CERN Traps Anti-Matter For 1000 Seconds

Antimatter is cool.

It lets us perform PET scans and powers the starship Enterprise. But it is extremely difficult to study.

That is because when anti-matter comes into contact with normal matter, they annihilate one another, emitting pure energy (photons). This is unfortunate for scientists because they would love to study anti-matter, but developing a trap for it is understandably tricky. The anti-matter particles can easily interact with background gases or the walls of the container.

But last year, researchers at CERN published a paper in Nature (which I also blogged about) describing how they managed to trap 38 atoms of anti-hydrogen (an antiproton orbited by a positron) for 172 ms.

They have not stopped working on improving their trap, however, and have now performed a study detailing how they were able to trap anti-hydrogen for 1000 seconds, an increase of nearly 4 orders of magnitude from their previous paper.

This is what they did:

First, CERN’s Antiproton Decelerator creates the antiprotons which will be used to create atoms of antihydrogen. The Anitproton Decelerator provides antiprotons in groups roughly 3 x 10⁷ in number. Only anti-protons which have an energy less than a certain amount (< 3 keV) are trapped. Typically the number of antiprotons less than this energy threshold is ~6 x 10⁴. These antiprotons are then cooled and compressed.

After this initial step, the antiprotons are then mixed with a cloud of positrons in an effort to get these two components to combine into atoms of antihydrogen. After mixing for about 1 second, the researchers end up with about 6 x 10³ atoms of antihydrogen.

All this takes place inside a magnetic trap. The trap is cylindrical in shape and has a length of 270 mm and a diameter of 44.5 mm.

A schematic diagram of the anti-hydrogen trap (a). The other graphs in this figure show the strength of the magnetic field at different points in the trap.

In order to actually “trap” the anti-hydrogen atoms, a magnetic field is generated inside this cylinder. The field is shaped such that the magnetic field is weakest in the middle of the trap (~ 1 T), and stronger along the edges of the trap (~ 2 – 3 T). In this way, a type of “well” is created which keeps the antihydrogen atoms in the middle of the apparatus, which prevents them from interacting with the walls of the trap and annihilating themselves.

After holding the antihydrogen atoms for a certain period of time, the researchers would shut down the magnets and wait for the atoms to annihilate themselves by hitting the walls of the trap. A special detector counts these annihilation events and allows them to detect the number of anithydrogen atoms remaining after the experiment.

Why don’t all the antihydrogen atoms remain? Most of them are lost through interactions with gases inside the trap, such as helium and molecular hydrogen.

They varied the experiment time from 0.4 seconds to 2000 seconds, and did several attempts for all time lengths. As you might expect, they detected more annihilation events per attempt for the short time lengths (e.g. 1.13 ± 0.13 events/attempt for 0.4 second time length) than the longer time lengths (0.77 ± 0.29 events/attempt for 1000 second time length).

Ah but now you are thinking, “but they did some experiments at 2000 seconds, why aren’t we hearing about that?”

The reason is that they only did  3 experiments at the 2000 second time scale, and while they did detect a few events, the results were not strong enough to say for sure that they were able to trap antihydrogen at that time scale.

The paper also discusses some of their computer simulations and how they compare to the actual experiment results, but I will leave that to the interested reader. 

So what are the implications of this work?

Being able to trap anti-matter for this period of time will allow for much easier ability to perform spectroscopy, since the density of atoms and intensity of radiation needed are dramatically reduced in the anti-matter can be held for a long period of time.

In addition, trapping anti-hydrogen for this long time scale will allow researchers to cool the anti-matter to very low levels, allowing them to probe the effect of gravity on anti-matter.

ALPHA Collaboration, G. B. Andresen, M. D. Ashkezari, M. Baquero-Ruiz, W. Bertsche, E. Butler, C. L. Cesar, A. Deller, S. Eriksson, J. Fajans, T. Friesen, M. C. Fujiwara, D. R. Gill, A. Gutierrez, J. S. Hangst, W. N. Hardy, R. S. Hayano, M. E. Hayden, A. J. Humphries, R. Hydomako, S. Jonsell, S. Kemp, L. Kurchaninov, N. Madsen, S. Menary, P. Nolan, K. Olchanski, A. Olin, P. Pusa, C. Ø. Rasmussen, F. Robicheaux, E. Sarid, D. M. Silveira, C. So, J. W. Storey, R. I. Thompson, D. P. van der Werf, J. S. Wurtele, & Y. Yamazaki (2011). Confinement of antihydrogen for 1000 seconds arXiv arXiv: 1104.4982v1