G - Fission

Question 1

92U235 + 0n1 —> 57La139 + 42Mo95 +2•0n1 + Q

Why does the above not balance?

What would be the typical Q value?

Answer 1

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

Question 2

What is fissionable, fissile and fertile element?

Answer 2

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

Question 3

Why does U235 readily fission and not U238?

Answer 3

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

Question 4

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?

Answer 4

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

Question 5

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?

Answer 5

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

Question 6

Explain how Cherenkov radiation works

Answer 6

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

Question 7

How to calculate the specific activity,A of a radioactive isotope? Is their one answer?

Answer 7

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