define//nuclear physics Flashcards
Describe how our knowledge and understanding of the structure of the nucleus has changed over time
-Democritus in Ancient Greece concludes all matter is made by tiny indivisible lumps called atoms.
-Dalton supports in 1804 the notion and argues different element’s are made by different types of atoms
- 100 years later JJ Thompson discovers electrons hence atoms are divisible, he comes up with the plum pudding model
- Rutherford discovers that atoms did not have uniform charge and density and suggests the idea of a nucleus
Describe the Rutherford scattering experiment and state its significance
The experiment set up consisted of an alpha particle (+ve) fired at a thin gold foil and a detector on the other side to detect how many particles deflected and by what angle.
The results showed that:
-most alpha particle went straight through suggesting nucleus is mainly empty space
-some alpha particles deflected through big angles, suggesting the nucleus is positive and repulsion occurred
-a small number deflected straight back at angles bigger than 90 suggesting that the nucleus is extremely small.
Through this experiment it was concluded that atoms consist of small dense positively charged nuclei.
How can you estimate the closest approach of a scattered particle?
Using a form of Coulomb’s law such as Eelec= Q1Q2/4pier
The distance of the closest approach is an estimate if the nuclear radius (its maximum value).
At the distance of closest approach Ek=0 as it is all transferred to Ep
State a typical value for a nuclear radius
The radius of an atom is about 5x10^-11 m (0.05 nm)
State the relationship between nuclear radius and the nucleon number
R=RoA^1/3
On formula booklet
State what is random and what is constant about radioactive decay
Random:
-when a particular nucleus of a sample will decay
-which particular nucleus will decay next
Constant:
-decay probability
-rate of decay
Define the activity of a nucleus
Activity refers to the number of nuclei that decay each second.
State how both the the activity and the number of unstable nuclei in an unstable nucleus change over time
Activity is proportional to the number of unstable nuclei, hence as they decrease so does the activity.
Hence N=Noe^(-lambda t) N can be substituted with A
Define half-life
The time taken for the initial number of nuclei to halve for a particular isotope
explain how half life affects applications of nuclear radiation
Knowledge about half lives can help with radioactive dating and medical diagnosis (Techenetium99 has a half life of at 6 hours hence limits radiation to the acceptable amount).
State the 4 types of nuclear radiation and describe their constituents
- Alpha- helium nucleus
- beta minus- electron
- beta plus - positron
- gamma - short wavelength high frequency em radiation
Desribe the relative ionising stengths and penetrating power of alpha radiation, and link these properties to its uses
Has slow speed but very ionising, is absorbed by paper or a few cm of air.
Used for smoke detectors, small amount of americium-241 used. When smoke enters the alarm, it absorbs alpha particle hence doesn’t get ionised, less current causes the alarm to sound.
Has a very long half life.
Explain why the meta stable form of the isotope of technetium-99 is a radioactive source suitable for use in medical diagnosis.
- has low toxicity
- only emits lambda rays which are weakly ionising
- half life of around 6 hours hence won’t remain in the body for too long after the examination
- can be prepared in a hospital
Why are gamma rays emitted?
Emitted by nuclei that need to lose energy
Beta radiation usage
Used to determine thickness of aluminium foil, paper, plastic and steel by measuring how much radiation passes through the metal using a Geiger counter. A source with a long half life must be chosen to avoid having to substitute often.
