5. Nuclear Physics Flashcards
Atom
The smallest unit of an element, made from neutrons, protons and electrons.
Proton - (Symbol, charge, mass, location)
Symbol = p
Charge = +1
Mass = 1
Location = In the nucleus
Nucleus
The central part of an atom that contains all the protons and neutrons, and so all the mass and positive charge. (pl. nuclei)
Neutron - (Symbol, charge, mass, location)
Symbol = n
Charge = 0
Mass = 1
Location = In the nucleus
Electron - (Symbol, charge, mass, location)
Symbol = e
Charge = -1
Mass = 0 (1/2000) (tiny mass)
Location = Orbit the nucleus
Testing the atomic model
In 1911, Ernest Rutherford proposed the atomic model based on the Rutherfords Gold Foil experiment (aka Alpha Scattering experiment.)
- Beam of alpha particles was directed through a slit in a flourescent screen towards a piece of thin gold foil (1 atom thick)
- Experiment carried out in vaccum so particles didn’t hit any air particles –> only collided with foil
- When the particles hit the circular flourescent screen surrounding the model there was a flash of light
Result of alpha scattering experiment
Most particles passed through the gold foil. Some were deflected slightly but still passed out the other side. Very few were completely deflected and hit behind where they were emitted.
All 4 atom features
- The number of protons and electrons are equal.
- Most of an atom is empty space. The size of the nucleus is incredibly small compared to the size of an atom.
- The nucleus is positively charged.
- The nucleus accounts for nearly all the mass.
Explanation of Alpha scattering results - (The number of protons and electrons are equal.)
Atoms do not have an overall charge
Explanation of Alpha scattering results - Most of an atom is empty space. The size of the nucleus is incredibly small compared to the size of an atom
Most of Rutherford’s α - particles went straight through the gold foil without changing their direction.
Explanation of Alpha scattering results - The nucleus is positively charged.
Alpha particles are positive. When some particles come close to the nucleus of the gold foil, they were repelled. The nucleus must therefore also be positive.
Explanation of Alpha scattering results - The nucleus accounts for nearly all the mass.
In one experiment the α - particle was deflected back in the opposite direction when a direct hit occurred. This change in momentum could only happen if the tiny nucleus also contains most of the mass.
Nuclide
An atom or nucleus characterised by a specific number of protons and neutrons.
Nuclide Notation
A notation using symbols for elements along with atomic number and nucleon number to describe the composition of an element’s nucleus.
A
Z X
A - Mass number
Z - Proton number
X - Element
Z (proton number) - In relation to charges
Z = relative charge of the nucleus
A (mass number) - In relation to mass
A = relative mass of the nucleus
Isotope
Atoms of the same element that have different nucleon numbers due to a varied number of neutrons.
Nuclear Fusion
Small nuclei fuse to form larger nuclei and release energy.
This is what happens in the sun
Eg.
2 2 4
1 H + 1 H –> 2 He
Total mass is conserved
Nuclear fission
Large nuclei split up into smaller nuclei and release energy.
Nuclei with very large nucleon numbers have heavy nuclei and are often unstable. This is why large nuclei often split up.
This can happen independently or can be caused by a single neutron colliding with the nuclei.
Total mass is conserved
Eg. A neutron is fired into the uranium-235 nucleus, causing the nucleon number to increase. A larger mass makes the nucleus unstable. Nuclear fission then occurs and the heavy nucleus splits into krypton and barium with three spare neutrons.
Background Radiation
The average level of radiation detectable as part of everyday life, due to a combination of natural and man-made sources.
Types of background radiation
Food - (eg. bananas –> dosage of 0.1 micosieverts)
Building materials + rocks - (eg. granite, concrete both contain elements such as uranium, radium and thorium)
Radon gas - (naturally found in atmosphere because of uranium decay)
Cosmic ray - (high radiation waves travelling from outside of our solar system)
Medical procedures - (eg. X-rays, CT scans)
Geiger–Müller (GM) tube
Detects alpha, beta and gamma radiation. A ‘click’ can be heard as the radiation is detected and a digital count is recorded.
Spark Counter
Detects alpha, beta and gamma radiation with an audible click.
Cloud Chamber
Shows the paths of alpha and beta particles by producing a vapour trail behind each particle.
How to calculate the radioactivity of a sample.
Count rate (counts/s) = Measured count rate (counts/s) - Background radiation (counts/s)
Alpha particles
A alpha particle is made up of 2 protons and 2 neutrons (like a helium nucleus)
Total mass = 4
Total charge = 2+
Strong ionising radiation –> large kinetic energy + large charge + concentrated mass
Least penetration - Will only travel approx. 5 cm in air + stopped by thin paper or skin
Alpha particle in an electric field is attracted to the negative plate bc of positive charge.
a (a-4) 4
z X –> (z-2) Y + 2 a
Beta Particle
A high energy electron
During beta decay a neutron decays into a proton and an electron is released. –> thus mass stays the same but the proton number increases.
Mass = almost none (approx. 1/2000)
Charge = -1
Mildly ionising
Mildly pentrative - Can travel through skin but stopped by a couple cm of aluminium foil.
Beta particle through an electrical field
Attracted to the positive plaete bc of negative charge.
a a + 0 0
z X –> z +1 Y + -1 e
a
z X
X - element symbol
a - mass number
z - proton number
Gamma rays
Gamma emissions are electromagnetic waves. Gamma emissions are of very high frequency and have very high energy.
Mass = 0
Charge = 0
Most penetrating - can only be stopped by several cm of lead or thick concrete
Weakly ionising - have high energy so still slightly ionising
Through an electrical field
No charge so the emissions aren’t attracted or repelled.
a a 0
z X –> z Y + 0 y
2nd is still the same element/ isotope as X
Half-life
Half-life is the time taken for the count rate of a radioactive source to decrease by half.
Half-life is the time taken for the number of radioactive nuclei to decrease by half.
Number of remaining nuclei (half-life formula)
Number of remaining nuclei = original amount / 2^n
n = number of half-lifes that have occurred.
(after approx. 5 halflifes 99% of the sample has decayed.)
Half life - (axis)
Y-axis = could be mass, nuclei number, count rate…
X-axis = is always Time (units)
How to handle / store alpha particles
Alpha particles can be stored in thin package sicne they are weakly penetrating.
Since they are highly ionizing protective clothing made of lead must be used just in case of penetration.
How to handle / store beta particles
Beta particles can be stored in lead / similarly dense metal container.
Protective clothing + gas mask worn.
How to handle / store gamma particles
Highly penetrative so buried deep underground to stop radiation causing problems.
Weakly ionising but still very penetrative so protective clothing should definetely be used. Usually robots handle gamma to protect humans.
Smoke Detectors - Uses of radiation
- Alpha particles emitted from sample –> land on detector. Bc of the 2+ charge a current flows.
- Smoke disrupts particles so breaks the current –> alarm gets set off.
Thickness measurement - Uses of radiation
- Beta particles emitted towards metal sheet. Detector placed underneath sheets. Depending on the amount of particles let through, the rollers are adjusted.
- Beta particles used because they are medium penetrative.
Fault detection - Uses of radiation
- Pipes wrapped in x-ray films. Radioactive sample inside the pipes.
- When radiation reaches the x-ray film it gets developed in those places.
- Gamma radiation used bc very penetrative + small half-life needed so as not to cause issue later on.
Irradiating food - Uses of radiation
- Large ‘blasts’ of UV-light (weakly ionising) directed at food.
- Kills single celled organisms –> extends shelf life + makes food safer for pateints with low immune systems to eat –> hospital food.
Cancer treatment - Uses of radiation
- Gamma or x-rays –> directed at tumour –> kills cancerous cells.
Organ function testing - Uses of radiation
- Radioactive tracer with small half-life injected into bloodstream. Patient put under radiation detector –> shows any places with cluster of radiation –> blood clots
- Gamma used bc its weakly ionising + penetrates skin.
Safety strategies when dealing with radioactivity
- Limit exposure time (less radiation + ionisation then)
- Sheilding (place barrier between radioactive material + human)
- Distance (increase distance from radioactive source –> less intense)
Sterilising medical equipment - Uses of radiation
- Gamma rays directed at equipment kills bacteria by breaking down their DNA.
- Gamma used bc its highly penetrative.
- This limits infections during surgery etc.
