So, I've just about finished up another internship conducting nuclear physics research. You can take a look, if you dare, at my research paper here. [Size: ~500 KB]
The layman's version? Read on...
Neutrons have a neutral charge. Thus, there is no "easy" way to detect neutrons. Therefore, indirect methods are used to accomplish this task. The neutron detector I tested, a fission ion chamber, indirectly counts the number of relative incident neutrons.
The way this happens is: neutrons collide into a fissile foil (uranium-238 in this case). When one event finally occurs (it doesn't always happen every time), the neutron collides into the uranium nucleus, producing an unstable uranium isotope. This isotope emits fission fragments. The chamber for which the fissile foil is contained also contains a gas (usually argon or methane). When the fission fragments (being very heavy) collide into the gas molecules, an ion path is created. These ions can then be collected on an electrode (also in the chamber), and the signal can be sent through electronics which detects a certain number of counts. The counts are represented by current (the amount of charge over a certain amount of time). These counts can then be related back proportionately to the incident number of neutrons. Thus, you can know how many neutrons were hitting the fissile foil.
Okay, so you managed to follow some or most of the above. Now, why would a neutron detector be important? Well, for several reasons. Using a particle accelerator (such as a cyclotron), we can simulate various scenarios that may occur in space, for example. So, to expand this example, neutrons bombarding with objects on space vehicles may cause a shut-down or malfunction of certain computer chips or parts. This can be a serious problem, of course. (By the way, this doesn't always happen in space, it's also known to happen via cosmic rays down to where we live, but the chance of it happening is greatly reduced because of the spacious atmosphere protecting us.) So, by using a cyclotron to generate neutrons and bombarding computer chips or parts with the neutrons at a facility, we can simulate if something is likely to malfunction or not BEFORE testing it in space. This, obviously, is a much more inexpensive way to test things, rather than spending billions of dollars on parts just to have it malfunction somewhere in space. There are additional uses of neutron detectors, but, personally, I feel this one is more important, so I won't go into the others.
I also did some efficiency tests using activation foils, but I won't explain it here, so see the paper for details on that.