1. L. Liszkay IRFU CEA Saclay, France GBAR collaboration
Reaction cross section measurements for the positive antihydrogen ion production in the GBAR experiment
L. Liszkay IRFU CEA Saclay, France
GBAR collaboration

2. Participants (France, ANTION project)
L. Liszkay, P. Comini*, P. Debu, P. Pérez, J.M. Rey, Y. Sacquin, B. Vallage, D. P. van der Werf** IRFU CEA Saclay (France) A. Maia-Leite, Liam Dodd (PhD students)
* present address: ETHZ (Switzerland) ** permanent address: Univ. Swansea (UK)
François Nez, Pierre Cladé
LKB (Laboratoire Kastler-Brossel, Paris)
David Lunney Audric Husson (PhD student)
CSNSM (Centre de Sciences Nucléaires et de Sciences de la Matière, Orsay)
Paul-Antoine Hervieux, Giovanni Manfredi
IPCMS (Institut de Physique et Chimie des Matériaux de Strasbourg)

3. The GBAR collaboration

4. Introduction - the GBAR project Antihydrogen ion creation for GBAR
Outline
Introduction - the GBAR project
Antihydrogen ion creation for GBAR
Reaction cross section - past experiments
Reaction cross sections - theoretical results
Preparations for new cross section measurements
Reaction zone
Detection
Positron pulse
Laser
Proton source
Antiproton beamline
Summary and outlook

5. The GBAR experiment at CERN
Direct observation of the gravitational free fall of antihydrogen (two experiments: AEGIS and GBAR)
Requires very cold antihydrogen (10 µK)
Distinctive idea: cool down posively charged antihydrogen ion, then photodetach the extra positron
Antiprotons from CERN AD + ELENA (100 keV)
Positrons from a linac-based source + high field trap (5T)
(GBAR was accepted by CERN research Board in 2012)

6. Scheme of GBAR at CERN Laser 𝑝 𝑝 𝑝 AD ELENA Decelerator 5.3 MeV
100 keV
1 keV
Linac e-
W target e+
Moderator e+
Buncher e+
e+ trap e+
9 MeV
~1 MeV eV
keV
keV
Positronium target cloud
Laser
Lasers
Experimental chamber with detectors 𝐻
𝐻 +
𝐻 +
𝐻 +
- e+
Cooling
Capture
20 µK
20 µK
1eV
1 keV
Positron source at CEA Saclay
Laszlo Liszkay, SLOPOS-13, 19 Sept.2013

7. Positronium target cloud for the GBAR experiment
Reactions in the cloud
107/pulse (~110 s)
~3 keV
Positron-positronium converter (mesoporous SiO2)
Target cavity

8. Cross section measurements: aims
Measurement of the cross section of the following reactions of positronium (Ps, positron-electron bound system):
1. Hydrogen and negative hydrogen ion production
2. Antihydrogen and antihydrogen ion production
With positronium (Ps) is in the fundamental, 3D or 2P state
Proton and antiproton in the 1-10 keV kinetic energy range

9. Earlier measurement 10-16 keV proton energy Ground state Ps only
Merrison et al,, Phys. Rev. Lett. 78, 2828(1997)
Method: positron detection

10. New theoretical calculations: proton reaction
Theoretical calculations by one of the partners (IPCMS)
2p & 3d states are good candidates
Optimal energy depends on the state
P. Comini and P. –A. Hervieux, N. J. of Phys. 15, (2013)

11. New theoretical calculations: second reaction
Calculation for ground state H
4-body problem, quantitative results may be unreliable
Cross section is much lower at excited state of H
Max. at lowest energy (above threshold)
1s, 2p, 3d states OK
P. Comini and P. –A. Hervieux, N. J. of Phys. 15, (2013)

12. New theoretical calculations: two reactions in the Ps cloud
Advantage of excited Ps state is not clear
Measurements are needed to optimize the energy & ps state
H* relaxation to ground state is important
P. Comini and P. –A. Hervieux, N. J. of Phys. 15, (2013)

13. New theoretical calculations: comparison with experiment
Calculation agrees well with measurement for the proton reaction
No test yet for the four-body reaction
P. Comini and P. –A. Hervieux, N. J. of Phys. 15, (2013)
Merrison et al,, Phys. Rev. Lett. 78, 2828(1997)

14. Cross section meas.: target chamber and detector
Faraday cup
Positron pulse
Fast phosphor screen
protons
grid
(anti)proton beam
(anti)atoms
CCD
camera
MCP
Positronium target cloud
Electric
quadrupole
(anti)ions
Viewport
Ion detection
Camera: fast shutter (1µs)
MCP: switched (~400 ns)

15. Detection of keV atoms with an MCP
Gated MCP + fast phosphor screen
fast ( 1 µs) CCD camera
Low background detection needed:
Suppress direct annihilation gamma background
Suppress MCP noise
Suppress camera dark noise
Separate charged particles
Detection efficiency corresponds to the sensitive area of the MCP above ~1 keV (hydrogen)

16. The slow positron beam at Saclay (CEA/IRFU)
Beam switch/user port (materials science)
Slow positron drift tube (~10 eV)
Penning trap for e+ (RIKEN)
e+/e- magnetic separator
W target + W mesh moderator
e- linac
(4.3 MeV)
Slow e+ rate
3x106 s-1

17. Multiring trap: positron cooling by trapped electrons
Electron cooling
Switched entry (synchronized with linac pulses)
Bunched exit (short, intense pulse)
5 T field
e+ beam

18. Positron optics after the trap
Electrostatic focussing
Exit from magnetic field with ~4 keV energy
~100 Gauss (?)
~10 mm diam.
4-5 keV
1 x 10 mm ellipse

19. Optical excitation of Ps
3D nm Doppler-free (in construction, GBAR)
2P nm frequency comb

20. Proton source
Penning-discharge source with electrostatic focussing
The same source is used to develop the antiproton decelerator

21. Experiments at IRFU (Saclay) and CERN
The positron beam intensity is sufficient only to measure the proton reaction
Energy dependence will be measured
Reaction with Ps in 3D and 2P states will be measured
The measurement will be continued at the stronger source at CERN
When antiprotons are available, the antiproton and antihydrogen reaction will be performand

22. Cross section of the (anti)hydrogen-Ps reaction
At the moment we have no tool to generate hydrogen atoms in the relevant energy range
The cross section will be deduced from the quantity of ions generated in the two reactions in one Ps target cloud
It is sufficient for GBAR but limits the precision of the cross section measurement

23. Second phase: measurements at CERN
New positron source --> measurement of the double reaction (CEA source is too weak)
Antiproton beam (AD+ELENA+GBAR decelerator) --> cross section of the reaction with antiproton and antihydrogen

24. 3x106 e+/s 1x108 e+/s Upgrade - GBAR at CERN 300 mm ~900 mm
4.3 MeV (magnetron)
200 Hz, 2.5 µs pulse
120 mA peak current
70 µA average current
3x106 e+/s
300 mm
9 MeV (Klystron)
300 Hz 300 mA peak current
200 µA average current
1x108 e+/s
Electron target + moderator: same construction
~900 mm

25. Antiproton deceleration after ELENA
Antiprotons at ~1-10 keV energy
Switched decelerator

26. Summary
Preparations for cross section measurements at IRFU
Continuation at CERN (ion creation, antiproton and antihydrogen reaction)
Essential information for GBAR (optimal Ps state, proton energy)
See also talk of Sebastian Wolf (next talk)
The work is supported by the Agence National de la Recherche, project number ANR-14-CE