Patent GB-1488169-A (GB)
Title: Electron multiplier circuits
IPC: H01J43/30
Filed: 1974-10-24
Abstract: 1488169 Photo-multiplier circuit MAXPLANCK-GES ZUR FORDERUNG DER WISSENSCHAFTEN EV 24 Oct 1974 [25 Oct 1973] 46025/74 Heading G1A A variable gain circuit, Fig. 1, for an electron multiplier having a source of primary electrons K, two or more dynodes, D 1 , D 2  ....., and an anode A, comprises a voltage divider comprising an impedance with tappings for each dynode and with its negative terminal connected to K, a load impedance R a , i.e. R a1  to R a4  connected between each dynode and its tapping, an amplifier OP whose input is selectably connected by switch Si to one of the dynodes, and which provides the output signal U s , the voltage of which is dependent on the current of the selected dynode, while its input voltage is substantially constant. The output of OP is coupled to the voltage divider of resistors Rb, i.e. R b1  to R b4  and to load resistors R a  by a second bank S 2  of the switch. The ratio R b1 /R a1 , R b2 /R a2  &c # V or = V - 2 where V = the dynode amplification factor. These ratios may apply to the second and higher dynode stages only. R a1  may have a greater impedance than R a2 , R a3  &c. The parts of the voltage divider between K and D, and between D 1  and D 2  may include a Zener diode. Fig. 4 shows a circuit in which the PD between K and D, is increased when the lowest number of active dynodes is selected by the switch. Fig. 2 shows diodes D b1  to D b5  connected in parallel with resistors Rb. Switch bank S 3  couples said positive terminal to D n  when S 1  couples D n-1  to OP. There is a cathode load impedance R ak  = R a1  and S 1  has a contact joined to the cathode. Zener diode Z, is in series with Rb 1. R b1 /R ak  # V-3. A constant current supply is used. The PD across each resistor Rd is independent of the setting of S 4 . Second load impedances, (R a ) higher than the first may be connected to some dynodes, each of their other ends being connected to a junction of a chain of capacitors, at a junction which is earthed by a switch bank when the first switch bank couples the corresponding dynode to OP. The first load impedances may be coupled to the dynodes or, provided that a resistor is coupled between the input and output of OP, to the junctions following resistors R a '. The second load impedances and capacitors are in an adapter which includes a switch bank and plugs into the photo-multiplier socket. The time constant of the second load impedance and its associated capacitance is not less than the rise time of amplifier OP. Diodes may be connected across capacitors in the capacitance chain, and the second load impedance of the dynode connected to OP is earthed. R a  for the most positive dynode, connected to Si may comprise a series of load resistors, in parallel with at least one of which is a diode, increasing in value from that nearest the dynode. OP may comprise two amplifiers with respectively low amplification and large bandwidth and vice versa. A first fast amplifier may have one input connected to the dynode selector switch and the other input to the output of a second amplifier via a voltage divider having a ratio equal to the feedback ratio from the second amplifier output to the selected dynode. There may be a current limiting resistor and means to maintain the voltage across it in the lead to K. Claims include: details of the bridging arrangements of the switch contacts; current limiting and regulating means in the supply lines and an amplifier overload indicator. The switch has a locking means so that it can only operate when the power source is off.