UCN(Ultracold Neutrons)

Jeff Martin

Outline• What is a UCN?• Interactions of UCN• How to make UCN• Fun things to do with UCN

Ultracold Neutrons• UCN are neutrons that are moving so slowly

that they are totally reflected from a variety of materials.

• So, they can be confined in material bottles for long periods of time.

• Typical parameters:– velocity < 8 m/s– temperature < 4 mK– kinetic energy < 300 neV

• Interactions:– gravity: V=mgh– weak interaction (allows UCN to decay)– magnetic fields: V=-B– strong interaction

Gravity• V = mgh• For a neutron on the planet Earth:m = 1 GeV/c2, g = 10 m/s2, h = 3 m

V = 300 neV• Recall, TUCN < 300 neV• Uses:

– UCN spectrometer– gravitational levelsexperiment

y

x

3 m

UCN

no UCN

Weak Interaction

n p + e- + + 782 keV

neutrons live for about 15 minutes

dW/dTe

Te

782 keV

Causes free neutrons to decay

proton

electron

electron anti-neutrino

Magnetic Interaction• The neutron has a magnetic moment:

= -1.9 N = - 60 neV/T• The potential energy of a magnetic moment in

a magnetic field is:V = - B

x

spin alignedV = |B|

7 T

Magnetic Interaction• The neutron has a magnetic moment:

= -1.9 N = - 60 neV/T• The potential energy of a magnetic moment in

a magnetic field is:V = - B

x

spin anti-alignedV = -|B|

7 T

Strong Interaction:QM in 3D

Central Potential V(r) ErV

m )(

22

2

ErVrrr

rrrm

)(sin

1sin

sin

11

2 2

2

2222

2

2

),()(),,( mlYrRr

ml

ml

ml YLYllY 2

22

2

2ˆ1

)1(sin

1sin

sin

1

mlz

ml

ml YLmYYi ˆ1

Euumr

llrV

dr

ud

m

2

2

2

22

2

)1()(

2

)()( rrRru

Solve by separation of variables:

Answer:

QM in 3DCentral Potential V(r)

Some interesting consequences:

... d, p, s, ... ,2 ,1 ,0 llllm ..., ,1 ,

0)0( ruQM in 3D is much like QM in 1D, but with an infinite wall at the origin.

Strong Interaction)(rV

r

potential Nucleus-neutron

MeV 40 fm 2 r0

)(isotropic scattering wave"-s" 0,~ ~ MeV, 1 For 0prlTn

length scattering a

a 4

MeV, 1 TFor 2

tot

n

Attractive Nuclear Force

Scattering Length

0 a

Weak potential

0 a Strong potential

Many different potentials cangive rise to the same value for

“a”Odds are, a > 0

Fermi Potential

)(2

)(2

rr m

aVeff

i

iieff am

V )(2

)(2

rrr For many nuclei in a solid,

For a all the same, and small lattice spacing cf. neutron ,

)()(2

)'('

2

4)( 0

023

0

2

VVVm

na

V

rdN

m

aVeff rrrrr

Let’s replace V(r) by an effective potential with the same a:

Fermi Potential(r)Veff

r

0V

Solid

Even attractive potential can lead to repulsive effective potential!

(the “Fermi Potential”)Just as long as a > 0

Largest Fermi potential is for Nickel-58 (58Ni)V0 = 335 neV

Absorption of UCN(r)Veff

r

0V

Solid

nm 50

depthn penetratio

UCNd

cm 11.01

0

loss

loss nd

Loss/bounce: f ~ d/dloss ~ 10-5 - 10-6

For a vessel of typical size L ~ 10 cm, the neutrons will bounce around for a time t = L/fv ~ 100 - 1000 s before being absorbed

How to make UCN

• Conventional Method:– Take neutrons from a reactor core

– En = 5-10 MeV

– bring into thermal equilibrium with nuclei

– Energy distribution of “cooled” neutrons follows Maxwell-Boltzmann distribution:

kT

E

eEN(E)

2

1

Low efficiency

Fraction of neutrons below 8 m/s is only:

10-11 at 300 K10-9 at 30 K

Use a few tricks to boost the UCN yield:

1. vertical extraction

2. turbine

ILL Neutron SourceInstitutLaue-LangevinGrenoble, France

Turbine operation:

neutron

neutron hits co-rotating blade and stops

highest UCN density achieved:~ 41 UCN/cm3

Superthermal SourceConsider a moderator with two energy levels: E = 0,

Neutrons coming into contact with the moderator canlose energy by exciting transitions in the moderator.

n mod UCN mod*+

down-scattering

up-scattering

In thermal equilibrium, these rates are equal.

The trick is to not allow the UCN to come into thermalequilibrium the moderator.

Superfluid 4He Superthermal Source

When energy of neutron is equal to energy of phonon, down-scattering can occur.

Another advantage:neutron-4He cross sectionis negligible!

under development at NIST for neutron lifetime measurement

Solid Deuterium

Cold n UCNPhonon

Cooling removes phonons

“Superthermal” Sources

• What makes a good superthermal UCN source?– Low neutron absorption– High single phonon energy

• Light atoms• Weak crystal

– Long elastic interaction length

• Solid deuterium has these properties!

UCN Losses in SD2

• Nuclear absorption on deuterium ~ 150 ms

• Phonon up-scattering ~ 150 ms @ TSD2 = 5 K

• Nuclear absorption on hydrogen cont. ~ 150 ms @ 0.2% H

• Conversion of para-deuterium ~ 150 ms @ 1% para-D2

• D2 molecule has two molecular states:

– Ortho (symmetric spin) + L=0,2,…– Para (antisymmetric spin) + L=1,3,…

• Energy difference between ground state (ortho) and excited state (para) is about 5 meV (80 K).

• At T=300K, D2 gas is 33% para and 67% ortho.

Ortho and Para-D2

Scattering from para-D2

UCN paraorthoCN

• At T=300K, 33% of D2 gas is para• At low T, conversion of para to ortho takes months• Use magnetic substance to speed conversion

• measure para-fraction using Raman scattering

Pulsed Neutrons

1 GeV proton beam on Tungsten target

produces~ 18 neutrons/proton

Neutrons are produced via proton-induced “spallation”

Can produce large bursts of neutrons, allowing SD2 to cool between pulses

Los Alamos Neutron Science Center LANSCE

UCN Source

Proton Linac

First SD2 UCN detection with

prototype source

Total flight path ~ 2 msec 0.33

m/s 6

1 m 2

50 ml SD2

0 ml SD2

Proton pulse at t = 0

detector

W

SD2

p beam

Bottling Mode

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70

Time after Pulse (s)

De

tect

ed

UC

N/1

60 m

s

UCN lifetime in SD2 Measured for the first time

0

10

20

30

40

4 8 12 16

SD

(mse

c)

T (K)0 0.1 0.2 0.3 0.4

Para Fraction

•Critical parameter in determining maximum UCN

•Strong dependence on T and ortho/para ratio in SD2

C. L. Morris et al.Phys. Rev. Lett. 89,272501 (2002).

World Record density achieved

ILL (1975)

Previous record for bottled UCN =

41 UCN/cm3

(at ILL)

A. Saunders et al, nucl-ex/0312021

Physics with UCNPrecise measurements of neutron interactions offer

window into fundamental physics:• Neutron Beta Decay

– Electroweak interaction tests– Probe for physics beyond the standard model, e.g. SUSY

• Neutron Electric Dipole Moment, Neutron-Antineutron oscillations– should not exist in standard model– Probe for physics beyond the standard model, e.g. SUSY

• Neutron Quantum States in Gravitational Field– Particle-in-well energy levels– Potential for precise tests of quantum mechanics,

equivalence principle, modifications of gravity.

The Standard Model• six quarks• six leptons• four gauge bosons• 17 parameters

+ Force carriers W, Z, γ, g

In the standard model, these are all the particles that exist

Shortcomings of the Standard Model

• Why so many parameters to fit?• Why is mass range so vast?• Why is the calc. Higgs mass unstable against corrections?• How to incorporate gravity?• Where’s the dark matter?• How to generate matter-antimatter asymmetry?

Belief: this is an effective theory below 100 GeVThese drawbacks motivate theoretical extensions to the standard model (SUSY, string theory), and motivate searches for cracks in the standard model.

Cabibbo, Kobayashi, Maskawa (CKM) Matrix

bsd

bsd

bsd

VVVVVVVVV

bsd

w

w

w

tbtstd

cbcscd

ubusud

w

w

w

99.004.0005.0

04.097.022.0

005.022.0975.0

Note: this matrix must be unitary!

Weak processes allow transitions between generationsWeak eigenstates of quarks are different from mass eigenstates

Weak eigenstates Mass eigenstates

Particle Data Group 2001 Central Values

Otherwise, something is missing from the theory.

A Precise Test of Unitarity

From data, we find: Vud

2=0.9487±0.0010 ( nuclear decays)

Vus2=0.0482±0.0010 ( from e.g. K+π0e+ νe )

Vub2=0.000011±0.000003 ( B meson decays)

WorldData2002

1222 ubusud VVV

In the standard model, we expect:

0014.09970.0222 ubusud VVV

off by 2.2 sigma

Neutron -decay

eνepn

MeV 0.782K.E.νe,p,

in the quark model…

min. 10 tmin., 15τ 2/1n

udd

udu

e

e-n

p

W-

Why measure GA and GV?

• GA related to strong interaction modifications (QCD) to quark axial-vector electroweak interaction

• GV is related to fundamental quark electroweak coupling (conserved vector current, CVC)

W- e-

GV=GFVud

-

W- e-

GF

d u

e e

u quark couples to dw

Universality (almost)

Status of -decay

Neu

tron

lifet

ime

A-Correlation

Supersymmetry

-

W- e-

e

~ ~0~ d

W- e-

e

d~ u~0~ uSensitive to loop

corrections-decay sensitive todifferences in squark/slepton couplings

The UCNA Experiment

T. J. Bowles4 (co-PI), R. Carr1, B. W. Filippone1, A. Garcia9, P. Geltenbort3, R. E. Hill4,

S. A. Hoedl9, G. E. Hogan4, T. M. Ito6, S. K. Lamoreaux4, C.-Y. Liu4, M. Makela8,R. Mammei8, J. W. Martin10, R. D. McKeown1, F. Merrill4, C. L. Morris4, M. Pitt8,B. Plaster1, K. Sabourov5, A. Sallaska9, A. Saunders4 (co-PI), A. Serebrov7, S.

Sjue9,E. Tatar2, R. B. Vogelaar8, Y.-P. Xu5, A. R. Young5 (co-PI), and J. Yuan1

1W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 911252Idaho State University, Pocatello, ID 83209

3Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France4Los Alamos National Laboratory, Los Alamos, NM 87545

5North Carolina State University, Raleigh, NC 276956University of Tennessee, Knoxville, TN 37996-1200

7St.-Petersburg Nuclear Physics Institute, Russian Academy of Sciences, 188350 Gatchina, Leningrad District, Russia

8Virginia Polytechnic Institute and State University, Blacksburg, VA 240619Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, WA

9819510University of Winnipeg, Winnipeg, MB R3B 2E9, Canada

Experimental Method to Measure A

EdPAdW cos1

PANN

NNEA

2

1exp

Endpoint energy 782 keV

Focus electrons onto detectors using a

strong (1 T) magnetic field

A(E) N+ - N-

N+ + N-

How to Measure a Beta-Asymmetry

Field defines n-polarization direction, “focuses” electrons onto detectors.

Layout in Area B

800 MeVprotons

UCNSource

beta-spectrometer

polarizermagnet

proton beamdirection

shield package

UCN port

remote extraction

UCNto expt

“prepolarizer” magnet

polarizer magnet

beta-spectrometer magnet

UCN guideinsertion

future UCNguide path

Status

UCNA Experiment

Experimental Parameters

• Goal precision: A/A = 0.2%– collection of 2108 decays

• Decay rate 100 Hz, 21 days data-taking

• UCN polarization > 99.9%• Systematics

– Total systematic corrections 0.17%– Total systematic uncertainty 0.04%

An Important Systematic Uncertainty: Backscattering

UCNA Experimental Goal:Asymmetry to 0.2%

Residual correction due to backscattering 0.1%

Calibration of low-energy electron backscattering in energy range of neutron beta-decay barely sufficient, will ultimately limit precision in future experiments.

New Measurements of Backscattering

Electron gun

Beam diagnostics

Backscattering chamber

Electron Beam

See: JWM et al, Phys. Rev. C 68,055503 (2003)and M. J. Betancourt et al, in preparation.

UCNA Schedule

• UCN source installed – April 2004• Shutdown over summer was

extended• UCNA commissioning – February

2005• Production running – summer 2005• More experiments to follow

•Silicon detectors, proton detectors

Future of UCNA

• Silicon Detectors– A, b, awm

• Proton Detectors– B, a

Conclusions

• UCN have many fascinating properties.

• Recent advances in UCN production will allow us to use these properties to make new precision measurements of the fundamental interactions of the neutron.

Possibilities for B and a• measurement of proton emission asymmetry, proton spectrum gives sensitivity to B and a, respectively

• accelerate protons into secondary electron emitter• detect secondaries in conventional detectors

Neutron Electric Dipole Moment

(EDM)• Existence of EDM implies violation of Time Reversal Invariance

• CPT Theorem then implies violation of CP conservation

+

-

J

+

-

J-

ddJJtt

00 KK • Observed in mixing, but not enough to explain matter/antimatter asymmetry of universe

Sources of EDM

• Present Exp. Limit < 10-25 e-cm• Standard Model value: 10-31 e-cm• Supersymmetry or Multi-Higgs models

can give 105xSM• Significant discovery potential with

new high sensitivity n EDM experiment (also atomic EDM’s - 199Hg)

Basic TechniqueBE

For B ~ 1mG = 3 Hz

For E = 50kV/cm and dn= 4x10-27e·cm = 0.2 Hz

|E|4

δΔνhδd

h|E|/4dΔν

|E|2d |B|2μhν

n

n

nn

J

New EDM Experiment

Superfluid LHe UCN converter with high E-field

2-3 orders-of-magnitude Improvement possible

Nature: 1/17/02

Nesvishevsky,et al

ILL Grenoble

Quantum States in Gravity Field

1-d Schrodinger potential problem

V

z

mgz

Height Selects Vertical Velocity

Quantized energylevels!

Classical expectation

Energy levels are observed at expectedabsorber heights.