Reaction term

Hi, I'd like to add reaction term just for the certain time in the diffusion model. For example,
0~3 s : no reaction
3~5 s : reaction
5~7 s : no reaction

I split studies into three parts (one reaction-activated study and two reaction-inactivated studies). However, resulted profile only showed the result from the last study (In the case of above example, study for 5~7s non-reaction study was only present).

Is there any way that I can see those three parts continuously??
I mean, if it's possible, I would like to inactive the reaction term for the certain time in one study..
I couldn't figure out how to deal with it..

Bests,

Woongsun Choi