Radioactivity

definition: spontaneous

• radioactivity happens independently external conditions

what is radiation

• alpha/beta/gamma radiaitons

what is unstable?

• nuclei are liable to decay/has too much energy

what is random nature of decay?

• cannot predict which nuclei will decay next/cannot predict when nuclei will decay

ionising

• kinetic energy lost

  • remove electrons from atoms

describe an experiment to determine whether certain source emit alpha/beta/gamma radiation or whether there’s background radiation

• use a GM counter

• place the source of radiation close to the detector

• measure the background radiation by reading from the counter when the radiation source is turned off

• put paper in between the source and detector, if it’s alpha radiation, there should be a rapid drop in activity/counts immediately. Otherwise, no change/minor change in counts

• put aluminium between the source and detector. If beta radiation, should see a rapid drop in counts. If gamma, no change

• put lead between the source and detector, if gamma, should see an immediate drop in counts

• source should be put in a lead box to prevent them penetrating and risk of ionising cells

  To find half life

• counts should be measured over a period of time and repeated and take an average

• graph of activity vs time plotted to find half life of source

• for both experiment background radiation doesn’t need to be measured as count rate is much greater

why beta particles ionise more molecules at the end of its journey than at the beginning?

• beta particles travel slower and have less kinetic energy so there are more collisions with atoms/molecules toward the end of the range as beta particles take longer to travel a given length

In the radiation experiment to determine count rate, why sometimes low proportion of decays are recorded

• emission of radiation in all direction

• so some radiation may be absorbed by the windows

• some maybe absorbed by other materials in the sample

Why most of times in experiments it is better to use samples with longer half life

So activity won’t change significantly over time

What are the four scenarios when an atom may become unstable

• the atom having too many neutrons

• the atom having too few neutrons

• the atom having too much mass

• the atom having too much energy

How do unstable nuclei become more stable

• emit radiation to become more stable

What does beta-plus radiation consist of? What are the penetrating capabilities of beta-plus radiations

• High-energy positrons, when a proton turns into a neutron emitting a positron and an electron neutrino

• no range

Under what circumstance is alpha radiation emitted?

When is gamma radiation emitted?

When a nucleus has too much mass

When a nucleus has too much energy

Under what circumstance is beta minus radiation emitted

when is beta plus radiation emitted

When a nucleus has too many neutrons

When a nucleus has too many protons

Explain the process of beta minus decay

Where a neutron in the nucleus turns into a proton, and release an electron and an electron antineutrino

Explain why the decay constant of an isotope can be determined even though nuclear decay is random

• probability of decay(at a specific time) may be determined

• probability may be applied accurately because very large numbers involved

How does the mass of a nucleus compare to the sum of the individual masses of its constituents

The mass of the nucleus is always less than the sum of the individual masses of its constituents

What is the name give to the difference in the mass of a nucleus and its individual constituents

the mass defect

Explain why there is a mass defect in nuclei

The mass defect is a result of some of the mass being converted into energy used to hold the nucleus together

Binding energy definition

why isn’t it useful to compare binding energies for different nuclei

the energy equivalent to mass deficit, when nucleus bind to form an atomic nucleus/energy required to separate completely the individual nucleons in nucleus

different nuclei have different numbers of nucleons

What is the relationship between the stability of a nucleus and its binding energy per nucleon

The larger the binding energy per nucleon, the more stable the nucleus will be

What are the two types of fission

spontaneous fission

induced fission

explain induced fission

A large nucleus absorbs a thermal neutron causing it to split to two smaller nuclei, releasing energy and at least one neutron

explain nuclear fusion

two smaller nuclei join together to form a larger one, and releases energy in the process

why is nuclear fusion not currently a feasible method of energy production

requires very high temperatures in order to overcome the electrostatic force between the nuclei

Criticise method of determining count rate

• should record background radiation

• too little repeats

• should record for more minutes and take a mean

why use a plastic bag to store alpha radiation source

alpha won’t penetrate plastic so gas won’t escape, and reduce risk of ionisation of cells

relationship between count rate and the distance between source and gm tube

what about area

if distance is doubled, count rate is quartered

greater the area, greater the count rate

if given count rate, how to find activity

activity = count rate in seconds

explain why there is an exponential decrease in the rate of decay for a sample containing a large number of unstable nuclei

• there is a fixed probability of an individual nucleus undergoing decay

• for a sample with large number of unstable nuclei, there is a predictable pattern

• the fraction of nuclei decaying in the next second is equal to the decay constant

• hence the number of nuclei decaying depends on the number of nuclei A=lamda*N

• the number of unstable nuclei decreases

Explain the conditions for nuclear fusion in a star

• high temperature

• nucleus have positive charge, so there is a strong force of repulsion between two nuclei

• the kinetic energy gained from high temperature provide energy to overcome this force of repulsion

• two nuclei need to be close enough to fuse

• high density/high pressure

• to maintain high collision rate to maintain fusion

on the binding energy per nucleon and nucleon number graph, where is fission and fusion

• fusion at the beginning before the peak - Iron-56

• fission at the end after the peak

• both fusion and fission have an increase in binging energy per nucleons during the process - for fission this is because nucleon number decreases in the process

• if binding energy per nucleon increases, energy is released