Final answer:
The RBE for low-energy β particles is higher because they have a shorter range within tissue and therefore cause more concentrated damage, whereas higher-energy β particles have a longer range that distributes ionizations over a larger volume, rendering them less biologically damaging with a lower RBE.
Step-by-step explanation:
When considering the Relative Biological Effectiveness (RBE) for different energies of beta (β) particles, the key factor is how the range of radiation in biological tissue depends on the energy of the radiation. Higher-energy β particles have a longer range in tissues because they have more energy to penetrate deeper before being stopped. In contrast, low-energy βs have shorter ranges in tissues which means they deposit their energy in a smaller volume, causing more concentrated damage, thus their RBE is higher.
Because higher-energy β particles spread their ionizations over a greater range, the damage they cause is less concentrated and more diffusely distributed, allowing for better repair by biological systems. This leads to a lower RBE, essentially suggesting that dose for dose, high-energy β-particles are less biologically damaging compared to their low-energy counterparts. Therefore, the correct answer is (b) Low-energy βs have shorter ranges in tissues.