A trap to study neutrons
17 February 1999
On 17 February 1932, James Chadwick submitted a paper to the journal Nature* describing his discovery of subatomic particles just a little heavier than protons, but without the positive charge of protons. He had discovered the neutron. Nowadays, neutrons have become valuable tools in research at facilities such as ISIS in the UK, and at the Institut Laue-Langevin (ILL) in Grenoble. But neutrons also remain objects to study in their own right. Physicists are particularly keen to discover if the neutron has an electric dipole moment (EDM). Although neutrons have no electric charge, they have are built from quarks which do have charge. If these internal charges are not arranged perfectly symmetrically, then the neutron will have an EDM - and this can happen only if the laws of physics are the not the same forwards and backwards in time.
This week's image shows the apparatus used in an experiment at ILL to make the most sensitive measurements of the neutron's EDM so far. Very slow moving ("ultracold") neutrons are stored in a quartz cylinder (40 cm across) at the heart of the apparatus - the neutrons enter/leave the cylinder through the pipe visible beneath its centre. The team measures how fast the neutrons' spin axis rotates in a weak magnetic field when they also apply a strong electric field via electrodes at the top (+ or - 100 V or so) and bottom (ground) of the cylinder. The rate of rotation should change in the strong electric field if the neutron has an EDM. So far the team has found no measurable effect**.
(*see J.Chadwick, Nature 129 312 (1932))
(** see Physical Review Letters 82 904 (1999))
Credit: Sussex University EDM team
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