Nuclear Physics: Radiation, Radioactivity & its Applications

Nuclear Physics: Radiation, Radioactivity & its Applications

Nuclear Energy • The Nucleus of an atom contains • Protons – Positively Charged • Neutrons – no charge • Atomic Mass Number – denoted by the letter A, this number represents the total number of protons + neutrons in the nucleus, telling you what isotope of the element you have. • Atomic Number – denoted by the letter Z, this number represents the number of protons in the nucleus, telling you what element you have.

Nuclear Energy • Atomic Symbol for a given isotope of an element is generally given as noted to the right. • A prime example is an alpha particle or helium nucleus

Nuclear Reactions • Two Types of Nuclear reactions produce vast amounts of energy according to Einstein’s famous equation E = mc2 • Fission – the splitting of an atom into smaller parts • Fusion- the joining of two small nuclei to produce one larger nucleus

Nuclear Reactions • Mass defect – is the amount of mass that is converted to energy during fission or fusion. • Calculation of Mass defect is the difference between the actual mass of the atom and the known mass of each of its parts • The amount of energy that this mass is converted into is called the binding energy

Sample Problem Calculate the mass defect and energy released in the creation of Carbon-13. Solution Expected Mass: Protons = 6 (1.007276 u) = 6.043656 u Neutrons = 7(1.008665 u) = 7.060655 u 13.104311 u Known Mass -13.003355 u Mass Defect .100956 u Energy Released = (931.5 MeV/u)(0.100956 u) = 94.04 MeV

Radioactivity • Three types of Radioactivity • Alpha – α – is the nucleus of a helium atom • Can be stopped by a sheet of paper, is harmful only if ingested • Beta – β – emission of an electron or positron • Can be stopped by a sheet of lead, is harmful to all living tissue • Gamma – γ – emission of a high energy photon • Cannot be completely stopped. Very harmful to all living tissue.

Nuclear Reactions • Alpha Decay • Beta Decay • Gamma Emission

Half Life • The half life of a radioactive material is the amount of time required for ½ of the sample to decay into another element or isotope. • Half lives are calculated according to the equation: a = a0(½)x

Half Life • a = amount of material left at any time • a0 = amount of material that you begin with • x = the number of half lives that have passed since you have begun counting • This type of decay is said to be exponential since it can be described graphically as a hyperbola

Sample Problem Carbon-14, a radioactive isotope of carbon, has a half life of 5730 years. If a 20 gram sample of carbon-14 is allowed to decay for 10,000 years, how much remains at the end of this period?

Solution a = a0(½)x a0 = 20 grams x = 10,000 yrs/5730 yrs/half life = 1.75 So a = 20 grams(½)1.75 = 5.95 grams

Detection of Radiation • Counters • Geiger Counter – Radiation causes a gas to emit electrons causing a voltage which makes the counter “click” • Scintillation counter – uses a solid, liquid, or gas scintillator – a material which is excited by radiation to emit light. The light is captured and amplified by a Photomultiplier (PM) tube – which turns it into an electric signal. • Semi-conductor detector – uses a p-n junction diode which produces a short electric pulse when irradiated

Detection of Radiation • Trackers • Photographic emulsion – the particle passing through the emulsion ionizes atoms in its path • Cloud chamber – a gas is cooled to a temperature slightly below its normal condensation temperature hence it condenses on any ionized molecule present this “tracks” the particle • Bubble chamber – a liquid is kept close to its boiling temperature and hence “bubbles” around any ionized particle – the bubbles are then left in the wake of the particle and photographed

Applications of Nuclear Processes • Energy can be released in a nuclear reaction by one of two processes: • Fission – the splitting of a nucleus into smaller nuclei • Fusion – the joining of two smaller nuclei into a larger nuclei

Fission • Usually caused by neutron bombardment of the nucleus, causing the nucleus to split • Mass is converted into energy • All current nuclear reactor technology uses fission • Fission is controlled by using a moderator, a material which absorbs neutrons to keep the chain reaction under control

Fusion • Fusion reactions take lighter nuclei, often an isotope of hydrogen called deuterium and fuse them together to make a heavier nuclei, often helium • This must occur at high energy and is very difficult to produce under laboratory conditions • Currently no workable fusion reactor has been produced on earth • The sun and stars all produce energy due to nuclear fusion

Measurement of Radiation: Dosimetry • Since radiation can harm the body it is important to quantify the amount of radiation received, or the dose. The study of this is called dosimetry and is an important part of an emerging field known as Health Physics • Dosimetry is most often concerned with the number of rads or millirads of radiation received. • A rad is defined as the amount of radiation which deposits energy at a rate of 0.01 J/kg in any absorbing material

Things to Know • Atomic number, Atomic mass number, atomic symbols, atomic equations • Mass defect, binding energy • Types of Radiation • Alpha • Beta • Gamma • Detection of Radiation • Nuclear Reactions • Fission • Fusion • Dosimetry

Nuclear Physics: Radiation, Radioactivity & its Applications