Bioradiation

Bioradiation

Radiation definition:   1-1

Radiation, when broadly defined, includes the entire spectrum of electromagnetic waves : radiowaves, microwaves, infrared, visible light, ultraviolet, and x-rays and particles.

Ionizing Radiation    1-2

      The common name for both radiation from x-ray machines and radioactive

sources is ionizing radiation. The name indicates that the radiation has sufficient energy to ionize atoms and molecules. An ionization takes place when an electron is removed from its position in the atom or molecule. Since a molecule usually has no net charge to begin with, the loss of a negative electron leaves behind a positive ion . The electron can then end up on another molecule which then becomes a negative ion.

Radioactive Elements  1-3

The atomic structure of most elements contains a nucleus that is stable. Under normal conditions, these elements remain unchanged indefinitely. They are not radioactive. Radioactive elements, in contrast, contain a nucleus that is unstable The unstable nucleus is actually in an excited state that can not be sustained indefinitely; it must relax, or decay, to a more stable configuration. Decay occurs spontaneously and transforms the nucleus from a high energy configuration to one that is lower in energy.

1-4The Nature of Radiation

The energy emitted by an unstable nucleus comes packaged in very specific

forms. In the years that followed the discovery of radioactivity, determining the

kind of radiation emitted from radioactive compounds was of great interest. It

was found that these radiations consisted of three types called: alpha (?), beta

(â) and gamma (?) radiations after the first three letters in the Greek alphabet The nuclear emission transforms the element into either a new element or a different isotope of the same element. A given radioactive nucleus does this just once. The process is called a decay or a disintegration

. The evidence for the three types of radiation comes from an experiment in

which the radiation from radioactive compounds was passed through a magnetic field. ?-rays passed through the field without disturbance, whereas the two other types were deflected from a straight line. Because it was known at that time that charged particles are deflected when they pass through a magnetic field, the conclusion was evident; ?-rays have no charge while ?- and â-radiations consist of charged particles. The ?-particles, deflected in one direction are positive whereas the â-particles, deflected in the opposite direction, are negative.

1-5Alpha radiation

In 1903, Ernest Rutherford (a New Zealander who worked in Cambridge, England most of his life) performed a simple and elegant experiment showing that the ?-particle is the nucleus of the helium atom. Rutherfordpositioned one

glass tube inside a second glass tube. The inner tube contained a radioactive

source that emitted ?-particles

1-6Beta and gamma radiation

Experiments have shown that the â-particle is a fast moving electron, whereas ?-radiation is an electromagnetic wave. Other examples of electromagnetic radiation are ultraviolet (UV), visible light, infrared and radio waves. Electromagnetic radiation is characterized by its wavelength or frequency. The wavelength is the distance from one wave peak to the next and the frequency is the number of waves passing a given point per second. Through quantum mechanics it is known that particles can be described as waves and vice versa. Thus, ?-rays and other electromagnetic radiation are sometimes described as particles and are called photons.

1-7Photoelectric effect is a process in which a photon interacts with a bound electron. The photon itself disappears, transferring all its energy to the electron and thereby imparting kinetic energy to the electron. This is the most important absorption process for radiation with an energy less than about 100 keV (which is the type of radiation used in medical diagnostics). The photoelectric effect varies dramatically with the electron density of the absorbing medium. Thus material that contains atoms with high atomic numbers, e.g., the calcium in bone, gives strong absorption due to the photoelectric effect.

1-8The Penetration of Radiation

When using a gun, the penetration by the bullet depends on the energy of the

bullet as well as the composition of the target. For example, a pellet from an air gun will be stopped by a few millimeters of wood but a bullet from a high

powered rifle will pass through many millimeters of steel. It is similar with

ionizing radiation. There are large differences in penetrating ability depending

on the type of radiation .

1-9What is Radioactivity?

X-rays and ?-rays will easily penetrate the human body. This property is utilized when x- and ?-rays are used for diagnostic purposes. ?- and â-particles, on the other hand, lose their energy within a short distance and cannot penetrate the body. Because of these penetration properties, ?-radiation is easy to observe whereas ?-and â-radiation are more difficult to detect.

The following conclusions can be drawn:

* If a radioactive source is on the ground, such as in a rock, the ?- and

â-radiation will be stopped by air and clothes. Only ?-rays would penetrate

into the body and deliver a radiation dose.

* When a radioactive source is inside the body, it is a different situation.

?- and â-particles are completely absorbed within a short distance in the

tissues, whereas only a certain fraction of the ?-radiation is absorbed. The

rest of the ?-radiation escapes and can be observed with counters outside

the body. Consequently, if you eat food containing radioactive compounds,

they can be easily measured if ?-rays are emitted.

It is possible then to measure the radioactivity that is inside animals and humans who have eaten food containing Cs-137 due for example to fallout from nuclear tests or nuclear accidents. For adults, approximately 50% of the ?-radiation escapes the body and the other half is absorbed by the body. Other important isotopes such as Sr-90 (strontium) and Pu-239 (plutonium) are very difficult to observe since they only emit â-particles and ?-particles.

1-10Biological Half-life

The radioactive isotopes that are ingested or taken in through other pathways

will gradually be removed from the body via kidneys, bowels, respiration and

perspiration. This means that a radioactive atom can be expelled before it has

had the chance to decay. The time elapsed before half of the compound has been removed through biological means is called the biological half-life and is usually written tb.

If a radioactive compound with physical half-life tp (t1/2) is cleared from the

body with a biological half-lifetb, the “effective” half-life (te) is given by the

expression:

If tpis large in comparison to tb, the effective half-life is approximately the same as tb.

The biological half-life is rather uncertain compared to the exact value of the

physical half-life. It is uncertain because the clearance from the body depends

upon sex, age of the individual and the chemical form of the radioactive substance. The biological half-life will vary from one type of animal to another and from one type of plant to another.

Cs-137, having a physical half-life of 30 years, is a good example. It was the

most prominent of the radioactive isotopes in the fallout following the Chernobylaccident in the Ukraine. Cesium is cleared rather rapidly from the body and the biological half-life for an adult human is approximately three months and somewhat less for children. Cs-137 has a biological half-life of 2 to 3 weeks for sheep, whereas for reindeer it is about one month.

Due to the fact that the biological half-life for animals like sheep is rather short, it is possible to “feed down” animals, with too high a content of Cs-137,

Before slaughtering. The animals can simply be fed non-radioactive food for a short period. Another possibility is to give the animals compounds such as “Berlin blue” which is known to speed up the clearance of cesium from the body. The result is a shorter biological half-life.

Some radioactive species like radium and strontium are bone seekers and,

consequently, are much more difficult to remove. The biological half-life for radium is long, and if this isotope is ingested, it is retained the rest of one’s life. It is possible to reduce the effects of a radioactive compound by simply preventing its uptake. Consider iodine. If people are to be exposed to radioactive iodine, it is possible to add non-radioactive iodine to their food. All iodine isotopes are chemically identical and the body can not discriminate one isotope from the other. There will be a competition between the different isotopes. If the amount of non-radioactive iodine is larger than the radioactive isotope the uptake of radioactivity is hindered. This kind of strategy can also be used to decrease the biological half-life.

1-11Radio-ecological Half-life

Radio-ecological half-life is less precise than the physical and biological half-life. Consider a region which has been polluted by a radioactive isotope (for example Cs-137). Part of the activity will gradually sink into the ground and some will leak into the water table. Each year, a fraction of the activity will be taken up by the plants and subsequently ingested by some of the animals in the area.

Radio-ecological half-life is defined as the radioactive half-life for the animals

and plants living in the area. It varies for the different types of animals and

plants. Knowledge in this area is limited at present, but research carried out

after the Chernobyl accident has yielded some information.

It is important to note that these ecological half-lifes are significantly shorter than the respective physical half-life, 30 years for Cs-137 and 2 years for Cs-134.