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ANTIMATTER DECELERATOR - THE SCIENCE
Producing the beams
Producing low energy antiproton beams is a delicate task. First of all the antimatter has to be produced. This is done by smashing a beam of matter particles into a target at high energy to produce a plethora of new particles and antiparticles. Then, because oppositely charged particles curve in opposite directions in a magnetic field, and because the degree of curvature is related to mass, antiprotons can be siphoned off from the other particles. These are then stored up and decelerated before being provided to the waiting experiments.
Physicists measure energy and momenta in units called electron volts, eV and electron volts divided by the speed of light eV/c. The energy of beams in the Laboratory's high energy accelerators in operation today are counted in billions of eV, or GeV for short. One GeV is about a thousandth of the energy carried by a mosquito in flight, in other words not very much energy at all. But since the fundamental particles CERN accelerate are millions and millions of times smaller than mosquitoes, the concentration of energy in those tiny volumes is enormous.
For the antimatter experiments low energies are required, so the job of the Antiproton Decelerator is to take the highly energetic antiprotons produced in collisions and slow them down. The starting point is a beam of protons at 26 GeV/c from an accelerator that has been in operation at CERN for over 40 years, the Proton Synchrotron, PS. This beam is smashed into a target, and the resulting antiprotons are collected at 3.57 GeV/c and injected into the AD.
The antiprotons arrive in the AD like a spray from an aerosol with a large spread of energy and position. They have to be forced to stay together in the beam. This is achieved using a technique called stochastic cooling, whereby measurements on the beam on one side of the ring are transmitted across the middle of the ring in time to tune AD components before the circulating beam arrives. Stochastic cooling soon has the beam circulating in an orderly manner.
Then the deceleration can begin. To complicate matters, decelerating the beam tends to exaggerate the spread of energy and beam size, an effect known as 'adiabatic blow up' so deceleration takes place in several steps interspersed with more cooling. First the antiprotons are decelerated to 2 GeV/c, then to 300 million eV/c, MeV/c. Here, a second cooling technique is applied. Known as electron cooling, a dense cool beam of electrons travels in the same direction as the antiprotons. It is similar to hot water being mixed with cold water to get lukewarm water.
This has the effect of lining the antiprotons up along the direction they are moving. With the beam thus ordered, the final deceleration can be performed, taking the antiprotons' momenta down to 100 MeV/c - slow enough to be captured by experiments. About three quarters of the original antiprotons are lost in the deceleration, but that still leaves over 10 million in each bunch delivered to the experiments. Bunches are delivered at the rate of about one per minute.
Commissioning the AD began in 1999 when tests were made with protons in the new machine. Protons were used in the setting-up process because it is easier to make highly intense beams out of matter then out of antimatter. They were used to gain experience in running the AD with low energy beams. At 100 MeV/c, the magnetic fields needed to steer the beam around the AD ring are so low that even the Earth’s magnetic field has to be taken into account. The first proton beam decelerated to the target energy came in July 1999. After gaining experience with low energy running, the switch to antiprotons was made, and on Thursday 29 June 2000 the AD team decelerated an antiproton beam to 100 MeV/c and ejected it to the experimental areas for the first time.
Following its successful commissioning, the AD will run continuously from Monday to Friday for about 3000 hours each year between April and October.
Trapping the Antimatter
The ASACUSA experiment looks at 'atomcules'. These unusual atoms consist of a helium atom in which one electron has been replaced by an antiproton. Theoretically, such atomcules were predicted to live for just a trillionth of a second before the antiproton fell into the helium nucleus and annihilated but early studies showed that a small fraction - about 3% - lived for several millionths of a second. That may not seem very impressive, but it would be like finding that most planets live only for a few hundred years before falling into their suns while just a few survived for a significant part of the history of the universe. This observation opened up a new window for comparing the world with the antiworld. ASACUSA's atomcules are easier to make than antihydrogen atoms, and spectroscopic techniques can easily be used to study them.
The goal of ATHENA, the direct comparison of the properties of antihydrogen and hydrogen atoms using very high precision laser spectroscopy. Antiprotons from the AD are further decelerated by passing them through thin foils before they reach the trap, a 2.5 centimetre diameter, 50 centimetre long electromagnetic cage. Once inside, powerful electromagnetic fields prevent the slow-moving antiprotons from escaping. The apparatus is designed to capture and to cool up to a million antiprotons to energies below 0.001 eV and mix them with positrons. These are collected from the decay of a radioactive isotope and trapped inside a second trap. About 100 million positrons can be collected within a minute and when all of those are mixed with the antiprotons, around 10-100 antihydrogen atoms per second will be produced. In the experiment's first phase, these will be left to annihilate and their annihilations measured. In a second phase, the anti-atoms will be trapped and precise laser spectroscopy will be used to compare antihydrogen with hydrogen.
The initial ATRAP experiments will take place in the world's most intricate Penning trap structure. The trap consists of a long series of gold-plated, copper rings, each of which has an interior diameter of 1.2 centimeters. The electrodes are cooled to 4 degrees above absolute zero (-269 degrees Celsius) during operation. The apparatus is located within a very strong (6 Tesla) magnetic field that is directed along the direction of the central axis of the trap.
Antiprotons enter the trap apparatus from below, and are captured in the lower section of the trap (below the place where the diameter of the support structure becomes larger.) When captured they oscillate within this lower structure, until collisions with cold electrons stored within lower their energy dramatically. They end up in the center of the lower region with a temperature that is also only 4 degrees above absolute zero.
Positrons from a carefully shielded radioactive source enter the trap apparatus from above. They will be accumulated in the upper section of the trap, at the same time that antiprotons are being accumulated in the lower section.
When cold antiprotons and cold positrons have both been accumulated, then a mechanical valve that separates the lower and upper regions of the trap will be opened, and the ingredients of cold antihydrogen will be allowed to interact. If antihydrogen is formed it will no longer be trapped in the Penning trap. It will drift to the walls of the trap and annihilate. We will look for simultaneous annihilations of an antiproton and a positron as the signature that cold antihydrogen has been formed for the first time.
The trap was constructed by ATRAP collaborators from Harvard University; this group is also coordinating the ATRAP apparatus and operation. A scintillating fiber detector built at the Institute for Nuclear Physics in Juelich, and a BGO detector built at the University of Bonn, will establish that cold antihydrogen has been formed. The beam is steered using a parallel plate avalanche detector built at IMEP in Vienna.
Once cold antihydrogen is formed, and its formation is optimized, then a new apparatus (under construction) will replace the trap. It will allow include a magnetic trap intended to capture the cold antihydrogen, and will allow the introduction of lasers for precise spectroscopy.
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