Action of Radiation on Matter
When an energetic particle moves through matter (living or nonliving),
what happens is a question of chemistry. Chemistry is the subject that
deals with the arrangement and rearrangement of electrons in atoms and
molecules. A chemical rearrangement generally requires an energy in the
neighborhood of a few electron-volts. (As we have seen, an electron-volt
is the energy released when an electron moves through a potential of one
volt, i.e., a little less than one per cent of the driving force in a
standard electric outlet.) An energetic particle, such as might be
emitted in a radioactive decay, typically has an energy of a few million
electron-volts. Thus a single such particle has the potentiality of
about a million chemical rearrangements.
Energetic particles may be charged or neutral, light or heavy, or
electromagnetic in nature. Because of this diversity one might think
there would be no common grounds for comparing the action on matter of
different particles. Each particle might conceivably make its own
inimitable variety of chemical rearrangements. Actually this is not the
case.
Unlike some chemical poisons, which seek out specific molecules in our
body, the energetic particles strike at whatever atoms or molecules
happen to get in their way. They act, in this sense, like a sledge
hammer. Their effects can be measured directly from the strength (or
energy) of the blow. Which particle delivers the blow is of little
consequence provided the same amount of energy is delivered and provided
the same tissues are affected (in the case of living matter). After the
blow, however, some specific chemical effects may occur. When water or
some other molecule in the body is broken up by radiation, the fragments
produced may themselves be chemical poisons and attack the biologically
important large molecules in a secondary way. In fact, it seems probable
that a considerable part of the radiation damage caused in living
systems, both healthwise and genetically, occurs in this manner.
Although the energetic particles are all similar in their ultimate
action on matter, namely in producing wholesale destruction of atoms and
molecules, they differ somewhat in the way in which they bring about
this destruction. Charged particles act in one way, gamma rays in
another, and neutrons in still another. It is simplest to begin our
discussion with the charged particles.
The most important charged particles are those connected with the
natural background of radioactivity and cosmic rays, and the fission
process. These include alpha rays, beta rays, mesons, and fission
fragments. For review, a table of the weights and charges of these
particles, as well as a few others, is shown. As usual, we have used the
weight and charge of the proton as units.
_Particle_ _Weight_ _Charge_
proton 1 1
alpha 4 2
electron beta rays 1/1840 -1
positron ” 1/1840 1
deuteron 2 1
triton 3 1
meson 1/8 1, -1
average light fission fragment 97 20
average heavy fission fragment 138 22
If the fission fragments were completely stripped of their orbital
electrons, they would have charges even greater than the values
indicated in the table. The reader will recall that the average charge
of the nucleus of the light fission fragment is 38, and of the heavy,
54. But such highly positively charged particles exert an enormous
attraction on electrons. Some of these remain attached even during the
fission process itself. As the fission products lose their speed during
passage through matter, they pick up more electrons and gradually lose
their charge.
When any of these energetic charged particles moves through matter, it
interacts with electrons in the atoms. As a result of this interaction,
the electrons may be dislodged from their usual states of motion. If the
interaction is gentle—either because the charged particle passes the
atom at a considerable distance or else because the particle is moving
so rapidly that the interaction lasts for only a short time—the electron
may be left undisturbed. If the interaction is more violent, however,
the electron may be excited to a more energetic state of motion while
still remaining in the same atom or molecule; or it may actually be
ejected, ending up at some other atomic site. In this latter event the
original atom is left with a residual positive charge and is said to be
_ionized_. At the same time the displaced electron is apt to unite with
whatever atom or molecule happens to be nearby, creating in this way a
negative ion. The whole process may be described as forming an ion pair.
In the wake of the charged particle one finds, therefore, ionized and
excited atoms and molecules. A rearrangement of atoms will now ensue
which leads to new chemical compounds. The important thing for us is,
however, that these chemical changes do not depend very much on the type
of particle which produced the ionization; the proportion between
ionization, excitation, and eventual chemical reaction remains more or
less the same. Roughly speaking, the more ion pairs that are formed in
living cells, the greater is the extent of biological damage.
To make an ion pair requires the expenditure of a certain amount of
energy. It might seem as though this amount should depend crucially on
the weight, charge, and energy of the particle, and also on the medium
through which the particle is moving. This is not so. There is some
dependence, of course, but only slight. Any charged particle,
irrespective of its energy, moving in any medium—air, water, soil, or
living tissue—creates ion pairs at the rate of about one per 32
electron-volts. A one-million-electron-volt particle produces about
30,000 ion pairs before losing all of its energy. (When it does lose its
energy, if it is a positively charged particle, it will pick up enough
electrons to become neutral. An alpha particle, for example, will become
an ordinary helium atom; a proton will become an atom of hydrogen.)
We have said that two charged particles having the same energy, produce
the same total number of ionizations. There is an important respect,
however, in which charged particles of the same energy may differ. That
is, in the density of ionization along their paths. In particular, the
more slowly the particle is moving and the greater its charge, the more
ionization and damage it will produce in a given distance. At the same
time it will lose energy at a greater rate. If we compare two charged
particles of the same energy plowing into matter, the one which leaves
the deeper furrow will be stopped more quickly.
For a greater charge it is easy to understand that the electrical
interaction is increased and hence each atomic electron is more strongly
disturbed. If, on the other hand, the particle moves more slowly (which
is usually the case if it is heavy) it spends a longer time in the
neighborhood of the atomic electrons. The electrical interaction thus
has a longer duration and is more effective in ejecting an electron. For
this same reason the density of ionization along the path of a
particular charged particle should tend to become greater and greater as
the particle slows down. Actually this tendency is opposed in the case
of a fission fragment by the increased likelihood of the particle’s
picking up electrons and reducing its charge. As a result, the
ionization density for these fragments is rather uniform. If a heavily
charged, slow particle moves through matter it leaves so many disturbed
and disrupted molecules behind that now these molecules may react with
each other. Therefore heavy ionization may lead to peculiar effects.
Nevertheless all ionizing particles give rise to roughly similar
chemical change and destruction.
Except for the beta rays, all the charged particles are very heavy
compared to the electron. Consequently, as they move through matter and
interact with the atomic electrons, their paths are not perceptibly
deflected from the original direction. The beta rays, on the other hand,
having the same weight as the atomic electrons, are appreciably affected
by their encounters and are frequently forced to change direction. Their
paths are thus winding and random.
Because the beta ray does not travel in a straight line, its ability to
penetrate matter must not be measured by its total path length. As a
rule of thumb, the _range_ of a beta particle, being the distance it
travels along the line of its original direction, is about one half of
its total path length. For heavier charged particles, however, no
distinction need be made between range and actual distance traveled.
The most important fact about the ranges of charged particles is that
they are small. An alpha particle, for instance, with a typical
radioactive energy of a few million electron-volts, has a range in water
(or living tissue) of a few thousandths of an inch. Such a particle
could not penetrate a sheet of paper. A fission fragment, despite its
great energy, is even less penetrating than the alpha particle. The
proton has a somewhat greater range than the alpha particle. But the
beta ray, because of its low weight, has by far the greatest range of
any of the charged particles. Even it, however, goes only a fraction of
an inch in solid or liquid materials.
The following table shows the ranges (in inches) of some of the charged
particles in air and water as a function of energy (in millions of
electron-volts):
_Range_
_Air_ _Water_
(Same as living tissue)
_Energy_ _5_ _1_ _2_ _5_ _5_ _1_ _2_ _5_
alpha 0.1 0.2 0.4 1.4 0.0001 0.0002 0.0004 0.0014
proton 0.3 0.9 2.8 13.4 0.0005 0.001 0.003 0.014
beta 49. 130. 300. 770. 0.063 0.16 0.38 1.0
The table shows that charged particles travel only short distances in
matter. For this reason these particles are not a serious external
radiation hazard. The protons and the alpha rays are usually stopped by
less than a foot of air. Ordinary clothing or even the outer layer of
our skin (which is composed of nonliving cells) will stop them
completely.
Beta rays are stopped by less than seventy feet of air or an inch or
less of solid material. (Actually most of the beta rays produced in the
fission process have energies less than a million electron-volts or so,
and hence their ranges are even smaller.) Radioactive contamination of
beta emitters directly on one’s clothes or body could cause trouble; but
a good scrubbing soon after exposure will eliminate this problem. The
interior of a house or building should be quite safe from any outside
source of charged particles emitted by radioactive substances except
possibly the most energetic beta rays. Only if the source of charged
particles is inside the body so that in spite of their limited ranges
the particles can find their way to sensitive tissues, is there any
danger. In this case, as we shall see in a later chapter, the danger may
be considerable.
Charged particles of one type stand pretty much by themselves. These are
the mesons found in cosmic rays. These particles move as fast as
energetic beta rays and, like the beta rays, carry unit charge. Their
biological effects are therefore the same as the biological effects of
beta radiation, with one important difference. The cosmic ray mesons
carry much more energy and therefore have a much greater range. Whereas
the beta rays are stopped in the skin, the mesons can cause damage
throughout the entire body. The mesons produce the same effects as a
substance which emits beta radiation uniformly in the whole body. This
fact is important. It puts us in the position to compare effects of
man-made radioactivity with effects of the cosmic rays to which we are
constantly exposed.
Not all the energy in cosmic rays is carried by mesons. We also find
showers of electrons. These are almost the same as beta rays except that
they have more energy and arrive frequently in fairly sizeable numbers
traveling along nearly parallel tracks. Their effects, however, are the
same as the effects of the mesons.
We have been talking now about the interactions between charged
particles and the atomic electrons. No mention has been made of
interactions between the charged particles and nuclei. Nuclear
interactions do occur sometimes, but by and large they have only a
negligible influence in slowing down the charged particle. They do
affect, however, beta rays.
When a beta ray collides with a highly charged nucleus, the beta
particle is violently deflected. The violence of this process is due to
the heavy charge of the nucleus and the small mass of the beta particle.
In the sudden change of velocity which occurs, part of the electric
force field which surrounds the electron breaks loose; the result is
high-frequency radiation called X-rays. The importance of such
electromagnetic radiation is that it can penetrate more deeply into
matter. In our bodies, for typical beta-ray energies, only a small part
of the beta-ray energy is converted into X-rays. But in many radioactive
processes gamma rays (which are physically the same as X-rays) are
produced quite abundantly. These rays may carry as much or more energy
than the beta rays.
Unlike charged particles, which constantly interact as they move through
matter, gamma rays can go for long distances without having a single
encounter. The actual distance depends on the energy of the gamma ray,
the medium in which it moves, and pure chance. On the average, a
one-million-volt gamma ray goes about six inches in water before
anything at all happens to it. A four-million-volt gamma ray goes about
a foot. In living matter the distances are approximately the same. Thus
gamma rays from an external source can find their way deep inside the
body.
Of course living matter is not injured by the mere presence of a gamma
ray. There is a small probability that the gamma ray could go right
through the body without a single encounter. If so, there would be no
biological effect. An effect is produced only when the gamma ray
interacts with the matter. There are three most important ways in which
such an interaction may occur.
One way is simple _absorption_ of the gamma ray by one of the atomic
electrons. The gamma ray disappears in this process, and the electron
acquires all of its energy. A tiny bit of this energy is used for the
electron to break its bond with the atom. The remainder goes into
kinetic motion of the electron. The electron is now on the loose and can
cause biological damage by exciting and ionizing other atomic electrons.
In fact it is now the same thing which we used to call a beta ray.
A second way in which the gamma ray may interact with matter is by
_scattering_. In this case the gamma ray does not disappear but merely
loses a part of its energy to the atomic electron. Again the electron is
free to cause biological damage, while the gamma ray goes on to its next
encounter.
The third way requires that the gamma ray be near a nucleus and have an
energy greater than a million electron-volts. (Ordinary X-rays such as
are used in medical practice are not energetic enough for this process
to occur.) Under these conditions the gamma ray may disappear, with the
simultaneous appearance of an electron and a positron. This is an
example of the creation of matter out of pure energy. In accordance with
the formula E = mc², a part of the gamma-ray energy is consumed in
producing particles with definite masses. This amounts to about one
million electron-volts. The remainder of the gamma-ray energy goes into
kinetic motion of the two particles. Again biological damage results
from the subsequent ionization due to the charged particles. After the
positron has expended its kinetic energy in the ionization process, it
will join with an electron in a disappearing act. The energy reappears
in the form of two or three gamma rays (each having less energy than the
original gamma ray).
In no case is the gamma ray directly responsible for any biological
damage. The damage is always made by electrons (or positrons) to which
the gamma ray has transferred some or all of its energy. But this only
makes gamma rays the more dangerous. They can first penetrate to the
sensitive tissues of the body, and then cause ionization.
We have already mentioned that X-rays are the same as gamma rays. The
latter are produced by an excited nucleus, the former in the collision
of an electron (or a beta ray) with a nucleus. The man-made X-rays are
obtained by first accelerating a stream of electrons and then letting
them impinge on a target containing highly charged nuclei.
The usefulness of X-rays is, of course, due to their power of
penetration; that is the same property which renders X-rays dangerous.
One can use X-rays to find out what happens to be inside the human body.
But this cannot be done without producing some disruption and
rearrangement in the tissues which lie in the path of the X-rays. The
damage is of the same kind as that caused by radioactivity or cosmic
rays.
The effects of neutrons on matter are rather similar to the effects of
gamma rays. Like gamma rays, neutrons can travel long distances in
matter without interacting. On the average, a million-volt neutron goes
a few inches in water before having a collision of any kind. Also like
the gamma rays, the neutrons are not themselves directly responsible for
any biological damage. Being neutral, they interact only with the atomic
nuclei to which they are strongly attracted. By far the most important
of these interactions is with the nuclei of hydrogen. There are a great
number of these in living tissue in the form of protein and water
molecules.
The collisions with hydrogen nuclei (i.e., protons) are important
because a large fraction of the neutron energy is transferred in the
process. This happens because the neutron and the proton have very
nearly the same weight. If the neutron hits a heavy nucleus, it loses
only a small fraction of its energy in the impact.[9] After colliding
with hydrogen or a heavier nucleus, the neutron continues on to other
such collisions. The nucleus, however, being charged and energetic, now
causes excitation and ionization of atomic electrons. Thus, like gamma
rays, energetic neutrons are exceedingly dangerous, because they can
first penetrate and then cause ionization.
Neutrons are dangerous even when they are not energetic. A nonenergetic
neutron may react with nuclei of living matter in a number of ways of
which two are particularly probable. Either the neutron may be captured
by a proton to form a deuteron, in which case the excess energy will be
emitted in the form of a two-million-volt gamma ray that will cause
further damage. Or the neutron may react with a nucleus of nitrogen¹⁴
(abundantly present in living matter) to produce a nucleus of carbon¹⁴
and an energetic proton. Thus a nonenergetic neutron will have a
biological effect equivalent to an energetic gamma ray, or to an
energetic proton plus an energetic carbon¹⁴ ion.
In summary, all particles, charged or not, have a similar action on
matter. Directly or indirectly, they produce excited atoms, molecules,
and ion pairs. These processes always occur in practically the same
proportions, and therefore the number of ion pairs formed can be used as
a measure of the radiation effects. The more ion pairs produced in
living matter, the greater the extent of biological damage. For this
reason it is customary to describe radiation effects in terms of the
number of ion pairs created per gram of living tissue in various parts
of the body. Since each ion pair corresponds to an energy transfer of
about 32 electron-volts, an alternative description may be given in
terms of the amount of energy deposited. The unit in common usage for
this purpose is the _roentgen_, which means specifically an energy
equivalent to lifting the body (in which the radiation is deposited) by
one twenty-fifth of an inch. This is equivalent to about 60 million
million ion pairs in each ounce. It is less exact but more significant
to say that one roentgen deposits in a cell of our body a few thousand
ion pairs.
Of course the amount of ionization within individual cells is not a
quantity that is easily measured. What one usually knows instead, is the
roentgen dosage to a piece of tissue, which consists of many cells. If
the charged particles inducing the ionization are electrons (as they are
when the primary radiation is a beta ray or a gamma ray), the ionization
will be distributed more or less uniformly among the cells in the
affected neighborhood. If the charged particle is heavy—a proton or an
alpha ray—the density of ionization which it produces is much greater,
so that some cells receive a good many more ion pairs, while others
nearby may receive none. For this reason it is sometimes important to
specify not just how many roentgens the tissue has been exposed to, but
also which kind of radiation has been responsible.
In a later chapter we shall discuss the biological effects of various
amounts of radiation. We may mention here, however, that 1000 roentgens
of X-rays or gamma rays delivered more or less uniformly over the whole
body of a human being in a time less than a few hours or so, will lead
to almost certain death. And it is a remarkable fact that nature has not
provided us with a warning. Radiation does not hurt. The greater is the
need that we understand this process which affects our well-being but
not our senses.