Plutonium and Neutron Emission

[daisey]

I have been reading up on radioactivity (Wikipedia). I keep coming across articles which talk about emission of Neutrons (during Plutonium decay, for example). Now I know alpha radiation is basically Helium-4 nuclei, but that consists of both Protons and Neutrons combined in a nucleus. What type of radiation consists of only Neutrons? Or are Neutrons ejected during decay not considered "radiation"?

 

[Kritmiss]

Hello daisey,
Some radioactive materials can decay by both alpha emission and a process called spontaneous fission - this is where neutrons are emitted when the nucleus splits apart spontaneously during radioactive decay. Examples of this are Pu-240 and Cf-252. Neutrons may also result from any light element impurities present in alpha emitting materials due to (alpha, n) reactions i.e. an alpha particle ejects a neutron from the nucleus of the target atom. Both types of neutron radiation can be recorded by neutron measuring instruments. A can containing several kilograms of plutonium isotopes (typically Pu-238, Pu-239, Pu-240 and Pu-241) will have significant neutron radiation associated with it due to both these mechanisms. However, Pu-241 does not contribute as it is a very weak beta emitter

 

[Bill_K]

Because of Coulomb repulsion, nuclei with large numbers of protons (Z) require even larger number of neutrons (N) to be stable. The most stable isotopes have a ratio of about N/Z = 1.5. This is called the valley of stability.

Two complementary situations:

1) Heavy ion fusion. When two heavy ions fuse, the composite system retains the average N/Z ratio of the two, much smaller than stability requires. The composite nucleus is said to be neutron-deficient.

2) Fission. The opposite situation occurs when a heavy nucleus fissions into two fragments. Again the fragments will carry the same N/Z ratio as the original nucleus, and will therefore have many more neutrons than required for stability. Among the decay modes of these fragments will be neutron emission. They are known as delayed neutrons since they are emitted some time after the fission takes place.

 

[daisey]

Gotcha, I think. So now, in addition to Alpha, Beta, and Gamma radiation, there is a type related to spontaneous fission events which result in neutron emissions.

From what I've read, the problems to humans with the former three types of radiation is their ionizing effects. What is the danger posed to humans subjected to a shower of free neutrons released in a spontaneous fission event?

Daisey

 

[Drakkith]

Neutrons can penetrate very deeply into substances thanks to their neutral charge. They are also about 1/4 the mass of an alpha particle. The speed at which they are emitted is usually very very fast, enough to cause the neutron to knock atoms out of their lattices when they hit a nucleus. This is similar to the effect of an alpha particle, but takes much longer to hit something since they must connect directly with the nucleus of an atom instead of just coming within the electromagnetic field of the atom like an alpha particle. While you can shield alpha particles with just about any thin substance, neutrons will go through several inches of steel easily depending on their speeds.

 

[Astronuc]

Gotcha, I think. So now, in addition to Alpha, Beta, and Gamma radiation, there is a type related to spontaneous fission events which result in neutron emissions.

From what I've read, the problems to humans with the former three types of radiation is their ionizing effects. What is the danger posed to humans subjected to a shower of free neutrons released in a spontaneous fission event?

Daisey

Neutrons are generally very penetrating, but they can collide with atoms knocking them around. If a neutron strikes a proton, it is scattered, like a billiard ball, and can lose up to almost all of its kinetic energy (if a straight dead-on-center collision). That proton can then ionize the atoms/molecules along its trajectory.

Neutrons are also absorbed by atoms such that the atom increases mass by 1 amu, and usually emits a gamma ray in the process. The resulting nuclide may or may not be radioactive. If it becomes a radionuclide, it will like be a beta emitter.

Spontaneous fission - http://en.wikipedia.org/wiki/Spontaneous_fission