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Radiation safety issues for top-up operation at SPring-8
Radiation Measurements 41 (2007) S236 – S241 www.elsevier.com/locate/radmeas
Radiation safety issues for top-up operation at SPring-8 Yoshihiro Asano ∗ , Tetsuya Takagi Japan Synchrotron Radiation Research Institute, Sayo-cho, Hyogo-ken 679-5198, Japan
Abstract Electron beam injection to top-up the stored beam current during the operation of the synchrotron radiation beamlines of an 8 GeV class synchrotron radiation facility, SPring-8, has been conducted for about a year. A description of operations during this period and of the safety systems is described here. In order to carry out the top-up operation safely, the two goals were set. One is that the electron beam shall never be allowed to invade into the beamlines during the operation. The other is that the dose rate outside the shield tunnel due to beam loss shall never be allowed to exceed set limits during the operations including top-up. To achieve these two goals, suitable guidelines for accelerator operation were decided and the interlock systems were reconstructed. Under these conditions the safety analyses were performed, and it was found that top-up operation was executed without any trouble during this period. © 2007 Elsevier Ltd. All rights reserved. Keywords: SPring-8; Top-up; Radiation safety; Shielding design; Synchrotron radiation; Interlock
1. Introduction In recent years, third generation synchrotron radiation facilities have been constructed for the purpose of reducing the stored electron beam emittance and increasing the number of the electrons per bunch, to obtain an extremely high brilliance photon beam. The life time of the stored electrons in these facilities is shortened for these reasons. Electrons are, therefore, injected into the storage ring at short intervals to supplement the decreased electrons during synchrotron radiation experiments. This so-called “top-up” operation is also effective from the view point of the prevention of the change of the heat load to the optical elements. Thus, this operation has increased importance for the third generation facilities, and many efforts have been made to achieve high quality performance, including beam stability, during injection (Ohkuma et al., 2003; Kimura, 2003). The safety shutter, the so-called main beam shutter (MBS), of the synchrotron radiation beamline, is opened during the topup operation, whereas it is generally closed during the beam injection to prevent the invasion of the high energy photons and neutrons into the beamline due to the stored electron beam loss.
∗ Corresponding author. Tel.: +81 791 58 2705; fax: +81 791 58 2740.
E-mail address: [email protected] (Y. Asano). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.01.015
Besides, most of the gaps of the insertion devices have been narrowed to increase electron beam loss during the injection. The information needed to establish safety criteria such as the beam loss scenario and the radiation shielding during top-up operation are, therefore, widely different from the normal operation. Many facilities such as Advanced Photon Source in USA (Job, 2002), European Synchrotron Radiation Facility in France (Berkvens et al., 2002), and ELLETRA in Italy (Casarin et al., 2002), are now executing or testing top-up operation. However, there are no common methods of safety analyses for the topup operation under these circumstances, and it is important to discuss safety analyses for this operation. Since May 20 in 2004, top-up operation for steady stored current at the storage ring of SPring-8 has been conducted (Tanaka et al., 2004). Before the top-up operation for the steady stored current mode, regular top-up injection was conducted for about a half year. In addition to the safety analyses of the normal operation, we recommended that two fundamental requirements for safe top-up operation be established. One is that the electron beam never invades into the beamlines during operation. The other is that the dose rate outside the shield tunnel due to the beam loss never exceeds set limits during operations including top-up. Based on these two fundamental requirements, the safety systems of SPring-8 were reconstructed for the top-up operation. Under these conditions, safety
Y. Asano, T. Takagi / Radiation Measurements 41 (2007) S236 – S241
analyses were performed and both of the top-up operations requirements were fulfilled without any trouble during the past year. In this paper, we present our measurements of the neutron leakage dose around the beamline during the top-up operations and summarized countermeasures to prevent misoperation including installation of interlock systems. 2. Safety considerations for top-up operation at SPring-8 2.1. Outline of SPring-8 The SPring-8, which is currently the facility with the highest stored electron energy, 8 GeV, has very low beam emittance. It is composed of an electron linear accelerator (linac), a booster synchrotron injector and a storage ring. The linac is about 140 m long and accelerates electrons up to about 1 GeV. The booster synchrotron with a circumference of 396 m accelerates the electrons injected from the linac up to 8 GeV. The electrons are then run up a path and injected into the storage ring, which is about 9 m higher altitude than the booster and capable of storing circulating currents of 8 GeV up to 100 mA. The storage ring with a circumference of 1496 m has 44 straight sections, 38 of which are available for insertion devices. Thirty-four sections are standard ones of 19 m long and other four sections are longer, 40 m. Further, 23 beamlines can be installed from bending magnets. The electrons emit synchrotron radiation while they are being deflected in the field of the ring bending magnets or in the insertion devices placed in the straight sections. The emitted synchrotron radiation is introduced to an experimental hall by a beamline through a ratchet shaped bulk shield wall of the storage ring. The BL01B1 beamline of SPring-8 is located along the continuation of the injection line. 2.2. Fundamental requirements There are no differences between the radiation safety requirements during the top-up operation and those during normal operation at SPring-8. Based on the ALARA principle, both the top-up and the normal modes must be operated maintaining the same radiation dose limits, namely 1.3 Sv/h at the boundary of the controlled area, and 50 Sv/y at the nearest point accessible to the general public. Further, the dose rates outside the shield wall or the hutch must never make it possible for the radiation exposure to exceed the limit for radiation workers, 1000 Sv/w. Since the injection efficiency is low and the electron beam loss is high during the injection, all the MBS are closed during the electron beam injection in normal operation. In this way, it is impossible for electrons which escape the precarious balance of magnets controlling them during the injection to creep into the beamlines. Further, both the shielding designs of storage ring and the beamlines were made positing the normal operation conditions. On the other hand, MBS are opened during top-up operation. Assuming the case of failure of top-up operation, the radiation doses outside the hutch due to invading 8 GeV electrons were estimated (Asano and Takagi, 2004) by using the convenient code SHIELD11 (Nelson, 2001), and the Monte Carlo
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code EGS4 (Nelson et al., 1985). As the results, the maximum dose outside the hutch due to the one invading electron can be estimated to be approximately 10−8 Sv and corresponds to 100 Sv if about 0.3% of stored electrons are injected and invade under top-up operation conditions. This is hazardous, so physical arrangements to avoid the situation are required. In addition to the MBS being open during the injection, the top-up operation has two main characteristics distinguishing it from normal operation at SPring-8. One is that high injection efficiency to avoid the disturbance of the synchrotron radiation beam as much as possible is required, and another is that the electron beam loss occurs mainly at the narrow gap of insertion devices such as in vacuum type undulators. Therefore, the injected electrons will be lost at a specific point. A scenario of the electron beam loss must be developed based on these two characteristics, and with a maximum stored beam current of 100 mA. The scenario of the beam loss consists of three parts in the case of normal operation, during the tuning, during the injection and during the user time (Asano and Takagi, 2002). Top-up is based on the normal operation, but is operated under distinctive limitations concerning such factors as high injection efficiency and localized beam loss. A scraper of the electron beam was installed into the transport line by the accelerator group to obtain high injection efficiency during the top-up operation. After that, more than 80% injection efficiency was secured. We decided that 80% be adopted as the injection efficiency, and the rest of the electrons were assumed to be lost at one point of the storage ring, conservatively. The estimation of leakage dose outside the shield wall was performed under these conditions. We assumed conservatively that the stored electron beam current of 100 mA (3.0 × 1012 stored electrons) is lost linearly with time over the course of 10 h. The lost electrons are compensated by top-up operation so that the maximum amount of the injection by the top-up operation is 1.3 × 1014 electrons during the working time of 34.1 h a week. The rest of 40 h a week is spent for the tuning and injection operations in the worst case. The beam loss rate due to top-up operation is 2.1 × 107 s−1 at any point during the user time. Consequently, we found that the leakage doses outside the shield wall of the storage ring or the optics hutch of the beamlines never exceeded the limits where injection efficiency is 80% during the top-up operation, even without the improvement of the shield (Asano and Takagi, 2004).
2.3. Countermeasures for top-up operation The machine should be designed so that there is no possibility that stored electrons invade into beamlines. According to beam dynamics analysis, the conditions under which the electrons can creep into beamlines are that the magnetic field strength of the bending magnets is not matched with the electron energy and that the field strength lowers to less than half of the standard value (Kumagai et al., 2003). Only a mismatched injection beam, therefore, can exist under such conditions. The BL01B1 beamline of SPring-8 is located along the continuation of the
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If the injection efficiency is less than the reference value that has been decided beforehand, the signal is sent to the electron gun to shutdown immediately. The average injection efficiency is checked every 8 h, and if the cumulative beam loss during the previous week is in excess of the preset value, permission for the top-up operation is cancelled. The radiation monitoring system of top-up operation is fundamentally operated by the same system as the normal operation (Takagi and Asano, 2002). The ion chamber for (X) ray and the helium-3 neutron counter are located in the injection area. The monitors have the function of raising an alarm, and these are connected to the interlock system to shutdown the operation. Some radiation monitors were installed temporally around the shield wall of storage ring in the first stage of the top-up operation. Furthermore, glass dosimeters are attached at the places where it is likely that the doses will be high, to measure the one week cumulative dose. The safety considerations and countermeasures for top-up operation at SPring-8 are summarized in Fig. 2.
Fig. 1. Conceptual flow diagram of the interlock system for the top-up operation at SPring-8.
injection line so that we must pay attention to the injection line as well. In order to permanently solve this problem, the power supply of the bending magnet at the transport line between the booster and the storage ring was connected directly to that of the storage ring. The transport line was placed under the ground with sufficient shielding. By these physical countermeasures, a mismatch never occurs physically, and all the electrons abort at the transport line when the extraction beam energy from the booster deviates from that of the injection energy of the storage ring. The safety interlock system for normal operation consists of the sub-systems of individual beamlines and the accelerators, and these are closed off from each other so that the operations and safety of each sub-system are fundamentally ensured that sub-system. The amount of the injection was limited to be always less than the stored current, 100 mA. In addition to this arrangement, the beam loss integrated system, in which the differentials between the beam current monitor (BCM) values of the beam extracted from the booster and the values of the DC current transformers (DCCT) for the stored electrons is observed, was installed for top-up operation. As illustrated in Fig. 1, the process of the interlock for top-up operation is as follows: (1) a request for the top-up operation is sent to the radiation safety interlock system first. The status of the MBS is checked and the interlock system is turned to allow a top-up operation injecting a beam without closing the MBS; (2) the status of the top-up operation, permitted/not permitted, is communicated to the accelerator control system, and the beam loss integrator is then started; (3) the status of the top-up operation is communicated to the safety interlock system and the beam loss integrator monitors the injection efficiency during top-up operation.
3. Injection efficiency at SPring-8 To obtain high injection efficiency and maintain an oscillation-free stored beam during injection, the SPring-8 accelerator group made great efforts to enhance the machine performance with regard to such items as the linac energy compression system, synchronous timing system, and phase and power stability of RF. Measurement results of the injection efficiency during top-up operation from June 24 to July 8, 2004 are shown in Fig. 3 including the efficiency of normal operation which includes tuning and commissioning. The average injection efficiency during top-up operation for 5464 samples was 87%. As shown in the graph, the efficiency during normal operation is clearly lower than that of top-up operation. One of the main reasons is that the beam is collimated by using a scraper, which is installed in the beam transport line from the synchrotron booster to the storage ring (SSBT line), to execute the top-up operation. 4. Neutron leakage dose measurements To confirm radiation safety, we measured the neutron leakage dose outside the beamline by using high sensitivity helium-3 counters with two different thick moderators of 2.6 and 8.6 cm during top-up operation These counters can detect neutrons due to natural background and we can estimate the effective dose rate with sufficient accuracy by using the ED2M method (Asano, 2001). It was calculated here to be 3.5 nSv/h ± 20%. The counters were set up outside the optics hutch of the BL01B1 beamline which is located in the forward continuation of the electron injection line from the booster synchrotron to the storage ring. Therefore, this area is considered to be one of the places where the highest leakage dose is generated due to electron beam loss of top-up operation. Fig. 4 shows the measurement results from May 14 to June 4, 2004 of neutron dose, stored current conditions, and injection beam loss during
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Fig. 2. Fundamental requirements and countermeasures for top-up operation at SPring-8.
: top–up : normal including tuning
Frequency probability ( percents )
30
20
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0 80
90 Injection efficiency ( percents )
Fig. 3. Measurements of injection efficiency for SPring-8 operation. Solid line indicates the injection efficiency during top-up operation, and dotted line is due to normal operation including tuning and commissioning.
the steady stored current mode of top-up operation. The measurement results during the regular injection mode are shown in Fig. 5. In both figures, the left-hand graph (A) shows the stored current conditions and the helium-3 counters outputs, and right-hand graph (B) shows the neutron dose rate and electron beam loss during injection corresponding to the left-hand side data. As shown in Fig. 4, the stored current was stable
from the beginning of the steady stored current mode of top-up operation. The outputs of helium-3 counters changed together with the stored beam current conditions and injection beam loss with good correlation. Besides, the counting rates slightly increased with the beginning of the top-up operation. During the normal injection, when the stored current was increased from 0 to 100 mA, the leakage doses became high, corresponding to high intensities of beam loss. Neutron leakage doses are shown in Fig. 6 as a function of the electron beam loss during the steady stored current mode and the regular injection mode of top-up operation corresponding to the data in Fig. 4. As shown in the figures, there are wide variations of neutron leakage doses. However, the neutron doses during May 28 to June 4 in Fig. 6 are slightly lower than that of the doses during May 21 to May 26, corresponding to the lower intensities of electron beam loss. There are no cases where the dose exceeds 10 nSv in the range of less than 1012 electron beam loss during either top-up or normal injection operations. 5. Summary Based on the two fundamental requirements for the topup operation of the 8 GeV class synchrotron radiation facility, namely that there be operation without the injected electron creeping into the beamlines and without the leakage doses outside the shield wall due to the beam loss exceeding set limits, the safety systems were discussed and constructed. In order to satisfy the requirements, we devised two countermeasures: the direct connection of the power supply of the bending magnets of the transport line with that of the storage ring to avoid the electrons creeping into beamlines physically, and measures to reliably maintain high injection efficiency of more than 80% for top-up operation. By maintaining this high injection
Y. Asano, T. Takagi / Radiation Measurements 41 (2007) S236 – S241
A
B
outside BL01B1 optics hutch (steady stored current)
outside BL01B1 optics hutch (steady stored current)
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Channel No. (date)
Fig. 4. Measurement results of neutron leakage dose during the top-up operation in the steady stored current mode from May 14 to June 4 2004. (A) shows the outputs of the high sensitivity He-3 counters and stored current. The solid line shows the stored current, the stars indicate the count rates of the He-3 counter with polyethylene moderator of 2.6 cm in thickness, and dark open circles are that of He-3 counter with the 8.6 cm moderator. The top-up operation was started from May 20. In (B) the intensity of the electron beam loss is shown by the closed circles, and neutron leakage doses are shown by the dark open circles corresponding, respectively, to the beam current and outputs of the He-3 counters in (A).
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outside BL01B1 optics hutch (regular injection)
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Fig. 5. Measurement results of neutron leakage dose during the top-up operation in the regular injection mode from March 25 to April 15, 2004. These marks and line indicate the same as in Fig. 4.
efficiency, we found that the leakage dose outside the shield wall or hutch never exceeded the limits, even if the rest of the electrons, 20% of injection were lost at one point. To achieve this high injection efficiency, the interlock system was reconstructed and a beam loss integrator was installed. The radiation monitors were set up and connected to the interlock system to confirm the dose level and provide against any emergency.
A radiation survey was performed before the routine operation of the top-up. The results were that the anticipated dose rates outside the shield wall and hutch were detected at several points, and these are almost entirely neutron doses with a rate of 0.1 SV/h. Gamma dose due to the beam loss was not confirmed because the levels were low and buried in the fluctuation of the background. Regular injection, normally twice a
Y. Asano, T. Takagi / Radiation Measurements 41 (2007) S236 – S241
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thickness. The results confirmed that the leakage dose outside the optics hutch of BL01B1 never exceeds 10 nSv where there is less than 1012 electron beam loss during top-up operation.
Neutron dose (pSv)
References
103
102
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1010 1011 Electron beam loss number
Fig. 6. Neutron leakage dose as a function of electron beam loss number during the steady stored current mode of the top-up operation. Diamonds indicate the data from May 28 to June 4, and open squares are from May 21 to May 26. Dots show all other data shown in Fig. 4.
day, was performed with the top-up operation from September 2003 till May 2004 without any trouble. After May 20, the topup operation to obtain steady stored current was performed. This “top-up” means that the electrons were injected into the storage ring, without closing the MBS of the beamlines, every 1–5 min. During the steady stored current mode of top-up operation, we measured the leakage dose outside the optics hutch of the BL01B1 beamline, which is located in the forward continuation of the electron beam injection line, by using high sensitivity helium-3 counters with moderators of different
Asano, Y., 2001. A study on radiation shielding and safety analysis for a synchrotron radiation beamline. JAERI-Research, 2001–2006. Asano, Y., Takagi, T., 2002. Overviews of induced activity at SPring-8. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 33. Asano, Y., Takagi, T., 2004. Safety considerations for top-up operation of an 8 GeV class synchrotron radiation facility. Proceedings of 11th International Congress on Radiation Protection Association IRPA 11, Madrid, Spain. Berkvens, P., Colomp, P., Bidault, F., 2002. Injection with front ends open at the ESRF. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 89. Casarin, K, Tromba, G., Vascotto, A., 2002. The new full-injector for the Elettra light source. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 80. Job, P.K., 2002. Experiences in radiation safety during the top-up operation at advanced photon source. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 76. Kimura, H., 2003. Present status of top-up operation at SPring-8 storage ring II. SPring-8 Information, vol. 8(5) (in Japanese). Kumagai, N., et al., 2003. Private communications, SPring-8. Nelson, W.R., 2001. The SHIELD11 Computer Code. Stanford Accelerator Center RP01-01. Nelson, W.R., Hirayama, H., Rogers, D.W.O., 1985. The EGS4 Code System. Stanford Accelerator Center SLAC-265. Ohkuma, H., et al., 2003. Beam-performance improvement of the SPring8 storage ring. Proceedings on Particle Accelerator Conference 2003, MPPB025. Portland, USA. Takagi, T., Asano, Y., 2002. Radiation safety systems at SPring-8. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 123. Tanaka, H., et al., 2004. Top-up operation at SPring-8-Towards maximizing the potential of a third generation light source. Ninth European Particle Accelerator Conference, EPAC. Lucerne, Swiss.
Radiation safety issues for top-up operation at SPring-8 Yoshihiro Asano ∗ , Tetsuya Takagi Japan Synchrotron Radiation Research Institute, Sayo-cho, Hyogo-ken 679-5198, Japan
Abstract Electron beam injection to top-up the stored beam current during the operation of the synchrotron radiation beamlines of an 8 GeV class synchrotron radiation facility, SPring-8, has been conducted for about a year. A description of operations during this period and of the safety systems is described here. In order to carry out the top-up operation safely, the two goals were set. One is that the electron beam shall never be allowed to invade into the beamlines during the operation. The other is that the dose rate outside the shield tunnel due to beam loss shall never be allowed to exceed set limits during the operations including top-up. To achieve these two goals, suitable guidelines for accelerator operation were decided and the interlock systems were reconstructed. Under these conditions the safety analyses were performed, and it was found that top-up operation was executed without any trouble during this period. © 2007 Elsevier Ltd. All rights reserved. Keywords: SPring-8; Top-up; Radiation safety; Shielding design; Synchrotron radiation; Interlock
1. Introduction In recent years, third generation synchrotron radiation facilities have been constructed for the purpose of reducing the stored electron beam emittance and increasing the number of the electrons per bunch, to obtain an extremely high brilliance photon beam. The life time of the stored electrons in these facilities is shortened for these reasons. Electrons are, therefore, injected into the storage ring at short intervals to supplement the decreased electrons during synchrotron radiation experiments. This so-called “top-up” operation is also effective from the view point of the prevention of the change of the heat load to the optical elements. Thus, this operation has increased importance for the third generation facilities, and many efforts have been made to achieve high quality performance, including beam stability, during injection (Ohkuma et al., 2003; Kimura, 2003). The safety shutter, the so-called main beam shutter (MBS), of the synchrotron radiation beamline, is opened during the topup operation, whereas it is generally closed during the beam injection to prevent the invasion of the high energy photons and neutrons into the beamline due to the stored electron beam loss.
∗ Corresponding author. Tel.: +81 791 58 2705; fax: +81 791 58 2740.
E-mail address: [email protected] (Y. Asano). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.01.015
Besides, most of the gaps of the insertion devices have been narrowed to increase electron beam loss during the injection. The information needed to establish safety criteria such as the beam loss scenario and the radiation shielding during top-up operation are, therefore, widely different from the normal operation. Many facilities such as Advanced Photon Source in USA (Job, 2002), European Synchrotron Radiation Facility in France (Berkvens et al., 2002), and ELLETRA in Italy (Casarin et al., 2002), are now executing or testing top-up operation. However, there are no common methods of safety analyses for the topup operation under these circumstances, and it is important to discuss safety analyses for this operation. Since May 20 in 2004, top-up operation for steady stored current at the storage ring of SPring-8 has been conducted (Tanaka et al., 2004). Before the top-up operation for the steady stored current mode, regular top-up injection was conducted for about a half year. In addition to the safety analyses of the normal operation, we recommended that two fundamental requirements for safe top-up operation be established. One is that the electron beam never invades into the beamlines during operation. The other is that the dose rate outside the shield tunnel due to the beam loss never exceeds set limits during operations including top-up. Based on these two fundamental requirements, the safety systems of SPring-8 were reconstructed for the top-up operation. Under these conditions, safety
Y. Asano, T. Takagi / Radiation Measurements 41 (2007) S236 – S241
analyses were performed and both of the top-up operations requirements were fulfilled without any trouble during the past year. In this paper, we present our measurements of the neutron leakage dose around the beamline during the top-up operations and summarized countermeasures to prevent misoperation including installation of interlock systems. 2. Safety considerations for top-up operation at SPring-8 2.1. Outline of SPring-8 The SPring-8, which is currently the facility with the highest stored electron energy, 8 GeV, has very low beam emittance. It is composed of an electron linear accelerator (linac), a booster synchrotron injector and a storage ring. The linac is about 140 m long and accelerates electrons up to about 1 GeV. The booster synchrotron with a circumference of 396 m accelerates the electrons injected from the linac up to 8 GeV. The electrons are then run up a path and injected into the storage ring, which is about 9 m higher altitude than the booster and capable of storing circulating currents of 8 GeV up to 100 mA. The storage ring with a circumference of 1496 m has 44 straight sections, 38 of which are available for insertion devices. Thirty-four sections are standard ones of 19 m long and other four sections are longer, 40 m. Further, 23 beamlines can be installed from bending magnets. The electrons emit synchrotron radiation while they are being deflected in the field of the ring bending magnets or in the insertion devices placed in the straight sections. The emitted synchrotron radiation is introduced to an experimental hall by a beamline through a ratchet shaped bulk shield wall of the storage ring. The BL01B1 beamline of SPring-8 is located along the continuation of the injection line. 2.2. Fundamental requirements There are no differences between the radiation safety requirements during the top-up operation and those during normal operation at SPring-8. Based on the ALARA principle, both the top-up and the normal modes must be operated maintaining the same radiation dose limits, namely 1.3 Sv/h at the boundary of the controlled area, and 50 Sv/y at the nearest point accessible to the general public. Further, the dose rates outside the shield wall or the hutch must never make it possible for the radiation exposure to exceed the limit for radiation workers, 1000 Sv/w. Since the injection efficiency is low and the electron beam loss is high during the injection, all the MBS are closed during the electron beam injection in normal operation. In this way, it is impossible for electrons which escape the precarious balance of magnets controlling them during the injection to creep into the beamlines. Further, both the shielding designs of storage ring and the beamlines were made positing the normal operation conditions. On the other hand, MBS are opened during top-up operation. Assuming the case of failure of top-up operation, the radiation doses outside the hutch due to invading 8 GeV electrons were estimated (Asano and Takagi, 2004) by using the convenient code SHIELD11 (Nelson, 2001), and the Monte Carlo
S237
code EGS4 (Nelson et al., 1985). As the results, the maximum dose outside the hutch due to the one invading electron can be estimated to be approximately 10−8 Sv and corresponds to 100 Sv if about 0.3% of stored electrons are injected and invade under top-up operation conditions. This is hazardous, so physical arrangements to avoid the situation are required. In addition to the MBS being open during the injection, the top-up operation has two main characteristics distinguishing it from normal operation at SPring-8. One is that high injection efficiency to avoid the disturbance of the synchrotron radiation beam as much as possible is required, and another is that the electron beam loss occurs mainly at the narrow gap of insertion devices such as in vacuum type undulators. Therefore, the injected electrons will be lost at a specific point. A scenario of the electron beam loss must be developed based on these two characteristics, and with a maximum stored beam current of 100 mA. The scenario of the beam loss consists of three parts in the case of normal operation, during the tuning, during the injection and during the user time (Asano and Takagi, 2002). Top-up is based on the normal operation, but is operated under distinctive limitations concerning such factors as high injection efficiency and localized beam loss. A scraper of the electron beam was installed into the transport line by the accelerator group to obtain high injection efficiency during the top-up operation. After that, more than 80% injection efficiency was secured. We decided that 80% be adopted as the injection efficiency, and the rest of the electrons were assumed to be lost at one point of the storage ring, conservatively. The estimation of leakage dose outside the shield wall was performed under these conditions. We assumed conservatively that the stored electron beam current of 100 mA (3.0 × 1012 stored electrons) is lost linearly with time over the course of 10 h. The lost electrons are compensated by top-up operation so that the maximum amount of the injection by the top-up operation is 1.3 × 1014 electrons during the working time of 34.1 h a week. The rest of 40 h a week is spent for the tuning and injection operations in the worst case. The beam loss rate due to top-up operation is 2.1 × 107 s−1 at any point during the user time. Consequently, we found that the leakage doses outside the shield wall of the storage ring or the optics hutch of the beamlines never exceeded the limits where injection efficiency is 80% during the top-up operation, even without the improvement of the shield (Asano and Takagi, 2004).
2.3. Countermeasures for top-up operation The machine should be designed so that there is no possibility that stored electrons invade into beamlines. According to beam dynamics analysis, the conditions under which the electrons can creep into beamlines are that the magnetic field strength of the bending magnets is not matched with the electron energy and that the field strength lowers to less than half of the standard value (Kumagai et al., 2003). Only a mismatched injection beam, therefore, can exist under such conditions. The BL01B1 beamline of SPring-8 is located along the continuation of the
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Y. Asano, T. Takagi / Radiation Measurements 41 (2007) S236 – S241
If the injection efficiency is less than the reference value that has been decided beforehand, the signal is sent to the electron gun to shutdown immediately. The average injection efficiency is checked every 8 h, and if the cumulative beam loss during the previous week is in excess of the preset value, permission for the top-up operation is cancelled. The radiation monitoring system of top-up operation is fundamentally operated by the same system as the normal operation (Takagi and Asano, 2002). The ion chamber for (X) ray and the helium-3 neutron counter are located in the injection area. The monitors have the function of raising an alarm, and these are connected to the interlock system to shutdown the operation. Some radiation monitors were installed temporally around the shield wall of storage ring in the first stage of the top-up operation. Furthermore, glass dosimeters are attached at the places where it is likely that the doses will be high, to measure the one week cumulative dose. The safety considerations and countermeasures for top-up operation at SPring-8 are summarized in Fig. 2.
Fig. 1. Conceptual flow diagram of the interlock system for the top-up operation at SPring-8.
injection line so that we must pay attention to the injection line as well. In order to permanently solve this problem, the power supply of the bending magnet at the transport line between the booster and the storage ring was connected directly to that of the storage ring. The transport line was placed under the ground with sufficient shielding. By these physical countermeasures, a mismatch never occurs physically, and all the electrons abort at the transport line when the extraction beam energy from the booster deviates from that of the injection energy of the storage ring. The safety interlock system for normal operation consists of the sub-systems of individual beamlines and the accelerators, and these are closed off from each other so that the operations and safety of each sub-system are fundamentally ensured that sub-system. The amount of the injection was limited to be always less than the stored current, 100 mA. In addition to this arrangement, the beam loss integrated system, in which the differentials between the beam current monitor (BCM) values of the beam extracted from the booster and the values of the DC current transformers (DCCT) for the stored electrons is observed, was installed for top-up operation. As illustrated in Fig. 1, the process of the interlock for top-up operation is as follows: (1) a request for the top-up operation is sent to the radiation safety interlock system first. The status of the MBS is checked and the interlock system is turned to allow a top-up operation injecting a beam without closing the MBS; (2) the status of the top-up operation, permitted/not permitted, is communicated to the accelerator control system, and the beam loss integrator is then started; (3) the status of the top-up operation is communicated to the safety interlock system and the beam loss integrator monitors the injection efficiency during top-up operation.
3. Injection efficiency at SPring-8 To obtain high injection efficiency and maintain an oscillation-free stored beam during injection, the SPring-8 accelerator group made great efforts to enhance the machine performance with regard to such items as the linac energy compression system, synchronous timing system, and phase and power stability of RF. Measurement results of the injection efficiency during top-up operation from June 24 to July 8, 2004 are shown in Fig. 3 including the efficiency of normal operation which includes tuning and commissioning. The average injection efficiency during top-up operation for 5464 samples was 87%. As shown in the graph, the efficiency during normal operation is clearly lower than that of top-up operation. One of the main reasons is that the beam is collimated by using a scraper, which is installed in the beam transport line from the synchrotron booster to the storage ring (SSBT line), to execute the top-up operation. 4. Neutron leakage dose measurements To confirm radiation safety, we measured the neutron leakage dose outside the beamline by using high sensitivity helium-3 counters with two different thick moderators of 2.6 and 8.6 cm during top-up operation These counters can detect neutrons due to natural background and we can estimate the effective dose rate with sufficient accuracy by using the ED2M method (Asano, 2001). It was calculated here to be 3.5 nSv/h ± 20%. The counters were set up outside the optics hutch of the BL01B1 beamline which is located in the forward continuation of the electron injection line from the booster synchrotron to the storage ring. Therefore, this area is considered to be one of the places where the highest leakage dose is generated due to electron beam loss of top-up operation. Fig. 4 shows the measurement results from May 14 to June 4, 2004 of neutron dose, stored current conditions, and injection beam loss during
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Fig. 2. Fundamental requirements and countermeasures for top-up operation at SPring-8.
: top–up : normal including tuning
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Fig. 3. Measurements of injection efficiency for SPring-8 operation. Solid line indicates the injection efficiency during top-up operation, and dotted line is due to normal operation including tuning and commissioning.
the steady stored current mode of top-up operation. The measurement results during the regular injection mode are shown in Fig. 5. In both figures, the left-hand graph (A) shows the stored current conditions and the helium-3 counters outputs, and right-hand graph (B) shows the neutron dose rate and electron beam loss during injection corresponding to the left-hand side data. As shown in Fig. 4, the stored current was stable
from the beginning of the steady stored current mode of top-up operation. The outputs of helium-3 counters changed together with the stored beam current conditions and injection beam loss with good correlation. Besides, the counting rates slightly increased with the beginning of the top-up operation. During the normal injection, when the stored current was increased from 0 to 100 mA, the leakage doses became high, corresponding to high intensities of beam loss. Neutron leakage doses are shown in Fig. 6 as a function of the electron beam loss during the steady stored current mode and the regular injection mode of top-up operation corresponding to the data in Fig. 4. As shown in the figures, there are wide variations of neutron leakage doses. However, the neutron doses during May 28 to June 4 in Fig. 6 are slightly lower than that of the doses during May 21 to May 26, corresponding to the lower intensities of electron beam loss. There are no cases where the dose exceeds 10 nSv in the range of less than 1012 electron beam loss during either top-up or normal injection operations. 5. Summary Based on the two fundamental requirements for the topup operation of the 8 GeV class synchrotron radiation facility, namely that there be operation without the injected electron creeping into the beamlines and without the leakage doses outside the shield wall due to the beam loss exceeding set limits, the safety systems were discussed and constructed. In order to satisfy the requirements, we devised two countermeasures: the direct connection of the power supply of the bending magnets of the transport line with that of the storage ring to avoid the electrons creeping into beamlines physically, and measures to reliably maintain high injection efficiency of more than 80% for top-up operation. By maintaining this high injection
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Fig. 4. Measurement results of neutron leakage dose during the top-up operation in the steady stored current mode from May 14 to June 4 2004. (A) shows the outputs of the high sensitivity He-3 counters and stored current. The solid line shows the stored current, the stars indicate the count rates of the He-3 counter with polyethylene moderator of 2.6 cm in thickness, and dark open circles are that of He-3 counter with the 8.6 cm moderator. The top-up operation was started from May 20. In (B) the intensity of the electron beam loss is shown by the closed circles, and neutron leakage doses are shown by the dark open circles corresponding, respectively, to the beam current and outputs of the He-3 counters in (A).
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Fig. 5. Measurement results of neutron leakage dose during the top-up operation in the regular injection mode from March 25 to April 15, 2004. These marks and line indicate the same as in Fig. 4.
efficiency, we found that the leakage dose outside the shield wall or hutch never exceeded the limits, even if the rest of the electrons, 20% of injection were lost at one point. To achieve this high injection efficiency, the interlock system was reconstructed and a beam loss integrator was installed. The radiation monitors were set up and connected to the interlock system to confirm the dose level and provide against any emergency.
A radiation survey was performed before the routine operation of the top-up. The results were that the anticipated dose rates outside the shield wall and hutch were detected at several points, and these are almost entirely neutron doses with a rate of 0.1 SV/h. Gamma dose due to the beam loss was not confirmed because the levels were low and buried in the fluctuation of the background. Regular injection, normally twice a
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thickness. The results confirmed that the leakage dose outside the optics hutch of BL01B1 never exceeds 10 nSv where there is less than 1012 electron beam loss during top-up operation.
Neutron dose (pSv)
References
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Fig. 6. Neutron leakage dose as a function of electron beam loss number during the steady stored current mode of the top-up operation. Diamonds indicate the data from May 28 to June 4, and open squares are from May 21 to May 26. Dots show all other data shown in Fig. 4.
day, was performed with the top-up operation from September 2003 till May 2004 without any trouble. After May 20, the topup operation to obtain steady stored current was performed. This “top-up” means that the electrons were injected into the storage ring, without closing the MBS of the beamlines, every 1–5 min. During the steady stored current mode of top-up operation, we measured the leakage dose outside the optics hutch of the BL01B1 beamline, which is located in the forward continuation of the electron beam injection line, by using high sensitivity helium-3 counters with moderators of different
Asano, Y., 2001. A study on radiation shielding and safety analysis for a synchrotron radiation beamline. JAERI-Research, 2001–2006. Asano, Y., Takagi, T., 2002. Overviews of induced activity at SPring-8. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 33. Asano, Y., Takagi, T., 2004. Safety considerations for top-up operation of an 8 GeV class synchrotron radiation facility. Proceedings of 11th International Congress on Radiation Protection Association IRPA 11, Madrid, Spain. Berkvens, P., Colomp, P., Bidault, F., 2002. Injection with front ends open at the ESRF. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 89. Casarin, K, Tromba, G., Vascotto, A., 2002. The new full-injector for the Elettra light source. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 80. Job, P.K., 2002. Experiences in radiation safety during the top-up operation at advanced photon source. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 76. Kimura, H., 2003. Present status of top-up operation at SPring-8 storage ring II. SPring-8 Information, vol. 8(5) (in Japanese). Kumagai, N., et al., 2003. Private communications, SPring-8. Nelson, W.R., 2001. The SHIELD11 Computer Code. Stanford Accelerator Center RP01-01. Nelson, W.R., Hirayama, H., Rogers, D.W.O., 1985. The EGS4 Code System. Stanford Accelerator Center SLAC-265. Ohkuma, H., et al., 2003. Beam-performance improvement of the SPring8 storage ring. Proceedings on Particle Accelerator Conference 2003, MPPB025. Portland, USA. Takagi, T., Asano, Y., 2002. Radiation safety systems at SPring-8. Proceedings of the Second International Conference on Radiation Safety at Synchrotron Radiation Sources. Grenoble, France, p. 123. Tanaka, H., et al., 2004. Top-up operation at SPring-8-Towards maximizing the potential of a third generation light source. Ninth European Particle Accelerator Conference, EPAC. Lucerne, Swiss.