Video Transcript
In radioactive contamination, what
must be a property of a contaminating substance? (A) The contaminating substance
must mix with other substances more easily than the substance that it
contaminates. (B) The contaminating substance
must absorb nuclear radiation. (C) The contaminating substance
must be more radioactive than the substance it contaminates.
Recall that an object is considered
to be radioactive if it contains atoms that can emit nuclear radiation, in other
words, unstable atoms. Recall also that contamination can
occur when substances with different properties are mixed together. For the most simple example of
radioactive contamination, we can consider two substances. Substance A is a radioactive
substance; it contains unstable atoms. Substance B is a completely
nonradioactive substance and so contains no unstable atoms. Both substances are soft and can
flow easily.
When these substances are mixed by
adding some of substance A to substance B, some of the unstable atoms from substance
A will have become part of substance B. The resulting mixture contains both
stable atoms and unstable atoms. As substance B initially contained
no unstable atoms and now contains some unstable atoms, substance B has become more
radioactive. For an object to be radioactively
contaminated, we must increase the radioactivity of the object.
What if we reverse the process,
adding substance B to substance A? As substance B is not radioactive,
adding substance B to substance A cannot make substance A more radioactive. Substance A does not become more
radioactive because it takes atoms from substance B. What about the case though for
mixing two substances that are both radioactive? Substance C is less radioactive
than substance D, even though they are both radioactive. What if we add some of substance C
to substance D? Was substance D contaminated? Well, we added some unstable atoms
to substance D, so it seems like we must’ve made it more radioactive. Is that correct though? It is not obvious.
Recalling our examples, let us
consider each of the options in the question. One of these options is
correct. Option (A) says that what matters
is how much more easily the contaminating substance mixes than the contaminated
substance. Option (B) says that what matters
is which of the substances absorbs nuclear radiation. Option (C) says that what matters
is which of the substances is more radioactive.
Suppose that option (A) is correct;
the substance that mixes more easily is the contaminating substance. When we say “mixes more easily,” we
can understand this to mean “gives more atoms to the other substance than it takes
from the other substance.” We saw that adding substance B to
substance A did not radioactively contaminate substance A, even though only
substance B gave atoms and only substance A took atoms. Which substance gives more atoms
and which substance takes more atoms cannot therefore be what determines whether
radioactive contamination occurs. We reject option (A).
Suppose that option (B) is
correct. The contaminating substance must
absorb nuclear radiation. Recall that when substance A was
added to substance B, substance B was radioactively contaminated. When this happened, substance A did
not absorb nuclear radiation. Substance A emitted nuclear
radiation. Radioactive substances are called
radioactive because they emit nuclear radiation. We can see then that option (B) is
incorrect, so we reject option (B).
Option (C) says that the
contaminating substance must be more radioactive than the contaminated
substance. When substance A was added to
substance B, substance B was radioactively contaminated, as substance A was more
radioactive than substance B. When substance B was added to
substance A, substance A was not radioactively contaminated, as substance B was less
radioactive than substance A. When substance C was added to
substance D, this would mean that substance D was not contaminated by substance
C.
The best way to understand this is
to consider substance D after substance C is added to it. If we take an amount of substance D
that was mixed with substance C and we compare this to an equal amount of substance
D that was not mixed with substance C, we can see that the amount that was mixed is
actually less radioactive. Adding substance C to substance D
decreased the part of substance D where mixing occurred. Recall that we have said that
radioactive contamination means that the radioactivity of the contaminated object
must increase. We see then that the contaminating
substance must be more radioactive than the contaminated substance. This is what option (C) says. Option (C) is correct.
