Americium radioactive decay

Americium toxicity results primarily from the damage done by the alpha particle emitted during radioactive decay. This alpha particle has very limited penetration in tissue, and hence, the cellular damage (including damage to genomic material) occurs only in the immediate vicinity of the sequestered americium. 

Americium isotopes are transformed by radioactive decay. However, the half-lives of the principal americium isotopes, 241Am and 243Am, are very long, 432 and 7,370 years, respectively, and there is only a small amount of transformation over a human lifetime. 241Am is formed by the decay of 241Pu (half-life 14.4 years) and this can lead to a significant transformation of that isotope to 241 Am in humans, especially for 241Pu that is fixed in the bone. 

The plutonium-241 that results from uranium-238 bombardment is itself radioactive with a half-life of 14.4 years, decaying by j3 emission to yield americium-241. (If the name sounds familiar, it s because americium is used commercially in making smoke detectors.) Americium-241 is also radioactive, decaying by a emission with a half-life of 432 years. 

We know that nuclear weapons are capable of mass destruction, yet radiation therapy, shown in Figure 4.18, is a proven cancer fighter. Smoke detectors, required by law in all homes, rely on the radioactive decay of americium-241. The human body itself is radioactive, due to the presence of radioactive isotopes including carbon-14, phosphorus-32, and potassium-40. Most people view radioactivity and nuclear reactions with a mixture of fascination, awe, and fear. Since radioactivity is all around us, it is important to understand what it is, how it arises, and how we can deal with it safely. 

Because there are few data on the results of human exposure to actinides, the health effects of these radioelements are more uncertain than those discussed above for ionizing radiation, radon, and fission products. Americium accumulates in bones and will likely cause bone cancer due to its radioactive decay. Animal studies suggest that plutonium will cause effects in the blood, liver, bone, lung, and immune systems. Other potential mechanisms of chemical toxicity and carcinogenicity of the actinides are similar to those of heavy metals and include (i) disruption of transport pathways for nutrients and ions (ii) displacement of essential metals such as Cu, Zn, and Ni ... 

Actinides in the environment can be classified into two groups (i) the uranium and thorium series of radionuclides in the natural environment and (ii) neptunium, plutonium, americium and curium which are formed in a nuclear reactor during the neutron bombardment of uranium through a series of neutron capture and radioactive decay reactions. Transuranics thus produced have been spread widely in the atmosphere, geosphere and aquatic environment on the earth, as a result of nuclear bomb tests in the atmosphere, and accidental release from nuclear facilities (Sakanoue, 1987). Most of these radionuclide inventories have deposited in the northern hemisphere following the tests conducted by the United States and the Soviet Union. 

The major difficulty with synthesizing heavy elements is the number of protons in their nuclei (Z > 92). The large amount of positive charge makes the nuclei unstable so that they tend to disintegrate either by radioactive decay or spontaneous fission. Therefore, with the exception of a few transuranium elements like plutonium (Pu) and americium (Am), most artificial elements are made only a few atoms at a time and so far have no practical or commercial uses. 

Radioactive decay of and Pu form Am and Am, which are also important and persistent sources of alpha radioactivity in discharge fuel. Another persistent americium radioisotope is 152-year Am, formed by neutron capture in Am. Its isomeric decay and the beta decay of its short-lived daughter result in 163-day Cm, which is the most intense source of alpha activity in discharged uranium fuel. Successive neutron captures lead to em, em, and Cm. Higher-mass curium nuclides are usually not important in power reactor fuel. 

The radioactivities of the plutonium radionuclides in the high-level wastes from fuel reprocessing are shown as a function of storage time in Fig. 8.8 [PI], Because the initial plutonium quantities are due only to the small fraction, e.g., 0.5 percent, of the plutonium that is lost to these wastes in reprocessing, larger quantities appear after a few years due to the decay of americium and curium. The Pu increases with time because of the decay of " Am and Cm, Pu increases from the decay of Am and Cm, and Pu increases due to the decay of Cm. Therefore, even though the total actinide activity in these wastes is dominated by plutonium after the americium has decayed, the plutonium in the wastes at this time is due mainly to the earlier decay of americium and curium and not to the small fraction of plutonium lost to the wastes in fuel reprocessing. 

Metallic americium has a face-centered cubic structure at its melting point and a double hexagonal closed-packed structure at temperatures below its melting point. The isotope americium-241 emits a-particles and y-rays in its radioactive decay, and is a source of y-radiation, used to measure the thickness of metals, coatings, degree of soil compaction, sediment concentration, and so on. The same isotope, mixed with beryllium, is used as a neutron source in oilwell logging and other applications. Americium-241... 

The nuclide gCf emits neutrons through spontaneous fission in 3% of all decays, the rest being a-decays. All the other neutron sources listed involve a radioactive nuclide whose decay causes a nuclear reaction in a secondary substance which produces neutrons. For example, ffSb produces neutrons in beryllium powder or metal as a result of the initial emission of 7-rays, in which case there is no coulomb barrier to penetrate. Radium, polonium, plutonium, and americium produce neutrons by nuclear reactions induced in beryllium by the a-particles from their radioactive decay. For the neutrons produced either by spontaneous fission in californium or by the a-particle reaction with beryllium, the... 

Americium-241 is used in smoke detectors. It has a first order rate constant for radioactive decay of k = 1.6 X 10" yr . By contrast, iodine-125, which is used to test for thyroid functioning, has a rate constant for radioactive decay otk = 0.011 day", (a) What are the half-lives of these two isotopes (b) Which one decays at a faster rate (c) How much of a 1.00-mg sample of each isotope remains after 3 half-lives (d) How much of a 1.00-mg sample of each isotope remains after 4 days ... 

Americium-241 is used in smoke detectors. It has a first-order rate constant for radioactive decay of fc = 1.6 X 10 r . ... 

Gamma rays are electromagnetic radiation, that is, the release of excess energy from the nucleus of an atom retained after radioactive decay. For instance, americium-241 decays by emission of an alpha particle as discussed above, but it also emits a gamma ray at the... 

What complicates the environmental situation is that plutonium is not the only transuranium element produced in nuclear reactors. Curium and americium are also formed by multiple neutron capture (see Fig. 14.1). The amounts of long-lived actinides in spent fuel as a function of time after removal from a reactor are shown in Table 14.12. The elements americium and curium formed in the reactor undergo radioactive decay to produce radioactive daughter species [57] ... 

The effect of radioactive decay of the americium and curium is an increase in the intensity of radioactivity with time after removal from the reactor. For a period... 

RADIOACTIVE MATERIAL. A substance is labeled radioactive when it emits radiant energy resulting from the disintegration of atomic nuclei, or radioactive decay. The radiant energy can be emitted as alpha particles, beta particles, ganuna radiation, or neutron radiation. Some materials are naturally radioactive, such as the elements uranium and radium. Other radioactive materials are man-made elements, such as plutonium and americium. See also ISOTOPE. 

AH of the 15 plutonium isotopes Hsted in Table 3 are synthetic and radioactive (see Radioisotopes). The lighter isotopes decay mainly by K-electron capture, thereby forming neptunium isotopes. With the exception of mass numbers 237 [15411-93-5] 241 [14119-32-5] and 243, the nine intermediate isotopes, ie, 236—244, are transformed into uranium isotopes by a-decay. The heaviest plutonium isotopes tend to undergo P-decay, thereby forming americium. Detailed reviews of the nuclear properties have been pubUshed (18). 

Most smoke alarms (Figure 19.1, p. 517) use a radioactive species, typically americium-241. A tiny amount of this isotope is placed in a small ionization chamber decay of Am-241 ionizes air molecules within the chamber. Under the influence of a potential applied by a battery, these ions move across the chamber, producing an electric current. If smoke particles get into... 

Americium is released into surface water primarily from plutonium production reactors, nuclear fuel reprocessing facilities, or in nuclear accidents. It may also be released from radioactive waste storage facilities. Since 241Pu decays into 241 Am,241 Am is also released as a result of 241Pu releases. Water sampling data were used to estimate effluent releases from the SRS from the plant s start up in... 

ISOTOPES There are 24 isotopes of americium. All are radioactive with half-lives ranging from 72 microseconds to over 7,000 years. Five of americium s isotopes are fissionable with spontaneous alpha decay. 

Americium does not exist in nature. All of its isotopes are man-made and radioactive. Americium-241 is produced by bombarding plutonium-239 with high-energy neutrons, resulting in the isotope plutonium-240 that again is bombarded with neutrons and results in the formation of plutonium-241, which in turn finally decays into americium-241 by the process of beta decay. Both americium-241 and americium-243 are produced within nuclear reactors. The reaction is as follows Pu + (neutron and X gamma rays) —> " Pu + (neutron and X gamma rays) —> Pu—> Am + beta minus ([ -) followed by " Am—> jNp-237 + Hej (helium nuclei). 

---