1.1 INTRODUCTION
Health Physics, is that
area of environmental health engineering that deals with the protection of the
individual and population groups against the harmful effects of ionizing
radiation. The health Physicist is responsible for the safety aspects in the
design of processes, equipment, and facilities utilizing radiation sources, so
that radiation exposure to personnel will be minimized, and will at all times
be within acceptable limits; and he must keep personnel and the environment under
constant surveillance in order to ascertain that his designs are indeed
effective. If control measures are found to be ineffective, or if they break
down, he must be able to evaluate the degree of hazard, and to make
recommendations regarding remedial action.
The scientific and
engineering aspects of health physics are concerned mainly with;(l) the
physical measurements, of different types of radiation and radioactive
materials, (2) the establishment of quantitative relationships between
radiation exposure and biological damage, (3) the movement of radioactivity,
through the environment, and (4) the design of radio logically safe equipment,
processes, and environments.
1.2 Relationship BETWEEN
Quality Factor and Linear Energy
Transfer
|
LET keV per micron in water
|
QF
|
|
3.5 or less
|
1
|
|
3.5-7.0
|
1-2
|
|
7.0-23
|
2-5
|
|
23-53
|
5-10
|
|
53-175
|
10-20
|
Quality
Factor Values for Various Radiations
|
Radiation
|
QF
|
|
Gamma-rays from radium in equilibrium With its decay products(filtered by 0.5 mm Platinum)
|
1
|
|
X-rays
|
1
|
|
Beta-rays and electrons of energy 0.03 MeV
|
1
|
|
Beta rays and electrons of energy 0.03 MeV
|
1.7
|
|
Thermal neutrons
|
3
|
|
Fast neutrons
|
10
|
|
Protons
|
10
|
|
Alpha-rays
|
10
|
|
Heavy ions
|
20
|
1.3 RADIATION
PROTECTION GUIDES
1.3.1 Organizations that Set Standards
1.3.2 International
Commission on Radiological Protection
The basic responsibility for providing guidance in matters of
radiation safety has been assumed by the international Commission on
Radiological protection (ICRP).This organization was established in 1928 by the
Second International Congress of Radiology as the International X-ray and
Radium Protection Commission.
1.3.3 INTERNATIONAL
ATOMIC ENERGY AGENCY
The international Atomic Energy Agency (IAEA), a specialized of
the United Nations that was organized in 1956 in order to promote the peaceful
uses of nuclear energy, is concerned with the partial application of the 1CRP
recommendations. " Under its Statute the International Atomic Energy
Agency is empowered to' provide for the application of standards of safety for
protection against radiation to its own operations and to operations making use
of assistance provided by it or with which it is otherwise directly associated,
1.3.4 INTERNATIONAL
LABOUR ORGANIZATION
The
international Labour Organization ( ILO), which was founded in 1919 and then
became part of the League of Nations, survived the demise of the League to
become the first of the specialized agencies of the United Nations: its concern
generally is with the social problems of labor. Included in its work is the
specification of international labor standards which deal with the health and
safety of workers. These specifications are set forth in the model, code of safety
regulations for industrial Establishments for the Guidance of Governments and
Industries, in the recommendations of expert committees, and in technical
manuals. In regard to radiation, the model code has been amended to
incorporate those recommendation of the ICRP that are pertinent to control of
occupational radiation hazards, and several manuals dealing with protection of
workers against radiation hazards have been published.
1.4 INTERNATIONAL
COMMISSION OR RADIOLOGICAL UNITS AND MEASUREMENTS
The International Commission on Radiological Units and
Measurements (ICRU), which works closely with the ICRP, has had, since its
inception in 1925, as its principle objective the development of
internationally acceptable recommendations regarding:
1) Quantities and units of radiation and radioactivity.
2) Procedures suitable for
the measurement and
application of these quantities
in clinical radiology and radiobiology.
3) Physical data needed in the application of theses procedures^ the
use of which tends to assure uniformity in reporting.
1.5 BASIC
RADIATION SAFETY CRITERIA
For purposes of radiation safety, the International Commission on
Radiological protection (ICRP) recognizes two categories of exposure:
1) Occupational exposure-to adults who are exposed to ionizing
radiation in the course of their work. Persons in this exposure category may be
called radiation workers. This category contains two sub-groups: (a) women of
reproductive capacity and (b) all other radiation workers.
2) Exposure of the general public.
For
occupational exposure, the basic recommendations for maximum exposure limits in
the case of routine, long-term (lifetime), continuous exposure are:
1) Whole body, gonads, lenses of the eye, or hemopoietic system: the
total accumulated lifetime dose may not exceed that given by the formula.
D= 5(N-18) rem,
Where N is the worker's age
in years. The only restriction to this formula is that the dose not exceed 3
rem for any consecutive 13-week period for workers who are not women of
reproductive capacity. For women of reproductive capacity, occupational
exposure is limited to 1.3 rem per consecutive 13-week period, under this
restriction, the dose to an embryo during the first 2 months of pregnancy would
normally be less than 1 rem. If the woman is pregnant the total accumulated
dose during the remaining 7 months is restricted to Irem.
1.6 RADIOACTIVITY
Radioactivity may be defined as spontaneous nuclear changes that
result in. the formation of new elements. These changes are accomplished by one
of several different mechanisms, including alpha particle emission, beta
particle and positron emission, and orbital electron capture. Each of these
reactions may or may not be accompanied by gamma radiation. Radioactivity and
radioactive properties of nuclides are determined by nuclear considerations
only, and are independent of the chemical and physical states of the
radioisotope.
1.6.1 Kinds of radiation Alpha radiation
An alpha particle is a highly energetic helium nucleus that is
emitted from the nucleus
of the radioactive isotope when the neutron to proton ratio is too low. It is a
positively charged, massive particle, consisting of an assembly of two protons
and two neutrons.
Alpha particles are
extremely limited in their ability to penetrate matter. The dead outer layer of
skin is sufficiently thick to absorb all alpha radiations from radioactive
materials. As a consequence, alpha radiation from sources outside the body do
not constitute a radiation hazard. In the case of internally deposited
alpha-emitting isotopes, however, the shielding effect of. the dead outer layer
of skin is absent, and the energy of the alpha radiation is dissipated in
living tissue. Alpha radiation is highly toxic when it irradiates the inside of
the body from internally deposited radioisotopes.
1.6.2 POSITRON
EMISSION
In those instances where the neutron to proton ratio is too low
and alpha emission is not energetically possible, the nucleus may, under
certain conditions, .attain stability by emitting a positron. A positron is a
beta particle whose charge is positive. In all other respects it is the same as
the negative beta particle, or an ordinary electron. Its mass is 0.000548
atomic mass units and its charge is + 4.8 x 10-10 sV)
Since
positrons are electrons, the radiation hazard from the positrons themselves is
very similar to the hazard from beta particles, the gamma radiation resulting
from the annihilation of the positron, however, makes all positron emitting
isotopes potential external radiation hazards.
1.6.3 ORBITAL
ELECTRON CAPTURE
If a neutron deficient atom
is to attain stability by positron emission, it must exceed the weight of its
daughter by at least two electron masses. If this requirement connot be met,
then the neutron deficiency is overcome by the process known as orbital
electron capture or, alternatively, as K capture, in this radioactive
transformation, one of the extra-nuclear electrons is captured by the nucleus,
and unites with an intra-nuclear proton to form a neutron. Whenever an atom
decays by orbital electron capture, an X-ray characteristic of the daughter
element is emitted as.an electron from an outer orbit falls into the energy
level occupied by the electron which had been captured. That characteristeic
X-rays of the daughter should be observed follows from the fact that the X-ray
photon is emitted after the nucleus captures the orbital electron and is
thereby transformed into the daughter. These low energy characteristic X-rays
must be considered by the health physicist when he computes absorbed radiation
doses from internally deposited isotopes which decay by orbital electron
capture.
1.6.4 BETA
EMISSION
A beta particle is an
ordinary electron that is ejected from the nucleus of a beta-unstable
radioactive atom. The particle has a single negative electrical charge (4.8 x
10-10 sC) and a very small mass (0.00055 atomic mass units). Beta radiation,
because of its ability to penetrate tissue to varying depths, depending on the
energy of the beta particle, may be an external radiation hazard, the exact
degree of hazard, of course, depends on the beta-emitting isotope, and must be
evaluated in every case. Generally, however, beta-rays whose energies are less
than 200 ke V and therefore have very limited penetrability, such as those from
tritium, 35S, and 14 C, are not considered as external radiation hazard. It
should be noted, however, that beta-rays give rise to highly penetrating X-rays
called bremsstrahlung when they are stopped by shielding. Unless shielding is
properly designed, and proper precautionary measures adopted, beta radiation
may indirectly result in an external radiation hazard through the production of
bremsstrahlung. Any beta-emitting isotope, of course, is potentially hazardous
when it is deposited in the body in amounts exceeding those thought to be safe.
1.6.5 GAMMA
RAYS
Gamma
rays are electromagnetic radiations with no charge they are of the
same nature as x rays and hence like x-rays they
produce Compton and photoelectric effects.
Gamma
rays coming from nuclei possess energies of the order of several
maga electron volts.(Mev)
High
energy gamma ray are very penetrating and can traverse a for greater
thickness of matter as compared to alpha and
beta particles.
1.6.6 NEUTRON
EMISSION
Neutron
carries no charge and has mass nearly equal to that of proton due to
the absence of electrostatic forces ionization of an
atom is cause only by direct collision. Thus a
neutron though highly penetrating is far less ionizing than charged particle.
1.7 HALF-LIFE
The
time required for any given radioisotope to decay to one-half of its
original quantity is a measure of the speed with
which the isotope undergoes radioactive decay.
This period of time is
called the half-life, and is characteristic
of the particular radioisotope. Each radioisotope has its own unique rate of decay, and no operation, either chemical or
physical, is known that will change the decay rate; the half-life of a radioisotope is an unalterable property of the isotope, Half-lives of
radioisotopes range groin microseconds to
billions of years. From the definition of the half-life, it follows that the fraction of a radioisotope
remaining after n half-lives is given by the relationship.
A/A0-1/2-
Where A0 is the
original quantity of activity, and A is the activity left after n half-lives.
1.8 RADIATION
DOSIMETRY
1.8.1 CURIE
The
curie is the activity of that quantity of radioactive material in which the
number of disintegrations per second is 3.7 x 10 10.
It
should be emphasized that, although the curie is defined in terms of a number
of disintegrating atoms per second,
it is not a measure of rate of decay. The curie is a measure only of quantity of radioactive material.
For
health physics, as well as for many other purposes, the curie is a very large
quantity of activity. Submultiples of the curie, as listed below, therefore are
used:
1
millicurie (mCi) = 10-3 Ci
1
microcurie ( u Ci) = 10-6 Ci
1
nanocurie ( nCi) = 10-9 Ci
1
picocurie (pCi) = 10-12 Ci
Multiples
of the curie that are frequently used are the kilocurie and the megacurie.
These quantities are generally not abbreviated.
1.8.2 ABSORBED
DOSE: THE RAD
Radiation
damage depends on the absorption of energy from the radiation, and is
proportional to the concentration of absorbed energy
in tissue. For this reason, the basic unit of radiation dose is expressed in
terms of absorbed energy per unit mass of tissue. This unite is called the
rad (Radiation Absorbed Dose)
and is defined as:
One
rad is an absorbed radiation dose of 100 ergs per gram.
The
rad is universally applicable to all types of radiation dosimetry-irradiation
due to external fields of gamma-rays,
neutrons, or charged particles as well as that due to internally deposited radioisotopes.
1.8.3 DOSE
EQUIVALENT: THE REM
The
rem is the unit of radiation dose equivalent (DE) that is used for radiation
safety purposes and for administrative purposes. The dose equivalent, expressed
in rems, considers the QF of radiation as well as the absorbed dose, plus other
factors, such as non-uniform distribution of an internally deposited radioisotope,
DF, that may influence the biologic effect of a given absorbed dose.
The dose equivalent is defined by
DE,
rems = dose, rads x QF x DF.
An absorbed dose of 1 mrad of X- or beta-rays is equal to 1 mrem,
and an absorbed heavy ion dose of 1 mrad is 20 mrem. The rad is based on only
physical factors; while the rem considers both physical and biological
factors..Maximum allowable radiation dose for the various radiations is given
in units of res or millirems. The use of the rem unit in routine radiation
surveying is illustrated by the following example:
The dose rate outside the shielding of a cyclotron is found to be
0,5 m R/hr gamma; 0.2 mrad/hr thermal neutrons, and 0.1 mrad/hr fast neutrons.
What is the total dose rate of the combined radiations.
Gamma-rays: 0.5
mR/. hr x 1 = 0.5
mrem/hr
Thermal
neutrons: 0.2 mrad/hr x 3 = 0.6
mrem/hr
Fast
neutrons 0.1 mrad/hr x 10 =
1.0 mrem/hr
Mixed
radiation dose rate = 2.1 mrem/hr
1.8.4 EXPOSURE:
THE ROENTGEN
For external radiation of any given energy flux, the absorbed dose
to any point within an organism depends on the type and energy of radiation,
the depth within the organism of the point at which, the absorbed. dose is
desired, and elementary constitution of absorbing medium at this point.
The ICRU defined exposure as " the quotient of Q by m, where
Q is the sum of the electrical charges on all the ions of one sign produced in
air when all the electrons (negatrons and positrons), liberated by photons in a
volume element of air whose mass is m, are completely stopped in air". The
special unite of exposure in air is the roentgen.
1 R - 2.58 x 10-4 Coulombs /kg
The operational definition of the roentgen may be easily converted
into the more fundamental units of energy absorbed per unit mass of air by
applying the fact that the charge on a single ion is 4.8 x 10-10 sC and that
the average energy dissipated in the production of a single ion pair in air is
34 eV. Therefore:
It
should be noted that the roentgen is an integrated measure of exposure, and is
independent of the time over which the exposure occurs. The strength of a
radiation field is usually given as an exposure rate, such as roentgens per
minute of milliroentgens per hour, (A milliroentgen, which is abbreviated
"mR" is equal to 0.001 roentgens.) the total exposure, of course, is
the product .of exposure rate and time.
1.9 BIOLOGICAL EFFECTS OF RADIATION
Radiation ranks among the most thoroughly investigated etiologic
agents associated with disease. Although much still remains to be learned about
the interaction between ionizing radiation and living matter more is known
about the mechanism of radiation damage on the molecular, cellular, and organ
system levels than is known for most other environmental stressing agents.
Indeed, it is precisely this vast accumulation of quantitative doseresponse
data that is available to the health physicist that enables him to/specify
environmental radiation levels for occupational • exposure, thus permitting the continuing
industrial, scientific, and medical exploitation of nuclear energy in safety.
1.9.1 DIRECT
ACTION
The ross biological effects resulting from overexposure to
radiation are the sequellae to a long and complex series of events that are
initiated by ionization or excitation of relatively few molecules in the
organism. For example, the L.D-50/30 day dose for man of gamma-rays is about
400 rad. Since 1 rad corresponds to an energy absorption of 100 ergs/g, or 6.25
x 10 18 eV/g, and since about 34 eV arc expended in producing a single
ionization, the lethal dose produces, in tissue, 400 rad x 6.25 x 1013 eV/g/rad =
7.65 x 10 l4 34 eV/ion
Ionized atoms per gram tissue. If we estimater that about nine
other atoms arc excited for each one ionized, we find that about 7.56 x 10
15 atoms/g of tissue arc directly affected by a lethal radiation dose. In soft
tissue, there are about 8 x m 22 atoms/g. the fraction of directly affected
atoms, therefore, is
1.9.2 INDIRECT ACTION
Direct effects of radiation, ionization, and excitation are
non-specific, and may occur anywhere in the body. When the directly affected
atom is in a protein molecule, or in a molecule of nucleic acid, then certain
specific effects dut to the damaged molecule may ensue. However, most of the
body is water, and most of the direct action of radiation therefore is on
water. The result of this energy absorption by water is the production, in the
water, of highly reactive free radicals that are chemically toxic ( a free
radical is a fragment of a compound or an element that contains an unpaired
electron) and which may exert their toxicity on other molecules. When pure
water-is irradiated we have.
H2O
H2O + e
And
positive ion dissociates immediately according to the equation
H 2 O+ H+ + OH,
While
the electron is picked up by a neutral water molecule
H2O + e- H2O-,
Which
dissociates immediately
H2O- H + OH-
The ions H+ and OH-are of no consequence, since all body fluids
already contain significant concentrations of both these ions. The free
radicals H and OH may combine with like radicals, or they ract with other
molecules in radiation. In the case of a high rate of linear energy transfer,
such as results from passage of an alpha particle or other particle of high
specific ionization, the free OH radicals are formed close enough together to
enable them to combine with each other before they can recombine with free H
radicals, which leads to the production of hydrogen peroxide.
OH + OH
H2O2,
Effects of radiation in which no threshold dose has been observed
are thought to be the result of a direct insult to a molecule by ionization and
excitation and the consequent dissociation of the molecule. Point mutations, in
which there is a change in a single gene locus, is an example of such an
effect. The dissociation, due to ionization or excitation, of an atom on the
DNA molecule prevents the information originally contained in the gene from
being transmitted to the next generation. Such point mutations may occur in the
germinal cells, in which case the point mutation is passed on to the next
individual; or it may occur in somatic cells, which results in a point mutation
in the daughter cell. Since these point mutations are thereafter transmitted to
succeeding generations of cells (except for the highly improbable instance
where one mutated gene may suffer another mutation), it is clear that for those
biological effects of radiation that depend on point mutations, the radiation
dose is cumulative; every little dose may result in a change in the gene burden
which is then continuously transmitted. When dealing quantitatively with such
phenomena, however, we must consider the probability of observing a genetic
change among the offspring of an irradiated individual. For radiation doses
down to about 25 rad, the magnitude of the effect, as measured by frequency of
gene mutations, is proportional to the dose. Below doses of about 25 rad, the
mutation probability is so low that enormous numbers of animals must be used in
order to detect a mutation that could be ascribed to the radiation. For this
reason, no reliable experimental data are available for genetic changes in the
range of 0 to about 25 rad.
2 RADIATION EFFECTS
In health physics, as in other areas of environmental control of
harmful agents, we are concerned with two types exposure: (1) a single
accidental exposure to a high dose of radiation during a short period of time,
which is commonly called acute exposure, and which may produce biological
effects within a short time after exposure;(2) long-term, low level
overexposure, commonly called continuous 01 chronic "exposure, where the
results of the overexposure may not be apparent for years, and which is likely
to be the result of improper or inadequate protective measures.
2.1 ACUTE EFFECTS
Acute radiation overexposure affects all the organs and systems of
the body. However, since not all organs and organ systems are equally sensitive
to radiation, the pattern of response, or disease syndrome, in an overexposed
individual depends on the magnitude of the dose. To simplify classification,
the acute radiation syndrome is subdivided into three classes; in order of
increasing seventy, these arc (1) the hemopoietic syndrome, (2) the
gastrointestinal syndrome, and (3) the central nervous system syndrome. Certain
effects are common to all categories; these include:
(a) Nausea and vomiting,
(b)
Malaise and fatigue,
(c) Increased temperature,
(d) Blood changes.
2.1.1 DELAYED EFFECTS
The delayed effects of radiation may be due either to a single
large over-exposure or can continuing low-level overexposure.
Continuing overexposure can be due to exposure to external
radiation fields, or can result from inhalation or ingestion of a radioisotope
which then becomes fixed in the body through chemical reaction with the tissue
proteinor , because of the chemical similarity of the radioisotope with normal
metabolites, may be systemically absorbed within certain organs and tissues.
In either case, the internally deposited radioisotope may continue
to irradiate the tissue for a .long time. In this connection, it should be
pointed out that the adjectives "acute" and "chronic", as
ordinarily used by toxicologists to describe single and continuous exposures
respectively, are not directly applicable to inhaled or ingested radioisotopes,
since a single, over " acute" exposure may lead to continuous, or
" Chronic" irradiation of the tissue in which the radioactive
material is located. In the case of either a single massive over-exposure or
continuous low level overexposure, the end results are the same among the
delayed consequences of overexposure which are of greatest concern are cancer,
shortening of life span, and cataracts.
6.1 HEALTH PHYSICS
INSTRUMENTATION
6.1.1
Radiation Detectors
Man possesses
no biological sensors of ionizing radiation. As a consequence he must depend entirely
on instrumentation for the detection and measurement of radiation.
Although there are many different instrument types, the
operating principles for most radiation-measuring instruments are relatively
few. The basic requirement of any radiation-measuring instrument is that the
instrument's detector interact with the radiation in such a manner that the
magnitude of the instrument's response is proportional to the radiation effect
or radiation property under measurement.
6.1.1 radiation effects used in the detection and
measurement of radiation
|
Effect
|
Type of Instrument
|
Detector
|
||
|
Electrical
|
1.
|
Ionization
chamber
|
1.Gas
|
|
|
|
2.
3. 4.
|
Proportional
counter
Geiger
Counter
Solid
state
|
2.
Gas
3.Gas
4.
Semiconductor
|
|
|
Chemical
|
1.
2.
|
Film
Chemical
dosimeter
|
1.
2.
|
Photographic
emulsion
Solid
or liquid
|
|
Light
|
1.
2.
|
Scintillation
counter Cerenkov counter
|
1.
2.
|
Crystal
or liquid
Crystal
or liquid
|
|
Thermo-luminescence
|
|
Thermoluminescent
dosimeter
|
|
Crystal
|
|
Heat
|
|
Calorimeter
|
|
Solid
or liquid
|
6.1.2 Particle-counting Instruments
Particle-counting
instruments are frequently used by health physicists to determine the
radioactivity of a sample taken from the environment, such as an air sample, or
to measure the activity of a biological fluid from someone suspected of being
internally contaminated. Another important application of particle-counting
instruments is in portable radiation-survey instruments. Particle-counting
instruments may be very sensitive they literally respond to a single ionizing
particle. They are, accordingly, widely used for searching for unknown
radiation sources, for leaks in shielding, and for areas of contamination. The
detector in particle-counting instruments may be either a gas or a solid. In
either case, passage of an ionizing particle through the detector results in
energy dissipation through a burst of ionization. This burst of ionization is
converted into an electrical pulse that actuates a readout device, such as a
sealer or a rate meter, to register a count.
6.1.3 Scintillation Counters
A
scintillation detector is a transducer that changes the kinetic energy of an
ionizing particle into a flash of light. Historically, one of the earliest
means of measuring radiation was by scintillation counting; Rutherford, in his
classical experiments on scattering of alpha particles, used a zinc sulfide
crystal as the primary detector of radiation; he used his eye to see the
flickers of light that appeared when alpha particles struck the zinc sulfide.
Today, the light is viewed electronically by photomultiplier tubes whose output
pulses may be amplified, sorted by size, and counted. The various radiations
may be detected with scintillation counters by using the appropriate
scintillating material.
6.1.3. scintillating materials
|
Phosphor
|
Density
|
Wavelength
|
Relative
|
Decay time (µsec)
|
|
|
(g/cm3)
|
of maximum
|
pulse
|
|
|
|
|
emission, Å
|
height
|
|
|
Nal (Tl)
|
3.67
|
4100
|
210
|
0.25
|
|
Csl (Tl)
|
4.51
|
Blue
|
55
|
1.1
|
|
KI (Tl)
|
3.13
|
4100
|
50
|
1.0
|
|
Anthracene
|
1.25
|
4400
|
100
|
0.032
|
|
Trans-Stilbene
|
1.16
|
4100
|
60
|
0.0064
|
|
Plastic
|
|
3550-4500
|
28-48
|
0.003-0.005
|
|
Liquid
|
|
3550-4500
|
27-49
|
0.002-0.008
|
|
p-Terphenyl
|
1.23
|
4000
|
40
|
0.005
|
Where
they are used to measure 14C and 3H
(With a suitable detector, a scintillation
counter may also be used as a beta-ray or an alpha-ray spectrometer.)
Gamma-ray
photons, passing through the crystal, interact with the atoms of the crystal by
the usual mechanisms of photoelectric absorption. Compton scattering, and pair
production. The primary ionizing particles resulting from the gamma-ray
interactions—the photoelectrons, Compton electrons, and positron-electron
pairs—dissipate their kinetic energy by exciting and ionizing the atoms in the crystal. The excited atoms return to the
ground state by the emission of quanta of light. These light pulses, upon striking
the photosensitive cathode of the photomultiplier tube, cause electrons to be
ejected from the cathode. These electrons are accelerated to a second
electrode, called a dynode, whose potential is about 100 V positive with
respect to the photocathode. Each electron that strikes the dynode causes
several other electrons to be ejected from the dynode, thereby
"multiplying" the original photocurrent. This process is repeated
about 10 times before all the electrons thus produced are collected by the
plate of the photomultiplier tube. This current pulse, whose magnitude is
proportional to the energy of the primary ionizing particle, can then be
amplified and counted. Figure 9.9 illustrates schematically the sequence of
events in the detection of a photon by a scintillation chamber.
A
photoelectric interaction within the crystal produces essentially
mono-energetic photoelectrons, which in turn produce light pulses of about the
same intensity. These light pulses, being of equal intensity, lead to current
output pulses of approximately the same magnitude).
6.1.4 Dose-measuring Instruments
Radiation flux is only one of the several factors that determine
radiation dose. That a flux measuring instrument does not necessarily measure
dose is shown by the following example:
Example 2.
Consider
two radiation fields of equal energy density. In one case we have a 0.1 MeV
photon flux of 2000 photons per cm2/sec. in the second case, the
photon energy is 2 MeV and the flux is 100 photons per cm2/sec. The
energy absorption coefficient for air for 0.1-MeV gamma radiation is 0.0233 cm2/g;
for 2 MeV gammas, the energy absorption coefficient is 0.0238 cm2/g.
The dose rates for the two radiation fields are given by:
= 8.5 X 10-8
jR/sec for the 0.1 MeV radiation, and
8.7
x 10-8 R/sec for the 2 MeV photons.
The dose rates for the two radiation fields are about the same. A
flux-measuring instrument, however, such as a Geiger counter, would register
about 20 times more for the 0.1 MeV radiation than for the higher-energy
radiation.
6.1.5 Pocket Dosimeters
To measure radiation dose, the response of the instrument must be
proportional to absorbed energy. A basic instrument for doing this, the free
air ionization chamber, was described in Chapter 6. In that chapter, too, it
was shown that an "jair wall" ionization chamber could be made on the
basis of the operational definition of the roentgen, and that such an
instrument could be used to measure exposure dose. Ionization chambers of this
type, which are often called "pocket dosimeters", are widely used for
personnel monitoring. Two types of pocket dosimeters are in common use. One of
these is the condenser type, as illustrated in Fig. 6.2. This type pocket
dosimeter is of the indirect reading type; an auxiliary device is necessary in
order to read the measured dose. This device, which is in reality an
electrostatic voltmeter that is calibrated in roentgens, is called a
"charger-reader" (because it is also used to charge the chamber). The
term minometer is often used synonymously with charger-reader. Figure 9.14
shows a photograph of a pocket dosimeter and its charger-reader. Commercially
available condenser-type pocket dosimeters measure integrated X- or gamma-ray
exposure doses up to 200 mR with an accuracy of about ± 15% for quantum energies
between about 0.05 and 2 MeV. For quantum energies outside this range,
correction factors, which are supplied by the manufacturer, must be used. These
dosimeters also respond to beta-rays whose energy exceeds 1 MeV. By coating the
inside of the chamber with befron, the pocket dosimeter can also be made
sensitive to thermal neutrons he standard type of pocket dosimeter, is designed
for measuring X and gamma radiation only. It is calibrated either with radium
or 60Co gamma-rays. Pocket dosimeters discharge slowly even when
they are not in a radiation field because of cosmic radiation and because
charge leaks across the insulator that separates the central electrode from the
outer electrode. A dosimeter that leaks more than 5 % of the full-scale reading
per day should not be used. Usually, two pocket dosimeters are worn. Since a
mal-function will always cause the instrument to read high, the lower of the
two readings in considered as more accurate. Because of leakage and possibility
of malfunction due to being dropped, pocket dosimeters are usually worn for one
day. Reading the instrument erases its information content. It is therefore
necessary to recharge the indirect reading pocket dosimeter after each reading.
6.1.6 Film Badges
Another very commonly used personnel monitoring device is the film
badge (Fig. 9.17), which consists of a packet of two (for X or gamma) or three
(for X, gamma, and neutrons) pieces of dental-sized film wrapped in light-tight
paper and worn in a suitable plastic or metal container. The two films for
X-and gamma-radiation include a sensitive emulsion and a relatively insensitive
emulsion. Such a film pack is useful over an exposure range of about 10 mR to
about 1800 R of radium gamma-rays. The film is also sensitive to beta-radiation,
and may be used to measure beta-ray dose, from betas whose maximum energy
exceeds about 400 keV, from about 50 mrad to about 1000 rad. Using appropriate
film and technics, thermal neutron doses of 10 mrem to 1000 rem, and fast
neutron doses from about 40 mrem to 100 rem may be measured.
Film badge dosimetry is based on the fact that ionizing radiation
exposes the silver halide in the photographic emulsion, which results in a
darkening of the film. The degree of darkening, which is called the optical
density of the film, can be precisely measured with a photoelectric
densitometer whose reading is expressed as the logarithm of the intensity of
the light transmitted through the film. The optical density of the exposed film
is quantitatively related to the magnitude of the exposure.
6.1.7 Thermoluminescent Dosimeter
Many crystals, including CaF2, containing Mn as an
impurity and LiF emit light if they are heated after having been exposed to
radiation; they are called thermoluminescent crystals. Absorption of energy
from the radiation excites the atoms in the crystal, which results in the
production of free electrons and holes in the thermoluminescent crystal. These
are trapped by impurities or imperfections in the crystalline lattice thus
locking the excitation energy into the crystal. Heating the crystal releases
the excitation energy as light. Which is obtained by heating the irradiated
crystal at a uniform rate, and measuring the light output as the temperature
increases. The temperature at which the maximum light output occurs is a
measure of the binding energy of the electron on the hole in the trap. More
than one peak on a glow curve indicates different trapping sites, each with its
own binding energy. The total amount of light is proportional to the number of
trapped, excited electrons, which in turn is proportional to the amount of
energy absorbed from the radiation. The intensity of the light emitted from the
thermoluminescent crystals is thus directly proportional to the radiation dose.
7.1 EVALUATION OF PROTECTIVE
MEASURES
The effectiveness of protective measures against radiation hazards
is evaluated by a surveillance program that includes observations on both man
and his environment. Such a surveillance program may employ one or more of a
variety of techniques, depending on the nature of the hazard and the consequences
of a breakdown in the system of controls. These techniques may include
pre-employment physical examinations, periodic physical examinations,
estimation of internally deposited radioactivity by bioassay and total body
counting, personnel monitoring, radiation and contamination surveys, and
continuous environmental monitoring.
7.1.1 Medical Surveillance
The great degree of overexposure required before clinical signs or
symptoms of overexposure appear precludes the use of medical surveillance of
radiation workers as a routine monitoring device. Nevertheless, medical
supervision may play an important role in protecting radiation workers against
possible radiation damage. Among the main tasks of medical supervision is the
proper placement of radiation workers according to their medical histories and
physical condition, and history of previous radiation exposure. Dermatitis,
cataracts, and blood dyscrasias, including leukemia are associated with
radiation exposure. A pre-employment physical examination, therefore, should be
given if the nature of the work, including consideration to possible accidental
overexposures, warrants it—in which special attention is paid to physical
conditions that may lead to, or be suggestive of, susceptibility to any of
these effects. Possible indirect effects from working with radioisotopes also
are considered by the examining physician. For example, sensitivity or allergy
may contraindicate work that requires the wearing of rubber gloves °r that may
require washing the hands or body with strong detergents or harsh chemicals in
order to decontaminate the skin. In addition to the pre-employment examination,
the radiation worker may be routinely examined at Periodic intervals to
ascertain that he continues to be free of signs that would contraindicate
further occupational exposure to radiation. The physician is thus instrumental
in preventing damage or injury that could otherwise have
arisen, either directly or indirectly, as a
consequence of working with radio-isotopes or exposure to radiation. Medical
supervision of radiation workers may also be necessary to evaluate
overexposure, to treat radiation injuries and to decontaminate personnel. These
activities of the physician are, of course, in addition to the routine health
services that he provides which are not connected to radiation hazards. It
should be pointed out that medical surveillance of workers is not unique to the
field of radiation health. All good occupational health programs include
pre-employment examinations, consideration of medical findings in job
placement, and continuing medical surveillance to help maximize the protection
of workers against the harmful effects of toxic substances.
7.1.2 Estimation of Internally Deposited
Radioactivity
One of the techniques for evaluating a contamination control
program is the determination of the body burdens of personnel who are at risk.
This determination is done indirectly by bioassay methods, and directly by
total body counting in the case of gamma-emitting radionuclides or beta
emitters that give rise to suitable bremsstrahlung. The underlying rationale
for bioassay is that a quantitative relationship exists among inhalation or
ingestion of a radionuclide, the resulting body burden, and the rate at which
the radio-nuclide is eliminated either in the urine or in the feces. From
measurements of activity in the urine and feces, therefore, we should be able
to infer the body burden. Unfortunately, the kinetics of metabolism of most
substances is influenced by a large number of factors, and, as a consequence,
the desirable quantitative relationships between body burden and elimination
rates are known for relatively few cases. In most instances, therefore,
bioassay data give only a very approximate estimate of the degree of internal
deposition of radioactivity. Although both urine and feces are available for
bioassay measurements in case of an accidental inhalation or ingestion of a
large amount of radioactivity, routine bioassay monitoring is usually done only
with urine samples, because of the ease of sample collection and also for
esthetic reasons.
For purposes of bioassay, we distinguish between readily soluble
and difficultly soluble compounds. This distinction is especially important in
the case of inhaled particulates that are relatively insoluble, in which the
difficultly soluble material is brought up from the lung and swallowed, while
at the same time some of the inhaled particulate matter is dissolving and being
absorbed into the body fluids. The resulting complexities due to varying
pulmonary deposition and clearance rates and the varying urinary to fecal
ratios of t radionuclide makes it difficult to quantitatively estimate the lung
dose from difficultly soluble air-borne contamination. Nevertheless, an
estimate of t minimum amount of difficultly soluble radioactive particulates
that was deposited in
the upper respiratory tract following a single accidental inhalation could be
made from the cumulative fecal activity during the first few days after the
inhalation. With this information, and using the ICRP model for lung clearance,
a less reliable estimate can then be made of the activity remaining in the deep
respiratory tract.
Readily soluble radionuclides may be grouped into three categories
according to their distribution and metabolic pathways: (1) those that are
uniformly distributed throughout the body, such as 3H in tritiated
water or radiosodium ions, (2) those that concentrate mainly either in specific
organs, such as iodine in the thyroid gland, mercury in the kidney, or in the
intracellular fluid, such as potassium or cesium, and (3) those which are
deposited in the skeleton.
7.1.3 Personnel Monitoring
Personnel monitoring is the continuous measurement of an
individual's exposure dose by means of one or more types of suitable
instruments, such as pocket meters, film badges, and thermoluminescent
dosimeters, which are carried by the individual at all times. The choice of
personnel monitoring instrument must be compatible with the type and energy of
the radiation being measured. For example, a worker who is exposed only to 14C
would wear no personnel monitoring instrument, since these isotopes emit only
beta-rays of such low energy that they are not recorded by any of the
commercially available personnel monitoring devices. Bioassay procedures would
be indicated if personnel monitoring were necessary.
7.2 Radiation and Contamination Surveys
A survey is a systematic set of measurements made by a health
physics surveyor in order to determine one or more of the following:
(a) an unknown radiation source,
(b) dose rate,
(c) surface contamination,
(d) atmospheric contamination.
In order to make these determinations, the health physics surveyor
must choose the appropriate instruments, and must use it properly.
7.3 Surface Contamination
Surface contamination can be located by scanning with a sensitive
detector, as a thin end window Geiger counter. After finding a contaminated
spot or area, a dose-measuring instrument may be employed to measure the dose
rate at some appropriate distance from the surface. The main hazard from
surface contamination is transmission of the contamination from the surface
into the body via inhalation or ingestion. To estimate this hazard, a smear
test is performed to determine whether the surface contamination is fixed
or loose, and therefore transmissible. A smear test consists of wiping the
suspected area with a piece of filter paper several centimeters in diameter and
then measuring the activity in the paper. The area to be smeared varies
according to the extent of the suspected contamination and the physical
conditions under which the survey is made; a wipe-area of 100 cm2 is
not uncommon. A smear survey, which is a systematic series of smears without
first using a scanning instrument to detect the contamination, is often done in
a work area that is subject to contamination, and where the background due to
radiation sources is high enough to mask the activity due to contamination. It should
be emphasized that a smear test is a qualitative, or at best a
semiquantitative determination whose chief purpose is to allow an estimate to
be made of the degree to which surface contamination is fixed. If significant
transmissible contamina-ation is found, and, if in the opinion of the health
physicist this contamination may be hazardous, then prompt decontamination
procedures are instituted.
7.4 Leak Testing Sealed Sources
Sealed gamma-ray, beta-ray, bremsstrahlung and neutron sources are
used in a wide variety of applications in medicine and in industry. In all
cases, the radioactive material is permanently enclosed either in a capsule or
another suitable container. Before being shipped from the supplier, all such
sources must pass inspection for freedom from surface contamination and
leakage. Either during transport from the supplier or in the course of time,
however, the capsule may develop faults through which the radioactive source
material may escape into the environment. Because of the serious consequences
of such an escape, a sealed source must be tested before being put into use and
periodically thereafter for surface contamination and leakage. The testing
cycle depends on the nature of the source and on the kind of use to which it is
put. However, it is usually recommended that such tests be performed at least
once every 6 months. The following technics may be employed to perform these
tests: to test for surface contamination, wipe all exposed external surfaces of
the source thoroughly with a piece of filter paper or a cotton swab moistened
with an appropriate solvent, then measure the activity on the paper or the
swab. The source is considered free of surface contamination if less
than
0.005 µCi alpha or less than 0.05 µCi beta activity was wiped off. To test for
leakage, one of the following tests may be performed:
1. Wipe the source with either a piece of wet
filter paper or a cotton swab. Repeat at least 7 days later. If less than 0.005
µCi alpha or less than 0.05 /xCi beta activity was wiped off each time, then
the source is considered free of leaks.
2. For high activity sources such as those used
in teletherapy, where
wiping
the source might be hazardous, accessible surfaces of the housing port or
collimator may be wiped while the source is in the "off" position.
3. Immerse the source in ethanediol, and
reduce the pressure on the liquid to 100 mm Hg for a period of 30 sec. A leak
is indicated if a stream of fine bubbles issues forth from the source. This
method is reliable only for such sources where enough gas would be trapped to
produce a stream of fine bubbles.
7.5 Air Sampling
Air sampling is considered an important part of a survey where
there is a possibility of significant atmospheric contamination. Allowable
working levels of contaminated air involve quantities of radioactivity very
much less than those which would be considered hazardous if the activity were
in a sealed source and if the hazard were limited only to external radiation.
Furthermore, even if only sealed sources are used, a program of air sampling is
recommended if the nature of the source is such that radioactive gaseous or
particulate matter could escape in the event that the source capsule develops a
flaw. An air sample, in such a case, might detect the contamination and the
leaky source before a significant amount of radioactivity escaped.
An
air sampling system consists of three basic elements: (1) a source of suction
(a vacuum pump) for drawing the air to be sampled through (2) a collecting
device, which usually separates the contaminant from the air, and (3) a
metering device for measuring the quantity of air sampled. After collection,
the sample of the contaminant is counted to determine the radioactivity
content, and then, when this information is combined with the size of the air
sample, the concentration of atmospheric radioactivity is calculated.
