Atoms and Radiation

Atoms and Radiation

1. Atomic Structure

   • Structure of the atom
     • Atoms contain three different types of subatomic particle called protons neutrons and electrons
       • The protons and neutrons are found at the centre of the atom in the nucleus
         • The nucleus is much smaller than the atom as whole
           • The electrons are arranged around the nucleus in energy levels
       • The proton has a relative mass of 1 and a charge of +1
       • The neutron has a relative mass of 1 and no charge
       • The proton has a relative of around a two thousandth and a charge of -1
       • The number of electrons in an atom is always equal to the number of protons so the atom has no overall charge
         • Atoms can lose or gain electrons turning them into electrically charged ions
           • If an electron is lost, the charge is positive
           • If an electron is gained, the charge is negative
   • Isotopes
     • The number of protons in an atom is called its atomic number
       • The atoms of a particular element have the same atomic number
       • Atoms of different elements have a different atomic number
     • The total number of protons and neutrons in an atom is called its mass number
     • Isotopes are different atoms of the same element with different numbers of neutrons
   • Evidence for atomic structure
     • An early model of the structure of the atom was called the plum pudding model
       • It was described as a sphere of positive charge with negatively charged electrons dotted around inside it
         • An experiment in 1905 proved this model to be incorrect
           • Scientist Ernest Rutherford designed an experiment to test the plum pudding model
             • A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected
               • It was expected that the alpha particles would pass straight through the foil but actually some of the particles emerged from the foil at different angles and some reflected back
                 • The scientists realised the alpha particles were being repelled and deflected back by a tiny concentration of positive charge in the atom
                   • As a result the new nuclear model of the atom was designed as a replacement

2. Nuclear Fission and Fusion

   • Nuclear Fission
     • Nuclear power reactors use nuclear fission with isotopes such as Uranium-235 and Plutonium-239
     • Nuclear fission is the process of splitting a nucleus
       • Uranium and Plutonium are used because their atoms are relatively large and easy to split
         • For fission to happen a neutron is fired at the nucleus so that neutron is absobred
           • Then the nucleus splits into two smaller nuclei releasing two or three neutrons
             • This releases energy
               • The neutrons will than go on to split other nuclei starting what is called a chain reaction
                 • Nuclear reactors control the chain reaction to stop it going too fast
   • Nuclear Fusion
     • Nuclear fusion involves two atomic nuclei joining to make a large nucleus which releases a huge amount of energy
       • This is how stars including the sun release energy using two hydrogen nuclei to form a helium nuclei

3. Nuclear Radiation

   • Background radiation
     • Background radiation is all around us
       • Some of it is natural i.e. cosmic rays, racks and soil, living things and food
       • Some of it is artificial i.e. radioactive waste, radioactive fallout, medical x rays
         • Despite the stigma attached to radioactive waste from power plants and radioactive fallout from nuclear weapons only 15 per cent of average background radiation comes from artificial sources nearly all of which is from x rays
   • Detecting radiation
     • Photographic film goes dark when it absorbs radiation simmilar to its reaction with visible light
       • People who work in places with radiation wear film badges which are checked regularly to onitor levels of radiation
         • Inside the badge there is a light-proof packet of photographic film inside the badge
           • To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film of which may include aluminium, copper, lead-tin alloy and plastic
             • There is also a open area at the centre of the badge
     • The Geiger-Muller tube detects radiation
       • Each time it absorbs radiation, it transmits an electrical pulse to a counting machine which either makes a clicking sound or displays the count rate
         • The greater the clicking frequency or the higher the rate the more radiation the Geiger-Muller tube is absorbing
   • Types of radiation
     • Alpha radiation consists of alpha particles which are identical to the nucleus of a helium atom comprising of two protons and two neutrons
       • An alpha particle, however, does not have any electrons
         • Alpha radiation is absorbed by paper
     • Beta radiation consists of high energy electrons emitted from the nucleus which have not come from the electron shells qround the nucleus
       • They form when a neutron splits into a proton and an electron which then shoots out of the nucleus at high speed
         • Beta radiation is absorbed by aluminium
     • Gamma radiation is very short wavelength, high frequency, electromagnetic radiation
       • It is stopped by lead
   • Half life
     • The nuclei of radioactive atoms are unstable which means they break down and change into a completely different type of atom
       • This is called radioactive decay
         • It is not possible to predict when an individual atom might decay but it is possible to measure how long it takes for half the nuclei of a piece if radioactive material to decay
           • Half life is described as the time it takes for the number of nuclei of the isotope in a sample to halve
   • Radioactivity safety
     • Alpha, beta and gamma radiation enters living cells and collides with molecules which causes ionisation; the damaging or destroying of molecules
       • Lower dosages don't destroy the cell but damage them which can give rise cancerous cells
         • Higher dosages kill cells completely causing radiation sickness if multiple cells are killed at once
           • The extent of the effects is dependent on both the amount of exposure and energy and penetration of the radiation
             • Outside the body Beta and Gamma radiation is the most dangerous because they can penetrate into the delicate and vital organs whereas alpha particles cannot penetrate skin
               • Inside the body alpha radiation is most dangerous because their damage is very localised whereas beta and gamma usually pass out the body before any damage is done
                 • To prevent radiation sickness precautions are taken including:
                   • If conducting experiments the radiation source should be used for as short a time as possible to minimise exposure
                   • Never touch a radiation source; always use tongs
                   • Hold the source at arm's length as, especially alpha, radiation does not travel far in the air
                   • Point the source away from the body
                   • Store the source in a lead box and if in contact regular (e.g. radiographers) wear an lead apron or stand behind a lead screen
                   • If someone needs an x-ray or radiotherapy, apply it only to the area being treated
   • Uses of radiation
     • Smoke detectors use alpha radiation
       • A weak source of alpha radiation is placed in the detector close to two electrons
         • The source causes ionisation and a current flows between the electrodes
           • If there is a fire then smoke will absorb the radiation so the current stops and the alarm sounds
     • Tracers in medicine use short half life beta or gamma emitters
       • Certain radioactive isotopes can be injected into people and their progress around the body can be followed using an external detector which produces a reading which is then converted into a display to show where the strongest reading is coming from
         • All isotopes which are taken into the body must be gamma or beta emitters so that the radiation passes out of the body
     • Radiotherapy uses gamma rays to treat cancer
       • High doses of gamma rays kill living cells so they are targeted directly at the cancerous cells at a high enough dosage to kill them
         • Some damage is done to normal cells making the patients feel sick but if it works then the cancer has gon
     • Steriiasation of food and surgical equipment can be done using gamma rays

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