Having successfully reproduced previously measured values o f total ionization cross- sections o f molecular hydrogen with this system attention was focussed on the ionization o f atomic hydrogen. In preparation for actually measuring its total positron impact ionization cross-section efforts were made to identify and resolve problems that arose from the operation of the R F discharge tube. This was done initially with positrons as projectiles and then, due to the higher beam intensity attainable, electrons. These studies will now be discussed.
4.3.1 Positron Im pact Ionization
The most immediate problem that was encountered in switching on the gas discharge was the extremely high count-rate observed on the Ceratron ion detector ( - 2 0 0 0 counts per second). Such a count-rate in the stop channel o f the timing system led to a large random background on the ionic time-of-flight spectrum which swamped the ion peaks making them indistinguishable from the background.
The count-rate was independent of the polarity o f the potentials on the ion extraction optics but disappeared when the discharge was turned off. A n examination o f the output pulses of the Ceratron revealed that they were considerably larger in magnitude than pulses origniating from R F pick-up which were successfully
discriminated against. This led to the conclusion that the observed count-rate was due to Lyman-a photons striking the Ceratron and being detected. The possibility that it may be due to photons from the visible part o f the spectrum was excluded as these were of insufficient energy to be detectable by the Ceratron, the threshold for this being approximately lOeV. Attempts were then made to ascertain the exact origin of these photons - it was unclear whether they emanated from the discharge itself or were emitted in the de-excitation of resonantly excited H atoms that had left the discharge tube. If the latter was true then they would be em itted on the line-of-sight o f the detector. Modifications to the ion extraction optics would then be necessary if the count-rate was to be reduced.
Initially, aluminum foil was used to block off the entrance aperture to the ion extraction optics and the region around the Ceratron cone. The Ceratron count-rate was then reduced to - 4 0 0 counts per second, still considerably more than the count- rate with the discharge off. This implies that photons are travelling around the sides of the extraction lenses and being reflected off the graphite covered surface o f the tube in which they are housed into the Ceratron. To make the internal surface o f this tube less reflective it was coated with carbon in the form o f soot. Rem oval o f the foil from the aperture and subsequent operation o f the discharge tube revealed that this had served to reduce the count-rate of Lyman-a photons to only 10 per second. This was sufficiently low for ion peaks to be seen on the time-of-flight spectra. It also demonstrates that som e photons escape the plasma and the discharge tube and that fluorescence o f the hydrogen target outside the discharge tube is negligible.
A typical time-of-flight spectrum that was obtained with the discharge on is shown in Figure 4.9; the peak at shorter time (centered on channel 209) is that due to the detection o f protons and at longer times the H 2 peak can be seen. This time difference betw een the two ionic species is a result o f their different charge-to-mass ratios and correspondingly different velocities in the electrostatic extraction field. It is therefore not necessary in this experiment to have a gas target which is entirely
atomic hydrogen as the atomic and molecular forms are distinguishable in this manner.
As explained in section 3.3.2 the inelastic processes that occur in the plasma include ionization - in a mixture of atomic and molecular hydrogen one would therefore expect protons to be formed on the ionization of atomic hydrogen in the discharge. Clearly neutral species can diffuse out of the plasma and the discharge tube and it is not inconceivable that protons could do the same. If this occurred then it is quite possible that these protons would be detected after the extraction field was initiated by a positron which had not been involved in an ionizing collision. It had to be ensured therefore that all detected protons resulted from the ionization o f atomic hydrogen by the positron beam.
This was tested by substituting for the MCP detector output pulses created by a pulse generator. These were of an amplitude sufficient to trigger the discriminator and pulser shown in Figure 3.9 and to start the timing sequence. By switching the positron beam off there would then be no ionization o f the neutral gas by positrons and any ions which were detected must have come from the plasma. H + yields obtained with this arrangement were negligible and it was concluded that these protons were being formed in the plasma and then diffusing out of the discharge tube - they were then present when an uncorrelated positron was detected and the extraction voltage pulse was applied. Thus they were being detected in coincidence with a positron in exactly the same way as those protons which were formed in positron impact ionization of atomic hydrogen.
4.3.2 Electron Im pact Ionization
Having demonstrated that the observation of H + ions formed on positron impact with this system was not without its attendant problems, their resolution was tantamount if an accurate determination of the total ionization cross-section was to
Fig 4.8
. _ ' n 1 ' 1 1 "r 1 1 1 * ' ( ' r r - r T i i i T f - r i i i i n r r
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