Nuclear physics

Nuclear physics studies atomic nuclei, their internal structure and the processes that change them. A nucleus contains protons and neutrons, whose short-range nuclear forces bind them while the Coulomb force repels the protons. The resulting nuclear energy levels and nuclear binding energies connect this subject to the low-energy consequences of quantum chromodynamics. Weak interactions additionally permit changes of nuclear composition through beta decay.

The mass defect makes the connection between structure and released energy quantitative. For a nucleus with $Z$ protons, $N$ neutrons and nuclear mass $M_{\mathrm{nuc}}$, its nuclear binding energy isA bound nucleus has less rest energy than its separated nucleons. In a nuclear reaction, the difference between the sums of initial and final rest energies supplies the Q value. The sign of that difference and the Coulomb barrier answer different questions: a reaction can release energy overall while still requiring a way to cross an entrance barrier.

Scattering experiments supplied an early structural clue. Rare large deflections of alpha particles by thin matter could be explained by close encounters with a highly concentrated central charge. Ernest Rutherford's 1911 analysis connected such angular measurements to a compact atomic centre, establishing a route from measured scattering cross-sections to internal structure.^[ref]

An nuclear physicist studies nuclear physics through mathematical theory, observation or experiment. The field determines the relevant physical systems and measurements, while the individual biographies below connect particular contributions with their scientific ideas.

A. W. Sunyar helped measure neutrino helicity with Maurice Goldhaber and L. Grodzins in 1958. Nuclear-recoil selection, magnetic polarization analysis and resonance-scattered gamma-ray counting linked a measurable photon signal to the emitted neutrino's handedness.^[ref]

The experiment inferred the neutrino property through angular-momentum correlations rather than requiring a direct track of the escaping neutrino.

L. Grodzins, with Maurice Goldhaber and A. W. Sunyar, investigated neutrino helicity through correlated nuclear recoil and gamma-ray polarization. Their 1958 result established negative helicity for the neutrinos selected by the experiment.^[ref]

The method combined several physical filters: a polarization-sensitive photon measurement became informative about an unobserved neutrino only through the selected nuclear process and conservation of angular momentum.

An experimental nuclear physicist who, with Hans Geiger, related the range of emitted alpha particles to a radioactive substance's transformation period. The Geiger-Nuttall law connects the particle's escape energy with the enormous variation in alpha decay lifetimes. It began as an empirical correlation, before quantum tunnelling supplied a mechanism for nuclear escape.^[ref]

Robert R. Wilson proposed using fast protons for radiation therapy by exploiting their depth-dependent energy deposition. A proton loses kinetic energy along its path and has enhanced stopping near the end of its range, producing a Bragg peak. His 1946 paper connected this measured interaction with a way to concentrate irradiation at a chosen depth. The physical idea links particle accelerator design, stopping power and spatial control of deposited energy.^[ref]

Physicist who proposed the use of fast protons for radiation therapy in 1946.^[ref]

Maurice Goldhaber, L. Grodzins and A. W. Sunyar inferred neutrino helicity indirectly from a nuclear-decay sequence. After electron capture, the recoiling excited nucleus emits a gamma ray; selecting resonantly scattered gamma rays preferentially selects a recoil direction. A magnetized absorber analyzes the circular polarization of that selected radiation. Combining the angular-momentum relation with the measured polarization gave evidence for negative neutrino helicity, rather than requiring a direct spin measurement on a detected neutrino.^[ref]

An experimental nuclear physicist who coauthored a 1964 comparison of allowed and unique first-forbidden beta-decay spectrum shapes. A double-lens spectrometer measured the shapes of several isotope spectra, testing whether their energy dependence followed the simplest transition models. The report concerns specific indium, potassium, rubidium, strontium and yttrium isotopes; its conclusions should not be transferred to an arbitrary isotope without checking the transition.^[ref]

A nuclear physicist who authored guidelines for evaluating measured nuclear-structure data. The evaluator's task is to reconcile measurements into an internally consistent set of energy levels, nuclear spins, parities and transition properties, while distinguishing a firm assignment from a tentative one. Such work makes a nuclear-data table an assessed physical model rather than a list of unexamined measurements.^[ref]

Victor Weisskopf developed a statistical treatment of particle emission from highly excited heavy atomic nuclei. His 1937 paper compares the process with evaporation: the excitation energy is shared among many internal quantum states, and the available final states constrain which particles can escape with which energies. This connects nuclear level density with emission probabilities and approximately thermal-looking outgoing spectra. Nuclear evaporation supplies a way to predict the aggregate outcome of a complicated nuclear reaction without following every individual internal trajectory.^[ref]

Gregory Breit and Eugene Wigner explained strong variations in slow-neutron capture through resonant nuclear excitation. In modern notation, an isolated nuclear resonance has a contribution proportional to ${\Gamma }_n{\Gamma }_{\gamma }/[(E-E_r{)}^2+{\Gamma }^2/4]$, apart from spin and kinematic factors. Here ${\Gamma }_n$ and ${\Gamma }_{\gamma }$ describe the entrance and radiative channels, while $\Gamma$ is the total decay width. With slowly varying prefactors, moving $\Gamma /2$ to either side of $E_r$ halves the peak, connecting its observed width with the unstable intermediate state.^[ref]

An experimental nuclear physicist who coauthored measurements of neutrons in quantized vertical states in the Earth's gravitational field. A horizontal mirror below and an absorber above formed a height-selective guide. Transmission was suppressed when the gap was too small to accommodate the lowest state, turning a tiny gravitational energy scale into a measurable transmission threshold.^[ref]

An experimental nuclear physicist whose electron speed-and-energy measurement provides a direct test of relativistic kinematics. Bertozzi's electron speed-energy experiment measures flight time and calorimetric energy independently.

Official profile: physics.mit.edu/faculty/william-bertozzi/

Léon Rosenfeld and Christian Møller related the observed electric quadrupole moment of the deuteron to orientation-dependent nuclear forces. A purely spherical charge distribution gives zero quadrupole moment, because averaging $3z^2-r^2$ over directions cancels. A measured nonzero value therefore constrains the internal quantum state and the angular dependence of the interaction. Their 1939 work used this small electrical observable to test descriptions of the force between a proton and a neutron.^[ref]

George Ewan helped introduce germanium detectors into experimental nuclear physics, improving the energy resolution of gamma ray measurements. His university credits him with the first nuclear experiment using such a detector. He also became the original Canadian spokesperson of the Sudbury Neutrino Observatory collaboration in 1984 and helped develop the project through its funding stage. These contributions connect instrument development with the organization of a large experiment capable of testing solar neutrino physics.^[ref]

A physicist whose research included nuclear physics and germanium gamma ray detection.^[ref]

An experimental nuclear physicist who coauthored the 1953 observation of a carbon-12 excited state associated with the Hoyle state. The team analyzed alpha particles from a nitrogen-14 reaction with deuterons using magnetic momentum selection, inferring the residual carbon-12 excitation from the spectrum. The paper explicitly acknowledges Fred Hoyle for pointing out the level's astrophysical significance.^[ref]

James Rainwater proposed that the motion of individual nucleons can favor a nonspherical atomic nucleus. Deforming the core changes the single-particle levels, so an occupied level can lower its energy enough to offset the collective cost of changing shape. This links the nuclear shell model to large measured electric quadrupole moments, which a fixed spherical picture has difficulty explaining. His proposal supplied a physical reason to let particle motion and the shape of the whole nucleus influence one another.^[ref]

A physicist.^[ref]

Ben Mottelson helped connect independent-particle nuclear motion with collective excitations. In a simple positive-parity, $K=0$ ground state nuclear rotational band of an even-even nucleus, the excitation energies approximately follow $E_J={\hslash }^{2}J(J+1)/(2\mathcal{I})$ for $J=0,2,4,\dots$, where $\mathcal{I}$ is a moment of inertia. Thus $E_4/E_2=10/3$ in this idealization. Comparing such patterns and transition strengths with observation distinguishes organized collective motion from an arbitrary list of separate particle excitations.^[ref]

A physicist.^[ref]

Aage Bohr developed a description connecting individual nucleon motion with collective nuclear rotational bands and changes of nuclear shape. A deformed atomic nucleus has an intrinsic orientation, while its laboratory quantum states have definite total angular momentum. Quantizing the collective orientation produces sequences of excited levels, or nuclear rotational bands. Their energies and electric quadrupole transitions supply complementary tests of the connection between the intrinsic shape and the observed spectrum.^[ref]

A physicist.^[ref]

Charles Louis Critchfield and Hans Bethe calculated the rate at which two protons can form a deuteron, initiating the proton–proton chain. In modern notation the reaction is $p+p\to d+e^{+}+{\nu }_e$: one proton changes into a neutron through the weak interaction. Their 1938 calculation combined penetration of the repulsive potential barrier with the probability of the nuclear transition. This joins two physically different obstacles to stellar nuclear fusion: getting the charged particles close together and converting their nuclear composition.^[ref]

A physicist.^[ref][ref]

Carl Friedrich von Weizsäcker developed a semi-empirical description of nuclear masses and mass defects. The semi-empirical mass formula connects nuclear binding energy with the number and arrangement of protons and neutrons, rather than treating every measured mass as an unrelated datum. In modern explanatory terms, bulk binding scales roughly with the nucleon number $A$, a surface correction with $A^{2/3}$, and the electrostatic cost with $Z^2/A^{1/3}$. These competing scalings explain why increasing nuclear size does not improve the binding per nucleon indefinitely.^[ref]

A physicist.^[ref][ref]

Hans Bethe connected laboratory nuclear reactions with the sustained power of stars. With Charles Critchfield, he calculated the slow two-proton reaction that begins the proton–proton chain, treating both barrier penetration and the nuclear transition.^[ref] His 1939 analysis also developed hydrogen-burning reactions in which carbon and nitrogen nuclei act as intermediate carriers and are regenerated. The net conversion releases energy by turning hydrogen into helium; the intermediate nuclei provide a cycle rather than a fuel supply that is consumed once.^[ref]

A physicist.^[ref][ref]

Eugene Wigner and Gregory Breit connected the capture of slow neutrons with resonant excited states of a target atomic nucleus. A capture probability can change rapidly when the projectile energy crosses one of these states. Their treatment relates the peak to the coupling into the intermediate state and its possible decay channels. It therefore explains why a small interval of incident energies can contain pronounced structure, rather than a smooth response determined by the target's size alone.^[ref]

A physicist.^[ref]

J. Hans D. Jensen developed a nuclear shell model in which strong spin-orbit coupling reorganizes the levels available to protons and neutrons. The mechanism connects the observed nuclear magic numbers with gaps in a one-particle spectrum: filling a level with total angular momentum $j$ accommodates $2j+1$ magnetic substates. His historical lecture describes how measured nuclear spins, binding patterns and other spectroscopic evidence constrained the model, rather than treating the choice of level ordering as purely mathematical.^[ref]

A physicist.^[ref]

Maria Goeppert Mayer helped explain nuclear magic numbers using a nuclear shell model with strong spin-orbit coupling. Coupling a nucleon's orbital angular momentum $\ell$ and spin $1/2$ gives levels with $j=\ell \pm 1/2$ and different energies. A large splitting changes the order and capacity of the shells, producing closures that a simple central potential alone fails to explain. For example, lowering the $j=7/2$ member of an $f$ level supplies eight states, accounting for the step from the closure at $20$ to the one at $28$.^[ref]

A physicist.^[ref]

Rudolf Mössbauer discovered that some gamma ray emission and absorption events in a solid occur without changing the crystal's phonon state. The momentum can then be transferred to the whole lattice, with an effective recoil energy far smaller than for a free nucleus. This permits extremely narrow nuclear emission and absorption lines to overlap, producing the Mössbauer effect. The narrow resonance makes very small energy shifts observable, including shifts due to the local electronic and magnetic environment of a nucleus.^[ref]

A physicist.^[ref][ref]

Robert Hofstadter investigated nuclear charge distributions through energetic electron scattering. His 1956 review compares $400\mathrm{MeV}$ electrons scattered from helium gas with predictions for point charges and several extended charge profiles. Beyond about $70^{\circ }$, the elastic scattering was over a hundred times smaller than the point-charge prediction. The extracted nuclear charge form factor distinguishes candidate spatial profiles, connecting measured angular counts with a model of nuclear size and shape.^[ref]

A physicist.^[ref]

An experimental nuclear physicist who coauthored the Wu experiment testing parity conservation in beta decay. The collaboration combined low-temperature nuclear orientation with directional radiation counting. Cooling the cobalt-60 sample made it possible to compare emitted electrons with a known nuclear orientation rather than average over random spins.^[ref]

An experimental nuclear physicist who coauthored the Wu experiment. Comparing beta decay from oriented cobalt-60 nuclei with the opposite orientation tested whether a mirror-reflected process would have the same probability. The observed directional asymmetry showed failure of parity conservation in the weak interaction.^[ref]

An experimental nuclear physicist who coauthored the 1957 test of parity conservation in cobalt-60 beta decay. Its directional counting measurement depended on orienting the nuclei at very low temperature. As the sample warmed and orientation was lost, the asymmetry disappeared, linking the observed signal to the nuclear orientation.^[ref]

An experimental physicist whose cryogenic expertise contributed to the Wu experiment. The National Bureau of Standards laboratory could align atomic nuclei at extremely low temperature, enabling a directional beta decay test in cobalt-60. The collaboration demonstrated a failure of parity conservation.^[ref]

Chien-Shiung Wu and collaborators tested parity conservation by measuring the beta decay of oriented cobalt-60 nuclei at very low temperature. Their apparatus placed a beta counter inside a demagnetization cryostat and used the accompanying gamma ray anisotropy to monitor nuclear orientation. The emitted electrons favored one direction relative to the nuclear spin. Because parity reverses momentum while leaving angular momentum unchanged, that directional asymmetry distinguishes a process from its mirror image and demonstrated parity violation in the weak interaction.^[ref]

A physicist.^[ref]

A theoretical nuclear physicist who studies nuclear forces using effective field theory. His joint review with Evgeny Epelbaum and Hans-Werner Hammer explains how the symmetries of quantum chromodynamics constrain low-energy interactions between nucleons. The approach organizes increasingly accurate calculations without treating a nucleus directly as a collection of interacting quarks.^[ref]

A theoretical nuclear physicist working on effective field theory descriptions of nuclear forces. His review with Evgeny Epelbaum and Ulf-G. Meißner compares descriptions at different resolution scales. Selecting the relevant low-energy degrees of freedom permits controlled calculations of few-nucleon systems while retaining constraints from quantum chromodynamics.^[ref]

A theoretical nuclear physicist who develops descriptions of nuclear forces through effective field theory. His joint review relates the allowed low-energy interactions of nucleons to symmetries of quantum chromodynamics. Organizing interactions systematically makes it possible to improve a nuclear calculation in stages and compare few-body predictions with measured nuclear properties.^[ref]

An experimental physicist who coauthored the Bloch nuclear induction experiment measurements of Felix Bloch and Martin Packard. A sample's precessing nuclear magnetization induces a radiofrequency signal in a pickup coil. Detecting that signal connects the collective motion of nuclear magnetic moments to nuclear magnetic resonance, making the same sample's magnetic dynamics repeatedly observable.^[ref]

Ernest Walton and John Cockcroft produced nuclear reactions with a controlled beam of electrically accelerated protons. Their 1932 lithium experiment detected energetic alpha particles from the target. The apparatus made both the incident proton energy and the outgoing reaction products experimentally accessible, replacing reliance on naturally emitted projectiles alone. The result demonstrated how accelerator control could be used to investigate the binding and transformation of atomic nuclei.^[ref]

A physicist.^[ref][ref]

John Cockcroft and Ernest Walton used accelerated protons to produce the disintegration of lithium in 1932. A high-voltage accelerator supplied the projectile beam, while the energetic alpha particles leaving the target provided the reaction signal. The reaction $p+{}^7\mathrm{Li}\to 2\alpha$ releases nuclear binding energy, so the outgoing particles can carry far more kinetic energy than the incident proton. This connected a controlled electrical accelerator with a quantitative nuclear reaction.^[ref]

A physicist.^[ref][ref]

M. Stanley Livingston and Ernest Lawrence developed the cyclotron as a practical source of energetic ions. Their 1932 paper explains how an alternating electric field gives a new acceleration on every gap crossing while the magnetic field supplies the return path. Keeping the half-orbit time synchronized with the field oscillation lets many small energy increments accumulate. The paper also emphasizes beam focusing and magnetic field correction, showing why successful acceleration required control of the trajectories as well as the resonance condition.^[ref]

A physicist.^[ref]

Ernest Lawrence and M. Stanley Livingston developed repeated acceleration of ions in a cyclotron. An oscillating electric field accelerates a particle each time it crosses the gap between two hollow electrodes; a perpendicular magnetic field bends it back toward the gap. In the nonrelativistic approximation, the orbit period $2\pi m/(|q|B)$ is independent of radius, allowing successive crossings to stay synchronized. Their 1932 report obtained proton energies of $1.22\mathrm{MeV}$ with a maximum applied voltage of $4,000\mathrm{V}$.^[ref]

A physicist.^[ref]

Emilio Segrè coauthored experiments showing that hydrogen-rich material can enhance neutron-induced radioactivity. Placing paraffin wax around the irradiation arrangement changed activation rather than merely shielding the target. Collisions with light nuclei slow the neutrons, altering their subsequent nuclear-reaction probabilities. This made the surrounding material part of the experiment and helped establish neutron moderation as a method for controlling induced radioactivity.^[ref]

A physicist.^[ref]

An experimental nuclear physicist who coauthored the study of hydrogenous substances in neutron-induced radioactivity. Placing paraffin wax around a source and target could enhance activation rather than merely absorb the radiation. Collisions with light nuclei slow the neutrons, connecting neutron moderation to the probability of subsequent nuclear reactions.^[ref]

Edoardo Amaldi coauthored the 1934 investigation of hydrogenous materials and neutron-induced radioactivity. The enhanced activation observed with paraffin wax showed that the material between source and target could improve the production of radioactive nuclei. Neutron moderation supplies the physical connection: collisions change the neutron energy distribution and hence the chance of a later nuclear reaction.^[ref]

A physicist.^[ref]

Enrico Fermi developed neutron bombardment as a way to create and investigate radioactive isotopes. Unlike a charged projectile, a neutron does not face the same repulsive potential barrier when approaching a positive atomic nucleus. Induced radioactivity supplies a delayed readout of reactions that occurred during irradiation. His Nobel lecture describes how comparing target materials, projectile conditions and the subsequent decay products connected experimental activation with the structure and transformations of atomic nuclei.^[ref]

A physicist.^[ref]

Hugo Atterling's scientific memoir of Manne Siegbahn explains how apparatus and source composition shaped early X-ray spectroscopy. One example traces two unexpected lines in a 1914 spectrum to a silver layer beneath a platinum coating, observed in first and second diffraction orders. The account connects identification of a chemical element with the interpretation of the instrument, rather than treating each observed line as a new property of the nominal target. His memoir also documents the development of precision and vacuum spectrometers.^[ref]

Physics.^[ref]

Frédéric Joliot and Irène Joliot-Curie demonstrated that irradiation can create material that still exhibits radioactivity after the irradiating source is removed. In their 1934 report, aluminium exposed to a polonium source continued to emit Positrons afterward, with an exponentially decreasing count rate. Similar behavior in other light elements showed that irradiation had produced unstable nuclear species. Measuring the subsequent radioactive decay separated this persistent activity from radiation produced only during the incoming-particle collisions.^[ref]

A physicist.^[ref]

An experimental nuclear physicist who, with Walther Bothe, investigated penetrating radiation produced by bombarding beryllium with alpha particles. Initially interpreted as gamma rays, the radiation motivated tests of the recoil it produced in other materials. James Chadwick's subsequent experiments identified the neutron, illustrating why observing penetrating radiation and correctly identifying its carrier were separate experimental steps.^[ref]

An experimental nuclear physicist who studied collisions of neutrons with nitrogen nuclei in 1932. Tracks in an expansion cloud chamber revealed both recoils and disintegration events, allowing charged reaction products to expose an otherwise invisible neutral projectile. The experiment established a concrete route to studying nuclear reactions initiated by neutrons.^[ref]

Irène Joliot-Curie and Frédéric Joliot identified induced radioactivity by following the Positrons emitted from irradiated light elements after the source was removed. Their 1934 paper reports continuing activity in aluminium, boron and magnesium, with different decay times. The falling count rate is a readout of unstable products left in the material, connecting controlled irradiation with the creation and measurement of radioactive isotopes.^[ref]

A physicist.^[ref]

Walther Bothe developed the coincidence counting for connecting signals from separate particle detectors. Requiring pulses within a short time window helps distinguish a common physical event from unrelated counts. For independent rates $r_1$ and $r_2$, accidental coincidences scale with $r_1{r}_2$ and the window duration; narrowing that duration therefore reduces a calculable background. The method turns timing information into evidence about particle paths and correlated processes, rather than relying on the count in one detector alone.^[ref]

A physicist.^[ref]

A physicist.^[ref]

A physicist whose uranium experiments helped establish radioactivity. His 1896 photographic plate tests separated exposure to the material from immediate illumination; the cautious interpretation in his report is part of the story of Becquerel's uranium-radiation experiment. He held the chair of applied physics at the Paris Museum of Natural History.^[ref]

A physicist and chemist whose investigation of radioactivity joined sensitive measurement to chemical separation. She moved from Warsaw to Paris to study, later becoming a professor of physics. The Curie ionization-current measurement shows the experimental craft behind the work: balancing a tiny electric current against a calibrated electric charge. She shared the 1903 Nobel Prize in Physics and received the 1911 prize in chemistry.^[ref]