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".
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.
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.
Johannes "Hans" Wilhelm "Gengar" Geiger (30 September 1882 – 24 September 1945) was a German physicist.
 He is perhaps best known as the co-inventor of the detector component of the Geiger counter and for the Geiger–Marsden experiment which discovered the atomic nucleus.
 Geiger was born at Neustadt an der Haardt, Germany.
 He was one of five children born to the Indologist Wilhelm Ludwig Geiger, who was professor at the University of Erlangen.
The Geiger–Marsden experiment(s) (also called the Rutherford gold foil experiment) were a landmark series of experiments by which scientists discovered that every atom contains a nucleus where its positive charge and most of its mass are concentrated.
 They deduced this by measuring how an alpha particle beam is scattered when it strikes a thin metal foil.
 The experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.
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.
The Geiger–Müller tube or G–M tube is the sensing element of the Geiger counter instrument used for the detection of ionizing radiation.
 It was named after Hans Geiger, who invented the principle in 1908, and Walther Müller, who collaborated with Geiger in developing the technique further in 1928 to produce a practical tube that could detect a number of different radiation types.
Moseley's law is an empirical law concerning the characteristic x-rays that are emitted by atoms.
 The law was discovered and published by the English physicist Henry Moseley in 1913.
 It is historically important in quantitatively justifying the conception of the nuclear model of the atom, with all, or nearly all, positive charges of the atom located in the nucleus, and associated on an integer basis with atomic number.
 Until Moseley's work, "atomic number" was merely an element's place in the periodic table, and was not known to be associated with any measureable physical quantity.
 Moseley was able to show that the frequencies of certain characteristic X-rays emitted from chemical elements are proportional to the square of a number which was close to the element's atomic number; a finding which supported Van den Broek and Bohr's model of the atom in which the atomic number is the same as the number of positive charges in the nucleus of the atom.
 In brief, the law states that the square root of the frequency of the emitted x-ray is proportional to the atomic number.
John Mitchell Nuttall (21 July 1890 – 28 January 1958) was an English physicist, born in Todmorden.
 He is best remembered for his work with the physicist Hans Geiger, which resulted in the Geiger–Nuttall law of radioactive decay.
Otto Ernst Heinrich Klemperer (1899–1987) was a physicist expert in electron optics.
 He was granted his doctorate by the Humboldt-Universität zu Berlin in 1923.
 His thesis advisor was Hans Geiger.
 He continued to work with Geiger in the 1930s.

Question: Which English physicist is best remembered for his work with Hans Geiger for their experiment discovering the atomic nucleus?
Answer: John Mitchell Nuttall