Principle
Using U-235 as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit two or three free neutrons (2.43 on average), called prompt neutrons. A subsequent fission fragment occasionally undergoes a stage of radioactive decay that yields an additional neutron, called a delayed neutron. These neutron-emitting fission fragments are called delayed neutron precursor atoms.
Delayed neutrons are associated with the beta decay of the fission products. After prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission occurs.
Delayed Neutron Data for Thermal Fission in U-235
| Group | Half-Life (s) | Decay Constant (s−1) | Energy (keV) | Yield, Neutrons per Fission | Fraction |
|---|---|---|---|---|---|
| 1 | 55.72 | 0.0124 | 250 | 0.00052 | 0.000215 |
| 2 | 22.72 | 0.0305 | 560 | 0.00546 | 0.001424 |
| 3 | 6.22 | 0.111 | 405 | 0.00310 | 0.001274 |
| 4 | 2.30 | 0.301 | 450 | 0.00624 | 0.002568 |
| 5 | 0.614 | 1.14 | - | 0.00182 | 0.000748 |
| 6 | 0.230 | 3.01 | - | 0.00066 | 0.000273 |
Read more about this topic: Prompt Neutron
Famous quotes containing the word principle:
“I do not mean to exclude altogether the idea of patriotism. I know it exists, and I know it has done much in the present contest. But I will venture to assert, that a great and lasting war can never be supported on this principle alone. It must be aided by a prospect of interest, or some reward.”
—George Washington (1732–1799)
“In case I conk out, this is provisionally what I have to do: I must clarify obscurities; I must make clearer definite ideas or dissociations. I must find a verbal formula to combat the rise of brutality—the principle of order versus the split atom.”
—Ezra Pound (1885–1972)
“The first principle of a free society is an untrammeled flow of words in an open forum.”
—Adlai Stevenson (1900–1965)