It is because of the hydrogen, but it’s not really to do with the mass, but rather because a “collision”/interaction is much more likely. For instance, 58Ni has the same likelihood to interact as hydrogen, while deuterium has only a fifth. 135Xe meanwhile has some millions of times the absorption power of hydrogen and it’s quite a lot heavier. It’s just not very practical as an absorber.
It is because of the hydrogen, but it’s not really to do with the mass, but rather because a “collision”/interaction is much more likely. For instance, 58Ni has the same likelihood to interact as hydrogen, while deuterium has only a fifth. 135Xe meanwhile has some millions of times the absorption power of hydrogen and it’s quite a lot heavier. It’s just not very practical as an absorber.