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A weak electrostatic field penetrating the interaction region was used to expedite the extraction of slow, scattered positrons. This technique was employed in an attempt to trigger the ion extractor within the time it takes a remnant ion to drift far enough to escape detection. This time is defined here as the effective ion lifetime, t.. In this work, r. has

been determined by measuring the yield of ions as a ftmction of a delay introduced in the signal triggering the ion extraction field. Figure 4.7 presents the results of such measurements for He^. It was thus determined that l<z;<4p,s for He^. This result is in

0.00035 0.00030 - 0.00025 - c D s i 0.00020 - T3 (D c ^ 0.00010 - 0.00015 - 0.00005 - 0.00000 5 6 0 1 2 3 4 Delay (^s)

Figure 4.7 The He"^ ion yield measured as a function of delay added to the signal triggering the ion extraction field.

agreement with a computer simulation for the present ion extraction/detection system, which predicted a lifetime of about 4|is for He^. These measurements and calculations were made for a 10mm diameter hole in the face of the negative ion deflection plate. Subsequently, this bole was widened to 20mm in an attempt to increase t. and

consequently the ion extraction efficiency. The ion lifetime r;. has not since been remeasured to monitor the effect of this modification, but measurements of the ion yield revealed an order of magnitude increase on widening the bole. This is consistent with an increase in z;., due to the ions having to drift farther in order to escape detection, as well as an increase in the "observable" volume of intersection between the positron and gas beams. The latter effect is not obvious when it is considered that the positron beam is of 8mm diameter and the gas jet emerges from a 6mm diameter nozzle. If the gas jet exhibits a similar geometry to that of the nozzle, both gas and positron beams are less than 10mm wide and opening the ion detector entrance bole to a 20mm diameter should not therefore affect the observable volume of intersection. However, the geometry of the gas jet is not known and it is possible that it is more than 10mm wide at the interaction region.

SEVUON (Dahl and Delmore 1988) has also been employed to devise the weak field penetrating the interaction region to aid the extraction of very slow positrons. The computer model was first used to design a system of electrodes capable of introducing such a field, resulting in the combination of R2, the RFA and the earthed cylindrical electrode represented in figures 3.1 and 3.5. SBVUON was then used to study positron flight-times as functions of the potential on R2 (F^) and the positions of R2, the RFA and the earthed cylinder in order to determine the optimum arrangement. A SIMION simulation of the system devised by this theoretical study and the experimental

observations discussed below is shown in figure 4.8. The electrostatic potential contours in the diagram show that for F^=-300V, the potential at the interaction region is lOmV, ensuring a negligible perturbation of the positron energy within the experimental resolution of the present work. The trajectory of a positron with £'=35eV and E'=\QAqV, following ionization of a He atom, is also shown.

The flight-times for positrons scattering with excess energies in the range O-lOeV and to angles between 0 and 180° were calculated from the model depicted in figure 4.8 and found to be between 0.03 and 1.34p,s. An estimate of the efi&ciency with which e^-ion coincidences are measured, as a fimction of the excess energy, E', has been deduced by comparison of positron flight-times with the measured ion lifetimes. The flight-times for positrons scattering to all angles were calculated to be less than the minimum effective He^ lifetime of Ips for £ ’'>leV, predicting a uniform, 100% efficiency for the measurement of e^-ion coincidences in this energy regime. For £ ’'<leV, however, not aU scattered positrons are predicted to reach the positron detector within Ifis of the colhsion event and the coincidence efficiency is less than unity. In such cases, the coincidence efficiency for a positron following a given trajectory was deduced by comparing the flight time for that trajectory with the dependence of the ion yield on the triggering signal delay shown in figure 4.7. For example, a delay of 2.5p,s results in a 50% decrease in yield, implying that for a 2.5|xs positron flight time, the probabihty of counting a e^-ion coincidence is 50%. The coincidence detection probabihty was determined in this way for positron trajectories with final energies between OeV and leV and scattering angles in the range 0-180°. For a given excess energy, the overaU coincidence efficiency was then deduced by calculating the average of the coincidence probabihties for ah the scattering

OmV— -5mV T L -20mV E=35eV -25mV -30mV -300V OV LA LJ

angles considered at that energy. The overall e-He^ coincidence efficiency is shown in figure 4.9, illustrating that the efficiency falls by less than 3% below E - lo V . Although this would result in an underestimate in 3% is negligible compared to the other experimental errors which arise from the present system in the close-to-threshold measurements. 1.1 c (D Ü 1.0 %

§

Excess energy, E (eV)

10

Figure 4.9 The probability of measuring a e^-ion coincidence as a function of the positron excess energy.

Experimental verification of the effect of the penetration field on the coincidence detection efficiency is inherently difficult because is very small near threshold. An attempt was made, however, to validate the technique experimentally by measuring the ion yield as a function of the potential on R2 within the limits -300V<F^<0V. An upper limit on the magnitude of the accelerating potential of about 300V was imposed since voltages much more negative than -300V were found to decrease the positron count rate. Two measurements of the ion yield for the direct ionization of H; for V ^= 0 \ and F^=-150V are shown in figure 4.10. On applying -150V to R2, a significant increase in the ion coincidence efficiency is apparent for energies more than about 0.5eV above the threshold

energy, E., in accordance with expectations. An attempt to verify this enhancement more accurately close-to-threshold has been made by measuring the ion yield at fixed energy as a fimction of These measurements, which were made with a He target and an incident positron energy of 27.5eV, 2.9eV above threshold, are presented in figure 4.11. They show that for F^<-200V the yield is greater by a factor 2.0±1.0 than that measured with F^>-100V, implymg a similar increase in the coincidence efficiency.

1.4e-4 1.2e-4 - 1.0e-4 - c 3 8.0e-5 - (Ô _{6.0e-5 —} T3 OJ '>- 4.0e-5 — g 2.0e-5 — O.Oe+0 - -2.0e-5 20 25 30 35 E,.=15.45eV

Positron Impact Energy, E(eV)

Figure 4.10 Near-threshold ion yields for showing the effect of on the efficiency of detecting e"^-ion coincidences.

Finally, it was necessary to ensure that the increased ion yield observed with F^=-300V was not a consequence of the positron impact energy being perturbed by the penetration field at the interaction region. This was achieved by measuring the total number of ions created for a fixed impact energy as a fimction of Using a Hj target, the positron

energy was fixed at Æ=10eV, an energy at which (jp^ is rising rapidly (e.g. Fomari et al

1983, Moxom et al 1995) and perturbation of this energy at the interaction region would result in a sharp increase in the number of ions created. To ensure that aU ions were detected, a weak DC ion extraction field was employed by applying ±2V to the ion deflector plates. For in the range 0 to -400V, the total ion count was found to be constant and it was therefore concluded that the weak electrostatic field penetrating the interaction region was not perturbing the positron energy. This result is consistent with the abovementioned computer simulation of the present system. These calculations predict that a potential of only -13mV with respect to the chamber earth is introduced at the centre of the interaction region on applying -400V to R2.

6.0e-7 E=27.5eV (E'=2.9eV) 5.0e-7 ss. ~ 4.0e-7 3 -2 & 3.0e-7 ■D 0> c 2.0e-7 o 1.0e-7 O.Oe+0 0 100 200 300 -V^(V)

Figure 4.11 Ion yields for He measured at a fixed, close-to-threshold energy to show in greater detail the enhancement in the coincidence efficiency as a function of V^.