Simultaneous positive and negative ion detector

A simultaneous positive and negative ion detector including a microchannel plate having segments which are biased to attract positively and negatively charged particles.

FIELD OF THE INVENTION 
The invention relates to devices for detecting ions. 
BACKGROUND OF THE INVENTION 
It is known to detect positive and negative ions by using a conversion 
anode to convert ions to secondary particles and then using a continuous 
dynode electron multiplier to provide a signal indicative of the abundance 
of ions. Stafford U.S. Pat. No. 4,423,324 discloses such a detector which 
may output a signal indicating the total of all positive and negative ions 
or may be multiplexed to produce a signal sequentially proportional to 
positive and then negative ions. 
It is also known to provide microchannel plates with distinct electrode 
patterns such as distinct strips. In operation, the strips are 
sequentially activated to provide a sequential view of particle position. 
SUMMARY OF THE INVENTION 
It has been discovered that providing a microchannel plate with segments 
which are biased to positive and negative potential at the input of the 
MCP advantageously provides an ion detector which can simultaneously 
detect positive and negative ions.

Structure 
Referring to FIGS. 1 and 2, simultaneous positive and negative ion detector 
system 10 for detecting positive and negative ions contained in ion cloud 
11 includes simultaneous positive and negative ion detector 12 which 
outputs signals to positive collector 14 and negative collector 16. 
Collectors 14, 16 each include electrically conductive anode 20, voltage 
supply 22 and output device 24 (an electrometer). Signal guard 26 is 
positioned between anodes 20, 20. 
Positive and negative ion detector 12 includes microchannel plate (MCP) 30 
having two electrically conductive metallic (Inconel) input segments 32, 
34 at MCP's 30 input locations and two output electrically conductive 
metallic (Inconel) segments 36, 38 at MCP 30 output locations. Between 
segments 32 and 34, as between segments 36, 38, is high resistance portion 
40 (a gap that is large enough to maintain the potential difference 
between the segments). 
Operation 
Referring to FIGS. 1 and 2. simultaneous PosItIve and negative ion detector 
system 10 is positioned to detect ions from ion cloud 11, which contains a 
combination of positive and negative ions. Input segments 32, 34 are 
biased to -1 kv and +1 kv, respectively, for attracting positive and ions, 
respectively. Additionally, output segments 36, 38 are biased at ground 
and +2 kv, respectively, to draw electrons generated by impacting ions 
through MCP 30, thus providing amplified output signals to anodes 20, 20 
of collectors 14, 16. Each anode is biased to a voltage which is 50 volts 
more positive than that of its respective output segment 36, 38 by 
providing voltage supplies 22 in series with the voltage provided to 
output segments 36, 38. 
Because detector 12 is simultaneously appropriately biased, system 10 may 
advantageously simultaneously detect positive and negative ions. Because 
MCP 30 is used, the detected ions may be simultaneously converted and 
amplified. Because separate output devices 24 are provided, the relative 
quantity of positive and negative ions may be simultaneously outputted. 
Other Embodiments 
Other embodiments are within the following claims. 
FIG. 3 shows a pulse counting simultaneous positive and negative ion 
detection system 50 which includes two cascaded, simultaneous positive and 
negative ion detectors 52, 54, positive collector 56 and negative 
collector 58. Collectors 56, 58 include metal anodes 60, 60, voltage 
supplies 62, 62 and output circuitry 64, 64 which provides outputs to a 
controller (not shown). Collectors 56, 58 are electrically separated by 
signal guard 66. Detector 52 includes MCP 70, electrically conductive 
metallic input segments 72, 74 and electrically conductive metallic output 
segments 76, 78. Detector 54 includes MCP 80, electrically conductive 
metallic input segments 82, 84 and electrically conductive metallic output 
segments 86, 88. In operation detectors 52, 54 are appropriately biased. 
More specifically, input segment 72 is biased to -1 kv, output segment 76 
and input segment 82 are biased to ground and output segment 86 is biased 
to +1 kv; also, input segment 74 is biased to +1 kv, output segment 78 and 
input segment 84 are biased to +2 kv and output segment 88 is biased to +3 
kv. The voltage of output segments 86, 88 is variable. Metal anodes 60, 60 
of collectors 56, 58 are also appropriately biased to a voltage which is 
50 volts higher than that of respective output segments 86, 88. The bias 
of input segments 72, 74 causes positive ions to be attracted to segment 
72 and negative ions to be attracted to segment 74. 
Also, other collectors used by detection systems 10 and 50 may be apparent 
to those skilled in the art. E.g., a scintillator/photo-diode combination 
may be used in place of the metal anode. 
Also, other charged particles, e.g., electrons, may be detected by system 
10. 
A multiplicity of detector-amplifier-collector units, rather than just two 
as shown, may be integrally arranged, to decrease lateral particle 
movement required.