UCN (Ultracold Neutrons) Jeff Martin Outline What is a UCN? Interactions of UCN How to make UCN Fun...
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Transcript of UCN (Ultracold Neutrons) Jeff Martin Outline What is a UCN? Interactions of UCN How to make UCN Fun...
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.