Neutron Emission & Capture

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I. Decay of a Free Neutron

A neutron, moving free in space (in other words, not in a nucleus) is unstable and decays into a proton, an electron, and an electron antineutrino. This is the reaction:

  0  1 n  --->   1  1 p   +  −1   0 e   +  νe — 

The half-life for this decay is about 10 minutes. Notice that this decay is equivalent to a beta decay. The mass number stays the same and the atomic number increases by 1. It's just that the neutron decays while free in space as opposed to decaying inside the nucleus of an atom.

You can find more information at the Wikipedia page for free neutron decay.


II. Neutron Emission

There appear to be around 75 isotopes that decay by neutron emission. In almost every case, these isotopes are products of fission and have extremely short half-lives.

Example #1:

  724 N  --->    723 N  +    0  1 n

The nitrogen-23 that is produced is, itself, unstable and it decays, also with a very short half-life. You can look up information about N-23 here. That same source gives the half-life of N-24 to be 52 nanoseconds.

Because there are so few examples of neutron emission, consider it as a possible test question.

Example #2:

  415 Be  --->    414 Be  +    0  1 n

You thought the half-life for N-24 was short? The half-life for Be-15 to decay is about 7.9 x 10¯22 seconds.

Example #3:

  416 Be  --->    414 Be  +  2  0  1 n

You have that correct. Be-16 decays by emitting two neutrons. H-5 and He-10 also decay by emitting two neutrons. H-6 emits three, sometimes four neutrons. The percent breakdown between three and four is not known. H-7 emits four neutrons. I think the neutrons are emitted in sequence as opposed to simultaneous emission.

Example #4:

  1  7 H  --->    1  3 H  +  4  0  1 n

There are some neutron emissions that are also accompanied by beta decay. Here's an example:

Example #5:

  53142 I  --->    54141 Xe  +  −1   0 e  +    0  1 n

Here is an alternate way to write the double decay. First, show the beta decay, then show the neutron emission (or decay):

  53142 I  --->    54142 Xe  +  −1   0 e  --->    54141 Xe  +  01 n

Typically, the beta decay is put first. There's no real reason except that usually how it is seen. Notice that the electron antineutrino is not included in the decay scheme. By the way, I-142 decays like this about 25% of the time. The other 75% is normal beta decay.

Here's a search, if you want to dig deeper. This document was a helpful resource for me when I put the above examples together.

I tried to determine if gamma radiation was also emitted during neutron emission. I was not able to determine that.


III. Neutron Capture

From the name, it seems pretty obvious what happens.

Example #1:

  48110 Cd  +    0  1 n  --->    48111 Cd

It turns out that cadmium has quite the propensity to absorb neutrons.

Example #2:

  48111 Cd  +    0  1 n  --->    48112 Cd

  48112 Cd  +    0  1 n  --->    48113 Cd

  48113 Cd  +    0  1 n  --->    48114 Cd

  48114 Cd  +    0  1 n  --->    48115 Cd

Cd-115 turns out to be unstable and it decays by beta to In-115.

Example #3:

  510 B  +    0  1 n  --->    511 B

Seems pretty ordinary, right? It's not.

The B-10 is injected into certain types of cancer and then a source of neutrons is directed at the cancer. The B-10 does the above, but then the B-11 decays via alpha emission:

  511 B  --->    3  7 Li  +    2  4 He  +  γ

The alpha particle and the gamma ray both deliver energy to the surrounding cancer, killing some of the cancer cells. It's called Boron Neutron Capture Therapy and it is very cool. The reaction below also plays a role in creating more gamma particles to help kill the cancer.

Example #4:

  1  1 H  +    0  1 n  --->    1  2 H  +  γ

Notice the inclusion of a gamma. Sometimes you might see this written:

  1  1 H  +    0  1 n  --->     12m H  --->    1  2 H  +  γ

Where m stands for metastable.

The above reaction has a short-hand way of writing it. I offer it here without comment:

  1  1 H (n, γ) 1 2 H

You also see the above written without the atomic numbers, but I'm sure you can visualize how that looks.

Example #5:

  92238 U  +    0  1 n  --->       92239m U  --->      92  239 U  +  γ

The U-239 is unstable and it decays by beta emission to Np-239 (the first tranuranic element, discovered in 1940). Np-239 also decays by beta emission to Pu-239 (the isotope used in the first atomic bomb exploded (the Triniy test in the US state of New Mexico as well in the bomb used at Nagasaki).


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