Nuclear Physics Flashcards
What is the radius of an atom?
1 × 10^−10 m
Provide some evidence for the distribution of mass in the atom
Scattering of alpha particles by thin metal foil
What happens to an electron if it gains energy by absorbing EM radiation?
They can move to higher energy level = further away from nucleus
What happens to an electron if it releases EM radiation?
It will move to lower energy level = that’s closer to nucleus
What happens if one or more electrons leaves an atom?
Atom becomes positively charged ion
What is an isotope?
Atoms with same no. of protons (same atomic number & same charge on nucleus) BUT different no. of neutrons (different mass number)
Do all elements have isotopes?
Yes
How many roughly stable isotopes do each element have?
1-2
What is radioactive decay?
When unstable isotopes decay into other elements & give out radiation as they try to become more stable
What can unstable isotopes also release when they decay?
Can release neutrons (n)
What is ionising radiation?
Radiation that knocks electrons off atoms, forming positive ions
What does the ionising power of a radiation source mean?
How easily it can knock electrons off atoms (forming positive ions)
What do radiation substances release?
Radioactive substances spit out one or more types of ionising radiation (e.g. beta, gamma, alpha)
Where is radiation given off in a radioactive substance
From the nuclei of their atoms
Why can’t you predict exactly which nucleus in a
sample will decay next, or any one of them will decay?
Radioactive decay is entirely random
How can radiation can be measured?
By using a Geiger-Muller tube and counter
How does Geiger-Muller tube and counter measure radiation?
Records the count-rate i.e. Number of radiation counts reaching it per second
What can half-life be used for?
Used to find rate at which a source decays - its ACTIVITY
What is activity measured in?
Becquerels, Bq (where 1 Bq is 1 decay per second)
Each time radioactive nucleus decays (to become stable nucleus), what happens to its activity?
Its activity as a whole will decrease
Do older sources emit more or less radiation?
Older sources emit less radiation
Why can’t we measure how long exactly it will take for a nucleus to decay?
The activity never reaches 0
What is half-life?
- The time taken for the number of radioactive nuclei in an isotope to halve
- The time it takes for the count rate from a sample containing the isotope to fall to half its initial level
What does it mean if a source has a short half-life?
The activity will fall quickly
Why would a source have a short half-life?
Because nuclei are very unstable and rapidly decay
Why are sources with short half-life dangerous?
Because they emit a high amount of radiation at start
What is good thing about sources with a short half-life
Can quickly become safe
What does it mean if a source has a long half-life?
Its activity falls more slowly = releases small
amounts of radiation for long time
Why would a source have a long half-life?
Because most of nuclei don’t decay for a long time
Why are sources with long half-life dangerous?
Nearby areas are exposed to radiation for (millions of) years
What do nuclear equations show?
Are a way of showing radioactive decay by using element symbols
What is the form nuclear equations are written in?
atom before decay → atom(s) after decay + radiation emitted
What is the golden rule to remember when writing nuclear equations?
The total mass and atomic numbers must be equal on both sides
What happens to an atoms atomic number and mass number when it emits an alpha particle?
Its atomic number reduces by 2 and its mass number reduces by 4
Why does the charge of the nucleus decrease when an atom emits an alpha particle?
Its atomic number reduced by 2 = a proton is positively charged and a neutron is neutral
How can an alpha particle be written as in a nuclear equation?
As a helium nucleus - see mind map for answer
Write out the nuclear equation for Uranium-238 which gives off thorium-234 and an alpha particle
See mind map for answer
What happens to an atom when beta decay occurs?
A neutron in nucleus turns in a proton and releases a
fast-moving electron (beta particle)
Why does the charge of the nucleus increase (become positive) when an atom emits a beta particle?
The number of protons in nucleus has increased by 1
Why does the mass of the nucleus not change during beta decay?
Because the nucleus has lost a neutron and gained a proton during beta decay ∴ mass of nucleus doesn’t change (protons and neutrons have same mass)
How can a beta particle be written as in a nuclear equation?
See mind map for answer
Write out the nuclear equation for Carbon-14 which gives off nitrogen-14 and a beta particle
See mind map for answer
How does a nucleus get rid of excess energy?
By releasing it as gamma rays
Do gamma rays change the atomic mass or atomic number of an atom?
No
How can gamma radiation be written as in a nuclear equation?
See mind map for answer
Practice Question on Half-life:
The initial activity of a sample is 640 Bq. Calculate the final activity as a percentage of the initial activity after 2 half-life.
= 25% (see mind map for working out)
What are alpha particles?
Helium Nuclei
What is alpha radiation?
When an alpha particle (α) is emitted from nucleus
What is an α-particle made up of (i.e. its no. of neutrons and protons)?
2 neutrons and 2 protons (like a helium nucleus)
What is the charge of an alpha particle?
2+
How far can an alpha particle travel in air?
Only can travel a few cm in air
What will absorb an alpha particle?
A sheet of paper
How ionising are alpha particles and why?
Strongly ionising because of their size
Give an example how alpha radiation can be used in a home
Alpha radiation can be used in a smoke detector. It ionises the air particles, causing current to flow. If there’s smoke in the air, it bind to the ions = meaning current stops and the alarm sounds.
What is a beta particle?
Beta particle (β) is a fast-moving electrons released by nucleus
How much mass does a beta particle have?
Virtually has no mass
What is the charge of a beta particle?
Charge of -1
How ionising are beta particles?
Moderately ionising
How far can a beta particle travel in the air?
A few meters
What will absorb a beta particle?
A sheet of aluminium (around 5 mm thick)
How can beta radiation be used (if a beta particle is moderately penetrating)?
Beta emitters are used to test the thickness of sheets of metal as particles are not immediately absorbed by material (like alpha radiation & do not penetrate as far as gamma rays). ∴ Slight variations in thickness affect the amount of radiation passing through the sheet.
What are gamma rays (γ)?
Waves of electromagnetic radiation released by nucleus
What the penetration of gamma rays and how far will it travel in the air?
They penetrate far into materials without being stopped and will travel a long distance through air
What is a gamma ray’s charge?
Has no charge
How ionising are gamma rays and why?
Weakly ionising because they tend to pass through rather than collide with atoms
What will absorb a gamma ray?
Absorbed by thick sheets of lead or meters of concrete
