Muon Liquid Handling System User Guide C. Johnson, S.P. Cottrell et al Version 0.
Contents 1 2 3 Getting Started 3 1.1 Layout of the liquid handling system 1.2 Layout of the in situ sample stick 1.3 Layout of the pump 3 4 4 General procedures 6 2.1 Sample loading 2.1.1 Loading Vessel 1 2.1.2 Loading Vessel 2 2.2 Evacuation of the system 2.2.1 Removing air from Vessel 1 2.2.2 Removing air from Vessel 2 2.2.3 Removing air from Vessel 3 2.2.4 Evacuation of the sample cell 2.3 Freeze-Pump-Thaw cycle 2.3.1 Sample in Vessel 1 2.3.2 Sample in Vessel 2 2.3.3 Sample in Vessel 3 2.
1 Getting Started This manual describes the liquid handling system and the in situ sample cell as used on the DEVA instrument with the “RF” spectrometer, for either normal muon spins relaxation or RF resonance experiments. 1.1 Layout of the liquid handling system The Muon Liquid Handling System has been designed to facilitate the in situ degassing and transfer of samples into and out of the sample cell. The main features of the liquid handling system are shown in Figure 1.
1.2 Layout of the in situ sample stick The liquid sample stick is designed to fit into the DEVA flow cryostat, details of which can be found in the DEVA manual. It consists of a shapol target cell 30 mm x 30 mm with a mylar window upon which may be mounted an RF coil. Two stainless steel capillaries provide a means of flowing liquid into and out of the cell along with feed throughs for an RF excitation signal and a pick-up coil.
Off/Run switch Roughing/ Turbo switch Pressure reading Turbo control panel Turbo Pump Rotary Pump Figure 2 The layout of the front panel on the vacuum pump used with the muon liquid handling system.
2 General procedures 2.1 Sample loading At this point, the system will be open to the atmosphere, ensure that valve V2 is closed to prevent air condensation in the cold trap. 2.1.1 Loading Vessel 1 • • • • Open the tap on the right-hand side of Vessel 1. Open tap A and valve V11. The liquid should be poured into the system very slowly to avoid liquid leaving the vessel via the side tap. When loading is complete, close the side tap, tap A and valve V11. 2.1.
2.2.1 Removing air from Vessel 1 Once the system has been pressurised, tap A on Vessel 1 can be opened. It is possible to bubbled helium gas through the sample liquid against the non-return valve, V13. If this is required, first ensure that valves V5, V7 and V8 are closed. Open valves V10 and V6, finally opening tap B slowly in order to regulate the flow. To fully evacuate the system and Vessel 1, the sample liquid must be frozen by raising a dewar of liquid nitrogen around it.
2.3 Freeze-Pump-Thaw cycle Using this system it is possible to operate freeze-pump-thaw cycles on sample liquids held in any of the three vessels. Procedures appropriate to each vessel are outlined below. 2.3.1 Sample in Vessel 1 • • • • • • • • All taps and valves should be closed. Open valve V2 and tap A on Vessel 1, and freeze the sample liquid by slowly raising a dewar of liquid nitrogen around it.
2.3.3 Sample in Vessel 3 • • • • • • • • All taps and valves should be closed. Open valves V2 and V3 along with tap C on Vessel 3 and freeze the sample liquid by slowly raising a dewar of liquid nitrogen around it. When it has been determined that the liquid has completely frozen switch the pump to roughing mode and open valve V1. Switch the pump to turbo mode and leave for several minutes. The pressure within the system can be read from the baratron gauge on the pump.
2.4.2 Vessel 3 to sample cell The transfer of liquid from Vessel 3 to the sample cell is brought about by the difference in pressure between that in the Vessel and a vacuum in the cell. However, in order to control the transfer of liquid the pressure difference should be as small as possible but not so low that the liquid starts to boil. • • • • • • • Switch the pump to its roughing mode. Open values V2 and V3 along with tap C and carefully open valve V1 until a suitable pressure is obtained (~0.5 bar).
2.4.4 Recovery of liquid in sample cell to Vessel 1 The liquid in the sample cell cannot be recovered to Vessel 1 using the liquid handling rig as presently configured. 2.4.5 Recovery of liquid in sample cell to Vessel 2 The liquid in the sample cell can be recovered to Vessel 2 using the following procedure. • • • • • • • Vessel 2 should be under vacuum along with the manifold between valves V5 and V6 prior to the recovery of the liquid from the sample cell.
• • • All the liquid may not return in a single pass and repeating the evacuation procedure may be required. However, some liquid may be present in the system and care should be taken during evacuation that the trap does not become blocked. The liquid may also be encouraged to leave the sample cell by using a small amount of gas. Close valve V3 and open valve V10 to allow helium gas into the lower part of the system.
3 Example Experiment The paramagnetic signal in liquid n-hexane is difficult to observe because the muonium polarisation is found to decay on a microsecond time scale, which is further reduced by the presence of dissolved oxygen. A series of commissioning experiments were carried out using n-hexane in order to gauge the ability of the liquid handling rig to degas the liquid. This experiment was carried out using a detachable liquid-sample cell.
a) b) 13.5 13.0 13.0 Initial asymmetry (%) Initial asymmetry (%) 12.5 12.5 12.0 12.0 11.5 11.5 11.0 11.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 Time (µs) c) 1.5 2.0 2.5 3.0 Time (µs) 13.5 Initial asymmetry (%) 13.0 12.5 12.0 11.5 11.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (µs) Figure 1 Muonium precession signal at 2G in n-hexane after a) 4, b) 7 and c) 10 freezepump-thaw cycles.
