The antimatter factory is ready for another successful year

CERN’s contribution to antimatter research is interspersed with important breakthroughs: from the creation of the very first anti-atoms in 1995 to the production of large quantities in 2002 and the invention in 2010 of the technique that freezes them down to allow precise studies of their properties. This week, antimatter experiments are on the starting blocks for a new run that promises to be just as exciting.


The Antiproton Decelerator (AD).

CERN’s Antimatter Decelerator (AD) is a unique antimatter factory that produces low-energy anti-protons for creating anti-atoms. The AD delivers its precious ingredients to several experiments that use them to study antimatter properties from many different angles. The 2011 run is about to start, and the experiments are ready to enter a new data-taking period. Their scientific goals for this year include applying spectroscopy techniques for the first time to probe the inner workings of antihydrogen atoms; evaluating the biological effects of antiprotons on living cells; and reaching a very high precision in the study of the antiproton.

Here are some detailed descriptions of the experiments and the results they expect in 2011.

The ACE experiment studies the biological effects of antiprotons on living cells. The potential use of antiprotons in cancer therapy was first discussed in the mid 1980s. Monte Carlo calculations showed that the energy deposited by antiprotons near the end of the range is twice as high as for protons, for identical energy deposition at the entrance to the target. This is caused by the secondary particles produced in the annihilation event, when an antiproton comes to rest. Some of these secondaries are low energy fragments and recoil ions resulting from a break-up of the target atoms and molecules. These have a high linear energy transfer (LET) and are therefore expected to have an enhanced biological effect, which would boost the energy enhancement. ACE's goal is to quantify this effect and study the biological effective dose of antiprotons annihilating biological targets.

In the ACE experimental set-up, living cells are embedded in a gelatine matrix and irradiated with antiprotons from the AD. After irradiation, cells are extracted from defined positions along the beam path, dissolved from the gelatine matrix and incubated so that they can be studied. The effective biological dose of antiprotons is extracted and compared with that of other particles, such as protons and carbon ions.

Early experiments with 46 MeV antiprotons showed a total enhancement factor of 4. Based on this encouraging result we have conducted a sequence of experiments using 126 MeV antiprotons, yielding a clinically relevant penetration depth of 10 cm in water. In contrast to carbon ion beams the ACE collaboration has observed a distinctively different dependency of biological effect with depth, which could be beneficial for certain deep-seated cancers surrounded by critical organs. Experiments planned for the near future aim at increasing the statistical significance of these findings and will allow ACE to perform virtual treatments of specific types of tumour in different locations using protons, carbon ions and antiprotons to identify those cancers where antiprotons could make a significant difference to the clinical outcome.

In parallel, the ACE team has been studying effects on cells situated outside the direct beam, which could indicate potential damage to cell DNA that could cause secondary malignancies due to the treatment. The group has also developed methods for absolute dosimetry of mixed radiation fields as generated by antiproton annihilations, and has studied methods to directly measure LET, an important indicator of increased biological efficiency. The possibility of real-time imaging of the dose distribution has been demonstrated in pilot experiments. These measurements will continue and are expected to yield a solid scientific basis for evaluating antiprotons as candidates for cancer therapy.

Detailed information about the ACE experiment can be found here.

The newest experiment at the AD, AEgIS (Antihydrogen Experiment: Gravity, Interferometry and Spectroscopy) aims to measure the gravitational interaction between matter and antimatter for the first time. The design involves forming a pulsed beam of cold antihydrogen atoms that will fly horizontally at a few hundred meters per second over about one meter through a classical atom interferometer. This so-called Moiré deflectometer allows us to measure the minute drop the atoms undergo over their parabolic trajectory, producing a shadow mask-like pattern on the high-resolution antihydrogen detector, a large-surface silicon microstrip detector. Such a beam - once it has been made – will also open the door to spectroscopic measurements of antihydrogen atoms in flight, but its formation requires a number of daunting challenges to be overcome, including the cooling of antiprotons to sub-Kelvin temperatures, forming positronium atoms (an electron and its antimatter particle – the positron – orbiting around their centre of mass) in large quantities, exciting them into a highly excited state via two laser pulses, bringing them into contact with the cold antiprotons, and finally accelerating the formed antihydrogen atoms into the interferometer.

The experiment is currently being built and installed in the AD, sharing the last remaining experimental area with the ACE experiment, and should be fully operational in 2012.

Detailed information about the AEgIS experiment can be found here.

After a memorable run in 2010 that featured the first trapping of antimatter atoms, the ALPHA collaboration at the Antiproton Decelerator is anxious to get started again in 2011. The current ALPHA device allows the study of interactions between trapped anti-atoms and microwaves. This will be the experimental emphasis in 2011.

The microwaves can be used to probe the hyperfine structure of antihydrogen - a first step down the path of precisely testing the CPT theorem, which requires that hydrogen and antihydrogen have identical spectra. The latest results from 2010 indicate that trapped antihydrogen could survive in ALPHA for a much, much longer time than the 172 ms reported in the proof-of-principle article on trapping. This suggests that it may be possible to perform such measurements on just a few trapped atoms. The collaboration will, however, devote a lot of time to improving the fraction of trappable antihydrogen atoms from the current best level of about 2 out of 10,000 produced atoms. In parallel, ALPHA’s scientists are designing and building an upgraded apparatus that will also allow laser access to the trapped anti-atoms, starting in 2012.

Detailed information about the ALPHA experiment can be found here.

ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) is a Japanese-European collaboration studying diverse aspects of antiproton physics. The most topical activity in 2011 will be the continuation of work to produce a beam of antihydrogen atoms in their ground state, at the extremely low energy of one or two milli-electron volts (meV). This will be used to measure the so-called 'Maser frequency' of antihydrogen. According to the CPT theorem this should be identical to the value for hydrogen. Studies of various promising trap designs for this purpose will be continued, including the Cusp trap (a pair of Helmholtz coils wound in opposite directions) and a radio-frequency RF Paul trap (which can confine both antiprotons and positrons simultaneously). The success of the Cusp trap in synthesizing antihydrogen shared first place with the ALPHA collaboration in Physics World’s top ten breakthroughs of 2010.

ASACUSA also aims to improve the precision of its measurement of the antiproton-electron mass ratio by continuing its laser spectroscopy measurements on antiprotonic helium. Going beyond the one part in a hundred million precision the collaboration recently achieved will also require ever-lower energy antiprotons, and will form an important part of the efforts in 2011. The precision of this measurement may then surpass that for the 'ordinary' proton, in which case we will 'know' the antiproton better than we know the proton.

Other items on the ASACUSA agenda for 2011 include systematic microwave spectroscopy of antiprotonic Helium-3, annihilation cross-section measurements of antiprotons on various metallic elements at 100 KeV, and differential cross-section measurements of antiprotons in hydrogen and helium using a reaction microscope.

Detailed information about the ASACUSA experiment can be found here.

ATRAP’s goal is precise spectroscopic comparisons of antihydrogen and hydrogen. The proposed method is to use antihydrogen atoms that are cold enough to be stored in a magnetic trap. The goal and method, proposed long ago while ATRAP was developing the required cold antiproton method at CERN’s LEAR, are now being vigorously pursued by multiple collaborations at CERN’s AD.

In 2009 ATRAP reported producing the first antihydrogen atoms at the minimum of a very strong magnetic field – the trapping field configuration proposed for storing and studying cold antihydrogen atoms. No trapped antihydrogen atoms were detected at a reported detection sensitivity of 20 atoms per trial (required to distinguish atom annihilations from the cosmic ray background).

The collaboration spent 2010 developing methods to increase the number of cold atoms rather than increasing its detection sensitivity, since more atoms per trial seemed important for achieving the spectroscopy envisioned at ATRAP. The result is that it is now possible to use 1000 times more antiprotons at a 3 times lower temperature than was ever used previously for making antihydrogen atoms.

In the first of two steps, ATRAP reported accumulating enough cold antiprotons to allow the first observation of a centrifugal separation of plasmas of trapped antiprotons and electrons. Second, a sequence of “embedded electron cooling” and “adiabatic cooling” methods was shown to further cool such antiprotons, yielding five million antiprotons at a temperature of 3 K or below. Whether this is the antiproton temperature or an upper limit on the temperature measurement technique remains to be established.

ATRAP hopes to use the much larger and much colder plasmas of antiprotons to produce enough trapped antihydrogen atoms for precise spectroscopy.

Detailed information about the ATRAP experiment can be found here.


by CERN Bulletin