Nuclear Physics

A short history Aristotle emphasised that nature consisted of four elements: air, earth, fire, and water. Atoms were initially proposed by John Dalton in the 1700’s. J.J. Thomson held that atoms are uniform spheres of positively charged matter in which electrons are embedded like a plum pudding.(1800’s) Rutherford established the presence of the nucleus(1911)

The Rutherford scattering Experiment Results The atom is mainly empty space. There is a small centre of positive charge. Results The atom is mainly empty space. There is a small centre of positive charge. http://www.youtube.com/watch?v=5pZj0u_XMbc

The Rutherford scattering Experiment Positive alpha particles are targeted at thin gold foil. Most pass through, a few are deflected. Conclusion The atom is mostly empty space. The atom has a small centre of positive charge.

Explanation of how spectra are formed. E=hf

Spectra When electrons are given energy they jump to a higher state. The absorbed energy is released when they return to the ground state. The greater the enegy jump the greater the frequency of the light. This causes spectral colours.

Spectra Absorption spectra- Certain wavelengths are absorbed from a continuous spectrum as it passes through a material. Emission spectra- Specific wavelengths are emitted by a hot substance. Types of spectra Line spectra= hot gas Continuous spectra= hot solids Spectroscopy is the method of identifying elements by their spectrum. Bunsen and Kirchoff discovered that no two elements emit the same spectrum.

Revision of some terms….. The atomic number is the number of protons in the nucleus.(Z) The mass number is the number of protons and neutrons in the nucleus.(A) Isotopes are forms of the same atom that have different numbers of neutrons.

Radioactivity Discovered by Becquerel in 1896 It is the spontaneous disintegration of the nucleus by emission of one or more types of radiation. There are 3 types of radiation which is proved experimentally by passing radiation through magnetic or electric fields. Alpha, Beta and Gamma

To distinguish between the 3 types of radiation

Properties of alpha α or Positive charge Size of a Helium nucleus (2 protons and 2 neutrons) Good ionisers Poor penetrators Alpha emission reduces the mass number by 4 and the atomic number by 2.

Properties of Beta β or Negative charge Medium ioniser. Medium penetrator. Beta emission increases the atomic number by one and the mass number remains unchanged. This is because a neutron forms a proton and an electron (beta)

Properties of gamma γ Electromagnetic radiation Good penetrator. Poor ioniser. Does not affect the mass or atomic numbers.

To demonstrate the ionizing affect of radioactivity Procedure: Bring a radioactive source close to the cap of a charged Gold Leaf Electroscope. Observation: Leaves collapse. Conclusion: The charge on the electroscope became neutralised by the ionised air.

The Geiger Muller tube Mica window Counter http://www.youtube.com/watch?v=bcjMOr-qiwA When radiation enters the detector it ionises the gas inside. A voltage exists between the central axis and the cylinder wall. Negative ions are attracted towards the positive anode and travel through the meter, registering a “count”.

To Measure background radiation Note the count after 1 minute on the geiger counter. Divide by 60 to find the activity in Becquerels.

The Penetrating power of alpha, Beta and gamma

To examine the penetrating power of the 3 types of radiation Get the background count rate. Place the alpha source in front of the detector and note the count rate. Move the detector away from the source in small steps and calculate the average count rate at each step. Continue until count rate equals background count rate. Repeat for Beta source and Gamma source. Note We could also have tested the penetrative ability of the different sources by placing different materials between source and detector. We would find that a few sheets of paper would stop Alpha, Aluminium would be required for Beta, while lead is necessary for Gamma radiation. the activity. a sheet of paper in front of the source and note the activity. It is less. Repeat putting a sheet of aluminium and then a block of lead. In each case the activity drops as the alpha, beta and gamma radiations are stopped by the paper, aluminium and lead respectively.

Activity The activity of a sample is the number of nuclei that decay per second. The unit of activity is the Becquerel.

The Law of Radioactive Decay The activity of a sample is proportional to the number of nuclei present. The Half Life This is the time taken for the activity to decrease by half.

Half Life

The equations

Note after n half lives of the original sample remain. A radioactive element has a half life of 5 years. What fraction of the sample will have decayed after 20 years? Note after n half lives of the original sample remain.

The mole A mole of any substance is the amount of the substance that contains as many particles as there are atoms in 12g of carbon 12. The atomic mass of any element expressed in grams contains 6.02X1023 atoms.

To examine the ionisation caused by nuclear radiation Charge an electroscope. Bring the radioactive source close to the cap. The leaves collapse as the ions neutralise the charge on the electroscope. Radioactive source

Nuclear Fission Fission is the splitting of a large nucleus with the release of energy. Uranium-235 is a fissile element.

Chain Reaction Each U-235 produces 3 neutrons when it splits. For a chain reaction to occur each fission must produce further fission. To achieve this Uranium enriched with U-235 is used.

The Fission Reactor Boron Graphite Uranium Concrete Graphite or heavy water.

The Fission Reactor Control rods absorb neutrons and so control the reaction The moderator slows the neutrons (thermal neutrons) to allow them to be absorbed by U235 The fuel is Uranium enriched with U235

Conservation of Mass Energy Einstein proposed that mass and energy were simply different ways of looking at the same thing. Mass can turn to energy and vice versa. He proposed that the amount of energy created from mass is found using the formula E = mc2 1g of matter was converted to energy in the nuclear bomb that exploded in Hiroshima in 1945

Environmental impact Mining Uranium releases Radon gas into the atmosphere Radioactive waste Accidents-Chernobyl and Fukushima Nuclear reprocessing-transport of spent rods

AND THIS ……..

Nuclear Fusion This is the joining of two small nuclei to form a larger nucleus with the release of energy.

Fusion Advantages There is a lot more energy released during fusion than fission Hardly any waste produced Disadvantages Fusion will only occur at temperatures of 10-15 million Kelvin Not currently profitable

Fusion powers the sun

Uses of Radioisotopes Medical imaging Medical therapy Food irradiation Carbon dating Smoke detectors Checking material thickness