Radioactivity Flashcards
Definition of an isotope
are atoms of the same element with the same number of protons but a different number of neutrons
Identifying a, B and y 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 a, B and y radiation
a – smoke detectors
B – thickness measurement of cardboard (in a paper mill)
y – 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
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 B emission associated with y rays of discrete frequencies
- Following B decay the nucleus is in an excited state
- Which are at discrete energies
- And emit y rays when they de-excite/fall down to lower states
- Reference to (delta)E=hf and stating y rays (or drop in energy level) have discrete energies.
Why is the gamma source technicium-99 used in medical diagnosis.
- It only emits y rays
- y 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)
