the right number of neutrons and protons that readily form stable spherical nuclei, so there are expected to be such magic numbers for forming pyramid nuclei too. Stable, in this case, means that the state persists for 10¹² to 10¹⁴ times longer than the typical timescale for nuclear reactions, namely 10^-21 seconds.
Dudek (contact jerzy.dudek@ires.in2p3.fr, +33-3-88-10-64-98) says that gadolinium-156 and ytterbium-160 are nuclei very conducive to residing in a stable pyramid configuration. Nuclei might exist also in stable octahedral (diamond) forms also. These nuclei would all possess a quantum property not seen before in nuclei: in the process of filling out an energy-level diagram for the nucleus, four nucleons of the same kind (neutrons or protons) could share a single energy level instead of the customary one or two permitted nucleons.
This rule-of-four would inhibit the normally observed decay patterns by which non-spherical nuclei throw off energy, usually by emitting gamma rays. In fact, in the case of nuclear pyramids it is expected to result in new and unprecedented decay rules. This inhibition would explain the puzzling results of earlier experiments.
Dudek and his colleagues plan to test these ideas in upcoming experiments.
Dudek et al., Physical Review Letters, 18 August 2006
Contact Jerzy Dudek
Université Louis Pasteur
jerzy.dudek@ires.in2p3.fr
Tel: +33-3-88-10-64-98

High-Flux, Short-Burst Gamma Ray Source

Bursts of gamma rays, the most energetic form of light, can be made by scattering laser light from an electron beam. Present examples -- including those at the SPring 8 machine in Japan and at Brookhaven National Lab in the United States -- deliver relatively long gamma pulses (greater than 100 picoseconds in duration) with relatively low brightness (yielding about a million gamma per second). A new proposal shows how a gamma source with pulses as short as 100 femtoseconds and fluxes as high as a billion per second could be built.
One of the most important things you can do with a bright stream of gammas is to pass them through a thin target where the gammas can generate electron-positron pairs. From this process, the positrons can be skimmed and, owing to their ability to probe certain processes within materials that cannot be probed by X-rays, be used to study such things as defects in bulk metals.
Basically, the positrons render valuable clues about a material sample (structural and magnetic) by taking up positions throughout the sample, where the positrons meet and annihilate with electrons, creating telltale radiation.
One of the researchers, Yuelin Lin (ylli@aps.anl.gov), says that because the positron bursts are so short -- as short as a trillionth of second -- they can be used to make slow-motion movies of ephemeral and hard-to-watch activities like metals melting at high temperatures.
Li et al., Applied Physics Letters, 10 July 2006
Contact Yuelin Lin
Argonne National Laboratory
ylli@aps.anl.gov

Back to Physics News Update