• Identify and discuss key elements of how units of radiation are measured
• Define common terms used in describing aspects of radiology including: curie, roentgen, rad, and rem
• Identify and explain the key aspects of radiation protection standards including: time, distance, and shielding
• Explain the physical and chemical effects of ionizing radiation
• Describe the fundamentals of dosimetry

The following is a recommended sample Radiation Safety Guide. While not all of its elements will apply to your situation, this Radiation Safety Guide gives you a very clear idea of what elements of radiation safety need to be dealt with by both organizations and staff.

In general, a radiation guide starts with some general "ground rules" for the use of equipment by authorized personnel. For example:

All authorized machine use personnel must:

1. Post areas where radiation-producing machines are used and stored. Rooms housing x-ray equipment must have warning signs on entrances specifically for x-ray.

2. Maintain security/control of ionizing radiation-producing equipment. Equipment itself or rooms housing x-ray equipment must be locked when not in use.

3. Keep a log of dates, use parameters, and users' names, as well as any performance checks done on equipment. The organization's Office of Environmental Health and Safety will also monitor work areas where x-ray machines are used to detect leakage or scatter radiation.

4. If you modify, transfer, dispose of, or purchase x-ray equipment, your State Department of Health Services must be advised by the Office of Environmental Health and Safety of these facts. Notify Environmental Health and Safety of these changes.

5. Wear dosimetry (film badges and/or finger rings) to document radiation exposures while working with x-ray equipment.

Machine Use Authorizations (MUA)

Requests to use radiation-producing equipment are separated into the following categories:

1. Analytical Use

2. Diagnostic Human and Non-Human Use

Separate machine use applications are required for each machine, and there may be requirements for additional shielding or state certification.

Units of Radiation Measurement

1. Activity (Unit: Curie)

Since the discovery by William Roentgen in 1895 that energetic electrons impinging on a target of high atomic number produce rays that easily penetrate matter and can expose photographic film (X rays), the scientific community has adopted special units to describe the amount and nature of ionizing radiation.

The International Commission on Radiological Units (ICRU) was formed to develop a system of units and nomenclature specific to the needs of physicians and other persons working with not only X rays, but other types of radiation found in nature or produced by man. The units that have been developed were named after pioneers in the field (Roentgen, Curie) or began as descriptive terms that turned into acronyms, then into units (rem-"roentgen equivalent man"). The ICRU designated units on the basis of observed quantities. Thus the special unit of activity, the curie, was equal to the number of disintegrations taking place per unit time from 1 gram of radium. The curie (Ci) was later redefined as the activity of that quantity of radioactive material in which the number of disintegrations per second is 3.7E10 (a number nearly the same as the number of disintegrations per second from 1 gram of radium).

We have since learned that a Curie of any radioisotope is a very appreciable amount, too great for most laboratory applications, so we commonly find activity expressed as millicurie (mCi, 1E-3 Curie) or microcurie (µCIF, 1E-6 Curie). It is essential that one not confuse the symbol for micro with that for milli. The 1,000-fold error that results may mean the difference between an almost inconsequential radiation problem and a major radiation hazard. A useful number to remember is 2.22E6 disintegrations per minute per microcurie. Most tracer applications require microcurie quantities, although it is not unusual to find millicurie quantities of 3H, 32P, and 125I in many laboratories.

2. Exposure ( Unit: Roentgen)

The ICRU defined the special unit of exposure in air to be the Roentgen (symbolized by R). R = 2.58E-4 coulomb kg air This unit is special in that it is defined only for X or gamma radiation in air. Thus, the Roentgen is not applicable to alphas, betas, or neutrons. Many survey instruments provide output data in terms of mR/hr (mR, 1E-3R). The Roentgen is not always useful for making accurate evaluations of energy absorbed due to radiation impinging on material. It is the absorbed energy that is a true index of biological damage. If one knows how well a certain material can absorb radiation as compared with air, the energy absorbed by that material when exposed to 1 R can be calculated. It is very easy to measure ionization in air with inexpensive equipment, so that the Roentgen can be measured directly. It is not so easy to measure the energy absorbed in material directly.

3. Absorbed Dose (Unit: rad)

The rad is the special unit of absorbed energy. It is defined as that amount of ionizing radiation that deposits 100 ergs/gram of material. The rad is applicable to all types of ionizing radiation, yet it is difficult to measure directly. Normally ionization in air or another gas is measured and the absorbed dose in a particular material calculated. One Roentgen results in 87.7 ergs being absorbed in 1 gram of air; if muscle tissue is placed in the same radiation beam, 1 R in air corresponds to about 95 ergs/gram. For most applications of x rays and gamma rays, it is reasonable to assume that 1 R = 1 rad. One Roentgen is a large exposure, therefore, we more often see the term millirad (mrad, 1E-3 rad).

4. Dose Equivalent (Unit: rem)

The rem is the unit of dose equivalent. The dose equivalent accounts for the difference in biological effectiveness of different types of radiation. It is the product of the absorbed dose (rad) times the quality factor (QF) of the radiation. The quality factor for x, gamma, and beta radiation is 1, therefore for these radiations 1 rad = 1 rem. The quality factor for alpha radiation is 20 and the quality factor for neutron radiation varies with energy from 2-11.

1. Introduction

Radiation protection standards apply to radiation workers or the general population. Standards for the general population are of importance since they serve as a basis for many of the considerations applicable to the siting of nuclear facilities and the design and implementation of environmental surveillance programs. Included in this section are a brief history of the development of radiation protection standards, a review of the goals and objectives sought, and a description of the approach being used to base such standards on the associated risk.

2. History of the Basis for Dose Limits

Shortly after the discovery of x-rays of 1895 and of naturally occurring radioactive materials in 1896, reports of radiation injury began to appear in the published literature (i). Recognizing the need for protection, dose limits were informally recommended with the primary initial concern being to avoid direct physical symptoms. As early as1902, however, it was suggested that radiation exposures might result in delayed effects, such as the development of cancer. This was subsequently confirmed for external sources and, between 1925 and 1930, it became apparent for internally deposited radionuclides when bone cancers were reported among radium dial painters (1).

With the publication by H.J. Muller in 1955 (ii) of the results of his experiments with Drosophila, concern began to be expressed regarding the possibility of genetic effects of radiation exposures in humans. This concern grew and dominated the basis for radiation protection from the end of World War II until about 1960, and led to the first consideration of recommendations for dose limits to the public. With the observances of excess leukemia among the survivors of World War II atomic bombings in Japan, and the failure to observe the previously anticipated genetic effects, however, the radiation protection community gradually shifted to a position in which somatic effects, primarily leukemia, were judged to be the critical (or governing) effects of radiation exposures. This belief continued until about 1970 when it was concluded that, although somatic effects were the dominating effects, the most important such effects were solid tumors (such as cancer of the lung, breast, bone, and thyroid) rather than leukemia (iii). Finally, in 1977 the International Commission on Radiological Protection (ICRP) initiated action to base radiation protection standards on an acceptable level of the associated risk (iv). This effort was provided additional support by the National Council on Radiation Protection and Measurements (NCRP) with the issuance of their updated "Recommendations on Limits for Exposure to Ionizing Radiation" in 1987 (v).

3. Basic Standards - Philosophy and Objectives

The primary source of recommendations for radiation protection standards within the United States is the National Council on Radiation Protection and Measurements (NCRP). Recommendations of this group are in general agreement and many of them have been given legislative authority through publication of the Code of Federal Regulations by the U.S. Nuclear Regulatory Commission.

a. Basic Philosophy

As a general approach, the main purposes in the control of radiation exposures are to ensure that no exposure is unjustified in relation to its benefits or those of any available alternative; that any necessary exposures are kept as low as is reasonably achievable (ALARA); that the doses received do not exceed certain specified limits; and that allowance is made for future developments.

b. Objectives of the Guides

In general, the objective or goal of radiation protection (and associated standards) is to limit the probability of radiation-induced diseases in exposed persons (somatic effects) and in their progeny (genetic effects) to a degree that is reasonable and acceptable in relation to the benefits of the activities that involve such exposures.

Radiation-induced diseases of concern in radiation protection are classified into two general categories: stochastic effects and non-stochastic effects.

i. A stochastic effect is defined as one in which the probability of occurrence increases with increasing absorbed dose, but the severity in the affected individuals does not depend on the magnitude of the absorbed dose. A stochastic effect is an all-or-none response as far as individuals are concerned. Cancers (solid malignant tumors and leukemia) and genetic effects are examples of stochastic effects.

ii. A non-stochastic effect is defined as a somatic effect which increases in severity with increasing absorbed dose in the affected individuals, owing to damage to increasing numbers of cells and tissues. Examples of non-stochastic effects attributable to radiation exposure are lens opacification, blood changes, and decreases in sperm production in the male. Since there is a threshold dose for the production of non-stochastic effects, limits can be set so that these effects can be avoided.

4. Radiation Protection Standards

a. Occupational Dose Limits

Standards provide for an upper boundary effective dose equivalent limit of 50 mSv/year (5 rem/year). On a cumulative basis, however, the newest NCRP recommendations have proposed that the average cumulative effective occupational dose equivalent not exceed 10 mSv (1 rem) times the age of the worker.5 UC Davis guidelines limit exposure to roughly one-half the state and federal limits. Two key changes or factors to be noted relative to these recommendations are:

i. The dose limit applies to the sum of the doses received from both external and internal exposures.

ii. The standards are expressed in terms of the effective dose equivalent, an approach which permits, on a mathematical basis, the summation of partial and whole body exposures.

5. Dose Limits for the General Population

For a variety of reasons, dose limits for the general population are set lower than those for radiation workers. Justifications for this approach include the following:

a. The population includes children who might represent a group of increased risk and who may be exposed for their whole lifetime.

b. It was not the decision or choice of the public that they be exposed.

c. The population is exposed for their entire lifetime; workers are exposed only during their working lifetime and presumably only while on the job.

d. The population in question may receive no direct benefit from the exposure.

e. The population is already being exposed to risks in their own occupations; radiation workers are already being exposed to radiation in their jobs.

f. The population is not subject to the selection, supervision, and monitoring afforded radiation workers.

g. Even when individual exposures are sufficiently low so that the risk to the individual is acceptably small, the sum of these risks (as represented by the total burden arising from somatic and genetic doses) in any population under consideration may justify the effort required to achieve further limitations on exposures.

6. Concept of Effective Dose Equivalent

a. Basic Objectives:

The objective in developing the concept of the effective dose equivalent was to obtain a system that would provide a unit for radiation protection standards that could be used to express, on an equal risk basis, both whole body and partial body exposures. In developing this approach, the ICRP sought to:

i. Base the limits on the total risk to all tissues as well as the hereditary detriment in the immediate offspring (first two generations);

ii. Consider, in the case of internally deposited radionuclides, not only the dose occurring during the year of exposure, but also the committed dose for future years.

Having stated this objective, the next goal of the ICRP was to set the occupational dose limits at such a level that the risks to the average worker incurred as a result of his/her radiation exposure would not exceed the risk of accidental death to an average worker in a "safe" non-nuclear industry.

Based on a review of data on a world-wide basis (see Table I), the ICRP concluded that, on the average, within a "safe" industry about 100 workers or less would be killed accidentally each year for one million workers employed. Thus, the associated risk of accidental death to the average worker in a "safe" industry would be about:

100/year/1,000,000 = 1E-4/year.

b. Risks of Death from Radiation Exposures:

Based on epidemiological studies with human populations and biological studies in animals, estimates can be made of the risk of a fatality from cancer or a genetic death for given levels of dose equivalent to various body organs. Some examples are given below to illustrate the thinking that goes into formulation of risk factors:

i. Studies of the survivors of the atomic bombings in Japan at the close of World War II indicate that for a collective dose of 10,000 person-Sv (1,000,000 person-rem) to the bone marrow, there will be, after latency period, an average of one excess case of leukemia occurring in the population each year. Assuming that each such case ultimately results in a death, and that the excess continues for a period of 20 years, there will be a total of 20 excess cases of leukemia and, therefore, 20 excess deaths due to this exposure. Thus, the risk of death due to leukemia resulting from exposure of the bone marrow can be estimated to be:

20 excess person deaths/10,000 person-Sv = 2E-3/Sv

ii. Similar studies among uranium miners have shown that there will be approximately 20 excess cases of lung cancer (and consequently 20 excess deaths, assuming all cases of lung cancer are fatal) for each 10,000 person-Sv (1,000,000 person-rem) to the lungs. Thus the risk of death from lung cancer can be estimated to be:

20 excess deaths/10,000 lung-Sv = 2E-3/Sv

iii. For breast cancer, epidemiological data have shown that there is an excess of about 100 breast cancers per 10,000 person-Sv (1,000,000 person-rem) to the female breasts. Assuming that breast cancer is fatal 50% of the time; and assuming that the population being exposed consists of 50% men and 50% women, then the risk of excess deaths due to exposure to the female breasts can be estimated to be:

100 excess cancers/10,000 breast-Sv x (0.5 fatality rate) x (0.5 of population being female)= 2.5E-3 / Sv

iv. For thyroid cancer, epidemiological data have shown that there is an excess of about 100 thyroid cancers per 10,000 Sv (1,000,000 rem) to the thyroids in humans. However, the fatality rate for thyroid cancer is only about 5%, so the risk of death due to cancer of the thyroid resulting from exposure to ionizing radiation is:

100 excess cancers/10,000 thyroid-Sv x (0.05 fatality rate)=5E-4/Sv

c. Similar calculations can be made to estimate the excess deaths due to exposures of other body organs, as well as genetic deaths due to exposure of the reproductive organs.

Table 1:

Fatalities From Accidents in Different Occupations (x 10,000 Per Year)

1. Physical and Chemical Effects of Ionizing Radiation

a. Ionizing radiation is so named because its initial interaction with matter is the ejection of an orbital electron from an atom, forming a pair of ions with opposite charges. Radiation passing through living cells will ionize or excite atoms and molecules in the cell structure. This produces ions and radicals within the cell (mostly from water molecules). When these radicals and ions interact with other cell materials, damage can result. Certain levels of cellular damage can be repaired by the cell. Further levels can result in cell death.

i. May directly involve and damage biologically important molecules in the cell - Direct Effects. Damage to the DNA molecule or a chemical change in other cellular material are the primary results. Damage to the DNA molecule can result in somatic mutations that may show up years after the exposure or genetic mutations that require several life spans to appear.

ii. May initiate a chain of chemical reactions, mediated through cellular water, leading to ultimate biologic damage - Indirect Effects. An hydroxyl poisoning effect on the cell membrane results in a change in its permeability. Inactivation and release of enzymes is the primary result.

b. The unit of radiation dose is the rad which equals 100 ergs of energy absorbed per gram of tissue.

c. Biological effects of all types of ionizing radiations are similar. Some radiations are more efficient than others, however, and produce more biological damage per rad dose.

i. The rem is the unit of biological dose called the units of Dose equivalence) which takes into consideration the differing efficiencies of the different radiations.

ii. The Dose Equivalence in rems is obtained by multiplying the dose in rads by the Quality Factor (QF) of the particular radiation. The QF is related to its ionization density.

1 for most gamma and x-rays, beta particles 2 - 11 for neutrons 20 for alpha particles

2. Cellular Effects of Ionizing Radiation

a. Cell killing is responsible for acute somatic effects of radiation. It occurs by two mechanisms:

i. Inhibition of mitosis which results from moderate doses and leads to delayed cell death.

ii. Immediate cellular death which results from very high doses.

b. Alteration of cellular genetic material consistent with continued cell proliferation: Usually manifests no visible change in cellular appearance but a point (recessive) mutation is formed, which may or may not be passed to future generations.

3. Systemic Biological Effects of Ionizing Radiation

a. Somatic effects:

Abnormality may become manifest only after many generations of cell replication: proposed mechanism for long-term somatic effects of radiation - carcinogenesis, nonspecific life shortening. (These are non-stochastic effects.)

b. Genetic effects:

If involves gonadal cells, mutations are passed on to offspring. Increase in number of "recessive" mutations in population pool leads to increased probability of abnormalities in offspring due to chance mating of individuals carrying same mutation. (These are stochastic effects.)

4. Acute Somatic Effects of Radiation Exposure in Humans

a. Related to killing of cells, generally in tissues where cells are rapidly proliferating. Observed effects usually occur 1-3 weeks after radiation exposure.

b. Systems of primary involvement:

i. Hematopoietic system - (fever, infections, hemorrhages)

Chief organ: bone marrow
Symptom latency: days to weeks
Death threshold: less than 500 rem
Characteristic symptoms: Malaise, fever, fatigue, infection, hemorrhage, and anemia. Low counts of platelets, lymphocytes and erythrocytes result in low resistance to infection and a decreased clotting ability.

ii. Gastrointestinal system - (abdominal pain, vomiting, severe diarrhea, fluid and electrolyte imbalance)

Chief organ: small intestine
Symptom latency: hours to days
Death threshold: 500-2000 rem
Characteristic symptoms: Malaise, nausea, vomiting, diarrhea, fever, dehydration, G.I. malfunction, and electrolyte loss. The intestinal epithelium is destroyed.

iii. General systemic effects "radiation sickness" - (central nervous system syndromes).

Chief organ: brain
Symptom latency: minutes to hours
Death threshold: 2000-5000 rem
Characteristic symptoms: lethargy, tremors convulsions, encephalitis, meningitis, and edema. Acute inflammation and vascular damage results in neuronal functional impairment.

5. Dose relationships:

a. 0-150 rem - none to minimal symptoms. Perhaps long-term effects many years later.

b. 150-400 rem - moderate to severe illness due to hematopoietic derangement.

c. 400-800 rem - severe illness. LD50 in man probably about 500 rem. GI damage at higher doses.

d. Above 800 rem - 100% fatal, even with best available treatment.

6. Partial body exposure.

Effects depend on particular tissue or organ exposed, but significant acute changes are usually seen only after a fairly high radiation dose (>1000 rem).

7. Long-Term Effects of Exposure to Ionizing Radiation

a. General characteristics: Usually occur many years after acute or chronic radiation exposure.

b. Biologic Effects of Ionizing Radiation:

i. Occur with much lower doses and dose-rates: insufficient to cause acute somatic effects.

ii. Probably related to irreparable damage to genetic material in cells which are capable of continued cell division.

8. Radiation Carcinogenesis in Humans

Genetic and proliferative alterations of cells require years to many lifetimes to develop.

a. Tumor development: Ionizing radiation in large amounts is an effective carcinogenic agent.

b. Sterility: Temporary sterility can be induced at exposure levels of approximately 150 rem. Females are more often permanently affected than males.

c. Cataracts: Due to the high sensitivity of the lens of the eye, opaque areas of the lens develop after exposure of 200-600 rem.

d. Life-shortening: The aging process is increased. Nutrition to the cell appears to be impaired. The total cell number is decreased and there is a modification of the composition of cellular material.

e. Fetal damage: The fetus is highly radiosensitive due to the rapid division of cells. No measurable fetal damage has been seen at exposures less than one rem.

f. Chromosomal damage: Detection of chromosomal damage requires many generations. An Oak Ridge study suggests that low intensity (1-10 rem/day) continuous exposure has only 1/4 - 1/10 the mutagenic efficiency of acute exposures.

9. Law of Bergonie and Tribondeau

Radiation sensitivity of cells generally varies directly with the rate of proliferation and the number of future divisions, and inversely with the degree of morphological and functional differentiation.

The following is listed from least to most radiosensitive:

Least Radiosensitive	Mature Red Blood Corpuscles
	Liver Cells Nerve Cells Pituitary Cells Thyroid Cells Muscle Cells Bone and Cartilage Cells Skin Epithelium Cornea Squamous Mucous Epithelium Renal Tubules Lung-Tissue Cells Lens Gonadal Germ Cells Bon-Marrow Cells
Most Radiosensitive	Lymphocytes

10. Factors that Influence the Severity of Absorbed Dose

a. Internal Radiation

i. Amount of Radioactivity

ii. Radioisotope

iii. Nature or type of the emission

iv. Critical Organ

v. Physical half-life

vi. Biological half-life

vii. Age, weight, sex

b. External Radiation

i. Amount of Radioactivity

ii. Nature or type of the emission

iii. Radiation Energy

iv. Time

v. Distance

vi. Shielding

vii. Age, weight, sex

viii. Area of the Body Exposed

Dose Equivalent Limits (Monitored Radiation Workers)

Common Radiation Exposures (Natural Sources and Human Made)

Biologically Significant Radiation Exposures (Absorbed/Acute Exposure)

Dosimeters are devices that quantitate the amount of radiation to which a person has been exposed.

A. Types of Dosimetry Used
1. Film Dosimeters

The dosimeter used most often is the film badge, comprised of one of two small x-ray films enclosed within a light-tight envelope and plastic holder. The badge is worn from one to four weeks on the trunk of the body, usually at waist level or on the collar. Photographic film in the form of thin, even layers of emulsion is spread on a thick paper support base. The emulsion consists of small silver halide crystals embedded in a gelatin matrix. When the badge is exposed to radiation, energy is transferred to the emulsion causing silver ions to cluster together. These silver clumps are called latent image centers. This film detects x and gamma rays, beta particles greater than 1 MeV, and neutron radiation, except fast neutrons. Fast neutrons require a separate type of film. the amount of exposure is related to the length of the track that is left on the film. The accuracy of film badges is plus or minus 10 mrem.

There are two types of film badges. One badge monitors x, gamma and beta radiation. The other contains an additional film which is sensitive to neutrons. Each holder has filters on the from and back sides containing an open window, then plastic aluminum, cadmium, and lead filters. The type of radiation (i.e., x, gamma, beta, or neutron) can be determined by observing the relative darkening of the film behind each filter.

2. Thermoluminescent Dosimeters (Whole Body Exposure Monitors)

In some situations, thermoluminescent dosimeters containing lithium fluoride or calcium fluoride chip and powder cartridges are used in place of x-ray film as a personnel monitor. Exposure of these materials to ionizing radiation results in the trapping of electrons in energy levels above those occupied normally. When the dosimeter is heated, these electrons are liberated from the traps. As the electrons return to their normal levels, visible light is released. The amount of light released is measured and is proportional to the exposure of the dosimeter to radiation. These materials are x, beta, and gamma sensitive and exposure is reported as being either deep and/or shallow energy penetration.

3. Finger Ring Dosimeters

To monitor hand exposure to radioactive materials thermoluminescent dosimeters in the form of finger rings are worn on the dominant hand with the TLD chip facing the source of radiation. The TLD process is described above. It important is assure chip placement in the dosimeter prior to each use.

B. The dosimetry reporting company, an independent contractor, will report exposures per individual for the finger ring and deep and/or shallow energy penetration for the whole body.

C. Precautions on Use of Dosimetry

When not in use, store your dosimetry in an area free of ionizing radiation. If you lose, contaminate, get your badge or rings wet or leave them in the sun for an extended period of time, please notify the Office of Environmental Health and Safety, Health Physics. While wearing a lead apron, place your badge outside the apron.

1. Film badges:

a. Fading of the latent image centers is produced with time, high humidity, and high temperature.

2. TLD:

a. The lithium fluoride chips and powder are highly sensitive to heat and moisture.

D. Distribution and Use of Film Badges

1. Dosimetry is issued by the Office of Environmental Health and Safety, Health Physics based on procedures used and the type of equipment used.

2. Badges may be exchanged weekly, monthly, or quarterly, depending upon the type of equipment or type and amount of materials used and experimental design.

E. Dosimetry Records

All dosimetry records are on file with the Office of Environmental Health and Safety, Health Physics. Upon your request, EH&S will supply you with your dosimetry history. If at any time your exposure exceeds the established guidelines or is unusually high, a health physics staff member will notify you of the incident.

When fast-moving electrons slam into a metal object, x-rays are produced. The kinetic energy of the electron is transformed into electromagnetic energy. The function of the x-ray machine is to provide a sufficient intensity of electron flow from the cathode to anode in a controlled manner. The three principal segments of an x-ray machine - ca control panel, a high-voltage power supply, and the x-ray tube are all designed to provide a large number of electrons focused to a small spot in such a manner that when the electrons arrive at the target, they have acquired high kinetic energy.

Kinetic energy is the energy of motion. Stationary objects have no kinetic energy; objects in motion have kinetic energy proportional to their mass and the square of their velocity.

The equation used to calculate kinetic energy is:

KE = 1/2 mv2

where m is the mass in kilograms, v is the velocity in meters per second, and KE is the kinetic energy in joules. In determining the magnitude of the kinetic energy of a projectile, the velocity is more important than the mass.

In a x-ray tube, the projectile is the electron. As its kinetic energy is increased, both the intensity (number of x-rays) and the energy (their ability to penetrate) of the created x-rays are increased.

The x-ray machine is a remarkable instrument. It conveys to the target an enormous number of electrons at a precisely controlled kinetic energy. At 100 mA, for example, 6 x 1017 electrons travel from the cathode to the anode of the x-ray tube every second.

The distance between the filament and the target is only about 1 to 3 cm. Imagine the intensity of the accelerating force required to raise the velocity of the electrons from zero to half the speed of light in so short a distance.

The electrons traveling from the cathode to anode in a vacuum tube comprise the x-ray current and are sometimes called projectile electrons. When these projectile electrons impinge on the heavy metal atoms of the target, they interact with these atoms and transfer their kinetic energy to the target. These interactions occur within a very small depth of penetration into the target. As they occur, the projectile electrons slow down and finally come nearly to rest, at which time they can be conducted through the x-ray anode assembly and out into the associated electronic circuitry.

The projectile electron interacts with either the orbital electrons or the nuclei of target atoms. The interactions result in the conversion of kinetic energy into thermal energy and electromagnetic energy in the form of x-rays.

By far, most of the kinetic energy of projectile electrons is converted into heat. The projectile electrons interact with the outer-shell electrons of the target atoms but do not transfer sufficient energy to these outer-shell electrons to ionize them. Rather, the outer-shell electrons are simply raised to an exceted, or higher, energy level. The outer-shell electrons immediately drop back to their normal energy state with the emission of infrared radiation. The constant excitation and restabilization of outer-shell electrons is responsible for the heat generated in the anodes of x-ray tubes.

Generally, more than 99% of the kinetic energy of projectile electrons is converted to thermal energy, leaving less than 1% available for the production of x-radiation. One must conclude, therefore, that, sophisticated as it is, the x-ray machine is a very inefficient apparatus.

The production of heat in the anode increases directly with increasing tube current. Doubling the tube current doubles the quantity of heat produced. Heat production also varies almost directly with varying kVp.

The efficiency of x-ray production is independent of the tube current. Regardless of what mA is selected, the efficiency of x-ray production remains constant. The efficiency of x-ray production increases with increasing projectile-electron endery. At 60 kVp, only 0.5% of the electron kinetic energy is converted to x-rays; at 120 MeV, it is 70%.

Characteristic Radiation

If the projectile electron interacts with an inner-shell electron of the target atom rather than an outer-shell electron, characteristic x-radiation can be produced. Characteristic x-radiation results when the interaction is sufficiently violent to ionize the target atom by total removal of the inner-shell electron. Excitation of an inner-shell electron does not produce characteristic x-radiation.

When the projectile electron ionizes a target atom by removal of a K-shell electron, a temporary electron hole is produced in the K shell. This is a highly unnatural state for the target atom and is corrected by an outer-shell electron falling into the hole in the K shell. The transition of an orbital electron from an outer shell to an inner shell is accompanied by the emission of an x-ray photon. the x-ray has energy equal to the difference in the binding energies of the orbital electrons involved.

Example:
A K-shell electron is removed from a tungsten atom and is replaced by an l_shell electron. What is the energy of the characteristic x-ray that is emitted?

Answer:

For tungsten, K electrons have binding energies of 69.5 keV, and L electrons are bound by 12.1 keV. Therefore, the characteristic x-ray emitted has energy of:

69.5 - 12.1 = 57.4 keV

In summary, characteristic x-rays are produced by transitions of orbital electrons from outer to inner shells. Since the electron binding energy for every element is different, the characteristic x-rays produced in the various elements are also different. This type of x-radiation is called characteristic radiation because it is characteristic of the target element. The effective energy characteristic x-rays increases with increasing atomic number of the target element.

Discrete X-ray Spectrum
We saw earlier that characteristic x-rays have precisely fixed, or discrete, energies and that these energies are characteristic of the differences between electron binding energies of a particular element. A characteristic x-ray from tungsten, for example, can have one of fifteen energies and no others

Bremsstrahlung Radiation
The production of heat and characteristic x-rays involves interactions between the projectile electrons and the electrons of target atoms. A third type of interaction in which the projectile electron can lose its kinetic energy is an interaction with the nucleus of a target atom. In this type of interaction, the kinetic energy of the projectile electron is converted into electromagnetic energy.

A projectile electron that completely avoids the orbital electrons on passing through an atom of the target may come sufficiently close to the nucleus of the atom to come under its influence. Since the electron is negatively charged and the nucleus is positively charged, there is an electrostatic force of attraction between them. As the projectile electron approaches the nucleus, it is influenced by a nuclear force much stronger than the electrostatic attraction. As it passes by the nucleus, it is slowed down and deviated in its course, leaving with reduced kinetic energy in a different direction. This loss in kinetic energy reappears as an x-ray photon. These types of x-rays are called bremsstrahlung radiation, or bremsstrahlung x-rays. Bremsstrahlung is the German word for slowing down or braking; bremsstrahlung radiation can be considered radiation resulting from the braking or projectile electrons by the nucleus.

A projectile electron can lose any amount of its kinetic energy in an interaction with the nucleus of a target atom, and the bremsstrahlung radiation associated with the loss can take on a corresponding range of values. For example, an electron with kinetic energy of 70 keV can lose all, none, or any intermediate level of that kinetic energy in a bremsstrahlung interaction; the bremsstrahlung x-ray produced can have an energy in the range of 0 to 70 keV. This is different from the production of characteristic x-rays that have specific energies.

Continuous X-ray Spectrum

If it were possible to identify and quantitate the energy contained in each bremsstrahlung photon emitted from an x-ray tube, one would find that these energies extend from that associated with the peak electron energy all the way down to zero. In other words, when an x-ray tube is operated at 70 kVp, bremsstrahlung photons with energies ranging from 0 to 70 keV are emitted. Thus, creating a typical continuous, or bremsstrahlung, x-ray emission spectrum.

This emission spectrum is sometimes called the continuous emission spectrum because, unlike in the discrete spectrum, the energies of the photons emitted may range anywhere from zero to some maximum value. The general shape of the continuous x-ray spectrum is the same for all x-ray machines. The maximum energy that an x-ray can have is numerically equal to the kVp of operation. The greatest number of x-ray photons is emitted with energy approximately one-third of the maximum photon energy. The number of x-rays emitted decreases rapidly at very low photon energies and below 5 keV nearly reaches zero.

Immediately following their discovery, x-rays were applied to the healing arts. It was recognized within months that they could cause harmful effects. Since that time, a great deal of effort has been devoted to developing equipment, techniques, and procedures to control radiation levels and reduce unnecessary radiation exposures.

Principles of Radiation Protection

All health physics activity in radiology is designed to minimize exposure to patients and personnel. Three cardinal principles of radiation protection developed for nuclear activities find equally useful application in diagnostic radiology; time, distance, and shielding. By observing the following principles, one can minimize radiation exposure:

A. Keep the time of exposure to radiation short.

B. Maintain a large distance between the source of radiation and the exposed person.

C. Insert shielding material between the source and the exposed persons.

Minimize Time

The dose to an individual is directly related to the duration of exposure. If the time during which one is exposed to radiation is doubled, then the exposure will be doubled. The equation for this relationship is:

Exposure = Exposure rate x Time

During radiography, the time of exposure is kept to a minimum to reduce motion unsharpness. During fluoroscopy, the time of exposure should also be kept to a minimum to reduce patient and personnel exposure. This is an area of radiation protection not directly controlled by the radiologic technologist. Radiologists are trained to depress the fluoroscopic foot switch in an alternating fashion, sequencing on-off rather than continuously on during the course of the examination. A repeated up-and-down motion on the fluoroscopic foot switch permits a high quality examination to be made with a considerably reduced exposure to the patient.

The 5-minute reset timer on all fluoroscopes reminds the radiologist that a considerable fluoroscopic time has elapsed. The timer records the amount of x-ray beam on-time. Most fluoroscopic examinations take less than 5 minutes. Only during difficult special procedures should it be necessary to exceed 5 minutes of exposure time.

Maximize Distance

As the distance between the source of radiation and an individual increases, the radiation exposure decreases rapidly. The decrease in exposure can be calculated using the inverse square law if the source of radiation can be considered a point source. Most radiation sources are point sources; the x-ray tube target, for example, is a point source of radiation. However, the scattered radiation generated within a patient appears to come not from a point but rather from an extended area. As a rule of thumb, even an extended source can be considered a point source if the distance from the source exceeds seven times the source diameter.

ID2 = id2

I = intensity at a distance (D) from a point source.
i = intensity as a different distance (d) from the same point source.

In radiography, the distance from the radiation source to patient is generally fixed by the type of examination, and the technologist is positioned behind a protective barrier.

During fluoroscopy, the radiologic technologist can exercise good radiation protection procedures by keeping the distance as great as possible for as long as possible. The technologist's exposure generally does not follow the inverse square law since during fluoroscopy, the patient serves as an extended source of radiation because of the scattered x-rays generated within the body. Therefore, during fluoroscopy, the technologist should remain as far from the examining table as practical.

Maximize Shielding
Placing shielding material between the radiation source and individuals exposed reduces the level of exposure. Shielding used in diagnostic radiology usually consists of lead, although sometimes concrete is used. The amount a protective barrier reduces radiation intensity can be estimated if the half-value layer (HVL) or the tenth-value layer (TVL) of the barrier material is known. The HVL is the thickness of material required to reduce the radiation intensity to one-half of its original value. Correspondingly, the TVL is the thickness of material required to reduce the radiation intensity to one-tenth of its original value. One TVL is equal to 3.3 HVLs. Table 1 shows approximate half-value and tenth value layers for lead and concrete for diagnostic x-ray facilities operating between 50 and 140 kVp. For comparison purposes, the HVL and TVL for several therapeutic sources of radiation are also shown.

Table 1. Approximate HVLs and TVLs of Lead and Concrete at Various Tube Potentials

Time, Distance, and Shielding
Usually, applications of the principles of radiation protection involve a consideration of all three. The typical problem involves a known radiation level at a given distance from the radiation source. One can calculate the level of exposure at any other distance, behind any shielding, for any length of time. The order that these calculations are made makes no difference.

EXAMPLE:
A radiographic installation is designed exclusively for chest radiography at 100 kVp. The output intensity is 4.6 mR/mAs at 100 cm SID (40 in TFD). The distance to the clerical support staff's desk on the other side of the wall that the x-ray beam is directed is 200 cm (80 in). The wall contains 0.96 mm Pb, and 300 mAs is anticipated daily. If the clerical support staff is to be restricted to 2 mR exposure per day, how long each day may the individual remain at their desk?

ANSWER:
Daily x-ray output = (4.6 mR/mAs) (300 mAs) = 1380 mR

Daily output at 200 cm = (1380 mR)(100 cm/200 cm)2 = 345 mR

Daily output behind 0.96 mm Pb or 4 HVLs = 22 mR

Time allowed = (2 mR) / (22 mR/day) = 0.09 day = 2.16 hours = 130 minutes

Designing for Radiation Protection
A number of features of modern x-ray equipment designed to improve radiographic quality have been previously discussed. Many of these features and a number of additional ones are also designed to reduce patient exposure during the x-ray examination. For instance, proper beam collimation is very effective in reducing patient exposure. It is also a primary contributor to enhanced radiographic quality. Filtration, on the other hand, is added to the x-ray beam only to reduce the patient dose.

Over 100 individual radiation protection devices and accessories are associated with modern x-ray equipment. Some are characteristic of either radiographic or fluoroscopic assemblies, and some are required for all diagnostic x-ray equipment. A few of those appropriate for all diagnostic x-ray equipment are as follow:

1. Diagnostic-type protective tube housing. Every x-ray tube must be contained within a protective housing that reduces the leakage radiation to less than 100 mR/hr at a distance of 1 meter from the housing.

2. Control Panel. The control panel must indicate the conditions of exposure and positively indicate when the x-ray tube is energized. These requirements are usually satisfied with kVp and mA meters. Sometimes visible and audible signals will indicate when the beam is on.

Radiographic Equipment
Discussed next are some of the more important aspects of radiation protection when using radiographic equipment:

1. Source-to-image receptor distance (SID) indicator. A SID indicator must be provided. It can be as simple as a tape measure attached to the tube housing or as advanced as laser lights, but it must be accurate to within 2 percent of the indicated SID.

2. Collimation. Light-localized variable-aperture rectangular collimators should be provided. The x-ray beam and light beam must coincide to within 2 percent of the SID. Cones and diaphragms may replace the collimator for special examinations. The attenuation of the useful beam by the collimator leaves must be equivalent to that by the protective housing.

3. Filtration. All general purpose diagnostic x-ray beams must have a total filtration (inherent plus added) of at least 2.5 mm Al. Radiographic tubes operated between 50 and 70 kVp must have at least 1.5 mm Al. Below 50 kVp, a minimum of 0.5 mm Al total filtration is required.

4. Beam Alignment. In addition to proper collimation, each radiographic tube head should be provided with a mechanism to ensure proper alignment of the x-ray beam and the film. It does no good to align the light beam and the x-ray with the film partly or completely out of the beam path.

5. Positive Beam Limitation (PBL). Automatic light-localized variable-aperture collimators are now required on all but special equipment in the United States. These PBL devices must be adjusted so that with any film size in use and at all standard SIDs, the collimator leaves are automatically adjusted to provide an x-ray beam equal to the image receptor. The PBL must be accurate to within 2 percent of the SID.

6. Reproducibility. For any given radiographic technique, the output radiation intensity should be constant from one exposure to another. The average variation in radiation intensity during repeated exposures should not exceed 5 percent.

7. Linearity. When adjacent mA stations are employed, for example, 100 and 200 mA, and exposure time is adjusted for constant mAs, the output radiation intensity must remain constant. The radiation intensity is measured in units of milliroentgens per milliampere-seconds (mR/mAs), and the maximum acceptable variation in the linearity is 10 percent.

8. Personnel Shield. It must not be possible to expose a radiograph while outside a fixed protective barrier, usually the console booth.

9. Portable X-ray Unit. A protective lead apron should be assigned to each portable x-ray unit. The exposure switch of such a unit must allow the operator to remain a least 180 cm (6 ft.) from the x-ray tube during the exposure(s).

Fluoroscopic Equipment
The features of fluoroscopic equipment discussed below are designed primarily to reduce patient and personnel exposure:

1. Source-to-Tabletop Distance. The source-to-tabletop distance must be not less than 38 cm (15 in.) on the stationary fluoroscopes and not less than 30 cm (12 in.) on mobile fluoroscopes. Increasing the distance between the fluoroscopic tube and the patient results in decreased patient dose because of the corresponding reduction in the difference between the entrance and exit dose of the patient.

2. Primary Protective Barrier. The image-intensifier assembly serves as a primary protective barrier and must be 2 mm Pb equivalent for equipment capable of operating above 125 kVp. It must be coupled with the x-ray tube and interlocked so that the fluoroscopic tube cannot be energized when in the parked position.

3. Filtration. The total filtration of the fluoroscope must be at least 2.5 mm Al equivalent. The tabletop, patient cradle, or other material positioned between the tube and the tabletop should be included as part of the total filtration. When the filtration is unknown, the half-value layer should be measured. If the half-value is not less than 2.4 mm Al when operated at 80 kVp, adequate filtration may be assumed.

4. Collimation. The fluoroscopic beam collimators must be adjusted so that an unexposed border on the fluoroscopic screen or input phosphor of the image intensifier is visible when the screen carriage is positioned 35 cm (14 in.) above the table top and the collimators are fully open. For automatic collimating devices, such an unexposed border should be visible at all heights above the tabletop.

5. Exposure Switch. The Fluoroscopic exposure switch should be the dead-man type; that is, if the operator should drop dead, the exposure would be terminated--unless, of course, the person fall on the switch. The conventional foot pedal satisfies this condition.

6. Bucky Slot Cover. During fluoroscopy, the Bucky tray is moved to the end of the examining table, leaving an opening in the side of the table approximately 5 cm (2 in.) wide at gonadal level. This opening should be automatically covered with at least 0.25 mm Pb equivalent.

7. Protective Curtain. A protective curtain or panel of at least 0.25 mm Pb equivalent should be positioned between the fluoroscopist and the patient. Without the protective curtain or Bucky slot cover, the exposure to the radiology personnel is many times higher.

8. Cumulative Timer. A cumulative timer that produces an audible signal or temporarily interrupts the x-ray beam when the fluoroscopic time has exceeded 5 minutes must be provided. This device is designed to make sure the radiologist is aware of the relative beam on-time during each procedure.

9. X-ray Intensity. The intensity of the x-ray beam at the tabletop of a fluoroscope should not exceed 2.1 R/min for each mA of operation at 80 kVp. Under no conditions should the intensity exceed 10 R/min during fluoroscopy. Tabletop intensities may exceed this level during cineradiography.

Design of Protective Barriers
When one is designing radiology departments or an individual x-ray examination room, it is not sufficient to consider only the general architectural characteristics. Attention must be given to the location of the x-ray machine(s) within the examination room and to the use of adjoining rooms. It is nearly always necessary to insert protective barriers, usually sheets of rolled lead, in the walls of the x-ray examination rooms. If the radiology facility is located on an upper floor, then it may be necessary to shield the floor also. For these reasons, it can be desirable to locate the radiology facility on the ground or basement level, with the examining rooms positioned along outside walls.

A great number of factors should be considered in designing a protective barrier. This discussion will include only the fundamentals and some basic definitions. Any time new x-ray facilities are being designed or existing facilities are being renovated, a certified physicist must be consulted for assistance in the proper design of the radiation shielding.

For the purpose of designing protective barriers, three types of radiation are considered. The first and most intense/hazardous is the primary radiation or useful beam. When a chest board is positioned on a given wall, it can be assumed that it will intercept the useful beam frequently. Therefore, it is sometimes necessary to provide shielding directly behind the chest board in addition to that specified for the rest of the wall. The wall so irradiated would be designated a primary protective barrier.

Lead bonded to sheetrock or wood paneling is most often employed as a primary protective barrier. Such lead shielding is available in various thicknesses, and it is specified for architects and contractors in units of pounds per square foot (lb/ft2). Rarely is it necessary to use in excess of 4 lb/ft2 in a diagnostic room. Concrete or concrete block may be used to replace lead. As a rule of thumb, 4 inches of concrete is equivalent to 1/16 inch of lead. Table 2 shows available lead thicknesses and equivalent thicknesses of concrete.

Table 2. Lead and Concrete Equivalents for Primary Protective Barriers

There are two types of secondary radiation: scatter radiation and leakage radiation. Scatter radiation results when the useful beam intercepts any object so that some x-rays are scattered. For the purpose of protective shielding calculations, the scattering object can be considered as a new source of radiation. During both radiography and fluoroscopy, the patient is the single most important scattering object. As a general rule of thumb, the intensity of scatter radiation 1 m (3.3 ft.) from the patient si 0.1 percent of the intensity of the useful beam at the patient. Leakage radiation is radiation emitted from the x-ray tube housing assembly in all directions other than that of the useful or primary beam. If the tube housing is properly designed, the leakage radiation will never exceed 100 mR/hr at 1 meter. This value is used for the barrier calculations.

Barriers designed to shield areas from secondary radiation are called secondary radiation barriers. Secondary radiation barriers are always less thick than primary protective barriers. Consequently, lead is rarely required for secondary barriers because the computation usually results in less than 0.4 mm Pb. In such cases, conventional gypsum board, glass, or lead acrylic is adequate. Many walls that are secondary barriers can be adequately protected with four thicknesses of 5/8 inch gypsum board. Most control booth barriers are secondary protective barriers; the useful beam is never directed at the control booth. Four thicknesses of gypsum board and 1/2 inch plate glass may be all that is necessary. In some cases, glass walls 1/2 to 1 inch thick can be used for control booth barriers. Table 3 contains equivalent thicknesses for secondary barrier material.

Table 3. Equivalent Material Thicknesses for Secondary Barriers