Sorry , i can't

What is the current range?

By far the best way to do this is with a NI daq system. Use a calibrated resistor, and an ADC, you can build the integrator trivially in labview.

Ranjin, good question. Before knowing the range any suggestion is pure guess.

Sorry I should have mentioned it in question itself , The current range I need is , from 10 pA - 1 uA .

I have been happy with the EG&G Ortec 439 model, which was very reliable for decades.

I think performing the integration using a device not designed for it, such as a calibrated resistor and an ADC, etc., can cause errors to be compounded when your current is not constant, since you will be sampling and adding the samples. Of course, random errors will cancel, but can you predict your beam current is really random with respect to time? Using a device such as the Ortec 439 is a better solution, because the integration is performed using analog circuitry. If the circuit is designed and calibrated well, then the error will be small, but the design is critical. But the resistor and ADC will work.

(I actually have a European patent on a similar circuit to integrate charge into and out of an auto battery.)

How much error can you tolerate?

EG&G Ortec 439 Digital Current Integrator is good one. However, You can design it according to your current beam (Ref. J. Phys. D: Appl. Phys. 40 (2007) 769–777, doi:10.1088/0022-3727/40/3/013)

Or if you like building circuit, you could use FET-input operational amplifier (OPA129 has 100fA bias current) connected as integrating circuit with high-grade polyester capacitor (w. external discharge) or simply current-voltage converter (normal metal-film resistors will be O.K., these values are up to 100Mohm).

Below 1 microA, it is advisable not to go for voltage measurement across resistor etc. you can use I to V converter op-amp circuit . but u will have to calibrate them correctly.

Keithley has very good pico ammeters which can measure from pA to 20 mA. KE 6487, KE6485.They have good noise filters etc. but the meters get damaged very easily since input has no protection which is done to keep the sensitivity in tact.

Specifically,if you want to buy integrator, i think danfysik can provide.

Your institute has many danfysik integrators ..http://www.ias.ac.in/jarch/bms/18/00000211.pdf

http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CHAQFjAI&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4915369%2F5112637%2F05112755.pdf%3Farnumber%3D5112755&ei=cM61T_CmKMLprAfCw8TWBw&usg=AFQjCNFpHAa0j5xhc7xuznMQXN-CW8l71Q

In VECC Mr Partha Bhaskar can make nano ammeters. You can contact Mr Partha Bhaskar in PMD lab. I have got one from them.

Thank you everyone for their valuable comments and advice.

@ Ranjini: Thank you very much for your advice. I am aware that we have many danfysik integrator in sinp, and in fact I was using danfysik integrator (model 554) which has gone bad so I need another one. I was looking for any alternative to danfysik's model and found ortec-439 model can be a replacement but it needs a counter. Thank you for suggesting name of Mr. Partha Bhaskar I will surely contact him.

Mr Partha bhaskar's meter has got integrator too.,. contact him soon .. his VECC telephone extension may be 2419.

for such low current resistor and ADC may not be a good solution as the input impedance of ADC will matter. In this case a I to V converter is the best option. in our company at Kolkata we make instruments which measures Million MegaOhm hence current of the order of several pico Ampere. The same instrument may be modified to a nano Ampere meter. If you don't get a suitable one readily available in the market we may try to do something for your project.

Maybe Grimm technologies may help you with an out-of-shelve product

http://www.dustmonitor.com/General/products.htm

See " plume dynamics and radiation emission from laser plasmas"

If you can afford to buy, then I'd recommend Keithley low current measurement devices. Some of them are amazingly good (we use them in the sub nA range).

If you need to build circuits, then watch out for designs of transimpedance amplifiers on the web.

If you want to build an integrating device and hence measure charge, there is a nice chip from texas instruments for this purpose:

http://www.ti.com/product/ivc102

or the Burr Brown variant

http://www.farnell.com/datasheets/1359857.pdf

Be sure to go through documentation and forum discussions. And yes, you will need an ADC circuitry to obtain your readings...

Usually, you can ask TI for a sample or two.

Dear Sir,

I fully agree with Kai Fauth (hello Kai !!!). For integrating the ion current during Ar ion etching (i.e. a similar application) we use a Keithley current-to-voltage amplifier and a homemade voltage-to-frequenzy converter. The latter was build by the people from our electronics workshop within 1-2 days.

The principle is described in the Appendix of

F. Müller et al., Langmuir 26 (2010) 18750, DOI 10.1021/la102325e

Regards, FM

A Keithley electrometer or similar device is an expensive, but reliable and tested solution, with high accuracy at very low currents.

If you can afford it, I agree that something like Keithley is best. It is not easy to build a transimpedance amplifier with sufficiently low leakage for pA or smaller measurements. Also, we have had issues with leakage from the Faraday cup, getting a factor of 2 different readings with two different collectors, switching either one on the same amplifier.

ORTEC 439 Digital charge Integrator good to detect nano coulomb.

very simple, yuu have to choose the very low resistance (carbon film resistors) and make circuit with the possible lowest resistance to give the potential and otherside of your coaxile cable with the oscilloscope with the impednce of 50 ohm current integrator to match with the impedence of coaxial cable. you can see the electron beam current signal but it will be noisy... so better you use some stopper in front of FC with suitable diameter of this stopper in the center..... if you feel more high potential problem , so try to use the Mylar sheet in front of stopper with the same diameter of hole of stopper to maintain the vacuum of FC side..... i think, it is very simple and easy way...

one thing more, if you choose high resistance then current signal will be much low intensity.... so better try to choose the low resistance value resistors and make their parallel combination to make more small to save the oscilloscope.

try it and i hope, you will get better result... as i am doing the same to see the electron beam current via FC.