Drive and measurement circuit for a photomultiplier
A method of measuring an anode current in an electron-multiplier device having an anode, a cathode, dynodes and a voltage divider network for applying voltages to the dynodes, which method includes applying an HV positive voltage to the anode and intermediate voltages to the dynodes, the cathode being at or near circuit ground potential, conducting dynode currents through or in parallel to the voltage divider to a point substantially at cathode potential, and deriving from those currents a current representative of the anode current.
1. A method of measuring an anode current in an electron multiplier device having a cathode at or near circuit ground potential, an anode at a relatively high positive potential, a series of discrete dynodes at successive intermediate potentials, and a voltage divider comprising a string of transistors connecting the dynodes to respective points in a resistor string of the voltage divider, the method comprising:
passing at least some of the dynode currents successively through the string of transistors;
summing currents flowing in the dynodes and optionally also a cathode current, and deriving the measured current from the summed currents; and
subtracting from the summed dynode currents a standing current in the voltage divider to measure substantially at ground potential a current representative of the anode current and derived from current flowing in the dynodes, without taking a measurement of a current between the anode and the dynode adjacent to the anode.
2. A method according to claim 1 , comprising diverting at least some of the dynode currents around the string of transistors which connect the voltage divider to the dynodes.
3. The method of claim 2 , wherein the transistors are field-effect transistors.
4. The method of claim 1 , wherein the transistors are field-effect transistors.
5. The method of claim 1 , wherein subtracting from the summed dynode currents a standing current in the voltage divider is to measure only substantially at ground potential a current representative of the anode current and derived from current flowing in the dynodes.
6. A biasing and measurement circuit for an electron multiplier having a cathode, an anode and a series of discrete dynodes therebetween, the circuit comprising a maintainer to maintain the cathode at or near circuit ground potential, a connector to connect the anode to a relatively high positive potential, a voltage divider comprising a string of transistors connecting the dynodes to respective points in a resistor string of the voltage divider to apply graduated intermediate voltages to the dynodes, and a measurer to measure substantially at ground potential a current representative of the anode current and derived from currents flowing in the dynodes, without taking a measurement of a current between the anode and the dynode adjacent to the anode;
wherein the measurer is configured to sum the currents flowing in all of the dynodes and optionally also a cathode current; and
the measurer comprises a subtractor to subtract from the summed dynode currents a standing current flowing in an active arm of the voltage divider.
7. A circuit according to claim 6 , wherein the subtractor comprises an operational amplifier.
8. A circuit according to claim 7 , wherein the operational amplifier is provided with a feedback loop to its inverting input of impedance R 2 , the non-inverting input being grounded via an impedance R 1 such that IDR 2 =IDVR 1 where IDV is the standing current through the resistor string of the voltage divider and ID is the standing current through the string of transistors.
9. A circuit according to claim 8 , wherein the transistors are field-effect transistors and comprising a director to direct at least some of the dynode currents through the source-drain paths of the field-effect transistors.
10. A circuit according to claim 9 , wherein the diverter comprises a coupling capacitor.
11. A circuit according to claim 6 comprising a diverter to divert a current from at least one dynode around the voltage divider.
12. The biasing and measurement circuit of claim 6 , wherein the measurer measures only substantially at ground potential.
13. An electron multiplier comprising a biasing and measurement circuit having a cathode, an anode and a series of discrete dynodes therebetween, the circuit comprising a maintainer to maintain the cathode at or near circuit ground potential, a connector to connect the anode to a relatively high positive potential, a voltage divider comprising a string of transistors connecting the dynodes to respective points in a resistor string of the voltage divider to apply graduated intermediate voltages to the dynodes, and a measurer to measure substantially at ground potential a current representative of the anode current and derived from currents flowing in the dynodes, without taking a measurement of a current between the anode and the dynode adjacent to the anode;
wherein the measurer is configured to sum the currents flowing in all of the dynodes and optionally also a cathode current; and
the measurer comprises a subtractor to subtract from the summed dynode currents a standing current flowing in an active arm of the voltage divider.
14. An electron multiplier according to claim 13 , wherein the electron multiplier is a photomultiplier.
15. The electron multiplier of claim 13 , wherein the measurer measures only at substantially ground potential.
16. A biasing and measurement circuit for an electron multiplier having a cathode, an anode and a series of discrete dynodes therebetween, the circuit comprising means for maintaining the cathode at or near circuit ground potential, means for connecting the anode to a relatively high positive potential, a voltage divider comprising a string of transistors connecting the dynodes to respective points in a resistor string of the voltage divider for applying graduated intermediate voltages to the dynodes, and means for measuring substantially at ground potential a current representative of the anode current and derived from currents flowing in the dynodes, without taking a measurement of a current between the anode and the dynode adjacent to the anode;
wherein the measuring means is configured to sum the currents flowing in all of the dynodes and optionally also a cathode current; and
the measuring comprises a subtractor to subtract from the summed dynode currents a standing current flowing in an active arm of the voltage divider.
17. The biasing and measurement circuit of claim 16 , wherein the means for measuring substantially at ground potential measures only at substantially ground potential.