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Activity and Half-Life Of Elements
The activity of a sample of radioactive element is the rate at which its constituent nuclei decay or disintegrate.
This is measured in disintegrations per second or Curie (Ci) units, where 1 Ci = 3.7 × 1010 disintegrations per second
1 micro Curie (µ C) = 3.7 × 104 disintegrations per second.
The law of radioactive decay states that “the activity of a sample is proportional to the number of undecayed nuclei present in the sample”.
Each radioactive nuclide has a characteristic, constant half-life (t1/2).The half-life of a radioactive element is the time required for its one-half of the sample to decay.
An isotope’s half-life allows us to determine how long a sample of a useful isotope will be available, and how long a sample of an undesirable or dangerous isotope must be stored before it decays to a low-enough radiation level that is no longer a problem.
For example, cobalt-60, an isotope that emits gamma rays used to treat cancer, has a half-life of 5.27 years
Since every half-life for a radionuclide is the same length of time, we can use the following equation to calculate how much radioactive nuclide is remaining after the passage of any number (n) of half-lives:
Practice Problem:
Question: The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?
Solution:
Step 1. Determine the number of half-lives that have passed: number of half-lives = time passed divided by the half-life (Be sure that the time units match!!)
Step 2. Use the Isotope Remaining equation to solve for how much isotope will remain after the number of half-lives determined in step 1 have passed.
Nuclear equations
Particles making an atom can be written using upper and lower subscripts where a proton, ‘p’ with charge +1 and mass 1ᴜ, is written as .
A neutron ‘n’ with no charge but with mass 1ᴜ, is written as , while an electron with a charge of -1 and negligible mass is written as . It is important to note that the principles of conservation apply in radioactive decay.
That means that the total number of nucleons (neutrons + protons) must be the same before and after decay. The L.H.S of the equation must be equal to the R.H.S for both total mass and charge.
Effects of Radioactive Decay On The Nucleus
Write an equation to show how a radioactive isotope of cobalt ( o) undergoes a beta decay followed by the emission of gamma rays to form a new nuclide X.
Hazards Of Radioactivity And Their Precautions
(i) Due to the ionizing radiation emitted by radiation materials, they affect living cells leading to serious illnesses. Symptoms of radiation exposures are immature births, deformations, retardedness, etc.
(ii) Their exposure to the environment through leaks may lead to environmental pollution leading to poor crop growth and destruction of marine life.