Reaction cross section measurements for the positive antihydrogen ion

production in the GBAR experiment

L. LiszkayIRFU CEA Saclay, France

GBAR collaboration

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 CladéLKB (Laboratoire Kastler-Brossel, Paris)

David LunneyAudric Husson (PhD student)CSNSM (Centre de Sciences Nucléaires et de Sciences de la Matière, Orsay)

Paul-Antoine Hervieux, Giovanni ManfrediIPCMS (Institut de Physique et Chimie des Matériaux de Strasbourg)

Participants (France, ANTION project)

The GBAR collaboration

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

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)

Scheme of GBAR at CERN

Positron source at CEA Saclay Laszlo Liszkay, SLOPOS-13, 19 Sept.2013 6

ELENA Decelerator

Linac W target Moderator Buncher e+ trap

100 keV

𝑝 𝑝

1 keV

e-

9 MeVe+

~1 MeVe+

eV

e+

keV

Posi

tron

ium

ta

rget

clo

ud

e+

keV

AD𝑝

5.3 MeV

Cooling

Lase

r

Experimental chamber with detectors

Lase

r

𝐻+¿1 keV

𝐻+¿20 µK

- e+𝐻

20 µKLa

ser

sCapture

1eV𝐻+¿

Positronium target cloud for the GBAR experiment

eHPsH

eHPspReactions in the cloud

Target cavity

107/pulse(~110 s)~3 keV

Positron-positronium converter (mesoporous SiO2)

eHPsH

eHPsp

eHPsH

eHPsp

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 stateProton and antiproton in the 1-10 keV kinetic energy range

Earlier measurement

eHPsp

Merrison et al,, Phys. Rev. Lett. 78, 2828(1997)

Method: positron detection

• 10-16 keV proton energy

• Ground state Ps only

New theoretical calculations: proton reaction

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013)

eHPsp• Theoretical calculations by one of the partners (IPCMS)

• 2p & 3d states are good candidates

• Optimal energy depends on the state

New theoretical calculations: second reaction

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013)

eHPsH• 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

New theoretical calculations: two reactions in the Ps cloud

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013)

eHPsH

eHPsp

• Advantage of excited Ps state is not clear

• Measurements are needed to optimize the energy & ps state

• H* relaxation to ground state is important

New theoretical calculations: comparison with experiment

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013) Merrison et al,, Phys. Rev. Lett. 78, 2828(1997)

eHPsp

• Calculation agrees well with measurement for the proton reaction

• No test yet for the four-body reaction

Cross section meas.: target chamber and detector

(anti)proton beam (anti)atoms

protons

(anti)ions

grid

MCP

Positron pulse

Electricquadrupole

CCDcamera

Fast phosphor screen

Viewport

Faraday cup

Ion detection

• Camera: fast shutter (1µs)• MCP: switched (~400 ns)

Positronium target cloud

Detection of keV atoms with an MCP

Detection efficiency corresponds to the sensitive area of the MCP above ~1 keV (hydrogen)

• 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

Penning trap for e+ (RIKEN)

Slow e+ rate 3x106 s-1

e- linac(4.3 MeV)

e+/e- magnetic separator

W target+ W mesh moderator

The slow positron beam at Saclay (CEA/IRFU)

Slow positron drift tube (~10 eV)

Beam switch/user port (materials science)

Multiring trap: positron cooling by trapped electrons

5 T field

e+ beam

• Electron cooling• Switched entry (synchronized with linac pulses)• Bunched exit (short, intense pulse)

~10 mm diam.4-5 keV

1 x 10 mm ellipse

Positron optics after the trap

~100 Gauss (?)

• Electrostatic focussing• Exit from magnetic field with ~4 keV energy

Optical excitation of Ps

3D - 410 nm Doppler-free(in construction, GBAR)

2P - 243 nm frequency comb

Proton source

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

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

eHPsp

eHPsH

eHPsp

eHPsH

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

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

Upgrade - GBAR at CERN

300 mm

4.3 MeV (magnetron)200 Hz, 2.5 µs pulse120 mA peak current70 µA average current

3x106 e+/s

9 MeV (Klystron)300 Hz 300 mA peak current200 µA average current

1x108 e+/s

~900 mmElectron target + moderator: same construction

Antiproton deceleration after ELENA

• Antiprotons at ~1-10 keV energy

• Switched decelerator

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-CE33-0008-01