facilities and methods Ultracold Neutrons—Physics and Production Introduction Free neutrons are the most funda- mental ones of the easily accessible, electrically neutral spin-1/2 systems. They take part in all the four known interactions: they strongly interact with nucleons and nuclear matter, they undergo weak β-decay, they have magnetic moments and gravitational mass. Although the neutron itself is a composite system, and thus, strictly speaking, not a fundamental particle, it is one of the finest probes for funda- mental physics. Neutrons can be and are used to study all interactions, to search for new interactions, to test fundamental quantum mechanics and symmetries of nature. Over the past five decades the field of slow neutron Figure 1. Dispersion relation of superfluid helium (c) and free neutron (a). precision physics developed and Neutrons with E » 1meV can excite a single phonon q » 0.7Å-1 with same energy could usually make major advance- and are thus down-scattered to the UCN energy range. The UCN production ments when new and more powerful rate (b) (circles) [16] shows the dominance of this single phonon process with neutron sources became available. respect to multiphonon processes. Very often experiments benefit from using slower neutrons and, thus, longer de Broglie wavelengths λ=2πh/(m · v), for example, thermal Besides numerous reviews, two new schemes for the production of neutrons at about 2,200 m/s average text books have been written on UCN UCN that have been studied in detail velocity have wavelengths of the typi- [2, 3]. For an update on the status of recently and are presently being real- cal atomic scale of about 1 Å. Fermi the field, readers are referred to two ized at various laboratories around the and Zinn [1] found that neutrons can recent workshops [4, 5]. In this article world. In particular, the UCN source be totally reflected from material we briefly review the peculiarities of project currently commissioning at surfaces under grazing angles of inci- ultracold neutrons, including their the Paul Scherrer Institut (PSI) in dence. To each material a critical generation and mentioning some of Villigen, Switzerland will allow for a (maximum) angle for total reflection their applications. next generation of more sensitive fun- of thermal neutrons exists that As several times already in the damental physics studies. becomes larger for smaller neutron past, we are today at a point at which velocities. Consequently, sufficiently fundamental physics applications slow neutrons will be reflected under require larger UCN intensities in Ultracold Neutrons all angles of incidence. These neu- order to further advance. Especially Ultracold neutrons are free trons are called “ultracold neutrons” the importance of the search for a (unbound) neutrons defined via their (UCN). Due to their specific proper- finite value of the electric dipole most important property: they can be ties (λ≥600 Å) they have opened a moment of the neutron pushes the stored! They have very low kinetic new door for the field of fundamental development of new and more power- energies below about 300 neV corre- physics. ful UCN sources. We describe the sponding to ~3.5 mK, hence their Vol. 20, No. 1, 2010, Nuclear Physics News 17 facilities and methods realistic storage containers (e.g., holes or slits) contribute considerably. UCN in Fundamental Physics The possibility to store UCN for relatively long observation times makes them unique and highly sensitive probes, testing our understanding of fundamental physics [2, 3, 6]. Most of these experiments today are statistically limited and further advancements strongly depend on new high-intensity sources for UCN. A key experiment is the search for a permanent electric dipole moment of the neutron (nEDM). A finite nEDM violates time-reversal invariance and, therefore, might help to understand the matter–antimatter asymmetry in our universe. It is tightly linked to some of the open problems in Figure 2. Experimentally determined temperature dependence of UCN modern physics, the so-called “strong production in deuterium [22]. The sharp increase with solidification is obvious. CP-problem” and the “SUSY CP- problem.” Its observation would be a clear indication for physics beyond the electro-weak Standard Model of name: ultracold. They can be confined In inhomogeneous magnetic particle physics [7–9]. Other impor- by the strong interaction (total reflec- fields, the kinetic energy change of tant studies with UCN include deter- tion at any angle of incidence from the neutrons can be expressed as mination of the neutron lifetime and Δ =± Δ| | surfaces of certain materials like Ni, Be, Ekin 60 · neV/T B , taking the decay parameters, strongly influenc- stainless steel), the magnetic moment positive sign if the spin component is ing our understanding of weak inter- interaction (repulsion of one spin com- antiparallel to the field B. actions and big bang nucleosynthesis ponent from field gradients due to the Also the gravitational interaction [6]. UCN are also being used to study neutron magnetic moment) and due to is on the same scale for UCN, fundamental quantum mechanics, search Δ =Δ Δ gravitation (limited vertical reach). Ekin h · 103 neV/m, where h is for exotic interactions, test baryon num- UCN reflection on walls via the the height difference. ber conservation, and measure proper- strong interaction can be well described The effects of all three interactions ties of the neutron itself, such as the by a potential step model solving the can be combined and used for UCN search for a tiny but finite charge of Schrödinger equation with a step traps. The energy spectrum of stored the neutron. height equal to the so-called Fermi UCN depends on peculiarities of the = π 2 (optical) potential: VF (2 h / m)·Na; trap and is a function of height with a where N is the number density in the material or magnetic field dependent The Production of UCN material assumed to be homogeneous, cut-off, typically below about Δh=3m. The typical kinetic energy of UCN a is the coherent scattering length, and m The storage time of UCN is fun- (~100 neV) is orders of magnitude is the mass of the neutron. For exam- damentally limited by the time con- below the typical neutron energy in a = β ple VF(Ni) 252 neV, VF (diamond) stant of neutron -decay of almost thermal moderator (~25 meV) of a = 304 neV. For kinetic energies below 15 minutes. Practically, also loss factors neutron source, such as a nuclear this threshold energy total reflection like nuclear absorption, inelastic reactor. The energy distribution of the occurs at any angle. up-scattering and imperfections of thermalized neutron gas is often 18 Nuclear Physics News, Vol. 20, No. 1, 2010 facilities and methods slowed down when climbing the grav- itational potential. Various developments have allowed one to increase the intensity of UCN considerably over the years. Clearly, the extracted intensity is proportional to the initial one, thus high initial neu- tron flux helps and could be increased by more than 4 orders of magnitude as compared to the first experiments. Also, the intensity in the tail of the Maxwell- Boltzmann distribution increases sig- nificantly when the temperature of the neutron gas is lowered, thus extract- ing UCN from a cold moderator of Figure 3. UCN production as function of incoming neutron energy [20]. The liquid hydrogen or deuterium gained dashed, colored curves display individual cold neutron energy distributions almost another 2 orders of magnitude. (intensity: vertical right scale) prepared from the “white” cold neutron beam Improvements were made on UCN using a neutron velocity selector and measured by time-of-flight. The measured guide quality by developing high UCN production cross-section (left vertical scale) for the individual energies is Fermi potential, very low roughness displayed as open circles. The red squares are the cross-sections calculated for surfaces, allowing a low-loss trans- the energy distributions, the solid black line is calculated as continuous port of UCN over many meters. function of energy. The cut-off at around 9 meV comes from cut-off of the Today’s highest UCN intensity is phonon density of states in the simple Debye model at the Debye temperature. obtained at the instrument PF2, which Above this energy, multiple phonon excitations still permit UCN production. has been in operation for more than 20 years at the high flux reactor of the Institute Laue-Langevin (ILL) in approximated by a Maxwell-Boltzmann control the background count rate of Grenoble. Vertical extraction over distribution and the amount of neutrons UCN detectors on a level of a few 17 m height through a curved replica in the very low energy tail is therefore counts per 1000 s allowed at that time guide system is used for initial deceler- rather small. Lower energy neutrons to detect similar UCN rates. Curved ation and background suppression. Then become further suppressed because of neutron guides were used to filter out a mechanical decelerator, the “neutron longer travel times in the moderator faster neutrons that could not fulfill turbine,” transforms very cold neutrons medium before extraction and the the necessary conditions for total of approximately 40 m/s into the UCN temperature of the extracted neutron reflection. Shapiro’s group in Dubna energy range. The neutron turbine gas is accordingly shifted to higher succeeded in detecting UCN this way (developed by A. Steyerl since 1975) values. Historically it was therefore in 1968 [10]. Steyerl (Munich) in consists of a set of curved blades mov- not even clear that a sizeable amount of 1969 [11] reported, independently, ing with a peripheral velocity 20 m/s ultracold neutrons could be extracted measured neutron cross-sections for in the same direction as the neutrons.