THE ELECTRIC AND MAGNETIC DIPOLE MOMENTS OF THE NEUTRON

In 1950 Purcell and Ramsey1 pointed out that the parity arguments then used to prove that particles and nuclei could not have electric dipole moments, must be based on an experimental rather than a theoretical basis. As a test of this assumption, Smith, Purcell and Ramsey2 used a neutron beam magnetic resonance apparatus to search for a neutron electric dipole moment and concluded that such a moment divided by the proton charge (µe/e) was experimentally less than 5 x 10−20 cm. Later, from the work of Lee and Yang3 and Wu, et al,4 it became apparent that the parity assumption was indeed invalid, but Landau5 and others pointed out that the parity argument against an electric dipole moment could be replaced by one based on time reversal invariance. However, Ramsey6 emphasized that time reversal invariance like parity at an earlier time, was merely assumed and must rest on an experimental basis. In 1964 Christenson, Cronin, Fitch, and Turlay7 discovered the CP violating mode in the decay of the KO L meson into two charged pions, which strongly suggested a violation of time reversal symmetry.


INTRODUCTION
In 1950 Purcell and Ramsey 1 pointed out that the parity arguments then used to prove that particles and nuclei could not have electric dipole moments, must be based on an experimental rather than a theoretical basis. As a test of this assumption, Smith, Purcell and Ramsey 2 used a neutron beam magnetic resonance apparatus to search for a neutron electric dipole moment and concluded that such a moment divided by the proton charge (µe/e) was experimentally less than 5 x 10-20 cm.
Later, from the work of Lee  -309 -

II. METHOD AND APPARATUS
The apparatus used in this experiment is essentially one to measure with high precision the precessional frequency of the neutron spin in a weak magnetic field with a neutron beam magnetic resonance apparatus similar to that used for measuring the magnetic moment of the neutron. A strong electrostatic field is then applied successively parallel and antiparallel to the magnetic field H. If the neutron had an electric dipole moment the torque due to this dipole moment in the electric field would make the precessional frequency of the neutron spin somewhat greater with the electric field in one direction and somewhat less in the opposite. By setting an experimental limit on the change in the precessional frequency, a limit is thereby set on the electric dipole moment of the neutron. The main requirements in the experiment are to achieve a very high sensitivity and to eliminate spurious effects that might either lead to a false apparent electric dipole moment or might obscure an actual moment.
A schematic view of the apparatus is shown in Fig. 2.
The neutron beam comes from the cryogenic moderator at the ILL reactor. The neutrons are conducted from the moderator through a neutron conducting tube of rectangular cross sections on whose surface they are totally reflected at glancing angles of two degrees or less. The use of such neutron conducting pipes which becomes possible with sufficiently slow neutrons, markediy enhances the intensity by overcoming the normal diminution of beam intensity with the inverse square i . .
-310of the distance from the moderator. This gain of intensity is badly needed to compensate in part for the even greater loss of intensity by the selection of extremely slow neutrons.
As shown in Fig. 2, the neutron beam goes through a portion of the pipe in which the walls consist of magnetized iron.
Depending upon the orientation of the neutron spin, there is netic field is provided in two separate segments with a 90 degree phase shift between them, the shape of the resonance curve is that of a dispersion curve with the steepest portion of the slope at the spin precession frequency as shown in Fig. 3.
If the frequency of the oscillator is set so that the detected neutron intensity is at the position of the steepest slope, the presence of a neutron electric dipole moment can be detected by successively reversing a strong electrostatic field.
If there is an electric dipole moment the torque due to the electric field will increase the precessional frequency of the neutron for one orientation of the field and decrease it for the opposite. At a fixed frequency of the oscillator, this change in the precessional frequency of the neutron spin will then be detectable with high sensitivity as a change in the neutron beam intensity.
The electric field is applied over a length of 196 cm. and typically has a value of about 100 kV/cm. The static magnetic field was about 17G and the neutron beam was 89% polarized.
Great care in the experiment must be taken to avoid One of the most bothersome spurious effects is that due to the motion of the neutrons with a velocity t through the electric field E since such motion produces an effective magnetic field Ex t;c. This effective magnetic field can then interact with the known neutron magnetic moment to produce an added precession frequency which will look like that due to an electric dipole moment since it will reverse with the

RESULTS
The results of the present phase of measurements at the    Although the new experiment being planned will have the above marked advantages, it must be recognized that it will still be an extremely difficult one. The limit has by now been pushed to such a low value that care must be taken to avoid all possible systematic effects. Although some of these are intrinsically reduced in an experiment with bottled neutrons, other serious problems will remain. For example, problems due to stray magnetic fields (especially when associated with reversals of the electric field) and to magnetic , field changes resulting from electrical sparks can be just as serious with bottled neutrons as with neutron beams.
These problems have already caused much difficulty in the beam version of the experiment and should be even more formidable in the bottled neutron experiment which seeks to lower the limit for the neutron electric dipole moment by a factor of 100 to 1000.
The apparatus is planned to be capable of being operated in either of two fashions. In one, pulsed ultra-cold neutrons will be admitted for a few seconds and then stored with the neutron valve closed for approximately 30 seconds before the valve is reopened so the neutrons can escape past the oscillatory field for a second time.
In the second mode of operation, the neutrons will continuously be introduced and permitted continuously to bounce out of the neutron bottle with a mean storage time of approximately 18 seconds. The precision of the two methods of observation are comparable and the two procedures should mutually compliment each other.
With an electric field of 30 kV/cm. and a multilayer Mumetal or Maly-Permalloy magnetic shield, it should be possible to achieve a limit on the electric dipole moment of 10-26 cm.
To go to a lower limit will probably require superconducting magnetic shields. These are currently contemplated but decisions on a subsequent phase of the experiment will not be taken until later. With superconducting shields and sufficiently long observation times, it should be possible to lower the limit to 10-27 cm. With a larger cell diameter and other improvements, sensitivity of the order of 10-28 cm.
might ultimately be reached.

V. OTHER NEUTRON BEAM MAGNETIC RESONANCE EXPERIMENTS
Since it will take more than a year before the ultracold neutron beam can be available at ILL and before the apparatus for the bottled neutron experiment can be ready and since the new apparatus will be required to achieve a significant improvement in the present limit, the collabora- The magnetic moment will be measured in an apparatus which is essentially the same as that now being used in the neutron electric dipole moment experiment. Permanent magnets, however, can be added to increase the magnetic field from 15 oerstead to 800 oerstead. The magnetic field can be calibrated in several alternative ways. One is by the use of a proton NMR magnetometer and another is by the use of a rubidium magnetometer.
A still different alternate is to pump water at high speed through a high magnetic field storage region to polarize the protons and then to have the water pass through the neutron beam pipe at high velocity, with the resonance being observed by the · separated oscillatory . 17 field method; in this case the second oscillatory field region has many of the characteristics of a volume filled with molecules in "super radiant" states. The flowing water method has the advantage of a close similarity between the averagings done by the neutrons and by the protons as each are confined to the neutron pipe. It is anticipated that all three methods will be used. The greatest possible care must be devoted to assuring that the magnetic field at the time of the proton calibration is the same as that during the measurements with the neutron.
With this technique, it appears that it should be rela- Although the possibility of doing these experiments was first discussed as a neutron beam magnetic resonance experiment, they can also be done by Mezei