Chapter 8: Nuclear Physics Flashcards
Describe the Rutherford scattering experiment.
- a beam of alpha particles was directed at a thin gold foil
- occurs in a vacuum so that no collisions between air particles and alpha particles can occur
- the experiment was done in order to determine structure of an atom
In the Rutherford scattering experiment it was observed that most of the alpha particles passed straight through. What can we infer from that?
the atom is mostly made from empty space
What evidence suggested that the nucleus had a positive charge?
Because the nucleus repels the alpha particles and caused it to deflect from its original path, some of them even bounced back,
Names 3 types of radiation.
- alpha
- beta
- gamma
Order the 3 types of radiation starting with the most ionising.
- alpha
- beta
- gamma
Order the 3 types of radiation starting with the most penetrating.
- gamma
- beta
- alpha
A sheet of paper can block which type of radiation?
Alpha radiation
When a nucleus decays through gamma radiation, how does the atomic number and mass number change?
They remain the same
Why is ionising radiation seen as dangerous?
Because it can kill or mutate cells, which could lead to mutations and therefore cancer
Which radiation is more harmful inside a human body, alpha or gamma?
Alpha — it has a high ionising power so it would damage more cells. It is also very poorly penetrating, therefore it is not able to leave the body, whereas gamma radiation is highly penetrating.
Give an example of a real life use of Beta decay and explain why Beta is chosen for this.
Beta radiation can be used to measure the thickness of paper or aluminium foil. Alpha isn’t used as it is less penetrative and wouldn’t reach the detector on the other side of the sheet. Gamma radiation is too penetrative and would pass through everything.
Which type of radiation follows the inverse square law?
gamma radiation
What does the inverse law state?
The intensity proportional to the square of the distance from the source.
What is intensity measured in?
Watts per metre squared
Describe and experiment which can be used to show the inverse square law and gamma rays.
- firstly measure the background radiation without gamma source
- put the gamma source at a set distance from GM tube and measure the count rate per minute. record 3 measurements and take an average.
- do this for many distances in 10cm intervals
- take away the background radiation for each reading
- square the distances and plot a graph of count rate per minute against 1/ distance squared
- if it is a straight line through origin direct proportionality confirmed
What is background radiation?
Radiation that is constantly in the surrounding from sources such as rocks and food
What is the decay constant? (λ)
The probability of a nucleus decaying per second
What are the units for the decay constant? (λ)
s^-1
What is half life?
The time it takes for half of the unstable nuclei in a substance to decay.
What equation can you use to work out the half life of an object?
T1/2 = ln(2)/λ
What is activity measured in?
Bq (decays per second)
True or false. Radio active isotope decay exponentially.
True.
Why is Technetium 99m useful in medicine?
- because it releases gamma radiation
- it has a short half life therefore it doesn’t stay highly radioactive for long
- half life of 6 hours: long enough to be detected
- can be made near to the hospital
- easy to detect outside the patient
- ‘clears away’ after a few days
What does the graph of N against Z show?
It shows the relation between proton number and neutron number. The graph shows a stability curve which starts as N=Z until N value of 20. After that the graph curves upwards and becomes steeper.
Where on the curve does β- decay occur and why?
Above the stability line, because the nuclei found there contains too many neutrons. Therefore when beta minus decay occurs the neutron turns into a proton and it becomes more stable.
What type of decay occurs below the stability line and why?
Beta plus decay. As the isotopes found here often have too many protons. Therefore when beta plus decay occurs the proton turns into neutrons.
How does heavier nuclei often decay?
Through alpha decay. This is because alpha decay emits a helium nucleus (2 protons and two neutrons) therefore causing the nuclei to become less heavy and more stable.
An alpha particle is fired at a nucleus, with the kinetic energy at the start known. How can you use energy conservation to find closest approach of the particle?
we can work out how close a particle will get to a nucleus: we known the Ek heads towards the nucleus and the electrostatic potential energy that repels it. Closest approach is when these two things are equal.
How is electron diffraction used to determine the diameter of a radius?
- an electron beam is fired at a thin sheet of the desired atom
- a diffraction pattern is produced on screen behind
- using the angle if a minimum we can use equations to calculate the diameter
What is the relationship between nuclear radius (R) and nucleon number (N)?
The nuclear radius is directly proportional to the cube root of the nucleon number.
True or false. The density or a nucleus is independent of its radius.
True
What equation is used to convert mass to its energy equivalent?
E=mc^2
What is mass defect?
The difference between the total mass of all the nucleons separately compared to the mass of the nucleus.
Why is there a mass defect?
Because energy is needed to bring the constituent parts of a nucleus together, therefore the mass equivalent of the energy is lost and the total mass decreases.
What is binding energy? Draw a graph to show this.
The energy required to separate a nucleus into its constituent parts.
What is nuclear fission?
- where an unstable nucleus splits into 2 smaller nuclei
- often occurs with the larger nuclei
- the binding energy per nucleon increases when fission occurs therefore the overall process releases energy
What is fusion?
When two small nuclei fuse together to create a larger nuclei. The new nucleus has a larger binding energy per nucleon that the old nuclei therefore energy is released in the process.
Why is it difficult to make fusion occur on earth?
There is a large repulsion between the two positively charged nuclei, therefore a lot of energy is required to overcome the repulsion and fuse them together.
It is hard to get a material that can withstand the heat and be cost effective.
How is fission used it nuclear reactors?
Rods of uranium-235 absorb neutrons and become unstable and then split into two daughter nuclei. It also releases 2/3 neutrons which go on to be reabsorbed by another uranium-235
What is the purpose of a moderator?
To slow down the neutrons so they travel slow enough to be absorbed by the uranium.
They do this through elastic collisions between the moderator and the nucleus.
Why are control rods essential for a nuclear power station?
They stop the chain reaction from being out of control. They absorb neutrons so that only 1 of the neutrons released in each reaction can go on to be absorbed by another uranium. If not then the nuclear reactor would overheat as too many reactions would happen at once.
Is boron used as a control rod or a moderator?
Control rod.
Give an example if a material that can be used as a moderator?
Water
What is the purpose of using water as a coolant?
It allows heat from the nuclear reactor to escape, which stops the reactor from overheating.
What is the critical mass?
The minimum mass of fuel needed for a chain reaction to occur.
Which waste products from a nuclear reactor cause the highest risk?
Spent fuel rods
How is high-level waste disposed of?
- they are first stored in cooling ponds
- then they are put in sealed steel containers and put deep underwater
What is low level waste contained in?
They are sealed in containers and put underground until it is safe again.
