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 Table of Contents 
REVIEW ARTICLE
Year : 2012  |  Volume : 35  |  Issue : 3  |  Page : 135-144  

Radiation safety for electron accelerators: Synchrotron radiation facility


RIKEN Spring-8 Center Safety Design Group, 1-1-1 Koto Sayo, Hyogo 679-5148, Japan

Date of Web Publication5-Sep-2013

Correspondence Address:
Yoshihiro Asano
RIKEN SPring-8 center, 1-1-1 Koto Sayo, Hyogo, 679-5148
Japan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.117670

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  Abstract 

Radiation safety is reviewed for electron accelerators, mainly synchrotron radiation facilities. Accelerator radiation safety systems consist on safety interlock system, radiation shielding and radiation monitoring. These systems depend strongly on the characteristics of machines such as the maximum electron energy. In this paper, conceptual safety systems and radiation sources for synchrotron radiation facilities are overviewed including the evaluation methods of shielding.

Keywords: Electron accelerator, radiation safety, shielding design, synchrotron radiation


How to cite this article:
Asano Y. Radiation safety for electron accelerators: Synchrotron radiation facility. Radiat Prot Environ 2012;35:135-44

How to cite this URL:
Asano Y. Radiation safety for electron accelerators: Synchrotron radiation facility. Radiat Prot Environ [serial online] 2012 [cited 2019 Jul 17];35:135-44. Available from: http://www.rpe.org.in/text.asp?2012/35/3/135/117670


  Introduction Top


Electron accelerators are used for various applications such as medical uses, industrial uses and science. In addition, the electron energy range is from about a few MeV to several 10 GeV or more. Radiation safety design and the system; therefore, depend on the characteristics of the machines. Recently, many synchrotron radiation facilities have been constructed and operated in the world. Synchrotron radiation facility is a typical electron accelerator complex, which consists on a linac or microtron injector, a booster synchrotron and a storage ring. In addition to these, synchrotron radiation beamlines are important for radiation safety.

Synchrotron radiation is a powerful tool to investigate material science, biological science, etc. Thus, many users access synchrotron radiation facilities. The users; however, do not have to well-understand radiation safety or radiation protection. Radiation safety was crucial issue for synchrotron radiation facilities and therefore safety systems including a safety interlock system are very important as well as the training of radiation protection for users. Radiation safety system mainly consists of the construction of radiation monitoring systems, access control systems and shielding design of accelerators and beamlines. These systems must be linked with a safety interlock system effectively and organically with high reliability. Based on the As Low As Reasonably Achievable* (ALARA) principle, [1] the safety design of these systems is performed and the safety systems must be constructed so as to be optimized for the machines as much as possible.

The electron energy of synchrotron radiation facilities ranges from a few 10 of MeV to 8 GeV and a synchrotron radiation beam has up to several 100 keV energy with extremely intense and quite strong directionality. Thus, the design of radiation shielding must be carried out carefully and make full use so-call local shielding. At synchrotron radiation facilities, the safety systems of beamlines, including radiation shielding are quite important as well as that of the electron machines. Radiation sources at synchrotron radiation facilities are: (1) High energy photons, neutrons and muons produced by the interaction of high energy electrons with the accelerator components, (2) gas bremsstrahlung, which generates the interaction of stored electrons with residual gas within the storage ring and (3) synchrotron radiation. Because the characteristics of radiation sources are quite different from each other, different methods of analyses are employed, generally.

ALAP principle is employed at some countries like England.


  Safety System Top


Design criteria for radiation safety

Based on the ALARA principle and recommendations of International Commission on Radiation Protection (ICRP) as shown in [Table 1], design criteria are employed for each facility. For example, the design criteria of the dose limitation at SPring-8 are 8 μSv/h (40 h for 1 week radiological worker occupancy), 2.5 μSv/h and 100 μSv/y, for a radiation controlled area, the boundary of the controlled area and the site boundary of SPring-8, respectively. At the Stanford National Linear Accelerator Center (SALC), the design criteria are 10 mSv/y (5 mSv for 2000 h radiological worker occupancy), 1 mSv/y (0.5 mSv for 2000 h non-radiological worker occupancy) for normal operation, 4 mSv/h for miss-steering conditions and 250 mSv/h for system failure. [2] In addition to these design criteria, it is important whether the experimental hall is set in a radiation controlled area or not, which relates directly to whether the users must be radiological workers or not.
Table 1: Main ICRP recommendations for dose limitation of effective dose E, and equivalent dose HT[1]

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Radiation monitoring

Radiation monitoring is classified into two categories: One for personnel monitoring and another for area monitoring. Personnel monitoring is used to measure and confirm individually the personal dose and thus badge types are employed to be equipped on radiation workers. Now, optically stimulated luminescence, thermoluminescence detectors, film badges, [3] and Glass dosimeters, [4] etc., are used to measure the γ(X) ray dose. For doses due to the high energy neutrons, poly-allyl diglycol carbonate, neutron track detector is employed, mainly. In general, γ(X) rays are dominant at the experimental hall and therefore users are only equipped with the γ(X) ray dosimeters, if necessary.

Area monitoring is used to clarify the circumstances of radiation fields at a facility and thus monitors will be assigned at positions where the dose will be increased in comparison with the dose limitation and be linked to safety interlock systems. Ionization chambers are usually used for γ(X) ray area monitors. Plastic scintillation counters are employed at some facilities. [5] For neutron monitoring, Helium-3 or BF 3 proportional counters with moderators are used. Some facilities employ superheated drop detectors (bubble detectors). [6] In general, the fast response is required for area monitors to follow the fast events such as accelerated electron beam loss. Integral dosimeters are also employed to confirm cumulative doses.

Safety interlock system

The safety interlock system must be constructed with the high reliability and a fail-safe system; however, the system depends strongly on each facility design and the safety philosophy. A safety interlock system for synchrotron radiation beamlines is one of the key issues as well as the accelerator interlock system. Because a synchrotron radiation beam is extremely intense and dangerous, the beamline safety interlock system, such as an access control to each beamline hutch, is required in connection with the status of each beamline shutter. The conceptual main frame of the safety interlock system is shown in [Figure 1] as an example. The beamline safety interlock system is independent of each other and the accelerator. In the figure, the main beam shutter of a front-end component installed in the shield tunnel is to control so as to lead the synchrotron radiation beam into the experimental hall (optics hutch); the beamline is completely separated from the accelerator while the main beam shutter is closed. The equipments of the interlock system are linked fundamentally to a programmable logic controller using the hard wires to stop the machine operation infallibly. Multiplex systems are often employed, such as a door keep system, emergency buttons and a beam shutter system. It depends on the level of importance or hazard and the credibility of the system to be constructed to the multiplex system.
Figure 1: Conceptual main frame of a safety interlock system (Programmable Logic Controller; Full arrows mean to connect the interlock system)

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  Shielding Design Top


Leakage doses outside the shield tunnel of accelerators directly depend on the distribution of the electron beam loss. The amount of induced activities also depends on the beam loss. For the shielding design of synchrotron radiation facilities, three important radiation sources: (1) High energy γ-rays and neutrons due to electron beam loss including muons; (2) gas bremsstrahlung and associated neutrons; and (3) synchrotron radiation, must be considered. In genera: (1) High energy γ-rays and neutrons due to electron beam loss are more important for the accelerator shielding. On the other hand, (2) gas bremsstrahlung and associated neutrons and (3) synchrotron radiation, are more important for synchrotron radiation beamlines. Based on the beam loss scenario, there are two methods for the shielding design of synchrotron radiation facilities. One is to use Monte Carlo simulation codes; the other is to use analytical methods including empirical formula. These methods have the strengths and limitations; one using the analytical and empirical formula must beware of the coverage so as to adapt. Normally, two or three methods including different codes are performed independently and the results are close checked for shielding designs.

Beam loss scenario

The beam loss scenario depends on the machine design and the operation mode, especially, the normal injection or the top-up injection mode. Evaluation of the beam loss scenario; therefore, is very important to design the accelerator shielding and it must be performed based on the results of beam dynamics simulations while considering dark currents. Most facilities of synchrotron radiation consist of a linac injector, a booster synchrotron and a storage ring with synchrotron radiation beamlines. A microtron is employed for the injector at some facilities, such as BESSY II. [7],[8] For a linac injector, the beam loss is generally assumed to occur at beam slits and chicanes, like bunch compressors. For a booster synchrotron, the beam losses mainly occur at the injection and extraction septum and the remainder occur at the points with raising higher momentum dispersion functions. For the storage ring, the beam loss depends on the operation mode and occurs mainly at the injection region (septum). The remainder occurs mainly at the higher momentum dispersion function points. The injection efficiencies are important to make the beam loss scenario and the construction of safety systems. The normal beam loss scenario for typical synchrotron facilities is roughly summarized in [Table 2]. The safety shutter, which is located inside the shield tunnel, is opened during top-up operation so that high injection efficiency is generally required during operation to protect superfluously increasing leakage doses at synchrotron radiation beamlines. The measurement data of injection efficiencies at SPring-8 as an example, is shown in [Figure 2] during normal and top-up operations [13] . The injection efficiencies have been observed by using the differentials between the beam current monitor values of the beam extracted from the booster and the values of the DC current transformers for stored electrons. The efficiency during top-up operation is clearly higher than that of normal operation. The main reason is that an electron beam is collimated by using a scraper, which is installed in the beam transport line from the synchrotron booster to the storage ring, to execute the top-up operation to be more than the injection efficiency of 80%.
Table 2: Rough summary of normal beam loss scenario during normal operation of storage rings

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Figure 2: Measurement data of the injection efficiency distributions during normal and top-up operations at SPring-8.[13] These data were measured from 24th June to 8th July, 2004

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Monte carlo code

The Monte Carlo codes can simulate radiation transport with accurate geometry. However, more time is required to obtain the results with sufficient accuracy. Typical Monte Carlo codes, which have been used for the shielding design of synchrotron radiation facilities, are summarized in [Table 3]. The codes have characteristic differences from each other, such as the available nuclear reaction and the maximum electron energy etc., so that one must pay attention when choosing the code.
Table 3: Monte Carlo code for using shielding design of synchrotron radiation facilities

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Analytical and empirical methods for high energy γ-rays and neutrons due to electron beam loss

The distribution of high energy γ-rays and neutrons due to the accelerated electron beam loss is anisotropic, but very high in the forward direction; the tendency is stronger with increasing the electron energy-rays and neutrons due to the accelerated electron beam loss is anisotropic, but very high in the forward direction; the tendency is stronger with increasing the electron energy. [19] The shielding design and analyses using analytical and empirical methods; therefore, are normally considered for two directions, forward and lateral directions, from the electron beam. In these calculations, one must pay attention to the domain functions for the application of the formula.

Forward direction

Leakage doses of γ-rays and neutrons can be expressed by a simple formula. [20] γ-ray leakage doses can be calculated by one attenuation coefficient for each electron beam energy range as follows:



Furthermore, the neutron leakage dose in the forward direction can be calculated by two neutron energy groups as follows and the dose due to high energy neutrons is negligibly small in the case of less than 50 MeV electron beam energy;



Here, is the γ-ray dose in Sv/h, P is the electron beam loss power in kW, r is the distance from the loss point to the measurement point in m, d is the thickness of the shielding material in cm; λ γ, λnh and λnt are the attenuation length of the γ-rays, high energy neutron and the giant resonance neutron, respectively. These attenuation lengths are summarized in [Table 4]. H nh and H nt in Sv/h mean the dose due to high energy neutrons and the dose due to giant resonance neutrons, respectively.
Table 4: Attenuation length of the shield materials for the forward direction

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Lateral direction

The neutron and γ-ray doses for the lateral direction can be calculated by using an analytical code, SHIELD11, which was developed by SLAC National Accelerator Laboratory. [21] This code can calculate the leakage dose due to the beam loss, ranging from over 1 GeV to about 20 GeV electron energy. There are some empirical formulas for leakage dose calculations. [22],[23],[24] For example, the γ ray doses due to electron beam loss can be calculated as follows: [22]



Also, the neutron doses can be calculated as follows: [22]



Here, λ γ, λ1 , λ2 and λ3 are the attenuation length of γ-rays, high energy neutrons, giant resonance neutrons and intermediate neutrons as summarized in [Table 5], respectively. J is the amount of electron beam loss in s−1 , f1 and f2 are correction factors of source reduction for high energy and intermediate energy neutrons to calculate a lower electron energy of less than about 5 GeV. [25] θ and Φ are the inclined degrees from the electron beam axis to a measurement point and the shield material, respectively. Z is the atomic number of a target. The calculations were compared with each other including the Monte Carlo simulations and SHIELD 11, showing reasonable agreements. [26],[27]
Table 5: Attenuation length for lateral direction

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In case of light nuclide targets, the photoneutron production ratio is lower than that of heavy nuclide targets so that neutron leakage dose is reduced to be about the half of the values, [20] which can be obtained by using formula (2) and (3) or (5), including the forward direction case. In case of thin roof shield, the skyshine dose must be considered. [28]

Muon

Muons are produced by high energy photons in the Colomb field of the target directly and decay products of photo-produced π and K mesons. The threshold energy of muon production is about 211 MeV; the distribution has an extremely high peak in the forward direction. The dose due to muons can be calculated as follows: [22]



Where X is the thickness of the shield material and X 0 is its radiation length as summarized in [Table 6]. X (E e ) is the maximum possible muon range of the shield material as summarized in [Table 7] for various energies. In general, the muon dose is not negligibly small at synchrotron radiation facilities in comparison with that of other radiation sources, especially, facilities with electron energy of more than about 10 GeV.
Table 6: Radiation length of the shield materials

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Table 7: Maximum possible muon range

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Gas bremsstrahlung and associated neutrons

The combination of a long straight section for insertion devices, such as undulators and high energy electrons, generates gas bremsstrahlung by interactions of the stored electrons with residual gas molecules in a vacuum chamber of the storage ring. Many simulations of gas bremsstrahlung have been presented. [29],[30],[31],[32] A simulation of gas bremsstrahlung is normally carried out with the residual gas density corresponding to atmospheric pressure, because of the low interaction probability at the operating pressure (less than 133 nPa). The results are linearly scaled to the operating pressure of the storage ring. However, the scaling procedure is not necessarily always possible. Since multiple Coulomb scattering and Møller scattering are practically negligible in the storage ring, whereas these scatterings occupy most of the interactions at atmospheric pressure. In order to obtain accurate fluence values and the emission angle distribution of the gas bremsstrahlung, it is necessary to eliminate multiple Coulomb scattering and Møller scattering; also an electron path length of less than about 10−2 g / cm 2 is required to fulfill the simulation conditions. [33] The emission angle distributions of gas bremsstrahlung for stored electrons of various energies are shown in [Figure 3] by using the EGS 4 Monte Carlo code. [34] These simulations were performed without considering the stored electron conditions. The electron beam size and the beam divergence, including the effect of insertion device magnets, must be also considered to reproduce the measurement data of gas bremsstrahlung. [35],[36]
Figure 3: Emission angle distribution of gas bremsstrahlung for various stored electron energies with a path length of 1 m and a gas pressure of 0.1 atm (0.013 g/cm2), allowing only a single interaction of the electron

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An analytical formula for the spectrum of gas bremsstrahlung is given as follows: [37]



Where, E is the photon energy and dN/dE is the number of the photons within dE; a is the fine structure constant (1/137), γe the classical electron radius (2.82 × 10−13 cm), Na the Avogadro constant (6.02 × 10 23 ) and A the atomic weight per unit mol and ν is E/E 0 (E 0 equal stored electron energy). Z is the effective atomic number of the residual gas components under the assumption of monatomic molecules and normally set to 10. The effective Z obtained by fitting the observed spectrum on the assumption of monatomic molecules is 8.1 ± 0.3 at SPring-8. [35]

Neutrons due to photo-nuclear reactions are generated by the interaction of gas bremsstrahlung with beamline components, such as photon stop. The neutron production process is the same as the process due to high energy electron beam loss. The neutron yield and the neutron dose due to the giant resonance production process were normalized by the stored current (A), the electron energy (GeV), the residual gas pressure (μPa) and by the length of the straight section (m). These are presented in a figure as a function of the atomic number of the thick target. [30] For example, 0.13, 0.22 and 0.58 μSv/h at 1 m for aluminum, iron and lead thick targets, respectively. The underestimation of the dose due to only considering the giant resonance production process is less than 20% in the lateral direction at the SPring-8 case. [33]

Synchrotron radiation

A synchrotron radiation beam has an extremely high power density and high brilliance with polarization and quite a narrow beam with strong directivity, so that the scattering area of the scatterer (most of the cases are optical elements, such as monochromator or mirror) within the beamline can be restricted. In addition, the spectrum depends strongly on the type of synchrotron radiation source device, such as wiggler, undulator or bending magnet; most of the synchrotron radiation energy is less than 100 keV. A general purpose radiation transport code, especially, the Monte Carlo code, is unqualified to adapt to beamline shielding calculations for synchrotron radiation because of the necessity of strong attenuation. The point kernel and single scatter methods are quite useful and some codes have been developed. [38] Using this code, the leakage dose outside the hutch of the beamline can be easily calculated while considering the polarization and buildup effects. [34] The spectra of synchrotron radiation beams, for example, are shown in [Figure 4]. The key parameters of the synchrotron radiation light sources of SPring-8 and SLAC Stanford Synchrotron Radiation Light source (SSRL) are indicated in [Table 8], including the distance from the synchrotron beam to the shield wall of the hutch and the thickness. Making the thickness of the side wall (T 0 ) sufficient to prevent leakage due to scattered photons is a practical consideration for the shielding design of each optics hutch. The distances from the synchrotron radiation beam to the hutch side wall and the wall thicknesses given in [Table 8] are tentative values. The leakage dose distributions were calculated by using the parameters in [Table 8]; the geometries are illustrated in [Figure 5]a. In this figure, the synchrotron radiation beam emitted from the ID hits the scatterer, such as optical elements and the scattered photons are shielded by the hutch wall. In the case of a calculation of the leakage dose due to scattered synchrotron radiation, the leakage dose outside the hutch is normally estimated while considering the polarization effect, buildup effect and self-shielding effect of the scatterer. These effects depend strongly on the photon energy; however, the buildup effect has the potential to cause misleading indications of serious conditions. The leakage dose distributions outside the shield wall with and without consideration of the buildup effect at the estimation points are shown in [Figure 5]b as a function of the scattering angle from the beam direction of travel. These distributions show a strong dependence on the scattering angle because of the slant length of the shield wall and the scattered photon energy dependency on the scattering angle. In comparing the SSRL with the SPring-8 beamlines, the effect of the buildup on the SSRL beamlines are obviously higher than that on SPring-8. In order to find the mechanism of the differences, the dependence of the buildup effect on the wall thickness is shown in [Figure 6] as the respective ratio of the maximum leakage dose with and without consideration of the build-up effect. In this figure, the horizontal axis shows the shield thickness of the hutch wall relative to T 0, indicated in [Table 8]; on this axis are the shield thicknesses of the actual facility of each beamline. The vertical axis shows the ratio of the maximum leakage dose considering the buildup effect and the maximum leakage dose and not considering the buildup effect. It is clear that there is some buildup effect in the SPring-8 beamline, but it is less than twice the dose calculated without considering the buildup effect. However, the buildup effect is obviously important in the SSRL beamline cases. This means that serious consequences may occur if the shielding design of 3 GeV class beamlines is performed without considering the buildup effect.
Table 8: Key parameters for buildup calculation of the light sources of Spring - 8 and SSRL

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Figure 4: Angle integrated synchrotron radiation flux for the SPring-8 and Stanford synchrotron radiation lightsource typical beamlines

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Figure 5: Leakage dose distribution outside the hutches with and without consideration of the buildup effect (b). Solid line with dot and dashed lines are the dose distributions with and without consideration of the buildup effect, respectively. The calculation conditions are those indicated in Table 8 for each beamline of SPring-8; the geometries are illustrated in (a)

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Figure 6: Dependence of the buildup effect on the shield thickness of the hutch side wall. T0 means the thickness of the actual facility as indicated in Table 8. The solid circles, open circles, double circles and open squares are for SPring-8 BL02B1, BL08W, BL45XU and BL47XU beamlines, respectively. The solid diamonds and double diamonds are for the SSRL SLM and BL-11 beamlines, respectively

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A synchrotron radiation beam with high-power density must take measures to protect against any thermal problem when a white beam has a probability to directly hit safety components of the beamline, such as the safety shutter and the photon stop, especially for components made of lead.

The high-power synchrotron radiation beam brings about new shielding problems, which have been usually neglected in shielding designs for the existing facilities. This is an issue of synchrotron radiation scattered by a concrete floor of the beamline hutch or so-called ground shine. The reason why the leakage dose due to the ground shine will be higher is that a high Z material, such as lead, is used for the hutch shield wall to save space. There are three ways to protect against ground shine: (1) One with a lead rectangular plate installed along the base of the inside of the hutch wall, (2) one with the bottom part of the hutch wall embedded in the concrete floor and (3) one with the same plate outside of the hutch wall as shown in [Figure 7] and (4) the plate installed along the base of the outside of the hutch wall is most effective in three configurations. [33]
Figure 7: Illustration of the concept of ground shine and three protection methods

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  Induced Activity Top


High energy electrons of about over 10 MeV can produce radioactive materials. Therefore, at a high energy, high-power machine must take care concerning induced activities within the machine components, including the dump, air, cooling water and shielding materials, including concrete. The convenient method to estimate the induced activities is summarized in reference. [20] Furthermore, the important nuclei of air and water- induced activity are ( 3 H, 7 Be, 11 C, 13 N, 15 O, 16 N, 38 Cl, 39 Cl, 41 Ar) and ( 3 H, 7 Be, 10 C, 11 C, 13 N, 14 O, 15 O), respectively. The amount of induced activity directly depends on the electron beam loss and neutron production. Therefore, it is important to reduce the unwanted electron beam loss and to reduce the neutron production at a beam dump by using light nuclei, such as graphite, for the dump core.


  Summary Top


Radiation safety for electron accelerators, mainly synchrotron radiation facilities is overviewed. At a synchrotron radiation facility, the radiation safety of synchrotron radiation beamlines is important, just as same as for accelerators. Especially, since many users access to the beamline hutches, a safety interlock system is required to be of high reliability. For the shielding design of the beamline, three radiation sources must be considered: High energy radiation and neutrons due to accelerated electron beam loss, gas bremsstrahlung and associated neutrons and synchrotron radiation. At a beamline, the leakage dose due to gas bremsstrahlung and synchrotron radiation are dominant and because both radiation sources have high directivity, it is effective to use a so-called local shield. To construct an effective and reliable safety system, close communication between radiation safety physicists, accelerator physicists and the beamline physicists is important.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]


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Abstract
Introduction
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Shielding Design
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