Chapter 26: Radioactivity

Why was the alpha particle placed in a lead container for Rutherford’s scattering experiment?

To produce a collated beam of alpha particles by restricting the direction of emissions

Why was gold chosen? Why did the gold need to be thin?

Gold-chosen due to its malleability, meaning it was easier to hammer into thin sheets. Needed to be thin, approximately x10-6 m, thick foil would stop the alpha particle completely.

Why are alpha particles the most ionising?

Highest charge of +2e Produced highest number of ion pairs per centimetre travelled in air Damage to cells than any other type of radiation

When was Rutherford’s gold foil experiment? What were the key findings?

1909- 1911 Atom is mostly empty space Central positively charged nucleus containing a majority of mass.

What were the main observations of Ruth Rutherford’s experiment? What did these tell us about the atom?

The majority of the alpha particles passed straight through the foil undeflected- suggested the atom was mostly empty space. Some particles were deflected through small angles-suggested the nucleus is positive as to like charges would repel. Very small number of particles deflected straight back at 90°. Suggests the nucleus is very small and is where most of the mass in charge of the atom is concentrated

What did Rutherford intend to investigate with the gold foil experiment?

The different angles of deflection of the alpha particles. The number of alpha particles that were deflected at each angle

What did Neils Bohr think about the atom? When?

1913 Improved Rutherford’s model. Electrons orbit the nucleus like planets around a star. Electrons occupied well defined shells/energy levels around nucleus.

What did John Dalton think about the atom? When?

1803 Suggested all matter was made of tiny solid particles- atom Couldn’t be broken down Different for every element. Chemical reactions occur – atoms rearranged to make different substances

What did JJ Thompson think about the atom? When?

1897 Discovered the electron Plum pudding model Sphere of positive charge- uniformly distributed. Negatively charged electrons- currants in the pudding

What did James Chadwick think about the atom? When?

1932. Discovered the neutron

What are beta plus and minus particles when are they emitted?

Beta plus- high energy positron emitted by the nucleus with too many protons Beta minus- high energy electron emitted by a nucleus with too many neutrons.

What apparatus was required for Rutherford scattering experiment

Alpha meter and lead container Thin sheet of gold foil Movable detector

What were the main observations of Rutherford scattering experiment?

Most Alpha particles were not deflected. Some alpha particles were deflected by a large angle. A small number of particles with deflected backwards.

Define the distance of closest approach.

An estimate the nuclear radius, for alpha particles it gives a maximum value.

How can we model gamma radiation as it is method?

Using the inverse square, the moment the radiation (modelled as light as it is part of the electromagnetic spectrum) it begins to spread a uniform as a sphere. As EM wave spread out the area over which it spreads is proportional to the radius squared I=k/x^2

Define background radiation.

The ionising radiation present in the environment.

Give some examples of sources of background radiation.

Red on gas Cosmic rays Food Medical Rocks and building materials.

Where does radon gas come from?

From rocks in the ground as well as building materials such as stone and brick. This is due to the presence of radioactive elements such as uranium occur naturally in small amounts in all rocks and soil .

How do cosmic rays lead to production of radiation?

Sun emits a huge number of protons every second. Some of these protons into the earths atmosphere at high speeds. When they collide with molecules in the air, it leads to the production of gamma radiation.

What is corrected count rate?

The measure count rate from a radioactive source minus the background count rate

How by choosing specific types of isotopes can we minimise the received radiation dose from a sample?

Choose a short lived isotope (an isotope with a short half life) compared to one with a long half life. Use a smaller sample of radioactive material, the smaller the batter.

What precautions must be taken to reduce the risk of harm when using radio sources?

Keeping sources shielded when not in use in a lead lined box Wearing protective clothing to protect the body and clothes from being contaminated. Keeping personal items outside of the room to rent them from being contaminated. Limiting exposure time. Handling radioactive material materials with long tongs to increase the distance. Monitor the exposure of workers such as a radiographers using detector badges.

When using radiation in medicine what are the precautions for the patient?

Patient should be protected with led to cover parts of the body not exposed. Exact day should be calculated carefully. Though should be directed accurately at the cancerous tissue to minimise damage to healthy tissue.

When using radiation in medicine what are the precautions for the radiographer?

Handle the source remotely with tongs or a machine. Stand behind a protective screen. Maintain distance from the source whilst the dose is given. Immediately still the sauce in a lead case once the dose is given.

Why are radioactive traces with a short half-life preferred?

Initial activities is very high as only small sample is needed. Short to the half life of the first of the ice decay Isotopes with short half-life pose a lower risk to the patient. Medical test doesn’t last long so a short half-life is sufficient.

What are two examples of radioactive traces?

Iodine- 131 Technetium- 99m

Summarise iodine 131, what is it useful for? What does it emit? How long is the half-life?

Specifically taken up by the thyroid gland, making it useful for monitoring and treating thyroid conditions. Emits beta particles meaning of state concentrated on the thyroid area and nowhere else in the body. A short half life of eight days meaning it won’t be around to cause prolonged exposure,

Summarise Technetium- 99m. What is it useful ? What does it emit? How long is the half-life?

Gamma emitter, my idea for detection Short half-life of six hours so is ideal for use as a tracer or not remain active for too long and can be tolerated by the body. Most penetrating so it can be detected outside the body. Weekly ionising and so causes minimal damage Can be prepared easily at hospital when required making it cost-effective treatment.

Why is gamma radiation widely used to sterilise medical equipment?

Most penetrating out of all types of radiation Penetrating enough to radiate all sides of the instruments. Instruments can be sterilised without removing the packaging.

Define radioactive decay.

Spontaneous disintegration of a nucleus deform a more stable nucleus resulting in the emission of an alpha beta or gamma particle. It is a random process meaning there’s an equal probability of any nucleus decaying and it cannot be known which particular nucleus would decay next, the rate of decades unaffected by the surrounding conditions.

Define average decay rate.

The average number of nuclei that are expected to decay per unit time.

Define decay constant.

The probability that an individual nucleus would decay per unit time

Define activity, what are the units?

The average number of nuclei the decay per unit of time Units = Becquerels

What are the key features of a radioactive decay graph?

Y axis- number of undecayed nuclei X axis- time The steep of the curve the larger the decay constant (lambda) Decay curves always start on the Y axis but never cross the X axis

How to calculate the number of moles

Mass / the molar mass

Define Avogadro’s constant.

The number of atoms in one mole of a substance.

How to calculate the number of nuclei

(Mass X Avogadro’s constant) / molar mass.

Define half life.

The average time taken for a given number of nuclei of a particular isotope to halve

What is the relationship between half-life and the decay constant?

The short of the half-life the larger of the decay constant and the faster the decay

What are some of the main uses of radioactivity?

Nuclear power Medicine- radiotherapy, tracers and sterilising equipment Radiocarbon dating of archaeological artefacts Uranium- lead dating of rock samples Radioisotope power systems

Which isotope is usually used in radioactive dating? What is it’s half life?

Carbon-14 5730 years

How does carbon dating work?

During organisms life, proportion of Carbon-14 is constant as carbon is constantly being replaced When organism dies- activity starts to fall Sample can be tested, ratios of carbon-14 to carbon-12 can be compared and then dated

What causes a nucleus to be unstable?

If it has: Too many protons or nucleons Too many nucleons ie it’s too heavy Too much energy

Where are the following isotopes found on the nuclear stability line? Beta minus emitters, beta plus and electron capture.

Beta minus- found to the left of the stability line where isotopes are neutron rich Beta plus- found to the right of the stability line where the isotopes are proton rich Electron capture- sound to right of stability line where the isotopes are proton rich

What happens when a nucleus has too much energy?

Gamma emission Usually occurs after different type of decay e.g. alpha or beta, nucleus becomes excited have has excess energy

What happens during beta minus decay?

Too many neutrons, electron and anti-electron neutrino released Neutron changes to a proton Neutron number decreases by 1, proton number increases by 1 Top number stays same, bottom number increases by 1

What happens during beta plus decay?

Too many protons Proton changes to a neutron, positron and neutrino are released Neutron number increases by 1, proton number decreases by 1 Top number stays same, bottom number decreases by 1

What is beta plus decay also known as?

Electron capture

What happens during alpha decay?

Nucleon number decreased by 4, proton number decreased by 2 Minus 4 from top number, minus 2 from bottom number

What is the half life of Molybdenum-99?

66 hours

What is the half life of Technetium-99m?

6 hours

How can we estimate the radius of a nucleus?

Alpha particles fired at sheet of gold foil At closest approach, the repulsive force reduces the speed of the alpha particle to zero momentarily At this point, the initial ke of the alpha particle is equal to the electric potential energy Making the two formulae equal to each other, r can be calculated r = closest approach

Why does the alpha scattering only give an estimate of closest approach?

Electrostatic force of repulsion between the positively charged alpha and nucleus prevents the two from getting too close Therefore provides an over-estimate

What are the advantages of the closest approach method?

Gives a good estimate for the upper limit for a nuclear radius Maths behind this approach are very simple Alpha particles are scattered by only by protons and not all the nucleons that make up the nucleus

What are the disadvantages of the closest approach method?

Does not give an accurate value for the nuclear radius, always gives an overestimate Alpha particles contain hadrons which are affected by the strong nuclear force Gold nucleus will recoil as alpha particle approaches Alpha particles have a finite size and mass where’s electrons can be treated as a point mass Very few particles rebound at exactly 180 degrees, so to detect these, a small collision region is required Alpha particles in the beam are assumed to have the same initial ke which may not be realistic Foil target must be extremely thin to prevent multiple scattering

How does the electron scattering provide an estimate for the radius of a nucleus?

Beam of electrons directed at thin target, each electron diffracts around nucleus (Diffract as de Broglie wavelength of electrons similar to size of nucleus) Forms diffraction pattern, graph of intensity against diffraction angle can be plotted Size of nucleus can be determines using the angle of the first minimum intensity

What are the advantages of the electron scattering experiment?

Much more accurate than the closest approach method Method gives direct measurement of the radius of the nucleus Electrons are leptons so they are not affected by the strong nuclear force

What are the disadvantages of the electron scattering experiment?

Electrons must be accelerated to very high speeds to maximise the resolution as significant diffraction takes place when the electron wavelength is similar in size to the nucleus Electrons can be scattered by both protons + neutrons, if excessive amount of scattering, can be difficult to determine first minimum of diffraction.

What does the graph of intensity against angle look like for electron diffraction?

INSERT IMAGE

What does the graph for nuclear radius vs nucleon number look like? What are the key features? What does this mean?

INSERT GRAPH Key Features: grasp starts with steep gradient at the origin then the gradient gradually decreases to almost horizontal Means that: As more nucleons are added, nucleus gets bigger, number of nucleons is not proportional to the size of r

What is the limit of proportionality R0 equal to?

1.05 fm

What do the letters stand for in the equation: R=R0/A^1/3?

R= nuclear radius R0= constant of proportionality (1.05 fm) A= nucleon number/ mass number

What do we know about the density of the nucleus?

It is constant Independent of the radius

What does the fact that the nuclear density is constant tell us?

That nucleons are evenly separated throughout the nucleus regardless of their size

What does the fact that nuclear density is significantly larger than the atomic density suggest?

The majority of the atom’s mass is contained at the nucleus The nucleus is very small compared to the atom Atoms must be predominantly empty space

What did Einstein propose about energy and mass?

Mass can be converted into energy Energy can be converted into mass

Give some examples of mass-energy equivalence

The fusion of hydrogen into helium in the centre of the sun The fission of uranium in nuclear power plants Nuclear weapons High energy particle collisions in particle accelerators

Define mass defect

The difference between an atom’s mass and the sum of the masses of its protons and neutrons

Define binding energy

The amount of energy required to separate a nucleus into its constituent protons and neutrons

Why do nuclear reactions produce more energy than chemical reactions?

Nuclear reactions involve changed in the nuclear binding energy whereas chemical reactions involve changes in the electron binding energy.

What is the atomic mass unit equivalent to?

The mass of exactly one-twelfth of an atom of carbon-12