Low-voltage Electron Beam and X-ray Dosimetry

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Objective for MPD D.8.1

  • Develop an end-user technique for confirming bio-burden kill when packaging materials are exposed to low-energy electron beams.

Background: The only major new market area for electron beam processing to have emerged in at least the past twenty-five years has been the use of low-energy electron beams to decontaminate the surfaces of food and medicinal packaging materials before they are filled in aseptic environments. In a few years, this has been by far the fastest growing end-use market for EB technology. Attempts to use low-energy electron beams with one-of-a-kind calorimeter systems have not been replicated and have proven to rely more upon modeling calculations than upon test data from which dose could be inferred. Likewise, dosimeters used in higher energy EB processes have been found wanting in the low energy area (80 to 300 keV). At the lower energy range, 80 to 150 keV, the electron beam is partially absorbed in the dosimeter and then the dose measurement is not performed appropriately Albeit, even with only partial beam penetration, alanine coated films offer the greatest sensitivity to very low energy beams, responding at as low as 80 keV and at very low beam currents, less than a milliamp. Other film dosimeters, such as those which rely upon color body formation, have been found to vary in gauge thickness and manufacturing consistency beyond the tolerances normally found in film manufacture. This encumbers end users with having to make corrections for flawed product manufacture.

Other more traditional end use area of low-energy EB processing, such as the curing of inks, coatings and adhesives, have their own specific product performance test methods. For example, ASTM International D-7244, “Standard Test Method for Relative Cure of Energy-Cured Inks and Coatings,” is an automated solvent rub test that can be used in the printing and coating areas. In these areas, manufacturing parameters are established with little need to rely upon an inferred dose.

Bio-burdens found on packaging materials would be down in the low micron thickness range, most likely being only a few cells thick. Techniques for assessing cell death have been developed by epidemiologists for the medical community. Such techniques rely upon assessments made of very thin biological matter. Given the limited beam penetration of the very lowest commercially available EB systems (70 to 80 keV), such beam penetration, provided the beam is not lost in an air gap, would be sufficient to affect such biological matter. Radiation effects on cellular matter can show up in changes in the fluorescence of some specific moieties or in the inability of cells to propagate in a polymerase chain reaction (PCR). Microbial fluorescence and real-time PCR are methodologies that have been evaluated in other areas. Here too in these new decontamination uses of low –energy EB one would no longer have to rely upon an inference of dose.

Action Items:

1 – Develop a real-time bio-burden kill methodology based upon epidemiological techniques used in other areas.

2 – Conduct inter-laboratory comparison studies involving low-voltage EB users and equipment suppliers to validate the direct use of biodosimetry for these surface decontamination applications.

3 – Develop dosimetric techniques useful in the electron energy range of 80 keV to 150 keV.

Resource Requirements:

1 –NIST should assemble an inter-disciplinary team to develop the protocols for using bio-dosimetry with low-energy electron beams. This is at least a two person-year effort.

2 – NIST should acquire a low-energy electron beam ($250,000) in order to implement this need in the low-energy EB area.