Question: What was the nationality was the person who established the essential nature of the atomic nucleus?

Context paragraphs:
Electric field gradient: In atomic, molecular, and solid-state physics, the electric field gradient (EFG) measures the rate of change of the electric field at an atomic nucleus generated by the electronic charge distribution and the other nuclei.  The EFG couples with the nuclear electric quadrupole moment of quadrupolar nuclei (those with spin quantum number greater than one-half) to generate an effect which can be measured using several spectroscopic methods, such as nuclear magnetic resonance (NMR), microwave spectroscopy, electron paramagnetic resonance (EPR, ESR), nuclear quadrupole resonance (NQR), Mössbauer spectroscopy or perturbed angular correlation (PAC).  The EFG is non-zero only if the charges surrounding the nucleus violate cubic symmetry and therefore generate an inhomogeneous electric field at the position of the nucleus.

Mass excess: The mass excess of a nuclide is the difference between its actual mass and its mass number in atomic mass units.  It is one of the predominant methods for tabulating nuclear mass.  The mass of an atomic nucleus is well approximated (less than 0.1% difference for most nuclides) by its mass number, which indicates that most of the mass of a nucleus arises from mass of its constituent protons and neutrons. Thus, the mass excess is an expression of the nuclear binding energy, relative to the binding energy per nucleon of carbon-12 (which defines the atomic mass unit).  If the mass excess is negative, the nucleus has more binding energy than C, and vice versa.  If a nucleus has a large excess of mass compared to a nearby nuclear species, it can radioactively decay, releasing energy.

Halo nucleus: In nuclear physics, an atomic nucleus is called a halo nucleus or is said to have a nuclear halo when it has a core nucleus surrounded by a "halo" of orbiting protons or neutrons, which makes the radius of the nucleus appreciably larger than that predicted by the liquid drop model.  Halo nuclei form at the extreme edges of the table of nuclides — the neutron drip line and proton drip line — and have short half-lives, measured in milliseconds.  These nuclei are studied shortly after their formation in an ion beam.

Cluster decay: Cluster decay, also named heavy particle radioactivity or heavy ion radioactivity, is a type of nuclear decay in which an atomic nucleus emits a small "cluster" of neutrons and protons, more than in an alpha particle, but less than a typical binary fission fragment.  Ternary fission into three fragments also produces products in the cluster size.  The loss of protons from the parent nucleus changes it to the nucleus of a different element, the daughter, with a mass number A = A - A and atomic number Z = Z - Z where A = N + Z.

James Chadwick: Sir James Chadwick, {'1': ", '2': ", '3': ", '4': "} (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932.  In 1941, he wrote the final draft of the MAUD Report, which inspired the U.S. government to begin serious atomic bomb research efforts.  He was the head of the British team that worked on the Manhattan Project during the Second World War.  He was knighted in England in 1945 for his achievements in physics.

Discovery of the neutron: The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century.  Early in the century, Ernest Rutherford developed a crude model for the atom, based on the gold foil experiment of Hans Geiger and Ernest Marsden.  In this model, atoms had their mass and positive charge concentrated in a very small nucleus.  Throughout the 1920s, the nucleus was viewed as composed of combinations of protons and electrons, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions.  The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932 and the determination that it was a new elementary particle, distinct from the proton.  Both the proton and neutron were presumed to be elementary particles until the 1960s when they were determined to be composite particles built from quarks.

Mass number: The mass number (symbol "A"), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.  It determines the atomic mass of atoms. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion.  The mass number is different for each different isotope of a chemical element.  This is not the same as the atomic number ("Z") which denotes the number of protons in a nucleus, and thus uniquely identifies an element.  Hence, the difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: "N=A−Z".

Borromean nucleus: A Borromean nucleus is an atomic nucleus that has a nuclear halo containing two neutrons.  Such a nucleus breaks into three components and never two when disrupted, in analogy to Borromean rings.  One example is the nucleus of carbon-22 C.

Alpha decay: Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into an atom with a mass number that is reduced by four and an atomic number that is reduced by two.  An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons.  It has a charge of +2e and a mass of 4u.  For example, uranium-238 decays to form thorium-234.  Alpha particles have a charge +2, but as a nuclear equation describes a nuclear reaction without considering the electrons – a convention that does not imply that the nuclei necessarily occur in neutral atoms – the charge is not usually shown.

Atomic nucleus: The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.  After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg.  Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the electron cloud.  Protons and neutrons are bound together to form a nucleus by the nuclear force.

Supporting paragraphs: Discovery of the neutron James Chadwick
Answer: English