Nuclear Physics Flashcards
Describe Rutherford’s scattering experiment
- plum pudding model
- he fired alpha particles at a thin gold foil and most passed through with a only a few deflecting as they pass through the gold foil
- He observed :
*that the atom has mostly empty space - the nucleus must have a large positive charge
- the nucleus must be tiny
- most of the mass is in the nucleus
state the strength of ionisation and the range (penetration power), effect in magnetic and electric field for
a) alpha radiation
b) beta radiation
c) gamma radiation
How to detect radiation
- use a geiger counter
- As radiation enters the Geiger-Müller tube and passes through the inert gas it leaves behind a trail of ions. These ions create a charged path between the metal rod in the centre of the detector and the metal casing. This briefly allows current to flow and the circuit to be complete. Each time the circuit is completed the counter registers a count on the screen.
problems with geiger tube
cannot identify what the particles are
in areas of high radiation, cannot distinguish between successive pulses
How to measure count rate
measure background radiation and subtract it from its measurements
How do you stop alpha, beta and gamma particles
alpha : stopped by a few cm of air or a few mm or paper
beta : stopped by few mm of aluminium
gamma : never stoped completely, intensity can be reduced by a few cm of lead or couple of metres of concrete
How can we use geiger counter to determine the radiation
try placing paper between source and geiger counter. If the count does not change then try a few mm of aluminium. If count does not change you have gamma radiation.
Which out of alpha beta and gamma is the most dangerous
alpha most dangerous, gamma least dangerous
alpha difficult to get out of body unlike beta and gamma
Discuss how a beta source may be used to control the thickness of sheet metal or paper
count rate is directly proportional to thickness of paper
use count rate to work out thickness of paper
Discuss how smoke alarms may use an alpha source such as Americium 241 to detect smoke.
- alpha particles ionise the air particles
- positive particles from the air attracted to the negative plate, and negative particles to the positive plate
- a current flows between
- the smoke absorbs the alpha radiation and prevents the air being ionised
- so current is much smaller triggering an alarm
Define activity and count rate
activity : the actual number of nuclei in a source that decay per unit of time.
count rate : Count rate is the number of ionisation pulses recorded per unit time by a detector. This is usually a small fraction of the overall activity.
If you are given radius, how do you calculate count rate
like inverse-square law, count rate is inversely proportional to radius squared so count rate = constant / radius ^2
What is the effect on radiation on the human body
could cause cells to die or become defective where defective cells divide and produce more defective cells causing cancer. If large number of cells die, body may not be able to reproduce them quickly enough
What to do and not do when handling radioactive sources
What are medical tracers used for
a gamma emitter is first injected into the body
organs absorbs radionuclides and emit gamma radiation
gamma sources use das they freely pass out of the body
these sources are used because they have a short half life (exposure to patient decreases rapidly)
what is radiotherapy used for
- gamma radiation can kill cancer cells
- they’re contaminated so they can focus on specifically on the cancerous region
Describe why the line of stability trends the way it does
As the number of protons increases the number of neutrons needed to keep the nucleus stable also increases, but at a slightly greater rate. This is because the larger the number of protons in the atom the greater the electrostatic repulsion. Neutrons act to separate the protons and reduce the repulsion
Beta plus decay is one way to get a proton to turn into a neutron, state another
Electron capture
- a proton captures an electron from the electron cloud and turns it into a neutron
Prove that the density of a nucleus always remains constant
Describe Rutherfords alpha scattering experiment to find the diammeter of a nucleus
alpha particles are repelled by electrostatic repulsion as both the nucleus and alpha particle are both positively charged
force of replulsion found using equation F = Qq / 4Pi Eo (r^2)
use the fact that the distance of closest approach is when all kinetic energy of alpha particle is converted into potential energy
Describe electron diffraction experiment to find the diameter of the nucleus
Describe the accuracy of the alpha scattering to find the diammeter of a nucleus
- In the alpha scattering experiment the calculations only produce the distance of closest approach of the alpha’s not the diameter
- The experiment can’t detect alphas scattered by 180o
- The alpha’s have their own size which must be taken into account
Describe accuracy of electron diffraction experiment to find the diammeter of a nucleus
- Electrons need high speed because they need a wavelength similar to the diameter of the nucleus
- Both experiments need monoenergetic beams and need a thin sample of target material.
Advantages and disadvantages of alpha scattering and electron diffraction
Alpha scattering
* Results upset by nuclear recoil
* Results upset by the strong force as alphas contain hadrons
* The alphas are only affected by protons not neutrons
Electron diffraction
* Not affected by the strong force since they are leptons
* The 1st minimum in the scattered intensity can be difficult to detect