TOPIC 2: Energy Deposition in Matter

Let’s explore energy deposition in matter.

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We now know the universe is naturally radioactive and that we are constantly irradiated from the land, air, and space. The average human body is made of about 4 octillion atoms, or 4×1027 atoms!  And there is radiation all around us…always.  So the atoms of our bodies are being hit by radiation…constantly. We have seen how ionizing radiation interacts with atoms by removing an orbital electron to create ions.

How does radiation deposit energy in human tissues? How do different types of radiation differ in their abilities to do damage to tissues and organs? How can we measure the deposited energy as a dose that accounts for all of the physical factors and the possible health effects as well?

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Radiation will enter your body and deposit energy whenever it hits tissue. In fact, it is doing so constantly no matter where you are and what you are doing. Its doing it right now.  So let’s figure out how to measure the dose and what it means for our health.

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We’ve already defined Exposure as the amount of photon energy deposited in air…which we called the Roentgen or the milliroentgens … abbreviated as the mR.  Exposure rate is a measure of how much energy is deposited per unit time, for example … milliroentgens per hour (mR/hr). This provides a universal indicator of the amount of energy transferred from radioactive material to air.  But the concern to individuals is not the amount of energy deposited into air, but rather the amount of energy absorbed into the human body.

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The exposure rate of a radiation field describes the ionization that could potentially take place in a human body. The real dose depends on how a person moves around in the radiation field and how long they are there. As the radiation energy is absorbed, either to the whole body uniformly or to only some parts of the body, we create a value for absorbed dose.  However, there are two aspects to dose.  The first aspect is the amount of energy that is absorbed by tissue.  The second is the damage that this energy does to the tissue.

Here is an example.  You are walking down a hall and you trip.  When you fall, the impact is partially absorbed by your arms and partially by bouncing on your chest.  You brush yourself off.  You look around and try to hide your embarrassment.  Then you continue on your way.  Now let’s try a different scenario.  Instead of catching yourself when you fall, you keep your hands to your sides and hit the ground with your forehead.  You are immediately knocked unconscious with a gaping head wound and are rushed to the hospital in a coma.

Both events have our body absorbing the same amount of energy.  In the first instance, the energy is absorbed by parts of the body that can dissipate that energy, harmlessly.  In the second instance, that energy is directed to one local sensitive point resulting in more damage.  The same “energy deposited” but with different “damage done.”

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As the name implies, Absorbed Dose, is the amount of energy absorbed (deposited) in tissue.  It is the amount of energy deposited into one gram of material by ionizing radiation.  The absorbed dose is quantified in units called RAD (radiation absorbed dose).  One RAD is defined as the absorption of 100 ergs of energy per gram of material (again – you don’t have to remember this number).  In the International System of Units, the absorbed dose is in units of gray (Gy).  One gray equals one joule per kilogram.  As a conversion, one gray equals 100 rad.  While the rad or gray describes the amount of energy deposited into the tissue it does not describe the biological damage done and the risk of serious health consequences.

One convenient conversion that you need to know for x-ray and gamma-ray is that one mR of exposure is almost equal to one mrad to tissue in the radiation field. For practical use, we round it out and say 1 mR of exposure will create 1 mrad of absorbed dose. 

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The LET represents the average amount of radiation energy lost by a charged particle  or electromagnetic energy when traversing a given distance in matter.  Radiation that produces significant damage over a short distance in tissue or other material is called high-LET radiation.  In contrast, low-LET radiation produces only a small amount of damage when evaluated over a short distance.

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Alpha particles represent high-LET radiation.  Alpha particles have a very short track length and produce a large number of primary and secondary ionizations. On an atomic scale, the alpha particle is massive.  It is like a bull thrashing around a porcelain shop.  It won’t take long to produce a lot of damage.

Because of their large mass, alphas tend to follow very straight paths deflected only by rare collision with a nucleus. They have considerable kinetic energy typically between 4 to 8 MeV. Kinetic energy is the energy a body possesses by virtue of its motion and consists of its Mass and velocity. When a particle stops, it has given up all of its kinetic energy.

Remember, the high-LET alpha particle has a relatively short track length but causes a lot of ionization as it gives up its energy.

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Beta particles, on-the-other-hand, represent low-LET radiation.  Its like a mouse thrashing around the same porcelain shop.  There may be some damage, but it will be much less.

A beta particle is only a fraction the mass of an alpha particle. Betas, with such a small mass, are quite easily deflected when passing atoms. Because the beta particle has a negative charge, it is repelled by the orbital elections also having a negative charge.  So, the beta tracks in matter are not straight, but instead are quite erratic.

The smaller mass and higher velocity of the beta particle reduces the probability of interaction with the orbital electrons of the atoms it passes.  Consequently, it has a much greater penetrating power and hence a much longer range through matter. For some energetic betas, the range can be over 30 feet in air and a few centimeters in tissue.

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LET was developed for charged particles, but it might help you understand the relative damage produced in tissue by different radiation types to include a discussion of electromagnetic radiation –gamma-rays. Gamma radiation has no mass or charge, so the range of gamma rays can be quite large. There really is no definite range, but typical gamma ranges are from inches for “weak” (low-energy) gammas and over 100 yards for very energetic (high-energy) ones. Gammas that can travel that far certainly are not interacting very much and are losing very little energy along their paths. These “low-LET” gammas would not produce much ionization along the same track length that expends all of the energy of the most energetic alphas.  Gamma rays with higher energy will travel further, thus have more potential for interactions with matter.

Hopefully you can see that to produce a given amount of damage, it takes a larger absorbed dose of low-LET radiation than for high-LET radiation.  For example, If a BB gun produced low-LET radiation, it would take a lot of hits from a BB gun to do the same amount of damage as a single hit from even a slow moving car.

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The Quality Factor, Q, is a proportionality factor that relates radiation damage to tissue to the actual radiation energy absorbed. It is related to the LET in concept.  It allows us to take into account the biological damage done by the absorbed energy and the risk of health effects from this radiation dose. The quality factor is multiplied by the absorbed dose to generate the dose equivalent.  The quality factor is also based on the relative biological effectiveness of different types and energies of radiation in producing damaging health effects.

For example, X-rays and gamma rays of all energies are normalized to a health effect, or quality factor, of 1. Beta particles have about the same ability to produce health effects within the body and consequently also have a quality factor of 1. However, alpha particles emitted inside the body can lose a lot of energy within a very short distance. This high-LET radiation is 20 times more damaging than X, gamma or beta radiation and is assigned a quality factor of 20.

Note that the situation is different when the radiation is emitted outside the body. In that case, X and gamma radiation may be more hazardous because they can travel through the body.  Beta radiation can barely penetrate the skin or eyes if it is very energetic, and alpha particles are of no consequence no matter how energetic they are because their ability to penetrate skin or eyes is negligible.

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