Enabling High Pressure Operation via a Novel Channel Electron Multiplier

Homeland security requirements create the need for portable mass spectrometers, with ion detectors capable of performing at elevated pressure.

PHOTONIS has developed a miniaturized channel electron multiplier that delivers high performance at pressures up to 10^-2 torr. The MegaSpiraltron™ detector is small and robust, and can achieve high gain while maintaining low noise.

The multi-channel configuration provides a six-fold increase in surface area compared to a single channel, yielding longer life. The internal spiral structure facilitates low noise performance by significantly reducing ion feedback generated from the high concentration of residual gas molecules in high pressure environments.

The ability to operate in poor vacuum was determined by measuring the background noise or dark current as a function of pressure. Life tests were performed by measuring the change in gain as a function of extracted charge under elevated pressures.

Experimental Conditions

The MegaSpiraltron was installed in a test chamber equipped with a turbopump and Edwards WRG-SL vacuum gauge. The system was pumped down to the 10^-7 torr range. Baseline data were collected for dark current and gain using an electrometer and a Canberra multi-channel analyzer. The pressure was increased using a variable leak valve to introduce laboratory air, pausing to insure equilibrium. Gain and dark current measurements were recorded as a function of chamber pressure. Life testing was conducted while continuously stimulating the multiplier with ions from an ion gun at 10^-2 torr and recording the output current.

Figure 1

Results and Conclusions

Figure 1 shows the dark current as a function of pressure. 1500V was selected to achieve a multiplier gain of 10⁵ . The detector operates quietly until reaching a pressure of about 10^-2 torr. Conventional channel electron multipliers become noisy around 10^-5 torr, but the MegaSpiraltron can operate several orders of magnitude above this. The life test, Figure 2, demonstrates that 0.6 Coulombs of total charge can be extracted without a substantial loss in gain. This translates into a lifetime of at least 1.9 years in a typical mass spectrometer under normal operating conditions. The initial gain drop is due to the contribution of adsorbed gases on the active surface of the multiplier.

Figure 2

MegaSpiraltron multipliers, Figure 3, operate effectively in high vacuum and poor vacuum making them an excellent choice for portable instrumentation and other high pressure applications.

Figure 3

PHOTONIS USA

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