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Physics with Cold Polarised Positronium David B. Cassidy & Allen P. Mills Department of Physics and Astronomy, University of California, Riverside, USA cassidy@physics.ucr.edu Jefferson Lab March 2009 Positronium

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physics with cold polarised positronium

Physics with Cold Polarised Positronium

David B. Cassidy & Allen P. Mills

Department of Physics and Astronomy,

University of California, Riverside, USA

cassidy@physics.ucr.edu Jefferson Lab March 2009

positronium
Positronium

Although electrons and positrons mutually annihilate each other, they can co-exist for a short time in a bound state called positronium (chemical symbol Ps)

Ps can exist in different configurations depending on the relative spin states of the positron and the electron. These are known as para-positronium (p-Ps), with total spin s = 0 and ortho positronium (o-Ps) with s = 1.

slide3

How long does positronium live?

The Ps lifetime against annihilation depends on the number of photons the atom decays into. For s=1 this must be an odd number and for s = 0 it must be even

The Ps decay rates are

The ground state Ps vacuum lifetimes (inverse of the decay rate) are

τp-Ps = 125 ps

τo-Ps = 142 ns

energy levels of hydrogen and positronium
Energy levels of hydrogen and positronium

Gross energy levels are half that of H. The large Ps magnetic moment makes the hyperfine splitting much larger than is the case for H

R. Ley, Appl. Surf. Sci. 194 301(2002)

porous silica and similar materials

How do we make Ps atoms?

Porous silica (and similar materials)

It’s actually quite easy to make Ps atoms: positrons directed onto a silica film will pick up an electron and make Ps in the bulk which may then diffuse to the surface or internal voids.

e+

Ps

SEQ

In some samples Ps may also become trapped in a surface state where they can interact with other Ps atoms and form Ps2 molecules.

a-SiO2

Ps2

we can also make ps on a metal surface
We can also make Ps on a metal surface:

Single crystal metal (Al(111))

Incident Positron pulse

~ 2 keV

Positron does not diffuse to surface and annihilates in the bulk

Positron trapped in a surface state t ~0.5 ns

e+ emission

Direct/thermal Ps emission

what can happen when ps atoms interact with each other
What can happen when Ps atoms interact with each other?
  • Nothing

o-Ps + o-Ps → o-Ps + o-Ps

  • Spin exchange quenching (SEQ)

o-Ps(m=1) + o-Ps(m = -1)→ 2 p-Ps + 2Eh

  • Positronium molecule (Ps2) formation

X+ o-Ps(m = 1) + o-Ps(m = -1)→ X+ Ps2 + Eb

a decoupled surko trap
A decoupled Surko trap

Nitrogen buffer gas

e+ lifetime ~ 2 seconds

efficiency ~ 20%,

4 Hz duty cycle

> 50,000 e+/pulse

SF6 cooling gas (~ 1 x 10-8 Torr)

e+ lifetime > 500 seconds

efficiency ~ 80%

Maximum 108 e+

Typical pulse, 5 x 107 e+

Pulse out ~ 15 ns FWHM

50 mCi 22Na source

Neon moderator

> 106 e+/sec

RSI 77, 073106 (2006)

slide9

The positron beam density can be controlled via the “rotating wall” electric field in the accumulator

n2D ~ 3×1010 cm-2

y (mm)

n2D ~ 8×109 cm-2

x (mm)

lead fluoride and lead tungstate detectors
Lead fluoride and lead tungstate detectors

Al(111) target

Prompt peak and later times recorded with different scope gain

NIM A 580 1338 (2007)

slide12

We can analyze each waveform separately, and/or we can automatically determine the parameter fd which is a measure of the amount of o-Ps created

fd

fd

single atom positronium decay in voids
Single atom positronium decay in voids
  • Single shot lifetime spectra measure the amount and the decay rate of o-Ps.
  • In porous materials the decay rate, g, depends on the pore size (pick-off).
  • Ps decays exponentially→

gA

gA< gB

gB

how do lifetime spectra change if ps atoms interact with each other
How do lifetime spectra change if Ps atoms interact with each other?
  • If SEQ occurs, o-Ps atoms are converted to p-Ps atoms and decay rapidly (g ~ 8 ns-1)
  • If Ps2 molecules are formed these will also decay rapidly (g ~ 4 ns-1)
  • Present resolution cannot tell the difference.
  • Decay becomes non-linear:

decay due to Ps-Ps interactions

Ordinary (pick-off) decay

parameter b describes strength of SEQ and/or Ps2 formation

slide15

If there is a Ps surface state then heating the sample will thermally desorb Ps, leading to an increase in the Ps fraction

Indicates no Ps surface state Ps2 formation suppressed

Sample has a Ps surface statePs2 formation allowed

We were lucky with our samples: they were each

suitable for one process and unsuitable for the other

slide17

Ps-Ps interactions indicated by density dependent changes in lifetime spectra: the “quenching” effect

Form of Q(T) indicates that the Ps-Ps interactions occur via two surface state atoms

Nature 449 196 (2007)

the near future
The (near) future……

Production of a Bose-Einstein condensate of positronium

This requires spin aligned Ps at high densities

slide21

How can we observe the formation of a Ps BEC?

Doppler broadening of thermal Ps ensemble will be much narrower for the condensed atoms. Can be observed by laser absorption

The lifetime difference between thermal and condensed Ps will be negligible.

concluding remarks
Concluding remarks
  • Ps2 formation observed, 60 years after it was predicted…
  • Observation via timing data and sample properties is compelling, but indirect
  • Excited state of Ps2 allows for a definitive spectroscopic measurement
  • New area of multi-positronium physics now possible. Beyond Ps2, Ps BEC formation should also be achievable using similar techniques, but polarised beam needed.

Thank you for your attention