A Path to a 0.1s Neutron Lifetime Measurement Using the Beam Method - - PowerPoint PPT Presentation

A Path to a 0.1s Neutron Lifetime Measurement Using the Beam Method

• F. E. Wietfeldt

Tulane University

The beam neutron lifetime method

neutron decay rate: Γ = − dN dt = N τ

The beam neutron lifetime method

neutron decay rate: Γ = − dN dt = N τ neutrons in detection volume :

Vdet

N = ρnVdet = φ v ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Abeam Ldet

The beam neutron lifetime method

neutron decay rate: Γ = − dN dt = N τ neutrons in detection volume :

Vdet

N = ρnVdet = φ v ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Abeam Ldet

neutron lifetime:

τ = Abeam Ldet Γ φ v ⎛ ⎝ ⎜ ⎞ ⎠ ⎟

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

vth =

2200 m/s reference thermal neutron velocity

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

neutron count rate:

Rn = εthAbeamvth φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

vth =

2200 m/s reference thermal neutron velocity

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

neutron count rate:

Rn = εthAbeamvth φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

vth =

2200 m/s reference thermal neutron velocity

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

neutron count rate:

Rn = εthAbeamvth φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

vth =

2200 m/s reference thermal neutron velocity

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

charged particle count rate:

Rp = ε pΓ

neutron count rate:

Rn = εthAbeamvth φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

vth =

2200 m/s reference thermal neutron velocity

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

charged particle count rate:

Rp = ε pΓ

neutron count rate:

Rn = εthAbeamvth φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

τ = Rnε pLdet Rpεthvth

vth =

2200 m/s reference thermal neutron velocity

for a “white” neutron beam:

τ = Abeam Ldet Γ φ(v) v

∫

dv

charged particle count rate:

Rp = ε pΓ

neutron count rate:

Rn = εthAbeamvth φ(v) v

∫

dv

neutron absorption cross section in thin “1/v” counter:

σ abs = σ th vth v

vth =

2200 m/s reference thermal neutron velocity

τ = Rnε pLdet Rpεthvth

most challenging

1400 1300 1200 1100 1000 900 800 700 neutron lifetime (s) 2010 2000 1990 1980 1970 1960 1950 year

neutron lifetime results current

beam method UCN bottle magnetic trap

900 895 890 885 880 875 870 neutron lifetime (s) 2010 2005 2000 1995 1990 year

neutron lifetime results since 1990

recent revisions

beam method UCN bottle

900 895 890 885 880 875 870 neutron lifetime (s) 2010 2005 2000 1995 1990 year

neutron lifetime results since 1990

τn = 880.0 ± 0.6 s χν

2 = 14.0/6 (3%)

beam method UCN bottle

900 895 890 885 880 875 870 neutron lifetime (s) 2010 2005 2000 1995 1990 year

neutron lifetime results since 1990

beam method UCN bottle τn = 879.6 ± 0.6 s τn = 887.3 ± 2.8 s

900 895 890 885 880 875 870 neutron lifetime (s) 2010 2005 2000 1995 1990 year

neutron lifetime results since 1990

beam method UCN bottle τn = 879.6 ± 0.6 s τn = 887.3 ± 2.8 s Δτn = 7.7 ± 2.9 s

900 895 890 885 880 875 870 neutron lifetime (s) 2010 2005 2000 1995 1990 year

neutron lifetime results since 1990

beam method UCN bottle τn = 879.6 ± 0.6 s τn = 887.3 ± 2.8 s Δτn = 7.7 ± 2.9 s

NIST beam neutron lifetime experiment

Measurement of the Neutron Lifetime Using a Proton Trap

J.S. Nico, M.S. Dewey, and D.M. Gilliam National Institute of Standards and Technology

• F. E. Wietfeldt

Tulane University

• X. Fei and W.M. Snow

Indiana University G.L. Greene University of Tennessee

• J. Pauwels, R. Eykens, A. Lamberty, and J. Van Gestel

Institute for Reference Materials and Measurements, Belgium

Fundamental Neutron Physics Program:

• 30 postdocs
• 31 Ph.D. theses
• 40 graduate students
• >50 undergraduate students
• 41 collaborating institutions

neutron interferometer 0.89 nm UCN NG6 end position

NIST Center for Neutron Research Cold Neutron Guide Hall

0.50 nm test beam NEW! NG-C high flux end position - curved SM guide

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

neutron beam proton detector trap electrodes door open (ground) mirror (+800 V) alpha, triton detector precision aperture Li deposit

6

B = 4.6 T

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

τ = Rnε pLdet Rpεthvth

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

τ = Rnε pLdet Rpεthvth

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

Ldet = nl + Lend

τ = Rnε pLdet Rpεthvth

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

Ldet = nl + Lend

# trap electrodes

τ = Rnε pLdet Rpεthvth

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

Ldet = nl + Lend

# trap electrodes length of electrode + spacer

τ = Rnε pLdet Rpεthvth

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

Ldet = nl + Lend

# trap electrodes length of electrode + spacer total effective end region length

τ = Rnε pLdet Rpεthvth

neutron beam proton detector alpha, triton detector precision aperture B = 4.6 T Li deposit

6

trap electrodes door closed (+800 V) mirror (+800 V)

Ldet = nl + Lend

# trap electrodes length of electrode + spacer total effective end region length

Rp Rn = τ −1 ε p εthvth ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ nl + Lend

( )

Proton Trap

1 10 100 1000 Counts 600 500 400 300 200 100 ADC Channel (7.47 ch. = 1 keV) Prot

• ton
• n Pulse Height Sp

Spectrum ( (32.5 .5 kV; 2 kV; 20 µ µg/cm /cm

2 Au)

Au) 32.5 keV

1 10 100 1000 Counts 500 400 300 200 100 TDC Channel (6.25 ch/µs) 3 Electrodes 4 Electrodes 5 Electrodes 6 Electrodes 7 Electrodes 8 Electrodes 9 Electrodes 10 Electrodes Proton Arrival Time Spectrum (32.5 kV; 20 µg/cm

2 Au)

• 40x10
• 6
• 20

20 40 Residuals 11 10 9 8 7 6 5 4 3 2 Electrode Number

4.0x10

• 3

3.5 3.0 2.5 2.0 1.5 Proton-Bkdg/Alpha 11 10 9 8 7 6 5 4 3 2 Electrode Number Nor

• rma

malized Prot

• ton
• n Cou
• unts vs. Trap Length

( (32.5 .5 kV; 2 kV; 20 µ µg/cm /cm

2 Au)

Au)

• 40x10
• 6
• 20

20 40 Residuals 12:00 AM 9/29/00 12:00 AM 9/30/00 12:00 AM 10/1/00 12:00 AM 10/2/00 12:00 AM 10/3/00 Date/Time

Fit of Rp Rn

• vs. number

trap electrodes

910 905 900 895 890 885 880 measured lifetime (s) 30x10

• 3

25 20 15 10 5 backscatter fraction

extrapolated result (stat. error only) 886.8 ± 1.2 s

27.5 kV 30 kV 32.5 kV

Lifetime vs. Backscatter

1/v neutron counter

neutron detection efficiency: εth = σ th 4π Ω x,y

( )

∫ ∫

ρ x,y

( )θ x,y ( )dxdy

1/v neutron counter

neutron detection efficiency: εth = σ th 4π Ω x,y

( )

∫ ∫

ρ x,y

( )θ x,y ( )dxdy

1/v neutron counter

Si detector solid angle

neutron detection efficiency: εth = σ th 4π Ω x,y

( )

∫ ∫

ρ x,y

( )θ x,y ( )dxdy

1/v neutron counter

Si detector solid angle areal density of Li foil

neutron detection efficiency: εth = σ th 4π Ω x,y

( )

∫ ∫

ρ x,y

( )θ x,y ( )dxdy

1/v neutron counter

Si detector solid angle areal density of Li foil neutron beam distribution

Error Budget

Error Budget

can be significantly reduced by an absolute calibration

• f the 1/v

neutron counter

Absolute neutron flux measurement to < 0.1% precision

• 10B alpha-gamma device

now working at NIST 0.06% precision recently achieved! (Andrew Yue, NIST)

Neutron fluence monitor

The Alpha-Gamma device

Monochromatic neutron beam HPGe detector

Alpha-Gamma device

PIPS detector with aperture

Totally absorbing

10B target foil

HPGe detector

Absolute neutron flux measurement to < 0.1% precision

• 3He gas scintillation chamber (Tulane, NIST) - in construction/testing
• 10B alpha-gamma device

now working at NIST 0.06% precision recently achieved! (Andrew Yue, NIST)

Neutron fluence monitor

The Alpha-Gamma device

Monochromatic neutron beam HPGe detector

Alpha-Gamma device

PIPS detector with aperture

Totally absorbing

10B target foil

HPGe detector

Absolute neutron flux measurement to < 0.1% precision

• 3He gas scintillation chamber (Tulane, NIST) - in construction/testing
• neutron radiometer (Indiana, Michigan) - under development
• 10B alpha-gamma device

now working at NIST 0.06% precision recently achieved! (Andrew Yue, NIST)

Neutron fluence monitor

The Alpha-Gamma device

Monochromatic neutron beam HPGe detector

Alpha-Gamma device

PIPS detector with aperture

Totally absorbing

10B target foil

HPGe detector

1) Improved result from previous data with recent absolute neutron calibration

1) Improved result from previous data with recent absolute neutron calibration ~ 0.6 s

1) Improved result from previous data with recent absolute neutron calibration ~ 0.6 s ~ 2.3 s

1) Improved result from previous data with recent absolute neutron calibration new number very soon! ~ 0.6 s ~ 2.3 s

~ 0.6 s 2) Repeat experiment using existing apparatus to ~1 s

~ 0.6 s 2) Repeat experiment using existing apparatus to ~1 s

~ 0.6 s 2) Repeat experiment using existing apparatus to ~1 s ~ 1.0 s result in ~2016

A Path to a 0.1 s Neutron Lifetime Experiment Using the Beam Method

A new, larger proton counting apparatus:

A new, larger proton counting apparatus:

• neutron beam: 35mm diameter, low divergence, 3x capture flux

A new, larger proton counting apparatus:

• neutron beam: 35mm diameter, low divergence, 3x capture flux
• new magnet: 5 T, larger bore and longer

A new, larger proton counting apparatus:

• neutron beam: 35mm diameter, low divergence, 3x capture flux
• new magnet: 5 T, larger bore and longer
• bigger proton trap: 16 segments, each 40 mm long and 60 mm ID

A new, larger proton counting apparatus:

• neutron beam: 35mm diameter, low divergence, 3x capture flux
• new magnet: 5 T, larger bore and longer
• bigger proton trap: 16 segments, each 40 mm long and 60 mm ID
• electrostatic lens system to reduce backscatter correction

A new, larger proton counting apparatus:

statistical improvement factor: (25x beam area)x(3x neutron flux)x(2x trap length)x(4x run time) = 600

• neutron beam: 35mm diameter, low divergence, 3x capture flux
• new magnet: 5 T, larger bore and longer
• bigger proton trap: 16 segments, each 40 mm long and 60 mm ID
• electrostatic lens system to reduce backscatter correction

Proton detector:

Segmented ion-implanted Si wafer (S. Wilburn, Nab collaboration)

• 1-2 mm thick, 60 mm dia., 127 pixels
• < 100 nm dead layer
• ns timing

Absolute neutron counting:

Best achieved so far with α, γ device: 0.6% A path to 0.1% exists with a new, improved apparatus:

• accept larger beam (35 mm)
• longer run times for neutron counting and wavelength measurement
• improved geometry to reduce γ attenuation

0.1% is also possible with 3He gas scintillator (we think)

Upgraded experiment setup

• Beam shaped by 40 mm and 35 mm dia. collimators
• 60 mm dia. targets on 76 mm dia. substrates
• 6LiF target - 5 g/cm2 areal density
• Thin 10B target – 30 g/cm2 areal density

slide courtesy A. Yue

Goal = ~0.1 s proton counting improvements improved 1/v foils improved absolute neutron flux calibration

Goal = ~0.1 s proton counting improvements improved 1/v foils improved absolute neutron flux calibration

Beam Neutron Lifetime at a Long Pulse Spallation Source:

Beam Neutron Lifetime at a Long Pulse Spallation Source:

major advantage: measure capture flux as a function of time-of-flight in situ

• wavelength resolution at the % level gives a strong handle
• n absorber thickness correction (~ 5 s).

Beam Neutron Lifetime at a Long Pulse Spallation Source:

major advantage: measure capture flux as a function of time-of-flight in situ minor advantage: synchronize proton trap with neutron pulses to improve signal/background

• wavelength resolution at the % level gives a strong handle
• n absorber thickness correction (~ 5 s).

Beam Neutron Lifetime at a Long Pulse Spallation Source:

major advantage: measure capture flux as a function of time-of-flight in situ minor advantage: synchronize proton trap with neutron pulses to improve signal/background requirements:

• frame overlap choppers
• maximum capture flux
• wavelength resolution at the % level gives a strong handle
• n absorber thickness correction (~ 5 s).

Merci! Thank you!

Extra Slides

Trap nonlinearity effects:

• Inhomogeneity of the axial magnetic

field - important

• Neutron beam divergence within the trap
• minor
• Nonuniform electrode and spacer widths
• negligible

5.0 4.5 4.0 3.5 3.0 axial magnetic field (T) 50 40 30 20 10 axial position (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

downstream upstream electrode positions

Trap Magnetic Field

800 600 400 200 potential (V) 50 40 30 20 10 axial position z (cm)

Trap Electrostatic Potential (10 electrodes)

1.000 0.995 0.990 0.985 0.980 0.975 0.970 p/n correction factor 10 9 8 7 6 5 4 3 trap length (electrodes)

Correction Cactors from Monte Carlo Calculation

Nonlinearity due to the inhomogeneous magnetic field

Proton detector efficiency effects :

• proton backscatter - important
• Neutron beam halo - minor

(ε p <1)

600 500 400 300 200 100 Vertical Pixels (0.1 mm) 600 400 200

Horizontal Pixels (0.1 mm) b)

10

• 7

10

• 6

10

• 5

10

• 4

10

• 3

10

• 2

10

• 1

10

Beam Fraction Outside a Radius

300 200 100

Radius from Beam Centroid (0.1 mm) LiF Deposit Radius

10

• 5

10

• 4

10

• 3

10

• 2

10

• 1

10

Beam Fraction Outside a Radius

100 80 60 40 20

Radius from beam Centroid (0.1 mm) Effective Detector Radius Active Si Radius

600 500 400 300 200 100 Vertical Pixels (0.1 mm) 600 400 200

Horizontal Pixels (0.1 mm) a)

Upstream end of trap Downstream end of trap

detector Li foil

Dy foil neutron flux measurements

Uncertainty budget

slide courtesy A. Yue

Sources of uncertainty that must be addressed in a 0.01% calibration:

• Statistical uncertainty (0.032%)
• 239Pu source counting uncertainties (0.031%)
• Wavelength determination (0.024%)
• Beam spot corrections to solid angle (AG - 0.015%, FM – 0.009%)
• -attenuation in thick (0.023%) and thin (0.012%) targets

The path to 0.01%

slide courtesy A. Yue

Effects that require more measurement time

• Wavelength measurement (0.024%)

– determine cause of point spread, G.L. Hansen measurement achieved

0.006% uncertainty

• Beam spot corrections to solid angle (AG – 0.015%, FM – 0.009%)

– more beam images, better control and assessment of image background

slide courtesy A. Yue

Effects that have been resolved

• 239Pu source counting uncertainties (0.031%)

– limited by uncertainty in source calibration stack solid angle (0.03%) –new stack characterization with better aperture and improved metrology techniques achieved 0.007% uncertainty

slide courtesy A. Yue

Effects addressed in an apparatus rebuild

• Statistical uncertainty (0.032%)

– build apparatus that accepts a much larger beam (~35 mm diameter)

• -attenuation in thick (0.023%) and thin (0.012%) targets

– new apparatus has only front-facing HPGe detectors

slide courtesy A. Yue

A 3He gas scintillation absolute neutron counter

(Tulane, NIST)

TPB acrylic 25 cm 13 cm PMT 1 atm He + N

3 2

5 mm dia, 5A neutron beam .004" Al window PMT PMT PMT PMT

Design features:

• absolute neutron counting to 99.95%
• 3He gas scintillates in XUV (70-90 nm)
• >50 kHz pulse counting rate
• XUV downshifted to visible by TPB
• >30 photoelectrons/neutron capture
• 1-10 torr N2 quenches long-lived triplet dimers

construction / testing now in progress