Tsoulfanidis N. Measurement and detection of radiation

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ACTIVATION

ANALYSIS

539

23.

Hankins,

A,, Babb, A. L., and Scribner, B. H.,

ANS Trans.

21:98 (1975).

24.

Ricci,

E.,

ANS

Trans.

21:99

(1975).

25.

Landsberger, S., and Wu,

D.,

ANS Transactions

64:lO (1991).

26.

Spyrou, N., and Arshed, W.,

ANS Transactions

64:ll (1991).

27.

Lindstrom, R. M., and Norman, B. R.,

ANS Transactions

64:12 (1991).

28.

Becker,

A.,

ANS Transactions

6412 (1991).

29.

Ruch,

R., Guinn,

P., and Pinker,

H.,

Nucl. Sci. Eng.

20:381 (1964).

30.

Guinn,

P., and Rumos,

C.,

ANS Trans.

14:105 (1971).

31.

Scheringer, H. L., and Lukens, H. R.,

ANS Trans.

14:106 (1971).

32.

Williamson,

G., and Harrison, W. W.,

ANS Trans.

14:107 (1971).

33.

Ricci,

E.,

"Charged Particle Activation Analysis," in

A. Lenihan, S.

Thomson, and

Guinn (eds.),

Advances in Activation Analysis,

Academic, London,

1972,

vol.

34.

Cline,

E.,

Putnam, M.

H.,

and Hermer, R. G., "GAUSS

VI,

A Computer Program for the

Automatic Analysis of Gamma-Ray Spectra from Ge(Li) Spectrometers,"

ANCR-1113 (1973).

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Nucl. Instrum. Meth.

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(1967).

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137525 (1976).

CHAPTER

SIXTEEN

HEALTH PHYSICS FUNDAMENTALS

16.1

INTRODUCTION

Health physics is the discipline that consists of all the activities related to the

protection of individuals and the general public from potentially harmful effects

of ionizing radiation. Ionizing radiation comes from two sources:

Natural or background radiation which is radiation emitted by radioisotopes

that exist on or inside the earth, in the air we breathe, in the water we drink,

in the food we eat, and in our bodies, as well as radiation incident upon the

earth from outer space (cosmic rays). Humans have been exposed to this

natural radiation for as long as they have existed on this planet.

Man-made radiation which is radiation emitted by all the radioisotopes that

have been produced through nuclear reactions (mainly fission), as well as

radiation produced by machines used in medical installations (e.g., X-ray

machines) or in scientific laboratories (e.g., accelerators).

Health physics is concerned with protection of people from radiation. Since

the background radiation has been, is, and will always be on our planet at about

the same level everywhere, there is not much a health physicist can do to protect

individuals or populations from background radiation. Hence, health physics is

concerned with protection of people from man-made radiation.

health physicist performs many tasks. He or she, most importantly,

Is responsible for the detection and measurement of radiation in areas of

work and in the environment

542

MEASUREMENT AND DETECTION

RADIATION

Is responsible for the proper operation and calibration of detection instru-

ments

Inspects at regular intervals the facilities where radiation sources are used

Enforces federal and state regulations dealing with proper handling of

radiation sources and establishment of acceptable levels of radiation fields at

places of work

5. Keeps records of exposure for all individuals under his or her jurisdiction

6. Knows how to clean areas that have been contaminated with radioactive

materials

Acts as the liaison representative between the regulatory agencies and his or

her organization

Although the term health physics was coined after 1940, and a health

physics society was established in 1955, the concern about the harmful effects of

radiation had been born much earlier-but probably not early enough. The first

recorded radiation damage case occurred in 1896, only a year after the discovery

of X-rays, yet the first limits concerning X-ray exposure were set in the 1920s.

Today, both national and international groups exist that act as advisory bodiest

to the appropriate regulatory agencies.

Since improperly handled radiation may produce deleterious effects to

humans, it is important that individuals who use radiation sources learn the

fundamentals of dosimetry, definition of dose units, biological effects of radia-

tion, standards for radiation protection, and operation of health physics instru-

ments. This chapter briefly discusses all these items. If more detailed treatment

of these topics is needed, consult the bibliography and references given at the

end of the chapter.

16.2

UNITS OF EXPOSURE AND ABSORBED DOSE

Protection of individuals against radiation necessitates the completion of two

tasks:

1. Development of safe radiation exposure limits

Construction of instruments that measure the intensity of radiation

Neither of these tasks can be accomplished without the means of quantitative

description of radiation, i.e., without defining radiation units.

The radiation effect is measured in terms of exposure or dose. Exposure is

defined as charge released per unit mass of air. Dose is defined as energy

absorbed per unit mass of material. The first radiation unit to be defined was

'~nternational Commission on Radiological Units and Measurements (ICRU); International

Commission on Radiological Protection (ICRP); in the United States, the National Council on

Radiation Protection and Measurements

(NCRP).

HEALTH

PHYSICS

FUNDAMENTALS

543

the roentgen (symbol R):

=exposure due to X-rays or gamma-rays of such intensity that the electrons

produced by this radiation in

cm3 of dry air, at standard temperature

and pressure, generate along their tracks electron-ion pairs carrying a

total charge of

esu of either sign

The

unit of exposure is defined as

C/kg air, without any new name

proposed for it. Numerically,

2.58

lop4

C/kg air

The roentgen suffers from two limitations:

1. It was defined in terms of electromagnetic radiation only.

It was defined in terms of air only.

Radiation protection may involve other types of radiation, and media other

than air. For this reason, another unit was defined called the radiation absorbed

dose or rad, defined as

rad

100 erg/g

The

unit of absorbed dose is the Gray (Gy), defined as

1 J/kg

100 rad

The rad (or the Gy) has a simple definition and is a unit independent of

both type of radiation and material. But the measurement of absorbed dose in

terms of rad (or Gy) is neither simple nor straightforward, because it is very

difficult to measure energy deposited in a certain mass of tissue. Fortunately,

one can bypass this difficulty by measuring energy deposited in air, which is

proportional to the exposure, and then relate it to the absorbed dose.

The measurement of exposure is achieved by using ionization chambers, and

the result is given in roentgens. Based on the definitions of the roentgen, the

following relationship can be established between roentgens and rads.

esu

lR= (2.082

lo9) ion pairs/esu

1.293

lop3

(34

eV/pair)l.602

10-l2 ergs/eV

88 ergs/g

0.88 rad

8.8 mGy

is the absorbed dose in air, and X is the exposure in air, the relationship

between the two is

0.88X (16.1)

For media other than air, the relationship is obtained as follows. The

absorbed dose rate in material

is (in terms of energy deposited per unit mass

per unit time)

544

MEASUREMENT

AND

DETECTION

RADIATION

The absorbed dose rate in air is

The ratio of Eq. 16.2 to Eq. 16.3 gives

Pa,i

.=-

Pa,

=--

alr

(0.88)Xair (16.4)

Pa,air

Pa,

air

Equations 16.1 and 16.4 express the fact that the measurement of absorbed

doset is a two-step process:

Exposure (or exposure rate) is measured.

2. Absorbed dose (or dose rate) is calculated from the measured exposure using

Eq.

16.4.

In practice, the instruments that measure radiation dose are, usually,

properly calibrated to read rad or Gy.

16.3

THE RELATIVE BIOLOGICAL EFFECTIVENESSTHE

DOSE EQUIVALENT

The units of absorbed dose defined in the previous section are quite adequate

for the quantitative assessment of the effects of radiation to inanimate objects,

like irradiated transistors or reactor fuel. For protection of people, however, the

important thing is not the measurement of energy deposited-i.e., the absorbed

dose-but the biological effects due to radiation exposure. Unfortunately,

biological effects and absorbed dose do not always have one-to-one correspon-

dence, and for this reason a new unit had to be defined: a unit that takes into

account the biological effects of radiation.

The ideal unit for the measurement of biological effect should be such that

a given dose, measured in that unit, produces a certain biological effect

regardless of the

type and enew of radiation

and also regardless of the

biologzcal

effect

considered. Unfortunately, such a unit cannot be established because of

the different modes by which radiation deposits energy in tissue, the intricate

way by which the energy deposition is related to a given biological effect, and

the complexity of biological organisms.

ideal unit may not exist, but some

unit that "equalizes" biological effects had to be defined.

'~~uations

16.2

and

16.3

give dose rate, not dose; the meaning of Eq.

16.4,

however, is the

same if one uses either dose or dose rate.

HEALTH PHYSICS FUNDAMENTALS

545

The first step toward that task was the introduction of a factor called the

relative biological effectiveness (RBE), defined as

[absorbed dose from X-ray or gamma radiation (200-300 keV)

producing a certain biological effect]

RBEi

[absorbed dose from radiation type

producing the same biological effect]

In understanding the meaning of RBE, note the following:

RBE is defined in terms of photons; therefore, it follows that RBE

for

electromagnetic radiation. Also, although the definition of RBE specifies the

energy of the photons to be 200-300 keV, RBE is taken as equal to

for

photons of all energies.

given type of radiation does not have a single RBE, because RBE values

depend on the energy of the radiation, the cell, the biological effect being

studied, the total dose, dose rate, and other factors.

3. It is a well-known fact that the biological damage increases as the energy

deposited per unit distance, the

linear energy transfer

(LET), increases. Thus,

heavier particles (alphas, heavy ions, fission fragments) are, for the same

absorbed dose, more biologically damaging than photons, electrons, and

positrons.

1963,

the International Commission on Radiological Units and Measure-

ments (ICRU) proposed the replacement of RBE

a new factor named the

quality

factor

(QF).

Here is an excerpt from their recommendation.

In radiation protection it is necessary to provide a factor that denotes the modification of the

effectiveness of a given absorbed dose by

LET

(Linear Energy Transfer). Unlike

RBE,

which is

always experimentally determined, this factor

must

assigned

on the basis of a number of

considerations and it is recommended that it be termed the

quality factor

(QF).

Provisions for

other factors are also made. Thus a

distribution factor

(DF)

may be used to express the

modification of biological effect due to nonuniform distribution of internally deposited

radionuclides. The product of absorbed dose and modifying factors is termed the dose equiva-

lent,

(H).

1973

the

ICRU'

recommended dropping the "F" from QF, a suggestion

that has now become practice. In

1977

the

ICRP~

recommended that the dose

equivalent

(H)

at a point in tissue be written as

NQD

(16.6)

where

quality factor

absorbed dose

product of all the modifying factors. The suggested value of

546

MEASUREMENT

AND

DETECTION OF RADIATION

RBE is now used only in radiobiology, whereas

is used in radiation

protection. A detailed discussion of similarities and differences between the two

factors is given in Ref.

For the radiations and energy ranges considered in this

book, RBE and

are practically the same, and from this point on, only the

factor

will be mentioned. Table 16.1 gives

values for various radiations

commonly encountered.

When the unit of absorbed dose is multiplied by the corresponding

value,

the unit of dose equivalent (H) is obtained. The H units are

rem

rad

and the SI unit

Sievert (Sv)

Thus

100

rem

Because it is only the dose equivalent that equalizes biological effects from

different types and energy of radiation,

only

(or rem) should

added, never

Gy (or rad).

Example

16.1

At the open beam port of a research reactor, the absorbed

dose rate consists of 10 mrad/h due to gammas, 10 mrad/h due to fast

neutrons, and 6

mrad/h due to thermal neutrons. What is the total dose a

person will receive by standing in front of the beam for

Answer

Calculate the dose equivalent H, as shown below:

Abs.

dose

rate

Gammas

10 1 10 0.1

Fast

neutrons

10 10 100 1 .O

Thermal neutrons

2 12 0.12

Total

122 1.22

Table

16.1

Quality Factors for Several Types of Radiati~n~,~

Radiation type

Gamma-rays

X-rays

Beta particles:

Electrons

Positrons

Protons

MeV)

Alpha particles

MeV)

Recoil nuclei

Neutrons:

Thermal

0.005

MeV

0.02

MeV

0.10

MeV

0.50

MeV

.OO

MeV

5.0

MeV

HEALTH

PHYSICS

FUNDAMENTALS

547

The total dose received by the individual is

122 mrem/h

0.17 mrem

1.7

(

3600

)

16.4

DOSIMETRY

FOR

RADIATION EXTERNAL TO THE BODY

The general dosimetry problem is defined as follows: Given the intensity of the

radiation field at a certain point in space, calculate the dose rate received by an

individual standing at that point. The radiation field, outside the body, is

assumed to be known in terms of the type, energy, and number of particles

involved. The calculation that follows disregards the possible perturbation of the

field from the presence of the human body. The calculation is different for

charged particles, photons, and neutrons.

16.4.1

Dose Due to Charged Particles

Consider a point in space where it is known that the charged-particle radiation

field is given by

E) dE

charged particles per m2 s with kinetic energy between E and E

person exposed to this field will receive a radiation dose because of energy

deposited

these charged particles. The dose equivalent rate is given by

where

dE/h

stopping power of tissue for particles of energy

density of tissue

Q( E)

quality factor for particles of energy E

The units of Eq. 16.7 are MeV/(kg s). To obtain the result in Sv/s, one needs to

transform MeV to J (1 MeV

1.602

10-l3 J).

Most of the time in practice, the radiation field is computed not as an

analytic function +(E) but as an energy group distribution, where

/Egl+(~) dE

number of particles per mi s with

energy between Eg and Eg-

(E,

Egp,

)

Using the multigroup structure,

Eq.

16.7

takes the form