Operation of the Buffer Gas System

Operation of the buffer gas system revolves solely around control of the driving pressures of the buffer gases. However, the required pressures are intrinsically linked to the duration of the phases of the trap cycle. While the partial pressures of the buffer gases inside the trap are not monitored, knowledge of the vacuum configuration, pumping speeds and driving pressures allows them to be estimated using the flow equations. However, these values are of little interest here and the following discussion will instead only consider the driving pressures for simplicity. Optimisation of the buffer gas pressures is performed with the goal of maximis- ing the count rate, while maintaining a desirable pulse shape with the shortest FWHM possible.

Due to the desire to cycle the trap quickly to keep the number of positrons per pulse low, moderated positrons must be trapped and cooled within 1 ms. This can be achieved by increasing the partial pressures of the buffer gases when compared to the operation of similar Surko traps. However, these pressures are limited by systematic effects on the pulse timing. These effects manifest as a tail in the timing spectra due to positrons lagging behind the pulse. This is parameterised by calculating the percentage of positrons which arrive after twice the FWHM has elapsed and is referred to here as the Itail parameter.

It was quickly established through systematic testing that this parameter is proportional to the pressures of the buffer gas. Initially, this was thought to be due to the positron pulse undergoing scattering in the transport stage of the apparatus. This adversely affects the energy distribution of the pulse as positrons which undergo scattering will likely lose vk, delaying their arrival at

the target. Simulations carried out in _Simionshowed qualitative agreement and, as a result, an additional turbomolecular pump was installed in the transport region. This reduced the pressure in this region by an order of magnitude, however little effect was seen on the pulse shape. As a result, it is thought that the scattering is taking place as the positrons travel through the ninth (and final) electrode, or shortly after they exit the trap. In order to reduce the pressure in this region as much as possible, the additional pump was relocated to the trap exit endstation. Unfortunately, this only resulted in a minimal improvement (1–2% reduced scattering) in the Itail parameter. However, it was found that the

driving pressures could be increased further without theItail parameter increasing

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3.4. Operation of the Trap Stage 73

as rapidly. This allowed the duration of the loading and cooling phases to be reduced, increasing the number of pulses per second. In order to eliminate this tail, a chopper may be implemented after the trap to truncate the pulse and discard any positrons which undergo scattering.

Optimisation of N2

At larger N2 pressures, the likelihood of a moderated positron undergoing an in-

elastic collision to become trapped within the potential well increases. However, this must be balanced against theItail parameter as well as the effect of reducing

the lifetime of the positrons inside the trap due to the increased probability of positronium formation and direct annihilation. The later of these two consider- ations appears to have little effect on the resulting count rate due to the timing regime and, instead, the pressure is ultimately limited by the increase in the tail due to scattering. Once the optimal N2 pressure is established, the duration of the

trap phase is reduced to keep the number of positrons per trap cycle satisfactorily low.

Figure 3.21 shows the effects of increasing the driving pressure from 70 mTorr up to 375 mTorr for fixed trap cycle parameters. It can be seen that at higher N2

pressures there is a corresponding increase in the count rate due to an increase in the number of positrons successfully loaded. For these measurements the duration of the loading phase remained constant; however it was sufficiently short that even at the higher pressures the effects of overlapping events per trap cycle were minimal. As discussed, the Itail parameter shows a steady increase proportional

to the driving pressure due to the increased probability of scattering upon release of the pulse. There is no significant effect on the FWHM of the pulse.

Optimisation of CF4

Optimisation of the CF4 pressure must be matched with the duration of the

cooling phase to give the isolated positrons sufficient time to thermalise. As discussed in section 3.4.2, if the N2 pressure, CF4 pressure or the duration of the

cooling phase are insufficient, the pulse broadens as the positron cloud hasn’t thermalised to the buffer gas temperature. Increasing the CF4 pressure allows a

shorter cooling phase and, therefore, the trap to be cycled faster. However, as with N2, the pressure must be balanced against theItail parameter and the lifetime

Jason

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Thesis

June

0.79 0.8 0.81 0.82 0.83 0.84 0.85 0.86 0.87 0.88 FWHM (ns) 18 18.5 19 19.5 20 20.5 21 21.5 22 22.5 23 Itail (% of total Intensity) 80 100 120 140 160 180 200 220 240 Count Rate (cps) 50 100 150 200 250 300 350 400 N₂ Pressure (mtorr)

Figure 3.21: Shown are the effects of the N2pressure on the characteristics of the

pulsed beam. During these measurements the CF4 pressure was 100 mTorr. The

lines show a logarithmic and linear fit to the count rate and Itail characteristics

respectively. No discernable systematic effect on the FWHM is observed.

0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 FWHM (ns) 19 20 21 22 23 24 25 26 27 28 Itail (% of total Intensity) 150 160 170 180 190 200 210 Count Rate (cps) 0 50 100 150 200 250 CF₄ Pressure (mtorr)

Figure 3.22: Shown are the effects of the CF4 pressure on the characteristics

of the pulsed beam. During these measurements the N2 pressure was 240 mTorr.

It can be seen that the count rate peaks slightly over 100 mTorr while a mini- mum in the Itail parameter is achieved slightly under this pressure. The FWHM

of the pulse approaches a minimum value of ∼_{0.8 ns as the pulse achieves full}

thermalisation.

Jason

Roberts

Thesis

June

2012

3.4. Operation of the Trap Stage 75

of the positrons inside the trap. Additionally, as CF4 provides the majority of

the cooling, the FWHM must also be kept in mind; this can be compensated for by adjusting the duration of the cooling phase. Unlike N2, the lifetime of

the positrons begins to detrimentally affect the count rate at higher pressures as positrons are lost to direct annihilation. Due to its closer proximity to the exit of the trap, the effect on theItail parameter is much more pronounced.

Figure 3.22 shows the effects of increasing the driving pressure of CF4 from

30 mTorr to 230 mTorr. It can be seen that the three characteristics discussed above are significantly affected. The count rate peaks around 125 mTorr, indicat- ing there is a contribution from CF4to the loading of positrons into the well at the

eighth electrode as discussed. Beyond this pressure the count rate decreases due to a reduction of the positron lifetime within the trap. At pressures lower than

∼_{100 mTorr, both the FWHM and Itail parameters are affected by insufficient}

cooling of the isolated positron cloud. As shown in figure 3.19, this results in a two component pulse, increasing both parameters. While the FWHM achieves a minimum value slightly over 100 mTorr, indicating that the positron pulse has been fully cooled, the Itail parameter begins to increase sharply at ∼90 mTorr.

Much like the N2, this is due to positrons within the pulse scattering, either elas-

tically or inelastically, upon release from the trap. During these measurements the duration of the cooling phase was held constant to allow the effects of the CF4 pressure to be investigated.