Questions tagged [nuclear-physics]

Nuclear physics is the study of the composition, behavior and interaction of atomic nuclei and their constituent parts.

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Why is the (free) neutron lifetime so long?

A neutron outside the nucleus lives for about 15 minutes and decays mainly through weak decays (beta decay). Many other weakly decaying particles decay with lifetimes between $10^{-10}$ and $10^{-12}$ ...
Forever_a_Newcomer's user avatar
45 votes
3 answers
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What is the origin of elements heavier than iron?

In all the discussions about how the heavy elements in the universe are forged in the guts of stars and especially during a star's death, I usually hear that once the star begins fusing lighter atoms ...
Zubin's user avatar
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Conversion of mass to energy in chemical/nuclear reactions

Is mass converted into energy in exothermic chemical/nuclear reactions? My (A Level) knowledge of chemistry suggests that this isn't the case. In a simple burning reaction, e.g. $$\mathrm{C + O_2 \to ...
one-more-minute's user avatar
15 votes
3 answers
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Why is nuclear force spin dependent?

Why nucleons with parallel spins have greater nuclear force than the ones with anti-parallel spins? I just want a clear and easy explanation.
Rohan's user avatar
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7 votes
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What makes the number of neutrons the number of proton similar?

In basic chemistry, we are taught that an atom has roughly the same number of neutrons and number of protons, this doesn't seems to hold for larger atoms, but it is always roughly proportional (i.e. ...
Andrew Au's user avatar
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10 answers
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Can Jupiter be ignited?

Our solar system itself contains two candidate "Earths" One is Jupiter's moon Europa and another is Saturn's moon Titan. Both of them have the problem of having at low temperature as Sun's heat ...
Xinus's user avatar
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25 votes
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How does rest mass become energy?

I know that there's a difference between relativistic and rest mass. Relativistic mass is "acquired" when an object is moving at speeds comparable to the speed of light. Rest mass is the inherent mass ...
QEntanglement's user avatar
35 votes
4 answers
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What stabilizes neutrons against beta decay in a neutron star?

Free neutrons are known to undergo beta decay with a half-life of slightly above 10 minutes. Binding with other nucleons stabilizes the neutrons in an atomic nucleus, but only if the fraction of ...
Slaviks's user avatar
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Why are alpha particles such a prominent form of radiation and not other types of nucleon arrangement?

It is said in many textbooks that alpha decay involves emitting alpha particles, which are very stable. Indeed, the binding energy (~28.3 MeV) is higher than for $Z$-neighboring stable isotopes. But ...
Yuval's user avatar
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Why are the dineutron and diproton unbound?

It is known that there are no diproton or dineutron nuclei. Does this mean that two protons or neutrons are not actually attracted to each other? Even if the attraction was weak, wouldn't it cause ...
Suzan Cioc's user avatar
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What are good examples to demonstrate Einstein's mass-energy relation [duplicate]

According to Einstein's mass-energy relation mass and energy are interchangeable. Can you provide some examples where: Mass gets converted into energy. Energy gets converted into mass.
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A proton's weak charge is .0719. Is this dimensionless? A ratio?

A recent piece of major news in the physics world is that the proton's weak-force charge was measured to be .0719. Is that a ratio? A dimensionless number, with no units? The articles I read ...
Kurt Hikes's user avatar
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9 votes
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Binding energy and mass

I’ve been told that a greater binding energy means the nucleus is more tightly bound, and therefore that decreases the mass of the nucleus with respect to its nucleons when separated. But why does a ...
lightweaver's user avatar
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28 votes
3 answers
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Why is the nucleus of an Iron atom so stable?

Lighter nuclei liberate energy when undergoing fusion, heavier nuclei when undergoing fission. What is it about the nucleus of an Iron atom that makes it so stable? Alternatively: Iron has the ...
martin clayton's user avatar
16 votes
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Protons' repulsion within a nucleus

Do the protons inside the nucleus repel each other by the electrostatic force? If they do, why doesn't the repulsion drive the protons apart so that the nuclei get disintegrated?
thecodeparadox's user avatar
20 votes
2 answers
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How do we know that some radioactive materials have a half life of millions or even billions of years?

If a radioactive material takes a very long time to decay, how is its half life measured or calculated? Do we have to actually observe the radioactive material for a very long time to extrapolate its ...
davitenio's user avatar
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Why doesn't the deuterium nucleus have spin $0$?

A deuterium nucleus is composed of a proton and a neutron. Both have spin $\tfrac12$ so I would expect the deuterium to have two possible spins: $1$ for the triplet and $0$ for the singlet. But ...
Ennaforox's user avatar
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Adding many more neutrons to a nucleus decreases stability?

If you take any large nucleus and add protons to it, the electrostatic repulsion between them will make the nucleus more unstable, because the electrostatic force between them is more repulsive at a ...
Jonathan.'s user avatar
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If the strong nuclear force is stronger than electrostatic repulsion, why don't nuclei collapse into a point?

Today in class we were discussing the strong nuclear force, and our teacher was explaining about how the strong nuclear force counteracts the repulsion force between protons in a nucleus. When asked ...
Mark Bensen's user avatar
14 votes
1 answer
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What is an intuitive picture of the motion of nucleons?

I understand the "motion" of electrons within an ordinary atom (say argon at room temperature and pressure). They are moving in "orbits" defined by quantum mechanical wavefunctions where the "orbits" ...
Jason Waldrop's user avatar
13 votes
2 answers
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Why is the spectrum of the $\beta$-decay continuous?

the spectrum of the Gamma and Alpha decays are both discrete, i.e. the $\alpha$-particles and the $\gamma$-rays take on only discrete values when emitted from a decaying nucleus. Why is it then, that ...
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11 votes
3 answers
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References on the non-compositeness of the known elementary particles

What paper(s) or theory(s) describe or prove that the elementary particles that we have determined today cannot be made up of smaller more fundamental particles?
44 votes
8 answers
13k views

Why doesn't an electron ever hit (and stick on) a proton? [duplicate]

Imagine there is a proton confined in a box and we put an electron at 10 cm distance: It gets an acceleration of thousands of meters/second^2 along a straight line joining the two CM's. One would ...
user avatar
39 votes
1 answer
2k views

Why are pear-shaped nuclei possible?

In a recent question, Ben Crowell raised an observation which really puzzled me. I obtained a partial answer by looking in the literature, but I would like to know if it's on the right track, and a ...
Emilio Pisanty's user avatar
17 votes
5 answers
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Age of the Earth and the star that preceded the Sun

One of the great unheralded advances made in the history of science was the ability to determine the age of Earth based on the decay of isotopic uranium. Based on the apparent abundance of uranium in ...
Humble's user avatar
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How can the nucleus of an atom be in an excited state?

An example of the nucleus of an atom being in an excited state is the Hoyle State, which was a theory devised by the Astronomer Fred Hoyle to help describe the vast quantities of carbon-12 present in ...
ODP's user avatar
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12 votes
2 answers
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How to understand elemental anomalies in nuclear binding energy? [closed]

What is the nature of nuclear energy? This is closely related to the correct explanation of mass defect. I did some research of that topic and cannot come to a single comprehensive and consistent ...
Paweł Załuski's user avatar
8 votes
5 answers
3k views

How can beta plus decay be possible?

$\beta +$ decay is where a proton gets turned into a neutron and a positron and a neutrino. However, a neutron is heavier than a proton, so obviously this reaction is endothermic. So then, why does ...
John Hon's user avatar
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7 votes
2 answers
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"Dear radioactive ladies and gentlemen" - Letter by Wolfgang Pauli

In 1930, Wolfgang Pauli wrote a letter to Lise Meitner for a convention in Tübingen, considering the problem of beta decay. Does anybody know, where to find the original letter online ?
mate64's user avatar
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1 answer
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Construction of interaction Lagrangian invariant under isospin $SU(2)$ transformations

The problem considered here is, as the title says, to construct a Lagrangian that is invariant with respect to $SU(2)$ transformations. I will present first the context and the reason of my ...
Constantine Black's user avatar
106 votes
8 answers
21k views

Why does the Sun's (or other stars') nuclear reaction not use up all its "fuel" immediately?

The temperature and pressure everywhere inside the Sun reach the critical point to start nuclear reactions - there is no reason for it to take such a long time to complete the reaction process. Just ...
user.3898215's user avatar
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17 votes
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Is the "protophobic fifth force" observed in nuclear decays a new fundamental force?

There is a recent paper in PRL, also on the ArXiv that claims the discovery of a fifth force, mediated by a 17 MeV boson, which explains an anomaly in nuclear decays. Is this a real effect? Could the ...
Rodriguez's user avatar
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12 votes
1 answer
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How do you go about guessing the ground-state spin and parity of a nucleus?

How do you go about guessing the ground-state spin and parity of a nucleus? Questions of this form seem to be asked frequently here, e.g., for 19F, 23Na, and 87Rb and 40K.
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12 votes
2 answers
923 views

Is this a correct demonstration for why elements above untriseptium cannot exist?

With the confirmation that elements 113, 115, 117, and 118 are indeed fundamental elements that are now to be named on the periodic table, the next question is: what is the highest atomic number ...
nordic_skier's user avatar
10 votes
3 answers
4k views

Mass defect- From where mass is being lost?

As a school student, I have wondered while studying mass defect the following mysterious problem My assumption Just like a car's mass is constituted by each part of it(i.e total mass of car will be ...
user avatar
5 votes
1 answer
24k views

What prevents an atom's electrons from "collapsing" onto its protons? [duplicate]

Forgive me if the answer to this is obvious. I have no formal physics training, and I remember that when I asked my physics teacher this, she just frowned and said "Good question." An electron is ...
asteri's user avatar
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42 votes
7 answers
5k views

Do nuclei emit photons?

Generally in text books they say that when a electron goes from high energy state to a lower energy state it emits photons. My question is, it is possible that a proton that goes from high energy ...
amilton moreira's user avatar
28 votes
3 answers
6k views

Are the protons and neutrons in the nucleus arranged in any particular way?

I was wondering this: suppose you have two oxygen atoms. They will both have 8 protons and 8 neutrons in the nucleus (at least if they are the most common isotope). Now, will all those particles be ...
Javier's user avatar
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28 votes
5 answers
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How do we know that the nucleus isn't a quark-gluon plasma?

The standard picture of the nucleus of atom is that is several distinct nucleons, which themselves are composed of quarks. However, it seems to me like a much simpler picture is that the nucleus is ...
Itai Bar-Natan's user avatar
20 votes
3 answers
9k views

What exactly does the weak force do?

I know that the weak force acts on nuclei and causes decay. But what exactly does the weak force do? Or to put it another way, why do we call it a force? Does it push the red particle of the picture ...
PNS's user avatar
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16 votes
2 answers
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What Happens to electrons after Alpha Decay and Nuclear Fission?

Where do the electrons go? In alpha decay do 2 electrons follow the alpha particle and make stable Helium or does the larger daughter nucleus become an anion? Also what do the electrons do in the ...
Holocron 314's user avatar
15 votes
2 answers
4k views

With what probability does nuclear fusion occur at energies far below the Coulomb barrier?

Even at the core of the sun, the temperature of $\sim 10^7$ K only results in $kT\sim1$ keV, which is about a thousand times less than the electrical potential energy of $\sim1$ MeV needed in order to ...
user avatar
9 votes
1 answer
477 views

Why do excited states in $^4$He not decay by photon emission?

Here's a level scheme for the $^4$He nucleus (source; click image to see full size): Notice that all of the confirmed decay modes are by disintegration — emission of a neutron, proton, or deuteron. ...
rob's user avatar
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7 votes
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Stable Nuclei - Deviation from equal protons and neutrons

While studying the semi-empirical mass formula for nuclei, I came across an "asymmetry term" whose function, as far as I understand, is to build in the fact that nuclei "prefer" to have equal numbers ...
Comp_Warrior's user avatar
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5 votes
7 answers
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Why can't electrons fall into the nucleus? [duplicate]

I read a book on pop sci book on quantum mechanics and the author said that electrons do not fall into the nucleus due to quantum mechanics- which principles suggest this (I think it was Heisenberg's ...
user58953's user avatar
  • 447
37 votes
6 answers
6k views

How many times has the matter in our Solar System been recycled from previous stars?

I've got a basic understanding of these facts: The Universe is a little over 13 billion years old. Our Galaxy is almost that old. Our Solar System is roughly 4.6 billion years old. The heavier ...
Clinton Pierce's user avatar
34 votes
4 answers
9k views

Is the speed of sound almost as high as the speed of light in neutron stars?

Have you ever wondered about the elastic properties of neutron stars? Such stars, being immensely dense, in which neutrons are bound together by the strong nuclear force on top of the strong gravity ...
JKL's user avatar
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30 votes
3 answers
24k views

How can a proton be converted to a neutron via positron emission and yet gain mass?

The mass of a neutron is greater than mass of a proton so how is it possible in positron emission for a proton to form a neutron and a positron?
user118899's user avatar
26 votes
2 answers
8k views

What happens to Protons and Electrons when a Neutron star forms?

What happens to Protons and Electrons when a Neutron star forms? At some point gravity overcomes the Pauli Exclusion Principle ( I assume) and they are all forced together. What happens in the process?...
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24 votes
3 answers
2k views

Long time deviations from exponential decay in radioactivity

Are there any examples of common substances whose decay is not exponential? We're used to thinking about radioactivity in terms of half-lives. This is a concept that makes sense only for a decay that ...
Carl Brannen's user avatar
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