ultra ultra low current amplifier of insulation
First you said "sense". Do you require to detect if current is present, detect if current passes some threshold or measure it? If measurement or comparison is required, what kind of resolution you hope to achieve?
It is obvious if you require 10pA range and if we asume 1pA resolution, that this is going to be very difficult to achieve. If you have noisy enviroment, you have very litle chance doing this succesfully. If you can't get enviroment that is low noise for this measurement, your whole aparatus will have to be enclosed in heavy gauge steel enclosure with RF shielding even maybe using special magnetic shielding materials if noise is at low frequencies. This you will have to find out experimentally.
Measurement aparatus will have to be very well isolated from surroundings electrically to avoid creation of current loops. Power supply should be either batteries or low noise, medical grade isolation power supply.
Source of signal and complete trace to first IC that processes signal has to be protect by driven shield and on top of that should be electromagnetic shield. Signal should be transported as balanced to reduce possibility of interference.
Whole aparatus will have to be protected from any vibrations that might produce triboelectric or microphonic effect in cables or piezoelectric effect in components. Setup should have minimum thermal differences in critical areas to reduce Tc effects.
Measurement could be done in number of ways, most common being integrator. Small capacitor is used as "bucket" to collect drips of current over time. You can measure time it takes to fill the capacitor to predeteremined level or measure level in the capacitor after predetermined time. You discharge capacitor at the end or charge it in the oposite way.
To grasp what you are measuring, 6x10^6 electrons every second is not that much. If you had 1V voltage across insulation that leaks less then 1pA, that insulation has to be over 1TΩ.
There are integrator chip available from Texas Instruments for such purpose.
ACF2101 and
IVC102 . They include integrating capacitors and discharge switches.
There are a lot of techniques involved in building electrometers in pA range, so first search on internet for electrometers to see ideas how it's done. Printed circuit is generally not good medium when you are in pA range. Air is very good isolator so a lot of high impedance ciruits bend pins of IC's into air and make connection o ceramic or other suitable materials posts of sufficient legth.
There are few articles arround...
Bob Pease: **broken link removed**
Keithley:
**broken link removed** (Hard Copy) and
**broken link removed**