Forum Technical Discussions Re: radiography Posted by: tj , E-mail: Address, on May 29, 2008 at 14:17 : In Reply to: Re: radiography posted by : DJ Kallhof , E-mail: Address, on May 16, 2008 at 21:14 : ----------- Start Original Message ----------- : Actually, the name is Dennis. Thank you for the math correction. I don't typically work with metric, the USA has caught up there yet, I mistakenly divided by 25.4 rather than multiplied. My bad. The point I was trying to make, is unless the collimator is the same size or smaller then the effective source size, it will not affect the beam spread. I didn't consider 360 degree beam spread being cut down to under 180 degrees as collimating, but I guess it is. : : As an afterthought, David you seem to be describiing image shadow formation (penumbra) or geometric unsharpness (Ug), which DOES depend on source size (as well as the set-up geometry) and IS affected by the source size. Different phenomenon from beam spread. : : : David : : : Could you clarify your sentence: : : : "The beam spread in industrial radiography is dictated by the effective source size of the isotope, the portal of the collimator simply allows that to occur." : : : In my opinion the source size is (almost) irrelevant in determining the geometric beam spread as the uncollimaed source emits omnidirectional radiation. Collimating attenuates radiation which travels into the body of the collimator - but the collimator's aperture emits essentially uncollimated radiation, the aperture dimensions will determine the extent of the beam spread. : : : The beam spread of a collimated source CANNOT be the same as that of an uncollimated dosurce. The collimator effectively turns an almost spherical radiation distribution from an uncollimated source into an approximately conical radiation beam : : : : : : Also something is wrong with your Imperial to metric conversions. 0.1" is 2,5mm. therefore the effective source size is the direct equivalent of 0.141", namely 3.54mm : : : Cheers : : : Nigel : : : : : : : I performed industrial radiography for 8 years before settling into aerospace. I may be able to offer some insight. The source size of your typical Ir 192 source is app. 0.100" X 0.100" (.00393mm X .00393mm) yielding an effective source size of 0.141" (.0055mm). If memory serves me, the portal on most collimators was app. somewhere between .250" -.375" (.0098mm - .01476mm). The beam spread in industrial radiography is dictated by the effective source size of the isotope, the portal of the collimator simply allows that to occur. The source simply floats somewhere in the opening of the much larger collimator portal. The function of the collimator is for the safety of the radiographer and anyone outside the ropes. : : : : : "No any change in beam spread"? What would be the purpose of collimators if they dont restrict the beam spread, Gowri? Perhaps you misunderstood Nandeesh's question. : : : : : Nandeesh, the collimator supplier should have provided a data sheet at the time of purchase detailing material, dimensions and beam spread. If you dont have that then theoretically you could make a set-up using a 40 x 30 film and precisely measure the source to film distance, expose for a couple of seconds to the collimated beam and measure the dimensions of the significant exposure on the developed film. At least it would give you a qualitative idea of the beam cone. : : : : : Hope that helps. Safe (as possible) shooting! : : : : : Nigel : : : : : : : Can u pls tell me how to estimate the beam spread in Gamma radiography when using A COLLIMATOR? : : : : : : Hi, : : : : : : This is Gowri from GJ. First of all we have to know that collimator is used to guide the radiation and for safe practice in dangerous Radiography. collimators are made up of high density metals eg. tungsten and heavy alloys to absorb the non required ionising radiation for safety. I think so there is no any change in beam spread when using collimators, rather than as usual. ------------ End Original Message ---------- Hello, You can think of the source as a light bulb on a lamp. If left bare you get radiation in 360 degrees. When you put a shade on the lamp the radiation, as you can see, is attenuated in areas covered by lamp shade. You can also see that the "beam" not attenuated by the shade is directly proportional to the dimensions of the shade or collimator. If you change the size of the lightbulb the effect remains the same (as long as the bulb fits in the shade!). Use a narrow lampshade for a narrow beam and vice/versa. Follow Ups: Post a Followup: Name: E-Mail: Subject: Comments: ----------- Start Original Message ----------- : : Actually, the name is Dennis. Thank you for the math correction. I don't typically work with metric, the USA has caught up there yet, I mistakenly divided by 25.4 rather than multiplied. My bad. The point I was trying to make, is unless the collimator is the same size or smaller then the effective source size, it will not affect the beam spread. I didn't consider 360 degree beam spread being cut down to under 180 degrees as collimating, but I guess it is. : : : As an afterthought, David you seem to be describiing image shadow formation (penumbra) or geometric unsharpness (Ug), which DOES depend on source size (as well as the set-up geometry) and IS affected by the source size. Different phenomenon from beam spread. : : : : David : : : : Could you clarify your sentence: : : : : "The beam spread in industrial radiography is dictated by the effective source size of the isotope, the portal of the collimator simply allows that to occur." : : : : In my opinion the source size is (almost) irrelevant in determining the geometric beam spread as the uncollimaed source emits omnidirectional radiation. Collimating attenuates radiation which travels into the body of the collimator - but the collimator's aperture emits essentially uncollimated radiation, the aperture dimensions will determine the extent of the beam spread. : : : : The beam spread of a collimated source CANNOT be the same as that of an uncollimated dosurce. The collimator effectively turns an almost spherical radiation distribution from an uncollimated source into an approximately conical radiation beam : : : : : : : : Also something is wrong with your Imperial to metric conversions. 0.1" is 2,5mm. therefore the effective source size is the direct equivalent of 0.141", namely 3.54mm : : : : Cheers : : : : Nigel : : : : : : : : : I performed industrial radiography for 8 years before settling into aerospace. I may be able to offer some insight. The source size of your typical Ir 192 source is app. 0.100" X 0.100" (.00393mm X .00393mm) yielding an effective source size of 0.141" (.0055mm). If memory serves me, the portal on most collimators was app. somewhere between .250" -.375" (.0098mm - .01476mm). The beam spread in industrial radiography is dictated by the effective source size of the isotope, the portal of the collimator simply allows that to occur. The source simply floats somewhere in the opening of the much larger collimator portal. The function of the collimator is for the safety of the radiographer and anyone outside the ropes. : : : : : : "No any change in beam spread"? What would be the purpose of collimators if they dont restrict the beam spread, Gowri? Perhaps you misunderstood Nandeesh's question. : : : : : : Nandeesh, the collimator supplier should have provided a data sheet at the time of purchase detailing material, dimensions and beam spread. If you dont have that then theoretically you could make a set-up using a 40 x 30 film and precisely measure the source to film distance, expose for a couple of seconds to the collimated beam and measure the dimensions of the significant exposure on the developed film. At least it would give you a qualitative idea of the beam cone. : : : : : : Hope that helps. Safe (as possible) shooting! : : : : : : Nigel : : : : : : : : Can u pls tell me how to estimate the beam spread in Gamma radiography when using A COLLIMATOR? : : : : : : : Hi, : : : : : : : This is Gowri from GJ. First of all we have to know that collimator is used to guide the radiation and for safe practice in dangerous Radiography. collimators are made up of high density metals eg. tungsten and heavy alloys to absorb the non required ionising radiation for safety. I think so there is no any change in beam spread when using collimators, rather than as usual. : ------------ End Original Message ---------- : Hello, : You can think of the source as a light bulb on a lamp. If left bare you get radiation in 360 degrees. : When you put a shade on the lamp the radiation, as you can see, is attenuated in areas covered by lamp shade. You can also see that the "beam" not attenuated by the shade is directly proportional to the dimensions of the shade or collimator. If you change the size of the lightbulb the effect remains the same (as long as the bulb fits in the shade!). Use a narrow lampshade for a narrow beam and vice/versa. ------------ End Original Message ------------ Optional Link URL: Example: "http://www....html". Alternative write "http://www.ndt.net/wshop/attach/filename.htm" if you transfered a filename.htm via FTP Page Title: is important! Optional Image URL: http://www....gif Attention! Please click the button only once. This may take a moment to process Forum Technical Discussions Help