Gas Bremsstrahlung at the Diamond Light Source. P. Bonner, R. Ryder

Transcription

1 Gas Bremsstrahlung at the Diamond Light Source. P. Bonner, R. Ryder Diamond Light Source Limited, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom. Abstract. The Diamond Light Source is currently under construction in the United Kingdom. It will be a 3GeV electron synchrotron which will produce pinpoint ultra-violet and X-ray beams of exceptional brightness for research purposes. Machines such as synchrotrons present challenging radiation safety problems. The same high energy electrons which produce the desired synchrotron radiation also interact with matter (for example, residual gas molecules in the vacuum vessel of the synchrotron), producing a spectrum of bremsstrahlung photons with energies up to the electron energy in this case 3GeV. The bremsstrahlung is highly penetrating, and it can produce exotic reactions in any material that it strikes, leading to the production of other radiations, including neutrons. As the demands upon synchrotron light sources have grown and stored beam currents have increased, so gas bremsstrahlung has ceased to be a theoretical health physics problem and has grown to be a dominant factor in determining radiation shielding requirements. Whilst the synchrotron radiation will be many orders of magnitude more intense than the gas bremsstrahlung, it is far less penetrating. Bremsstrahlung photons will pass down the beam line and can scatter off any material that they strike, including mirrors, slits and detectors. At some operational accelerators, scattered bremsstrahlung dose rates outside the beam line hutches are so high that the beam line cannot be used for several weeks after being commissioned. This is due to the vacuum not having reached its optimal level as the storage ring is still conditioning. This report reviews the current literature on gas bremsstrahlung shielding, and compares several different techniques used to estimate the expected levels of gas bremsstrahlung. It then attempts to quantify the potential dose rate and angular distribution of the bremsstrahlung, estimate the effect of vacuum pressure and conditioning time for beam lines, and assess the associated shielding requirements. Recommendations are made regarding the dimensions of beam shutters, stops and hutch walls for Diamond, to enable the facility to operate safely. 1. Introduction Gas bremsstrahlung is a well known phenomenon in electron storage rings. This highly penetrating radiation can be a serious radiation hazard, and adequate shielding is essential. This report attempts to quantify the potential dose rate and angular distribution of the bremsstrahlung, estimate the effect of vacuum pressure and conditioning time for beam lines, and assess the associated shielding requirements. A typical arrangement of a beam line is shown in figure 1.. Direct Gas Bremsstrahlung When high energy electrons interact with matter, bremsstrahlung photons will be produced (dominant process above 10MeV). The emitted photons will have an energy spectrum ranging up to the total electron energy in Diamond s case photons with energies up to 3GeV will be produced. Photons of these energies are highly penetrating, and thus can often be the dominant factor in determining radiation shielding. Gas bremsstrahlung needs to be shielded at various locations around the storage ring and on beam lines. Around the storage ring, the bulk shield is made of high density concrete. On beam lines, tungsten, lead or steel may be used as appropriate. In all cases, the shutters or stops on beam lines will need to be thick enough to reduce the dose rate to 0.5µSv per hour or less. 1

2 Ratchet Wall (Concrete) Experiment Hutch Flight tube Optics Hutch End Wall (Concrete) Dipole Insertion Device Dipole Port shutter Front end Beam Stop Experiment Experiment Shutter Ratchet Wall (Concrete) Optics Optics Shutter This diagram is not to a particular scale. The distance from the Insertion Device to the Beam Stop is typically of the order of metres. Hutches are usually between 5 and 10 metres long, and to 4 metres wide, and are of either steel or lead and steel construction. FIG. 1. Schematic of Beam Line, from Dipole to Beam Stop.

3 Throughout this report, the standard Diamond operating parameters of 3GeV beam energy and 500mA current are used for calculations unless stated otherwise. The poorest working vacuum pressure has been taken as 10-8 millibars (10-6 Pa). There are two main methods for analysing the gas bremsstrahlung around particle accelerators:.1. The first of these methods (Method 1) uses expressions, applied to specific accelerator situations to assess photon fluxes and thus shielding requirements. These treatments make use of the concept of Radiation Length. This is a thickness of material (grams per square centimetre) which will reduce the energy of an electron beam by a factor of e. The specific value of the radiation length depends on the atomic number of the residual gas in the vacuum vessel. As accelerators operate at conditions of high vacuum, radiation lengths are typically of the order of metres. It is important to bear in mind that gas bremsstrahlung treatment applies for thin targets only, so while radiation length is similar in concept to half value thickness as used in shielding applications, this is not the same. A typical formula for radiation length is [1]: 1 4N A =. Z( Z + 1). r X 0 137A where X 0 is the radiation length N A is Avogadro s constant Z is atomic number of residual gas r e is classical electron radius,.8 x m e 183.ln 3 Z (1) Method 1 tends to produce a multi-step process. First, radiation length is evaluated based on accelerator conditions. This is then applied to calculate radiation energy loss from the beam. The result is converted to equivalent quanta. At a specified distance downstream from the interaction point, photon flux is determined from knowledge of the bremsstrahlung divergence. A conversion factor is then used to evaluate dose-rate. It is possible to merge the steps into one equation, an example of which is shown below [] PIE0 L D = () X d.. The second method (Method ) applies established computer modelling codes to accelerators in an attempt to derive an expression for the dose rate in a semi-empirical manner. This treatment tends to produce a single equation which provides a figure for dose-rate as a function of beam energy, current, pressure, length of vacuum vessel straight and downstream monitoring point. In all cases, but particularly Method, where the approach is semi-empirical, the results tend to be best fitted to the conditions which are found on the author s local machine. Particular differences can be seen in energy dependence and geometry for length of vacuum vessel and downstream distance. A typical expression used in the Method 1 treatment is shown above. Some typical formulae for doserate by semi-empirical methods are as follows [3], [4]: 0 D = E m 0 0c.67 L P.. I. d( L + d) P 0 (3) 3

4 L / D =.4 10 PIE0 l (4) d Where: D = dose rate in micro-grays per hour. E 0 = electron energy in MeV. L = length of target straight in ring, metres d = distance downstream. The point from which d is measured varies discussion below. I = beam current (ma) P = pressure within ring (Pascals) P 0 = Pa Other variables as previously defined. Expressions (3) and (4) have a strong dependence on beam energy this is a consequence of the equations being best fit solutions to modelling predictions. Also, the dose-rate equations do not display a pure inverse square behaviour with distance. The semi-empirical method makes the approximation that the residual gas in the storage ring will be air, whilst the other methods attempt to assess the gas composition. Radiation length has a strong dependence on the atomic number, Z, of the gas. Typical average values of Z range from 4.6 to 10 [5]. Whilst it would be preferable to make an accurate measurement of the composition of the residual gas, this will not be possible until after the machine is operational. For this reason, it has been usual to see the approximation that Z=10 applied, as this can safely be assumed to be a conservative estimate. Current indications are that the residual gases in the Diamond storage ring are expected to be mostly methane and hydrogen, with carbon monoxide and carbon dioxide present in much smaller quantities. However, as there is no certainty as to the composition of the gas, where a radiation length has had to be calculated for application to Diamond machine conditions, Z has been approximated to 10. As mentioned above, it is often not explicit where the conceptual source point for the radiation is, some authors treating the mid-point of the straight as the source, others acknowledging that the point moves depending on the length of the straight section. Holbourn [], Tromba and Rindi [4], treat the source as being at the mid-point of the straight, and d is the distance from this. In Ipe s [3] treatment, the source position is variable, between half way and three quarters of the way to the downstream end, but the distance, d, is measured from the downstream end of the pipe. Where different treatments have been applied to Diamond, care has been taken to ensure that d is the same factor, always measured from the downstream end of the long straight. Table I below shows the gas bremsstrahlung dose rates predicted by 7 different methods (0 metres downstream from the end of an metre long straight under good vacuum conditions of mbar) and the thickness of lead or tungsten beam stop required to reduce that rate to 1µSv per hour. The estimated dose rates vary by just more than a factor of four. This is considered reasonable considering the wide range of variables used by the different authors. Not surprisingly, the beam stop thickness varies only by about 10 percent 70mm to 300mm for lead, 184mm to 04mm tungsten. For the shielding calculations an attenuation coefficient of 47.0 per metre is used for lead, 69.0 for tungsten. At relativistic energies, the photons will be emitted in a narrow cone about the forward direction of the electron beam. The divergence (half angle) of the bremsstrahlung photons is given thus [6]: µ θ = radians. (5) E 0 4

5 Where µ is electron rest energy and E 0 is total electron energy. In the case of Diamond, E 0 is 3GeV, and θ is 0.17 milliradians. Method (Author) Gas mixture Table I. Dose Rates and Shielding Requirements Calculated by Seven Different Methods. Holbourn [1] Rindi [7] Ipe [3] Holbourn [] Esposito et al [8] Tromba & Rindi [4] Ferrari et al [9] 60% CO Z=10 Air 60% CO Air Air Air 40 % H 40 % H N/A N/A N/A Radiation length, m Dose rate, mgy hr -1 Lead, mm Tungsten, mm Scattered Gas Bremsstrahlung In the early days of synchrotron radiation light sources, gas bremsstrahlung was treated as a theoretical health physics problem. Recently, stored beam currents have increased and there has been increased pressure to commission beam lines before the ring has pumped down to normal vacuum pressures. Typically, storage rings work at vacuums of around 10-9 mbar, but this level is only reached after much pumping and conditioning, the process whereby the vessels off-gas as current runs through the storage ring. The level of conditioning is measured in Ampere-hours of beam current. Typically 100 Amperehours of conditioning is required before the storage ring vacuum starts to tend towards the expected levels of 10-9 mbar. In the early stages of operation, the average vacuum around the ring will be poorer, around 10-7 or 10-8 mbar. As the pressure decreases, so the rate of pumping also decreases and it takes longer for each incremental improvement in the vacuum. Additionally, vacuum is not uniform around the storage ring. For example, the vacuum in the vicinity of a pump will be better than in the middle of a long narrow gap vessel. When the beam line shutters are opened, the bremsstrahlung will pass down the beam line. The high energy photons will scatter from any material that they strike, including mirrors, monochromators, slits and detectors etc. Some operational accelerators have experienced scattered bremsstrahlung dose rates which are so high outside the beam line hutches that the beam line cannot be used for several weeks following the commissioning period or whenever the storage ring vacuum vessel is let up to atmospheric pressure for maintenance or modification. Gas bremsstrahlung from relativistic electrons is very preferentially emitted in the forward direction. However, the photons can scatter from any solid object which the beam strikes, so shielding off axis needs to be considered. Scattered radiation is very much less intense than that in the forward direction, but it can still pose a significant hazard. This will be of greatest concern in beam line hutches, where the beam is deliberately directed onto scattering targets such as slits or mirrors. Furthermore, whilst gas bremsstrahlung is forward directed, scattered bremsstrahlung will travel in all directions hence all hutch walls and roofs need to be completely shielded. Several cases have been studied: 1. Ipe et al [10] used an EGS 4 (Electron Gamma Shower) simulation of the conditions at the Advanced Photon Source to assess beam line shielding requirement. This considered a simulated optics hutch 7.7m long, 1.m from beam line to lateral wall, 1.5m to roof, situated 31 metres downstream from a 15 metre long straight with the main beam scattering from a 5cm long copper block. The APS machine parameters used were a beam of 7GeV and 300mA. The study found that the level of scattered gas bremsstrahlung incident on the back wall outside of the beam stop would require the back wall to be shielded with more than 110mm of lead. In order to avoid this, it was recommended that the beam line was collimated, so as to shadow the back wall from scattered 5

6 photons. This would then reduce the shielding required on the back wall to 50mm of lead all over, with 100mm over the central 1m. Collimating the beam would also have the effect of reducing the scattered gas bremsstrahlung dose rate at the lateral walls and roof, to the extent that synchrotron radiation would dominate the shielding requirement there. The recommended shielding for lateral walls and roof under these conditions was 19mm and 1mm respectively.. Ipe and Fasso [3] studied the APS again, this time using the FLUKA code. The hutch model was the same as the previous example. Figure shows the variation of the calculated dose rates with scattering angle calculated in Ipe s studies (dose rate assessed 1 metre from the scattering point, machine conditions are 7GeV, 300mA, mbar). A typical beam stop only covers a few degrees of beam width, yet as can be seen from the graph, significant amounts of radiation will be scattered through angles of 10-0 degrees or more. This has a major impact on the shielding required for the back wall of the hutch. Dose rates at 90 degrees to the beam are also significant. 5mm of lead is needed all over for the lateral walls and roof of the hutches, with more in the forward and less-than-45-degrees-fromforward directions. Scattered Dose Rate at Various Angles from Different Materials Dose Equivalent (micro-sieverts m/hr) 1.E+04 1.E+03 1.E+0 1.E+01 1.E+00 1.E Angle, degrees Tungsten Copper Lead FIG.. Variation of calculated dose rates with scattering angle. 4. Neutron Shielding When high energy bremsstrahlung photons are incident upon an absorbing material, neutrons can be produced via the (γ,n) reaction (Giant Resonance reaction). This can be an issue in beam stops. Generally, neutron production rates are quite low. Most authors do not regard the dose rates as significant, although in the case of the tungsten beam stop considered by Nisy Ipe [10], it was recommended that the beam stop should be shielded locally with 150mm of polythene. Liu, Nelson and Kase [11] do provide a simple treatment for calculating neutron dose from power radiated. Using parameters for the Diamond ring, the neutron dose rate should be less than 1µSv per hour 1 metre from the scattering point (i.e. within the hutch). Neutrons are not anticipated to be a significant hazard at Diamond. 5. Experimental Measurements Holbourn [] made measurements of dose-rate at ESRF, comparing the predictions of four different calculation methods against actual dose-rate. Thermoluminescent dosimeters (TLD s) were used to measure the dose-rate directly behind a beam stop. The four methods predicted dose-rates for direct 6

7 gas bremsstrahlung ranging from 168 up to 590µGy h -1 ma A rate of 66µGy h -1 ma was measured. Comparisons by other authors also tend to show that for direct gas bremsstrahlung, measured dose-rates are usually lower than predicted. Experience [1] at ESRF with stainless steel vacuum vessels has demonstrated a problem with hutch wall shielding, to the extent that all the hutches on insertion device beam lines needed to have an additional 0mm of lead retro-fitted to the walls, as the initial 10mm proved inadequate. Insertion device vacuum vessels at ESRF have now been coated with Non Evaporable Getter (NEG) which has solved some of the problems by reducing the pressure in areas of the storage ring which were previously difficult to pump by conventional methods. 6. Shielding Recommendations for Diamond As in all radiation shielding considerations, there is a cost-benefit to be balanced. In this case, the level of radiation generated by the machine will be a function of vacuum and current. It is known that the vacuum will not be at ideal levels initially, and this is where the trade off needs to be made between the expense of over shielding against the time cost of waiting for the vacuum to improve to the level where the radiation generated is within tolerable limits. The scale of the bremsstrahlung problems experienced around the hutches can be effectively reduced by collimating the bremsstrahlung beam to the minimum consistent to the width required by the experiment. This should be done within the front end if possible. This will require thicker collimators and absorbers than those required to limit the synchrotron radiation beam. Below, the radiation safety components of a beam line are considered in turn. The Diamond machine parameters used for these calculations are 3GeV, 500mA and 10-8 millibars. It is expected that the average storage ring vacuum will be between mbar and mbar. This is a dynamic quantity which is also difficult to measure accurately, as constraints on space mean that it is rarely possible to place a vacuum gauge at every location in the ring where it would be desirable to measure the vacuum. The amount of gas bremsstrahlung produced is directly proportional to the vacuum pressure, so a conservative value of mbar has been chosen for the calculations. After around 100 Amperehours of conditioning, a storage ring vacuum level of around mbar should be attained. A poorer vacuum level is used in the calculations to make some allowance for the extra radiation which will be produced in the early conditioning of the machine, as has previously been discussed Shutters and Beam Stops Generally, the shutter or beam stop needs to reduce the dose rate to less than 0.5µSv hr -1 under normal operating conditions. The thickness of lead or tungsten needed to achieve this has been calculated and is given in table II. The port shutter is the first shutter which the beam encounters on leaving the storage ring. As mentioned above, the storage ring will not achieve a vacuum pressure of mbar until it has conditioned. During conditioning, much more bremsstrahlung is likely to be produced as vacuums poorer than mbar may be experienced. Thus the port shutter thickness has been calculated with an average storage ring vacuum of mbar. The port shutter is located metres downstream from the end of the insertion device straight, which has a length of metres. All other shutters and the beam stop are specified as having the same thickness, irrespective of the downstream distance. Table II. Calculated Shutter and Stop thicknesses in tungsten and lead. Shutter Minimum shutter or stop thickness, mm tungsten Minimum shutter or stop thickness, mm lead Port Other / Stop

8 6.. Ratchet Wall Bremsstrahlung radiation will be most intense at the end of long straights, but as radiation will be emitted from dipoles all the way around the ring, shielding will be required to completely enclose the ring this is the so-called ratchet wall shielding. The lower intensity but wider distribution of this radiation means that concrete is likely to be used as the shielding material. It has been previously been calculated that a thickness of 1.55 metres of barytes concrete will be required on the ratchet end walls of insertion device beam lines [13]. Normally, bremsstrahlung photons will go straight down the beam line and should be adequately shielded by shutters and stops. If there are any circumstances where the bremsstrahlung from a straight section can be incident on the ratchet end wall, the wall s shielding will need to be supplemented with 100mm of lead Hutch Walls This is the most difficult thickness to quantify. There is very poor agreement between reported calculations and measurements. The scale of problems between different light sources also varies. Based on experiences at other facilities as detailed in section 3, recommendations are made for the required quantities of lead needed in walls and roofs of hutches on Diamond insertion device beam lines. These figures are given in table III. As an alternative to lead, steel is being considered for hutch wall shielding. At high energies, steel is approximately half as effective as lead as a radiation shield. However, steel may have some advantages in terms of engineering and cost, so figures for steel are also quoted. These thicknesses are probably an overestimate but have been proposed to reduce operational problems and eliminate the need for any retro fitting of extra shielding. Table III. Proposed Hutch Wall Shielding Specification Shielding Type Upstream Lateral Wall Roof Downstream Wall Wall Lead 50mm 50mm 0mm 75mm (15mm locally) Steel 100mm 100mm 40mm 150mm, (50mm locally) 7. Conclusions There is strong evidence that the extent of gas bremsstrahlung problems varies widely on synchrotrons around the world. Severe problems have been encountered at the ESRF, whilst Elettra has not had such difficulty. Gas bremsstrahlung decreases as operators and health physics groups gain experience at established synchrotron radiation sources. Occasionally, more intense problems arise. An example of this is the much wider angular distribution of gas bremsstrahlung seen on beam line 10 at the Daresbury Synchrotron Radiation Source [14]. Many factors can affect the intensity of the scattered radiation. While simulations have been shown to predict dose-rates to a fair degree of accuracy, certain factors cannot be predicted. Of particular concern is the level of vacuum in long narrow insertion device straights of the storage ring. For these calculations, a vacuum of 1x10-8 millibars has been used. The vacuum will be worse than this during the initial conditioning of the machine, until an integrated beam current of several tens of Amperehours has been accumulated. Gas bremsstrahlung levels will also be elevated during periods of short beam lifetime, such as following long shut downs. In the early stages whilst the storage ring is undergoing conditioning, it will be necessary to run the machine with the shutters closed and possibly to restrict access to hutches until sufficient experience has been gained of the machine s performance. The machine is likely to require several tens of Ampere-hours of conditioning before the vacuum has reached an adequate level to permit the port shutters to be opened without producing excessive 8

9 radiation. This may appear restrictive, but it would be prohibitively expensive to completely shield out all of the radiation, and these effects should be transient. Extensive measurements will need to be made once the machine starts operation. Full conditioning of the machine will not be completed until at least 100 ampere hours are reached. The hutch shielding thicknesses presented here may appear excessive. It must be remembered that they have been quantified to reduce operational constraints due to high bremsstrahlung dose rates in and around the hutches. It is hoped that this shielding will be more than adequate and thus avoid the need for additional shielding to be retro-fitted. References. 1. Holbourn, M. P. Gas Bremsstrahlung Production in the SRS. Health Physics Note HP81/139, Holbourn, M. P. Gas Bremsstrahlung Measurements at the ESRF, Ipe, N and Fasso, A. Impact of Gas Bremsstrahlung on Synchrotron Radiation Shielding at the Advanced Photon Source. SLAC-PUB-6410, Tromba, G and Rindi, A. Gas Bremsstrahlung From Electron Storage Rings: A Monte Carlo Evaluation and Some Useful Formulae. Nuclear Instruments and Methods in Physics Research A9, Asai, J. Design Study of Gas Bremsstrahlung Beam Stop for Beam lines at the Canadian Light Source. KEK Proceedings 00-18, p.37-47, Heitler, W. 1954, The Quantum Theory of Radiation, Oxford Press. 7. Rindi, A. Gas Bremsstrahlung from Electron Storage Rings. LNF-80/56(P), INFN Laboratori Nazionali di Frascati, October Esposito, A and Pelliccioni, M. Gas Bremsstrahlung Production in the ADONE Storage Ring. LNF-86/3(NT), Fracsati, Italy, Ferrari, A. Pelliccioni, M. and Sala, P.R. Estimate of Fluence Rate and Absorbed Dose Due to Gas Bremsstrahlung from Electron Storage Rings. LNF-93/0168, INF, Frascati, Italy, Ipe, N, Haeffner, D. R, Alp, E. E, Davey, S. C, Dejus, R. J, Hahn, U, Lai, B, Randall, K. J, Shu, D. Guide to Beam line Radiation Shielding at the Advanced Photon Source. ANL/APS/TB-7, Liu, J.C, Nelson, W. R, and Kase, K. R. Gas Bremsstrahlung and Associated Photon-Neutron Shielding Calculations for Electron Storage Rings. Health Physics 68 () 05-13, Ryder, R and Holbourn, M. Report of Visit to Elettra and ESRF. CCLRC. January Diamond Synchrotron Light Source. Report of the Design Specification. Volumes 1 and. CCLRC Rutherford Appleton Laboratory, Chilton, OX11 0RQ Ryder, R. Personnal communication. 9