Interaction of Radioactivity with Matter

Read this article to learn about the interaction of radioactivity with matter.

Alpha Particles:

Fig. 13.5 illustrates the passage of an alpha-particle through matter. Because alpha-particles (made up of 2 protons and 2 neutrons) are comparatively heavy and doubly charged, they cause a great deal of ionization as they collide with atomic electrons in the material, knocking them out of their atoms.

Because they are so much heavier than an electron, they do this without deviating from a straight path, but each collision results in a small loss of energy to the alpha particle, so that it steadily slows down.

The density of ionisation tends to increase towards the end of the particle’s path. Occasionally an alpha-particle may suffer a collision with an atomic nucleus, but this is compara­tively rare because the nucleus is so small. However, if an alpha-particle does hit a nucleus it will be deviated significantly from its forward path and will also send the nucleus recoiling in a different direction.

The recoil nucleus can then go on to cause additional ionisation in the material. Because the ionisation produced is so dense, the alpha particle will soon lose all its energy as a result of many electron collisions and rapidly come to a stop. The distance travelled before it finally stops is called the particle’s range. The range depends on the particle’s energy and the material through which it travels, but for an alpha-particle it is always very short.

For example an alpha-particle with an energy of 1 MeV will have a range of 5 mm in air and only 7 microns in tissue. From this it is clear that an alpha-particle source outside the body will do little harm, because all the alpha-particles will be absorbed in the superficial layers of the skin which are dead anyway.

However, if an alpha source was allowed to get inside the body its radiation would be absorbed in a few cells and could produce very damaging effects. This is why alpha emitting radionuclides, such as ²³⁸Pu, are so dangerous, if inhaled. Alpha emitters are not used in medicine.

Negatrons:

Compared with α-particles, negatrons are very small and rapidly moving particles that carry a single negative charge. They interact with matter to cause ionization and excitation exactly as with a-particles. However, due to their speed and size, they are less likely than α-particles to interact with matter and, therefore, are less ionizing and more penetrating than α-radiation.

Another difference between α-particles and negatrons is that, whereas for a given α-emitter all the particles have the same energy, negatrons are emitted over a range of energy, i.e., negatron emitters have a characteristic energy spectrum. The maximum energy level (E_max) varies from one isotope to another, rang­ing from 0.018 MeV for ³H to 4.81 MeV for 38Cl. The difference in E_max affects the penetration of radiation; β-particles from ³H can travel only a few millimetres in air, whereas those from ³²P can penetrate over 1 m of air.

The reason for negatrons of a given isotope being emitted within an energy range was explained by W. Pauli in 1931, when he postulated that each radioactive event occurs with an energy equivalent to E_max, but that the energy is shared between a negatron and a neutrino. The proportion of total energy taken by the negatron and the neutrino varies for each disintegration. Neutrinos have no charge and negligible mass and do not interact with matter.

Beta-particles:

Fig. 13.6 illustrates the passage of a beta particle through matter. Because beta particles (electrons) are lighter and only singly charged, they produce less dense ionization than alpha-particles and are much more easi­ly deviated from a straight line as they ionize atoms in the material through which they pass.

Frequently the collisions with an atomic electron are sufficiently violent to cause the beta particle to deviate through a large angle and then the atomic electron with which it collided acquires enough energy to move off on its own This electron is called a delta-ray and it goes on to produce further ionisation.

Occasionally, if a beta-particle happens to encounter an atomic nucleus in a material of high atomic number, it will be deviated very violently and in doing so gives off bremsstrahlung X-rays (from the German for breaking radiation). After a very zigzag track, beta-particles will eventually come to rest and so, like alpha-particles, they exhibit a definite range.

However, since they produce less dense ionisation, they slow down more gradually than alpha-particles and will have a longer range. For example, 600 keV is a typical beta-particle energy; this is the maximum energy of beta particles from ₁₃₁I decay and the average energy from decay of ₉₀Sr and its daughter ₉₀Y.

A 600 keV beta-particle has a range of 2.5 metres in air or 3 mm in tissue. Because all beta sources emit a range of beta-particle energies, rather than just a single energy, there will always be a spread in the range of particles emitted. The number of beta-particles, therefore, falls off quite rapidly with thickness of material traversed, until none remains after a thickness equal to the range of the maximum energy present.

If a beta source is close to, or even inside the body, its radiation will be absorbed within a few millimetres of the source. This means that all the energy is absorbed in local tissues and, since beta-particles are mod­erately ionising, there is potential for damaging effects to these tissues. In diagnosis this may be looked upon as a hazard to be minimised, but it can be put to good use in therapeutic applications.

For example the high energy beta particles emitted from the decay of ₉₀Sr (and its daughter ₉₀Y) can be used in an external application for therapeutic doses to surface tissues. The beta-particles from ₁₃₁I are also used in therapy of the thyroid. Since iodine is concentrated by thyroid tissue, a patient administered with radioactive iodine ₁₃₁I will receive a larger radiation dose to the thyroid than to other organs.

Nuclides such as ₃H, which emit low energy beta particles, result in a smaller radiation dose which means that, in small amounts, they may be safely administered internally for in vivo diagnostic studies. However, since the beta-particles will not escape from the patient, measure­ments of the activity present must be made by collecting blood or urine samples and then counting these in the laboratory.

Even then detecting the low energy beta-particles is not easy and the samples must be intimately mixed with the detecting medium in a liquid scintillation counter. Slightly higher energy beta particles, such as those from ₃₅S, are useful for autoradiography. When tissue containing ₃₅S is placed on a photographic film, the beta-particles will only blacken the film locally, producing an image of the activity distribution in the sample.

γ-Rays and X-rays:

Gamma-rays and X-rays are not particles like alpha and beta, but are examples of electromagnetic radiation (like high energy light) and consequently interact with matter in a rather different way. Fig. 13.7 illustrates the passage of several gamma-rays through matter. Unlike alpha and beta, where each particle undergoes many individual interactions, each gamma-ray only encounters one, or possibly two interactions and many gamma-rays pass through with no interaction at all.

Gamma-rays and X-rays do not produce ionisation directly, but instead they do so by first producing secondary electrons. These arise from two types of process; scattering and absorption. Scattering occurs through the process of Compton scattering in which a gamma-ray interacts with a free electron in the material.

The gamma-ray passes some of its energy to the electron and continues on its way as scattered radiation with a lower energy and travelling in a different direction (for exam pie rays 1 and 4 in Fig. 13.7). There are two possible absorption processes in which the gamma-ray disappears altogether.

Photoelectric absorption occurs when a gamma ray gives up all of its energy at once to an atomic electron which is then ejected from the atom (rays 3 and 6 in Fig. 13.7). Gamma-rays with energy greater than 1 MeV can also be absorbed by pair production, in which an electron and positron pair are spontaneously pro­duced (ray 7 in Fig. 13.7).

The positron will subsequently annihilate with an atomic electron produc­ing two gamma-rays of 511 keV. After any of these processes the secondary electrons produced go on to produce ionisation of the material, just like a beta-particle. Unlike alpha- and beta-particles, which are stopped after many interactions, gamma-rays and X-ray each undergo only a few interactions.

Gamma-rays do not, therefore, have a definite range, but instead the intensity of a gamma-ray beam is attenuated by a combination of scattering and absorption processes so that it falls off steadily with distance. The distance required to halve the number of the gamma-rays is called the half-value layer (HVL). This is analogous to the half-life of radionuclide decay and the same expo­nential mathematics applies.

In Fig. 13.7 the incident radiation consists of ten gamma rays entering at the left. During passage through one half value layer of the material 3 of these are absorbed and 2 scattered leaving an attenuated beam containing only 5 gamma-rays. In the next half value layer, half of these would again disappear.

The half value layer depends on the energy of the radiation and the nature of the material. For a gamma-ray of 140 keV the HVL in lead is 0.25 mm. Therefore, 0.25 mm of lead shielding will reduce the intensity of 140 keV radiation to half its original value, 0.5 mm will reduce it to one quarter, 1 mm (4 HVLs) to one sixteenth and 3 mm (12 HVLs) by a factor of 1/4096.

In tissue the HVL is much greater, being 47 mm for 140 keV gamma-rays. Thus, if a radionuclide such as ^99mTc, which emits 140 keV gamma-rays, is present inside a person it is clear that sufficient numbers of gamma-rays will be able to penetrate body tissues to permit external detection of the whereabouts of the radionuclide for diagnostic imaging purposes.

Gamma-rays that do not escape will distribute their energy throughout several organs, leading to a distributed radiation dose which is much less damaging than the local doses from beta emitters. This is why pure gamma emitting radionuclides such as ^99mTc are so useful for diagnostic imaging purposes.

Radiation Dose:

When ionising radiations (alpha, beta, gamma or X-rays) pass through matter they pass on some or all of their energy to the material by ionising and exciting the atoms of the material through the processes described above. The damage done by this depends both on the energy deposited and the amount of material involved.

The radiation damage increases as the amount of energy depos­ited increases and decreases if it is spread throughout a greater amount of material. The radiation absorbed dose is, therefore, defined as the energy absorbed divided by the mass of material involved. One joule of energy absorbed in each kilogram of material is defined as an absorbed dose of one gray (written 1 Gy).

Usually we are dealing with doses smaller than this so we use units of one thousandth of a gray, or one milligray (written 1 mGy). The concept of absorbed dose applies to all types of material but, when we need to assess the effect on biological tissues, we also need to take account of the fact that some types of radiation are more harmful than others.

For example, because they are so densely ionising, alpha-particles are about twenty times as effective at killing cells as beta-particles, gamma-rays or X-rays. Therefore, when measuring the dose to biological tissues, we use a quantity called equivalent dose which is defined as the absorbed dose multiplied by a radiation weighting factor.

This radiation weighting factor is 20 for alpha-particles but 1 for beta- particles, gamma-rays and X-rays. Confusingly although equivalent dose has essentially the same units as absorbed dose, it is given a different special name of the sievert (written Sv) or millisievert (mSv). Since we never have to deal with alpha particles in medical applications it happens that the equivalent dose (in Sv or mSv) is always numerically the same as the absorbed dose (in Gy or mGy).

There is one final complication to measuring the effect of radiation on a person; not all tissues in the body are equally sensitive to radiation damage. For example the bone marrow is particularly suscep­tible to damage whereas the skin is relatively insensitive. Therefore, in situations where different parts of the body might receive different doses, it is usual to calculate a weighted sum of the equivalent doses to each organ.

The organ weighting factors take account of the susceptibility of each organ to damage. Thus bone marrow gets a larger weighting factor than skin. This weighted sum of organ doses is called the effective dose. Because the weighting factors for all organs in the body add up to one, if every organ receives the same equivalent dose, the effective dose will be the same as the equivalent dose.

Therefore, the effective dose can be thought of as the uniform whole body dose which would have the same effect (in terms of the risk of doing harm) as the actual non-uniform dose. Effective dose is measured in units of sievert (Sv) or millisieverts (mSv) just the same as equivalent dose.