Random Flashcards
Gamma irradiation of foods
Uses gamma rays from 60Co or 137Cs to prevent growth of bacteria to increase shelf life
Radon gas
Released from naturally occurring U deposits
Breathed in and decays in the lungs into high energy alpha-emitters e.g. 218/214Po, 214Pb, 214Bi which can cause DNA damage
Why is neutron emission rare?
Because neutrons are bound by 8 MeV
Proton emission is even more rare
Ionisation
The process by which a neutral atom acquires a positive or negative charge
Gamma interactions
Cataclysmic
Have 3 possibilities
1. Pass through with no interaction
2. Interact, lose energy, change direction (Compton effect)
3. Transfer all energy and disappear (photoelectric effect)
Compton effect
When an incident photon interacts with an orbital electron to produce an ejected electron (recoil electron) and a scattered photon of less energy than the incident photon
Why is alpha radiation the least penetrating?
Large nucleus so will interact with matter quickly
Alpha particles have…
…“high linear energy transfer”, can cause great biological damage
Effect of shielding high energy beta particles with lead
Can generate more radiation than it shields
“Bremsstrahlung X-rays” - a type of secondary radiation produced as a result of slowing/stopping the primary radiation (beta-particles)
Metastable nucleus
= can remain in an excited state for a prolonged period of time
Photon emission from X-rays
- An electron from a lower energy level is removed
- Electrons from higher energy levels then occupy the resulting vacancy by photon emission - a negatively-charged, high speed electron is attracted towards the positively-charged nucleus which causes it to lose some velocity
- This deceleration means the electron loses some of its kinetic energy, which is given off in the form of a photon
* Bremmstrahlung = “braking radiation” - the X-rays produced when accelerating electrons are suddenly decelerated upon collision with a metal target
Element with most binding energy per nucleon
56Fe
4 nuclei with stable odd/odd numbers of p/n
2D, 6Li, 10B, 14N (18F is radioactive)
Magic numbers
2, 8, 20, 28, 50 82, 126
4 fundamental fores in nature
Strong
Electromagnetic
Weak
Gravity
What holds quarks together in protons/neutrons?
The strong force
Neutrons
2 down, 1 up
Free neutrons are unstable (t1/2 = 15 mins)
Down quark splits into up quark and W- then e- i.e. n —> p + e + v
Protons
2 up, 1 down
Free protons are stable (t1/2 = >10^33 years)
Unstable nuclei with excess energy can convert an up quark and an electron to a down quark i.e. p + e + energy —> n or p+ —> n + beta+ + v
Residual strong force
Holds protons and neutrons together, determines the size of nuclei
What holds protons and neutrons together?
Residual strong force
The strong force is distance-dependent
Repulsive at <0.7 fm
Very strong at 1 fm
Negligible at >2.5 fm
Why is 2He (diproton) an extremely unstable He isotope?
2 protons, 0 neutrons
Pauli exclusion principle forces the 2 protons to have anti-aligned spins which gives the diproton a negative binding energy
Would be more stable if the strong force was 2 % greater
Forces in the nucleus
Strong force between quarks by exchange of gluons (very short range)
Residual strong force between protons and neutrons (short range)
Electrostatic repulsion between protons (weaker than the strong force, but can become stronger at >2 fm)
Weak force holds the neutron together (very short range, leads to beta- decay)
All members of a decay series…
…differ in A by 4.
Thorium decay series
4n
232Th —> 208Pb
Neptunium decay series
4n + 1
237Np —> 209Bi
Uranium
4n + 2
238U —> 206Pb
Actinium
4n+3
235U —> 207Pb
14C
Created by cosmic radiation in the thermosphere
t1/2 = 5730years
7.5 kg produced per year - incorporated into CO2 and becomes part of the carbon cycle
Beta- emission from living plants
13.6 disintegrations min-1 g-1 (14C)
238U:206Pb ratio of 1:1
Means 1 half life has passed
