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May 1994
Instructional Subject: Interactions of Fundamental Particles With Matter
ºInteraction of Particles
Photons
Electrons
Heavy Charged Particles
Neutrons
ºRadiation Dose Units
Instructor: To be determined
Instructional Goal: To provide the participant with a basic knowledge of radiation concepts.
Instructional Objectives: The participant should be able to accomplish the following:
ºRecognize the differences of radiation producing particles
ºKnow where to go for information
ºUnderstand the interactions of particles with matter and energy transfer mechanisms
ºBe able to identify and use radioactive dose units
Training Support Material: Handouts
Study Guide
Video Tapes: None
Slides: As determined by the instructor
Equipment Required: As determined by the instructor
Reference Material: Radiation Protection Training Manual and
Study Guide - Chapter 1
Instructional Subject: Accelerators and Their Ionizing Radiations
¨ Primary Radiation
¨ Secondary Radiation
Instructor: To be determined
Instructional Goal: To provide the participant with a basic knowledge
accelerator and x-ray radiation potentials.
Instructional Objectives: Upon completion of this instructional period, the
participant should be able to accomplish the following:
¨ Recognize potential radiation hazards
Training Support Material: Handouts
Study Guide
Video Tapes: As determined by instructor
Slides: As determined by the instructor
Equipment Required: As determined by the instructor
Reference Material: Radiation Protection Training Manual and
Study Guide - Chapter 2
Instructional Subject: Sources and Effects of Radiation
¨ Biological Effects of Radiation
¨ Radiation Exposure Limit
¨ Radiation from Background Medical and Consumer Products
Instructor: To be determined
Instructional Goal: To provide the participant with information concerning
the effects and limits of radiation exposure
Instructional Objectives: Upon completion of this instructional period, the
participant should be familiar with the effects and limits allowed
for radiation exposure in the use of radioisotopes and/or equipment
and:
¨ Be familiar with methods of protection
Training Support Material: Handouts
Study Guide
Video Tapes: None
Slides: As determined by the instructor
Equipment Required: As determined by the instructor
Reference Material: Radiation Protection Training Manual and
Study Guide - Chapter 3
Instructional Subject: Radiation Detection and Measurement
¨ Survey Instruments
¨ Use of Radiation Survey Instruments
¨ Calibration of Survey Instruments
¨ Personnel Monitoring Instruments
¨ Proper Use of Personnel Monitors
Instructor: To be determined
Instructional Goal: To provide the participant with a basic knowledge of
instruments available and how they are used
Instructional Objectives: Upon completion of this instructional period, the
participant should be able to accomplish the following:
¨ Identify the different types of instruments and detectors
¨ Be able to perform surveys with the appropriate instrument
¨ Understand how instruments are calibrated and how to determine the
efficiency
¨ Have a basic understanding of statistics
Training Support Material: Handouts
Study Guide
Video Tapes: None
Slides: As determined by the instructor
Equipment Required: As determined by the instructor
Reference Material: Radiation Protection Training Manual and
Study Guide - Chapter 4
Instructional Subject: Radiation Protection and Control of Exposures
¨ External Radiation Protection
¨ Hazards Associated with Accelerators
¨ Radiation Safety Procedures -
Warning and Interlock Systems
¨ Radiation Survey Procedures
¨ Control Measures for Radiation Levels
¨ Planning in Emergencies - Radiation Accidents
Instructor: To be determined
Instructional Goal: To provide information to the participant on
requirements of radiation protection
methods; How to determine shielding needs; A view of possible hazards; A view
of protective and cautioning devices; Methods of surveys and controls
Instructional Objectives: Upon completion of this instructional period, the
participant should be able to accomplish the following:
¨ Be knowledgeable required safety warning and control devices
¨ Be able to hazards associated with accelerator and x-ray equipment.
Training Support Material: Handouts
Study Guide
Video Tapes: As determined by instructor
Slides: As determined by the instructor
Equipment Required: As determined by the instructor
Reference Material: Radiation Protection Training Manual and Study Guide - Chapter 5
Instructional Subject: Radiation Protection Program
¨ Rules and Regulations
¨ Course Review
¨ Course Evaluation
¨ Examination
Instructor: To be determined
Instructional Goal: To provide information to the participant on
requirements of a radiation protection
program; Review all material covered and provide RSO with an evaluation of the
course
Instructional Objectives: Upon completion of this instructional period, the
participant should be able to accomplish the following:
¨ Be knowledgeable of the rules and regulations and the requirements to
have a radiation training program
¨ Be able to demonstrate acceptable knowledge of the information
presented to easily pass the test for the course
Training Support Material: Handouts
Study Guide
Video Tapes: As determined by instructor
Slides: As determined by the instructor
Equipment Required: As determined by the instructor
Reference Material: Radiation Protection Training Manual and
Study Guide - Chapter 6
I. Interactions of Fundamental Particles With Matter
II. Accelerators and Their Ionizing Radiation
III. Sources and Effects of Radiation
IV. Radiation Detection and Measurement
V. Radiation Protection and Control of Exposures
VI. Radiation Protection Programs
Appendix II Rules of Thumb and Useful Equations
Appendix III Penetration Ability of Beta Radiation
Appendix IV Reference Data for Selected Radioisotopes
The concepts and ideas presented in the text require a fundamental understanding of biology, physics and mathematics. While the material is presented in its most basic form, undoubtedly some ambiguities will remain. A bibliography is provided for the interested student to further his or her knowledge in this area.
The Radiation Safety Office is available to answer any questions or aid any user in the radiation science field. We welcome all inquiries and would appreciate comments and suggestions on how to improve this Study Guide, Training Course or our Radiation Safety Program.
The Radiation Protection Training Course has been established to satisfy the training requirements for University of Maryland personnel who use radiation producing devices.
The course will be presented periodically. Personnel who are unable to attend the formal lecture presentation may pursue a course of study on their own by using the Accelerator Radiation Protection Training Manual and Study Guide.
Upon completion of the study course or lecture, an examination will be given to authenticate completion of the program requirements. Receipt of the completed examination by the Radiation Safety Office will be the only completion documentation accepted.
A Certificate of Achievement will be presented to those who have successfully completed the course and a permanent record of training completion will be on file in the Radiation Safety Office.
Return to the Table of Contents
The primary cause of biological damage from ionizing radiation is the production of ions in living tissues. For establishment of techniques in radiation protection, it is necessary to understand the manner in which radiation interacts with matter and transfers its energy. This chapter deals with the various mechanisms of energy loss by radiation while traversing through matter.
Energy from radiation is transferred to matter via two major ways: Ionization and Excitation. Ionization is the process of removal of an electron from an atom whereby the atom is left with a net positive charge. In excitation, the energy of incoming radiation is added to the atomic system, transferring it from the ground state to an excited state.
The interaction of all types of radiation with matter will ultimately have the same effect. However, the initial stages of energy loss for each type of radiation are different. In an accelerator, there are four principal types of radiation of concern:
The electrons, positrons, protons, deuterons and alpha are grouped as the charged particulate radiation while the x-ray, gamma and neutrons are the uncharged radiation.
Since photons are chargeless, they do not interact by electrostatic forces as is the case with charged particles. Charged particles cause ionization of matter directly along their path of travel and as such are directly ionizing radiation whereas photons are indirectly ionizing radiation. That is, photons have sufficient energy to release high energy secondary charged particles (electrons) from matter through one of three basic interactions:
The high speed electrons resulting from these interactions then cause the ionization of the medium.
The Photoelectric Effect is the interaction of low energy photons*** with matter, whereby all of the energy of the photon is transferred to an inner shell electron (usually the K shell). The electron is ejected from the atom and the atom is left with an inner shell vacancy. This shell vacancy creates an excitation energy which corresponds to the Binding Energy (BE) of the ejected photoelectron.
KEphotoelectron = EX or Eg - BE of inner shell electron ejected
The resultant atom is now in an excited state and will decay to the ground state by emission of X-rays and fluorescent radiation with the total energy equal to the BE of the photoelectron. The energies of the secondary radiations are usually much lower than the primary photon energy.
For this reaction to occur, the photon must have sufficient energy to knock the inner shell electron.
*** A photon, as described by the Quantum Theory, is a "particle" or "quantum" that contains a discrete quantity of electromagnetic energy which travels at the speed of light, or 3 X 108 meters per second.
Photons with energies much greater than the BE of the electrons in an atom may interact through essentially elastic scattering interactions in which the total KE of the system is conserved. In this interaction, the electron appears to the photon as a free electron.
The primary g loses part of its energy to the Compton electron which gets scattered at an angle q from the original direction of the incident g, while the compton scattered g (g') is scattered at an angle w. In this process, the scattered photon and Compton electron share the energy of the incident g.
The KE carried off by the Compton electron may be deposited locally (i.e., absorbed immediately by the surroundings). However, the energy carried off by the Compton scattered photon is not deposited locally. Therefore, this scattered photon can significantly contribute to the dose outside a shielding apparatus.
High energy photons transfer their energy primarily by pair production. A high energy photon passing close to a nucleus suddenly disappears and an electron and a positron appear in its place. This interaction must take place in the neighborhood of a nucleus to conserve momentum. 
Eg = KE+ + KE- + 2 moc2 moc2 represents the rest mass energy of an electron (0.51 MeV)
Since both particles are created from energy supplied by the incident photon, the process is energetically possible only if Eg or EX is greater than 1.02 MeV.
When the positron slows down (i.e., loses its KE), it will annihilate itself by combining with an electron. This produces two photons with an energy of 0.51 MeV each. This "annihilation radiation" represents the energy equivalent of the rest mass of two electrons which is converted to pure energy according to the principle of Einstein's theories, in particular, E = mc2; where
E = energy of two 0.51 MeV photons
m = the rest mass of two electrons (1/1840 amu)
c = the velocity of light (3 X 108 m/sec)
For electrons, two mechanisms contribute to the energy loss process. In addition to atomic collisions (that result in ionizations or excitations), radiation losses occur when an electron is deflected by the electromagnetic field of a nucleus. The total energy loss, (dE/dX)tot, is written as:
(dE/dX)tot = (dE/dX)coll+ (dE/dX)rad
The quantity (dE/dX) is also referred to as the stopping power of the medium for the particle. The first term is the energy loss due to collisions and the second term represents energy loss from radiation, primarily bremsstrahlung. At low energies, < 1 MeV, the energy loss due to ionization is predominant. At very high energies the radiative term dominates and gives rise to electron-photon cascade showers. As the high energy electrons are deflected from their path in the vicinity of a nucleus, high energy photons referred to as the bremsstrahlung photons are emitted. These high energy photons, in turn, produce Compton electrons and electron- positron pairs, which then produce additional bremsstrahlung photons, and so on. These repeated interactions result in electron-photon cascade shower.
The ratio of energy loss by radiation to energy loss by ionization in an absorber Z is approximately equal to EZ/800, where E is the electron energy. Thus, the probability of bremsstrahlung increases with the increasing atomic number of the absorber. As a result a low Z material is used for shielding these radiations.
The electrons lose a large fraction of their energy in single collisions with matter, undergoing large deflections in the matter and travelling in a zig-zag manner.
The interactions of heavy charged particles are fundamentally similar to that of electrons. The positive ions or heavy charged particles loose energy by interacting with electrons of the matter causing ionization and excitation of atoms. Unlike electrons, a heavy charged particle transfers only a small fraction of its energy in a single electronic collision and its deflection in the collision is negligible. Thus, a heavy charged particle travels almost in a straight path through matter, losing energy continuously in small amounts. Generally, the rate of energy loss of all charged particles moving with the same velocity in a given absorber is proportional to the square of their charges. Heavy atoms or high Z absorbers are less efficient for slowing down heavy charged particles because many of their electrons are too tightly bound in the inner shells.
Like photons, neutrons are uncharged particles and can travel long distances in matter without interacting. Neutrons interact with an atomic nucleus via two basic processes; scattering and absorption. In this section we will deal with only scattering interactions: elastic scattering and inelastic scattering.
Q = 4mME
max (M+m)^2
An accelerator is used to impart kinetic energy to electrically charged particles. In general, relatively small amount of energy is required to accelerate the electrons to nearly the speed of light. The increase in kinetic energy increases solely the mass of the particle and has little effect on its velocity. The accelerating potential is produced by waves travelling at constant velocity. These waves are generated in radiofrequency, RF, waveguides. The RF power source which drives the waveguide supplies very high power in short pulses. For example, the accelerating waveguides in a linear accelerator consists of a circular tube containing a series of disks along its length. Each disk has a central hole through which the electron beam passes. In cyclotrons and synchrotron, the particles are repeatedly returned to the same accelerating electrode by means of a magnetic field which causes them to move in circular or nearly circular path.
Ionizing radiation from accelerators can be classified according to its source:
The primary and secondary radiations are grouped under prompt radiation fields and are described in this chapter. The remaining two types of radiations: stray radiation and induced radioactivity are explained in Chapter V.
Prompt Radiation Fields
Primary radiation is the beam of particles accelerated by the particle accelerator and may comprise of electrons, protons, deuterons, alphas and other heavy particles. The primary radiation is always directed and is focused into a beam. Inside the machine or the vacuum chamber it exists as an "internal beam" and as it emerges from the chamber usually by passing through a thin metal foil it is referred to as the "external beam".
The accelerator beam is usually collimated and travels in a straight line unless deflected by a magnet. Thus, the primary radiation field occupies a very small volume, and its energy is concentrated to a smaller area. However, some accelerators such as industrial processing units, have the ability to spread primary radiation over a larger area with a scanning horn. In these units, the primary radiation intensity is also very high but is concentrated to a larger area.
In either case, any exposure to primary radiation, i.e. the beam itself, presents extreme external hazards. When the beam strikes a solid object, e.g a target, target holder or beam stopper, the beam may scatter in the backward direction or produce secondary radiation which may be very penetrating.
Secondary radiation is produced when the primary beam strikes a target or other material. As a result of this interaction, either charged particles are created or electromagnetic radiation is produced. The charged particles such as electrons, protons and positive ions produced from the interaction have short ranges while the x-rays, gamma rays and neutrons created in the process are very penetrating and are responsible for thick shielding around the accelerators. Often this secondary radiation is the principal useful output of the machine, e.g. x-rays for radiography and neutrons for activation analysis.
X-rays are electromagnetic radiations which originate in the electron field outside the nucleus of an atom. A high energy electron knocks an orbital electron from the innermost shell of an atom thereby leaving the atom in an excited state. An X-ray is released when the electron from an outer shell fills the vacancy and the atom returns to the ground state. The X-rays can also be produced by rapid deceleration of fast moving high energy electrons in the vicinity of a nucleus. The electrons are deflected from their original path and while slowing emit x-rays, also known as the "bremsstrah- "-lung radiation". Bremsstrahlung is a German word for "braking radiation".
The efficiency of conversion of kinetic energy of an electron into bremsstrahlung radiation increases with increasing electron energy and with increasing atomic number of the target material.
The fraction, f, of the electron's kinetic energy which is converted to x-ray energy is given by the following equation:
f = 1.1 x 10-6 E Z
where Z is the atomic number of the target, and
E is the electron energy
For example, in a thick tungsten target, a 10 MeV electron converts about 30% of its energy into X-rays whereas a 100 MeV electron converts over 75% of its energy.
The energy of x-rays is usually expressed as the maximum or peak energy of the impinging electrons. In a typical X-ray production process the X-rays are emitted in a broad energy spectrum ranging up to the maximum energy of the accelerated electron used.
Neutrons are chargeless particles having a mass nearly equal to protons. Neutrons are classified according to their kinetic energies as follows:
epithermal
thermal
intermediate
fast
Neutrons are usually generated in all directions with a high energy component in the forward direction. As the accelerator energy increases, the high energy component becomes more important and is the basis of shielding around accelerators. In electron accelerator, an intermediate target is used to create bremsstrahlung beam which in turn, strikes a second target and neutrons are produced.
Living organisms are a collection of complex systems of many symbiotic parts arranged and packaged in a manner to allow maintenance of their internal environment and self-reproduction. The basic units are composed of cells. Cells of similar origin and structure are further grouped to form tissues. The four main groups of tissues are: muscle, nerve, connective and epithelial. Associated cells and tissues form organs which, taken collectively, function to create and control the necessary internal conditions suitable for life.
A great diversity exists among the different kinds of cells found in the body. Many have a brief lifespan, undergoing division (a process called mitosis) in a period of hours, while others (such as nerve cells) do not divide at all after birth. Mitosis represents production of the chromosome, on which the genes containing all the genetic information necessary for cell function resides. Any alteration of the genetic information, or of the processes associated with mitosis can result in either a permanent change in the nature of the cell (mutation), or in the cell's death. When a cellular component is damaged by any agent (chemicals, radiation, excessive heat, etc.), a multitude of measurable effects can result. The changes may initially be restricted to a single or a few types of cells. In time, whole organs or organ systems may be affected due to the absence of a required function that upsets the equilibrium or control of the whole interrelated system. Gross physiological or morphological changes may result from an initial damage to a sufficient number of many kinds of cells. The type of cell damage will depend upon what the specific agent is that the cell is exposed to, and the amount of damage will be related to how much of the agent reaches that particular kind of cell. Biological effects from radiation are produced as a result of the transfer of energy from the radiation to the cells through ionization and excitation as described in the next section.
Radiation passing through living cells causes ionization or excitation of atoms and molecules contained in the cell. Since most of the human body is water, water molecules are a likely target for being hit by photons or charged particles. The reaction which occurs when this happens is an ionization to form a positive ion and an electron:
H2O ----> H2O+ + e-
and the H2O+ is rapidly hydrated to form:
H2O+ + H2O ----> H2O+ OH·
Here the OH× is a "free radical", a species that contains an unpaired orbital electron, and is highly reactive chemically. The free electron will also react with a water molecule (after it slows down from bumping into other molecules) to yield another free radical, this time hydrogen:
e- + H2O+ ---> OH- + H·
The overall reaction is thus:
H2O ---> H· + OH-
with the products separated by a considerable distance so that immediate back reactions to form water are not favored. Such radicals can combine with each other and with dissolved oxygen to give a variety of potent oxidizing agents such as hydrogen peroxide, superoxide, molecular oxygen and the perhydroxy radical.
Both the initial radicals and these products can migrate to biologically important molecules (like DNA - the structural material of genes) and cause bond breakage and/or oxidation of attached groups. In this way, energy of the radiation is transferred to biologically significant molecules, changing their structure. This mode of energy-transfer is known as the Indirect Effect and can account for an appreciable fraction of damage. Note that the presence of oxygen can magnify this pathway due to additional radical formation.
In addition to the indirect effect, radiation may itself cause ionization in DNA or other biological molecules. The energy of ionization is far greater than the bond energy in organic molecules, thus causing bond breakage. The amount of this Direct Effect occurring depends on the number of a particular type of molecule in the cell, and its size. The larger the molecule, the better target it makes. Since DNA is the largest molecule in the cell as well as the site of all the genetic information, its response has a central role in the mediation of radiation effects.
Depending on how it is damaged, different results will occur. If the damage results in a strand break in its back bone (breaking the molecule in half), subsequent mitoses may fail resulting in cellular death. If the break is in one of its side groups (bases), it will then transmit different genetic data during subsequent division resulting in some kind of a mutation. Both direct and indirect effects contribute to the overall number of such damaging events to the DNA and will vary for individual cell types.
The radiosensitivity of a particular cell depends on a number of factors. An early observation of this difference is reflected in the "Law of Bergonie and Tribondeau" which states "the radiosensitivity of a tissue is directly proportional to the reproductive activity and inversely proportional to the degree of differentiation". Tissues consisting of rapidly dividing stem cells (like blood or sperm cell precursors) are quite sensitive to radiation whereas cells that do not divide or only rarely divide (like nerve or muscle cells) are considerably more resistant. From microscopic examination, cells appear to get stuck in the division process after radiation exposure, which is consistent with the "Law" above. Other factors involved include metabolic rate, state of nourishment, oxygen level and presence of particular enzymes within the cell. The latter are most likely involved with the repair of some of the radiation damage.
The following table gives a summary of how various cells, tissues, organs and organ systems are affected by radiation. The doses reported are for X or gamma rays only and represent a single, acute exposure.
| Type | Biological Response |
| -Extremely Radiosensitive - | |
| Blood-forming Organs lymph nodes, thymus, spleen, bone marrow | Exposures as low as 50 rad can affect the white cell population within 15 minutes. Red cell count falls 2 to 3 weeks later. Results in a feeling of general weakness, anemia, and a lower resistance to infection. |
| - Moderately Radiosensitive - | |
| Reproductive Organs female, male | Exposures below 100 rad can reduce fertility. Temporary sterility can occur lasting 12 to 15 months following 200-300 rad. On the average, a larger exposure is needed to produce sterility in the male than in the female. Damage to the germ cells can lead to somatic and/or hereditary changes. |
| Radiosensitive - | |
| Digestive Organs small intestine,lower intestine,pharynx,esophagus | Degenerative changes occur as soon as 30 minutes after exposure of 500-1000 rad. Initial effects are: impaired secretion of necessary fluids. Cell breakdown results in failure of food and water absorption leading to infection and dehydration from diarrhea. |
| - Moderately Radioresistant - | |
| Vascular System arteries (lg & sm) capillaries, heart, | Sensitivity varies for the vascular system. Damage is great only in the 600-1500 rad range. This damage by radiation veins contributes to some of the changes in other tissues. |
| - Radioresistant - | |
| Skin | Exposures between 500-1000 rad can produce skin changes. However, as little as 100 rad can cause cell death in the germinal layer. |
| Bone and Teeth | Some parts of bone can be damaged by 700-1500 rad. Regeneration can begin 2 to 6 weeks after exposure. |
| - Relatively Radioresistant - | |
| Respiratory System | Inflammation of the lungs can occur at 1000-2000 rad. Possible hemorrhaging due to changes produced in blood vessels. |
| Urinary System | Secondary effects can show up years after exposure in the 500-2000 rad range due to changes in blood vessels. |
| - Very Radioresistant - | |
| Muscle and Connective Tissues | Massive exposures (over 2000 rad) are needed to cause slight changes in these tissues. |
| - Extremely Radioresistant - | |
| Nervous Tissue | Massive exposures are required (over 3000 rad) to bring about morphological changes in these tissues. |
The most radiation-sensitive state of any individual is during embryonic development. If irradiated at a time when a particular tissue or organ is being differentiated, exposures as small as 25- 50 rad can lead to gross malformations. In humans, this corresponds to 2-6 weeks of gestation. This sensitivity is due to the presence of only few cells at this stage which ultimately will give rise to a particular tissue or organ. If these are destroyed, other cells cannot replace them.
Biological effects of radiation may be subdivided into two categories; genetic effects and somatic effects.
Somatic effects may be further subdivided into two groups: short-term or acute effects, and long term or latent effects.
Short term effects arise from large acute exposures in excess of about 100 rad, and are observed in a few days or weeks after exposure. These effects may be characterized by the following features:
Death, radiation sickness, prodromal response, central nervous system death, response of the skin and acute radiation syndrome are some of the examples where these characteristics can be observed.
Lethal effects are observed in mammals within a period of 30 days from acute exposures in the few-hundred rad range. Acute exposure refers to delivery of radiation dose in a short time period, generally within minutes. Expression of this response is known as the LD50/30 or the dose which yields 50% lethality in an irradiated group measured at 30 days. At doses appreciably below the LD50/30, very little lethality occurs; whereas at doses appreciably above, 100% lethality occurs.
Acute Lethal Responses Species RAD guinea pig 175-409 dog 350 goat 350 man 350-450 mouse 550 rat 590-970 monkey 600 rabbit 800 fowl 1000 goldfish 2300The ranges shown represent an uncertainty only in the case of man, where precise experimental data does not exist. Other ranges represent a difference depending on the particular strain of the species used. The cause of death at the LD50/30 is due to response of the blood forming organs. Interestingly, at the tissue level, a given dose yields about the same observable damage in any species. Some species, however, are better able to cope with the damage and so survive. When organisms are exposed at or above the acute LD50/30 value, characteristic physiological responses are seen. These responses are known as "radiation sickness" and "acute radiation syndrome". The following tables illustrate the symptoms and their timing from various whole-body dosages.
Acute Dose Probable Effect
( rad )
0 - 50 No obvious effect, except possibly minor blood changes.
80 - 120 Vomiting and nausea for about 1 day in 5 to 10 percent of exposed
personnel. Fatigue but no serious disability.
130 - 170 Vomiting and nausea for about 1 day, followed by other symptoms of
radiation sickness in about 25 percent of personnel. No deaths
anticipated.
180 - 220 Vomiting and nausea for about 1 day followed by other symptoms of
radiation sickness in about 50 percent of personnel. No deaths
anticipated.
270 - 330 Vomiting and nausea in nearly all personnel on first day, followed
by other symptoms of radiation sickness. About 20 percent deaths
within 2 to 6 weeks after exposure; survivors convalescent for about 3 months.
400 - 500 Vomiting and nausea in all personnel on first day, followed by
other symptoms of radiation sickness. About 50 percent deaths within 1
month; survivors convalescent for about 6 months.
550 - 750 Vomiting and nausea in all personnel within 4 hours from exposure,
followed by other symptoms of radiation sickness. Up to 100 percent
deaths; few survivors convalescent for about 6 months.
1000 Vomiting and nausea in all personnel within 1 to 2 hours. Probably no
survivors from radiation sickness.
5000 Incapacitation almost immediately. All personnel will be fatalities
within 1 week.
Time after Survival improbable Survival possible Survival probable
exposure (700R or more) (550R to 300R) (250R to 100R)
--------------------------------------------------------------------------------------------
Nausea, vomiting, Nausea, vomiting, Possibly nausea, and
diarrhea in first diarrhea in first diarrhea on first day.
few hours. few hours.
------------------------------------------------------------------------------
1st week No definite symptoms
in some cases(latent
period).
-------------------------
-------------- Diarrhea No definite symptoms
Hemorrhage (Latent Period).
Purpura
Inflammation of
mouth and throat No definite symptoms fever
2nd Week Rapid emaciation ----------------------- (Latent Period)
Death Epilation
(Mortality probably Loss of appetite
100 percent). and general malaise
-------------- Fever
Hemorrhage ----------------------------
Epilation
Purpura Loss of appetite and
Petechiae malaise
3rd week Nosebleeds Hemorrhage
Inflammation of Petechiae
mouth and throat Pallor
Diarrhea Sore throat
-------------- Emaciation Moderate emaciation.
4th week Death in most Recovery likely in
serious cases about 3 months unless
(Mortality 50% for complicated by poor
450 R). previous health or
superimposed injuries
--------------------------------------------------------------------------------------------
The effects of Nuclear Weapons U.S. Government Printing Office, May 1957
Response Dose,rad Syndrome
Hematopoetic 700 Death in 10-21 days caused by
Death to blood changes resulting in
1000 infection or hemorrhaging.
Gastro-intestinal 1000 Death in 4-7 days. Nausea,
Death to vomiting and diarrhea; food
10000 and water intake depressed.
Death by severe morphological
changes in gastrointestinal
tract.
Central Nervous 10000 Death within 2 days. Minutes
System Death to after exposure disorientation,
100000 incoordination and semiconsciousness
develops. Coma and death occurs
from central nervous system damage.
Molecular Death over Immediate death. Death caused
100000 by inactivation of substances
required for basic metabolic
processes.
Reddening of the skin was the first biological response to radiation noted in man; it still seems to be the most frequently observed injury in the sublethal range. The biological response of the skin, in order of increasing severity is:
The total radiation exposure, the length of exposure time, radiation quality, the distribution of dose in the irradiated tissue and the region of the body exposed are all the variables that influence the progression and severity of radiation injury. The clinical observations of skin erythema in man may be summarized as follows:
Radiation, given either acutely or chronically, increases the incidence of a number of conditions observable from 5-20 years after the exposure was delivered. None of these responses are unique to radiation exposure, they occur with some normal incidence in the general population, but are increased in frequency in irradiated populations. The following have been shown to be associated with radiation:
The above late effects can only be predicted for large populations. For an individual in an irradiated group, the exact cause of death cannot be identified, either natural or from one of the many environmental agents capable of producing the same effect. That is because each agent will contribute to the risk in proportion to its amount and effectiveness, as well as factors related to the genetic resistance or sensitivity of the individual exposed.
If a given radiation exposure is delivered over a longer time period, the effect observed is less. Experiments utilizing the "split-dose" technique have shown that radiation damage is repaired by the organism as long as any single exposure is less than the LD50/30. For example, if animals are given one-half of the LD50/30 (called a "conditioning dose") followed some time later by another equal dose (called the "test dose") with sufficient separation of the two doses (say, a few weeks), the animals will survive. If no time elapses between them, death occurs within 30 days. Spreading the dose over weeks or months at a low rate reduces the effect appreciably. For the induction of mutations in mice, the mutation yield for chronic exposure is about half that for acute exposure. Many other responses appear to follow this reduction in effectiveness under chronic exposure conditions.
Studies have compared the projected loss of life expectancy resulting from exposure to radiation with other health risks. Estimates are calculated by looking at large group of individuals, recording the age at which death occurs from apparent causes, and estimating the number of days of life lost as a result of these early deaths. The total number of days of life lost is then averaged over the total group observed.
Estimate of Dats of Health Risk Life Expectancy Lost Smoking 20 cigarettes/day 2370 (6.5yr) Overweight by 20% 985 (2.7yr) Auto accidents 200 5 rems/year for 30 years(cal) 150 Alcohol consumption (US av) 130 Home accidents 95 Safest jobs (such as teaching) 30 1 rem/year for 30 years (cal) 30 Natural background radiation 8 Medical X-rays 6 Natural disasters 3.5 1 Rem occupational dose(cal) 1 -------------------------------------------------- Adapted from USNRC Regulatory Guide 8.29
These estimates illustrate that health risks from occupational radiation exposure are of the same order of magnitude as risks that we have historically encountered in normal day-to-day activities. Exposure to radiation should be considered in this perspective when considering its risk. As long as radiation exposure is kept at a value where its contribution to risk is a small part of the total sum of all risks, then it should not be of major concern.
Soon after the discovery of X-rays and radium, the dangers of radiation exposure became well known. Standard setting organizations like the International Council on Radiation Protection (ICRP) and the National Council on Radiation Protection and Measurements (NCRP) were formed to recommend limits on the exposure of radiation. Prior to 1928, the radiation exposure limit was based on the amount of radiation needed to produce reddening of the skin (erythema). When the Roentgen (R) was defined in 1928, this "erythema exposure" was calculated to range from 0.04 R - 2 R per day. In 1935, the NCRP's first recommendation for exposure limitation was 0.1 R/day (31 R/year). This was an arbitrary limit, based on no observable effects of three technicians' exposure to radium gamma rays. In 1949, the NCRP reduced the limit to 0.05 R/day (0.3 R/week; 15 R/year) because radiations then being used were more penetrating. A major revision adopted by both the NCRP and ICRP took place in 1957 and will be in effect until January 1, 1994. This limit allowed an individual to receive up to 3 Rem in 13 consecutive weeks, provided that the accumulated dose does not exceed 5(N-18) rem, where N is the individual's age. The latest revision, was completed in 1991, and will be effective on January 1, 1994 and eliminates what has been called a "bank" of available dose before exceeding the 5(N-18) dose.
Occupationally exposed individuals are allowed higher radiation exposures than the general population for the following reasons:
Current State and Federal guidelines describe the radiation exposure limits to an occupational radiation worker as follows:
1) Total whole body exposures: 5 rems/yr
(external + internal exposures)
2) Any individual organ or tissue: 50 rems/yr
other than the lens of the eye
(external + internal)
3) Lens of the eye: 15 rems/yr
4) Skin or extremity: 50 rems/yr
(shallow dose)
Major Organs and Thyroid Gland: 15 rem/yr
Fetus: 0.5 rem
The dose limit to the whole body for non-radiation workers, in addition to natural and medical sources is 0.1 rem/year. The dose limit to the whole body for the U.S. population from all sources of radiation other than natural and medical sources is 0.1 rem/year per person.
The population as a whole is exposed to radiation whether it's from naturally occurring radioactivity present in the earth, inter- stellar space, medical sources, or from radioactivity contained in consumer products.
Naturally occurring radiation arises from three sources: cosmic rays entering the earth's atmosphere, naturally occurring radioactive materials in the earth's crust, and naturally occurring radioactive materials within the body.
Primordial nuclides are those that are long lived and have existed in the earth's crust throughout history. The main contributors to external exposure from primordial nuclides are K-40, U-238, and Th-232, and their decay products. The concentrations of primordial nuclides in soil are dependent on the process by which the soil was formed. The table below shows the typical activity of these nuclides in various types of rock:
Typical Activity Concentration (pCi/gm) Absorbed dose rate in air Type of Rock K-40 U-238 Th-232 (µrad/hr) Igneous Acidic (e.g. granite) 27 1.6 2.2 12 Intermediate (e.g. diorite) 19 0.62 0.88 6.2 Mafic (e.g. basalt) 6.5 0.31 0.30 2.3 Ultrabasic (e.g. durite) 4.0 0.01 0.66 2.3 Sedimentary Limestone 2.4 0.75 0.19 2.0 Carbonate --- 0.72 0.21 1.7 Sandstone 10 0.5 0.3 3.2 Shale 19 1.2 1.2 7.9 Source: UNSCEAR 1977 Report
In various parts of the world, there are areas with high natural radiation levels. At the beach of the Black Sands in Guarppari, State of Espirto Santos, Brazil, it is possible to receive a radiation exposure of 5 mrad/hr due to the monazite (Thorium bearing minerals) sands. At Pocos de Caldas, State of Gerais, Brazil, the average range of radiation exposure is 0.1 - 3 mrad/hr.
Naturally occurring radionuclides can give rise to external doses when contained in raw materials used to construct roads and buildings. Uranium and thorium are commonly found in cement, concrete blocks, and masonry products. For example, the possible annual dose near a granite wall at the "Redcap Stand" in Grand Central Station, New York is 200 mrem (assuming an occupancy of 8 hrs/day).
Naturally occurring radionuclides enter the body through inhalation and ingestion. Of the cosmogonic nuclides only H-3, C-14, and Na-22 contribute to internal exposure. The major contribution to internal exposure from primordial nuclides are K-40 and the decay products of the uranium and thorium series.
The following table summarizes the estimated annual tissue absorbed dose from natural sources:
mrad
Source of Irradiation Gonads Lungs
External Irradiation
Cosmic Rays:
Ionizing component 28 28
Neutron component 0.35 0.35
Terrestrial Radiation: (g) 32 32
Internal Irradiation
Cosmogonic radionuclides:
H-3 (b) 0.001 0.001
Be-7 (g) ----- 0.002
C-14 (b) 0.5 0.6
Na-22 (b+g) 0.02 0.02
Primordial radionuclides:
K-40 (b+g) 15 17
Rb-87 (b) 0.8 0.4
U-238, U-234 (a) 0.04 0.04
Th-230 (a) 0.004 0.04
Ra-226, Po-214 (a) 0.03 0.03
Pb-210, Po-210 (a+b) 0.6 0.3
Rn-222, Po-214 (a) inhalation 0.2 30
Th-232 (a) 0.004 0.04
Ra-228, Tl-208 (a) 0.06 0.06
Rn-220, Tl-208 (a) inhalation 0.008 4
Total (rounded) 78 110
Source: UNSCEAR 1977 ReportTechnologically enhanced exposure to natural radiation is defined as exposure to natural radiation to which man would not be exposed if some kind of technology had not been developed. For example, travel by air, using natural gas for cooking or heating, and living near a coal fired power plant increase an individual's exposure to naturally occurring radiations. Air travel increases the exposure due to cosmic rays and solar flares when flying at high altitudes. The following table shows calculated doses for various routes:
Subsonic Flight Supersonic Flight at 11 km at 19 km Dose per per Flight round Flight round duration trip duration trip Route (hr) (mrad) (hr) (mrad) Los Angles-Paris 11.1 4.0 3.8 3.7 Chicago-Paris 8.3 3.6 2.8 2.6 New York-Paris 7.4 3.1 2.6 2.4 New York-London 7.0 2.9 2.4 2.2 Los Angles-New York 5.2 1.9 1.9 1.3 Sydney-Acapulco 17.4 4.4 6.2 2.1 Source: UNSCEAR 1977 Report
The table below shows the doses received by astronauts on various space missions. The largest part of the dose was received when the spacecraft passed through the earth's radiation belts. The belts contain protons, electrons, and alpha particles trapped by the earth's magnetic fields.
Mission or Launch Date Duration of Type of Orbit Dose Mission Series (Yr-Mo-Dy) Mission (Hr) (mrad) Apollo VII 68-08-11 260 Earth Orbital 157 Apollo VIII 68-12-21 147 Circumlunar 150 Apollo IX 69-02-03 241 Earth Orbital 196 Apollo X 69-05-18 192 Circumlunar 480 Vostok 18-6 Earth Orbital 2-80 Voskhad 1,2 Earth Orbital 30,70 Soyuz 3-9 Earth Orbital 62-234 Source: UNSCEAR 1977 Report
Individuals living around coal-fired power plants are exposed to enhanced levels of Ra-226, Ra-228, U-238, Th-228, Th-232, and K-40 from gaseous and particulate combustion products of coal. The major contribution to the dose is from the alpha radiation of Pb-210, Th-228, and Th-232.
Phosphate products contain high concentrations of the nuclides in the U-238 decay series. About 1/2 of the phosphate rock that is mined is converted into fertilizer, the rest goes into commodities such as phosphoric acid, gypsum, and land fills. Thus, the use of phosphate fertilizers result in radiation exposures from the following:
Radiation exposure from consumer products are considered "Technologically Enhanced" since the radioactive material is deliberately incorporated into the product to serve a specific purpose.
The following tables describe various consumer products containing radioactive materials and some annual population dose rates:
Product Nuclides Amount
--------------------------------------------------------
Radioactive Material Contained in Paint or Plastic:
Time Pieces H-3 1-25 mCi
Pm-147 65-200 µCi
Ra-226 0.1-3 µCi
Compasses H-3 5-50 mCi
Pm-147 10 µCi
Thermostat Dials and Pointers H-3 25 mCi
Automobile Shift Quadrants H-3 25 mCi
Speedometers Pm-147 0.1 mCi
Radioactive Material Contained in Sealed Tubes:
Time pieces, marine navigational
instruments H-3 0.2-2 Ci
Exit signs, stepmarkers, public
telephone dials, light switch
markers H-3 0.2-30 Ci
Electronic and Electrical Devices:
Fluorescent lamp starters Ra-226 1 µCi
Vacuum tubes, electric lamps,
germicidal lamps Natural
Thorium 50 mg
Glow lamps H-3 0.01 mCi
High voltage protection devices Pm-147 3 µCi
Low voltage fuses Pm-147 3 µCi
Miscellaneous:
Smoke and fire detectors Am-241 1-100 µCi
Ra-226 0.01-15 µCi
Kr-85 7 mCi
Incandescent gas mantles Natural
Thorium 0.5 gm
Ceramic tableware glaze Natural
Uranium 20% by
or weight of
Thorium the glaze
Adapted from UNSCEAR 1977 Report
Product mrem TV Receivers 0.50 Airport X-Ray 0.001 Luminous Watches 0.05 Tobacco Products 2000.00 Coal Combustion 1.00 Natural Gas Combustion 5.00 Uranium in Dentures 10000.00 Adapted from NCRP Report No. 56
Technique mrad Sacral Spine 2180 Barium Enema 1320 Upper GI Series 710 Dental Bite-Wing 400 Skull 330 Chest 44Source: Bureau of Radiological Health
The table below summarizes the annual dose rates received from natural background, medical and other sources of radiation. The values indicated are averages and may vary slightly with other reported values:
Annual Dose Rates to Population in USA BEIR III (1980)
Natural Background mrem/yr
Cosmic 28
Terrestrial 26
Internal - C-14, Ra-226, Pm-222, K-40 28
82
Medical
Diagnosis 77
Dental 1.4
Radiopharmaceutical 13.6
92
Other
Weapon Tests (Fallout) 5
Power Plant and Nuclear Industry < 1
Building Materials (brick, masonry) 5
TV Receivers 0.5
Airline Travel 0.5
12
Total 186 mrem/yr
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Radiation Detection and Measurement
The instruments used for measurement of radiation from accelerators can be subdivided into two groups; survey instruments and personnel monitoring instruments. The following sections describe these types in detail.
The major principle for sensing and measuring radiations in survey instruments is based on the ionizations radiation produces when interacting in a gas filled detector. Radiation passing through matter creates ion pairs. These ion pairs are in turn, collected to form an electrical signal through the use of an electric field. The signal, either a current or a pulse, is then used to register the presence or amount of radiation. There are a number of different types of radiation detectors, each operating on this basic principle, but designed for specific purposes. The three major types of portable radiation survey instruments, the Ion Chamber, Geiger Counter and a Neutron Detector are discussed below.
Basic Diagram of a Gas Filled Detector
Ionization produced in the gas converts neutral molecules to positive ions and electrons within the sensitive volume. This volume is contained between charged electrodes; one positive and the other negative. The charged species are collected at the electrodes of opposite sign.
Either a photon (X or gamma ray), producing primary electrons along its path, or a particle (alpha or beta) producing secondary electrons, will create ions that will travel to the electrodes and be collected. A sufficient potential must be applied across the electrodes to prevent ion recombination and make collection possible.
As the ions are collected, a current will flow. This will be measured on a sensitive measuring circuit "C" shown in the diagram above. Alternatively, the current may be measured as a pulse by a pulse counter "P" from the collection of each primary particle.
Radiation instruments are designed with specific purposes in mind. The instrument selected depends on particular needs of an individual. Generally, Geiger Counters are more sensitive than Ion Chambers and can monitor low levels of contamination in the laboratory. For measurement of radiation levels in the laboratory, the Ion Chamber is the proper instrument to use. Each instrument comes with an operating manual that describes its function and limitations such as warm up time, battery life, operating temperature range, minimum sensitivities, etc. Outlined as follows are simple instructions on the proper use of portable radiation survey instruments.
Read the instrument's operating manual. Gain familiarity with the controls and operating characteristics.Check the batteries. Most instruments have a battery check indicator. Replace weak batteries. Turn off the instrument when not in use. When storing the instrument for extended periods, remove the batteries to prevent damage from battery acid leakage.
Check the operability of the detector. Pass the detector over a radioactive check source (sometimes attached to the side or end of the instrument) to verify that the detector responds to radiation.
Determine the instrument's response time. By passing the detector at varying speeds over a check source, you can determine how long it takes for the detector to respond to the radiation.
Determine the operating background. Note the instrument's response in an area free of radiation levels. Subtract this background value from the "gross" reading to obtain the "net" results in the radiation area: Snet = Sgross - Sbackground.
When using portable instruments, caution should be used in extending the detector cord as this may generate electrical noise and register as "counts".
Pocket dosimeters are small devices (about the size of a marking pen) that can be carried in a shirt or lab coat pocket to record exposure to radiation. The dosimeter is set to zero prior to use by a separate battery or AC line operated charging device. When radiation passes through the sensitive volume of the dosimeter, the charge is dissipated in proportion to the amount of radiation received. "Self reading" dosimeters have an optical system to allow the wearer to view the amount of radiation received by looking through the dosimeter like a telescope. "Indirect reading" dosimeters require a separate readout device (which also serves as the dosimeter charger). Several exposure ranges are available, the most common being from 0 to 200 mr.
The advantage of a pocket dosimeter is that it can provide an on-the-spot result of an individual's exposure to radiation. However, pocket dosimeters are susceptible to erroneous readings when exposed to excessive moisture, dust, or physical abuse. In each case, the dosimeter will read high. For this reason, two dosimeters are usually worn for periods of one day or less. The lower reading dosimeter is considered to be the more accurate. Another disadvantage is the dosimeter's limited exposure range. If the dosimeter is exposed to radiation beyond its range, then the total exposure received cannot be determined.
The badge holder contains filters that allow different radiation types (beta, X, gamma, neutron) and energies to be distinguished on the film. An "open" window (i.e., no filter) allows all radiations of sufficient energy to pass and expose the film. A plastic filter absorbs most low energy beta radiation. Other filters such as copper or lead absorb most high energy beta radiation and all but high energy gamma radiation. Fast neutrons interact with a cadmium filter to produce film blackening. Slow neutrons interact with the nitrogen atoms in the film's gelatin layer and the resulting proton tracks are counted.
Advantages of film badges are:
Disadvantages are:
To eliminate this latter disadvantage, pocket dosimeters can be worn along with film badges. If the pocket dosimeter indicates a possible high exposure, the film badge can be evaluated on an emergency basis, usually within twenty-four hours after the receipt by the vendor.
Advantages of TLDs are:
Disadvantages are:
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The three basic methods used to reduce the external radiation hazard are time, distance, and shielding. Good radiation protection practices require optimization of these fundamental techniques.
The amount of dose an individual accumulates will depend on how long the individual stays in the radiation field:
Dose = Dose Rate x Time
mrem = mrem/hr x hr
Therefore, to limit a person's dose, one can restrict the time spent in the area. The length of time a person can stay in an area without exceeding a prescribed limit is called the "stay time" and is calculated from the simple relationship:
Stay Time = Limit (mrem)
Dose Rate (mrem/hr)
Example: How long can a radiation worker stay in a 1.5 rem/hr radiation field if we wish to limit a dose to 100 mrem?
Stay Time = 100 mrem = 0.667 hr = 4 minutes
1500 mrem/hr
The amount of dose an individual receives will also depend on the distance between the person and the source.
The Inverse Square Law - Point sources of X and gamma radiation follow the inverse square law, which states that the intensity of the radiation decreases in proportion to the inverse of the distance squared:
I proportional to 1/d^2
To represent this in a more useful formula:
I proportional to 1/d^2; I= (1/d²)
I1 = K(1/d1²) Therfore,
I2 = K(1/d2²)
K
I1 = d1²
I2 K
d2²
I1 = d2²
I2 d1²
I1d1² = I2d2²
I1 I2
source* ----------|--------|
--------->| |
I1 = the radiation intensity at distance d1 from the radiation source.
d1 = the shorter distance from the source where the radiation intensity is I1.
I2 = the radiation intensity at distance d2 from the radiation source.
d2 = the longer distance from the source where the radiation intensity is I2.
Therefore, by knowing the intensity at one distance, one can find the intensity at any given distance.
Example: The exposure rate one foot from a source is 500 mrem/hr. What would be the exposure rate three feet from the source?
I1 = 500 mrem/hr
d1 = 1 foot
d2 = 3 feet
I2 = I2d2² = (500 mrem/hr)(1 Foot)²= 500 mrem/hr= 55.6 mrem/hr
d2² (3 Foot)² 9
1) Charged particles
Low-Z materials such as Al, or ordinary concrete are sufficient to minimize the production of x-rays as a result of electron interaction with matter. High-Z materials such as tantalum or platinum are used as ion-beam stoppers and for minimizing neutron production at energies below 5 Mev.
2) X and Gamma Radiation
Monoenergetic X or gamma rays collimated into a narrow beam are attenuated exponentially through a shield according to the following equation:
I=Ioe-µx
Where:I is the intensity outside of a shield of thickness x Io is the unshielded intensity
µ is the linear attenuation coefficient
x is the thickness of shielding material
The linear attenuation coefficient is the sum of the probabilities of interaction per unit path length by each of the three scattering and absorption processes; photoelectric effect, compton effect, and pair production. Note that µ has dimensions of inverse length. The reciprocal of µ is defined as the mean free path which is the average distance the photon travels in an absorber before an interaction takes place. Because linear attenuation coefficients are proportional to the absorber density, which usually does not have a unique value but depends somewhat on the physical state of the material, it is customary to use "mass attenuation coefficients" which remove density dependence:
Mass attenuation coefficient µm = µ/For a given photon energy, µm does not change with the physical state of a given absorber. For example, it is the same for water whether present in liquid or solid f rm. If the absorber thickness is in cm, then µm will have units of:
cm-¹ which = cm²/gm
gm/cm³
Values of the mass attenuation coefficient for lead are given in Appendix IV.
Example: The intensity of a point source is 1 rad/hr. What would be the intensity on the outside of a 2 inches thick lead shield? Density of lead: 11.35 gm/cm³
I = Ioe-µx
Io = 1 rad/hr
x = 2 inches x 2.54 cm/inch = 5.08 cm
I = (1 rad/hr) x e-[(1.29 cm-¹)(5.08 cm)] = 0.0014 rad/hr = 1.4 mrad/hr
3) Buildup Factor
In case of thicker shields, a phenomenon of buildup from scattering exists which must be accounted for. The thicker and taller the shield, the larger the build up of scatter component. Also, the energy of the source affects the contribution of the scatter factor to the exposure rate. Thus, for a thick shield, we insert a build-up factor in the above equation:
I = BxIoe(-µx)
where Bx is the buildup factor
In case of primary x-rays, the shielding calculations are conducted by the formula:
Bx = 1.67 x 10-5 Hd² (1)
DT
Bx = shielding transmissionThe value of Bx obtained is related to shielding thicknesses in terms of the number of tenth-value layers of the shielding material that is required to reduce the radiatio levels to dose- limit values. A tenth-value layer (n) is that thickness through which the x-ray dose equivalent is reduced by a factor of 10. Once the tenth-value lay r is known, the s ielding-barrier thickness, S, can be estimated pretty conservatively. Thus
H = max. permissible dose equiv. (mrem/h)
d = dist. between x-ray source and ref. pt (m)
D = abs. dose index rate (rad m2/min)
T = area occupancy factor
Bx = 10-n (2)
S = T1 + (n-1)Te (3)
T1 = first tenth-value layer in shielding thickness
Te = subsequent tenth-value layer
Values of T1 and Te for concrete, steeland lead are obtained from plots in Appendix I.
Example: Calculate the concrete shielding-barrier thicknesses for the forward directed (0°) and sideward directed (90°) x-rays from 1-cm diameter, 3 MeV, 2 mA electron beam incident on a thick high Z (tungsten) target at a distance of 5 m from the barriers. Assume occupancy factor of 1.
Solution: From equation (1),
Bx(0°) = 4.7 x 10-7
Using the Appendix I, the corresponding tenth-value layers are obtained; T1 = 26 cm and Te = 23 cm.
From equation (2), the number of these layers are:
n = 6.33
thus, S = 26 + 5.33 x 23 = 149 cm or 59 in.
A concrete thickness of 167 cm (66in) is recommended. Similarly, we can determine the barrier thickness for sideward directed beam. Once the calculations are done, the S value is found to be S = 126 cm or 50".
Thus, a concrete thickness of 137 cm (54in) is recommended. An additional thickness equivalent to at least one-half value layer is recommended in the above calculations. (NCRP-51)
Dark current is produced when poor vacuum conditions exist or when accelerator vacuum components are being outgassed. The name "dark current" is analogous with the current observed through a photo tube in the absence of light. When the machine voltage or radio frequency generating system is turned on, the dark currents of electrons are accelerated even if the electron source is not turned on. Electron linear accelerators and direct accelerators are prone to this phenomenon during warm-up or conditioning of accelerator components. In direct accelerator, radiation may be emitted due to capacitance until the charge has been brought to zero.
the process of skyshine occurs. Skyshine is not a problem with the accelerators that are provided with adequate roof shielding. Wall shielding, in the absence of roof shielding, will reduce radiation levels near shield wall, but the radiation levels further away may be high because of the skyshine. It should not be assumed that if radiation doses are all within acceptable limits close to the shield of an accelerator, that they are always lower further away. A survey made of the area close to the shield wall and further away from the wall may indicate the need for additional roof shielding.
If the beam strikes any material in the accelerator or if the material is exposed to intense secondary radiation, it will become radioactive. The beam in electron accelerator usually strikes other objects such as vacuum chamber walls and electrode supports. Radiation from these areas or sources does not become a personnel hazard until the machine is turned off and personnel enter these areas for accelerator maintenance, target changes or outine adjustments. Activation of cooling water and other cooling media in targets may be a problem. The major concern is the residual activity present on the water system. It may be hazardous during maintenance work after the accelerator is shut down. This means that shielding might be required around circulating pumps, heat exchanger and holding tanks. Surveys of activation of several locations around the accelerator should be made after each major beam current or energy change. The results obtained from these surveys can be helpful for maintenance work. The most radioactive parts of the accelerator must be removed and shielded.
These hazards are controlled by the installation of a separate ventilation system. The ventilation system is designed so that the accelerator room and the irradiation room are at lower pressure than other parts of the building. The airborne radioactive concentrations in the hazardous areas should be monitored with the beam on and off.
When particle accelerators are used for the production of intense beams of neutrons by bombarding with deuterium, tritium gas is released from the target. This tritium outgassing is the major cause of tritium contamination within the beam tube assembly, vacuum system and exhaust system. Tritium is usually present as tritium gas, tritium oxide or a mixture of both gas and oxide. It is not an external hazard due to its low range about 0.6 mg/cm2, however once inside the body there is no protection to the living tissues. With tritium in the body fluids, the entire energy may be absorbed within a single cell nucleus. It is eliminated by the body with an effective half-life of approx. 12 days. Tritium is not produced in significant amounts by accelerator. It is present only as a material absorbed in a tritium target or otherwise.
Interlocks are electrical systems which are used to turn off electrical power in hazardous situations. The most important part of an accelerator interlock system is the one that deals with radiation hazards. The radiation interlock system serves two functions: to prevent access to areas in which unsafe radiation levels are being produced by the accelerator; and to prevent operation of the accelerator if unsafe radiation levels will be produced in occupiable areas. This is accomplished by installing radiation detection and monitoring instruments in the control room and other occupiable areas. If the permissible levels have exceeded, the instruments turn off the beam and give a warning.
A part of the interlock system deals with extreme electrical hazards to personnel, particularly high-voltage DC power supples. Interlock switches on the access doors to high- voltage compartments interrupt the input voltage to the compartment when the doors are opened, and discharge the capacitors.
One or more disabling switches in target rooms or experimental rooms are usually installed in case a person gets trapped in these areas and can turn off the accelerator. Also, in most accelerator facilities before the beam can be turned on, audible and light warnings are produced by the control circuit.
The last point to remember is that the interlocks are not to be used as a means of shutting down the accelerator. The accelerator should be turned on/off only and only from the control console.
Caution - Radiation Area: In areas where the level of radiation could cause a major portion of an individual's body to receive an exposure from external radiation that exceeds 5 mrem/hr at 30 cm from source or container. "Radiation Area" posting should be at the point of entrance to the area.
Caution - High Radiation Area or Danger - High Radiation Area: In areas where the level of radiation could cause a major portion of an individual's body to receive an exposure from external radiation that exceeds 100 mrem/hr at 30 cm from source or container. "High Radiation Area" posting should be at the point of entrance to the area.
Grave Danger - Very High Radiation Area: In areas where the level of radiation could result in an individual receiving an absorbed dose in excess of 500 rad/hr at 1 meter from radiation source or from any surface that the radiation penetrates.
| Accelerator Type: | 3-MV Potential drop (industrial unit) |
| Date: | 11 December 1991 |
| Location: | Maryland |
| No. Individuals Exposed: | One |
| Esposure: | 0.40 - 0.13 Gy/s |
| Primary Causes: | Failure to follow established operating and safety procedures. Inadequate training in radiation safety and nuclear physics. |
| Description of Accident: The accident occurred during maintenance on the lower window pressure plate. The filament voltage of the electron source was turned "off", but the high voltage terminal was on full accelerating potential. The operator's body, especially his extremities and head were exposed to the electron dark current. Approximately 3 months after the accident, the victim's four digits on the right and left hand were amputated. | |
| Accelerator Type: | Linear Accelerator |
| Location: | University |
| No. of Individuals: | Two |
| Exposure: | Not stated |
| Primary Causes: | Failure to follow established operating procedures. Target area not completely barricaded. |
| Description of Accident: Two research assistants entered the target area of a LINAC without notifying the operator. They also failed to remove an interlock key and ignored warning lights, signs, and the sound of the accelerator klystron pulsars. One worker leaned over, adjusted the target by hand, and tried to make the alignment by eye. While aligning he noticed a bluish iridescence in his eyeglasses. Both realized what had happened and left target area immediately. No biological damage occurred. | |
| Accelerator: | Linear Accelerator |
| Location: | California |
| No. Individual Exposed: | Six graduate students |
| Exposure: | 920-2690 mrem - X-radiation |
| Description of Accident: Six graduate students entered a high radiation area by overriding the interlocks on the doors that should have prevented access to the area. These students were involved in the initial tune-up of the LINAC. During the tune-up of the RF phase of the LINAC operation, the students were exposed to x-radiation. | |
From the above listed incidents, it's obvious that the accidents and overexposures occurred as a result of human errors - bypassing interlocks, failure to follow established safety and operating procedures and lack of sufficient safety training. It is vital that all operators participate in emergency drills and safety training.
It is essential to anticipate the possibility of an overexposure and a clear procedure should be established should any overexposure occur. A set of emergency procedures listing phone numbers of Radiation Safety Office and the Principal Investigator should be posted at the console. In case of an overexposure, the following steps should be taken:
The above steps should be taken promptly even though symptoms of overexposure do not exist. Usually, some time must elapse before any indication of overexposure is observed.
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Radiation Protection Programs
A Radiation Safety Officer, and at the University of Maryland, a Radiation Safety Committee, has the responsibility to implement the radiation protection program. UM's program includes:
Routine and unannounced inspections of laboratories and other use areas for compliance with applicable rules and regulations are performed by radiation safety personnel. Those working with radioactive materials and radiation producing devices have the responsibility to report promptly to authorities any condition which may lead to or cause a violation of radiation safety regulations or cause unnecessary exposure to radiation or radioactive material. Thus, workers must be familiar with the conditions of their radiation producing device authorization, applicable State or Federal regulations.
Part Title
A General Provisions
Section A.1- Scope
Section A.2- Definitions
Section A.3- Exemptions
Section A.4- Records
Section A.5- Inspections
Section A.10- Prohibited Users
B Registration of Radiation Machine Facilities and Services
C Licensing of Radioactive Material
Section C.1- Purpose and Scope
Section C.3- Source Material
Section C.20- Types of Licenses
D Standards for Protection Against Radiation
Section D.1- Purpose and Scope
Section D.101- Radiation Protection Programs
Section D.201- Occupational Dose Limits for Adults
Section D.502- Conditions Requiring Individual Monitoring of External and
International Occupational Dose
Section D.901- Caution Signs
Section D.902- Posting Requirements
Section D.903- Exceptions from Posting Requirements
Section D.1205- Notification and Reports to Individuals
I Radiation Safety Requirements for Particle Accelerators
J Notices, Instructions and Reports to Workers and Inspections
The Safety Manual also includes a description of the administrative organization of the safety program and other useful information applicable to the safe use of radioactive materials on Campus. Each user must become familiar with the requirements in the appropriate sections of the manual.
The Radiation Safety Office staff performs periodic inspections of laboratories using radioisotopes and radiation installations to assure compliance with the safety manual, license and State Code. Violations of established rules, regulations and procedures may results in the loss of privilege to use radioactive material as well as cause an undue hazard to both the user and the people in the surrounding work area. Therefore, radiation safety can only succeed when each user follows both the spirit and actual rules described by the Radiation Safety Manual and this Training Manual and Study Guide.
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Cember, H., "Introduction to Health Physics", New York: Pergamon Press Inc., 1969. Second Edition, 1983.
Grosch, D.S. and Hopwood, L.E., "Biological Effects of Radiations", New York: Academic Press, 1979.
Joe, H.J., "Radiation Safety Technical Training Course:, Argonne, IL: Argonne National Lab (ANL-7291 Rev 1), 1972.
Knoll, G.F., "Radiation Detection and Measurement:, New York: John Wiley and Sons, 1979.
National Academy of Sciences, "The Effects on Populations of Exposure to Low Levels of Ionizing Radiation: (BEIR III Report), Washington, D.C.: National Academy Press, 1980.
National Council on Radiation Protection and Measurements (NCRP)) Reports, Washington D.C.:
# 39 Basic Radiation Protection Criteria, 1971.
# 45 Natural Background Radiation in the United States, 1975.
# 51 Radiation Protection Design Guidelines for 0.1 - 100 MeV particle Accelerator Facilities, 1977.
# 56 Radiation Exposure from Consumer Products and Miscellaneous Sources, 1977.
# 58 Handbook of Radioactivity Measurement Procedures, 1978, Second Edition, 1985.
Patterson, H.W.; Thomas, R.H; "Accelerator Health Physics", Academic Press, New York, 1973.
Schauer, D.A, et. al., "A Radiation Accident at an Industrial Accelerator Facility", Vol. 65, No. 2, pg. 131-140, Aug. 1993.
Snyder, W.S., Ford, M.R., Warner, G.G., Watson, S.B., "Absorbed Dose Per Unit Cumulated Activity for Selected Radionuclides and Organs", (MIRD Pamphlet No. 11) Maryville, TN.: MIRD Committee, 1975.
State of Maryland, "COMAR 26.12.01 - Ionization Radiation Protection".
Taylor, L.S., "Radiation Protection Standards", Cleveland, OH.: The Chemical Rubber Company, 1971
United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), "Sources and Effects of Ionizing Radiation", New York: 1977 Report to the General Assembly.
United States Department of Health, Education, And Welfare, "Particle Accelerator Safety Manual", October, 1968.
United States Public Health Service, Bureau of Radiological Health, "Radiological Health Handbook", US Government Printing Office, January, 1970.
United States Nuclear Regulatory Commission, "Rules and Regulations, Title 10, Chapter 1: Code of Federal Regulations, Part 20 - Standards for Protection Against Radiation:, US Government Printing Office, as amended. Return to the Table of Contents
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Rules of Thumb and Useful Equations
The average energy of a beta-ray spectrum is approximately one-third the maximum energy.
The range of beta particles in air is about 12 ft/MeV. Thus, the maximum range of P-32 is: 1.71 MeV x 12 ft/MeV = 20 Ft.
The dose rate in rads per hour in a solution by a beta emitter is 2.12 EC/r, where E is the average beta energy per disintegration in MeV, C is the concentration in microcuries per cubic centimeter, and r is the density of the medium in grams per cubic centimeter. The dose rate at the surface of the solution is one-half the value given by the relation. Example: For P-32 average energy of approximately 0.7 MeV, the dose rate from 1 µCi/cc (in water) is 1.48 rads/hr.
The surface dose rate through the nominal protective layer of skin from a uniform thin deposition of 1 µCi/cm² is about 9 rads/hour for energies above about 0.6 MeV.
For a point source of beta radiation (neglecting self and air absorption) of millicurie strength, the dose rate at 1 cm is approximately equal to 200 x mCi rads/hour and varies only slowly with beta energy. Example: The dose rate for 1 mCi P-32 at 1 cm is : 200 x 1 mCi = approximately 200 rads/hour at one centimeter.
For material with a half-life greater than six days, the change in activity in 24 hours will be less than 10%.
Useful Equations
Radioactive Decay
A = Aoe-gamma · t
Efficiency
Efficiency = observed cpm/actual dpm
Minimum Detectable Activity
MDA = Bkg cpm ÷ (3 x (Bkg)½/t) ÷ Eff = dpm
Stay Time
Stay time = limit (mR)/rate (mR/hr)
Inverse Square Law
I1d1² = I2d2²
Gamma Exposure Rate
R/hr at one foot = 6 C Sum of enReturn to the Table of Contents
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Reference Data for Selected Radioisotopes
----BETA---- ----Gamma----
Max At