NLC - The Next Linear Collider Project
NLC BackgroundsWhat’s New?
Tom Markiewicz
LC’99, Frascati, Italy
October 1999
Tom Markiewicz
NLC - The Next Linear Collider Project
Background Calculations Summary
Overall status presented by Takashi Maruyama/SLAC at LCWS 4/99 at Sitges, Spain
New Developments by Jeff Gronberg/LLNL
•New Be ring mask effectively eliminates backscattered hits in VXD-L1
•Allows 1.2 cm VXD Layer 1 for fields down to 3 Tesla
•Study potential problem with scattering angle approx. used by FLUKA
Synchrotron Radiation:
•Stan Hertzbach updates analysis from 1996: OK for 8 x x 40 y
•Takashi Maruyama adds 200m of incoming beamline to GEANT
•Study SR, off-energy beam striking apertures, beam-gas scattering
•SR from disrupted beam in extraction line not a problem
Muon Background: Lew Keller/SLAC
•9m long tunnel filling spoilers are the best insurance against muons. Current plan is to build ONLY ONE cavern downstream of last collimator, but to leave it empty until needed.
Tom Markiewicz
NLC - The Next Linear Collider Project
Background Simulations
Machine Parameters:
1 TeV c.o.m. NLC “B”
1.0E10 e-/bunch; 95 b/train; 120 Hz
Generators: “Guinea-Pig” Program
Simulators:
GEANT3: e+/e- & photons
FLUKA98: neutrons
Geometry:
LCD Small Detector w/ real field
Important that layout is as realistic as possible
Complete Extraction Line and Dump
200 m of INCOMING beam line
Tom Markiewicz
NLC - The Next Linear Collider Project
Radiative Bhabhas125K per bunch @ <E>=370 GeV
IP Backgrounds
e+,e- pairs from beams. interactions44K per bunch @ <E>=10.5 GeV (0.85 W)
Tom Markiewicz
NLC - The Next Linear Collider Project
Maximum Radius of Pair e-,e+ vs. z( 1 TeV, Constant Bz, 20 mrad c )
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Small Detector w/ 6 Tesla Solenoid
1.2 cm
Tom Markiewicz
NLC - The Next Linear Collider Project
IR Layout Details
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X (cm)
Y(cm)
Q1
Extraction Beampipe
Maximum Radius of Pair Background
x-y Distribution of Pair e-,e+ at z = 2 m1 TeV, 6 Tesla Field Map
Pairs deposit ~ ½ Watt DC 109 rad/year
Plan View - 6 Tesla Detector
Low Z absorber
Pair Energy Monitor
Collar
Tom Markiewicz
NLC - The Next Linear Collider Project
Detector Backgrounds(LCD Small Detector)
Detector Issue Current Results
VXD hit density Goal: < 1 hit/mm2/trainFind: < 10 hits/mm2/train at r=1.2 cmMain source: Charged secondaries from e+,e- pairs hitting Q1 face
Tracking Chamber Hits Goal: < 104 /trainFind: ~105 /train at r = 30 cm secondaries from e+e- pairs hitting beam pipe and VXD
VXD Lifetime Goal: 3 x 109 neutrons/cm2
Find: 2 x 109 neutrons/cm2/year (1.5 year lifetime)Main Source: Neutron secondaries from e+e- pairs
hitting 2m < z < 6m
Hit Density a very rapid function of radius: @ r = 2 cm, goal is met
Work in Progress: Addition of low Z absorbers in TESLA & JLC designs have eliminated backscatters
Tom Markiewicz
NLC - The Next Linear Collider Project
Be Ring Maskplaced along field lines that reach VXD-L1
Reduces backscattered component of VXD-L1 hits to 16% of total
Direct hits now dominateVXD-L1 whereas before were only 11% of total
VXD-L1 Hit Density = 2.0 hits/mm2/train
Under study: why did # VXD hits/primary e- increase to make density go from expected 1.2 to observed 2.0?
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Tom Markiewicz
NLC - The Next Linear Collider Project
1.2 cm VXD L1 in BOTH L & S Detectors
Black: Layer 1Turquoise: Layer 2 Green: Layer 3 Blue: Layer 4 Red: Layer 5
B (Tesla)
Hit
s / b
unch
~3.5 x more Layer 1hits at 3 Tesla
With few backscattered hits, LCD group currently feels aggressive 1.2 cm VXD is also possible for Large Detector (3-4 T) detector
2.0 hits/mm2/train
84% from multiple hits by primary pair electrons
Tom Markiewicz
NLC - The Next Linear Collider Project
Neutron Backgrounds
•e+/e- pairs and radiative Bhabhas hitting beam-pipe and magnets in the extraction line.•Disrupted beam lost in the extraction line.
•0.25 % beam loss in recent redesign•Disrupted beam and beamstrahlung photons in the dump
Neutron hit density in VXD
Beam-Beam pairs 1.7 x 109 hits/cm2/yr
Radiative Bhabhas 0.02 x 109 hits/cm2/yr
Beam loss in extraction line 0.01 x 109 hits/cm2/year
Backshine from dump 0.25 x 109 hits/cm2/yr
TOTAL 2.0 x 109 hits/cm2/yr
Tom Markiewicz
NLC - The Next Linear Collider Project
Neutrons from the Beam Dump
Tom Markiewicz
NLC - The Next Linear Collider Project
Possible Problem with FLUKA may UNDERESTIMATE
Beam Dump Contribution to Neutron Background
More detailed study of source of VXD neutron hits indicated that NOTHING is coming down the beampipe
Leads to study of scattering angles in FLUKA
Learn of polar scattering angle approximation:
•OK for transport through bulk material (shielding)
•Not so good for small angle scattering down beam pipe
Severity of problem unknown at this time
Investigation under way with COG (a LLNL neutron transport code)
May also try MARS code
Tom Markiewicz
NLC - The Next Linear Collider Project
Quadrupole Synchrotron Radiation Update(Stan Hertzbach)
Apertures:
Q1: 5.9 mm and 5.4 mm radius SR “stay-clear” considered (6.4 mm spec. in DIMAD)
Extraction line: 10 mm radius beam pipe for z < 6 m, then 9 mm in quad magnets
Criteria:
No SR hits inner bore of Q1 or Be Ring Mask protecting VXD-L1
This is stricter than used in the past, where hits were allowed and we studied rescattering. It may be too strict, but # photons rises quickly!
Assumptions:
Flat beam halo assumed to fill collimation aperture
Consider 8 x x 40 y and greater collimation depth
Geometry:
Horizontal beamline with detector rotated 10 mrad
Tom Markiewicz
NLC - The Next Linear Collider Project
QSR Masking Conclusions
SR masks at 11m (y) and 12m (x) satisfy goal of NO Q1 / NO BeRM hits for 8 x x 40 y
collimation and current Q1 aperture
Increasing y collimation depth not possible
Additional masks at 4m (y) and 8m (x) required to maintain goals if
• want to increase x collimation depth to 12x
•Q1 SR stay-clear needs to be smaller (magnet engineering)
•Current feeling is that this is too close to the detector (50 mW of SR with an average photon energy of 49 GeV)
Try to design Q1 so that aperture is increased for last 25% of its length closest to IP
Maintain full diameter aperture of extraction beam pipe:
•Implications for Q1 support structure near L*=2m
Tom Markiewicz
NLC - The Next Linear Collider Project
Maruyama’s Conclusions on QSR from GEANT
Extraction line generates 63.1 /e of <E> = 31.1 MeV
•36 kW per beam
•160 W per beam strikes extraction line, mostly 90 < z < 130 m
Incoming beam analysis in progress
Misunderstanding in feeding beam parameters to GEANT. Need to check against Hertzbach’s work and include masks at 11 and 12m. Ignoring this:
QSR generates 17.2 /e of <E> = 28.5 MeV
•8.9 kW per beam
•6 W per beam strikes extraction line, mostly 6 < z < 9 m
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