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

Outer Vacuum Chamber Delivered

The nEDM@SNS experiment uses the equivalent of an oversized travel mug to surround the large volume of liquid helium at its heart with a vacuum layer to prevent it from heating up. The so-called Outer Vacuum Chamber (OVC) was recently delivered to ORNL. nEDM@SNS Outer Vacuum Chamber arriving from GNB

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Radio-frequency Shield Room Installed

For optimal performance the experiment’s SQUID magnetometers must reside in a hermetically sealed Faraday cage. The aluminum Cryovessel serves this function, but temperature sensors and other devices with wiring that penetrates the Cryovessel must terminate in a connected Faraday cage (the Radio-Frequency Screen Room, or RFSR) outside the Magnetic Shield

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