G4beamline Simulations for H8

Transcription

1 G4beamline Simulations for H8 Author: Freja Thoresen EN-MEF-LE, Univ. of Copenhagen & CERN Supervisor: Nikolaos Charitonidis CERN (Dated: December 15, 2015) Electronic address:

2 2 Abstract Detailed simulations of the H8 beam line at the North Area, using the G4beamline software were performed in the framework of this study. The conventions used by the program are analysed. Having modelled precisely the beam line, several studies examining the beam transmission and composition were performed. The results were compared with measurements, where a satisfactory agreement was found. The muon production and transport is studied in details throughout the beam line.

3 3 I. INTRODUCTION CERN, the European Organization for Nuclear Research, is the biggest research facility located in Switzerland and France. 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 to a 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 produced on a fixed target, or test beams that use secondary beams produced from a target. This beam is transported to the facilities of the North Area through different beam lines. In this study the H8 beam line at the Experimental Hall North 1 (EHN1) is studied using the G4beamline software. A schematic of the CERN accelerator complex is shown in Figure 1. FIG. 1: The CERN accelerator complex. The H8 beam line is located at the North Area.

4 4 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 production from the target has been studied extensively in [2]. The T4 target station is shown in Figure 2. Before and after the target, three horizontal bends exist for the purpose of enhancing the particle production through a wobbling of the beam, as described in [3]. The horizontal bend located after the target makes therefore a first selection by bending the particles in different angles corresponding to their energies. For a schematic of the target station and the particle trajectory 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 bent according to their energy. The particles with the energy such that the deflection kick in the bend corresponds to the right angle will be able to continue through the TAX collimator. In a usual configuration the energy of the particles moving 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. A schematic of the H8 beam line is shown in Figure 3. After the target and the B3T bend, two vertical coupled bends are placed. These bends will bend the beam up, and the muon halo created in the target will be dumped in the ground, following a straight line from the target. After the vertical bend up, momentum selection collimators will select the momentum of the particles transported. Finally the beam will be bend down again, and to the right before reaching 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 line is a magnetic spectrometer, since with the correct choice of the magnet currents and collimator apertures, a precise selection of the secondary particles momentum can be made.

5 FIG. 3: Simple schematic of the H8 beamline layout. The secondary beam starts at the T4 target. Then the beam will be bend up by B1 and B2, and later it will be bend down by B3 and B4. With the bends and the momentum slits, a precise momentum selection is performed. 5

6 6 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 beamline can be simulated without the need to write a lot of C++ code. In G4beamline the beam line can be described in a single ASCII file and the output from simulated detectors is given in.root format. This offers the advantage that the histograms can be made using HistoRoot[4]. But most importantly, all the common beam line elements are implemented in G4beamline. In the next section the placement and the particular conventions for the bends will be described. 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 in G4beamline 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 be performed. Three coordinate systems are available : (a) Centerline coordinates, which are the coordinates of the beam trajectory, (b) local coordinates, which are the coordinates of any local transformation possibly occuring in one element or (c) global coordinates, which are the coordinates of the model s world. From the three coordinate systems, centerline is the most convenient to use. Therefore the placement of the magnetic elements is done in this coordinate 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] = 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 order to correctly follow the trajectory of the beam, without causing extra losses. For an illustration of the effect, see Figure Offset The offset is a distance which the bend should be moved by, in order to be optimally placed. More specifically, the rotation of the bend will affect the bending of the beam. To correct the bending of the beam, one can either adjust the magnetic field after the rotation or perform do a minor shift of the magnet.

7 7 (a) Magnetic field parameter set. (b) Magnetic field and geometry rotation set. (c) Magnetic field, geometry rotation and shift set. 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, rotation and 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 the rotation 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 the left, 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. The direction of the beam is from the left to the right.

8 8 2. Conclusion on parameters For the vertical bends, similar investigation was performed. The signs of the parameters for both horizontal and vertical 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

9 9 IV. COMPARISON OF SIMULATED AND MEASURED BEAM PROFILES Measurements of triggers (scintillators), FISC and wire chambers was performed at ENH1, with a beam of 180 GeV. 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 purpose of this study, the secondary beam with parameters matching the Atherton s formula in [2] has being generated and used 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 divergence of 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] 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 be seen.

10 10 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 charge of the particles impinging on the filament, thus estimating the beam profile. Several FISC s are installed throughout the beam line. A selective comparison between the simulations and the profiles obtained by the FISCs can be seen in Figure 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 horizontal plane there a very good agreement between simulated and measured results. In the vertical plane the similarity is even better. The small inconvenience in the symmetry observed in the vertical level, can be explained by the fact that the convention of left and right in the simulation, is opposite of the detector.

11 11 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 can determine the position of a particle in both the vertical and horizontal plane. The results of the comparison of the beam profile between the wire-chambers and the simulated profiles can be seen in Figure 8 for wire chamber XDWC3,4, 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. The tale of the histogram would probably be clearer in the simulation, if there were more statistics. The small offset of the center of the histograms is due to the effect of the small corrector magnet that results in a misteered beam in the H8 beam line, which is not simulated. In the vertical plane there is also a very good agreement.

12 12 D. Scintillators Several scintillators (triggers) are placed along the beam line in order to monitor the beam intensity in several points of the beam line. The scintillators can only determine the number of particles hitting the detector. A relative comparison between the measured and simulated scintillator counts was performed. The results can be seen in Figure 9. FIG. 9: The simulated values are normalized to the first point of the measured data, and the data is shown on a logarithmic scale. In this plot the transmission of the beam line is shown at 180 GeV/c. The measured and simulated rates are quite similar.

13 13 V. PARTICLE PRODUCTION AND BEAM COMPOSITION STUDIES Different simulations were performed, in order to better understand the secondary beam composition or contamination of the particle beam by other than the desired secondary particles. More specifically, particles like muons were extensively studied, in order to understand the parameters that may affect the beam composition reaching the experimental zones. A. Secondary beam composition and transmission The produced secondary beam, in a zero order approximation, consists of 70 % protons and 30 % pions. Assuming this 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 the beam-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.

14 14 B. Collimator Settings The effect of the collimators is very big in the secondary particle beams. In order to examine how the collimator settings affect the muon count of the beam line, studies with three different collimator settings were performed. The result is shown in figure 11. In all cases, the starting beam has the profile shown in Figure 6. Three cases of collimation were 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 therefore named 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 no collimators, 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.

15 15 C. Muons To determine whether the number of muons in the simulation was correct, the theoretical number of pions decaying into muons was calculated. The result was compared with a simulation of a pencil pion beam, see figure 12. The theoretical 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 the beam 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 muons does 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.

16 16 D. Different Starting Beam Compositions To examine further the muons in the beam line, an experiment with different starting beams in the simulation was performed in figure 13. The interest was in seeing how many muons was created in a mixed beam, compared to other beams. 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.

17 17 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 serves as absorber, for the non-interacted pions of the experiments in order not to contaminate the area and the zones. In order to estimate the number of muons that survive and pass through it, it was modeled in G4BL. The area of block was m 2. A big detector (50 50 m 2 ) was placed right after the beam dump in the simulation, in order to estimate 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 dump is 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, a number of factors must be considered. E.g. the efficiency in the simulated detectors is 100 %, and in the real world the efficiency differs greatly. Also a lot of the background particles that might get detected in the real world, will not be 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.

18 18 VIII. BIBLIOGRAPHY [1] G4beamline User s Guide, 2.16, /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,