In his 1920 Bakerian Lecture on the Nuclear Constitution of Atoms, Ernest Rutherford speculated on the properties of the then undiscovered neutron, remarking that "it may be impossible to contain it in a sealed vessel". Nevertheless, trapping of very low energy---or "ultracold"---neutrons (UCN) is possible due to an effective "optical potential" experienced by neutrons in matter. At present, storage times in the best material traps are essentially limited only by the free neutron lifetime, approximately 880 seconds, offering a precise method of measuring this quantity, whose value has important consequences in theoretical physics. Another important use of stored UCN is in experimental efforts to measure a nonzero neutron electric dipole moment (EDM). In contrast to neutron lifetime measurements, EDM experiments require storage materials that are capable of supporting large electric fields. In many cases, continued progress in these experiments will also be driven by the increased outputs expected from new UCN sources that will use novel storage materials suitable to cryogenic environments.
In this work, we report on UCN storage in vessels coated with the fluoropolymer CYTOP and with deuterated polyethylene (dPE), carried out at the SUN-2 prototype UCN source at the Institute Laue-Langevin (ILL) in Grenoble, France. The two materials were investigated for their potential use as coatings for the UCN production volume of the SuperSUN source, and for part of the storage cells of the PanEDM neutron EDM experiment, respectively, both of which are currently under construction at the ILL.
In Chapter 1, we motivate the development of improved coatings for UCN storage by discussing the use of neutron EDM measurements as probes of physics beyond the Standard Model. This is followed in Chapter 2 by a brief review of UCN physics, and a discussion in some detail of several models for nonideal UCN storage. The CYTOP and dPE storage results are then presented in Chapter 3, in addition to a measurement of the optical potential of CYTOP, which is found to be 115.2(2) neV. In Chapter 4, we present the results of simulations that can be used to extract an intrinsic storage property of the CYTOP and dPE coatings, the loss factor, η, from the experimental storage data. This analysis is discussed in Chapter 5, in which conservative estimates of η are made: the loss factor of CYTOP is found to be η=2.7(2)×10−5 at 11.7 K and η=1.1(1)×10−4 at 295.8 K, and the loss factor of dPE is found to be η=3.1(1)×10−5 at 11.9 K and η=2.6(1)×10−4 at 295.1 K
