- 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 [[Isotope]]s: same element, different neutron count, different mass and stability.+ 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 [[Isotope]]s: same element, different neutron count, different [[mass]] and stability.- - **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.+ - **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.- - **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.+ - **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.- 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.+ 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 ray]]s. 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.
History of Neutron
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+ ## 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.++ - [[Neutron star]] — a nucleus the size of a city
- | electric charge | 0 | measured as (−0.2 ± 0.8) × 10⁻²¹ e — zero to 21 decimal places |+ | electric charge | 0 | measured as (−0.2 ± 0.8) × 10⁻²¹ e — zero to within a part in 10²¹ |
+ 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 [[Isotope]]s: 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 21 decimal places |+ | 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 μ<sub>N</sub> | 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 [[Quark]]s 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.++ ## 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