G4beamline Simulations for H8

Author: Freja Thoresen∗

EN-MEF-LE, Univ. of Copenhagen & CERN

Supervisor: Nikolaos CharitonidisCERN

(Dated: December 15, 2015)

∗Electronic address: frejathoresen@gmail.com

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Abstract

Detailed simulations of the H8 beam line at the North Area, using the G4beamline software were performed in theframework of this study. The conventions used by the program are analysed. Having modelled precisely the beamline, several studies examining the beam transmission and composition were performed. The results were comparedwith measurements, where a satisfactory agreement was found.

The muon production and transport is studied in details throughout the beam line.

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I. INTRODUCTION

CERN, the European Organization for Nuclear Research, is the biggest research facility located in Switzerland andFrance. The CERN accelerator complex consists of several machines, the most important of which are Booster, PS,SPS and LHC.

The Super Proton Synchrotron (SPS) is the second largest accelerator at CERN. The beam is accelerated in toa final momentum of 400 GeV/c. The SPS beam is extracted to several facilities. One of them is the North Area,where the SPS beam can be used for ”fixed target” experiments, i.e experiments that measure the particles producedon a fixed target, or test beams that use secondary beams produced from a target. This beam is transported to thefacilities of the North Area through different beam lines. In this study the H8 beam line at the Experimental HallNorth 1 (EHN1) is studied using the G4beamline software. A schematic of the CERN accelerator complex is shownin Figure 1.

FIG. 1: The CERN accelerator complex. The H8 beam line is located at the North Area.

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II. THE TARGET T4 AND H8 BEAM LINE

A. Target Station

As already mentioned in the introduction, the primary SPS beam is extracted and transported on a target station.There, the proton beam with a momentum of 400 GeV/c impinges on a thin beryllium target. The particle productionfrom the target has been studied extensively in [2]. The T4 target station is shown in Figure 2. Before and after thetarget, three horizontal bends exist for the purpose of enhancing the particle production through a ”wobbling” of thebeam, as described in [3]. The horizontal bend located after the target makes therefore a first selection by bendingthe particles in different angles corresponding to their energies. For a schematic of the target station and the particletrajectory towards H8 beam line, see Figure 2.

FIG. 2: T4 Target station. The primary proton beam hits the T4 target, and the produced secondaries will be bentaccording to their energy. The particles with the energy such that the deflection kick in the bend corresponds to theright angle will be able to continue through the TAX collimator. In a usual configuration the energy of the particlesmoving through H8 is 180 GeV [3].

B. H8 beam line

The H8 beam line is about 600 m long, and consists of various bends, quadropoles, collimators and detectors. Aschematic of the H8 beam line is shown in Figure 3. After the target and the B3T bend, two vertical coupled bendsare placed. These bends will bend the beam up, and the muon halo created in the target will be dumped in theground, following a straight line from the target. After the vertical bend up, momentum selection collimators willselect the momentum of the particles transported. Finally the beam will be bend down again, and to the right beforereaching the experimental zones through several magnetic elements.

In the following schematic the vertical bends and some of the main collimators are shown. H8 secondary beam lineis a magnetic spectrometer, since with the correct choice of the magnet currents and collimator apertures, a preciseselection of the secondary particles’ momentum can be made.

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FIG. 3: Simple schematic of the H8 beamline layout. The secondary beam starts at the T4 target. Then the beam willbe bend up by B1 and B2, and later it will be bend down by B3 and B4. With the bends and the momentum slits, aprecise momentum selection is performed.

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III. SIMULATION OF THE H8 BEAM LINE USING G4BEAMLINE

G4beamline is a program based on GEANT4. G4beamline is widely used in accelerator physics where the beamlinecan be simulated without the need to write a lot of C++ code. In G4beamline the beam line can be describedin a single ASCII file and the output from simulated detectors is given in .root format. This offers the advantagethat the histograms can be made using HistoRoot[4]. But most importantly, all the common beam line elements areimplemented in G4beamline. In the next section the placement and the particular conventions for the bends will bedescribed.

A. Magnetic Bends

Bending dipoles are used to bend particles of a specific energy at a specific angle. The bend has a magnetic field,which gives a force on the particles given by the Lorentz force, and changes therefore their trajectory.

In order to correctly place the elements in the program, an examination of the effect of different parameters inG4beamline had to be made. The parameters to be defined in the program when placing a bend is: Magnetic field,geometry rotation and offset.

In G4BeamLine, the user can choose the coordinate system with which the placement of the elements will beperformed. Three coordinate systems are available : (a) Centerline coordinates, which are the coordinates of thebeam trajectory, (b) local coordinates, which are the coordinates of any local transformation possibly occuring inone element or (c) global coordinates, which are the coordinates of the model’s ”world”. From the three coordinatesystems, centerline is the most convenient to use. Therefore the placement of the magnetic elements is done in thiscoordinate system, i.e the beam is impinging on the ”center” of the element’s aperture.

1. Magnetic Field

The bending of a particle track is determined by the Lorentz force applied to it, due to an external magnetic field.The rotation (or ”bending angle”) of the particle track after a bend with length L and field B can be expressed by

θ[mrad] =299.79 ·B[T ] · L[m]

p[GeV/c](1)

where p is the momentum of the particle passing through the (assumed homogeneous) external magnetic field.

2. Geometry rotation

When the magnetic field bends the particles, the magnet metallic structure itself also needs to be rotated, in orderto correctly follow the trajectory of the beam, without causing extra losses. For an illustration of the effect, see Figure4.

3. Offset

The offset is a distance which the bend should be moved by, in order to be optimally placed. More specifically, therotation of the bend will affect the bending of the beam. To correct the bending of the beam, one can either adjustthe magnetic field after the rotation or perform do a minor shift of the magnet.

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(a) Magnetic field parameter set. (b) Magnetic field and geometry rotation set. (c) Magnetic field, geometry rotation and shiftset.

FIG. 4: A schematic figure showing the effect of the three parameters in G4beamline.

B. Conventions in G4beamline for bends

Apart from understanding the meaning of the parameters needed in G4BeamLine in order to define the bends,the correct sign of these parameters had to be understood. Specifically, ”left” and ”right” is a matter of convention.Therefore as reference, the system of CERN was used. In this document, ”left” means ”beam left” and ”right” means”beam right”. As previously mentioned, 3 parameters must be defined when placing a bend: Magnetic field, rotationand shift. The correct signs of the numbers in these three parameters are analyzed in the section below.

1. Horizontal Bends

The signs of the parameters for a horizontal bend is here investigated to define ”left” and ”right” in G4beamline.a. Magnetic field With a negative value for the magnetic field the beam will go left, and with a positive value

for the magnetic field the beam will go right, with respect to the previous position.b. Geometry Rotation For the horizontal bend, a rotation around the Y-axis is performed. An example of this

rotation can be seen in Figure 5. So if the rotation parameter is positive, the bend will rotate to the left, and if therotation is negative the bend will rotate to the right.

c. Offset The shift/offset will move the bend to the left or right. With a positive shift the bend will move to theleft, and a negative shift will move the bend to the right.

(a) Positive rotation (b) Negative rotation

FIG. 5: Top view of a vertical bend. In figure (a) the bend rotates left, and in figure (b) the bend rotates right. Thedirection of the beam is from the left to the right.

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2. Conclusion on parameters

For the vertical bends, similar investigation was performed. The signs of the parameters for both horizontal andvertical bends, alongside with their effect can be found in Table I and II.

TABLE I: Parameters for horizontal bends.

Right Left

Magnetic Field Positive Negative

Rotation (around y-axis) Negative Positive

Shift Positive Negative

TABLE II: Parameters for vertical bends

Up Down

Magnetic Field Negative Positive

Rotation (Z90, around x-axis) Negative Positive

Shift Negative Positive

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IV. COMPARISON OF SIMULATED AND MEASURED BEAM PROFILES

Measurements of triggers (scintillators), FISC and wire chambers was performed at ENH1, with a beam of 180GeV. The measurements can be compared to the results from the simulation in G4beamline.

A. Simulated Beam Parameters

As already discussed in the previous sections, the primary SPS beam is impinging on the Be-target. For the purposeof this study, the secondary beam with parameters matching the Atherton’s formula in [2] has being generated andused as source. More specifically, the simulated secondary beam has a composition of roughly 70 % protons and 30 %pions, and the spot-size is given in Table III. The x and y spot-sizes as well as the horizontal and vertical divergenceof the starting secondary beam can be seen also in Figure 6.

TABLE III: Parameters for the secondary beam produced at the Be-Target

x x-prime y y-prime

RMS[mm/mrad] 1.997 1.244 0.5857 1.236

FIG. 6: The produced secondary beam at the T4 target, based on calculations by Atherton et al.

In the following sections, a comparison between the several beam instruments and the simulation results can beseen.

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B. Comparison of the FISC profiles

The FISC (Filament Scanner) is a profile monitor of the beam. Using a moving filament, it records the total chargeof the particles impinging on the filament, thus estimating the beam profile. Several FISC’s are installed throughoutthe beam line. A selective comparison between the simulations and the profiles obtained by the FISCs can be seen inFigure 7.

(a) x-profile (G4BL) (b) x-profile (measurement)

(c) y-profile (G4BL) (d) y-profile (measurement)

FIG. 7: Comparison of the beam profile at FISC12, located at a distance of 477 m from the target. In the horizontalplane there a very good agreement between simulated and measured results. In the vertical plane the similarity is evenbetter. The small inconvenience in the symmetry observed in the vertical level, can be explained by the fact that theconvention of left and right in the simulation, is opposite of the detector.

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C. Comparison of Wire Chambers’ profiles

Wire chambers are gas detectors. The wire chamber consists of wires in both the x- and y-direction, so it candetermine the position of a particle in both the vertical and horizontal plane. The results of the comparison ofthe beam profile between the wire-chambers and the simulated profiles can be seen in Figure 8 for wire chamberXDWC3,4, which is placed 543 m after the target.

(a) x-profile (G4Bl) (b) x-profile (measurement)

(c) y-profile (G4BL) (d) y-profile (measurement)

FIG. 8: In the horizontal plane, there is a very good aggrement between the measured and the simulated profile. Thetale of the histogram would probably be clearer in the simulation, if there were more statistics. The small offset of thecenter of the histograms is due to the effect of the small corrector magnet that results in a misteered beam in the H8beam line, which is not simulated. In the vertical plane there is also a very good agreement.

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D. Scintillators

Several scintillators (triggers) are placed along the beam line in order to monitor the beam intensity in severalpoints of the beam line. The scintillators can only determine the number of particles hitting the detector. A relativecomparison between the measured and simulated scintillator counts was performed. The results can be seen in Figure9.

FIG. 9: The simulated values are normalized to the first point of the measured data, and the data is shown on alogarithmic scale. In this plot the transmission of the beam line is shown at 180 GeV/c. The measured and simulatedrates are quite similar.

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V. PARTICLE PRODUCTION AND BEAM COMPOSITION STUDIES

Different simulations were performed, in order to better understand the secondary beam composition or contam-ination of the particle beam by other than the desired secondary particles. More specifically, particles like muonswere extensively studied, in order to understand the parameters that may affect the beam composition reaching theexperimental zones.

A. Secondary beam composition and transmission

The produced secondary beam, in a zero order approximation, consists of 70 % protons and 30 % pions. Assumingthis composition as a starting point, the total number of particles through the beam-line can be seen in Figure 10.It can be seen that after the first triplet (which defines the acceptance of the beam line), the particle loss in thebeam-line is negligible.

FIG. 10: Loss of secondary particles in the beam line. In this plot it is shown that with a beam starting with 70 %protons and 30 % pions, almost no other secondaries are created. There is a great loss of particles in the first 50 m,which is expected because of the selection of particles in the first ”acceptance defining” quadrupole triplet.

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B. Collimator Settings

The effect of the collimators is very big in the secondary particle beams. In order to examine how the collimatorsettings affect the muon count of the beam line, studies with three different collimator settings were performed. Theresult is shown in figure 11. In all cases, the starting beam has the profile shown in Figure 6. Three cases of collimationwere studied: (a) All collimators fully open, (b) No collimators at all, and (c) Collimator slits as shown in Table IV.

TABLE IV: Settings for the slit sizes of the collimators. These values are the from the H8 beam line, and are thereforenamed the default values.

Name of Collimator Slit Size [mm]

C1 16

C2 6

C3 4

C4 6

C5 16

C6 6

C7 8

C8 8

C9 6

C10 16

C11 16

FIG. 11: Simulated muons throughout the beam line, with different collimator settings. When there are nocollimators, there are more muons. This means that the muons are not mainly created in the collimators, but rather

in the first quadropoles, which are placed around Coll-03, Coll-05 and Coll-06. With the collimators more closed,less muons will pass through the beam line.

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C. Muons

To determine whether the number of muons in the simulation was correct, the theoretical number of pions decayinginto muons was calculated. The result was compared with a simulation of a pencil pion beam, see figure 12. Thetheoretical number of muons was only calculated using the decay time of pions and the distance of a pion travelled.It does not take into account the selection of particles in the beam line.

FIG. 12: Number of muons in theoretical analysis and simulation with a pion pencil beam. In the beginning of thebeam line, the number of muons are in aggreement with the number of theoretically decayed muons. At the

momentum selection by the two bends, B3 and B4, a number of muons is not bent (since they have the ”wrong”energy), and therefore the muon count in the next detectors is smaller than before. The theoretical number of muonsdoes not take into account the elements in the beam line, so it is therefore reasonable that the muon count does not

correspond to the simulated muon count after the momentum selection.

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D. Different Starting Beam Compositions

To examine further the muons in the beam line, an experiment with different starting beams in the simulation wasperformed in figure 13. The interest was in seeing how many muons was created in a mixed beam, compared to otherbeams.

FIG. 13: Number of muons vs. beam type. The pure pion beam creates more muons than the other beams, as expected.The peak around 30 m is due to the interaction of the beam with the first ”acceptance” quadrupoles.

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E. Muons after the Beam Dump

At the end of the H8 beam line a beam dump (a thick, iron block of 3.2 m length) is placed. This block servesas absorber, for the non-interacted pions of the experiments in order not to contaminate the area and the zones. Inorder to estimate the number of muons that survive and pass through it, it was modeled in G4BL. The area of blockwas 5 × 3.2 m2. A big detector (50 × 50 m2) was placed right after the beam dump in the simulation, in order toestimate the muon population in a relatively big area, publicly accessible.

FIG. 14: Simulated muon counts. It can be seen from the plot that the muon population in the zone behind the dumpis approx. 1 % of the initial secondary beam.

VI. DISCUSSION

From the comparison of the simulations with the measured quantites, performed in the framework of this study, anumber of factors must be considered. E.g. the efficiency in the simulated detectors is 100 %, and in the real worldthe efficiency differs greatly. Also a lot of the background particles that might get detected in the real world, will notbe detected in the simulation.

VII. SUMMARY AND CONCLUSION

The correct signs of parameters for placing a bend in G4BeamLine is shown in table I and III.Detailed simulation studies of the performance and operation of the H8 beam line in the North Area at CERN were

made. The simulated results are in a satisfactory aggreement with the measured results.

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VIII. BIBLIOGRAPHY

[1] G4beamline User’s Guide, 2.16, http://muonsinc.com/muons3/G4beamline /G4beamlineUsersGuide.pdf[2] H.W. Atherton, C. Bovet, N. Doble, G. von Holtey, L.Piemontese, A. Placci, M. Placidi, D.E. Plane, M. Reinharz and E.

Rossa, Precise Measurements of Particle Production by 400 GeV/c Protons on Beryllium Targets, (1980).[3] Ilias Efthymiopoulos, Target Station T4 Wobbling - Explained, (Feb. 2003)[4] HistoRoot, http://historoot.muonsinc.com/