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Nucifer and the neutrinos

July 4, 2012

French physicists have built a special detector called Nucifer, which they hope will lead to the discovery of a new, subatomic particle and shed light on dark matter.

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Image: CEA de Saclay

It is yet another particle that could revolutionize the world of physics.

French physicists say they are on the verge of discovering a fourth type of neutrino, a ghost particle that might give us insight into the dynamics of the universe.

Neutrinos are a lot like electrons - except that they are electrically neutral and don't interact much with their environment.

At the French Atomic Energy Commission (CEA) center at Saclay near Paris, physicists are working on a special detector called Nucifer, which will detect neutrinos emitted from a nearby open-core reactor called Osiris.

Nucifer was originally built for the International Atomic Energy Agency (IAEA) as an anti-proliferation tool.

They are everywhere

Physicists Thierry Lasserre and David Lhuillier now hope Nucifer will spot the fourth type of neutrino. Neutrinos are everywhere - everyday millions pass through our bodies.

Nuclear worker above the open-core reactor Osiris at CEA-Saclay near Paris
A nuclear worker above the open-core reactor Osiris at CEA-Saclay near ParisImage: CEA de Saclay

The three existing types of neutrinos - or "flavors" - are electrons, taus and muons.

But Lasserre and Lhuillier think a fourth, heavier type might have escaped detection.

"This neutrino would be different because it would not interact with normal matter, it would only mix with the other neutrinos," says Lasserre to explain why he thinks this fourth so-called sterile neutrino has never been found.

Small revolution

Scientists recently discovered that they had for decades underestimated the number of neutrinos emitted by nuclear reactors by 7 percent.

The detection of a new particle would be "a big discovery."

"Neutrinos are the most abundant particles in the universe. If there is a new one, it would a real big contribution to matter in the universe," says Lhuillier. "The discovery of a new particle that is not included in the standard series of elementary particles and fundamental interactions would be a small revolution."

Fifteen projects have been launched across the globe to find the missing particle. The scientists at CERN - in the news this week with its discovery of a new boson particle - are also conducting tests.

CERN recently drew a lot of attention for its work on neutrinos. It reported it had observed neutrinos travelling faster than the speed of light. But the findings later turned out to be wrong.

Getting up close

At CEA-Saclay, the physicists have positioned their neutrino detector at seven meters from the core of the reactor, Osiris.

"It's the closest ever neutrino experiment to a nuclear core," says Lasserre.

Several stories underground, the detector Nucifer has been placed inside a solid lead box to exclude all background noise, gamma rays and neutrons that might interfere with the experiment.

Once active, the weakly-interacting neutrinos will go straight through the lead and into Nucifer, where a very small percentage of them will be detected.

The device Nucifer detects neutrinos emitted by the reactor at CEA-Saclay near Paris
The Nucifer device will detect neutrinos emitted by the reactor at CEA-Saclay near ParisImage: CEA de Saclay

Nucifer is filled with a liquid scintillator, a liquid that emits light when it is struck by an incoming particle.

When a neutrino hits a hydrogen atom inside the liquid it emits a neutron and a positron, an antiparticle that is associated with an electron. The two particles will then emit two flashes of light, a larger and a smaller one.

It is this signal, or signature, that Lhuillier and Lasserre hope to detect.

If Nucifer detects a particle with a higher energy, it would indicate a heavier neutrino - and possibly the fourth type.

Light on dark matter

While the discovery of a new neutrino would unlikely change our everyday lives, it could transform our understanding of the universe.

Physicists think it possible that ghost neutrinos - such as the one they hope to find in the Paris experiments - account for dark matter, a type of matter that cannot be seen with a telescope, but which has a gravitational effect on stars and other cosmic things.

Scientists evoked the concept of dark matter when they realized that the masses of stars and cosmic bodies in a galaxy do not constitute the entire mass of the galaxy.

Some mass was missing.

"We are missing about 80 percent of the mass that is not visible, and this is known as the dark matter problem," explains Lasserre, "If a fourth neutrino is found, it would certainly contribute to dark matter."

Scientists say they know very little about dark matter and can only see its effects on the visible world. Any new insight would be a major step forward.

Lhuillier and Lasserre hope to start recording their first results by the end of 2012.

Author: Clea Caulcutt, Paris
Editor: Zulfikar Abbany