Protactinium isotopes

Subsequently, a wide array of developments in TIMS methods for uranium-series measurement occurred during the past decade including initiation of methods for measurement of long-lived radium (Volpe et al. 1991 Cohen and O Nions 1991) and protactinium isotopes (Pickett et al. 1994 Bourdon et al. 1999), development of improved sources or ionization methods for TIMS analysis, and introduction of commercially available multi-collector TIMS instruments designed specifically for uranium and thorium isotopic measurement. 

In a study of the long-lived protactinium isotopes produced from thorium bombarded by high energy deuterons or helium ions (175), pieces of thorium metal of 25 mil thickness were used to increase the total yield of the protactinium. On the other hand, when the time for chemical separations had to be shortened in order to study the short-lived protactinium Isotopes, thinner pieces of thorium, 5 mils or less in thickness, were used to ensure rapid dissolution. In some cases, thorium nitrate powders wrapped in aluminum were used as the target in order to reduce further the time for dissolution of the target. 

Twenty-nine isotopes of protactinium with measured half lives are known. All are radioactive. (A more detailed explanation of protactinium isotopes can be found in the Discovery and Naming section.)... 

The actinium decay series consists of a group of nuclides whose mass number divided by 4 leaves a remainder of 3 (the 4n + 3 series). This series begins with the uranium isotope which has a half-life of 7.04 X 10 y and a specific activity of 8 X 10 MBq/kg. The stable end product of the series is ° Pb, which is formed after 7 a- and 4 /3-decays. The actinium series includes the most important isotopes of the elements protactinium, actinium, ftancium, and astatine. Inasmuch as U is a conqx>nmt of natural uranium, these elem ts can be isolated in the processing of uranium minerals. The longest-lived protactinium isotope, Pa (ti 3.28 X 10 y) has been isolated on the 100 g scale, and is the main isotope for the study of protactinium chemistry. Ac (t 21.8 y) is the longest-lived actinium isotope. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were previously discussed by Lally (1992), Ivanovich and Murray (1992), and Chen et al. (1992). Because ICPMS methods were not discussed in preceding reviews and have become more commonly used in the past decade, we also include some theoretical discussion of ICPMS techniques and their variants. We also primarily focus our discussion of analytical developments on the longer-lived isotopes of uranium, thorium, protactinium, and radium in the uranium and thorium decay series, as these have been more widely applied in geochemistry and geochronology. 

Radium and Protactinium. TIMS protocols for both radium and protactinium currently involve cycling the isotopes Ra and Ra (tracer or normal), and Pa and... 

The age equation. The Pa age equation, calculated assuming no chemical shifts in protactinium or uranium and an initial Pa/ U = 0, is analogous to the Th age equation (Eqn. 1), but simpler. There is no term analogous to the term because there is no long-lived intermediate daughter isotope between and Pa ... 

We will show below that the initial Pa/ U = zero assumption holds for a number of corals that have typical low Th concentrations. Initial Pa/ U values for most other carbonates have not been studied in detail. Furthermore, in contrast to thorium, there is no long-lived isotope of protactinium that can be used as an index isotope although some work has employed corrections for initial Pa. Such corrections essentially assume that Th is an isotope of protactinium and assume a bulk earth Th/ U ratio and secular equilibrium between Pa and U. The term for applying this correction is analogous to the initial Th term in Equation (3). 

Initial Pa/ U levels are more difficult to assess, primarily because there is no long-lived or stable isotope of protactinium that can be used as an index isotope. Edwards et al. (1997) analyzed a set of surface coral sub-samples younger than 1000 years by both °Th and Pa techniques. For all samples, Th concentrations were less than 100 p g so that initial °Th/ U values were negligible. Each sub-sample yielded °Th and a ages identical within analytical errors (Fig. 8), indicating that initial Pa/ U was negligible. This suggests that surface corals with typical Th values do not require corrections for initial Pa. Whether or not corals with elevated Th require such corrections is an open question. 

Cheng H, Edwards RL, Mttrrell MT, Benjamin TM (1998) Uranium-thorittm-protactinium dating systematics. Geochim Cosmochim Acta 62 3437-3452 Cherdyntsev W, Kazachevskii IV, Kttz mina YA (1965) Age of carbonate determined from the isotopes of thorium and uranium. Geochem Int 2 749-756... 

Radioactive, silvery metal of which only about 125 g exists worldwide, isolated from reactor material. Protactinium occurs in the decay series of 238U (K. Fajans) as 234Pa. It also occurs in that of 235U this isotope, 231Pa, was discovered by L. Meitner and 0. Hahn. The element is only of scientific interest. 

The element with 91 protons is protactinium (Pa). The isotope jPA also undergoes beta decay,... 

ISOTOPES There are a total of 30 isotopes of protactinium. All are radioactive, and none are stable. Their decay modes are either alpha or beta decay or electron capture. Their half-lives range from 53 nanoseconds to 3.276x10+ ears. 

Protactinium is a relatively heavy, silvery-white metal that, when freshly cut, slowly oxidizes in air. AH the isotopes of protactinium and its compounds are extremely radioactive and poisonous. Proctatinium-231, the isotope with the longest half-life, is one of the scarcest and most expensive elements known. It is found in very small quantities as a decay product of uranium mixed with pitchblende, the ore of uranium. Protactiniums odd atomic number (gjPa) supports the observation that elements having odd atomic numbers are scarcer than those with even atomic numbers. 

All the isotopes of protactinium are highly radioactive poisons and therefore very dangerous. 

The most important radioactive isotope of neptunium is Neptunium-237, with a half-life of 2.l44xl0+ years, or about 2.1 million years, and decays into protactinium-233 through alpha decay. Neptunium s most important use is in nuclear research and for instruments designed to detect neutrons. 

The most common isotope of protactinium is Pa (tj/2 = 3.3 x 10 years), which occurs in pitchblende in the amount of 300 mg/ton, about the same as radium. The heroic efforts of British researchers resulted in the isolation of some hundred grams of Pa from the sludge left over from uranium processing without this supply, little or nothing would... 

Protactinium is a very dangerous substance to work with. It is highly toxic and presents a radiation hazard (alpha emitter). The Pa-231 isotope is a long-lived alpha-emitter which is not excreted out readily. Exposure can cause cancer. 

X 10 yr) and ends with stable ° Pb, after emission of eight alpha (a) and six beta (jS) particles. The thorium decay series begins with Th (ti/2 = 1.41 X 10 °yr) and ends with stable ° Pb, after emission of six alpha and four beta particles. Two isotopes of radium and Th are important tracer isotopes in the thorium decay chain. The actinium decay series begins with (ti/2 = 7.04 X 10 yr) and ends with stable Pb after emission of seven alpha and four beta particles. The actinium decay series includes important isotopes of actinium and protactinium. These primordial radionuclides, as products of continental weathering, enter the ocean primarily by the discharge of rivers. However, as we shall see, there are notable exceptions to this generality. 

Pa, protactinium, was first identified in 1913 in the decay products of U-238 as the Pa-234 isotope (6.7 h) by Kasimir Fajans and Otto H. Gohring. In 1916, two groups, Otto Hahn and Lisa Meitner, and Frederick Soddy and John A. Cranston, found Pa-231 (10 years) as a decay product of U-235. This isotope is the parent of Ac-227 in the U-235 decay series, hence it was named protactinium (before actinium). Isolation from U extraction sludges yielded over 100 g in 1960. 

To remove radium and other radioactive constituents from pitchblende, Hahn and Meitner treated pulverized pitchblende repeatedly and for long periods of time with hot concentrated nitric acid. From the insoluble siliceous residue they separated a new radioactive substance, which they called protoactinium. This name has subsequently been shortened to protactinium. When they added a little tantalum salt to a solution containing protactinium, the reactions of the new substance so closely resembled those of tantalum that Hahn and Meitner were unable to separate the two substances (118). Since tantalum is not radioactive, the protactinium could thus be obtained free from other radioelements. Since protactinium is not an isotope of tantalum, it should be possible to separate them from each other (119). By working up large quantities of rich pitchblende residues from the Quinine Works at Braunschweig, Hahn and Meitner were able to extract more active preparations of the new element (49). 

Uranium-238 emits an alpha particle to become an isotope of thorium. This unstable element emits a beta particle to become the element now known as Protactinium (Pa), which then emits another beta particle to become an isotope of uranium. This chain proceeds through another isotope of thorium, through radium, radon, polonium, bismuth, thallium and lead. The final product is lead-206. The series that starts with thorium-232 ends with lead-208. Soddy was able to isolate the different lead isotopes in high enough purity to demonstrate using chemical techniques that the atomic weights of two samples of lead with identical chemical and spectroscopic properties had different atomic weights. The final picture of these elements reveals that there are several isotopes for each of them. 

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