help needed ,charge sensitive amplifier design

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hello, i am designing radiation measurement readout front-end electronics, as part of my project ,in very 1st step to understand working of CSPA(charge amplifier) i had done little experiment by connecting photodiode with opa657 .here i have used general purpose ir photodiode and Rf=147k ohm, Cf= 1pf. in feedback with opa657. i am giving 5v pulse train having 50ns pulse width and 2Mhz repetition rate to IR LED which is placed very near to IR photodiode which simulated as radiation source and photodiode will work as detector. now problem is as per theory output of cspa have long decaying pulse having time constant equals to Rf.Cf , but in my experiment cspa output is same like input pulse having 70ns width and 350mV peak amplitude .can you please tell where i am making mistake? i don't understand whats wrong with this circuit.