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Experience with a new polymer-alanine dosimeter in a high-energy particle accelerator environment
Experience with a New Polymer-Alanine Dosimeter in a High-Energy Particle Accelerator Environment F. CONINCKX CERN,
1211
and H. SCHQNBACHER Geneva 23, Switzerland
The experience and results obtained with a new alanine dosimeter in the mixed radiation field of the proton and electron accelerators of the European Organization for Nuclear Research (CERN) are described.
KEYWORDS: dosimeters.
alanine dosimeter;
dose distribution
measurements;
radiophotoluminescent
INTRODUCTION The efficiency and reliability of DL-alanine, measured by the electron spin resonance technique, are well known. Generally, paraffin or cellulose is used as a binder for alanine. Kojima et al. (1986) developed a dosimeter consisting of a mixture of alanine and polystyrene, which is commercially available under the name ‘Aminogray.’ In collaboration with industry, a polymer-alanine dosimeter was developed at the European Organization for Nuclear Research (CERN) for use as a routine dosimeter for high-dose measurements. For practical applications, this mixture of polymer and alanine was extruded as a cable. The alanine dosimeter cable can be used for the measurement of dose distributions in particle accelerator tunnels, nuclear reactors, gamma sources, and irradiation facilities for radiation processing. The dosimeter cable can be cut into pieces after irradiation, for dose measurements by ESR.
DESCRIPTION
OF THE DOSIMETER
The dosimeter is based on a homogeneous mixture of polymer-alanine, which is extruded like a flexible cable and cut into rods of 30 mm length for dosimetry use. The dosimeter is 33% ethylene-propylene rubber (EP-rubber), and 67% DL-a-alanine by weight. Several polymers of the EP-rubber type were mixed with the alanine and extruded as cables to produce a compound without an appreciable ESR signal at zero dose. The finally selected alanine mixture was molded with a string of 0.2 mm diameter polyester fiber in the core of the cable, to improve the mechanical properties. Traction tests were performed on the different samples. Heat treatment after irradiation of the final sample at 50°C for two hours gave the same ESR peak-to-peak readings as those with no heat treatment. No fading was observed for storage at ambient temperature over ten months. Apart from the known advantages of alanine dosimeters, i.e., tissue- equivalent material, wide dose range, stability and almost energy- independent response, others are due to the polymeric binder, i.e., flexibility, availability of different lengths and diameters, high degree of homogeneity, etc. The dosimeter is commercially available under the trade name ‘Elcugray.’ 67
ESR dosimetry and applications
68
CALIBRATION
OF THE DOSIMETER
Irradiation of the dosimeters with @Co y-rays in the range of 1x10’ to 5x10’ Gy showed good reproducibility and a very satisfactory dose response. In an intercomparison between the alanine dosimeters of CERN and the Istituto Superiore di Sanita (ISS) in Rome (Coninckx et al., 1990) the same results were obtained for precision and homogeneity. The overall uncertainty was less than +4% at a 95% confidence level for the two laboratories. The error due to the geometry of the sample inside the cavity was resolved by choosing a dosimeter length greater than the height of the cavity. In comparison with other dosimetric systems, this dosimeter type proved to be most suitable for application in high-energy electron and proton accelerator environments (Coninckx et al., 1989).
EXPERIENCE
AT CERN HIGH-ENERGY
PARTICLE
ACCELERATORS
Routine Annlications in LEP. About 1000 measuring points are located in the 27 km circumference of the tunnel in the Large Electron-Positron storage ring (LEP). The aim of this program is to check on the degradation of the materials and components exposed to synchrotron radiation and to make predictions from the results. The position of the dosimeters is as close as possible to the radiation-sensitive items such as magnet coils, electrical cables, electronic equipment, etc. In general, the dosimeters are read once a year during the shutdown of the accelerator. Results of the dose measurements during the year 1989, on 118 magnet units in octant 1 of LEP with Elcugray are shown in Fig. 1. As expected, very uniform dose distribution was found in the arcs and no appreciable dose in the straight sections (189-197 and 204-210 magnetic units).
HALF-CELL
Fig. 1. Results of dose measurements 1989.
DIPOLE
POSITION
2
on 118 Magnet Units in LEP Octant 1 during the year
Figure 2 shows a comparison of dose measurements with Elcugray and radiophotoluminescence (RPL) dosimeters in LEP. Owing to the low-energy component of the synchrotron radiation (< 500 keV), it is evident that, because of its energy dependence, RPL overestimates the dose in a high-energy electron accelerator by a factor varying from 2 to 20, depending on the energy spectrum of this radiation. The energy-dependence of alanine above 80 keV (relative to water) is below 10% (Coninckx et al., 1989).
69
ESR dosimetry and applications 1 o6 Irradiations
made
in synchrotron
radiation
of LEP 6
10”
1 o4 0” b ,g lo3 2 cx : 0 0
lo*
Fzzi
10’
CERN
10” 10”
Fig. 2. Comparison
10’
10’ Dose
of dose measurements
10’
10’ ALANINE
10”
lo6
in gray
with Elcugray alanine and RPL dosimeters in LEP.
Fig. 3. Elcugray dosimeter cable wound around the LEP vacuum chamber and leading to the ceiling of the accelerator tunnel at 2 m distance.
70
ESR dosimetry and applications
Dose Distribution Measurements in the LEP Tunnel. Dose distribution measurements were carried out at different locations in the LEP tunnel. The Elcugray dosimeter cable was installed across the tunnel, around the tunnel walls, and around the vacuum chamber (see Fig. 3). Measurements were also made with and without the lead shielding around the vacuum chamber. Figure 4 gives the dose rates per unit beam current thus obtained across the tunnel and around the vacuum chamber, respectively. It is evident that the use of an alanine cable, instead of individual dosimeters for dose distribution measurements, is much more convenient for practical reasons. With this system, the installation time and the precision of the dose distribution measurements can be considerably improved.
Doses in Gy/Ah
LEP center
5.9
El
4.9
El
Fig. 4. Results of dose rate distribution measurements with Elcugray a) in tunnel cross-section; b) around the vacuum chamber.
Doses in Gy/Ah
dosimeter
cable in LEP:
ESR dosimetry and applications
71
Dose measurements in the SPS tunnel. An Elcugray dosimeter cable of 125 m length was installed in the tunnel of the 450-GeV Super Proton Synchrotron (SPS) adjacent to an optical-fiber cable, in order to measure the integrated dose along its passage through the radiation area. Another 125 m Elcugray cable was used along the cable trays in the dump area of the SPS to check the efficiency of shielding to reduce radiation damage to the cables. In both cases very satisfactory results were obtained and Elcugray proved its superiority over RPL dosimeters as regards accuracy, in particular in the key dose range above 5x102 Gy. Dose distribution measurements in an irradiation facilitv for material testing. Studies on radiation hardness of materials are carried out within R&D programs of different projects at CERN. Particular target areas are used as distinct radiation sources for the irradiation of the materials. One of the irradiation facilities is situated on the top of the target area of the 28 GeV Proton Synchrotron (PS). It consists of a hole leading down to the target area, above the secondary beam line, upstream from a beam dump. The facility consists of a tube in which samples are lowered down to the target area for irradiation. In this tube, as well as in the irradiation container, an Elcugray dosimeter cable was exposed in order to measure the dose distribution. The dose rate in the irradiation container was found to be 54 Gy/h. This value was in good agreement with readings by other dosimeter types, e.g., TLD, RPL, and Fricke.
CONCLUSION After successful intercomparison and intercalibration of the Elcugray alanine dosimeter cables, very satisfactory field experience has been gained for more than two years by using the dosimeter in the CERN routine high-dose dosimetry program. The superiority of alanine compared to other dosimeter types has been confirmed. By constructing the dosimeter in the form of a polymer cable, considerable improvement in effectiveness and precision is obtained.
REFERENCES Coninckx F., Schiinbacher H., Bartolotta A., Onori S. and Rosati A. (1989) Alanine dosimetry as the reference dosimetric system in accelerator radiation environments. Appl. Radiat. Isof., 40, 977-983.
Coninckx F., Schonbacher H., Onori S. and Bartolotta A. (1990) Intercomparison of CERN and ISS alanine dosimetric systems. Proc. Int. Symp. High Dose DOS. Radiat. Processing, Vienna, (IAEA, Vienna, 1991), pp. 411-118. Kojima T., Tanaka R., Morita Y. and Seguchi T. (1986) Alanine dosimeters using polymers as binders. Appl. Radiat. Isot., 37, 517-520.
1211
and H. SCHQNBACHER Geneva 23, Switzerland
The experience and results obtained with a new alanine dosimeter in the mixed radiation field of the proton and electron accelerators of the European Organization for Nuclear Research (CERN) are described.
KEYWORDS: dosimeters.
alanine dosimeter;
dose distribution
measurements;
radiophotoluminescent
INTRODUCTION The efficiency and reliability of DL-alanine, measured by the electron spin resonance technique, are well known. Generally, paraffin or cellulose is used as a binder for alanine. Kojima et al. (1986) developed a dosimeter consisting of a mixture of alanine and polystyrene, which is commercially available under the name ‘Aminogray.’ In collaboration with industry, a polymer-alanine dosimeter was developed at the European Organization for Nuclear Research (CERN) for use as a routine dosimeter for high-dose measurements. For practical applications, this mixture of polymer and alanine was extruded as a cable. The alanine dosimeter cable can be used for the measurement of dose distributions in particle accelerator tunnels, nuclear reactors, gamma sources, and irradiation facilities for radiation processing. The dosimeter cable can be cut into pieces after irradiation, for dose measurements by ESR.
DESCRIPTION
OF THE DOSIMETER
The dosimeter is based on a homogeneous mixture of polymer-alanine, which is extruded like a flexible cable and cut into rods of 30 mm length for dosimetry use. The dosimeter is 33% ethylene-propylene rubber (EP-rubber), and 67% DL-a-alanine by weight. Several polymers of the EP-rubber type were mixed with the alanine and extruded as cables to produce a compound without an appreciable ESR signal at zero dose. The finally selected alanine mixture was molded with a string of 0.2 mm diameter polyester fiber in the core of the cable, to improve the mechanical properties. Traction tests were performed on the different samples. Heat treatment after irradiation of the final sample at 50°C for two hours gave the same ESR peak-to-peak readings as those with no heat treatment. No fading was observed for storage at ambient temperature over ten months. Apart from the known advantages of alanine dosimeters, i.e., tissue- equivalent material, wide dose range, stability and almost energy- independent response, others are due to the polymeric binder, i.e., flexibility, availability of different lengths and diameters, high degree of homogeneity, etc. The dosimeter is commercially available under the trade name ‘Elcugray.’ 67
ESR dosimetry and applications
68
CALIBRATION
OF THE DOSIMETER
Irradiation of the dosimeters with @Co y-rays in the range of 1x10’ to 5x10’ Gy showed good reproducibility and a very satisfactory dose response. In an intercomparison between the alanine dosimeters of CERN and the Istituto Superiore di Sanita (ISS) in Rome (Coninckx et al., 1990) the same results were obtained for precision and homogeneity. The overall uncertainty was less than +4% at a 95% confidence level for the two laboratories. The error due to the geometry of the sample inside the cavity was resolved by choosing a dosimeter length greater than the height of the cavity. In comparison with other dosimetric systems, this dosimeter type proved to be most suitable for application in high-energy electron and proton accelerator environments (Coninckx et al., 1989).
EXPERIENCE
AT CERN HIGH-ENERGY
PARTICLE
ACCELERATORS
Routine Annlications in LEP. About 1000 measuring points are located in the 27 km circumference of the tunnel in the Large Electron-Positron storage ring (LEP). The aim of this program is to check on the degradation of the materials and components exposed to synchrotron radiation and to make predictions from the results. The position of the dosimeters is as close as possible to the radiation-sensitive items such as magnet coils, electrical cables, electronic equipment, etc. In general, the dosimeters are read once a year during the shutdown of the accelerator. Results of the dose measurements during the year 1989, on 118 magnet units in octant 1 of LEP with Elcugray are shown in Fig. 1. As expected, very uniform dose distribution was found in the arcs and no appreciable dose in the straight sections (189-197 and 204-210 magnetic units).
HALF-CELL
Fig. 1. Results of dose measurements 1989.
DIPOLE
POSITION
2
on 118 Magnet Units in LEP Octant 1 during the year
Figure 2 shows a comparison of dose measurements with Elcugray and radiophotoluminescence (RPL) dosimeters in LEP. Owing to the low-energy component of the synchrotron radiation (< 500 keV), it is evident that, because of its energy dependence, RPL overestimates the dose in a high-energy electron accelerator by a factor varying from 2 to 20, depending on the energy spectrum of this radiation. The energy-dependence of alanine above 80 keV (relative to water) is below 10% (Coninckx et al., 1989).
69
ESR dosimetry and applications 1 o6 Irradiations
made
in synchrotron
radiation
of LEP 6
10”
1 o4 0” b ,g lo3 2 cx : 0 0
lo*
Fzzi
10’
CERN
10” 10”
Fig. 2. Comparison
10’
10’ Dose
of dose measurements
10’
10’ ALANINE
10”
lo6
in gray
with Elcugray alanine and RPL dosimeters in LEP.
Fig. 3. Elcugray dosimeter cable wound around the LEP vacuum chamber and leading to the ceiling of the accelerator tunnel at 2 m distance.
70
ESR dosimetry and applications
Dose Distribution Measurements in the LEP Tunnel. Dose distribution measurements were carried out at different locations in the LEP tunnel. The Elcugray dosimeter cable was installed across the tunnel, around the tunnel walls, and around the vacuum chamber (see Fig. 3). Measurements were also made with and without the lead shielding around the vacuum chamber. Figure 4 gives the dose rates per unit beam current thus obtained across the tunnel and around the vacuum chamber, respectively. It is evident that the use of an alanine cable, instead of individual dosimeters for dose distribution measurements, is much more convenient for practical reasons. With this system, the installation time and the precision of the dose distribution measurements can be considerably improved.
Doses in Gy/Ah
LEP center
5.9
El
4.9
El
Fig. 4. Results of dose rate distribution measurements with Elcugray a) in tunnel cross-section; b) around the vacuum chamber.
Doses in Gy/Ah
dosimeter
cable in LEP:
ESR dosimetry and applications
71
Dose measurements in the SPS tunnel. An Elcugray dosimeter cable of 125 m length was installed in the tunnel of the 450-GeV Super Proton Synchrotron (SPS) adjacent to an optical-fiber cable, in order to measure the integrated dose along its passage through the radiation area. Another 125 m Elcugray cable was used along the cable trays in the dump area of the SPS to check the efficiency of shielding to reduce radiation damage to the cables. In both cases very satisfactory results were obtained and Elcugray proved its superiority over RPL dosimeters as regards accuracy, in particular in the key dose range above 5x102 Gy. Dose distribution measurements in an irradiation facilitv for material testing. Studies on radiation hardness of materials are carried out within R&D programs of different projects at CERN. Particular target areas are used as distinct radiation sources for the irradiation of the materials. One of the irradiation facilities is situated on the top of the target area of the 28 GeV Proton Synchrotron (PS). It consists of a hole leading down to the target area, above the secondary beam line, upstream from a beam dump. The facility consists of a tube in which samples are lowered down to the target area for irradiation. In this tube, as well as in the irradiation container, an Elcugray dosimeter cable was exposed in order to measure the dose distribution. The dose rate in the irradiation container was found to be 54 Gy/h. This value was in good agreement with readings by other dosimeter types, e.g., TLD, RPL, and Fricke.
CONCLUSION After successful intercomparison and intercalibration of the Elcugray alanine dosimeter cables, very satisfactory field experience has been gained for more than two years by using the dosimeter in the CERN routine high-dose dosimetry program. The superiority of alanine compared to other dosimeter types has been confirmed. By constructing the dosimeter in the form of a polymer cable, considerable improvement in effectiveness and precision is obtained.
REFERENCES Coninckx F., Schiinbacher H., Bartolotta A., Onori S. and Rosati A. (1989) Alanine dosimetry as the reference dosimetric system in accelerator radiation environments. Appl. Radiat. Isof., 40, 977-983.
Coninckx F., Schonbacher H., Onori S. and Bartolotta A. (1990) Intercomparison of CERN and ISS alanine dosimetric systems. Proc. Int. Symp. High Dose DOS. Radiat. Processing, Vienna, (IAEA, Vienna, 1991), pp. 411-118. Kojima T., Tanaka R., Morita Y. and Seguchi T. (1986) Alanine dosimeters using polymers as binders. Appl. Radiat. Isot., 37, 517-520.