This article reviews the status of an experiment to study the  neutron spin-electron angular correlation with the Los Alamos Ultra-Cold Neutron (UCN) source. The experiment will generate UCNs from a novel solid deuterium, spallation source, and polarize them in a solenoid magnetic field. The experiment spectrometer will consist of a neutron decay region in a solenoid magnetic field combined with several different detector possibilities. An electron beam and a magnetic spectrometer  will provide a precise, absolute calibration for these detectors. An A-correlation measurement with a relative precision of 0.2% is expected by the end of 2002

Alduschenkov, A.

Asahi, K.

Bowles, T.

Filippone, B. W.

Fowler, M.

Geltenbort, P.

Hartmann, F.

Hill, R.

Hime, A.

Hino, M.

Hoedl, S.

Hogan, G.

Ito, T. M.

Jones, C.

Kawai, T.

Kharitonov, A.

Kirch, K.

Kitagaki, T.

Lamoreaux, S.

Lassakov, M.

Liu, C-Y.

Makela, M.

Martin, J.

McKeown, R.

Morris, C.

Pichlmaier, A.

Pitt, M.

Rudnev, Yu.

Saunders, A.

Seestrom, S.

Serebrov, A.

Smith, D.

Soyama, K.

Tipton, B.

Utsuro, M.

Vasilev, A.

Vogelaar, B.

Walstrom, P.

Wilhelmy, J.

Young, A. R.

Yuan, J.

English

Caltech Authors - Main

A Proposed Measurement of the ft
Asymmetry in Neutron Decay with the
Los Alamos Ultra-Cold Neutron Source
B.Tipton*, A. Alduschenkov, K. Asahi, T. Bowles, B. Filippone, M.
Fowler, P. Geltenbort, F. Hartmann, R. Hill, A. Hime, M. Hino, S.
Hoedl, G. Hogan, T. Ito, C. Jones, T. Kawai, A. Kharitonov, K.
Kirch, T. Kitagaki, S. Lamoreaux, M. Lassakov, C-Y. Liu, M.
Makela, J. Martin, R. McKeown, C. Morris, A. Pichlmaier, M. Pitt,
Yu. Rudnev, A. Saunders, S. Seestrom, A. Serebrov, D. Smith, K.
Soyama, M. Utsuro, A. Vasilev, B. Vogelaar, P. Walstrom, J.
Wilhelmy, A.R. Young, J. Yuan
* Kellogg Radiation Laboratory 106-38
1200 E. California Blvd.
Pasadena, CA 91125
E-mail: tipton@krl.caltech.edu.edu
Abstract. This article reviews the status of an experiment to study the neutron spin-
electron angular correlation with the Los Alamos Ultra-Cold Neutron (UCN) source.
The experiment will generate UCNs from a novel solid deuterium, spallation source, and
polarize them in a solenoid magnetic field. The experiment spectrometer will consist
of a neutron decay region in a soleniod magnetic field combined with several different
detector possibilities. An electron beam and a magnetic spectrometer will provide a
precise, absolute calibration for these detectors. An A-correlation measurement with
a relative precision of 0.2% is expected by the end of 2002.
INTRODUCTION
Fundamental studies of neutron beta decay provide new constraints on elec-
troweak interactions at low energies. The leading order expression for neutron beta
decay is
[l
 + a!^  + A^ + B^ + D^^]. (1)
The parameters a, B, and D represent the neutrino-electron correlation, the spin-
neutrino correlation, and the time-reversal violating correlation. The parameter A
CP539, Symmetries in Subatomic Physics: 3rd Int'l Symp., edited by X.-H. Guo, A. W. Thomas, and A. G. Williams
© 2000 American Institute of Physics 1-56396-964-5/00/S17.00
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measures the parity violating correlation between the neutron spin and the electron
momentum directions. The A correlation is related to the ratio of the fundamental
nucleon vector and axial vector weak coupling constants, A = GA/GV through the
relation,
Neutron lifetime measurements are sensitive to a different mixture of Gy and GA\
the lifetime and A-correlation measurements may be combined to constrain these
couplings separately. With the Fermi coupling constant, G/r, known from muon
decay, the fundamental CKM matrix element Vud may be extracted. Precision
determinations of Vud in this manner allow unitarity tests of the CKM matrix and
possible access to new physics. A diagram of the current constraints on Vud is
shown in Figure 1.
A new experiment is proposed at the Los Alamos National Laboratory to study
the parity- violating spin-electron correlation in neutron beta decay. The experiment
relies on a superthermal deuterium source and a spallation proton target to generate
a high flux of ultra-cold neutrons (UCNs). UCNs are defined as neutrons with
velocity low enough that they undergo total external reflection from the surface of
materials containing them. For the case of 58Ni guides, Vneutron <8 m/s.
UCN from pulsed spallation source offers significant advantages over other tech-
niques of determining the neutron beta-asymmetry. The total external reflection
property allows the UCNs to be transported far from the spallation source, through
significant shielding. This separates the beta spectrometers from the high back-
ground environment at the spallation target. The timing structure of the pulsed
beam allows one to reject prompt beam background during pulses; the produced
UCNs may be stored to deliver nearly constant UCN flux in between pulses. Fi-
nally, a several Tesla magnetic field is sufficient to overwhelm the UCN kinetic
energy, allowing a solenoid magnet to select one polarization state. Nearly 100%
neutron polarization is achievable.
A SOLID DEUTERIUM SOURCE OF UCN
At the heart of the proposed experiment is new type of ultra-cold neutron source
using a solid deuterium (SD2) crystal above a spallation source of cold neutrons.
Figure 2 shows a schematic diagram of the proposed source. Up to 100/^C of 800
MeV protons from LANSCE will impinge on a tungsten spallation target. Produced
neutrons will reach a thin film of SD2 after being moderated in polyethylene at
liquid nitrogen and liquid helium temperatures. These neutrons may phonon scatter
in the SD2 crystal to come nearly to rest. Above the SD2, a structure of a 58Ni
coated guide surrounded by a cold beryllium reflector and a polyethylene moderator
will serve both to contain the cold neutrons near the SD2 and to transport produced
UCN away. A storage volume above the SD2 may be connected by an open valve
287
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.980 -
.975
.970 -
.965
1.255 1.260 1.280
FIGURE 1. Constraints on the CKM matrix element Vud for recent extractions of the parameter
A from the A-correlation. The A values come from PNPI 97 [2], PERKEO [1], ILL-TPC [3], and
PERKEO 99 [4]. The Vud constraint from the ft measurement in superallowed beta decays comes
from [5]. The dark oval represents the expected size of the la allowed region when the results
proposed experiment are combined with the world average neutron lifetime measurement.
to the UCN source during one second proton pulses; the valve will be closed for
nine seconds in between pulses while the UCN are delivered to experiments.
In runs at Los Alamos over the past year, this design was tested with a proto-
type UCN SD2 source connected to a 3He spectrometer. The performance of the
test source indicates that an equilibrium stored UCN density of 350 UCN/cm3 is
achievable for the A-correlation experiment.
THE SPECTROMETER
A schematic design of the A-correlation experiment is shown in Figure 3. UCNs
will be transported from the storage to the experiment through diamond-coated
guides. On their way, they will pass through a six Tesla solenoid field, which
selects one neutron spin state with near 100% efficiency. The neutrons may then
be rotated periodically to the opposite spin state by subsequent passage through
an adiabatic fast passage spin flipper, with expected efficiency > 99.9%.
The polarized UCNs will then be fed into a 10-cm diameter bottle inside a one
Tesla superconducting solenoid. The bottle is open ended and will store the neu-
trons for a average of five seconds. While less than 1% of the flowing UCNs decay
288
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800 MeV protons
FIGURE 2. A schematic diagram of the UCN source.
inside the bottle, most are absorbed in 6Li coated end chambers or flow out of the
end region to neutron flux analyzing detectors.
When a UCN decays, its emitted beta will spiral in the solenoid field towards
the end of the bottle; this field geometry will have nearly 4yr beta collection effi-
ciency. Before reaching electron detectors, the betas will encounter a magnetic field
expansion region, which will both orient the trajectories more perpendicular to the
detector face and act as a mirror to contain the electrons in the detector region.
The impact of backscattering will then be reduced.
Two electron detector designs are being developed for the experiment. In one
design a 15-cm diameter Time Projection Chamber(TPC) will cover a thin scin-
tillator. The chamber allows a precision measurement of the electron trajectory
for better fiducial volume definition and statistical sensitivity. The requirement
of a TPC-scintillator coincidence in each event also eliminates backgrounds, such
as from cosmic rays. Alternatively, a double-sided silicon strip detector may be
placed in the experiment to provide excellent energy resolution. The combination
of both detector technologies with differing sensitivities and systematics will allow
cross-checks in the experiment.
A total of 108 Hz of neutrons will decay in the experiment, with 87 Hz analyz-
able after efficiencies and cuts are considered. Backgrounds in the experiment are
essentially negligible. With the experiment comissioning by the end of 2001 the
anisotropy in neutron beta decay is then expected to be measured to the 0.16%
level statistically by the end of 2002.
The anticipated largest source of systematic uncertainty comes from knowledge
289
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To UCN Flux and/or
Polarization Monitor
MWPC
XXXXXXXXXXXXXXXXXX^
Decay Volume
Solenoid Magnet (IT)
Light Guides
(to PMTs)
Plastic Scintillator
Polarizer Solenoid (6T)
To UCN Source
Emitted Betas Spiral
Along Magnetic Field Lines
FIGURE 3. An overhead view of the spectrometer for the A-correlation experiment.
of the detector performances. The experimental goal is to keep the size of this
systematic below 0.03% for a total systematic uncertainty of 0.042%.
An ambitious program has been begun to allow detailed study of the electron
detectors after they are constructed. A combination of a custom built 150 keV
electron gun and the 1 MeV Dynamotron at NASA's Jet Propulsion Laboratory
provide continuous, stable, tunable electron beam from 5 keV to 1 MeV. A double
focusing magnetic spectrometer has been constructed at Caltech to calibrate the
electron beam energy to better than 0.3%. The combination of these devices will
allow an extremely high precision characterization of the detector efficiency, cali-
bration, linearity, resolution, and backscattering probability before and during the
experiment.
CONCLUSION
A new experiment is proposed at Los Alamos to provide the world's best mea-
surement of the A-correlation in neutron beta decay. The experimental collabo-
ration plans to measure A to a precision of 0.2% by then end of 2002. This is
an improvement of over a factor of four compared to the most recent A-correlation
experiment [4]. The black dot in Figure 1 shows the impact of this measurement on
the determination of Vu^ when combined with world data on the neutron lifetime.
290
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ACKNOWLEDGMENTS
The author would like to acknowledge the support of the U.S. National Science
Foundation.
REFERENCES
1. Bopp, P et. a/., Phys. Rev. Lett. 56, 919 (1986).
2. Erozoliminskii, B. G.et. a/,, Phys. Lett. B236, 33 (1991).
3. Schreckenbach, K. et a/., Phys. Lett B349, 427 (1995).
4. Reich, J. et a/., Phys. Lett. 440, 535 (2000).
5. Hardy, J.C., Towner, I.S., nucl-th/9807049.
291
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A proposed measurement of the ß asymmetry in neutron decay with the Los Alamos Ultra-Cold Neutron Source

et a/.,

et a/., Phys. Lett B349,

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A proposed measurement of the ß asymmetry in neutron decay with the Los Alamos Ultra-Cold Neutron Source

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