How do neutrons make nonradioactive elements radioactive?

Answer 1

The process of making materials radioactive by neutron bombardment is usually referred to as neutron activation. Neutrons are generally found in abundance within the cores of operating nuclear reactors where it is the neutron flux that is causing the fission chain to continue. These neutrons can be used to activate stuff. There are whole books on this subject. We'll tackle it here with a few paragraphs, so let's go. There is a funny group of rules associated with the way an atomic nucleus reacts to an incoming neutron. It isn't necessarily like breaking a rack of billiard balls like in so many science shows. It's more like the neutron manages to press up against the nucleus and the nucleus captures it. Then what happens, happens. One of the concepts to be considered is the energy of the incoming neutron. How fast is it going? (Faster isn't necessarily better for increasing the probability of capture.) Another is the size of the target that a given nucleus presents to that neutron. We call that a neutron absorption cross section, and it's measured in barns. (Yes, like the broad side of a barn, as in hitting the broad side of a barn. And who said physicists weren't funny and couldn't make jokes!) There are tables to look this stuff up on because each given isotope has a given neutron capture cross section, just like you'd imagine. One little (or no-so-little) thing is that different isotopes of the same element have different neutron capture cross sections. That ups the challenge for investigators. What happens is that the neutron is captured and all the nucleons (the protons and neutrons that are in the nucleus) must now make up a new contract as to how they are all gonna share this tiny little space where they are all crammed in. You remember that the protons and neutrons in an atomic nucleus are all smooshed together and all of them have given up a bit of mass and that mass has been converted into binding energy or nuclear glue to hold the whole thing together, right? Well, all that went out the window when the new kid arrived on the block. New arrangements have to be made. Depending on the element and the particular isotope of that element that captures a neutron, a number of different things can result. If the neutron is actually captured and there is no stable isotope of that element possible with the "new" configuration, it disintegrates. Disintegration schemes vary, as one might guess. If a stable isotope results, so be it. (It's stable.) If an unstable isotope results, it is radioactive, and rules apply for its decay. The table of nuclides holds the particulars. If a neutron is captured by a fissionable isotope, it fissions. The result will be fission fragments, a neutron or neutrons, and some other stuff. Hit the links and check out the particulars. An example of irradiation by neutrons (activation) might be cobalt. A little slug of cobalt is welded up inside a skin of stainless steel and then the pellet is dropped into an operating nuclear reactor to give it a neutron bath. Cobalt metal is almost entirely cobalt59 which is the only stable isotope. And it presents a big target to an incoming neutron. When it collects an extra neutron, presto! Cobalt60, which is highly radioactive with a 5.2 year half life. We reel in our little pellet of cobalt60 and put it inside a lead lined casket that has a little door. Now we stick labels all over the thing warning folks to leave it the heck alone. Then we truck it out to a job site where welding work has to be x-rayed. We set the thing up pointing at the weld, stick some photographic film on the other side, then run off and open the little door by remote control. The source is looking at the weld and is sending out x-rays (because of the decay of the cobalt60) which passes through the weld and the photographic plate is exposed. We then shut the door and develop the film. One weld x-ray finished. The advantage of this is that we don't have to drag an x-ray machine out there and then try to find a place to plug it in. Big advantage. Oh, as an aside, cobalt is sometimes alloyed into steel to harden it. Cobalt hardened steel could be used in, say, valve seats. But we don't use cobalt hardened steel for valve seats in nuclear plant primary systems. Guess why! Now you're an expert. Be sure to check that your name is spelled correctly on the role sheet and sign it so you can get credit for the class.

Answer 2

when the strong nuclear force can no longer overcome the repulsive electric forces among protons then nucleus become radioactive. The kind of radiation given off by a nucleus depends on the nature of the imbalance in the nuclear forces:

1) Alpha Particles

2) Beta Paricles

3) Gamma and X-Rays; the most penetrating radiation

4) Radioactive Decay