3he ionization chambers as neutron beam flux monitors for...

Beam Monitors ofthe NPDGamma

ExperimentChad Gillis

BackgroundThe NPDGammaExperimentExperimental SetupA Gamma AsymmetryMeasurementThe BeamMonitorsWhat they areHow they workA transmissionmeasurementBeam MonitorAnalysisPolarimetryPolarizer TransmissionMeasurementThe NPDGammaCollaborationSummary andConclusion

3He Ionization Chambers as Neutron Beam

Flux Monitors for the NPDGamma

Experiment

Chad Gillis

Indiana University NSERC Canada

APS DNP MeetingNashville, Tennessee

October 2006

The NPDGamma ExperimentA precision measurement of the coupling constant for weak pion exchangebetween nucleons

ReactionThe NPDGamma

(2.2 MeV)

γ800 MeV Linactarget

strikes production Pulsed (20 Hz) proton beam

Cold (meV) neutrons from LH moderator2

The Los Alamos Neutron Science Center

The NPDGamma Apparatus

beampolarizer

Helmholtz coils provide uniform vertical magnetic field.

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

gamma detectors

targetLH

beampolarizer

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

gamma detectors

targetLH

beampolarizer

gamma detectors

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

targetLH

gamma detectors

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

beampolarizer

targetLH

beampolarizer

gamma detectors

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

targetLH

beampolarizer

gamma detectors

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

targetLH

beampolarizer

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

gamma detectors

targetLH

beampolarizer

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

gamma detectors

targetLH

A Gamma Asymmetry Measurement

A Pn =1

[N() N( + )]

[N() + N( + )]

A = gamma asymmetryPn= beam polarization

n nnn nn

π + θ

A Pn =1

[N( + ) N()]

[N( + ) + N()]

π + θ

A Pn =1

[N( + ) N()]

[N( + ) + N()]

π + θ

The Beam Monitors

I Ionization chambers containing 3He, 4He, N2.

I Function by application of a high voltage.

I Placed directly in the beam.

I Run in current mode.

I Output current signal depends only on neutron energy and

incident neutron rate

I Very low noise

hellooo

The Beam Monitors

I Very low noise

hellooo

The Beam Monitors

I Very low noise

hellooo

The Beam Monitors

I Very low noise

hellooo

The Beam Monitors

I Very low noise

hellooo

The Beam Monitors

I Very low noise

hellooo

The Beam Monitors

The n + 3He reaction

I To detect low energy neutrons, use an exothermicreaction that produces charged particles.

n + 3He p + t + 764 keV

Jπ 0+=

0.5 MeV wide

20.6 MeV

energy

ground state

20.2 MeV

I Dominated by: n + 3He ! p + t+ 764 keV KE

I Cross section is huge and has a very clean energy dependence.

I Products do not leave the chamber.

I Gas can be mixed to produce required attenuation and response.

I Response to gamma background is negligible.

10-6 10-4 10-2 100 102 104 106 108

Cross Sectionbarns

neutron energy E (eV)

/ 1pE/ tof

! energy range

of interest

10-6 10-4 10-2 100 102 104 106 108

Cross Sectionbarns

/ 1pE/ tof

! energy range

of interest

10-6 10-4 10-2 100 102 104 106 108

Cross Sectionbarns

/ 1pE/ tof

! energy range

of interest

10-6 10-4 10-2 100 102 104 106 108

Cross Sectionbarns

/ 1pE/ tof

! energy range

of interest

10-6 10-4 10-2 100 102 104 106 108

Cross Sectionbarns

/ 1pE/ tof

! energy range

of interest

Workings of the Beam Monitors

voltage signal

0.5 cmtriton recoil

neutron beam

proton recoil1.6 cm

R = 5−350 MΩ

Beam Monitor Signal

Source is pulsed at 20 Hz.

Frame overlap chopper prevents overlap of neighbouring pulses.

Time-dependent signal allows time of ight analysis.

E = 12m

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

E = 12m

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

E = 12m

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

V = NPQR

output voltage

neutron rate

capture probability

charge per neutron

amplier gainConstant time of ight:V / N

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

V = NPQR

output voltage

neutron rate

capture probability

charge per neutron

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

V = NPQR

output voltage

neutron rate

capture probability

charge per neutron

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

V = NPQR

output voltage

neutron rate

capture probability

charge per neutron

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

V = NPQR

output voltage

neutron rate

capture probability

charge per neutron

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

V = NPQR

output voltage

neutron rate

capture probability

charge per neutron

130 120 110 100 90 80 70 60 50 40 30 20 10

time (ms)

Beam Monitor Signal

0 5 10 15 20 25 30 35 40 45 50

tof (ms) at 21 meters

frame overlap chopper cuts beam

Bragg scattering from aluminum in flight path

0 5 10 15 20 25 30 35 40 45 50

tof (ms) at 21 meters

Monte Carlo calculationNormalized Flux From Monitor Signal

V = NPQR

Q;R constant

P increases with tof( / tof)

A Transmission Measurement

1)(E 2(E2)

1(E1) 2(E2)R1 R2

attenuator

~~ ~2 V =T1= V N f

~αN f

1= V N f1 αN2 V = f

monitor

preamplifier

1)(E 2(E2)

1= V N f1 αN2 V = f

monitor

preamplifier

attenuator

~~ ~2 V =T1= V N f

~αN f2(E2)

1)(E 2(E2)

1= V N f1 αN2 V = f

monitor

N~~ α N

~~ ~2 V =T1= V N f

~αN f2(E2)

preamplifier

attenuator

1)(E 2(E2)

1= V N f1 αN2 V = f

monitor

N~~ α N

~~ ~2 V =T1= V N f

~αN f2(E2)

preamplifier

attenuator

3He As a Spin Filter

I Resonance requires J = 0+

I Polarized 3He preferentially attenuates neutrons of one spin

direction

I The attenuation is observed using the beam monitors

p + t + 764 keV

Jπ 0+=

20.2 MeV

20.6 MeV

energy

ground state

0.5 MeV wide

n + 3He

π = +J 12

=Jπ 0+

direction

p + t + 764 keV

Jπ 0+=

20.2 MeV

20.6 MeV

energy

ground state

0.5 MeV wide

n + 3He

π = +J 12

=Jπ 0+

direction

beampolarizer

neutron guide

monitorbeam

spinflipper

pulsed coldneutrons

gamma detectors

targetLH

Beam Polarization measurement

I The polarization-dependent transmission of the 3He cell

can be seen by observing the ux-normalized signal ofthe second monitor.

0 5 10 15 20 25 30 35

time of flight in ms

Signal of downstream monitor

polarizedunpolarized

0 5 10 15 20 25 30

time of flight at 21.4 meters

Unpolarized Cell Transmission

entire cellcorrected for glass walls

best fit: nσ = 0.0714 ms-1 tof

J.D. Bowman (Spokesperson), G.S. Mitchell, S.I. Penttila, A. Salas-Bacci,W.S. Wilburn and V.W. Yuan Los Alamos National LaboratoryS.J. Freedman and B. Lauss University of California, Berkeley

G.L. Jones Hamilton College P.-N. Seo North Carolina State UniversityT. Ino, Y. Masuda and S. Muto High Energy Accelerator Research

Organization, JapanR.C. Gillis, M. Leuschner, B. Lozowski, J. Mei, H. Nann, D.R. Rich, S. Santra

and W.M. Snow Indiana UniversityM. Gericke, S.A. Page and W.D. Ramsay University of Manitoba and TRIUMF

E.I. Sharapov Joint Institute for Nuclear Research, DubnaT.E. Chupp, K. Coulter, M. Kandes and M. Sharma University of Michigan

S. Covrig, M. Dabaghyan, F.W. Hersman, T.-U. Lee and H. Zhu University ofNew Hampshire

W. Chen and T.R. Gentile National Institute of Standards and TechnologyC. Crawford, D. Desai, G.L. Greene, T. Ito, R. Mahurin and A. Yue University

of Tennessee and ORNLR. Alarcon, L. Barron Palos Arizona State University

R.D. Carlini Thomas Jeerson National Accelerator FacilityT.B. Smith University of Dayton

I NPDGamma: to provide a precise measurement of h1,the coupling constant for weak meson exchange.

I To perform this measurement, the neutron beampolarization must be known.

I The beam monitors: ionization chambers that provide asignal proportional to incident beam ux.

I Beam monitors are reliable, low noise and lowbackground.

I Beam monitors have been used to measure the beamtransmission through components of the apparatus.

I Beam monitors allow for a continuous measurement ofbeam polarization.

I Beam monitors will be useful for similar measurementsin the future.

3he ionization chambers as neutron beam flux monitors for...

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