Industrial Irradiators for Radiation Processing

Transcription

1 Industrial Irradiators for Radiation Processing Radiation processing beacame a reality with the availability of particle accelerators and artificially produced radioactive sources (60Co and 137CS)

2 Industrial Irradiators Electron accelerators widely used, 0.1 to 10 MeV X-rays, mostly used in medical diagnostics and radiography; some used in radiation processing (3 to 10 MeV) Isotope sources; medical sterilization and food irradiation Heavy ion accelerators, mostly for ion implantation Synchrotrons, mostly for resist work Nuclear reactors, for producing radioisotopes

3 Radioisotope vs Electron Accelerator Source Electrical input Electron. acceleration Scanned electron I)eam 60CO or 137CS photon emission, continuous, in all directions X-ray conversion plate Electron or X-ray beam available when needed, in the desired direction

4 Components ofan Irradiation Facility 1. Radiation Source Electron accelerator of specified power and electron energy 60Co source of specified strength 2. Radiation Shielding Concrete «3 m, for 10 MeV electrons) or lead shielding, or pool of water between the irradiator and workers 3. Target Room The area where actual irradiations are done

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6 Components ofan Irradiation Facility (contd) 7. Shipping and Receiving Areas They should be well separated from each other to prevent mixing of irradiated and unirradiated products 8. Safety Devices and Monitors Radiation monitors, set to shut - off the system at predetermined dose Air conditioning - temperature fluctuations detrimental to processing Large air flow - to maintain ozone and NO x levels low Ozone monitors - to show when it is safe to enter the target room

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13 Electron Accelerators Advantages II Various power and electron energy levels available.. Very high dose rates.. Generally, short processing time 41 Cost increases only marginally with power 41 Cost increases. with electron energy 41 Can be switched off when not required II Can be used for electrons or X..rays 41 Directional beam (horizontal or vertical).. Better utilization o'f beam energy >95% availability reported

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17 The penetration ofelectrons increases with increasing electron energy as shown by the depth/dose curves Depth (g1cm 2 ) The dose uniformity increases with increasing electron energy

18 The penetration of electrons increases with increasing electron energy as shown by the depth/dose curves Q) (f) o Q) > 0- +-' -co Q) a: Depth (g/cm 2 ) The dose uniformity increases with increasing electron energy

19 The penetration of electrons increases with increasing electron energy as shown by the depth/dose curves (I) en o C (I). > - -«S (I) a:: Depth (g/cm 2 ) The dose uniformity increases with increasing electron energy

20 The penetration of electrons increases with increasing eleclron energy as shown by the depth/dose curves (1) CJ) o c (1) >. - co -(1) a: Depth (g/cm 2 ) The dose uniformity increases with increasing electron energy

21 Electron Beam Penetration Typical Depth/Dose Curve for 10 MeV Electrons Q) 140 ic;;;;i,vptimum Thickness of Sample ~ 120 j ::==-= Maximum Relative Dose ~ 100 :='::::::::::::::::: Minimum Required Dose 80.. '. - > 60,,..... ~....:::: 40 :: : Q) 20 :: :: B a: 0 I:::::::::::::::': o Depth (g/cm 2 ) 5 6 Dose first increases with penetration and then decreases Penetration proportional to 1/density At optimum thickness, dose uniformity is ±12.5 %

22 ELECTRON BEAM PENETRATION One-Sided vs Two-Sided Irradiation for 10 MeV Electrons By optimizing two-sided irradiation, the effective penetration of e- beam can be increased by a factor of >2 140 Q) 120 ~ 100 C (1) 80.~ Q) 40 a:: 20 o o... \~ 1 ~ /., -, "!~ lid ~ ~ til " " " " "., ::l ~ '-' '-' " " " " ~ '-' '" '" '-' ~ ~ ~ "- 7 8 Dose Depth (g/cm)

23 Dose Distribution in Water as a Function ofdepth (Gamma Radiation from 60Co; Saylor, 1997) 1.2 0)1 > ; ~ 0.6 -Q) 0.4 tn o C 0.2..One-Sided Two-Sided TOne-Sided O+-----r r--.--~----,---.., o Distance from 60CO Source (cm)

24 Comparison ofrelative Dose vs Depth For 60CO y-rays and 5 MeV X-Rays CI> tn o C CI>. > --CO CI> a: Depth in Product (em) Fpr sterilizations of typical packages of medical disposables

25 I Radioactive Decay of 137Cs and 60Co I CS ~ > > <C 30 60CO Time (years)

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28 Capital Cost per kw ofelectron Accelerator. (lomev) ~ ~ Power (kw)

29 Cost vs Power of 10 MeV Accelerators ;; ::) ~ t/) < o kw

30 Electron Processing Rubber Coatings 10% 12% Other 60/0 Foam r=.::::::::::;~~ 4% H.eat Shrmkable 34% Wire Insulation 34% -500 Accelerators Worldwide (Saunders,1988; now -1000) Sources Woldwide for Medical Sterilization and Food Irradiation

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32 Energy Spectrum ofx-rays from 5 MeV Electrons..- ri.l... 6.~ := 5 ;:J 4 ~ ~ 3 ea ~....~ 2,!:J ~ 1 < 0 ~ E (MeV) The average energy of X-rays is 1.06 MeV

33 Nominal Equivalence ofelectron Accelerators and 60Co 50 kw of electron beam = 3.38 MCi 60CO X-rays from 5 MeV, 200 kw electron accelerator =16 kw =1.1 MCi 60CO X-rays from 10 MeV, 50 kw electron accelerator = 10 kw =0.67 MCi of 60CO

34 Types of Scanning in Low-Energy Electron Accelerators Scanning coil -.,,,,, Electrons o ~ Product...,',,, CURTAIN TYPE Filament ~ a: a ~. &i l l t Ii t l Ii ~.J,",,--~,, 1 I I 1 I I I, I /.~ ~ ~ ~ ~-~--~_~_~ I, ~ ~--' ,," '\ W."ndow I I I " I I \ \ I \ \ \ /, I It, ' I 'I I, I \ I I 1 I I " - - -,,'" 7 1- ~ ~ \' ~',..J.\t!!!:.:::!:::!::!::!::!::::!:::::!::::!::::!:=::;;;, I I \ \,, \,,, \ 1 \ \ I, I I,\ 1/ J I \ \ I" I \, _...;._-' ,;_ SCANNING TYPE )' '''''''1\111,._ j 1 I I 1 I \ I 1 I, I 1 \,, I I I I I,,, 1 t Il I I, \ I I 1 1,

35 Direct Electron Accelerator Principle Of Operation (Cleland,1992) Low Voltage AC Power DC Power I -Q., Q.,-----'--1 High Voltage '----I Generator High Voltage,..--i:T~Gl~=rrElectron Gun a+--i-~ Accelerator Tube,--I--L-." ~i!:::t~electronbeam Control System ~ Scan Magnet ~~ Chamber Scanned Electron Beam ~ Foil Window

36 Traveling-Wave Linear Electron Accelerator Klystron 3000 MHz Buncher Vacuum esonator Tank Electrol'l-----'1-.-I Gun Cavity Resonators Load Resistance --e!:;::: ~~~~~~~&-~,(? ~-~oil Synchronization rn~~~ Electron Beam Window atching

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40 Silicon Lithography hv or e - '<;l- Mask "'t- Silicon Oxide "'t- Silicon Treated :::::::::;.~*~B=E::r"'t-Resist Areas Negative R~~ Development ~ Patterns \\ ~.-After Etching Positive

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42 Characterization ofthe Irradiator Source: e- Determine depth/dose curves wih a wedge Yes Yes h > Expected enetration Alunimum -<d=s Wedge '<d"=" Film "'- OJr Dosimeter Determine dose profile ~~ Beam spot Yes Yes

43 Characterization ofthe Irradiator Source: e- 'Y Determine Dose Profile 00 o -==Scan Yes Yes Determine Nominal Dose Received by Product Yes Yes Product Conveyor System

44 Conclusions Gamma irradiation would continue to be an important component of industrial radiation processing Industrial electron irradiation would continue to grow for most of the current products Areas of major growth for electron accelerators are most likely to include environmental (water purification, sewage sludge irradiation, flue gas irradiation), viscose, and advanced composites The availability of a good variety of electron accelerators in a wide energy ranae (0.2 to 10 MeV) is conducive to growth of the radiation processing industry Continued effort to increase understanding and usefulness of the technology would also help the growth of this industry