Pair production experiment
The Astra Gemini Laser: April 2010
Dr Christopher D Murphy
University of Oxford
15th July 2009
Overview
 The Astra Gemini Laser
 Detection of positron production
o  Positrons and electrons
o  Annihilation Gammas
o  Positronium
 Experimental Parameters
 Two Experimental Layouts
o  High-Z foil pair production
o  Vacuum Pair Production
 Possible Conclusions
The Astra Gemini Laser
 6 week experimental campaign successfully proposed
 Timetabled for April 2010
 Laser Specifications
o  2 Beams
o  One beam currently operational. Second beam commissioning
February 2010
o  15J of energy per beam
o  Readily obtainable. Could possibly be increased in April ʻ10
o  40 fs pulse duration
o  40 fs has been reached, but currently 45 fs is more typical
o  2 micron spot containing 50% of the energy
15J × 50%
1
21
−2
I=
=
6
×10
Wcm
40 ×10−15 π (1×10−4 ) 2
> One shot every minute
Detection Possibilities
Need to detect:
 Positrons directly
o  Positron-electron magnetic spectrometer
 Gamma from positron annihilation
o  Energy resolved gamma detection
o  Shielding sub 200keV to improve signal to noise in gamma
scintillators
o  CdTe energy resolving semiconductor detectors
 Signature from positronium (p-Ps or o-Ps)
o  Time delayed gamma emission
o  Spectral line emission from electron transitions
The Astra Gemini Target Area
 Vacuum Specifications
o  Little or no effort has been put into improving the vacuum
o  The last experimental run operated at 10-5 mbar
o  They did differentially pump a small chamber to 10-6 mbar
o  This would be configurationally difficult at focus
 What does this mean?
o  Considering the volume over which the intensity is above
1019 Wcm-2
o  Few 105 molecules are present
o  Could use a prepulse to clear the focal volume:
o  Need nanosecond prepulse to clear the focal
volume?
o  Is readily available from the laser
Pair production in high-Z foils
 Proposed two-foil experiment
 Limiting factors:
o  Laser matter and electron
matter interaction will
produce gammas at t=0  Possibilities for detection:
o  e+ e- pairs produced in the solid
foil will be directly observable
o  Lifetime of Ps will be
reduced in high density or
high field environment
o  Ps produced in the inter-foil
space will be spatially separated
to improve observation
probability
o  Target is opaque to Ps
emission line n=2 to n=1
o  Ps lifetime in inter-foil space will
be closer to the vacuum lifetime
Vacuum Experiment
 ʻInteractionʼ gamma is small
 Many NaI gamma detectors
observe coincident gammas
 Number of positrons will be small
 Need a trap to take positrons
away to a low noise region
 Transient quasi-particles are too
transient to be probed?
 Positronium production unlikely
Experimental plan
Week 1
Setup (synchronization, diagnostic setup)
Week 2
Pair production with high-Z foils
Week 3
Pair production with high-Z foils
Week 4
Ultrahigh vacuum generation
Week 5
Vacuum pair production (ϒϒ/refectivity)
Week 6
Vacuum pair production (ϒϒ/refectivity)
Possible Conclusions
 Solid Target Experiment
o  Direct detection of e+
o  Epp spectrometer
o  Gamma flash renders
scintillators ineffective for ~ 3 ns
after the laser shot
o  Useless for p-Ps
o  Possible for o-Ps
o  Observation of line emission
from Ps in vacuum?
o  Requires highly shielded
position sensitive detection
 Vacuum Experiment
o  Direct detection of e+ is
difficult
o  Possible with a trap?
o  ʻInteractionʼ gamma flash
small
o  Direct detection of
coincident gammas
feasible
o  Line emission /
shadowography unlikely
due to low numbers