Neutron

The neutron is the uncharged particle that sits in the Atomic nucleus alongside the Proton. It is a hair heavier than a proton and carries no Electric charge at all. Neutrons are what make Isotopes: same element, different neutron count, different mass and stability.

Inside a nucleus a neutron can last forever. On its own it does not — a free neutron survives about 15 minutes before falling apart.

Properties

property	value	note
electric charge	0	measured as (−0.2 ± 0.8) × 10⁻²¹ e — zero to within a part in 10²¹
rest mass	1.674 927 500 56 × 10⁻²⁷ kg	CODATA 2022
rest mass (energy)	939.565 421 94 MeV	CODATA 2022
rest mass (atomic units)	1.008 664 916 0 u	PDG 2024
heavier than the proton by	1.293 332 4 MeV	PDG 2024
mean life (free neutron)	878.4 ± 0.5 s ≈ 14 min 38 s	PDG 2024 average
magnetic moment	−1.913 042 7 μN	PDG 2024
mean square charge radius	−0.1155 ± 0.0017 fm²	PDG 2024
spin	½	a fermion
quark content	u d d	PDG 2024

Two of those rows quietly prove the neutron has parts. A truly structureless neutral particle would have no magnetic moment and no charge radius. The neutron has both, because inside it there are charged Quarks moving around.

Why it decays and the proton doesn't

The neutron is heavier than the proton by 1.293 MeV. That gap is bigger than an Electron's mass of 0.511 MeV, so there is energy to spare for this:

n → p + e⁻ + ν̄ₑ

That is beta-minus decay, and it accounts for essentially 100 % of free neutron decays. Underneath, one down quark turns into an up quark. Run it the other way and you would need to add energy, which is why a lone proton has nothing to decay into.

Bound in a stable nucleus, that decay is blocked: the resulting nucleus would have more energy than the one you started with. So the neutrons in the carbon in your hand are as permanent as the protons.

The neutron lifetime puzzle

There are two ways to measure how long a neutron lives, and for 20 years they have not agreed.

• The bottle method — trap ultracold neutrons in a magnetic or material bottle, wait, count the survivors. It measures how many neutrons are left.
• The beam method — send a neutron beam through a detector and count the protons that appear. It measures how many neutrons died.

measurement	method	mean life (s)
GONZALEZ 21	bottle (UCN magnetic trap)	877.75 ± 0.28
PATTIE 18	bottle (UCN magnetic trap)	877.7 ± 0.7
EZHOV 18	bottle (magneto-gravitational)	878.3 ± 1.6 ± 1.0
SEREBROV 18	bottle (gravitational trap)	881.5 ± 0.7 ± 0.6
YUE 13	beam (in-beam, trapped protons)	887.7 ± 1.2 ± 1.9
PDG 2024 average (bottles only)	—	878.4 ± 0.5

The beam answer is about 9 seconds longer than the bottle answer — roughly 1 % — and the error bars do not overlap. The PDG's own average of the bottle results already carries a scale factor of 1.8, its way of saying the inputs disagree more than they should. Add the beam result and the scale factor rises to 2.2, with an average of 878.6 ± 0.6 s.

Nobody has settled it. Either one class of experiment has an unfound systematic error, or free neutrons sometimes decay into something the beam method cannot see.

What free neutrons do

Having no charge is the neutron's superpower: nothing electrical pushes back, so it walks straight into a nucleus that would swat a proton away.

• Chain reactions. A slow neutron absorbed by uranium-235 splits the nucleus and releases two or three fresh neutrons — each of which can split another. Every fission reactor is that sentence, managed: a moderator (water, graphite) slows the neutrons down, because slow neutrons are far better at being captured.
• Neutron stars. When a massive star's core collapses, gravity crushes electrons and protons together into neutrons. What remains is Neutron star matter: roughly a Sun and a half of mass in a ball the size of a city — the density of an Atomic nucleus, scaled up to kilometres.
• Seeing inside things. A neutron beam passes through centimetres of metal but scatters strongly off light nuclei like hydrogen — the opposite of X-rays. Neutron imaging sees water in a running fuel cell and hydrogen in ice; and because the neutron carries that magnetic moment from the table above, neutron beams also map magnetism inside materials.

Discovery

James Chadwick proved the neutron in 1932. Bothe and Becker had fired alpha particles at beryllium in 1930 and found a strange penetrating radiation; the popular guess was high-energy gamma rays. Chadwick showed instead that it was a neutral particle with about the mass of a proton. He took the 1935 Nobel Prize in Physics for it.

Where to read next

• Proton — the charged nucleon, and the mass it does not get from its quarks
• Quark — the u and d quarks that make both nucleons
• Isotope — what changing the neutron count does
• Atomic nucleus — why nucleons stay together at all
• Neutron star — a nucleus the size of a city