Introduction Flashcards
Segre chart (or chart of nuclides) summarizes all the known nuclides. What are the main characteristics?
- Stable nuclides run up the middle of the chart.
- Below line of stability, beta - dominates
3, Above line of stability, electron capture and beta + dominate - At high atomic numbers, alpha decay becomes significant and fission or proton / neutron emission can take place.
Differences between isotopes, isotones, isobars?
- Isotopes: Same Z (along horizontal on Segre diag.)
2.Isotones: Same N (along vertical on Segre diag.) - Isobars: Same A=Z+N (along diagonals on Segre diag.)
Terms of SEMF?
- Volume
- Surface
- Coulomb
- Symmetry (better binding with matching numbers of protons and neutrons)
- Pairing (effects of nucleons pairing up and is positive for nuclei with even atomic and neutron numbers, zero for those with odd mass numbers, and negative for nuclei with odd atomic and neutron numbers)
Energy release from binding energy curve per atomic mass?
- Fusion: move up the slope of the curve, making less tightly bound nuclei into more well bound we can release energy from the conversion of mass
- Fission: splitting heavy nuclides into lighter ones in order to move up the curve
slope is much steeper on the left than the right, indicating that we will gain significantly more energy
per nucleon involved from fusion than fission.
What can you notice from log scaling the Segre diagram?
Makes the lighter elements more pronounced. Stable elements that are revealed: Helium-4, berylium-8, carbon-12, oxygen-16.
What is half-life?
For half the atoms to decay. After 10 half-lifes the can be treated a completely decayed
Units of activity A?
Bq = Decays per second
Characteristics of alpha decay
- With Z over 82, Coulomb repulsion is strong enough such that a stable nucleus is not formed.
- Emits a helium-4 nucleus, which is energetically favorable due to its high binding energy.
- Q-value is positive for alpha particle but negative for lighter elements
Describe Geiger-Nuttall plot
y-axis: log of half life
x-axis: Q value
Linear relationships are formed that show the same negative gradient along different decay lines.
Characteristics of beta decay
- The balance of protons and neutrons in the nucleus affects stability due to symmetry term, for low A particles equal number of protons and neutrons is preferred.
- For neutron rich nuclei, this is achieved by beta minus decay in which a neutron is converted into a proton, emitting an electron and an anti-neutrino in the process
- In proton rich nuclei, there are two processes that occur:
a) Beta + decay: proton becomes neutron, emitting a positron and a neutrino. has energy requirment
b) Electron capture: nucleus captures one of
the inner shell electrons and combines it with a proton to form a neutron and a neutrino. The outer shell electrons then cascade down into the vacancy left in the inner shell, emitting characteristic X-rays.
beta energy spectra?
Energy from the beta decay process is shared between the beta particle and the neutrino (as well as the nucleus), resulting in the spectrum of energies. The shape of the spectrum is modified by the Coulomb field of the nucleus due to either positive or negative emission. the maximum is at the Q-value.
gamma ray emissions?
When nuclei are produced in α or β decay, or in a nuclear reaction, they are generally formed in excited states. These decay rapidly by gamma ray emission.
gamma ray energies?
- discrete energies which are characteristic of the daughter nucleus. Well defined energies.
- Typical γ energies are around 500 keV, but they can overlap the X-ray region and extend up to some MeV.
- Spectroscopy can be used to identify the composition of a source.
Decay chains?
A nucleus has to take several steps to reach stability. This can involve different forms of decay at different stages, and some nuclides can decay by either alpha or beta emission
The Bateman equations?
Describe the general solution to decay chains. Fist is an exponential and the others rise to a maximum and then drop. For each subsequent decay peak it shifts more tho the right.
Fission characteristics
- Coulomb force, which works at range, while the nucleus is held together by the strong nuclear force, which is much shorter ranged. with increasing numbers of protons, each experiences that much more repulsion.
- deforms to a dumbbell shape, with two large lobes held together by a thin neck.
- If the Coulomb repulsion between the two lobes
is large enough to overcome the strong nuclear force holding the neck together, then the nucleus will split.
Activation energy?
energy required to overcome the barrier to fission is the activation energy or fission barrier.
Neutron reactions from a capture?
- might be released as γ radiation and the neutron
is captured (e.g. uranium-238) - excitation energy from the addition of the neutron may exceed the activation energy, causing fission (e.g. uranium-235). Produces more neutrons that induce more fission (chain reaction).
Assign a probability to each process. Uranium-235 has an odd number of neutrons, so adding one more changes it from even-odd to even-even and the extra pair gives more binding energy unlike uranium-238
What is criticality?
- uncontrolled chain reaction in fission fuel can lead to an extremely large and fast energy release.
- controlled chain reaction is achieved by ensuring that the right number of neutrons from fission induce further fission - criticality
3, Ensure that each fission only results in one fruther fission, to maintain constant power
Pressurized water reactors characteristics?
- A chain reaction is sustained in the fuel, and the heat generated is removed by high pressure water
- The pressure is maintained by the pressuriser
and the chain reaction is managed by use of the control rods - The water coolant is pumped to the steam generator, where its heat is used to boil water in the secondary circuit and turn it into steam
- Steam is then used to drive a turbine, producing electricity
- Colder steam is condensed back into water and pumped back to the steam generators to
repeat the process
Natural reactors?
- Uranium plus ground water produced conditions which allowed natural nuclear reactors to run stably for hundreds of thousands of years due to feedback mechanisms.
- Waste products remained closely confined to reactor localities ever since, suggesting that burring is a good long term solution for radioactive waste
