Scientist sets out to prove neutrinos have mass, antiparticle

BY WENDY PITLICK, Black Hills Pioneer
http://www.zwire.com/site/news.cfm?newsid=18249287&BRD=1300&PAG=461&dept_id=156925&rfi=8

LEAD - Many experiments proposed for the deep underground science and engineering laboratory are designed for scientists to learn about the properties of neutrinos.

Dr. Steve Elliott, of Los Alamos National Laboratory in New Mexico wants to take those questions one step further as he speculates that neutrinos have mass and are their own antiparticles.

Officially dubbed the Majorana experiment, named after the Italian physicist Ettore Majorana who studied the phenomena of particles that are their own antiparticles back in the 1930s, Elliott said his project is based on double beta decay. The experiment will use a large collection of germanium detectors, a chemical tin-like element that is an important semiconductor and has electrical properties that are between a metal and an insulator. Germanium crystals will be used in the experiment. According to Elliott, a collaboration of scientists from 15 different universities and laboratories from four different countries - Russia, Japan, the United States and Canada - will use extremely purified germanium crystals to find an extremely rare nuclear decay in the element. That process, he said can only occur if neutrinos have mass, and if they are their own antiparticles - which is Elliott's hypothesis.

But before Elliott said the experiment can start the detectors must be completely free of any interfering elements or particles. Uranium and thorium, which are very common types of radioactive elements found in everything, can create signals that would mask those which would come from double beta decay. Cosmic rays, extremely high-energy particles that constantly pass through the earth, can also interfere with the experiment. In order to remove these disturbances, Elliott said the experiment would best be located at least 5,000 feet underground, and must be conducted with the most purified germanium crystals as possible.

"You have to grow crystals to make these detectors," he said. "That crystal growing process naturally purifies the detectors."

In order to further shield the experiment from outside interferences, Elliott said his detector would also be lined with electroformed (highly purified) copper on the inside, and covered with lead bricks on the outside.

In order to detect double beta decay of the germanium, Elliott said energy deposits would ionize atoms of the germanium crystals. The electrons in those ionizations will drift, producing an electronic pulse. That pulse might indicate double beta decay, and is exactly what Elliott will be looking for to determine if neutrinos have mass, and are their own antiparticles.

"The decay can only occur if neutrinos are massive Majorana particles," he said. "If neutrino is its own antiparticle it is referred to as a Majorana particle. The decay rate itself, if you measure the rate at which these things decay, that is directly related to what the magnitude of the neutrino mass is."

If he is right, Elliott said the discovery of massive Majorana neutrinos could be a missing link in the standard model of particle physics. "One of the key missing pieces of information is understanding the origin mass of neutrinos," he said. "So they would play a great role in trying to understand how mass is incorporated into the standard model of particle physics. Neutrinos also play an important role in cosmology. Whether they have mass can affect large-scale structures. They affect how supernovas explode. There are a lot of reasons why we want to understand all we can about neutrinos."

So what does this potential discovery mean for the average person?

According to Elliott, unless people are particularly interested in cosmology and understanding how the universe evolves, more information about neutrinos will not affect life for the general public. However, technology that is used to gather this new information is extremely relevant.

"I know that the Pacific Northwest National Laboratory, which is also a collaborator in this experiment, does an immense amount of Homeland Security research that uses these same technologies," he said. "The low background sciences, the detector development, the material purification, all of these things have other applications (mostly in radiation detection.) So from that standpoint they impact the world as a whole and they're doing that now."

Since Elliott and his collaborators have been working for the last four or five years trying to understand how their detector would be built, they are anxious to get underground to set up their experiment.

Though they are still awaiting funding for the project that could come from the Department of Energy, Elliott said his most aggressive timeline has the detector in the established DUSEL by 2010. While he has no preferences about where that DUSEL might be, as long as it is at least 5,000-feet deep, he has been a staunch supporter for the DUSEL as a whole.

"I think it's extremely important that this field of science has such a laboratory," he said. "I'm more concerned about that fact than where it is actually located. If Homestake goes forward there is a fair amount of money (from T. Denny Sanford's $70 million pledge to the project) already in the bank to make quick progress. That early access would be a great thing if Homestake could be up and providing space quickly."