Nuclear Physics Flashcards
(94 cards)
1
Q
Describe how the alpha particles were detected in the Rutherford scattering experiment
A
- Over a period of months, Geiger and Marsden counted the number of alpha particles deflected at different angles theta
- The alpha particles were detected by a fluorescent screen and each time an alpha particle hit the screen a small flash of light was emitted which was seen through a microscope and they counted hundreds of thousands of such flashes of light
- The vast majority of the alpha particles were detected through large angles of about 150 degrees or more
2
Q
What were the conclusions of the Rutherford scattering experiment? (6)
A
- The atom has a very small positively charged nucleus
- Rutherford suggested that the positive charge on the nucleus is responsible for the repulsive force on the positively charged alpha particle, which causes it to change direction
- The fact that only a very small number of particles undergo a large deflection tells us that the nucleus is much smaller in diameter than the atom - The nucleus contains nearly all the mass of the atom and consideration of the conservation of momentum tells us that the alpha particle would knock a small cells out of the way but that the alpha particle will bounce back after an encounter with a nucleus much heavier than itself
- Using our knowledge of electrostatic theory it is possible to calculate the maximum size of the gold nucleus. If an alpha particle is tuned round by 180 degrees, it much have encountered a gold nucleus head on and there must have been a moment when the alpha particle stopped moving, Then all of the alpha particle’s kinetic energy has been transferred to electrical potential energy
3
Q
What is a femtometer?
A
Nucelar radii and diameters are measured in femtometers, 10-15. The unit is abbreviated to fm
4
Q
How do you figure out the momentum of electrons?
A
p = E (electron energy) / c (speed of light)
5
Q
How do you calculate the wavelength of the electrons?
A
lamda = h (planck constant) / p (the electron’s momentum)
6
Q
How do you calculate the diameter of the nucleus?
A
sin theta (angle of the first diffraction minimum) = 1.22 lamda (wavelength of the light) /d (diameter of the particles)
7
Q
What is the empirical formula of the radius of a nucleus?
A
r=r0Atothepowerof1/3
8
Q
What does empirical mean?
A
The equation is based purely on experimental results. it is not exact but it gives an approximate value for a nuclear radius
9
Q
What is an atomic mass unit?
A
One atomic mass unit (1u) is equal to 1.67 x 10-27 kg
10
Q
What is a becquerel?
A
The activity of a radioactive source is equal to the number of particles emitted per second. the unit of activity is the becquerel (Bq). 1 becquerel (1Bq) = an emission of one particle per second
11
Q
Describe alpha particles
A
- Helium nucleus: 2 protons and 2 neutrons
- Mass: 4u , 6.6 x 10 -27 kg
- Charge: +2e
- Strongly ionising, the strong charge on the alpha particle pulls electrons out of atoms, creating pairs of positive and negative ions along the particle’s path
- Travel few centimetres in air and can be stopped by a sheet of paper
- Deflected slightly in strong electric and magnetic fields
- Speed: 5% of c (typically alpha particles have kinetic energies of a few MeV as they leave the parent nucleus e..g 5MeV travels at about 5% of the speed of light
- Produces 10,000 ion-pairs per mm of air
12
Q
Describe beta particles
A
- Fast electron
- Mass: 9.1 x 10-37 kg
- Charge: -e
- Less ionising than alpa
- Travel several meters in air and can be stopped by a few mm of lead
- Significant deflection in electric and magnetic fields
- Speed: 98-99% of c
- Produce about 100 ion-pairs per mm of path travelled in air
13
Q
Describe gamma rays
A
- Electromagnetic photon (electrical neutral emissions).
- 0 mass and charge, but typically has an energy if about 1MeV, which corresponds to a wavelength of about 10^-12m
- Very weakly ionising, producing about one ion-pair per mm of path travelled in air
- No deflection in electric and magnetic fields as not charged
- Speed: c
- Very penetrating, few centimetres of lead
- Gamma rays can transfer their energy to electrons in metals (rather like a photoelectric effect); then the moving electrons create ion-pairs)
14
Q
What did scientists think before the scattering?
A
- Scientists thought that atoms were like a plum pudding
1. The idea of atoms has been around since the time of the Ancient Greeks in the nth Century BC. Democritus proposed that all matter was made up of little, identical lumps called ‘atomos’
2. Much later, in 1804, John Dalton put froward a hypothesis that agreed with Democritus - that matter was made up of tiny spheres (‘atoms’) that couldn’t be broken up. He reckoned that each element was made up of a different type of ‘atom’
3. Nearly 100 years later J.J Thomson discovered that electrons could be removed from atoms. So Dalton’s theory was not quite right (atoms could be broken up)
4. Thomson suggested that atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
5. Until this point though nobody had proposed the idea of the nucleus. Rutherford was the first to suggest atoms did not have uniformly distributed charge and density
15
Q
How did Rutherford’s scattering show the existence of a nucleus?
A
- In 1909, Rutherford and Marsden tried firing a beam of alpha particles at thin gold foil
- A circular detector screen surrounding the gold foil and the alpha sources was used to detect alpha particles deflected by any angle
- They expected that the positively charged alpha particles would be deflected by the electrons by a very small amount if the plum pudding model was true
- Instead most of the alpha particles went straight though the foil, while a small number were deflected by a large angle
- Some were even deflected by more than 90 degrees, sending then back the way they came - this was confusing at the time and called for a change to the model of the atom
16
Q
What did the results of the Rutherford scattering model suggest?
A
- That atoms must have a small, positively charged nucleus at the centre;
1. Most of the atom must be empty space because most of the alpha particles passed straight though the foil
2. The nucleus must have a large positive charge, as some positively-charged alpha particles were repelled and deflected by a large angle
3. The nucleus must be very small as very few alpha particles were deflected back
4. Most of the mass must be in the nucleus, since the fast alpha particles (with high momentum) are deflected by the nucleus
17
Q
How can you estimate the closet approach of a scattered particle?
A
- When you fire an alpha particle at a gold nucleus, you know its initial kinetic energy
- An alpha particle that ‘bounces back’ and is deflected through 180 degrees will have revered direction a short distance from the nucleus. It does this at the point where its electric potential energy equals its initial kinetic energy
- It is just conservation fo energy and you can use it to find how close the particle can get to the nucleus (NOTES)
- To find the charge of a nucleus you need to know the atom’s proton number, Z that tells you how many protons are in the nucleus. A proton has a charge of +e (where e is the size of the charge on an electron), so the change of a nucleus must be +Ze
- The distance of closet approach is an estimate of nuclear radius - it gives a maximum value for it. However, electrons diffraction gives much more accurate values for nuclear radii
18
Q
How can you use electron diffraction to estimate nuclear radius?
A
- Electrons are a type of particles called lepton. Leptons do not interact with the strong nuclear force (whereas neutron and alpha particles do). Because of this, electron diffraction is an accurate method for estimating the nuclear radius
- Like other particles, electrons show wave-particle duality, she electron beams can be diffracted
- A beam of moving electrons has an associated de Broglie wavelength, lamda, which at high speeds (where you have to take into account relativistic effects is approximately lamda = hc/E
- The wavelength must be tiny (around 10^-15m) to investigate the nuclear radius, so the electrons will have to have a very high energy
- If a beam, of high energy electrons is directed onto a thin film of material in front of a screen, a diffraction pattern will be seen on the screen
- The first minimum appeared where sin theta = 1.22lamda / 2R
- Using measurements from this diffraction pattern you can rearrange the above equation to find the radius of the nucleus
19
Q
How does intensity vary?
A
- Intensity varies with diffraction angle
1. The diffraction pattern is very similar to that of a light source shining through a circular aperture - a central bright maximum (circle) construing the majority of the incident electrons, surround by other dimmer rings (maxima)
2. The intensity of the maxima decreases as the anlel of diffraction increases. The graph shows the relative intensity of electrons in each maximum - You might see a logarithmic plot of this graph where the different in the peak heights is less pronounced
20
Q
What is the nuclear radius like in comparison to the atomic radius?
A
- The nuclear radius is very small in comparison to the atomic radius
- By probing atoms using scattering and diffraction methods, we know thatL
1. The radius of an atom is about 0.05nm (5 x 10^-11)
2. The radius of the smallest nucleus is amount 1fm (1 x 10^-15 - So nuclei are really tiny compared with the size of the whole atom
21
Q
What is the typical radius of a nucleus?
A
1 x 10^-15m
22
Q
What is the nucleus made up of?
A
- The nucleus is made up of nucleons
1. The particles that make up the nucleus ( protons and neutrons) are known as nucleons
2. The number of nucleons in an atoms is called the nucleon (or mass) number, A
3. As more nucleons are added tot he nucleus, to gets bigger
4. You can measure the size of nucleus by firing particles at it
23
Q
What is the nuclear radius proportional to?
A
- Nuclear radius is proportional to the cube root of the nucleon number
1. When data from nuclear radii experiment is plotted on a graph of nuclear radius R against the cube root of the nucleus number A^1/3, the line of best fit gives a straight line
2. This shows a linear relationship between R and A^1/3 . As the nucleon number increase, the radius of the nuclear increases proportionally to the cube root of A
3. R=R0A^1/3 where R0 is roughly 1.4fm
24
Q
What is the density of nuclear matter like?
A
- The density of nuclear matter is enormous
1. The volume that each nucleon (i.e. a proton or a neutron) takes up in a nucleus is about the same
2. Because protons and neutrons have nearly the same mass (we’ll call it mnucleon) it means that all nuclei have a similar density (rho), which you can quickly prove
3. If you substitute the instant into this formula, you’ll get that the nuclear density is run 1.45 x 10^17 kgm^-3
4. Nuclear matter is no ordinary stuff. Its density is enormous, much larger than atomic density. This suggests that an atom contains lots of empty space, with most of its mass being in a small nucleus
25
What are unstable nuclei like?
- Unstable nuclei are radioactive
1. If a nucleus is unstable, it will break down to become more stable. Its instability could be because having too many neutrons, not enough neutrons or just too much energy in the nucleus
2. The nucleus decays by releasing energy and/or particles until it reaches a stable form, this is called radioactive decay
3. When a radioactive particle hits an atom it can knock off electrons, creating an ion - so, radioactive emissions are also known as ionising radiation
4. An individual radioactive decay is random - it cannot be predicted
26
How can you use penetrating power to investigate radiation types?
- Different types of radiation have different penetrate power, and so can be stopped by different types of material:
1. Record the background radiation count rate when no source is present
2. Place an unknown source near to a Geiger counter and record the count rate
3. Place a sheet of paper between the source and the Geiger counter, record the count rate
4. repeat step 2, replacing the sorer with a 3mm thick sheet of aluminium
- Depending on whether the count rate significantly decrease, you can collate what kind of radiation the source was emitting. For example, if paper has no effect and aluminium cause a significant (but not complete) reduction n count rate, the source must be emitting beta and gamma radiation
27
How can you control how thick material is using radiation?
1. When creating sheets of material, like paper foil or steel, ionising radiation can be sued to control its thickness
2. The material is flatted as it is fed through rollers
3. A radioactive source is placed on one side of the material, and a radioactive detector on the other. The thicker the material, the more radiation art absorbs and prevents from reaching the detector
4. If too much radiation is absorbed, the rollers move closer together to make the material thinner. if too little ration is being absorbed, they move further apart
28
Describe the different ionising properties of alpha and beta particles
- What a radioactive source can be used for often depends on its ionising properties
1. Alpha particles are strongly positive, and so they can easily pull electrons off atoms
2. Ionising an atom transfers some of the energy from the alpha particle to the atom. The alpha particle quickly ionises many atoms (about 10,000 ionisation our mm in air for each alpha particles) and loses all its energy. This makes alpha-sources suitable for use in smoke alarms because they allow current to flow. but won't travel very far
3. Although alpha particles can't penetrate your skin, sources of alpha particles are dangerous if they are ingested, They quickly ionise body tissue in a small area, causing lots of damage
4. The beta-minus particle has a lower mass and change than the alpha particle, but. higher speed, Tis means it can still knock electrons off atoms. Each beta particle will ionise about 100 atoms per mm in air, losing energy at each interaction
5. This lower number of interactions means that beta radiation causes much less damage to body tissues
6. Beta radiation is commonly used for controlling the thickness of a material
29
How are gamma rays used in medicine?
- Gamma radiation is even more weakly ionising that beta radiation, so will do even less damage to body tissue. This means that it can be used in medicine:
1. Radioactive tracers are used to help diagnose patients without the need for surrey. A radioactive source with a short half-life to revert prolonged radiation exposure is either eaten or injected into the patient. A detector, e.g. a PET scanner, it then used to detect the emitted gamma rays
2. Gamma rays can be used in the treatment of cancerous tumours - damaging cells and sometimes curing patients of cancer. Radiation damages all cells, cancerous or not and so sometimes a rotating beam of gamma rays is used. This lessens the damage done to the surrounding tissues, whilst giving a high does of radiation to the tumour at the centre of radiation
3. Damage to other, healthy cells is not completely prevented however and treatment can cause patients to suffer side effects - such as tiredness and reddening or soreness of the skin
4. Exposure to gamma radiation can also cause long term side effects like infertility for certain treatments
5. As well as patients, the risks towards medical staff giving these treatments mist be kept as low as possible. Exposure time to radioactive sources is kept to a minimum, and generally staff leave the room (which is itself shielded) during treatment.
- Simply put, radiation use in medicine has benefits and risks. The key is trying to use methods which reduce the risk (shielding, rotating beams etc.) while giving you the results you want. It's all one big balancing act
30
How do you detect background radiation?
- Put a Geiger counter anywhere and the counter will click, it is detecting background radiation
- When you are a reading from a radioactive source, you need to measure the background radiation separately and subtract it from your measurement
31
What are the different sources of background radiation? Describe them
1. The air
2. The ground and buildings
3. Cosmic radiation
4. Living things
5. Man-made radiation
32
What is the air's background radiation?
-Radioactive radon gas is released from rocks
-It emits alpha radiation
-Because we can inhale this gas, it is dangerous as radiation can get inside our lungs
-There are a lot of rocks in the Earth that contain
radioactive uranium, thorium, radon and potassium and so we are always exposed to some ionising particles
-The concentration of this gas in the atmosphere varies a lot from place to place, but it is usually the largest contributor to background radiation
33
What are the ground and building background radiation?
-All rock contains radioactive isotopes
34
What is cosmic radiation background radiation?
- Cosmic rays are particles (mostly high-energy protons) from space
- The sun emits lots of protons, which can also create ions in out atmosphere
- When they collide with particles in the upper atmosphere, they produce nuclear radiation
35
What are living thing's background radiation?
- All plants and animals contain carbon, and some of this will be radioactive carbon-14
- They also contain other radioactive material such as potassium-40
36
What are man made background radiation?
- In most areas, radiation from medical or industrial source makes up a tiny, tiny fraction of the background radiation
- In some jobs workers are at a higher risk, X-rays used in hospitals also cause ionisation and so radiographers make sure that their exposure to X-rays is as small as possible
- In nuclear power stations, neutrons are produced in nuclear reactors
- The damage caused by neutrons is a source of danger for workers in that industry
37
What does the intensity of gamma radiation obey?
-The intensity of gamma radiation obeys the inverse square law
1. A gamma source will emit gamma radiation in all directions
2. This radiation spreads out as you get further away from the source
3. This means the amount of radiation per unit area (the intensity) will decrease the further you get form the source
4. If you took a reading of intensity, I at a distance x from the source you find that it decrease by the square of the distance from the source
-This can be written as the equation I=k/x^2
where k is a constant
38
How do you consider the inverse square law when working with radioactive sources?
1. Using a radioactive source becomes significantly more dangerous the closer you get to the source. This is why you should always hod a source away from your body when transporting it through the lab
2. Long handling tongs should also be used to minimise the radiation absorbed by the body
3. For those not working directly with radioactive source, it is best to just keep as far away as possible
39
How can you investigate the inverse square law?
1. Set up the equipment as shown in the diagram, leaving out the source at first (DIAGRAM)
2. Turn on the Geiger counter and take a reading of the background radiation count rate (in counts per sec). Do this three times and take an average
3. Place the tube of the Geiger counter so it is lined up with start of the rule
4. Carefully place the radioactive source at a distance d from the tube
5. Record the count rate at that distance. Do this 3 times and take an average
6. Move the source so the distance between it and the Geiger counter doubles (2d)
7. Repeat steps 5 and 6 for distances of 3d, 4d etc.
8. Once the experiment is finished, put away your source immediately - you don't want to be exposed to more radiation than you need to be
9. Correct your data for background radiation and then plot a graph of corrected count rate against durance of the counter from the source, You shoulder that as the distance doubles, the corrected count rate drops to a quarter of its starting value, supporting the inverse square law
40
How does every isotope decay?
- Every isotope decays at a different rate
1. Radioactive decay is completely random. You can't predict which atom's nucleus will decay when
2. Although you can't predict the decay of an individual nucleus, if you take a very large number of nuclei, their overall behaviour shows a pattern
3. Any sample of a particle isotope has the same rate of decay, i.e. the same proportion of atomic nuclei will decay in a given time
- Isotopes of an element have the same number of protons, but different number of neutrons in their nuclei
41
How is the rate of decay measured?
- The rate of decay is measured by the decay constant
1. The activity of a sample, the number of nuclei (N) that decay each second, is proportional to the size of the sample
2. For a given isotope, a sample twice as big would give twice the number of decays per second
3. Activity is measured in Bequerels (Bq) and 1 Bq = 1 decay per second
- The number of radioactive nuclei decaying per second (activity) is proportional to the number of nuclei remaining
42
What is the decay constant?
The decay constant (lamda) is the probability of a given nucleus decaying per second, the bigger the value of lamda, the faster the rate of decay, its unit is s^-1
43
What is the definition of half-life?
The half like (T1/2) of an isotope is the average time it takes for the number of unstable nuclei to halve
-Measuring the number of unstable nuclei isn't the easiest job in the world and in practice, half-life isn't measured by counting nuclei, but by measuring the time it takes the activity to halve, The longer the half-life of an isotope, the linter it stays radioactive
44
What is the equation for half life?
T1/2 = ln2 / lamda = 0.693 / lamda
45
How do you figure out the number of unstable nuclei remaining?
-The number of unstable nuclei remaining N depends on the number originally present, N0 and the number remains can be calculated:
N=N0e^-lamdat (t is time in seconds)
46
How do you calculate the activity of a sample
-As a sample decays, its activity goes down:
| A=A0e^-lamdat (t is time in seconds)
47
What are the different uses for different half lives?
- Radioactive substances are extremely useful. You can sue them for all sorts - to date organic material, diagnose medical problems, sterilise foods and in smoke alarms. Knowledge about half lives can be used for:
1. Radioactive dating of objects
2. Medical diagnosis
48
How are half-lives used in radioactive dating of object tis?
1. The radioactive isotope carbon-14 is used in radioactive dating
2. Living plants take in carbon dioxide form the atmosphere as part of photosynthesis, including the radioactive isotope carbon-14
3. When they die the activity of carbon-14 in the plant starts to fall, with a half life of around 5730 years
4. Archaeological finds made from once living material (like wood) can be tested to find the current amount of carbon-14 in them, and date them
49
How are half-lives used in medical diagnosis?
1. Radioactive tracers are used to help diagnose patients
2. Technetium-99m is suitable for this size because it emits gamma radiation, has a half-life of 6 hours (long enough for data to be recorded, but short enough to limit the radiation to an acceptable level) an decays to a much more stable isotope
50
Why are long half-lives dangerous?
1. As well as being useful radioactive substances can be dangerous too
2. This is an even bigger problem if the substances stay radioactive for a long time
3. Some isotopes found in waste products of nuclear power generation have incredibly long half-lives
4. This means that we must plan ahead about how nuclear waste will be stored, e.g. in water tanks or sealed underground, to precent dame to the environment or people not only now but into the future too
51
Why are some nuclei more stable than others?
- The nucleus is under the influence of the strong nuclear force holding it together and the electromagnetic force pushing the protons apart
- It is a very delicate balance and it is easy for a nucleus to become unstable. You can get stability graph by plotting Z (atomic number) against N (number of neutrons)
52
When will a nucleus become unstable?
- A nucleus will be unstable if it has:
1. Too many neutrons
2. Too few neutrons
3. Too many nucleons altogether i.e. it's too heavy
4. Too much energy
53
Describe alpha emission
- Alpha emission happens in heavy nuclei
1. When an alpha particle is emitted, the proton number decrease by two, and the nucleon number decreases by four
2. Alpha emission only happens in very heavy atoms, like uranium and radium, that means they are rich in protons (to right of curve)
3. The nuclei of these two atoms are too massive to be stable
54
Describe beta minus emission
- Beta minus emission happens in neutron rich nuclei
1. Beta-minus (usually just called beta) decay is the emission of one electron form the nucleus along with an antineutrino
2. Beta decay happens in isotopes that are "neutron rich" (i.e. they have many more neutron than protons in their nucleus)
- Beta minus decay occurs in elements to the left of the line of instability and these elements have too few protons to be stable and each of the decays tends to move the nucleus towards the line of stability
3. When a nucleus ejects a beta particle, one of the neutrons in the nucleus is change into a proton
4. When a beta minus particle is emitted: the proton number increases by one and the nucleon number stays the same
55
What happens in beta plus emission?
- Two types, beta plus and K capture
- Nuclei that are rich in protons, with an atomic number less than 82, tend to decay by positron Beta+ emission or by K-capture
- A positron is the antiparticle of an electron and has the same mass as an electron but has a positive decay
- In Beta plus a proton gets changed into a neutron
1. In Beta plus decay the atomic number decreases by 1
2. A neutrino, which is the antiparticle of a antineutrino is emitted together with the positron
3. Positron
56
When is gamma radiation emitted?
- Gamma radiation is emitted form nuclei with too much energy
1. After alpha, beta plus/minus decay, or K capture the nucleus often has excess energy - it is in an excited state. When the nucleon drops back to a lower level, a photon is emitted and the energy of these photon is often measured in MeV, and these high energy photons are the gamma rays that we detect
2. During gamma emission, there is no change to the nuclear constituents - the nucleus just loses excess energy
3. Usually the excited nucleus releases its energy very quickly after a decay with a half life shorter than 10^-9s
3. Another way that gamma radiation is produced is when a nucleus captures one of its own orbiting electrons
4. Electron capture causes a proton to change into a neutron and this means the nucleus unstable and it emits gamma radiation
- The artificial isotope technetium-99m is formed in an excited state form the decay of another elements and it is used as a tracer in medical imaging
57
What are the conservation rules in nuclear reactions?
In every nuclear reaction energy, momentum, charge and nucleon number must be conserved
58
What is the mass defect equivalent to?
- The mass defect is equivalent to the binding energy
1. The mass of a nucleus is less than the mass of its constituent parts - the difference is called the mass defect
2. Einstein's equation E=mc^2, says that mass and energy are equivalent and it applies to all energy changes
3. So, as nucleons join together the total mass decreases - this 'lost' mass is converted into energy and released
4. The amount of energy released is equivalent to the mass defect
5. If you pulled the nucleus completely apart, the energy you'd have to use to do it would be the same as the energy released when the nucleus formed
- The energy needed to separate all of the nucleons is called the binding energy (measure in Mev) and it is equivalent to the mass defect
6. A mass defect of 1 u is equivalent to about 931.5 Mev of being energy
59
When is the average binding energy per nucleon at a maximum?
- The average binding energy per nucleon is at a maximum around N=50
- A useful way of comparing binding energies of different nuclei is to look at the average binding energy per nucleon
1. A graph of average binding energy per nucleon against nucleon number, for all elements shows a curve, high average biding energy per nucleon means that more energy is needed to remove nucleons from the nucleus
2. In other words, the most stable nuclei occur around the maximum point on the graph, which is at a nucleon number 56 (i.e. Iron Fe)
3. Combining small nuclei is called nuclear fusion and this increases the average binding energy per nucleon dramatically, which means a lot of energy is released during nuclear fission
4. Fission is when large nuclei are split in two, the nucleon numbers of the two new nuclei are smaller than the original nucleus, which means there is an increase in the average binding energy per nucleon. So, energy is released during nuclear fission ( but not as much energy per nucleon as in nuclear fusion)
60
What does the change in average binding energy give?
- The change in average binding energy gives the energy released
- The average binding energy per nucleon graph can be used to estimate the energy released from nuclear reactions
61
How do you illustrate the ionising power of alpha?
- A charged gold leaf electroscope
1. An alpha source is held above a positively charged electroscope
2. The alpha particle produce positive and negative ions.
3. The positive ions are repelled from the positively charged electroscope, but the negative ions are attracted to the electroscope and it is discharged
62
How do you detect ionising radiation?
1. You use the ionising properties of alpha, beta and gamma radiation to detect them using a Geiger-Muller (GM) tube (GM used in schools whereas solid-state detectors are more widely used)
63
How does a GM tube work?
1. A metal tube is filled with argon gas at low pressure
2. A voltage of about 450V is applied between a central anode and the outside of the tube
3. When radiation enters the tube, atoms are ionised and a small current flows
4. Each current pulse is amplified, so that ew can record the rate at which particles enter the tube
5. It is important that we can detect and then understand the effects of ionising radiation. When ionising particles enter the body; the ions that are produced cab damage or destroy cells in our bodes, with serious consequences for our health
64
Why are there biological effects of radiation?
1. Our bodies are made up of many different types of complicated molecules and if an electron is removed or added to a molecules, it has been changed chemically and will therefore behave differently in any interaction with another molecule
2. Typically it requires a few eV of energy to remove an electron from a molecule
3. Such energy is carried by photons of ultraviolet light
4. Alpha and beta particles, and gamma-ray photons all carry energy measured in MeV
5. Such radiations cause ionisation, and ionising radiation is dangerous to us because it can change the chemistry of our bodies
6. The functions of enzymes can be altered, cells can be managed and mutations can occur to our DNA which can lead to cancer
65
What is ionising radiation?
Ionising radiation is dangerous because ions are produced in our bodies, which damage cells and the functions of enzymes are changed
66
What is a dose of radiation?
1. Dose is the energy absorbed per kilogram of a body
2. A Gray is the unit of dose Jkg^-1
- An absorbed dose of radiation is defined as the energy absorbed per kilogram of a body
67
How does the type of radiation impact its biological effect?
1. If we receive a high dose of radiation, we are at a higher risk of becoming ill due to the damage caused to our bodies
2. However, the impact of the radiation on our bodies also depends on how the dose is administered, and this depends on the type of radiation we are exposed to
3. The different types of radiation have different penetrating power, and the lack of penetration of alpha particles is explained by their very high ionising power
68
Why is alpha less penetrating?
1. Alpha particles lose energy over a shorter range than beta particles and gamma rays, because alpha particles are relatively slow-moving and they carry a high charge
2. By contrast, beta and gamma rays lose their energy over much longer distances
3. Consequently, alpha particles are much more damaging to our bodies because many ions are produced in a small volume
69
How do you measure the damage done by radiation? What is the definition?
1. We measure the damage done by radiation in dose equivalents, which is defined as H=WRD, were WR is the radiation weighing factor which is a dimensionless number that depends on the type of radiation. Dose equivalent has the same units as dose, Jkg-1, but to distinguish dose form dose equivalent, the latter is given the unit sievert
- Dose equivalent is the measure of damage done by radiation
- Sievert is the unit of dose equivalent is also Jkg^-1, but for some equivalent ht unit is called the sievert (Sv)
70
What are the safety precautions with radioactive material?
- When we handle radioactive sources we must take care to minimise the risk
1. A radioactive source should be enclosed in a container that is leak-poor to a void the escape of any radioactive liquid or gases
2. Sources are kept in lead-lined boxed and looked away in metal cupboards
3. When in use, sources are used for a short period of time
4. Sources are kept away from our bodies and are handled with long tongs
- There are strict regulation for the handling of radioactive materials in laboratories, hospitals and industry
- A leak of radioactive gas or liquid is particularly hazardous because we can inhale a gas or swallow liquid which could then allow radiation to be emitted inside our bodies
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How are tumours treated in radiotherapy?
1. Beams of gamma rays directed towards a tumour
- Hazard from directing it from outside the body is that rays also pass through healthy tissue which could be damaged by the radiation
2. To reduce the impact of the gamma rays on the healthy tissues, the source is rotated around the body, but always directed towards the tumour
- In this way the tumour receives a high dose, and healthy tissues receive a low dose, from which the healthy tissues can recover
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How else are tumours treated?
1. Tumours are also treated by short-range internal radiotherapy
- Under aesthetic, a surgeon can place a small needle or wire of a radioisotope into the tumour itself
- The radioisotope emits beta particles, which are strongly ionising and short-ranged
- Now the radiation is directed straight into the tumour, and the beta particles do not penetrate as far as any healthy tissue
- When the correct dose has been administered, the wire is removed
2. Alpha radiation is also used in targeted alpha therapy (TAT), e.g. leukaemia can be treated in this way; an alpha emitter is attached to an organic compound which then adheres to cancerous cells
3. Short-half-life alpha emitters can be used as a source of energy
- Alpha particles that are emitted inside a solid material are self absorbed and this means that the alpha particles cannot escape through the solid
- The particles release their energy to the material and it heats up
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What is nuclear instability?
1. Every element in the periodic table has many different isotopes, when all these isotopes are added together they provide a total of several thousand nuclei, however most of thee isotopes are unusable and they decay by the emission of radiation to become more stable
2. In total there are only 253 stable nuclides and all other nuclides decay and their half lives vary from billions of years to fractions of a microsecond
- the highest atomic number for a stable nucleus is 82 (lead)
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What happens when an unstable nucleus decays?
1. It emits radiations, which change the nucleus so numbers A and Z may change
2. When Z changes the symbol X changes too as a new element as been formed
3. This product of the decay is called a daughter nucleus (the product of the decay of a radioactive ('parent') nucleus
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What is the metastable state?
When an atom or nucleus is in a metastable state, it exists for an extended time in a state other than the system's state of least energy
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When is the nucleus left in a metastable state?
When the half-life of gamma emission is much more than 10^-9s, we say that the nucleus is left in a metastable state. The half-life for metastable states varies from seconds to many years
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How do you distinguish a metastable state from a stable state? Describe metastable technetium-99m
- We use the letter 'm'
- Technetium-99m is a decay product of molybdenum-99, which can be produced in nuclear reactors
- The half life of m is long enough for it to be transported to be hospitals where it is then put to good use
- The T-99m is then chemically speared from the m and it is then used as a diagnostic tracer
- The short half-life of 6 hours makes a t-99m ideal; it produces a relatively high activity but for a short time
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What must be conserved in every nuclear reaction?
- energy
- momentum
- charge
- nucleon number
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How can a diffraction pattern be produced?
- By shining a green laser thorough a thin film of lycopodium powder, which contains very small particles of about 30micrometers in diameter
- Series of diffraction rings, caused by the scattering and interference of the light off the particles
1. High energy electrons are directed at thing targets of an element
2. The nuclei act to scatter electrons in the same way that the lycopodium power scatters light
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What was the set up of the Rutherford scattering experiment?
- Investigate the scattering of alpha particles by:
1. A thin foil of gold
2. An Alpha source placed in long thin lead container to produce a well-directed beam of alpha particle
3. Gold foil and the whole apparatus was evacuated so that the alpha particles could travel without being stopped by air
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What is the empirical formula for the radius of a nucleus?
r=r0A^1/3
- r is radius of nucleus
- r0 is 1.2fm
- A is mass number or nucleus number of nucleus
- Know graph of nuclear radius against nucleon number
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How can you prove the inverse square law for gamma radiation?
1. This relationship (I=k/x^2) can be proved by taking measurements of intensity at different distances from a gamma source, using a Geiger counter
2. If the distance form the source is doubled, the intensity is down to fall to a quarter which verifies the inverse square law
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How does stability affect N and Z?
1. For small values of N and Z, stable nuclei are roughly equal numbers of protons and neutrons, however as Z increases the number of neutrons becomes higher than the number of protons
2. The physical reason for this is that the electrostatic repulsion of the protons becomes more significant as the nucleus gowns
3. This repulsive effect balanced in a stable nucleus by extra neutrons, which provide extra attractive nuclear interaction
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What happens in K capture?
1. K capture where the electrons that orbit the nucleus in the lowest shell, the k one are tightly bound to the nucleus can be captured by the the nucleus so that a proton is turned into a neutron
2. Element and electron -> Element (decrease in proton number but same mass number) + neutrino
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What does E=mc^2 means?
- Helium nucleus contains two neutrons and two protons
| - The mss of the helium nucleus is very slightly smaller than the mass of its separate nucleus
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What happens as the helium nucleus forms?
Some mass is converted to energy and released
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What creates this extra mass?
1. Work must be done to overcome the very strong nuclear forces that bind the nucleons together and pull a helium nucleus apart
2. The energy put in to do this creates the extra mass
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How do you calculate the mass defect?
1. A nucleus of Z protons and N neutrons has a mass that is less than the mass of the protons and neutrons that make it up
2. DelaM = Zmp+Nmn - Mnucelus and this is measured in kg or au
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What is binding energy? How do you calculate it?
1. Since mass and energy are interchangeable we can also express mass as energy
2. Binding energy is the energy that corresponds to the mass defect and is binding energy = mass defect x c^2
3. Binding energy is the energy that would have to be supplied to the nucleus to separate it back into its constituent protons and neutrons, expressed in J or MeV
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What is an atomic mass unit equivalent to?
1. An atomic mass unit is 1/12 of the mass of an atom of 12C or 1.661 x 10^-27 kg
2. Using E=mc^2 a mass of 1.661x10^-27 is equivalent to 1.495x10^-10J or 931.5MeV
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How do you find the binding energy per nucleon for stable nuclei?
binding energy per nucleon = total binding energy / number of nucleons
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What happens to nuclei with a high binding energy per nucleon? Why?
- Are most stable
- More energy per nucleon is needed to pull the nucleons apart
- This info allows us to predict the stability of nuclei of different masses
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Describe the graph of binding energy per nucleon against nucleon number
1. Binding energy per nucleon increases rad-idly with nucleon number for lighter element and is about 8MeV per nucleon for helium and elements heavier than lithium
2. Helium nuclei are very stable relative to other low-mass nuclei, which explains why alpha decay is more common than proton emission
3. Binding energy per nucleon has its highest value for 56Fe at 8.79MeV our nucleon and decreases with increasing nucleon number for any stable nucleus heavier than 56Fe
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Why might binding energies be negative?
Stable nuclei have less nuclei potential energy than the free nucleons, so you may see binding energy quoted with a negative value, but usually positive as binding energy is the energy that has to be supplied to break the nucleus apart
