Transcript Slides - Agenda INFN

New physics with intense
positron beams
Alfredo Dupasquier
Frascati, 20 gennaio 2010
OUTLINE
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Why do we need positrons?
How do we get positrons?
How do we produce tunable positron beams?
What can we do with intense positron beams?
Acknowledgements
Many slides of this presentation were stolen from the lectures given
by
A.P. Mills, C. Surko, M. Charlton, Hui Chen, M. Giammarchi, A. Alam,
C. Hugenschmidt, R. Krause-Rehberg
at the Enrico Fermi School “Physics with many positrons”
(Varenna 2009)
Why do we need positrons?
• Medical uses: PET
• Atomic physics: Positron interaction with gas molecules
(theory validation free of exchange effects); Spectroscopy
of positronium (quantum electrodynamic tests)
• Solid State Physics: Electron momentum spectroscopy and
Fermi surfaces
• Materials Science: Positron spectroscopy of lattice defects
Positron annihilation
as a probe for electronic structure
2D-ACAR
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Annihilation of thermalised positron in a solid: mostly 2γ
photons, each with energy mc2.
Centre of mass frame: ; spin and momentum conservation → γ’s
in 180º
Positron is thermalised, thus p+ << p-.
Laboratory frame:
frame: small
small deviation from 180º due to finite
Laboratory
(electron)from
momentum
→to
small angular deviation say in x, y
deviation
180º due
directions
if z = mean
direction
finite
(electron)
momentum
→ of γ-emergence.
small angular deviation say in x,
y directions if z = mean direction
of γ-emergence.
2D-ACAR in quartz at 86 K
The narrow peaks are due to
thermalized para-Ps. Side peaks
are Umklapp components.
The broad distribution is due
to the quartz electrons
Data from the Bristol group (Alam et al.)
3D reconstruction of
the Fermi surface of
ZrZn2 obtained from
2D-ACAR data
(Major et al,
Phys.Rev. Lett.
92, 107003
(2004)
3D representation of
the Fermi surface of
ZrZn2 from LMTO
calculations
Positron annihilation
as a probe for lattice defects
PALS and CDB
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Positrons are trapped by open volume defects in solids.
Trapped positrons survive more time than delocalized bulk
positrons
Positron annihilation with fast core electrons is less likely when
positrons are trapped (less Doppler broadening of the annihilation
radiation)
Trapped positrons probe the local chemistry , thus help to detect
defect-impurity complexes.
PALS
CDB
The width of the
annihilation line
comes from the
Doppler broadening
due to the motion of
the annihilating pair
Trapped positrons do
not overlap with fast
core electrons. The
annihilation line
becomes narrower.
Positron study of microstructural
transformation in alloys
CDB
Defect depth profiling with a tunable
monoenergetic positron beam
pz [a.u.]
S parameter
0.55
0.5
1.0
1.5
2.0
2.5
SiGe "as grow"
SiGe annealed
Ge defects
interface
An example regarding
a SiGe/Si/SiO2
multilayer
grown on Si
2.0
 / Si
0.56
1.5
0.54
1.0
0.53
0.52
Si SiO2
SiGe
0.51
0.1
1
10
100
Si
1000
Mean implantation depth [nm]
How do we get positrons?
• Nuclear beta+ decay
advantages:
a) usable in on-campus laboratories;
b) produces polarized positrons, which are important
for some experiments)
• Pair production from nuclear gamma rays
advantages: high intensity;
disadvantages: need a reactor or an accelator)
• Pair production from Brehmstrahlung
advantages: high intensity;
disadvantages: need an accelerator
A possibility to be explored
Positron production by ultra-intense laser pulses
Simulation of
electron-photonpositron shower for
25 MeV electrons
on gold
Green: electrons
Yellow: Brehmstrahlung
Red: positrons
Positron yield vs. laser intensity
(predicted)
Recent results at the Jupiter Laser Facility (LLNL)
• Titan laser (wavelength
1024 nm, energy 120 to
250 J, pulse length 0.7 to
10 ps)
• 60% total energy in a focal
spot 8 μm
• Pre-pulse intensity
contrast 10-5
• Extracted positrons per
pulse 1.6x1010 forwards,
2x109 backwards
• Positron peak energy 6
MeV, r.m.s. spread 2.8
MeV
This is not a simulation!
How do we produce tunable positron beams?
Positron thermalisation in a solid
Most used moderators
W single crystals
Pt
Solid Ne
Requirements:
Negative positron workfunction
Low defect density
Stable surface
Monoenergetic positron beam
Positron kinetic energy: 0.1 – 20 keV
• Radioactive source (Na22 - 30 mCi) and
moderator (W - 1 m)
• Electrostatic positronic optics
• Sample, cryostat/furnace (10 K – 1100
K) and gamma detectors (HpGe)
Work in progress at FRII Munich
(Koegel)
Surko’s positron trap
Progress in positron traps: multicells
MCT layout
Recent advances and current research
requiring high intensity positron sources
• Fundamental physics with antihydrogen
(ATHENA, ATRAP, AEGIS).
• First evidence of Ps2 molecules
Method I: Antiproton + Positron (ATHENA)
104 antiprotons
Method II: Antiproton + Rydberg Ps (ATRAP)
C. H. Storry et al., First Laser-Controlled Antihydrogen Production, Physical
Review Letters 93, 263401 (2004)
108 e+
• Spontaneous radiative recombination
Two-stage
Rydberg
charge exchange
• Three body recombination
14 ± 4 antihydrogen atoms
In Aegis: Antiproton + Rydberg Ps (obtained by Ps and laser excited)
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p  ( Ps)  H  e 
*
   a 2 n4
0
Varenna - July 2009
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Large cross section
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Quantum states of antihydrogen related to Ps
quantum number
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Reaction suitable for cold antihydrogen
production (cold antiprotons!)
A E g I S in short
Acceleration of antihydrogen.
Formation of antihydrogen atoms
The antihydrogen beams will fly
(with v~500 m/sec) through a
Moire’ deflectometer
•Positronium: 107 atoms
•Antiprotons: 105
•Antihydrogen: 104/shot
Antiprotons
Positrons
The vertical displacement (gravity fall) will be
measured on the last (sensitive) plane of the
deflectometer
Such measurement would represent the first direct determination of the gravitational effect on antimatter
Varenna - July 2009
Vacuum
Positronium yield from materials:
requirement of 10% (reemitted, cold) out
of 108 in ortho-Ps.
Solid
Positron beam
(lectures by R. Brusa and A. Dupasquier)
Ps
Silicon nanochannel material: 10-15 nm
pores: max o-Ps formation observed 50%
e+
Ps
Ps
Velocity of reemitted Ps: 5 x 104 m/s
Ps
(corresponding to thermalized at 100 K)
Positronium emission
Laser excitation of the Positronium to Rydberg states (more on this later on)
Varenna - July 2009
Cold Ps production (Brusa, Mariazzi)
100
0.140 eV
1000
0.5
T = 300 K
T = 200 K
T = 150 K
0.4
0.3
0.4
0.2
0.1
0.3
0.2
0.1
T = 300 K
0
5
10
15
20
counts [arbitrary units]
0.096 eV
prompt
FWHM = 23 ns
0
100
200
300
400
o-Ps time of flight [ns]
7 keV, T = 300 K
4 keV, T = 300 K
a)
0.096 eV
0.076 eV
0.068 eV
7 keV, T = 300 K
7 keV, T = 200 K
7 keV, T = 150 K
25
Positron implantation energy [keV]
1
7 KeV, T = 300 K
7 KeV, T = 200 K
7 KeV, T = 150 K
T=305±10K
T=1515±15K
T=195±10K
T=1425±25K
T=145±10K
T=1260±15K
b)
0.0
0.1
log(dN/dE) [arbitrary units]
10
counts [arbitrary units]
1
F3
F3 o-Ps fraction/implanted positrons
Positron implantation depth [nm]
10
Positron implantation energy [keV]
0
50
100
150
200
250
o-Ps time of flight [ns]
0.0
0.1
0.2
0.3
o-Ps kinetic energy [eV]
Ps emitted at 150 K 27%; thermal fraction 9% of 27% = 2.4%
Cold Ps production
(Ferragut, Calloni, Dupasquier)
75
Xerogel 85 mg/cm
3
Positronium Fraction %
70
65
Aerogel 150 mg/cm
60
55
50
45
AerogelC 20 mg/cm
3
40
1
10
Positron implantation energy (keV)
3
20 mg/cc
85 mg/cc
100 mg/cc
Here we show that when intense positron bursts are implanted into a thin
film of porous silica, Ps2 is created on the internal pore surfaces. We found
that molecule formation occurs much more efficiently than the competing
process of spin exchange quenching, which appears to be suppressed in the
confined pore geometry. This result experimentally confirms the existence of
the Ps2 molecule and paves the way for further multipositronium work. Using
similar techniques, but with a more intense positron source, we expect to
increase the Ps density to the point where many thousands of atoms interact
and can undergo a phase transition to form a Bose–Einstein condensate6. As
a purely leptonic, macroscopic quantum matter–antimatter system this
would be of interest in its own right, but it would also represent a milestone
on the path to produce an annihilation gamma-ray laser.
Ps2 formation
Future (near)
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Positronium BEC
Annihilation gamma-ray laser
New experiments with antihydrogen
Multipositronic atoms
Injecting 105 positrons in a cavity of 10-13 cm3 gives a density of 1018 e+/cm3
leading to a critical temperatire of the order of tens of K
Positronium BEC
Motivations
• Special system of weakly interacting bosons,
gives good opportunity for studying critical
phenomena near the critical temperature
• Necessary step for implementing an
annihilation gamma laser
Annihilation gamma laser
A large number of positronium atoms
annihilating in a coherent mode
How the annihilation laser works
• Store >1012 polarized positrons in
a multicell trap
•Deliver the positrons in a small
linear cavity (typically 0.2 μm diam
1 mm length) where they form
positronium
•Triplet positronium BEC forms
after cooling below 100 K
•Trigger coherent annihilation by
converting tripler to singlet
by a microwave pulse at the
hyperfine splitting frequency.
Annihilation gamma laser
Motivations (as proposed by Mills)
• High precision measurement of the electron
Compton wavelength
• Resonant photon-photon scattering producing
positronium
• Trigger of the deuterium-tritium fusion
reaction
• Detecting high-Z materials concealed in large
low-Z containers
• Military uses
That’s the end of the talk
Thanks