TOPIC 10 (F5)

  5.1 UNDERSTANDING THE NUCLEUS OF AN ATOM

1. Composition of the nucleus of an atom
a)  Each atom has a very small and very dense core called nucleus.
b) Most of the mass of atom is contained in the nucleus
c)  A nucleus consists of a number of protons and neutrons.
d) Protons and neutrons also known as nucleons.
e)  A proton has a unit positive charge.
f)   A neutron is an uncharged particle of about the same mass as the proton.
g) The electrons move in orbits around the nucleus.
h) An atom is neutral because it contains an equal number of positively 
    charged (protons) and negatively charged (electrons).  So the net 
    charge is zero   
    
2. Proton number, Z,
a)  is defined as the number of protons in a nucleus
b) The number of electrons = the number of protons in neutral atom.
c)  An element is identified by its proton number
     
3. Nucleon number, A
a)  is defined as the total number of protons and neutrons in a nucleus. 
b) Number of neutrons, N = A - Z

4. Nuclides
a)  Is an element with same number of protons but different number 
    of neutrons.
b) Nuclide notation
 
            c)   Example
 
5. Isotopes
a)    are atoms with the same proton number but different nucleon number.
b)   Isotopes of an element contain the
                        i.        same number of protons
                      ii.        same number of electrons
                     iii.        have same chemical properties reactions (involve the electrons)
                     iv.        have different physical properties (mass is different.)
c)    Can exist naturally or made artificially.
d)   Isotope with unstable nucleus
                        i.      is called radioisotopes
                      ii.      tend to decay
e)    Example:


5.2  ANALYSING RADIOACTIVITY

1.    Radioactivity is the spontaneous disintegration of unstable nucleus into more stable nucleus with the emission of energetic particles or photons.
2.    The decay occurs randomly
3.    Unstable nucleus has a big number of proton and number of nucleon
4.    Three types of nuclear radiation emitted during decay
            a)    Alpha, α
            b)   Beta, β
            c)   Gamma, γ

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   5.    Due to their different charges and masses, they have different
           a)    Ionising power (ions per mm in air)
              Radiation which has a stronger ionizing power will have a lower 
              penetrating effect.

           b)   The nuclear radiation can be stopped by:

 
           c)  Effect on electric field


        d)  Effect of magnetic field

6.  Nature and properties of the nuclear radiation


7.  Common detectors for radioactive emissions

 
RADIOACTIVE DECAY

1.    Radioactive decay is the process by which unstable atomic nuclei emit subatomic particles or radiation
2.    During the decay, its nucleus breaks up, emits an alpha particle or beta particle and energy, and forms a new atom of a different element.
3.    Parent nuclide X changes into a daughter nuclide Y.
4.    Three types of radioactive decay : 
       - alpha decay, 
       - beta decay 
       - gamma decay
 
8.    Radioactive decay series
a)    Sometimes the daughter nuclide of a radioactive is still unstable. It will eventually decay into another nuclide which is also unstable.
b)   This process continues as a radioactive decay series until a stable nuclide is reached.
c)    Each decay will emit either an alpha particle or a beta particle and may be gamma rays.


HALF-LIFE

1. Half-Life is the time taken for half the atoms in a given sample to decay
2. The concept of Half-Life
    a) The decay is a random process.
    b) As the time increases,
         - the number of atoms remaining decreases 
         - the number of atom disintegrating increases
3. After a certain period of time, half of initial number of atoms have 
    disintegrated and the other half remains.
4. A typical decay curve for radioisotope

5. Example:
    a) 
 
b) A radioactive source gives a reading of 640 counts per second when
    measured using G-M tube. One day later, the reading drops to 40 counts
    per second. What is the half life of this source?


Exercise:

1.    An artifact recovered from an archeological site was found to have 
    1/64 of its initial amount of radioactive carbon. If the half-life of the radioactive carbon is 5600 years, determine the age of the artifact. 
      (33600 years)
2.    A radioactive material has half-life of 48 seconds. Its activity is 16 count/second. What was its activity 6 minutes 24 seconds ago? 
     (4096)


5.3 UNDERSTANDING THE USES OF RADIOISOTOPE

1. Properties of radioisotopes which make them suitable in many application
    a)    Emits radioactive radiation.
    b)   Radioactive radiation can kill cells.
    c)    Radioactive radiations have different penetrating ability with materials 
         of  different thickness and densities.
   d)   Can cause cell mutation
   e)    Can ionise molecules
   f)     Activity decreases with time

2. Applications of radioisotopes are in the field of
    a)    Industries
    b)   Medicine
    c)    Agriculture
    d)   Archaeology

 
APPLICATION IN INDUSTRIES
1. Thickness control



a)    To control the thickness of paper, plastic, clothes and metal.
b)   Place the radioactive source (gamma ray for  metal sheets, beta particles for paper, plastic and clothes) at one side of the material
c)    The detector (G-M tube) at the other side


Food containers control
a)    To ensure  that the food containers (cans and food packages) fill to the specific amount.
b)   Place the radioactive source (beta particles) at one side of the material 
c)    The detector (G-M tube) at the other side
d)   When the containers is inadequately filled, the detector will register a higher reading


Detecting leaks in underground water pipes


a)    adding sodium-24 into the water in the storage tank.
b)   moving the detector (G-M tube) above the underground pipe
c)    the leakage can be detected when the G-M tube detects a higher reading.

Determine the degree of wear and tear of engine parts 
- added radioisotopes to engine oil


Smoke detectors


a)    Contain a weak radioactive source such as Americium-241.
b)   Alpha particles emitted from the source and ionized air molecules.
c)    The ionized air molecules conduct electricity and a small current flows in the smoke detector.
d)   When smoke enters the detector, the soot particles in the smoke absorb some of the alpha particles.
e)    the current that flows in the smoke detector decrease and trigger the alarm.
f)     Americium-241 has a long half-life, 460 years so that the substance will last longer.


APPLICATION IN MEDICAL FIELD


Tests and treats overactive thyroid glands or certain kinds of thyroid cancer.
- Intravenous injection of iodine-123 (half-life 13 hours) or Iodine-131 
  (half-life 8 days)

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Detect the position of blood clots / thrombosis
- Inject sodium-24 into the bloodstream

Detect and treats brain tumor
- Using phosphorous-32

Cobalt-60 (emits gamma ray) is used to
- Destroy cancer cell in radiotherapy 
- Sterillise medical equipments


It is important to administer the correct dosage because
- Excessive radiation will damage too many healthy cells.
- Insufficient radiation will not stop the cancer from spreading.

 
APPLICATION IN AGRCULTURE       

Kill pests using radioactive rays
- Used gamma rays

Stop the pest from reproducing.
- Expose the pests to low dosage of gamma rays.
- Induce mutation in the pest.

Study the effectiveness of fertilisers 
- Used nitrogen-15 or phosphorus-32
- Added radioisotopes to the soil water.
- The radioisotopes can track uptake of fertiliser from root to leaves in plant.
- Scientists can find out how much fertilizer has been absorbed by the plant.

Induce genetic mutation in plant
- Use radioactive radiation to induce genetic mutation.
- In order to produce a better strain (has a higher resistance against diseases).


APPLICATION IN ARCHEOLOGY       

Determine the age of artifacts (carbon dating)

a)    By comparing the activity of the dead sample with the activity of same mass of living sample.
b)   Carbon-14 is a radioisotope with a half-life of 5730 years and decays 
     by emitting beta particles.
c)    Living animals and plants have a known proportion of carbon-14 in 
     their tissues which remains constant.
d)   When living things die, the amount of carbon- 14 in their body decreases
     at a known rate.
e)   The amount of carbon-14 left in a decayed plant or animal can be used
      to tell its age


5.4 NUCLEAR ENERGY

Nuclear fission

1. is the splitting of a heavy nucleus into two lighter nuclei.
2. occurs when the nucleus of an atom is bombarded with a neutron.



3. The energy of the neutron causes the target nucleus to
    - split into two (or more) nuclei that are lighter than the parent nucleus,
    - releasing a large amount of energy during the process.

4. Examples:
   - A neutron hitting a uranium-235 nucleus, causing it to split producing
     xenon-143, strontium-90 and three neutrons + energy.
  - Or a neutron hitting a uranium-235 nucleus, causing it to split producing barium-141, krypton-92 and three neutrons + energy


5. In the fission reactions,

a)    One neutron starts the fission process,
b)   The nucleus is divided into two large fragments of roughly equal mass
c)    Three neutrons are produced
d)   There is a significant mass defect
e)    If one of these neutrons bombards another uranium-235 nucleus
f)     more fissions will occur and releasing more neutrons.
g)   A chain reaction is produced.
h)    Example:
                      i.        As uranium atoms continue to split,
                    ii.        a significant amount of energy is released during each reaction,
                   iii.        the heat released is greatest
                   iv.        A controlled chain reaction is used in nuclear power stations to
                 generate electrical energy.
          v.    An uncontrolled chain reaction is used in nuclear bombs



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Nuclear fusion
 1. Nuclear fusion is believed to be the process  by which energy is 
     released by the Sun.
 
3. Hydrogen bomb uses the principle of  nuclear fusion for its design.

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Atomic mass unit , a.m.u 
a)    is the unit of mass for atoms and subatomic particles such as the proton,
b)   neutron and electron
c)    1 atomic mass unit or 1 u is   1   of the mass of the carbon-12 atom
                                                         12
  
d)   The mass of one carbon-12 atom   

e)    Nuclear fission and fusion release a large amount of energy.
f)     The source of this energy is from the loss of mass in nuclear reactions.
g)   The sum of the masses of the particles before the reaction is more than
    the sum of the masses of the particles after the reaction.

i)     A loss of mass in a nuclear reaction means that the mass had
   changed to energy.
j)     The relationship between the mass and the energy:

 

 



         








Example 1

Below is an equation for the decay of radium-226 (Ra-226)



(a) Find the mass defect in
            (i)  a.m.u
          (ii)  kg
(b) Calculate the amount of energy released in
            (i)  J
          (ii)  eV


Answer:

(a) mass defect
            (i)  In a.m.u
= 226.02536 – (222.01753 + 4.00260)
= 226.02536 – 226.02013
= 0.00523 a.m.u


 (ii) In Kg
= 0.00523 x 1.66 x 10-27 Kg
= 0.00868 x 10-27 Kg
= 8.68 x 10-30 Kg

(b) energy released in
            (i)    Joule
  E = mc2
= 8.68 x 10-30 x (3.0 x 10 8)2
= 78.1362 x 10-14
= 7.81 x 10-13 J

          (ii)    eV
= 7.81 x 10-13 J
   1.66 x 10-19 J
= 4.70 x 106 eV


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