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Neutrons has no charge so they are hard to interact with materials. That's what I'm thinking.

But they collide with atoms and it causes nuclear fission.

And they can also be a dangerous radioactive which bring out biological damage.

Why? You know that the most of the atom is empty space.

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"But they collide with atoms and it causes nuclear fission" only if they hit the right nuclei with the right energies. Other times they are captured by the nucleus they hit. Most often they just bounce off like billiard balls. –  dmckee Sep 13 '13 at 16:53
    
@dmckee How do they bounce? Which force applied to them? –  user28936 Sep 20 '13 at 16:12
    
The residual strong force (i.e. strong nuclear force), but at this level you might do just as well thinking of it like billiard balls. –  dmckee Sep 20 '13 at 16:17
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Well you have two effects.

For starters, free neutrons are unstable, and they decay to a proton and an electron, with a half life of about 11 minutes. So then you have two charged particles that fly off in opposite directions wreaking havoc. Secondly, neutrons, being uncharged, are not deflected by Coulomb forces, so they can score a direct hit on the atomic nucleus, and be captured. This then usually results in a proton being ejected from the nucleus, so the atom changes to a different element, of one less atomic number.

The ejected proton ("knock on" proton) also has high kinetic energy; often around 14 MeV, so it is the energetic charged particle products, that do the damage.

Neutron damage in biological tissue, is a strong function of neutron KE. Special "tissue equivalent" monitor detectors, are used to monitor neutron hazards, in locations where they could be produced. Impacting Deuterons, on a heavy ice target, will result in neutrons, or protons, in the 14 MeV range. The detectors, often proportional gas counters, containing organic materials either solid or gas, or both, that collectively mimic the capture cross-section for neutrons in biological tissues. These detectors, are like Geiger counters, but below the critical avalanche Voltage range

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In many, many materials the neutrons are liable to thermalize and capture on time scales much shorter than that for decay. In which case it is the gamma(s) associated with the capture reaction that cause the secondary effects. –  dmckee Sep 13 '13 at 23:44
    
Well often it is the thermalization that leads to the neutron wandering around in the material, like a ticking time bomb, before decay to the charged pair. Your comment raises the issue of why do gammas cause damage themselves. Gammas don't interact with the valence electrons, they too interact with the nucleus, resulting in the ejection of an electron or positron, and the ionization cause by those particles does the damage, by breaking chemical bonds, particularly in organic materials. Detection of neutrons or gammas usually involves detection of those charged protons or electrons. –  user26165 Sep 14 '13 at 0:07
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The average time to thermalize and capture for a few MeV neutron in a vat of mineral oil is about 200 microseconds (taking KamLAND as an example). It's probably a little longer than that in a human body, but only by a small factor. It is a several orders of magnitude larger in air, but even then an appreciable fraction don't decay: they capture. And the gamma interact with both nuclei and all the electrons. Indeed at modest energy it is the electron density of the material that dominates their cross-section. Check the PDB chapter on the passage of radiation through matter. –  dmckee Sep 14 '13 at 0:16
    
I would add that my information is well over 50 years old; so probably Wikipedia, would differ from what I have said; that's progress. –  user26165 Sep 15 '13 at 0:11
    
Neutron have a mean lifetime of 881.5±1.5 s (about 14 minutes, 42 seconds). –  user28936 Sep 20 '13 at 16:09
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Atoms are very small, you are correct. But if you have enough neutrons, the probability of at least one hitting what it needs to hit increases dramatically. That is why a nuke goes off when it reaches a critical mass (where there are enough neutrons produced to reliably trigger additional collitions). It is also why you body has a hard time defending against them as they can pass through most tissue and randomly hit DNA or other important components. Again, most pass through as you are correctly predicting but if you are exposed to enough of them, the damage that the minority of them cause builds up.

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they interact purely by linear energy transfer (LET. think billiard ball collisions) since they are chargeless. because of this neutron "radiation" is only indirectly radioactive.

when free neutrons strike fissile material unstable isotopes form, which quickly decay, producing ionizing radiation and sometimes yet more free neutrons--a neutron chain reaction if you will. sometimes the struck material doesnt have to be radioactive itself, but turns radioactive upon neutron bombardment, such as cobalt-59(present in stainless steel) turning into dangerous cobalt-60 from neutron capture.

but if you are talking biological damage it would make sense to talk more about hydrogen and oxygen, of which we are full of (in the form of water). in the former case, LET effectively kicks the proton and turns the hydrogen nucleus (we are full of hydrogen in the form of water) into a free proton which can directly ionize and severely damage the body from within. oxygen-16 releases a gamma photon upon neutron capture, and quickly ejects a free proton to form nitrogen-16. within seconds this decays back to O-16 by beta decay. nothing that was released as byproducts in these processes are good for the body. most importantly, the danger of indirect radioactivity in biology is ionizing irradation from inside the body rather than from outside.

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This is a good answer except for the statement "they interact purely by linear energy transfer". That gets contradicted by the later part of this very answer. While nuclear scattering is a major component of how it's transformed into ionizing radiation, it's not the only way. –  AlanSE Sep 14 '13 at 1:20
    
later part is inelastic collision--KE is not conserved, but partially converted to gamma radiation. where's the contradiction? –  gregsan Sep 14 '13 at 9:55
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