Nuclear Physics Flashcards
Describe the basic structure of an atom
nucleus containing protons and neutrons, around which electrons orbit in fixed energy levels/shells
Describe the plum pudding model of the atom
Sphere of positive charge with negative electrons embedded in it
What charges do protons, neutrons and electrons carry?
Protons = positive
Neutrons = no charge
Electrons = negative
Why do atoms have no overall charge?
Equal numbers of positive protons and negative electrons
What is the radius of an atom?
around 1x10^-10m
What is ionisation?
Process which adds or removes electrons from an atom
What is the mass number of an element?
Number of protons + number of neutrons
Which particle do atoms of the same element always have the same number of?
protons
What are isotopes?
Atoms of the same element (same number of protons) with different numbers of neutrons
What were the two main conclusions from the alpha particle scattering experiment?
- most of the mass of an atom is concentrated in the nucleus
- nucleus is positively charged
What are the three types of nuclear radiation?
alpha, beta and gamma
Which type of nuclear radiation is the most ionising?
alpha
What is the range in air of alpha, beta and gamma radiation?
a few cm, 1m and unlimited, respectively
what are the equation symbols for alpha and beta particles?
4 0
a and β
2 -1
What is meant by the half-life of a radioactive source?
time taken for half the unstable nuclei to decay or the time taken for the count rate to halve
What is radioactive contamination?
unwanted presence of substances containing radioactive atoms on or in other materials
Where does background radiation come from?
rocks
cosmic rays
fallout from nuclear weapons testing
nuclear accidents
Why are gamma-emitting sources used for medical tracers and imaging?
gamma rays pass through the body without causing much damage to cells
What is nuclear fusion?
when two light nuclei join to make a heavier one
How does nuclear fission occur?
an unstable nucleus absorbs a neutron, it splits into two smaller nuclei, and emits two or three neutrons plus gamma rays
Why is Rutherford’s alpha scattering experiment so important?
It led to the discovery of the nucleus and changed the model of the atom
Complete the sentence on the alpha scattering experiment: Most of the alpha particles passed straight through …
therefore the atom is mostly empty space
Complete the sentence on the alpha scattering experiment: Some of the alpha particles are deflected …
therefore the central nucleus is where charge is concentrated
Complete the sentence on the alpha scattering experiment: A very small number are backscattered/have a large angle scattering …
Nucleus is very small + massive (lots of mass) as well as being where charge is concentrated
Definition of an energy level
- exists at a particular distance from the nucleus and contains electrons with a certain defined energy (all the same)
What happens to the energy levels the further they are from the nucleus?
They have higher energy
Can electrons move between energy levels?
Yes, by gaining/losing exactly the right amount of energy
What is nuclear radiation?
Particles of energy (photons) emitted (given out) by an unstable nucleus
What does an alpha particle consist of?
2 protons and 2 neutrons
What is the charge of an alpha particle?
+2
What is the range in air of an alpha particle?
a few cm
Penetration of alpha particles
stopped by a sheet of paper (layer of dead skin cells on outside)
Ionising power of alpha particles
very high
What do beta particles consist of?
fast electron (close to speed of light)
Charge of beta particle
-1
Range in air of beta particle
10s of cm
beta particle penetration of materials
stopped by couple of mm of Al
Ionising power of beta particle
quite ionising
What does a gamma ray consist of?
high frequency energy photon
charge of gamma ray
0
Range in air of gamma ray
near infinite
Gamma ray penetration of materials
reduced by v thick lead or several feet of concrete
ionising power of gamma ray
low
Neutron consists of
a neutron
charge of a neutron
0
Which types of radiation are uncharged?
neutron and gamma
which type of radiation consists of positively charged particles?
alpha
Which type of radiation consists of negatively charged particles?
beta
Which type of nuclear radiation does not ionise substances?
neutron
Which type of nuclear radiation has the greatest ionising effect?
Alpha
Which type of nuclear radiation travels the furthest in air?
neutron
Definition of activity
The total number of radioactive events (alpha, beta, gamma or neutron radiation) taking place per second
Definition of count rate
Number of radioactive events picked up by the detector per second
Which is bigger, activity or count rate, and why?
Activity is bigger
- not all radiation enters the detector
- some radiation passes through without ionising - not picked up
Units for activity and count-rate (they are the same)
Becquerel (Bq)
How is count rate detected?
With a Geiger counter
Definition of half-life
The time taken for the activity of a sample to drop to half of the original value
Place the types of radiation in order of most to least damaging if someone is irradiated by it
Most:
gamma
beta
alpha (stopped by skin)
Place the types of radiation in order of most to least damaging if someone is contaminated by it
Most:
alpha (all of the radiation/energy absorbed by the body)
beta
gamma
Definition of fission
the process during which a large unstable nucleus splits to form 2 smaller fragment nuclei and neutrons
What is a chain reaction in fission?
A single reaction causes further reactions in this case by releasing neutrons which trigger further fission events. Usually chain reactions result in a very rapid increase in the rate of release of energy - this needs to be controlled to be safe
Definition of fusion
The process by which 2 light nuclei join together producing a more massive nucleus and lots of energy
Advantages of fusion reactors
- The fuel (hydrogen) is present in sea water
- the product in an inert, harmless gas - Helium
- BUT it is very difficult
Nuclear fusion in the sun
Hydrogen fuses to form helium
