Radioactivity Flashcards
How do radioactive nuclei achieve stability?
They undergo spontaneous decay
Define half life
Time required for half of the nuclei in a sample to undergo a decay event
Define decay constant
The decay constant k is characteristic of the particular radioactive nuclei and does not depend on the amount of sample. Rate of decay depends on the number of nuclei present
Define activity
Specific measurement of rate of decay. The number of nuclei that disintegrate per second. Measured in Becquerel
Give the equation for activity
Activity is proportional to the number of nuclei in the sample
A = kN
Final and initial activity are also related by exponential decay
A = A0e ^-kt
Define specific activity
Activity per gram of radioactive nuclide
Define molar activity
Activity per mile of radioactive nuclide
Derive the equation for half life
No/2 = Noe^-kt(1/2) ln2 = kt(1/2) t(1/2) = ln2/k
Explain how radiocarbon dating works
C14 constantly produced in upper atmosphere by reactions of neutrons with N14: 14N + n –> 14C + p
C14 accumulates in all living organisms. Ratio 12C:14C is constant when organisms are alive, but increases after death as C14 decays to 14N: 14C –> 14N + e-
And activity of 14C decreases
t = 8033 x ln (Ao/At)
Ao activity of modern day sample
At activity of archaeological sample
Stability of the nucleus involves the competition between which two forces?
Coulomb or electrostatic repulsion between protons acts to push these nucleons apart over a long range
The strong nuclear force is a short range attraction between all nucleons
How does the competition between forces in the nucleus explain our observations on their stability
In nuclides with too few neutrons, the electrostatic repulsion overwhelm the strong nuclear attractions
As the nucleus gets larger, the long range electrostatic repulsion between protons accumulates and eventually overwhelms the strong nuclear attraction
Nuclides with M>208 (Uranium) are unstable
Name describe and give an example of the 6 different decay mechanisms
Alpha decay Beta decay Positron decay Electron capture Neutron emission Gamma emission
Define the radioactive decay sequence
The series of decay events that lead to stability - usually represented as a graph of atomic number bs neutron number
What factors determine the stability of the nucleus
The size of the nucleus
The N:Z ratio
Describe the relationship between N:Z ratio and zone of stability
All known stable nuclides fall inside the zone of stability. The zone has a N:Z ratio near 1 but bends towards more neutrons per proton as the nucleus gets larger
Identify 3 harmful effects of radiation
Radiation can have harmful interaction with biological tissue:
Radiation sickness
Cancers
Nuclear weapons
Name 2 positive effects of radiation
Radiation is probably responsible for saving many more lives than it takes
Cancer therapy
Medical imaging
Explain why radiation is damaging
Radiation produced by radioactive decay is very high energy and can cause the ionisation of matter by ejecting an electron from an atom (low penetrating e.g. Alpha) a large proportion of these reactions will be with water due to body’s composition. Water will be ionised into a cation and electron which can both go on to react further to produce free radicals
What are the effects of free radicals in the body
Free radicals are very reactive and damage
DNA strands - genetic damage, cancer
Cell membranes - cells break apart
Proteins - enzymes lose function
How serious will the radiation damage be depends on which 3 factors?
Type of radiation (energy and penetration and relative biological effectiveness)
Length of exposure Short term (acute) radiation poisoning - high doses for short periods of time cause acute cell damage and often death Long term (chronic) radiation induced cancer. Anything that interrupts DNA can lead to cancer
Source of exposure
Internal exposure - ingestion or inhalation. Alpha and beta are most dangerous. Most gamma escapes the body
External exposure. Alpha and beta can’t penetrate through air and skin. Gamma radiation can - more dangerous
What unit is used to measure the biological effect of radiation
The sievert (Sv)
Takes into account
Radiation type
The energy of radiation
Activity of the source
Name 3 sources of natural radiation
Radon - part of 238U decay series
40K - present in bananas, sunflower seeds and kidney beans
Cosmic rays in upper atmosphere
Identify 2 methods used to ensure only cancer cells are killed in radiation therapy
Focussing ionising radiation onto the tumour - uses gamma radiation as it must penetrate air and skin
Internal administration of radiopharmaceutical - must be targeted to the tumour. Uses alpha or beta emitters that have short range effect. Attaching radionuclides to antibodies can target radiopharmaceutical to other cells or organs
Iodine -131 used to treat thyroid cancer
Describe how radioimaging is obtained and it’s purpose
Radioimaging uses radiation emitted from within the body to map the body. The radiation must be highly penetrating so it can be detected and is not harmful to the patient - requires gamma radiation. Distribution of the radioisotope is imaged by scintillation counting. CAT can give a 3D construction of the body. Radiation can come from a number of sources including Tc99m and positron emitters
List reasons why Tc99m is ideal for imaging
It can easily be incorporated into many drugs
It can easily be prepared from Mo99
It does not change its chemistry when it decays
It emits only highly penetrating gamma rays (not harmful alpha and beta particles)
Explain how PET imaging works
Pet uses a radionuclide that emits positrons. Positron reacts with electrons in the body to produce 2 high energy gamma rays which are detected outside the body. Most common PET imaging agent is fludeoxyglucose (FDG)
