Endüstriyel amçlı gama ışınlama sistemlerinde ışınlama verimiin hesaplanması

Abstract

ÖZET Bu çalışmada tıbbi malzeme üreten bir firmadan edinilen üretim kapasitesi esas alınarak bir gama sterilizasyon tesisi için gerekli paremetreler hesaplanıp, plaka şeklindeki bir kaynak önünden geçen malzemenin etkileneceği doz ve tesi sin ışınlama verimi hesaplanmıştır. Malzemenin taşındığı taşıyıcı kap boyutları küçültülerek ve adım sayısı arttırılarak, malzemenin etkileneceği doz ve tesis ışınlama verimi hesaplanarak bir karşılaştırma ya pılmıştır. Böylece bu çalışma ile endüstriyel amaçlı sterilizasyon tesislerinde hangi faktör ve parametrelerin dikkate alınması gerektiği vurgulanmış ve hesaplama yöntemleri gösterilmiştir. Ancak, bu tür bir tesisin kapasite çalışmasıyla ve ekonomik fizibilitesiyle ilgilenilmemiş, yalnızca radyasyon doz miktarlarının ve ışınlama veriminin üzerinde durularak konuya bilimsel yaklaşılmıştır. VI

SUMMARY In this work, the physical characteristics of the ir radiation process and the principal features in the design of the gamma irradiator discussed. Dose, dose distribution, and irradiating efficiency facilities are calculated, and it is shown, how the dose distribution and irradiating efficiency varies with product density in a industrial gamma irradiator. The steady world-wide growth of gamma processing in dustry since the early I960' s, has encouraged commercializa tion of a broad range of applications. From the early com mercial success of medical product sterilization, the scope now includes such applications as plastics, modification, sa nitation of cosmetics, and pharmaceutical products, treatment of wastes and food irradiation. The growth in this applica tions has presented the designers of gamma processing equip ment with new challenges. The use of raddiation for the elimination of harmful microorganisms, the extesion of shelf life and the disinfes- tation of food and agricultural crops has been demonstrated. VIIAccess to the irradiation room is through a labyrinth which reduces radiation scattering or leakage, thus entrance to the cell can be closed a light-weight door. Small amounts of ozone are expected to be produced in the irradiation room during exposure. Ozone being a toxic gas, is removed by exhaust ventilation fans. The ventilation system is interlocked with the source mechanism so that the unit cannot be operated unless the fans are functioning. A water-filled pool is provided for storage and replenishment - of the Co-60 source. The pool is approximetly 5 m in length and 6 m in depth. One end of the pool, 1.5 m wide, is used during unloading of the source from its transport container, while the other and, 0.9 m wide, is used for source storage. To minimize corosion, water in the pool is continuous ly circulated through a deionizer. A pool skimmer is provi ded to remove floating matter. A float switch, interlocked with the deionizer and fresh water supply, maintains the pre set water level of the pool. The major compenents of the irradiator are; a) Co-60 source and source mechanism b) Biological shield ( irradiation room ) c) Product turntables d) Monitoring devices VIIIe) Control cosole f ) Safety features The Co-60 is doubly encapsulated in stanless steel pencils 45 cm long by 1 cm diameter. Each pencil can contain up to 0.37 PBq. Co-60. Biological shield is normally constructed from stan dart concrete. Its thicness depends on the projected strenght of the radiation source and is typically in the order of 2 m. The turntables are automatically rotated specific turn increments during the radiation cycle. Thus ensuring that the four sides of the boxes are equally exposed to the sour ce. If, for any reason the rotation of the turntables is restricted or jammed the sources will automatically return to the pool to avoid excessive doses to the samples. On radiation monitor is wall-mounted adjacent to the irradiation room entrance and interlocked with the maze door such the personnel cannot enter the irradiation room unless all sources are in the pool and there is no abnormal radiat ion in the radiation room. IXSafety interlock and devices are installed to ensure person nel protection. The personnel access door at the entrance of the maze is electrically interlocked with the source and the monitor so that the source cannot be raised, if the door is o- pen and convesely., the door cannot be opened if the source is exposed or, if the source is in the safe position but there is an abnormal radiation field in the irradiation room. The radiation level inside the irradiation room with all sources in storage position is normally below 2.5 mGy/h- with 18 PBq. of Co-60. în the event of fire or excessive rise in temperature in the irradiation room, a sensor automaticaaly lowers the source safe position. The throughput of any gamma irradiation facility for the sterilization matter dependent on a) Pareme ter s related to the source b) Paremeters related to the object being irradiated. A knowledge of the dose distribution in the irradia tion field is indispensable in planning an irradiation pro cess.The dose distribution in the irradiation field is de pendent on source activity and source geometry. Dose rate distribution is different for different source geometries, especially at distances however, dose rate of any source geo metry can be calculated according to the 1/r2 relationship i.e. Actual dose rate will be lower than the calculated one due to absorption in the source and other mechanical parts. On the other hand, the depth dose distribution in the object being irradiated is dependent not only on parameters related to the source but also on product-source distance, product density and dimension. Dose homogeneity in the products is very much depen dent on the product bulk density. To compansate for these dose differences and thus to improve dose homogeneity in the product, the boxes are usu ally related around the source. Dose uniformity ratio, i.e. The ratio of the maximum dose to the minimum dose. XIîn this work all these calculations for irradiation effici ency related different bulk densities are based on a 1 PBg. Co-60 line source, 0.45 m long located at the center of the turntable. If a different uniformity ratio is required, then the dimensions of the product box should be altered accordingly, thus affecting the throughput. It should be noted that actu- el dose in the products may be different from the calculated ones since some of the assumptions made may not be true for `a particular cituation. The dose-rate distribution was also checked by five different phycical and four different chemical dosimeters. Irradiation efficiency, as the percentage of the total amount of gamma irradiation emitted from the Co-60 source, that is absorbed in the product. It is desirable to minimize this self absorption because absorbed radiation is converted to heat which must, in turn, be removed from the irradiation chamber by the ventilation system. Irradiation efficiecies are computed for each type of irradiator designs. XIIThe efficiencies depend on the manner in which product surrounds the Co-60 source and are specified as a function ofproduct density. Irradiation efficiences for medical pro duct sterilizers can range upto 45 %. în many cases, the design objective is to minimize the overdosing ratio while maximazing the efficiency. To minimi ze the overdosing ratio it is necessary to minimize the box width consistent with customer requirements and to increase' the number of product layers and passes on either side of source. Dose distributions and irradiation efficiency of in dustrial gamma irradiator were calculated along the horizan- tal axis of a box, 0.35 x 0.35 x 0.44 m width, containing disposable medical products to be sterilized, related to dif ferent bulk densities. Following that, dose distributions and irradiation efficiency for the same system were calcula ted decreasing the dimensions of box. As a result, when comparing is made for calculations of two different dimensions of box, it is shown that the ef ficiency can be increased by decreasing the dimensions of box. XIII