What are you trying to model? Just the behavior of the circuit with
constant illumination, or are you trying to model the time-domain behavior
for sudden illumination changes (as when you suddenly block the view of a
hot object with a cold one)?
For the circuit behavior part, 'just' model this using the source
impedance of the FET in parallel with the published capacitance -- too bad
they don't tell you what that is! Alternately, measure the FET source
impedance (it ought to be at least fairly constant), then select a FET
from SPICE that shows the same impedance (or diddle with the area scaling
parameter in a SPICE Model until things work right). This should tell you
all you need to know about stability and responsiveness.
For the time-domain behavior of the pyroelectric stack, you'll probably
have to measure and model. It's thermal, which means that it's complex
and weird, but you should be able to come arbitrarily close with enough RC
low-pass stages. You'll probably have to measure this and characterize
it, and you should remember that the thermal path from the device case to
ambient will matter, as will any hot-object illumination of the heat sink,
as pyroelectric devices measure the temperature difference not the
absolute temperature (but you knew that -- right?).
(They really should make these with a built-in sensor to measure the base
temperature).
