Nuclear Physics Flashcards
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
Q
What are unstable nuclei like?
A
- 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
Q
How can you use penetrating power to investigate radiation types?
A
- 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
Q
How can you control how thick material is using radiation?
A
- When creating sheets of material, like paper foil or steel, ionising radiation can be sued to control its thickness
- The material is flatted as it is fed through rollers
- 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
- 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
Q
Describe the different ionising properties of alpha and beta particles
A
- 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
Q
How are gamma rays used in medicine?
A
- 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
Q
How do you detect background radiation?
A
- 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
Q
What are the different sources of background radiation? Describe them
A
- The air
- The ground and buildings
- Cosmic radiation
- Living things
- Man-made radiation
32
Q
What is the air’s background radiation?
A
-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
Q
What are the ground and building background radiation?
A
-All rock contains radioactive isotopes
34
Q
What is cosmic radiation background radiation?
A
- 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
Q
What are living thing’s background radiation?
A
- 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
Q
What are man made background radiation?
A
- 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
Q
What does the intensity of gamma radiation obey?
A
-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
