Antimatter: The Factory

Over the past 20 years scientists at CERN have been using antiparticles in many different ways for their daily work. 

Antiparticles can be generated by colliding subatomic particles. Before being delivered to the various physics experiments, they must be isolated, collected and stored in order to tune their energy to the appropriate level.

Until now, each of these steps has been carried out by a dedicated machine with the main purpose of providing high energy antiparticles. 


But now the first "self-contained antiproton factory", the Antiproton Decelerator (or AD), is operational at CERN . It will produce the low energy antiprotons needed for a range of studies, including the synthesis of antihydrogen atoms - the creation of antimatter.


What is the AD?
The Antiproton Decelerator is a very special machine compared to what already exists at CERN and other laboratories around the world. So far, an "antiparticle factory" consisted of a chain of several accelerators, each one performing one of the steps needed to produce antiparticles. The CERN antiproton complex is a very good example of this.

At the end of the 70's CERN built an antiproton source called the Antiproton Accumulator (AA). Its task was to produce and accumulate high energy antiprotons to feed into the SPS in order to transform it into a "proton-antiproton collider".
As soon as antiprotons became available, physicists realized how much could be learned by using them at low energy, so CERN decided to build a new machine: LEAR, the Low Energy Antiproton Ring. Antiprotons accumulated in the AA were extracted, decelerated in the PS and then injected into LEAR for further deceleration. In 1986 a second ring, the Antiproton Collector (AC), was built around the existing AA in order to improve the antiproton production rate by a factor of 10.
The AC is now being transformed into the AD, which will perform all the tasks that the AC, AA, PS and LEAR used to do with antiprotons, i.e. produce, collect, cool, decelerate and eventually extract them to the experiments. 

CERN's Antiproton Collector (AC) and CERN's Antiproton Accumulator (AA)

What does the AD consist of?

The AD ring is an approximate circle with a circumference of 188 m. It consists of a vacuum pipe surrounded by a long sequence of vacuum pumps, magnets, radio-frequency cavities, high voltage instruments and electronic circuits. Each of these pieces has its specific function:
- Antiprotons circulate inside the vacuum pipe in order to avoid contact with normal matter (like air molecules), and annihilate. The vacuum must be optimal, therefore several vacuum pumps, which extract air, are placed around the pipe.
- Magnets as well are placed all around. There are two types of magnets: the dipoles (which have a North and a South pole, like the well-known horseshoe magnet) serve to change the direction of movement and make sure the particles stay within their circular track. They are also called "bending magnets". Quadrupoles (which have four poles) are used as 'lenses'. These "focussing magnets" make sure that the size of the beam is smaller than the size of the vacuum pipe. 

- Magnetic fields can change the direction and size of the beam, but not its energy. To do this you need an electric field: this is provided by radio-frequency cavities that produce high voltages in synchronicity with the rotation of particles around the ring.
- Several other instruments are needed to perform more specific tasks: two cooling systems "squeeze" the beam in size and energy; one injection and one ejection system let the beam in and out of the machine. 

How does the AD work ?
Antiparticles have to be created from energy (remember: E = mc²). This energy is obtained with protons that have been previously accelerated in the PS. These protons are smashed into a block of metal, called a target. We use Copper or Iridium targets mainly because they are easy to cool (but a piece of English beef would serve the same purpose - it would just roast very quickly and is rather messy). 

Then, the abrupt stopping of such energetic particles releases a huge amount of energy into a small volume, heating it up to such temperatures that matter-antimatter particles are spontaneously created (this is explained in our briefing room).
In about one collision out of a million, an antiproton-proton pair is formed. But given the fact that about 10 trillion protons hit the target (about once per minute), this still makes a good 10 million antiprotons heading towards the AD. 

The newly created antiprotons behave like a bunch of wild kids; they are produced almost at the speed of light, but not all of them have exactly the same energy (this is called "energy spread"). Moreover, they run randomly in all directions, also trying to break out 'sideways' ("transverse oscillations"). Bending and focussing magnets make sure they stay on the right track, in the middle of the vacuum pipe, while they begin to race around in the ring.
At each turn, the strong electric fields inside the radio-frequency cavities begin to decelerate the antiprotons. Unfortunately, this deceleration increases the size of their transverse oscillations: if nothing is done to cure that, all antiprotons are lost when they eventually collide with the vacuum pipe. 

To avoid that, two methods have been invented: 'stochastic' and 'electron cooling'. Stochastic (or 'random') cooling works best at high speeds (around the speed of light, c), and electron cooling works better at low speed (still fast, but only 10-30 % of c). Their goal is to decrease energy spread and transverse oscillations of the antiproton beam.
Finally, when the antiparticles speed is down to about 10% of the speed of light, the antiprotons squeezed group (called a "bunch") is ready to be ejected. One "deceleration cycle" is over: it has lasted about one minute.

A strong 'kicker' magnet is fired in less than a millionth of a second, and at the next turn, all antiprotons are following a new path, which leads them into the beam pipes of the extraction line. There, additional dipole and quadrupole magnets steer the beam into one of the three experiments. 

The AD experiments

Three experiments are installed in the Antiproton Decelerator's experimental hall:
ASACUSA " Atomic Spectroscopy and Collisions using Slow Antiprotons";
ATHENA "Antihydrogen Production and Precision Experiments" and
ATRAP "Cold Antihydrogen for Precise Laser Spectroscopy". 

ATHENA and ATRAP's goal is to produce antihydrogen in traps, by combining antiprotons delivered by the AD with positrons emitted by a radioactive source.
Antihydrogen atoms were first observed at CERN in 1995, and later (1997) at Fermilab. In both cases they were produced in flight, that means they moved at nearly the speed of light, i.e. much too fast to allow precise measurements on any of their proprieties! They made unique electrical signals in detectors that destroyed them almost immediately after they formed.
Now the idea is to produce slow antihydrogen atoms and store them into "traps", allowing extremely accurate comparisons of the properties of hydrogen and antihydrogen.
ASACUSA, on the other hand, will synthesize "exotic" atoms, in which an electron is replaced by an antiproton. Precise laser spectroscopy of these exotic atoms is expected to reveal lots of information on the behavior of atomic systems.

Source : CERN