Radiation measurement system with mosfet electrometer circuit which is protected against voltage transients

Claim 2

In a radiation measurement system wherein a radiation detector provides a low-level d.c. input signal, an electrometer circuit for amplifying said signal comprising a metal-oxidesemiconductor element having a control terminal and load terminals, input circuit means coupled to the said control terminal and responsive to said input signal and to the undesired voltage transients which may appear therewith, means for coupling said load terminals to a source of power, and output circuit means coupled to said load terminals to supply an amplified output signal corresponding to the low-level input signal, said input circuit means comprising limiting means and filtering means, said limiting means limiting the magnitude of voltages applied to said control terminal to less than a predetermined magnitude, said limiting means having a response time, and said filtering means attenuating high frequency components of voltage applied thereto to prevent fluctuations in voltage applied to said input circuit means from producing a voltage on said control terminal exceeding a predetermined level within said response time of said limiting means.

Claim 3

The electrometer circuit of claim 1 wherein said limiting means comprises a diode having an abrupt voltage threshold of conduction at said predetermined magnitude and negligible leakage current in its non-conductive state, said diode being coupled from the control terminal of the metal-oxide-semiconductor element to a reference potential approximately equal to the normal potential on said control terminal.

Claim 4

The electrometer circuit of claim 1 wherein said limiting means comprises another diode of the same general type as the first mentioned diode connected in parallel with said first mentioned diode and being oppositely poled with respect thereto.

Claim 5

The electrometer circuit of claim 1 wherein the time constant of said filtering means is greater than the response time of said limiting means and less than that which would cause instability of the circuit.

Claim 6

The electrometer circuit of claim 1 wherein said filtering means comprises a resistor serially coupled to said control terminal in the signal circuit path and a capacitor having one of its ends connected between the resistor and said control terminal and the other of its ends connected to a reference potential.

Claim 7

The electrometer circuit of claim 5 wherein said limiting means comprises a pair of oppositely poled, parallel connected cold cathode diodes, having the anode and cathode of each diode connected between said control terminal and said reference potential.

Claim 8

A radiation measurement system for measuring a property of a material, comprising a radiation source providing radiation directed toward the material to be measured; a radiation detector for detecting radiation from said material and including means for providing low-level d.c. signal indicative of the property to be measured; an electrometer circuit coupled to the radiation detector for amplifying said low-level d.c. signal, said electrometer circuit comprising a metal-oxide-semiconductor element having a control terminal and load terminals, input circuit means coupled to said control terminal and responsive to said low-level signal and to undesired voltage transients which may appear therewith, means for coupling said load terminals to a source of power, and output circuit means coupled to said load Terminals to supply an amplified output voltage corresponding to the low-level signal, said input circuit means comprising limiting means and filtering means, said limiting means limiting the magnitude of voltages applied to said control terminal to less than a predetermined magnitude, said limiting means having a response time, and said filtering means attenuating high frequency components of voltage applied thereto to prevent fluctuations in voltage applied to said input circuit means from producing a voltage on said control terminal exceeding a predetermined level within said response time of said limiting means, whereby said metal-oxide-semiconductor element is protected from said undesired voltage transients without impairing the operation of said electrometer circuit to provide an output signal indicative of the measured property.

Claim 9

A radiation measurement system for measuring a property of a material, comprising a radiation source providing radiation directed toward the material to be measured; a radiation detector for detecting radiation from said material and including means for providing low-level d.c. signal indicative of the property to be measured; an electrometer circuit coupled to the radiation detector for amplifying said low-level d.c. signal, said electrometer circuit comprising a metal-oxide-semiconductor element having a control terminal and load terminals, input circuit means coupled to said control terminal and responsive to said low-level signal and to undesired voltage transients which may appear therewith, means for coupling said load terminals to a source of power, and output circuit means coupled to said load Terminals to supply an amplified output voltage corresponding to the low-level signal, said input circuit means comprising limiting means and filtering means, said limiting means limiting the magnitude of voltages applied to said control terminal to less than a predetermined magnitude, said limiting means having a response time, and said filtering means attenuating high frequency components of voltage applied thereto to prevent fluctuations in voltage applied to said input circuit means from producing a voltage on said control terminal exceeding a predetermined level within said response time of said limiting means, whereby said metal-oxide-semiconductor element is protected from said undesired voltage transients without impairing the operation of said electrometer circuit to provide an output signal indicative of the measured property.

Claim 10

The radiation measurement system of claim 7 wherein a source of potential greater than said predetermined magnitude is applied to said radiation detector, said system further comprising means for selectively connecting and disconnecting the output of said radiation detector, respectively, to and from said input circuit means, and means for connecting the output of said radiation detector to a reference potential substantially less than said predetermined magnitude whenever said radiation detector output is disconnected from said input circuit means.

Claim 11

A radiation measurement system including an electrometer circuit for converting a minute current from a radiation detector at an input terminal of said circuit to an appreciable voltage at an output terminal, said circuit comprising a differential amplifier including a pair of MOSFETs, each having a control terminal and load terminals a resistor having a resistance value in the order of megohms coupling said input and output terminals, input circuit means coupled to the control terminal of one of said MOSFETs and responsive to voltages at said electrometer input terminal for preventing voltages in excess of a predetermined value from being applied to said control terminal of said one MOSFET, means for applying a selected potential to the control terminal of the other of said MOSFETs, means coupled to a load terminal of each of said MOSFETs for providing at said electrometer output terminal an output voltage signal proportional to the difference between the voltage applied to the control terminal of said one MOSFET and said selected potential, wherein said input circuit means comprises diode means for limiting the magnitude of voltages applied to the control terminal of said one MOSFET to less than a predetermined magnitude, said limiting means having a response time, and filtering means attenuating high frequency components of voltage applied to said input terminal to prevent fluctuations in voltage applied to said input circuit means from producing a voltage on said control terminal of said one MOSFET exceeding a predetermined level within said response time of said limiting means.

Claim 12

A radiation measurement system including an electrometer circuit for converting a minute current from a radiation detector at an input terminal of said circuit to an appreciable voltage at an output terminal, said circuit comprising a differential amplifier including a pair of MOSFETs, each having a control terminal and load terminals a resistor having a resistance value in the order of megohms coupling said input and output terminals, input circuit means coupled to the control terminal of one of said MOSFETs and responsive to voltages at said electrometer input terminal for preventing voltages in excess of a predetermined value from being applied to said control terminal of said one MOSFET, means for applying a selected potential to the control terminal of the other of said MOSFETs, means coupled to a load terminal of each of said MOSFETs for providing at said electrometer output terminal an output voltage signal proportional to the difference between the voltage applied to the control terminal of said one MOSFET and said selected potential, wherein said input circuit means comprises diode means for limiting the magnitude of voltages applied to the control terminal of said one MOSFET to less than a predetermined magnitude, said limiting means having a response time, and filtering means attenuating high frequency components of voltage applied to said input terminal to prevent fluctuations in voltage applied to said input circuit means from producing a voltage on said control terminal of said one MOSFET exceeding a predetermined level within said response time of said limiting means.

Claim 13

The system of claim 9 wherein said input circuit means comprises means for limiting the magnitude of voltages applied to the control terminal of said one MOSFET to less than a predetermined magnitude, said limiting means comprising a diode having an abrupt voltage threshold of conduction at said predetermined magnitude and negligible leakage current in its nonconductive state, said diode being connected between the control terminal of said one MOSFET and a reference potential approximately equal to the normal potential on said one MOSFET control terminal.

Claim 14

The system of claim 10 wherein said limiting means further comprises another diode like said first mentioned diode connected in parallel therewith and with opposite polarity thereto.

Claim 15

The system of claim 11 wherein both of said diodes are of the cold cathode type.

Claim 16

The system of claim 9 having a common circuit potential and comprising means for zero-standardizing said radiation measurement system, said zero-standardizing means including means for selectively establishing zero input signal to said input terminal and means for adjusting said selected potential to provide a given system output signal corresponding to said zero input signal, said means for selectively establishing zero input signal comprising means for disconnecting the output of said radiation detector from said input terminal and means for shunting said radiation detector output to the common potential of said circuit.

Claim 17

The system of claim 9 comprising means for limiting the magnitude of voltages which may be applied to the electrometer output terminal to values below a given voltage, whereby transients appearing at the output terminal are prevented from being fed back to said MOSFETs.