UCN%20(Ultracold%20Neutrons)

1
UCN(Ultracold Neutrons)
Jeff Martin

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

2
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 lt 8 m/s
• temperature lt 4 mK
• kinetic energy lt 300 neV
• Interactions
• gravity Vmgh
• weak interaction (allows UCN to decay)
• magnetic fields V-??B
• strong interaction

3
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 lt 300 neV
• Uses
• UCN spectrometer
• gravitational levels
• experiment

no UCN
3 m
y
UCN
x
4
Weak Interaction
Causes free neutrons to decay
electron
proton
electron anti-neutrino
dW/dTe

• n ? p e- ? 782 keV
• neutrons live for about 15 minutes

Te
782 keV
5
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

spin aligned
7 T
V ?B
x
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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

spin anti-aligned
V -?B
x
7 T
7
Strong InteractionQM in 3DCentral Potential
V(r)
Solve by separation of variables
Answer
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QM in 3DCentral Potential V(r)
Some interesting consequences
QM in 3D is much like QM in 1D, but with an
infinite wall at the origin.
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Strong Interaction
Attractive Nuclear Force
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Scattering Length
Weak potential
Strong potential
Many different potentials can give rise to the
same value for a
Odds are, a gt 0
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Fermi Potential
Lets replace V(r) by an effective potential with
the same a
For many nuclei in a solid,
For a all the same, and small lattice spacing cf.
neutron ?,
12
Fermi Potential
Even attractive potential can lead to repulsive
effective potential! (the Fermi Potential) Just
as long as a gt 0
Largest Fermi potential is for Nickel-58
(58Ni) V0 335 neV
13
Absorption of UCN
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
14
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

15
Low efficiency
Fraction of neutrons below 8 m/s is only 10-11
at 300 K 10-9 at 30 K Use a few tricks to boost
the UCN yield 1. vertical extraction 2. turbine
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ILL Neutron Source
Institut Laue-Langevin Grenoble, France
Turbine operation
highest UCN density achieved 41 UCN/cm3
neutron hits co-rotating blade and stops
neutron
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Superthermal Source
Consider a moderator with two energy levels E
0, ?
Neutrons coming into contact with the moderator
can lose energy by exciting transitions in the
moderator.
down-scattering
?
n
mod
UCN
mod

?
up-scattering
In thermal equilibrium, these rates are
equal. The trick is to not allow the UCN to come
into thermal equilibrium the moderator.
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Superfluid 4He Superthermal Source
When energy of neutron is equal to energy of
phonon, down-scattering can occur. Another
advantage neutron-4He cross section is
negligible!
under development at NIST for neutron lifetime
measurement
19
Solid Deuterium
UCN
Cold n
Phonon
Cooling removes phonons
20
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!

21
UCN Losses in SD2

• Nuclear absorption on deuterium
• ? 150 ms
• Phonon up-scattering
• ? 150 ms _at_ TSD2 5 K
• Nuclear absorption on hydrogen cont.
• ? 150 ms _at_ 0.2 H
• Conversion of para-deuterium
• ? 150 ms _at_ 1 para-D2

22
Ortho and Para-D2

• D2 molecule has two molecular states
• Ortho (symmetric spin) L0,2,
• Para (antisymmetric spin) L1,3,
• Energy difference between ground state (ortho)
  and excited state (para) is about 5 meV (80 K).
• At T300K, D2 gas is 33 para and 67 ortho.

23
Scattering from para-D2
CN
para
UCN

• At T300K, 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

24
Pulsed Neutrons
Neutrons are produced via proton-induced
spallation Can produce large bursts of
neutrons, allowing SD2 to cool between pulses
1 GeV proton beam on Tungsten target produces
18 neutrons/proton
25
Los Alamos Neutron Science Center LANSCE
Proton Linac
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First SD2 UCN detection with prototype source
Total flight path 2 m
50 ml SD2
detector
SD2
0 ml SD2
p beam
W
Proton pulse at t 0
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Bottling Mode
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UCN lifetime in SD2 Measured for the first time

• Critical parameter in determining maximum rUCN
• Strong dependence on T and ortho/para ratio in SD2

C. L. Morris et al. Phys. Rev. Lett. 89, 272501
(2002).
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World Record density achieved





Previous record for bottled UCN 41 UCN/cm3 (at
ILL)
ILL (1975)
A. Saunders et al, nucl-ex/0312021
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Physics with UCN

• Precise 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.

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The Standard Model

• six quarks
• six leptons
• four gauge bosons
• 17 parameters

In the standard model, these are all the
particles that exist
Force carriers W, Z, ?, g
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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?
• Wheres the dark matter?
• How to generate matter-antimatter asymmetry?
• Belief this is an effective theory below 100 GeV

These drawbacks motivate theoretical extensions
to the standard model (SUSY, string theory), and
motivate searches for cracks in the standard
model.
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Cabibbo, Kobayashi, Maskawa (CKM) Matrix
Weak processes allow transitions between
generations Weak eigenstates of quarks are
different from mass eigenstates
ö
æ
ö
æ
ö
æ
d
V
V
V
d

ç

ç

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ub
us
ud
w
Weak eigenstates
Mass eigenstates

s
V
V
V
s

ç

ç

ç
cb
cs
cd
w

ç

ç

ç
b
V
V
V
b
ø
è
ø
è
ø
è
tb
ts
td
w
ö
æ
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æ
d
d
005
.
0
22
.
0
975
.
0
Particle Data Group 2001 Central Values

ç

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w

s
s
04
.
0
97
.
0
22
.
0

ç

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w

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b
b
99
.
0
04
.
0
005
.
0
ø
è
ø
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ø
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w
Note this matrix must be unitary!
Otherwise, something is missing from the theory.
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A Precise Test of Unitarity
In the standard model, we expect

• From data, we find
• Vud20.94870.0010 ( nuclear decays)
• Vus20.04820.0010 ( from e.g. K?p0e
  ?e )
• Vub20.0000110.000003 ( B meson
  decays)

World Data 2002
off by 2.2 sigma
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Neutron ?-decay
in the quark model
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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)

Universality (almost)
GVGFVud
GF
m-
nm
u
d
e-
e-
W-
W-
u quark couples to dw
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Status of ?-decay
Neutron lifetime
A-Correlation
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Supersymmetry
Sensitive to loop corrections ?-decay sensitive
to differences in squark/slepton couplings
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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
91125 2Idaho State University, Pocatello, ID
83209 3Institut Laue-Langevin, BP 156, F-38042
Grenoble Cedex 9, France 4Los Alamos National
Laboratory, Los Alamos, NM 87545 5North Carolina
State University, Raleigh, NC 27695 6University
of Tennessee, Knoxville, TN 37996-1200 7St.-Peter
sburg Nuclear Physics Institute, Russian Academy
of Sciences, 188350 Gatchina, Leningrad District,
Russia 8Virginia Polytechnic Institute and State
University, Blacksburg, VA 24061 9Center for
Experimental Nuclear Physics and Astrophysics,
University of Washington, Seattle, WA
98195 10University of Winnipeg, Winnipeg, MB R3B
2E9, Canada
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Experimental Method to Measure A
Endpoint energy 782 keV
Focus electrons onto detectors using a strong (1
T) magnetic field
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How to Measure a Beta-Asymmetry
Field defines n-polarization direction, focuses
electrons onto detectors.
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Layout in Area B
800 MeV protons
UCN Source
polarizer magnet
beta-spectrometer
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shield package
UCN port
proton beam direction
remote extraction
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Status
beta-spectrometer magnet
UCN guide insertion
polarizer magnet
future UCN guide path
UCN to expt
prepolarizer magnet
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UCNA Experiment
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Experimental Parameters

• Goal precision ?A/A 0.2
• collection of 2?108 decays
• Decay rate 100 Hz, 21 days data-taking
• UCN polarization gt 99.9
• Systematics
• Total systematic corrections 0.17
• Total systematic uncertainty 0.04

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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.
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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.
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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

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Future of UCNA

• Silicon Detectors
• A, b, awm
• Proton Detectors
• B, a

51
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.

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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

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Neutron Electric Dipole Moment (EDM)

• Existence of EDM implies violation of Time
  Reversal Invariance
• CPT Theorem then implies violation of CP
  conservation

-
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Sources of EDM

• Present Exp. Limit lt 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 EDMs - 199Hg)

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Basic Technique
B
E
n
J
For B 1mG n 3 Hz For E 50kV/cm and dn
4x10-27ecm Dn 0.2 mHz
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New EDM Experiment
Superfluid LHe UCN converter with high E-field
2-3 orders-of-magnitude Improvement possible
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Nature 1/17/02
Nesvishevsky, et al ILL Grenoble
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Quantum States in Gravity Field
1-d Schrodinger potential problem
mgz
V
z
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Height Selects Vertical Velocity
Quantized energy levels!
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Energy levels are observed at expected absorber
heights.
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