The ASACUSA experiment (Atomic Spectroscopy Using Slow Antiprotons)

Foreword

Techniques to produce substantial quantities of anti-matter have been developed at CERN for the needs of large accelerators: the SPS was accelerating protons and antiprotons, and LEP is accelerating electrons and positrons (anti-electrons).

Anti-particles are identical to normal particles except for a) opposite electric charge b) opposite type of matter. The two types of matter annihilate, into pure energy, when they get in touch.
Besides high energy experiments, aiming at studying the sub-structure of 'elementary' particles like the proton, another family of very-low energy experiments consist in slowing down anti-particles to the lowest possible energy, and form with them atoms and molecules that can be studied with classical spectroscopy methods.

At CERN, these experiments are carried on at the Antiproton Decelerator (AD) machine, which from July 1999 will replace the former LEAR (Low Energy Antiproton Ring).

The AD will decelerate antiprotons from 100 MeV, as produced by the PS, down to 5.8 MeV; at this energy the antiprotons can be stopped in a dense target, and form exotic atoms.

Antiproton-Helium atoms

When low energy antiprotons (p-) are introduced into ordinary matter, a chain of processes takes place:
- they slow down
- they form 'antiprotonic' atoms (some atomic electron is replaced by an anti-p)
- the p- of the antiprotonic atoms annihilate immediately with the protons of the nucleus, a process favored by atomic collisions in media dense enough.

A noticeable (and hitherto unique) exception to this fast decay is represented by antiprotonic Helium atoms, p- He+, in which an antiproton replaces an orbital electron.
About 3% of all the p- He+ atoms created when a bunch of antiprotons stops in a Helium target are metastable, and survive for a few microseconds before the antiproton falls into the nucleus and annihilates.
There is time, while the metastable state decays by multi-step radiative processes, to submit the surviving p- He+ atoms to laser spectroscopy.
The decaying atoms are excited by laser pulses; when the frequency, or wavelength of the laser light coincide with atom energy-level jumps, the decay rate is enhanced by the laser light, and the corresponding short peak in annihilation products release can be detected.
By superimposing a moderate RF excitation to the laser pulses, it is possible to induce triple resonances and measure the hyperfine structure of atomic energy levels, related to the small interaction of the electron's spin with the antiproton angular momentum.
The theoretical activity has also been largely stimulated by the need of calculating multi-body energy levels with a precision comparable to that of experimental measures (a few parts per million).

The ASACUSA program

The ASACUSA or AD-3 experiment is a Japanese-European collaboration (about 40 physicists), largely founded by the Japanese Ministry of Science.

The experiment will study the properties of p-He+ atoms at various energies, as well as slow antiproton properties in matter (stopping power in thin foils and gases, channeling, ionization of atoms and molecules, drift velocity in gases, cross section for antiprotonic atom formation).

ASACUSA Phase I - Fast-extracted bunches of antiprotons are stopped in a (dense) helium gas target and produces instantaneously a sample of metastable p-He+. Laser-microwave triple resonance will be used to measure the p-He+ atoms energy levels and transitions.

Phase II (2000) - A decelerating Radio Frequency Quadrupole (RFQ) will be added to the AD beam to reduce the antiproton energy from MeV to keV values. This will allow investigations of the atomic interactions of antiprotons at very low energies.

Phase III (> 2000) - The addition of a Penning trap will allow to capture antiprotons at rest (see also the ATHENA experiment), and re-accelerate them to eV energies.
This will allow the creation of p-He+ and other exotic atoms with low energy and longer lifetimes, in quasi-vacuum conditions. Protonium atoms, p-p+, very attractive for theoretical calculations, are also expected.

The experiment

Antiproton beam characteristics: 10^7 antiprotons/bunch, 1 bunch/sec; the bunches are 200nsec-long, ~ 1mm2 in cross-section.
The X-Y profile of the beam is measured upstream of the target with a resolution of 0.5mm by a parallel plate ionization chamber, with Camac read-out.
Cryogenic gas target: He at 5 degrees Kelvin.

About 97% of the incoming antiprotons annihilate promptly in the helium gas; the prompt decay products (mostly pions), detected by a plastic Cherenkov counter, supply a measure of the intensity of each beam pulse.
A special gateable-gain photomultiplier (Hamamatsu) is used to measure both the first light pulse and the smaller signal produced over several msec's by the decays of the remaining 3% of metastable p- He+ atoms.

A fast LRS digital oscilloscope is used to record the overall decay signal (up to 20msec), as well as the small 5 ns spikes related to the laser/RF induced decays of the metastable population.

The control and DAQ system of the experiment is based on LabVIEW and a network of PCs (WinNT and/or Linux); most of the instruments are controlled via GPIB or Ethernet.

The experimental equipment includes:

• YAG-pumped dye lasers for optical spectroscopy (2 sets)
• RF equipment (Microwave synthesizer and power amplifier ) for microwave resonance spectroscopy.
• Optical tables and accessories; chemical laboratory for the laser solutions.
• Cryostats and cryogenic targets (He at 5 K)
• Gas supply and mixing system
• Camac readout for p- beam profile monitor
• Annihilation detector (Cherenkov counters + fast gating PM)
• LRS digital oscilloscope, Ethernet controlled
• Data acquisition computers.

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I'm indebted to J. Eades, CERN contact person for the experiment, and E. Widmann, project leader for antiprotonic helium spectroscopy experiments, for their very kind explanations.