Neptunium decay series

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

Table 4.4. Neptunium decay series (neptunium family) A = An+. ...

The final members of the decay series are stable nuclides ° Pb at the end of the thorium family, Pb at the end of the uranium-radium family, Pb at the end of the actinium family, and Bi at the end of the neptunium family. In all four decay series one or more branchings are observed. For instance, Bi decays with a certain probabihty by emission of an a particle into Tl, and with another probability by emission of an electron into Po. os-pj decays by emission of an electron into Pb, and Po by emission of an a particle into the same nuclide (Table 4.1), thus closing the branching. In both branches the sequence of decay alternates either a decay is followed by P decay or p decay is followed by a decay. 

The major decay paths for the naturally occurring isotopes of uranium and thorium are shown in Table 1. Other actinides of environmental importance include Np, Pu, Pu, and " Am. These have decay series similar to and overlapping those of uranium and thorium. Neptunium-237 (fy2 = 2.14 X 10 yr, a) decays to ° Bi through a chain of intermediates, emitting seven a- and four j3 "-particles. Plutonium-238 (h/2 = 86 yr, a) decays into an intermediate daughter on the decay series. 

Neptunium-237 is the parent nuclide of a decay series that starts with a emission, followed by p emission, and then two more a emissions. Write a balanced nuclear equation for each step. 

The terrestrial occurrence of Ac, Pa, U, and Th is due to the half-lives of the isotopes 235U, 238U and 232Th which are sufficiently long to have enabled the species to persist since genesis. They are the sources of actinium and protactinium formed in the decay series and found in uranium and thorium ores. The half-lives of the most stable isotopes of the trans-uranium elements are such that any primordial amounts of these elements appear to have disappeared long ago. However, neptunium and plutonium have been isolated in traces from uranium13 minerals in which they are formed continuously by neutron reactions such as... 

FlC- 1-2, The three naturally occurring radioactive decay series and the man-made neptunium series. Although (which is the parent to the actinium series) and (which is the parent to the thorium series) have been discovered in nature, die decay series shown here begin with the most abundant Icmg-Uved nuclides. 

A fourth long radioactive decay series, the neptunium series (Fig. 1.2), is conqxrsed of nuclides having mass numbers which divided by 4 have a remainder of 1 (the 4n + 1 series). The name comes from the longest lived A = 4n + I nuclide heavier than Bi, Np, which is considered as the parmt species it has a half-life of 2.14 X 10° y. Inasmuch as this half-life is considerably shorter than the age of the earth, primordial Np no longer exists on earth, and, therefore, the neptunium series is not found as a natural... 

There are lour radiooc.livt, decay series lhat, by sequences of u and emissions, result in the eventual tormalion ot a stable isotope of lead or bismuth Members of each series have mass numbers which are exactly divisible by 4 (the thorium series, 4n), or alter being divided by lour leave remainders of I (the neptunium series, 4n i-1),... 

The only nonnaturaUy occurring radioactive decay series, the neptunium series, has a formula ... 

Neptunium-237 decays by a series of steps to bismuth-209. How many a and [i particles are produced by this decay process ... 

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. 

Very small amounts of Np, as well as of Pu, have been discovered on earth the half-lives of Pu (in the 4/i -t- 3 series) is 2.411 x 10 y. Both isotopes are too short-lived to have survived the 4 eons since the solar system was formed. However, they are always found in minerals containing uranium and thorium and it is believed that the neutrons produced in these minerals through (o(,n) and (y,n) reactions with U and Th as well as by spontaneous fission of form the neptunium and plutonium through n-capture and 3-decay processes. The n-production rate in the uraniuniJ eisl pitchblende (containing —50% U) is about 50 n/kg s. The typical value for the " Pu/" U ratio in minerals is 3... 

Within a day, Abelson recalls, I established that the 2.3-day activity had chemical properties different from those of any known element. [It] behaved much like uranium. Apparently the transuranics were not metals like rhenium and osmium but were part of a new series of rare-earth-like elements similar to uranium. For a rigorous proof that they had found a transuranic the two men isolated a pure uranium sample with strong 23-minute U239 activity and demonstrated with half-life measurements that the 2.3-day activity increased in intensity as the 23-minute activity declined. If the 2.3-day activity was different chemically from any other element and was created in the decay of U239, then it must be element 93. McMillan and Abelson wrote up their results. McMillan had already thought of a name for the new element— neptunium, for the next planet out beyond Uranus—but they chose not to offer the name in their report. They mailed the report, Radioactive element 93, to the Physical Review on May 27, 1940, the same day Louis Turner sent Szilard his transuranic theories anticipation and discovery can cut that close in science. 

FIGURE 1.6 The artificial radioactive decay chain of the 4n -t 1 series, beginning with neptunium-237, deeaying through uranium-233 and ending with thallium-205. 

Elements with atomic numbers greater than uranium ( U), the element with the highest atomic number found in nature, are called transuranium elements. All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the University of California at Berkeley or at Argonne National Laboratory. By colliding uranium ( U) with neutrons, they produced uranium ( U), which days later decayed to neptunium (" "Np). 

---