P6 RADIOACTIVE MATERIALS

P6 RADIOACTIVE MATERIALS

1. NUCLEAR ATOM

   • an atom is the smallest part of an element. Neutrons + Protons form the nucleus + electrons orbit it at high speed in shells. Nearly all mass of an atom is concentrated in the nucleus. Proton number decides what element an atom is and neutron decides what isotope an atom is.
   • a strong nuclear force holds the protons + neutrons together as it has to balance the electrostatic repulsive force between the protons

2. STRUCTURE OF AN ATOM

   • 1906 experiment with alpha particle scattering by Rutherford-Geiger-Marsden where they fired alpha particles at a gold sheet.
     • observations- most alpha particles passed straight through, undeflected. Some alpha particles were scattered at small angles after passing through the gold sheet, and very few alpha particles were back scattered back
     • conclusions- that the atom is mostly empty space as most alpha particles passed straight through, that the mass + charge of an atom is concentrated in the nucleus + that the nucleus is positive because the positive alpha particles were repelled.

3. RADIOACTIVE ELEMENTS

   • are unstable + constantly emit ionising radiation which makes them more stable. Ionising radiation knocks out electrons from atoms to form a positive ion. Ionising radiation can either be high energy electromagnetic waves or high energy particles.
   • background radiation is low level ionising radiation that is all around us E.g. gamma radiation from ground + buildings, cosmic rays from space, food + drink, medical sources + radon gas from under ground
   • radioactivity is a random process, + you can never tell when an unstable nucleus will decay, but the amount of radiation emitted is dependant on the amount of radiation present. Radioactive material is UNAFFECTED by chemical + physical processes like weathering etc.

4. TYPES OF IONISING RADIATION

   • ALPHA
     • 2+ charge, heavy mass, deflected by magnetic + electric fields, highly ionising, low penetrating.
       • are massive + easily knock off electrons- so they lose energy faster + can only travel a few cm in air. They are stopped by skin or paper. They are HELIUM NUCLEI= 2 protons + 2 neutrons, charge of 2 + mass of 4.
   • BETA
     • -1, very light, deflected by the opposite magnetic + electric fields to alpha, medium ionisation + medium penetration
       • range in air about 1m but stopped by 3mm of aluminium. Is a fast moving electron= atomic mass of 0 + charge of -1--> 0/-1 e.
   • GAMMA
     • neutral charge, no mass, not deflected by electric or magnetic fields, low ionising, high penetrating.
       • not stopped by air, thick (m) of concrete/lead and is high frequency EM waves

5. HAZARDS OF IONSING RADIATION

   • ionising radiation collides with living cells + knocks electrons out of their atoms leaving positive ions. The intensity + scale of damage is dependant o intensity of radioactive source. High intensity radiation may kill living cells + cause tissue damage-> radiation sickness/ cell sterilisation. Lower intensity radiation-> affect genes of cells-> mutations->cancer
   • Alpha- not penetrating so safer outside than inside body as high ionising.
   • the equivalent dose of radiation absorbed is measured in Sievert (Sv) and 1 Sv of gamma, beta or alpha has the SAME biological affect on the body. Its measure of possible harm done to your body
   • oxygen, nitrogen + hydrogen are all susceptible to ionising radiation + are heavily abundant in the body. The charged ionising ions break + form chemical bonds to atoms + this can change their molecular structure

6. RADIOATCIVE DECAY

   • when a radioactive element emits an alpha or beta particle the number of protons + neutrons changes + the atom becomes a new element. The radioactivity of a material decreases after time because the amount of radiative nuclei decreases. This is called radioactive decay. The number of stable nuclei present determines the radioactivity/ activity of an element
   • many large nuclei are unstable + emit alpha particles to become more stable, they often also emit gamma rays but this doesn't cause it to change into a new element unlike Alpha particles. The new element is known as the daughter product + may or may not be radioactive. Most radioactive elects belong to a decoy chain where the 1st decays into the 2nd-> 3rd etc... Radioactive elements have unstable nuclei + have too much energy. Even after emitting alpha/beta it may still have too much energy so could emit gamma rays too.

7. HALF LIFE

   • is the time taken for half the nuclei in the sample of a radioactive element to decay. Its specific to each element + can take from seconds--> millions of years.
   • the activity of an element is a measure of its rate of decay. The activity can never reach zero its just gets more negligible

8. USING IONISING RADIATION

   • used in radiotherapy to kill cancer cells, gamma radiation usually used + honed in on tumour- some surrounding tissue can be damged.
   • sterilisation of medical equipment- better than boiling which can damage equipment. Kills microorganisms through irridation
   • sterilisation of food as microorganisms began the decaying process immediately. irritated with gamma radiation to kill microorganisms
   • radioactive tracer- e.g. for finding brain tumours. Gamma emitter with half life of a few hours is injected, radiation detected from outside + builds tumour picture.

9. KEEPING PEOPLE SAFE

   • IRRIDATION- exposure to radiation without contact with source CONTAMINATION- surface/person has come into contact with radioactive material
   • low level waste- sent to landfills
   • intermediate level waste- e.g. cases of used fuel rods stored in concrete blocks + steel canisters
   • high level waste- radioactive so generates heat, cooled for 50yrs in glass _ steel then buried deep underground
   • lead aprons,gloves, walls, film badge-> black + level of exposure

10. NUCLEAR FUSION

    • strong force keeps positive repulsive protons in nucleus with neutrons. When 2 nuclei combine-> larger nucleus=nuclear fusion. E.g. hydrogen+ hydrogen->helium
    • nuclei can only fuse if they overcome the repulsive electrostatic force + get close enough for the strong force to hold them together= lot of energy-> V HIGH TEMP.
    • energy is released + can be calculated by: E= mc2
      • c=constant speed of light 3 x 10 ^8

11. NUCLEAR FISSION

    • used in nuclear power stations to generate electricity.
    • a nuclear fuel such as uranium/plutonium releases large amounts of energy when its nuclei split. In fission neutron is fired at the nucleus of the fuel to make it unstable. The nucleus breaks down into 2 smaller nuclei + emits more neutrons. This starts a chain reaction, which can only be controlled by limiting neutrons
    • nuclear fission + fusion release more energy than any other chemical reaction because the binding energy that holds the nucleus together is larger than the force that holds electrons in place.
    • the fuel i kept in fuel rods which capture + emit neutrons as the fuel rod nuclei are split. The chain reaction is controlled by control rods which are lowered in + out + absorb neutrons+ slow down reaction. Coolants like water used to take away heat produced in fission as heat--> water->steam_.drives turbines+generator=electricty