New physics with intense positron beams Alfredo Dupasquier Frascati, 20 gennaio 2010

OUTLINE 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 Annihilation of thermalised positron in a solid: mostly 2γ photons, each with energy mc 2. Centre of mass frame: ; spin and momentum conservation → γ’s in 180º Positron is thermalised, thus p + << p -. Laboratory frame: small deviation from 180º due to finite (electron) momentum → 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.)

(Major et al, Phys.Rev. Lett. 92, (2004 ) 3D reconstruction of the Fermi surface of ZrZn 2 obtained from 2D-ACAR data 3D representation of the Fermi surface of ZrZn 2 from LMTO calculations

Positron annihilation as a probe for lattice defects PALS and CDB 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

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. CDB

Positron study of microstructural transformation in alloys CDB

Defect depth profiling with a tunable monoenergetic positron beam An example regarding a SiGe/Si/SiO2 multilayer grown on Si

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

Green: electrons Yellow: Brehmstrahlung Red: positrons Simulation of electron-photon- positron shower for 25 MeV electrons on gold

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 Extracted positrons per pulse 1.6x10 10 forwards, 2x10 9 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

Radioactive source (Na mCi) and moderator (W - 1  m) Monoenergetic positron beam Positron kinetic energy: 0.1 – 20 keV 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 Ps 2 molecules

Method I: Antiproton + Positron (ATHENA) Varenna - July antiprotons 10 8 e + Spontaneous radiative recombination Three body recombination 14 ± 4 antihydrogen atoms Method II: Antiproton + Rydberg Ps (ATRAP) Two-stage Rydberg charge exchange C. H. Storry et al., First Laser-Controlled Antihydrogen Production, Physical Review Letters 93, (2004) In Aegis: Antiproton + Rydberg Ps (obtained by Ps and laser excited) Large cross section Quantum states of antihydrogen related to Ps quantum number Reaction suitable for cold antihydrogen production (cold antiprotons!)

Varenna - July 2009 Such measurement would represent the first direct determination of the gravitational effect on antimatter A E g I S in short Acceleration of antihydrogen. Formation of antihydrogen atoms Antiprotons Positrons The antihydrogen beams will fly (with v~500 m/sec) through a Moire’ deflectometer The vertical displacement (gravity fall) will be measured on the last (sensitive) plane of the deflectometer Positronium: 10 7 atoms Antiprotons: 10 5 Antihydrogen: 10 4 /shot

Varenna - July 2009 Ps VacuumSolid Positron beam Ps Positronium emission e+ Positronium yield from materials: requirement of 10% (reemitted, cold) out of 10 8 in ortho-Ps. Velocity of reemitted Ps: 5 x 10 4 m/s (corresponding to thermalized at 100 K) Laser excitation of the Positronium to Rydberg states (more on this later on) (lectures by R. Brusa and A. Dupasquier) Silicon nanochannel material: nm pores: max o-Ps formation observed 50%

Cold Ps production (Brusa, Mariazzi) Ps emitted at 150 K 27%; thermal fraction 9% of 27% = 2.4%

Cold Ps production (Ferragut, Calloni, Dupasquier)

Here we show that when intense positron bursts are implanted into a thin film of porous silica, Ps 2 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 Ps 2 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.

Ps 2 formation

Future (near) Positronium BEC Annihilation gamma-ray laser New experiments with antihydrogen Multipositronic atoms

Injecting 10 5 positrons in a cavity of cm 3 gives a density of e + /cm 3 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 >10 12 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