Atomic Energy, Vol. 100, No. 1, 2006
RADIATION STERILIZATION OF MAIL
O. A. Zinov’ev,1 V. S. Skobkin,1 N. S. Lobanov,1 O. K. Chugunov,1 G. Ya. Pizhov,1 A. Ya. Naidenov,2 and T. P. Dubinina2
UDC 615.478.73
The results of sterilization of mail in typical packages using 60Co γ rays with average energy 1.33 MeV and in an electron accelerator with electron energy 9 MeV are presented.
It has now become necessary to develop radiation sterilization of mail. It is believed that radiation sterilization of mail entering from the quarantine zone can be accomplished if only the exposure dose of ionizing radiation is known. The first experiments showed that this is inadequate to determine the commercial regulations for the process. In addition, the possibility that pathogenic microorganisms could be used as biological weapons [1–3] has predetermined experimental testing of any technology for commercial sterilization of mail by radiation methods. The present paper presents the results of the sterilization of mail in typical packages using a plasma-beam discharge in an electron accelerator and in the field of 1.33 MeV 60Co γ-rays. To sterilize mail, it is necessary to provide asceptic conditions under which the technological zones are divided into into two zones – a “dirty” zone which mail enters for sterilization and a clean zone where sterilized mail is stored. These zones must be separated by a continuous barrier with an independent exit which is carefully monitored to prevent mixing of sterilized and dirty mail [4]. An object (books) enters in a standard package, a 600 × 300 × 300 mm cardboard or plastic box, sealed in polyethylene packaging. The thickness of the film is >150 µm. The objects to be sterilized are placed in the reaction zone using a specially developed scheme taking account of the positional height of the object to be sterilized and the distribution of the γ-ray intensity over the zone. To accumulate the required absorbed dose, the objects are irradiated for several days. For uniform irradiation, they must be successively rotated by 180°, as a result of which with repeated irradiation the absorbed dose is increased and evened out. The sterilized objects are moved into the clean room, where they are examined, additional monitoring is performed, and an established protocol is followed. Microbiological testing systems have been developed to determined the required radiation sterilization regime. These systems consist of polyethylene packets containing samples of sublimation dried test cultures of bac thuringiensis, which simulate the organisms bac anthracis that cause malignant anthrax. The number of spores in each sample was 2·108. Models containing 16 books, each with 304 pages, which were tightly packaged in a cardboard box, were used as mail samples. Sterilization on a g Apparatus. Packets each containing 1 gram of the test culture were placed in a model between books at different distances from the source. Film dosimeters were placed in the same order. The arrangement of the biological samples and dosimeters in the model is shown in Fig. 1. The objects were irradiated with γ-ray exposure dose rate 1 2
Russian Science Center Kurchatov Institute. Biokhimmash. Translated from Atomnaya Énergiya, Vol. 100, No. 1, pp. 63–68, January, 2005. Original article submitted April 15,
2005. 1063-4258/06/10001-0067 ©2006 Springer Science+Business Media, Inc.
67
I 1
2 3
C
4 5 6 • 8 •
7 9 10
II
Fig. 1. Arrangement of microbiological test systems (1–10) inside a model of mail: → – direction of the flux of γ radiation (I) and after rotation (II); C – container.
TABLE 1. Results of Irradiation of Test Samples with E = 1.33 MeV, P = 6 R/sec γ-Irradiation regime Dexp, MP
*
Remark
2·108
5
No paper
1.15
2·106
5
»
5
5
»
5
»
Irradiation time, days
Survivability
0.3
1
0.57
0.6
1
0.9
1
1.73
6·10
1.2
1
2.31
5·104
1.5
1
2.8
40
5
»
2.1
0.0016
4
Sterile
3
150 sheets of paper
5
0.0578
4.5
»
3
300 sheets
6
0.0578
4.5
»
3
1500 sheets
7
0.0578
4.5
»
3
2100 sheets
10
0.0578
4.5
»
3
4100 sheets
0
2·109
2
Control *
Number of samples Kγ = Dabs /Dexp
Dexp – exposure dose of radiation, recorded by control dosimeters [5]; Dabs – dose absorbed by biological materials and paper in the mail, determined using the coefficient Kγ = Dabs /Dexp for real conditions.
6 R/sec to accumulate an absorbed dose 10 Mrad. This exposure dose rate was chosen because modern commercial γ setups with 60Co have essentially a residual irradiation power analogous to this (see Table 1). It is evident from the experimental data that complete suppression of growth of the spore culture bac thuringiensis is already observed with an exposure dose of 2.1 MR for a period of 4 days of irradiation on the GUT 200 M apparatus. Since the setup has operated for 18 years, to decrease the sterilization time for mail periodic monitoring is required and the radioactive sources 60Co must be replaced. Sterilization Using the Fakel Electron Accelerator. Mail is sterilized with a high-energy electron beam produced by a plasma-beam discharge in the atmosphere and a solid body [6]. Electrons with energy 9 MeV can penetrate to a depth up to 0.5 m in a solid body whose density is less than 1 g/cm3, exciting along its path a plasma-beam discharge. 68
Fig. 2. Block diagram of a sterilization apparatus based on the Fakel electron accelerator: 1) power source for the electron gun and the microwave sections of the accelerator; 2) electron injector; 3) modulator of azimuthal motion of the electron beam; 4) power supply for the motor of the milling wheel and the modulator of the electron beam; 5) milling wheel with the objects to be sterilized (side view); 6) motor of the milling wheel; 7) electron beam; 8) view of the milling wheel (in the plane).
The sterilization of mail using the Fakel accelerator differs from sterilization on a γ apparatus only in that there is no need to rotate the samples. For sterilization on a Fakel linear accelerator, two microwave sections and an electron injector were used. The electron beam was modulated along the azimuth so as to cover a zone along the sterilized object (600 mm). The object itself, which is located in a cell of a carousel conveyer (milling wheel), was moved along the vertical axis. A block diagram of the sterilization apparatus, together with the biological test systems and the milling wheel, is shown in Fig. 2. The working sterilization regime of the electron accelerator is as follows: 9 MeV electrons, 1 µsec pulse duration with 400 Hz frequency and ~1 A pulse current, ~25 mm effective electron beam diameter on target, and ~1.5 m beam path length from the source. The instantaneous exposure dose rate was 4.25 MR/sec in the zone of the output foil and 1 MR/sec after traversing the air path. The absorbed dose of the electron irradiation was measured with color polyethylene dosimeters with error ≤25%. If the radiation regime of the electron beam on target is taken into account, then the average absorbed dose rate in the working zone, i.e., in the box with the mail, can be calculated. It is ~5.55 krad/sec. This result makes it possible to calculate the time characteristics of the plasma-beam discharge taking account of the coefficient of beam energy transfer to the target: K = Pmax /P = 0.1. Since total suppression of spore growth is observed at absorbed dose Dabs ~ 10 Mrad, the approximately required duration of irradiation is T = Dabs /Pmax = 104 krad/5.55 krad/sec = 1800 sec. In Fig. 3, the dependence of the survivability of the spores on the depth of the test system in the model was constructed from the experimental data taking account of the computed irradiation time 30 min. It is evident that the curve of spore survivability does not have a monotonic (classical) form, as happens when microorganisms are irradiated with γ radiation. The nonmonotonic absorption of the energy of a plasma-beam discharge in the bulk of the paper and the relative survivability as a function of the location depth of the test systems Babs /B0 = ƒ(D) are due to the ionization reaction in the bulk of the paper material. As electrons slow down and enter deeper into the object being sterilized, ionization losses increase. In addition, the number of secondary and back-scattered electrons from deeper layers of the material increases. Consequently, for a low depth the absorbed energy increases. At a certain depth, the decrease in the number of electrons moving in the initial direction becomes substantial. The number of back-scattered electrons also becomes smaller; this causes the absorbed 69
Babs/B0 8–10 7 10–3
6 5
10–5
3
4
2
1
2
10–7 0
4
8
12 Dabs, MR
6
4
2
0
δ
Fig. 3. Spore survivability versus the exposure dose of ionizing radiation: 1–10) number of test system and film dosimeter; δ – number of sheets in a journal: δ = 228, 5δ = 1140.
dose to decrease in the deeper layers of the material. These processes result in the formation of a maximum on the absorption curve. The dropoff on the leading and trailing edges of the peak is explained by the reflection and scattering of electrons at the air–material interface. A similar picture of the absorbed dose is observed in the bulk of a polyethylene film [6]. It is evident in Fig. 3 that after passing through the bulk of the paper in the direction of motion of the high-energy electrons the absorbed dose of the electron irradiation decreases to a minimum, corresponding to the prescribed minimum for sterilization (Babs /B0 = 3.3·10–6). The largest suppression of spore growth is observed in layers (3–4)δ, where δ = 128 sheets each 0.15 mm thick. The absorbed dose reaches its maximum value in these layers. Thus, for irradiation for 30 min of mail by 9 MeV electrons the prescribed sterilization criterion 3.3·10–6 is achieved up to paper layer thickness (3–4)δ. The experiments showed that the prescribed depth of sterilization (layer thickness) of paper can be increased by increasing the exposure dose rate, the energy of the electron beam, and the irradiation time. As the exposure dose increases, the absorbed lethal dose for many spore microorganisms decreases [7]. This makes it possible to decrease the average absorbed dose in sterilization and thereby preserve the strength and other properties of the paper. For ecological considerations, the energy of an electron beam can be increased only up to 10 MeV. The best procedure was found to be increasing the irradiation time. The experiments showed that in this manner the depth of the sterilized layer in a box can reach (5–6)δ. Investigation of Residual Phenomena in Paper After Exposure to Radiation. After radiation treatment in regimes which ensure sterilization of test systems, the mechanical properties of the paper and degree of preservation of the information on the paper were investigated. Public magazines, newspapers, and filter paper were used in the experiments. It was found that a mechanical property of the public magazine booklets – strength on bending – decreases by not more than 5% compared with the unirradiated samples. Examination under a microscope showed that the volume mass and swelling of the paper also did not change much. The information was completely preserved. The whiteness of the paper did not change – either immediately after irradiation or in time after radiation sterilization. The news paper was found to be more sensitive to irradiation. The rupture and bending strength of the paper decreased by 15% for sheets thicker than 85 µm. The decrease in the mechanical strength is explained by the radiation destruction of cellulose microfibers as a result of breaking of interatomic bonds by electrons. Examination showed that the microfibers became shorter at the rupture sites. Filter paper was found to be most sensitive to radiation. It lost 20% of its mechanical properties. The paper has a complicated heterogeneous composition, characterized by structural anisotropy. Consequently, in paper with a constant mass (deviation not more than 20%) the main properties can fluctuate – thickness ±5%, resistance to rupture ±20%. 70
In summary, a 9–10 MeV electron beam from a linear accelerator can reliably sterilize in 30 min mail in the form of standard packages of paper correspondence in cardboard boxes with dimensions 600 × 600 × 600 mm and thickness up to 768 sheets contaminated with spore cultures. Sterilization of mail using a γ-ray source with average energy 1.33 MeV is accomplished in 4 days using the GUT-200M apparatus. The method is also applicable for long-term storage of archival, museum, and other materials. The experimental data presented in this work have been used as a basis to propose a technology for commercial sterilization of mail. The present work was performed by an interdepartmental team consisting of staff from the Russian Science Kurchatov Institute, the Biokhimmash company, and the Karpov Industrial Association Mosmedpreparaty. V. D. Rusanov, N. V. Znamenskii, V. V. Petrenko,Ya. I. Shtrombakh, and L. V. Neumyvakin provided great experimental assistance during the development of the method of radiation sterilization. They provided the required radiation apparatus and technical equipment. Teams of experimenters, including V. N. Denisenko, D. D. Maslennikov, N. P. Altukkhov, A. V. Gal’, V. L. Shiryaevskii, L. F. Bezborodova, N. G. Misyureva, G. I. Tikhonov, and A. A. Fridman, participated in the work. The authors are grateful to them all.
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