< 5.0)
is covered by a separate very forward calorimeter satisfying different design
criteria. This system is described in Chap. 6. < 3.0)
is covered by the barrel and endcap hadron calorimeters which sit inside the
4 T field of the CMS solenoid. In the central region around
= 0 a hadron shower 'tail catcher' is installed outside the solenoid coil
to ensure adequate sampling depth. The active elements of the barrel and endcap
hadron calorimeter consist of plastic scintillator tiles with
wavelength-shifting (WLS) fibre readout. Layers of these tiles alternate with
layers of copper or stainless steel absorber to form the sampling calorimeter
structure. The tiles are arranged in projective towers with fine granularity to
provide good di-jet separation and mass resolution.
The pseudorapidity range (3.0 <
< 5.0)
is covered by a separate very forward calorimeter satisfying different design
criteria. This system is described in Chap. 6.
The choice of lead tungstate crystals as the active element of the
electromagnetic calorimeter influences the HCAL design and performance. The
optimisation of the HCAL/ECAL combination requires prototype work in test
beams. Studies using a realistically sized HCAL prototype are also required to
study the effect of cracks and dead areas that may be introduced when building a
calorimeter. For these prototypes to be realistic it is essential that the
mechanical design be fully developed, taking into account the need for hermetic
calorimetry and the provision of supports for inner elements of the CMS
detector.
= 0.087 x 0.087
for
< 2.0
to match that of the electromagnetic calorimeter and the muon chambers. This
granularity is sufficient for good di-jet separation and mass resolution. The
calorimeter readout must have a dynamic range from 20 MeV to 2 TeV to
allow the observation of single muons in a calorimeter tower while maintaining
adequate response for the highest energy hadronic showers. The muon signal will
be used for calibration and assist in muon identification.
The physics programme most demanding of good hadronic resolution and
segmentation is the detection of narrow states decaying into pairs of jets. The
di-jet mass resolution includes contributions from physics effects such as
fragmentation as well as detector effects such as angular and energy
resolution. When the jet
is small, mass resolution is dominated by physics effects. For high
jets arising from decays of boosted light objects, angular resolution plays a
more important role than energy resolution. The influence of hadron calorimeter
transverse segmentation has been studied for hadronic decays of boosted W and Z
bosons. Segmentation coarser than
= 0.1 x 0.1
significantly degrades the mass resolution, particularly for W and Z bosons
having
> 500 GeV,
while the energy resolution has relatively little effect. It is only in the
case of back-to-back high
jets, arising from the decay of heavy objects, that physics and angular effects
are suppressed to the point where energy resolution plays a significant role.
Thus the only physics process in which the hadron energy resolution is expected
to be important is in the detection of a heavy Z' decaying into two jets.
Further details are given in Sect. 11.6.2.
The existing test beam data set, with optimal weighting for leakage, yields a
resolution, for single hadrons at
= 0,
of
.
Initial Monte-Carlo studies substantially confirm this result
[1].
Detailed simulations of the cracks, dead material, etc. of the calorimeter
system must be developed to obtain energy resolution as a function of
and 
,
and missing
resolution.
The 25 ns time interval between beam crossings sets the scale for the time resolution needed in the calorimeter. At LHC design luminosity there are approximately twenty minimum bias interactions per crossing. The calorimeter must help distinguish the rare interesting events from this background (which is strongly suppressed in the calorimeter by the 4 T field) and must have the granularity and time resolution to suppress multi-bunch crossing pile up.
For adequate performance, the hadron calorimeter response must be uniform and stable with time at the level of a few percent. During the life of the experiment the response of the calorimeter may change as a result of radiation damage or ageing. Rigorous quality control during manufacture, combined with test beam calibration, and a sophisticated, redundant system for monitoring the response of each layer of each tower, will be needed to meet this performance target.
Fig. 5.1: The tower structure of HCAL for the barrel (end view).
Copper has been chosen as the absorber material because it has a shorter
interaction length than steel. Furthermore, it is easy to construct mechanical
structures by electron beam welding (EBW) starting from relatively flat plates.
EBW tests on four 0.5 x 0.5 x 0.04
plates have been very encouraging [2]. but other options are still being
pursued. One example is to cast the main body of the wedge and electron beam
weld only the inner and outer stainless steel plates.
(see Fig. 5.1) to avoid radial gaps in the active coverage at wedge
boundaries. Even numbered wedges are larger at the outer radius while odd
numbered wedges are larger at the inner radius. Each wedge module is assembled
from flat plates of copper and stainless steel which are electron beam welded
into a complete unit. The outer surface is machined to the required precision
after welding. Electron beam welding causes no local distortions in the plates.
The stresses and deformations of such a construction have been analysed by a
finite element analysis and shown to be acceptable [3,4].
The active readout scintillator tiles in each layer are divided into segments of
= 0.087 x 0.087.
This granularity gives good shower resolution and matches the trigger
granularity of the electromagnetic calorimeter and of the muon system.
Each hadron endcap (HF) consists of eighteen identical wedges (14 tonnes
per wedge), matching the barrel segmentation, and constructed in a similar way
out of plates perpendicular to the beam direction. The endcap wedges are also
segmented longitudinally into two different compartments HF1 and HF2, in this
case having a thickness of 50 mm and 100 mm respectively. There are
nine layers of fine sampling and twelve of coarse sampling for a total of
21 sampling layers. The final details will be fixed after further R&D
studies [5]. Again, each wedge is stepped in
to maintain active coverage. The standard
= 0.087 x 0.087
granularity of each layer is modified to match that of the endcap
electromagnetic calorimeter.
The region 2.6 <
< 3.0
is a high radiation environment (20 kGy/year, 2 Mrad/year) and the
active elements here may have to be replaced every few years. Given the
encouraging radiation tolerance of lead tungstate crystals (see
Sect. 4.2.3), one option is to extend the ECAL to cover this region, thus
protecting the endcap HCAL behind it. An alternative is to construct the first
six layers of the endcap HCAL in this region in the form of an easily removable
ring (Fig. 5.2).
In the central pseudorapidity region the barrel hadron calorimeter within the
solenoid is too thin for complete shower containment. Further scintillator
sampling layers with the same tower granularity are therefore added outside the
coil and cryostat, which constitute an absorber layer with a thickness of
approximately 1.3
.
The first additional scintillator layer has an individual readout for each
segment and wraps around the full length of the cryostat in a 36-fold symmetry.
In the central pseudorapidity region of
< 0.43,
an additional interaction length of absorber and two scintillator layers are
added, forming a section called the tail catcher. The tail catcher matches the
muon chamber segmentation and hence has a 12-fold symmetry.
The electromagnetic calorimeter has a thickness of about 1.1
.
The effective absorber plate thickness of the HCAL increases with the polar
angle as
.
It follows that the stochastic resolution term in the barrel depends only on
the physically relevant variable
.
The barrel thickness varies from a thickness of 5.46
at
= 0
to 10.82
at
= 1.3.
A smooth transition is made to the endcap region at
= 1.5.
However, two
segments in this region are traversed by a 100 mm gap to provide cable and
fibre paths out of the detector. The total absorber thickness in the endcap
averages about 11
,
to allow for the logarithmic increase in depth needed for higher energy shower
containment.
Although the use of this technique in CMS has been simulated in detail [10,11,12,13] and tests with prototypes have been started [14], extensive R&D is still required [12], especially in the optimisation of the tail catcher. The green WLS light will be channelled via clear fibres to the endcap region where the photodetectors will be placed. Photodetectors with gain that can operate in a 4 T axial field are required.
The hadron calorimeter will consist of a
large number of towers (
3400). In order
to limit the number of individual elements,
the tiles in a given layer are arranged as
a single mechanical unit called a
'megatile'. For a barrel wedge the (
)
segmentation is 17 () x 4 () and the
68 tiles in one layer of a wedge constitute
a megatile. An example of an endcap
megatile is shown in Fig. 5.3. There are
several possible construction techniques.
In one, the separate tiles are cut out of
scintillator, the edges painted white, and
the tiles attached to a plastic substrate
using plastic rivets. In another, the tiles
are cut from a single large piece of
scintillator, and glued in place with white
epoxy to form the megatile. The light from
each tile is collected by a green WLS fibre
placed in a machined groove in the
scintillator. After exiting the
scintillator the WLS fibre is spliced to a
clear fibre which transports the light to
the edge of the megatile. The clear fibre
terminates into one side of a multifibre
optical connector at the megatile boundary.
Multifibre optical cables carry the light
from the megatiles to decoder boxes. Here,
the fibres from the corresponding tiles in
different layers are organised into
bundles, enabling the light from all tiles
belonging to the same tower segment to be
optically mixed into a photodetector.
Fig. 5.3: An example of the megatile concept for the endcap.
The megatile, along with the readout fibres, will be packaged as a tray which will be inserted into the calorimeter absorber structure. After insertion, the multifibre optical cables will be connected between the trays and the decoder boxes.
The advantage of this scheme are that the scintillator trays can be built and tested remotely from the installation area. Once the calorimeter absorber is assembled, the trays can be inserted very rapidly. Conversely, the trays can be removed and refurbished without removal of the absorber structure in the (unlikely) event of catastrophic radiation damage to the scintillator.
The construction of a scintillator tray unit (Fig. 5.3) begins with a plastic cover plate whose thickness is 0.5 mm, followed by the 4 mm thick scintillator megatile wrapped in a thin sheet of Tyvek 1073D (a plastic insulating material) for reflectivity and light tightness. The top of the megatile is covered with a white polystyrene plate with a thickness of 2 mm. Individual tiles are grooved to hold the WLS fibres and the cover is grooved to provide routing for the fibres to the outside of the tray. The fibres rise out of the scintillator into matching grooves on top of the white plastic. The white plastic layer is also grooved to accept tubes for the wire-driven radioactive calibration sources.
The clear fibres, which are spliced to the WLS fibres, run to an optical connector at the edge of the tray. Finally, a second plastic cover plate is placed on top of the white plastic and the entire unit is riveted together. The sides of the trays are then made light tight.
= 2.5
(a total of
1 kGy
(0.1 Mrad) in the barrel and 20 kGy (2 Mrad) in the endcap)
without the necessity of replacement. The total optical system should produce
enough light to easily identify minimum-ionising tracks penetrating the
calorimeter (for use in muon identification as well as calibration/monitoring).
Well controlled scintillator thickness and fibre diameters are critical for the
optimal performance of the calorimeter. Attenuation lengths of the fibres also
must be well controlled.
The baseline choice of material for the HCAL optical system satisfies these
requirements. For the barrel, Kuraray SCSN81 plastic scintillator will be
used. This material has been shown to be radiation hard [5] and have good
long-term stability. For the WLS fibre, the baseline choice is
Kuraray Y-11 double-clad fibre. Double-clad WLS fibres have good
mechanical properties as well as delivering
1.5 times
more light than single-clad. The baseline clear fibre is Kuraray double-clad
clear fibre. For the endcaps it is proposed to use the Kharkov (Ukraine)
plastic scintillator [15] with the same parameters. An alternative choice for
the scintillator may be PSM-115(A), a polystyrene scintillator manufactured by
injection molding at IHEP, Protvino. Minsk and Moscow (INR) are also
investigating the production of double-clad WLS fibre.
These choices will satisfy the requirements of much of the endcap region. However, for the more forward region, we will need to use more radiation hard materials. Work from SDC has shown that polystyrene scintillator using the 3HF dye, and wavelength-shifting fibre using the Kuraray orange dye, O2, will survive the expected radiation dose in the forward part of the endcap. Figure 5.4 [5] illustrates the improvement obtained using the green/orange combination. A new Bicron blue scintillator (BC499-S2) also looks very promising.
Fig. 5.4: Light yield as a function of
dose for blue/green and green/orange
tile/WLS combinations. 'MFM' stands for
multiple WLS fibres embedded in the
scintillator; '
' stands for one single WLS
fibre with a shape.
It is well documented that the light yield from scintillator increases when
embedded in a magnetic field [16,17]. Recent measurements at Fermilab and
Florida State indicate that this effect saturates above 2 T [18] for the
baseline SCSN81 scintillator as well as for other scintillators (see
Fig. 5.5). The effect appears to be due to the primary excitation as can
be seen by comparing the response to
and UV sources.
Fig. 5.5: Fractional light increase as a function of magnetic field
for various scintillator tile/WLS combinations.
Fibres are spliced together by controlled melting of the ends inside a restricting tube (thermal fusion). This technique has been optimised for factors such as long-term mechanical stability, strength to withstand repeated flexing, high optical transmission and very small variation in transmission for different splices. The mean value of the transmission through a splice (normalised to the uncut fibre) is measured to be 91% with an r.m.s. of 1.8%.
Multifibre optical connectors were developed by the CDF collaboration. These
connectors allow the optical signals to be treated in a similar way as
electrical signals. The scintillator tile trays can be quickly connected to and
disconnected from multifibre optical cables that look strikingly like
multiconductor electrical cables. The optical connectors are made via precision
injection molding of mechanically stable plastic. In this manner, all
connectors are identical, and there is no need for pair-matching of the
connectors. The reproducibility of the optical connector transmission for many
make/break operations has been measured to have a mean transmission of 83%,
with an r.m.s. of 0.6%, for a single fibre. Considering all fibres in the
connector, the r.m.s. is
2
to 3%.
Variation in transverse uniformity of tiles in a tower, or variation in tile-to-tile light yield for tiles in a tower, will increase the constant term in the energy resolution. We have carried out detailed studies to identify the requirements on the optical system so that these variations do not contribute substantially to the constant term. We have found that tile-to-tile variation of less than 10% is acceptable. The CDF plug upgrade calorimeter group has built several thousand tiles. The measured tile-to-tile variation of the light yield from a set of 1000 finished tiles is found to be 6.4%. This is adequate for a good hadron calorimeter.
The transverse uniformity of a tile is dominated by the placement of the WLS fibre. Based on knowledge from the CDF group, we expect our transverse non-uniformity to be a few per cent. This non-uniformity will not affect the resolution constant term appreciably.
The moving source system was developed for the CDF and SDC calorimeter projects. It consists of a set of tubes placed in the scintillator trays plumbed to a 'source-mover'. The source is inside a long flexible stainless steel tube. The source mover can (via computer control) push the source down any of the tubes and thus expose any of the tiles to the source. The same system will be used for the initial quality control testing at the site of the manufacture of the scintillator tray.
This quality control strategy is the same as used by CDF in their calorimeter upgrade project. Their experience gives us confidence that the strategy will work for CMS as well.
10 
fibre bundles corresponding to a tower are required to have a linear dynamic
range of
and operate in a uniform 4 T magnetic field. For calibration purposes, the
detectors must have the capability of measuring the signal generated by a
radioactive source as a DC current to a precision of 3%. In addition, the
photodetectors are located inside the detector, adjacent to the HCAL itself,
where service access is infrequent thus placing an additional requirement on
mean-time-to-failure. The useful lifetime of the photodetector must correspond
to ten years of operation at a luminosity of
.
A final requirement on the ratio of the signal to noise follows from the need
to measure the signal from a minimum ionising particle. Progress is being made
on the development of two types of photodetectors that can operate in magnetic
fields and still provide gain. These are the proximity focused hybrid
photodiode (PFHPD) [19,20] and the semiconductor avalanche photodiode (APD)
[21].
PFHPDs exhibit a gain that is linear with applied voltage,
2000 at
10 kV. In beam tests the gain has been measured to decrease by only 2% in
an axial field of 3 T of the RD-
5
magnet [14]. The devices are linear to 2% over the required
dynamic range and exhibit a fast response that is determined by the diode
source capacitance. The outstanding questions for these devices are use of
fibre optic windows, development of non-magnetic packaging, and reduction of
the dark current to levels suitable for measurement of the DC current signal
from the calibration sources and of the signal from a minimum ionising
particle.
Several tens of these APDs have been acquired for beam tests. The issues of gain stability and sensitivity to neutron irradiation for these devices will be addressed in collaboration with the ECAL photodetector development group. APDs with associated preamplifiers are also under development in Moscow and Minsk. The characteristics of these photodetectors will be compared to the presently available commercial devices.
Electronics boxes containing the decoder/mixer boxes, the photodetectors and
associated HV supplies, as well as their preamplifiers and their low voltage
distribution, will be distributed around the outer radius of the
= 1.5
transition region from barrel to endcap, close to the HCAL detector itself. The
FERMI system will also reside in this region. They will be attached to either
the barrel or the endcap and will be able to move along with their own
sub-detector. Source driver boxes for both the endcap and the barrel also sit
close to the coil in the
= 1.5
region. The barrel and endcap are serviced via the 100 mm gap between the
two sub-detectors in the
= 1.5
region. This region also contains cables from the electromagnetic calorimeter
and the tracking detectors.
Hadron calorimeter-related services include optical cables from the barrel and endcap megatiles, source tubes servicing each of the megatiles, and possibly quartz fibre bundles transmitting laser signals to each of the individual tiles of a megatile. The electronic boxes and source drivers are connected to the outside world via a cable path that snakes around the barrel to reach the outside centre of the detector.
A thorough investigation will be made of the choice of absorber materials and their suitability in the design and manufacturing process in order to confirm the use of copper and electron beam welding. A test pyramid 0.5 m x 0.5 m in section, but having the full radial depth of the HB and the present segmentation, is being manufactured in industry.
The barrel engineering design must include:
1) For the barrel: a determination of the vertical sag of the assembled half-barrel as well as horizontal distortions under self supporting conditions, and a design of the assembly cradle which will also be used for lifting in the underground area and inserting into the coil vacuum tank.
2) For the endcap: a determination of the cantilever stresses and deflections of the assembled endcap as well as horizontal distortions under self supporting conditions.
3) For the barrel and endcap: a study of the individual wedge plate connections including the determination of the required depth of electron beam welding. The definition of the procedure for individual wedge handling during assembly including the determination of lifting points. A study of the half-barrel vertical assembly including the determination of the gap size for the top wedge. A study of the wedge-to-wedge connections.
The endcap will be assembled as a horizontal ring (not a vertical ring) on a back plate. The stresses of lifting the endcap into a vertical position will have to be studied.
Despite the use of non-magnetic materials, a fast discharge of the superconducting solenoid can produce large transient electrical and magnetic forces due to eddy currents in the absorber. The effect of these forces must be calculated and taken into account for the calorimeter structure and support system. To decrease the effect, the wedges will be insulated electrically from one another on a large fraction of their contact surfaces.
Other design items include cable routing layouts for both barrel and endcap, scintillator tray insertion/removal techniques for both barrel and endcap, assembly scheme layouts and tolerance studies.
We do not intend to calibrate each wedge in a test beam. Instead, we intend to transfer the absolute calibration from a number of wedges exposed to a particle beam to all wedges using the radioactive source calibration. This scheme is currently under test.
Radioactive
Sources
= 1 mm)
that will route
radioactive sources throughout the system. This is a system similar to the one
used by CDF and proposed by SDC. A wire with a point-like Cs source will be
pushed through these tubes by a remotely controlled system of drivers. The DC
current induced by the source traversing one tile of a tower will provide an
accurate measurement of the response of the entire measuring chain. The
experience of CDF shows that this measurement can be maintained at the level of
1%. Change of response due to photodetector or electronics will show up as a
change of the response of all tiles of a given tower and can be compensated by
an adjustment of the overall calibration factor. Change of response due to
radiation damage will lead to a change of the measured current that is
dependent on the depth of the layer.In order for the moving radioactive source calibrations to be done periodically during collision runs, it will be important that the front-end electronics allow simultaneous digitisation of the DC current from the photodetector and of fast pulses, both beam-related and from laser light injection. Photodetectors with high leakage currents would compromise the ability to do the source calibrations.
The most convenient location of the source tube drivers is in the transition region between the barrel and endcap. However, this region is in the 4 T magnetic field and conventional driver motors may not work. Piezoelectric wave stepping motors may do the job, and even aircore conventional motors might work if the axle is aligned with the external field.
J taking into account the total number of towers, photodetector efficiency and
allowing for reasonable losses of light in the distribution process.
For the GEANT shower simulation the geometry of the hadron calorimeter is
modeled in detail, including major mechanical structure and cracks [11]. Shower
particles are traced down to low energy, typically 1 MeV. This GEANT
simulation provides information on shower profiles in the calorimeter and on
the influence of the mechanical structure and cracks on the measured energy in
the calorimeter. The initial Monte-Carlo studies of the proposed calorimeter
layout suggest that, without energy weighting,
in the barrel and
in the endcap for single particles.
For the fast simulation all the particles, including leakage of shower particles from ECAL, are transported by GEANT to the front face of HCAL and then transferred to the fast shower simulation code [9]. The transverse and longitudinal shower profiles determined from test beam data are parametrised and used to simulate the energy deposit in read-out towers. The original shower code was developed for SDC and extensively used to optimise its calorimeter design [7,22].
Another fast shower simulation code has been developed based on the ALEPH parametrisation of the electromagnetic showers. Hadrons in showers are transported by GEANT in the same way as in full shower simulations, while energy deposit due to electrons, positrons and photons in the showers are simulated using the parametrisation code, if they are expected to be contained in the HCAL volume. This code, although slower than the code developed by SDC, is better able to account for the effects due to cracks and dead material in HCAL.
.
The ten-year integrated dose is estimated to be 1 kGy (0.1 MRad) at
the front of the HB. The maximum dose expected at a rapidity of 2.5 is
estimated to be about 20 kGy (2 Mrad). It is known that up to 30%
peak damage in the HCAL will not induce a constant term in the energy
resolution which is unacceptable [3].In common with most commercial polystyrene based scintillators, SCSN81 together with a K27 doped WLS fibre such as Kuraray Y11, suffers a light yield reduction of about 30% at 10 kGy (1 Mrad) and an unacceptable 70% at 50 kGy (5 Mrad). The baseline HCAL design uses this combination in the barrel region. In Table 5.1 the performance of this combination is compared with that of several other options.
Table 5.1
A comparison of the performance of various combinations of scintillator
and WLS fibres.
--------------------------------------------------------------------------------------
WLS fibre Light yield Decay time Light loss [%] for a given dose
Scintillator (a. u.) ns 1 Mrad 2 Mrad 5 Mrad
--------------------------------------------------------------------------------------
SCSN81* Kuraray Y11 1 10 30 50 70
SCSN81 G2 Kuraray - 5 - 40 -
3HF(green) O2 Kuraray 0.25 15 10 20 35
BC499-S2 Y11 1.7 - - 35 -
--------------------------------------------------------------------------------------
*Ukraina and PSM-115 scintillators and Y11 WLS fibre combinations give
similar results as SCSN81/Y11 combination.
In the endcap region up to
2
either SCSN81 or Ukraina scintillator with Y11 doped fibre is used. Between
2.0 <
< 3.0
a green scintillator (3HF) with an orange WLS fibre (O2 - Kuraray)
could be used. However the low number of photons, combined with the longer
decay time constant and the instability of 3HF under illumination, make this a
difficult choice. The new Bicron blue scintillator BC499-S2 together with
double-clad Kuraray Y11 WLS fibre appears to be a promising combination
for the region between 2.0 <
< 3.0.
The scintillator produces 70% more photons than SCSN81 and shows a light yield
loss of 35% at a dose of 50 kGy (5 Mrad).
The light yield in the endcap elements may turn out to be smaller than in the
barrel. However, since most of the physics resides in transverse energy, the
stochastic term in the resolution, which scales as
,
improves with decreasing angle. This effect will more than compensate for the
loss in photoelectron statistics.
For the region 2.6 <
< 3.0
in the front-endcap ring, yearly re-masking and/or periodic replacement of the
scintillator has to be envisaged.
An important consideration is the decay time of the scintillator/fibre combination. Typical decay times for blue scintillators with K27 doped fibres are about 10 ns. This means that after 25 ns about 30% of the photons are still to come. There exist newer fluors such as Bicron's G2 which when combined with a blue scintillator lead to a decay time of about 4 to 5 ns so that a much larger fraction (85%) of the light will be in the 25 ns interval. This fluor, in the liquid state, did not show any radiation damage at 1 MGy (100 Mrad). However the damage was worse, amounting to 40% at 20 kGy (2 Mrad), when used with SCSN81.
Radiation damage at low dose rates can be substantially worse than at high dose rates. A major effort is being made at the University of Michigan where some samples have already been irradiated for over one year. This will provide an indication of the damage that is expected over a ten-year period. Similar work has been started at the Institute of Nuclear Physics, Tashkent.
We will work with industry, and in Protvino and Kharkov, to produce radiation-hard blue scintillator and fast, radiation-hard WLS fibre by exploring different polymerisations of the plastic. A vigorous R&D programme is under way to get the best scintillator and fibre combination given that there are at least two years before a choice has to be made.
Another concern is the fluorescence decay time of the WLS dye. The O2 dye is about 8 ns slower than the K27 dye. Given the 25 ns bunch spacing, the reduced speed of the O2 dye is still an issue.
30 kGy
(3 Mrad).
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