Nuclear Physics (Decay) Flashcards
What is the definition of radioactive decay?
Spontaneous disintegration of a nucleus to form a more stable nucleus
Resulting in the emission of alpha/beta/gamma rays
How can the random nature of radioactive decay be proved?
Observing count rate of Geiger muller tube
When near radioactive source counts are irregular == cannot be predicted
(Note: the count rate is a measure of the decay of unstable nucleus)
Describe how the closest approach can be used to estimate the radius of a atomic nucleus (long)
During Rutherfords experiment alpha particles are fired at a thin gold foil
These particles have an initial kinetic energy as moves towards positive nucleus
At the point of closest approach the electrostatic force reduces speed of the alpha particle to zero immediately
At this point kinetic energy is zero and electrical potential energy is equal to the inital kinetic energy
== Calculate using equation given
Describe the forces/energy at the point of closest approach
Maximum electromagnetic NOT STATIC force of repulsion is felt
Minimum kinetic energy
Maximum potential energy
Maximum acceleration
Advantages of closest approach method?
- Good estimate of upper limit of nuclear radius
- Simple mathematics
- Alpha particles are scattered only by protons
Disadvantages of closest approach?
- This method doesn’t give accurate result (always over estimate since measures distance not radius)
- Alpha particles are hadrons therefore are affected by strong nuclear force
- Alpha particles in beam must all have the exact same inital kinetic energy
Why is electron diffraction preferred over closest approach?
Because electrons are leptons therefore they will not interact via strong nuclear force
But alpha particles will since they are hadrons
Therefore electron diffraction far more accurate
During electron diffraction explain why the electrons must have high speed
Electrons must have a high speed so that there de broglie wavelength is around 10^-15
Gap = wavelength gives most diffraction
What are electrons diffracted through during electron diffraction?
Crystal lattice
Describe the pattern given by electron diffraction
Concentric circles with a central bright spot
This gets dimmer as you move away from the centre
Describe the graph of intensity against angle
- Peak at maximum in centre of graph
- Sharply decreases for other maximums
Where is the first minimum on the graph of intensity against angle?
The first dip closest to max
What is the general radius of a nucleus
1x10^-15
How can you calculate the radius of any nucleus
Radius = (1.4x10^-15) x (Nucleon number)^1/3
As the nucleon number increases, what happens to the nucleon number
Increases
Since nuclear density is constant, what is its general calculated value
1.45 x 10 ^ 17
(revise derivation of equation for nuclear density btw)
Describe Rutherford’s apparatus
- Alpha source and gold foil in an evacuated chamber
- Covered by a fluorescent coating
(The fluorescent coating is so you can see where the alpha particles hit inside the chamber)
-Microscope moved around outside chamber to observe path of alpha particles
What were the conclusions from the Rutherford experiment?
- Most alpha particles passed straight through the foil with no deflection
= Suggested atom is mostly empty space
(Not uniform density as suggest by thompson model)
A small amount of particles were deflected by a large angle - this suggested that the
centre of the atom is positively charged, since repelled
● Very few particles were deflected back by more than 90° - this suggested that the that the centre of the atom was very dense as it could deflect fast moving alpha particles but also very small as only small amount deflected
General conclusion of atom following Rutherford experiment
Atom has a small, dense, positively charged nucleus at its centre
3 uses of gamma radiation
- Detector = radioactive source with a short half life to reduce exposure = injected into patient detected using gamma cameras
- Sterilise surgical equipment
- Radiation therapy = kill cancerous cells in targetted area however damages healthy cells
Why is alpha radiation so dangerous
It is highly ionising
= If ingested it can ionise body tissue
Explain why Technetium 99 is used as a suitable source of radiation in medical diagnosis
- Only emits gamma rays
- Which are weakly ionising
- Therefore will not cause harm to tissues in body
- Short half life
- Will not remain active in body after use
- However remain active long enough during diagnosis
Why was Rutherford experiment in a evacuated chamber (vacuum)?
- To ensure that any collisions the alpha particles undertook where due to the gold foil and not any other substance such as air
