Moderation of Fast Neutrons
慢化快中子
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The neutrons emitted in nuclear fission reactions have high energies, typically in the range of 1 MeV. But the cross section for neutron capture leading to fission is greatest for neutrons of energy around 1 eV, a million times less. Neutrons with energies less than one electron volt are commonly referred to as "thermal neutrons" since they have energies similar to what particles have as a result of ordinary room-temperature thermal energy.
核裂变反应中发出的中子具有高能量，通常在1 MeV的范围之内。但导致裂变的中子捕获截面在中子能量约为1 eV时最大，这是比1 MeV小百万倍的量级。能量低于1电子伏特的中子通常被称为“热中子”，因为它们的能量与普通室温热能相似。 |
It is necessary to slow down the neutrons for efficient operation of a nuclear reactor, a process called moderation. While neutrons are efficiently slowed by inelastic scattering from U-238 , the non-fissionable isotope of uranium, when their energies are higher than 1 MeV, the remainder of the process of slowing them down must be done by elastic scattering from other nuclei. When a neutron collides elastically with another nucleus at rest in the medium, it transfers some of its energy to it. The maximum transfer of energy occurs when the target nucleus is comparable in mass to the projectile. Water and carbon (graphite) are commonly used moderators. Water is a good moderator, but the hydrogens in the water molecule have a fairly high cross section for neutron capture, removing neutrons from the fission process. Heavy water, used as moderator in Canadian reactors, avoids this loss.
为了原子反应堆的有效运行，必须减慢中子，这一过程称为慢化。虽然中子通过与铀-238（铀的非裂变同位素）的非弹性散射被有效减慢，但当中子能量高于1 MeV时，减慢过程的其余部分必须通过与其他核的弹性散射来完成。当一个中子与静止的另一个核发生弹性碰撞时，它会将部分能量传递给该核。当目标核的质量与入射核相当时，能量传递最大。水和碳（石墨）通常用作慢化剂。水是良好的慢化剂，但水分子中的氢具有相当高的中子捕获截面，会将中子从裂变过程中移除。重水用于加拿大反应堆的慢化，以避免这种损失。
Conceptually, the effectiveness of water as a moderator can be compared to what happens on a pool table when the cue ball strikes another ball on the table head-on. The head-on elastic collision with an equally massive target ball at rest stops the cue ball and sends the target ball forward with the cue ball's original speed. The hydrogens in the water play the role of the target ball and are effective in dramatically slowing the fast neutrons, even when the collision is not head-on.
从概念上讲，水作为慢化剂的效果可以类比于当击球球在台球桌上正对另一只静止球碰撞时的情况。当一个质量相等的静止目标球与击球球发生正碰弹性碰撞时，击球球会停止，而目标球则以击球球原来的速率向前运动。水中的氢原子起到目标球的作用，即使碰撞不是正对的，也能有效显著地减慢快中子。
Another conceptual image which may help with understanding the need for moderation is the nature of a short putt on the green of a golf course. The original experiments in the laboratory of Otto Hahn in Germany tried unsuccessfully to get uranium to absorb neutrons by bombarding them with fast neutrons - 235U just has a very small probability of absorbing fast neutrons. But it has a high probability of absorbing slow ones. If your golf ball is a few centimeters from the hole, you don't get out your driver and hit it as hard as possible - it just will not go into the cup that way. But a gentle tap with your putter has a high probability of success. Moderation to slow the neutrons by collisions with nuclei of similar mass dramatically increases the probability of neutron capture leading to fission.
另一个有助于理解为何需要慢化的重要概念形象是，高尔夫球场上一个距离洞口只有几厘米的球。奥托·哈恩在德国实验室进行的最初实验尝试失败了，他们试图让铀吸收中子，通过用快中子轰击铀——但铀-235对快中子吸收的概率非常小。然而，它对慢中子的吸收概率却很高。如果你的高尔夫球只在洞口几厘米外，你不会拿出球杆全力击球——这样球就无法进洞。但用球杆轻轻一推，就有很高的成功概率。通过与质量相近的核碰撞来减慢中子，显著提高了中子被吸收导致裂变的概率。
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