# Tag Info

27

Neutrons (and protons) being spin 1/2 fermions, must fit antisymmetric wavefunctions. This "wavefunction" doesn't always involve waves, though. For nucleons - the generic term for neutron or proton - this wavefunction for the pair is a produce of (1) a spatial part, (2) a spin part, and (3) an isospin part. The isospin part is a clever way to describe ...

19

A neutron is not a proton and an electron lumped together (as your question seems to suggest you think) A hydrogen atom is a bound state of an electron and a proton (bound by the electromagnetic force) whereas a neutron is a bound state of three quarks (bound by the strong force). You might be tempted to think that a neutron is also a bound state of an ...

18

Neutrons have spin 1/2 and therefore obey the pauli exclusion principle, meaning two neutrons cannot occupy the same space at the same time. When two neutrons' wavefunctions overlap, they feel a strong repulsive force. See http://en.wikipedia.org/wiki/Exchange_interaction .

14

Isotones are nuclides having the same number of neutrons. Magic proton or neutron numbers give the nucleus greater stability. Magic 82-isotone nuclides for instance: Isobars are nuclides having the same mass number (i.e. sum of protons plus neutrons). The number of protons in beta-plus (beta-minus) decay decreases (increases) by a unit and the number of ...

14

Spontaneous processes such as neutron decay require that the final state is lower in energy than the initial state. In (stable) nuclei, this is not the case, because the energy you gain from the neutron decay is lower than the energy it costs you to have an additional proton in the core. For neutron decay in the nuclei to be energetically favorable, the ...

11

Conservation of energy and the electron-degenerate pressure. For the neutron to decay you must have $$n \to p + e^- + \bar{\nu}$$ or $$n + \nu \to p + e^- \quad.$$ In either case that electron is going to stay around, but in addition to the neutrons being in a degenerate gas, the few remaining electrons are also degenerate, which means that adding a ...

10

The anti-particle corresponding to a neutron is an anti neutron! The neutron is made up of one up quark and two down quarks. The anti-neutron is made up of an anti-up quark and two anti-down quarks. Both have zero charge because the charges of the quarks within them balance out. You are correct that elementary particles with no charge are often their own ...

10

Masses and coupling between quarks are free parameters in the standard model, so there is not real explanation to that fact. About the measurment: you can have a look at this wikipedia article about Penning traps which are devices used for precision measurements for nucleus. Through the cyclotron frequency (Larmor factor) we can obtain the mass of the ...

8

The neutron is made of two down quarks and an up quark; the proton of two up quarks and a down quark. This leads to two effects that differentiate their masses. One is that the up and down quark themselves have different masses. The other is that the proton is charged, and so quantum corrections involving virtual photons affect its mass. The details are ...

8

Neutron sources You can buy a commercial off-the shelf "neutron generator", or you can use a radioactive source. Neutron generators are accelerator-based fusion reactors1 and have the advantage of being able to simply turn the neutron supply on and off. The most common source is AmBe (Americium-241/Beryllium), though Californium-252 and tritium both have ...

7

OK, here is something concrete and quantitative, "Guidelines for predicting single-event upsets in neutron environments": Neutrons in the atmosphere result from cosmic-ray spallation interactions with nitrogen and oxygen nuclei. A typical reaction is a 1 GeV proton fragmenting a nitrogen necleus into lighter charged particles and simultaneoously emitting ...

7

Thermal neutrons capture on hydrogen and carbon with reasonable (i.e. not large, but significant) cross-sections (this is the delayed event detection methods of most organic liquid scintillator anti-neutrino detectors--i.e the one that don't dope their scintillator with Gadolinium). So though a "cloud"--meaning a localized diffuse gas--of neutrons can ...

6

Two nuclides (a type of a nucleus with some values of $Z$ and $N$) that have the same number of neutrons $N$ have the same number of neutrons. The previous sentence is a tautology but I wrote it to show that the equality doesn't mean anything special, aside from the things that are obviously implied by it. Two nuclides that have the same total number of ...

6

A neutron is a fermion, a hydrogen atom is a boson. This is related to the fact that a neutron decays into three fermions rather than two which is what you seem to think. A neutron is composed of three valence quarks, $u,d,d$, while a hydrogen atom is made out of $u,u,d,e^-$. The internal size of a neutron is about $10^{-14}$ meters while the internal size ...

6

In spite of the name, neutron stars also contain protons and electrons. This is required for equilibrium with respect to weak-interaction processes which can convert neutrons into protons and electrons. Since neutron stars also contain protons and electrons, they can contain electric currents which generate magnetic fields. It is thought that the protons ...

6

The neutron has no net charge, but it does have a net magnetic moment. As an aside, this simple fact provides strong evidence that the neutron is a composite particle (made of smaller things like quarks and gluons), because if it were a neutral elementary particle we would not expect it to have any magnetic moment. But we know that the neutron is composite, ...

6

A proton is made of two up quarks and a down quark, whereas a neutron is made of two down quarks and an up quark. The quark masses contribute very little of the actual mass of the proton and neutron, which mostly arises from energy associated with the strong interactions among the quarks. Still, they do contribute a small fraction, and the down quark is ...

6

The magnetic moment generates a magnetic field on it's own, even if the neutron is not moving. This field will Lorentz transform just like any other Electromagnetic field. More interestingly, you'll find that a moving neutron will generate an electric field in addition to its magnetic field.

5

Yes, gravity is negligible, the neutrons are color-neutral so there are only leftover strong-interaction forces which are tiny due to confinement (QCD-related forces between color-neutral objects decrease more quickly than a power law), and the range of the weak force is even shorter (1,000 times shorter than the size of the proton). At 10 nanometers, a ...

5

In a nucleus whose N/Z ratio is too large, the Pauli exclusion principle forces many of the neutrons to be in states with high energies. This makes the system less stable. For a fixed N, adding protons also makes such highly neutron-rich systems more stable, because the interaction between the protons and the neutrons is attractive, and the protons can go ...

5

A neutron bomb is still a hydrogen bomb, just designed in such a way as to allow much of the neutron radiation to escape, instead of remaining trapped to enhance the chain-reaction. A neutron bomb explosion would be basically the same as a hydrogen bomb, just with a little less explosive energy, and a little more neutron radiation---making it more harmful ...

5

Yes, heavy shielding is needed primarily for gamma radiation. Neutron radiation (with energies seen in fission reactors) is easily stopped with boron-10 (isotopically enriched boric acid in water). While alpha and beta radiation is easier to shield, it is even more dangerous if alpha and beta active particles (dust) is consumed by human, because they will ...

5

Neutron decays into a proton, electron and electron anti-neutrino. Not only electric charge but also (electronic) lepton number has to be conserved (I'm not very sure in this statement). In short: you start and have to finish with 1 matter particle (anti-matter counts as -1). Mean time $\neq$ half-life. More on wikipedia. Physically there is no difference ...

5

The Pauli Exclusion Principle states that no two identical fermions (neutrons and protons are fermions - they have half-integer spins and obey Fermi-Dirac statistics) can occupy the same quantum state at the same time. If the neutron were to $\beta$-decay as: $$n \longrightarrow p + e^- + \bar{\nu_e}$$ then this freshly minted ...

5

Between what I've learned about cosmic rays and what I can find online (example: http://www.fisica.unlp.edu.ar/~veiga/experiments.html), it seems that the primary source of neutrons in cosmic ray showers is the disintegration of the atomic nuclei that are struck by the cosmic ray or its decay products. As you may know, cosmic rays enter the atmosphere with ...

4

The neutron decays into a proton, an electron and an antineutrino. So even the end components are different from Hydrogen which is just a proton with an electron orbiting around it. The binding forces are also different. The proton and the electron are bound by the electromagnetic force. The neutron by the strong to the rest of the nucleons in a nucleus. ...

4

When physicists say that a particle has electric charge, they mean that it is either a source or sink for electric fields, and that such a particle experiences a force when an electric field is applied to them. In a sense, a single pair of charged particles are a battery, if you arrange them correctly and can figure out how to get them to do useful work for ...

3

Neutron generators use a solid target, metal lattice filled with D2 or tritium, metal hydrides. The fusion reactions D + T → n + 4He En = 14.1 MeV D + D → n + 3He En = 2.5 MeV happen within the lattice, most of the neutrons escape since they do not interact electromagnetically and generate the neutron source. There is no way a self sustaining reaction ...

3

Well, look at the other terms...in particular consider the Coulomb term as that one depends only on the proton number. Walecka's book writes it as $$E_3 = \frac{3}{5} \frac{Z (Z-1)}{4\pi R_C} e^2 \approx a_3 \frac{Z^2}{A^{1/3}} \,.$$ This term is strictly positive and grows rapidly as the atomic number increases, while being slightly decreased by growing ...

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