nuclear science

Atomic nucleus


The nucleus of an atom is the very dense region, consisting of nucleons (protons and neutrons), at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons.

The diameter of the nucleus is in the range of 1.6 fm (1.6 × 10−15 m) (for a proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as uranium). These dimensions are much smaller than the size of the atom itself by a factor of about 23,000 (uranium) to about 145,000 (hydrogen).

The branch of physics concerned with studying and understanding the atomic nucleus, including its composition and the forces which bind it together, is called nuclear physics.

Contents [hide]
1 Introduction
1.1 Etymology
1.2 Nuclear makeup
2 Protons and neutrons
3 Forces
4 Nuclear models
4.1 Liquid drop models
4.2 Shell models and other quantum models
4.3 Consistency between models
5 See also
6 References
7 External links



[edit] Introduction

[edit] Etymology
The term nucleus is from Latin nucleus (“‘kernel’”), derived from nux (“‘nut’”). In 1844, Michael Faraday used the term to refer to the "central point of an atom". The modern atomic meaning was proposed by Ernest Rutherford in 1912.[1] The adoption of the term "nucleus" to atomic theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis stated, in his famous article The Atom and the Molecule, that "the atom is composed of the kernel and an outer atom or shell".


A figurative depiction of the helium-4 atom. In the nucleus, the two protons are shown in red and neutrons blue. This depiction shows the particles as separate, whereas in an actual helium atom, the protons are superimposed in space and most likely found at the very center of the nucleus, and the same is true of the two neutrons. Thus all four particles are most likely found in exactly the same space. Classical images of separate particles thus fail to model known charge distributions in very small nuclei
[edit] Nuclear makeup
The nucleus of an atom consists of protons and neutrons (two types of baryons) bound by the nuclear force (also known as the residual strong force). These baryons are further composed of subatomic fundamental particles known as quarks bound by the strong interaction. Which chemical element an atom represents is determined by the number of protons in the nucleus. Each proton carries a single positive charge, and the total electrical charge of the nucleus is spread fairly uniformly throughout its body, with the exception of hydrogen and helium, where the charge is concentrated most highly at the central point, as would be expected for fermions in 1s states without orbital angular momentum.[2] As each proton carries a unit of charge, the charge distribution is indicative of the proton distribution, and the neutron distribution probably is similar.[2]


[edit] Protons and neutrons
Protons and neutrons are fermions, with different values of the isospin quantum number, so two protons and two neutrons can share the same space wave function. They sometimes are viewed as two different states of the same particle, the nucleon.[3][4]

In the rare case of a hypernucleus, a third baryon called a hyperon, with a different value of the strangeness quantum number can also share the wave function. However, the latter type of nuclei are extremely unstable and are not found on Earth except in high energy physics experiments.

The neutron has a positively charged core of radius ≈ 0.3 fm surrounded by a compensating negative charge of radius between 0.3 fm and 2 fm. The proton has an approximately exponentially decaying charge distribution with a mean square radius of about 0.8 fm.[5]


[edit] Forces
Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker between neutrons and protons because it is mostly neutalized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms internally together (i.e., for example, the forces that hold the electrons in an inert gas atom bound to its nucleus).

The nuclear force is highly attractive at very small distances, and this overwhelms the repulsion between protons which is due to the electromagnetic force, thus allowing nuclei to exist. However, because the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential), only nuclei smaller than a certain size can be completely stable. The largest known complete stable nucleus is lead-208 which contains a total of 208 neutrons and protons. Nuclei larger than this maximal size of 208 particles are unstable and (as a trend) become increasingly short-lived with larger size, as the number of neutrons and protons which compose them increases beyond this number.

The residual strong force is effective over a very short range (usually only a few fermis, roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons (i.e., between protons and neutrons, and also between protons and protons, and neutrons and neutrons). The effective absolute limit of the range of the strong force is represented by halo nuclei such as lithium-11 or boron-14, in which dineutrons, or other collections of neutrons, orbit at distances of about ten fermis (roughly similar to the 8 fermi radius of the nucleus of uranium-238). These nuclei are not maximally dense, and such halos are always composed of neutrons, which have no charge to disrupt them. Such nuclei are also always unstable and short-lived; for example, lithium-11 has a half-life of less than 8.6 milliseconds.


[edit] Nuclear models
Main article: Nuclear size
The nuclear radius (R) is considered to be one of the basic things that any model must explain. It is roughly proportional to the cube root of the mass number (A) of the nucleus, and particularly in nuclei containing many nucleons, as they arrange in more spherical configurations:

The nucleus has approximately a constant density and therefore the nuclear radius R can be approximated by the following formula,

R = r0A1 / 3

where, A = Atomic mass number (the number of protons, Z, plus the number of neutrons, N) and r0 = 1.25 fm = 1.25 × 10−15 m. In this equation, the constant r0 varies by 0.2 fm, depending on the nucleus in question, but this is less than 20% change from a constant.[6]

In other words, packing protons and neutrons in the nucleus gives approximately the same total size result as packing hard spheres of a constant size (like marbles) into a tight bag.


[edit] Liquid drop models
Main article: Liquid-drop model
Early models of the nucleus viewed the nucleus as a rotating liquid drop. In this model, the trade-off of long-range electromagnetic forces and relatively short-range nuclear forces, together cause behavior which resembled surface tension forces in liquid drops of different sizes. This formula is successful at explaining many important phenomena of nuclei, such as their changing amounts of binding energy as their size and composition changes (see semi-empirical mass formula), but it does not explain the special stability which occurs when nuclei have special "magic numbers" of protons or neutrons.


[edit] Shell models and other quantum models
Main article: Shell model
A number of models for the nucleus have also been proposed in which nucleons occupy orbitals, much like the atomic orbitals in atomic physics theory. These wave models imagine nucleons to be either sizeless point particles in potential wells, or else probability waves as in the "optical model", frictionlessly orbiting at high speed in potential wells.

In these models, the nucleons occupy orbitals in pairs, due to being fermions, but the exact nature and capacity of nuclear shells differs somewhat from those of electrons in atomic orbitals, primarily because the potential well in which the nucleons move (especially in larger nuclei) is quite different from the central electromagnetic potential well which binds electrons in atoms. Nevertheless, the resemblance to atomic orbital models may be seen in a small atomic nucleus like that of helium-4, in which the two protons and two neutrons separately occupy 1s orbitals analogous to the 1s orbitals for the two electrons in the helium atom, and achieve unusual stability for the same reason. This stability also underlies the fact that nuclei with 5 nucleons are all extremely unstable and short-lived.

For larger nuclei, the shells occupied by nucleons begin to differ significantly from electron shells, but nevertheless, present nuclear theory does predict the magic numbers of filled nuclear shells for both protons and neutrons. The closure of the stable shells predicts unusually stable configurations, analogous to the noble group of nearly-inert gases in chemistry. An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other element. Similarly, the distance from shell-closure explains the unusual instability of isotopes which have far from stable numbers of these particles, such as the radioactive elements 43 (technetium) and 61 (promethium), each of which is preceded and followed by 17 or more stable elements.


[edit] Consistency between models
Main article: Nuclear structure
As with the case of superfluid liquid helium, atomic nuclei are an example of a state in which both (1) "ordinary" particle physical rules for volume and (2) non-intuitive quantum mechanical rules for a wave-like nature apply. In superfluid helium, the helium atoms have volume, and essentially "touch" each other, yet at the same time exhibit strange bulk properties, consistent with a Bose-Einstein condensation. The latter reveals that they also have a wave-like nature and do not exhibit standard fluid properties, such as friction. For nuclei made of hadrons which are fermions, the same type of condensation does not occur, yet nevertheless, many nuclear properties can only be explained similarly by a combination of properties of particles with volume, in addition to the frictionless motion characteristic of the wave-like behavior of objects trapped in Schroedinger quantum orbitals.

Hans Blix

Hans Martin Blix (help·info) (born 28 June 1928 in Uppsala, Sweden) is a Swedish diplomat and politician. He was Swedish Minister for Foreign Affairs (1978 - 1979). Blix was also the head of the United Nations Monitoring, Verification and Inspection Commission from March 2000 to June 2003, when he was succeeded by Demetrius Perricos. In 2002, the commission began searching Iraq for weapons of mass destruction, ultimately finding none. He is the son of professor Gunnar Blix and Hertha Wiberg and grandson of professor Magnus Blix. He comes from a family of Jamtlandic origin.

Early career
Blix studied at Uppsala University and Columbia University, earning his Ph.D. from the University of Cambridge (Trinity Hall).[1] In 1959, he became a Juris Doctor in International Law at the University of Uppsala, where he was appointed Associate Professor in International Law the next year. [2]

Between 1962 and 1978 Blix was a member of the Swedish delegation at the Disarmament Conference in Geneva. He held several other positions in the Swedish administration between 1963 and 1976, and from 1961 to 1981 served on the Swedish delegation to the United Nations. From 1978 to 1979, Blix was the Swedish Foreign Minister.

Blix chaired the Swedish Liberal Party's campaign during the 1980 referendum on nuclear power, campaigning in favor of retention of the Swedish nuclear energy program.


[edit] Head of the International Atomic Energy Agency (1981-1997)
Blix became Director General of the International Atomic Energy Agency between 1981 and 1997 after Sigvard Eklund. He personally made repeated inspection visits to the Iraqi nuclear reactor Osiraq before its attempted destruction by the Iranians, in 1980, and its eventual destruction by the Israeli Air Force in 1981 during Operation Opera. Although most agreed that Iraq was years away from being able to build a nuclear weapon, the Iranians and the Israelis felt any raid must occur well before nuclear fuel was loaded to prevent nuclear fallout. The attack was regarded as being in breach of the United Nations Charter (S/RES/487) and international law and was widely condemned.

Iraq was alternately praised and admonished by the IAEA for its cooperation and lack thereof. It was only after the first Gulf War that the full extent of Iraq's nuclear programs, which had switched from a plutonium based weapon design to a highly enriched uranium design after the destruction of Osiraq, became known.


[edit] Iraq disarmament crisis (2002-2003)
During the Iraq disarmament crisis before the 2003 invasion of Iraq, Blix was called back from retirement by UN Secretary General Kofi Annan to lead United Nations Monitoring, Verification and Inspection Commission in charge of monitoring Iraq. Kofi Annan originally recommended Rolf Ekéus, who worked with UNSCOM in the past, but both Russia and France vetoed his appointment. Newt Gingrich stated that approving Hans Blix as chief UN weapons inspector was one of the biggest mistakes the United States ever made.[citation needed]

Hans Blix personally admonished Saddam for "cat and mouse" games [3] and warned Iraq of "serious consequences" if it attempted to hinder or delay his mission [4].

In his report to the UN Security Council on 14 February 2003, Blix claimed that "If Iraq had provided the necessary cooperation in 1991, the phase of disarmament -- under resolution 687 -- could have been short and a decade of sanctions could have been avoided." [5]

Blix's statements about the Iraq WMD program came to contradict the claims of the George W. Bush administration, [6] and attracted a great deal of criticism from supporters of the invasion of Iraq. In an interview on BBC TV on 8 February 2004, Dr. Blix accused the US and British governments of dramatising the threat of weapons of mass destruction in Iraq, in order to strengthen the case for the 2003 war against the regime of Saddam Hussein. Ultimately, no stockpiles of weapons of mass destruction were found. [7]

In an interview with London's Guardian newspaper, Hans Blix said, "I have my detractors in Washington. There are bastards who spread things around, of course, who planted nasty things in the media" [8].

In 2004, Blix published a book, Disarming Iraq, where he gives his account of the events and inspections before the coalition began its invasion.

Blix said he suspected his home and office were bugged by the United States, while he led teams searching for Saddam Hussein's supposed weapons of mass destruction.[3]Although these suspicions were never directly substantiated, evidence of bugging of UN security council representatives around the time the US was seeking approval from the council came to light after a British government translator leaked a document "allegedly from an American National Security Agency" requesting that British intelligence put wiretaps on delegates to the UN security council.[4]


[edit] Weapons of Mass Destruction Commission
Please help improve this article or section by expanding it. Further information might be found on the talk page. (July 2007)

Since 2003 Blix has been chair of the Weapons of Mass Destruction Commission (WMDC), an independent body funded by the Swedish government and based in Stockholm [9].

In December 2006, the Weapons of Mass Destruction Commission said in a report that Pakistan’s infamous and controversial nuclear proliferator Abdul Qadeer Khan could not have acted alone or “without the awareness of the Pakistani government”.[5]


[edit] Honours
Doctorate Honoris causa of the University of Moscow in 1987.
Recipient of the Henry de Wolf Smyth Nuclear Statesman Award (Washington, D.C.), 1988.
Honorary membership in the Cambridge Union Society.
Gold Medal for distinguished service in the field of nuclear affairs by the Uranium Institute (now World Nuclear Association) in 1997.
Doctorate Honoris causa of the Vrije Universiteit Brussel in 2003.
"Commander of the Légion d'Honneur" in 2004.
Doctorate Honoris causa of the University of Padova in 2004.
Sydney Peace Prize 2007[6].
Doctorate Honoris causa of the University of Cambridge in 2007.[7]

[edit] Cultural references
Hans Blix is parodied in Team America: World Police, where he is fed to sharks by Kim Jong-il, President of North Korea.
Hans Blix appeared in the documentaries The World According to Bush [10] and Europe & USA: Behind the Scenes of a Political Rupture [11] .
Hans Blix appears in David Hare's political play Stuff Happens.
Hans Blix is mentioned in Pain Of Salvation's song "America", from the album Scarsick.

[edit] Bibliography
Disarming Iraq, Hans Blix, Pantheon (9 March, 2004). ISBN 0-375-42302-8
Why Nuclear Disarmament Matters, Hans Blix, The MIT Press (30 April, 2008). ISBN 0-262-02644-9