Flashcards in Radioactivity (Unit 5) Deck (31):
Definition of an isotope
are atoms of the same element with the same number of protons but a different number of neutrons
Properties of alpha, beta and gamma radiation
See table on sheet
Identifying alpha, beta and gamma radiation
Put the detector within a few cm of the source and put some paper between the source and the detector. If the count rate drops significantly, then the source is alpha.
If the count rate doesn’t change, remove the paper and replace it with a few mm of Aluminium. If the count rate drops significantly, then the source is beta. If the count rate doesn’t change, then the source is gamma.
Applications of alpha, beta and gamma radiation
alpha – smoke detectors
beta – thickness measurement of cardboard (in a paper mill)
gamma – detecting leaking pipes / medical tracer
Safe handling of radioactive sources in a laboratory
• handle with (long) (30 cm) tweezers because the radiation intensity decreases with distance
• store in a lead box (immediately) when not in use to avoid unnecessary exposure to radiation
Examples of background radiation
• Cosmic rays
• Ground, rocks and buildings
• Radon (in atmosphere)
• Nuclear fallout (from weapons testing/nuclear accidents)
• Discharge/waste from Nuclear power
Experimental verification of inverse square law for gamma rays
• Count rate measured by GM tube from a gamma source (gamma rays not stopped by air)
• Measured count rate equals counts from source PLUS background counts
• Measure background count rate and subtract this from the measured rate with the source present. This gives the corrected count rate (counts just from the source).
• Vary the distance of the GM tube from the source and plot a graph of corrected count rate against 1/(distance)2 to establish an inverse square relationship between intensity and distance.
What is meant by 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.
• the rate of decay is unaffected by the surrounding conditions.
Definition of decay constant
the probability of (a nucleus) decay per unit time (usually per second).
Definition of activity
The number of nuclei of an isotope that decay each second.
(remember each decay produces one radioactive particle that can then be detected with a Geiger Muller tube)
Units of activity
Bq (Becquerel) – number of decays per second.
Definition half life
Time taken for half the nuclei of a particular isotope present to decay OR time taken for the activity of a particular isotope to half
Decay constant from a log graph
decay constant = - gradient
Plot sketch of graph of N against Z for stable nuclei
Decay equations for alpha, beta+, beta- and electron capture
Decay modes for alpha, beta+, beta- and electron capture
Existence of nuclear excited states within nucleus
• Nuclei can be in excited states (eg following radioactive decay).
• Nuclei can de-excite (and lose energy) by emitting a photon.
• As energy levels differences are really big in nuclei, the photons have very large energies.
• And hence high frequencies and short wavelengths (gamma part of spectrum).
• Gamma emission is therefore often associated with alpha and beta decay.
Why is beta emission associated with gamma rays of discrete frequencies
• Following beta decay the nucleus is in an excited state
• Which are at discrete energies
• And emit gamma rays when they de-excite/fall down to lower states
• Reference to E=hf and stating gamma rays (or drop in energy level) have discrete energies.
Why is the gamma source technicium-99 used in medical diagnosis.
• It only emits gamma rays
• gamma rays can be detected outside the body/are weakly ionising and cause little damage
• It has a short enough half-life and will not remain active in the body after use
• It has a long enough half-life to remain active during diagnosis
• The substance has a toxicity that can be tolerated by the body
• It may be prepared on site (at hospital)
Features of Rutherford scattering
• Air must be removed (vacuum) because alpha particles only travel short distances in air due to collisions with air molecules.
• Gold foil must be thin so that alpha particles are not absorbed by the foil (have more than one collision).
• Majority of alpha particles pass straight through (without any deflection).
• A small number of alpha particles are scattered through very large angles, some even come back towards the alpha source (scattered through 180 degrees).
• Conclusions about structure of atom
• The atom is mostly empty space because most alpha particles do not pass close enough to the nucleus to be deflected.
• The nucleus is very small and positively charged to provide necessary electric field strength to repel alpha particle.
• Nucleus contains most of the mass (of the atom) because the alpha particles are scattered through very large angles
• Nucleus is much smaller compared to the separation between nuclei (and hence to the size of the atom itself).
Maximum size of nuclear radius from estimate of closest approach
conversion of initial ke (of alpha particle) to electrical potential energy at point of closest approach.
Ek=[1/2mv2]=1/(4pi(epsilon 0)) x QaQN/r
Where Qa is charge on alpha particle, QN is charge on nucleus and r is effectively the distance of closest approach to nucleus.
Why are other methods for measuring the nuclear radius other than alpha scattering used?
• strong force acting between alpha particle and nucleus complicates results
• scattering is produced by the distribution of protons, not the whole nucleon distribution
• alpha particles are relatively massive, causing recoil of nucleus which complicates results
Advantage of using electrons to measure nuclear radius
• electrons are not subject to the strong force so, electron scattering patterns are easier to interpret.
• electrons give greater resolution (or are more accurate) because they get closer to the nucleus and alpha particles cannot get so close to the nucleus due to electrostatic repulsion so only provide information on the closest distance of approach, not radius.
• electrons produce less recoil in nucleus because electrons are much less massive (than nucleus).
• high energy electrons are easier to produce because electrons have a lower specific charge so are easier to accelerate.
Experimental method for determining size of nucleus
Typical size of nuclear radius
1x10-15 m (1 fm)
Information that can be gained about the nucleus using alpha particles
• maximum diameter of the nucleus
• proton number and nuclear charge
• that the mass of the nucleus is most of the mass of the atom
Information that can be gained about the nucleus using high energy electrons
• Nuclear radius (diameter)
• Nuclear density
Determination of nuclear radius from electron scattering
Derivation of radius from experimental data