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Measurement of the Neutron Magnetic  Dipole Moment Using Schwinger Scattering


This experiment will measure Schwinger scattering in silicon. Previous attempts done by Shull and others did not produce the expected results. The successful realization of Schwinger scattering experiment in Si will give the value of the neutron magnetic dipole moment.


Schwinger scattering is caused by the interaction between the neutron's magnetic dipole moment (MDM) and the atomic electric field silicon crystal. The atomic electric field of the silicon induces a tiny magnetic field in the rest frame of the moving neutron which rotates the neutron polarization by a very small angle (about 3.2´10-4 radians).  To magnify this rotation a neutron beam is Bragg reflected down a narrow slot cut from perfect silicon.  At each of consecutive reflection a magnetic field will rotate of the neutron polarization by π/2 in order for the Schwinger scattering effect to accumulate.  For 135 successive reflections off of the (220)-planes of our crystal a 3.84 Å neutron will produce a small but measurable total rotation of 0.043 radians.

Figure 1 shows the slotted Si crystal for the experiment. Here the (220)-planes of the Si oriented perpendicular to the slot wall.  A constant magnetic field is provided by four coils. Additional pairs of cancel any external magnetic fields. Schwinger interaction will rotate the neutron polarization creating measured neutron polarization along beam direction. The Schwinger setup can be seen in Figure 2. A supermirror polarizer polarizes monochromatic neutron beam which can be flipped by an RF flipper. The neutron polarization is analyzed by a Heusler crystal at the end of the experimental apparatus.

A successful Schwinger scattering experiment provides "proof of principle" for measuring of the neutron electric dipole moment (EDM) using a similar technique. This technique is completely different from standard neutron EDM experiments which use UCN in high magnetic fields thus providing a different prospective on systematic errors of EDM experiments.

Measurements of the Neutron Magnetic Dipole Moment (MBM) Using Schwinger Scattering
Figure 1: The crystal for the MDM experiment. Neutrons are reflected down the slot. Figure 2: Four verticle coils provide a smooth magnetic field around the crystal region. (Photographs by: NI&D Group)

Lead Organizational Unit:



Neutron Physics Group

Muhammad Arif

NIST Associates: 

Murry Peshkin
Argonne National Laboratory

Thomas A. Dombeck
University of Hawaii

Donald Koetke
Valparaiso University