109

Free neutrons in flight are not deflected by electric fields. Objects which are not deflected by electric fields are electrically neutral. The energy of the strong proton-neutron interaction varies with distance in a different way than the energy in an electrical interaction. In an interaction between two electrical charges, the potential energy varies ...


105

How long is long? So "half life only of about 12 min" is actually really a strange idea to most of your readers. 12 minutes is a very long time, atomically speaking! Like, the charged pions have a half-life of 18 nanoseconds, the uncharged one is 58 nano-nanoseconds (attoseconds). You might say "well those are mesons, not baryons like the proton and neutron,...


72

What you are looking for is isotopes with neutron–proton ratio N/Z less than 1. You can find these isotopes, for example, in this list from Wikipedia. As you can see, you are looking for members of the table with N less than Z. In these table you are looking for isotopes that are roughly above the gray zone (also known as band or belt of stability). The ...


59

The alpha particle is a quantum mechanical system, and it is not clear what we might mean by drawing pictures of billiard balls arranged according to classical polyhedra.In particular, the alpha has quantum numbers $J^\pi=0^+$, so it has complete spherical symmetry. In a shell model picture, which provides a simple guide to the exact 4-body wave function, ...


42

According to Wikipedia: Other than protium (ordinary hydrogen), helium-3 is the only stable isotope of any element with more protons than neutrons.


40

Basically, the answer is no, it's not possible. When we produce neutrons for research purposes, we have to produce them using nuclear reactions. They come out of the nuclear reactions with energies that are determined by the reaction, are not otherwise under our control, and that are on the MeV energy scale of nuclear physics. Examples of a neutron source ...


37

A neutron is not "a proton and an electron". A neutron is not composed of a proton and an electron inside of the neutron. In quantum mechanics, particles can appear and disappear or change into other particles. With the neutron, one of the down quarks can decay change into an up quark by emitting a W boson, turning into a proton. The W boson quickly decays ...


37

You're asking about two distinct phenomena. The difference between them is subtle, and I think there is some context missing from the second question that you quote, which makes things more confusing than they need to be. When the neutron star forms, most of the protons and electrons combine together to form neutrons This is mostly correct. The process ...


32

If you shoot an electron or a proton at a nucleus at moderate energies (a few hundred $\mathrm{MeV}$ to a few $\mathrm{GeV}$) it will usually either bounce off the whole nucleus or break up the nucleus. But every once in a while (and this gets rarer and rarer the harder you throw it in) it will actually bounce off of a single nucleon. At the right energies ...


28

NB: I feel like this is a pretty half-assed job, and I apologize for that but having opened my mouth in the comments I guess I have to write something to back it up. We start with Fermi's golden rule for all transitions. The probability of the transition is $$ P_{i\to f} = \frac{2\pi}{\hbar} \left|M_{i,f}\right|^2 \rho $$ where $\rho$ is the density of ...


28

This does not violate the exclusion principle because the exclusion principle merely states that there cannot be more than one fermion in the same quantum mechanical state. In the case of two protons and two neutrons, the different particle species don't exclude each other to begin with (because a neutron state is different from a proton state). Furthermore,...


27

There are two very relevant facts that inform this answer: (1) The rest mass energy of a neutron is 1.29 MeV higher than that of a proton. $(m_n - m_p)c^2 = 1.29$ MeV. (2) The total number of neutrons plus protons (essentially the only baryons present) is a constant. Neutrons and protons can transform into one another via reactions moderated by the weak ...


25

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 ...


23

It's boron-10 that is the good neutron absorber. Boron-11 has a low cross section for neutron absorption. The size of the nucleus isn't terribly relevant because neutrons are quantum objects and don't have a precise position. The incident neutron will be delocalised and some part of it will almost always overlap the nucleus. What matters is the energy of ...


22

Although a neutron is electrically neutral, it has a non-zero magnetic dipole moment. It interacts with a magnetic field to give a potential $$ U = \vec{\mu} \cdot \vec{B} $$ A gradient of magnetic field strength will give a force $$ \vec{F} = \nabla|\vec{\mu} \cdot \vec{B} | $$ It's not possible to produce large, sustained field gradients, nor is it ...


22

Suppose that the strong nuclear force were instead caused by Coulomb interactions. Since we know how strong the binding energies are (of the order of $\sim 1\ \text{MeV}$, as can be gleaned by say, looking at a table of alpha particle energies) and how far apart the nucleons are (about a proton radius, or $a_p\sim1\ \text{fm}$) we know how charged the ...


20

A neutron isn't a proton and an electron. The reaction involved in beta decay is $$n \to p + e^- + \bar{\nu}_e$$ where $\bar{\nu}_e$ is an electron anti-neutrino. But even that doesn't mean a neutron is a proton plus an electron plus a anti-neutrino. It means that a neutron's quantum numbers are the same as a state consisting of a proton an electron and ...


20

This is the illustration of the Chadwick experiment (from Wikipedia): The key point is that the scattering is close to the forward direction. So if we draw the diagram for the Compton scattering as: The angle $\theta$ is small. This diagram shows the hypothetical incoming gamma ray hitting a proton in the paraffin and scattering it. The equation for the ...


20

The neutron is magnetic. It is a tiny little magnet. In more formal language, it carries a magnetic dipole moment of size $$ \mu_n = −9.6623647(23) \times 10^{−27} {\rm J\,T}^{−1}. $$ This is what allows it to interact with electromagnetic waves---or, to say the same thing another way, with photons. This also means that when accelerated then yes, it will ...


19

The neutron is in no way "composed" of a proton and an electron. It can decay to a proton, electron, and an antineutrino. But that doesn't mean that these three particles literally co-exist inside the neutron at the beginning. Instead, the decay involves some real transmutation of elementary particles. The only thing that one can say because of the decay is ...


18

It is a misnomer (at best) to characterize a neutron star as all neutrons. There are protons and electrons too. Imagine compressing a bunch of regular matter at some point it requires less energy for a proton and electron to combine to form another neutron rather than for the electron to try to fill a very high energy state. That means there are so many ...


18

It seems that within the standard model of particle physics A permanent electric dipole moment of a fundamental particle violates both parity (P) and time reversal symmetry (T). These violations can be understood by examining the neutron's magnetic dipole moment and hypothetical electric dipole moment. Under time reversal, the magnetic dipole moment ...


18

It's not exactly a myth that protons and electrons combine to form neutrons, but it's not very accurate. A proton and electron can react to produce a neutron, but a neutron isn't simply a composite particle consisting of a proton joined to an electron. Protons and neutrons are hadrons, which means they consist of quarks. A proton has 2 up quarks & 1 ...


18

Rob's answer is the simplest and probably best, but let me add another approach. We know that nuclei are made out of protons and neutrons. Protons repulse each other, but somehow, if you get them close enough, they stick together extremely strongly. This already suggests that there is another force in play! So even if you completely ignored neutrons, you ...


17

Protons and neutrons are referred to collectively as nucleons. Nucleons interact via the strong nuclear force, and this interaction can't be expressed by any simple equation. The reason is that nucleons are not fundamental particles. They're actually clusters of quarks. Short range The low-energy structure of nuclei is amazingly insensitive to the details ...


17

The main difference is gonna be the stability of the various isotopes. Most elements technically have a very large number of isotopes (carbon isotopes range from carbon 8 to carbon 22), but most of these have a very short half-life due to the poor stability of a number of neutrons too large (or too small). The list of isotopes will usually be either somewhat ...


14

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 ...


14

Pauli's exclusion principle states that two fermions can't occupy the exact same quantum state simultaneously. Two fermions can have spatial wavefunctions that overlap with nonzero values at common locations. That is fine - the point is that the entire spatial wavefunctions (along with spin states) can't be the same for both particles.


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