GS Lecture-2 - 01_07_2013

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RADIOGRAPHY TECHNIQUES FOR THICKNESS QUALIFICATION

AND LATITUDE IMPROVEMENT

Dr. GURSHARAN SINGH

Associate DirectorRadiochemistry and Isotope GroupBhabha Atomic Research Centre

Mumbai - 400 085

INTRODUCTION

If a specimen is not of uniform thickness, the thickness difference betweenthe thinnest and thickest sections, which can be covered in a singleexposure, within the permitted density range, is called the thickness latitude.This value depends upon the type of material and the radiation energy used

for radiography testing.

The experimental studies detailed here can be used to develop thickness latitudetechnique with gamma ray sources, using the property of intensificationaction of lead screens.

2. FACTORS AFFECTING THICKNESS LATITUDE2.1 Radiographic Contrast

This is defined as optical density difference between two areas on the

radiograph and is expressed as:

D=D2 - D1

The permitted density range in a radiograph as per ASME Section V, forgamma radiography is between 2 and 4. Radiographic contrast consists of twoparts:

Subject contrast and Film contrast

2.1.1Subject ContrastConsider a specimen of maximum thickness T1and minimum thickness T2. If such aspecimen is exposed to radiation beam of intensity T0. The transmitted intensitiesat thicknesses T1and T2 will be T1and T2.
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Figure 2.1Concept of Subject Contrast

Subject contrast, depends upon the following factors;

Type of material to be radiographed Thickness difference in the object Radiation energy Scatter radiation

2.1.2Film ContrastPlot of log10exposure vs. the resulting optical densities of a x-ray film is called itsCharacteristic curve. For the radiation, to which it was exposed. A typicalcharacteristic curve is shown in figure 2.2. The slope of this curve at a given densityis measure of its film contrast or gradient at that density. The slope of the straightline, joining two points of specified densities, on the characteristic curve, ismeasure of the average gradient of the film, between these density values.
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Figure 2.2

Characteristic Curves of Various Types ofFilms

Screen type fast salt screenNon-Screen type very slow speed

Gradient of film, at density D = Tan

From the shape, and position of the characteristic optical density axis, we canobtain information about

Speed of the film Gradient of the film

When an object with varying thicknesses is radiographed different portions of theradiographed so obtained, will have different optical densities as shown in figure 2.3

Figure 2.3

Optical Densities for varying Thickness object

If a radiographs is so exposed that the thickness T1 is recorded as a filmdensity of about 2, with a technique giving good sensitivity, it is likely thatthe film density under T2will be so great that nothing can be seen throughthis part of the radiograph when viewed with a normal film-illuminator.Similarly if the film density under thickness T2 is exposed to be about 2, bythe use of a shorter exposure time , the part of the radiograph

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corresponding to the thicker section will have a very low density, At this lowfilm density, the film gradient is smaller and so the sensitivity obtained onthis part of the radiograph will be poor.

Figure 2.4 shows variation of film contrast with optical density. The film contrast orgradient controls the details visibity of the defects on the radiograph.

Figure 2.4Film Gradient Vs Film Density

The film contrast is affected by:

Type of the film Radiograph density Film processing Activity of developer

As stated above the film gradient varies with film density. To a first

approximation, for fine-grain film, the gradient is proportional to the density,over the density range used in industrial radiography. As a thickness rangein the specimen will result in a density range on the film, a differentsensitivity will therefore be obtained in each thickness. The problem of specifyinga suitable technique therefore must be decided in terms of sensitivities which willbe acceptable.

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Latitude problems deal with the problem of a high subject contrast. Some of thecommonly used methods to deal with such problems are mentioned below;

3. Latttude techniques with X-ray sources

3.1 Maximum density at thinnest section

The thinnest part of the specimen should be exposed to have as high afilm density as can be satisfactorily viewed. with fine-grain film there is noloss in sensitivity due to the use of very high film densities, except fromdeficiencies in the viewing equipment" A, local area of a radiograph, up to adensity of about 4.0 can usually be viewed satisfactorily using a maskedPhotoflood-type bulb. There will be a fall-off of sensitivity in the thickersections, which becomes progressively worse as the film density obtainedbecomes smaller, If the latitude required is not too large, this simplemethod may be satisfactory.

3.2 Use of harder radiation

Use of higher energy radiation will have the effect of reducing the sensitivity,

especially in the thinner sections. In the thicker regions, there are twoeffects which act in opposite directions" The use of harder radiation in itselfreduces sensitivity, but the greater penetration obtained means that thedensity difference between the thickest and thinnest section is less. Thus for agiven maximum film density on the radiograph. the density representing thethick section is not, so low, and the corresponding higher film gradient leadsto a better sensitivity. Experiments show that the latter effect tends to be

predominant, and usually the use of a slightly higher kilovoltage results inhardly any loss 1n sensitivity in the thicker portions of an irregular-sectionspecimen with, as already stated, a slight loss of sensitivity in the thinnerregions.

The use of a slightly higher kilovoltage has the secondary advantage ofshortening the exposure time. Obviously this technique cannot cope with allcases. If too large an increase in kilovoltage is used, there must, of course,be an overall loss in sensitivity. This technique is frequently used withadvantage in the radiography of thin butt welds having the excess weld-metalleft on, where a satisfactory film density is obtained through the thickness ofthe weld metal without an inconveniently high film density under the plate

metal, It is also applicable to the radiography of pipe-welds, when the filmis inside the pipe and bent away from the source of radiation, Without thismodified technique, the length of weld covered in each exposure on a smalldiameter pipe, tends to be uneconomically short.

This then is the first a important technique for dealing with a thickness rangewhich, with a normal technique, would result in too great a density range on the

film. This is illustrated in Figure 2.5.

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Figure 2.5Variation of Density with Thickness

a. Diagram of radiographic set-upb. Density variation along film lengthc. Useful length of film using normal techniqued. Useful length of film using higher energy radiation

3.3 Use of Filters with X-rays

Another way of effectively increasing the kilovoltage is to use a filter in theX-ray beam, close to the X-ray tube. The purpose of this filter, which is usually oflead is to absorb the softer components of the x-ray spectrum before these reachthe specimen. If the specimen is relatively thick, such filtration will have littleeffects as the specimen acts as its own filter, but if there are thin sections, filtrationcan be a very effective technique. Lead filters are usually used as they tend tocontribute a minimum of scattered radiation themselves and the filter is placed

close to the x-ray tube in order to reduce the proposition of scattered radiationgenerated in the filter which reaches the film.

Typical thickness of filters are :

200 kv x-rays: 0.01 0.02 in lead400 kv x-rays: 0.025-0.05 in lead
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As described above, the use of a filter close to the x-ray tube to produce a harderbeam of radiation is one of the simpler aspects of filtration techniques, as used inindustrial radiography.

3.3.1 Use of filters close to the source of radiation

The main purpose of such a filter is to absorb the softer components of theemergent X-ray beam, and so harden the radiation and reduce contrast.Reduction of contrast for a given working film density range clearly means anincrease in thickness latitude.

The very soft radiation emitted by the x-ray tube which the filter absorbs is theradiation which is most easily absorbed by the film and which has the greatestphotographic effect: it only reaches the film through the thinnest parts of thespecimen, or where there is no specimen, and in these regions it produces anintense film blackening. Thus in the case of radiography of a cylinder there isundercutting of the edges of the image of the cylinder, and detail in the specimen

at A and B will be obscured. The use of a filter on the x-ray tube can reduce thisundesirable undercutting

Another way of regarding the mode of operation of such a filter is to consider it

as adding an increment of thickness over the whole specimen. The use of filterationat the tube seems to have its principal application in the x-ray energy range 150 to400 kev. And even in this region there ios some advantage in combining it with theuse of a filter between the specimen and the film.

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Figure 2.6

Radiography of Cylinders

3.3.2 Use of filters close to the film.

The radiation reaching the film through a specimen is a mixture of the higher

energy components of the primary x-ray beam and scattered radiation generated inthe specimen itself: in the energy range in which Compton-scatter is produced thislatter will consists of lower energy radiation. A thin lead filter under the specimenwill tend to absorb a larger proportion of this softer scattered radiation, and if the

filter itself is not a serious source of additional scattered radiation, the ratio ofscattered-to-direct radiation at the film should be less when a filter between thespecimen and film is employed.

Is/Id can be measured experimentally by using film as the radiation detector,and the effect of such a filter can be determined. That the use of a filter doesreduce is/Id has been confirmed. There is a marked reduction with 400kv x-raysand a smaller effect with 200 kv x-rays. Experimental values obtained with Ir-192and Co-60 radiations and steel are plotted below:

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Figure 2.7

Is/Id vs Thickness

On uniform thickness specimens any gain in sensitivity which might be expected

from a reduction in Is/Id, from filtration, is nullified by the corresponding decreasein the effective absorption coefficient due to the hardeningof the radiation, but onirregular-section specimens such as, for example, the cylinder in figure 2.6 differentconsiderations apply. Near the edges a large proportion of the softer components ofthe radiation penetrates the specimen and also as these parts of the specimen are

some distance from the film scattered radiation generated here can spread intoother parts of the image. A filter in the cassette tends to absorb in the thinnestregions, near the edges of the specimen, is greatly improved.

The thicknesses of suitable filters are very similar to those already quoted for filterson the x-ray tube:

150 kV X-rays - 0.01 in lead200-250 kV - 0.02 in lead400 kV - 0.04 in lead1,000 kV - 0.05 in lead

Sometimes the use of a thick lead front intensifying screen, of the appropriatethickness, instead of thinner 0.001-0.003 inch thick lead screen required forintensification purposes.

In theory, thin tin screen under the lead filter would absorb the characteristic K-radiation of lead and so reduce further the radiation generated in the filter itself.But such refinements are rarely used in the industrial radiography filed.

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3.4 Separate Exposures

Of course, in an extreme case where the maximum attainable sensitivity isrequired in each thickness it may be necessary to take a separate radiograph ofeach part using a suitable high sensitivity technique in each case, but if anapparatus which can produce a large range of x-ray energy is available, such alaborious method will rarely be needed.

3.5 Problem of cylindrical objects

Theproblemof the radiography of specimens of irregular thickness has alreadybeen mentioned. But the radiography of specimens of varying thickness, whichhave their edges within the film area e.g. a cylinder with the X-ray beam at rightangles to the principal axis, represents a much more difficult problem. Maskingmethods are exceedingly laborious, and almost impossible to perform satisfactorily

if detail close to the edge of the specimen is required. But a sati

satisfactory radiograph can be obtained quite easily by the use of an appropriatefiltration technique.

Summarizing, if the specimen generates an excessive amount of scattered radiation,or has portionswhichare not close to the film, a filter between the specimen and

film is likely to give the best results. If however, an increase of latitude isrequired rather than a reduction of the effects of scattered radiation it may beadvantageous to put the filter close to the X-ray tube.

4. Latitude Technique with Ir-192 Source and Lead Screens.

The techniquedevelopmentwith gamma rays is different than that with x-ray, aswithgammaray sources, the filters are not effective. In this technique the results

of extensive experiments carriedouton the intensification action of metal screens

have been used. The studies have been carried out with the most commonly usedsource for open field radiography i.e Ir-192 and medium speed and mediumcontrast films tike Agfa D-7 and NDT- 65.

4.1 The need for Radiography Screens.

X-ray and Gamma ray radiography is usually carried out by sandwiching a doublecoated x ray film between a pair of intensifying screens. Let us consider the

factors which make them essential in radiography.

As we are aware, when an object is radiographed, the interaction of the absorbedradiation with the object material results in the production of :

- Low energy scattered radiation- Secondary electrons and- Secondary X-rays

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These radiations along with the primary transmitted radiation affect thePhotographic film placedbelowthe object. As these are of lower energythanthe

transmitted primary radiation, and are non-image forming, these will degrade the

image of the object.

One more point to be noted is that only the radiation,which isabsorbed in the

sensitive layer of film, causes photographic actionand not the radiation which just

passesthrough it. It is found that even with doule coated x-ray films,usedthese

days, the fraction of the total gamma -ray energyabsorbedin the sensitive layer

of film is very small". For example,with iridium-192 radiation, less than 1% of

the energy is absorbed; rest of the energy does not perform any useful

photographicwork.

Thus itfollowsthat there are two major disturbing factors, when radiographing an

object without use of radiography screens.

There is low radiation absorption in sensitive layer of film, which results inlonger

Exposure time. Low energy transmitted scattered radiation absorbs preferentially in the film

and reduces the radiograph quality.

Hence, to reduce exposure time and to improve image quality of radiographs, useof some mechanism is essential.

4.2 Types of Radiography Screens

There are two types of radiography screens used in radiography testing.

- Metallic Foil Screens [ mostly used] and- Fluorescent screens [ rarely used]

These screens differ in their basic characteristics e.g., speed, contrast and scatteredradiation reduction effects. Few years back, a new type of screens calledfluorometallicscreens were invented in Japan. These make use of best points ofboth these screens and cover up their weak points too.

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4.2.1 Metallic Foil Screens

4.2.1.1 Reduction of Scattered Radiation

As has already been pointed out, the absorbed primary radiation in object results inproduction of Low energy scattered. Radiation which falls along with the

transmittedprimaryX-rays or gammarays on the film. Since high atomic numberelements have more absorption capacity for low energy radiations,soif a metal

foil of high atomic number isplacedbetween object and film, it will absorb mostof the low energy radiation, before it falls on the film. Hence effect of the lowenergy, scattered radiation, obtained due to back scattering of the primaryradiation, from objects placed behind the specimen, a high atomic number sheetis placed behind the film. Hence a double coated x-ray film when sandwitchedbetween a pair of metallic foil screens, receives a reduced amount of scatteredradiation which results in an improved definition of the radiograph.

4.2.1.2 Intensifying Action

Intensifying action of a radiography screen is expressed in terms of IntensificationFactor (IF) which is defined as ratio of :

Exposure time required to produce a film Density without screen and Exposuretime for same density using screen

4.2.1.3 Reason for Intensification

Exposure to film, without use of screens is caused mainly due to electrons,

generated from photon interactions with the film emulsion. When metallic screensare employed, additional electrons are emitted, due to photon interaction withmetal (mainly due to photoelectric effect).

As the x-ray film is more sensitive to electrons than x-rays or gamma rays, thephotoelectrons ejected out of front lead screen when using Ir-192 gamma raysource. 40% due to direct gamma rays and 3% due to scattered and secondaryphotons. Hence it implies that intensification action due to metallic screens ismainly due to photoelectrons.

4.2.1.4 Dependence of Intensification factor on various factors

Intensification factor due to metallic screens depends upon:

- Metal of the foil- Thickness of the foil- Energy of radiation, and- Specimen thickness

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4.2.1.5 Dependence of Intensification factor on Atomic Number of Metallic

Foil

For a given radiation source, the number of electrons produced, depends uponnature of metal foil. It could be seen from Figure 2.8 that intensification factorincreases with atomi number of the metal. When atomic number of these metalsare very near to each other, then intensification factors increases with increase indensity. For gamma radiography, in most of the cases lead screens are used. Theother reasons for selecting lead over high atomic number materials areconsiderations of its low cost, its malleability and flexibility properties.

4.2.1.6 Dependence of Intensification factor on thickness of foil.

For a given source, when the thickness of foil is very small, the number ofelectrons reaching the film is not great and hence intensification factor is small. Asthe thickness of foil goes on increasing. I.F. increases grows to a maximum (this

thickness corresponds to range of photoeletronics in that metal) and then remainspractically constant. As thickness is further increased greater number of gammaphotons will be attenuated in upper layers and this will reduce the production ofphotoelectrons in the lower layers near the film, and hence, intensification factor

will start decreasing with increase in thickness. Figure 2.9 shows the practicalvalues of intensification factor obtained using various thicknesses of lead foils withIr-192 gamma source.

Figure 2.9Intensification Factor vs Lead Screen Thickness

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4.2.1.7 Dependence of Intensification factor on Energy of Radiation

The maximum effectiveness of an intensifying screen for recording informationfrom a beam of photons, passing through a specimen is supposed to occur whenenergy of transmitted photons is highest, for which the photoelectric effect inscreen material predominates. As the energy of photons increases from a lowvalue, the attenuation of photns diminishes sharply and reaches minimum atenergies ranging from 2-3 MeV. At energies of 1 MeV and above, absorptioncoefficients depend to a lesser extent on atomic number of material and hencethe difference between intensification due to a metal with high atomic number,and one with low, diminishes sharply. This is due to decreasing role ofphotoelectric process to Compton process even for high Z number materials. Atthese energies photon interactions become dependent mainly on mass per unitarea of material. Hence for all materials, used as front screens, equalintensification effect should be anticipated. But the stronger electron scattering inhigh atomic number materials, than in low atomic number materials causes the

difference. For gamma energies exceeding 2-3 MeV, the number of electronpositron pairs formed as gamma rays pass through matter increases greatly. Thisprocess of pair production depends upon atomic number of materials to beradiographed, so again the difference between intensification due to high and low

atomic numbers increases.

Table 1 gives the practical values of I.F. obtained, using Ir-192, Cs-137 and Co-60sources with lead screens.

TABLE 1Film Agfa Gavaert D-7

Source Front6 thou screen

I.F. For Density 2.0

Back 6 thouLead Screen

Double LeadScreen

Ir 192 2.2 2.2 3.8

Cs 137 1.1 1.8 2.0

Co-60 0.9 1.7 1.6

4.2.1.8 Dependence of Intensification factor on object Thickness

A specimen placed between source and film performs two functions:

- It filters the primary radiation- Gives low energy scattered radiation

The radiography screen can have different sensitivities for primary radiation and theradiation given by above two effects, hence the change in intensification factor withobject thickness can be expected. Figure 2.10 shows the practical plot ofintensification factor with material thickness. Screen thickness for maximum I.F with

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Ir-192 source, as obtained from figure 2.9 was used for the purpose. It shows thatas the material thickness increases, the intensification effect due to low energyscattered radiation is greater than the effect due to high energy filtered radiation.

Figure 2.10Intensification Factor vs Material Thickness

4.2.1.9 Various screen materials for higher energies

The use of various screen materials for radiography of steel, using Co-60 gamma

source, has been reported in literature. It has been reported that for Co-60radiography of steel, copper and steel screens gave equal radiographic sensitivityas did lead and tantalum screens, but copper screens gave better sensitivity thanlead screens, using same film. However lead screens gave Double theintensification factor than copper screens. It was found that by using a slow filmwith lead screens, which required double the exposure time than the film used withcopper screen, the sensitivity was improved, but still it was less than copper screensensitivity. Hence for radiography of steel using Co-60 source and linearaccelerators in the range 3-8 MeV, use of copper screens has been recommended.

Two possible reasons for difference in radiographic sensitivity could be sharpness,as affected by electron scattering and recorded subject contract. The high atomicnumber materials have increased electron scattering properties. An electrondirectional effect associated with stronger electron scattering in lead then coppermay account for difference in radiography sensitivity. Detail visibility also dependson recorded subject contrast which again is inversely proportional to scatter buildup factor.

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Table -2

Film: AGFA GAEVERT D 7

Source I.F For Film Density = 2.0

Front(1)

Screen(2)

Back(1)

Screen(2)

Double(1)+(2)

Screen2) + (2)

Ir-192 2.1 3.0 2.75 3.1 4.1 4.8

Cs-137 1.25 1.4 1.45 2.0 2.0 2.5

Co-60 1.4 1.4 1.9 2.3 2.3 3.0

4.3 Summary

The lead screens improve image quality and also give intensification action. Salt

screens give slightly more intensification in gamma radiography comparedto lead screens. However, the image quality with use of salt screens ispoor. Hence, salt screens are very rarely used in industrial radiography,that too where their disadvantages are clearly understood and aretolerable.

For development of latitude techniques, radiographic image quality ateach thickness of the extended range, is important. Hence, lead screensare useful for the purpose.

4.4 Technique Principle

From table-1 and figure 2.10, it can be observed that use of lead screens, forradiography of steel with Ir-192 source results in the following intensificationfactors;

- 2.2 with only front screen- 2.2 with only back screen- 3.8 4.2 with double lead screens, depending upon the object thickness.-

For all practical purposes, I.F can be regarded as radiation multiplication factor forthe transmitted radiation; this forms the basis for latitude technique development.

As we are aware, when an object with varying thickness is radiographed, it willresult in different optical densities at different sections of thickness as shown infigure 2.3. Depending upon the contrast of the D-ray film used, this densitydifference affects the exposure range covered in a single exposure as shown infigure 2.13. The contrast of most of the industrial X-ray films is constant andmaximum between densities 2.0 to 4.0 hence this is the permitted density range by

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most of the radiography codes of practice, as this gives the best visibility of thedefect image.

Figure 2.13Film Contrast vs Exposure Range Coverage

the above example, film A is faster than film B and has lower contrast, but as canbe observed, it covers a much longer exposure range than film B.

If the contrast of the film is lower, it affects the defect visibility too, hence for mostof the industrial radiography inspections, where normal radiographic sensitivity(2%) is desired, medium speed and medium contrast films like Agfa D-7 or NDT-65are used. For the development of latitude technique with lead screens, the studieswere thus carried out using Agfa D-7 film.

Figure 2.14 gives experimentally made characteristic curves of D-7 and NDT-65films with no screens, which gives maximum intensification action as per figure 2.9have been used. The average gradient of D-7 film between densities 2 to 4, has

been observed as 4.01

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Figure 2.14

Characteristic curves of AGFA D-7 and NDT 65 Films

Average gradient of the film is defined as:

Gradient = D2D1/ Log10E2Log10E1

This means, for steel specimen, if an exposure is taken on Agfa D-7 film with Ir

192 source so as to keep density difference at thinnest and thickest portionsbetween 2-4, a thickness difference of 20.9 mm can be inspected in a singleexposure. As the shape of characteristic curve of industrial films does not changewith lead screen exposures, as can be seen from figure 2.14, it implies that thethickness difference which can be inspected in a single exposure using single leadscreen and double lead screens will also remain to be 20.9mm.

4.4.1 No screen exposure

As described above, for no screen exposures, if an exposure at the thinnest portion(A) of the specimen, shown in figure 2.15 gives optical density 4.0, the point (B)

corresponding to optical density 2.0 will be 20.9 mm away from point (A).

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Figure 2.15

Varying Thickness Specimen

4.4.2 Single Lead Screen Exposure

From the above, we can see that;Radiation reduction factor at point B=E/3.16 now if we put a single lead screen, it

gives I.F = 2.2 this means that radiation amplification factor = 2.2

2n = 2.2n= 1.137, n is number of HVTSn x 12.5 = 14.2mm

This means that;- At point A, which is 14.2mm away from point A, optical density on the

radiograph will be 4.0 with SLS exposure.- At point B, 20.9 mm away from A, O.D will be + 2.0- The portion A B represents the overlap area between no-screen and

single screen exposures.

-4.4.3 Double lead screen exposure

From figure 2.10, we can see that at higher thicknesses, I.F due to double leadscreen exposure = 4.2

Net radiation amplification w.r.t. single lead exposure= 4.2 / 2.2 = 1.9

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2n= 1.9

n = .9329 n x 12.5 = 11.66 mmThis means that;

- at point Awhich is 11.66 mm away from A, O.D = 4.0- at point B, which is 20.9 mm away from Aagain the optical density will

be = 2.0- Portion between ABrepresents overlap area of screen and double lead

screen exposures.

Hence net thickness difference where O.D values are between 4 and 2 is thedistance between point A and B.

This is equal to: T + 14.2 + 11.66 + 20.9

= T + 46.76 mm

T is the minimum thickness at point A.

This implies that a thickness difference of about 46mm can becovered in a single exposure using this technique.

4.4.4 Recommended thickness range for Ir-192

For every radiation source and material, there is a minimum recommendedthickness for acceptable sensitivity values. The recommended values, for Ir-192

and Co-60 sources and steel, as per ASME SectionV are given in table -3. 3.

T A B L E 3

_____________________________________________________

Material Minimum thickness_____________________________

Ir 192 Co 60______________________________________________________

Steel 19 mm 38 mm

The maximum thickness for the use of sources is primarily dictated by exposuretime. Therefore, the upper limits are not show, for normal technique, the uppersteel thickness for ir-192 source is approximately 65mm. from the above table if weconsider T=19mm, we can conclude that the thickness difference = T+ 46.76 =

65.76mm can be radiographed in a single exposure.
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4.4.5 Applicability of the technique with Cs-137 and Co-60 sources

From table-1 we can observe that I.F for Cs 137 and Co-60 are very less withsingle and double lead screen exposures. Hence this technique cannot be used withthese sources.

4.4.6 Sensitivity values

The results of the experimental radiographic sensitivities, obtained under differentareas, using the above mentioned technique are given in table 4.

TABLE 4

Area Thicknessfor IQI

selection

Radiographic Sensitivity

DIN IQI ASTM PLATEIQI

Min VisibleWire dia.

S% Number HoleVisible

Equiv.S%

N S 12mm 0.2mm 1.66 10 4T 2.8

SLS 29mm 0.5mm 1.72 20 4T 2.4

DLS 40mm 0.8mm 2.0 30 2T 1.87

4.5 Conclusion

It can be observed from the experimental results that by using a single source, filmand commonly used lead screens, it is possible to extend the radiographic thicknesslatitude with acceptable radiographic sensitivities obtained at each thickness.

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GS Lecture-2 - 01_07_2013