Zero Sum Game for Neutrons

A recent measurement of a "spinning" helium-3 nucleus revealed that under certain conditions, the entire nucleus behaves much like a single neutron. The experiment ran in Jefferson Lab's Hall A (pictured here). The result was published in the July 11 Physical Review Letters in a paper titled 3He Spin-Dependent Cross Sections and Sum Rules.

A study of polarized neutrons showed that the helium nucleus can be described in a surprisingly simple way.

A recent measurement of a "spinning" helium-3 nucleus revealed that under certain conditions, the entire nucleus behaves much like a single neutron. The result was reported recently in the journal Physical Review Letters.

It's difficult to pin down an individual neutron for study. Freed from the nucleus of the atom, neutrons naturally disintegrate in about 15 minutes. To overcome this issue, physicists have to come up with creative ways of studying the neutron's properties.

One solution is to set "spinning" the protons and neutrons inside a nucleus. In one isotope of helium, helium-3, there are two protons and one neutron. In helium-3, the intrinsic angular momentum, or spin, of the protons will naturally align in opposite directions. Their contributions to the spin of the nucleus cancel out, leaving the nucleus' spin resting almost solely on the spin of the neutron. By measuring quantities related to spin in this situation, researchers can collect information about the neutron's spin and about the nucleus in which it resides.

Karl Slifer
Assistant Professor, Physics Department
University of New Hampshire

A team of nuclear physicists used this process in an experiment conducted in Jefferson Lab's Hall A. They also gave the spinning neutron inside the helium-3 nucleus some additional energy by pinging it with electrons from the lab's CEBAF electron-beam accelerator. According to Karl Slifer, an assistant professor at the University of New Hampshire, the researchers then watched how the neutron, and the nucleus as a whole, responded.

"Depending on how much energy this electron gives to your target neutron, you're going to see different things happen to that neutron. So what we see is that the neutron enters into a series of excited states," he explains. But understanding these excited states has proved difficult.

"We found that each of these states can be very complicated," Slifer says. It seems that the theory that physicists need to use to help them understand their measurements of the excited states is so complicated that it can only be solved for very simplified conditions.

Luckily, researchers have a way around that complication. A relation called the Burkhardt–Cottingham sum rule predicts that regardless of the complexity of the individual excited states, the sum of these possible states is zero. This rule only holds if the electron is "spinning" in its direction of travel and the neutron is spinning perpendicular to that, which was how the Hall A experiment was designed.

"You get a very simple situation. So it's a way of kind of getting around the complexity of the individual states and looking at the global reaction of the neutron," Slifer explains.

The researchers used this information in an earlier experiment to measure the properties of the neutron. In this experiment, they focused on measuring changes in the nucleus in which the neutron resides.

"What we found is that even though this is a more complicated situation, and you have more excited states, it still all sums to zero. So the helium-3 nucleus is behaving, at least in this particular situation, very much like the neutron is by itself."

The result was published in the July 11 Physical Review Letters in a paper titled 3He Spin-Dependent Cross Sections and Sum Rules.

By Kandice Carter
Science writer