αChapter 3 of 9

Alpha (α) decay: the nucleus throws out a chunk

A heavy nucleus spits out a bundle of 2 protons + 2 neutrons. No quark changes type. It's a tug-of-war between the strong force and electric repulsion, and quantum tunneling decides when it happens.

23892U→23490Th+42He+4.27 MeV

Top number = mass number A (protons + neutrons): 238 = 234 + 4. Bottom = protons Z: 92 = 90 + 2. Uranium becomes thorium because the proton count decides the element.

Step through an alpha decay

Step 1A uranium-238 nucleus: 92 protons and 146 neutrons, all constantly moving around. Nothing outside is touching it.

Step 2Inside, 2 protons and 2 neutrons briefly clump into a tight bundle: an alpha particle. That bundle (a helium-4 nucleus) is unusually tightly bound, so forming it frees up energy. Throwing out a single proton or neutron would cost energy instead, which is why the nucleus ejects a 4-pack.

Step 3The alpha rattles around and hits the edge roughly 10²¹ times per second. Past the edge is a wall of electric repulsion it doesn't have enough energy to climb (see the energy diagram below). Each hit has about a 1 in 10³⁸ chance of getting through anyway.

Step 4Eventually (after about 4.5 billion years on average for U-238) the alpha turns up outside the wall. It didn't climb over. It tunneled through, which is possible because quantum particles are spread-out waves rather than hard little balls.

Step 5Out here the strong force can't reach it, and its 2 protons are repelled by the 90 protons left behind. That push launches it at about 14,000 km/s (≈5% of light speed) with 4.2 MeV of energy. The thorium nucleus recoils the other way, the way a gun kicks back when it fires.

Step 6The alpha is a bare helium nucleus with charge +2. Moving through matter, it rips electrons off the atoms it passes (ionization), which is what makes radiation harmful, and it slows down fast: it stops within a few cm of air or a sheet of paper. Then it grabs 2 electrons and becomes an ordinary helium atom. Almost all the helium on Earth was made this way, underground.

Why only heavy nuclei do this

The strong force is about 100× stronger than electric repulsion, but it only reaches the nucleons touching each one. Electric repulsion reaches everywhere. Making a nucleus bigger doesn't add glue per nucleon (each one still has about a dozen neighbors), but the repulsion keeps piling up: in uranium every proton is pushed by 91 others. Past lead (82 protons), the repulsion wins often enough that pushing out a chunk becomes worthwhile.

The wall and the tunnel: an energy diagram

This graph shows how much energy the alpha would need to be at each distance from the nucleus's center. Inside, the strong force makes a deep pit. Right at the edge the strong force cuts off and electric repulsion takes over, making a tall hill that slopes down the farther out you go.

The alpha only has 4.27 MeV, but the hill is about 28 MeV tall. In everyday physics a ball can't roll over a hill taller than its energy allows, ever. In quantum physics the alpha is a wave, and a wave hitting a wall doesn't stop dead. It fades away inside the wall. If the wall is thin enough, a tiny bit of the wave reaches the other side, and that's the probability of finding the alpha out there. Once it's outside, electric repulsion pushes it down the hill, and that's where its speed comes from.

This explains the enormous spread in half-lives. Polonium-212 emits 8.95 MeV alphas, so its wall is thinner, and it decays in 0.3 microseconds. Uranium-238's 4.27 MeV alphas face a thicker wall: 4.5 billion years. Twice the energy gives roughly 10²³ times faster decay.

At the quark level: nothing changes

Count the quarks before and after. Every up and down quark is still there, just regrouped. Alpha decay is the only decay here where no particle changes identity and the weak force isn't involved. The strong and electric forces do all the work.

	protons	neutrons	● up quarks	● down quarks
Before: U-238	92	146	330	384
After: Th-234	90	144	324	378
After: α	2	2	6	6
After total	92 ✓	146 ✓	330 ✓	384 ✓

(Each proton has 2 up + 1 down; each neutron has 1 up + 2 down.)