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Nuclear physics

Beaming GANIL turns 25

Twenty-five years of consistent scientific breakthroughs is a good track record for any facility. Researchers continue to break new ground in particle physics at the GANIL heavy-ion accelerator in Caen, even as they await the SPIRAL2 facility, scheduled to go online by 2013.


© M. Desaunay/Ganil

This “pipeline” directs the SPIRAL beams towards the experiment rooms.

As a heavy-ion accelerator, the high energy beams of the heavy-ion accelerator GANIL 1, one of the four largest such facilities in the world, are used to create exotic nuclei, and to probe the forces governing the inner workings of atoms. Recent research by some of the 700 physicists around the world who use the facility continues to add experimental muscle to physics theory, illuminating the tiny universe of atomic particles and the larger phenomena occurring inside dying stars.
Bertram Blank 2 recently quantified the nature of the decay of iron 45 (45Fe), tracking its energy and trajectory with a dynamic, gas-based target 3. This unstable heavy ion is unique in that it emits two protons as it decays. Blank explains that this kind of radioactivity was predicted over forty years ago, though not confirmed until Blank’s team managed to create a handful of atoms in 2001. The two protons emitted by 45Fe may shed light on the “pairing” force – the tendency of protons to pair with other protons and neutrons. “If you have two particles being emitted, there is a question of sequence: are they being released at the same time, or one after the other? If it is together, then they may have a correlation – you might say they talk to each other, sharing energy and relative angle,” he explains.

ganil petite

© J.-S. Bousson/CNRS Photothèque

Working on part of SPIRAL2 in the clean room.

Pairing is also of interest to Jean-Antoine Scarpaci 4. In breaking up the nucleus of helium 6 (6He), he hoped to shed light on the correlations between nucleons inside atomic nuclei. 6He is made of a 4He core surrounded by two loosely bound extra neutrons, held to the nucleus by a weak thread of nuclear force. Scarpaci snapped that thread at GANIL. As Scarpaci points out, the neutrons on 6He seem to have a relationship with each other: “If one is not there, the other is not there.” As the 6He passed close to a lead target, the loosely bound neutrons were attracted to the target and were emitted at an angle from the beam that depended on the shape of the primary 6He. The relative positions of the two neutrons on the nucleus could thus uncover much about nuclear forces.
Scarpaci’s work used equipment supplied by Sweden and the Netherlands, available thanks to GANIL’s status as a large-scale facility for European research. These resources, and the 10-15 million electron volts (MeV) per nucleon beams that will be at his disposal with the arrival of the new accelerator SPIRAL2 in 2013, make GANIL ideally suited to probe these mysteries further. “To be able to understand these exotic phenomena, this is the place to do it,” says Scarpaci.
Of course, GANIL researchers are not waiting until 2013 to do innovative research. Hervé Savajols created hydrogen 7 (7H) ions, the most neutron rich atom ever created, in which six neutrons are associated with a single proton 5. “The finding of the heaviest hydrogen isotope tells us how many neutrons a single proton can hold, a long-standing problem from both experimental and theoretical points of view,” says Savajols. He and his colleagues managed to create the rare ion by firing helium 8 (8He) nuclei at a 12C target at high speed. On colliding, 8He forms heavy 7H when the beam gives one of its protons to the target to form nitrogen 13 (13N). As 7H only exists for 10-21 seconds, its creation had to be confirmed from the kinematical characteristics of 13N. To do this, instead of hoping that detectors placed near the high speed beam’s target would catch whatever bounced off, Savajols directed the helium ions into a gas that acted as both a target and detector. This is similar to the system used by Blank. The sensitivity of this system, called MAYA, is unparalleled, allowing the scientists to see an ion that might shed light on the inner workings of neutron stars. “The sensitivity is almost 100 percent. When we create 7H, we also create 13N, but this is such a low energy atom that we would have no chance to see it without tracking it inside the target,” explains Savajols. “We were pioneering in that system.”
Recent work by Elias Khan 4 using MAYA also helps us understand how stars function. He and his colleagues became the first scientists to compress an extraterrestrial nucleus: nickel 56 (56Ni) 6. 56Ni is believed to exist chiefly in conditions of extreme pressure, when stars are about to explode. “If you can compress nuclei, you can have an idea of how a star explodes to make a neutron star,” says Khan.
Khan directed a beam of 56Ni to collide with the gas detector, compressing the ion in the process. Compression causes an action in the nuclei not unlike breathing, called a “giant monopole resonance.” While the energy required to do this is very high, the energy residue of this process is extremely low, making it difficult to detect. This work could be the foundation to explore the characteristics of some of the other 95 percent of all ions that are not found on Earth. “To do this, you really need an active target, and there are not many places where this is possible. There are places in the United States and elsewhere in Europe that are starting, but we have a few years’ advance on the rest,” says Khan.

Mark Reynolds

Notes :

1. Grand accélérateur d'ions lourds, Caen. View web site
2. Centre d'études nucléaires de Bordeaux Gradignan (CNRS / Université Bordeaux-I).
3. J. Giovinazzo et al., “First Direct Observation of Two Protons in the Decay of 45Fe with a Time-Projection Chamber.” Phys. Rev. Lett., 2007. 99: 102501.
4. Institut de physique nucléaire d'Orsay (CNRS / Université Paris-XI).
5. M. Caamaño et al., “Resonance State in 7H.” Phys. Rev. Lett., 2007. 99: 062502.
6. C. Monrozeau, et al., “First Measurement of the Giant Monopole and Quadrupole Resonances in a Short-Lived Nucleus: 56Ni.” Phys. Rev. Lett., 2008. 100: 042501.

Contacts :

Bertram Blank,
Elias Khan,
Jean-Antoine Scarpaci,
Hervé Savajols,


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