PERÍMETRO

The positron sources used in the course of this project are ^*Co sources of 3.7GBq initial activity and a 3.7MBq ^^Na source and were supplied by DuPont. The ^^Na source consists of a 4mm spot of ^^Na deposited on a 4mm thick Ft disc of diameter 18mm and sealed with a 10pm Ti window. This source was mounted in the source/moderator arrangement used in the experiment of Jones et al (1993) and was used to set up and characterise the e^ beam. For the near-threshold measurements a more intense positron source was required to compensate for the high fraction of the beam discarded in reducing its energy spread. The ^*Co sources each consist of a 3mm diameter, unsealed spot of ^*Co deposited on a Rh disc of diameter 10mm and thickness 1mm. A new source and moderator holder was constructed for these sources for reasons discussed below.

The fast positrons emerging from the sources were moderated by frve superimposed layers of annealed 90% transmission W mesh. In this experiment, aU grids were composed of 90% transmission W mesh. The moderator grids were annealed in a manner similar to that described by Zafar et al (1988, 1989). This annealing technique involves heating the meshes to a temperature of around 2000 °C in an oven consisting of two W foil strips, while maintaining a pressure less than approximately 10 ^ Torr with a rotary backing pump. The temperature is slowly increased by resistively heating the W strips until the background pressure increases to nearly 10'^ Torr. Before this limit is reached the temperature is reduced to allow the backing pump to re-evacuate the chamber. Once the pressure has fallen again to less than 10'^ Torr and stabiUsed, this process is repeated until it is possible to increase the temperature to 2000 °C without an appreciable increase in pressure.

The source/moderator holder used with the ^^Na sources is described briefly here and in detail in the thesis of Jones (1991). Figure 3.2 shows a section diagram of this arrangement.

The platinum disc onto which the source is deposited has a M2 threaded hole allowing it to be mounted on a piece of M2 studding mounted on one end of the brass plug. The plug is externally threaded with a 20mm diameter and is screwed into a cylindrical PTFE block. The PTFE is mounted on two lengths of stainless steel studding screwed into the face of a 70mm diameter stainless steel flange. A PTFE washer of internal diameter 10mm isolates the source from two brass washers of similar dimensions between which the moderator

Brass plug

Cylindrical

PTFE block "Na source _{on Pt disc}

PTFE washers

Annealed W moderator grids

Figure 3.2 A cutaway diagram of the source and moderator holder used in the experiment of Jones et al (1993) and described in detail in the thesis of Jones (1991).

gilds are held. Holes are tapped around the edge o f these washers so that they may be

positioned by the steel rods and fixed in place with nuts. Electrical contact to the

moderator is achieved with a wire soldered to one o f the brass washers and to a

feedthrough on the flange. A similar connection is also made to the brass plug by way o f

a solder tag fixed on the base o f the plug with a small screw. Tliis allowed the source to

be biased to a potential slightly higher than to repel any positrons ejected in its

the accelerating field is not divergent. Such a field could cause spiralling of the e^ trajectories as well as the loss of slow e^ emerging fiom the moderator. Again this mesh is held between two brass washers fixed on the steel rods. These washers are electrically isolated fiom the moderator by a further PTFE washer.

A new source/moderator holder has been constructed for the present measurements to accommodate the ^*Co source foils. In designing the new arrangement, an effort was made to ensure that the moderator and earth grids would be positioned normally to the direction of the longitudinal magnetic field. An additional aim was to minimize the exposure time in fitting the source. A section through the new holder is shown in figure 3.3.

The moderator grids are clamped between two brass split rings each of 1mm thickness and internal diameter 12mm. These rings are split so that they fit tightly into a PTFE holder. Before the moderator is mounted, a 90% transmission W mesh of about 15mm diameter is positioned on the lip of a cylindrical aluminium block of diameter 32mm and fixed in place by a further brass ring of 12mm internal diameter. The PTFE moderator holder is then pushed into place on top of the brass ring. The outer diameter of the holder is such that it fits tightly into the upper aluminium block and so fixes itself in place. This block is then positioned on 3 lengths of stainless steel M3 studding screwed into the face of a 70mm diameter stainless steel flange and is held in place between stainless steel nuts. Electrical contact to the moderator was made by soldering a wire onto one of the adjacent brass rings and routing this through channels cut into the PTFE holder and the aluminium block and down the side of the arrangement to a feedthrough in the flange.

Earthed Grid Stainless Steel / Nuts PTFE Holder Brass Split Rings Aluminium Blocks Source and Foil Moderator Meshes Insulating Ceramic

Bushes Stainless Steel

/ Studding

Aluminium Plunger

Steel Flange

Below the moderator holder a second cylindrical Al block of diameter 32mm is held in place on the studding. Accurate positioning of both blocks is achieved by adjusting the nuts on the studding. Placing the source as close as possible to the moderator maximizes the fast e^ incident on the moderator and therefore the slow e^ flux. It is, however, necessary to leave a gap («1mm) between the blocks to electrically isolate them fi'om each other. The source foil is positioned in the recess of the aluminium plunger which is then screwed into the lower cylindrical block, into which several turns of M l6 thread have been cut. Only one or two turns of M16 thread are cut into the plunger and block to keep the plunger travel to a minimum and therefore expedite installation of the source. The foil is clamped in place between the plunger and a lip of 0.5mm thickness on the block. Contact to the source is made with a wire and solder tag screwed to the base of the lower block. The fi’ont grid is earthed through its contact with the upper Al block and the studding. In this way, all electrical connections can be made prior to mounting the source and these joints receive no stresses when the source is installed.

3.4

The Vacuum System

The vacuum system is represented schematically in figure 3.1. The experiment has been performed at a background pressure of approximately 4x10'^. The source and moderator holder sits in a 125mm long stainless steel tube of 37mm internal diameter which is bolted onto a 300mm long stainless steel tube of internal diameter 50mm via an adaptor flange. A stainless steel four-way piece of 50mm internal diameter and length 203mm couples the 300mm tube and the 500mm long brass tube of 95mm internal diameter which houses a trochoidal velocity selector. The pumping impedance introduced by the lead aperture

placed after the ExB plates is compensated for by an Edwards E02 oü vapour dijBRision pump bolted to the four-way. The E02 is backed by an Edwards E2M-18 rotary pump. The brass tube is fixed to a stainless steel five-way of internal diameter 100mm and length 250mm along the beam axis. This is connected to an Edwards E04 difiusion pump backed by a Varian SD300 rotary pump. Bolted on to the four-way via an adaptor flange is a stainless steel tube of length 500mm and internal diameter 75mm in which the RFA sits. Joining this tube to the target chamber is a brass tube of length 78mm and internal diameter 75mm. The target chamber is constructed from a 152mm cubic aluminium block with a vertical 127mm diameter central clearance hole. Into each of the four remaining faces of the block, 75mm diameter clearance holes are cut. Coupled to the bottom face of the target chamber is an Edwards E06 difihision pump and an Edwards ED500 rotary backing pump. Orthogonal to the beam axis, a brass tube of length 150mm and internal diameter 75mm is bolted to one face of the target chamber. This tube houses the ion detector arrangement. The opposite face is sealed with a 140mm diameter brass flange fitted with three electrical feedthroughs. These feedthroughs provide the necessary connections to the retarding field analyser (RFA) and the two ion deflection plates shown in figure 3.1. Atop the chamber sits a 200mm stainless steel flange on which are mounted 13 stainless steel mini-flanges of 34mm diameter. The target gas nozzle is fed through the central mini-flange; two adjacent mini-flanges support an ionization gauge and a manipulator arm for the insertion and removal of two apertures used to align the e^ beam. To the remaining face of the target chamber, another 78mm long brass tube of 75mm internal diameter is connected. This is joined to a stainless steel T-piece, of length 200mm and internal diameter 60mm, which is bolted to an Al tube of internal diameter 95mm and

length 140mm. The 152mm diameter Al flange which seals the other end of this tube also supports the positron detector arrangement.

Magnetic valves are included in the backing lines between each difiRision and backing pump. The power to these valves and all the pumps is supplied by two vacuum control units. The system is protected against high chamber pressures or a break in the supply of water cooling the difiusion pumps: power to the diSusion pumps and all sensitive electronics would be cut and the magnetic valves closed in such instances.