SLAC MEMORANDUM RSC-07-03
August 28, 2007
September 19, 2007 (revised * )
TO:
FROM:
Distribution
Johannes Bauer
SUBJECT: Radiation Safety Committee Meeting — May 30, 2007
LCLS Front End Enclosure: Radiation Shielding by Wall 1 and Wall 2
Attendees:
Members: W. Wisniewski, S. Rokni, T. Fieguth, M. Kelsey, M. Saleski,
J. Hastings, J. Schmerge, D. Schultz ** , B. Hettel
Others: A. Prinz, T. Fornek, P. Emma, H. Tompkins, M. Santana,
J. Welch, S. Mao, J. Liu, T. Sanami, J. Bauer
Summary
The main subjects of the meeting were the designs of the bulk shielding of Walls 1 and 2, which define the upstream and downstream boundaries of the Front End Enclosure
(FEE). Located 16 feet underground, Wall 1 separates the beam dump area at the end of the Undulator Hall (UH) from the FEE. Further downstream, Wall 2 separates the FEE from the Near Experimental Hall (NEH). The committee was asked to review and approve the design and implementation of the bulk shielding of Wall 1. The design for
Wall 2 was only shown for information, not for approval. Also presented for review, not approval, was the estimation of Bremsstrahlung power reaching the FEE.
After an introduction by Sayed Rokni, Jim Welch laid out the construction plans for
Walls 1 and 2. Paul Emma described the sources of beam loss. The results of the dose rate calculations were then presented by Mario Santana together with the estimates of
Bremsstrahlung power reaching the FEE.
The committee approved the design and implementation of the bulk shielding for Wall 1.
The committee requested LCLS to implement adequate quality assurance procedures for the construction of the walls, especially to ensure the gaps in the shielding are properly staggered. They suggested configuration control for the beam finding wire system to ensure that only approved wires would be placed into the beam. The committee also asked LCLS to provide over time more accurate estimates of the usage of and power loss in inserted devices. Radiation Physics was asked to submit an explanation for a strong drop in dose rate seen at Wall 1.

S. Rokni: Introduction
In his introductory remarks, Sayed Rokni reminded the committee that the Front End
Enclosure is like the optical hutches in SSRL, the area right upstream of the experimental hutches. In the FEE, the dose rate limit is set to 0.5 mrem/h, since this area has to be accessed only by people, who may be required to have at least GERT training. Behind the FEE, in the Near Experimental Hall, the dose rate limit is much tighter, 0.05 mrem/h, since the users there shall not be required to have any training
An important question looms over the whole analysis: How much Bremsstrahlung will be emitted by the beam? Since no facility similar to LCLS has been built, we cannot base our estimates on any experimental guidance.
Wall 2 is presented for information only, not yet for approval, since for Wall 2 more calculations and also more meetings with LCLS will be needed.
Jim Welch: Wall 1 and Wall 2 Engineering Design
Jim Welch presented in his talk the engineering design for Wall 1 and Wall 2 [1]. Wall 1 is proposed as a 4 ½ feet thick steel wall followed by a 3 feet thick concrete wall. Due to gaps, the thickness of the steel is equivalent to 4 feet of pure steel. Jim Welch pointed out that the concrete borders at the edges and earthquake braces are missing from the drawings he presented.
The steel part of the wall will be made out of big 1-inch thick steel plates, which are already cut and ready for installation. The hole for the beam lines will be tapered by having different size holes in the steel plates. With a complementary shaped steel plug inserted, the cracks between the wall and the plug will be staggered and will not line up.
The opening in the concrete will look similar and will be filled with concrete plugs out of heavy concrete.
A question was raised about the tolerances of the steel plates. The plates, Jim Welch said, were flame-cut and hence have a tolerance of about ¼ inches, which should be good enough for the ¾ inch distance between the gaps. The individual plates are not bolted together. Instead a restraint system is holding the plates together. Alignment is done via rails on both sides.
Wall 2 looks very similar, but besides the +0° beam lines, two more low-energy beam lines will pass through it. The steel will be only an equivalent of 3 feet thick. The same staggered holes with plugs will be inserted into the steel walls, but holes for beam lines will be drilled through the concrete wall after construction. The steel plates for Wall 2 already arrived and are also already cut to shape.
Sayed Rokni stressed that good quality assurance for the wall construction, especially regarding the gaps, will be essential. If any gap would be detected in a survey, the beams will have to stop already at the Beam Switch Yard until a remedy has been implemented.
In the discussion, Sayed Rokni was informed that formal QA is not yet part of the contract, but that it will be part of the upcoming contract discussions.
Action Item 1: LCLS will implement adequate quality assurance procedures for the construction of the walls to ensure the gaps in the shielding are properly staggered.
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One foot space next to either wall is reserved for additional shielding, if such shielding turns out to be needed. Any equipment close to that wall, like a collimator, must respect that contingency for the shielding and must be placed far enough from the wall. Jim
Welch stated that if certain earthquake restraints will be in the way, these parts of the concrete can be removed with a jack-hammer and relocated.
Paul Emma: Shielding Design Basis at the Wall 1
Paul Emma’s talk [2] focused on the sources for beam loss, against which Wall 1 will have to provide shielding, and introduced the scenarios (including their expected frequencies) under which such losses would occur.
For lateral radiation shielding design, a loss of maximal 5 W is assumed, except at the
Beam Transport Hall (BTH) collimators, where with local shielding 20 W loss will be permitted.
In the forward direction, a total loss of 20 W is assumed in the forward direction, created by several devices between the end of the Linac and the FEE. Some of these losses will occur during normal operation: the energy-adjustable collimator CED, the horizontally adjustable collimator CX35, the fixed-aperture protection collimator PCMUON, and the vertical dump dipole magnet BYD1. Other beam losses (listed in Table 1) will occur only during times of beam diagnostics, when devices are inserted into the beam for short times, or when the beam is terminated by a dump upstream.
Device name wire scanners
WS31 to WS34
Beam finder wires
BFW
Table 1: Estimated frequency of beam losses due to diagnostic devices during commissioning
Number of devices
Frequency of usage during commissioning
(per device)
Fraction of beam interacting with device
4
33
10 scans/day
1 scan/day
0.15%
0.05%
Optical Transition
Radiation (OTR) screens
Abort Kicker and Single
Beam Dump (SBD)
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1
Tune-up Dump (TDUND) 1
5 insertions/day
1 abort/sec
2 hours/day
0.12%
3.3%
8%
In front of the undulator will be four wire scanners, WS31 to WS34, with 20µ m carbon wires. They will be used during commissioning for 10 scans per day in X and Y at 30 Hz beam rate, leading to about 0.15% of all beam pulses hitting one such wire.
During alignment, Beam Finder Wires (BFW) will look for the beam inside the undulators during alignment. Two wires, one each for X and Y directions, will be inside each of the 33 undulator segments. Initially the BFW systems will be equipped with the
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thicker 40µ m carbon wires for a large signal, but later hopefully 20µ m thick carbon wires will suffice. Such a scan will not be done frequently, since the whole undulator part will have to be moved during the scan. When learning that only about 20 pulses would interact with each scan wire during one scan, Tom Hettel commented that this number is quite low and that hence a bigger safety margin might be needed for this estimate.
Each of three Optical Transition Radiation (OTR) screens with very thin Aluminum foils are to be used about five times each day to measure energy spread and emittance. Such screens already work fine in the injector and are envisioned to be frequently used during commissioning. Responding to a question, Paul Emma said that these screens are currently in the beam for about one minute. Soon the procedure will be automated such that the image is being digitized for later off-line studies, which will reduce the time for the OTR screen to be inside the beam.
Quite far away, 250m upstream of the undulator entrance, will be the Abort Kicker and
Single Beam Dump (SBD), while the Tune-up Dump (TDUND) will be very close to the undulator entrance. The latter dump will be used much less frequently after commissioning. David Schultz wondered whether stoppers at the FEE would need to be inserted, if the Tune-up Dump would be inserted all day. Stan Mao: No, the stoppers would not need to be used in that case.
The discussion after the talk touched upon the difficulty to provide at this time reliable estimates for the frequency that beam diagnostics will be used. Since these numbers are essential input for radiation shielding estimates, Sayed Rokni promised that Paul Emma will return in the future with updated estimates.
Mario Santana: Dose Rate Levels and Bremsstrahlung Power in the Front End
Enclosure
This talk [3], Mario Santana’s first during the meeting, covers Wall 1. It also touches upon the estimation of Bremsstrahlung power reaching the FEE, but only a review by the
RSC, not an approval, is requested.
Bulk shielding of Wall 1:
Table 2 lists the beam power at five fixed and inserted devices, which Mario Santana assumed to be present during normal operation. He pointed out a few simplifications he introduced for the simulation. For example, only the beam finding wire of the last undulator section, Section 33, is included, and it also covers the OTR screen further upstream. Some beam loss scenarios were analyzed with the FLUKA, others with the
MARS15 simulation program. All calculations were reviewed in April by an external review committee [4].
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Table 2: Assumed beam power at loss points during normal operation
Loss point Beam power Comments
2 Tune-up Dump
3 Beam Finder Wire
20 W due to misaligned beam grazing at
0.1 mrad angle
420 W at 10 Hz
5,000 W
4 BYD1
5 W due to misaligned beam grazing at
0.1 mrad angle
5 Main Electron Dump 5,000 W
Continuous loss, causes
Bremsstrahlung, simulated with FLUKA
Simulated with FLUKA
Small loss by conversion to Bremsstrahlung, simulated with FLUKA
Continuous loss, causes
Bremsstrahlung, simulated with MARS15
Simulated with MARS15
Mario Santana mentioned that simulation with materials other than carbon as Beam
Finder Wires result in much higher dose rates. Paul Emma pointed out that the quoted
5,000 W beam would be for a 120 Hz repetition rate, but that one would always lower the frequency before such a scan. If necessary, the BCS would have to limit the frequency during such scans.
Among the studied situations are also some, in which components are misaligned. Such a misalignment of the last undulator section (with BFW33) is shown in the plot that displays all components included in the calculation. That plot also indicates the position of three PPS stoppers, TD-23 type stoppers out of Tungsten and Copper. In additional plots, details of the geometry were presented. The concrete at the side of the steel plates is in the simulation assumed to be heavy concrete, but no significant difference is seen to regular concrete. The RSC members were assured that the question, what would happen if the beam dump magnets might fail, will be part of another RSC meeting.
The summary on the dose rates is given in Table 2 together with the fractions of the time that the different dose rates are expected to appear.
The highest total dose in FEE is 0.34 mrem/h and occurs when the Tune-up Dump is out and a Beam Finder Wire is in. The largest dose due to collimators C31 to C38, maximal about 0.01 mrem/h, is caused by CY38, but this collimator will not be built due to budget constraints. Although the Tune-up Dump is contributing less than 0.05 mrem/h to the dose inside FEE, it was pointed out that the beam loss at this device is important because radiation damage to the undulator magnets might limit their lifetime. This topic will be studied in separate simulations.
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Loss point
Table 2: Results of dose rates at Wall 1 inside FEE
Dose rate in mrem/h
Beam power in W
Tune-up dump in
Tune-up dump out
BFW out BFW in
20 0 < 0.01 < 0.01
2 Tune-up Dump
3 Beam Finder Wire
4 BYD1
5 Main Electron Dump
420
5,000
5
5,000
< 0.05
0
0
0
0 0
0 < 0.10
< 0.03 < 0.03
< 0.20 < 0.20
Total
Time fraction
< 0.05
0.08
< 0.24 < 0.34
0.91 0.01
The dose rates due to BFW33 were also simulated with this undulator section in the rollout position and revealed no significant change in dose rate. Also simulated, but not shown, are results with beam finder wires out of Tungsten instead of Carbon. Since
Tungsten wire causes a 100 times higher dose, the question was raised how it could be avoided that over the 30-year lifetime of the facility someone accidentally installs a
Tungsten wire as a beam finder wire. Even though Jim Welch and David Schultz stressed that using such a wire would not be in the interest of LCLS and that the BSOICs would trip, the committee agreed that a control system, similar to ones already set up at other facilities, would be beneficial in reducing the likelihood of such mistakes.
Action Item 2: LCLS will implement configuration control for the BFW system to avoid that non-approved wires are placed into the beam.
David Schultz wondered about the strong reduction in dose rate, a drop by a factor 100, at the wall inside FEE, while the area below does not see a similarly large drop in dose rate.
It was pointed out that this might be due to certain cutoffs used in the simulation program, and also that at synchrotron light facilities similar drops by few orders of magnitudes are seen. While downstream the dose might be dominated by the hard-toshield muons, the dose at the side might be mainly due to easily stoppable low-energy photons. Radiation physics will submit a more detailed answer to the RSC.
Action Item 3: Radiation Physics will provide to the RSC an explanation for a strong drop of the dose observed at the Wall 1 shielding.
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Bremsstrahlung power into FEE:
All calculations presented in this section are based purely on Monte Carlo, since no experimental data exist. Besides the sources mentioned in Table 2, also the three OTR screens and four wire scanners WS31 to WS34 are considered.
Different situations were studied, like different size apertures, wire material, and magnetic field settings. Details can be found in RP Note RP-07-04 [5]. Using Tungsten wire would give too much Bremsstrahlung. Instead, Carbon would be the preferred material. When quadrupole magnets would be off, they would not focus the beam and instead let it interact with the beam pipe. The BCS system, the RSC was assured, would detect such losses along the line.
Continuous creation of Bremsstrahlung is dominated by beam loss at vertical bend magnet BYD1. As long as this loss does not exceed 5W, not more than 400 mW of
Bremsstrahlung power will enter the FEE. The loss due to inserted devices can reach up to 1,225 mW, when the beam is scanned at 120 Hz due with a BFW system. Since such measurements would, however, only be taken at 30 Hz, only 400 mW should be added to the 400 mW of continuously produced Bremsstrahlung power.
In conclusion, Mario Santana stated that the shielding by Wall 1 will keep the dose inside
FEE below the limit of 0.5 mrem/h, since the simulation predicts less than 0.24 mrem/h for normal continuous operation and less than 0.34 mrem/h with inserted devices (true as long as only carbon wires are used in the BFW systems). With not more than 5 W of loss at BYD1, the continuous Bremsstrahlung power reaching the FEE will stay below
400 mW, and any additional instantaneous Bremsstrahlung power will also be below
400 mW, as long as carbon wires are used at maximal 30 Hz beam frequency.
In the discussion, the validity of the time and power loss estimates for the inserted devices were questioned, especially in light of the 0.34 mrem/h dose rate, though still below, being quite close to the 0.5 mrem/h limit. Paul Emma thinks that the estimates are quite conservative, and that the 0.34 mrem/h dose rate is only possible during a short period of time. In order to ensure the dose rate in FEE will not exceed the limit of
0.5 mrem/h, LCLS is suggested to provide over time more accurate estimates how much power is lost through inserted devices and how frequently these devices will be used.
Ted Fieguth then raised questions on the assumption that the BCS can limit the loss at the various points, like the 5 W loss at the BYD1 magnet. The calibration of LIONs, he said, is not easy, a 5 W loss would require a more sensitive device like an ion chamber, and beam current comparisons with toroid measurements are too difficult.
Since no major objections were raised, the RSC approved the design and implementation of the bulk shielding of Wall 1.
Approval: The Radiation Safety committee approved the design and implementation of the bulk shielding of Wall 1.
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Mario Santana: Radiation Shielding for FEE Downstream Wall (Wall 2)
The presenter of this talk [6] should have been Alberto Fassò, but he is currently on leave of absence. Only a review of the design and implementation of Wall 2 is requested at this time, but not approval by the RSC. The design of Wall 2 is very similar to the one of
Wall 1, except that only an equivalent of 3 feet instead of 4 feet of steel is required. The hall following Wall 2 will be 15 feet tall with a second floor above a 3 feet thick ceiling.
The goal for the shielding at Wall 2 is a dose rate in the Near Experimental Hall of not more than 0.05 mrem/h, which corresponds to 100 mrem/year and does not require
GERT training for access. Before the X-ray beam passes Wall 2 and enters NEH, it is deflected inside FEE by two mirrors and passes through several collimators and a shadow wall. The simulation program FLUKA was used to estimate the dose rate from 400 mW of Bremsstrahlung reaching the FEE and producing showers there, while the simulation program MARS11 was applied to beam loss originating upstream of FEE, which causes radiation to spray through Wall 1 and Wall 2. Synchrotron Light and the Free Electron
Laser Light itself is not an issue for the Wall 2 design, but it will be for NEH.
400 mW Bremsstrahlung reaching FEE (FLUKA Simulations)
Different situations were evaluated, like normal beam situations, misaligned collimators, beam loss at different components, radiation passing through “real” walls, i.e. walls with gaps and penetrations necessary for earthquake braces, or beams being stopped by hutch shutters. It was pointed out while details on the designs are still changing, these calculations benefit the on-going discussions between Radiation Physics and LCLS.
Some calculations on whether the hutch shutters are strong enough to take the beams without being damaged are still missing, but Sayed Rokni stated that this issue will be handled separately, and that the hutch shutters only need to absorb a few watts anyway.
Three sets of calculations were presented: (A) every device perfectly aligned,
(B) collimator C2 misaligned by 10 mm, (C) collimator C2 misaligned by 10 mm with the wall modeled realistically in all details. Misalignment of C2 was chosen as representative case, since among all the collimators it is the one most easily creating radiation inside NEH.
For Situation A (normal operation with perfect alignment of everything), the goal of
0.05 mrem/h can be easily reached, if a small exclusion zone is established by a fence right where the beam enters NEH.
When collimator C2 is misaligned by 10 mm (Situation B), the dose inside NEH is not acceptable. This situation raises the question when what type of access may be allowed to NEH, but further discussion on remedies was referred to another meeting of the RSC.
The simulation shows that the wall shields well enough, but that mainly muons pass through the beam pipe, which makes shielding difficult. A design that tolerates some misalignment is, of course, most desired. Hal Tompkins asked whether a misalignment by 10 mm is the worst-case scenario, or whether configurations in between were also studied. No, such configurations were not studied, since one hopes to find a design that is able to deal with even the extreme case of a 10 mm mis-alignment. Of course, once the hutch shutters go in, the dose rate in NEH drops below the limit.
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The results for Situation C (same misalignment of C2, realistic wall model) reveals the same pattern as for Situation B, and one may conclude that the holes in Wall 2 have no significant effect on the dose rate in NEH.
In the upper floor of NEH, the dose rate is always far below the limit, even with collimator misalignment and no local shielding at the beam pipe.
5 W continuous beam loss at BYD1 (MARS15 Simulations)
The results from the MARS15 simulation were easily presented, since a continuous 5 W beam loss at the BYD1 magnet will lead to a very low dose rate inside FEE of not more than 0.0025 mrem/h.
In summary, Mario Santana stated that the design for Wall 2 is leading to a dose rate of less than 0.05 mrem/h in NEH if the beam loss at BYD1 is not exceeding 5 W, if interlocks ensure that diagnostic devices are inserted into beam at not more than 30 Hz beam repetition rate, if no high-Z material is inserted, if a 1-foot thick shadow wall is placed inside the FEE (at a location to be optimized via ray trace), if a small radiological exclusion zone is established in NEH right where the beam passes through the wall, and if the design is able to accommodate possible collimator misalignment with hutch shutters open. The hutch shutters themselves are suitable for the task.
During the discussion following the talk, it was commented that the question of how much Bremsstrahlung is produced is difficult to answer. Only a few weeks ago it was realized that the power loss has to be limited to 5 W at any loss point with a total loss of not more than 20 W. These limits, Paul Emma worries, might be too low.
The question was raised whether one can require the beam stoppers (PPS stoppers in front of Wall 1) to be in whenever the BTW system is used. Yes, this can be done, but then every BTW becomes part of BCS, creating a complicated situation.
Another question was under which circumstances one would use shielding and not fences for the area around the beam pipe in NEH. The answer was that it depends on the situation. Right now shielding with 14 inches of lead with a 45 cm lateral exclusion zone is proposed, but according to Jerry Hastings this design is still being discussed.
Sayed Rokni pointed out that the design also has to be convenient for the experiments.
Alyssa Prinz reminded the audience that mainly muons pass through the opening, and these are difficult to shield.
Sayed Rokni does not expect that Wall 2 will need to be thicker than currently proposed.
The same quality assurance as for Wall 1 will required. Also, more discussion will be needed on which areas may be occupied and accessed by whom. Even though the current plan is to allow free access to NEH, beam lines at 0° are still risky for users. Sayed
Rokni shared about an experience at FFTB, where once a collimator was placed into the beam line without proper authorization, which increased the dose rate downstream.
In an outlook to future meetings, Sayed Rokni promised that many more items not discussed so far, like BCS, configuration control etc., will be submitted to the RSC over time.
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Summary of Action Items:
Action Item 1: LCLS will implement adequate quality assurance procedures for the construction of the walls to ensure the gaps in the shielding are properly staggered.
Action Item 2: LCLS will implement configuration control for the BFW system to avoid that non-approved wires are placed into the beam.
Action Item 3: Radiation Physics will provide to the RSC an explanation for a strong drop of the dose observed at the Wall 1 shielding.
Summary of Approvals:
Approval: The Radiation Safety committee approved the design and implementation of the bulk shielding of Wall 1.
Attachments:
(available at https://www-internal.slac.stanford.edu/esh/committees/rsc/m_2007.htm):
1.
Presentation to RSC, J. Welch, “,Wall 1 & Wall 2 Engineering Design,”
May 30, 2007.
2.
Presentation to RSC, P. Emma, “ Shielding Design Basis at the Wall-1 ,”
May 30, 2007.
3.
Presentation to RSC, M. Santana Leitner, “ Dose Rate Levels and Bremsstrahlung
Power in the Front End Enclosure ,” May 30, 2007.
4.
Review of LCLS Radiation Shielding Calculations, Committee Report, April 5-6,
2007.
5.
M. Santana Leitner, " Studies on Bremsstrahlung sources in the BTH and LCLS undulator; Irradiation of the FEE by Bremsstrahlung beams from these sources, "
RP-07-04, in preparation.
6.
Presentation to RSC, M. Santana Leitner, “ Radiation Shielding for FEE
Downstream Wall (Wall 2) ,” May 30, 2007.
* Changes for Revision September 19, 2007: Clarifying differences between incident power and actual power lost at loss points. Removing typo on thickness of aluminum foil in OTR screens.
** Not voting on LCLS topic
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