rt training course and important notes for qa

7/28/2019 RT Training Course and Important Notes for QA

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RT Training course and important notes for QA/QC Engineers

In Industrial Radiography, the usual procedure for producing a radiograph is to have a penetrating radiation (X-Rays or Gamma Rays) on one side of the specimen to be examined and a detector of the radiation (the film) on

the other side. (Fig 1)The energy of the radiation must be chosen so that sufficient radiation is transmitted throughto the detector.The detector is usually a sheet of photographic film, held in a light-tight envelops (dark

plastic cover) or cassette having a very thin front surface which allows the x- ray to passthrough. The source of radiation is chosen to be physically small ( a few millimeters indiameter) and as X-rays travel in straight lines from the source through the specimen to thefilm, a sharp image is formed on the film of any defects, edges, or discontinuities in thespecimen. The geometric image formation is exactly analogous to shadow formation with avisible light source and the sharpness of the image depends, in the same way, on the radiationsource diameter and its distance away from the surface on which the image is formed.

Cavity

SOURCE

Specimen

Lead Screens

Film

The film in its cassette is normally placed behind the specimen and the X-rays are switchedon for an appropriate time (the exposure time) after which the film is taken away and

processed photographically, i.e. developed, fixed , washed and dried. The film(radiograph isthen placed on an illuminated screen , when the differences in X-ray intensity in the X-ray

beam transmitted through the specimen( Differential absorption ) can be seen to bereproduced on the film as differences in film blackening (Photographic Density) , which areseen on the illuminated film as differences in brightness.When the radiograph is placed on an illuminated screen for visual inspection, the parts of thefilm which have received more radiation during exposure- the regions under cavities for

example-appear darker, that is, the density is higher.The quality of the film can be assessed in terms of three factors, namely contrast, definitionand graininess.Density= Log A1/A2 where A1 is the intensity of incident light of the illuminator and A2 is theintensity of the transmitted light after passing through the film.

Strength of the source or activity is expressed in Curies1 Curie is equal to 3 x 1010 disintegrations per second. SI unit is Becquerel and is1disintegration per second 1 curie= 37 GBqUnit of exposure is expressed in Roentgen and it is the quantity of radiation that produces 1esu of electric charge in 1cc of air at NTPSI unit is Grey and 1 grey = 100 Roentgen


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Rem is the unit for biological damage to human tissues and for gamma and X-Ray Roentgenand Rem are equalSI Unit of rem is Sievert and 1Sv = 100 R

Distance Square Law E1/E2 = d22 / d12 Where E1 is the exposure at d1 meters and E2 the

exposure at d2 meters.Half value layer is the thickness of material which reduces one half of the original intensity.For steel and for IR192 source of radiation, the HVL is 12.5 mmRadiation level dose rate at cordon off distance should be 0.75 mR/Hr.

RHM (Roentgen per hour at 1 meter per curie) factor is the radiation output defined for eachisotope of specific energy. For IR 192, it is 500 mR/Hr

Radiation Rate meters are survey meters which shows the rate of exposure.Personnel monitors are pocket dosimeters, film badges and TLDs. Exposure is readcumulative.

Wire diameters when using DIN 1-7, 6-12, 10-16 penetrametersDesignated

wire Number

1 2 3 4 5 6 7 8

Diameter of

the wire

3.2mm

2.5mm

2.0mm

1.6mm

1.25mm

1.0 mm 0.8 mm 0.63mm

Designated

wire Number

9 10 11 12 13 14 15 16

Diameter of

the wire

0.5mm

0.4mm

0.32mm

0.25mm

0.2mm

0.16mm

0.125mm

0.1 mm

Techniques in radiography is Single wall single image, double wall double image, double

wall single image and panoramic.Penetrameter sensitivity is T/100 x TH/2 Where T is the thickness of the penetrameter andH is the hole diameter visible on the radiograph.Radioactivity is the property possessed by certain elements of emitting , and rays.Artificial radioisotopes are obtained by fission or irradiation in a nuclear reactor.The half life of a radioactive source is the period of time in which the intensity of theradiation emitted will fall to one-half of its initial vale.HVL for steel is 12.5 mm with IR 192 isotope and TVL is 40 mmHVL for concrete is 43 mm with IR192 isotope and TVL is 138 mmTo calculate the present strength of a Iridium 192 radio nuclei, the formula is originalstrength at the time of procurement x 2-No. of days passed/75

Defect Descernability;The image is affected by sharpness and definitionUn-sharpness is due to geometry, radiation energy (inherent) and filmTotal unsharpness = Ug + Ui + Uf Ug = s x a /(F-a) :s is the source size, a is the thickness of the specimen and F the source to film distance.Exposure time is directly proportional to the square of the distanceET1/ET2 = d12/d22 Where ET1 is the exposure time at d1 meters and E2 the exposure time at d2meters.For un-sharpness due to radiation energy, the following table gives the rough estimate forfilms of AGFA make.

Radiation Energy Un-sharpness in mm

50 KV X-Rays 0.03100 KV X-Rays 0.05


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200 KV X-Rays 0.10400 KV X-Rays 0.152 MV X-Rays 0.328 MV X-Rays 0.6031 MV X-Rays 1.00

Ir192 rays 0.17Co60 rays 0.35Yb169 rays 0.07-0.13

Choice of Source to film distance (s.f.d.)

To obtain a radiograph which is as sharp as possible, so as to show maximum details, thetotal un-sharpness should be kept minimum. This is done by determining the value of U fromthe above table, and then making Ug = UT which enables the desirable s.f.d F to becalculated from F= t (UT + s)/ UTWhereUT is the total un-sharpness (mm) t is the specimen thickness (mm) s is the effectivesource diameter (mm)

As per code 0 to 2 thick specimen should have 0.51 mm un-sharpness.F for 2 thk with source size of 2 mm = 50 x 2.51/0.51 = 246 mm or 10 inches minimum.Home work Questions

1.When radio graphed a weld of thickness 50 mm , designated wire number of 1 to 4 (1.6mm) is visible, 5,6 and 7 are not visible with the DIN ISO 1-7

penetrameter. What is the sensitivity achieved?2 calculate the dose rate output at 15 meters from a 50 Ci source of IR 192 ( RHM for IR192is 0.5 )3 A 10 Ci source is used to take a pipe joint. Calculate the cordon off distance to be markedso that the dose rate is to be 0.75 mR/Hr as per RPD(IAEC)

regulation.

4. A 100 Ci source kept in a shielding container of lead of diameter thickness 18 mm. Thecontainer is again safely kept inside a room of 10 feet x 10 feet withconcrete wall thichness of 129 mm. Imagine the container is at the centre of the room. A

person working at a distance of 10 feet from the room and the worktime is around 30 minutes. Find out how much of dose he would have received.HVL Lead is 3 mm and for concrete is 43 mm RHM for IR 192 is 0.5

5. calculate the geometric un sharpness when radio-graphing a weld of thickness 25 mm witha SFD of 20 inches and with a source size of 2 mm.6. A source of Ir 192 of strength 20 Ci today will be _______ Ci after 225 days. (half Life 75days)7. From an exposure chart for a SFD of 20 inches an exposure time of 10 minutes indicated.

What would be the exposure time if the SFD is increased to 1.5meters.

8.We obtained a radiographic density of 2 using D7 film with a weld of 20 mm. theExposure time is 10 minutes.Now the client wants 2.5 Density. With all other parameters

being the same what is the new exposure time. From the characteristic curve of D7 we find alog relative exposure value for 2 density is 2.5 and that for density 2.5 is2.65.9.I have taken a radiograph of a weld, 5 months ago with Ir 192 source of 10 curie giving 5minutes exposure time. Now the same weld to be taken with the same settings and parametersWhat will be the exposure time.10.For radiographing a 12.5 mm thick weld, an exposure time of 30 min is given under someconditions. Now with the same conditions and parameters I want to do 25 mm thick weld.

Calculate the new exposure time.

rt training course and important notes for qa

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