Radioactivity Flashcards
What is meant by the random nature of radioactive decay
There is equal probability of any nucleus decaying
It cannot be known which particular nucleus will decay next
It cannot be known at what time a particular nucleus will decay next
The rate of decay is unaffected by the surrounding conditions
It is only possible to estimate the proportion of nuclei decaying in the next time interval
Define isotope
Different forms of the same element
Same proton number Z
Different nucleon number A as different number of neutrons
Define radioactive half-life
Half-life is time for number of nuclei to halve for a particular isotope
Define radioactive decay constant
Probability of decay in unit time
Electron capture forms an argon atom which subsequently releases a photon - explain how this occurs
Orbital electron vacancy due to electron capture
Outer electron fills vacancy and emits photon
State one assumption made when calculating the approximate density of nuclear matter
Nucleus is spherical
Ignoring gaps between nucleons
All nuclei have same density
Total mass = mass of constituent nucleus
Explain what is meant by induced fission
Large nucleus splits into two smaller nuclei and is brought about by bombardment or collision
Explain what is meant by thermal neutrons
Low energy or speeds < 1eV
How do neutrons produced by fission differ from the initial neutron
Released neutrons have higher energies or speeds
State which type of radiation produces the greatest number of ion pairs per mm in air
alpha
State which type of radiation could be used to test for cracks in metal pipes
gamma
Why is a gamma emitting source commonly used for tracers and why should it not have a very short half-life or a very long half-life
The radiation needs to pass through the body to be detected
Not short otherwise the activity of the source becomes too weak during measurements
Not long otherwise the decaying source may remain in the body for a long time and could cause damage
State two physical features or properties required of the shielding to be placed around the reactor at a nuclear power station
Which material is usually used for this purpose
Thick, high density, withstands high temperature
Material giving minimal fatigue problems after irradiation
Reinforced concrete
Describe the effect of the shielding on the gamma rays, neutrons and neutrinos that reach it from the core of the reactor. Also explain why the shielding material becomes radioactive as the reactor ages
Effect of shielding:
Gamma rays - intensity reduced
Neutrons - some absorption
Neutrinos - very little effect
Why shielding becomes radioactive:
Neutron absorption by nuclei or atoms
Makes nuclei neutron rich or unstable
Becomes beta minus emitters and/or gamma emitters
State and explain two precautions that should be taken when working with a sample of magnesium in a school laboratory
Handle with tweezers because the radiation intensity decreases with distance
Store in a lead box when not in use to avoid unnecessary exposure to radiation
What is more hazardous, beta minus or gamma radiation
Gamma rays are more penetrating and are therefore more hazardous to the internal organs of the body
Beta minus particles are more hazardous because thye are more ionising
Name one type of radiation or particle that may be used in scattering experiments and describe the main physical principle of the scattering process
State the information which can be obtained from the results of this scattering
Alpha particles
Electrostatic repulsion
Obtains proton number, nuclear charge, upper limit to nuclear radius
Or
Electrons
Diffraction of de Broglie Waves by nucelus
Obtains nuclear radius/density
Give three origins of the radiation contributing to background radiation
Cosmic rays Ground/rocks/buildings Air Nuclear weapons/power Medical waste
Discuss the dangers of exposing the human body to a source of alpha radiation and compare the dangers when the source is held outside the body compared to inside
Alpha radiation is highly ionising hence causes cancer/damages cells
Outside: less damage - absorbed by dead skin some particles directed away from body
Inside: more damage - all particles absorbed living tissue will absorb alpha radiation can reach vital organs
Electron intensity (y) angle of diffraction (x) for electron diffraction
Electron intensity decreasing with angle of diffraction to a non-zero first minimum
Discuss the merits of using high energy electrons to determine nuclear radii rather than using alpha particles
Electrons are not subject to SNF so scattering is easier to interpret
Electrons give greater resolution so are more accurate as can get closer
Electrons give less recoil and are easier to produce
Discuss two ways in which a proton-rich nuclide may decay
e- capture/e+: N increases by one, Z decreases to one as p –> n
a: move closer to line of stability i.e. proton to neutron ratio is reduced
Suggest two reasons why alpha particles cause more ionisation than a beta particle of the same initial kinetic energy
a particle has more mass/momentum than b particle
a particle has twice as much charge as b particle
a particle travels much slower than a b paritcle
Explain why the isotope T-99 is often chosen as a suitable source of radiation for use in medical diagnosis
Only emits gamma rays - easily detected outside body and causes little damage, short enough half life to not remain active in body after use but long enough to remain active during diagnosis
Explain why beta minus emitting isotopes are produced when the fuel rods are in the reactor
Fission nuclei are neutron-rich and therefore unstable
Neutron-proton ratio is much higher than for a stable nucleus
Beta minus particle emitted when a neutron changes to a proton in a neutron-rich nucleus
Discuss the relative importance of a nuclear power station producing radioactive waste rather than greenhouse gases when deciding if nuclear power stations are needed
- reduction in greenhouse gases to stop global warming
- long term storage of radioactive waste is essential as radiation damages living cells
- radioactive isotopes with very long half-lives are in the used fuel rods
- nuclear power is reliable because it does not use oil or gas from other countries
- radioactive waste needs to be stored in secure and safe conditions for many years
Nuclear power is needed; reduction of greenhouse gases is a greater problem than the storage of radioactive waste because global warming would cause sea levels to rise and safe storage of radioactive waste can be done
Nuclear power is not needed; storage of radioactive waste is a greater problem than reduction of greenhouse gases because radioactive waste has to be stored for thousands of years and greenhouse gases can be reduced using renewable energy sources
Explain why it is not necessary to consider the beta particle emissions when determining the radiation dose received by a scientists body from a source
Beta particle:
- range in air < 1.5m
- absorbed by air
- lose energy in air more rapidly than gamma photons
- ionise air much mre than gamma photons
What is true about an alpha particles when closest to the nucleus
its speed is least
its kinetic energy is least
What is the decay constant for a radioisotope
The constant of proportionality which links rate of decay to the number of undecayed nuclei
For a nuclear reactor in which the fission rate is constant, which is true:
A) There is a critical mass of fuel in the reactor
B) For every fission event, there is on average on further fission event
C) a single neutron is released in every fission event
D) No neutrons escape from the reactor
B
What materials are commonly used for:
a) moderating
b) controlling
c) shielding
a) graphite
b) boron
c) concrete
Which one of the following statements is not true about the control rods used in nuclear reactors:
a) they must absorb neutrons
b) they must slow down neutrons to thermal speeds
c) they must retain their shape at high temperatures
d) the length of rod in the reactor must be variable
B
Why is a moderator required in a thermal nuclear reactor?
To reduce the kinetic energy of fission neutrons
What is the effect on the fission neutrons (Number and EK) in the reactor of a thermal nuclear reactor being shut down by inserting control rods fully into the core
Number of fission neutrons reduces, and their average kinetic energy remains unchanged
The neutrons required for induced fission of U-235
should be slow neutrns
Artificial radioactive nuclides are made radioactive in a reactor as a consequence of bombardment by
neutrons
Why are moderators in nuclear reactors made from graphite
To decrease the neutron speeds
Nuclear binding energy is
The energy equivalent of the difference between the total mass of the individual nucleons and their mass when they are contained in the nucleus
Alpha particles always deflect _____ from the nucleus because
away
of the electrostatic repulsive force
Never bends towards/around nucleus
Can go straight pass nucleus if far enough away
Alpha particle between the electrostatic force boundary of two nuclei will
travel in a straight line between both of the nuclei
Sketch A=A0e^-lambda*t
cuts y axis at A0
tends to x=0
A =
lambda * N
Unit of N
nuclei
SState the effect of the emission of alpha particles and gamma, state the effect of their emission on the parent nucleus
alpha particle will cause their parent nucleus to lose two protons and two neutrons therefore making it lose mass and form a new element two places lower in the periodic table. Gamma rays cause no change in the atomic number, however it does bring the components of the nucleons closer together due to the SNF. Both these emissions leave the parent nucleus in a more stable state.
Explain why alpha particle emitters are principally dangerous when they come into contact with the body or are ingested into it
alpha particles are dangerous if they ingested because they can quickly ionise body tissue in a small area, causing lots of damage such as lung cancer
Explain why gamma ray emitters can be more dangerous as remote sources than alpha particle emitters
alpha particles are not penetrating enough to penetrate through human skin unlike gamma rays and so as a remote source, gamma rays can reach can reach air cells whilst alpha particle cannot. This means gamma rays can damage our cells, killing them/causing mutations leading to long term side effects such as infertility.
Isotope =
different number of neutrons, same number of protons
Half life =
time taken for the number of nuclei of a given isotope to halve from the original amount
T1/2 =
ln2/lambda
State and explain what decay product can be detected outside the body in a treatment emitting a beta particle, an antineutrino and a gamma ray
Gamma ray because it can easily penetrate the skin and is easily detectable whereas not all beta minus particles can.
State why an alpha emitter is not suitable to be a tracer
strongly ionising so would cause damage inside the body and it cannot penetrate the skin to be detected by the detectors.
State why the half-life of the technetium isotope (~6hrs) makes it suitable to be a tracer
half life is long enough for medical procedure to occur i.e. monitoring the blood flow and short enough to minimise the risk of side-effects i.e. preventing prolonged radiation exposure to limit damage to living cells.
Explain how the presence of a radioactive tracer may do some damage to the individual
damage to healthy cells is not completely preventing – gamma rays can negatively affect cells leading to side effects such as tiredness and reddening or soreness of the skin. The isotope is gamma emitting which is not largely dangerous in short exposure times but prolonged use can be damaging – it is possible for side effects to occur despite a relatively short half-life as it is still ionising just fairly weakly.
Explain why the total activity of a radioactive sample could be different from a calculated value
the sample could decay into a daughter nucleus that is radioactive in itself which means there will be further radioactive decay meaning a greater amount of decays per second i.e. activity until it has finally reached a stable state.
Isotope decays by ____ of its original amount every n half lives
2^-(n*T1/2)
How is technetium-99 used in medical diagnosis
it is a gamma emitter used by doctors as a tracer in the human body. It is injected into the patient’s bloodstream. Scanners outside of the body measure the gamma activity, enabling the blood flow to be monitored. This radioactive source has a short half-life to prevent prolonged exposure. A detector i.e. a PET scanner is then used to detect the emitting gamma rays. It can give information on the functioning of organs, metabolism and blood flow.
Activity equation
A=A0e^-lambda*t
For alpha particles and for gamma rays, state the effect of their emission on the parent nucleus
Alpha particles cause their parent nucleus to lose two protons and two neutrons therefore making it lose mass and form a new element two places lower in the periodic table.
Gamma rays cause no change in the atomic number, however it does bring the components of the nucleus closer together due to the SNF.
Both these emissions leave the parent nucleus in a more stable state
Explain why alpha particles are principally dangerous when they come into contact with the body or are ingested into it
Alpha particles are dangerous if they are ingested because they can quickly ionise body tissue in a small area, causing lots of damage such as lung cancer
Explain why gamma ray emitters can be more dangerous as remote sources than alpha particle emitters
Alpha particles are not penetrating enough to penetrate through human skin unlike gamma rays and so as a remote source, gamma rays can reach air cells whilst alpha particles cannot. This means gamma rays can damage our cells, killing them/causing mutations leading to long term sides effects such as infertility
