What speed of response do you need? Worth checking the response time of any SCR, they aren't necessarily
brilliant, and phototransistor is probably ruled out, photodiode inherently much faster especially
if reverse biased.
A cheap fast comparator like a LM339 sounds a much more flexible component for this kind of circuit, in fact
its exactly the kind of thing comparators are meant to do.
An SCR could be driven by the comparator output (don't forget they are open-collector outputs),
or an SR flipflop could be the latching component.