Transcript ppt

Neutron-Antineutron Oscillation Search with Cold
Neutrons
M. Snow
Indiana University/CEEM
NANO Workshop
ILL beam experiment
Improved experiment with horizontal beam:
(1) Reactor source, (2) pulsed source
Vertical experiment: DUSEL proposal
Thanks for slides and calculations to:
Yuri Kamyshkov, Geoff Greene, Hiro Shimizu
Neutron-Antineutron transition probability
2
2



V
2
P
t


sin



2
2
nn
E  V 
 V
h

where V is the potential difference for neutron and anti-neutron.
E V
For H  
 
 
2

t

Present limit on   10 23 eV
Contributions to V:
<Vmatter>~100 neV, proportional to density
<Vmag>=B, ~60 neV/Tesla; B~10nT-> Vmag~10-15 eV
<Vmatter> , <Vmag> both >>
 2 V2
For 
h


t  <<1 ("quasifree condition")

Figure of merit=
NT
2
 t 
 
   t  
h 
  nn 
2
Pnn
2
N=#neutrons, T=“quasifree” observation time
Neutron Cooling: MeV to neV
3
10
T30K
T293K
2
10
1
10
0
W(En)
10
-1
10
-2
10
-3
10
-4
10
UCN
-5
10
Very cold
Cold
ThermEpitherm
-6
10
-8
10
-7
10
-6
10
-5
10
-4
10
En [eV]
-3
10
-2
10
-1
10
0
10
N-Nbar search at ILL
(Heidelberg-ILL-Padova-Pavia)
Schematic
layout of
Heidelberg - ILL - Padova - Pavia nn search experiment
at Grenoble 89-91
(not to scale)
Cold n-source
25 D2
fast n, background
58
HFR @ ILL
57 MW
No GeV background
No candidates observed.
Measured limit for
a year of running:
Bended n-guide Ni coated,
L ~ 63m, 6 x 12 cm 2
H53 n-beam
~1.7. 1011 n/s
Discovery potential :
N n  t 2  1.5 10 9 sec
Focusing reflector 33.6 m
Flight path 76 m
< TOF> ~ 0.109 s
Magnetically
shielded
95 m vacuum tube
Detector:
Tracking&
Calorimetry
with L ~ 90 m and t  0.11 sec
measured
Measured limit :
18
7
Pnn nn1.6

10
8.6 10 sec
  8.6  10 sec
7
Annihilation
target 1.1m
E~1.8 GeV
~1.25 1011 n/s
Baldo-Ceolin M. et al., Z. Phys. C63,409 (1994).
Beam dump
Quasifree Condition: B Shielding and Vacuum
Bt<<ћ ILL achieved |B|<10 nT over 1m diameter, 80 m beam,one layer
1mm shield in SS vacuum tank, 1% reduction in oscillation efficiency (Bitter et
al, NIM A309, 521 (1991). For new experiment need |B|<~1 nT
If nnbar candidate signal
seen, easy to “turn it off”
by increasing B
Voptt<<ћ:
Need vacuum to eliminate
neutron-antineutron optical
potential difference.
P<10-5 Pa is good enough,
much less stringent than LIGO
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
The conceptual scheme of antineutron detector
Better Cold Neutron Experiment (Horizontal beam)
• need cold neutrons from high flux source, access of neutron
focusing reflector to cold source, free flight path of ~200-300m
Improvement on ILL experiment by factor of ~1000 in transition
probability is possible (but expensive) with existing n optics
technology and sources
D ~ 2-3 m
L = 300 m
concept of neutron supermirrors: Swiss Neutronics
neutron reflection at grazing incidence (< ≈2°)
@ smooth surfaces
@ multilayer
@ supermirror
refractive index n < 1
  2d sinq
d2>d
d1
d3>d
1.0
1.0
0.9
0.9
0.9
0.8
0.8
0.8
0.7
0.7
0.7
0.6
0.5
0.4
1.0
reflectivity
reflectivity
reflectivity
total external reflection
e.g. Ni qc = 0.1 °/Å
0.6
0.5
0.4
0.6
0.5
0.4
0.3
0.3
0.3
0.2
0.2
0.2
0.1
0.1
=5Å
0.0
0.0
0.2
0.4
0.6
0.8
q [°]
1.0
1.2
1.4
0.1
=5Å
0.0
0.0
0.2
0.4
0.6
0.8
q [°]
1.0
1.2
1.4
=5Å
0.0
0.0
0.2
0.4
0.6
0.8
q [°]
1.0
1.2
1.4
Supermirror Neutron Optics: Elliptical Focusing Guides
Muhlbauer et. al., Physica B 385, 1247 (2006).
Under development for neutron scattering spectrometers
Can be used to increase fraction of neutrons delivered from cold source
(cold source at one focus, nbar detector at other focus)
“Supermirrors”: qcritical mqcritical
Commercial Supermirror Neutron Mirrors are Available With m ≈ 3 - 4.
Phase space acceptance for straight guide m2, more with focusing reflector
1
Reflectivity
~ 1000 layers
qc
mq
c
Multilayer mirror
“Items of commerce”
Supermirror Neutron Optics: Higher m and reflectivity
H. Shimizu, KEK/Japan
m=10!
From H. Shimizu
Prototype supermirrors with m~6 produced
Useful neutron flux scales roughly as m2
Image Courtesy; H. Shimizu
New Experiment at Existing Research Reactor?
• need close access to
cold source to fully
illuminate elliptical
reflector
• Requests to all >20
MW research reactors
with cold neutron
sources
• Can a reactor be
found? Not yet…
Cutaway view HFIR reactor at ORNL
Advantages of a Next-Generation Pulsed Neutron
Source (ESS) for
A Neutron-Antineutron Oscillation Experiment
Possibility to use active neutron optics to partially correct for
gravitational defocusing
Possibility to optimize the target/moderator system with this
experiment in mind, and take advantage of a “colder” cold
source if moderator research is successful.
Possibility to upgrade reflector with other developments in
neutron optics (higher m higher reflectivity supermirrors, …)
At a pulsed neutron source like ESS, neutrons of a given speed reach the
mirror at a known time. We can therefore imagine an array of mirrors
tiling an ellipse and phased to the source to condition the beam
Piezodrivers
This tilting can be used to counteract the defocusing of the beam
from gravity, thereby reducing the beam/detector size and therefore
reduce the cost of the experiment.
Nt2 distribution vs vertical Y in the target plane
m=4; | xtarget | < 1 m.
Radius of beam is smaller by~factor of 2
->cost of experiment is smaller (scales generically as the area)
Supermirror Neutron Optics: Future Possibilities
“In the future one may consider varying the shape of the guides
actively by means of piezo actuators. If used at pulsed sources,
beam size and therefore the divergence for each wavelength
during a neutron pulse can be optimized. This corresponds to a
kind of active phase space transformation that will allow the
circumvention of Liouville’s theorem. The combination of fast
mechanical actuators with supermirror technology may become
useful for active phase space transformation.”
P.Boni, NIM A586, 1 (2008).
One could design the experiment to be able to take advantage
of such advances in active neutron optics technology through
modification of the reflector
Scheme of Vertical N-Nbar experiment
· ~3 MW TRIGA research
reactor with vertical hole and
cold neutron moderator ® vn ~
1000 m/s
· Vertical shaft ~1000 m deep
with diameter ~ 4-5 m at
proposed US DUSEL facility
· Large vacuum tube, focusing
reflector, magnetic shielding
· Detector (similar to ILL N-Nbar
detector) at the bottom of shaft
Letter of intent to DUSEL
submitted
3.4 MW Annular Core TRIGA reactor
3E+13 n/cm2/s thermal flux
Deuterium
moderator
Focusing
Reflector
L~150 m
Vacuum
Tube
L~1000m
D~3-4m
Magnetic
Shield
Neutron
trajectory
Annihilation
Target
D~2 m
Approximate
scales
100 m
Detector
1m
Beam dump
Annular core TRIGA reactor (General Atomics)
for N-Nbar search experiment
~ 1 ft
• GA built ~ 70 TRIGA reactors 0.01¸14 MW (th)
• 19 TRIGA reactors presently operating in US
(last commissioned in 1992)
• 25 TRIGA reactors operating abroad
(last commissioned in 2005)
• some have annular core and vertical channel
Well-established technology
annular core TRIGA reactor 3.4 MW
with convective cooling, vertical channel,
and large cold LD2 moderator (Tn ~ 35K).
Courtesy of W. Whittemore
(General Atomics)
Cold Neutron
Source
Example
Made in PNPI, Russia
Liquid hydrogen at 20K
Inserted vertically into
research reactor
Delivered to new Australian
research reactor, 18 MW power
3.4 MW annular core
research TRIGA reactor
with Liquid D2 cold
neutron moderator
TRIGA =
Training
Research
Isotopes
from
General
Atomics
Vertical flight path
Shaft diameter
1-1.5 km
15-20 ft
Focusing mirror reflector
Vacuum chamber with
Active + passive magnetic shield
Annular core TRIGA reactor
LD2 cryogenic cold moderator; neutron temperature
4qc
10 5 Pa
1 nT
3.4 MW
35K
Running time
Robust detection signature nC  several pions
Annihilation properties are well understood
Active magnetic shielding allows effect
Free-n sensitivity increases more than
3-5 years
1.8 GeV
LEAR physics
ON/OFF
1000
Expected background at max sensitivity
0.01 event
1km Vertical Space Working Group
NNbar: search for neutron to antineutron transitions (Yuri Kamyshkov/UT)
Study of diurnal Earth rotation (Bill Roggenthen/SDSMT)
Physics of cloud formation (John Helsdon/SDSMT)
Search for transitions to mirror matter (n  n) (Anatoli Serebrov / PNPI)
Cold atom interferometry for detection of gravitational waves
(Mark Kasevich / Stanford U)
Experiment
Length
Dia
Pressure
Mag.
shield
NNbar
1.5 km
4-5 m
<10 Pa
~ 1 nT
Mirror neutrons
1.5 km
4-5 m
<10
Pa
~ 1 nT
n disappearance
Atom interferometry
1-4 km
0.3 m
< .1
Pa
~ 1 nT
grav. wave detection
Cloud Form Physics
0.5-1 km
3-5 m
0.2 atm
N/A
atm. physics facility
Diurnal rotation
0.1-1 km
1m
<10
N/A
E&O
Pa
Purpose
Talks posted at http://hepd5s.phys.utk.edu/nnbar/April/
Sources of x1000 Improvement on ILL Experiment
with Cold Neutrons
-increased phase space acceptance of neutrons from source
(using m=3 supermirrors): x~60
-increase running time: x~3
-increase neutron free-flight time (t2):
x~100 (vertical), ~4-10 (horizontal)
-source brightness :
x~1/20 (vertical 3.4 MW TRIGA)
X~1/2 (horizontal, 20-60MW research reactor)
For horizontal experiment: greater source brightness
~counteracted by (dispersive) gravitational defocusing of
Maxwellian neutron spectrum
Sources of x1000 Improvement on ILL Experiment
with Cold Neutrons from CW Source
-increased phase space acceptance of neutrons from source
(using m=3 supermirrors): x~60
-increase running time: x~3
-increase neutron free-flight time (t2):
x~4-10 (horizontal)
-source brightness :
x~1/2 (horizontal, 20-60MW research reactor)
For CW horizontal experiment: greater source brightness
~counteracted by (dispersive) gravitational defocusing of
Maxwellian neutron spectrum