The n He Experiment 3 Parity Violation The

The n He Experiment 3 Parity Violation The

The n He Experiment

Parity Violation
The weak interaction is 10 times smaller than its strong counterpart.
However, experiments can probe this small component of the
hadronic interaction by observing a unique property of it, parity
violation (PV). Weak interactions look different under spatial
inversion (looking at them in a mirror.)

PV was discovered by C.S. Wu in
1957, by observing a correlation
between the polarization of Co nuclei
and the direction of beta emission.

RF Spin Rotator

Probing the Hadronic Weak Interaction
M.A. Brown, C.B. Crawford, E. Martin
University of Kentucky

G.L. Greene, S. Kucuker
University of Tennessee

S. Baessler
University of Virginia

J.D. Bowman, S. Penttila
Oak Ridge National Laboratory

V. Gudkov, Y. Song
Unviersity of South Carolina

M. Viviani
Istituto Nazionale di Fisica Nucleare,
Sezione di Pisa

P.N. Seo
Triangle Universities Nuclear Laboratory

A. Barzilov, I. Novikov
Western Kentucky University

University of New Hampshire

L. Baron
Universidad Nacional Autnoma de
M.T. Gericke, S. Page, WTH. Van Oers,
R. Mahurin, V. Tvaskis,
M. McCrea, D. Harrison
University of Manitoba

N3He is using a spin flipper with transverse windings which allows for both
longitudinal and transverse spin rotation. It is being developed at UK based on
calculations with the magnetic scalar potential. For this experiment,
longitudinal polarized neutrons are required which called for the change from
the NPDGamma RFSF. The spin rotator is based on NMR, where the neutron
spin precesses at the Larmor frequency around a magnetic field. The spin
rotator creates an RF field BRF which rotates in resonance with the Larmor
frequency making it appear static, thus causing the neutron spin to also
precess around this field, rotating the spin 180o. The RFSF is ramped inversely
proportional to the time of flight of the neutrons within the flipper to ensure
that all the neutrons within a pulse are rotated efficiently.

Abstract: Although QCD has had tremendous success in describing the strong interaction at high energy,
the structure of nuclear matter remains elusive due to the difficulty of QCD calculations in the low energy
frontier. Thus nuclear structure has typically been explored through electromagnetic interactions, like
electron scattering. The hadronic weak interaction (HWI) is an attractive alternative because it involves
only nucleons, but the weak component is short-range and precisely calculable at low energies. While the
HWI is dominated by the strong force by a factor of 107, it can be isolated due to its unique property of
parity violation (PV). N3He is a precision experiment designed to measure the proton asymmetry through
the reaction n + 3He p + t.

Spallation Neutron Source

The goal of the n3He is to determine the single-spin proton
asymmetry in the reaction
The asymmetry is evident in the direction of the proton emission
with respect to the polarity of the incoming neutron. Studying such a
PV circumstance will shed light on the Hadronic Weak Interaction.

Hadronic Weak Couplings
In the DDH meson exchange model, the strength of the HWI is
specified by coupling constants at the vertex where (when) an
exchange meson is emitted or absorbed. The fundamental weak
interaction occurs at the vertex. There are six unique couplings
characterized by the type of meson exchanged and details of the
vertex. By investigating the many different hadronic nuclear reactions
with varying sensitivities to these couplings, experimental values can
be obtained to test the DDH theory and eventually the EFT theories
once the calculations have been completed in that context.

The n3He experiment is one of the experiments which will allow for
values of the coupling constants to be obtained. Once a number of
the HWI experiments have been completed, you can develop a
system of equations involving the asymmetries along with the
coupling constants and their theoretically calculated counterparts
(Ex. Below).

The SNS, located at Oak Ridge National
Laboratory, is an intense neutron beam
produced by pulsing a high energy
proton beam on a mercury target. The
velocity, energy, and wavelength can all
be extrapolated from the TOF of the
neutrons in the 60 Hz pulse structure.
The energy spectrum of the neutrons is
nearly thermal, slightly higher than the
temperature of the LH2 moderator,
located prior to the guide for the FnPB
where the n3He experiment will be


TOF Spectrum

The target/ion chamber will serve both as an unpolarized 3He target and an in
situ detector of the proton current as a function of emission direction. As
neutrons capture on the 3He they create an excited 4He nucleus which then
decays into a proton and triton. Both particles ionize the gas as they travel. The
negative ions collect on the sense wires at ground, while the positive ions
travel to the high voltage field wire. The asymmetry is detected in current
Ionization distribution for a single
mode by looking at the where the ion current is greater. If the current is higher
capture event due to the proton
downstream, then the proton was emitted in the forward direction.
carrying 3 times as much energy as
the triton and depositing its energy
at the end of its track

Experimental Setup
bender polarizer

FnPB cold
neutron guide


He Beam

He Target / Ion Chamber

10 Gauss

transition field
(not shown)

shim coils
(not shown)

RF spin

He target /
ion chamber


Supermirror Polarizer
The supermirror polarizer is the same polarizer used in NPDGamma. The SM polarizer has a polarization
efficiency of 95%. Neutrons striking the mirror are reflected if their energy is less then the nuclear
potential of the neutrons within the Fe/Si coating. This is due to the repulsive potential of many nuclear
cores felt simultaneously by the spread out neutron wave packet. If the SM coating is magnetic, the
nuclear potential is modified by the magnetic dipole interaction which repels one spin state and attracts
the other into the coating. The transmitted neutrons are absorbed by boron in the glass, producing a soft
0.5 MeV gamma. The mirror uses an Fe/Si magnetic coating to reflect the neutrons rather than FeCoV/TiN
coating in order to prevent activation of Co-60. The polarizer channels are curved slightly
more than the optimal angle in order to prevent any direct
line of sight between the
moderator and the spin rotator,
which prevents any neutrons from distorting the beam
The difference in R+ and R- leads to the high neutron

The statistical uncertainty is dependent on the detector efficiency, the neutron
flux, and the polarization.
N = 2.2x1010 n/s x 107 s
P = 96.2%
d = 6

Beam Fluctuations
RFSF Efficiency
Alignment (beam,
field, chamber)


1.3 10


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