Session 1WE: Workshop on Physics with Neutrons I

Investigation of New Approaches to Ultra-cold Neutron Production at IUCF

Ultra-cold neutrons (UCN) can be produced beyond the thermal limits using superfluid He and solid deuterium. We have started systematic investigations into other systems that could potentially be an efficient UCN converter at the Indiana University Cyclotron Facility (IUCF). The candidate material needs to have a large density of states which allows the incident cold neutrons to couple to leading to fast downscattering, and more importantly, the source needs to have a small neutron absorption that allows the number density of UCN to accumulate. Our team has recently demonstrated experimentally that solid oxygen can be used to produce UCN through mechanisms different from the previous sources. In this talk, I will first summarize the experimental results, and then address the seemingly different physics of UCN production in the low-temperature magnetic phases of solid oxygen. The contrast between the alpha and beta phases strongly suggests the magnetic origin of the UCN production in solid oxygen.   

He-II UCN source in Japan and Canada

Ultracold neutrons (UCN) can be confined in a material/magnetic bottle. This unique property is very useful for various kind of experiments, for example neutron EDM, $\beta$ decay and gravity experiments. Confined neutrons distribute in a phase space with equal weight. A phase space density is one of the most important parameter for the experiments. In the traditional UCN source like the turbine UCN source at Grenoble, the phase space density is limited by Liouville's theorem. We use phonon phase space of superfluid helium (He-II) for neutron cooling, where neutron phase space density is not limited by Liouville's theorem. UCN density in He-II is represented as the product of a UCN production rate, which is the product of a production cross section and an incident neutron flux, and a UCN storage lifetime. The UCN storage lifetime is very long in He-II. We constructed a He-II spallation UCN source in a 392 MeV 1 $\mu$A proton beam line at RCNP. He-II was placed in a cold neutron source. UCN, which were produced in the He-II, were transported to an experimental volume through UCN guides. We obtained a UCN density of 15/cm$^{3}$ at an experimental port, where the maximum UCN energy was 90 neV. The UCN density is large compared with the turbine UCN source. We are constructing a new UCN source based on the experiment at RCNP. We will increase the UCN production rate by better coupling between the He-II and the cold neutron source. We will increase the UCN storage lifetime and transport efficiency from the UCN production volume to the experimental volume. We will place the new UCN source at TRIUMF, where we can use a 500 MeV 40 $\mu$A proton beam. We expect the improvement of UCN density by three order of magnitude.   

Fundamental Neutron Physics at NIST

The program in fundamental neutron~physics at the National Institute of Standards and Technology (NIST) began nearly two decades ago. Currently, five neutron beam lines are dedicated~to studies of fundamental neutron interactions. The neutrons are provided by the NIST Center for Neutron Research (NCNR), a national user facility for studies that include condensed matter physics, materials science, nuclear chemistry, and biological science. The beam lines for fundamental neutron physics experiments include a high intensity polychromatic beam, three monochromatic beams (0.496 nm,~0.89 nm, and .384 nm), and a neutron interferometer and optics facility (0.2 nm -- 0.49 nm). The presentation will discuss the broad program in fundamental neutron physics with a brief description of some of the experiments performed at the NCNR. In addition, the status of the new guide expansion project that includes a new neutron guide that will provide an additional very high intensity polychromatic beam for fundamental physics research will be presented.   

Cold neutron interferometry

Neutron interferometry is a powerful technique for studying fundamental physics. A large dimensional interferometer for long wavelength neutrons is extremely important in order to investigate problems of fundamental physics, including tests of quantum measurement theories and searches for non-Newtonian effects of gravitation, since the sensitivity of interferometer depends on the wavelength and the interaction length. Neutron multilayer mirrors enable us to develop the large scale interferometer for long wavelength neutrons. The multilayer mirror is one of the most useful devices in cold neutron optics. A multilayer of two materials with different potentials is understood as a one-dimensional crystal, which is suitable for Bragg reflection of long wavelength neutrons. Cold and very cold neutrons can be utilized for the interferometer by using the multilayer mirrors with the proper lattice constants. Jamin-type interferometer by using beam splitting etalons (BSEs) has shown the feasibility of the development of large scale interferometer, which enables us to align the four independent mirrors within required precision. The BSE contains two parallel multilayer mirrors. A couple of the BSEs in the Jamin-type interferometer separates and recombines the two paths spatially. Although the path separation was small at the first test, now we have already demonstrated the interferometer with perfectly separated paths. This has confirmed that the multilayer mirrors cause no serious distortion of wave front to compose a interferometer. Arranging such mirrors, we are capable of establishing even a Mach-Zehnder type with much larger size. The interferometer using supermirrors, which reflects the wide range of the wavelength of neutrons, can increase the neutron counts for high precision measurements. We are planning the experiments using the interferometer both for the very cold neutrons and for the pulsed neutrons including J-PARC.   

The Hadronic Weak Interaction and Parity Violation in Cold Neutron-Nucleus Capture

The study of the hadronic weak interaction has a long tradition, starting with the first observation of parity violation in the nucleon-nucleon (NN) interaction in cold neutron capture experiments, in the early 60's (Y. Abov \textit{et al.,} 1964). Since then, there has been intense effort in gaining a better understanding of the weak NN interaction, both on the theoretical side, as well as on the experimental side. The existence of the NN weak interaction was first predicted in the generalization of Fermi's theory of nuclear beta decay (Feynman, Gell-Mann, Sudarshan, and Marshak) to include a universal charged weak current. In other words, a consistent theory for nuclear beta decay required the existence of the NN weak interaction. This basic framework has survived within the Standard Model (SM), with the crucial addition of the neutral weak hadronic currents. To this day, the latter remains a very poorly tested (and poorly understood) sector of the SM. The basic weak currents, as they occur in the SM, are modified by the strong interactions at low energy. At the same time, the large mass of the weak bosons requires close proximity of the quarks engaged in the interaction. The precision measurement of parity violating observables in few body NN systems can therefore provide important benchmarks for models that aim to describe low-energy, non-perturbative QCD, as well as effective models that seek to describe the NN weak interaction itself. Progress in measuring parity violating observables in cold neutron capture experiments has historically been hampered by a lack in high intensity neutron sources and other technological problems. Recently, significant technological advancements on all fronts and, especially, the completion of new, high intensity neutron sources have spurred renewed experimental activity in this area. I will present a brief overview of recent theoretical efforts and talk about current and proposed experimental work with cold neutrons.   

Precision measurement of quantum states of neutrons in the terrestrial gravity

Quantum states of matter in gravitational fields are expected as well as those in electromagnetic fields and nuclear fields. However, the gravitational force is extremely week compared with the forces from the others fields. Therefore, the observation of the quantum effect of gravity is very challenging. UCNs are the best candidates as a probe for the gravitational force because of their neutral charge and long lifetime. They can be reflected on a normal material surface, so can be trapped and make quantum states on the bottom mirror in the terrestrial gravity. The scale of the quantum effect is around 10 microns in length. It is in measurable order. By observing the discriminative spatial distribution in vertical, the quantum effect can visibly be demonstrated. Currently, only a few experiments that demonstrate quantum effects are reported. Keys of the experiments are UCN's flux, position resolution of a UCN detector, and fine neutron guides that select proper quantum states. I will present the initiating experiment done at the Institute Laue-Langevin in France. Then I will show our ongoing experiment using the position sensitive detector with fine spatial resolution of 3 microns. Details of the detector development will be presented. The quantum states are sensitive to non-Newtonian gravity and/or sensitive to the gravity-like force which reaches approximately 10 microns. The precision measurement has potential to search for such exotic forces.