June 13, 2003 1
Geant4 Simulations of the MICE Beamline
Tom RobertsIllinois Institute of Technology
June13, 2003
June 13, 2003 2
Introducing the g4beamline Program• A general tool for simulating beamlines, using Geant4 5.1p1.• All problem-specific aspects of the simulation are given in a
simple ASCII file.• The basic idea is to define elements, and then to place them
into the system (perhaps multiple times).• Centerline coordinates can be used, simplifying layout for
beamline-like configurations.– Centerline coordinates are piecewise-straight, with the z axis down
the nominal centerline of the beamline.– The centerline coordinates {x,y,z} rotate at a corner (bending
magnet), as do all elements placed after the corner.• By default, objects are simply lined up along the centerline;
specific locations and rotations can also be given.• The complexity of the description matches the complexity of
the problem.
June 13, 2003 3
The MICE Beamline Simulation• Decay Solenoid:
– Accurate magnetic map computed via infinitely-thin sheets– Map parameters (# sheets,nR,nZ,dR,dZ,length) are determined
automatically, given the required accuracy (0.0002 relative accuracy used)
• Quadrupole Magnets:– Perfect and constant block fields used.– No fringe fields.
• Bending Magnets:– Fringe field computation - Laplace’s Equation for magnetic
potential– Assume infinitely-wide– Computation done using Excel,
1 mm grid– Solution extended in Y and Z
via symmetry
Pole
Pole
Solution RegionSolution RegionSolution Region
June 13, 2003 4
RAL Type I bending Magnet ModelBend Type 1
(pole half-length=457, Eff-half-length=519) B fields
-0.2000
0.0000
0.2000
0.4000
0.6000
0.8000
1.0000
1.2000
0 200 400 600 800 1000
By on AxisBz Halfway up
June 13, 2003 5
micebeam.in (Input to g4beamline)coil Decay innerRadius=200.0 outerRadius=250.0 length=5000.0 material=Cu solenoid DecayS coilName=Decay current=47.94 color=1,0,0tubs SolenoidBody innerRadius=250 outerRadius=1000 length=5000 kill=1group DecaySolenoid length=5000
place DecayS z=0place SolenoidBody z=0
endgroup
idealquad default ironRadius=381 ironLength=1104.9 kill=1idealquad Q1 fieldLength=863.6 fieldRadius=101.6 gradient=2.0 ironColor=0,.6,0 idealquad Q2 fieldLength=863.6 fieldRadius=101.6 gradient=-3.0 ironColor=0,0,.6idealquad Q3 fieldLength=863.6 fieldRadius=101.6 gradient=0.8 ironColor=0,.6,0
mappedmagnet B1 mapname=RALBend1 Bfield=-0.9646 \fieldWidth=660.4 fieldHeight=152 fieldLength=2000 fieldColor='' \ironLength=1397 ironHeight=1320 ironWidth=1981 ironColor=1,1,0 kill=1
mappedmagnet B2 mapname=RALBend1 Bfield=-0.3512 \fieldWidth=660.4 fieldHeight=152 fieldLength=2000 fieldColor='' \ironLength=1397 ironHeight=1320 ironWidth=1981 ironColor=1,1,0 kill=1
detector MICEdiffuser1 radius=250 length=1.0 color=0,1,1
place Q1 z=3000place Q2 z=4400place Q3 z=5800place B1 z=7855.8 rotation=Y30 x=250corner B1c z=8000 rotation=Y60place DecaySolenoid z=12200place B2 z=16135 rotation=Y15.8 x=175corner B2c z=16185 rotation=Y31.7place MICEdiffuser1 z=18840
Group Elements together
A corner in the centerlineY60 is a 60° rotation around Y;
Multiple rotations: Y60,Z45,X90
Kill=1 makes a Perfect Shield.
“tubs” is Geant4-speak for atube or cylinder
A detector generates an NTuple
The beam and physicsspecifications are omitted for clarity, asis other basic stuff.
Every elementhas a name
Color is R,G,BOmitted=invisible
A solenoid is a coil plus a currentThe coil has a sharable map
June 13, 2003 6
MICE Beamline layout
June 13, 2003 7
Pictures of Simulated TracksColors of Tracks:
Green pi+Blue mu+White e+
Other particles are killed.
Colors of Objects:Green Focusing QuadBlue Defocusing QuadYellow Bending MagnetRed Decay SolenoidWhite Wide detector atMICE Z Position
• The target is at the lower left, with protons not shown – if they were shown they would head 25 degrees down to the lower right.
• The detector at MICE diffuser1 is much larger than the experimental acceptance, so I can see what’s out there.
• For quads and the solenoid, only the ends are shown.• These pictures are 2-d plan views (not 3-d as the previous picture).
June 13, 2003 8
Good Muon
June 13, 2003 9
π+ μ+ e+
Positrons are quite rare.
June 13, 2003 10
Pion
There are also a gazillion protons.
June 13, 2003 11
There are many ways for muons to miss
June 13, 2003 12
There are many ways for muons to miss
June 13, 2003 13
There are many ways for muons to miss
June 13, 2003 14
But some are just lucky
June 13, 2003 15
Pions – Beam Loss position along Centerline
June 13, 2003 16
Pions at the MICE Z Position
June 13, 2003 17
Muons at the MICE Z Position
June 13, 2003 18
Protons at the MICE Z Position
June 13, 2003 19
Pion Momentum at the MICE Z position
June 13, 2003 20
Muon Momentum at the MICE Z Position
June 13, 2003 21
Proton Momentum at the MICE Z Position
Scale is different – this is quite similar to the π+ momentum distribution.
June 13, 2003 22
Conclusions• Visualization is essential to verify the layout is correct.• g4beamline is a flexible and useful tool for simulations
like this.• The MICE detector will have significant backgrounds
from the beamline – not to mention strays that cannot be accurately modeled, and of course Cosmic Rays.
• We need to compute normalized fluxes for protons, pions, and muons.
• Diffuser1 is clearly not needed to “spread out the beam”; Diffuser2 is still required to break the angle-position correlation.