I know that a particle (for example proton) should need a minimum kinetic energy(due to the coulombic repulsion) to react with a nucleus. But why we do not write the kinetic energy of the particle bombarded while calculating the energy released or Q-value. Maybe the kinetic energy would be used in overcoming barrier height. But what if kinetic energy is more than the barrier height then why do we not include the excess kinetic energy in calculations. I am confused that what happens exactly when a particle which is to be bombarded reaches the barrier height, does the fusion immediately takes place or something else? Also why we do not include Barrier height energy in calculations? Which energy to be included and which not? For context, I had doubt in the below question. Actually there is no theory given about calculation of energy during bombardment of particle in my book. Only questions are given about the calculation. I also searched on internet for such theory but I am not getting that. I am a high school student so I am confused. The questions are given for practice to score in exam. My doubt in the above image is that the kinetic energy of photons and nuclei matters in calculations but then why not the barrier height as it is also the initial kinetic energy given to the particle. I just want to know more about what happens after particle enters the barrier height. How does the reaction proceed(does the particle bombarded directly hit the nucleus and embed into it or something else)and also please help me about energy calculations in such questions or cases.