The x-ray safety program is monitored by the Laboratory Safety Committee.
The following sections outline the structure of the x-ray safety program including duties and responsibilities.
All x-ray instruments need to be registered with the Ministry of Labor and an internal permit issued by the Safety Office (X-Ray Safety Officer). Please contact Greg Friday (firstname.lastname@example.org or ext. 35755) prior to purchasing an x-ray instrument.
University of Waterloo's x-ray safety program encompasses all work with x-ray emitting devices under University of Waterloo (UW) control.
This program reflects the requirements in Ontario Regulation 861/1990 X-ray Safety and the Federal Radiation Emitting Devices Act.
This program outlines procedures and controls to ensure safe working conditions when working with or near x-ray emitting devices at the University of Waterloo.
All staff, faculty and students working with x-ray producing equipment must complete the X-ray safety training.
The training consists of 2 parts:
The theoretical part is the following x-ray safety training material and questions located at the end of the course. Minimum scores of 80% correctly answered questions covering each section is required to complete this training. Once the theoretical training is completed, the X-ray Safety Officer notifies the x-ray permit holder that the person has completed the theoretical training and is eligible for them to administer the practical training.
The practical part of the training is administered by the Permit Holder of the x-ray unit.
Once the permit holder is satisfied the worker is competent to operate the x-ray unit, a Record of training (PDF) is completed by the supervisor and worker then forwarded to the X-ray Safety Officer.
The worker may then begin working with the x-ray unit.
The construction and certification of x-ray producing equipment is controlled by Health Canada under the Radiation Emitting Devices Act.
Health Canada has also published various safety codes as guides for construction and use of radiation emitting equipment.
The operation of all x-ray equipment for non-human use in Ontario is covered under the Occupational Health & Safety Act, X-ray Safety Regulation.
Work designation - X-ray worker/student designation
An "x-ray worker/student" is a person who, as a necessary part of employment/assignment, may be exposed to x-rays and may receive a dose equivalent in excess of 1.0 mSv per year whole body.
Workers/students who meet this criteria will be designated as an "x-ray worker/student" and must complete a Notification of x-ray worker/student designation (PDF)
Workers/students who are designated as "x-ray workers/students" must be:
- Informed in writing as to their status as "x-ray worker/student".
- Informed in writing of the limits on dose equivalent that may be received.
- Informed in writing of the limits of dose equivalents to pregnant x-ray worker/student.
- Provided with appropriate dosimetry to ensure dose limits are not exceeded.
X-ray diffraction cabinets are the most common type of x-ray emitting devices used at UW. The design and construction of these cabinets ensures the dose to a person is kept well below 1.0 mSv per year, therefore these persons will not normally be designated as x-ray workers/students and would not have to wear dosimetry.
Worker's duties and responsibilities
- Complete x-ray safety training and are authorized to work with x-ray emitting devices.
- Comply with x-ray safety regulations and UW x-ray safety program.
- Report immediately incidents of exposure or malfunction to the X-ray Safety Officer (ext. 35755) and supervisor.
- Wear the appropriate radiation dosimeter.
Laboratory Safety Committee
Terms of reference
The Laboratory Safety Committee's (LSC) overall responsibility is to monitor the use of radionuclides, biohazardous materials, chemicals, lasers, x-rays and other safety issues related to teaching and research laboratories*. The Committee is advisory to the Vice-President, University Research, and to the Safety Office and provides the following functions:
- Oversees strategies to ensure ongoing and adequate surveillance, hazard identification, and risk evaluation of laboratory related activities.
- Assesses requirements for laboratory users training and laboratory safety procedures. Recommends revisions, when indicated.
- Reviews reports related to laboratory safety services, activities, incidents, and interventions in laboratory areas. Recommends corrective actions, when indicated.
- Maintains subcommittees based on areas of expertise to receive, review and approve reports and applications required by legislation and regulatory agencies.
- Reports as required to the Vice-President, University Research.
Membership of the Committee will include:
- Appointments by Vice-President, University Research, in consultation with Faculty Deans:
8 - Faculty members with the following areas of laboratory expertise appointed for renewable three-year terms:
- 2 - radionuclides (subcommittee required)
- 2 - biohazards (subcommittee required)
- 2 - chemicals
- 2 - lasers and x-rays
2 – Faculty Laboratory directors/managers - renewable three-year terms
Ex-officio members will include:
- Director, Health Services or designate
- Director, Office of Research Ethics or designate
- Director, Safety Office or designate
- Safety Officer, Safety Office
- Committee Chair is appointed by the Vice-President, University Research.
- Committee will meet at least annually and as required.
- The Safety Office
Permit holder's duties and responsibilities
The x-ray permit holder shall provide facilities, equipment and supervision according to x-ray safety regulations and UW x-ray safety program. The X-Ray Safety Officer (Greg Friday email@example.com or ext. 35755) should be contacted prior to purchasing any x-ray equipment so that it can be properly registered with the Ministry of Labor and an internal (UW) permit issued.
The x-ray permit holder shall ensure workers and students working under their supervision:
- Complete x-ray safety training and are authorized to work with x-ray emitting devices.
- Comply with x-ray safety regulations and UW x-ray safety program.
- Report immediately incidents of exposure or malfunction to the X-ray Safety Officer (ext. 35755).
- Notify the X-ray Safety Officer of any change in location or modification to any x-ray emitting device under their supervision.
- Wear the appropriate radiation dosimeter.
X-ray officer's duties and responsibilities
The X-ray Safety Officer shall administer the x-ray safety program by overseeing and coordinating all aspects of x-ray safety within the institution.
The X-ray Safety Officer shall:
- Act as the agent of the institution in respect to x-ray registration.
- Establish, implement, maintain a safety control and assessment program in conjunction with the Radiation Safety Committee.
- Annually review and survey x-ray emitting devices for radiation leakage.
- Implement a personnel monitoring program.
- Ensure radiation safety instruments are calibrated and serviced as required.
- Conduct a quarterly review of occupational radiation exposures and recommends ways of reducing exposures in the interest of ALARA.
- Control the purchasing, use and disposal of x-ray emitting devices through the issuance of internal permits.
- Ensure appropriate radiation protection training is provided on a regular basis as part of an ongoing "radiation protection awareness program" for all users and those who come into contact with x-ray emitting devices.
- Maintain required records.
- Ensure that each internal permit is amended when necessitated by changes to facilities, equipment, policies, procedures or personnel.
- Investigate and report to the Ontario Ministry of Labour all overexposures or accidents involving x-rays.
Duties of the X-ray Safety Officer with respect to the the Laboratory Safety Committee
The X-ray Safety Officer shall:
- Function as the link between the Laboratory Safety Committee and x-ray emitting devices users within the institution.
- Prepare or review in consultation with the Laboratory Safety Committee a comprehensive x-ray safety program.
- Have major input in matters pertaining to:
- facility and equipment design
- work practices and procedures
- evaluation, issuance and enforcement of internal permits
- disciplinary action necessitated by noncompliance
- X-ray safety training
Understanding how x-rays are produced and their interaction with materials and people requires a rudimentary knowledge of matter and radiation.
This program uses a model of the atom developed by Ernest Rutherford. Rutherford suggested that the atom consists of a positively charged nucleus surrounded by negatively charged electrons.
Later experimental results suggested that the nucleus is composed of two types of particles, positively charged protons and uncharged neutrons.
Electrons in the atom move around the nucleus in specific shells. The nucleus has a diameter of about 10-14 meters whereas the diameter of the space occupied by electrons is 10,000 greater or 10-10 meters.
Negatively charged electrons in atoms are placed in 7 shells around the nucleus with the inner most shell (K) being the lowest energy level and the outer most (Q) the highest.
In a neutral atom, there are the same number of electrons (negative charge) and protons (positive charge).
Example: Lithium Atom
A lithium atom has 3 protons and 3 neutrons inside the nucleus with 3 electrons orbiting around the nucleus as shown below.
As stated in the previous sub-section, the nucleus is composed of two types of particles, positively charged protons and uncharged neutrons. Both particles have a mass of approximately one Atomic Mass Unit (amu = 1.66 X 10-24 g) and make up almost the entire mass of the atom.
Electrons, on the other hand make up almost none of the mass of the atom as they weigh only 0.00055 amu.
|Particle||Weight in amu|
The sum of the number of neutrons and protons is called the mass number or A.
This number is displayed to the top left of the atomic symbol. The mass number for an element containing 3 protons and 3 neutrons such as Lithium is 6.
The number of protons in an atom is called the Z number or atomic number of the atom. This number is displayed to the bottom left of the atomic symbol.
Elements are atoms that contain specific numbers of protons and electrons. The unique chemical nature of any element is defined by the number of electrons and protons. The chemical symbol for an atom containing 3 protons (atomic number of 3) is Li for Lithium.
Layout for chemical nomenclature
Example of chemical nomenclature using lithium
Since the chemical symbol for each atomic number (Z) is unique, it is often omitted.
The unit of energy used when discussing atomic structure is the electron volt.
One electron volt is equal to the amount of energy gained by an electron traveling through an electrical potential difference of 1 V (Volt). One electron volt is equivalent to 1.60207 × 10-19 J(Joule). Electron volts are commonly expressed as million electron volts (MeV) and billion electron volts (GeV).
Radiation is kinetic energy (energy in transit) and may be in the form of electromagnetic radiation or high speed particles (particulate radiation).
The unit of energy used to describe radiation energy is the electron volt.
The electron volt may also be described as the amount of energy acquired by an electron when it moves through a potential of one volt.
Electron volt = 1.602 X 10-19 Joule
Electromagnetic radiation consists of oscillating electric and magnetic fields which are propagated in matter or free space. Electromagnetic radiation may be described as either a wave or a particle depending on the phenomena described.
When electromagnetic radiation is defined as a wave, it can be arranged in a spectrum according to frequency or wavelength as shown in electromagnetic spectrum, where wavelength is inversely proportional to frequency as defined by the formula:
l= wave length in meters
v = frequency in Hz
c= the speed of light (3 X 108 m/s)
Electromagnetic radiation may also be described as a particle or a discrete bundle of energy called a photon. The energy of any photon is proportional to frequency as described by the following formula:
E = hv
E= energy (electron Volts =1.602 X 10-19 Joule)
h = Plank's constant (6.6 X 10 -34 J second)
v = frequency in Hz (cycles/second)
Interaction with matter
The following sections will review electromagnetic radiation and how it interacts with matter. We can use this information to help us understand the effects of ionizing radiation on humans.
The interaction of radiation with matter brings about changes in its physical, chemical and biological behaviour. The transfer of energy from x-ray photons to atoms or molecules leads to ionization or excitation of these atoms or molecules.
Radiation with sufficient energy (about 33.9 eV in dry air) interacting with an orbital electron may impart enough energy to eject one or more orbital electrons from an atom, producing a cationic species and a free electron commonly referred to as an ion pair (see figure below ionization). Ion pairs themselves can interact with surrounding matter producing more ion pairs or secondary ionization.
If the energy transferred is not sufficient to cause ionization, excitation occurs. Excitation is brought about by the transfer of energy to an orbital electron and raising the electron to a higher orbital or energy level (see figure below excitation). Electrons in an excited state may form or break molecular bonds or simply revert to their original energy level by the emission of electromagnetic radiation.
Mechanism of interaction
X-ray photons interact in a probabilistic manner. This means that an individual photon has no definite maximum range. However, the total fraction of photons passing through an absorber decreases exponentially with the thickness of the absorber. There are two major mechanisms by which x-rays interact with matter: photoelectric effect and compton scattering.
The photoelectric effect occurs when the electromagnetic radiation or photon imparts all its energy to an orbital electron. The photon simply vanishes, and if the energy of the photon is greater than the binding energy of the electron, it will be ejected from the atom causing it to produce an ion pair. The ejected electron (often called a photo electron) will then produce secondary ionization events with its surrounding atoms in a similar manner to beta particles. The photoelectric effect has the highest probability with lower energy photons and atoms having a high atomic numbers.
When only a portion of the electromagnetic radiation (photon) is transferred to an electron, an ion pair is produced while a less energetic photon continues in an alternate direction. This photon then continues to interact with other electrons until its energy is depleted and the electron produces secondary ionization events. Compton scattering causes a change in direction of the photons and may appear to bend around corners making shields less effective.
The University of Waterloo has many varied types of x-ray equipment used for various teaching or research programs.
Regardless of the type of equipment, production of x-rays is based on the same principles.
X-ray production is the result of interactions between high speed electrons and target atoms. Different types of x-rays are produced depending on how the electrons interact with the target atoms.
When an electron is propelled towards the nucleus of an atom, its velocity increases then decreases as it moves away from the nucleus due to electrostatic attraction of positively charged protons in the nucleus. The electron accelerates then suddenly de-accelerates, causing electromagnetic radiation equal to that of the change in kinetic energy of the electron. This type of emission is referred to as Bremsstrahlung radiation (Bremsstrahlung meaning braking).
The energy of the E= mc2 has a continuous spectrum up to a maximum equal to the maximum kinetic energy of the electron. The production of Bremsstrahlung radiation increases with the atomic number of the target atom; thus, it increases electrostatic attraction.
The second method of x-ray production is by orbital electron transitions. Characteristic x-rays are emitted at specific energies dependent on the type of target material.
High energy electrons interact with an electron in an inner orbital shell and if the energy is high enough may eject that electron, producing a vacancy in the inner orbital shell.
The vacancy is filled by an outer orbital electron, the excess energy is emitted as an x-ray with discrete energy equal to the difference between the energy states of the inner and outer electron shell.
Typical characteristic x-ray energies:
Copper target - 8.05 keV
Molybdenum target - 17.46 keV
Silver target - 22.15 keV
Chromium target - 5.39 keV
X-ray tube output
The combined output of Bremsstrahlung and Characteristic x-rays results in an x-ray spectrum shown to the right.
Electron source (cathode)
A current is applied to the tungsten filament heating it to 2000oC , the greater the current the higher the number of electrons produced. One milliamp (mA) is equal to 6.25 electrons/sec.
A voltage potential is applied between the cathode and the anode. The acceleration (kinetic energy) of the electrons generated by the heated filament is proportional to the applied potential.
Electrons from the cathode are accelerated towards the anode, the electrons are focused on a very small target attached to a heat sink. About 1% of the kinetic energy of the electrons is converted to x-rays, the other 99% is converted to heat which is removed by the heat sink.
The target is placed at an angle to the electrons and only those x-rays emitted at 90o to the tube access are used. The result is a very narrow beam of x-rays in a cone of about 30 degrees and include both characteristic and bremsstrahlung x-rays. Collimators are used to reduce the beam size to about 1 mm diameter and monochromators are used to filter out undesirable wavelengths.
An evacuated glass enclosure allow the electrons to be accelerate to the target without colliding with gas molecules, resulting reduction of kinetic energy or being deflected away from the target.
Demonstration of x-ray tube
Since the effects of ionizing radiation are directly proportional to the amount received, a system had to be set up to measure the quantity of ionizing radiation. The following section will cover units of measure for ionizing radiation, conversion from SI units and the dosimetry requirements for persons using x-rays at UW.
The term "radiation dose" is a somewhat general term applied to the amount of ionizing radiation and its effects on any given material. To avoid ambiguity, it is necessary to distinguish between exposure, absorbed dose and dose equivalent.
The roentgen (R), a unit adopted in 1928 as a unit of exposure, is defined as the ability of photons to produce ionization in air. More specifically, it is the amount of photon energy required to produce 1.610 x 1012 ion pairs in one cubic centimetre of dry air at 0 degrees Celsius. The roentgen applies only to photons (gamma and x-rays) less than 3.0 MeV and their ionization of air. The roentgen was however preferable to the previous unit the "erythema dose", which was measured by the quantity of gamma or x-radiation required to produce visible reddening on the skin of the hand or arm.
Absorbed dose (gray)
To overcome the shortfalls of the roentgen, a unit of absorbed dose was adopted in 1953 which was defined as the amount of energy absorbed in a given mass and is called a "gray". 1 gray (Gy) = 1 joule per kg of mass and it is necessary to define the absorbing material when using gray as a unit for absorbed dose. One gray will produce various levels of tissue damage depending on the type of radiation.
Equivalent dose (sievert)
To develop a unit of dose that reflects biological effects of radiation, it was necessary to modify absorbed dose by using a "Radiation Weighting Factor" The equivalent dose or sievert (Sv) is the amount of energy absorbed in a given mass multiplied by a "Radiation Weighting Factor" (WR) which is specific for each type of radiation.
1 Sv = 1 Gy x WR where, "x-rays have a WR of 1"
"Radiation weighting factor" for various type of ionizing radiation reflect how the absorbed energy produces tissue damage are listed below:
|Item||Type of radiation and energy range||Weighting factor (WR)|
|1||Photons, all energies (x-rays)||1|
|2||1Electrons and muons, all energies||1|
|3||2 Neutrons of energy < 10 keV||5|
|4||2 Neutrons of energy 10 keV2 to 100 keV||10|
|5||2Neutrons of energy > 100 keV2 to 2 MeV||20|
|6||2Neutrons of energy > 2 MeV2 to 20 MeV||10|
|7||2 Neutrons of energy > 20 MeV||5|
|8||Protons, other than recoil protons, of energy > 2 MeV||5|
|9||Alpha particles, fission fragments and heavy nuclei||20|
1 Excluding Auger electrons emitted from nuclei bound to DNA.
2 Radiation weighting factors for these neutrons may also be obtained by referring to the continuous curve shown in Figure 1 on page 7 of the 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, published in 1991.
Effective dose is a reflection of the difference in radio-sensitivity of various organs. A weighting factor is applied to the equivalent dose for organs or tissues that are more sensitive to radiation.
Conversion to SI units
Canada and most of the world use the SI units of measurement, all regulatory documents and reports are required to have values in SI units. However, one of our closest trading partners, the USA, does not. To assist in conversion to SI units the following table has been supplied.
|1 gray (Gy)||100 rad (rad)|
|1 milligray (mGy)||100 millirad (mrad)|
|1 microgray (uGy)||100 microrad (urad)|
|1 nanogray (nGy)||100 nanorad (nrad)|
|1 kilorad (krad)||10 gray (Gy)|
|1 rad (rad)||10 milligray (mGy)|
|1 millirad (mrad)||10 microray (uGy)|
|1 microrad (urad)||10 nanogray (nGy)|
|1 kilorem (krem)||10 siervert (Sv)|
|1 rem (rem0||10 millisievert (mSv)|
|1 millirem (mrem)||10 microsievert (uSv)|
|1 microrem (urem)||10 nanosievert (nSv)|
|1 sievert (Sv)||100 rem (rem)|
|1 millisievert (mSv)||100 millirem (mrem)|
|1 microsievert (uSv)||100 microrem (urem)|
|1 nanosievert (nsv)||100 nanorem (nrem)|
|1 kilocurie (kci)||37 tetrabecquerel (TBq)|
|1 curie (Ci)||37 gigbecquerel (Gbq)|
|1 millicurie (mCi)||37 megabecquerel (MBq)|
|1 microcurie (uCi)||37 kilobecquerel (kBq)|
|1 nanocurie (nCi)||37 becquerel (Bq)|
|1 tetrabecquerel (TBq)||27 curie (Ci)|
|1 gigabecquerel (CBq)||27 millicurie (mCi)|
|1 megabecquerel (MBq)||27 microcurie (uCi)|
|1 kilobecquerel (kBq)||27 nanocurie (nCi)|
|1 becquerel (Bq)||27 picocuries (pCi)|
The table below lists the maximum annual dose equivalent for designated x-ray workers and other workers/students.
|Part of body irradiated||Exposure conditions and comments||Dose equivalent annual limit (millisieverts)|
|X-ray workers||Other workers|
|Whole body or trunk of body||Uniform irradiation||50||5|
|Partial or non-uniform irradiation of body||The limit applies to the Effective Dose Equivalent defined in Note (A)||50||5|
|Lens of eye||Irradiated either alone or with other organs or tissues||150||50|
|Skin||The limit applies to the mean dose equivalent to the basal layer of the epidermis for any area of skin of 1 square centimetre or more||500||50|
|Individual organs or tissues other than lens of eye or skin||The limit on effective dose equivalent applies, with an overriding limit on the dose equivalent to the individual organ or tissue`||500||50|
|Abdomen of pregnant worker||5|
The effective dose equivalent, HE, is determined by the following formula:
HE = SigmaTWTHT
- T is an index for tissue type
- HT is the annual dose equivalent in tissue T
- WT is a weighting factor which has the following values:
- 0.25 for the gonads
- 0.15 for the breast
- 0.12 for the red bone marrow
- 0.12 for the lungs
- 0.03 for the bone surfaces
- 0.03 for the thyroid
- 0.06 for each of the five other organs or tissues receiving the highest dose equivalents, but excluding the skin, extremities and eye lenses.
The exposure of all other remaining tissues can be neglected. When the gastro-intestinal tract is irradiated, the stomach, small intestine, upper large intestine and lower large intestine shall be considered as four separate organs; and:
SigmaTWTHT is the sum of the WTHTvalues for all irradiated tissues which receive more than 1 millisievert in a given year.
The annual limits do not include any dose equivalent received by a worker from background sources or received as a patient undergoing medical diagnostic or therapeutic procedures.
The annual limits include any dose equivalent received by a worker, as a consequence of his or her occupation, from all sources of ionizing radiation.
This next section will discuss the relationship between exposure (dose) and response as well as describe various effects of ionizing radiation.
For example, a large quantity of X-rays has the potential of producing an undesired effect on a person. However, used properly with appropriate shielding the x-rays will have no biological effect.
Paracelsus (16 th Century): "It's the dose that makes the poison".
Types of exposures
Chronic exposure is continuous or intermittent exposure to low levels of radiation over a long period of time.
Acute exposure is exposure to a large, single dose of radiation, or a series of doses, for a short period of time. Large acute doses can result from accidental or emergency exposures or from special medical procedures.
Response to x-rays
Response on a molecular level
To describe how atoms and molecules react to ionizing radiation, it is convenient to separate responses according to their time frames.
Physical stage (10 -18 to 10 -13 sec)
The interactions of radiation with matter described earlier generate ionized and excited atoms and molecules in the irradiated material. These are generally very short-lived and within 10-13 seconds either dissociate to free radicals, react with neighbouring ions or molecules, or lose their excitation energy in the form of heat. The exact nature of the processes taking place depends on the type of material. For example, in graphite and metals almost all the absorbed radiation energy appears as heat, while with water and organic materials most of the absorbed energy is used in breaking chemical bonds to give free radicals and new chemical products.
Chemical stage (10 -13 to 1 sec)
The free radicals produced are also short-lived and within a few seconds, or less, react amongst themselves, or with the substrate to produce chemical changes that lead to the biological effects observed much later.
Biochemical stage (1 sec to 11 days)
Chemical changes lasting less than a second can bring about biological effects many years later, such as possible impairment of biochemical functions resulting from damage to membranes and enzymes.
Biological stage (11 days to 32 years)
Damage to various cellular constituents may result in loss of viability, sterility, cancer and genetic damage.
Not all cells in the human body respond the same to radiation. In 1906, the law of Bergonie and Tribondeau stated: The radio-sensitivity of a tissue is directly proportional to its reproductive capacity and inversely proportional to its degree of differentiation. In short, this means that actively dividing cells or those not fully mature are at most risk from radiation.
The most radio-sensitive cells are those which have one or more of the following characteristics:
- High division rate
- High metabolic rate
- Are non-specialized type
- Are well nourished
Examples of various tissues and their radio-sensitivity:
|Radio-sensitive cells||Radio-resistant cells|
|Reproductive cells||Bone, cartilage, muscle|
|Blood forming tissues||Liver|
|Epithelium of skin||Kidney|
|Epithelium of gastrointestinal tract||Nerve tissue|
Dose response relationship
Linear vs non-linear dose response
For most biological effects the relationship between dose and effect is not a linear one. For example, if arsenic were administered to a group of subjects, at low doses there would be no response, but as the dose is increased the effect would be proportional to the dose. This is called a non-linear dose response as shown in the Dose Response figure below.
Radiation protection adopts a linear non-threshold model to express response to radiation as shown in the Dose Response figure below. Increasing effects were observed at high doses of radiation, but effects at low doses were statistically insignificant. It was felt that there must be some risks from radiation at low doses so the effects from high doses were extrapolated back to zero dose and the linear non-threshold dose response was developed for radiation protection.
Types of effects
In earlier sections we have learned how ionizing radiation interacts with matter. However, in radiation protection what really matters is the effect of ionizing radiation on the human body.
Biological effects of radiation may occur in the unexposed as well as the exposed population. These effects are termed as "Stochastic effects" and are not unequivocally linked to the exposure. The response is proportional to the dose, but exposure does not guarantee an effect. For example, lung cancer is a disease that both smokers and non-smokers get, but the more cigarettes a person smokes the higher the likelihood of lung cancer. However even heavy smokers are not guaranteed to get lung cancer.
Biological effects may be group into three major classes:
The affected cells are somatic or non-reproductive cells such as skin, liver or lung cells.
Acute effects result from a large, single dose of radiation, or a series of doses, for a short period of time. Large acute doses can result from accidental or emergency exposures or from special medical procedures. Listed below are some of the major acute biological effects of ionizing radiation.
Blood consists of three major cell types:
- Red blood cells to transport oxygen and carbon dioxide
- White blood cells for fighting infections
- Platelets to assist in blood clotting
Reduction in the levels of the various cell types may happen at exposure levels as low as 140 milligray (mGy) and will almost certainly appear above 500 mGy.
The various types of blood cells are produced in bone marrow which replaces damaged cells over a period of time. If the bone marrow receives a dose greater than 2 gray (Gy), the rate of production is lowered. Production stops if the marrow is subjected to a dose of 5-6 Gy.
Cells embedded in the stomach produce glandular secretions which aid in digestion and are sensitive to doses of a few sievert (Sv). These cells will reduce or cease to function, but they will resume after a recovery period. The most radiation sensitive organ in the gastrointestinal tract is the small intestine. At the present, this is the organ which will determine if a person survives an acute massive whole body attack of radiation. In a healthy individual, the cells lining the small intestine are constantly worn away and are replaced by crypt cells. With an acute dose of more than 10 Sv, the crypt cells are killed, and the lining of the small intestine ruptures and death follows.
An acute dose of 2.5 Sv to the gonads will produce temporary sterility in males and doses in excess of 5 Sv will cause permanent sterility. Females experience a 1 to 3 year sterile period with an acute dose of 1.70 Sv, while permanent sterilization may occur with a dose in excess of 3 Sv, depending upon age at time of exposure.
After an acute over-exposure to radiation, an erythema may occur. This is a reddening of the skin, which may be accompanied by changing in pigmentation, blistering, and ulceration.
Central nervous system
50 Sv to the central nervous system will damage it, resulting in unconsciousness within minutes of exposure and death occurring shortly after.
In the previous section we found that the most radio-sensitive cells were those that fit the following criteria:
- High division rate
- High metabolic rate
- Non-specialized type
- Well nourished
The cells of the foetus meet all four of these criteria and special care must be taken by the mother and co-workers to reduce exposure of the foetus to ionizing radiation.
Radiation effects on the developing foetus vary with time. For example, there is a much larger effect during the time of organ development (fourth to eighth weeks).
The process of development of an embryo is highly complex and in many cases unsuccessful. Congenital abnormalities are responsible for the loss of 40% of human embryos in the first 20 weeks. About 6% of live born children have abnormalities.
Evidence for prenatal or neonatal death caused by radiation in humans is sparse, and estimates of the relevant LD50 have been extrapolated from rodent data.
Severe mental retardation
Data from the atomic bomb survivors in a study of 1600 children exposed in utero has shown about 30 cases of severe mental retardation (normal rate 0.8%). The most sensitive gestational period was 8-15 weeks, and in this period the fraction of those retarded increased by 0.4 per Sv. For the period 16-25 weeks the rate was 0.1 per Sv. There appeared to be a threshold of around 0.2 Sv.
Mental impairment of lower severity is also apparent in children exposed in utero. There is a dose related decrease in IQ of about 30 units per Sv again in the 8-15 week period.
Irradiated foetuses seem to be more sensitive to the induction of cancer before the age of 10. There is disagreement between the Japanese data and that derived from prenatal irradiation for medical purposes. Current best estimates are 2.5 X 10-2 per Sv for leukemia and 3.5 X 10-2 per Sv for other cancers.
Genetic effects of radiation show up in the offspring of the exposed person. Mutations occur due to an alteration in the DNA passed to the offspring. These genes may be altered in three ways:
- The omission of needed DNA
- The addition of extra DNA
- The rearrangement of genes within a DNA strand
It has been shown that larger radiation doses produce an increase in offspring mutation. However, at lower doses the risk of mutation is more difficult to predict. It is not clear whether the risk of mutation at lower doses is linear (proportional to dose) or threshold (no effect at lower dose).
Radiation exposure may come from either natural or artificial (person-made) sources.
It is important to be aware of the effects of radiation exposure, but it is also important to have some perspective as to the expected dose received from the use of radioisotopes compared to that received from the environment.
Ionizing radiation is simply a form of energy derived from nuclear interaction and is always around us.
Natural radiation comes from soil and rocks, the food we eat, the houses we live in, cosmic rays and even our own bodies. Potassium-40, a naturally occurring radioisotope in fruits and vegetables is the most significant contributor to our dose from internal radiation emitters.
Your physical location on earth affects your annual radiation dose. Radiation exposure increases with altitudes (electromagnetic radiation from cosmic rays increases with altitude) or you may live in an area with the bedrock close to the surface (uranium and thorium concentrations would be higher).
Approximated average dose from natural sources include:
|Natural source of radiation||Average dose per year|
|Cosmic rays (NCRP 93)||0.27 mSv|
|Soil (NCRP 93)||0.28 mSv|
|Air (Radon) (NCRP 93)||2.00 mSv|
|Internal emitters (NCRP 93)||0.39 mSv|
|10,000 km jet flight||0.05 mSv|
Person-made sources of radiation include television, smoking, and x-rays for medical reasons. Some common examples and corresponding approximate average doses to the public are given below.
|Source of radiation||Average dose per year|
|Diagnostic X-ray (NCRP 93)||0.39 mSv|
|Nuclear medicine (NCRP 93)||0.14 mSV|
|Consumer products (NCRP 56)||0.01 mSV|
|Nuclear fallout (NCRP 93)||0.01 mSv|
|Dose to the public nuclear reactors (USA)||< 0.01 mSv|
Comparison of typical ionizing radiation exposures
|Ionizing radiation exposures from the world or around us|
|Source of exposure||Exposure|
|Natural background||0.06 uS/hr|
|Coal burning power plant||1.65 uS/yr|
|X-rays from TV set (1 inch)||5 uS/hr|
|Airplane ride (39,000 ft)||5 uS/hr|
|Nuclear power plant (normal operation at plant boundary)||6 uS/yr|
|Weapons fallout||~0.01 mSv/yr|
|Building materials (concrete)||0.03 mSv/yr|
|Drinking water||0.05 mSv/yr|
|Coast to coast airplane round trip||0.05 mSv|
|Pocket watch (radium dial)||0.06 mSv/yr|
|Eyeglasses (containing thorium)||0.06 - 0.11 mSy/yr|
|Natural gas in home||0.09 mSy/yr|
|Terrestrial background (Atlantic coast)||0.16 mSy/yr|
|Cosmic radiation (sea level)||0.26 mSy/yr|
|Radionuclides in the body (ie. potassium)||0.39 mSy/yr|
|Cosmic radiation (Denver)||0.50 mSv/yr|
|Terrestrial background (Rocky Mountains)||0.63 mSv/yr|
|Three Mile Island (dose at plant duration of the accident)||0.80 mSv|
|Ionizing radiation exposures from medical procedures|
|Source of exposure||Exposure|
|Chest x-ray||0.05 - 0.20 mSv/exp|
|Dental x-ray||0.10 mSv/exp|
|Head/Neck x-ray||0.020 mSv/exp|
|Lumbar/spinal x-rays||1.30 mSv/exp|
|Pelvis/hip x-ray||1.70 mSv/exp|
|Upper GI series||2.45 mSv/exp|
|Cumulative natural background||3 mSv/exp|
|Lower GI series||4 mSv/exp|
|Diagnostic thyroid exam||900 mSv/exp|
|Therapeutic thyroid exam||10,000 mSv/exp|
|Maximum ionizing radiation exposures from work|
|Source of exposure||Exposure|
|Occupational exposure limits for foetus||4 mSv|
|Occupational limits||50 mSv/yr|
|Occupational limits (skin)||500 mSv/yr|
|Occupational limits (extremities)||500 mSv/yr|
|Occupational limits (lens of eye)||150 mSv/yr|
To insure that x-ray radiation dose rates are not exceeded, various ionizing radiation detectors have been developed and sold commercially.
This section deals with the most common types of radiation detectors used at the University of Waterloo.
Thermoluminescent dosimetry (TLD)
Thermoluminescent materials such as lithium fluoride (LiF) provide a simple inexpensive method of measuring radiation dose over an extended period of time. Normally when energy is applied to an electron, it will move up to a higher energy state (orbital), then drop back to its ground state releasing the excess energy.
Crystals of LiF have met a stable energy state in which excited electrons may be trapped for periods up to 80 years. Heat applied to the crystal will raise the electron out of the meta-stable trap, allowing it to revert back to it ground state by emitting the excess energy as photons.
The amount of light emitted is proportional to the amount of radiation absorbed and can be quantified using a photo-multiplier tube.
Thermoluminescent dosimeter (TLD)
Construction of a TLD consists of two lithium fluoride (LiF) crystals of different thickness mounted on a metal plaque. These crystals are shielded by either a metal foil or an aluminum planchet. The difference in the shielding and thickness of the crystals allows the differentiation between whole body dose and skin dose.
- TLD badges should be worn at the chest position (if worn at waist bench tops will shield the badge from radiation giving low results) .
- The TLD badge should be stored away from light, radiation and dust when it is not used.
- TLDs are changed every three months except for badges worn by pregnant women. These badges change every two weeks.
- Lithium fluoride (LiF) is considered to be approximately tissue equivalent, with little energy dependence for photon energy in excess of 100 keV.
- LiF over responds by 40% relative to the 20- to 70 keV range.
- Lithium fluoride TLDs will detect a minimum of 0.2 mSv dose for the wearing period (usually 3 months).
Ring badges consist of a single lithium fluoride crystal inside a plastic holder. This type of badge is used to measure extremity dose and is required when working closely with the x-ray beam.
- Ring badges should be worn on the dominant hand.
- Ring badges should be stored away from light, radiation and dust when it is used.
- Ring badges are changed every three months.
Gas filled detectors
The most common type of radiation detector is the gas filled detector (see figure below). These detectors consist of a gas filled tube, a positive electrode (cathode) and negative electrode (anode). Ionizing radiation enters the tube, atoms of gas are ionized, producing positive and negative ions. If a potential is applied across the tube, the positive ions will migrate towards the anode and the negative ions towards the cathode (Coulomb attraction). The migration of ions to the electrodes result in a current flow which can be measured; thus, reflect the amount of ionizing radiation striking the tube.
When the potential applied across the tube is very low, ion pairs produced by ionizing radiation will not move fast enough towards their respective electrodes, the ion pairs will then reform into neutral gas molecules and produce little or no current flow. Increasing the voltage potential to a level which electrostatic attraction accelerates 100% of the ions to their respective electrodes (Ionization chamber plateau) producing a current flow proportional to the radiation striking the tube (see figure below on signal vs. voltage). The number of ion pairs produced is dependent on the amount of ionizing radiation entering on the tube. This type of detector can be used to measure dose rate.
Ionizing chamber detectors produce a very small current requiring high signal amplification.
As the voltage potential across the gas filled chamber is increased past the ionization chamber plateau, it enters into the proportional counter region. The voltage potential is now high enough to accelerate the ion pairs to a speed that cause secondary ionization (see figure Signal vs. Voltage). Secondary ionizations themselves produce more ionizations; this avalanche process multiplies the signal as high as 106 times for each original ionizing event. Proportional counters must wait for the avalanche process to finish and counted before counting the next event which is described as dead time (about ½ microsecond).
The output of proportional counters is proportional to the primary ionizing energy, the higher the energy of the ionizing radiation the higher the output of the detector.
Proportional counters can be used to discriminate between various types and energies of ionizing radiation.
Geiger-Mueller counters are the most common detectors at UW and operate at the last voltage plateau of the gas filled detectors (see figure to the right on signal vs voltage).
The increased voltage potential not only causes an avalanche of secondary ionizations as in proportional counters but also UV photons are produced when electrons strike the collecting anode. These UV photons produce another avalanche of ionizations, which would be self-perpetuating except that a quenching gas is added to ensure the process ceases.
Geiger-Mueller Counters produce a very large signal for each ionizing event that is the same for all energies of ionizing radiation. A detector of this type is extremely sensitive and is normally used for leakage surveys (They must not be used to measure dose rate as they give the same response no matter what the energy level of the ionizing radiation).
When an ionizing particle interacts with an atomic electron, it may fail to completely detach the electron from its atom (ionization), but merely transfer sufficient energy to raise the electron to a higher energy state (excitation). When the electron subsequently falls to a lower level, the excess energy is emitted as electromagnetic radiation, often in the visible energy range. If the light is not absorbed by the scintillation medium it may be observed as a glow such as produced by a luminous radium watch dial.
Pure sodium iodide mixed with a small amount of thallium becomes one of the most commonly used scintillation detectors. The addition of thallium atoms into the structure of a sodium iodide crystal produce energy levels to which electrons are more easily excited to, and on de-excitation, produce visible light of a wavelength not readily absorbed by the sodium iodide. The NaI crystal is sealed in a cylindrical aluminum container with a face of beryllium or Mylar to prevent it from absorbing atmospheric moisture and to permit low energy radiation entry.
A photo-multiplier tube collects the photons emitted by the NaI crystal, converts them to an electric signal which can be counted.
The sequence of events:
- The ionizing photon enters the crystal and imparts some or all of its energy to an atomic electron.
- The excited electron dissipates its energy by emitting light.
- The light strikes the photo-multiplier tube. The light is amplified and converted to a signal proportional to the energy deposited in the NaI(TI) crystal.
NaI(TI) detectors were widely used for x-ray energy measurements, and because of their higher detection efficiency and lower cost, they are still common for such applications where energy resolution is not paramount.
Calibration of survey meters
Gamma survey meters (dose rate meter)
These meters are to be sent away annually for calibration. A calibrated gamma dose rate meter is available at the Safety Office.
Radiation dose is directly proportional to exposure time. Therefore, one of the simplest methods of reducing exposure is the reduction of the time spent exposed to the radiation.
For example, if you were to work in a radiation field of 25 mSv/hr for one hour your dose received will be 25 mSv. If you were to spend 6 hours in a 25 mSv/hr field your dose received would total 25 X 6 = 150 mSv.
Working distance for x-ray radiation
The intensity of x-ray radiation emitted from a point source is inversely proportional to the distance (follows the inverse square law).
Calculation of working distance for x-ray radiation
To calculate a dose at a given working distance uses the following formula derived from the inverse square law:
Dose vs. distance
Ia = Intensity of radiation at a distance A
Ib = Intensity of radiation at a distance B
Ib = 100 mSv/hr
A = 3 meters
B = 1 meter
Ia = 11. mSv/hr
Shielding is the most effective method of reducing exposures to x-rays in a non-medical institution.Time and distance are usually dictated by the experiment.
X-ray radiation is diminished in intensity by any given absorber but not completely stopped. Materials having a high atomic number can absorb more x-rays than lighter elements. A frequently used shielding material is lead. It is important to remember that X-rays can be scattered in the shielding material and emerge at odd angles.
The most convenient way to calculate the amount of shielding necessary is to use the concept of Half Value Layer (HVL), which is the amount of material that is able to reduce the incident radiation by one half.
As described in the program section under legislation, operation of non medical x-ray emitting devices is under the Ontario Ministry of Labour and the construction is controlled by Health Canada.
- The X-ray Safety Officer shall be notified as to the location of all x-ray emitting equipment.
- Prior to start up the X-ray Safety Officer will submit an application to the Ontario Ministry of Labour for approval by the Director (Director of the Health and Safety Support Services Branch of the Ministry of Labour).
- If the x-ray emitting equipment is moved or modified, the x-ray Safety Officer shall be notified and a new application submitted for approval prior to start up.
- Upon approval by the Ministry of Labour the X-ray Safety Officer will issue a permit for the x-ray equipment.
The permit will contain the following information:
- Name and contact information for permit holder
- Location of x-ray emitting device
- Type of x-ray emitting device
- Make model and serial number of x-ray emitting device
- List of trained workers
- The Permit will be posted in the room containing the x-ray emitting device.
- Typical x-ray permit (PDF)
X-ray diffraction cabinets
- X-ray warning sign next to the x-rays “ON” switch.
- A label with:
- Name of the manufacturer
- Serial number
- Date of manufacture
- City and country of manufacture
- X-ray warning sign on all doors.
- A power “ON/OFF” switch.
- A warning light that indicates when the power is “ON”.
- A lock of a type that requires the insertion of a key before x-rays can be produced and the removal of the key to terminate production of x-rays.
- A warning light that indicates when x-rays are being generated. If x-rays are pulsed the light must remain on for at least one-half second.
- A warning light on the control panel, interlocked in such a manner that x-rays cannot be produced if the warning light malfunctions.
- Shielded doors or panels over all access openings.
- Interlocking each of the doors with at least two independent interlocks to prevent the generation of x-rays if any door or panel is open.
- A beam limiting device that:
- Ensures that the primary x-ray beam is aligned with the x-ray detector
- Restricts the size of the primary x-ray beam, at the plane of the x-ray detector, so that it does not exceed the maximum size of the detector
- Exposure rate from leakage radiation, averaged over a time period that is not less than five minutes, does not exceed 0.5 milliroentgen per hour at a distance of five centimetres from the cabinet average over an area not less than 10 square centimetres.
X-ray room requirements
Open beam/diagnostic x-ray sources
Open beam and diagnostic x-ray equipment is to be used in a room solely designated for that purpose.
X-ray diffraction cabinets
These cabinets may be located in the general laboratory environment.
Each entrance to an area where a x-ray emitting device is located will be marked with an x-ray warning sign.
Permit (PDF) for the x-ray emitting device is to be displayed.
Health Canada has published various safety codes as a guide for construction and use of radiation emitting equipment.
Safety Code 35. Safety Procedures for Installation, Use and Control of the X-ray Equipment in Large Medical Radiological Facilities, 2008, 43 p.
Included in this code are sections for the specific guidance of the radiologist, the physician, the operator and the medical or health physicist concerned with safety procedures, equipment performances and protection surveys.
Safety Code 26. Guidelines on Exposure to Electromagnetic Fields from Magnetic Resonance Clinical Systems, 1987, 20 p.
Reviews biological effects of various fields used in magnetic resonance devices and provides general guidance on exposure levels to the patient and to the operator.
Safety Code 28. Radiation Protection in Veterinary Medicine, 1991, 38 p.
The safety procedures, equipment and installation guidelines detailed in this code are primarily for the instruction and guidance of persons employed in Federal Public Service departments and agencies, as well as those under the jurisdiction of the Canada Labour Code. Facilities under provincial jurisdiction may be subject to requirements specified under provincial statutes.
Safety Code 32. Safety Requirements and Guidance or Analytical X-ray Equipment, 1994, 33p.
Provides requirements and guidance to ensure radiation risks from analytical x-ray equipment remain low. Specific responsibilities for the equipment owner, user and maintenance personnel are outlined. Information on safety procedures, standards, surveillance and monitoring is also provided.
Safety Code 34. Radiation Protection and Safety for Industrial X-ray Equipment, 2003, 55 p.
Provides regulatory requirements and guidance in accordance with IAEA and ICRP radiation protection objectives to ensure the safe use of industrial x-ray equipment for radiography purposes. Specific responsibilities and roles are outlined for the equipment manufacturer, owner, radiation safety officer, certified operators and clients seeking industrial radiography services. Guidance information on radiation safety training, personnel monitoring, survey meters and emergency procedures is provided.
If a worker/student has received a dose equivalent in excess of the annual limits in a period of three months, the x-ray Safety Officer shall investigate the cause of the exposure.
The x-ray Safety Officer shall provide a report in writing of the findings of the investigation and of the corrective action taken to:
- The Director of the Health and Safety Support Services Branch of the Ministry of Labour
- The Joint Health and Safety Committee
- Permit holder
- The worker/student
Any accident, failure of any equipment or other incident occurs that may have resulted in a worker/student receiving a dose equivalent in excess of the annual limits, the x-ray Safety Officer shall notify the Director of the Health and Safety Support Services Branch of the Ministry of Labour and the Joint Health and Safety Committee.
A written report of the accident or failure shall be sent to the Director of the Health and Safety Support Services Branch of the Ministry of Labour within forty-eight hours.
- This quiz is limited to persons working x-ray emitting device and members of the UW community.
- The current quiz no longer maintains records. For this reason, the Safety Office is developing a Learn based X-ray Safety Course which will be available by the end of March 2020. When complete, this course will meet the theoretical portion of our training program.
- If you cannot wait for the Learn course, you can access the current training program here. Note, if you take the quiz in this format, you will need to take the Learn course as well to ensure your training record is updated.