neutron lifetime measurements - nist · lifetime measurements ne ... nist beam lifetime ne 1 10 100...

Neutron LifetimeMeasurements

Paul HuffmanNorth Carolina State UniversityOak Ridge National Laboratory

2009 Neutron Physics Summer School

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eOutline

• History of the lifetime(short - only about 15 minutes)

• Physics highlights (not previously covered)

• Measurements that constitute the world average

• Measurements either in progress or under development

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eMeasuring the Lifetime: The Early Years

It took many years from the discovery of the neutron by Chadwick in 1932 and the conjecture of its instability by Chadwick & Goldhaber in 1935 until its radioactive decay was observed in 1948.

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eWhy is it so Difficult?

• Long lifetime -> low decay rate

• Limited numbers of neutrons

• Difficult to obtain a “well-defined” sample

• Many ways to either lose neutrons from your container or miss counting them

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eThe early years

Proton countern lifetime between 13 and 26 minutesProton counter

n lifetime must exceed 21 minutes

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e

1st “precise” lifetime experimentRobson et al., 1951

Chalk River reactor; 3 cm diameter beam thermal beam with 2x109 n/cm2/s flux

e-p coincidenceτn = 1108 (216) s

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e

A major step forwardChristensen et al. in 1972

e-spectrometer; τn = 918 (14) s

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e1950-1972

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e

Proton counting experimentsat KI in Moscow

1978 KI result: τn = 877 (11) s

In 1980 Byrne et al. found τn = 937 (18) s [withdrawn in the meantime]. They concluded in a Letter to Nature 310, 212 (1984)“… a third direct measurement has given the value τn = 877 ± 11 s, which is totally at variance with all other evidence. We suggest here that …. exclude values of τn outside the range 911 ± 10 s …

1972 Christensen result: τn = 918 (14) s

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-

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n

p

n ! p+ + e

! + !̄e + 782 keV

Neutron Decay

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• τn: Big Bang Nucleosynthesis - determines primordial helium abundance

• gv: determines Vud, test of CKM unitarity

• ga: axial vector coupling in weak decays

• D: search for new CP violation

• a, A, B: precise comparison is sensitive to non-SM physics:• right handed currents• scalar and tensor forces• CVC violation• second class currents

n

pD

4He

D

( )Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

= )(d

s

b)(d

s

b

'

'

'

Importance of Neutron Decay Parameters

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eNeutron Beta Decay

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eBig Bang NucleosynthesisProton

Neutron

MatterAntimatter

Annihilation1 µs

NucleonFreeze Out

1 s

Light ElementFormation

10 min

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eLight Element Abundances

3He/H p

4He

2 3 4 5 6 7 8 9 101

0.01 0.02 0.030.005

BM

C

NB

B

Baryon-to-photon ratio 10 10

Baryon density Bh2

0.24

0.23

0.25

0.26

0.27

10 4

10 3

10 5

10 9

10 10

2

57Li/H p

Yp

D/H p

PDG BBN Review (2007)

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eLight Element Abundances

G.J. Mathews, T. Kajino, T. Shima, PRD 71 (2005) 021302

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eCKM Unitarity

• |Vus| and |Vub| obtained from high-energy experiments

• |Vud| obtained from:

1. 0+ → 0+ nuclear beta decay2. neutron beta decay3. pion beta decay

( )Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

= )(d

s

b)(d

s

b

'

'

'

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• Corrected ft (Ft) values should be constant

• |Vud|2 ∝ 1/<Ft>

• |Vud|2 = 0.9490 ± 0.0005

0+→ 0+ Nuclear Beta Decay

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egA and gV

g A(G

eV-2

)

gV(GeV-2)

Electron Asymmetry

Nuclear Lifetimes

Neutron Lifetime

|Vud| ∝ gv

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-14.75x10-6

-14.70

-14.65

-14.60

-14.55

-14.50

-14.45

gA(GeV

-2)

11.60x10-6

11.5511.5011.4511.40gV(GeV

-2)

PDG 2005

Neutron Lifetime (PDG)

0+ →

0+

CK

M U

nita

rity

(Vu

s)

Beta Asymmetry (PDG)

CKM Unitarity

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-14.75x10-6

-14.70

-14.65

-14.60

-14.55

-14.50

-14.45

gA(GeV

-2)

11.60x10-6

11.5511.5011.4511.40gV(GeV

-2)

Beta Asymmetry (PDG)

CKM Unitarity

CK

M U

nita

rity

(Vu

s)0+

→ 0

+


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-14.75x10-6

-14.70

-14.65

-14.60

-14.55

-14.50

-14.45

gA(GeV

-2)

11.60x10-6

11.5511.5011.4511.40gV(GeV

-2)

Beta Asymmetry (PDG)Perkeo II

CKM Unitarity

CK

M U

nita

rity

(Vu

s)0+

→ 0

+


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-14.75x10-6

-14.70

-14.65

-14.60

-14.55

-14.50

-14.45

gA(GeV

-2)

11.60x10-6

11.5511.5011.4511.40gV(GeV

-2)

Beta Asymmetry (PDG)Perkeo II

Neutron Lifetime (Serebrov)

CKM Unitarity

CK

M U

nita

rity

(Vu

s)0+

→ 0

+


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-14.75x10-6

-14.70

-14.65

-14.60

-14.55

-14.50

-14.45

gA(GeV

-2)

11.60x10-6

11.5511.5011.4511.40gV(GeV

-2)

Current SituationBeta Asymmetry (PDG)Perkeo II (preliminary)

CKM Unitarity

CK

M U

nita

rity

(Vu

s)0+

→ 0

+


Neutron Lifetime (Serebrov)

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eEnergy Scales/Nomenclature

Energy Wavelength Temperature Velocity

Fast > 500 keV > 1 x 107 m/s

Epihermal 500 keV - 25 meV

1 x 107 m/s- 2200 m/s

Thermal 25 meV 0.18 nm 300 K 2200 m/s

Cold 25 meV - 0.05 meV

0.18 nm - 4 nm

300 K - 0.6 K

2200 m/s - 100 m/s

Very Cold 50 ueV - 0.2 ueV

4 nm - 64 nm

0.6 K - 0.002 K

100 m/s -6 m/s

Ultracold < 0.2 ueV > 64 nm < 2 mK < 6 m/s

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eUltracold Neutrons

sin sin c = ( V / E )1/2

V ~ 10-7eV

V=2 ! h

2

m Na

• Strong Interaction

! "#$%&'()*

("+*",• Gravitational Interaction

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eUltracold Neutrons• Magnetic Interaction

N

S

n"High Field Seekers"

"Low Field Seekers"

Vm = - • B

10-7

eV/T

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eTypes of Measurements

• Cold Beam

• Material Bottle

• Magnetic Storage

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eCold Beam Technique

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eUCN Bottle Technique

UCN →

Detector

τ = T / log (N0/N)1/τ = 1/τn + 1/τwall + ...

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• Originally proposed in 1961 by Vladimirskii

• First realized in 1983 by Abov et al. using a combination both magnetic and gravitational interactions.

Magnetic Storage

dN(t)/dt = –(N0/ ) e–t/

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eLifetime Measurements

➊ Neutron BeamDetect decay products from a beam with a well defined neutron fluence rate

➋ Material BottleMeasure change in number of confined neutrons as a function of time

➌ Magnetic BottleMeasure change in number of confined neutrons as a function of time

➍ Magnetic TrapCount decay products of magnetically trapped neutrons as a function of time and measure the slope.

!dN

dt= N!

N1/N2 = e!!(t1!t2)

ln(N/N0) = !!t

Absolute neutron flux (10-3)

Understanding neutron energy spectrumLoss mechanisms (walls)

Complicated OrbitsSpin Flips

Complicated OrbitsTo date: poor signal to noise

Technique Challenges

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eExisting Measurements

maeb nortuen

notorp

rotceted

,ahpla notirt

rotceted

noisicerp

erutrepa.4 = B T 6

iL

tisoped

6

sedortcele part desolc rood

)V 008+(

rorrim

)V 008+(

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maeb nortuen

notorp

rotceted

,ahpla notirt

rotceted

noisicerp

erutrepa.4 = B T 6

iL

tisoped

6

sedortcele part desolc rood

)V 008+(

rorrim

)V 008+(

Beam Lifetime

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eBeam Lifetime

maeb nortuen

notorp

rotceted

sedortcele part nepo rood

)dnuorg(

rorrim

)V 008+(

,ahpla notirt

rotceted

noisicerp

erutrepa

iL

tisoped

6

.4 = B T 6

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eILL Beam Lifetime

J. Byrne et al., Phys. Rev. Lett. 65, 289 (1990)

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eILL Beam Lifetime

J. Byrne et al., Phys. Rev. Lett. 65, 289 (1990)

τn = 893.6 ± 5.3 s

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eNIST Beam Lifetime

J.S. Nico et al., Phys. Rev. C 71, 055502(2005)

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eNIST Beam Lifetime

1

10

100

1000

stn

uo

C

6005004003002001000

ADC Channel (7.47 ch. = 1 keV)

Proton Pulse Height Spectrum

(32.5 kV; 20 µg/cm2 Au)

32.5 keV

1

10

100

1000

stn

uo

C

5004003002001000

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/cm2 Au)


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eBeam Lifetime

maeb nortuen

notorp

rotceted

sedortcele part nepo rood

)dnuorg(

rorrim

)V 008+(

,ahpla notirt

rotceted

noisicerp

erutrepa

iL

tisoped

6

.4 = B T 6


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eNIST Beam Lifetime

1

10

100

1000

stn

uo

C

6005004003002001000

ADC Channel (7.47 ch. = 1 keV)

Proton Pulse Height Spectrum

(32.5 kV; 20 µg/cm2 Au)

32.5 keV

1

10

100

1000

stn

uo

C

5004003002001000

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/cm2 Au)


-40x10-6

-20

0

20

40

sla

udi

se

R

111098765432

Electrode Number

4.0x10-3

3.5

3.0

2.5

2.0

1.5

ah

plA/

gd

kB-

not

orP

111098765432

Electrode Number

Normalized Proton Counts vs. Trap Length

(32.5 kV; 20 µg/cm2 Au)

-40x10-6

-20

0

20

40

sla

udi

se

R

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

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eNIST Beam Lifetime


τn = 885.5 ± 3.4 s

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eBottle Experiments

• MamBo I - material bottle

• MamBo II - material bottle

• Bottle w/Upscatter - material bottle

• ILL Bottle - material bottle

• Gravitrap - material bottle

• NESTOR - magnetic storage ring

• ILL permanent magnet

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eMamBo I

• Fill with UCN

• Vary surface area to volume ratio

• 1/τ = 1/τn +

1/τwall + ...

• Extrapolate to infinite volume

W. Mampe et al., PRL, 63 (1989) 593

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eMamBo I

τn = 887.6 ± 3 s

W. Mampe et al., PRL, 63 (1989) 593

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τn = 881 ± 3 s(unpublished)

Pichlmaier, PhD thesis, TU Munich

MamBo II

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eRotating Gravitational Bottle

V. Nesvizhevsky et al., JETP 75(3) (1992) 405

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eRotating Gravitational Bottle

V. Nesvizhevsky et al., JETP 75(3) (1992) 405

τn = 888.4 ± 3.3 s

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eBottle w/Upscattering

W. Mampe et al., JETP Lett, 57 (1993) 82

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eBottle w/Upscattering

W. Mampe et al., JETP Lett, 57 (1993) 82

τn = 882.6 ± 2.7 s

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eILL Bottle Lifetime

Arzumanov et al., Phys. Lett. B483 (2000)

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Arzumanov et al., Phys. Lett. B483 (2000)

V = 65 l V = 20 lT = 300 KT = –9 °CT = –26 °C

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circles - inner chambertriangles - outer chamber

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τn = 885.4 ± 0.9 ± 0.4 sArzumanov et al., Phys. Lett. B483 (2000)

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eMeasurement Summary

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

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eILL Gravitrap

A. Serebrov et al., Phys. Lett. B605 (2005) 72

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eILL Gravitrap

, s

-1

54321

5431

, s

-1

2


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eILL Gravitrap


τst = 872.2 ± 0.3 s

τst = 865.6 ± 0.6 s

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eGravitrap

τn = 878.5 ± 0.7 ± 0.3 s


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900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

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C. Amsler et al. (Particle Data Group),

PL B667, 1 (2008) and 2009 partial update

for the 2010 edition (URL: http://pdg.lbl.gov)

. .

. .. .. .. .

. . .

. . .

. . .

. .

. . .

. .

. . .

. . .

. .

. . .

The most recent result, that of Serebrov et al., is so far from other results that it makes no sense to

include it in the average.

In 1980 Byrne et al. found τn = 937 (18) s [withdrawn in the meantime]. They concluded in a Letter to Nature 310, 212 (1984)“… a third direct measurement has given the value τn = 877 ± 11 s, which is totally at variance with all other evidence. We suggest here that …. exclude values of τn outside the range 911 ± 10 s …

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eReanalyses

Fomin and Serebrov, 7th UCN Workshop (2009)

• Recent reanalysis by Fomin and Serebrov

• Incorporated quasi-elastic scattering from the walls

• Shifted the MamBo value down by 7.3 ± 1.6 s

• Also reanalyzed Mampe ’93 result, which also shifts lifetime to a lower value.

τn = 880.3 ± 3 s

τn = 881.5 ± 2.4 s

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eGravitational-Magnetic Trap

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• Permanent magnets (1 T at surface)

• Filled from either below or on top

• Depolarization characterized by coating inner walls with Fomblin to retain spin-flipped neutrons

• Estimate 0.5 s in 50 days at ILL

Permanent Magnet Trap

V.F. Ezhov, 7th UCN Workshop (2009)

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τn = 878.2 ± 1.9 s

03

53

04

54

05

55

06

56

07

57

ensity

00520002005100010050

5-

0

5

01

51

02

52

03

Inte

emiT

Permanent Magnet Trap

V.F. Ezhov, 7th UCN Workshop (2009)

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eMagnetic Storage Ring

τn = 877 ± 10 s

W. Paul et al., Z Physics C, 45 (1989) 25

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900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

900

895

890

885

880

200420022000199819961994199219901988 2006 2008

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ePlanned Experiments• Beam

- Improved flux measurement for NIST expt. (Nico et al.)

- J-Parc ion chamber (Otono et al.)

• Material Bottle- Accordion Bottle (Steryl et al.)

- Updated gravitational trap (Serebrov et al.)

• Magnetic Trapping (Magneto-Gravitational)- PENeLOPE (Picker et al.)

- LANL permanent magnet (Bowman et al.)

• Magnetic Trapping (4π magnetic confinement)- Halback octupole magnet (Zimmer et al.)

- NIST Ioffe trap (Mumm et al.)

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eNIST Neutron Fluence

estimated accuracyof τn will be ± ~2.2 s

± 1.5 s with increased running

Alpha-Gamma

Flux Monitor

λ analyzer

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eBeam Ion-Chamber

R. Kossakowski et al., Nucl. Physics A503 (1989) 473

τn = 878 ± 31 s

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eIon-Chamber

H. Shimizu, 7th UCN Workshop (2009)

goal: 0.1%measurement

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eAccordion Bottle

Albert Steryl, private communication (2008)

estimated accuracyof τn will be ± 1 s

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eGravitrap II

estimated accuracyof τn will be ± 0.2 s

Serebrov, 7th UCN Workshop (2009)

Will run at PF2

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eMagnetic Trapping

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• Superconducting analog to permanent magnet trap (2 T at wall)

• Rings alternate in current sense

• Decay protons guided to scintillator

• Marginally trapped neutrons and Majorana spin-flips are a problem

PENeLOPE

PENeLOPE

S. Materne, 7th UCN Workshop (2009)

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ePENeLOPE


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ePENeLOPE


• anticipated statistical precision: ~ 0.1 s

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eHalback Gravitational

• Shallow Halbach array + gravity for trap, trap door loading

• Guide field for decay betas

• Marginally trapped neutrons experience chaotic orbits and are ejected rapidly

• Goal precision ± 0.1 s

• Presently under construction

P.L Walstron et al., NIMA, 599 (2009) 82

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e4π Magnetic Confinement• Halbach Octupole PErmanent (H.O.PE.)

magnetic trap

• 1.3 T at surface, 8 l volume

9 cm internal bore

K. Leung, 7th UCN Workshop (2009)

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eH.O.PE.

Radial/Transverse Axial/LongitudinalSuperconducting coilsProton extraction electrodes

Permanent octupole magnets


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eH.O.PE.


• Initial testing to begin soon, aim to begin measurement in 2010

• anticipated statistical precision: < 0.5 s

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eNIST UCN Lifetime• Produce UCN using the

“superthermal” technique

• Confine low field seekers within a magnetic bottle

• Detect each neutron as it decays using scintillation techniques

S

S

N N

TMP

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eIoffe-Type Magnetic Trap

5.6 T

3.75 T

1.88 T

-200 200-100 1000

-200

200

-100

100

0

Magnetic Field

Strengthx (mm)

)m

m( y

-600 -300 0 300 600

-200

0

200

z (cm)

)mc( y

-200 200-100 1000

5

1

4

2

3

r (mm)

)T( |

B|

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e

Energy Dissipation:Superthermal Process

• 0.89 nm (12 K or 0.95 meV) neutrons can scatter in liquid helium to near rest by emission of a single phonon.

• Upscattering (by absorption of a 12 K phonon)

- ~ Population of 12 K phonons

- ~ e–12 K/Tbath

0

5

10

15

20

0 10 20

Momentum Q (nm -1)

Q2

2m

Elementary Excitationsin Liquid Helium

( y

gren

Ek/

B)

K ni

n

ucn

phononSuperfluid Helium

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eDetection of Decay Events• Recoiling charged particle

creates an ionization track in the helium.

• Helium ions form excited He2* molecules(ns time scale) in both singlet and triplet states.

• He2* singlet molecules decay, producing a large prompt

• (< 20 ns) emission of extreme ultraviolet (EUV) light.

• EUV light (80 nm) converted to blue using the organic fluor (d)TPB (tetraphenyl butadiene).

liquid helium

2He *

e--

- 80nm

- 430nm

Organic Fluor

n → p+ + e– + νe + 782 keV

d

u

d

u

e-

W

e

Neutronsremain in trap

until they decay

Turn offneutronbeam

Experimental Method

Turn onneutronbeam Accumulate

neutrons intrap

Detect pulseof light from

each decay event

d

u

d

u

e-

W

eProof-of-principle Data2.0

1.5

1.0

0.5

0.0

Count R

ate

[s

-1]

25002000150010005000

Time [s]

2.0

1.5

1.0

0.5

0.0

Co

un

t R

ate

[s

-1]

25002000150010005000

Time [s]

2.0

1.5

1.0

0.5

0.0

Co

un

t R

ate

[s

-1]

25002000150010005000

Time [s]

A = (1.92 ± 0.03) s-1

τ = (677 +13/–12) sA = (1.10 ± 0.06) s-1

τ = (844 +53/–47) sNo trapped neutrons

W = –(A/τ) e–t/τ

d

u

d

u

e-

W

eNew High-Current Trap

• Quadrupole on loan from the KEK institute, solenoids wound in-house

• Conservative approach:

- design 30% under load line

• Tested to yield a trap depth and size of:

- B ≥ 3.0 T, design 3.1 T

- ø ≥ 11 cm, l ≥ 42 cm

• x20 more trapped neutrons

d

u

d

u

e-

W

eNIST UCN Summary

• Apparatus presently ready to take data

• Expect a ± 2 s measurement in ~2yr perios

• < ± 0.5 s measurement possible at upgraded NIST cold source or at the SNS

New Dewar on Beamline at NIST

d

u

d

u

e-

W

eSummary

• Neutron lifetime is still an important parameter for understanding both the weak interaction and the light element production in BBN

• Experiments are very difficult

• Significant discrepancies in current measurements

• Many experiments are current either in progress or in the planning stages

neutron lifetime measurements - nist · lifetime measurements ne ... nist beam lifetime ne 1 10 100...

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