CHEM 1R03 Lecture Notes - Lecture 33: Scintillation Counter, Particle Accelerator, Positron
Chemistry Lecture Twenty-Three
Radioactivity (continued)
•A positron has a charge of +1 CU and negligible mass (anti-matter electrons)
•When an atom loses a positron from the nucleus, its mass number remains the
same and atomic number decreases by 1
•Positrons result from a proton changing into a neutron
•In this situation, the mass number stays the same and again the atomic number
decreases by one
•The following is an example:
•Electrons can also be captured which occurs when an inner orbital electron is
pulled into the nucleus
•No particle emission occurs but the atom changes which results similarly to positron
emission
•The proton combines with the electron to make a neutron where the mass number
will stay the same but the atomic number decreases by one
•For example:
•The following are examples on writing nuclear equations:
•Next there is artificial nuclear transformation and this involves changing one
element into another by bombarding it with small nuclei, protons, or neutrons
•Artificial transformation previously used to be done using alpha and beta particles
but now these reactions are done in a particle accelerator
•Elements with atomic numbers 93 and up are man-made (the transuranium
elements)
•There are also ways to detect radioactivity since the rays can cause air to become
ionized
•Geiger-Muller counter works by counting electrons generated when Ar gas atoms
are ionized by radioactive rays
•These rays cause certain chemicals to give off a flash of light when they strike the
chemical and a scintillation counter is able to count the number of flashes per
minute
Half-Lives and Other Uses
•Half-life: the length of time it takes one half of the radionuclides to decay
•Not all radionuclides in a sample decay all at once
•Half of the radioactive atoms decay after each half-life
•The radionuclide with the shortest half life will have the greater number of decays
per minute
•Even though the number of radionuclides changes, the length of time it takes for
half of them to decay does not —> the half life of a radionuclide is constant and
each radionuclide has it’s own unique half-life
•Radioactive dating is used in many fields, such as geography (minerals) and
archeological (fossils)
•It also has medical uses for example: radio-tracers —> certain organs absorb most
or all of a particular element
•One can measure the amount of tracer absorbed by using a suitable radioisotope of
the element and a radiation counter