New physics with intense positron beams

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

2. OUTLINE • Why do weneedpositrons? • How do wegetpositrons? • How do we produce tunablepositronbeams? • What can we do with intense positronbeams? 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)

3. Why do we need positrons? • Medicaluses: PET • Atomicphysics: Positroninteraction with gas molecules (theoryvalidation free ofexchangeeffects); Spectroscopyof positronium (quantum electrodynamictests) • Solid State Physics: Electron momentumspectroscopy and Fermi surfaces • Materials Science: Positronspectroscopyof lattice defects

4. Positron annihilation as a probe for electronic structure 2D-ACAR • 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: small deviation from 180º due to finite (electron) momentum → small angular deviation say in x, y directions if z = mean direction of γ-emergence. 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.

5. 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.)

6. 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

7. 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.

8. PALS

9. CDB The widthof the annihilationlinecomesfrom the Doppler broadening due to the motionof the annihilatingpair Trappedpositrons do notoverlap with fast coreelectrons. The annihilationlinebecomesnarrower.

10. Positronstudyof microstructural transformation in alloys CDB

11. Defectdepthprofiling with a tunablemonoenergeticpositronbeam An example regarding a SiGe/Si/SiO2 multilayer grown on Si

12. How do wegetpositrons? • Nuclearbeta+decay advantages: a) usable in on-campus laboratories; b) producespolarizedpositrons, which are importantfor some experiments) • Pair production fromnuclear gamma rays advantages: high intensity; disadvantages: need a reactor or anaccelator) • Pair production fromBrehmstrahlung advantages: high intensity; disadvantages: needanaccelerator

21. A possibility to be exploredPositron production by ultra-intense laser pulses

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

23. Positronyield vs. laser intensity(predicted)

24. 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

25. This is not a simulation!

26. How do we produce tunable positron beams?

27. Positron thermalisation in a solid Most used moderators W single crystals Pt Solid Ne Requirements: Negative positron workfunction Low defect density Stable surface

28. 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)

30. Work in progress at FRII Munich (Koegel)

33. Surko’s positron trap

34. Progress in positrontraps: multicells

35. MCT layout

37. Recentadvances and currentresearchrequiring high intensitypositronsources • Fundamental physics with antihydrogen (ATHENA, ATRAP, AEGIS). • First evidence of Ps2 molecules

38. Method II: Antiproton + Rydberg Ps (ATRAP) 104 antiprotons 108 e+ Method I: Antiproton + Positron (ATHENA) C. H. Storry et al., First Laser-Controlled AntihydrogenProduction, Physical Review Letters 93, 263401 (2004) • Spontaneous radiative recombination • Three body recombination Two-stage Rydberg charge exchange 14 ± 4 antihydrogen atoms 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

39. AEgIS 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 The vertical displacement (gravity fall) will be measured on the last (sensitive) plane of the deflectometer Antiprotons Positrons Such measurement would represent the first direct determination of the gravitational effect on antimatter Varenna - July 2009

40. e+ Vacuum Solid Positron beam Positronium emission Ps Ps Ps Ps Positronium yield from materials: requirement of 10% (reemitted, cold) out of 108 in ortho-Ps. (lectures by R. Brusa and A. Dupasquier) Silicon nanochannel material: 10-15 nm pores: max o-Ps formation observed 50% Velocity of reemitted Ps: 5 x 104 m/s (corresponding to thermalized at 100 K) Laser excitation of the Positronium to Rydberg states (more on this later on) Varenna - July 2009

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

42. Cold Ps production (Ferragut, Calloni, Dupasquier)

43. 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.

44. Ps2 formation

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

46. 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

47. Positronium BECMotivations • 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

48. Annihilation gamma laser A large number of positronium atoms annihilating in a coherent mode

49. 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.

50. Annihilation gamma laserMotivations (asproposedbyMills) • High precisionmeasurementof the electron Compton wavelength • Resonantphoton-photonscatteringproducing positronium • Trigger of the deuterium-tritiumfusionreaction • Detectinghigh-Zmaterialsconcealed in largelow-Zcontainers • Militaryuses