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Radiation protection considerations in radioactive ion beam facilities
Nuclear Inst. and Methods in Physics Research B 455 (2019) 96–107
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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Radiation protection considerations in radioactive ion beam facilities☆ a,b
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Yao Yang , Youwu Su , Wuyuan Li Wang Maoa, Lijun Wanga a b
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, Weiwei Yan , Lina Sheng , Yang Li , Bo Yang ,
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Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE INFO
ABSTRACT
2010 MSC: 00-01 99-00
Radiation protection considerations related to radioactive ion beam facilities are discussed. Both the in-flight type and ISOL type are included in this paper. A description of the facility is first provided to better understand the problem. Then, two key components, the production target and beam dump are introduced for their impact on radiation safety. The study of prompt radiation field or shielding design is also presented for different facilities. In addition, the induced radioactivity in accelerator components is addressed for possible interventions and final decommissioning. Finally, the remote handling system in each facility is also reviewed, which is dedicated designed to conduct the maintenance of highly activated components. The aim of this work is to provide valuable experience for other under construction or planned accelerator facilities.
Keywords: Radioactive ion beam facility Shielding design Radioactivity Remote handling
1. Introduction The use of radioactive ion beams (RIBs) has greatly enhanced the development of nuclear physics and astrophysics, and it becomes also widely available in material science, biomedical research, etc [1–3]. Two methods are commonly applied to produce RIBs, in-flight separation (also called the projectile fragmentation) technique and isotope separation on line (ISOL) [1–4]. The former using energetic heavy ions hitting a thin target, then the secondary beam is purified through the magnetic rigidity (in dipole) and atomic energy loss analysis (in degrader), and finally transported to the experimental areas. In this process, the unreacted primary beam and the unwanted fragments are collected at the beam dumps. For the ISOL method, the thick target is impinged by the high energy light ions (usually the protons) from accelerators or nuclear reactors, the products are thermalized and transported to the ion source for ionization, thereafter accelerated and mass separated in the post accelerator. According to the generation principle of RIBs mentioned above, at both the in-flight type and ISOL type facilities, the principal beam losses locate at the production target, beam dumps and experiment terminals, moreover the dipole magnets also act as beam stops when bending the beam. The degraders are also included at the in-flight facility. Prompt radiations are produced through collisions, and at the same time, the
materials are activated by the primary and secondary particles. The accelerator workers, general public and environment must be protected from these radiations, moreover, the radiation sensitive accelerator equipment also needs to be considered. For example, magnets, driving motor, camera, and vacuum pump, etc. Besides, in possible failure of devices, removal of the activated components or install the new one also poses challenges due to the radiation hazard. Thus, proper design of local shielding and using of remote handling systems are necessary in this situation. Currently, there are many radioactive ion beam facilities in operation or planned to be built worldwide. In this paper, the radiation protection considerations in five typical radioactive ion beam facilities are reviewed. Including the FRS at GSI [2,3,5,6], Super-FRS at GSI FAIR [2,3,7], BigRIPS at RIKEN RIBF [2,3,8], ARIS at MSU FRIB [3,9], which are belong to in-flight type as well as an ISOL type facility, the ISOLDE at CERN [3,4,10]. Among them, the FRS, BigRIPS and ISOLDE are already in operation, whereas the Super-FRS and ARIS are under construction. Firstly, an introduction of the facility is given, and subsequently the production target module and beam dump system are also introduced due to their impact on radiation safety. Furthermore, the shielding design and induced radioactivity are detailed discussed. Finally, the remote-handling (RH) system for each facility is briefly reviewed.
Fully documented templates are available in the elsarticle package on CTAN. Corresponding author at: Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail address: [email protected] (W. Li).
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https://doi.org/10.1016/j.nimb.2019.06.031 Received 7 May 2019; Received in revised form 17 June 2019; Accepted 20 June 2019 Available online 25 June 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 455 (2019) 96–107
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Fig. 1. Layout of the FRS at GSI, as well as the synchrotron SIS and Experimental Storage and cooler Ring (ESR).
2. RIB facilities 2.1. FRS at GSI The projectile FRagment Separator (FRS) is installed at GSI, which has operated since 1990. The heavy ion synchrotron SIS can accelerate all ions from hydrogen to uranium with energies ranging from 1 to 4.5 GeV/u. The radioactive isotopes can be injected into the Experimental Storage and cooler Ring (ESR) or experimental terminals for further studies (see Fig. 1) [5,6]. The main parameters of FRS with maximum magnetic rigidity B max of 18 Tm, momentum resolving and p/p of power of 1500 ( = 20 mm mrad) and acceptances , ±7.5 mrad, ±7.5 mrad and ±1%, respectively [5]. Fig. 2 shows the production target ladder of FRS. In order to meet different experimental requirements, a total of 75 targets of different elements with different thicknesses can be installed at the target station. All targets are cylinders with a diameter of 2 cm, can be moved into the path of the ion beam with millimeter precision using step motor control [11]. If required, the target holder can also be exchanged by remote control [11,12]. Fehrenbacher and Festag [13] have measured the neutron doses along the FRS using the Thermo Luminescence Dosimeters (TLDs) in 2002. The uranium beams with energy between 100 and 1000 MeV/u and mean intensity of 107 ions/s, the total operation time is 21 d. Moreover, a single source model is adopted to estimate the spatial dose pattern near the FRS. The model is based on the results induced from 1 GeV/u uranium ions hit on a thick iron target. The result is shown in Fig. 3. The results indicate that the target area and the first dipole area (the first focal plane) are the two main radiation sources. And near the target area, an averaged dose rate of 20 mSv/h is observed. As discussed above, the unreacted primary ions and unwanted fragments will deflect
Fig. 3. Dose distributions at FRS determined with measurements and model calculations. The dose values are given in µ Sv/h.
in the magnetic field and deposit in the first dipole, thus caused the strong radiation field. The activation level in the target area and first focal plane is in the order of mSv/h, even after a few weeks of decay [12]. Meanwhile, the working place is limited due to the whole beam line is surrounded by 4 m thick concrete. As a consequence of the above, the RH system are necessary and have been developed and implemented at FRS. Two industrial robots KUKA KR 350 are installed at the target area and the first focal plane as shown in Fig. 4[12,14]. At the target area, the robot moves on a 5 m long rail system that enables to operate different components installed on the big vacuum chamber. While at the first focal plane, the robot is mounted on a fixed concrete base. All the actions like install, align, maintain, etc. are finished only with two tools: the hook and the gripper, which are put in the shelves in both locations. The vacuum sealing is Viton O-rings together with pillow seals to connect beam pipes and vacuum chamber. This design is inherently safe, meanwhile, that is also convenient for robots to operate. Finally, a lead-shielded container located behind the shield wall, the activated beam line inserts can be stored. For more details regarding the RH system of FRS, refer to the reports [12,14]. Although the present FRS facility has achieved great success in nuclear physics, there are four major limitations: low intensity of the primary beam, low transmission for projectile fission fragments, low transmission of fragments to experimental areas, and small acceptance of fragments by the storage cooler ring [7]. Thus, a new powerful inflight fragment separator SUPERconducting FRagment Separator (Super-FRS) is proposed at GSI [7,15,16].
Fig. 2. Production target ladder of FRS. 97
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Fig. 4. The RH system at target area and first focal plane of FRS.
2.2. SuperFRS at GSI FAIR
reacting primary beam will be stopped and also to protect the subsequent parts of the separator. Fig. 6(b) gives the layout of the beam dump plug [19,20]. The graphite and beryllium absorbers are used to stop the ions in slow extraction and fast extraction mode, respectively. Same as the target plug design, the hook, drive motors (vertical and horizontal), stainless steel shielding and beam dumps are included, and the total weight is about 7.5 t. Furthermore, there is a 60 cm long iron block behind every beam dump, which serves to stop the protons and neutrons when ions lost on the absorber. Radon et al. [21] have reported the shielding design of the target area of Super-FRS using the Monte Carlo program FLUKA [22,23]. The 238 U beam is adopted with an energy of 1.5 GeV/u and intensity of 1012 ions/s. Fig. 7 gives the shielding design and prompt dose rate distribution at the pre-separator area. About 10% of ions are lost in the target, 85% in the first beam dump and the other five dumps receive 1% on each side. Massive iron blocks (2 m) and up to 6 m of concrete is also added in order to meet the design goal of 0.5 µ Sv/h for public access. Besides, the working platform at the top of the iron shielding is provided for faster access when the beam is off. Since the major beam losses are appeared on the pre-separator, the main-separator is designed to be an open tunnel by the concrete shielding together with the compacted soil, and finally at 7 m of soil outside the building [20]. Fig. 8 gives the shielding design and prompt dose rate distributions at the main-separator area. The 114Pd ions are taken as a typical fragment of the 238U, and with energy up to 1.3 GeV/u and intensity up to 5.5 × 1010 ions/s. The ions are deposited on two tungsten degraders (degrader 1: 3 × 1010 ions/s; degrader 2: 1010 ions/s) and two aluminum slits (front slit: 1010 ions/s; back slit: 5 × 109 ions/s). The supply tunnel is used to provide control racks for cryogen and vacuum support, data acquisition, etc. The induced radioactivity in the pre- and main-separator of SuperFRS is also evaluated with the same beam parameters listed above. Firstly, the activation of pre-separator is evaluated in four irradiation periods with 90 d of irradiation and 120 d of cooling [19,21]. The total activity of the target, graphite part of the beam dump and iron part of the beam dump is 2.8 × 1010 Bq, 1.4 × 1012 Bq and 2.4 × 1013 Bq, respectively. The importance nuclides in graphite are 3H, 7Be and 10Be, and in iron are 54Mn, 46Sc, 56Co, etc. The residual dose rate distributions in the pre-separator of Super-FRS is given in Fig. 9. In close of the
The Super-FRS project is one of the essential parts in the planned international Facility for Antiprotons and Ion Research (FAIR) at GSI. The new driver accelerator SIS100/200 will provide uranium ions at 1.5 GeV/u in energy and 1012 ions/s in intensity. The Super-FRS facility contains a two-stage magnetic system, the pre- and the main- separator, and each equipped with a degrader. The separated RIBs will be delivered to three branches for different experimental goals, see in Fig. 5[7]. The main parameters of Super-FRS with maximum magnetic rigidity B max of 20 Tm, momentum resolving power of 1500 ( = 40 mm , mrad) and acceptances and p/p of ±40 mrad, ±20 mrad and ±2.5%, respectively [7,15,16]. The production targets of Super-FRS are dedicated designed for different operating modes: in slow extraction mode, a rotating multislice graphite wheel is used; whereas in fast extraction mode, where high power densities appears, the beam spot need to be expanded for the safe use of the graphite target, also a windowless liquid lithium target is an alternative solution [15–18]. Fig. 6(a) shows the production target design at Super-FRS, the entire plug including the hook, drive motors, vacuum seal, iron shielding and target wheel [19]. Six beam dumps are located behind the dipole as depicted in Fig. 5. The non-
Fig. 5. Layout of the proposed Super-FRS facility at FAIR. The pre-separator and main-separator are equipped with degrader, beam dumps are located behind the dipole as illustrated, as well as three branches: Low-Energy Branch, High-Energy Branch and Ring Branch. 98
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Fig. 6. Schematic view of the (a) production target plug and (b) beam dump plug at Super-FRS. Both are mounted at the bottom of the shielding, and moving by the drive motors.
amounts to about 106 Bq. Nuclides like 7Be, 32P and 35S can be removed by filtering, however, for gaseous activities (mainly 3H, 37Ar and 14C) can be released to the environment in a week through a chimney. Secondly, the activation of main-separator is evaluated under 14 d of irradiation and 1 d of cooling [14], see in Fig. 10. The results indicate that the hot points are the two degraders and the two slits, the level
first beam dump, the dose rate can reach up to 5 Sv/h; in the working platform at levels of about few tens of µ Sv/h, which allows hands-on maintenance in case of repair or exchange. Cooling water for the beam dump will also be activated to about 1010 Bq by 3H and 7Be. Moreover, activated air in the working platform must be contained until the short lived nuclides have decayed. After 7 d of decay, the total activity of air
Fig. 7. Prompt dose rate distributions in the pre-separator area of Super-FRS. Left: horizontal plane. Right: vertical cross-sectional plane through the first beam dump. 99
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Fig. 11(b) [19,20]. More details about the RH system and shielding flask design can be found in [24,25]. Finally, remote manipulators will be used to conduct the maintenance work. While for the main-separator, due to the separate locations of the beam line inserts, the RH system is more complex than that in the pre-separator. A total of 15 beam line inserts, including the detector ladder, slit, degrader, plastic scintillator, TOF detector, etc. in three focal planes FPF2, FPF3 and FPF4, will require remote maintenance, as shown in Fig. 11(c) [14]. Multiple plugs are installed on the top of the big vacuum box of each focal planes. Fig. 11(d) shows the view of beam line inserts in FPF2 [14]. When the beam is turned on, the RH system (mobile platform, robot, etc.) will be placed in the parking area (see in Fig. 11(c)) to avoid the direct irradiation; and when the beam is shut down, the mobile platform will carry the robot and other tools to the focal plane to conduct the maintenance work, see more information in [14,24].
Fig. 8. Prompt dose rate distributions in the main-separator area of Super-FRS. Four beam loss points, two tungsten degraders and two aluminum slits, are marked.
2.3. BigRIPS at RIKEN RIBF The BigRIPS (Big RIKEN Projectile fragment Separator) is designed to be a two-stage superconducting in-flight facility at RIKEN RIBF (Radioactive Ion Beam Factory). Fig. 12 shows the schematic view of the BigRIPS at RIKEN RIBF [8,26]. The main parameters of BigRIPS with maximum magnetic rigidity B max of 9.5 Tm, momentum resolving , power of 1270/3420, and acceptances and p/p of ±40 mrad, ±50 mrad and ±3%, respectively [26]. The BigRIPS can provide all kinds of heavy ions up to 350 MeV/u, and the designed goal of beam intensity is 1 pµ A (6.24 × 1012 ions/s). Two types of targets are used in the BigRIPS: the water-cooled fixed targets (see in Fig. 13(c)) are used for the low beam power situation, as well as the water-cooled rotating multi-slice targets are used for the high beam power situation (see in Fig. 13(b))[27,28]. Besides, the combination of two rotating targets can also obtain the thickness variety to meet different requirements. Fig. 13 gives the target flange mounted on the remote-handling maintenance cart, the ladder of fixed targets and the step-shaped rotating beryllium target on BigRIPS, respectively. For the beam dump system, due to the varieties in the magnetic rigidity, the beam stopping location changes widely, consequently the beam dump is specially designed at the BigRIPS. Fig. 14 shows the beam dump structure at the BigRIPS [29]. The dumps are installed at both the inside (including the inner and outer side-wall parts) and the exit (exit dump) of the first dipole magnet. The inner
Fig. 9. Residual dose rate distributions in the pre-separator area of Super-FRS after a certain irradiation and cooling period.
near the beam line is in the order of few tens of mSv/h. The RH system will be equipped in the pre- and main- separator, and also in the hot-cell regions see in Fig. 11(a) [14,24]. The hot cell region is situated close to the target area. The whole plug will be moved by the crane from the beam line into a shielding flask (up to 30 cm thick of iron), then transported to the hot cell or storage places, see in
Fig. 10. Residual dose rate distributions in the main-separator area of Super-FRS after 14 d of operation and 1 d of cooling. 100
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Fig. 11. (a) Layout of the Super-FRS tunnels. (b) The RH system at the target hall. (c) The RH system at the main-separator. (d) Schematic view of the beam line inserts in the focal plane FPF2.
Fig. 12. Schematic view of the BigRIPS at RIKEN RIBF.
side-wall dump is fabricated with rectangular screw tubes made of oxgen-free copper, and for the outer part, swirl tubes made of Cu-Ag alloy. Behind the inner and outer dump, copper blocks are used to shield the light charged particles. The exit dump is a V-shaped Cu-Cr-Zr plate equipped with screw tubes for cooling. Plates are vertically inclined by 6°, and the position is tunable in 0–125 mm. Uwamino et al. [30,31] give the safety design of the RIBF at RIKEN. The estimation of neutron yield is calculated on the assumption that the yield is in proportion to E2·A, where E is the beam energy in MeV/u and A is the mass number. The results show that the neutron production is highest for the 84Kr30+ beam, thus considered as the base for the shielding design. The BigRIPS is shielded by the ordinary concrete, with 2 m in the north, west, south and roof wall, as well as 2.5 m in the east and 4/5.5 m in the floor wall. Outside of the south, east and floor wall is soil. Tanaka et al. [32] have measured the neutron dose rate outside of the concrete shield of BigRIPS, and the results are compared with that
Fig. 13. (a) The target flange mounted on the remote-handling maintenance cart. (b) The step-shaped rotating beryllium target. (c) Ladder of fixed targets.
by the PHITS [33] simulations. Fig. 15 shows the PHITS calculated neutron dose map induced by 48Ca 100 pnA beam incidents on a 15 mm beryllium target. At three positions (A, B and C), agreement shows within a factor of 2.5. In order to estimate the lifetime of each target units, Yoshida et al. [27] have simulated the dose rate around the target chamber for uranium beam of 350 MeV/u and 1 pµ A at a 5 mm beryllium target, see in Fig. 16. For the rotating actuator, the dose rate is below 1 kGy/month, and at the O-ring, the maximum value is about 100 kGy/month, which is needed to be exchanged within several months. Besides, at the O-ring of the pump unit is 1 kGy/month, which is not a crucial problem. 101
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flange unit and the vacuum pump unit are removed using a remotehandling maintenance cart (see in Fig. 17(a)), then the cart drives along the rails in the access tunnel to a temporal storage area (see in Fig. 17(b)) [27]. The cables connected to the target flange are disconnected by the operator in the access tunnel. Meanwhile, a 10 cm lead shield is mounted on the cart to protect the operator. The removed components will be stored in a shielded container for cooling, and the new one will be installed on at the chamber simulator located at the entrance of the access tunnel [27]. 2.4. ARIS at MSU FRIB The Facility for Rare Isotopes Beams (FRIB) at Michigan State University (MSU) is a next-generation facility for rare isotope science, in which the Advanced Rare Isotope Separator (ARIS) will be used to separate the radioactive nuclear beam of interest from the reaction [9]. The ARIS has been designed as a three-stage (stage 1 also called the preseparator, stage 2 and stage 3 called the main-separator) in-flight facility, Fig. 18 shows the schematic view of the ARIS at MSU FRIB [9]. The main parameters of ARIS with maximum magnetic rigidity B max of 8 Tm, momentum resolving power of 1720/3000–4000, and ac, ceptances and p/p of ±40 mrad, ±40 mrad and ±5%, respectively [26]. The ARIS can provide all kinds of heavy ions at energy greater than 200 MeV/u and beam power up to 400 kW. The ARIS production target uses a multi-slice graphite with rotating speed of 5000 rpm, and the target is put into a water-cooled heat sink. The whole target module consists of five parts: the graphite target, drive shaft, heat exchanger, ferrofluidic rotary feedthrough and motor, see in Fig. 19[35]. About 100 kW of beam power deposited in the target, and the residual 300 kW on the beam dump. The lifetime of the target is expected in the order of two weeks, due to the annealing of the radiation damage. The beam dump designed for ARIS is a water-filled rotating drum with 70 cm in diameter and 400 rpm in speed [9,36]. The drum shell is 0.5 mm thick titanium alloy, and the water is used as the absorber in 10 cm of thickness. Fig. 20 gives the structural concept of the beam dump for ARIS [35,36]. In order to minimize the amount of activity in the cooling water, the water is circulated through the shielded ion-exchange columns. The beam dump lifetime is about one operational year, 5500 h. The target hall has the strongest radiation environment since the target and beam dump are the two major sources as discussed above. Fig. 21 shows the shielding design for ARIS [37]. The location of beam line components is also indicated, three vacuum vessels (target, beam dump and wedge) are used to house the components. The multi-shaped cast iron blocks are put on top of the vacuum vessels. The bottom is made of stainless steel, and combination with ordinary concrete. Meanwhile, a metal (bronze) shield is placed after the target module to protect the subsequent magnets. The unshielded activation levels at 30 cm distance from three typical inserts, the target module, beam dump module and bronze shield, are estimated based on the components lifetime (irradiation time) and waiting time (cooling time) for preparation activities. The 48Ca beam at 549 MeV/u (upgraded) and 400 kW of power is used for calculations. The unshielded dose rates at 30 cm from the components are given in Table 1[38]. The results indicate that the parts need adequately shielded when the workers access to the target hall. In this condition, the allowed time is about 80 h per year for conducting activities in the target hall, and the personnel doses below the MSU ALARA limit of 5 mSv/a [38]. Basic activities include manual utility disconnect, reentrant lid bolting and general cell maintenance. While when the beam is on or the shielding is removed, the RH system is needed in this situation. The RH system at the ARIS consists of a 20 ton crane, a window workstation with master slave manipulators (MSMs), vision system, equipment lift, etc. see in Fig. 22[37–39]. Dexterous maintenance of component modules is performed by a pair of MSMs at the window workstation. The lead glass window is used
Fig. 14. Schematic view of the beam dump at the BigRIPS.
Fig. 15. The PHITS calculated neutron dose rate distributions at BigRIPS induced by 48Ca 100 pnA beam incident on a 15 mm beryllium target. Three measurement points A, B and C are also shown.
Fig. 16. The PHITS calculated dose rate distributions around the target chamber of BigRIPS.
For a typical operation period at the BigRIPS, the residual dose rate is expected to 1–3 Sv/h at the end of irradiation and about 0.1–0.3 mSv/h after 2 d of cooling [27]. The main activity is 24Na in the aluminum made unit, which is induced by the fast neutrons from reaction 27Al(n, )24Na. To facilitate the remote handling at BigRIPS, the pillow-seal systems are introduced [34]. For maintenance, the target 102
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Fig. 17. (a) Structure view of the target chamber, maintenance cart and vacuum pump unit at BigRIPS. (b) The target maintenance area at BigRIPS.
Fig. 18. Overview of the Advanced Rare Isotope Separator (ARIS) at MSU FRIB. The location of target, beam dump, degrader, etc. have indicated.
Fig. 20. The structure concept of the beam dump for ARIS.
indicate that the target and beam dump contributed dose rate is about 1 µ Sv/h and < 1 µ Sv/h, respectively, while the bronze shield contributed is > 1 µ Sv/h, however, the shield is no need for exchange or repair and expected to last for 30 years of facility lifetime [38]. Other information about the waste storage and target hall canyon shielding design can be found in [38].
for shielding of gamma rays and also for inspection, and the thickness is equivalent to the surrounding concrete wall. The dose rate at the personnel working station is also evaluated for the present and upgraded condition. The design goal is 1 µ Sv/h, which comes from 5 mSv/a of the MSU ALARA goal and 5000 operational hours in a year. The results
Fig. 19. Left: Prototype of the production target at ARIS. Right: Schematic view of the target module at ARIS. Five parts of ferrofluidic rotary feedthrough and motor are shown. 103
target,
drive shaft,
heat exchanger,
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Fig. 21. Shielding design for ARIS. Beam line components and three vacuum vessels are indicated. Table 1 The unshielded activation levels at 30 cm distance from components. Component
Irradiation time
Cooling time
Dose rate (µ Sv/h)
Target module Beam dump Bronze shield
14 days 1 year 30 years
4h 16 h 2 days
1.3 × 105 5.1 × 105 1.547 × 107
Fig. 23. The ISOLDE facility at CERN.
Fig. 22. RH system at the hot cell of ARIS.
2.5. ISOLDE at CERN The ISOLDE (Isotope Separator On Line Device) facility at CERN uses ISOL method for the production of RIBs. The light particles (usually proton) and thick target are used for the reaction, then transported to the ion source and accelerated again as discusses earlier. Fig. 23 shows the ISOLDE facility at CERN [10]. The protons from the PS Booster with 1.4 GeV of energy and 2 µ A of intensity, and then delivered to two separate isotope separators: the General Purpose Separator (GPS) and the High Resolution Separator (HRS). The former equipped with an H-magnet with 1.5 m of bending radius and 70° of
Fig. 24. The target unit at ISOLDE.
bending angle, the resolution m/ m is 800; and the latter with two Cmagnets both with 1 m of bending radius and 90° and 60° of bending angle, respectively, and both the m/ m at 6000 [40]. The production target for ISOLDE shows a wide choice of materials and simultaneously guided by the production rate and release efficiency 104
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[41]. A standard target container at ISOLDE is made from a tantalum cylinder with 20 cm in length and 2 cm in diameter. Fig. 24 gives the target unit at ISOLDE [3]. The transfer tube is used to deliver the RIBs to the ion source. Moreover, a quartz tube is employed to absorb the rubidium atoms. The often used targets in ISOLDE including molten metal targets (Ge, Sn, La, etc.), solid metal targets (Ti, Nb, Ta, etc.), carbides (Al4C3, SiC, VC, etc.) and oxides (MgO, Al2O3, CaO, etc.), more information can be found in [41]. Furthermore, in order to keep the diffusion path short, the powders or thin foils (typically 20 µ m thick) are used for the solid metal target. For specific experimental requirement, up to 30 target changes are performed per year at ISOLDE [40]. The unabsorbed primary beam will deposit on the beam dump (see in Fig. 23). Currently, two beam dumps are installed at ISOLDE, both are steel blocks with 1.6 m × 1.6 m × 2.4 m at GPS and 0.4 m × 0.4 m × 1.0 m at HRS, finally surrounded by the concrete shielding [40]. The beam dumps are no need for exchange or repair in the lifetime of the facility, unless it is upgraded or decommissioned. The shielding design for ISOLDE is finished before 1990 with protons at 1 GeV of energy and 1013 protons/s of intensity [42]. The target area has been shielded with concrete and earth shielding, and the total thickness is equivalent to 8 m of earth. Meanwhile, approximately 4 m of earth shielding as well as 0.8–1.2 m of iron between the target and separator areas. For the separator area, which are shielded from the experimental area by 1–3 m thick concrete walls. Obviously, the shielding needs to be improved for currently beam parameters and also for the future upgraded facility HIE-ISOLDE. A full study of the radiological protection and safety impact of the ISOLDE and HIE-ISOLDE is given in [43]. The activated air also caused a serious problem, one approach is to separate the tunnel ventilation system from its neighboring, and the other is add additional shielding around the beam dumps [40]. The target/ion source units at GPS and HRS are the highly activated components, which need safety handling [10,40]. The residual dose rate in the Faraday cages around the two production targets is typically 4–5 mSv/h, thus the RH system is needed for intervention [42]. In early operation, two mobile industrial robots are taken over from the previous machines are used for remote handling. In 2014, two new robots KUKA KR 60L45-3 are commissioned to replace the former ones. The new robots are re-designed and modified in order to adapt to the strong radiation environment. One novel change is that the new robots are installed on the ceiling rails (see in Fig. 25) [14,40]. The irradiated
targets will be transported to the alpha-gamma hot cells for further treatment. The mechanical disassembly of targets and oxidation of uranium carbide will be finished at two compartments in the hot cell, respectively. The operator is protected by 15 cm thick lead bricks from floor to ceiling and 35 cm thick lead glass window for visualization. 3. Conclusions Radioactive beam physics has facilitated the construction of new facilities and the discovery of new physical phenomena. Meanwhile, it is important to ensure the radiation safety of the facility. In this work, the radiation protection considerations in multiple radioactive ion beam facilities were discussed. Including the introduction of the accelerator and its key equipment, radiation shielding design, activation evaluation as well as the remote-handling system. The following conclusions can be drawn from the present review. (1) Production target differs greatly between in-flight type and ISOL type facility. Obviously, this was due to the different production mechanism of RIBs. For in-flight facility, where the graphite was commonly used, since it with high production cross-section for fragments and high operating temperature. Also, it can be mounted on a rotating wheel to spread the power deposition. In order to meet different experimental requirements, the target was usually designed to be multi sliced. However, the selection of target material was more widely and flexible in ISOL facility depending on the interested isotope. (2) The major role of the beam dump was to stop the unreacted primary beam and at the same time to protect the subsequent beam line parts. For choosing of absorber materials, the high melting point and good heat conduction were considered. In in-flight facility, the low-Z materials can be adopted due to the short projectile range of heavy ions. Nevertheless, the high-Z materials were commonly selected for ISOL type, because of its long projectile range of protons. Finally, water cooling can be employed to remove the heat deposition. (3) The dominated beam losses appear in an in-flight type consist of the production target, beam dump, slit and degrader; though in ISOL type, only the target and beam dump were the main radiation sources. The neutrons were the principal contributors for the prompt radiation field as well as for the activation. For radiological safety design, firstly, the strongest radiation field needs to be found for multiple beam-target combinations. Then, compared with empirical formulas, the Monte Carlo codes like FLUKA, PHITS were more applicable for complex geometry conditions. In the first layer, iron/ steel was generally adapted to moderate the high energy neutrons, and in the second layer where concrete was commonly used. Also, the soil was readily available for shielding and the costs can be minimized. (4) Induced radioactivity in accelerator components was the dominant source of occupational radiation exposure. The produced radionuclides depends on beam parameters (particle type and energy, irradiation time, cooling time) and irradiated material. Fortunately, the typical radionuclides in mental material like aluminum, iron and copper, etc. and also in the cooling water and air were studied adequately in the early related reports. For example, 11 C, 13N, 15O and 41Ar were the concerned radionuclides in the air. Detailed estimation of activation level in components and working areas was essential for personnel intervention and final waste treatment and disposal. (5) The residual dose rates were usually in the order of mSv/h or even higher near the beam loss position, the hands-on maintenance was restricted. Therefore, the remote handling (RH) system becomes necessary, which services to conduct disconnect, removal, exchange, etc. In addition, components can also be transported to the hot-cell, which services to repair, maintain of inserts and store the
Fig. 25. (a) GPS and HRS KUKA robots on overhead rails. (b) GPS KUKA robots transporting a target unit. 105
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radioactive waste. The radiation sources were less and more centralized in ISOL facility when compared with the in-flight type. Consequently, it provides convenience for designing of radiation protection and remote handling system.
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Finally, as the accelerator develops toward high energy and high intensity, which poses great challenges for radiation protection, not only for radioactive ion facilities but also for other types of accelerators. On the one hand, sufficient shielding needs to be added to protect personnel, meanwhile, radiation sensitive devices also need to be considered due to the radiation damage. On the other hand, high levels of activation limit the possibility of hands-on maintenance. Hence, the RH system needs to be planned and dedicated designed, which was a costly and challenging work. Acknowledgement This work was supported by the National Key R&D Program of China [Grant No. 2017YFC0107700]. The authors would like to thank E. Kozlova (GSI) and A. Zeller (MSU) for providing information for this work. References [1] H. Geissel, G. Munzenberg, K. Riisager, Secondary exotic nuclear beams, Annu. Rev. Nucl. Part. Sci. 45 (1) (1995) 163–203, https://doi.org/10.1146/annurev.ns.45. 120195.001115. [2] D.J. Morrissey, B.M. Sherrill, In-flight separation of projectile fragments, The Euroschool Lectures on Physics with Exotic Beams, vol. I, Springer, Berlin, Heidelberg, 2004, pp. 113–135, , https://doi.org/10.1007/978-3-540-44490-9_4. [3] Y. Blumenfeld, T. Nilsson, P.V. Duppen, Facilities and methods for radioactive ion beam production, Phys. Scr. T152 (2013) 014023, , https://doi.org/10.1088/00318949/2013/t152/014023. [4] P. Van Duppen, Isotope Separation On Line and Post Acceleration, Springer, Berlin Heidelberg, Berlin, Heidelberg, 2006, pp. 37–77, https://doi.org/10.1007/3-54033787-3_2. [5] H. Geissel, P. Armbruster, K. Behr, A. Brünle, K. Burkard, M. Chen, H. Folger, B. Franczak, H. Keller, O. Klepper, B. Langenbeck, F. Nickel, E. Pfeng, M. Pfützner, E. Roeckl, K. Rykaczewski, I. Schall, D. Schardt, C. Scheidenberger, K.-H. Schmidt, A. Schröter, T. Schwab, K. Sümmerer, M. Weber, G. Münzenberg, T. Brohm, H.G. Clerc, M. Fauerbach, J.-J. Gaimard, A. Grewe, E. Hanelt, B. Knödler, M. Steiner, B. Voss, J. Weckenmann, C. Ziegler, A. Magel, H. Wollnik, J. Dufour, Y. Fujita, D. Vieira, B. Sherrill, The GSI projectile fragment separator (FRS): a versatile magnetic system for relativistic heavy ions, Nucl. Instrum. Methods Phys. Res. Sect. B 70 (1–4) (1992) 286–297, https://doi.org/10.1016/0168-583x(92)95944-m. [6] FRS setup. URL:https://www-win.gsi.de/frs-setup/ (accessed 10.02.2019). [7] H. Geissel, H. Weick, M. Winkler, G. Münzenberg, V. Chichkine, M. Yavor, T. Aumann, K. Behr, M. Böhmer, A. Brünle, K. Burkard, J. Benlliure, D. Cortina-Gil, L. Chulkov, A. Dael, J.-E. Ducret, H. Emling, B. Franczak, J. Friese, B. Gastineau, J. Gerl, R. Gernhäuser, M. Hellström, B. Jonson, J. Kojouharova, R. Kulessa, B. Kindler, N. Kurz, B. Lommel, W. Mittig, G. Moritz, C. Mühle, J. Nolen, G. Nyman, P. Roussell-Chomaz, C. Scheidenberger, K.-H. Schmidt, G. Schrieder, B. Sherrill, H. Simon, K. Sümmerer, N. Tahir, V. Vysotsky, H. Wollnik, A. Zeller, The Super-FRS project at GSI, Nucl. Instrum. Methods Phys. Res. Sect. B 204 (2003) 71–85, https:// doi.org/10.1016/s0168-583x(02)01893-1. [8] H. Sakurai, RI beam factory project at RIKEN, Nucl. Phys. A 805 (1–4) (2008) 526c–532c, https://doi.org/10.1016/j.nuclphysa.2008.02.291. [9] M. Hausmann, A. Aaron, A. Amthor, M. Avilov, L. Bandura, R. Bennett, G. Bollen, T. Borden, T. Burgess, S. Chouhan, V. Graves, W. Mittig, D. Morrissey, F. Pellemoine, M. Portillo, R. Ronningen, M. Schein, B. Sherrill, A. Zeller, Design of the advanced rare isotope separator ARIS at FRIB, Nucl. Instrum. Methods Phys. Res. Sect. B 317 (2013) 349–353, https://doi.org/10.1016/j.nimb.2013.06.042. [10] E. Kugler, The ISOLDE facility, Hyperfine Interact. 129 (1–4) (2000) 23–42, https:// doi.org/10.1023/a:1012603025802. [11] Relativistic beams of exotic nuclei a powerful tool for nuclear structure physics. URL:https://www.win.gsi.de/frs/introduction/suemmerer_gsinachrichten_3_97_13e.pdf (accessed 12.02.2019). [12] K.-H. Behr, B. Achenbach, A. Brünle, H. Geissel, M. Gleim, W. Hüller, C. Karagiannis, G. Münzenberg, C. Scheidenberger, K. Sümmerer, H. Weick, M. Winkler, Remote-handling systems at the high-radiation areas of the GSI fragment separator FRS, Nucl. Instrum. Methods Phys. Res. Sect. B 266 (19–20) (2008) 4280–4283, https://doi.org/10.1016/j.nimb.2008.05.062. [13] G. Fehrenbacher, J.G. Festag, Neutron dose distribution at the GSI fragment separator, Radiat. Prot. Dosim. 110 (1–4) (2004) 739–742, https://doi.org/10.1093/ rpd/nch210. [14] F. Amjad, H. Weick, J. Mattila, E. Kozlova, C. Schloer, On design and development of Super-FRS main tunnel remote handling system concept, 17th International IEEE Conference on Intelligent Transportation Systems (ITSC), IEEE, 2014, , https://doi.
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Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Radiation protection considerations in radioactive ion beam facilities☆ a,b
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Yao Yang , Youwu Su , Wuyuan Li Wang Maoa, Lijun Wanga a b
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a
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, Weiwei Yan , Lina Sheng , Yang Li , Bo Yang ,
T
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE INFO
ABSTRACT
2010 MSC: 00-01 99-00
Radiation protection considerations related to radioactive ion beam facilities are discussed. Both the in-flight type and ISOL type are included in this paper. A description of the facility is first provided to better understand the problem. Then, two key components, the production target and beam dump are introduced for their impact on radiation safety. The study of prompt radiation field or shielding design is also presented for different facilities. In addition, the induced radioactivity in accelerator components is addressed for possible interventions and final decommissioning. Finally, the remote handling system in each facility is also reviewed, which is dedicated designed to conduct the maintenance of highly activated components. The aim of this work is to provide valuable experience for other under construction or planned accelerator facilities.
Keywords: Radioactive ion beam facility Shielding design Radioactivity Remote handling
1. Introduction The use of radioactive ion beams (RIBs) has greatly enhanced the development of nuclear physics and astrophysics, and it becomes also widely available in material science, biomedical research, etc [1–3]. Two methods are commonly applied to produce RIBs, in-flight separation (also called the projectile fragmentation) technique and isotope separation on line (ISOL) [1–4]. The former using energetic heavy ions hitting a thin target, then the secondary beam is purified through the magnetic rigidity (in dipole) and atomic energy loss analysis (in degrader), and finally transported to the experimental areas. In this process, the unreacted primary beam and the unwanted fragments are collected at the beam dumps. For the ISOL method, the thick target is impinged by the high energy light ions (usually the protons) from accelerators or nuclear reactors, the products are thermalized and transported to the ion source for ionization, thereafter accelerated and mass separated in the post accelerator. According to the generation principle of RIBs mentioned above, at both the in-flight type and ISOL type facilities, the principal beam losses locate at the production target, beam dumps and experiment terminals, moreover the dipole magnets also act as beam stops when bending the beam. The degraders are also included at the in-flight facility. Prompt radiations are produced through collisions, and at the same time, the
materials are activated by the primary and secondary particles. The accelerator workers, general public and environment must be protected from these radiations, moreover, the radiation sensitive accelerator equipment also needs to be considered. For example, magnets, driving motor, camera, and vacuum pump, etc. Besides, in possible failure of devices, removal of the activated components or install the new one also poses challenges due to the radiation hazard. Thus, proper design of local shielding and using of remote handling systems are necessary in this situation. Currently, there are many radioactive ion beam facilities in operation or planned to be built worldwide. In this paper, the radiation protection considerations in five typical radioactive ion beam facilities are reviewed. Including the FRS at GSI [2,3,5,6], Super-FRS at GSI FAIR [2,3,7], BigRIPS at RIKEN RIBF [2,3,8], ARIS at MSU FRIB [3,9], which are belong to in-flight type as well as an ISOL type facility, the ISOLDE at CERN [3,4,10]. Among them, the FRS, BigRIPS and ISOLDE are already in operation, whereas the Super-FRS and ARIS are under construction. Firstly, an introduction of the facility is given, and subsequently the production target module and beam dump system are also introduced due to their impact on radiation safety. Furthermore, the shielding design and induced radioactivity are detailed discussed. Finally, the remote-handling (RH) system for each facility is briefly reviewed.
Fully documented templates are available in the elsarticle package on CTAN. Corresponding author at: Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail address: [email protected] (W. Li).
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https://doi.org/10.1016/j.nimb.2019.06.031 Received 7 May 2019; Received in revised form 17 June 2019; Accepted 20 June 2019 Available online 25 June 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 455 (2019) 96–107
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Fig. 1. Layout of the FRS at GSI, as well as the synchrotron SIS and Experimental Storage and cooler Ring (ESR).
2. RIB facilities 2.1. FRS at GSI The projectile FRagment Separator (FRS) is installed at GSI, which has operated since 1990. The heavy ion synchrotron SIS can accelerate all ions from hydrogen to uranium with energies ranging from 1 to 4.5 GeV/u. The radioactive isotopes can be injected into the Experimental Storage and cooler Ring (ESR) or experimental terminals for further studies (see Fig. 1) [5,6]. The main parameters of FRS with maximum magnetic rigidity B max of 18 Tm, momentum resolving and p/p of power of 1500 ( = 20 mm mrad) and acceptances , ±7.5 mrad, ±7.5 mrad and ±1%, respectively [5]. Fig. 2 shows the production target ladder of FRS. In order to meet different experimental requirements, a total of 75 targets of different elements with different thicknesses can be installed at the target station. All targets are cylinders with a diameter of 2 cm, can be moved into the path of the ion beam with millimeter precision using step motor control [11]. If required, the target holder can also be exchanged by remote control [11,12]. Fehrenbacher and Festag [13] have measured the neutron doses along the FRS using the Thermo Luminescence Dosimeters (TLDs) in 2002. The uranium beams with energy between 100 and 1000 MeV/u and mean intensity of 107 ions/s, the total operation time is 21 d. Moreover, a single source model is adopted to estimate the spatial dose pattern near the FRS. The model is based on the results induced from 1 GeV/u uranium ions hit on a thick iron target. The result is shown in Fig. 3. The results indicate that the target area and the first dipole area (the first focal plane) are the two main radiation sources. And near the target area, an averaged dose rate of 20 mSv/h is observed. As discussed above, the unreacted primary ions and unwanted fragments will deflect
Fig. 3. Dose distributions at FRS determined with measurements and model calculations. The dose values are given in µ Sv/h.
in the magnetic field and deposit in the first dipole, thus caused the strong radiation field. The activation level in the target area and first focal plane is in the order of mSv/h, even after a few weeks of decay [12]. Meanwhile, the working place is limited due to the whole beam line is surrounded by 4 m thick concrete. As a consequence of the above, the RH system are necessary and have been developed and implemented at FRS. Two industrial robots KUKA KR 350 are installed at the target area and the first focal plane as shown in Fig. 4[12,14]. At the target area, the robot moves on a 5 m long rail system that enables to operate different components installed on the big vacuum chamber. While at the first focal plane, the robot is mounted on a fixed concrete base. All the actions like install, align, maintain, etc. are finished only with two tools: the hook and the gripper, which are put in the shelves in both locations. The vacuum sealing is Viton O-rings together with pillow seals to connect beam pipes and vacuum chamber. This design is inherently safe, meanwhile, that is also convenient for robots to operate. Finally, a lead-shielded container located behind the shield wall, the activated beam line inserts can be stored. For more details regarding the RH system of FRS, refer to the reports [12,14]. Although the present FRS facility has achieved great success in nuclear physics, there are four major limitations: low intensity of the primary beam, low transmission for projectile fission fragments, low transmission of fragments to experimental areas, and small acceptance of fragments by the storage cooler ring [7]. Thus, a new powerful inflight fragment separator SUPERconducting FRagment Separator (Super-FRS) is proposed at GSI [7,15,16].
Fig. 2. Production target ladder of FRS. 97
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Fig. 4. The RH system at target area and first focal plane of FRS.
2.2. SuperFRS at GSI FAIR
reacting primary beam will be stopped and also to protect the subsequent parts of the separator. Fig. 6(b) gives the layout of the beam dump plug [19,20]. The graphite and beryllium absorbers are used to stop the ions in slow extraction and fast extraction mode, respectively. Same as the target plug design, the hook, drive motors (vertical and horizontal), stainless steel shielding and beam dumps are included, and the total weight is about 7.5 t. Furthermore, there is a 60 cm long iron block behind every beam dump, which serves to stop the protons and neutrons when ions lost on the absorber. Radon et al. [21] have reported the shielding design of the target area of Super-FRS using the Monte Carlo program FLUKA [22,23]. The 238 U beam is adopted with an energy of 1.5 GeV/u and intensity of 1012 ions/s. Fig. 7 gives the shielding design and prompt dose rate distribution at the pre-separator area. About 10% of ions are lost in the target, 85% in the first beam dump and the other five dumps receive 1% on each side. Massive iron blocks (2 m) and up to 6 m of concrete is also added in order to meet the design goal of 0.5 µ Sv/h for public access. Besides, the working platform at the top of the iron shielding is provided for faster access when the beam is off. Since the major beam losses are appeared on the pre-separator, the main-separator is designed to be an open tunnel by the concrete shielding together with the compacted soil, and finally at 7 m of soil outside the building [20]. Fig. 8 gives the shielding design and prompt dose rate distributions at the main-separator area. The 114Pd ions are taken as a typical fragment of the 238U, and with energy up to 1.3 GeV/u and intensity up to 5.5 × 1010 ions/s. The ions are deposited on two tungsten degraders (degrader 1: 3 × 1010 ions/s; degrader 2: 1010 ions/s) and two aluminum slits (front slit: 1010 ions/s; back slit: 5 × 109 ions/s). The supply tunnel is used to provide control racks for cryogen and vacuum support, data acquisition, etc. The induced radioactivity in the pre- and main-separator of SuperFRS is also evaluated with the same beam parameters listed above. Firstly, the activation of pre-separator is evaluated in four irradiation periods with 90 d of irradiation and 120 d of cooling [19,21]. The total activity of the target, graphite part of the beam dump and iron part of the beam dump is 2.8 × 1010 Bq, 1.4 × 1012 Bq and 2.4 × 1013 Bq, respectively. The importance nuclides in graphite are 3H, 7Be and 10Be, and in iron are 54Mn, 46Sc, 56Co, etc. The residual dose rate distributions in the pre-separator of Super-FRS is given in Fig. 9. In close of the
The Super-FRS project is one of the essential parts in the planned international Facility for Antiprotons and Ion Research (FAIR) at GSI. The new driver accelerator SIS100/200 will provide uranium ions at 1.5 GeV/u in energy and 1012 ions/s in intensity. The Super-FRS facility contains a two-stage magnetic system, the pre- and the main- separator, and each equipped with a degrader. The separated RIBs will be delivered to three branches for different experimental goals, see in Fig. 5[7]. The main parameters of Super-FRS with maximum magnetic rigidity B max of 20 Tm, momentum resolving power of 1500 ( = 40 mm , mrad) and acceptances and p/p of ±40 mrad, ±20 mrad and ±2.5%, respectively [7,15,16]. The production targets of Super-FRS are dedicated designed for different operating modes: in slow extraction mode, a rotating multislice graphite wheel is used; whereas in fast extraction mode, where high power densities appears, the beam spot need to be expanded for the safe use of the graphite target, also a windowless liquid lithium target is an alternative solution [15–18]. Fig. 6(a) shows the production target design at Super-FRS, the entire plug including the hook, drive motors, vacuum seal, iron shielding and target wheel [19]. Six beam dumps are located behind the dipole as depicted in Fig. 5. The non-
Fig. 5. Layout of the proposed Super-FRS facility at FAIR. The pre-separator and main-separator are equipped with degrader, beam dumps are located behind the dipole as illustrated, as well as three branches: Low-Energy Branch, High-Energy Branch and Ring Branch. 98
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Fig. 6. Schematic view of the (a) production target plug and (b) beam dump plug at Super-FRS. Both are mounted at the bottom of the shielding, and moving by the drive motors.
amounts to about 106 Bq. Nuclides like 7Be, 32P and 35S can be removed by filtering, however, for gaseous activities (mainly 3H, 37Ar and 14C) can be released to the environment in a week through a chimney. Secondly, the activation of main-separator is evaluated under 14 d of irradiation and 1 d of cooling [14], see in Fig. 10. The results indicate that the hot points are the two degraders and the two slits, the level
first beam dump, the dose rate can reach up to 5 Sv/h; in the working platform at levels of about few tens of µ Sv/h, which allows hands-on maintenance in case of repair or exchange. Cooling water for the beam dump will also be activated to about 1010 Bq by 3H and 7Be. Moreover, activated air in the working platform must be contained until the short lived nuclides have decayed. After 7 d of decay, the total activity of air
Fig. 7. Prompt dose rate distributions in the pre-separator area of Super-FRS. Left: horizontal plane. Right: vertical cross-sectional plane through the first beam dump. 99
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Fig. 11(b) [19,20]. More details about the RH system and shielding flask design can be found in [24,25]. Finally, remote manipulators will be used to conduct the maintenance work. While for the main-separator, due to the separate locations of the beam line inserts, the RH system is more complex than that in the pre-separator. A total of 15 beam line inserts, including the detector ladder, slit, degrader, plastic scintillator, TOF detector, etc. in three focal planes FPF2, FPF3 and FPF4, will require remote maintenance, as shown in Fig. 11(c) [14]. Multiple plugs are installed on the top of the big vacuum box of each focal planes. Fig. 11(d) shows the view of beam line inserts in FPF2 [14]. When the beam is turned on, the RH system (mobile platform, robot, etc.) will be placed in the parking area (see in Fig. 11(c)) to avoid the direct irradiation; and when the beam is shut down, the mobile platform will carry the robot and other tools to the focal plane to conduct the maintenance work, see more information in [14,24].
Fig. 8. Prompt dose rate distributions in the main-separator area of Super-FRS. Four beam loss points, two tungsten degraders and two aluminum slits, are marked.
2.3. BigRIPS at RIKEN RIBF The BigRIPS (Big RIKEN Projectile fragment Separator) is designed to be a two-stage superconducting in-flight facility at RIKEN RIBF (Radioactive Ion Beam Factory). Fig. 12 shows the schematic view of the BigRIPS at RIKEN RIBF [8,26]. The main parameters of BigRIPS with maximum magnetic rigidity B max of 9.5 Tm, momentum resolving , power of 1270/3420, and acceptances and p/p of ±40 mrad, ±50 mrad and ±3%, respectively [26]. The BigRIPS can provide all kinds of heavy ions up to 350 MeV/u, and the designed goal of beam intensity is 1 pµ A (6.24 × 1012 ions/s). Two types of targets are used in the BigRIPS: the water-cooled fixed targets (see in Fig. 13(c)) are used for the low beam power situation, as well as the water-cooled rotating multi-slice targets are used for the high beam power situation (see in Fig. 13(b))[27,28]. Besides, the combination of two rotating targets can also obtain the thickness variety to meet different requirements. Fig. 13 gives the target flange mounted on the remote-handling maintenance cart, the ladder of fixed targets and the step-shaped rotating beryllium target on BigRIPS, respectively. For the beam dump system, due to the varieties in the magnetic rigidity, the beam stopping location changes widely, consequently the beam dump is specially designed at the BigRIPS. Fig. 14 shows the beam dump structure at the BigRIPS [29]. The dumps are installed at both the inside (including the inner and outer side-wall parts) and the exit (exit dump) of the first dipole magnet. The inner
Fig. 9. Residual dose rate distributions in the pre-separator area of Super-FRS after a certain irradiation and cooling period.
near the beam line is in the order of few tens of mSv/h. The RH system will be equipped in the pre- and main- separator, and also in the hot-cell regions see in Fig. 11(a) [14,24]. The hot cell region is situated close to the target area. The whole plug will be moved by the crane from the beam line into a shielding flask (up to 30 cm thick of iron), then transported to the hot cell or storage places, see in
Fig. 10. Residual dose rate distributions in the main-separator area of Super-FRS after 14 d of operation and 1 d of cooling. 100
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Fig. 11. (a) Layout of the Super-FRS tunnels. (b) The RH system at the target hall. (c) The RH system at the main-separator. (d) Schematic view of the beam line inserts in the focal plane FPF2.
Fig. 12. Schematic view of the BigRIPS at RIKEN RIBF.
side-wall dump is fabricated with rectangular screw tubes made of oxgen-free copper, and for the outer part, swirl tubes made of Cu-Ag alloy. Behind the inner and outer dump, copper blocks are used to shield the light charged particles. The exit dump is a V-shaped Cu-Cr-Zr plate equipped with screw tubes for cooling. Plates are vertically inclined by 6°, and the position is tunable in 0–125 mm. Uwamino et al. [30,31] give the safety design of the RIBF at RIKEN. The estimation of neutron yield is calculated on the assumption that the yield is in proportion to E2·A, where E is the beam energy in MeV/u and A is the mass number. The results show that the neutron production is highest for the 84Kr30+ beam, thus considered as the base for the shielding design. The BigRIPS is shielded by the ordinary concrete, with 2 m in the north, west, south and roof wall, as well as 2.5 m in the east and 4/5.5 m in the floor wall. Outside of the south, east and floor wall is soil. Tanaka et al. [32] have measured the neutron dose rate outside of the concrete shield of BigRIPS, and the results are compared with that
Fig. 13. (a) The target flange mounted on the remote-handling maintenance cart. (b) The step-shaped rotating beryllium target. (c) Ladder of fixed targets.
by the PHITS [33] simulations. Fig. 15 shows the PHITS calculated neutron dose map induced by 48Ca 100 pnA beam incidents on a 15 mm beryllium target. At three positions (A, B and C), agreement shows within a factor of 2.5. In order to estimate the lifetime of each target units, Yoshida et al. [27] have simulated the dose rate around the target chamber for uranium beam of 350 MeV/u and 1 pµ A at a 5 mm beryllium target, see in Fig. 16. For the rotating actuator, the dose rate is below 1 kGy/month, and at the O-ring, the maximum value is about 100 kGy/month, which is needed to be exchanged within several months. Besides, at the O-ring of the pump unit is 1 kGy/month, which is not a crucial problem. 101
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flange unit and the vacuum pump unit are removed using a remotehandling maintenance cart (see in Fig. 17(a)), then the cart drives along the rails in the access tunnel to a temporal storage area (see in Fig. 17(b)) [27]. The cables connected to the target flange are disconnected by the operator in the access tunnel. Meanwhile, a 10 cm lead shield is mounted on the cart to protect the operator. The removed components will be stored in a shielded container for cooling, and the new one will be installed on at the chamber simulator located at the entrance of the access tunnel [27]. 2.4. ARIS at MSU FRIB The Facility for Rare Isotopes Beams (FRIB) at Michigan State University (MSU) is a next-generation facility for rare isotope science, in which the Advanced Rare Isotope Separator (ARIS) will be used to separate the radioactive nuclear beam of interest from the reaction [9]. The ARIS has been designed as a three-stage (stage 1 also called the preseparator, stage 2 and stage 3 called the main-separator) in-flight facility, Fig. 18 shows the schematic view of the ARIS at MSU FRIB [9]. The main parameters of ARIS with maximum magnetic rigidity B max of 8 Tm, momentum resolving power of 1720/3000–4000, and ac, ceptances and p/p of ±40 mrad, ±40 mrad and ±5%, respectively [26]. The ARIS can provide all kinds of heavy ions at energy greater than 200 MeV/u and beam power up to 400 kW. The ARIS production target uses a multi-slice graphite with rotating speed of 5000 rpm, and the target is put into a water-cooled heat sink. The whole target module consists of five parts: the graphite target, drive shaft, heat exchanger, ferrofluidic rotary feedthrough and motor, see in Fig. 19[35]. About 100 kW of beam power deposited in the target, and the residual 300 kW on the beam dump. The lifetime of the target is expected in the order of two weeks, due to the annealing of the radiation damage. The beam dump designed for ARIS is a water-filled rotating drum with 70 cm in diameter and 400 rpm in speed [9,36]. The drum shell is 0.5 mm thick titanium alloy, and the water is used as the absorber in 10 cm of thickness. Fig. 20 gives the structural concept of the beam dump for ARIS [35,36]. In order to minimize the amount of activity in the cooling water, the water is circulated through the shielded ion-exchange columns. The beam dump lifetime is about one operational year, 5500 h. The target hall has the strongest radiation environment since the target and beam dump are the two major sources as discussed above. Fig. 21 shows the shielding design for ARIS [37]. The location of beam line components is also indicated, three vacuum vessels (target, beam dump and wedge) are used to house the components. The multi-shaped cast iron blocks are put on top of the vacuum vessels. The bottom is made of stainless steel, and combination with ordinary concrete. Meanwhile, a metal (bronze) shield is placed after the target module to protect the subsequent magnets. The unshielded activation levels at 30 cm distance from three typical inserts, the target module, beam dump module and bronze shield, are estimated based on the components lifetime (irradiation time) and waiting time (cooling time) for preparation activities. The 48Ca beam at 549 MeV/u (upgraded) and 400 kW of power is used for calculations. The unshielded dose rates at 30 cm from the components are given in Table 1[38]. The results indicate that the parts need adequately shielded when the workers access to the target hall. In this condition, the allowed time is about 80 h per year for conducting activities in the target hall, and the personnel doses below the MSU ALARA limit of 5 mSv/a [38]. Basic activities include manual utility disconnect, reentrant lid bolting and general cell maintenance. While when the beam is on or the shielding is removed, the RH system is needed in this situation. The RH system at the ARIS consists of a 20 ton crane, a window workstation with master slave manipulators (MSMs), vision system, equipment lift, etc. see in Fig. 22[37–39]. Dexterous maintenance of component modules is performed by a pair of MSMs at the window workstation. The lead glass window is used
Fig. 14. Schematic view of the beam dump at the BigRIPS.
Fig. 15. The PHITS calculated neutron dose rate distributions at BigRIPS induced by 48Ca 100 pnA beam incident on a 15 mm beryllium target. Three measurement points A, B and C are also shown.
Fig. 16. The PHITS calculated dose rate distributions around the target chamber of BigRIPS.
For a typical operation period at the BigRIPS, the residual dose rate is expected to 1–3 Sv/h at the end of irradiation and about 0.1–0.3 mSv/h after 2 d of cooling [27]. The main activity is 24Na in the aluminum made unit, which is induced by the fast neutrons from reaction 27Al(n, )24Na. To facilitate the remote handling at BigRIPS, the pillow-seal systems are introduced [34]. For maintenance, the target 102
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Fig. 17. (a) Structure view of the target chamber, maintenance cart and vacuum pump unit at BigRIPS. (b) The target maintenance area at BigRIPS.
Fig. 18. Overview of the Advanced Rare Isotope Separator (ARIS) at MSU FRIB. The location of target, beam dump, degrader, etc. have indicated.
Fig. 20. The structure concept of the beam dump for ARIS.
indicate that the target and beam dump contributed dose rate is about 1 µ Sv/h and < 1 µ Sv/h, respectively, while the bronze shield contributed is > 1 µ Sv/h, however, the shield is no need for exchange or repair and expected to last for 30 years of facility lifetime [38]. Other information about the waste storage and target hall canyon shielding design can be found in [38].
for shielding of gamma rays and also for inspection, and the thickness is equivalent to the surrounding concrete wall. The dose rate at the personnel working station is also evaluated for the present and upgraded condition. The design goal is 1 µ Sv/h, which comes from 5 mSv/a of the MSU ALARA goal and 5000 operational hours in a year. The results
Fig. 19. Left: Prototype of the production target at ARIS. Right: Schematic view of the target module at ARIS. Five parts of ferrofluidic rotary feedthrough and motor are shown. 103
target,
drive shaft,
heat exchanger,
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Fig. 21. Shielding design for ARIS. Beam line components and three vacuum vessels are indicated. Table 1 The unshielded activation levels at 30 cm distance from components. Component
Irradiation time
Cooling time
Dose rate (µ Sv/h)
Target module Beam dump Bronze shield
14 days 1 year 30 years
4h 16 h 2 days
1.3 × 105 5.1 × 105 1.547 × 107
Fig. 23. The ISOLDE facility at CERN.
Fig. 22. RH system at the hot cell of ARIS.
2.5. ISOLDE at CERN The ISOLDE (Isotope Separator On Line Device) facility at CERN uses ISOL method for the production of RIBs. The light particles (usually proton) and thick target are used for the reaction, then transported to the ion source and accelerated again as discusses earlier. Fig. 23 shows the ISOLDE facility at CERN [10]. The protons from the PS Booster with 1.4 GeV of energy and 2 µ A of intensity, and then delivered to two separate isotope separators: the General Purpose Separator (GPS) and the High Resolution Separator (HRS). The former equipped with an H-magnet with 1.5 m of bending radius and 70° of
Fig. 24. The target unit at ISOLDE.
bending angle, the resolution m/ m is 800; and the latter with two Cmagnets both with 1 m of bending radius and 90° and 60° of bending angle, respectively, and both the m/ m at 6000 [40]. The production target for ISOLDE shows a wide choice of materials and simultaneously guided by the production rate and release efficiency 104
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[41]. A standard target container at ISOLDE is made from a tantalum cylinder with 20 cm in length and 2 cm in diameter. Fig. 24 gives the target unit at ISOLDE [3]. The transfer tube is used to deliver the RIBs to the ion source. Moreover, a quartz tube is employed to absorb the rubidium atoms. The often used targets in ISOLDE including molten metal targets (Ge, Sn, La, etc.), solid metal targets (Ti, Nb, Ta, etc.), carbides (Al4C3, SiC, VC, etc.) and oxides (MgO, Al2O3, CaO, etc.), more information can be found in [41]. Furthermore, in order to keep the diffusion path short, the powders or thin foils (typically 20 µ m thick) are used for the solid metal target. For specific experimental requirement, up to 30 target changes are performed per year at ISOLDE [40]. The unabsorbed primary beam will deposit on the beam dump (see in Fig. 23). Currently, two beam dumps are installed at ISOLDE, both are steel blocks with 1.6 m × 1.6 m × 2.4 m at GPS and 0.4 m × 0.4 m × 1.0 m at HRS, finally surrounded by the concrete shielding [40]. The beam dumps are no need for exchange or repair in the lifetime of the facility, unless it is upgraded or decommissioned. The shielding design for ISOLDE is finished before 1990 with protons at 1 GeV of energy and 1013 protons/s of intensity [42]. The target area has been shielded with concrete and earth shielding, and the total thickness is equivalent to 8 m of earth. Meanwhile, approximately 4 m of earth shielding as well as 0.8–1.2 m of iron between the target and separator areas. For the separator area, which are shielded from the experimental area by 1–3 m thick concrete walls. Obviously, the shielding needs to be improved for currently beam parameters and also for the future upgraded facility HIE-ISOLDE. A full study of the radiological protection and safety impact of the ISOLDE and HIE-ISOLDE is given in [43]. The activated air also caused a serious problem, one approach is to separate the tunnel ventilation system from its neighboring, and the other is add additional shielding around the beam dumps [40]. The target/ion source units at GPS and HRS are the highly activated components, which need safety handling [10,40]. The residual dose rate in the Faraday cages around the two production targets is typically 4–5 mSv/h, thus the RH system is needed for intervention [42]. In early operation, two mobile industrial robots are taken over from the previous machines are used for remote handling. In 2014, two new robots KUKA KR 60L45-3 are commissioned to replace the former ones. The new robots are re-designed and modified in order to adapt to the strong radiation environment. One novel change is that the new robots are installed on the ceiling rails (see in Fig. 25) [14,40]. The irradiated
targets will be transported to the alpha-gamma hot cells for further treatment. The mechanical disassembly of targets and oxidation of uranium carbide will be finished at two compartments in the hot cell, respectively. The operator is protected by 15 cm thick lead bricks from floor to ceiling and 35 cm thick lead glass window for visualization. 3. Conclusions Radioactive beam physics has facilitated the construction of new facilities and the discovery of new physical phenomena. Meanwhile, it is important to ensure the radiation safety of the facility. In this work, the radiation protection considerations in multiple radioactive ion beam facilities were discussed. Including the introduction of the accelerator and its key equipment, radiation shielding design, activation evaluation as well as the remote-handling system. The following conclusions can be drawn from the present review. (1) Production target differs greatly between in-flight type and ISOL type facility. Obviously, this was due to the different production mechanism of RIBs. For in-flight facility, where the graphite was commonly used, since it with high production cross-section for fragments and high operating temperature. Also, it can be mounted on a rotating wheel to spread the power deposition. In order to meet different experimental requirements, the target was usually designed to be multi sliced. However, the selection of target material was more widely and flexible in ISOL facility depending on the interested isotope. (2) The major role of the beam dump was to stop the unreacted primary beam and at the same time to protect the subsequent beam line parts. For choosing of absorber materials, the high melting point and good heat conduction were considered. In in-flight facility, the low-Z materials can be adopted due to the short projectile range of heavy ions. Nevertheless, the high-Z materials were commonly selected for ISOL type, because of its long projectile range of protons. Finally, water cooling can be employed to remove the heat deposition. (3) The dominated beam losses appear in an in-flight type consist of the production target, beam dump, slit and degrader; though in ISOL type, only the target and beam dump were the main radiation sources. The neutrons were the principal contributors for the prompt radiation field as well as for the activation. For radiological safety design, firstly, the strongest radiation field needs to be found for multiple beam-target combinations. Then, compared with empirical formulas, the Monte Carlo codes like FLUKA, PHITS were more applicable for complex geometry conditions. In the first layer, iron/ steel was generally adapted to moderate the high energy neutrons, and in the second layer where concrete was commonly used. Also, the soil was readily available for shielding and the costs can be minimized. (4) Induced radioactivity in accelerator components was the dominant source of occupational radiation exposure. The produced radionuclides depends on beam parameters (particle type and energy, irradiation time, cooling time) and irradiated material. Fortunately, the typical radionuclides in mental material like aluminum, iron and copper, etc. and also in the cooling water and air were studied adequately in the early related reports. For example, 11 C, 13N, 15O and 41Ar were the concerned radionuclides in the air. Detailed estimation of activation level in components and working areas was essential for personnel intervention and final waste treatment and disposal. (5) The residual dose rates were usually in the order of mSv/h or even higher near the beam loss position, the hands-on maintenance was restricted. Therefore, the remote handling (RH) system becomes necessary, which services to conduct disconnect, removal, exchange, etc. In addition, components can also be transported to the hot-cell, which services to repair, maintain of inserts and store the
Fig. 25. (a) GPS and HRS KUKA robots on overhead rails. (b) GPS KUKA robots transporting a target unit. 105
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radioactive waste. The radiation sources were less and more centralized in ISOL facility when compared with the in-flight type. Consequently, it provides convenience for designing of radiation protection and remote handling system.
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