Chapter 1 Rt Training

8/11/2019 Chapter 1 Rt Training

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INTRODUCTION

When x- rays or Gamma rays are passed

through material, some of the radiation

i s

absorbed or changed and the amount of

absorption

i s

dependent upon the thickness of

the material, the density of the material and

the atomic number of the absorber. Some

kind of detector such as film, a fluorescent

screen, or a Geiger counter is used to record

the variations in intensity of the emerging

beam as visual images or signals. The image

produced on the film

i s

evaluated for

identifying the discontinuities Industrial

radiography i s primarily concerned with

recoding images on filrn

The two types of radiations, rays

Gamma rays, are used in radiography and

Iia\/c similar characteristics, but differ in their

origin of their generation. Both of them are

hazardous for human beings if they are not

controlled properly. The three basic essentials

in producing a radiograph are, source of

radiation, object to be tested and a cassette

containing the film.

The bjsic properties of the penetrating

radiation are

1)

They travel in straight lines,

2)

they penetrate materials,

3

they affect

photographic films, 4 they interact with body

tissues,

5)

they ionize gases,

6)

They cannot

be sensed by human sense organs.

The advantage of RT over other NDT methods

is that it presents a permanent record, simple

to read out, relatively less costly, can be used

in sites, and the system

i s

portable. 'But it

i s

highly dangerous to handle radiatiori sources

without proper training.

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SOURCES OF RADIATION

X-RAYS The basic principle on which x ray is

generated is that when a stream of high sp~ec

electrons is stopped abruptly heat ,jnd

\:

radiation are generated. The heat is re no\,.ld

and the

x

ray

i s

used for rad~ugrayhy

The X ray unit consists of

a) filament cathode, which produces electrons

when it is heated by filament current


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into heat and x rays. The heat i s removed by

either natural air co oling b y suitable design or

by water cooling the cathode. Tungsten is

chosen for increasing the efficiency of the

production of

X

rays and copper for rem oving

the heat.

The area on which the electrons impinge on

the tungsten block i s called the actual focal

spot The projected area of the actual focal

spot perpendicular to the direction of x rays i s

called the effective focal spot and plays a vital

role in de ciding the quality of the radiograph.

The important parameters are in

X

ray testing

are

1) tube voltage and the voltage between

the anode and cathode wh ich

i s

between 50-

550 kV; the kV decides the thickness to

which the penetration i s possible; higher the

voltage thicker the component that can be

tested.

2) tube current, which i s in mil l i amperes

mA

) .

the quantity of x radiation depends on

the tube current.

3) time of exposure in minutes or seconds

The image produced on the radiograph

i s

dependen t on the quantity of radiation as we ll

as the time to which it

i s

exposed and i s

called the exposure. Exposure milliamp ere

GAMMA RAYS: Gamma rays are produced

as spontaneous emission from the nucleus of

radioactive isotopes.

When a radioactive element decays its

nucleus emits three types of emissions:

1

alpha p article,

2

beta particle 3) gamma ray.

Alpha particle i s a helium ion with two

neutrons and two protons. Beta particles are

negatively charged ions. Both the particulate

radiations travel very short distances and are

stopped easily by materials. Gamma ray

i s

a

radiation, high ly penetrating in nature and i s

used in radiography.

The Roentgen R) i s the unit measure of or

Gamma radiation in air. It i s defined as

quantity f radiation that will produce one

electrostatic un it esu) of charge in one cubic

centimeter of air at STP. One Roentgen of

radiation approximately equals 83 ergs of

radiation energy pe r gram of air.

amma ray Intensity:

This

i s

measured in

roentgens per h ou r at one meter, a measure of

radiation emission over a period of time at a

fixed distance. The activ ity amoun t of

radioactive material

)

of a gamma ray source

determines the intensity of radiation. The

measure of activ ity is curies 1 curie 3.7 x

10 disintegrations per second ).


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emission consists of 1.33 and 1.1

7

MeV,

which

is

similar i n energy content to

2

MeV x

ray machines. The radioisotope s supplied in

the form of capsuled pellet. Because of its

penetrating radiation requires thick shielding

for safety purposes and handling

s

difficult.

The sm allest available size s 3 mm.

Iridium 192: Also produced by neutron

bombardment of Ir 191 has a half l ife of 7

days. The penetrating capability

s

equal to

600

kV x ray machine. Its relatively low

energy radiation and high specific activity

makes it an easily shielded strong radiation

source o f small phys ical size. Isotope camera :

Because of the eve r present rad iation hazard,

isotope sources must be handled with

extreme care, stored and locke d in adeq uately

shielded containers when not in use. The

equipment, which accomplishes safe

handling and storage of isotope sources

together wit h a source, is called a camera.

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RADIOGRAPHIC

FILM

Radiographic fi lm s normally used as the

recording medium in radiography and

resembles a photographic fi lm. The film

consists of a) base, and b) emulsion. A cross

section of the f i lm

s

shown in f ig 5. The base

s a transparent me dium made of polyesters of

about 50 microns thick on wh ich the sensitive

To protect the delicate emulsion from

mechanical damage, a thin layer of gelatin

s

given as a protection layer.

DENSITY: The function of emulsion coating

is to produce opaque silver metal deposits

when exposed to light or radiation. The

degree of darkness

s

expressed as density.

Density, D Log

li

1

It )

li lntensity of Incident light t lntensity

of the transmitted light.

From this it can be seen that, darker the film,

higher the density. The density of the

radiograph, called the transmission density, s

measured either by the special instrument

called densitometer which directly gives an

electronic readout or compared with the

standard radiograph film which has different

calibrated densities The scale for density is

0

to 15, while 15 is almost opaque. Optimum

density for an industrial radiograph is

2

to

4.

Exposure: It is the product of intensity of

radiation and duration for which the energy

was acting on the film. To express

radiographic exposure, one has to give

Kilovoltage, Milliam peres and Time in case of

X-rays and type o f source, its strength in

curies and exposure time in minutes for an


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Chapter 1 Rt Training