Computed pion yields from a tantalum rod target
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m. Computed Pion Yields from a Tantalum Rod Target. Comparing MARS15 and GEANT4 across proton energies. 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

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Computed Pion Yields from a Tantalum Rod Target

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Computed pion yields from a tantalum rod target

m

Computed Pion Yields from a Tantalum Rod Target

Comparing MARS15 and GEANT4 across proton energies

Stephen Brooks, Kenny Walaron

NuFact’05


Contents

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 Walaron

NuFact’05


Benchmark problem

Benchmark Problem

Pions

  • Pions counted at rod surface

  • B-field ignored within rod (negligible effect)

  • Proton beam assumed parallel

    • Circular parabolic distribution, rod radius

Protons

1cm

Solid Tantalum

20cm

Stephen Brooks, Kenny Walaron

NuFact’05


Possible proton energies

Possible Proton Energies

Stephen Brooks, Kenny Walaron

NuFact’05


Total yield of p and p

Total Yield of p+ and p−

Normalised

to unit

beam power

NB: Logarithmic scale!

Stephen Brooks, Kenny Walaron

NuFact’05


Angular distribution mars15

Angular Distribution: MARS15

What causes the strange kink in the graph between 3GeV and 5GeV?

Stephen Brooks, Kenny Walaron

NuFact’05


Mars15 uses two models

MARS15 Uses Two Models

  • 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 Walaron

    NuFact’05


    Angular distribution geant4

    Angular Distribution: +GEANT4

    GEANT4 appears to have its own ‘kink’ between 15GeV and 30GeV

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Geant4 hadronic use cases

    GEANT4 Hadronic “Use Cases”

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Total yield of p and p geant4

    Total Yield of p+ and p−: +GEANT4

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Raw pion yield summary

    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 Walaron

    NuFact’05


    Tracking through two designs

    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 Walaron

    NuFact’05


    Fate plots

    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 Walaron

    NuFact’05


    Fate plot for chicane linac

    Fate Plot for Chicane/Linac

    (Pion distribution used here is from a 2.2GeV proton beam)

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Fate plot for phase rotation

    Fate Plot for Phase Rotation

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Probability grids

    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 Walaron

    NuFact’05


    Probability grids1

    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 Walaron

    NuFact’05


    Chicane linac transmission mars15

    Chicane/Linac Transmission (MARS15)

    Energy dependency is much flatter now we are selecting pions by energy range

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Phase rotator transmission mars15

    Phase Rotator Transmission (MARS15)

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Chicane linac transmission geant4

    Chicane/Linac Transmission (GEANT4)

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Phase rotator transmission geant4

    Phase Rotator Transmission (GEANT4)

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Conclusions energy choice

    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:

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Conclusions codes data

    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 Walaron

    NuFact’05


    References

    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 Walaron

    NuFact’05


    Computed pion yields from a tantalum rod target

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Energy deposition in rod heat

    Energy Deposition in Rod (heat)

    • Scaled for 5MW total beam power; the rest is kinetic energy of secondaries

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Total yield of p and p1

    Total Yield of p+ and p−

    From a purely target point of view, ‘optimum’ moves to 10-15GeV

    • Normalised to unit rod heating (p.GeV = 1.6×10-10 J)

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Angular distribution

    Angular Distribution

    2.2GeV

    6GeV

    Backwards

    p+ 18%

    p− 33%

    8%

    12%

    15GeV

    120GeV

    8%

    11%

    7%

    10%

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Possible remedies

    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 Walaron

    NuFact’05


    Simple cuts

    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:

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Simple cuts1

    Simple Cuts

    • Acceptance of the decay channel in (pL,pT)-space should look roughly like this:

    pT

    Larmor radius = ½ aperture limit

    pTmax

    Pions in this region transmitted

    qmax

    pL

    Angular limit (eliminate backwards/sideways pions)

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Simple cuts2

    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 Walaron

    NuFact’05


    Simple cuts3

    Simple Cuts

    • So, does it?

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Simple cuts4

    Simple Cuts

    • So, does it? Roughly.

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Simple cuts5

    Simple Cuts

    • So, does it? Roughly.

    • If we choose:

      • qmax = 45°

      • pTmax = 250 MeV/c

    • Now we can re-draw the pion yield graphs for this subset of the pions

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Cut yield of p and p

    Cut Yield of p+ and p−

    • Normalised to unit beam power (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 Walaron

    NuFact’05


    Cut yield of p and p1

    Cut Yield of p+ and p−

    This cut seems to have moved this optimum down slightly, to 8-10GeV

    • Normalised to unit rod heating

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Chicane linac transmission

    Chicane/Linac Transmission

    6-10GeV now looks good enough if we are limited by target heating

    • Normalised to unit rod heating

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Phase rotator transmission

    Phase Rotator Transmission

    • Normalised to unit rod heating

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Computed pion yields from a tantalum rod target

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Rod with a hole

    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 Walaron

    NuFact’05


    Rod with a hole1

    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 Walaron

    NuFact’05


    Yield decreases with hole

    Yield Decreases with Hole

    30 GeV

    2.2 GeV

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Yield per rod area with hole

    Yield per Rod Area with Hole

    30 GeV

    2.2 GeV

    This actually decreases at the largest hole size!

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Rod with a hole summary

    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 Walaron

    NuFact’05


    Variation of rod radius

    Variation of Rod Radius

    • We will change the incoming beam size with the rod size and observe the yields

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Variation of rod radius1

    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

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Variation of rod radius2

    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:

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Total yield with rod radius

    Total Yield with Rod Radius

    Stephen Brooks, Kenny Walaron

    NuFact’05


    Cut yield with rod radius

    Cut Yield with Rod Radius

    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 Walaron

    NuFact’05


    Note on rod tilt

    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 Walaron

    NuFact’05


    Note on rod length

    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 Walaron

    NuFact’05


    Conclusion

    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 Walaron

    NuFact’05


    Future work

    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

    Stephen Brooks, Kenny Walaron

    NuFact’05


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