Radioactivity – nuclear equations and decay chains

A presentation for Wed. in the

last week of Physics 125

Nuclear notation

• Z = atomic number or proton number, is the number of protons in the nucleus.

• N = neutron number, is the number of neutrons in the nucleus.

• A = Z + N = mass number, is the number of nucleons in the nucleus.

• In general, the notation is Z X N

• For example, 6 C6 has atomic mass 12.000

A

12

Periodic table – links to isotope data

• The standard periodic table is a useful way to organize isotope data:

• Lawrence Berkeley Lab interface: http://ie.lbl.gov/education/isotopes.htm (link)

• Lund University interface: (link)

• http://nucleardata.nuclear.lu.se/toi/perchart.htm

Chart of the nuclides

• All the nuclides may be charted on a single large chart, with the neutron number on the horizontal axis and the proton number on the vertical axis (or vice-versa):

• This has an advantage because it will allow us to visualize the decay schemes in a new way.

• Wall-size charts are available.

• Web-based charts are cheaper, and can link to massive amounts of data.

Beta decay on the chart of nuclides• Carbon-14 decays by negative beta decay to

nitrogen-14 and an electron (and a neutrino):

• 6C 7N + -1e + (- decay)

14 14 0

Z increases by one

Z

NN decreases by one

Beta decay on the chart of nuclides• Negative beta decay creates a daughter nuclide

to the upper left of the parent:

• 6C 7N + -1e + (- decay)

14 14 0

Z increases by one

Z = 7

N = 8N decreases by one N = 7

Z = 6

Beta-plus decay on the chart of nuclides• Oxygen-15 decays by positive beta decay to

nitrogen-15 and a positron (and a neutrino):

• 8O 7N + +1e + (+ decay)15 15 0

Z decreases by one

Z

N increases by one N

Beta-plus decay on the chart of nuclides• Positive beta decay creates a daughter nuclide

to the lower right of the parent:

• 8O 7N + +1e + (+ decay)15 15 0

Z decreases by one

Z = 8

N increases by one N = 7

Z = 7

N = 8

Electron capture (EC)• 4Be + -1e 3Li (and an X-ray)

• This process has the same result as + decay, except that no beta particle is emitted.

7 70

Alpha decay• Polonium-210 decays by alpha decay to lead-

206 and an alpha particle:

• 84Po 82Pb + 2He

• The proton number decreases (84 82) and the neutron number decreases (126 124).

• Each alpha decay reduces Z by 2 and N by 2.

• The daughter nuclide is diagonally down and to the left on the chart of nuclides.

• (link to BNL NUDAT) (decay mode shown here)

210 206 4

Decay chains• Heavy elements may decay by a series of alpha decays

in a decay chain. Beta decays also occur in the chain, and the chain may have branches when a nuclide has two modes of decay.

• There are three major decay chains, labelled by an important isotope on the chain: U-238, Th-232, U-235

• All these chains end with a stable isotope of Pb (lead).

• Natural ores of uranium and thorium contain all of the nuclides on the chain in some equilibrium concentration. The waste from processing these ores can be quite hazardous due to high activity of these nuclides.

Radioactive decay chain: Thorium-232

Radioactive decay chain: Uranium-238

Radioactive decay chain: Uranium-235

Decay chains for U-238 and Th-232

Decay chains• A web site has an animation of these: (disappeared!)

• Natural ores of the heavy radioactive elements will contain these three long-lived isotopes of uranium and thorium (U-238, U-235, and Th-232). Because the natural ore has been in the ground for a long time, the daughter products have increased to equilibrium concentrations which depend on the half-lives. Each of the products in the chain may contribute a similar amount to the total activity of the natural ore.

Qualitative picture of chart of nuclides• The stable nuclides are in a pattern that runs diagonally

through the unstable nuclides. • Nuclides far from the diagonal are less stable: they have

shorter half-lives. • Nuclides to one side decay by - decay, on the other side

they decay by + decay. • Heavy nuclides decay by alpha decay directly toward

nuclides closer to the center of stability. • Exotic decays: spontaneous fission, p, or n.

• Qualitative picture: (these use java controls) http://www.nndc.bnl.gov/nudat2/index.jsp (U.S. NUDAT site) http://amdc.in2p3.fr/jvnubase/jvNubase_en.html (France)

Chart of nuclides with half-lives (note those with t ½ over 1 hour)

A table of nuclides with half-lives t ½ over 1 hour is distributed by the Brookhaven National Laboratory in the U.S., and called “Nuclear Wallet Cards for Radioactive Nuclides”, by Jagdish K. Tuli. This is available as a printed booklet, a pdf file, and in a form to be loaded into a PDA (Palm Pilot) for use by emergency workers in the field. See http://www.nndc.bnl.gov/wallet/nwccurrent.html

This also contains two lists of isotopes of special interest:

“Nuclear Wallet Cards” from www.nndc.bnl.gov

“Nuclear Wallet Cards” from www.nndc.bnl.gov

Some more resources for radiation safety:

CDC radiation emergencies – isotopes, see: http://www.bt.cdc.gov/radiation/isotopes/index.asp

CDC radiation emergencies – http://www.bt.cdc.gov/radiation/index.asp

EPA radiation protection - information - http://www.epa.gov/radiation/

EPA link to fact sheets - http://www.epa.gov/radiation/radionuclides/index.html

Nuclear technology and its consequences.

I would like to make a few additional remarks about the nuclear fuel cycle and the basic physics involved in the production of fission products.

Some of these might show up in the environment, and will be very consequential if a nuclear accident or conflict occurs.

Sources of nuclear contamination

• Mining of uranium ores (and some other special cases).• Processing of uranium ores into uranium, thorium, and

other naturally-occurring actinides (radium, etc.) • Routine operation of nuclear power plants. • Spent fuel rods from nuclear power plants. • Isotopes produced in nuclear reactors by neutron

activation, used mostly in medical research and industrial radiography.

• Accidents in nuclear power plants. • Nuclear weapons: production, use, abuse, disposal.

Mining and processing of uranium ores

• Mining of uranium ores generates tailings with lower levels of uranium and its decay products. This creates problems similar to some other mining operations, except that the contamination is radioactive, rather than merely toxic (like lead or copper mining).

• Processing of uranium ores into uranium, thorium, and other naturally-occurring actinides (radium, etc.) can produce concentrated wastes which contain relatively long-lived isotopes in moderate concentration.

• To understand this issue, look at the decay chains and remember that most of the daughters will be present if the uranium is extracted, each with comparable activity.

Routine operation of nuclear power plants

• Nuclear power plants operate by the fission of uranium (and possibly plutonium) into fission products.

• This results in two main types of products: • Fission products, which occur when the U-235 is split

into two fragments with a range of masses. • Neutrons are generated, some of which participate in the

fission chain reaction by being absorbed by other uranium nuclei. But many neutrons are absorbed by other materials in the reactor, causing neutron activation of these elements. The neutron-activated material can build up huge activities (MCi) of long-lived isotopes.

• For detail, I have found some material in an older book on nuclear power. (use overhead).

Spent fuel, medical and industrial isotopes

• Spent fuel rods from fission reactors is, of course, a big problem, but unless an accident occurs, the material is being contained in ponds and will go to storage. Accidents in transport are usually not catastrophic.

• Accidents with medical isotopes have been notorious and well-publicized, but localized with only moderate numbers of casualties.

• (This is my opinion, I believe accidents in nuclear power reactors and nuclear weapons issues are of much more concern. Some pictures from Chernobyl are included on the next slides.)

Chernobylnuclearreactor,

Ukraine

April 26, 1986

Chernobyl reactor encased in

concrete and steel sarcophagus

Deserted city with reactors in the background. The area abandoned

is half the size of Colorado.

4400 people have died so far, about 7 million have ill health, and

over 150,000 abandoned their homes.