Uranium alpha decay

Plutonium has a much shorter half-life than uranium (24.000 years for Pu-239 6,500 years for Pu-240). Plutonium is most toxic if it is inhaled. The radioactive decay that plutonium undergoes (alpha decay) is of little external consequence, since the alpha particles are blocked by human skin and travel only a few inches. If inhaled, however, the soft tissue of the lungs will suffer an internal dose of radiation. Particles may also enter the blood stream and irradiate other parts of the body. The safest way to handle plutonium is in its plutonium dioxide (PuOj) form because PuOj is virtually insoluble inside the human body, gi eatly reducing the risk of internal contamination. 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

ISOTOPES There are 37 isotopes of radon. All are radioactive. None are stable. They range in mass numbers from Rn-196 to Rn-228. Their half-lives range from a few microseconds to 3.8235 days for Rn-222, which is the most common. It is a gas that is the result of alpha decay of radium, thorium, or uranium ores and underground rocks. 

Actinium is the last (bottom) member of group 3 (IIIB) of elements in the periodic table and the first of the actinide series of metallic elements that share similar chemical and physical characteristics. Actinium is also closely related in its characteristics to the element lanthanum, which is located just above it in group 3. The elements in this series range from atomic number 89 (actinium) through 103 (lawrencium). Actiniums most stable isotope is actinium-227, with a half-life of about 22 years. It decays into Fr-223 by alpha decay and Th-227 through beta decay, and both of these isotopes are decay products from uranium-235. 

Uranium is the fourth metal in the actinide series. It looks much like other actinide metallic elements with a silvery luster. It is comparatively heavy, yet malleable and ductile. It reacts with air to form an oxide of uranium. It is one of the few naturally radioactive elements that is fissionable, meaning that as it absorbs more neutrons, it splits into a series of other lighter elements (lower atomic weights) through a process of alpha decay and beta emission that is known as the uranium decay series, as follows U-238—> Th-234—>Pa-234—>U-234—> Th-230 Ra-226 Rn-222 Po-218 Pb-2l4 At-218 Bi-2l4 Rn-218 Po-2l4 Ti-210—>Pb-210—>Bi-210 Ti-206—>Pb-206 (stable isotope of lead,... 

The most stable isotope of plutonium is Pu-244, with a half-life of S.OOxlO+ years (about 82,000,000 years). Being radioactive, Pu-244 can decay in two different ways. One way involves alpha decay, resulting in the formation of the isotope uranium-240, and the other is through spontaneous fission. 

Problem 8 The next step in the uranium-238 decay scheme is the emission of an alpha particle from thorium-230. Describe the mass number, atomic number, and element name for the resulting nucleus. 

When thorium emits alpha particles, it disintegrates into other daughter radionuclides (radioactive materials), such as radium-226 and radon-222 (from thorium-230 in the uranium-238 decay series) or radium-228 and thoron (radon-220 from thorium-232 in the thorium decay series). It eventually decays to stable lead-208 or -206, which is not radioactive. More information about the decay of thorium can be found in Chapter 3. The toxicological characteristics of radon, radium, and lead are the subject of separate ATSDR Toxicological profiles. 

Conservation of mass and charge are used when writing nuclear reactions. For example, let s consider what happens when uranium-238 undergoes alpha decay. Uranium-238 has 92 protons and 146 neutrons and is symbolized as After it emits an alpha particle, the nucleus now has a mass number of 234 and an atomic number of 90. 

Ernest Rutherford, however, used the alpha decay of uranium, which produces helium, to estimate the ages of several uranium ores in 1906. By measuring the ratio of helium to uranium and the current rate of helium production (that is, the current decay rate of uranium), he deduced that the minerals were at least 440 million years old. All this was, of course, before anyone knew anything about isotopes. 

Write out the complete formula for the alpha decay of a uranium nucleus with 238 nucleons. 

Uranium-238 decays to lead-206 through a series of alpha (blue) and beta (red) decays. 

Plutonium was the second transuranium element to be discovered The isotope 2 sPu was produced in 1940 by Seaborg, McMillan, Kennedy, and Wahl at Berkeley, California hy deuteron bombardment of uranium in a 150-cm cyclotron. Plutonium exists in trace quantities in naturally occurring uranium ores. The metal is silvery in appearance, but tarnishes to a yellow color when only slightly oxidized. A relatively large piece will give off sensible heal as the result of alpha decay. Large pieces are capable of boiling water. 

The radium isotope of mass number 226 occurs in the uranium (2n + 2) alpha-decay series. Its half-life is 1.620 years, and it yields radon-222 by o-disintegration. Other naturally occurring isotopes of radium are "v Ra in... 

Figure 7.5 Uranium and thorium decay series and the half-lives of each isotope. The chart begins with the most abundant U isotope (238U) and ends with stable lead isotopes (206,207,208p Alpha decay is denoted by the vertical arrows and beta decay by the diagonal arrows. (Modified from Griffin et al., 1963.)...

Because helium forms no compounds and is almost absent in the Earth s atmosphere, it was unknown for a long time. The first clue leading to its discovery was an unidentified yellow emission line in the solar chromospheric spectrum observed by French astronomer Pierre Janssen during an eclipse of the Sun in 1868. Lockyer named the unknown element helium for the Greek sun god, helios. Subsequendy it was discovered to be rather abundant in radioactive rocks, where it is trapped after emission from uranium series alpha decays. Ramsay and Soddy showed that the alpha rays were helium atoms whose electrons had been stripped away. In his biography of Lord Rutherford, A. S. Eve wrote ... 

The isotopes ofHe do notalways occur in all natural samples in their usual proportion. Because helium has only two stable isotopes, variations in their abundance ratio are usually attributed to 3He. But in cases where radioactive alpha decays have enriched 4He, that reason for 4He richness is usually fairly obvious. One example is He in rocks containing uranium. The 4He/3He ratio is about 100 times greater than solar in the Earth s atmosphere because the history of radioactive decay of uranium in the Earth (Rutherford ) and its outgassing has enriched our atmosphere in daughter 4He. 

Uranium was the first element shown to be radioactive. Complete the following reaction representing the alpha decay of uranium-238. 

When Uranium-238 undergoes one alpha decay and then one beta decay, the resulting isotope is... 

One prominent and well-known kind of nuclear component is that which is produced by the decay of naturally occurring radionuclides (see Table 1). The best- and longest-known examples are He, produced by alpha decay of the natural isotopes of uranium and Th, and " Ar, produced in one branch of the beta decay of " K. (There are several other natural radionuclides which produce He by alpha decay, but whether because of low parent abundance and/or very slow decay, only in very unusual samples is the production of He not strongly dominated by uranium and thorium.) Since radioactive decay laws are well known, the ratio of daughter to parent isotope(s) in a closed system is a simple function of time, whence this phenomenon has been long and extensively exploited as a geochronometer (e.g., see Chapter 1.16). 

Uranium-235 decays by alpha emission (half-life = (7.0381 0.0096) X 10 yr Jaffey et al., 1971) to Th, which decays (half-life = 1.06 d) to Pa (half-life = 32,760 220 yr Robert et al., 1969). Because of its short half-life, Th can be ignored for Pa dating applications. 

Uranium-238 decays through alpha decay with a half-life of 4.46 x 10 y. How long would it take for seven-eighths of a sample of uranium-238 to decay ... 

The primary use for plutonium (Pu) is in nuclear power reactors, nuclear weapons, and radioisotopic thermoelectric generators (RTGs). Pu is formed as a by-product in nuclear reactors when uranium nuclei absorb neutrons. Most of this Pu is burned (fissioned) in place, but a significant fraction remains in the spent nuclear fuel. The primary plutonium isotope formed in reactors is the fissile Pu-239, which has a half-life of 24 400 years. In some nuclear programs (in Europe and Japan), Pu is recovered and blended with uranium (U) for reuse as a nuclear fuel. Since Pu and U are in oxide form, this blend is called mixed oxide or MOX fuel. Plutonium used in nuclear weapons ( weapons-grade ) is metallic in form and made up primarily (>92%) of fissile Pu-239. The alpha decay of Pu-238 (half-life = 86 years) provides a heat source in RTGs, which are long-lived batteries used in some spacecraft, cardiac pacemakers, and other applications. 

Radon derives its name from the element radium. The gas is radioactive and is formed by radioactive decay processes deep in the earth. Uranium-238 decays very slowly to radium-226, which further decays by alpha particle emission to radon-222 (see Chapter 17). The half-life of radon-222 is 3.825 days. Other shorter-lived isotopes are formed from the decay of thorium-232 and uranium-235. Every square mile of soil to a depth of 6 inches is estimated to contain about 1 g of radium, which releases radon in tiny amounts into the atmosphere. 

Uranium-238 decays by alpha emission to thorium-234 in the first step of one series. Thorium-234 subsequently emits a beta particle to produce protactinium-234 in the second step. The series can be summarized as shown in Table 26-4a. The net reaction for the series is... 

Radioactive uranium-238 decays spontaneously in a long series of steps, the first of which is an alpha decay. When an alpha particle is emitted from a nucleus of uranium-238, the uranium atom loses two protons and two neutrons. The loss of protons makes the product another element, thorium, as shown in the following equation and in Figure 21.4. 

When a nucleus of uranium-238 undergoes alpha decay, a nucleus of thorium-234 and a nucleus of helium-4 are produced. The atomic numbers balance on the two sides of the equation, as do the mass numbers. 

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