nEDM - Neutron Electric Dipole Moment

The goal of the nEDM@SNS experiment at the Fundamental Neutron Physics Beamline at ORNL’s Spallation Neutron Source is to make the world’s best measurement of the neutron’s electric dipole moment, improving the current precision by two orders of magnitude.


Is the neutron round? If not, so what?


It turns out that the “roundness” of the neutron’s electric charge distribution (measured by the electric dipole moment) is connected at a deep level to a rather fundamental question – “Why is there any matter in the Universe?”


Why Corned Beef Sandwiches — And The Rest Of The Universe — Exist​

Matter and anti-matter were created in equal amounts during the Big Bang. Almost all of it (all but about half a part per billion) was annihilated in subsequent matter/anti-matter collisions. Clearly (and thankfully) a small excess of matter survived to form our Universe. Theories attempting to explain this almost invariably predict that the neutron is not quite perfectly round.


The neutron electric dipole moment (or nEDM) was first measured in 1950 by Smith, Purcell and Ramsey at the ORNL’s Graphite Reactor – the world’s first intense neutron source. This first measurement showed that the neutron was very nearly round (to better than one part in a million). 

In the last seventy years the precision of the measurement has improved by six orders of magnitude; the neutron is now known to be round to better than one part in a trillion!  – Historical Development of nEDM Precision 


The goal of the nEDM@SNS experiment at the Fundamental Neutron Physics Beamline at ORNL’s Spallation Neutron Source is to further improve the precision of this measurement by two orders of magnitude. Click here to see a visual representation of this level of precision.

Latest News

Light Collection Readout Production Electronics Tested

The nEDM@SNS experimental signal is a burst of scintillation light following the capture of a neutron by a Helium-3 nucleus. Processing electronics that use silicon photomultipliers to count the number of emitted photons and record their arrival time have now been completely tested.

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Experimental Building – Final Design Complete

Unusual requirements for the nEDM@SNS experimental building include a magnetic keep-out zone near the apparatus and a deep basement for access to the cryogenic interior. Final design of the building was recently completed. 3D CAD model of nEDM@SNS experimental building. Drawing of the experimental building and environs from final design

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