Nuclear halo
Encyclopedia
In nuclear physics
, an atomic nucleus
is called a halo nucleus or is said to have a nuclear halo if its radius is appreciably larger than that predicted by the liquid drop model, wherein the nucleus is assumed to be a sphere of constant density.
For a nucleus of mass number
A, the radius r is (approximately)
where
is 1.2 fm.
Typically, an atomic nucleus is a tightly bound group of protons and neutrons. However, in some isotopes, there is an overabundance of one species of nucleon. In some of these cases, a nuclear core and a halo will form.
Often, this property may be detected in scattering experiments which show the nucleus to be much larger than the otherwise expected value. Normally the cross section (corresponding to the classical radius) of the nucleus is proportional to the cube root of its mass. (This is the same relation as would be seen with a solid sphere.)
One example of a halo nucleus is 11Li which has a half life of 8.6 ms. It decays into 11Be by the emission of an antineutrino and an electron. Its cross-section of 3.16 fm is close to that of 32S
, or even more impressively that of 208Pb
, both much heavier nuclei. It contains a core of 3 protons and 6 neutrons, and a halo of two independent and loosely bound neutrons.
Nuclei which have a neutron halo include 11Be and 19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C. Two-neutron halo nuclei break into three fragments and are called Borromean
because of this behavior. 8He and 14Be both exhibit a four-neutron halo.
Nuclei which have a proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and 27S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive forces of the excess proton(s).
Halo nuclei form at the extreme edges of the chart of the 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.
Experimental confirmation of nuclear halos is recent and ongoing. Additional candidates are suspected. Several nuclides have a halo in the excited state
but not in the ground state
.
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
, an atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
is called a halo nucleus or is said to have a nuclear halo if its radius is appreciably larger than that predicted by the liquid drop model, wherein the nucleus is assumed to be a sphere of constant density.
For a nucleus of mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. 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...
A, the radius r is (approximately)
where
is 1.2 fm.Typically, an atomic nucleus is a tightly bound group of protons and neutrons. However, in some isotopes, there is an overabundance of one species of nucleon. In some of these cases, a nuclear core and a halo will form.
Often, this property may be detected in scattering experiments which show the nucleus to be much larger than the otherwise expected value. Normally the cross section (corresponding to the classical radius) of the nucleus is proportional to the cube root of its mass. (This is the same relation as would be seen with a solid sphere.)
One example of a halo nucleus is 11Li which has a half life of 8.6 ms. It decays into 11Be by the emission of an antineutrino and an electron. Its cross-section of 3.16 fm is close to that of 32S
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, or even more impressively that of 208Pb
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, both much heavier nuclei. It contains a core of 3 protons and 6 neutrons, and a halo of two independent and loosely bound neutrons.
Nuclei which have a neutron halo include 11Be and 19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C. Two-neutron halo nuclei break into three fragments and are called Borromean
Borromean rings
In mathematics, the Borromean rings consist of three topological circles which are linked and form a Brunnian link, i.e., removing any ring results in two unlinked rings.- Mathematical properties :...
because of this behavior. 8He and 14Be both exhibit a four-neutron halo.
Nuclei which have a proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and 27S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive forces of the excess proton(s).
Halo nuclei form at the extreme edges of the chart of the 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.
Experimental confirmation of nuclear halos is recent and ongoing. Additional candidates are suspected. Several nuclides have a halo in the excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
but not in the ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
.
List of known isotopes with nuclear halo
| Atomic number |
Name | # of nuclear halo isotopes |
Nuclear halo isotopes |
|---|---|---|---|
| 2 | helium Helium Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table... |
2 | helium-6 helium-8 |
| 3 | lithium Lithium Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly... |
1 | lithium-11 |
| 4 | beryllium Beryllium Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl... |
2 | beryllium-11 beryllium-14 |
| 5 | boron Boron Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the... |
3 | boron-8 boron-17 boron-19 |
| 6 | carbon Carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds... |
2 | carbon-19 carbon-22 |
| 10 | neon Neon Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or... |
1 | neon-17 |
| 15 | phosphorus Phosphorus Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks... |
1 | phosphorus-26 |
| 16 | Sulfur Sulfur Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow... |
1 | sulfur-27 |
External links
- http://arxiv.org/abs/0809.2607
- http://www.uni-mainz.de/eng/13031.php
- http://arxiv.org/abs/nucl-ex/0111001v2