G - Fission Flashcards
92U235 + 0n1 —> 57La139 + 42Mo95 +2•0n1 + Q
Why does the above not balance?
What would be the typical Q value?
The above doesn’t balance because there products are often unstable and a series of beta decay occur — hence the final stable products are often shown.
Standard models of fission predict symmetric fission with an increase in BE of <1MeV per nucleon. The approximate Q value is around 200MeV
• the above is 208MeV
What is fissionable, fissile and fertile element?
Fissionable - a nucleus which can be made to fission by the impact of neutron.
• don’t necessarily have to be self sustaining
• ex: U238 and Th232 (not self sustaining)
Fissile - subset of fissionable materials that can also maintain a self-sustaining chain reaction using low-energy neutrons.
• ex: U235, Pu239, U233
• thermal neutrons can be described to have very little to no (kinetic) energy
Fertile - nuclei that cannot fission but transmute to produce fissile materials
• ex: U238, Th232
Why does U235 readily fission and not U238?
When calculating the mass defect, M(Z,A) we need to look at the delta term in the SEMF, B(Z,A) equation.
The fissioning structures are U236 and U239 respectively.
U236 is even-even and more tightly bound than the even (z) - odd (N) U239.
Hence there is a greater mass defect and thus more binding energy available for U235.
• due to the critical threshold for fission process, the nucleon pairing term becomes important
What would happen to a neutron if it were in a vacuum? How is a neutron moderated? How would one calculate the energy of a neutron after moderation?
Vacuum - travel until eventually decaying into a proton
Moderation - collisions with particles in the medium will reduce the (kinetic) energy of the neutron until it becomes captured (absorption) by a nucleus
Energy after n events:
E_after/E_before = exp(-epsilon)•n
Epsilon = 1 - [(A-1)^2/2A] • ln((A+1)/(A-1))
A = mass of nucleus under collisions
What is the absorption cross-section proportional to (for neutrons)? How is the slowing down (scattering) capacity and the absorption capacity linked for a moderator?
Absorption is proportional to 1/velocity
Slowing down power is epsilon•E_s
• E_s = macro scattering cross-section
• E_a absorption cross-section (cm^-1)
Moderating ration = epsilon•E_s/E_a
• C - 160 | H20 - 60 | D20 - 5500
Explain how Cherenkov radiation works
Very similar affect to a sonic boom - but with particles instead of sound waves
SONIC BOOM
plane moves faster than sound in air — sound vibrations bunch up behind plane — as the plane moves past you so does the bunched up sound waves — this causes a temp change in pressure which is observed as loud boom to the observer
BLUE GLOW
Fission reaction produces high energy beta particles — these move faster than light in water (light travelling at .75c) — beta particles also interact with some water molecules which release photons — these photons bunch up behind the electron which form a cone of blue light
How to calculate the specific activity,A of a radioactive isotope? Is their one answer?
Activity: (dn/dt) A = lambda•N
• units - Bq
Specific activity (per unit mass) : a_m = (lambda•N)/mass = lambda•(Na/molar-mass)
• units - Bq•g^-1
Activity A is the rate of decay of a sample and is what we’re used to.
Specific activity, A_m, is the specific activity per unit mass!!
**u may also be asked to find the specific activity per unit volume (depending on units given etc) be careful and read the question
