Part 2, Non-catalyzed reactions with the wood cell wall Lignin fate and characterization during ionic liquid biomass pretreatment for renewable chemicals and fuels production Chemical Changes during Anaerobic Decomposition of Hardwood, Softwood, and Old Newsprint under Mesophilic and Thermophilic Conditions. Part 1, Catalyzed reactions with wood models and wood polymers Soy protein adhesives.
Nuclear Spectroscopy and Reactions 40-A
Description Solution-state NMR provides a powerful tool to observe the presence or absence of covalent bonds between wood and adhesives. Finely ground wood can be dissolved in an NMR compatible solvent system containing dimethylsulfoxide-d 6 and N-methylimidazole-d 6 , in which the wood polymers remain largely intact. High-resolution solution-state two dimensional NMR correlation experiments, based on 13 C— 1 H one-bond heteronuclear single quantum coherence, allow structural analysis of the major cell wall components. This technique was applied to loblolly pine that was treated with polymeric methylene diphenyl diisocyanate pMDI related model compounds under controlled moisture and temperature conditions.
Chemical shifts of carbamates formed between the pMDI model compounds and loblolly pine were determined.
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Publication Notes We recommend that you also print this page and attach it to the printout of the article, to retain the full citation information. This article was written and prepared by U. Government employees on official time, and is therefore in the public domain. Citation Yelle, Daniel J. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, e. Gamma rays are produced in many processes of particle physics.
Typically, gamma rays are the products of neutral systems which decay through electromagnetic interactions rather than a weak or strong interaction. For example, in an electron—positron annihilation , the usual products are two gamma ray photons. Similarly, a neutral pion most often decays into two photons. Many other hadrons and massive bosons also decay electromagnetically. High energy physics experiments, such as the Large Hadron Collider , accordingly employ substantial radiation shielding. Since gamma rays are at the top of the electromagnetic spectrum in terms of energy, all extremely high-energy photons are gamma rays; for example, a photon having the Planck energy would be a gamma ray.
A few gamma rays in astronomy are known to arise from gamma decay see discussion of SNA , but most do not. Photons from astrophysical sources that carry energy in the gamma radiation range are often explicitly called gamma-radiation. In addition to nuclear emissions, they are often produced by sub-atomic particle and particle-photon interactions. Those include electron-positron annihilation , neutral pion decay , bremsstrahlung , inverse Compton scattering , and synchrotron radiation.
In October , scientists from various European universities proposed a means for sources of GeV photons using lasers as exciters through a controlled interplay between the cascade and anomalous radiative trapping. Thunderstorms can produce a brief pulse of gamma radiation called a terrestrial gamma-ray flash. These gamma rays are thought to be produced by high intensity static electric fields accelerating electrons, which then produce gamma rays by bremsstrahlung as they collide with and are slowed by atoms in the atmosphere.
Gamma rays up to MeV can be emitted by terrestrial thunderstorms, and were discovered by space-borne observatories. This raises the possibility of health risks to passengers and crew on aircraft flying in or near thunderclouds. The first confident observation occurred in Extraterrestrial, high energy gamma rays include the gamma ray background produced when cosmic rays either high speed electrons or protons collide with ordinary matter, producing pair-production gamma rays at keV. Alternatively, bremsstrahlung are produced at energies of tens of MeV or more when cosmic ray electrons interact with nuclei of sufficiently high atomic number see gamma ray image of the Moon at the beginning of this article, for illustration.
The gamma ray sky see illustration at right is dominated by the more common and longer-term production of gamma rays that emanate from pulsars within the Milky Way. Sources from the rest of the sky are mostly quasars. Pulsars are thought to be neutron stars with magnetic fields that produce focused beams of radiation, and are far less energetic, more common, and much nearer sources typically seen only in our own galaxy than are quasars or the rarer gamma-ray burst sources of gamma rays.
Pulsars have relatively long-lived magnetic fields that produce focused beams of relativistic speed charged particles, which emit gamma rays bremsstrahlung when those strike gas or dust in their nearby medium, and are decelerated. This is a similar mechanism to the production of high-energy photons in megavoltage radiation therapy machines see bremsstrahlung.
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Inverse Compton scattering , in which charged particles usually electrons impart energy to low-energy photons boosting them to higher energy photons. Such impacts of photons on relativistic charged particle beams is another possible mechanism of gamma ray production. Neutron stars with a very high magnetic field magnetars , thought to produce astronomical soft gamma repeaters , are another relatively long-lived star-powered source of gamma radiation. More powerful gamma rays from very distant quasars and closer active galaxies are thought to have a gamma ray production source similar to a particle accelerator.
High energy electrons produced by the quasar, and subjected to inverse Compton scattering, synchrotron radiation , or bremsstrahlung, are the likely source of the gamma rays from those objects. It is thought that a supermassive black hole at the center of such galaxies provides the power source that intermittently destroys stars and focuses the resulting charged particles into beams that emerge from their rotational poles. When those beams interact with gas, dust, and lower energy photons they produce X-rays and gamma rays. These sources are known to fluctuate with durations of a few weeks, suggesting their relatively small size less than a few light-weeks across.
Such sources of gamma and X-rays are the most commonly visible high intensity sources outside our galaxy. They shine not in bursts see illustration , but relatively continuously when viewed with gamma ray telescopes. The power of a typical quasar is about 10 40 watts, a small fraction of which is gamma radiation.
Much of the rest is emitted as electromagnetic waves of all frequencies, including radio waves. The most intense sources of gamma rays, are also the most intense sources of any type of electromagnetic radiation presently known. They are the "long duration burst" sources of gamma rays in astronomy "long" in this context, meaning a few tens of seconds , and they are rare compared with the sources discussed above.
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By contrast, "short" gamma-ray bursts of two seconds or less, which are not associated with supernovae, are thought to produce gamma rays during the collision of pairs of neutron stars, or a neutron star and a black hole. The so-called long-duration gamma-ray bursts produce a total energy output of about 10 44 joules as much energy as our Sun will produce in its entire life-time but in a period of only 20 to 40 seconds. The leading hypotheses for the mechanism of production of these highest-known intensity beams of radiation, are inverse Compton scattering and synchrotron radiation from high-energy charged particles.
These processes occur as relativistic charged particles leave the region of the event horizon of a newly formed black hole created during supernova explosion. The beam of particles moving at relativistic speeds are focused for a few tens of seconds by the magnetic field of the exploding hypernova. The fusion explosion of the hypernova drives the energetics of the process.
If the narrowly directed beam happens to be pointed toward the Earth, it shines at gamma ray frequencies with such intensity, that it can be detected even at distances of up to 10 billion light years, which is close to the edge of the visible universe.
Ultra-bright scintillators for planetary gamma-ray spectroscopy
Due to their penetrating nature, gamma rays require large amounts of shielding mass to reduce them to levels which are not harmful to living cells, in contrast to alpha particles , which can be stopped by paper or skin, and beta particles , which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with high atomic numbers and high density, which contribute to the total stopping power.
Protective clothing, goggles and respirators can protect from internal contact with or ingestion of alpha or beta emitting particles, but provide no protection from gamma radiation from external sources.
The higher the energy of the gamma rays, the thicker the shielding made from the same shielding material is required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half the half value layer or HVL.
For example, gamma rays that require 1 cm 0. Depleted uranium is used for shielding in portable gamma ray sources , but here the savings in weight over lead are larger, as a portable source is very small relative to the required shielding, so the shielding resembles a sphere to some extent.
The volume of a sphere is dependent on the cube of the radius; so a source with its radius cut in half will have its volume and weight reduced by a factor of eight, which will more than compensate for uranium's greater density as well as reducing bulk. The loss of water or removal of a "hot" fuel assembly into the air would result in much higher radiation levels than when kept under water. When a gamma ray passes through matter, the probability for absorption is proportional to the thickness of the layer, the density of the material, and the absorption cross section of the material.
The total absorption shows an exponential decrease of intensity with distance from the incident surface:. As it passes through matter, gamma radiation ionizes via three processes: the photoelectric effect , Compton scattering , and pair production. Additionally, gamma rays, particularly high energy ones, can interact with atomic nuclei resulting in ejection of particles in photodisintegration , or in some cases, even nuclear fission photofission. Gamma spectroscopy is the study of the energetic transitions in atomic nuclei, which are generally associated with the absorption or emission of gamma rays.
As in optical spectroscopy see Franck—Condon effect the absorption of gamma rays by a nucleus is especially likely i. The immobilization of nuclei at both ends of a gamma resonance interaction is required so that no gamma energy is lost to the kinetic energy of recoiling nuclei at either the emitting or absorbing end of a gamma transition.
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Such loss of energy causes gamma ray resonance absorption to fail. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy is also sufficient to excite the same energy state in a second immobilized nucleus of the same type. Gamma rays provide information about some of the most energetic phenomena in the universe; however, they are largely absorbed by the Earth's atmosphere.
Instruments aboard high-altitude balloons and satellites missions, such as the Fermi Gamma-ray Space Telescope , provide our only view of the universe in gamma rays.
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Gamma-induced molecular changes can also be used to alter the properties of semi-precious stones , and is often used to change white topaz into blue topaz. Non-contact industrial sensors commonly use sources of gamma radiation in refining, mining, chemicals, food, soaps and detergents, and pulp and paper industries, for the measurement of levels, density, and thicknesses . Gamma-ray sensors are also used for measuring the fluid levels in water and oil industries . Typically, these use Co or Cs isotopes as the radiation source.
These machines are advertised to be able to scan 30 containers per hour. Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include the sterilization of medical equipment as an alternative to autoclaves or chemical means , the removal of decay-causing bacteria from many foods and the prevention of the sprouting of fruit and vegetables to maintain freshness and flavor. Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer , since the rays also kill cancer cells.
In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues. Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques.