Study on the influence of density in the dose uniformity
of the CDTN gamma irradiator
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ABSTRACT
This paper describes an experimental study carried out to determine the dependence of the
dose uniformity with the irradiated product bulk density. The knowledge of this dependence helps
to ensure that the irradiation process is carried out with acceptable doses uniformity. The
measurements were made in the Gamma Irradiation Facility at Nuclear Technology Development
Center (CDTN) an R&D Institute connected to the Nacional Nuclear Energy Commission (CNEN),
Brazil. Four different bulk densities were investigated. The results showed that the dose uniformity
decreases with the product bulk density increasing. It is interesting to note that the DUR obtained
values are consistent with a high dose uniformity that is requiring, e.g. in R&D studies. It is
important to note that these results apply to the specific geometry conditions studied.
Keywords: dose uniformity, gamma processing, gamma ray, irradiator
1. INTRODUCTION
In cases where it is indicated, the irradiation process promotes improvements of products and
materials functional properties. Irradiation can be used, for example, to improve mechanical
properties of polymers and plastics or to reduce the pathogen burden in foods, making them safer.
The main parameter involved in the process is the energy transferred by radiation to the
material. This amount of energy per material mass unit characterizes the absorbed dose.
Specific improvements are associated with characteristics dose ranges. The doses necessary
to achieve the desired effect are within these ranges that in the most cases are determined
through experimental dose-effect studies.
The minimum scale values are those from which the desired effects are obtained, whereas
the maximum values correspond to those from which there is a risk that the functional properties
of the product are adversely affected (e.g., deteriorating mechanical properties of plastic parts for
medical / pharmaceutical or organoleptic changes in foodstuffs).
Each process/product set has its typical limits that, in the case of products regulated as
foodstuffs and products for healthcare, are prescribe by government authorities [1].
During the irradiation process, several types of interactions occur between the radiation and
the material. These interactions attenuate the radiation by depositing a portion of its energy in the
material. This attenuation produces a decrease in absorbed dose with depth whose rate depends
on the product composition and density and on the energy carried by the radiation. Besides this
depth variation there is also a lateral variation that depends on the irradiation geometry. Such
variations affect the product absorbed dose uniformity and its knowledge is essential for the
quality assurance of the irradiation process.
To study these variations is adopted the concept of Dose Uniformity Ratio (DUR), defined as
the ratio between maximum and minimum values of absorbed dose in all the extension of the
irradiated product [2].
This paper presents experimental results of a study conducted within the framework of Quality
Assurance Program of the CDTN Gamma Irradiation Facility [3]. The purpose of this study was
to determine the dependence of the DUR with the irradiated material bulk density.
The adopted methodology has used five cardboard boxes irradiated at 41 cm from the source.
The first one empty and the other four filled with different bulk densities products. DUR were
calculated using the maximum and minimum values of the absorbed dose measurements.
2. EXPERIMENTAL SETUP
2.1. The Irradiation Facility
The CDTN irrradiator is a GammaBeam-127 type, manufactured by MDS Nordion [4]. The GB127 is a dry storage, panoramic irradiator that uses Nordion’s F-127 shipping container for source
storage and operation. Products to be irradiated are placed inside the biological shield, either on
turntables or directly on the floor. After safety interlocks have been engaged, the source is raised
by a pneumatic hoist for the prescribed amount of time and then returned to the safe position
inside the container. Source to product distance can be varied easily, resulting in good dose
uniformity across a wide variety of densities. This batch processing approach lends itself well to
research and testing, food irradiation, sterile insect technique and many other applications. The
Table 1 shows the irradiator specifications. The irradiator is a box chamber with high-density
concrete walls. Figure 1 presents a close up of the irradiation facility.
Table 1: Irradiator Specifications
Maximum Source Activity
60 kCi (2.220 TBq)
Dose Rate @ 1 m
600 Gy/h
Product Configurations
12 turntables (50 cm diameter, 2 rpm)
Maximum Product Weight (on turntables)
700 lbs. (320 kg)
Source to Product Distance
Variable up to 300 cm
Floor Area
33 ft. x 39 ft. (10.1 m x 11.9 m)
Mode of Operation
Manual batch
Figure 1: Irradiator close up
2.2. Dosimeters
Dose measurements were carried out using 15 Red Perspex 4034 Batch LS dosimeters [5].
These routine dosimeters are made of polymethyl methacrylate (PMMA) with an overall
uncertainty of ± 8.2% at a 95% confidence level, in the range of 5-50 kGy, calibrated against
Fricke reference dosimeters at the facility [6]. Therefore, all the values of absorbed dose
measurements are affected by this uncertainty and the DUR calculated values are affected by
± 11.6%.
2.3. Product Irradiation
The products were irradiated within carton boxes (30x30x20 cm) over turntables. The boxes
centers were shifted by 41 cm from the source and its middle levels at 55 cm from the floor. They
were divided into five vertical planes and three horizontal levels. The 15 dosimeters locations are
shown in Figure 2. In order to optimize the dose homogeneity in the product, the turntable has
turned at 2 rpm during the irradiation cycle.
Figure 2: Carton with dosimeter positions (dots)
Dosimeter
Vertical planes
3
Source
center
2
41cm
1 2
3 4
1
55cm
Horizontal levels
5
Carton packing
Turntable
3. RESULTS AND DISCUSSION
Five carton boxes were irradiated in order to study the bulck density effect in the dose
uniformity. The first one empty and the other four filled with sawdust, pharmaceutical gauze, dry
vermiculite and wet vermiculite). At the irradiation position, the dose rate at the center point of
the box was 1.4 kGy/h.
It is considered the “global dose uniformity” as the ratio between their maximum and minimum
dose in the boxes [7].
3.1. Dose Measurements
3.1.1. Empty carton box
Experimental dose measurements with empty carton box (bulk density 0.02 g/cm3) are shown
in Figure 3-A and 3-B. For all levels, the results show a symmetrical distribution relatively to the
vertical axis passing through the middle of the box (Fig. 3-A). The maximum dose value found
was (24.7 ± 2.0) kGy, located on level 2 (middle level) while the lowest was (22.0 ± 1.8) kGy, on
level 3 (DUR=1.12 ± 0.13).
Figure 3-A: Horizontal dose profile (empty)
For all vertical planes, the results show an asymmetrical distribution relatively to the horizontal
axis passing through the middle of the carton box (plane 2 in Fig. 3-B). The observed asymmetry
is due to displacement of the geometric center of the source of their design position. This
displacement resulted from a mistake in the mounting phase of the source and was observed
through measurements of dose rates performed in a previous study [8].
Figure 3-B: Vertical dose profile (empty)
3.1.2. Pharmaceutical gauze filled box
The experimental dose measurements carried out with pharmaceutical gauze filled box (bulk
density 0.08 g/cm3) at 41 cm from source, are shown in Figure 4-A and 4-B. The results have
the same behavior as the empty box case in both horizonal and vertical axis, but with more
pronounced dose profiles resulting from higher density. The maximum dose value found was
(24.5 ± 2.0) kGy, located on level 2 while the lowest was (21.5 ± 1.8) kGy, on level 3
(DUR=1.14 ± 0.13).
Figure 4-A: Horizontal dose profile (gauze)
Figure 4-B: Vertical dose profile (gauze)
3.1.3. Sawdust filled box
The experimental dose measurements carried out with sawdust-filled box (bulk density 0.2
g/cm3) at 41 cm from the source, are shown in Figure 5-A and 5-B.
Figure 5-A: Horizontal dose profile (sawdust)
The results have the same behavior as the previous case, in both horizonal and vertical axis,
but with a more pronounced profiles resulting from higher density. The maximum dose value
found was (26.8 ± 2.2) kGy, located on level 2 while the lowest was (23.2 ± 1.9) kGy, on level 3
(DUR=1.16 ± 0.14).
Figure 5-B: Vertical dose profile (sawdust)
3.1.4. Dry vermiculite filled box
The experimental dose measurements carried out with dry vermiculite-filled box (bulk density
0.3 g/cm3) at 41 cm from the source, are shown in Figure 6-A and 6-B.
Figure 6-A: Horizontal profile (dry vermiculite)
The results have the same behavior as the previous case, in both horizonal and vertical axis.
The maximum dose value found was (25.0 ± 2.1) kGy, located on level 2 while the lowest was
(21.0 ± 1.7) kGy, on level 3 (DUR=1.19 ± 0.14).
Figure 6-B: Vertical profile (dry vermiculite)
26,0
Dose [kGy]
25,0
24,0
Plane 1
Plane 2
23,0
Plane 3
22,0
Plane 4
Plane 5
21,0
20,0
1
2
3
Horizontal levels
3.1.5.Wet vermiculite filled box
The experimental dose measurements carried out with wet vermiculite-filled box (bulk density
0.6 g/cm3) at 41 cm from the source, are shown in Figure 7-A and 7-B.
Figure 7-A: Horizontal profile (wet vermiculite)
The results have the same behavior as the previous case, in both horizonal and vertical axis.
Here the profile is the most pronounced of all, since it is the case of higher density.
The maximum dose value found was (21.6 ± 1.8) kGy, located on level 2 while the lowest was
(17.2 ± 1.4) kGy, located on level 3 (DUR = 1.25 ± 0.15).
Figure 7-B: Vertical profile (wet vermiculite)
3.2. Global Dose Uniformity
The DUR calculated values are shown in Table 2 for all cases. The effect of bulk density on
global DUR is shown in Figure 7.
Table 2: Global DUR calculated values
Filling
Vermiculite
dry
wet
Empty
Gauze
Sawdust
Density
[g/cm3]
0.02
0.08
0.2
0.3
0.6
DUR
1.12 ± 0.13
1.14 ± 0.13
1.16 ± 0.14
1.19 ± 0.14
1.25 ± 0.15
Figure 8: Effects of bulk density on DUR
4. CONCLUSIONS
The results showed that the dose uniformity decreases with the product bulk density
increasing. It is interesting to note that the DUR obtained values are consistent with a high dose
uniformity that is requiring, e.g. in R&D studies. It is important to note that these results apply to
the specific geometry conditions studied.
These facts are important because a priori knowledge of dose distribuition guarantees an
appropriate irradiation protocol and ensures total quality results.
5. ACKNOWLEDGEMENTS
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