Dose Distributions For Containers Electron Sterilized At Energies From 150 - 250 Kev

The development of a flexible electron processor has been necessary for
the validation of its use in the sterilization of a variety of container
geometries (1). A.250 kV selfshielded accelerator used for this purpose,
which provides a 30 cm wide beam at currents to 20 ma: is shown in Figure
1. The ability to separate the electron source from the lower,
horizontal, selfshielded product handling assembly permits the insertion
of separating shield members at the interface to accommodate a variety
of product heights. One of the major commercial applications of the
processor shown in Figure 1 is the controlled depth of sterilization
(CDS) process which permits the bilateral sterilization of pouched
delivery devices filled with labile, sterile pharmaceuticals. Details
of this art have been taught in earlier publications (2,3) and the
process has been applied commercially to orthopedic/ophthalmic
injectablea at this company since 1987.

*This paper was presented as S09-42 at the Eighth International Meeting
on Radiation Processing in Beijing, China, 13-18 September, 1992, and
will be published in the proceedings in the Journal of Radiation
Physics and Chemistry.

This facility has been used for the dose distribution mapping of a
number of open-mouthed container geometries — primarily for aseptic
fill-seal applicationé. The major concern for these (largely) polymer
systems is the dose distribution over the interior (product contact)
surface as well as the distribution in the upper regionof the outer
surface. Since most aseptic fillers work in an environment flushed with
sterile air or nitrogen until heat sealed, the sterilization of the
outer surface a few centimeters below the seal line is considered
adequate. In optimizing the sterilizer design for a given container
geometry, anelectron energy must be selected which will provide
acceptable top:bottom peripheral inner dose ratios which will provide
acceptable Sterility Assurance Levels. For example, in order to achieve
a 9D or 10 (-9) reduction in the bioburden of the most occluded surface
(which might require 10 kilo Grays) one would wish to maintain a maximum
upper seal lip dose which is well below the threshold of polymer
physical degradation (typically 100 kiloGrays). Clearly, high electron
energies can provide acceptable dose ratios; low energies are desirable
for reasons of radiation shielding and system economics. We have
therefore performed several mapping/optimization studies at sterilizer
operating voltagesat orbelow 250 kV,which are considered to be
readily selfshielded at the low power levels (beam currents) required
for in-line sterilization.