Background

In 1986, Gullikson and Mills discovered that rare gas solids, RGS (Ne, Ar, Kr and Xe), are very effective for moderating particles emitted in the decay of radioactive isotopes. Since then, RGS moderators have become routinely used by several research groups around the world.

Mills and Gullikson (1986) explained the positron reemission from rare gas solids in the following way: When fast positrons are implanted into solids, they rapidly lose energy via inelastic collisions. In the case of RGS, fast positrons lose energy via inelastic collisions until they do not have enough energy to form electron-hole pairs, excitons or Ps. Below this energy, in the band gap region, phonon emission is the only available energy loss process. In RGS the band gap energy is large, ranging from 9.3 to 21.4 eV. Phonon emission removes only a few meV energy per collision, resulting in a longer positron

diffusion length in RGS (-0.5pm) than in metal solids (-0.2 pm). Therefore,

unthermalised positrons may reach the surface of the RGS with enough energy to overcome their positive positron work function (Gullikson and Mills, 1986). Positrons which can thus escape from the solid are called epithermal, or ‘hot’, and the associated reemission picture developed by Mills and Gullikson (1986) is usually referred to as the ‘hot positron’ model.

Figure 4.1 shows the positron reemission probability versus positron implantation energy for a thick Ne target investigated by Mills and Gullikson (1986). For energies above Eg (the band gap of solid Ne, 21.4 eV) and Ex (the exciton threshold energy which is equal to 17.5 eV for solid Ne) a positron may escape from the solid, after creating an electron-hole pair and exciton, respectively. Eth, is the inelastic threshold energy and above this energy Ps formation in the solid becomes possible. The dip at very low energies arises from positrons which can become trapped in the solid or at the surface of the solid.

08 _{SOLIO N e} TaSK Q — 1 W _ > V - z 0 6 |- § F t L cn

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Figure 4.1 Slow positron reemission probability versus implantation energy for a thick target o f solid Ne (Mills and Gullikson, 1986),

Mills and Gullikson (1986) also investigated the energy distribution of reemitted slow positrons from solid Ne, as shown in figure 4.2. They studied three different implantation energies and observed that the energy distribution of the reemitted positrons becomes narrower with increasing implantation energies. They proposed that positrons implanted at higher energies would penetrate deeper and, therefore, they would have a higher probability to lose energy before re-emerging from the surface.

A generic arrangement for a RGS moderator is shown in figure 4.3 in which rare gases could be deposited on the moderator cup and source assembly, which was cooled

down to below rare gases sublimation temperatures. Different moderator cup

configurations, moderator growing temperatures and pressures, annealing processes, and vacuum conditions have been investigated.

Chapter 4 - Rare Gas Solid Moderators iCt Ne E^«800eV I Oh % *♦ D'­ Or U i ' , : i ! i » ♦ * I ♦ ♦ * • t •» * r A ! - r"AE'0 58eV_{>- ♦} L . \ o«*

L

Figure 4.2 Slow positron emission spectrum from solid Ne (Mills and Gullikson, 1986).

Electrical Heat Shield

Insulator Radioactive Source

RGS

Cold Head Cold Finger Moderator Cup

Figure 4.3 A generic arrangement for a RGS moderator.

The first Ne moderator was investigated by Mills and Gullikson (1986) with three different moderator geometries; a fiat face and two copper cylindrical cups. They attained efficiencies of 3 xlO’^, 6.4 xlO'^, 7 xlO'^ using moderators with a fiat face, a cylindrical Cu cup and a longer cylindrical cup, respectively, in a vacuum of - 1 0'^® torr, and they estimated that the moderator efficiency should be stable for at least a day. They also tested Ar, Kr and Xe moderators, and their results are summarised in table 4.1.

Ne Ar Kr Xe

Efficiency (%) 0.70 0.13 0.14 0.13

AE(eV) 0.58 1.7 1 . 8 3.2

Table 4.1 Efficiencies and energy spreads (full width half maximum) o f RGS moderators. Mills and Gullikson (1986).

Following Mills and Gullikson, Khatri et al (1990) compared cylindrical and cone configurations using solid Ne and found that the efficiency for the conical cup was 2.7 times higher than that for the cylindrical one. They achieved an efficiency of 4.6 xlO"^ with a conical copper cup and 1.7 xlO'^ with a cylindrical copper cup. The experiment used an encapsulated ^^Na source, which had a 5 pm Cugg/Bcz window and although no correction was applied to the above results for the attenuation caused by the window, they estimated that with a bare source their efficiency would be 1.4 xlO'^ using a conical configuration and solid Ne. They also studied solid Ar, Kr and Xe moderators and obtained lower efficiencies compared to solid Ne.

Massoumi et al (1991) was the first to investigate RGS moderators (Ar, Kr and Xe) in an electrostatically guided positron beam. Their estimated moderator efficiencies using a stainless steel cylindrical cup were, e ~ 7.6 xlO"^, 4.7 xlO"^, and 3.0 xlO"^ for Ar, Kr and Xe respectively. These numbers include corrections for source effects, grid transmission, geometrical losses and beam transport. The moderators were grown at 50 K with a pressure of 8 x1 0’^ torr and the beam intensities were found to be stable to within ~ 3.2 %, 11.7%, and 1.8% per hour for Ar, Kr and Xe, respectively, in a vacuum of ~5 xlO'^ torr.

Chapter 4 - Rare Gas Solid Moderators

Solid Ne was also investigated by Weber et al (1992), in a magnetically guided positron beam, with a conical cup and an approximate efficiency of 1.5 x l0 ‘^ was achieved. Their beam intensity dropped by approximately 12%, in a vacuum of ~10'^ torr, during a two week period.

Merrison et al (1992), investigated Ar and Kr moderators and reported that the efficiency of these moderators could be increased by up to a factor of three by coating them with a thin layer of O2 and then charging it with low energy electrons. The charged layer of O2 gives rise to a field within the bulk which gives the positrons a drifl: velocity in the direction of the exit surface. Therefore the positron diffusion is no longer random on average, and more positrons are reemitted. They obtained an efficiency of 2x10'^ with Ar after electron bombardment, whilst the previous efficiency was 7x10"^.

Grund et al (1992) investigated RGS moderators, in an electrostatically guided positron beam, under poor vacuum conditions ( -1 0"^ torr) and investigated the effect of annealing the RGS. They reported that Kr and Xe had efficiencies nearly as good as that of Ne. Using a conical moderator cup the highest efficiency was 8 xlO"^ for Ne, followed by 2 xlO^ for Ar, 6.3 xlO^ for Kr and 4.8 xlO^ for Xe. They found a dependence of the moderator efficiency on its operating temperature: increasing with increasing temperature for Kr and Xe. They conjectured that this is due to lower coverage of the moderator with impurities.

Following Grund et al (1992), Mills et al (1994) showed that the annealing of a Kr moderator results in an efficiency of 90% that of solid Ne. They reported efficiencies of 2.1 xlO'^ for Kr, 2.3 xlO'^ for Ne and 1.6 xlO'^ for Xe. The higher efficiency they obtained for Kr, in comparison to Grund et al (1992), is attributed to the better vacuum of their system (1 xlO'^ torr). Kr was also studied by Vasumathi et al (1995) in an electrostatic beam and an efficiency of 4 xlO"^, comparable with the results of Massoumi

et ûf/ (1991), was achieved with an energy spread (full width half maximum, FWHM) of 2.5+0.1 eV.

Recently, Greaves and Surko (1996) studied Ne moderators with a parabolic copper cup. Moderators were grown at ~7-8 K and annealed for a few minutes at 10 K. They achieved a moderation efficiency of 5 xlO'^ and an energy distribution (FWHM) of