In What Is Radiation? we met ionizing radiation. But "ionizing radiation" is not one thing; it contains very different types. One is stopped by paper, another needs thick lead. This difference is the basis of both protection and of why some types are more dangerous.
Alpha and beta
These are particle radiation — actual material particles.1
- Alpha (α): a relatively heavy, doubly positively charged particle made of two protons and two neutrons (in fact a helium nucleus). It ionizes very strongly but penetrates very little: a sheet of paper or the dead outer layer of skin stops it. So it is not dangerous from outside; but if an alpha-emitting substance is inhaled or swallowed (e.g. radon), it is risky because it damages internal tissue directly.
- Beta (β): a high-speed electron ejected from the nucleus (negatively charged, far lighter than alpha). More penetrating than alpha, but still stopped by a few millimeters of aluminum or plastic. It can penetrate skin but usually not deep tissue.
Gamma and x-ray
These are not particles but photons — electromagnetic radiation, with no charge and no mass.1 The two are physically identical; the only difference is their origin: gamma (γ) comes from the atomic nucleus, while x-rays arise from electron interactions (e.g. in an x-ray tube). Because they are uncharged, they interact little with matter and are highly penetrating: they can pass right through the body — which is exactly what makes medical imaging possible. To attenuate them, dense materials (lead, concrete) are needed.
Neutron
The neutron is an uncharged nuclear particle; it mainly arises in nuclear reactors and some special sources and does not appear in diagnostic radiology. Being uncharged it is highly penetrating, but it interacts with nuclei regardless of charge; it is stopped most effectively by hydrogen-rich materials (water, paraffin) and thick concrete.1
What stops it?
What stops a given type is the most intuitive summary of its penetration:
References
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 2–3: radyasyon türleri (alfa, beta, gama, X-ışını, nötron), yükleri, enerjileri ve madde içindeki menzilleri/zayıflaması.
- İlişkili: Radyasyon Nedir? · Radyasyon Birimleri · Radyasyondan Korunma