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9.3 Facility requirements (access, personal protection, radiation
shielding and beam containment, toxic materials)
9.3.1 Requirements and calculations
The radiation concerns related to the LCLS project fall into three distinct areas:
• radiation safety
• radiation background in experiments
• machine protection
In each of these areas one or more of the following radiation sources needs to be
considered:
1) bremsstrahlung from beam/halo interactions with beamline components
2) gas bremsstrahlung
3) synchrotron radiation
4) neutrons
5) muons
6) induced activity
7) secondary electrons and positrons
9.3.1.1 Photons
We are concerned with radiation effects in the areas downstream of the undulator,
taking into account any relevant radiation source, located upstream, inside, or
downstream of the undulator. Potentially all sources, 1 through 7 above, can contribute
to the radiation background in experiments. Machine protection mainly concerns
possible radiation damage to mirrors and collimators which will intercept the
spontaneous synchrotron radiation from source number 3, accompanying the laser beam
proper. The radiation safety items include shielding, beam containment system (BCS)
and personnel protection system (PPS). Since the LCLS electron beam power will be
comparable to that of the FFTB, the existing enclosure and dump shielding as well as
PPS and BCS should be adequate without major modifications. New designs of the three
safety items are only required for the optics enclosure and experimental hutch
downstream of the electron beam dump, taking into account radiation sources 1 through
6. The ultimate goal is to have sufficiently accurate estimates of radiation sources 1
through 7 and their effects. Initial modeling of some of the dominant sources has been
completed and the status of the calculations is summarized in the text below. More
comprehensive calculations, as described below, will be continued in the future.
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9.3.1.1.1 Bremsstrahlung from collimators
Two copper collimators, each 10 cm long and with an internal diameter of 0.2 cm,
will be placed between the dog-leg and the undulator. The purpose of the first collimator
is to reduce the electron beam halo, while the second should intercept any mis-steered
beam which could hit and damage the undulator. The first collimator, continuously
intercepting about 1% of the beam, will be a constant source of forward-directed
bremsstrahlung and muon radiation. The second one should interact with the beam only
in exceptional cases and is not expected to contribute substantially to the radiation field
under normal operating conditions.
Bremsstrahlung radiation produced in the first collimator will hit the PPS photon
stoppers which must be inserted into any beamlines when access is allowed in any
downstream enclosure. Two calculations have been made with the FLUKA code [1] for
the first of such photon stoppers, assumed located 20 m downstream of the undulator:
electrons in the beam halo were assumed to hit the collimator on its front edge in one
case, and inside the aperture in the second case. The first case was found to generate the
higher energy deposition. For a 1% loss of a 14.35 GeV electron beam of 1.722 kW (1
nC/pulse at 120 pps) the calculated energy deposition rate in the stopper was less than
0.02 mW. The shielding requirements for Giant Resonance photo-neutrons generated in
the stoppers located in the on-line hutches of the Near-Field Hall are being evaluated by
means of the SHIELD11 code considering an equivalent electron beam depositing the
same power in the stoppers. However, bremsstrahlung from collimators is neither the
only nor the main source of radiation to be considered for shielding design: other
radiation components (bremsstrahlung from profile monitors, muons, X-rays) must also
be taken into account.
9.3.1.1.2 Bremsstrahlung from profile and intensity monitors
A Profile Monitor will be inserted into the electron beam upstream of the undulator
for diagnostic purposes. It consists of a wire scanner with tungsten wires of 20 m
diameter. During a scan, 20% of the electron beam will be intercepted on average. A
FLUKA simulation was made assuming electrons hitting an equivalent wire of square
cross section 17.72 m m thick 3 m upstream of the undulator. For a 14.35 GeV beam of
1.722 kW at 120 Hz and 20% interception, the absorbed power in a photon stopper is
0.46 mW. This would cause unacceptable radiation levels in hutches downstream of the
stopper when access is allowed. Therefore, it has been decided to allow use of the profile
monitor only with a 10 Hz beam, thus reducing the power absorption to 0.04 mW. This
condition will be part of the PPS interlock logic for access. The calculated radiation level,
together with that due to collimator-halo interaction, will be used to evaluate shielding
requirements for on-line hutches.
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X-ray Intensity Monitors will be used at several locations along the undulator as
diagnostic devices for the photon beam. However, the electron beam will also be
intercepted. There will be 10 or 12 of them, but calculations have been made for the one
located in the last 10 m section of the undulator. The material is diamond, 0.5 mm thick:
but because the beam strikes it at an angle of 45˚, the effective thickness traversed is
0.707 mm. The 5 W deposited in the photon stopper result in radiation levels which are
too high to allow access downstream of these devices. Therefore, they will be
interlocked with the PPS system. Scattered bremsstrahlung and photo-neutrons produced
in the stoppers must be taken into account in the stopper shielding design.
9.3.1.1.3 Gas bremsstrahlung
Interaction of the electron beam with residual low-pressure gas molecules in the
vacuum pipe will give rise to forward-directed gas bremsstrahlung. This type of radiation
has been thoroughly investigated at circular storage rings, where the beam current is
much more intense. However, at LCLS the straight length over which bremsstrahlung is
produced will be much longer (120 m between the dog-leg and the first bending magnet
before the electron dump). The residual gas pressure and the electron energy will also be
higher.
A preliminary estimation of gas bremsstrahlung dose rate, made by an empirical
formula reported by Ferrari et al. [2], gives 60 mrem/h at a 20 m distance from the end of
the undulator, in absence of any shielding. The dose rate behind the photon stopper
shielding would be much smaller and would represent a negligible contribution compared
with that of bremsstrahlung due to collimators and to beam diagnostic devices, but since
the formula was derived for very different conditions (lower energies and much smaller
straight lengths), it is planned to get a more accurate estimate by carrying out more a
detailed Monte Carlo calculation.
Bremsstrahlung, will co-propagate with any insertion device radiation until it hits a
medium with an adequately short extinction depth. This will be either a mirror or a
crystal. The impact will, in general, impulsively excite an intense thermal neutron field
which will decay with a time constant on the order of milliseconds. To suppress any
possible interference with LCLS experiments operating over similar time intervals, the
layout includes a lead/polyethylene shield wall between the crystal and specular take-off
optics tanks and the experimental end stations. Since both beam lines pass through this
wall at off-axis angles and via small apertures, the suppression of this source of
interference by the wall is expected to be very effective.
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9.3.1.2 Muons
Muons produced by electron interactions in the Beam Switchyard and upstream of it
are already ranged out at present by 55 feet of iron and cannot constitute a concern.
Muons can be created in the diagnostic area (by losses upstream of and inside the dogleg, in the collimators and in the profile monitors); there will be other possible muon
sources inside the undulator (X-ray intensity monitors) and the electron dump. These
muons will be either bent away by magnets downstream of the undulator or shielded by
iron shielding located on the top of the electron dump.
However, persons accessing the on-line hutches and the research yard downstream of
the Near-Field Hall and possibly the Far-Field Hall could be exposed to several other
muon sources, which are produced when high-energy bremsstrahlung hits the photon
stoppers. Muons can also constitute an important radiation background for experiments.
The presently available calculation tools (programs MUCARLO [3] and MUON89 [4]
can only address muon production in thick targets and have been applied to estimate the
contribution of the electron dump. The FLUKA code can simulate with great accuracy
muon transport, but not yet muon production by photons. Work is in progress to include
this effect and it is planned to use this code for a detailed simulation as soon as the
upgrade will take place. It is expected that not more than 3 m of iron will be needed to
shield any of the known muon sources.
9.3.1.3 Neutrons
Photo-neutrons can be generated on the zero-degree line in any object hit by electrons
and by bremsstrahlung (and, to a much lesser degree, by muons). Such objects include
the electron dump, the transport line to the dump, photon stoppers outside and inside the
experimental Halls, and any optical device in the X-ray line. Neutrons generated outside
the Near-Field Hall can penetrate to the Hall through the concrete shielding or streaming
through the X-ray beam pipe. Neutron shielding design must consider both personnel
protection and reduction of experimental background. For the latter, time-dependent
calculations may be necessary in order to take advantage of the slowing down time which
could allow to gate out thermal neutron contribution. Preliminary estimations can be
easily made for the Giant Resonance neutron component, which is practically isotropic
and whose yield is known to be proportional to deposited power. Higher energy
components require a more detailed study which can be carried out by the FLUKA code.
9.3.1.4 Toxic materials
Both Li and Be are materials under consideration for optical components, Add
section on how to deal with Be
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9.3.2 Personnel protection system (PPS) and access
In light of the above sections, the access and personnel protection system (PPS) is
designed to prevent access to experimental areas where beams are present and to prevent
beams from entering an area during personnel access.
The experimental hall shielding will consist of fixed and moveable parts. The
experimental hall perimeter walls and central beamline walls are planned to be fixed
shielding consisting of appropriate material for the energy spectra of expected radiation.
The experimenter hutches may have movable walls to adjust for experimental
requirements. The moveable wall configuration will activate the current radiological
configuration control system when changing the hutch shielding. The experimental walls
will have the capability of adjusting to the different angles of any hutch branch lines.
The access control system will be capable of retaining integrity and reliability, while
compensating for wall placement.
The PPS functions as an access control system to the central beamline and each
experimenter hutch. The PPS access control components consist of entry modules (from
the experimental control area into the hutch, and entry points from the experimenter
hutch to the central beamline), interlocks for photon stopper and electrical hazards, logic
for access, and status and control both locally and remotely.
The hutch will be designed to contain all radiation background so that the dose rates
outside the hutch are acceptable when photons from the FEL are inside the hutch. Access
to the hutch will be controlled by a Hutch Protection System (HPS) modeled after
existing SSRL hutches. The key parts of the HPS are a keyed access door, photon
stopper interlocks, and area security system. The HPS allows either permission for
personnel access or for beam to enter the hutch. It contains the logic interlock circuits
that govern the sequence of access operations centered on the status of the stoppers. It
also captures or releases the hutch door keys, acknowledges completion of a personnel
security search, and keys the experiment enclosure on-line or off-line. Access to the
hutch is permitted only if all photon stoppers are closed.
Access to the front end enclosure requires the FFTB stoppers inside the FFTB tunnel
to be in. The LCLS HPS will control the operation of photon stoppers per other areas or
hutches, which are required to be in. Each stopper is protected by two ion chambers and
a burn-through monitor. A list of possible PPS violations and system responses is
contained in Table 9.3.21.
Table 9.3.2.1 PPS violation and response.
1. FFTB tunnel
security fault
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Any FFTB tunnel security fault, while dump D2 and stoppers ST60 and ST61 are not closed,
will shut-off the LCLS beam, close FFTB stoppers, turn off the electrical hazards.
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2. Inside tunnel
BSOICs trip
These BSOICs continuously monitor the radiation inside the tunnel. If the detected radiation
level exceeds the preset limit, the PPS will shut-off the LCLS and any other beams in the
Linac. The BSOIC interlocks are by-passed when the FFTB tunnel is in the No Access state.
3. Outside tunnel
BSOICs trip
These BSOICs continuously monitor the radiation outside the tunnel. If the detected radiation
level exceeds the preset limit, the PPS will shut-off the LCLS beam and close the beam
stoppers.
4. HPS security
fault
Any HPS security fault while the hutch photon stoppers and the FFTB beam stoppers are not
closed will shut-off the LCLS beam and close the hutch and FFTB stoppers.
References
[1] A. Fassò, A. Ferrari, J. Ranft, P. R. Sala, “New developments in FLUKA modeling hadronic and EM
interactions”, SARE3 Workshop, KEK 7-9 May 1997. Ed. H. Hirayama, KEK Proceedings 97-5 (1997), p.
32-43
[2] A. Ferrari, M. Pelliccioni, P. R. Sala, “Estimation of fluence rate and absorbed dose rate due to gas
bremsstrahlung from electron storage rings”, Nucl. Instr. Meth. B83, 518 (1993)
[3] L. P. Keller, “Muon background in a 1.0 TeV Linear Colider”, SLAC-PUB 6385, 1993
[4] W. R. Nelson and Y. Namito, Computer Code MUON89, SLAC Radiation Physics Dept., 1989
9.3.3 PPS beam stoppers
Two Personnel Protection System (PPS) beam stoppers will be required to allow
entry into the experimental hutches while the e- beam is being delivered to the undulator
and deflected into the dump. The function of these stoppers is to block and absorb any
coherent or incoherent  or X-radiation from the undulator, as well as bremsstrahlung
from anywhere in the beam transport system. These stoppers are patterned after an SLC
design used in Sector 10 of the SLAC linac and in the PEP-II extraction lines [1]. The
design energy is 12-15 GeV and the assumed power for continuous exposure is Pav ~5
kW. The absorbing element in each stopper is a Cu/W block assembly with an overall
length of 20.32 cm [~22 radiation lengths (Xo)]. The beam first encounters the 3.7 Xo
long Cu section. The remaining 12.3 Xo are provided by a block of free-machining
tungsten (W~90%,  ~17 g/cm3) which is brazed to the Cu. The shower maximum of the
electromagnetic cascade for 12-15 GeV occurs at a depth of 5.5-5.7 Xo. A built-in burnthrough monitor is located at that z-depth. It consists of a pair of cavities separated by a
Cu diaphragm. The first cavity is pressurized with dry N2. Its return line contains a
pressure switch with the trip level set to 15 psig. Should excessive beam power be
deposited in the stopper block, the diaphragm will perforate, allowing the N2 to escape
into the second cavity, which is open to atmospheric conditions on the outside. The
pressure switch will interrupt beam delivery within 2-3 linac pulses. The transverse
dimensions of the stopper block are 3.16 cm wide x 2.9 cm high. The minimum radial
attenuation distances are then, respectively, 3.5 Xo and 3.1 Xo in the Cu section and 3.5
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Xo and 3.1 Xo in the free-machining W. This is more than adequate to attenuate the
radial shower.
The absorber block can be vertically inserted or removed from the beam channel by
means of a remotely controlled air cylinder. The block has a water-cooled heat sink
attached to the top surface which allows continuous and safe dissipation of up to 5 kW of
beam power.
A third stopper will be installed which is essentially an ordinary pneumatic valve with
no special heat-removal provisions. It is intended for protective insertion when the linac
is running in an ultra-low peak or average current mode, for which the average radiation
power is on the order of mW or less.
References
[1] D. Walz, "Beam Stopper for PEP II Injection," SLAC Memorandum, Sept. 28, 1994.
9.3.4 Burn through monitors
A built-in burn-through monitor is located in the PPS stoppers, and described as part
of that system. It consists of a pair of cavities separated by a Cu diaphragm. The first
cavity is pressurized with dry N2. Its return line contains a pressure switch with the trip
level set to 15 psig. Should excessive beam power be deposited in the stopper block, the
diaphragm will perforate, allowing the N2 to escape into the second cavity, which is open
to atmospheric conditions on the outside. The pressure switch will interrupt beam
delivery within 2-3 linac pulses.
9.3.5 Shielding
9.3.6 Beam containment system (BCS)
9.3.7 Lead end stops
9.3.8 Muon stops
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