When considering E-beam irradiation of a product, several key concepts should be taken into account in order to figure out the specifications of the accelerator;
Dose |
The use of ionizing radiation in any process depends on the transfer of energy from the radiation beam to the material being processed.
The quantity measured is called the dose. Dose is then defined as the energy absorbed by the material or product, not what is emitted by radiation source or E-beam accelerators.
The dose measurement is a critical part of radiation processing.
Electron beam dose is measured officially in terms of a unit called the gray.
One Gray is defined as the absorption of 1 Joule of energy per Kilogram of material.
The practical unit is the kGy, Kilo Gray, (one thousand Gray).
-Typical Doses
Penetration |
Energy
The total distance which electrons can penetrate into a given material is more or less a linear function of the energy of the electrons (more energy = more penetration).
Density
The total distance which electrons can penetrate into a given material is a linear function of the density of the material (higher density = less penetration).
-More about Penetration
Practically, the penetration of the electrons into the material will depend on the energy of the electrons, the density of the material considered and the implementation of single/double side irradiation.
As an illustration, using an energy of 10 MeV, the single side penetration of the electrons in a product having a density of 1 (water = 1 g/cm3) is 3,6 cm (considering the “equal-entrance-exit point” concept). The same product irradiated from both sides could have a penetration of up to 9 cm.
Throughput |
Once the energy level has been set – and is sufficient - for a given product, in a given process, the power of the whole system must be evaluated in order to meet the quantitative requirements. Evidently, it takes more power to irradiate a couple units of products per days than several thousands per hour.
The formulae most often used to figure out the power requirements of the equipment is based on the “area method”
A/T = K F I / D
- A = Area (m2) ;
- T = Time (min);
- K = Area Processing Coefficient (function of the energy as per table below)
Energy (MeV) |
0,3 |
0,4 |
0,5 |
0,6 |
0,8 |
1,0 |
1,5 |
2,0 |
3,0 |
4,0 |
5,0 |
10,0 |
K (Mrad m2/mA min) |
3,09 |
2,26 |
1,86 |
1,64 |
1,42 |
1,31 |
1,20 |
1,16 |
1,13 |
1,13 |
1,14 |
1,17 |
- F = Fraction of electrons which are actually absorbed by the product.
- I = beam Current (mA) ; this is a major specification of the accelerator.
- D = Dose (Mrad) - note : 1Mrad = 10 kGy
More about Throughput
Accelerator Requirements for E-beam Processing
Examples of Industrial E-beam Applications |
In order to illustrate the above concepts, a couple of industrial E-beam irradiation cases can be looked at shortly.
Keep in mind however that these are simplified approaches in order to illustrate the concept. Real evaluation require additional analyses and thorough studies
Exemple of Medical Device Sterilization
Wire Jacket Irradiation
Crosslinking |
The benefits of crosslinking are to change thermoplastic materials into thermosets and to cure rubber. The molecules in these materials tend to slip and slide over one another fairly freely. As temperature rises, these uncrosslinked materials soften and finally melt. When they are crosslinked, molecular movement is severely impeded; and the form is stable against heat. This locking together of molecules is the origin of all the benefits of crosslinking, including:
- Increased Tensile Strength.
- Increased Form Stability.
- Resistance to Deformation.
- Resistance to Solvents.
- Shrink-Memory.
- Resistance to Stress Cracking.
The effects of the Amazing Electron Beam on polymeric materials vary with the type of polymer. Although there are exceptions, most polyolefins and halogenated polyolefins will crosslink. Crosslinking also occurs in many elastomers and other aliphatic polymers, although individual checking is required. Aromatic polymers, on the other hand, generally resist crosslinking.
Biological Effects |
There are two applications for electron beam irradiation involving biological systems. The first, known as disinfestation, entails the elimination of live insects from grain, tobacco, and other unprocessed bulk crops. Disinfestation requires a very low dose, typically on the order of 1K Gray or less. The second applications involves partial or complete sterilization of medical products and aseptic packaging materials for foods. Adequate and certain microbial reduction is essential in such products to eliminate the possibility of transmitting infectious micro-organisms to people. The dose required in these applications may range up to 35K Gray depending on the organisms involved and their resistance to radiation.
Although radiation sterilization most certainly works, the specific process is not well understood. it is clear, however, that the deposition of energy by decelerating electrons is an extremely effective method of cellular inactivation. Current belief attributes this inactivation to either irreparable damage to the cell membrane or to DNA alterations caused by bond breakage.
Chain-Scissioning |
When a polymer is subjected to electron radiation, many of the carbon-carbon bonds may be broken; and the resulting radicals tend to re-link with hydrogen atoms, creating shorter polymer chains. This chain scissioning degrades materials.
Although this is often not the desired effect, it frequently benefits. Teflon, for example, will degrade under radiation so it can be ground into a fine powder, making it ideal for the manufacture of printing inks.
Material degradation also is the object in the treatment of toxic wastes. Many of these wastes are complex hydrocarbons, which degrade under an electron beam. The resulting polymers have no value other than their inability to cause organic problems.
Cellulose also will degrade rapidly when exposed to electron beam radiation. The ability to degrade celluloid-based materials could be important in the conversion of these materials to glucose and alcohol.
Dose Measurement |
The chemical or sterilizing effect of electron beam irradiation varies greatly based on the amount of electron energy or dose that a product receives. Because of this, dose measurement is a critical part of radiation processing.
A radiation dose is measured in terms of the amount of radiation energy absorbed per unit mass of the material. The Traditional unit of dose is the RAD, defined as the absorption of 100 Ergs of energy per Gram of material. The MRad (one million RAD) is the practical unit. the S.I. unit is the Gray, defined as the absorption of 1 Joule of energy per Kilogram of material. The practical unit is the KGray (one thousand Gray). Absorbed dose can be translated into more common industrial terms associated with heat and electron energy.
What are the other ionisation techniques? |
- Gamma rays and X rays, which originate from different technologies.
Is there any risk of products treated by ionisation being radioactive? |
- None. The radiation source has no effect upon the treated products..
Is a radioactive source used for the electron accelerator? |
- No, the phenomenon is produced electrically.
In order to increase electron energy, is it simply a matter of increasing the power of the accelerator? |
- On the contrary, the one is generally inversely proportional to the other.
Is it advantageous for the electron energy to be as powerful as possible? |
No. The authorised maximum is 10 MeV, in order to avoid any risk of the disintegration of
the atomic core in the treated product. In any case this is more than sufficient for the majority of applications.
