Nuclear physics

Nuclear physics

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Area of ​​Expertise - physics

The discovery of radioactivity by Becquerel in 1896 opened up a new subject for physics: the investigation of the properties of atomic nuclei.

The aim of nuclear physics experiments and their theoretical interpretation is to explain the core properties and their temporal changes with the help of quantum physics. This is made more difficult by the fact that the atomic nucleus itself is made up of elementary particles. Therefore, there are similarities with the methods of elementary particle physics and high-energy physics.

Information from the interior of the nucleus is obtained by analyzing the natural radiation during radioactive decay or after the initiation of nuclear reactions.

With the application of nuclear fission for energy generation, reactor physics and nuclear energy with primarily physical-technical issues became independent subject areas.

Nuclear physics

the Nuclear physics is the branch of physics that deals with the structure and behavior of atomic nuclei.

The high energy physics and elementary particle physics have developed out of the nuclear physics and were therefore counted earlier with it the actual nuclear physics was then sometimes called to differentiate Low energy-Core physics called.

The technologies based on nuclear fission for generating energy (see also nuclear energy) and for weapon purposes have developed from certain research results in nuclear physics. But it is misleading to call this technical-economic-political area "the nuclear physics".

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Nuclear physics

Generally speaking, this is Nuclear physics the branch of physics that studies the structure and interactions of atomic nuclei. It is thus between atomic physics and High energy- or elementary particle physics lies.

The experimental methods of nuclear physics include v. a. mass spectroscopy, with the help of which the Nuclear binding energies can be determined who Nuclear spectroscopy (Spectroscopy of the gamma radiation emitted by atomic nuclei) and the Nuclear magnetic resonance (Generation of excited nuclear spin states). In scattering experiments, so-called projectiles (e.g. electrons, protons or entire nuclei) are “shot” at a nucleus. The nucleus can be excited and emit radiation, which can then be examined spectroscopically. If the projectile is also a core, it can close Nuclear reactions come.

The applications of nuclear physics range from the Food irradiation about medical diagnosis and therapy (Nuclear medicine, Magnetic resonance imaging), materials research, age determination and the exploration of the stars up to solving crimes.

Another area of ​​application that has meanwhile been of great political and commercial importance is nuclear technology, i.e. the generation of energy in nuclear power plants. Today v. a. Issues relating to the treatment and storage of radioactive waste play a role. Even after many decades, the technical use of nuclear fusion is still a pure research area.

The manufacture and control of nuclear weapons is also a (high risk) application of nuclear physics.

Atomic and Nuclear Physics

Atomic and nuclear physics is a branch of physics. It deals with the structure of the atomic shell and the atomic nucleus, the properties and the detection of radioactive radiation as well as the generation of nuclear energy by fission and fusion.

Atomic and nuclear physics is a branch of physics. It deals with the structure of the atomic shell and the atomic nucleus, the properties and the detection of radioactive radiation as well as the generation of nuclear energy by fission and fusion.

Further sub-areas that atomic and nuclear physics are concerned with are:

  • the radioactivity of substances and their effects,
  • the Radiation protection, in particular the protection of people from radioactive radiation,
  • the laws of nuclear decay,
  • Energy balances in nuclear reactions,
  • Elementary particles and their properties,
  • the use of nuclear physics knowledge, e.g. B. the generation of energy in nuclear reactors.

Status: 2010
This text is being edited.

Similar questions

If the Higgs field has an effect on the entire atom, a proton cannot be heavier than an electron. In theory correct right? But why is the proton heavier?

Hello, I wrote a paper in physics class yesterday in which you should (among other things) specify the direction of the electrical current. One electron and one proton was shown. But I always thought that only electrons can flow and be like that electric current. Because protons can only be ionized hydrogen molecules, can't they? How are you supposed to get that flowing?

As far as I know, everything is made up of atoms. These consist of protons and electrons and other small particles. But what is a proton or electron made of? Then what does it consist of? Can you then actually ask the question & quotAnd what does it consist of? & Quot and get an answer to it?

Hello everyone, next week I'm going to take a bio exam and I'm not quite sure about the respiratory chain.

Well, an H atom has 1 proton and 1 electron.
But now 1 NADH + (H +) has 3 protons and 2 electrons - right? And in what distribution?
Because if it has 2 protons and 2 electrons for the H, it would only give off one proton, because in the end NAD + remains, and then it would be this from the H +, which I suspect is then only 1 proton Give up proton too.
So only 2 protons would be released, but then only 20 ATP would be produced, but 30 are definitely due to the 10 NADH.
Then to the electrons. In the production of water you already have 2 protons (= 2 H +), which therefore still need 2 electrons to become 2 H (which would contradict the fact that NADH has 2 P and 2 E). In any case, you then give up the 2 electrons, and with 10 NADH, 10 H2O are created, 2 more then through the electrons of FADH2.

Or are the electrons and protons of NADH + (H +) divided in such a way that NADH has 1 proton and 1 electron (only one electron gives off) and H + has 2 protons and 1 electron?
In order to have an NAD + in the end, 2 protons and 2 electrons could then be given up, then the production of 30 ATP would not go up.
Or is it the case that only the NADH get into the respiratory chain and the H + is not used? This was shown in another graphic.
I'm really puzzling over it: D
It would be great if someone understood me and could help!

For tasks on proton / electron accelerators one always starts from an idealized space anyway, i.e. vacuum and so on. But what about gravitational force on freely moving / resting electrons or protons?

I know that in a neutron star, electrons and protons fuse to form a neutron. My question is why there is also a NEutrino.

A sodium atom has 11 protons, 11 electrons and 1 external electron,

so a sodium ion has 11 protons 10 electrons and no external electron.

The chlorine atom has 17 electrons, 17 protons and 7 external electrons.

Does the chlorine ion then have 18 electrons 17 protons and no external electron?

Do you have to do it with the atom, if you want to calculate the attraction to the nucleus, let's say 3 protons attract 3 electrons !! And what if you say what force acts on a single electron, the electrons divide the force of attraction so the first electron attracts the first proton and the 2 attracts the 2 and the 3 attracts the 3 proton. I have heard that if you ionize an atom then the force on the remaining electrons is greater. Please understandable answers.

How do I find the number of protons, electrons and neutrons in sodium?

Can someone explain it to me?

As an example in the picture there are 20 protons + and 20 electrons - is it always the case that there are always as many electrons as protons?

It always means charges flowing through a cable (so it is electricity). But it also means that only electrons flow, not protons! And then what are elementary charges. I don't understand the information on the Internet !?

Isobar (nuclear physics)

Isobars (from ancient Greek ἴσος isos, German & # 8218 equal & # 8216 and βαρύς barýs, German & # 8218 heavy & # 8216) are nuclides of two different chemical elements, i.e. with different atomic numbers, whose atomic nuclei have the same number of nucleons (same mass number). & # 911 & # 93 That means that they have different numbers of protons and accordingly also different numbers of neutrons. & # 911 & # 93

If the atomic number of two isobars only differs by 1, one of these nuclides is always radioactive (Mattauch's isobar rule). & # 911 & # 93 & # 912 & # 93

Particle and Nuclear Physics

In the particle and nuclear physics groups, scientists are concerned with understanding nature at the subatomic level through theory and experiment: the elementary particles as the building blocks of matter, the fundamental forces that act between them, and atomic nuclei.

With this in mind, particle physicists investigate open questions and possible extensions of the Standard Model, which describes the current state of particle physics knowledge. One such quandary is the origin of the mass of elementary particles.

The neutrino experiment SNO Photo: Sudbury Neutrino Observatory

Furthermore, the researchers deal with the theoretical foundations of physics beyond the Standard Model, such as a new symmetry - the so-called supersymmetry - between particles and forces.

In experiments at the Large Hadron Collider (LHC) at CERN in Geneva, the fundamental components of matter are generated and their properties are studied. A Higgs particle has been discovered in 2012, whose properties have to be measured exactly. Moreover, one hopes, for example, to possibly find the new particles predicted by supersymmetry.

In addition, the physicists participate in the SNO +, COBRA and GERDA experiments and investigate the properties of neutrinos. One goal is to examine whether neutrinos are their own anti-particles.

Nuclear Physics - Basics

In the basic knowledge we get straight to the point. Here you will find the most important results and formulas for your physics class. And so that fun is not neglected, there are the popular LEIFI quizzes and varied exercises with detailed sample solutions. This is how you can check whether you have understood everything.

Structure of atomic nuclei

  • Atomic nuclei consist of electrically positive protons and electrically neutral neutrons. Protons and neutrons are nucleons.
  • A chemical element has a fixed number of protons (Z ), but can have several isotopes with different numbers of neutrons (N ).
  • The notation [ rm <> ^ is used to clearly identify cores_ quad text quad rm <> ^ <14> _ <6>]

Key figures of cores

Nuclide map of stable nuclei

  • Different atomic nuclei are often represented in a (N ) - (Z ) - nuclide map.
  • Different elements are in different lines, isotopes of the same element are in the same line.
  • Small, light nuclei have roughly the same number of protons as neutrons, while large, heavy nuclei have a significantly larger number of neutrons than protons.

Scattering experiment

  • Scattering experiments can be used to investigate the structure and structure of the smallest particles.
  • The object to be examined is bombarded with fast particles that are scattered on the object.
  • Conclusions about the structure of the object are drawn from the spatial distribution of the scattered particles.

Table of contents

Nuclear physics is carried out both theoretically and experimentally. Your most important theoretical aid is quantum mechanics. Experimental tools are e.g. B. particle detectors and radiation detectors, particle accelerators and vacuum technology.

The task of “pure” nuclear physics in the sense of basic research is the clarification and explanation of the corestructure, so the details of the structure of the atomic nucleus.

Many have emerged from the study of radioactivity and reactions with nuclei Applications developed, for example

  • Energy generation from nuclear reactions using nuclear reactors and nuclear fusion reactors,
  • medical diagnostic and therapeutic procedures (such as scintigraphy, brachytherapy), collectively referred to as nuclear medicine,
  • chemical applications in radiochemistry or nuclear chemistry,
  • Procedure for preventive damage detection in pipelines using gamma radiation,
  • Production of material surfaces with special properties by means of ion implantation,
  • Auxiliary methods for other research areas such as radiocarbon dating in archeology or cosmochemistry.

Typical orders of magnitude in the area of ​​atomic nuclei and nuclear processes are

The building blocks of the nuclei are the nucleons: neutrons and protons. The number Z the number of protons in a nucleus is equal to the number of electrons in a neutral atom. Z determines the chemical properties of the atoms and is therefore called the atomic number (or, based on the atomic nucleus, also the atomic number). The mass of the atomic nucleus is given by the number A. of all nucleons and is therefore also called mass number. As you can see is the neutron number N = A.Z. Types of atoms with the same atomic number but different numbers of neutrons are called isotopes of the respective element. The physical properties of the nucleus depend on both the atomic number and the neutron number, the chemical properties (almost) only on the atomic number.

When describing nuclear reactions and scattering processes, the term cross-section is important. The cross-section for a certain process is a measure of the probability that this process will occur in individual cases.

The representation is mainly based on Friedrich Hund: History of physical terms [1] and Jörn Bleck-Neuhaus: Elementary particles. [2]

Radioactivity edit

Natural radioactivity was discovered in 1895 by Antoine Henri Becquerel based on the blackening of a photo plate, but it was not until 1915 that it was properly classified as a nuclear physical phenomenon. Important discoveries, initially mainly by Pierre Curie and Marie Curie, were that there are various radioactive elements, some of which are only created during the process. Becquerel and the Curie couple received the 1903 Nobel Prize in Physics for their experiments on radioactivity, which could be described as the historic beginning of nuclear research. The rays were initially only differentiated by their different penetration capabilities and in 1900 Ernest Rutherford named them alpha, beta and gamma rays. In 1900 he discovered that the radioactive elements transform into other elements, as well as the decay law, which describes the exponential decay of a pure radioactive substance. Since radioactivity cannot be accelerated or decelerated by chemical or physical influences, he correctly concluded that the conversions must be purely random processes. The random character of the whole phenomenon was demonstrated by Egon von Schweidler in 1905 on the basis of the expected static fluctuations and could also be made visible by scintillation (William Crookes 1903) and particle traces in the cloud chamber (Charles Wilson 1911). Such observations have made a significant contribution to turning the hypothesis that atoms exist into a scientific certainty. In 1909, Rutherford further shows that alpha particles are doubly ionized helium atoms. According to the displacement theorems found by Kasimir Fajans and Frederick Soddy in 1912/13, it was clear that the chemical element changes by Δ Z = - 2 < displaystyle Delta Z = -2> with alpha radioactivity and by Δ Z = + 1 , and the same remains for gamma radioactivity (Δ Z = 0 < displaystyle Delta Z = 0>). The high energy of the individual alpha or beta particles was inexplicable, orders of magnitude higher than the energy turnover (per atom) in chemical reactions. In 1914, Rutherford identified gamma rays as extremely short-wave electromagnetic waves through diffraction on crystals.

The long lifespan of alpha emitters was traced back in 1928 by George Gamov to the quantum mechanical tunnel effect, which was primarily able to explain their dependence on the energy of the alpha particles. To explain beta radioactivity, Enrico Fermi postulated his own weak interaction in 1934, in which the emitted electrons together with the (then hypothetical) neutrinos according to the Einstein formula E = m c 2 < displaystyle E = m , c ^ <2>> arise. The formation of gamma radiation, like the emission of light by atoms, was described by the quantum electrodynamics proposed by Paul Dirac in 1928 and further elaborated by Fermi in 1932. The z. In 1935 Carl Friedrich von Weizsäcker attributed the sometimes extremely long lifetimes, which also occur here with “isomeric nuclei”, to the fact that, unlike in the atomic shell, in these cases the gamma quanta concerned have to be generated with a high angular momentum.

Discovery of the atomic nucleus

The key experiment that led to the surprising discovery of the atomic nucleus was carried out by doctoral student Ernest Marsden in the laboratory of Nobel Prize winner Ernest Rutherford on December 20, 1910. During control experiments to produce a sharply delimited beam of α-particles, he had noticed that the particles were pushed through thin metal foils 99.99% of the time you can walk through it with almost no distraction, but in isolated cases you can be distracted by more than 90 °. The strong deflection contradicted the expected result: According to Thomson's model of the atom ("raisin cake model", English plum pudding model), the atom would have consisted of electrons floating in a diffuse, positively charged cloud. It was known that α-particles are ionized atoms of the noble gas helium and could not deviate that far from their orbit either from the positively charged clouds or from numerous collisions with the electrons. The purpose of the experiments was actually to investigate the properties of this cloud more closely. Rutherford interpreted the unexpected result to mean that the atoms of the foil largely consisted of empty space, which allowed the alpha particles to pass unhindered, while small, electrically charged and very massive particles existed in it, which the alpha particles very strongly in one of the rare particularly close collisions could throw their course. Brief rough calculations showed Rutherford that these “nuclei” were at least 1000 times smaller than the atom, but had to contain practically all of its mass.

This idea was supported by Henry Moseley, who proved in 1915 for 40 elements that the photons of the characteristic X-ray radiation with the highest energies exactly met the formula that had been established by Niels Bohr in 1913 for the innermost orbits of the electron in the Coulomb field of a point charge, if the correct atomic number has been used for the nuclear charge. These orbits are a corresponding multiple smaller than the atomic diameter.

The finite size of the atomic nucleus had also been demonstrated by Rutherford in 1919 by the fact that the deflection of alpha particles, which had come closer than a few fm to the center of the Coulomb potential, no longer followed the frequency distribution calculated for point charges. This phenomenon is called "anomalous Rutherford scattering" and was used until the 1950s to determine the core radii more precisely.

Components and size of the atomic nucleus edit

In 1919, Rutherford found evidence on a photograph taken with the cloud chamber that an energy-rich alpha particle had knocked out a hydrogen nucleus from a nitrogen nucleus. He saw in the hydrogen nucleus a universal building block of all nuclei and gave it the name "Proton". Since it had already been established by mass spectrometry around this time that the atoms of all elements were almost whole-numbered atomic weights A. Rutherford assumed, the nuclei with mass numbers A. and atomic number Z are from a number A. Protons and (A − Z) Composed of electrons. This proton-electron model was accepted as valid for a long time until the neutron was discovered by James Chadwick in 1932.

Nuclear spin, magnetic dipole moment, electric quadrupole moment edit

The hyperfine structure, a splitting of the spectral lines on the order of 1:10 −5 (in the optical range) was discovered in 1924 and interpreted by the existence of a nuclear spin, which causes a magnetic moment in the nucleus. These extremely low additional energies result depending on the setting angle to the angular momentum or the magnetic moment of the atomic shell. From the number of lines created by the splitting and their displacement in an additionally applied magnetic field (Zeeman effect), the spin of a heavy nucleus could be determined for the first time in 1927 (83 209 B i, I = 9/2 < displaystyle <> _ <83> ^ <209> mathrm , I = 9/2>).

The spin 1/2 of the proton was detected in 1927 on the basis of an apparently very remote phenomenon, an anomaly in the temperature dependence of the specific heat of hydrogen at temperatures below 100 K. The explanation is based on the fact that the H2-Molecule exists in one of two stable allotropic forms at these temperatures, in which the spins of the two protons couple to 0 or 1. The energy difference between the two forms is extremely small (approx. 10 −12 eV), but the quantum mechanical states of the molecules have opposite symmetry and therefore show different rotational spectra. These are expressed in the specific heat at low temperatures.

The magnetic moment of the proton was suggested by Otto Stern et al. demonstrated by working on an extremely narrowly focused H.2-Molecular beam could observe a slight softening in an inhomogeneous magnetic field. The experiment is analogous to the splitting of an atomic beam with atoms with an unpaired electron (Stern-Gerlach experiment from 1923). The deflection of the molecules by the force on the moments of the two protons was approx. 700 times smaller than that caused by the magnetic moment of the two electrons, and was only visible at all because the two electrons in the H2-Molecule align their magnetic moments exactly antiparallel. The determined protonG-Factor of at least G = 5 (instead of the electron G = 2 or even as traditionally expected G = 1) showed that proton and electron are fundamentally different elementary particles. In 1937 Isidor Rabi expanded the apparatus in such a way that the magnetic energy splitting could be verified by means of a resonance method. With this he increased the accuracy to 4 decimal places and also measured the magnetic moment of other nuclei, among others. that of the deuteron, which is (approximately) the sum of the moments of the proton and neutron. The spin of the neutron had already been determined to be 1/2 by observing the optical hyperfine structure on suitable nuclei.

From 1935 it was discovered in the hyperfine structure of nuclei with nuclear spin ≥ 1 that the level differences did not exactly follow the linear dependence that applies to the interaction of dipoles, but rather had a quadratic contribution. The possible explanation was the electric quadrupole moment due to a permanent deviation from the spherical shape. This was not gradually accepted until the 1940s.

Mass Defect, Binding Energy, Fusion, Fission Edit

Around 1920, the increasingly precise mass determinations of the nuclei in mass spectrometers showed that all nuclei are somewhat lighter than the sum of the masses of their building blocks (then assumed as protons and electrons). Arthur Eddington first suspected a connection with the binding energy, this was the first practical application of Einstein's formula E = m c 2 < displaystyle E = m , c ^ <2>> to measured data. Eddington saw the fusion of hydrogen to helium as the source of the otherwise physically inexplicable energy radiation from the sun. However, most of the protons in the sun do not fuse directly, but rather via the carbon-catalyzed Bethe-Weizsäcker cycle, named after its discoverers in 1938, Hans Bethe and Carl Friedrich von Weizsäcker.

According to this, nuclear fission was energetically possible, but was considered impossible until it was surprisingly discovered by Otto Hahn, Fritz Strassmann and Lise Meitner in 1938/39. Intensive government research began in Germany, Great Britain and the USA with the aim of exploiting the energy release of over 200 MeV per uranium nucleus for a bomb of unprecedented destructive power. Most of the wealth of data and knowledge gained was not made available to general science until the 1950s. In 1942 the first nuclear reactor was put into operation in the USA. The first new chemical element, plutonium (Z = 94), which was also suitable for a bomb, was produced by the ton in reactors (by capturing neutrons on uranium nuclei). The fusion of hydrogen to helium (in the form 1 3 H + 1 2 H → 2 4 H e + 0 1 n < displaystyle <> _ <1> ^ <3> mathrm + <> _ <1> ^ <2> mathrm to <> _ <2> ^ <4> mathrm + <> _ <0> ^ <1> mathrm >) was realized in 1952 in the first H-bomb. In further test explosions with increasingly powerful bombs, further transuranic elements, newly formed by multiple neutron capture, were detected in the remains (around 1960 about to Z = 103). For details, see Nuclear Weapons and Nuclear Weapons Technology.

Edit core models

With regard to the composition of the nuclei, the proton-electron model of 1920 was replaced by the proton-neutron model after the discovery of the neutron. For the unique strength of the forces of attraction, a separate strong interaction was postulated, the possible occurrence of which, together with its short range, was first interpreted by Hideki Yukawa in 1937 through the constant generation, exchange and absorption of a hypothetical particle. This particle was discovered by Cecil Powell in 1947 in cosmic radiation and called a pion.

For the binding energy and thus the mass defect, Carl Friedrich v. Weizsäcker presented the droplet model in 1935, in which the interaction of strong nuclear force and electrostatic repulsion is modeled purely phenomenologically. A further understanding of the structure of the nuclei was only possible in 1949 through the shell model by Maria Goeppert-Mayer and (independently) J. Hans D. Jensen, who, analogous to the shell model of the atomic shell, considered the nucleons as bound particles in a common spherical potential well. In doing so, they ignored the fact that the creation of this common potential could not be justified from the short-range nuclear power. The shell model was extremely successful, among others. when explaining the “magic numbers” that characterized the nuclei with their particularly strong bond, as well as the sequence of nuclear spins and magnetic moments when the nuclei are successively built up from protons and neutrons. However, with its spherically symmetrical potential well, it was unable to interpret either the quadrupole moments or the collective excitations that showed themselves in more and more excitation spectra of nuclei between the magic numbers with the development of gamma spectroscopy with scintillation counters. Based on ideas from James Rainwater, Aage Bohr and Ben Mottelson proposed the collective model with stable ellipsoidal deformation in 1954. At first it seemed difficult to reconcile this with the shell model as a model of independent particle movements. The eventually successful unified model was elaborated in the 1960s.

Nuclear reactions edit

After the elastic collision of helium nuclei on gold nuclei in the Rutherford experiment of 1910, a real reaction between two nuclei was first observed in a cloud chamber photograph in 1919, also by Rutherford. It led to the discovery that the hydrogen nucleus is contained in other nuclei as a building block, which is why it was given its own name: proton (reaction 7 14 N + 2 4 α → 8 17 O + 1 1 p < displaystyle <> _ < 7> ^ <14> mathrm + <> _ <2> ^ <4> alpha to <> _ < 8> ^ <17> mathrm + <> _ <1> ^ <1> mathrm

>). At the same time the first core radii were determined by anomalous Rutherford scattering (see above). In 1930 James Chadwick showed for the first time, through elastic scattering of alpha rays in helium gas, the doubling of the 90 ° deflection, which is predicted by quantum mechanics solely on the basis of the indistinguishability of the impacting particles of the boson type. In 1933 Christian Gerthsen showed, according to the theoretical prediction, the opposite effect of the collision of identical fermions with the scattering of protons in hydrogen. In 1932, Chadwick detected the neutron as a building block of the nucleus by analyzing the radiation of heavy neutral particles after bombarding Be with alpha particles (reaction 4 9 B e + 2 4 α → 6 12 C + 0 1 n < displaystyle <> _ <4> ^ <9> mathrm + <> _ <2> ^ <4> alpha to <> _ < 6> ^ <12> mathrm + <> _ <0> ^ <1> mathrm >). The sharp maxima in the cross-section (resonances) observed during the scattering of neutrons by nuclei at certain energies were explained in 1936 by Niels Bohr's model of the short-term formation of an excited compound nucleus, which can then give off its energy in various forms. Among them is the final capture of the neutron, which leads to a heavier isotope than occurs in nature, and which is transformed into the nucleus of a heavier element in a subsequent beta decay (from 1939, first 93 239 N p, 94 239 P u < displaystyle <> _ < 93> ^ <239> mathrm , <> _ < 94> ^ <239> mathrm >, later in the remains of H-bomb explosions up to about Z = 100). In 1938 it was discovered by Hahn, Straßmann and Meitner that neutron capture can also trigger nuclear fission. From 1946 onwards, new elementary particles were discovered that had been created in nuclear reactions of cosmic radiation and at particle accelerators (pion, lambda particles, etc.). The ability to excite a nucleus through the electric field when a fast second nucleus flies past (Coulomb excitation), was used from 1949 to study collective ideas. At the same time, research into particle transfer in reactions with fast projectiles (e.g. Stripping of a neutron from a passing deuteron), which is caused by the new reaction mechanism of the direct response erklärt wurde und eine Fülle von Daten zur Struktur der Kerne in den so gebildeten Zuständen hervorbrachte. Ab den 1960/70er Jahren wurden zunehmend an Schwerionenbeschleunigern die Reaktionen von zwei schweren Kernen bei hochenergetischen Stößen untersucht. Als neuer Typ zeigte sich dabei die tiefinelastische Reaktion, bei der das Projektil tief in den Targetkern eindringt, die Kernmaterie gleichsam zum „Aufkochen“ bringt und Anzeichen eines Phasenwechsels analog zur Verdampfung eines Flüssigkeit hervorruft. Mit solchen Reaktionen wurden auch die extrem schweren und meist kurzlebigen Kerne oberhalb von etwa Z = 99 hergestellt. Ist das hochenergetische Projektil hingegen ein Proton, findet häufig eine Spallation statt, das ist im ersten Schritt eine Verteilung der Einschussenergie auf alle Nukleonen des getroffenen Kerns, gefolgt von Abregung durch Verdampfung von (vorzugsweise) Neutronen oder Spaltung.

Kernspaltung Bearbeiten

Otto Hahn und sein Assistent Fritz Straßmann entdeckten im Dezember 1938, dass durch Bestrahlung mit Neutronen Urankerne gespalten werden (induzierte Kernspaltung). Später wurde nachgewiesen, dass bei diesem Prozess ein großer Energiebetrag sowie weitere Neutronen freigesetzt werden, sodass eine Spaltungs-Kettenreaktion und damit die Freisetzung technisch nutzbarer Energiemengen in kurzer Zeit, also bei hoher Leistung, möglich ist. Darauf begannen, etwa gleichzeitig mit dem Zweiten Weltkrieg, Forschungsarbeiten zur Nutzung dieser Energie für zivile oder militärische Zwecke. In Deutschland arbeiteten unter anderem Carl Friedrich von Weizsäcker und Werner Heisenberg an der Entwicklung eines Kernreaktors die Möglichkeit einer Kernwaffe wurde gesehen, aber nicht ernsthaft verfolgt, weil die voraussehbare Entwicklungsdauer für den herrschenden Krieg zu lang erschien. In Los Alamos forschten im Manhattan-Projekt unter der Leitung von Robert Oppenheimer die Physiker Enrico Fermi, Hans Bethe, Richard Feynman, Edward Teller, Felix Bloch und andere. Obwohl dieses Projekt von Anfang an der Waffenentwicklung diente, führten seine Erkenntnisse auch zum Bau der ersten zur Energiegewinnung genutzten Kernreaktoren.

Öffentliche Diskussion Bearbeiten

Kaum ein Gebiet der Physik hat durch seine Ambivalenz der friedlichen als auch zerstörerischen Nutzung die öffentliche Diskussion mehr angeheizt: Für Fortschrittskritiker war die Kernphysik die Büchse der Pandora, für Fortschrittsgläubige eine der nützlichsten Entdeckungen des 20. Jahrhunderts. Die Kernspaltungstechnik war der Auslöser einer neuen Wissenschaftsethik (Hans Jonas, Carl Friedrich von Weizsäcker). Die politische Auseinandersetzung um den vernünftigen und verantwortbaren Umgang mit der Kernenergie findet bis heute in der Auseinandersetzung um den Atomausstieg Deutschlands statt.