A new type of ultra-cold neutron (UCN) source based on the spallation process is under construction at PSI. The essential elements are a pulsed proton beam with highest intensity ( I p ≥ 2 mA ) and a low duty cycle (1%), a lead spallation target, a large D 2 O moderator and a solid deuterium ( sD 2 ) converter system. Spallation neutrons are thermalized in the D 2 O , further cooled and partially downscattered into the ultra-cold neutron range in the sD 2 . The expected UCN density is higher than 1000 UCN / cm 3 in typical experiments, an increase of almost two orders of magnitude over the best source currently available (at ILL). The new UCN source is expected to come into operation in 2009.

The PSI ultra-cold neutron source

A new type of ultra-cold neutron source, based on the spallation process, is scheduled to start operation at PSI in 2008. The UCN source will be operated in a pulsed mode using the 1.2 MW (590 MeV, 2 mA) proton beam delivered from the ring cyclotron. In this paper we discuss the neutronic and the thermo-mechanical behavior of the target assembly under pulsed operation.

The spallation target of the ultra-cold neutron source UCN at PSI

The production rates of ultracold neutrons (UCN) from cold neutrons on gaseous, liquid, and solid deuterium targets have been measured. The comparison of the measured and calculated UCN production on gaseous ${}^{2}{H}_{2}$ is used to calibrate the simulated target extraction and transport efficiencies of the experimental apparatus. The production cross section in solid ${}^{2}{H}_{2}$ at 8 K for UCN with energies between 0 and 250 neV is ${R}_{\mathrm{solid},8 K}={\ensuremath{\sigma}}_{\mathrm{solid},8 K}^{\mathrm{CN}\ensuremath{\rightarrow}\mathrm{UCN}} \ensuremath{\rho}=(1.11\ifmmode\pm\else\textpm\fi{}0.23)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This value is consistent with other experiments in which UCN had been extracted from ${}^{2}{H}_{2}$. The value also agrees with calculations using the incoherent approximation and a simple Debye model and corroborates predictions for UCN densities expected at the high-intensity UCN source at the Paul Scherrer Institut. The temperature dependence of the UCN production in solid ${}^{2}{H}_{2}$ down to 8 K can be explained within the same model when multiple-phonon excitation is included.

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Production of ultracold neutrons from a cold neutron beam on a 2 H 2 target

At the Paul Scherrer Institute (PSI), a very intense source of ultracold neutrons (UCN) is being built. The UCN converter of solid deuterium must be contained in a vessel. Produced UCN leave that vessel through its top lid. To decide on the design of the vessel and the top lid, we have measured the transmission of neutrons with velocities between 3 and 20 m/s through different material foils. Contrary to expectations, we found that transmission through aluminium and aluminium alloys is equal or even higher compared to zirconium and reactor-grade zirconium alloys, respectively.

Transmission of very slow neutrons through material foils and its influence on the design of ultracold neutron sources

We report on gold foil activation measurements performed along a vertical channel along the tank of the ultracold neutron source at the Paul Scherrer Institute. The activities obtained at various distances from the spallation target are in very good agreement with MCNPX simulations which take into account the detailed description of the source as built.

Neutron production and thermal moderation at the PSI UCN source

The new type of ultra-cold neutron source presently under construction at PSI requires the possibility of transporting the full high-intensity proton beam, currently 2 mA, for about 8 s every 800 s onto a spallation target located at the centre of the source. For this purpose, a fast kicker magnet with a rise time (5–95%) of 0.5 ms has been built. The required magnet and power supply have been manufactured at PSI and were installed during a shutdown. A first test of the fast beam switching process, however, for safety reasons with a reduced beam intensity of 20 μ A , was performed successfully.

A fast kicker magnet for the PSI 600 MeV proton beam to the PSI ultra-cold neutron source

The present status of the continuously upgraded control system for the three accelerators is presented. Some of the major improvements in the 20 year history of using the control system are highlighted. The experience of the development and the transition from a group of isolated systems based on minicomputers to a message-based distributed system is presented. The set of tools provided for operation and development of the accelerators is described.

Status report of the PSI accelerator control system, the tools provided for operation of the accelerator

The goal is to build an intelligent subsystem connected via Ethernet to the distributed control system of the PSI accelerator complex. It is based on VME, using multiple CPUs for communication and device handling, and will drive the present serial CAMAC loops. The system will transform the logical requests from the network into physical functions for CAMAC, and will include the necessary device handling.

I. Jirousek

Papers

A fast kicker magnet for the PSI 600 MeV proton beam to the PSI ultra-cold neutron source

Status report of the PSI accelerator control system, the tools provided for operation of the accelerator

An intelligent VME-driver for a serial CAMAC loop