Natural Radioactivity

Just how old is the Galaxy? Most people know that an answer to that question requires a collaborative effort between many groups of scientists. First, there are the scientists who develop the instruments that are capable of detecting the particles and energy streaming toward Earth from all of the corners of the Milky way Galaxy. In addition, there are the engineers who develop rockets to power those instruments into space, and the scientists who spend their time analyzing the data collected by the instruments. The purpose of this Educational Brief is to focus on one of the tools which they use as they conduct their research. That tool is cosmic ray clocks.

RADIOACTIVITY
In the everyday world of physical changes and chemical changes it is the electrons that circle the nucleus that are important. Each chemical reaction can be explained in terms of nature’s attempt to have every element possess a stable arrangement of those electrons. The interactions that occur result in negatively charged electrons being lost, gained, or shared by the elements. While physical changes and chemical changes are important, for the purposes of this Educational Brief we need to focus on nuclear changes (changes that happen in the nucleus of the atom).
For nuclear changes the number and arrangement of electrons can be ignored. Instead, it is the composition of the nucleus that is important. Other than hydrogen-1 which consists of just one positively charged proton (and its corresponding electron), each atom of every other element possesses a nucleus consisting of more than one positively charged proton, and one or more neutral neutrons. These protons and neutrons are referred to as nucleons.
For the most part the nuclei of the elements that make up the matter around us are stable, but some spontaneously change into other elements in one of a variety of ways. When these unstable atomic nuclei spontaneously breakdown, they are said to have decayed. During these changes the nuclei become new elements by emitting smaller atoms, sub-atomic particles, and energy; or by capturing nearby electrons. Collectively these processes are called natural radioactivity.
The major particles released during these changes are the alpha particle, beta particle and the positron. In addition to the previously mentioned particles, energy is released in the form of neutrinos, and gamma radiation. Each of the particles and energies are explained here:

ISOTOPES AND RADIOACTIVITY
All atoms having a nucleus with the same number of protons form a unique chemical element. If the number of protons in an atom changes, a new element is formed. But, when atoms of an element have different numbers of neutrons they form what is referred to as an isotope of that element.
Most of the elements occur naturally as more than one isotope. For example, helium occurs naturally as both helium -3 and helium -4. A helium-3 nucleus consists of two protons and one neutron while a helium-4 nucleus consists of two protons and two neutrons. The number of protons in an atom is called its atomic number, while the sum of the protons and neutrons is called its mass number. As you will see later on, it is the ratio of the number of neutrons to the number of protons that allows scientists to determine how stable an atom is.

STRONG NUCLEAR FORCE
As mentioned above, the protons found in the nucleus have a positive charge while the neutrons are neutral. These nucleons are very close together inside the nucleus. Immediately the question arises as to how more than one proton can co-exist with other protons and not be repelled by the electric force. The gravitational attraction between neighboring nucleons is much too small to account for their stability. Scientists have determined that another force, called the strong nuclear force, holds the nucleus together. This force is an attractive force which acts between all nucleons, both protons and neutrons alike.
For atoms with low atomic numbers, the ratio of neutrons to protons is very close to one. For the larger atoms as the number of protons increases the number of neutrons increases at a greater rate until the ratio is much greater than one. For the elements with more than 82 protons the electrical repulsion between the protons is too great for the strong nuclear force to hold the nucleus together. Consequently there are no completely stable nuclides above lead.

USING RADIOACTIVITY TO TELL TIME
The nice thing about natural radioactivity is that the decay of each radioactive isotope follows its own specific mathematical timetable. For each isotope scientists can determine the timetable. Understanding the timetable centers around learning about the concept of a half-life. Although it is not possible to determine exactly when a specific atom will decay, it is possible (using statistics) to predict the decay of a large number of radioactive nuclei. As it turns out, for a large number of radioactive nuclei, one-half of an original sample of the nuclei will decay in a specific period of time. That period of time is called the half-life for that isotope. At that point,
the isotope begins its second half-life. During the second half-life, one-half of the radioactive atoms present at the beginning of that half-life will decay. This process continues until (for all practical purposes) all of the radioactive atoms have decayed.
One important example of this process is used to determine the age of objects made from once-living matter. As the diagram below shows, cosmic ray protons (high energy protons that bombard the Earth ) strike the nucleus of an atom and generate a free neutron. The free neutron strikes a nitrogen-14 nuclei causing it to emit a proton and change into a radioactive carbon-14 atom. The carbon-14 atom then becomes part of the food cycle when it combines with an oxygen molecule to form a carbon dioxide molecule. Although the carbon-14 decays into nitrogen-14 by emitting an alpha particle (its half-life is 5730 years), the amount of carbon-14 remains constant while the organism is alive. The amount stays constant because the organism replenishes the decayed carbon-14 during its respiration (animals), or photosynthesis (plants).

The following table shows sample data for an organism that had ten grams of carbon-14 in its cells at the time that it died.

Similar information is available for other radioactive isotopes. In fact, another important radioactive isotope used in dating rocks here on Earth and meteorites from space is uranium -238. The succession of nuclear decays that eventually end with stable lead-206 contains more than a score of possible radioactive products (called daughter atoms). Such a sequence of decays is called a decay series. The uranium-238 series is shown below. Nuclei in the series are shown by a dot representing the number of neutrons (y-axis), and the number of protons (x-axis). Note that a diagonal line to the right indicates a beta decay, while a diagonal line to the left indicates an alpha decay.

It is actually these decay series which account for certain radioactive elements that are found in nature that would not otherwise be expected.
Once a specific radioactive decay series is known, and the half-life for a particular isotope is determined, scientists can use this information to calculate the age of a sample containing that isotope. This is done by comparing the amounts of the original radioactive atom (parent atom) present to the amount of daughter atoms present.

COSMIC CLOCKS
How then can we use our knowledge of radioactive decay and half-lives to determine the age of the Milky Way galaxy? As it turns out the answer may be found in cosmic ray particles. As mentioned earlier, cosmic ray particles are the particles that bombard the Earth from any location outside its atmosphere. Scientists classify cosmic rays depending on their source. The three categories are :

The cosmic ray particles that are radioactive can become excellent indicators of their origin, their age, and the mechanism which brought them to our solar system. For example, the atoms that decay by electron capture (a proton captures an electron and becomes a neutron) need to have orbital electrons (electrons found in the lower energy levels) available to make their transmutation (change to a new element). These atoms will not have the orbital electrons once they have been accelerated to speeds approaching the speed of light since the electrons will all have been stripped away. Once that occurs, these nuclei travel through space as a cosmic ray nucleus which is unable to decay. The amount of this isotope available in a given sample gives clues about the age and the origin of the sample.
In the case of the cosmic ray clocks one of the interesting isotopes is beryllium-10. The percentage of beryllium-10 found on Earth and in the solar system is quite small. Astrophysicists believe that it was not created during the Big Bang nor during stellar nucleosynthesis. ( An accompanying Educational Brief: Fusion & Nucleosynthesis, provides further explanation and can be found at: http://education.gsfc.nasa.gov/experimental/99invest.Site/science-briefs/ace/ed-fusion.html ). Instead, they believe it is created when cosmic rays (mainly carbon, nitrogen, and oxygen nuclei) collide with hydrogen and helium ions as they travel through space. Since beryllium-10 is radioactive (with a half-life of 1.6 million years), determining its relative abundance as compared with other cosmic ray isotopes could be a significant step toward calculating the age of the cosmic rays and the size of the Milky Way galaxy.

ACE AND COSMIC CLOCKS
The Advanced Composition Explorer (ACE) spacecraft is playing a role in reading the beryllium-10 clocks. ACE was launched in August of 1997 to make observations that are being used to test current theories on the creation and evolution of the Galaxy. The nine ACE instruments are performing measurements of particles and cosmic rays over a wide range of energies and nuclear masses. The instruments on ACE detect many of the heavier isotopes which originated during the formation, evolution, and subsequent explosion of stars. Since many of the instruments on ACE have a collecting power ten to one thousand times greater than instruments from previous missions, the data from ACE should go a long way toward answering questions about the origin and evolution of the universe.
To access a site that has animations about radioactivity go to:l
http://physics.bu.edu/cc104/decay_start.html

Dr. Eric R. Christian ACE Deputy Project
Scientist

Beth Jacob ACE Outreach Specialist