The optimal potential of the MCP front side with respect to ground depends on the particles to be detected. Ions should be
pre-accelerated onto the detector with a potential of -2000V or higher. For most ion species it is suitable to operate the MCP
back side on or near ground potential, thus the front side is in the range of –2kV to –3kV (ion mode). Electrons should be
accelerated to at least 200eV to ensure high detection efficiency. Thus the MCP front should be around +200V or higher with
respect to the electron source for low energetic electrons (electron mode). For UV photon detection or fast particles with
>10keV/u the MCP front side potential is arbitrary.
If you want to operate the detector at different bias than the now verified “ion mode” you may simply change your voltage
settings accordingly. It is important to never exceed relative voltages of 100V between the reference and signal wire and of
500V between holder and signal wires or holder (respectively the timing anode) and MCP back. Otherwise discharges can occur
and damage the detector and electronics.
Before applying different voltage settings for the first time or after manipulating on the detector hardware / in-vacuum cables
in some way you have to make sure that these voltages (e.g. high potential with respect to ground on MCP back and the anode
contacts) can be safely set without discharge occurring. You should slowly increase the voltage towards the target values and
observe MCP back/front signal and the MCP resistance carefully during this procedure, as in described for the initial startup
procedure. It is advisable that all voltages for the anode and MCP back are drawn from only one (or as few as possible) high
voltage supply channels by using a voltage dividing scheme (e.g. with the
RoentDek HVZ and/or BA3).
Again you must verify that set voltages on high voltage supplies for MCP back and MCP front are maintained which is
sometimes not the case, especially when biased with the same polarity. Also
RoentDek modules show this effect and need
an additional terminating resistor (as in the HVT, see separate manual).
For achieving optimal results it is now necessary optimizing MCP gain and amplifier gain for the follow-up timing electronics.
You should refer to the specific manuals. Generally it is advisable to operate at a high MCP gain because only this ensures
optimal signal-to-noise ratio. Although it is also possible to increase the signal height by increasing the amplifier gain, it will
also increase the noise level. Therefore it is better to operate at a moderately high MCP gain, i.e. where the MCP stack can still
produce a decent pulse height distribution and no excessive after-pulses (ion-feedback). However, there are reasons to operate
at lower MCP gain (and compromise on position and time resolution) if the highest MCP signals in the distribution saturate
the amplifier (at its lowest gain setting), if ion feedback must be reduced (e.g. for multi-hit applications) or if MCP life-time is
an issue (i.e. for very high particle flux).
MCP Delay Line Detector Manual (11.0.1304.1) Page 33 of 83
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