Sobre la supuesta violación al principio de simplicidad de la substancia

The decay rates and branching ratios for p^ emission and electron capture for ^^Na are shown in figure 2.1. This radioactive source, used throughout the work, was supplied by DuPont Pharmaceutical and came within a capsule in the form o f a 4mm spot deposited on a 4mm thick platinum disc with a diameter of 18mm. The source was sealed with a 10pm titanium window. It was estimated by Massoumi et al (1988) that around 40% of the p^ particles liberated from the source are allowed to escape in the

( liuptcr 2 Expevimtnnal Apparafii.s 2 2 Na 22 Ne 22 Ne Il 2 = 2.6 years Electron Capture = 10% Emission = 90% Y = 1.28 MeV (emitted within 1 0 ps)

Figure 2.1. The decay schemes and branching ratio o f ^^Na.

forward direction. ^^Na has a half-life o f 2 . 6 years approximately, which gives a range

of source activities from 9 to 5.2mCi during the experimentation period.

The source and the moderator arrangement are placed in the middle o f a

specially designed vacuum chamber, shown in figure 2.2, which consists o f two

concentric stainless steel cylinders with an axis perpendicular to that of the beam-line. The source is mounted on the end of the retractable shaft enabling the source to be

Inner R otatable C ylinder R em o v a b le Lead P lu g Partially Lead , F illed O uter C ylin d er i M oderator H ou sin g

Figure 2.2. A cutaway schematic o f the housing chamber fo r the source and the moderator assembly.

( liaplcr 2_____________________________________________hlxpcrinienlaJ Apparatus

withdrawn from the centre o f the chamber and the inner cylinder to be rotated. In this way the lead plug, on which the moderator assembly is mounted, can be safely removed from the system. The high level o f radiation from the source is greatly reduced by partial filling o f the inner cylinder with lead. Further reduction o f the background radiation is achieved by shielding the outside o f the chamber with lead bricks.

The source holder and the moderator assembly are shown in figure 2.3. Typically, the moderator consists o f three, superimposed 90% transmission, annealed tungsten (W) meshes, with a diameter o f approximately 14mm. The annealing process, which is described fully by Zafar et al (1988 and 1989), is carried out within a vacuum chamber in an “oven”, shown in figure 2.4. This is made from two pieces o f 99.95% pure tungsten sheet, typically o f dimensions 2cm by 5cm. Two W-alloy clamps hold the strips o f tungsten, outwardly bowed in the centre, such that the prepared meshes can be placed in the middle and heated by the current passing through the walls o f the oven. The temperature o f the meshes can be monitored through a pyrex window by means o f a pyrometer. The annealing process consists o f repeatedly raising the temperature o f the oven to around 2000°C for a few seconds in a pressure o f less than 10’*Torr. At the beginning o f the annealing process, the increase in temperature is accompanied by an increase in pressure in the vacuum chamber. This is caused by the impurities being released from the moderator and the oven itself. The annealing is carried out for up to several hours, until no increase o f pressure in the chamber is seen. Later, the meshes are allowed to cool down to room temperature under vacuum and are then transferred in air to the e^-beam system. Despite the fact that the W meshes are pretty robust, any additional handling increases the likelihood o f stress and thus damage to the crystal structure.

The moderator holder consists o f a PTFE sleeve o f diameter 20mm and length 9mm, into which annular PTFE washers and brass rings are inserted to provide support and insulation for the grids, as shown in figure 2.3. The electrical contacts to the grids are made through breiss rings. The source is held at the same potential as the moderator.

The moderator meshes are placed as close as possible to the source, in order to

increase the solid angle and thus to improve the positron beam intensity. The slow positrons are emitted perpendicularly to the surface o f the moderator if no inelastic

processes at the surface take place. The potential applied to the moderator meshes (Fm)

( liuptcr 2 ]\,xperimental A pparalns Lead plug Manipulator n w Moderator holder Earth grid Retarding grid A nnealed W m esh moderator Brass ring /

PTFE source PTFE insulator

holder 22^^^ source

Figure 2.3. A schematic diagram o f the ^^Na source and the moderator holder (to scale).

W -a llo y clam p

O v en

M esh es

( liapler 2____________________________________________ l^xpc ri mental A pparalns

E = e V^ +^ ^ (2.1)

where is o f the order o f a few eV and arises from the positron work function o f the

moderator and any possible contact potentials. For a clean W surface, equals 2.8 eV

(Jacobsen a/, 1990).