Stochastic Effects

Stochastic Effects

Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects. Stochastic effects often show up years after exposure. As the dose to an individual increases, the probability that cancer or a genetic effect will occur also increases. However, at no time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. In addition, because stochastic effects can occur in individuals that have not been exposed to radiation above background levels, it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure.

While it cannot be determined conclusively, it often possible to estimate the probability that radiation exposure will cause a stochastic effect. As mentioned previously, it is estimated that the probability of having a cancer in the US rises from 20% for non radiation workers to 21% for persons who work regularly with radiation. The probability for genetic defects is even less likely to increase for workers exposed to radiation. Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur.

Radiation-induced hereditary effects have not been observed in human populations, yet they have been demonstrated in animals. If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation, hereditary effects could occur in the progeny of the individual. Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and, during certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high.

More on Specific Stochastic Effects

Cancer

Leukemia

Genetic Effects

Cataracts

Interaction Radiation and Matters

Interaction of Electromagnetic Radiation and Matter

It is well known that all matter is comprised of atoms. But subatomically, matter is made up of mostly empty space. For example, consider the hydrogen atom with its one proton, one neutron, and one electron. The diameter of a single proton has been measured to be about 10^-15 meters. The diameter of a single hydrogen atom has been determined to be 10^-10 meters, therefore the ratio of the size of a hydrogen atom to the size of the proton is 100,000:1. Consider this in terms of something more easily pictured in your mind. If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm), its electron would be approximately 10 kilometers away. Therefore, when electromagnetic waves pass through a material, they are primarily moving through free space, but may have a chance encounter with the nucleus or an electron of an atom.

Because the encounters of photons with atom particles are by chance, a given photon has a finite probability of passing completely through the medium it is traversing. The probability that a photon will pass completely through a medium depends on numerous factors including the photon’s energy and the medium’s composition and thickness. The more densely packed a medium’s atoms, the more likely the photon will encounter an atomic particle. In other words, the more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur  Similarly, the more material a photon must cross through, the more likely the chance of an encounter.

When a photon does encounter an atomic particle, it transfers energy to the particle. The energy may be reemitted back the way it came (reflected), scattered in a different direction or transmitted forward into the material. Let us first consider the interaction of visible light. Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate. If the material is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object. If the material is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material, but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave. The light may be reemitted from the surface of the material at a different wavelength, thus changing its color.

X-Rays and Gamma RaysX-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei. However, X-rays and gamma rays have enough energy to do more than just make the electrons vibrate. When these high energy rays encounter an atom, the result is an ejection of energetic electrons from the atom or the excitation of electrons. The term "excitation" is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state.

Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles. With each interaction, the energy may be directed in a different direction. The higher the energy of a photon, the more likely the energy will continue traveling in the same direction. As the radiation moves from point to point in matter, it loses its energy through various interactions with the atoms it encounters. If the radiation has enough energy, it may eventually make it through the material.

Photon Interaction with Matter is KeyFrom the previous paragraph, it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power. Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planck’s constant (E = hƒ). The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =c/λ ). However, it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating. The key is its interaction with matter, or more specifically, whether the photon's energy is right to excite some transition of a charged particle. For instance, microwaves penetrate glass very easily, but they are strongly absorbed by water. Move up to slightly higher frequency, and infrared is strongly absorbed by both glass and water, but both substances transmit visible light. Ultraviolet is stopped by glass, but not so readily by water.

Nature of Radiation

Nature of Radiation

Radiation is a form of energy. There are two basic types of radiation. One kind is particulate radiation, which involves tiny fast-moving particles that have both energy and mass. Particulate radiation is primarily produced by disintegration of an unstable atom and includes Alpha and Beta particles.

Alpha particles are high energy, large subatomic structures of protons and neutrons. They can travel only a short distance and are stopped by a piece of paper or skin. Beta particles are fast moving electrons. They are a fraction of the size of alpha particles, but can travel farther and are more penetrating.

Particulate radiation is of secondary concern to industrial radiographers. Since these particles have weight and are relatively large, they are easily absorbed by a small amount of shielding.However, it should be noted that shielding materials, such as the depleted uranium used in many gamma radiography cameras, will be a source of Beta particles if the container should ever develop a leak. If a leak were to occur, the material could be transferred to the hands and other parts of a radiographer’s body, causing what is known as particulate contamination. This is the reason periodic “leak” and “wipe tests” are performed on equipment.

The second basic type of radiation is electromagnetic radiation. This kind of radiation is pure energy with no mass and is like vibrating or pulsating waves of electrical and magnetic energy. Electromagnetic waves are produced by a vibrating electric charge and as such, they consist of both an electric and a magnetic component. In addition to acting like waves, electromagnetic radiation acts like a stream of small "packets" of energy called photons. Another way that electromagnetic radiation has been described is in terms of a stream of photons. The massless photon particles each travel in a wave-like pattern. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Electromagnetic radiation travels in a straight line at the speed of light (3 x 10⁸m/s).

Light waves, radio waves, microwaves, X-rays and Gamma rays are some examples of electromagnetic radiation. These waves differ in their wavelength as shown in the electromagnetic spectrum image above. Although all portions of the electromagnetic spectrum are governed by the same laws, their different wavelengths and different energies allow them to have different effects on matter. Radio waves, for example, have such a long wavelength and low energy that our eyes cannot detect them and they pass through our bodies. It takes a special antenna and electronics to capture and amplify radio waves.

The wavelength of visible light is on the order of 6,000 angstroms, while the wavelength of X-rays is in the range of one angstrom and that of Gamma rays is 0.0001 angstrom. This very short wavelength is what gives X-rays and Gamma rays their power to penetrate materials that light cannot. Unlike light, X- and gamma rays cannot be seen, felt, or heard. The fact that they cannot be detected with our normal human senses and can damage our cells is why they must be treated carefully.

Ionization and Cell Damage

Ionization and Cell Damage

As previously discussed, photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials. This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material. This creates electrons, which carry a negative charge, and atoms without electrons, which carry a positive charge. Ionization in industrial materials is usually not a big concern. In most cases, once the radiation ceases the electrons rejoin the atoms and no damage is done. However, ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other. This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed. Ionization may cause unwanted changes in some materials, such as semiconductors, so that they are no longer effective for their intended use.

Ionization in Living Tissue (Cell Damage) In living tissue, similar interactions occur and ionization can be very detrimental to cells. Ionization of living tissue causes molecules in the cells to be broken apart. This interaction can kill the cell or cause them to reproduce abnormally.

Damage to a cell can come from direct action orindirect action of the radiation. Cell damage due to direct action occurs when the radiation interacts directly with a cell's essential molecules (DNA). The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA). DNA is found in every cell and consists of molecules that determine the function that each cell performs. When radiation interacts with a cell wall or DNA, the cell either dies or becomes a different kind of cell, possibly even a cancerous one.

Cell damage due to indirect action occurs when radiation interacts with the water molecules, which are roughly 80% of a cells composition. The energy absorbed by the water molecule can result in the formation of free radicals. Free radicals are molecules that are highly reactive due to the presence of unpaired electrons, which result when water molecules are split. Free radicals may form compounds, such as hydrogen peroxide, which may initiate harmful chemical reactions within the cells. As a result of these chemical changes, cells may undergo a variety of structural changes which lead to altered function or cell death.

Various possibilities exist for the fate of cells damaged by radiation. Damaged cells can:

• completely and perfectly repair themselves with the body's inherent repair mechanisms.
• die during their attempt to reproduce. Thus, tissues and organs in which there is substantial cell loss may become functionally impaired. There is a "threshold" dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome. Exceeding the threshold dose increases the level of harm. Such outcomes are called deterministic effects and occur at high doses.
• repair themselves imperfectly and replicate this imperfect structure.  These cells, with the progression of time, may be transformed by external agents (e.g., chemicals, diet, radiation exposure, lifestyle habits, etc.). After a latency period of years, they may develop into leukemia or a solid tumor (cancer). Such latent effects are called stochastic (or random).

Exposure of Living Tissue to Non-ionizing RadiationA quick note of caution about non-ionizing radiation is probably also appropriate here. Non-ionizing radiation behaves exactly like ionizing radiation, but differs in that it has a much greater wavelength and, therefore, less energy. Although this non-ionizing radiation does not have the energy to create ion pairs, some of these waves can cause personal injury. Anyone who has received a sunburn knows that ultraviolet light can damage skin cells. Non-ionizing radiation sources include lasers, high-intensity sources of ultraviolet light, microwave transmitters and other devices that produce high intensity radio-frequency radiation.

Survey Meters

Survey Meters The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation. A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter. There are many different models of survey meters available to measure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector. Gas filled detectors consists of a gas filled cylinder with two electrodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin taut wire along the axis of the cylinder acts as the other electrode. A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge). The gas becomes ionized whenever the counter is brought near radioactive substances. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value. Depending on the voltage applied between the anode and the cathode, the detector may be considered an ion chamber, a proportional counter, or a Geiger-Müller (GM) detector. Each of these types of detectors have their advantages and disadvantages. A brief summary of each of these detectors follows. Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode, which results in a collection of only the charges produced in the initial ionization event. This type of detector produces a weak output signal that corresponds to the number of ionization events. Higher energies and intensities of radiation will produce more ionization, which will result in a stronger output voltage. Collection of only primary ions provides information on true radiation exposure (energy and intensity). However, the meters require sensitive electronics to amplify the signal, which makes them fairly expensive and delicate. The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies. This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator. An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used. Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode. Due to the strong electrical field, the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas. The electrons produced in these secondary ion pairs, along with the primary electrons, continue to gain energy as they move towards the anode, and as they do, they produce more and more ionizations. The result is that each electron from a primary ion pair produces a cascade of ion pairs. This effect is known as gas multiplication or amplification. In this voltage regime, the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle. Hence, these gas ionization detectors are called proportional counters. Like ion chamber detectors, proportional detectors discriminate between types of radiation. However, they require very stable electronics which are expensive and fragile. Proportional detectors are usually only used in a laboratory setting. Geiger-Müller (GM) CounterGeiger-Müller counters operate under even higher voltages between the anode and the cathode,usually in the 800 to 1200 volt range. Like the proportional counter, the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas. However, this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions. This all happens in a fraction of a second and results in an electrical current pulse of constant voltage. The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse. In other words, the size of the current pulse is independent of the size of the ionization event that produced it. The GM counter was named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in developing it further in 1928. The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute. If the instrument has a speaker, the pulses can also produce an audible click. When the volume of gas in the chamber is completely ionized, ion collection stops until the electrical pulse discharges. Again, this only takes a fraction of a second, but this process slightly limits the rate at which individual events can be detected. Because they can display individual ionizing events, GM counters are generally more sensitive to low levels of radiation than ion chamber instruments. By means of calibration, the count rate can be displayed as the exposure rate over a specified energy range. When used for gamma radiography, GM meters are typically calibrated for the energy of the gamma radiation being used. Most often, gamma radiation from Cs-137 at 0.662 MeV provides the calibration. Only small errors occur when the radiographer uses Ir-192 (average energy about 0.34 MeV) or Co-60 (average energy about 1.25 MeV). Since the Geiger-Müller counter produces many more electrons than a ion chamber counter or a proportional counter, it does not require the same level of electronic sophistication as other survey meters. This results in a meter that is relatively low cost and rugged. The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge). Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage. In the ion chamber region, the voltage between the anode and cathode is relatively low and only primary ions are collected. In the proportional region ,the voltage is higher, and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected. In the GM region, a maximum number of secondary ions are collected when the gas around the anode is completely ionized. Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and proportional regions. Radiation at different energy levels forms different numbers of primary ions in the detector. However in the GM region, the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated the event. The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse. This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters.

Film Badge

Film Badges

Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film.

A special film is used which is coated with two different emulsions. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film is coated with a fine grain, slow emulsion that is less sensitive to exposure. If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted, the fast emulsion is removed and the dose is computed using the slow emulsion.

The film is contained inside a film holder or badge. The badge incorporates a series of filters to determine the quality of the radiation. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles. Beta particles are effectively shielded by a thin amount of material.

The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 millirem. The major disadvantages are that it must be developed and read by a processor (which is time consuming), prolonged heat exposure can affect the film, and exposures of less than 20 millirem of gamma radiation cannot be accurately measured.

Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket. The clip-on badge is worn most often when performing X-ray or gamma radiography. The film badge may also be worn when working around a low curie source. Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation. A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body.

Thermoluminescent dosimeters (TLD)

Thermoluminescent Dosimeter

Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a film badge, it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received, if any. Thermoluminescent dosimeters can measure doses as low as 1 millirem, but under routine conditions their low-dose capability is approximately the same as for film badges. TLDs have a precision of approximately 15% for low doses. This precision improves to approximately 3% for high doses. The advantages of a TLD over other personnel monitors is its linearity of response to dose, its relative energy independence, and its sensitivity to low doses. It is also reusable, which is an advantage over film badges. However, no permanent record or re-readability is provided and an immediate, on the job readout is not possible.

How it works
A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material. Some of the atoms in the material that absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.

Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor.

Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The "glow curve" produced by this process is then related to the radiation exposure. The process can be repeated many times.