Contents
Radiation dosimetry
Radiation dosimetry is a process of measurement of absorbed doses of ionizing radiation. The instrument which is used for this purpose is called dosimeters.
Measurement of Doses
An international commission of radiological protection (ICRP) and the International Commission on radiation units and Measurement (ICRU) have published recommendations and data to evaluate and calculate the effect of various radiation doses to the human risk and health assessment purposes. Various physical quantities associated with measurement of radiation doses are Absorbed dose, equivalent dose, effective dose, and KERMA.
Absorbed Dose
It is the basic or fundamental dose. It is a measure of average energy emitted by ionizing radiation to per unit mass of absorber (D=dE/dM). The absorbed dose, sometimes also known as the physical dose. Its SI unit is Gray.
(Gy) Where 1 Gy = 1 joule/1kg. It is also used to describe the localized effect such as Dose absorbed by the cancerous cells in radiotherapy. Generally localized levels are in the range of ( 0-50 mGy).
Equivalent Dose
It is the biological effects of absorbed dose i.e. the average dose absorbed effectively by organ multiplied by weighing factor. It is its SI unit is sievert (SV).
( For the practical purpose of Assessing and regulating the hazard of ionizing radiation to workers and the general population, weighing factor are useful.)
A radiation weighing factor is an estimate of the effectiveness per unit dose of the given radiation relative to low – linear energy transfer (LET) standard.
Weighing factors are dimensionless multiplicative factors used to convert physical dose (Gy) to equivalent dose(Sv); i.e. to place biological effect from exposure to a different type of radiation on a common scale.
According to ICRU to the weighing factor is different for a different type of radiation :
Effective dose
It provides the equivalent dose to the whole body that gives the same as the localized exposure. So it is the sum of equivalent doses to each organ multiplied by its weighing factor.
(*) Remaining tissues: Adrenals, extrathoracic region, gall bladder, heart, kidney, lymphatic nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen, thymus, uterus cervix.
Kerma:
It is the kinetic energy released by ionizing radiation per unit mass of absorber.
Natural radioactivity
The spontaneous emission of particles and radiation from an unstable nucleus is called radioactivity.
Detectable radioactive elements are found throughout in nature i.e. soil, water, air and vegetables. These are responsible for the natural radioactivity.
Three main sources of radioactivity are:
- Radon: The biggest source of natural background radiation is airborne radon and its progenies (about 55%), a radioactive gas that emanated from the ground. It is a decay product of Uranium, which is relativity in Earth’s crust. Its value is more in one ore bearing rocks.
- The second source is the cosmic radiation that is continuously bombarding from the outer space. Primarily they consist of positively charged Ions ranging from protons to larger nuclei drive from sources outside of our solar system. These radiations interact with the atmosphere to create secondary radiations including X-rays, muons, protons, Alpha particles, pions, electrons, and neutrons.
- The third type is terrestrial radioactivity which is caused by radiations due to radionuclides such as potassium, uranium, and thorium.
Biological Effect of Radiation
The source of radiation is natural or artificial when it is absorbed by organs. There will be certain biological effects. The biological effects start with the consequences of the interaction of radiation with the atoms forming the cell.
Radiations affected the cell mainly by two processes
- Direct process in which radiation interacts with the atom of DNA molecule or other cellular components critical to the Survival of the cell.
- Indirect interaction in which enough number of atoms are affected that there is a significant alteration in the information carried by DNA molecule.
According to radiation protection manual the biological effect of radiation are:
Acute and delayed effect
A single accidental exposure to a high dose of radiation during a short period of time is referred to as acute exposure and may produce biological effects within a short period after exposure. These effects include: Skin damage, Nausea, and Vomiting, Malaise and Fatigue, increased temperature, Blood changes, Bone marrow damage, damage to cells lining the small intestine, damage to a blood vessel in the brain.
The above list is given for information purposes only. The dose that can produce such effect are extremely unlikely even in the event of an accident at the U of T.
The delayed effect of radiation is due to both acute exposure and continuous exposure (chronic exposure). In this case, the negative effect may not be apparent for years. Chronic exposure is likely to be the result of improper or inadequate protective measures.
In this case of inhalation or ingestion of radioactive material, a single “acute” event may cause a long period of “chronic” internal body expose due to radiation of tissues where radioactive material has been fixed.
The most common delayed effects are various forms of cancer (leukemia, bone cancer, thyroid cancer, lung cancer ) and genetic defect ( malformation in children born to Parents exposed to radiation). In any radiological situation involving the induction of Cancer, there is a certain time period between the exposure to radiation and the onset of diseases. This is known as the “latency period” and is an interval in which no symptoms of diseases are present. The minimum latency period for leukemia produce by radiation in 2 years and can be up to 10 years or more for other types of cancer.
Dose effect relationship
The connection between the effect of exposure to radiation and dose ( i.e. dose-response relationship ) is classified into two categories: non-stochastic and stochastic
The non-stochastic effect also referred to as a deterministic or tissues and organ effect, is specific to each exposed individual. They are characterized by:
- A certain minimum dose but must be exceeded before the particular effect is observed. Because of this minimum dose, the non-stochastic effects are also called threshold effects. The threshold may differ from individual to individual.
- The magnitude of the effect increases the size of the dose received by the individual.
- There is a clear relationship between exposure to radiation and observer effect on the individual.
The stochastic effects are those that occur by chance. They are more difficult to identify since the same type of effect may appear among individuals not working with radioactive materials. The mains stochastic effects are cancer and genetic defects. According to Current Knowledge of molecular biology, a Cancer is initiated by damaging chromosomes in a somatic cell. Genetic defects are caused by damage to chromosomes in a germ cell (sperm or ovum). There is no known existing threshold for stochastic effects. One single Photon or electron can produce the effect. For these reasons, a stochastic effect is called a Linear or zero threshold dose-response effect.
The stochastic effect can also be caused by many other factors, not only by radiation. Since everybody is exposed to natural radiation, and to other factors, the stochastic effect can arise in all for us regardless of the type of work (working with radiation or not). whether or not an individual develops the effect is simply a question of chance.
There is a stochastic correlation between the number of cases of cancers developed inside a population and the dose received by the population at a relatively large level of radiation. Attempts have been made to extrapolate the data from these level of dose to a low level of dose ( close to the levels received from background radiation). There is no scientific evidence to prove the result of these attempts.
Since there is no evidence of a lower threshold for the appearance of Stochastic effects, the prudent course of action is to ensure that all radiation exposure follow a principle known as ALARA ( as low as responsible achievable). We will be referring to the application of principal at U of T in subsequent modules.
Effects of Radiation on Foetus
It is well known that the Foetus is more sensitive to the effect of radiation than the adult human. If irradiation occurs in the first 30 weeks of pregnancy, the delayed effect may appear in the child. These include mental and behavior retardation, with a delay period of approximately 4 years.
Because of these possible effects, dosimetry during pregnancy differs from the usual protocol. Special attention is paid to both external and internal radiation. A radiation safety officer of the U of T must review procedures for handling radioactive materials when a pregnant worker performs such work.
It is not possible to accurately measure the doors to the Fortis and so it must be inferred from the exposure to the mother. The radiation protection principle limits exposure to the mother in order to achieve meaning to the foetus.
Effect of very low radiation levels
Exposure to the very low level of radiation is a controversial issue, originating many debates throughout the scientific community. What happens at a very low level of radiation exposure (a few percentages of the background), on top of the day to day natural irradiation), it is not known.
As was explained earlier, everybody is exposed to a label of radiation called the natural radiation or background radiation. Also, it was proved that the background label varies on earth by a factor greater than 10.
There are not enough data to support extrapolations of the effect from high level of exposure (like the survivor of Hiroshima and Nagasaki bombing, uranium minor, medical exposer etc.). However, these extrapolations are made. There are four mathematical models are used to describe the effect of a low level of radiation. All are supported (more or less) by controversial epidemiological studies, or by extrapolation conclusion obtained from studies with other mammals to humans.
Radiation Monitors
These instruments are used to monitor the external exposure from radiation. According to IAEA (International atomic energy agency), radiation oncology.
Radiation monitoring is essential because it is used to
- Assess the condition and exposure to the individual.
- To ensure safety condition and workplaces.
- To create the data of monitoring over a prolonged period of exposure.
Radiation monitoring is based upon measurement of the following factors:
- The radiation level at workplaces.
- The label around sources (including therapy instruments).
- Equivalent doses received by individual working at radiation prone areas.
Radiation monitors are basically of two types:
- Area monitors e.g. Gas-filled detectors (ionization chamber, proportional counter, GM counter) and solid-state detectors ( scintillation counter and semi-conductor counters)
The gas detectors are based on the principle of separation and measurement of the charge carrier produced by the radiation after ionization of gas atom in their path. Depending upon the operating voltages they are further separated into their other categories.
The scintillation counter is based on light emission from certain organic and inorganic crystals after the absorption of radiation. A photomultiplier tube optically coupled to the scintillators to convert light pulses into electric pulses.
The semiconductor counters are based on separation and detection of charge carriers in the depletion layer of the diode when light radiation of suitable frequency falls on it.
- Personal dosimeters ( individual Dosimeters) e.g. Thermoluminesce dosimeter (TLD) Radiophotoluminesce dosimeter (RLD), Optically stimulated dosimeters (OSD).
The cumulative doses from alpha, beta, and gamma are evaluated by measurement of track densities of the film under the different optical filters and then comparing these densities with the calibrated films.
The monitoring of neutron doses is done by the following process:
Thermal neutron
A cadmium window absorbs a thermal neutron and the resulting Gamma radiation blackens the film below this window as an indication of the neutron dose.
Fast Neutron
Nuclear track emulsion is used. In these emulsions neutron increase with hydrogen nuclei in the emulsion and surrounding materials, producing required Photon by elastic collisions. These particles create a latent image, which leads to the darkening of the film along their tracks after processing.
The principle of an RPL glass dosimeter :
The material used in silver activated phosphate glass. When silver activated phosphate glass is exposed to radiation, stable luminescence centres are created in silver irons, donated as Ag0 and Ag+. This luminescence centres emit light upon excitation. The readout technique uses pulsed ultraviolet laser excitation. A photomultiplier tube register the orange fluorescence emitted by the glass.
Advantages of RPL glass system :
- The RPL signal is not erased during the readout, thus the dosimeter can be re-analysed several times, and the measured data reproduced. Accumulation of the dose is also possible that may be used for registration of the lifetime dose.
- Commercially available RPL dosimeters typically cover the doors range of 30 μSv to 10 Sv.
- RPL signal exhibits very low fading and is not sensitive to the environmental temperature making it convenient in individual monitoring.
The Working Principle of OSL Dosimeter
OSL Dosimeters contain a thin layer of Aluminium oxide. During analysis, the Aluminium oxide is stimulated with selected frequencies of laser light producing luminescence proportional to radiation exposure.
Advantages of OSL Dosimeter
OSL dosimeters are highly sensitive e.g. system can be used down to 10μ Sv with a precision of +- 10μ Sv. This high sensitivity is particularly suitable for individual monitoring in a low radiation environment.
TLD Dosimeter
A TLD badge consists of a set of TLD chips enclosed in a plastic holder with filters. The most frequently used TDL materials ( also referred to as phosphors).