Subject: How to Handle the pA DC current sensing

We Recently have got a problem statement of sensing very low current from photo diode of type InGaAs with NEP being 2e-13 but with frequency being zero or continuous illumination, imagine it being light from a point source in far field that i want to sense

To put more detail, We have a point source illuminating radiant intensity around 120W/sr , the diode active dia is 3mm with capacitance of 9nF

We would like to detect the Above source from 10Km so the power at our detector would be very less, this makes the current from the photo diode to be as less as 2pA.

In order to detect this low current i would need at-least a gain of 50X10E+9 , keeping 100mV as my threshold at a frequnecy 0Hz

is say the frequency is 0Hz because the illumination from source would stay for almost 12Sec

As we are already having LT6268-10 with 4G GBWP, can i use the opamp in full Gain mode ? will it cause  any further implications

If above is true i can think of using a 10G GBWP opamp also

If you can suggest one it would be more helpful, i tried of using ADI online tool due to peak current limitations i was not able to make out any conclusions from it

Kindly throw some light in this regard