5th radioactivity

KS4 RadioactivityThe nucleus
The best model of the atom was known as the Thomson or “plum pudding” model. The atom was believed to consist of a positive material “pudding” with negative “plums” distributed throughout.
© Boardworks Ltd 2003
This experiment changed the way we think of the atom.
Since particle accelerators were yet to be developed, naturally occurring high energy particles were used as projectiles. Alpha particles are spontaneously emitted by certain heavy elements. These particles have speeds of the order of 107 m/s.
These were made incident on thin films of metals of high atomic weight, such as gold.
Isotopes
The number of protons in an atom is crucial. It gives you the charge of the nucleus and therefore it gives you the number of electrons needed for a neutral atom.
Change the number of protons and you change the element.
The number of neutrons in the nucleus is less crucial. You can change the number of neutrons without changing the chemical properties of the atom. So it behaves in the same way. Atoms with the same proton number but different numbers of neutrons are called isotopes.
H, H, H are isotopes of hydrogen
Food irradiated by exposing it to the gamma rays of a radioisotope -- one that is widely used is cobalt-60. The energy from the gamma ray passing through the food is enough to destroy many disease-causing bacteria as well as those that cause food to spoil, but is not strong enough to change the quality, flavor or texture of the food. It is important to keep in mind that the food never comes in contact with the radioisotope and is never at risk of becoming radioactive.
Some meats are irradiated. Pork, for example, is irradiated to control the trichina parasite that resides in the muscle tissue of some pigs. Poultry is irradiated to eliminate the chance of food borne illness due to bacterial contamination.
Archaeological dating
Recalling that all biologic organisms contain a given concentration of carbon-14, we can use this information to help solve questions about when the organism died. It works like this:when an organism dies it has a specific ratio by mass of carbon-14 to carbon-12 incorporated in the cells of it's body. (The same ratio as in the atmosphere.) At the moment of death, no new carbon-14 containing molecules are metabolized, therefore the ratio is at a maximum. After death, the carbon-14 to carbon-12 ratio begins to decrease because carbon-14 is decaying away at a constant and predictable rate.
Thickness control
Beta radiation sources can be used to measure the thickness of
materials. Beta radiation can penetrate paper, plastic and other thin materials. However the count will be reduced, and from this reduction the thickness of the material can be gauged.
Radioactivity
Some atoms are unstable. They have too much energy or the wrong mix of particles in the nucleus. So to make themselves more stable, they breakdown (or decay) and get rid of some matter and/or some energy. This is called radioactive decay and isotopes of atoms that do this are called radioisotopes.
The process is spontaneous and random. You can’t do anything to speed it up or slow it down- spontaneous.
You can’t predict when it will happen-random (Can’t predict which atom will decay at any given time. The only reason we can do any calculations on radioisotopes is because there are huge numbers of atoms in most samples so we can use statistics to accurately predict what’s most likely to happen.
Radioactive Decay is a Random Process
You can NEVER tell when an individual atom is going to decay. You can figure out approximately how many atoms in a group are going to decay in a certain time, but you can’t tell which ones are going to blow.
The timescale for radioactive decay is described by the quantity called a “half-life”.
Half-lives can be VERY short (helium-5 decays in 7.6 x 10-22 seconds), or very long (thorium-232 decays in 1.4 billion years).
Precautions for Using Radioactive Material
Label all containers with a radioactive material label and specify the isotope
No eating, drinking or smoking in the laboratory
Use spill trays and absorbent covering
Use fume hoods for handling potentially volatile material
Use glove/ box for handling large quantities of volatile material
Wear laboratory coat, disposable gloves, and laboratory safety glasses
Use gloves appropriate for the chemicals to be handled
Use automatic or remote pipetting devices. NEVER pipette by mouth.
Cloud Chamber
The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water or alcohol vapour. When an alpha particle or beta particle interacts with the mixture, it ionises it.
Helium nuclide
Stopped by paper or the skin. Range in air is short
4
+2
β
negligible
-1
γ
Electro-magnetic radiation
Reduced by many cms of lead or a few metres of concrete
0
0
Background radiation
Background radiation is the radiation all around us. Working in pairs try to think of five possible sources of background radiation.
You have FIVE minutes!!
Safety first
There are several types of radiation. They differ in what effects they have and their nature.
All radioactive sources must be handled safely.
Do you know what the hazard symbol for radiation is?
As well as the normal laboratory safety instructions you follow are there any extra rules concerning radioactivity?
Springfield Nuclear Power Plant Safety Rules:
Any employee who fails to adhere to the rules below will be suspended:
Do not handle radioactive sources directly use tongs or a robotic arm.
Never point a radioactive source at a fellow worker or yourself.
When not in use store radioactive sources in lead-lined containers.
Always wear radiation protection suits.
Radiation badges should be worn to record exposure to radiation.
Mr. Burnz
Task:
Working in pairs write down the three safety rules from above that would be most relevant in your school saying why you chose them. Also say which safety rule you think is the most important and why.

The effects of a field on radiation
Gamma radiation has no mass or charge so it is not deflected.
Beta radiation has a –1 charge and a small mass so is strongly deflected
Alpha radiation has a +2 charge but a RAM of 4 so is only weakly deflected.
The effect of a magnetic or electric field on radiation depends upon the nature of the radiation.
Using your results from the previous three investigations, fill in the table below:
least
medium
most
Electric Charge:
Ionisation effect:
Strongly ionising
Ionisation effect:
Weakly ionising
Penetration power:
Reduced by several cm’s of lead or several metres of concrete
Ionisation effect:
Reduced by lead
Stopped by aluminium
If the exposure is high, it can kill the cell.
If the exposure is lower it can cause cancer.
The higher the exposure, the higher the risk of cancer.
Alpha is the most ionising radiation, gamma is the least.
What happens if radiation is incident upon a living cell?
Ionising radiation can be used to kill cancer cells.
Radiation can ionise cells which causes cellular damage.
What two effects on living cells can ionisation have?
Which type of radiation is the most ionising?
Which type of radiation is the least ionising?
When a neutral atom loses or gains electrons and hence charge.
By losing electrons.
By gaining electrons.
Alpha radiation.
Gamma radiation.
The most penetrating?
The least penetrating?
High energy electrons?
Only reduced in intensity by lead and concrete?
Gamma
Alpha
Alpha
Alpha
Beta
Beta
Beta
Gamma
Gamma
Sterilisation
Gamma rays are used to kill bacteria, mould and insects in food. This can be done even after the food has been packaged. It can affect the taste, but supermarkets like it because it lengthens the shelf life.
Gamma rays are also used to kill bacteria on hospital equipment. It is particularly useful with plastic equipment that would be damaged by heat sterilisation.
Gamma Source
Radiotherapy
A carefully controlled beam of gamma rays can be used to kill cancer cells. It must be directed carefully to minimise the damage to normal cells.
However, some damage is unavoidable and this can make the patient ill.
It is therefore a balancing act - getting the dose high enough to kill the cancerous cells, but as low as possible to minimise the harm to the patient.
Leak detection in pipes
The radioactive isotope is injected into the pipe. Then the outside of the pipe is checked with a Geiger-Muller detector, to find areas of high radioactivity. These are the points where the pipe is leaking. This is useful for underground pipes that are hard to get near.
The radioactive isotope must be a gamma emitter so that it can be detected through the metal and the earth where the pipe leaks. Alpha and beta rays would be blocked by the metal and the earth.
The isotope must have a short half life so the material does not become a long term problem.
GM tube
Beta Source
A radioactive source is on one side of the material and a detector on the other.
If too much radioactivity is getting through, then the material is too thin and the rollers open up a bit to make the material thicker.
If not enough radioactivity is detected then the rollers compress to make the material thinner.
This method is used in the manufacture of lots of sheet materials: plastics, paper, sheet steel.
Photographic film
1. What happens to film when radiation is incident upon it?
It darkens.
2. Can photographic film tell you the type of radiation incident upon it?
No, just the amount of radiation received.
3. What can this be used for?
Can be used in radiation badges, that record the exposure of workers to radiation. Different windows detect different types of radiation.
radiation
124
125
The detector is a metal tube filled with gas. The tube has a thin wire down the middle and a voltage between the wire and the casing.
When the radioactivity enters the tube, it ionises the gas in the tube. This produces a pulse of current which is amplified and passed to a counter.
The Argon contains a little bromine to act as a quenching agent and prevent continuous discharge.
Good at detecting alpha and beta, not as good at detecting gamma.
Argon gas
The Spark Detector
The spark detector consists of a metal grid and a metal strip. A high voltage is applied between the grid and the strip. The voltage is increased until electrical arcing (sparking) across the gap just occurs.
When ionising radiation is placed close to the detector there is a marked increasing in the amount of sparking.
High voltage
Why?
Cloud chamber
Cloud chambers show the actual paths of the ionising particles. They rely on ionisation. The cloud chamber is cooled and then is super-saturated with alcohol. If an ion is formed a droplet of condensation appears. Best for alpha radiation as alpha most ionising; then Beta which shows faint traces, but cloud chambers are not as good for gamma as gamma is only weakly ionising.
Solid carbon dioxide
What is a Half-Life?
The half-life (t½) is the amount of time that it will take half of the atoms to decay. This does not mean that in twice that amount of time, all the atoms will decay. Since this is a random process, there is no history and you have to start over, so in the second half-life, half of the remaining atoms will decay, leaving a quarter of the original atoms.
. . .
N/8 undecayed atoms
N/4 undecayed atoms
N/2 undecayed atoms
N/2 something else
T = t½
25%
12.5%
Nuclide Half life
219Th90 1 s
40Cl17 1.4 min
32P15 14.3 d
14C6 5730 y
235U92 7.04x108 y
238U92 4.46x109 y
Half life is not affected by chemical and physical state of matter.
Anthropologists, biologists, chemists, diagnosticians, engineers, geologists, physicists, and physicians often use radioactive nuclides in their respective work.
Radioactive Decay
What happens to the nucleus of an atom when it emits a radiation?
Alpha Decay
When an unstable nucleus emits an alpha particle it loses 2 protons and 2 neutrons
For example,
226
88
Ra
222
86
Rn
4
2
He
Note: The atomic and mass numbers on both sides of the equation balance.
Beta decay
Beta decay is more complicated. A beta particle is an electron. But where does this electron come from?
When an unstable nucleus emits an electron a neutron in its nucleus changes into a proton and an electron. The electron is emitted.
For example
218
84
Po
218
85
At
0
-1
e
Note: The atomic and mass numbers on both sides of the equation balance.
Gamma Decay
After an alpha or beta particle has been emitted from the nucleus of an isotope, the nucleus has too much energy.
Too get rid of that excess energy, a gamma wave is emitted.
Alpha
Beta
Gamma
Which type of radiation is the most damaging inside the body?
Alpha
Beta
Gamma
Which type of radiation is the most dangerous outside the body?
Alpha
Beta
Gamma
Which of the following is not a use of radiation?
Pre-natal scans
Einstein’s formula above tells us how the change occurs
In the equation above:
Energy
Light
Speed
The equation may be read as follows:
Energy (E) is equal to Mass (m) multiplied by the Speed of Light (c) squared
This tells us that a small amount of mass can be converted into a very large amount of energy because the speed of light (c) is an extremely large number
A dollar bill has a mass of 1 gram.
m = 0.001 kg; c = 3 x 108 m/s; E = ?
E = mc2
E = 9 x 1013 J = 90,000,000,000,000 J
E = 12 kilotons of TNT-equivalent
Fiss vs. Fuse
Fiss = break down
Start with a larger atom and finish with two or more smaller atoms
Fuse = build up
Start with two smaller atoms and finish with one larger atom
Fission
When atoms are bombarded with neutrons, their nuclei splits into 2 parts which are roughly equal in size.
Nuclear fission in the process whereby a nucleus, with a high mass number, splits into 2 nuclei which have roughly equal smaller mass numbers.
During nuclear fission, neutrons are released.
1. Spontaneous Fission
2. Induced Fission
Some radioisotopes contain nuclei which are highly unstable and decay spontaneously by splitting into 2 smaller nuclei.
Such spontaneous decays are accompanied by the release of neutrons.
Nuclear fission can be induced by bombarding atoms with neutrons.
Induced fission decays are also accompanied by the release of neutrons.
The nuclei of the atoms then split into 2 equal parts.
The Fission Process
A neutron travels at high speed towards a uranium-235 nucleus.
235
92
U
1
0
n
235
92
U
1
0
n
235
92
U
1
0
n
The neutron strikes the nucleus which then captures the neutron.
The Fission Process
The nucleus changes from being uranium-235 to uranium-236 as it has captured a neutron.
The Fission Process
The Fission Process
It transforms into an elongated shape for a short time.
The Fission Process
The Fission Process
It then splits into 2 fission fragments and releases neutrons.
The Fission Process
The Fission Process
The Fission Process
The Fission Process
Both the fission fragments and neutrons travel at high speed.
The kinetic energy of the products of fission are far greater than that of the bombarding neutron and target atom.
EK before fission << EK after fission
Energy is being released as a result of the fission reaction.
Energy from Fission
Calculate the total mass before and after fission takes place.
The total mass before fission (LHS of the equation):
The total mass after fission (RHS of the equation):
3.9014 x 10-25 + 1.6750 x 10-27 = 3.91815 x 10-25 kg
2.2895 x 10-25 + 1.5925 x 10-25 + (2 x 1.6750 x 10-27) = 3.9155 x 10-25 kg
Energy from Fission
3.91815 x 10-25 kg
3.91550 x 10-25 kg
Energy from Fission
mass difference, m = total mass before fission – total mass after fission
m = 3.91815 x 10-25 – 3.91550 x 10-25
m = 2.65 x 10-28 kg
This reduction in mass results in the release of energy.
E = mc2
E = energy released (J)
m = mass difference (kg)
c = speed of light in a vacuum (3 x 108 ms-1)
E
m
c2
m = 2.65 x 10-28 kg
c = 3 x 108 ms-1
E = E
E = 2.385 x 10-11 J
U
235
92
Cs
138
55
n
1
0
2
n
1
0
Rb
96
37
Energy from Fission
The energy released from this fission reaction does not seem a lot.
This is because it is produced from the fission of a single nucleus.
Large amounts of energy are released when a large number of nuclei undergo fission reactions.
Energy from Fission
Each uranium-235 atom has a mass of 3.9014 x 10-25 kg.
The total number of atoms in 1 kg of uranium-235 can be found as follows:
No. of atoms in 1 kg of uranium-235 = 1/3.9014 x 10-25
No. of atoms in 1 kg of uranium-235 = 2.56 x 1024 atoms
Energy from Fission
If one uranium-235 atom undergoes a fission reaction and releases 2.385 x 10-11 J of energy, then the amount of energy released by 1 kg of uranium-235 can be calculated as follows:
total energy = energy per fission x number of atoms
total energy = 2.385 x 10-11 x 2.56 x 1024
total energy = 6.1056 x 1013 J
Nuclear Fusion
In nuclear fusion, two nuclei with low mass numbers combine to produce a single nucleus with a higher mass number.
H
2
1
He
4
2…

5th radioactivity

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