Upload
oliver-ray
View
217
Download
0
Embed Size (px)
Citation preview
Stephen Brooks, Kenny WalaronNuFact’05
1 of 24
Computed Pion Yields from a Tantalum Rod Target
Comparing MARS15 and GEANT4 across proton energies
Stephen Brooks, Kenny WalaronNuFact’05
2 of 24
Contents
• Benchmark problem
• Physics models and energy ranges– MARS15 model– GEANT4 “use cases”– Effects on raw pion yield and angular spread
• Probability map “cuts” from tracking– Used to estimate muon yields for two different
front-ends, using both codes, at all energies
Stephen Brooks, Kenny WalaronNuFact’05
3 of 24
Benchmark Problem
• Pions counted at rod surface
• B-field ignored within rod (negligible effect)
• Proton beam assumed parallel– Circular parabolic distribution, rod radius
20cm
1cmSolid Tantalum
Protons
Pions
Stephen Brooks, Kenny WalaronNuFact’05
4 of 24
Possible Proton EnergiesProton Driver GeV RAL StudiesOld SPL energy 2.2
3 5MW ISIS RCS 1
[New SPL energy 3.5GeV] 45 Green-field synch.
6 5MW ISIS RCS 2
FNAL linac (driver study 2) 8 RCS 2 low rep. rate
10 4MW FFAG
[FNAL driver study 1, 16GeV] 15 ISR tunnel synch.
[BNL/AGS upgrade, 24GeV] 20JPARC initial 30 PS replacement
40JPARC final 50
75100
FNAL injector/NuMI 120
Stephen Brooks, Kenny WalaronNuFact’05
5 of 24
8G
eV
10
Ge
V
15
Ge
V
20
Ge
V
4G
eV
3G
eV
2G
eV
5G
eV
6G
eV 3
0G
eV
40
Ge
V
50
Ge
V
75
Ge
V
10
0G
eV
12
0G
eV
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
0.1600
1 10 100 1000
Proton Energy (GeV)
Pio
ns/
(Pro
ton
*GeV
)
GEANT 4 Pi+
GEANT 4 Pi-
MARS15 Pi+
MARS15 Pi-
Total Yield of + and −
Normalisedto unit
beam power
NB: Logarithmic scale!
Stephen Brooks, Kenny WalaronNuFact’05
6 of 24
Angular Distribution: MARS15
2.2
Ge
V
3G
eV
4G
eV
5G
eV
6G
eV
8G
eV
10
Ge
V
15
Ge
V
20
Ge
V
30
Ge
V
40
Ge
V5
0G
eV
75
Ge
V
10
0G
eV
12
0G
eV
0
20
40
60
80
100
120
1 10 100 1000
Proton Energy (GeV)
An
gle
fro
m A
xis
(°)
pi+ 1st Quartile
pi+ Median
pi+ 3rd Quartile
pi- 1st Quartile
pi- Median
pi- 3rd Quartile
What causes the strange kink in the graph between 3GeV and 5GeV?
Stephen Brooks, Kenny WalaronNuFact’05
7 of 24
MARS15 Uses Two Models<3GeV 3-5 >5GeV
MARS15 CEM2003 Inclusive– The “Cascade-Exciton Model” CEM2003 for E<5GeV– “Inclusive” hadron production for E>3GeV
• Nikolai Mokhov says:
A mix-and-match algorithm is used between 3 and 5 GeV to provide a continuity between the two domains. The high-energy model is used at 5 GeV and above. Certainly, characteristics of interactions are somewhat different in the two models at the same energy. Your results look quite reasonable, although there is still something to improve in the LANL's low-energy model, especially for pion production. The work is in progress on that. A LAQGSM option coming soon, will give you an alternative possibility to study this intermediate energy region in a different somewhat more consistent way.
Stephen Brooks, Kenny WalaronNuFact’05
8 of 24
Angular Distribution: +GEANT4
GEANT4 appears to have its own ‘kink’ between 15GeV and 30GeV
Stephen Brooks, Kenny WalaronNuFact’05
9 of 24
GEANT4 Hadronic “Use Cases”
<3GeV 3-25GeV >25GeV
LHEP GHEISHA inherited from GEANT3
LHEP-BERT Bertini cascade
LHEP-BIC Binary cascade
QGSP (default) Quark-gluon string model
QGSP-BERT
QGSP-BIC
QGSC + chiral invariance
Stephen Brooks, Kenny WalaronNuFact’05
10 of 24
Total Yield of + and −: +GEANT4
0.00
0.05
0.10
0.15
0.20
0.25
0 1 2 3 4 5 6 7 8 9 10
Proton Energy (GeV)
Pio
n/(
Pro
ton
*En
erg
y(G
eV))
GEANT4 Pi+ LHEP-BIC
GEANT4 Pi- LHEP-BIC
GEANT4 Pi+ QGSP
GEANT4 Pi- QGSP
GEANT4 Pi+ QGSP_BIC
GEANT4 Pi- QGSP_BIC
GEANT4 Pi+ QGSP_BERT
GEANT4 Pi- QGSP_BERT
GEANT4 Pi+ LHEP
GEANT4 Pi- LHEP
GEANT4 Pi+ LHEP-BERT
GEANT4 Pi- LHEP-BERT
GEANT4 Pi+ QGSC
GEANT4 Pi- QGSC
MARS15 Pi+
MARS15 Pi-
Stephen Brooks, Kenny WalaronNuFact’05
11 of 24
Raw Pion Yield Summary
• It appears that an 8-30GeV proton beam: – Produces roughly twice the pion yield…– …and in a more focussed angular cone
...than the lowest energies.
• Unless you believe the BIC model!
• Also: the useful yield is crucially dependent on the capture system.
Stephen Brooks, Kenny WalaronNuFact’05
12 of 24
Tracking through Two Designs
• Both start with a solenoidal channel
• Possible non-cooling front end:– Uses a magnetic chicane for bunching,
followed by a muon linac to 400±100MeV
• RF phase-rotation system:– Line with cavities reduces energy spread to
180±23MeV for injecting into a cooling system
Stephen Brooks, Kenny WalaronNuFact’05
13 of 24
Fate Plots
• Pions from one of the MARS datasets were tracked through the two front-ends and plotted by (pL,pT)
– Coloured according to how they are lost…– …or white if they make it through
• This is not entirely deterministic due to pion muon decays and finite source
Stephen Brooks, Kenny WalaronNuFact’05
14 of 24
Fate Plot for Chicane/LinacMagenta Went backwards
Red Hit rod again
Orange Hit inside first solenoid
Yellow/Green Lost in decay channel
Cyan Lost in chicane
Blue Lost in linac
Grey Wrong energy
White Transmitted OK
(Pion distribution used here is from a 2.2GeV proton beam)
Stephen Brooks, Kenny WalaronNuFact’05
15 of 24
Fate Plot for Phase RotationMagenta Went backwards
Red Hit rod again
Orange Hit inside first solenoid
Yellow/Green Lost in decay channel
Blue Lost in phase rotator
Grey Wrong energy
White Transmitted OK
Stephen Brooks, Kenny WalaronNuFact’05
16 of 24
Probability Grids
• Can bin the plots into 30MeV/c squares and work out the transmission probability within each
Chicane/Linac Phase Rotation
Stephen Brooks, Kenny WalaronNuFact’05
17 of 24
Probability Grids
• Can bin the plots into 30MeV/c squares and work out the transmission probability within each
• These can be used to estimate the transmission quickly for each MARS or GEANT output dataset for each front-end
Stephen Brooks, Kenny WalaronNuFact’05
18 of 24
Chicane/Linac Transmission (MARS15)
3
GeV
4
GeV
8
GeV
4
0GeV
5
0GeV
7
5GeV
1
00G
eV
120
GeV
3
0GeV
6G
eV
2
.2G
eV 1
5GeV
1
0GeV
5
GeV
2
0GeV
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
1 10 100 1000
Proton Energy (GeV)
Pio
ns p
er P
roto
n.G
eV (e
st. C
hica
ne/L
inac
)
pi+/(p.GeV)
pi-/(p.GeV)
Energy dependency is much flatter now we are selecting pions by energy range
Stephen Brooks, Kenny WalaronNuFact’05
19 of 24
Phase Rotator Transmission (MARS15)
3GeV
4GeV
8GeV
40G
eV
50
GeV
75G
eV
100G
eV
12
0GeV
30G
eV
6GeV
2.2G
eV
15G
eV
10G
eV
5GeV
20G
eV
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
1 10 100 1000
Proton Energy (GeV)
Pio
ns p
er P
roto
n.G
eV (e
st. P
hase
Rot
ator
)
pi+/(p.GeV)
pi-/(p.GeV)
Stephen Brooks, Kenny WalaronNuFact’05
22 of 24
Conclusions (energy choice)• While 30GeV may be excellent in terms of
raw pion yields, the pions produced at higher energies are increasingly lost due to their large energy spread
• Optimal ranges appear to be:
According to: For +: For −:
MARS15 5-30GeV 5-10GeV
GEANT4 4-10GeV 8-10GeV
Stephen Brooks, Kenny WalaronNuFact’05
23 of 24
Conclusions (codes, data)
• GEANT4 ‘focusses’ pions in the forward direction a lot more than MARS15– Hence double the yields in the front-ends
• Binary cascade model needs to be reconciled with everything else
• Other models say generally the same thing, but variance is large– Need HARP data in the range of interest!
Stephen Brooks, Kenny WalaronNuFact’05
24 of 24
References
• S.J. Brooks, Talk on Target Pion Production Studies at Muon Week, CERN, March 2005; http://stephenbrooks.org/ral/report/
• K.A. Walaron, UKNF Note 30: Simulations of Pion Production in a Tantalum Rod Target using GEANT4 with comparison to MARS; http://hepunx.rl.ac.uk/uknf/wp3/uknfnote_30.pdf
Stephen Brooks, Kenny WalaronNuFact’05
26 of 24
Energy Deposition in Rod (heat)
• Scaled for 5MW total beam power; the rest is kinetic energy of secondaries
2.2G
eV
3GeV
4GeV
5GeV
6GeV
8GeV
10G
eV
15G
eV
20G
eV 30G
eV
40G
eV50
GeV
75G
eV
100G
eV12
0GeV
0
200000
400000
600000
800000
1000000
1200000
1400000
1 10 100 1000
Proton Energy (GeV)
Po
wer
Dis
sip
atio
n (
Wat
ts,
no
rmal
ised
to
5M
W
inco
min
g b
eam
)
Stephen Brooks, Kenny WalaronNuFact’05
27 of 24
Total Yield of + and −
2.2G
eV
3GeV 4G
eV
5GeV
6GeV 8G
eV
10G
eV
15G
eV
20G
eV
30G
eV
40G
eV
50G
eV
75G
eV
100G
eV12
0GeV
0
0.2
0.4
0.6
0.8
1
1.2
1 10 100 1000
Proton Energy (GeV)
Pio
ns
per
Pro
ton
.GeV
of
rod
hea
tin
g
pi+/(p.GeV_th)
pi-/(p.GeV_th)
• Normalised to unit rod heating (p.GeV = 1.6×10-10 J)
From a purely target point of view, ‘optimum’ moves to 10-15GeV
Stephen Brooks, Kenny WalaronNuFact’05
28 of 24
Angular Distribution2.2 GeV
0
50
100
150
200
250
0 20 40 60 80 100 120 140 160 180 200
Off-axis angle (°)
Pio
ns piplus
piminus
6 GeV
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100 120 140 160 180 200
Off-axis angle (°)
Pio
ns piplus
piminus
15 GeV
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 20 40 60 80 100 120 140 160 180 200
Off-axis angle (°)
Pio
ns piplus
piminus
120 GeV
0
5000
10000
15000
20000
25000
30000
0 20 40 60 80 100 120 140 160 180 200
Off-axis angle (°)
Pio
ns piplus
piminus
2.2GeV 6GeV
15GeV 120GeV
Backwards+ 18%− 33%
8%12%
8%11%
7%10%
Stephen Brooks, Kenny WalaronNuFact’05
29 of 24
Possible Remedies
• Ideally, we would want HARP data to fill in this “gap” between the two models
• K. Walaron at RAL is also working on benchmarking these calculations against a GEANT4-based simulation
• Activating LAQGSM is another option
• We shall treat the results as ‘roughly correct’ for now, though the kink may not be as sharp as MARS shows
Stephen Brooks, Kenny WalaronNuFact’05
30 of 24
Simple Cuts
• It turns out geometric angle is a badly-normalised measure of beam divergence
• Transverse momentum and the magnetic field dictate the Larmor radius in the solenoidal decay channel:
z
T
eB
pr
z
T
z
T
B
p
B
cpr
3]T[
)/10](MeV/c[]cm[
8
Stephen Brooks, Kenny WalaronNuFact’05
31 of 24
Simple Cuts
• Acceptance of the decay channel in (pL,pT)-space should look roughly like this:
pL
pTLarmor radius = ½ aperture limit
Angular limit (eliminate backwards/sideways pions)
Pions in this region transmitted
max
pTmax
Stephen Brooks, Kenny WalaronNuFact’05
32 of 24
Simple Cuts
• So, does it?
• Pions from one of the MARS datasets were tracked through an example decay channel and plotted by (pL,pT)
– Coloured green if they got the end– Red otherwise
• This is not entirely deterministic due to pion muon decays and finite source
Stephen Brooks, Kenny WalaronNuFact’05
35 of 24
Simple Cuts
• So, does it? Roughly.
• If we choose:– max = 45°
– pTmax = 250 MeV/c
• Now we can re-draw the pion yield graphs for this subset of the pions
Stephen Brooks, Kenny WalaronNuFact’05
36 of 24
Cut Yield of + and −
• Normalised to unit beam power (p.GeV)
3GeV
4G
eV
5G
eV
8GeV
10G
eV
20G
eV
40G
eV
50G
eV
75G
eV
100G
eV
12
0GeV
15G
eV
2.2G
eV
6GeV
30G
eV
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
1 10 100 1000
Proton Energy (GeV)
Pio
ns
per
Pro
ton
.GeV
(w
ith
in 4
5°,
pt<
250M
eV/c
)
pi+/(p.GeV)
pi-/(p.GeV)
High energy yield now appears a factor of 2 over low energy, but how much of that kink is real?
Stephen Brooks, Kenny WalaronNuFact’05
37 of 24
2.2
Ge
V
3G
eV
4G
eV
5
Ge
V
6G
eV
8G
eV
1
0G
eV
15
Ge
V
20
Ge
V
30
Ge
V
40
Ge
V
50
Ge
V
75
Ge
V
10
0G
eV
1
20
Ge
V
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
1 10 100 1000
Proton Energy (GeV)
Cu
t P
ion
s p
er P
roto
n.G
eV(h
eat)
pi+/(p.GeV_th)
pi-/(p.GeV_th)
Cut Yield of + and −
• Normalised to unit rod heating
This cut seems to have moved this optimum down slightly, to 8-10GeV
Stephen Brooks, Kenny WalaronNuFact’05
38 of 24
Chicane/Linac Transmission
2
.2G
eV
3
Ge
V
4
Ge
V
5G
eV
6
Ge
V
8
Ge
V
1
5G
eV
2
0G
eV
3
0G
eV
4
0G
eV
5
0G
eV
7
5G
eV
1
00
Ge
V
12
0G
eV
1
0G
eV
0
0.01
0.02
0.03
0.04
0.05
0.06
1 10 100 1000
Proton Energy (GeV)
Ch
ican
e/L
inac
Pio
ns
per
Pro
ton
.GeV
(hea
t)
pi+/(p.GeV_th)
pi-/(p.GeV_th)
• Normalised to unit rod heating
6-10GeV now looks good enough if we are limited by target heating
Stephen Brooks, Kenny WalaronNuFact’05
39 of 24
Phase Rotator Transmission
• Normalised to unit rod heating
2G
eV
3G
eV
4G
eV
5G
eV
6G
eV
8G
eV
15
Ge
V
20
Ge
V
30
Ge
V
40
Ge
V
5
0G
eV
75
Ge
V
10
0G
eV
12
0G
eV
10
Ge
V
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1 10 100 1000
Proton Energy (GeV)
Ph
ase
Ro
tato
r P
ion
s p
er P
roto
n.G
eV(h
eat)
pi+/(p.GeV_th)
pi-/(p.GeV_th)
Stephen Brooks, Kenny WalaronNuFact’05
41 of 24
Rod with a Hole
• Idea: hole still leaves 1-(rh/r)2 of the rod available for pion production but could decrease the path length for reabsorption
Rod cross-section r
rh
Stephen Brooks, Kenny WalaronNuFact’05
42 of 24
Rod with a Hole
• Idea: hole still leaves 1-(rh/r)2 of the rod available for pion production but could decrease the path length for reabsorption
• Used a uniform beam instead of the parabolic distribution, so the per-area efficiency could be calculated easily
• r = 1cm
• rh = 2mm, 4mm, 6mm, 8mm
Stephen Brooks, Kenny WalaronNuFact’05
43 of 24
Yield Decreases with Hole
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
0
0.2
0.4
0.6
0.8
1
1.2
pi+/(p.GeV)
pi-/(p.GeV)
Area fraction
30 GeV
2.2 GeV
Stephen Brooks, Kenny WalaronNuFact’05
44 of 24
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009
pi+ eff.
pi- eff.
Yield per Rod Area with Hole
30 GeV
2.2 GeV
This actually decreases at the largest hole size!
Stephen Brooks, Kenny WalaronNuFact’05
45 of 24
Rod with a Hole Summary
• Clearly boring a hole is not helping, but:
• The relatively flat area-efficiencies suggest reabsorption is not a major factor– So what if we increase rod radius?
• The efficiency decrease for a hollow rod suggests that for thin (<2mm) target cross-sectional shapes, multiple scattering of protons in the tantalum is noticeable
Stephen Brooks, Kenny WalaronNuFact’05
46 of 24
Variation of Rod Radius
• We will change the incoming beam size with the rod size and observe the yields
Stephen Brooks, Kenny WalaronNuFact’05
47 of 24
Variation of Rod Radius
• We will change the incoming beam size with the rod size and observe the yields
• This is not physical for the smallest rods as a beta focus could not be maintained
Emittance x Focus radius Divergence Focus length
25 mm.mrad extracted from proton machine
10mm 2.5 mrad 4m
5mm 5 mrad 1m
2.5mm 10 mrad 25cm
2mm 12.5 mrad 16cm
Stephen Brooks, Kenny WalaronNuFact’05
48 of 24
Variation of Rod Radius
• We will change the incoming beam size with the rod size and observe the yields
• For larger rods, the increase in transverse emittance may be a problem downstream
• Effective beam-size adds in quadrature to the Larmor radius:
2
2
z
Teff eB
prr
Stephen Brooks, Kenny WalaronNuFact’05
49 of 24
Total Yield with Rod Radius
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 0.5 1 1.5 2 2.5 3 3.5 4
Rod Radius (cm)
Pio
n Y
ield
pi+/(p.GeV)
pi-/(p.GeV)
30GeV
2.2GeV
Stephen Brooks, Kenny WalaronNuFact’05
50 of 24
Cut Yield with Rod Radius
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
0 0.5 1 1.5 2 2.5 3 3.5 4
Rod Radius (cm)
Pio
n Y
ield
Wit
hin
45
° o
f A
xis
an
d T
ran
sv
ers
e M
om
en
tum
Be
low
25
0M
eV
/c
pi+/(p.GeV)
pi-/(p.GeV)
30GeV
2.2GeV
Rod heating per unit volume and hence shock amplitude decreases as 1/r2 !
Multiple scattering decreases yield at r = 5mm and below Fall-off due to
reabsorption is fairly shallow with radius
Stephen Brooks, Kenny WalaronNuFact’05
51 of 24
Note on Rod Tilt
• All tracking optimisations so far have set the rod tilt to zero
• The only time a non-zero tilt appeared to give better yields was when measuring immediately after the first solenoid
• Theory: tilting the rod gains a few pions at the expense of an increased horizontal emittance (equivalent to a larger rod)
Stephen Brooks, Kenny WalaronNuFact’05
52 of 24
Note on Rod Length
• Doubling the rod length would:– Double the heat to dissipate– Also double the pions emitted per proton– Increase the longitudinal emittance
• The pions already have a timespread of RMS 1ns coming from the proton bunch
• The extra length of rod would add to this the length divided by c
Stephen Brooks, Kenny WalaronNuFact’05
53 of 24
Conclusion
• Current results indicate 6-10GeV is an optimal proton driver energy for current front-ends– If we can accept larger energy spreads, can go to a
higher energy and get more pions
• A larger rod radius is a shallow tradeoff in pion yield but would make solid targets much easier
• Tilting the rod could be a red herring– Especially if reabsorption is not as bad as we think
• So making the rod coaxial and longer is possible
Stephen Brooks, Kenny WalaronNuFact’05
54 of 24
Future Work
• Resimulating with the LAQGSM added
• Benchmarking of MARS15 results against a GEANT4-based system (K. Walaron)
• Tracking optimisation of front-ends based on higher proton energies (sensitivity?)
• Investigating scenarios with longer rods– J. Back (Warwick) also available to look at
radioprotection issues and adding B-fields using MARS