The Stefan Meyer Institut contribution to the PSI UCN Source project is connected with the investigation of the properties of solid deuterium (D2) as the UCN converter and comparison with other possible converter materials: solid oxygen (O2) and solid heavy methane (CD4). The experimental program was performed at the FUNSPIN beamline of the Swiss Spallation Neutron Source (SINQ) at PSI and the properties of D2, O2 and CD4 in the temperature range between 8 K and room temperature have been measured. The detailed description of the obtained results can be found in the PhD thesis of M. Kasprzak.

The development of high intensity ultracold neutron (UCN) sources is important for improving the accuracy of the experiments investigating fundamental properties of the neutron, e.g. the search for the electric dipole moment. Presently there are several projects to build new UCN sources in order to provide the desired increase in intensity. At the Paul Scherrer Institute (PSI) we are setting up a high intensity UCN source with the aim to increase the available flux and densities by two orders of magnitude compared to the strongest source currently available [UCN densities of about 50 UCN per cm3 are available at Institut Laue-Langevin (ILL)].

One of the ways to increase the UCN intensity is to use an appropriate material as UCN converter, i.e. a medium that converts cold neutrons (CN) into UCN by inelastic scattering; the converter must have specific properties such as energy levels and excitations that enable the downscattering to take place. This mechanism differs from a typical scheme of neutron moderation used in the CN sources, i.e. the neutrons do not reach thermal equilibrium with the moderator material. This method of UCN production, superthermal UCN production, was first proposed by Golub and Pendlebury and is used in the PSI UCN source as well as in the other new UCN sources.

Overview of the PSI UCN source

In the PSI UCN source fast neutrons of average energy of about 2 MeV are produced by the spallation reaction of protons with an energy of 590 MeV hitting a lead target. The proton beam with an intensity of about 2 mA is delivered from the ring cyclotron with a low duty cycle ~1%, i.e. with 4 to 8 s beam on every 400-800 s. The protons generate neutrons on the target consisting of lead filled in zircaloy tubes. The spallation neutrons (about 10 neutrons per proton) are first moderated in a 3.3 m3 tank of heavy water at room temperature and then further cooled and downscattered into the UCN energy range in 30 dm3 of solid deuterium (sD2) at low temperature ~5 K. The neutrons exit sD2 and gain energy because of the material optical potential (for sD2 at 5 K it is 105 neV) and then are further transported vertically 1.1 m upwards loosing energy due to gravity and reach the storage volume where the neutrons with energies below 250 neV can be trapped and guided to the experiments. The sD2 converter and the UCN storage tank are separated from each other using a valve to reduce neutron loss during storage. During the proton pulse, the valve to the storage volume is open and the UCN from the sD2 converter fill the storage vessel. After the proton pulse is over, the shutter closes and UCN are transported to the experiments. The storage volume has a size of about 80 × 80 × 240 cm3 and serves as intermediate UCN storage between the proton beam pulses, thus allowing for quasi continuous availability of the UCN from the source. Storage of UCN relies on the possibility to totally reflect these neutrons under all angles of incidence from suitable materials i.e. diamond-like carbon (DLC) coated materials.

Overview of the experimental results

Our investigations of UCN production started in the summer 2004 at the Swiss Spallation Neutron Source (SINQ) at PSI, using the polarized CN beamline for fundamental physics (FUNSPIN). Two experiments based on the same concept of measuring UCN produced from the CN beam were performed. The first experiment took 10 days of beam time and we have successfully measured the absolute UCN production cross sections in gaseous, liquid, and solid deuterium and the temperature dependence of the UCN production in sD2. Additionally the polarization of UCN produced from polarized CN in sD2 was measured. The second experiment was conducted in autumn 2005 at the same beamline. During 5 weeks of beam time we have measured the production of UCN from the CN beam in D2, O2, and CD4 as well as the CN transmission through all three all three materials. Moreover, in order to understand underlying processes of UCN production in gaseous and solid D2, the CN energy dependent UCN production was measured.

The idea of the performed experiments is simple: cold neutrons arrive via a “flight tube” to the target cell, the CNs and UCNs leaving the cell are measured by a detector system. The target cell is mounted on a cryostat and can be filled with D2, O2 or CD4, as appropriate, in the temperature range between 8 K and room temperature. The CNs interact with the material in the cell and some of them get downscattered to UCN energies.

Both the CNs and the UCNs enter the guide system following the target cell and after roughly 0.6 m the UCNs are separated from the CNs by a mirror which is usually a silicon wafer coated with UCN reflecting material with high Fermi potential. The UCNs are reflected upwards by this mirror and after 1 m are again reflected by a second mirror into a horizontal guide section. This section consists of a storage tube and two UCN shutters: the entrance shutter and the exit shutter. The UCNs that passed the storage tube are later detected by the 3He UCN detector.

The assembly of items 5-8 is rotated by 90 degrees around the vertical guide so that the UCN detector is moved out of the CN beam axis. The CNs are transmitted through the lower mirror and then pass through a chopper, enter the time of flight tube, and are detected in the CN detector.