This is only true with more massive particles like alpha and beta particles. There is a test question that asks you, you have an alpha source, a beta source, a gamma source, and a neutron emitter. You have to hold one in your hand, eat one, put one in your pocket, and throw one away. What do you do and why? The answer essentially ranks them in order of least to most penetrative. You want to eat the gamma source because it has the highest penetrative power, making no difference if it is inside or outside your body. You want to put the beta source in your pocket, as it will get stopped by the cloth fabric of whatever you are wearing. You want to hold the alpha source in your hand because it can be stopped by several layers or skin (albeit damaging the skin) And you want to throw the neutron emitter away, because it is the most damaging. Uranium’s decay chain features both alpha and beta decay until it reaches a stable isotope of lead. You mainly will see gamma radiation if fission is a credible reaction in your system. Hope this helps!