Experiments with UCNs can offer an order-of-magnitude improvement in the precise measurement of neutron properties that are sensitive to physics beyond the Standard Model of elementary particle physics. UCNs are isolated neutrons that have been slowed and cooled to the point where their kinetic energy is less than 335 nanoelectron volts, their temperature is less than 4 milliKelvin, and their velocity is less than 8 meters per second (human running speed is ~10 meters per second). Cold neutron scattering on condensed matter, which is sensitive to magnetic structure, provides complimentary information to x-ray scattering.

Neutrons are neutral, elementary, subatomic particles that are constituents of all elements' nuclei, with the exception of hydrogen. Neutrons are unstable and spontaneously undergo beta decay (with a half-life of ~10 minutes). They also can pass through matter with no detectable primary ionization. They interact with matter primarily through collision with other nuclei. Fast neutrons have a long history in scientific research. Fast neutron radiography is a nondestructive testing method with a variety of industrial applications, including elemental analysis and materials classification, and has the potential for element-sensitive imaging for contraband and explosives detection. Another common use for fast neutrons is in soil-moisture measurements taken with neutron probes. In the past couple of decades, physicists have made increasing use of slow neutrons for studying the structure and dynamics of matter on atomic, molecular, and macro-molecular scales.

The UCN Team's efforts at the Los Alamos National Laboratory (LANL) to develop a high-density UCN source are a great step in the field of slow-neutron research. UCNs could be characterized as extremely slow (remember, they travel at ~8 meters per second). This lower energy level gives physicists more control over how they use the neutrons as probes in experiments and the ability to better control the conditions of their experiments. UCNs can be stored for up to their beta-decay lifetime (their half-life is ~10 minutes), which is a relatively long coherence time for measurements. UCN energy is so low that reflective walls can trap them--the probability that a UCN will be absorbed as it bounces off a container wall has been measured to be less than one in 10,000--allowing physicists to create much more precise experiments in traps with well-defined geometry. Because they can be trapped by walls, they can be transported to and stored in areas well shielded from radiation in order to perform very-low-background experiments. Also, in principle, experimentalists can achieve 100% neutron polarization with straightforward magnetic filtering.

The process of "creating" a UCN starts when energetic protons in the LANSCE accelerator strike a tungsten target (Figure 2 illustrates the process described in this and the following two paragraphs). In a process known as spallation, a proton interacts with the nucleus of a tungsten atom in the target. As a result of the collision, many particles, including many neutrons, are ejectedfrom the nucleus (see a spallation movie). From here, medium-speed neutrons pass into the UCN-source device and through two moderators: first, one of polyethylene (long chains of CH2 molecules) at 77 Kelvin and a second, colder one of polyethylene at 4 Kelvin.

In each stage of the moderation, the neutrons collide with the "static" nuclei of the molecules in the moderators. Kinetic theory tells us that these neutrons should lose some kinetic energy to the static nuclei. In this process, the neutron energy is maximally transferred to a target of equal mass, and thus, most of the neutron moderator is hydrogen based. Inside neutron moderators, neutrons experience multiple collisions, lose their energy, and eventually come to the same temperature as the moderator. This is why, in Figure 2, we show the neutrons at 10meV in the first moderator, and 3 meV in the second.

At this point, the slow neutrons pass through and interact with the solid deuterium in the UCN "source." It is during this interaction that the slow neutrons lose enough energy to truly reach the UCN regime (i.e., they reach an energy level of < 335 neV, < 4 mK, and < 8 m/s). On the road to perfecting their UCN source (see Figure 1), the UCN Team encountered difficulty in reaching theoretically possible UCN yields.

In hydrogen, the spins of the two protons in the H2 nucleus can either be aligned (total spin = 1) or opposite to each other (total spin = 0). The first state is called ortho-hydrogen, and the second is called para-hydrogen. In deuterium, the situation is slightly more complicated. The nucleus of the deuterium molecule (D2) is made of two deuterons. Each deuteron nucleus is made of one proton (spin 1/2) and one neutron (spin 1/2), so the total nuclear spin of the deuteron is 1. The total spin of the two spin-1 deuterons in a D2 nucleus can be either 0, 1 or 2. The two states with even total spins have a symmetric wavefunction and are called ortho-deuterium. The state with odd total spin has an antisymmetric wavefunction and is called para-deuterium. At low temperatures, ortho-deuterium is the ground state (the state with the lowest possible energy); para-deuterium still holds a finite rotational energy of about 7 meV higher than ortho-deuterium. Both the ortho and para deuterium can create UCNs efficiently with almost the same rate, however, para deuterium can also efficiently "annihilate" a UCN because it can give away its (unquenched) rotational energy to the UCN, and in the process becomes an ortho deuterium. The UCN Team devised a way of filtering out most of the para deuterium molecules within the source allowing them to achieve the highest UCN density (98 ± 5 UCN/cm3) ever stored!

With UCNs, researchers can study phenomena such as neutron beta-decay, which is purely dominated by the weak interaction--giving the researcher a view of the weak interaction unimpeded by other forces and interactions. Neutron b-decay is the only one of the four basic forces of the universe (electromagnetism,
strong interaction, weak interaction, and gravity) that violates parity. Physicists have been trying to measure precisely the parameters of the weak interaction since parity violation was verified in the late 1950s. UCNs can also be used to measure the permanent electric dipole moment (EDM) of the neutron--to test time-reversal violation symmetry, which corresponds to reversal of motion. Invariance under time implies that whenever a motion is allowed by the laws of physics, the reversal of that motion is also allowed.

In the beginning of the universe, there was energy, not in the form of matter or antimatter. Later, the universe cooled down, and the matter-generating process occurred. Physicists believe that in this process, matter is preferentially generated over antimatter, and this process has to violate CP symmetry (which means time-reversal symmetry is violated, through another theorem stating that CPT has to be conserved). Actually measuring the EDM of the neutron or electron can tell us how much (albeit very small) violation there is.

Finally, the theoretical possibility exists for neutron-antineutron oscillation. Neutron-antineutron oscillation can happen only if a physics process that violates the baryon number conservation (another necessary ingredient to explain the matter-antimatter asymmetry) exists. A high-density UCN source provides us with the means to explore the possibility of neutron-antineutron oscillation.