| MadSci Network: Physics |
Hi Steve, The answer is sort of yes, sort of no. You are correct in that neutrons are necessary for the production of Pu-239 from U-238, however, the neutrons themselves are not accelerated. In order to get free neutrons, you generally need to take an H- ion (yes Hydrogen with an extra electron) and accelerate it into a high Z target, usually tungsten or some other heavy metal and through this process (called spallation) you get semi-directionally dependent neutrons out of the metal target. There is a new facility being built that will use liquid Mercury for the target. (See the web links below.) The functional problem with using a Van de Graaff, is that generally they don't have sufficient acceleration capability to produce a neutron that will be absorbed by the U-238. There are some that do, it's just that they are put to better use for different things. A cyclotron or linear accelerator would be the device of preference here. If you don't care what the neutron spectrum looks like and/or want less than 10MeV neutrons, a fabulous source of neutrons would be a nuclear reactor. Numerous research reactors exist in the US and abroad that fill this role in particle research. On to Pu production. In order for U-238 to become Pu-239 it has to absorb a neutron and then undergo a beta decay (some gamma rays may be produced as well). All isotopes of all elements have what is called a neutron cross section. This is essentially the probability of an incident neutron being absorbed by the target nucleus and is neutron energy dependent. The individual unit of cross section is the barn and it is 1x10E-24cm^2. For most isotopes of most elements, the cross section is very small; well under one barn for most neutron energies. However, some have a high enough cross section to soak up a neutron. What happens next depends on the isotope. Some fission, some eject an alpha particle, some emitt gamma rays, some do other things not related the the topic right now, and some are capable of doing several of the above (but not more than one at a time). In the case of U-238 it might fission, but that cross section is ~5microbarns; it might eject an alpha particle, but that cross section is ~1microbarn. The most probable interaction is that it will give off a gamma ray after neutron absorbtion (becomming U-239 with a half-life of 23.5 minutes after which it will beta decay into Pu-239). The cross section for this case ranges from about 2.7 to 277 barns depending on the neutron energy. In this rare case, as the neutron energy goes up, so does the cross section. Thus higher energy neutrons are prefered. For an excellent example of how to make Pu, look up fast breeder reactors (US nuclear weapons program). You also asked about heat. In a nuclear reactor, obviously, very hot (>300F) for a commercial power reactor, and rather cool for a research reactor (<200F generally). If you use an accelerator, it depends on what mode you operate. If the accelerator is pulsed and the target has proper cooling, the heat build up will generally be fairly small (<100F) as you don't want to damage the target that makes the neutrons. If the beam is steady state, you can eaisly melt a metal target in a very short span of time with or without cooling (this is what tends to make VdG's a poor choice). Lastly, hazardous or radioactive waste. The answer is that very little is produced, however, anything subjected to neutrons will absorb a certain fraction of them and become radioactive, even if only for an instant. In the case of some things, they were radioactive to begin with and are merely something different that's also radioactive. When sheilding for neutrons, you generally want to pick an isotope that wants to absorb a neutron and will rapidly decay into something nonradioactive. Boron-10 is an excelent example. It gladly absorbs a neutron (cross section of over 3800 barns) becomming B-11 and spontaneously alpa decays (an alpha particle you will remember is merely a helium nucleus witout electrons) into Li-7 which is stable. There are many other examples, but that is a nuclear physics lesson for another day. Please check out the selection of websites below or just type in the words -neutron source- in Yahoo and go to the web page matches (the first few pages will have what you want). Good hunting! Scott Kniffin Senior Engineer, Orbital Sciences Corporation NASA Goddard Space Flight Center Office of Systems Safety and Mission Assurance Radiation Effects and Analysis Group, Code 562 http://www.sns.gov/aboutsns/source.htm http://www.isis.rl.ac.uk/ http://www.pns.anl.gov/
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