Helium production from radioactive decay

An alplia p uticle is an energetic helium nucleus. The alplia particle is released from a radioactive element witli a neutron to proton ratio tliat is too low. The helium nucleus consists of two protons and two neutrons. The alplia particle differs from a helimn atom in that it is emitted witliout any electrons. The resulting daughter product from tliis tj pe of transformation lias an atomic number Uiat is two less tluin its parent and an atomic mass number tliat is four less. Below is an e. aiiiple of alpha decay using polonium (Po) polonium has an atomic mass number of 210 (protons and neutrons) and atomic number of 84. 

Joseph Lockyer (1836-1920) was one of the pioneers of solar spectroscopy. In examining the spectra of solar prominences in 1869, Lockyer noticed an absorption line that he could not identify. Reasoning that it represented an element not present on Earth, he proposed a new element - helium, from the Greek word helios for Sun. This idea failed to achieve acceptance from Lockyer s scientific colleagues until a gas having the same mysterious spectral line was found 25 years later in rocks. The helium in terrestrial uranium ore formed as a decay product of radioactive uranium. Thus, this abundant element was first discovered in the Sun, rather than in the laboratory. Lockyer s cosmochemical discovery was recognized by the British government, which created a solar physics laboratory for him. Lockyer also founded the scientific journal Nature, which he edited for 50 years. 

Helium-3 is a decay product of radioactive tritium (3H, half-life = 12.44 years) that has been produced by nuclear bombs as well as naturally by cosmic rays in the upper atmosphere. Because virtually all 3He atoms escape from the surface ocean to the atmosphere, the 3He/tritium ratio in subsurface seawater samples indicates the time since the water s last exposure to the atmosphere. Both 3He and tritium are measured by gas mass spectrometry. Alternatively, tritium may be measured by gas counting with a detection limit of 0.05 to 0.08 tritium unit, where 1 tritium unit represents a 3H/H ratio of lxl0 18. A degassed water sample is sealed and stored for several months to allow the decay product 3He to accumulate in the container. The amount of 3He is then measured by mass spectrometry, yielding a detection limit of 0.001 to 0.003 tritium unit when 400-gram water samples are used. With this technique, the time since a water mass left the surface can be determined within a range from several months to 30 years. 

Helium is the second most abundant element in the universe. In the Earth, it is continuously formed by radioactive decay, mostly of uranium and thorium. Its present concentration in the atmosphere is probably the equilibrium concentration between the amount being released from the Earth s crust and the amount of hehum escaping from the atmosphere into space. The atmosphere represents the major source for neon, argon, krypton, and xenon. They are produced as by-products during flactional distillation of liquid air. Radon is obtained from the radioactive decay of radium. 

Radioactive elements in rocks (see Section 2.8), most importantly potassium (K) and heavy elements such as radium (Ra), uranium (U) and thorium (Th), can release gases. Argon (Ar) arises from potassium decay and radon (Rn, a radioactive gas that has a half-life of 3.8 days) from radium decay. The uranium-thorium decay series results in the production of a particles, which are helium nuclei. Once these nuclei capture electrons, helium has effectively been added to the atmosphere. 

Atmospheric 4He is a product of radioactive processes taking place in the Earth s crust. About 98 % of the helium is produced by the decay of two radioactive isotopes 238U and 232Th. The rate of 4He formation can be estimated from the lithospheric quantity of these isotopes and the escape rate of the helium formed. We can assume that helium formed in the Earth s mantle cannot escape into the air. Then only 4He produced in the crust reaches the atmosphere. This quantity is estimated to be 7 x 106 m3 STP per year (Junge, 1963). 

Radium-224 is radioactive and decays by emitting an a-particle. (a) Write an equation for this process, (b) The decay of radium-224 produces helium gas. Rutherford and Geiger determined that a-particles were emitted from ggRa at a rate of 7.65 x lO s moU, and that this corresponded to a rate of helium production of... 

The noble gases occur as minor constituents of the atmosphere (Table 17-1). Helium is also found as a component (up to 7%) in certain natural hydrocarbon gases in the United States. This helium undoubtedly originated from decay of radioactive elements in rocks, and certain radioactive minerals contain occluded helium which can be released on heating. All isotopes of radon are radioactive and are occasionally given specific names (e.g., actinon, thoron) derived from their source in the radioactive decay series 222Rn is normally obtained by pumping off the gas from radium chloride solutions. Ne, Ar, Kr and Xe are obtainable as products of fractionation of liquid air. 

The presence of radioactive decay products In lead sources or tracers obtained from natural sources is undesirable for some applications. A number of lead Isotopes can be produced in carrier-free form and In high yield by helium-Ion bombardment of mercury or by protnn or deuteron bombardment of thallium targets. The isotopes prepared in this way and the relevant reactions are indicated In Table I. 

Helium is produced in radioactive decay processes and emitted as a-radiation. Therefore it is found in all uranium minerals. The helium-method for the age determination of minerals is also based on the formation of a-particles. Consequently, as a decay product of radioactive processes, helium is also found in many natural gases. The largest deposits of He-rich natural gases are located in the USA, Siberia, Algeria and Canada (see Table 4.1). In Europe, helium occurs in Polish natural gas with a molar fraction of about 0.4% and in gas from the North Sea with a molar fraction of up to 0.12% [4.2]. Table 4.2 gives an overview of the worldwide production of He and its reserves. 

Radionuclides differ from other nuclei in that they emit ionizing radiation—alpha particles, beta particles, and gamma rays. The most massive of these emissions is the alpha particle, a helium nucleus of atomic mass 4, consisting of two neutrons and two protons. The symbol for an alpha particle is shown as the product of Reaction 4.10. An example of alpha production is found in the radioactive decay of uranium-238 ... 

Principal gaseous products from fission and radioactive decay are xenon, krypton, and helium. Using the SCALE/ORIGEN program, the whole core inventories of these elements after 10 years of operation were estimated to be 217 g of xenon, 24 g of krypton, and 1 mg of helium. A considerable amount of these elements would be expected to be entrained in the foam. Consequently, a core clad breach due to pressurization is not credible, and consequences of clad mpture would be relatively insignificant as compared to a commercial power reactor. 

The fusion of hydrogen into helium provides the energy of the hydrogen bomb. The helium content of the atmosphere is about 1 part in 200,000. While it is present in various radioactive minerals as a decay product, the bulk of the Free World s supply is obtained from wells in Texas, Oklahoma, and Kansas. The only known helium extraction plants, outside the United States, in 1984 were in Eastern Europe (Poland), the USSR, and a few in India. 

---