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Technical Discussions
rajan
rajan
08:32 Aug-15-2008
expose chart for selinium75

pleas anybody can give me a expose formula or chart for seliniun isotope using gamma radiography


 
 Reply 
 
collin maloney
NDT Inspector, - Plant Inspector
Applus RTD, Australia, Joined Nov 2000, 147

collin maloney

NDT Inspector, - Plant Inspector
Applus RTD,
Australia,
Joined Nov 2000
147
01:30 Aug-21-2008
Re: expose chart for selinium75
----------- Start Original Message -----------
: pleas anybody can give me a expose formula or chart for seliniun isotope using gamma radiography
------------ End Original Message ------------

If you multiply your Ir192 exposures by 4 you will achieve acceptable images.


 
 Reply 
 
Scott Cargill
Scott Cargill
02:53 Aug-26-2008
Re: expose chart for selinium75
----------- Start Original Message -----------
: : pleas anybody can give me a expose formula or chart for seliniun isotope using gamma radiography
: If you multiply your Ir192 exposures by 4 you will achieve acceptable images.
------------ End Original Message ------------

Selinium75 has a constant of 2.2 R per curie compared to Iridium 192's 5.2 R.

Multiply your exposures by a factor of 2.4 x




 
 Reply 
 
Slawomir Mackiewicz
Slawomir Mackiewicz
01:48 Sep-12-2008
Re: expose chart for selinium75
Hi
Rules of thumb given by Scot and Collin (i.e. multiply the exposure time calculated for Ir-192 by a factor 4 or by a factor 2.4) are both equally wrong or… good !

The ratio of exposure constants (dosage rates) for Ir-192 and Se-75 is only a part of the story. These two radionuclides have different energy spectra. For Se-75 main emission lines are between 136 and 280 keV whereas for Ir-192 they are between 296 and 468 keV. As a result Selenium radiation is much ‘softer’ and its linear attenuation coefficient is higher than for Iridium. Their HVL values in steel are respectively 6.2 and 9.2 mm.

It means that exposure chart for Se-75 and Ir-192 have different slopes. As a result there is no single factor connecting exposure times for Ir-192 and Se-75. For example factor 2.4, mentioned by Scott, is good for steel thickness, say 10-15mm, whereas factor 4 mentioned by Collin may be good for thicker plates, say 25-30 mm.

The Rajan request for Se-75 exposure chart can not be simply answered as the exposures required for particular testing conditions are determined by many changing factors, as type/brand of film, thickness of Pb screens, type of processing, mode of processing, type/brand of used chemicals etc. So there are no one universal chart, sliding ruler or computer program good for all RT laboratories.

The solution is purchasing a professional software for exposure calculations (necessary with tuning up/auto-correction function) or preparation the exposure chart reflecting all working conditions of own laboratory.

----------- Start Original Message -----------
: pleas anybody can give me a expose formula or chart for seliniun isotope using gamma radiography
------------ End Original Message ------------




 
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