Question: Where are free protons emitted from?
About the article "Proton", section "Abstract".

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The A2 Collaboration studies reactions induced by high-energy photons incidenton nucleons or nuclei.
A beam of photons with known energy and flux is produced via bremsstrahlung using a dedicated tagging spectrometer provided by the University of Glasgow.
A polarized electron beam produces circularly polarized photons.
Linearly polarized photons can be obtained from coherent bremsstrahlung in an oriented crystal radiator.
The central part of the detector system is a hermetic calorimeter consistingof the Crystal Ball detector (672 NaJ crystals) in combination with the TAPS detector (384 BaF2 crystals)in forward direction.
For charged-particle tracking and identification two layers of coaxial multi-wire proportional chambers and a barrel of 24 scintillation counters surrounding the target are installed inside the cavity of the Crystal Ball sphere.
A frozen-spin target for polarized protons and deuterons is of particular importance for studying spin degrees of freedom.
The main physics goals are:  Protons and neutrons are excited when they absorb a photon.
If the photon energy is sufficiently high, mesons are emitted.
The probabilities for such meson production reactions as well as their angular and spin dependence incorporate indispensable information about nucleon excited states and meson-nucleon dynamics.
Electric and magnetic polarizabilities are well known concepts in classical physics, which describe the influence of static electric and magnetic fields on composite systems.
In case of protons and neutrons scalar and spin-dependent polarizabilities can be measured in low energy Compton scattering.
At MAMI formula_4 and formula_5 mesons are photoproduced with high rate.
With the Crystal Ball detector, decay modes of these mesons can be studied in an almost background-free environment.
The charge distributions inside nuclei has been measured with high accuracy in electron scattering experiments.
Information about the matter distribution can be obtained from the coherent photoinduced production of pions from nuclei, where photon and pion interact coherently with all protons and neutrons inside a nucleus.
A publication database can be found here. 
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge.
One or more protons are present in the nucleus of each atom.
Protons and neutrons are collectively referred to as "nucleons".
The number of protons in the nucleus of an atom is referred to as its atomic number.
Since each element has a unique number of protons, each element has its own unique atomic number.
The name "proton" was given to the hydrogen nucleus by Ernest Rutherford in 1920, because in previous years he had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by collision, and was thus a candidate to be a fundamental particle and building block of nitrogen, and all other heavier atomic nuclei.
In the modern Standard Model of particle physics, the proton is a hadron, and like the neutron, the other nucleon (particle present in atomic nuclei), is composed of three quarks.
Prior to that model becoming a consensus in the physics community, the proton was considered a fundamental particle.
In the modern view, a proton is composed of three valence quarks: two up quarks and one down quark.
The rest masses of the quarks are thought to contribute only about 1% of the proton's mass.
The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together.
Because the proton is not a fundamental particle, it possesses a physical size—although this is not perfectly well-defined since the "surface" of a proton is somewhat fuzzy, due to being defined by the influence of forces that do not come to an abrupt end.
The proton is about 1.6–1.7 fm in diameter.
The free proton (a proton not bound to nucleons or electrons) is a stable particle that has not been observed to break down spontaneously to other particles.
Free protons are found naturally in a number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity.
Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons.
Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances.
Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay.
Protons also result (along with electrons and antineutrinos) from the radioactive decay of free neutrons, which are unstable.
At sufficiently low temperatures, free protons will bind to electrons.
However, the character of such bound protons does not change, and they remain protons.
A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom.
The result is a protonated atom, which is a chemical compound of hydrogen.
In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, which is chemically a free radical.
Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies.
When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H2), which are the most common molecular component of molecular clouds in interstellar space.
Such molecules of hydrogen on Earth may then serve (among many other uses) as a convenient source of protons for accelerators (as used in proton therapy) and other hadron particle physics experiments that require protons to accelerate, with the most powerful and noted example being the Large Hadron Collider.
In the MPR the fusion neutrons are collimated into a neutron beam.
The neutron beam is directed onto a thin plastic film (Polyethylene) where the neutrons scatter elastically on the protons of the foil.
The recoil protons emitted in the forward direction enter a magnetic part of the spectrometer where they are momentum analyzed and focused onto the focal plane.
An array of plastic scintillators coupled to photomultiplier tubes (PMTs) register the spatial distribution of the protons.
This proton distribution is then related to the neutron energy spectrum. 
A solar proton event (or proton storm) occurs when protons emitted by the Sun become accelerated to very high energies either close to the Sun during a solar flare or in interplanetary space by the shocks associated with coronal mass ejections.
Besides protons, the events can include other nuclei like helium ions and HZE ions, meaning that the event is sometimes a solar particle event.
These high energy protons and ions cause several effects.
They can penetrate the Earth's magnetic field and cause ionization in the ionosphere.
The effect is similar to auroral events, the difference being that electrons and not protons are involved.
Energetic solar protons are also a significant radiation hazard to spacecraft and especially astronauts, who can receive large amounts of absorbed dose from the ionizing radiation. 
Metastable isomers may also decay by internal conversion — a process in which the energy of nuclear de-excitation is not emitted as a gamma ray, but instead used to accelerate one of the inner electrons of the atom, so that it leaves at high speed and energy.
This process is only possible because inner atomic electrons penetrate the nucleus, where they are subject to the intense electric fields which result when the protons of the nucleus re-arrange in a different way.
In nuclei which are far from stability in energy, still other decay modes are known.