Caltech Senior Physics Laboratory
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The 7th edition of the Table of the Isotopes contains so much material that it is difficult to read (even more so with the 8th). This guide describes 60Co in some detail, with the relevant tables and charts for 60Co reproduced and attached to this writeup. Short comments are supplied for other commonly encountered isotopes, namely, 137Cs, 57Co, 22Na, 24Na, l06Ru, and 228Th, with selected entries for them also attached. Be sure to examine the introductory section of the "TABLE" for detailed descriptions of format, notation, and abbreviations. Please note that the program "PCNudat" contains much of this information in electronic format. Also do familiarize yourself with the 8th edition CD-ROM. The WWW version of this guide (that you are reading now) provides some graphical examples from the CD-ROM.
There are several entries for each isotope, both tables of data and diagrams. The first useful data is at the beginning of the Mass=60 section. It is a simple diagram that tells you which of the A=60 nuclei exist or have been artificially produced. It also gives some information about the decay of each system; for example, 60Co decays to 60Ni and is about 2.8 MeV/c2 more massive (as can be seen by reading the scale to the left of the diagram). This diagram also tells you that an excited, isomeric level of 60Co decays to 60Ni with a half-life of 10.47 minutes, while the ground state decay has a half life of 5.271 years. The isomeric level decays 99.76% of the time to the 60Co ground state via an isomeric transition, IT, and only .24% of the time to 60Ni. (References: (1) p. 369-373, (2) p. 229-234, (3) p. 274)
Now you want to look at the diagrams for the daughter isotope, 60Ni. There are three diagrams, do not look at the main entry (part 2). We are interested in the diagram which lists the isotopes decaying to this one (part 1). It includes the parent and daughter nuclei, the levels in the daughter which are the final product of the decay, and the subsequent g-ray emissions. The decays are labelled by the type of decay [e.g., b -, b +, and electron capture (EC)] and the relative probability of each mode. The 60Co level with the 10.47 minute half life is labeled as a meta-stable state m , which means that it is not the 60Co ground state but is a long - lived excited state. The bold entry over the g-ray transitions are their energies, the italic entries are the branching ratios for the decay if more than one g decay is possible. The spin, and parity of each level in 60Ni is listed on the left side of the line (e.g.,. 0+, 2+ etc.). If the lifetime of a level is known it will also be listed. This diagram is not complete. It only includes the levels in the daughter which can be reached by the radioactive decay of 60Co and 60Cu. There may be other levels in-between those shown. You will have to look at the main entry if you want a complete listing of levels.
The main diagram for 60Co does not list any information about radioactive decay (ref. (3), p. 275). It does list the nuclear levels, their energies, spins, parities, and lifetimes. The vertical lines connecting states identify the known g ray transitions; their energies, if not given, can be calculated by taking the difference between the energies of the levels. We are seldom interested in this information because long lived radioactive decays come from the ground state of the parent nucleus. The higher energy states have very short lifetimes (pico-seconds, femto-seconds, or shorter) unless they are meta-stable, but even these will have decayed in a few minutes.
137Cs has an additional subtlety that you must be aware of. You can start in the usual way, which is to look at the A=137 diagram (ref. (3), p. 1242). From this you will find that 137Ba is the daughter and the important decay information will be listed there. The diagram lists the various decays and their branching ratios, but it does not point out that the de-excitation of the .661 MeV level in Ba is rather unusual. It involves a large spin change (11/2- to 3/2+) and will be hindered so that another de-excitation mechanism, Internal Conversion, is possible. Instead of emitting a g-ray, the nucleus de-excites by knocking out one of the atomic electrons with kinetic energy equal to the decay energy. It is usually the K shell electron that is internally converted because it has the greatest wavefunction overlap with the nucleus. The hole that is left behind will be repopulated by an electron from a higher atomic shell, usually the L shell, and this L to K transition is accompanied by an X - ray. To find out how probable internal conversion is you have to look at the tabular information for 137Ba under ~prod~ or products of the decay. The 0.662 MeV g-ray is a product of the decay and is produced 90.11 % of the time. The remaining 10% goes into internal conversion, listed under e/g.
134Cs b decays to 134Ba with a half-life of 2.065 years. Because the ground state of 134Cs is 4+, it decays predominantly to the 4+ states of 134Ba. 27.28% of the decays go to the 1.970 MeV level, and 70.23% to the 1.401 MeV level. These levels subsequently decay by g - ray emission, yielding strong lines at 0.569, 0.802, and 0.796 MeV. (The 4+, 2+, 0+ sequence is a very common decay sequence in nuclei. look for it in the attached diagram. ref. (3), p. 1208)
57Co decays by electron capture to the 136.47 keV level of 57Fe. This level subsequently decays by g emission 10.68% of the time to the ground state, and 85.6% of the time to the 14.4 keV level. The de-population of the 14.4 keV level proceeds by a combination of g emission and internal conversion (e/g = 8.2) yielding an X - ray with each electron. This is a standard Mössbauer Effect source, and that is also frequently used for excitation of X - ray fluorescence. Reference (3), p. 254.
22Na decays by positron emission and electron capture yielding a single g - ray at 1.274 MeV. 22Na in vacuum is an excellent source of positrons, otherwise (especially when in close proximity to material) the positrons annihilate and produce .511 Mev g-rays. [See Experiment 14 ] Reference (3), p. 43.
24Na is made by neutron activation, (23Na + n -- > 24Na). Only the ground state lives long enough to be used as a calibration source (14.96 hours). It is a source of a uniquely high energy g - ray (Eg = 2.754 MeV). Reference (3), p. 52.
207Bi decays by electron capture to 207Pb with a half-life of 31.55 years. The decay leads to an unusual level in 207Pb that is almost stable, even though it is the excited state of the nucleus. This isomer of 207Pb has an 0.805 second lifetime due to its unusual spin (13/2). The g - ray decay to the ground state is hindered because it is an M4 transition. Instead of emitting a g - ray, the isomeric level decays by Internal Conversion (See 137Cs ) and generates a monoenergetic electron which has 1.06366 MeV of kinetic energy. 207Bi also produces g-rays at 0.569 and 1.770 MeV, but these are of less interest than the electron line.
106Ru and 228Th are examples of radioactive decay series. The parent has a long half life, but may not directly yield any interesting (useful) emissions. The daughters, on the other hand, produce useful decay products. In the case of 106Ru, the daughter's (106Rh) half life is only 29.8 seconds (ref. (3), p. 802), so short that it is frequently overlooked in the Table of Isotopes. 228Th, another parent of a decay series, is a long lived daughter (1.91 years) of yet an even longer decay series starting with 232Th (1.405x1010 y). 238U (4.468x109 y) is still another series. The Uranium and Thorium series are shown in the attached diagrams. The 238U series involves a sequence of 16 daughters before it reaches an end point at 209Pb. The 232Th series has 9 daughters (including 228Th) and terminates at 208Pb. The parent nuclei each have very long half-lives ( > a billion years). but the daughters have a wide variety of half-lives (milli-seconds to years), and are found in the environment in various equilibrium concentrations. The Uranium and Thorium series are responsible for the Radon gas that emanates from stone and concrete buildings, and are responsible for most of the naturally occurring lead found on the earth.
106Ru b - decays 100% of the time to the ground state of 106Rh and does not yield any useful particles other than very low energy electrons. But the daughter, l06Rh, is unstable and decays to l06Pd yielding electrons, with a maximum end-point energy of 3.54 MeV, which can be used for calibration of plastic scintillators and spectrometers. Reference (3), p. 807.
In this decay, a neutron inside the nucleus is decaying to a proton + an electron + an anti-neutrino (n - > p +e - +
228Th decays with a half life of 1.91 years, proceeding through a chain of five a and two b decays before arriving at stable 208Pb. All of the intermediate daughters have short half-lives (the longest being 10.6 hours), so that their abundance is not determined by their half lives, but rather by the abundance of the parent 228Th (i.e., the daughters are in equilibrium - they are produced at the same rate that they decay.) A detailed review of the radiations emanating from a 228Th source requires that each and every daughter be looked up in the Table of Isotopes.
Briefly, the main g - rays emitted occur at the following energies (energies in MeV and percent abundance):
0.084 (25%), 0.115 (5%), 0.239 (88%), 0.300 (5%), 0.510 (8%), 0.583 (30%), 0.727 (3%), 0.860 (8%), and 2.615 (36%).
The a - particles are emitted at the following energies:
5.6856 (95%), 6.06 (35%), 6.2883 (99%), 6.7785 (100%), and 8.7844 (64%).
Check the latest literature for updated numbers.
This document last updated 12 July, 2000
137Cs - [g - ray, X - ray, Conversion Electrons, b - spectrum]
134Cs - [g - rays]
57Co - [g - rays, X - rays, Conversion Electrons]
22Na - [g - rays, Positronium/Annihilation Radiation]
24Na - [g - rays]
207Bi - [electrons and g - rays]
Radioactive Series - 106Ru and 228Th
106Ru - [b - spectrum, g - rays]
). The total energy released in the decay will depend on the difference in binding energy between the parent and daughter nuclei with this energy manifested as the kinetic energy of the electron and anti-neutrino. (The proton also receives part of the decay energy but is so massive in comparison to the electron that the recoil energy is negligible.) The spectrum of energies available to the electron ranges between 0 and the total energy of the decay (3.54 MeV). It is most probable for the neutrino and electron to carry away about the same amount of energy and so the spectrum is peaked at mid-energies. 228Th - [g - rays and a - particles]
Glossary Of Terms:
References:
Selected Decay Schemes
