Radioactivity

Question 1

How do radioactive nuclei achieve stability?

Answer 1

They undergo spontaneous decay

Question 2

Define half life

Answer 2

Time required for half of the nuclei in a sample to undergo a decay event

Question 3

Define decay constant

Answer 3

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

Question 4

Define activity

Answer 4

Specific measurement of rate of decay. The number of nuclei that disintegrate per second. Measured in Becquerel

Question 5

Give the equation for activity

Answer 5

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

Question 6

Define specific activity

Answer 6

Activity per gram of radioactive nuclide

Question 7

Define molar activity

Answer 7

Activity per mile of radioactive nuclide

Question 8

Derive the equation for half life

Answer 8

No/2 = Noe^-kt(1/2)
ln2 = kt(1/2)
t(1/2) = ln2/k

Question 9

Explain how radiocarbon dating works

Answer 9

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

Question 10

Stability of the nucleus involves the competition between which two forces?

Answer 10

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

Question 11

How does the competition between forces in the nucleus explain our observations on their stability

Answer 11

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

Question 12

Name describe and give an example of the 6 different decay mechanisms

Answer 12

Alpha decay
Beta decay
Positron decay
Electron capture
Neutron emission
Gamma emission

Question 13

Define the radioactive decay sequence

Answer 13

The series of decay events that lead to stability - usually represented as a graph of atomic number bs neutron number

Question 14

What factors determine the stability of the nucleus

Answer 14

The size of the nucleus

The N:Z ratio

Question 15

Describe the relationship between N:Z ratio and zone of stability

Answer 15

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

Question 16

Identify 3 harmful effects of radiation

Answer 16

Radiation can have harmful interaction with biological tissue:

Radiation sickness
Cancers
Nuclear weapons

Question 17

Name 2 positive effects of radiation

Answer 17

Radiation is probably responsible for saving many more lives than it takes

Cancer therapy
Medical imaging

Question 18

Explain why radiation is damaging

Answer 18

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

Question 19

What are the effects of free radicals in the body

Answer 19

Free radicals are very reactive and damage

DNA strands - genetic damage, cancer

Cell membranes - cells break apart

Proteins - enzymes lose function

Question 20

How serious will the radiation damage be depends on which 3 factors?

Answer 20

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

Question 21

What unit is used to measure the biological effect of radiation

Answer 21

The sievert (Sv)

Takes into account
Radiation type
The energy of radiation
Activity of the source

Question 22

Name 3 sources of natural radiation

Answer 22

Radon - part of 238U decay series

40K - present in bananas, sunflower seeds and kidney beans

Cosmic rays in upper atmosphere

Question 23

Identify 2 methods used to ensure only cancer cells are killed in radiation therapy

Answer 23

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

Question 24

Describe how radioimaging is obtained and it’s purpose

Answer 24

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

Question 25

List reasons why Tc99m is ideal for imaging

Answer 25

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)

Question 26

Explain how PET imaging works

Answer 26

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)