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Flashcards in Radioactivity Deck (26):

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
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)