Neutron activation

Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting gamma rays, or particles such as beta particles, alpha particles, fission products and neutrons (in nuclear fission). Thus, the process of neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years.

Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. All naturally-occurring materials, including air, water, and soil, can be induced (activated) by neutron capture into some amount of radioactivity in varying degrees, as a result of production of neutron-rich radioisotopes. Some atoms require more than one neutron to become unstable, which makes them harder to activate because the probability of a double or triple capture by a nucleus is below that of single capture. Water, for example, is made up of hydrogen and oxygen. Hydrogen requires a double capture to attain instability as hydrogen-3, tritium, while natural oxygen (oxygen-16) requires three captures to become unstable oxygen-19. Thus water is relatively difficult to activate, as compared to sea salt (NaCl), in which both the sodium and chlorine atoms become unstable with a single capture each. These facts were realized first-hand at the Operation Crossroads atomic test series in 1946.

Examples

Main article: neutron capture

An example of this kind of a nuclear reaction occurs in the production of cobalt-60 within a nuclear reactor:

59
27Co
 + 1
0n
 → 60
27Co

The cobalt-60 then decays by the emission of a beta particle plus gamma rays into nickel-60. This reaction has a half-life of about 5.27 years; and due to the availability of cobalt-59 (100% of its natural abundance) this neutron bombarded isotope of cobalt is a valuable source of nuclear radiation (namely gamma radiation) for radiotherapy.[1]

In other cases, and depending on the kinetic energy of the neutron, the capture of a neutron can cause nuclear fission—the splitting of the atomic nucleus into two smaller nuclei. If the fission requires an input of energy, that comes from the kinetic energy of the neutron. An example of this kind of fission in a light element can occur when the stable isotope of lithium, lithium-7, is bombarded with fast neutrons and undergoes the following nuclear reaction:

7
3Li
 + 1
0n
 → 4
2He
 + 3
1H
 + 1
0n
 + gamma rays + kinetic energy

In other words, the capture of a neutron by lithium-7 causes it to split into an energetic helium nucleus (alpha particle), a hydrogen-3 (tritium) nucleus and a free neutron. The Castle Bravo accident, in which the thermonuclear bomb test at Enewetak Atoll in 1954 exploded with 2.5 times the expected yield, was caused by the unexpectedly high probability of this reaction.

In the areas around a pressurized water reactors or boiling water reactors during normal operation, a significant amount of radiation is produced due to the fast neutron activation of coolant water oxygen via a (n,p) reaction. The activated oxygen-16 nucleus emits a proton (hydrogen nucleus), and transmutes to nitrogen-16, which has a very short life before decaying back to oxygen-16.[2]

16
8O
 + 1
0n
 → 1
1p
 + 16
7N
 (Decays rapidly)
16
7N
 →
γ
 + 0
-1e-
 + 16
8O

This activation of the coolant water requires extra biological shielding around the nuclear reactor plant. It is the high energy gamma ray in the second reaction that causes the major concern. This is why water that has recently been inside a nuclear reactor must be shielded until this radiation subsides. One to two minutes is generally sufficient.

Occurrence

Neutron activation is the only common way that a stable material can be induced into becoming intrinsically radioactive. Neutrons are only free in quantity in the microseconds of a nuclear weapon's explosion, in an active nuclear reactor, or in a spallation neutron source.

In an atomic weapon neutrons are only generated for from 1 to 50 microseconds, but in huge numbers. Most are absorbed by the metallic bomb casing, which has not yet or only just started to be affected by the explosion within it. The neutron activation of the soon-to-be vaporized metal is responsible for a significant portion of the nuclear fallout in nuclear bursts high in the atmosphere. In other types of activation neutrons may irradiate soil that is dispersed in a mushroom cloud at or near the Earth's surface, resulting in fallout from activation of soil chemical elements.

Effects on materials over time

In any location with high neutron fluxes, such as within the cores of nuclear reactors, neutron activation contributes to material erosion; periodically the lining materials themselves must be disposed of, as low-level radioactive waste. Some materials are more subject to neutron activation than others, so a suitably chosen low-activation material can significantly reduce this problem (see International Fusion Materials Irradiation Facility). For example, Chromium-51 will form by neutron activation in Chrome steel (which contains Cr-50) that is exposed to a typical reactor neutron flux.[3]

Carbon-14, most frequently but not solely, generated by the neutron activation of atmospheric nitrogen-14 with thermal neutron, is (together with its dominant natural production pathway from cosmic ray-air interactions and historical production from atmospheric nuclear testing) also generated in comparatively minute amounts inside many designs of nuclear reactors which contain nitrogen gas impurities in their fuel cladding, coolant water and by neutron activation of the oxygen contained in the water itself. Fast breeder reactors (FBR) produce about an order of magnitude less C-14 than the most common reactor type, the Pressurized water reactor, as FBRs do not use water as a primary coolant.[4]

Uses

Radiation safety

For physicians and radiation safety officers, activation of sodium in the human body to sodium-24, and phosphorus to phosphorus-32, can give a good immediate estimate of acute accidental neutron exposure.[5]

Neutron detection

One way to demonstrate that nuclear fusion has occurred inside a fusor device is to use a Geiger counter to measure the gamma ray radioactivity that is produced from a sheet of aluminium foil.

In the ICF fusion approach, the fusion yield of the experiment (directly proportional to neutron production) is usually determined by measuring the gamma-ray emissions of aluminium or copper neutron activation targets.[6] Aluminium can capture a neutron and generate radioactive sodium-24, which has a half life of 15 hours[7][8] and a beta decay energy of 5.514 MeV.[9]

The activation of a number of test target elements such as sulfur, copper, tantalum and gold have been used to determine the yield of both pure fission[10][11] and thermonuclear[12] weapons.

Materials analysis

Neutron activation analysis is one of the most sensitive and accurate methods of trace element analysis. It requires no sample preparation or solubilization and can therefore be applied to objects that need to be kept intact such as a valuable piece of art. Although the activation induces radioactivity in the object, its level is typically low and its lifetime may be short, so that its effects soon disappear. In this sense, neutron activation is a non-destructive analysis method.

Neutron activation analysis can be done in situ. For example, aluminium (Al-27) can be activated by capturing relatively low-energy neutrons to produce the isotope Al-28, which decays with a half-life of 2.3 minutes with a decay energy of 4.642 MeV.[13] This activated isotope is used in oil drilling to determine the clay content (clay is generally an alumino-silicate) of the underground area under exploration.[14]

Historians can use accidental neutron activation to authenticate atomic artifacts and materials subjected to neutron fluxes from fission incidents. For example, one of the fairly unique isotopes found in trinitite, and therefore with its absence likely signifying a fake sample of the mineral, is a barium neutron activation product, the barium in the trinity device coming from the slow explosive lens employed in the device, known as Baratol.[15]

See also

References

  1. ↑ Manual for reactor produced radioisotopes from the International Atomic Energy Agency
  2. ↑ Neeb, Karl Heinz (1997). The Radiochemistry of Nuclear Power Plants with Light Water Reactors. Berlin-New York: Walter de Gruyter. p. 227. ISBN 3-11-013242-7.
  3. ↑ http://ie.lbl.gov/toi/nuclide.asp?iZA=240051
  4. ↑ "IAEA Technical report series no.421, Management of Waste Containing Tritium and Carbon-14" (PDF).
  5. ↑ ORNL Report on determination of dose from criticality accidents
  6. ↑ Stephen Padalino; Heather Oliver & Joel Nyquist. "DT neutron yield measurements using neutron activation of aluminum". LLE Collaborators: Vladimir Smalyukand, Nancy Rogers.
  7. ↑ http://www.aanda.org/articles/aa/full/2001/10/aah2362/node4.html
  8. ↑ http://kubchemistry.weebly.com/uploads/6/9/8/7/6987088/chapter_22_nuclear_reactions.ppt
  9. ↑ http://www.site.uottawa.ca:4321/astronomy/index.html#sodium24
  10. ↑ Kerr, George D.; Young, Robert W.; Cullings, Harry M.; Christy, Robert F. (2005). "Bomb Parameters". In Robert W. Young, George D. Kerr. Reassessment of the Atomic Bomb Radiation Dosimetry for Hiroshima and Nagasaki – Dosimetry System 2002 (PDF). The Radiation Effects Research Foundation. pp. 42–43.
  11. ↑ Malik, John (September 1985). "The Yields of the Hiroshima and Nagasaki Explosions" (PDF). Los Alamos National Laboratory. Retrieved March 9, 2014.
  12. ↑ US Army (1952). Operation Ivy Final Report Joint Task Force 132 (PDF).
  13. ↑ http://www.site.uottawa.ca:4321/astronomy/index.html#aluminium28
  14. ↑ "Aluminum activation log".
  15. ↑ "Radioactivity in Trinitite six decades later. Journal of Environmental Radioactivity Volume 85, Issue 1, 2006, Pages 103–120".

External links

Further reading

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