Week 3 - Radioactivity Flashcards
Radioactivity
- Nuclides that are unstable will spontaneously rearrange their nuclei to attempt to become stable
o Radioactive decay - Unstable nuclides may undergo multiple decays before reaching the stability
- Radioactive decay usually involves the ejection of a particle or ray
o Alpha decay
o Beta decay
o Gamma decay
Alpha Decay
2 protons and 2 neutrons are ejected from unstable nucleus (generally large nucleus)
o Products –> New nuclide (due to loss of proton) and helium nucleus
Beta Decay
- Nucleus that has excess neutron and is unstable
- Neutron decays to a proton
- Negative high energy election (Beta-Minus) emitted
- Mass less anti-neutrino also emitted
o Has kinetic energy - Conservation of energy
Positron Decay (Beta Positive)
- A nucleus that is in excess of protons relative to the stable configuration
- Proton decays to a neutron
- Positive high energy positron Beta emitted
- Massless Neutrino also emitted
- Conservation of energy
- Important in Position Emission Tomography (PET)
Gamma Decay
- Many alpha and beta decays leave the nucleus in an excited stated
- Excited nucleus decays rapidly to the ground state
- Emission of one or more gamma rays
- Gamma rays are high energy electromagnetic waves (similar to x-rays)
- Typical energies of 0.1 to 10 MeV
- Every nuclide has a unique set of gamma energies
- Characteristic or signature
- Gamma Spectroscopy
Similarities and Differences Between X-Rays and Gamma Rays
- Both X-rays and gamma rays are photons of Electromagnetic Radiation
- Gamma rays usually have a higher energy but there is a considerable overlap of energies
- X-rays originated from electron transitions/interactions
o Bremsstrahlung x-rays - Gamma rays originate from the nucleus
Radioactivity
- A magnetic field will separate the different types of radioactive decay particles
Other Types of EM Wave Emission
- If electrons in an atom are disrupted from their normal configuration by an excitation or ionisation proves
- Electrons rearrange themselves to return the atom to its ground state
o Happens very quickly - When the atom transitions from the excited state to the ground state
o Electron moves from a lower to a higher energy level - Characteristic X-rays are emitted (unique for each atom)
o Excess energy released - As the atomic number of an atom increase
o Electron binding energy increases
o Characteristic x-ray energies increase
Radioactive Decay
- Radioactive decay and emission is spontaneous
- A sample of Uranium contains billions of atoms (nuclei)
o 1 gram of Uranium-238 contains 6x10^23 - Different nuclides will have different rates of decay
Rate of Nuclear Decay
Half-life
o The time take for half the nuclei in a sample to undergo decay
- Uranium-238 has a half-life of 4.468 billion years ago
Can be measured
- Activity of a Sample – number of disintergrations per second (dis/sec)
- SI Unit = Becquerel (Bq) = 1 disintegration/second
- Historical Unit = Curie (CI)
o 1CI = 3.7 x 10^10 Bq
Nuclear Decay: Activity of a Sample Depends on
o Number of radioactive nuclei in the sample (N)
o Characteristics of the nuclide
Lambda = the decay parameter
Probability of a given nucleus decaying in a particular time interval
o Every nuclide has an unique lambda decay parameter
o Bigger Lambda = Greater probability of decay
Radioactive Decay and Half-Life
- Measure the activity of a radioactive sample at regular time intervals
- Exponential decay of the number of nuclei
- N = Number of nuclei at time
- N0 = Numer of nuclei at t=0
- Lambda = the decay parameter
- Consider t = ½
Statistics and Radioactive Decay Measurements
- Radioactive decay is a random process
- Any measurement based on observing the radiation emitted in nuclear decay is subject to some degree of statistical fluctuation
- Fluctuations are an unavoidable source of uncertainty
- Counting Statistics includes the framework of statistical analysis required to process the results of nuclear counting experiments
- Makes prediction about the expected precision of quantities derived from these measurements
Counting Statistics for Radioactivity Measurements
- Make a measurement for 1 minute
- Will be number of counts / minute
- Repeat 3 times
- Would get a different count rate each time
- Should always measure multiple times to make predictions
- Standard deviation is the uncertainty
Counting Statistics: Fast Decaying Source
o Insufficient time to take repeated measurements
o The first and only measurement N (best estimate of the mean)
o Best estimate of the standard deviation (Square Roots of the number of counts – N)
