The FLUKA study of the secondary particles fluence in the AD-Antiproton Decelerator target area.

Transcription

1 The FLUKA study of the secondary particles fluence in the AD-Antiproton Decelerator target area. M. Calviani and E. Nowak EN/STI CERN, Geneva, Switzerland Keywords: CERN AD, antiproton production, dog-leg magnets, FLUKA Abstract In this paper we present Monte Carlo FLUKA simulations [1, 2] carried out to investigate the secondary particles fluence emerging from the antiproton production target and their spatial distribution in the AD target area. The detailed quantitative analysis has been performed for different positions along the magnet dog-leg as well as after the main collimator. These results allow tuning the position of the new beam current transformers (BCT) in the target area, in order to have a precise pulse-by-pulse evaluation of the intensity of negative particles injected in the AD-ring before the deceleration phase. This is an internal CERN publication and does not necessarily reflect the views of the CERN management.

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3 Contents 1 Introduction The ADT FLUKA geometry and BCT positions The FLUKA simulation results of the particles fluence in the ADT Conclusion References... 11

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5 1 Introduction The AD (Antiproton Decelerator) target area is the source of low-energy antiprotons for the antimatter experiments at CERN. In order to guarantee the reliable production of antiprotons for the AD physics program in the ELENA-era, an ambitious consolidation program has been launched in the AD target area. In order to support specific requirements of the upgrade project, a set of FLUKA simulations have been already carried out. The presented studies are the continuation of the extensive work on antiproton production described in CERN-ATS-Note TECH [3]. This note extends the study with a more detailed evaluation of particle fluence in various areas of the dog-leg. 2 The ADT FLUKA geometry and BCT positions The antiproton production assembly together with the focusing horn, collimators, beam dump and bending/focusing magnets present in the AD-target area are shown in Fig. 1. [7]. Based on the detailed FLUKA model of the AD-target area (Fig. 2.) described in ref. [3], further extensive investigation on the particles fluence distribution in the AD-target zone have been carried out. Fig. 1: The AD-target area technical drawing with the antiproton beam production and bending/focusing elements [7]. Fig. 2: The AD-target area FLUKA model. The existing BCT positions (downstream the QFO9052 magnet and the COL6005 collimator) and the considered for future installation ones (along the dog-leg) are indicated in red. 1

6 Antiprotons emerge from the target along with many other particles such as: secondary protons, electrons, positrons, neutrons and positive and negative pions, kaons and muons, hence the knowledge about their spatial distribution in the ADT zone and their respective contribution to the overall fluence is of interest for different considerations. The main aim of this work was to support the new beam current transformer (BCT) installation in the AD target area. The BCTs at future location should measure the overall fluence of antiprotons accompanied by remaining negatively charged particles of the momentum required by the AD ring, which are produced in the AD target zone and transferred to the AD machine. As presently the intensity of antiprotons is measured in the AD-ring after the first stochastic cooling deceleration phase, the intensity measurement in the dog-leg would allow measuring directly the production from the target area, without additional perturbations. In addition, the existing beam transformer present in the AD-target area (TFA6006), which is installed right after the main collimator, does not provide useful information on pbar production, as it is primarily detecting the high intensity primary proton beam (partially attenuated going through the target but still several orders of magnitude more intense than secondary beams). 3 The FLUKA simulation results of the particles fluence in the ADT For the FLUKA analysis we have chosen the following positions, all of them technically exploitable for the BCT installation: 1) after the main collimator (COL6005), 2) after BHZ6025, 3) after BHZ6035 and 4) after BHZ6045 (i.e. the last magnet of the AD-target dog-leg). The fluence of various particle species in the different locations as obtained from the simulations are presented in figures [3-6]. The results are then summarized in Table 1, where the integral values in the energy range from 100 MeV to 26 GeV as well as the fluence ratio with respect to pbar are indicated. Fig. 3: The figure shows the proton, negative pion and antiproton fluence after the main collimator. One can see the large proton contribution (integrated fluence per pulse of ) peaking at 23 GeV (green points), which originates from the primary proton beam ( protons per pulse) only partially attenuated by the 2

7 primary target. Three orders of magnitude of difference is expected between protons and antiprotons fluence (pink points) at this position. Significant fluence component originates also from negative pions, which represent the main component of the particles fluence emerging after the dog-leg (blue points). Fig. 4: The figure shows the particles fluence after the BHZ6025 magnet (the second dipole of the dog-leg, which initiate separation of the negative particles from the primary proton beam directed towards the dump. The figure reveals the dominating negative pion component accompanied by other negative particles such as electrons, muons and kaons. The reduction of the proton contribution compared to the spectrum after the main collimator can be observed. 3

8 Fig. 5: The figure shows the particles fluence after the QFO6035 magnet (the middle dog-leg dipole). This figure shows the negative particles spectra which start to peak according to the momentum selection. Fig. 6: The figure shows the particle fluence after the BHZ6045 dipole (the last magnet of the dog-leg) featured by the spectrometer momentum selection. For negative pions and muons the maximum of fluence is observed at around 3.4 GeV kinetic energy, for kaons at around 3.1 GeV while for antiprotons at around 2.75 GeV consistently with 3.57 GeV/c momentum and different rest masses (E k = p 2 c 2 + m 0 2 c 4 - m 0 c 2 ). 4

9 Table 1. The table presents particles fluence at different positions along the dog-leg magnets and the ratio to the antiproton fluence for the respective positions. The fluence is integrated over the energy range between [0.1, 26] GeV per one pulse of protons. particle p e- e+ K- K+ Pi- Pi+ mumu+ pbar After the collimator After the BHZ6025 After the BHZ6035 After the BHZ6045 p/cm 2 /pulse R p/cm 2 /pulse R p/cm 2 /pulse R p/cm 2 /pulse R 1.27 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x The results clearly indicate that the position after the dog-leg (after the BHZ6045) is the optimal one to measure the negatively charged particles with the right momentum required by the AD ring acceptance. Among the considered BCT locations only this one can provide a very narrow antiproton spectrum fully contained in the energy range of [2, 3] GeV. The particles spectra after the dog-leg are dominated by the negative pion contribution (see Table 1). The antiproton fluence at this location is two orders of magnitude lower, while non-negligible contribution from neagtive kaons, muons and electrons components is expected. The positive particles contribution (in particular protons) to the overall particle fluence is negligible. Negative pions of 3.57 GeV/c momentum (corresponding to a kinetic energy of 3.43 GeV) are transported together with the antiproton beam in the AD ring where mainly decay after few turns. We can anticipate that electrons injected at the same time lose their energy by synchrotron radiation, spiral towards the central orbits and are lost on the various shutters belonging to injection and cooling devices [4], [5]. As expected, at the TFA6006 position, the antiproton fluence is more than three orders of magnitude lower than the proton one (Table 1). Fluence maps have also been produced (see Figures [7-16]) averaged over a vertical range of ±20 cm centered on the beam axis, for each of the species listed in Table 1. Due to the specific shape of the QDE6030 quadrupole (see Ref. [3]) a sizeable particles fraction, for most of the considered particle species, is deflected towards the target area lateral shielding. 5

10 Fig. 7: The figure shows the negative pion fluence spatial distribution in the AD target area averaged ±20 cm around the beam line. The significant pion fraction is guided along the magnet spectrometer towards the extraction line. Fig. 8: The figure shows the negative muon fluence spatial distribution in the AD target area averaged ±20 cm around the beam line. 6

11 Fig. 9: The figure shows the electron fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. Fig. 10: The figure shows the antiproton fluence spatial distribution in the ADT area averaged over ±20 cm around the beam line. The small antiproton production against other emerging particles can be appreciated. 7

12 Fig. 11: The figure shows the negative kaon fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. Fig. 12: The figure presents the positron fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. 8

13 Fig. 13: The figure presents the proton fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. Protons are mainly distributed between the target station and the dump, due to the effect of the BHZ6024/6025 bending dipoles. Further details about the role of these magnets in bending negative and positive particles can be found in ref. [6]. Fig. 14: The figure shows the positive pion fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. 9

14 Fig. 15: The figure shows the positive muon fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. Fig. 16: The figure shows the positive kaon fluence spatial distribution in the AD target area averaged over ±20 cm around the beam line. 4 Conclusion The aim of this study is to further support the consolidation work which is presently ongoing in the Antiproton Decelerator target area. In view of BCT upgrading and additional installation, it became important to quantitatively asses the particles population in various locations of the target area. The 10

15 position after BHZ6045 appears to be the most convenient one, as the contribution of the antiproton current to the measured value is the highest one and corresponds to negative particles in the energy range of interest. 5 References [1] G. Battistoni, S. Muraro, P. R. Sala, F. Cerutti, A. Ferrari, S. Roesler, A. Fass`o, J. Ranft, The FLUKA code: Description and benchmarking, Proc. of the Hadronic Shower Simulation Workshop 2006, Fermilab 6 8 September 2006, M. Albrow, R. Raja eds., AIP Conference Proceeding 896, 31-49, (2007) [2] A. Fass`o, A. Ferrari, J. Ranft, and P. R. Sala, FLUKA: a multi-particle transport code, CERN (2005), INFN/TC 05/11, SLAC-R-773 [3] M. Calviani, E. Nowak, FLUKA implementation and preliminary studies of the AD-target area, CERN-ATS-Note TECH (2012), [4] A.H. Sullivan, Shielding for ACOL CERN/PS/AA/ACOL Note ( ) [5] E. Johnes, Antiproton production and collection CERN Accelerator School Antiproton for colliding beam facilities, CERN-84-15, p [6] M. Calviani, Memorandum - Justification for the removal of the AD anti-proton production target position as external condition for the definition of the operational mode of AD CERN- EN-STI/ , EDMS/ , [7] EDMS document