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Year : 2013  |  Volume : 36  |  Issue : 4  |  Page : 150-159  

Study on the dosimetric characteristics of indigenously developed computer-controlled multisource gamma irradiation system

Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Web Publication8-Oct-2014

Correspondence Address:
A K Bakshi
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CT and CRS, Anushakti Nagar, Mumbai - 400 094, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-0464.142388

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A computer-controlled multisource gamma ( 60 Co, 137 Co, and 241 Am) irradiation system has been developed indigenously for the purpose of strengthening the quality assurance program of individual monitoring service in India. The system can be used for the irradiation of personnel dosimeters both in panoramic and collimated modes in a well-reproduced geometry. Measurements of output (air-kerma rate) of the sources in both collimated and panoramic modes and transit dose were carried out. The study also includes radiation protection survey of the installation. Depending upon the distance from the source, the Monte Carlo-calculated air-kerma rates of the 60 Co and 137 Cs sources compare within 5-10% against measurements. In the collimated mode, the calculated beam uniformity is within 3% in the area of 20 cm × 20 cm centered on the collimator axis.

Keywords: Gamma irradiation system, Monte Carlo, output measurement

How to cite this article:
Bakshi A K, Sahoo S, Srivastava K, Selvam T P, Joshi V J, Chougaonkar M P, Babu D. Study on the dosimetric characteristics of indigenously developed computer-controlled multisource gamma irradiation system. Radiat Prot Environ 2013;36:150-9

How to cite this URL:
Bakshi A K, Sahoo S, Srivastava K, Selvam T P, Joshi V J, Chougaonkar M P, Babu D. Study on the dosimetric characteristics of indigenously developed computer-controlled multisource gamma irradiation system. Radiat Prot Environ [serial online] 2013 [cited 2019 Nov 13];36:150-9. Available from: http://www.rpe.org.in/text.asp?2013/36/4/150/142388

  Introduction Top

In India, individual monitoring using film badges was carried out from 1952 to 1975. Following the indigenous development of thermo-luminescent dosimeter (TLD) based on CaSO 4 :Dy teflon TLD discs [1] and the related technological development, the centralized film badge monitoring service was gradually phased out and replaced by TLD badge service. With the increasing use of radiation in various fields and the enforcement of regulation by the competent authority, it was necessary to decentralize the monitoring service. For this purpose, methodologies were developed, and a program of accreditation of laboratories was initiated in order to share the workload. Presently, all the radiation workers in the field of medicine, industry, agriculture and research applications are being monitored by three accredited laboratories. The accredited laboratories are provided with technical and scientific support by this center.

Quality assurance (QA) of the monitoring system are very important part in such a widespread program to ensure uniform standard of accuracy and reliability of the service. We have classified QA program into two components: (a) Quality control of dosimeters during production and (b) overall performance of the monitoring system during field use. Procedures were developed and documented to facilitate successful implementation of the QA program. At present, individual monitoring in India is being carried out by 15 processing laboratories situated at various parts of the country. Periodic QA check of the monitoring units was introduced in the year 1985. With increasing participation of private and public sector laboratories in conducting individual monitoring activities of radiation workers, it has become important to carry out regular QA checks of these laboratories to maintain accuracy and uniformity of performance. The performance of the processing laboratories is evaluated half yearly based on the criteria outlined in American National Standard Institute. [2]

Quality assurance in individual monitoring requires precise delivery of air-kerma/personal dose equivalent to multiple dosimeters in a reproducible geometry. As large number of dosimeters is to be exposed to different categories of radiations, including various radiation field magnitudes, an automated gamma irradiation system capable of irradiating multiple dosimeters in a reproducible geometry is considered to be the best arrangement. In view of the above and to conduct large scale QA program in a time bound manner, an automated multisource gamma irradiation system has been developed indigenously. We report the technical features, dosimetric studies and Monte Carlo simulation of some of the beam characteristics of this irradiation system.

  Materials and methods Top

Technical features of the gamma irradiation system

Irradiator shielding

A schematic diagram of the shielding part of the gamma irradiator is shown in [Figure 1]. It is made of lead and mild steel (MS). There is a rotating drum at the center of the irradiator to house three gamma sources 60 Co (1 Ci), 137 Cs (1 Ci), and 137 Cs (2 Ci) and a channel for the movement of each of the sources during irradiation. The facility has provisions of both panoramic and collimated exposures. For the collimated exposure, there is a conical aperture whose diameter is 9.2 cm at a distance of 20 cm from the vertex of the cone. This corresponds to full cone angle of 24° with respect to the cone axis. During the collimated exposure, the source ( 60 Co or 137 Cs) is at the vertex.
Figure 1: Schematic diagram of gamma irradiation system. The height of the system is 47 cm and diameter is 47 cm. Additional 4.5 cm lead is introduced in the middle of the system, makes the diameter is 56 cm to reduce the leakage radiation

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There is a separate arrangement for exposures involving 241 Am source (2 Ci). 241 Am source is positioned in a rectangular box made of MS of wall thickness 1 cm. This box is positioned at the end of the collimator and is away from the central axis of the collimator opening by about 2.5 cm. It blocks the collimated beam of 60 Co or 137 Cs sources partially.A photograph showing the partial blockage of the collimated beam by a 241 Am source shielding is presented in [Figure 2]. During the ON position, the source comes to the central axis of the collimator end through an electromechanical control.
Figure 2: Photograph of collimated part of the irradiator with shielding part of 241Am partially blocking the path of collimated beam

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The shielding design of the irradiator is based on analytical calculation. The basis of shielding calculation is for a point 60 Co source (1 Ci) in OFF position and the leakage radiation level at the surface of the irradiator <10 μSv/h. This is expected to minimize the contribution of leakage radiation (<0.1% of the useful beam) (ISO-4037-I, 1996) to the dosimeter at the time of panoramic exposure involving 137 Cs source. The tenth value layer values of shielding range between 3.8 and 6.1.

Panoramic part

The irradiator system has provision of irradiation of dosimeters through panoramic and collimated mode of operation at different distances from the source. Panoramic part of the irradiator system has been incorporated to expose a large number (60-70) of personal dosimeters at a time for testing individual sensitivity, reproducibility, dose linearity, etc., The panoramic table is made of polymethyl methacrylate (PMMA) of 20 mm thickness and 1240 mm diameter and has a central hole of diameter 34 mm for sources to come out and remain seated in the exposed condition for the preset time for panoramic irradiation. The table has a provision to rotate slowly at a speed of 1 rpm either in the anticlockwise or in a clockwise direction to nullify the spatial variation in the output of the sources. This table has circular graduation at 25, 30, 35, 40, 45, 50, and 60 cm from the center of the source to facilitate the positioning of dosimeters at different distances from the source. Rotational motion of the panoramic table is controlled by DC motors without any limiting switch.

Linear positioning system

Linear positioning system is basically required for the exposure of the personal dosimeter on phantom at different distances from the source. It can also serve the purpose of calibration of portable gamma survey meter in terms of ambient dose equivalent. A phantom stand, with the upper surface made of 20 mm thick PMMA sheet is part of the linear positioning system. The stand can move from 600 to 2800 mm from the source along X-axis. Provision for rotation in clockwise and anti-clockwise direction is made to change the angle of the phantom with respect to the source up to ±180°. The height of the stand is adjustable by 300 mm to align the center of the phantom with respect to the source in the collimated exposure mode. All the motions of the phantom stand are controlled by DC brake motors and limiting switches.

Additional accessories of the system

A hand control unit is provided with the system to control the motion of the phantom stand and panoramic stand remotely. Six control keys are provided in this unit. The functions switches provided in the unit are rotation of the panoramic table clockwise and anti-clockwise, up and down motion of the phantom stand, rotational motion of the phantom stand, motion of the phantom stand along X-axis to change the distance of the phantom with respect to the source in the collimated mode of exposure.

Cross laser beams for the accurate alignment of the dosimeter in collimated geometry are provided along with the system. A close circuit camera with a provision to move up-down left-right up to 180° and a TV monitor is also part of the system to view the source position from the control console of the system. A three-dimensional model of the system and a photograph of the system are shown in [Figure 3] and [Figure 4].
Figure 3: Three-dimensional model of gamma irradiation system comprising panoramic and collimated irradiation facility

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Figure 4: Photograph of panoramic gamma irradiator

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Details of the operational software

Based on our inputs, the supplier M/s Panacea Medical Technologies Pvt Ltd, Bangalore has also developed the operational software and provided along with the irradiation system for overall control of the system and calculation of irradiation time for a preset exposure/air-kerma/dose equivalent. The software can be used for the following functions of the system using a computer.

The software can be operated through the following sequence of commands: Start Programs → Panacea → Irradiator → Launch Irradiator.exe. At the start of the software, a window will appear with five menus namely system, irradiation, configuration, maintenance, print, and help. Each menu has several submenus for different functions. Brief description of each menu is given below:

  • System: It has seven submenus such as login, user, change password, backup, restore, logoff, and exit. This menu is mainly used to login into the program through a login name and password and create a password and exit the program
  • Irradiation: Under this menu, one can operate the system through collimated mode, panoramic mode and see the earlier history of exposure. In collimated mode, one can select three different sources namely 137 Cs, 60 Co, and 241 Am, delivered exposure/air-kerma/dose equivalent at a predetermined distance. Software can calculate the time of exposure of its own considering the date of exposure and distance of the dosimeter. Similarly, under panoramic mode one can use two different sources namely 137 Cs and 60 Co, distance of exposure and rotational mode of the panoramic table. The software will calculate the time of exposure of its own based on the air-kerma/exposure chosen. As part of safety in the operation of the system and to check the status of hardware, there are five safety controls provided in the software. These are: (i) Power to the system, (ii) battery power, (iii) door open, (iv) emergency switch, and (v) key switch. If the status of any one of them is shown in red, it indicates that there is a malfunction in the hardware and system cannot be operated without rectifying the problem. All interlocks listed above are validated before moving the source out of the system. In the case of the failure of any one of the above, during irradiation, the source will go back to a safe position. Screenshots of the software window for irradiation in collimated and panoramic mode of operation are shown in [Figure 5] and [Figure 6], respectively
    Figure 5: Screenshot of collimated mode of operation for exposure of dosimeter

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    Figure 6: Screenshot of panoramic mode of operation for exposure

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  • Configuration: This menu is used to re-enter or update the details of the radioactive sources, which in turn is used for air-kerma rate/dose rate calculation
  • Maintenance: This menu consists of four submenus such as the hardware address, configure hardware, managers, and source details. Each submenu has its own functions. For example, submenu - source details can be used to change the half-life of the source, its exposure rate, date of installation of the source/date of measurement. Similarly, the submenu - configure hardware can be used to enable/disable several hardware of the system. The submenu - manager can be used to create users with any given privilege like manager, supervisor or operator. Furthermore, manager can revoke any existing user and can take back up and restoration of the irradiation both collimated and panoramic
  • Print menu: At present, it has been kept inactivated mode as no printer is attached to the computer
  • Help menu: Help menu provides all sorts of instructions to tackle the problems related to the operation of the system.

Gamma sources

Presently, three sources are loaded in the system. The activities of the sources are about 946 mCi of 137 Cs, about 877 mCi of 60 Co and about 2 Ci of 241 Am as on the date of loading of the sources. The uncertainty associated with the activity of each source is about 10%. The 137 Cs, 60 Co source capsules were custom made having double encapsulation using stainless steel (SS-316L) with wall thickness 2 mm. The 137 Cs source is in the form of CsCl 2 powder and 60 Co in the form of pellet were manufactured as per the customs design by Board of Radiation and Isotope Technology, Department of Atomic Energy, Mumbai, India. The 241 Am is a disk source made from a mixture of americium oxide and aluminum powder encapsulated in a hermitically sealed capsule made from SS, an imported source. In the source OFF position, 137 Cs, 60 Co sources are stored inside the shielded drum of the irradiator with a provision to rotate and move out though a cylindrical tube by pneumatic control system. Customized transport containers for 137 Cs, and 60 Co sources were fabricated for loading and unloading the sources into the system.

Ionization chamber-based measurements of the output and transit air-kerma

For the measurement of the output of three gamma sources and transit air-kerma of 137 Cs and 60 Co in panoramic mode, an ionization chamber procured from Physicalisch Technische Werkstatten (PTW), Freiburg, Germany was used, measuring volume of the chamber is 30 cc having almost flat (±2%) energy response in the energy range 40 keV to 1.25 MeV. The chamber can measure, air-kerma rate, photon dose equivalent and photon dose equivalent rate within an accuracy of ±2% traceable to national standard. The chamber wall was reportedly made from graphite lined PMMA of thickness 1 mm. The chamber along with an electrometer namely UNIDOS, manufactured by PTW was used for the measurement. The leakage current of the electrometer was found to be in the order of ±1 × 10−14 A.

Measurement of radiation levels around the system and surrounding areas

The irradiator system has been installed in an existing room in the basement of a huge laboratory building of this center. Although the activity of 60 Co source was 877 mCi at the time of loading, however due to some technical glitches, the system could not be made fully operational for its routine use in panoramic mode for 2 years. Till that time the system has been used only through collimated mode. Therefore, at the time of radiation protection survey the activity of 60 Co source was estimated to have decayed to 666 mCi. For the measurement of leakage radiation levels in the control room and adjacent room, a 500 cc ionization chamber based digital survey meter RAM ION DIG procured from M/s Rotem Industries, Israel, Model No. BAK 1940 (minimum measurable dose equivalent rate of 0.1 μSv/h) was used. Measurement of leakage radiation levels with source off condition at the surface and away (at 1 m) from the surface was carried out with a calibrated Automess Survey Meter, model 6150AP6/H manufactured by Southern Scientific Ltd, UK (with minimum measurable dose equivalent rate of 0.01 μSv/h). Measurement of radiation levels in the control room and adjoining room was carried out with 60 Co source in the irradiation position in the panoramic mode of operation considering this to be the most hazardous situation. It may be noted that the adjacent rooms are unoccupied and upper floor room of the irradiator room is rarely occupied (occupancy factor 1/16). A schematic drawing of the room housing the gamma irradiator, control room and adjacent room are shown in [Figure 7]. The details of the dimension of the rooms, wall thickness, its material, measurement points, etc., are shown in the same figure [Figure 7].
Figure 7: Schematic diagram of the room along with wall thicknesses housing gamma irradiator system

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Monte Carlo modeling of the irradiator

The 60 Co and 137 Cs sources and the irradiator system were modeled using the Monte Carlo-based MCNP code. [3] The simulation includes modeling of the collimator. In the calculations, the transverse of the source coincided with the axis of the collimator axis. In the Monte Carlo calculations, 1.17 and 1.33 MeV gamma energies of 60 Co and 0.662 MeV gamma energy of 137 Cs were considered. Modeling of sources was carried out as per the schematic diagram presented in [Figure 8]. The Monte Carlo study included calculation of: (a) Air-kerma along the axis of the collimator as a function of source-to-detector distance d (50-280 cm) and (b) beam uniformity at d = 1 m. The calculations also include modeling of sources without the shielding material of the irradiator. This is to study the influence of shielding material on the calculated air-kerma.
Figure 8: The schematic diagram of 60Co and 137Cs source capsules with stainless steel encapsulation. (a) The active part of 60Co source is 5.5 mm in length and 3.4 mm in diameter. The overall dimension of the 60Co source is 9.5 mm in length and 7.4 mm in diameter. (b) The active part of 137Cs source is 7 mm in length and 6 mm in diameter. The overall dimension of the 137Cs source is 14 mm in length and 10 mm in diameter

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Beam uniformity was calculated along the directions perpendicular to the collimator axis. This is important as the shielding part of the 241 Am source (Section "Irradiator shielding"), obstructs the collimated beam. This obstruction is expected to affect the uniformity of beam along the horizontal direction. Therefore, it was modeled in the simulation to understand beam uniformity of 60 Co and 137 Cs sources. In the calculations, photon energy fluence spectrum was calculated which was subsequently converted to air-kerma rate by using the mass-energy-absorption coefficient of air. [4] Photon histories up to 10 6 were followed in the calculations. The cutoff energy for the photon transport was taken at 10 keV. The statistical uncertainties on the calculated air-kerma values were found <1%.

  Results and discussion Top

Output of the sources

[Figure 9] and [Figure 10] compare the measured and Monte Carlo-calculated output (air-kerma rate in mGy/h) of the 137 Cs and 60 Co sources at different d from the sources along the axis of the collimator. The agreement between measurements and calculations is within 5% up to d = 150 cm and within 10% for d ≥ 200 cm. In the Monte Carlo calculations the activities of 877 mCi of 60 Co and 946 mCi of 137 Cs were utilized to convert air-kerma per photon to air-kerma rate. The agreement between calculations and measurements is reasonable considering an uncertainty of about 10% on the activity of the sources. [Figure 11] shows the plot of measured air-kerma rate with distance for 137 Cs, and 60 Co sources for collimated geometry along with the inverse square law fitted function. It is seen from this plot that the measured values follow inverse square law within 5% and hence acceptable as per ISO. [5] Hence, one can irradiate dosimeter at any distance between 100 and 280 cm, can derive the output by applying inverse square law on measured output at one of the distances such as 100 cm from the source.
Figure 9: Plot of air-kerma rate with distance for collimated geometry for 137Cs source

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Figure 10: Plot of air-kerma rate with distance for collimated geometry for 60Co source

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Figure 11: Plot of air-kerma rate with distance for collimated geometry

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In panoramic mode, the output of the 137 Cs and 60 Co sources was measured at 25 and 50 cm distances. For 241 Am, it was measured at 50 cm from the source using a 112 cm 3 flat ionization chamber (procurement from PTW). The measured data for the three sources are presented in [Table 1].
Table 1: Output of gamma ray sources 60Co, 137Cs and 241Am measured by 30 cc ionization chamber at different distances for both panoramic and collimated modes

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Transit air-kerma

Transit air-kerma is the air-kerma received by the dosimeter due to the source in transit while coming out from the source storage or going back to the storage. It is important to measure this air-kerma as it may be significant and can invite uncertainty in the accurate delivery of air-kerma, for situations, such as delivery of low air-kerma (<1 mGy) to the dosimeters in panoramic mode. The panoramic mode of exposure was used as in this mode dosimeter is placed closer (25/50 cm) to the source movement channel and hence susceptible to receive measurable air-kerma. The duration of irradiation was varied from 5 s to 20 s in steps of 5 s. Five measurements were made for each time duration. [Figure 12] presented the measured (average of five measurements) air-kerma against irradiation time and extrapolated to zero. The intercept of the plot on the Y-axis (air-kerma) is considered to be the transit air-kerma. It is estimated as 1.12 μGy for 60 Co and 2.67 μGy for 137 Cs source. The transit air-kerma values are insignificant (0.025% for 137 Cs and 0.0025% for 60 Co) with respect to the outputs of the 137 Cs and 60 Co sources at 50 cm distance.
Figure 12: Extrapolated plot of air-kerma versus irradiation time for the determination of transit air-kerma of panoramic gamma irradiation system using 30 cc Physicalisch Technische Werkstatten ionization chamber

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Beam uniformity in collimated geometry

Beam uniformity at 1 m from the 60 Co and 137 Cs sources was analyzed by normalizing the Monte Carlo-calculated off-axis air-kerma values to the on-axis value. The normalized air-kerma along the vertical axis is shown in [Figure 13]. The output is symmetric and is uniform within 5% at 15 cm off-axis. [Figure 14] shows the normalized response (normalized air-kerma with respect to center of beam) along the horizontal axis of the collimator for 60 Co and 137 Cs sources. Note that in this case, part of the beam is attenuated by the presence of a shielding block, which is housing the 241 Am source. Hence, the air-kerma profile is likely to be asymmetric. In this case, beam is uniform within 5% at off-axis distances of -10 cm and +15 cm for 60 Co and 137 Cs sources. It may be noted that ISO recommends beam uniformity of 5%. [5] The investigation suggests that the central square area of −10 cm to +10 cm can safely be used for exposing the multiple dosimeters.
Figure 13: Beam uniformity along the vertical axis of collimated beam for 137Cs and 60Co sources generated through simulation

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Figure 14: Beam uniformity of 137Cs and 60Co sources along horizontal axis in collimated geometry generated through simulation

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Photon fluence spectrum

Monte Carlo-calculated photon fluence spectra at 1 m along the transverse axis of the sources 60 Co and 137 Cs sources with and without the irradiator are presented in [Figure 15] and [Figure 16], respectively. The photon fluence in each of the energy bins (bin size is 5 keV) is plotted against the mid-point of the respective energy bins. For an encapsulated source, scattering of photons in all polar angles will occur. The scattered photons will include both single scattered and multiple scattered. Hence, the fluence spectrum will include the source photons and the Compton scattered photons. During Compton scattering, a gamma-ray photon is scattered with a scattering angle in the range of 0-180°. When the source photon energy is higher than the pair production threshold of 1.022 MeV, a peak at 511 keV photons is also expected due to annihilation of electron and positron pair. The large angle scattered photons in the polar angle range 150-180° will be in the energy range of 194-184 keV for 137 Cs and 225-212 keV for 60 Co. Enhanced photon fluence in the energy range corresponding to a large angle of scattering is due to large angle single scattered photons and multiply scattered photons. The photon fluence distribution is corresponding to the sources without the irradiator assembly shown in [Figure 15] and [Figure 16] is consistent with the above discussion.
Figure 15: Simulated photon fluence spectra at a distance of 1 m from 60Co source with and without irradiator body

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Figure 16: Simulated photon fluence spectra at a distance of 1 m from 137Cs source with and without irradiator body

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In the case of sources surrounded by irradiator assembly, there will be additional scattering contribution. For the 60 Co source, the irradiator assembly enhances the scattered component by about 17% whereas for the 137 Cs source; it is about 10%. The photon fluence spectra also include K-edge fluorescent photons (tallied in the energy bin 75-80 keV) resulting from photoelectric effect with the lead material of the irradiator.

Leakage radiation levels

For each location, five measurements were taken and average of them is presented in [Table 2] against the locations. It can be seen that the highest dose equivalent rate measured is 200 ± 20 μSv/h at location A which is close to the entrance door of the room housing the irradiation system, whereas at the sitting positions of the operator near control console (location B and C) it is 35-40 μSv/h. The dose rate at location A in the collimated mode of operation with 60 Co source is in the range of 10-15 μSv/h and that of B and C are in the range of 1-2 μSv/h. Considering the workload for panoramic (about 1 h/week) and collimated mode (2-3 h/week) of operation, the annual dose to the operator will be well within the annual recommended limit (30 mSv/year) as stipulated by the Safety Directives of Atomic Energy Regulatory Board of India (http://www.aerb.gov.in). However, from the point of view of safety of the operator, it has been instructed that once the panoramic irradiation is made ON, the operator should not stay in the control console room till the source goes back to OFF position.
Table 2: Measurement of radiation levels at different position on the surface of the irradiator and at different location in the operator's room

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The dose rates at the surface of the irradiator with source in the "OFF" condition are also presented in [Table 2]. It can be seen that the dose equivalent rate at the top surface [Figure 17]a is in the range of 0.1-0.9 μSv/h. Along the side surface 1 [Figure 17]b, the dose equivalent rates are in the range of 0.1-1.3 μSv/h and whereas on the side surface 2 [Figure 17]c, dose equivalent rate are in the range of 0.1-0.4 μSv/h. The reason for higher dose equivalent rate on side surface 1 is attributed to the presence of source movement channel on this side of the irradiator and slight leakage radiation through the shielding of 241 Am gamma source, which is attached on the body of the irradiator on this side. Similarly the highest dose equivalent rate at A, B, and C on the top surface is attributed to the leakage radiation through the channel of source movement and leakage radiation from 241 Am gamma source shielding. At 1 m from the surface of the irradiator the dose rate was always <1 μSv/h.
Figure 17: (a) Top surface of panoramic irradiator (b) Right side view of the irradiator (side surface 1) (c) Left side view of the irradiator (side surface 2)

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  Conclusions Top

A multisource computer-controlled gamma irradiation system was developed indigenously for accurate delivery of air-kerma and Hp (10) to personal dosimeters required for QA program in individual monitoring. Personal dosimeters can be irradiated in both panoramic mode in air and on phantom in the collimated mode of operation. The output (air-kerma rate) of the 137 Cs, 60 Co, and of 241 Am gamma sources was measured using ionization chambers. Depending upon the distance from the source, the measured output of the 137 Cs and 60 Co sources compare to the Monte Carlo calculated values, to within 5-10%. In the collimated mode, beam uniformity at 1 m from the sources is within 3% in the 20 cm × 20 cm area centered on the collimator axis. In the panoramic mode, the measured output around the 60 Co and 137 Cs source at 50 cm radial distance on the transverse plane is within 2%. The transit doses of the 60 Co and 137 Cs sources are 0.0025% and 0.025% of the output at 50 cm, respectively.

  Acknowledgments Top

Authors sincerely acknowledge the contribution of M/s Panacea Medical Technologies Pvt. Ltd., White Field, Bangaluru, India for fabricating the computer-controlled gamma irradiation system as per our design and installing the same at our laboratory. The authors are also thankful Dr. D. N. Sharma Director, Health Safety and Environment Group for his encouragement for/during the work.

  References Top

1.Vohra KG, Bhatt RC, Chandra B, Pradhan AS, Lakshmanan AR, Shastry SS. A personal dosimeter TLD badge based on CaSO4:Dy teflon TLD discs. Health Phys 1980;38:193-7.  Back to cited text no. 1
2.American National Standard Institute (ANSI). American National Standard for Dosimetry-Personnel Dosimetry Performance-Criteria for Testing. USA: ANSI/HPS N13.11;2009.  Back to cited text no. 2
3.Los Alamos Monte Carlo Group MCNP-3A. General Monte Carlo Code for Neutron and Photon Transport Version 3.1. Radiation Safety Information Computational Center (RSICC), P. O. Box 2008, Oak Ridge, TN, 37831-6362;1983.  Back to cited text no. 3
4.Hubbell JH, Seltzer SM. Tables of X-Ray Mass Attenuation Coefficients and Mass Energy Absorption Coefficients. Gaithersburg, MD: National Institute of Standards and Technology; 1995. Available from: http://www.physics.nist.gov/physRefData/XrayMassCoef.  Back to cited text no. 4
5.International Organisation for Standardization (ISO). X and Gamma Reference Radiation for Calibrating Dosemeters and Dose Rate Meters and for Determining their Response as a Function of Photon Energy, Part-I: Radiation Characteristics and Production Methods. ISO-4037-I;1996.  Back to cited text no. 5


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17]

  [Table 1], [Table 2]


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