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:
“A state of war or anarchy, in which law has little force, is so far valuable, that it puts every man on trial. The man of principle is known as such, and even in the fury of faction is respected.”
—Ralph Waldo Emerson (1803–1882)
“A little grit in the eye destroyeth the sight of the very heavens, and a little malice or envy a world of joys. One wry principle in the mind is of infinite consequence.”
—Thomas Traherne (1636–1674)
“We seem to be pariahs alike in the visible and the invisible world, with no foothold anywhere, though by every principle of government and religion we should have an equal place on this planet.”
—Elizabeth Cady Stanton (1815–1902)