Precision Neutron PolarimetryELISE MARTIN, CHRISTOPHER CRAWFORD, MARY ESTES, BINITA HONA

University of KentuckyLIBERTAD BARRON-PALOS

Universidad Nacional Autonoma de MexicoTIM CHUPP, MONISHA SHARMA

University of Michigan

AREG DANAGOULIAN, SCOTT WILBURN, ANDI KLEIN, AARON COUTURELos Alamos National Laboratory

GORDON JONESHamilton CollegeSEPPO PENTTILA

Oak Ridge National Laboratory

Polarized neutrons are needed for neutron decay correlation experiments, such as abBA. In this experiment the correlation factors A~σ

n ·k

e and

B~σn ·k

ν must be found to ~ 0.1%. Because of the

correlation between neutron spin and k, the neutron polarization must be known to ~ 0.3%.

Steps to polarize neutrons:• Optically pump Rb vapor with

circular polarized laser• Polarize 3He atoms via spin-

exchange collisions• Polarize 3He nuclei via the

hyperfine interaction• Polarize neutrons by spin-

dependent transmission • neutrons spinning parallel to 3He will pass• neutrons anti-parallel to 3He are filtered

n + np

pnp

n

+p

σ0< 3 b

σ1=5300 b

Neutron Beam Monitorsn + n

ppn

p

n

+p

-HV -HV

signal

e-

3He+n

● Incoming neutrons create 760 keV p + t that ionize the 3He gas.

● The ions are attracted by the negative high voltage and create a signal proportional to the number of neutrons.

● The 3He gas is unpolarized and therefore transmission is spin independent.

Experimental Setup

M1 Polarizer M2 SF Analyzer M3

Energy Spectrum

Fast FastSlow Slow

Chopper open

pulsepulse

50 ms 100 ms

Deconvolution

F {M∗R}=F {M }F {R}M*R = measured signalM = neutron fluxR = response function = t A e− t

volta

ge

Lambda (ang)

RF Spin Flipper

holding field

sn

BRF

The spin flipper uses NMR techniques to flip the spin of every other neutron pulse. The spinning neutrons (S

n) precess around the holding field at

the Larmor frequency. The RF field (BRF

) matches the precession of the neutrons. The neutrons then begin to precess around the RF field which causes the spin to flip back and forth.

The neutrons are first detected in monitor 1 (M1). They then pass through a polarizer, where they are filtered by spin state. The number of neutrons is measured again in M2, the neutrons go through the spin flipper (SF), where every other pulse the spin state is switched. The neutrons are filtered again through the analyzer and detected in M3.

The purpose of this experiment is to measure the polarization of neutrons through the polarizer by finding the transmission through the polarizer. This neutron polarization can be found again by using the transmission through the analyzer. These two neutron polarizations can then be verified against each other. The spin flipper finds the polarization when polarized up and negative polarization when polarized down, which can also be compared.

The setup can also be seen in the photograph below. The end of the neutron guide is at the left, followed by M1, the polarizer, M2, and the spin flipper. The coils for the magnetic holding field can also be seen.

M 2

M 1

The monitor signals (M2 & M3) depend on the number of neutrons (N) transmitted through the polarizer and analyzer, which depends on the spin filtering efficiency, which depends on the 3He polarization (P

He). Since both transmission

and polarization depend on N, as seen below, we can find polarization by measuring transmission, which is found by taking ratios of the monitor signals. The following trigonometricfunctions are then used to verify the experimental findings.

Finding Polarization

T n=A cosh PHe Pn=B tanh PHe

Each pulse of neutrons, from the neutron source at LANSCE, has a thermal spectrum. The fast neutrons appear first, the chopper opens for some time, then begins to close, and finally the slowest neutrons appear. The pulses are 50 ms long. Many pulses (1000-10000) are taken and then averaged together, as seen in the next graph.

The spectra for each of the three monitors is seen here, converted into wavelength (angstroms). When this is done correctly the Bragg edges line up as seen. The red circle shows that after the chopper has closed, where the signal should be zero, there remains an exponentially decaying signal. The real signal was extracted by deconvoluting the signal into the actual neutron flux and the exponential response function.

A sample of a signal before and after deconvolution. The signal now goes to zero after the chopper is closed, as it should.

Sample plots of transmission and polarization, fitted to known functions. Fitting is done floating A and B, which should be 1. If A and B are constrained to 1 then the residuals are as shown. Future data is expected to be even more precise by attempting to eliminate the exponential response and reduce noise in the beam monitors.

The ideal response should be a delta function. If the response function is not a delta function it must be deconvolved from the measured signal to extract the physical signal.(i.e. the response function convolves with the physical flux to produce the measured signal)

Using the convolution theorem and fourier transforms, the data can be deconvoluted:

Abstract: Polarized neutrons are needed for decay correlation experiments. A method of preciselypolarizing neutrons using 3He spin filtering techniques is shown. The transmissions of neutrons through a polarizer and an analyzer are used to find the polarization of the filtered neutrons.

Polarizer and Analyzer