Physics · nuclei · NEET
| Nature | Alpha: helium nucleus (2p + 2n) | Gamma: high-energy photon (EM wave) |
| Charge | Alpha: +2e | Gamma: 0 (beta is -1e) |
| Mass | Alpha: about 4 u (heavy) | Gamma: 0 (massless); beta about 1/1836 u |
| Ionising power | Alpha: highest | Gamma: lowest (order a > b > g) |
| Penetrating power | Alpha: lowest, stopped by paper | Gamma: highest, needs thick lead (order g > b > a) |
| Speed | Alpha: about 0.05c to 0.07c | Gamma: c = 3 x 10^8 m/s |
It depends on the situation. Alpha has the highest ionising power, so if an alpha source gets inside the body (eaten or breathed in) it does the most damage to nearby cells. But from outside the body, alpha is stopped by the dead skin layer, so it is fairly harmless. Gamma passes deep through the body from outside, so as an external source gamma is more dangerous. For NEET, remember: ionising power alpha > beta > gamma, penetrating power gamma > beta > alpha.
Gamma is a photon with no charge and no mass. Because it has no charge, it rarely interacts with atoms, so it can travel far through matter (high penetration) but knocks out very few electrons (low ionisation). Alpha is heavy and doubly charged, so it strongly pulls on electrons in every atom it passes and quickly loses energy. That gives alpha huge ionising power but a very short range.
No. Gamma rays carry no charge, so a magnetic or electric field exerts no force on them and they go straight through undeflected. Alpha (positive) and beta (negative) both bend, but in opposite directions. Alpha bends less than beta because alpha is about 7000 times heavier, so it is harder to deflect.
A beta-minus particle is just a fast-moving electron. Its charge is -1 (in units of e) and its mass is about 1/1836 of a proton, which is roughly 0.00055 u. It is created inside the nucleus when a neutron changes into a proton, so it is NOT one of the original electrons orbiting the atom. Beta-plus (a positron) has the same tiny mass but charge +1.
Alpha is stopped by a sheet of paper or a few centimetres of air. Beta passes through paper but is stopped by a few millimetres of aluminium. Gamma passes through both and needs several centimetres of lead or thick concrete to reduce it. This ladder (paper, aluminium, lead) is a very common NEET one-liner.
Ionising power depends on charge, mass and speed. Alpha has charge +2 (double), is very heavy, and moves relatively slowly, so it spends more time near each atom and pulls electrons out strongly. Beta has charge -1 and is light and fast, so it ionises less. Gamma has no charge at all, so it ionises the least.
Gamma radiation has the greatest penetrating power. It is an uncharged photon, so it passes through paper and aluminium and needs thick lead or concrete to be absorbed. The order of penetration is gamma > beta > alpha.
Alpha (positive) and beta (negative) bend in opposite directions, and gamma (neutral) goes straight. Beta bends much more than alpha because a beta particle is far lighter. This three-way split in a field is a classic exam figure.
Alpha moves at roughly 5 to 7 percent of the speed of light. Beta can reach close to the speed of light because it is very light. Gamma is an electromagnetic wave, so it always travels at the speed of light, c = 3 x 10^8 m/s.
Alpha is a helium-4 nucleus (2 protons + 2 neutrons). Beta-minus is an electron and beta-plus is a positron, both created in the nucleus during decay. Gamma is a high-energy photon released when a nucleus drops from an excited state to a lower energy state.
Alpha decay lowers atomic number Z by 2 and mass number A by 4. Beta-minus raises Z by 1 (A unchanged); beta-plus lowers Z by 1. Gamma emission changes neither Z nor A, because only energy leaves the nucleus, not particles.
