NIST - Neutron Spin Rotation

Neutron Spin Rotation

Collaborators:

B.R. Heckel, E.G. Adelberger, H.E. Swanson - University of Washington, Seattle
P.R. Huffman - NIST
D.M. Markoff, D.G. Haase - North Carolina State University
W.M. Snow, G.L. Hansen, H. Nann, C.D. Bass - Indiana University

Figure 1: parity-violating meson exchange between two nucleons
Figure 1

Parity violation has only been observed in the weak force. The model for describing parity-violation in nucleon-nucleon scattering is shown in Figure 1. An intermediate meson (e.g., pion, rho, or omega) is exchanged between two nucleons, with one parity-conserving (PC) strong interaction vertex and one parity-non-conserving (PNC) weak vertex. We are interested in determining the coupling strength at the weak vertex.

One consequence of parity-violation is that the neutron scattering cross-section in an unpolarized target will have a small helicity dependence. The forward scattering amplitude, f, which is related to the total cross section via the optical theorem can be separated into a PC and PNC term,

where in general, f_PNC << f_PC. A transversely polarized neutron beam can be considered as a coherent superposition of positive and negative helicity component states. The presence of parity violation in f(0) causes these two components to propagate with slightly different velocities. Therefore one observes a slow rotation of the polarization vector as the beam passes through the target. This rotations, denoted as phi _PNC), are expected to be quite small, about 10^-7 radians per meter for neutrons in a liquid helium target.

Description of the Experiment

The experiment is designed to distinguish small PNC neutron spin rotations from rotations arising from residual magnetic fields. Two liquid helium target positions are used, one in front and the other behind a central solenoid called the

-coil, as illustrated in Figure 2. Neutrons polarized vertically (y axis) by a super mirror polarizer enter the apparatus in Figure 2 from the left (along the z axis). The neutrons pass nonadiabatically from a uniform 1 mT field in the current sheet coils into the low field target region. The magnitude of the

-coil magnetic field is such that the neutron spins to undergo a Larmor precession of

rad about the y axis upon passage through the coil. When the helium target is located in front of the

-coil, any spin rotation in the x-y plane that occurs within the target due to PNC effects will be reversed as the neutron passes through the

-coil. If the target is positioned behind the

-coil, no reversal will occur. To the extent that moving the target alters neither the ambient magnetic field nor the average neutron trajectory, any rotations from magnetic fields will not change as the targets moved. The analyzing super mirror polarizer detects the ąx component of the neutron beam polarization, depending upon the sense of current in the rear current sheet coil before the analyzer. Data taken with the

-coil turned off is used as a null test.

(Spin rotation apparatus picture)
Figure 2. Neutron spin rotation experiment overall view.

The liquid helium target consists of four chambers, two side by side in front of the -coil, and two side by side behind the -coil. Each chamber is 5 cm high by 2.5 cm wide by 50 cm long. The chambers are connected through a common centrifugal pump and valve assembly that allows one front and one rear target on the opposite side to be filled simultaneously, while the opposing targets are emptied. In effect, each side is an independent experiment that produces a PNC signal always opposite in sign to the other side. A comparison of the two sides allows effects from drifts in the magnetic field and reactor power fluctuations to be removed from the data, thus providing an important check for consistency of the data. Neutron guide tubes transport the beam from the cryostat to the analyzing supermirror polarizer. The neutrons transmitted through the analyzer are counted in a ³He ionization chamber detector. The detector is split to count the two target sides separately, and there are three collector plates on each side separated along the length of the detector. The neutron capture cross section in ³He follows the inverse velocity law, so by counting the number of neutrons captured as a function of length in the detector, one can study the velocity dependence of the signals. In particular, because rotations from magnetic fields are inversely proportional to the neutron velocity, we can use the neutron spin signal to null the magnetic field by requiring that the any rotation signal detected be independent of velocity.

Experimental Results and Future Prospects

This experiment took an initial data set in 1996 on the fundamental physics polychromatic cold neutron beam. A null result result of phi

_PNC = (3.7 ± 6.6(stat) ± 1.1(syst)) × 10^-7 rad in a 46 cm target was obtained. This result is statistics, with systematic effects . We are presently in the midst of an upgrade to the apparatus that will lead to improved cryogenic performance, optimized collimation to bring more beam into the target, lower target temperature to reduce neutron scattering, and reduced target fill and drain times. As a result of these changes we expect to collect a much larger data set during the next run, scheduled for 2004. Our anticipated statistical accuracy for phi

_PNC from the upcoming run is 10^-7 rad/m.

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Inquiries or comments: david.gilliam@nist.gov
Online: October 2003