MCP based bipolar detection of high masses using 20kV post acceleration
A time-of-flight mass analyser has no inherent mass limitation, but the detection efficiency of a micro channel-plate detector assembly decreases with mass, because, at a given ion energy, heavier ions have a lower conversion efficiency to electrons. The relatively modest ion acceleration used in the standard detector assembly limits the mass range to around 5,000 amu for inorganic cluster-ions and less for large organic ions .
It is possible to increase the sensitivity for high-mass ions by using post-acceleration to increase the energy of the initial impact between ion and detector. One way to do this is to float the detector to a high voltage with respect to ground. There is then the question of managing the high electric fields created so as to avoid arcs/discharges and finding a way to couple the ion signal to a ground based detection system.
The first photograph shows the in-vacuo assembly which mounts a 25mm active area dual microchannelplate assembly from a high voltage insulator so it can be floated at up to 20kV.
The second photo shows the grounded external safety cover. Inside are bias chains, capacitative coupling/decoupling and a fast preamplifier for driving a 50 Ohm signal cable.
To keep costs low, a single high-voltage supply is provided. A resistor chain inside the safety cover divides this voltage to provide post-acceleration and excitation for the multiplying detector. As the detector ages with time the excitation will need to be increased, which will tend to increase the post-acceleration also. Eventually, a simple resistor change is required to keep the highest voltage at no more than 20kV.
To switch the polarity of ions detected , the high voltage power supply is reversed and two leads on the detector are simply swapped.
Technical Specifications Summary
Type of Detector
- 25mm Active area, Dual MCP (chevron type) with housing and electrical connections.
- Post-acceleration voltage 20kV (variable)
High voltage Power supplies
- 20kV high voltage power supply with suitable current rating for post-acceleration
- Power supply for MCP through a resistive chain network
- Input : 230 VAC ±5%, 50 Hz, Single phase (110V, 60Hz also possible)
- Ultrahigh vacuum compatible 10-8 mbar.
- Electrostatic shielding provided to avoid any disturbance in the flight path of the ions due to high voltages.
- Suitable electrical feedthroughs with rating > 20kV for providing electrical voltages
- Completely shielded and grounded with suitable openings for pumping
- Vertically mounted on a CF 100 (152 mm OD) Conflat flange
- Height of the detector from the flange will be adjusted as per the conditions of the system
- Local pre-amplifier for maximum count rates and fast rise and fall times (<2ns)
- Low voltage, low noise, power supplies to operate the pre-amplifier included
- Dark noise with all components at operational voltages and with pre-amplifier will be < 10 cps
Last updated:15:12 24/07/2019
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