- Email: [email protected]
Radiation mapping inside the Bunkers of medium energy accelerators using a robotic carrier
Applied Radiation and Isotopes 80 (2013) 103–108
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radiation mapping inside the Bunkers of medium energy accelerators using a robotic carrier R. Ravishankar c,n, T.K. Bhaumik a, T. Bandyopadhyay c, M. Purkait a, S.C. Jena a, S.K. Mishra c, S. Sharma b, V. Agashe b, K. Datta a, B. Sarkar a, C. Datta a, D. Sarkar a, P.K. Pal b a
Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Kolkata 700064, India Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India c BARC, Variable Energy Cyclotron Centre,1/AF,Bidhannagar, Kolkata 700064, India b
H I G H L I G H T S
Measurement and estimation of neutron and gamma radiation field prevailing inside active bunkers of Cyclotron during operation. We can evaluate accident or worst-scenario radiation exposure to a human operator, utilizing the radiation field mapping. The work has reduced un-wanted beam loss at undesirable spots. Use of robotic carrier helped radiation field mapping during machine operation and reduced human labor.
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2012 Received in revised form 6 May 2013 Accepted 11 June 2013 Available online 27 June 2013
The knowledge of ambient and peak radiation levels prevailing inside the bunkers of the accelerator facilities is essential in assessing the accidental human exposure inside the bunkers and in protecting sensitive electronic equipments by minimizing the exposure to high intensity mixed radiation fields. Radiation field mapping dynamically, inside bunkers are rare, though generally dose-rate data are available in every particle accelerator facilities at specific locations. Taking into account of the fact that the existing neutron fields with a spread of energy from thermal up to the energy of the accelerated charged projectiles, prompt photons and other particles prevailing during cyclotron operation inside the bunkers, neutron and gamma survey meters with extended energy ranges attached to a robotic carrier have been used. The robotic carrier movement was controlled remotely from the control room with the help of multiple visible range optical cameras provided inside the bunkers and the wireless and wired protocols of communication helped its movement and data acquisition from the survey meters. Variable Energy Cyclotron Centre, Kolkata has positive ion accelerating facilities such as K-130 room Temperature Cyclotron, K-500 Super Conducting Cyclotron and a forthcoming 30 MeV Proton Medical Cyclotron with high beam current. The dose rates data for K-130 Room Temperature Cyclotron, VECC were collected for various energies of alpha and proton beams losing their total energy at different stages on different materials at various strategic locations of radiological importance inside the bunkers. The measurements established that radiation levels inside the machine bunker dynamically change depending upon the beam type, beam energy, machine operation parameters, deflector condition, slit placement and central region beam tuning. The obtained inference from the association of dose rates with the parameters like beam intensity, type and energy of projectiles, helped in improving the primary beam transmission and minimizing the ambient radiation fields inside the bunkers. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Accelerators Bunkers Radiation measurements Neutron, gamma
1. Introduction The radiation environment in a medium energy accelerator facility is complex and is a composition of radiations of different varieties dominated by neutrons, photons, high energy charged
n
Corresponding author. Mobile: +91 9433092466; fax: +91 3323346871. E-mail address: [email protected] (R. Ravishankar).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.06.017
particles and other induced activities in air and in other accelerator components. It is essential to estimate the ambient and peak radiation levels prevailing inside the bunkers of the accelerator facilities which are kept strictly inaccessible. The knowledge of such radiation levels are very essential in many aspects. The mandatory requirement of keeping a record of peak and ambient dose rates levels at various locations for different operating conditions of the Cyclotron with different types and energies of projectiles stopping at different positions inside the bunkers is of
104
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
vital consideration. In emergency situations and accidental personnel exposure situations, the knowledge of prevailing radiation levels will help in handling such scenarios and will help in reducing the initial panicky which may normally arise out of such situations and deciding the proper course of further actions like blood counts and biological samples analysis. It is an established fact that the electronic components like microprocessor chips, optical devices, control and electrical cables are susceptible for radiation damage and they have a cumulative effect of radiation exposure. If the ambient levels inside the bunkers are measured and necessary steps are taken to minimize with optimization of the parameters of beam accelerating, extraction and transport systems, then the durability of the components can be increased (K-130 VECC Safety Report). The radiation field data inside the bunkers of medium energy positive ion accelerators are available generally but they are at a very few locations for a few specific beam conditions. Nevertheless the dynamic radiation fields in each accelerator facility are unique in its own respect. Using computer simulations with general purpose Monte Carlo radiation transport codes can give an estimate of dose rates inside bunkers but the actual geometry construction is too complex to create and nevertheless experimental measurements will give the best solution for dose-rate estimations. The attempt for mapping the prevailing dose rates for various beam type and energy combinations are very important in radiological safety point of view. Also a systematic study of relationships of beam intensity, type, beam-energy and type of beam stopper material dynamically for a particular accelerator facility has its own scientific merit and practical values. Mapping radiation fields inside the bunkers during cyclotron operation for multiple combinations of conditions are tedious and laborious if they carried out manually with limited number of survey meters available. Hence it was carried out using a mobile robotic carrier with remote controlled and data transfer and communication facilities without any human exposure and with minimal intervention. Radiation dose rates of both neutron and gamma radiations have been mapped for predetermined strategic locations of radiological safety importance inside vault and experimental caves of K-130 Room Temperature Cyclotron facility for α beams of 30, 35, 40, 50 and 60 MeV and proton beams of 10,12, 15 and 18 MeV with beam dumping at specific locations. The data were collected remotely using the communication features of the robotic carrier as read from the attached survey meters. The analysis of the results showed the linearity with beam intensity and other relationship with beam type and energy.
Fig. 1. The Robotic Carrier along with Ludlum 2363 Neutron Gamma Survey meter inside the experimental area.
2. Methods and Materials Fig. 1 shows the picture of the robotic carrier along with the Ludlum 2363 Gamma Neutron Survey meter.
Fig. 2. The various predetermined strategic locations of radiological safety importance inside vault.
Fig. 3. The various predetermined strategic locations of radiological safety importance inside experimental cave.
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
Figs. 2 and 3 show the various predetermined strategic locations of radiological safety importance inside vault and experimental cave respectively. General Neutron Survey Instruments (response normally upto 14 or 20 MeV) and personnel monitoring devices are not available for measuring high energy neutrons above 20 MeV. Computer simulations based on the Monte-Carlo methods may be an alternative for the estimation of dose to the persons exposed by chance around those facilities but nevertheless the experimental verification cannot be substituted and are very vital in validating the virtual experiment viz. simulation. Neutron survey instruments are used to detect neutrons with a wide range of energies (thermal to 14 MeV) and directions. Practically most of the survey instruments are deficient in terms of energy and angular dependence of response to some extent. Mostly two types of neutron survey meters are in use: Andersson–Braun (Andersson and Braun, 1962) and Leake type (Leake, 1965, 1968). They use BF3 or 3He proportional counter located inside a moderator of CH2 that has a perforated thermal neutron attenuating layer located within it. Such instruments present an ergonomic challenge being heavy and bulky and have caused injuries during radiation surveys. These conventional survey meters though designed for measuring neutrons up to 17 or 20 MeV have been found to have poor response above 10 MeV
105
which make them insufficient tool in high energy accelerator facilities (Patterson and Thomas, 1973) (Radiation protection for particle accelerator facilities, 2003). Ludlum 2363 Gamma Neutron Survey meter from Ludlum Measurements INC with Proton Recoil Scintillator Los Alamos (PRESCILA) neutron probe is a low weight alternative ( 2 Kg) capable of extended energy response ( up to 100 MeV), high sensitivity with moderately good gamma discrimination capability. Also it is capable of measuring both neutron and gamma radiation simultaneously with both analog and digital display. The instrument contains an internal energy compensated G-M detector for gamma and 42–41 PRESCILA probe for neutron. An array of ZnS(Ag) based scintillators is located inside and around a Lucite light guide which couples the scintillations to bi-alkali PMT. The use of both thermal and fast scintillators allows the instrument to be used for a wide range of energy spectrum (beyond 20 MeV). The light guide and borated polyethylene frame provide moderation for thermal scintillator element. The inherent pulse height advantage of proton recoils over electron tracks in the scintillator makes it possible to use standard pulse height discriminator instead of any specialized and sophisticated one. The probe is having excellent sensitivity for neutron (350 cpm per mrem/h) and gamma (1050 cpm per mrem/h). The directional response is uniform (715%) over a wide range of energies (Instruction Manual Ludlum Model 2363; Private communication). Response linearity has been characterized to over 20 mSv/h. Gamma rejection is effective in gamma fields up to 2 mSv/h. The internal view of PRESCILA is given in Fig. 4. The mobile robot shown in Fig. 1 has two drive wheels and one castor for support. Steering is achieved through differential motion of the drive wheels. A supervisory microcontroller is connected to a single board computer (SBC) through serial port. The mobile
Dose rates(μSv/h)/nA
1.0E+03
1.0E+02
1.0E+01
1.0E+00
1.0E-01 5
15
25
35
45
55
65
Projectile Energy (MeV) Fig. 4. The internal arrangement of PRESCILA neutron probe of Ludlum 2363 neutron gamma survey meter.
Fig. 6. n and γ-Dose rates Vs Energy of the projectiles (per nA intensity) (Alpha and Proton)–inside the experimental cave.
Fig. 5. Block diagram of Radiation Mapping System.
106
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
60
Dose -rate(μSv/h)
50
Position_C Position_D
40
Position_E
30
Position_G Position_H
20 10 0 25
35
45
55
65
Primary particle Energy-MeV Fig. 7. n Dose-rate at salient positions (per nA Alpha: Primary).
16
Dose -rate (μsv/h)
14 Position_C
12 10
Position_D
8
Position_E
6
Position_G
4
Position_H
2 0 25
35 45 55 Primary particle energy- MeV
65
Dose -rate (μsv/h)
Fig. 8. γ Dose-rate at salient positions (per nA Alpha: Primary).
10 9 8 7 6 5 4 3 2 1 0
Position_C Position_D Position_E position_G Position_H
9
11
13
15
Primary Particle Energy- MeV Fig. 9. n Dose-rate at salient positions (per nA Proton: Primary).
robot is controlled by high level programs running on the SBC. The microcontroller receives commands from the SBC and sends command packets to two different PID controllers for the two motors, controlling the motion of the robot. Two DC motors (with gearbox and encoder) drive the wheels of the robot and also send feedback about its position to the PID controller. For wireless communication with a
wireless access point, the robot uses a wireless adapter module (Fig. 2). The Gamma Neutron Survey Meter is mounted on the mobile robot at a height of 1.2 m to match with the height of the beam lines. This meter communicates with the SBC using an RS-232 serial link (Fig. 5). The experiments have been carried out at K-130 VECC with beam stopping at various Faraday Cups (thick Ta disc) in vault and
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
107
14
Dose-rate (μSv/h)
12 Position_C
10
Position_D
8
Position_E
6
Position_G
4
Position_H
2 0 8
10 12 14 16 Primary Particle Energy- MeV
18
Fig. 10. γ Dose-rate at salient positions (per nA Proton: Primary).
3. Results The dose-rate measurements for both neutron and gamma have been carried out in machine bunker and in experimental bunker for the locations specified in Figs. 2 and 3 respectively. Figs. 6–10 show the dose-rate measurements for both neutron and gamma. The dose rates inside the cave at position F for unit nano ampere intensity of proton and alpha beams of different energy levels measured 1.2 m away from the target are shown in Fig. 6. The modified exponential trend lines are also shown in Fig. 6. The Dose rates can be reasonably fitted with projectile energies using modified exponential type of the form, y ¼ a eðb=xÞ
ð1Þ
Where y corresponds to dose rates in mRem/h per nA beam intensity and x corresponds to projectile energy in MeV. The values of the constants a and b can be estimated separately for n and γ dose rates for both type of projectiles. The estimated values have been found to be in order with measured values with reasonably less standard error and good correlation. The modified exponential form will approximately give the order of dose-rate
Dose-rate(x1.0e-01 μSv/h)
8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 25
35
45
55
65
Primary Particle Energy-MeV Fig. 11. Neutron Dose-rate with std error (normalized to 1 nA) at G position in Experimental Cave.
0.4
Dose-rate(x1.0e-01 μSv/h)
at thick Ta target in Experimental Cave bombarded by 30, 35, 40, 50 and 60 MeV α beams; 10, 12, 15 and 18 MeV proton beams for different beam currents. The range of 40 and 60 MeV α in Ta are 0.013 cm and 0.024 cm respectively. The Ta target was kept in a target holder-collimator arrangement in such a way the secondary neutrons undergo insignificant degradation in energy and number. Measurements were carried out at various predetermined strategic locations inside the vault (shown in Fig. 2) and in the cave at 1.2 m from target for neutron dose-rate estimation. Also Neutron fluence rates were measured with Area Neutron Monitors (BF3 based) located inside the bunkers and the corresponding dose-rates have been estimated using flux to dose conversion factors (Conversion coefficients for use in radiological protection against external radiation, 1996). The experiments were carried out for different projectile currents (100 nA, 200 nA, 300 nA, 400 nA and 500 nA) and different energies. The measurements have been done for twelve locations inside the machine bunker leading to 648 set of values due to beam type, energy, current and beam stopping combinations. In experimental bunker measurements have been carried out for 8 locations leading to 144 set of values. For each set of measurements, multiple readings of both neutron and gamma have been taken with each measurement lasting for 10 sec and the mean value in each set was taken for consideration.
0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 20
30
40
50
60
70
Primary Particle Energy-MeV Fig. 12. Gamma Dose-rate with std error (normalized to 1 nA) at G position in Experimental Cave.
values in case of worst case scenario in active bunkers which is the main purpose of the work. The position F was chosen for discussion and analysis as this location has the least chance of having errors due to external factors like multiple scattering and divergence. The measurements inside the vault for beam stopping at FC-01 at salient locations are shown in the following Figs. 7–10. From the graph it is evident the position G is dominant in all the cases. The position G is the location corresponding to central viewing port without the attenuation effects due to shielding of the magnetic Iron Core. The gamma dose rates in case of primary proton beam tends to saturate for proton energies greater than 12 MeV but in case of
108
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108 10.00
% part w.r.t. to Observed value
9.00 8.00
std_err
7.00 6.00
std_deviation
5.00 4.00 3.00 2.00 1.00 0.00 20
30
40
50
60
70
Primary Particle Energy-MeV Fig. 13. Percentage part of Std. Err and Std. Dev. in observed values (taking the highest values).
primary alpha the saturation trend may be much greater than 60 MeV. The neutron dose rates increase exponentially with primary beam energy. The trend suggests the neutron dose rates may be tending towards saturation at much higher energies both for alpha and proton primaries. Also from Fig. 10, it is observed the gamma dose rates are three to five times higher at positions near to central port regions (position G and H) which suggests the dominance of prompt photon emissions from the nuclear interactions of primary within the cyclotron machine (like central regions, DEE and Deflector). All these dose-rate curves can be reasonably fitted with modified exponential function as given in Eq. (1). The standard error and standard deviations of the huge measured values have been studied. Figs. 11 and 12 give the neutron and gamma dose rates (for primary alpha) with Std. error for the position G in the experimental cave. The standard error and standard deviations have been found to be reasonably good for both types of alpha and proton primaries. The position G was considered here as it is more susceptible for systematic errors due to oblong direction with primary beam but less susceptible for rather hard predictable errors as compared to locations like C. Fig. 13 shows the percentage part the maximum estimated standard errors and standard deviations in the corresponding observed values (with 95% confidence levels). The results show not more than 10% in any case which is reasonably good for radiation protection purpose. 4. Conclusion The work finds its utility in radiation field mapping under different machine operating conditions (variation in projectile
type and energy) which is very vital for accidental human radiation exposure estimation. The use of mobile robot for the radiation mapping was extremely useful as otherwise it could have been cumbersome considering the huge volume of data obtained. The work was also found to be helpful in locating the maximum beam loss positions and thereby improving beam optics resulting in better beam transmission. It was observed during mapping that on optimization of beam internal parameters, there was significant reduction in the ambient dose rates inside the machine bunker areas. These circumstances also decrease the radiation damage to electronic components of control circuits and PLC inside bunkers. It was also observed immediately after machine shutdown, a significant reduction of minimum waiting time from a few hours to a few minutes, before entry into the bunker areas, (nevertheless a minimum waiting time of 10 min based on ventilation system is always enforced) which resulted in the improvement of overall machine up time. Further work is in progress in developing the robotic carrier to have two dimensional and three dimensional radiation mapping of the bunker rooms.
Acknowledgments The authors acknowledge the contribution and active support provided by the K-130 Room Temperature Cyclotron Operation Group and Workshop, VECC. The authors are also thankful to Dr. R. K. Bhandari, Ex. Director, VECC, Kolkata, Dr. D. N. Sharma, Director, Health, Safety and Environment Group, BARC, Mumbai and Shri Manjit Singh, Director, DM&A Group, BARC, Mumbai for their constant support and encouragements.
References Andersson, I.O., Braun, J.A., 1962. Neutron rem counter with uniform sensitivity from 0.025 eV to 10 MeV. In: Proceedings of the Symposium on Neutron Dosimetry, vol. 2. Harwell, Vienna: IAEA, pp. 85–89. Conversion coefficients for use in radiological protection against external radiation, 1996. InternationalCommissionon Radiological Protection-ICRP Publication 74. Ann. ICRP 26(3–4), Elesevier Science, Oxford. Instruction Manual Ludlum Model 2363, Ludlum Measurements Inc., Texas. K-130 Cyclotron Safety Report, 2010. VECC. Kolkata, India. Leake, J.W., 1968. An improved spherical dose equivalent neutron detector. Nucl. Instrum. Methods Phys. Res. Sect. A 63, 329–332. Leake, J.W.A., 1965. Spherical dose equivalent neutron detector. Nucl. Instrum. Methods Phys. Res. Sect. A 45, 151–156. Patterson, H.W., Thomas, R.H., 1973. Accelerator Health Physics. Academic Press, New York. Private communication, Ludlum Measurements Inc., Texas. Radiation protection for particle accelerator facilities, 2003. National Council on Radiation Protection and Measurements. NCRP Report no. 144, USA.
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Radiation mapping inside the Bunkers of medium energy accelerators using a robotic carrier R. Ravishankar c,n, T.K. Bhaumik a, T. Bandyopadhyay c, M. Purkait a, S.C. Jena a, S.K. Mishra c, S. Sharma b, V. Agashe b, K. Datta a, B. Sarkar a, C. Datta a, D. Sarkar a, P.K. Pal b a
Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Kolkata 700064, India Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India c BARC, Variable Energy Cyclotron Centre,1/AF,Bidhannagar, Kolkata 700064, India b
H I G H L I G H T S
Measurement and estimation of neutron and gamma radiation field prevailing inside active bunkers of Cyclotron during operation. We can evaluate accident or worst-scenario radiation exposure to a human operator, utilizing the radiation field mapping. The work has reduced un-wanted beam loss at undesirable spots. Use of robotic carrier helped radiation field mapping during machine operation and reduced human labor.
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2012 Received in revised form 6 May 2013 Accepted 11 June 2013 Available online 27 June 2013
The knowledge of ambient and peak radiation levels prevailing inside the bunkers of the accelerator facilities is essential in assessing the accidental human exposure inside the bunkers and in protecting sensitive electronic equipments by minimizing the exposure to high intensity mixed radiation fields. Radiation field mapping dynamically, inside bunkers are rare, though generally dose-rate data are available in every particle accelerator facilities at specific locations. Taking into account of the fact that the existing neutron fields with a spread of energy from thermal up to the energy of the accelerated charged projectiles, prompt photons and other particles prevailing during cyclotron operation inside the bunkers, neutron and gamma survey meters with extended energy ranges attached to a robotic carrier have been used. The robotic carrier movement was controlled remotely from the control room with the help of multiple visible range optical cameras provided inside the bunkers and the wireless and wired protocols of communication helped its movement and data acquisition from the survey meters. Variable Energy Cyclotron Centre, Kolkata has positive ion accelerating facilities such as K-130 room Temperature Cyclotron, K-500 Super Conducting Cyclotron and a forthcoming 30 MeV Proton Medical Cyclotron with high beam current. The dose rates data for K-130 Room Temperature Cyclotron, VECC were collected for various energies of alpha and proton beams losing their total energy at different stages on different materials at various strategic locations of radiological importance inside the bunkers. The measurements established that radiation levels inside the machine bunker dynamically change depending upon the beam type, beam energy, machine operation parameters, deflector condition, slit placement and central region beam tuning. The obtained inference from the association of dose rates with the parameters like beam intensity, type and energy of projectiles, helped in improving the primary beam transmission and minimizing the ambient radiation fields inside the bunkers. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Accelerators Bunkers Radiation measurements Neutron, gamma
1. Introduction The radiation environment in a medium energy accelerator facility is complex and is a composition of radiations of different varieties dominated by neutrons, photons, high energy charged
n
Corresponding author. Mobile: +91 9433092466; fax: +91 3323346871. E-mail address: [email protected] (R. Ravishankar).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.06.017
particles and other induced activities in air and in other accelerator components. It is essential to estimate the ambient and peak radiation levels prevailing inside the bunkers of the accelerator facilities which are kept strictly inaccessible. The knowledge of such radiation levels are very essential in many aspects. The mandatory requirement of keeping a record of peak and ambient dose rates levels at various locations for different operating conditions of the Cyclotron with different types and energies of projectiles stopping at different positions inside the bunkers is of
104
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
vital consideration. In emergency situations and accidental personnel exposure situations, the knowledge of prevailing radiation levels will help in handling such scenarios and will help in reducing the initial panicky which may normally arise out of such situations and deciding the proper course of further actions like blood counts and biological samples analysis. It is an established fact that the electronic components like microprocessor chips, optical devices, control and electrical cables are susceptible for radiation damage and they have a cumulative effect of radiation exposure. If the ambient levels inside the bunkers are measured and necessary steps are taken to minimize with optimization of the parameters of beam accelerating, extraction and transport systems, then the durability of the components can be increased (K-130 VECC Safety Report). The radiation field data inside the bunkers of medium energy positive ion accelerators are available generally but they are at a very few locations for a few specific beam conditions. Nevertheless the dynamic radiation fields in each accelerator facility are unique in its own respect. Using computer simulations with general purpose Monte Carlo radiation transport codes can give an estimate of dose rates inside bunkers but the actual geometry construction is too complex to create and nevertheless experimental measurements will give the best solution for dose-rate estimations. The attempt for mapping the prevailing dose rates for various beam type and energy combinations are very important in radiological safety point of view. Also a systematic study of relationships of beam intensity, type, beam-energy and type of beam stopper material dynamically for a particular accelerator facility has its own scientific merit and practical values. Mapping radiation fields inside the bunkers during cyclotron operation for multiple combinations of conditions are tedious and laborious if they carried out manually with limited number of survey meters available. Hence it was carried out using a mobile robotic carrier with remote controlled and data transfer and communication facilities without any human exposure and with minimal intervention. Radiation dose rates of both neutron and gamma radiations have been mapped for predetermined strategic locations of radiological safety importance inside vault and experimental caves of K-130 Room Temperature Cyclotron facility for α beams of 30, 35, 40, 50 and 60 MeV and proton beams of 10,12, 15 and 18 MeV with beam dumping at specific locations. The data were collected remotely using the communication features of the robotic carrier as read from the attached survey meters. The analysis of the results showed the linearity with beam intensity and other relationship with beam type and energy.
Fig. 1. The Robotic Carrier along with Ludlum 2363 Neutron Gamma Survey meter inside the experimental area.
2. Methods and Materials Fig. 1 shows the picture of the robotic carrier along with the Ludlum 2363 Gamma Neutron Survey meter.
Fig. 2. The various predetermined strategic locations of radiological safety importance inside vault.
Fig. 3. The various predetermined strategic locations of radiological safety importance inside experimental cave.
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
Figs. 2 and 3 show the various predetermined strategic locations of radiological safety importance inside vault and experimental cave respectively. General Neutron Survey Instruments (response normally upto 14 or 20 MeV) and personnel monitoring devices are not available for measuring high energy neutrons above 20 MeV. Computer simulations based on the Monte-Carlo methods may be an alternative for the estimation of dose to the persons exposed by chance around those facilities but nevertheless the experimental verification cannot be substituted and are very vital in validating the virtual experiment viz. simulation. Neutron survey instruments are used to detect neutrons with a wide range of energies (thermal to 14 MeV) and directions. Practically most of the survey instruments are deficient in terms of energy and angular dependence of response to some extent. Mostly two types of neutron survey meters are in use: Andersson–Braun (Andersson and Braun, 1962) and Leake type (Leake, 1965, 1968). They use BF3 or 3He proportional counter located inside a moderator of CH2 that has a perforated thermal neutron attenuating layer located within it. Such instruments present an ergonomic challenge being heavy and bulky and have caused injuries during radiation surveys. These conventional survey meters though designed for measuring neutrons up to 17 or 20 MeV have been found to have poor response above 10 MeV
105
which make them insufficient tool in high energy accelerator facilities (Patterson and Thomas, 1973) (Radiation protection for particle accelerator facilities, 2003). Ludlum 2363 Gamma Neutron Survey meter from Ludlum Measurements INC with Proton Recoil Scintillator Los Alamos (PRESCILA) neutron probe is a low weight alternative ( 2 Kg) capable of extended energy response ( up to 100 MeV), high sensitivity with moderately good gamma discrimination capability. Also it is capable of measuring both neutron and gamma radiation simultaneously with both analog and digital display. The instrument contains an internal energy compensated G-M detector for gamma and 42–41 PRESCILA probe for neutron. An array of ZnS(Ag) based scintillators is located inside and around a Lucite light guide which couples the scintillations to bi-alkali PMT. The use of both thermal and fast scintillators allows the instrument to be used for a wide range of energy spectrum (beyond 20 MeV). The light guide and borated polyethylene frame provide moderation for thermal scintillator element. The inherent pulse height advantage of proton recoils over electron tracks in the scintillator makes it possible to use standard pulse height discriminator instead of any specialized and sophisticated one. The probe is having excellent sensitivity for neutron (350 cpm per mrem/h) and gamma (1050 cpm per mrem/h). The directional response is uniform (715%) over a wide range of energies (Instruction Manual Ludlum Model 2363; Private communication). Response linearity has been characterized to over 20 mSv/h. Gamma rejection is effective in gamma fields up to 2 mSv/h. The internal view of PRESCILA is given in Fig. 4. The mobile robot shown in Fig. 1 has two drive wheels and one castor for support. Steering is achieved through differential motion of the drive wheels. A supervisory microcontroller is connected to a single board computer (SBC) through serial port. The mobile
Dose rates(μSv/h)/nA
1.0E+03
1.0E+02
1.0E+01
1.0E+00
1.0E-01 5
15
25
35
45
55
65
Projectile Energy (MeV) Fig. 4. The internal arrangement of PRESCILA neutron probe of Ludlum 2363 neutron gamma survey meter.
Fig. 6. n and γ-Dose rates Vs Energy of the projectiles (per nA intensity) (Alpha and Proton)–inside the experimental cave.
Fig. 5. Block diagram of Radiation Mapping System.
106
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
60
Dose -rate(μSv/h)
50
Position_C Position_D
40
Position_E
30
Position_G Position_H
20 10 0 25
35
45
55
65
Primary particle Energy-MeV Fig. 7. n Dose-rate at salient positions (per nA Alpha: Primary).
16
Dose -rate (μsv/h)
14 Position_C
12 10
Position_D
8
Position_E
6
Position_G
4
Position_H
2 0 25
35 45 55 Primary particle energy- MeV
65
Dose -rate (μsv/h)
Fig. 8. γ Dose-rate at salient positions (per nA Alpha: Primary).
10 9 8 7 6 5 4 3 2 1 0
Position_C Position_D Position_E position_G Position_H
9
11
13
15
Primary Particle Energy- MeV Fig. 9. n Dose-rate at salient positions (per nA Proton: Primary).
robot is controlled by high level programs running on the SBC. The microcontroller receives commands from the SBC and sends command packets to two different PID controllers for the two motors, controlling the motion of the robot. Two DC motors (with gearbox and encoder) drive the wheels of the robot and also send feedback about its position to the PID controller. For wireless communication with a
wireless access point, the robot uses a wireless adapter module (Fig. 2). The Gamma Neutron Survey Meter is mounted on the mobile robot at a height of 1.2 m to match with the height of the beam lines. This meter communicates with the SBC using an RS-232 serial link (Fig. 5). The experiments have been carried out at K-130 VECC with beam stopping at various Faraday Cups (thick Ta disc) in vault and
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108
107
14
Dose-rate (μSv/h)
12 Position_C
10
Position_D
8
Position_E
6
Position_G
4
Position_H
2 0 8
10 12 14 16 Primary Particle Energy- MeV
18
Fig. 10. γ Dose-rate at salient positions (per nA Proton: Primary).
3. Results The dose-rate measurements for both neutron and gamma have been carried out in machine bunker and in experimental bunker for the locations specified in Figs. 2 and 3 respectively. Figs. 6–10 show the dose-rate measurements for both neutron and gamma. The dose rates inside the cave at position F for unit nano ampere intensity of proton and alpha beams of different energy levels measured 1.2 m away from the target are shown in Fig. 6. The modified exponential trend lines are also shown in Fig. 6. The Dose rates can be reasonably fitted with projectile energies using modified exponential type of the form, y ¼ a eðb=xÞ
ð1Þ
Where y corresponds to dose rates in mRem/h per nA beam intensity and x corresponds to projectile energy in MeV. The values of the constants a and b can be estimated separately for n and γ dose rates for both type of projectiles. The estimated values have been found to be in order with measured values with reasonably less standard error and good correlation. The modified exponential form will approximately give the order of dose-rate
Dose-rate(x1.0e-01 μSv/h)
8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 25
35
45
55
65
Primary Particle Energy-MeV Fig. 11. Neutron Dose-rate with std error (normalized to 1 nA) at G position in Experimental Cave.
0.4
Dose-rate(x1.0e-01 μSv/h)
at thick Ta target in Experimental Cave bombarded by 30, 35, 40, 50 and 60 MeV α beams; 10, 12, 15 and 18 MeV proton beams for different beam currents. The range of 40 and 60 MeV α in Ta are 0.013 cm and 0.024 cm respectively. The Ta target was kept in a target holder-collimator arrangement in such a way the secondary neutrons undergo insignificant degradation in energy and number. Measurements were carried out at various predetermined strategic locations inside the vault (shown in Fig. 2) and in the cave at 1.2 m from target for neutron dose-rate estimation. Also Neutron fluence rates were measured with Area Neutron Monitors (BF3 based) located inside the bunkers and the corresponding dose-rates have been estimated using flux to dose conversion factors (Conversion coefficients for use in radiological protection against external radiation, 1996). The experiments were carried out for different projectile currents (100 nA, 200 nA, 300 nA, 400 nA and 500 nA) and different energies. The measurements have been done for twelve locations inside the machine bunker leading to 648 set of values due to beam type, energy, current and beam stopping combinations. In experimental bunker measurements have been carried out for 8 locations leading to 144 set of values. For each set of measurements, multiple readings of both neutron and gamma have been taken with each measurement lasting for 10 sec and the mean value in each set was taken for consideration.
0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 20
30
40
50
60
70
Primary Particle Energy-MeV Fig. 12. Gamma Dose-rate with std error (normalized to 1 nA) at G position in Experimental Cave.
values in case of worst case scenario in active bunkers which is the main purpose of the work. The position F was chosen for discussion and analysis as this location has the least chance of having errors due to external factors like multiple scattering and divergence. The measurements inside the vault for beam stopping at FC-01 at salient locations are shown in the following Figs. 7–10. From the graph it is evident the position G is dominant in all the cases. The position G is the location corresponding to central viewing port without the attenuation effects due to shielding of the magnetic Iron Core. The gamma dose rates in case of primary proton beam tends to saturate for proton energies greater than 12 MeV but in case of
108
R. Ravishankar et al. / Applied Radiation and Isotopes 80 (2013) 103–108 10.00
% part w.r.t. to Observed value
9.00 8.00
std_err
7.00 6.00
std_deviation
5.00 4.00 3.00 2.00 1.00 0.00 20
30
40
50
60
70
Primary Particle Energy-MeV Fig. 13. Percentage part of Std. Err and Std. Dev. in observed values (taking the highest values).
primary alpha the saturation trend may be much greater than 60 MeV. The neutron dose rates increase exponentially with primary beam energy. The trend suggests the neutron dose rates may be tending towards saturation at much higher energies both for alpha and proton primaries. Also from Fig. 10, it is observed the gamma dose rates are three to five times higher at positions near to central port regions (position G and H) which suggests the dominance of prompt photon emissions from the nuclear interactions of primary within the cyclotron machine (like central regions, DEE and Deflector). All these dose-rate curves can be reasonably fitted with modified exponential function as given in Eq. (1). The standard error and standard deviations of the huge measured values have been studied. Figs. 11 and 12 give the neutron and gamma dose rates (for primary alpha) with Std. error for the position G in the experimental cave. The standard error and standard deviations have been found to be reasonably good for both types of alpha and proton primaries. The position G was considered here as it is more susceptible for systematic errors due to oblong direction with primary beam but less susceptible for rather hard predictable errors as compared to locations like C. Fig. 13 shows the percentage part the maximum estimated standard errors and standard deviations in the corresponding observed values (with 95% confidence levels). The results show not more than 10% in any case which is reasonably good for radiation protection purpose. 4. Conclusion The work finds its utility in radiation field mapping under different machine operating conditions (variation in projectile
type and energy) which is very vital for accidental human radiation exposure estimation. The use of mobile robot for the radiation mapping was extremely useful as otherwise it could have been cumbersome considering the huge volume of data obtained. The work was also found to be helpful in locating the maximum beam loss positions and thereby improving beam optics resulting in better beam transmission. It was observed during mapping that on optimization of beam internal parameters, there was significant reduction in the ambient dose rates inside the machine bunker areas. These circumstances also decrease the radiation damage to electronic components of control circuits and PLC inside bunkers. It was also observed immediately after machine shutdown, a significant reduction of minimum waiting time from a few hours to a few minutes, before entry into the bunker areas, (nevertheless a minimum waiting time of 10 min based on ventilation system is always enforced) which resulted in the improvement of overall machine up time. Further work is in progress in developing the robotic carrier to have two dimensional and three dimensional radiation mapping of the bunker rooms.
Acknowledgments The authors acknowledge the contribution and active support provided by the K-130 Room Temperature Cyclotron Operation Group and Workshop, VECC. The authors are also thankful to Dr. R. K. Bhandari, Ex. Director, VECC, Kolkata, Dr. D. N. Sharma, Director, Health, Safety and Environment Group, BARC, Mumbai and Shri Manjit Singh, Director, DM&A Group, BARC, Mumbai for their constant support and encouragements.
References Andersson, I.O., Braun, J.A., 1962. Neutron rem counter with uniform sensitivity from 0.025 eV to 10 MeV. In: Proceedings of the Symposium on Neutron Dosimetry, vol. 2. Harwell, Vienna: IAEA, pp. 85–89. Conversion coefficients for use in radiological protection against external radiation, 1996. InternationalCommissionon Radiological Protection-ICRP Publication 74. Ann. ICRP 26(3–4), Elesevier Science, Oxford. Instruction Manual Ludlum Model 2363, Ludlum Measurements Inc., Texas. K-130 Cyclotron Safety Report, 2010. VECC. Kolkata, India. Leake, J.W., 1968. An improved spherical dose equivalent neutron detector. Nucl. Instrum. Methods Phys. Res. Sect. A 63, 329–332. Leake, J.W.A., 1965. Spherical dose equivalent neutron detector. Nucl. Instrum. Methods Phys. Res. Sect. A 45, 151–156. Patterson, H.W., Thomas, R.H., 1973. Accelerator Health Physics. Academic Press, New York. Private communication, Ludlum Measurements Inc., Texas. Radiation protection for particle accelerator facilities, 2003. National Council on Radiation Protection and Measurements. NCRP Report no. 144, USA.