Nuclear Reactions– changing the hearts of atoms

Syntheses of Transuranium Elements

Uranium has no stable isotopes, but both ²³⁵U and ²³⁸U isotopes remain on Earth because they have very long half-lives of 7.0410⁸ y and 4.510⁹ y respectively. A third isotope ²³⁴U is present as a decay product of ²³⁸U (t_1/2 2.4510⁵ y). Elements heavier than uranium are called transuranium elements^*, and until they were synthesized, they were mysterious and unknown.

• 
  Can transuranium elements be made?
  How to make them?
  How can they be identified?
  What are their chemical and physical properties?
  What are their nuclear properties?
  How unstable are they?
  

Looking at the periodic table at the dawn of nuclear age, making the unknown transuranium elements were a frontier that has never been explored. Their syntheses were a challenge, but their success would have been great scientific achievements. Using the particle accelerator, the Berkeley group in the United States made a great stride in this endeavour.

From 1940 to 1962, about 11 radioactive transuranium elements (almost 100 nuclides) have been synthesized, about one every two years. Representative isotopes of the 11 elements are neptunium (Np⁹³), plutonium (Pu⁹⁴), americium (Am⁹⁵), curium (Cm⁹⁶), berklium (Bk⁹⁷), californium (Cf⁹⁸), einsteinium (Es⁹⁹), fermium (Fm¹⁰⁰), mendelevium (Md¹⁰¹), nobelium (No¹⁰²) and lawrencium (Lw¹⁰³).

At this point, the Berkeley group led by Seaborg^ was particularly proud, because they have synthesized new elements to complete the actinide series, analogous to the 14 elements of the lanthanide series:

La⁵⁷ , Ce⁵⁸, Pr⁵⁹, Nd⁶⁰, Pm⁶¹, Sm⁶², Eu⁶³ , Gd⁶⁴ , Tb⁶⁵ , Dy⁶⁶, Ho⁶⁷, Er⁶⁸, Tm⁶⁹, Yb⁷⁰, Lu⁷¹

Ac⁸⁹, Th⁹⁰, Pa⁹¹, U⁹² , Np⁹³ , Pu⁹⁴ , Am⁹⁵, Cm⁹⁶, Bk⁹⁷, Cf⁹⁸ , Es⁹⁹, Fm¹⁰⁰, Md¹⁰¹, No¹⁰², Lw¹⁰³

Among these, large quantities (tons) of ²³⁹Np⁹³ and its decay fissionable product ²³⁹Pu⁹⁴ have been made in nuclear reactors by the reaction ²³⁸U (n, ) ²³⁹Np (see G.T. Seaborg and A.R. Fritsch, Scientific American, April 1963).

Beginning in the 1950s, substantial quantities of ²³⁹Pu were irradiated in nuclear reactors with high neutron fluxes leading to the successive capture of neutrons interspersed with negative  decays. This led to heavier and heavier isotopes of all the elements, in decreasing quantities. Newly synthesized nuclides were used as target material for neutron irradiation in order to make even heavier nuclides. They have synthesized most of the heavy elements including fermium ²⁵⁷Fm (half-life 100 d) this way.

Elements 93, 94, 96, 97, 98, and 101 were first created using neutrons from nuclear reactions that were made possible by a 60 inch cyclotron at the University of California at Berkeley from 1939 to 1961. Another heavy ion linear accelerator (HILAC) and an 88 inch cyclotron there enabled them to accelerate heavier particles. They used the nuclei of carbon and boron for the creation of heavy elements such as nobelium and lawrencium,

²⁴⁶Cm + ¹²C  ²⁵⁴No¹⁰² + 4 n,

²⁵²Cf + ¹⁰B  ²⁴⁷Lw¹⁰³ + 5 n,

²⁵²Cf + ¹¹B  ²⁴⁷Lw¹⁰³ + 6 n.

Multiple neutron captures occur virtually instantaneously in a thermonuclear explosion, increasing the mass number of the original uranium 238 atoms by various amounts. As a result, many transuranium nuclides are formed.

Skill Developing Questions

1. 
   What are transuranium elements?
   Why are their syntheses important?
   
2. 
   What isotope of transuranium elements has the longest half life?
   What are the nuclear reactions used to make isotopes of transuranium elements?
   (Check a table of nuclides, and you will be surprised by the number of long-lived nuclides in this group. Check other properties of some of the nuclides too.)
   
3. 
   How were elements 102 and 103 made?
   What was the significance of making these elements?
   

Syntheses of Transactinide Elements

Elements with atomic number greater than those of actinides are called transactinide elements. These are super heavy elements, and their syntheses are even more of a challenge because their half-lives are very short, making their isolation and detection very difficult. However, the difficulties have not discouraged humans from trying, and trying they did.

• 
  How can transactinide elements be synthesized?
  What are their nuclear properties?
  Which element might have long enough half-life for a successful detection?
  How to isolate the newly synthesized nuclides?
  

A

Transactinides

²⁴²Pu (²²Ne, 4n) ²⁶⁰Rf¹⁰⁴ rutherfordium
²⁴⁹Cf⁹⁸ (¹²C⁶, 4n) ²⁵⁷Rf<sup>104

249</sup>Cf (¹⁵N, 4n) ²⁶⁰Ha¹⁰⁵ hahnium

²⁴⁹Cf (¹⁸O, 4n) ²⁶³Sg¹⁰⁶ seaborgium

²⁶⁸Mt¹⁰⁹ ( , ) ²⁶⁴Ns¹⁰⁷ nielsbohrium

²⁰⁹Bi (⁵⁵Mn, n) ²⁶³Hs¹⁰⁸ hassium

²⁰⁸Pb (⁵⁸Fe, n) ²⁶⁵Hs<sup>108

272</sup>E¹¹¹ ( , ) ²⁶⁸Mt¹⁰⁹ meitnerium

²⁰⁸Pb (⁶⁴Ni, n) ²⁷¹Uun¹¹⁰ ununnilium

²⁰⁹Bi (⁶⁴Ni, n) ²⁷²Uuu¹¹¹ unununium
trivial question is often an important one. The chemical properties of transactinide elements should be similar to those of transition metal, because they are not actinides. For example, element 104 is in the group 4B (or 4 according to the International Union of Pure and Applied Chemistry) which consists of Ti, Zr, and Hf and element 104. The synthesis of element 104 was attempted in the former U.S.S.R. and the U.S.A.

In 1964, workers at Dubna (U.S.S.R.) bombarded plutonium with neon ions, and they suggested the reaction ²⁴²Pu (²²Ne, 4n) ²⁶⁰E¹⁰⁴. They expected ²⁶⁰E¹⁰⁴ to form a relatively volatile compound with chlorine (a tetrachloride), and they performed experiments aimed at chemical identification. They named it kurchatovium (Ku) in honor of Igor V. Kurchatov (1903-1960), late head of Soviet nuclear program.

I
²⁴⁹Cf⁹⁸ (¹²C⁶, 4n) ²⁵⁷Rf¹⁰⁴

n 1969 the Berkeley group reported that they had identified two, and possibly three isotopes of Element 104. Their attempts that far have not been able to produce ²⁶⁰E¹⁰⁴ reported by the Soviet groups in 1964. The Berkeley group used the reaction ²⁴⁹Cf⁹⁸ (¹²C⁶, 4n) ²⁵⁷Rf¹⁰⁴, which decays by emitting  particles with a half life of 4 to 5 s. The International Union of Pure and Applied Physics has proposed using the neutral temporary name, "unnilquadium", but the U.S. group named it rutherfordium (Rf).

I
²⁴⁹Cf (¹⁵N, 4n) ²⁶⁰Ha¹⁰⁵

n 1967 G.N. Flerov reported that a Soviet team at Dubna might have produced a few atoms of element 105 with masses 260 and 261 by bombarding ²⁴³Am with ²²Ne. Their evidence was based on time-coincidence measurements of alpha energies. The Soviet group had not proposed a name for 105. In late April 1970, Ghiorso, Nurmia, Haris, K.A.Y. Eskola, and P.L. Eskola, working at the University of California at Berkeley, announced their identification of Element 105. The synthesis was made by bombarding ²⁴⁹Cf with a beam of 84-MeV ¹⁵N ions from the Heavy Ion Linear Accelerator (HILAC). The reaction was ²⁴⁹Cf (¹⁵N, 4n) ²⁶⁰Ha¹⁰⁵. Its half-life was 1.6 s. They proposed the name hahnium (symbol Ha), after Otto Hahn (1879-1968). Other isotopes of Ha have been synthesized since then.

I
²⁴⁹Cf(¹⁸O, 4n)²⁶³Sg¹⁰⁶

n June 1974, members of the Dubna team reported their synthesis of Element 106. In September 1974, workers of the Lawrence Berkeley and Livermore Laboratories also reported the creation of Element 106 . These groups used the Super HILAC to accelerate ¹⁸O ions for the reaction ²⁴⁹Cf(¹⁸O, 4n)²⁶³Sg¹⁰⁶, which decayed by alpha emission to rutherfordium. At Dubna, 280-MeV ions of ⁵⁴Cr from the 310-cm cyclotron were used to strike targets of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, in separate runs. Foils exposed to a rotating target disc were used to detect spontaneous fission activities, the foils being etched and examined microscopically to detect the number of fission tracks and the half-life of the fission activity.

The syntheses of tranactinides have been summarized in the CRC Handbooks of Physics and Chemistry and some reactions are given in the Table. The stories and politics about the work on these elements are fascinating. More new elements are still being made, and there is an optimism for venturing even further into the region of super-heavy nuclides.

Skill Developing Questions

1. 
   What are transactinide elements?
   Why are their syntheses considered important?
   
2. 
   What are the reactions used to make isotopes of element 104 to 106?
   

Activation Analysis

Activation analysis (AA) is a method used to determine amounts of elements in samples. The method consists of irradiating the sample with subatomic particles and then measuring certain types of the induced radioactivity. The measured radioactivity is directly proportional to the amount of certain nuclide. A neutron, proton, alpha, or photon (gamma) source is usually used to irradiate the sample. Particles are used to induced X-ray emission or gamma-ray emission. Energy of neutrons varies from slow to fast depending on the element or nuclide to be determined. In sophisticated establishment neutrons of any desirable energy is available in order to get the best results. Neutron activation analyses (NAA) are particularly common.

• 
  What is activation analysis?
  How is activation analysis done?
  What are the applications of activation analysis?
  

Activation analysis determines elemental content regardless of the chemical states, chemical composition or physical location. Art work or other samples can be analyzed by NAA without destruction of the sample, and NAA is often called a non-destructive method.

The sample is first made radioactive by bombardment with suitable subatomic particles, then the radioactive isotopes created are identified and the element concentrations are determined by the gamma rays they emit. NAA is capable of detecting many elements at extremely low concentrations.

For an NAA quantitative determination, the sample is first weighted into a plastic or quartz container, sealed to prevent contamination, and then irradiated for a suitable period of time. Some isotopes of an element to be determined usually capture neutrons and become a radioactive isotope. The activated isotope is radioactive and by measuring the decays emitted, its quantity can be determined by comparison with known standard samples. In the core of nuclear reactors, trillions of neutrons pass through every square centimeter of the sample every second during the irradiation. Neutrons have no charge and will pass through most materials without difficulty. Therefore the center of the sample becomes just as radioactive as the surface with a few matrix problems.

Today, detectors used for AA and NAA are able to measure the energies and number of various particles (including photons) emitted from the sample. The measured spectra give reliable results after correcting for decay, sample size, counting time and irradiation time.

Improvements in detectors and radiation techniques have reduced, if not eliminated the requirement of chemical separations. Detection limits depend on the element as well as on other factors. Elements become very radioactive and can be determined at  low levels of parts per trillion. Using thermal neutrons, about 70 elements can be determined. For arsenic, a 5 nanogram in a sample can be determined.

E

xperiments used to explore chemical compositions of lunar and Martian surfaces are elegant applications of AA. Alpha particles from ²⁴²Cm and portable neutron sources have been used. In these applications, the source and detector can be mounted on the same wagon, and the radioactivity is measured immediately after the radiation.

For further information on AA, visit the following sites: http://www.chem.tamu.edu/services/naa/index.html

http://web.missouri.edu/~murrwww/archlab.htm.
http://www.research.cornell.edu/VPR/Ward/NAA.html

Skill Building Questions

1. 
   Describe the neutron activation analysis (NAA).
   
2. 
   What are some of the applications of NAA? (Trace element "fingerprinting" of archaeological specimens to determine their provenance (source) by neutron activation analysis; http://web.missouri.edu/~murrwww/archlab.htm.
   Hundreds of different types of material have been analyzed by the neutron activation analysis facilities at Ward Center. The following list includes some of the scientific, engineering, and industrial disciplines that have used neutron activation analysis at Ward Center: http://www.research.cornell.edu/VPR/Ward/NAA.html
   
3. 
   Chlorine has two stable isotopes, 75.77% ³⁵Cl and 24.23% ³⁷Cl. The thermal neutron cross sections are 44 and 0.4 b respectively. The half-lives for products ³⁶Cl and ³⁸Cl are 3x10⁵ y and 37.2 m respectively. Neglect the decay during irradiation, estimate the radioactivity when 10 nanograms of Cl is irradiated by neutrons whose intensity is 1x10¹⁵ neutrons cm^–2 s^–1 for 10 seconds. (Hint: rate =  N I if decay during irradiation is negligible, else, the reactivity = m/M N I (1 - e^–t); m is the weight of the sample, and M is the atomic mass.)
   

Problems

1. 
   What are nuclear reactions and how are they different from chemical and physical reactions? Give two examples of nuclear reactions and explain how the products can be identified.
   
2. 
   Is the reaction ¹⁴N + ⁴He  ¹⁷O + ¹H endothermic or exothermic? How much energy is absorbed or released in the reaction? Masses: H, 1.007825; n, 1.008665; He, 4.00260; ¹⁴N, 14.00307; and ¹⁷O, 16.99914. Conversion factor and constant: 1 amu = 1.66 x 10 27 kg, c = 3.0 x 108. m s 1 (velocity of light).
   
3. 
   Calculate the binding energy in J of ¹⁴N⁷ and ¹⁷O. How much energy is released in the formation of 14.0 g of N₂? Discuss your results. (1.678 x 10^-11 J for each atom of ¹⁴N)
   
4. 
   What methods have been used to produce neutrons? Give an example for each of the methods you have given.
   
5. 
   How can the nuclides ¹⁴C, ²⁴Na, ³²S, and ⁶⁰Co be produced?
   
6. 
   Describe the components of cosmic rays, and some nuclear reactions induced by cosmic rays.
   
7. 
   What is the mass of ¹⁴C if the  decay energy is 0.156 MeV? Calculate the energy of the ¹⁴N (n, p) ¹⁴C reaction. Masses: H, 1.007825; n, 1.008665; He, 4.00260; ¹⁴N, 14.00307; (Mass of ¹⁴C = 14.00307 + 0.156/931.4; and energy of reaction, 0.626 MeV)
   
8. 
   What are the products of these reactions, ¹⁴B ( ,), ¹⁸N ( , ), ⁹Be (⁶Li, p), ⁹Be (⁷Li, d), ¹¹B (, p), ¹²C (, d), ¹²C (t, p), ¹³C (t, d) and ¹³C (t, )?
   
9. 
   The total cross section for the reaction ⁵⁹Co (n, ) ⁶⁰Co reaction is 37 b (data from CRC Handbook of Chemistry and Physics). Calculate the mass of ⁶⁰Co produced when 1 kg of ⁵⁹Co metal is irradiated for 24 hours in a nuclear reactor where the neutron flux is 10¹⁵ neutron per square centimeter per second. Neglect the decay of ⁶⁰Co in your calculation.
   
10. 
    What elements with atomic number less than 83 do not have stable isotopes? How can these elements be produced?
    
11. 
    Describe how one of the elements with atomic number (Z) between 95 and 109 is made. You may have to search the literature in this case.
    

Further reading and work cited

Gibson, W.M., (1980), The physics of nuclear reactions. ?? (QC794.G48, 1980)

Satchler, G.R., (1990), Introduction to nuclear reactions. Macmillan 2nd Ed.

Hodgson, P.E., (1971), Nuclear reactions and nuclear structure. (QC794.H69, 1971)

McCarthy, I.E., (1980), Nuclear reactions. Pergamen Press (QC794.M17.1970)

R.B. Shirley and V.S.Hirley (1996), Table of Isotopes John Wiley & Sons, Inc.

Web Sites:

Useful Nuclear Reaction Data

National Nuclear Data Center, Brookhaven National Laboratory, Upton, NY 11973-5000 provides excellent data on nuclear reaction in great details.

http://www.nndc.bnl.gov/nndc/nndcnrd.html

Web sites about Activation Analysis:
http://www.chem.tamu.edu/services/naa/index.html
http://web.missouri.edu/~murrwww/archlab.htm.
http://www.research.cornell.edu/VPR/Ward/NAA.html

 John Douglas Cockcroft (1897-1967) and Ernest T.S. Walton (1903 1995) received the 1951 Nobel Prize for Physics for the development of the first nuclear particle accelerator, known as the Cockcroft-Walton generator.

* Particle collision researches led to the discovery of mesons and hyperons in the sub-disciplines nuclear physics and particle physics (or high-energy physics). The former studies the reactions induced by subatomic particles and properties of multi-nucleon systems whereas the latter studies the interaction among subatomic particles.

* Chadwick James (1891-1974) was awarded the Nobel Prize for Physics in 1935 for the discovery of neutrons.

 Enrico Fermi (1901-1954) developed the mathematical statistics, discovered neutron-induced radioactivity, directed the first controlled chain of nuclear fission, and received the 1938 Nobel Prize for Physics.

 Emilio Gino Segrè (1905-1989) cowinner with Owen Chamberlain (1920-) of the Nobel Prize for Physics in 1959 for the discovery of the antiproton.

* For more on transuranium elements visit the URL: www.tricity.wsu.edu/~ustur/

 Element 106 created at LBL in 1974 and confirmed in 1993 has been named seaborgium in honor of Nobel Laureate (1955, chemistry with Edwin Mattison McMillan) Glenn Theodore. Seaborg (1912-1999), with its chemical symbol of Sg in 1994. See
http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1994/seaborgium-mag.html