Measurement of 12 C ions fragmentation cross sections on a gold thin target with the FIRST apparatus Marco Toppi, on behalf of the FIRST collaboration 54 th International Winter Meeting on Nuclear Physics 25-29 January 2016, Bormio (Italy) 1
FIRST experiment purposes • The study of the nuclear fragmentation processes in the interaction of highly energetic ions in matter is of great interest in: 1. Basic research: To improve our knowledge of nucleus-nucleus collisions (e.g. hadronic shower induced by ions in the atmosphere) 2. Applied physics: in particular in particle therapy for the treatment of tumors and in space radiation protection • Accurate measurements of fragmentation cross sections of light ions interacting with elemental and composite targets are crucial to benchmark and improve the nuclear interaction models implemented in Monte Carlo (MC) simulation codes. d 2 σ /d Ω dE [b sr (MeV/u) -1 ] ) 4 -1 He at 17 ° (MeV/nucleon) -2 10 • The current discrepancies between Data MC codes and experimental data are QMD BIC -3 mainly due to the lack of available 10 INCL -1 dE (b sr data and to their limited precision. -4 10 Ω /d Data: E600 (GANIL)C+C @ 95 MeV/nucleon σ 2 d MC simulation: GEANT -5 10 0 50 100 150 J.Dudouet et al., Phys. Rev. C 89, (2014) (MeV/nucleon) E (MeV/nucleon) Energy [MeV/nucleon] (d) 2
FIRST experiment purposes • The study of the nuclear fragmentation processes in the interaction of highly energetic ions in matter is of great interest in: 1. Basic research: To improve our knowledge of nucleus-nucleus collisions (e.g. hadronic shower induced by ions in the atmosphere) 2. Applied physics: in particular in particle therapy for the treatment of tumors and in space radiation protection • Accurate measurements of fragmentation cross sections of light ions interacting with elemental and composite targets are crucial to benchmark and improve the nuclear interaction models implemented in Monte Carlo (MC) simulation codes. • The FIRST ( F ragmentation of I ons R elevant for S pace and T heraphy) experiment at SIS accelerator of GSI measured the fragmentation cross sections of a 12 C beam on thin targets • Collected 5 M events of 12 C @ 400 MeV/nucleon impinging on a 0.5 mm Gold target • Differential cross sections measured for fragments emitted in the forward region (with polar angle θ wrt the beam axis < 5°) 3
FIRST detector optimization FLUKA simulation: Energy distribution N frag /N C [(MeV/u) -1 ] n (Z=0) • A MC simulation of a 12 C beam at n, H H (Z=1) He (Z=2) 400 MeV/nucleon on a 8 mm Li (Z=3) carbon target has been developed Be (Z=4) B (Z=5) using the FLUKA code, to design C (Z=6) the detector: ➡ Z > 2 fragments ~ same velocity of the 12 C ions. Emitted in forward direction Energy [MeV/nucleon] FLUKA simulation: Angular distribution ➡ Protons & neutrons are the most N frag /N C [sr -1 ] abundant fragments: wide β n (Z=0) H (Z=1) spectrum 0< β <0.6 and wide He (Z=2) angular distribution Li (Z=3) Be (Z=4) • The dE/dx loss by the fragments B (Z=5) C (Z=6) spans from 2 to 100 m.i.p. n, H Angle [°] 4
The FIRST apparatus The TPC didn’t work during the data acquisition. The KENTROS detector (scintillators and fibers for ToF, E loss and tracking measurements) has not been used in this analysis focused on forward emitted fragments only ( θ < 5°) Start Counter (SC) : thin scintillator. N C, ToF and trigger Beam Monitor (BM) : drift chamber for beam direction and impact point on target Vertex Detector (VTX) : 4 layers of pixel silicon detectors. Tracks direction ( θ < 40°) ToF Wall (TW) : two layers of plastic scintillator (192 vertical slats). X, Y, Z, E loss and ToF The FIRST reconstruction challenge is to match the VTX and the TW (~6 m apart) information for the forward fragments (passing through the magnet region). 5
The Vertex detector • High tracking efficiency and vertex Frag. vertex reconstruction efficiency (~ 99%) • Excellent tracking resolution < 10 μ m (x,y) and vertexing resolution < 10 μ m Beam (x,y) and < 50 μ m (z): fundamental particle when extrapolating the fragment Target tracks along ~6 m to the ToF Wall 4 layers of • The VTX slow pixel silicon Number of pixels/cluster detectors integration time (115 µs) causes some pile-up 15 that was taken into account 10 • The VTX can provide also information on 5 the fragment charge looking at the number 0 of fired pixels per cluster 1 2 3 4 5 6 Atomic number Z 6
The ToF-Wall detector • Two planes of 192 plastic scintillators (slat dimension: 1.10 m x 2.5 cm x 1 cm) • The resolutions have been estimated (on data) by comparing the position, ToF and Energy loss values measured for hits in the two TW planes that are associated to the same incoming fragment • X & Y hit position resolution: σ X ~ 0.7 cm, σ Y ~ 2 – 9 cm 2 ) [cm] (ToF) [ns] 12 • ToF resolution: 1.8 DATA DATA 1.6 σ ToF ~ 800 ps ADC MC MC 10 1.4 (y σ σ 8 σ Y ~ 2 – 9 cm σ ToF ~ 800 ps 1.2 • E loss resolution: 1 6 σ E ~ (2–12) MeV 0.8 4 0.6 0.4 2 0.2 0 0 0 20 40 60 80 100120 140 0 20 40 60 80 100120 140 E [MeV] E [MeV] loss loss 7
The ToF-Wall detector • Two planes of 192 plastic scintillators (slat dimension: 1.10 m x 2.5 cm x 1 cm) • The resolutions have been estimated (on data) by comparing the position, ToF and Energy loss values measured for hits in the two TW planes that are associated to the same incoming fragment • X & Y hit position resolution: σ X ~ 0.7 cm, σ Y ~ 2 – 9 cm 200 [MeV] Z = 6 180 • ToF resolution: loss 160 Fragment charge σ ToF ~ 800 ps 2 10 E Z = 5 140 identification (ZID) • E loss resolution: 120 is performed using 100 σ E ~ (2–12) MeV Z = 4 an algorithm based 10 80 on detected dE/dX 60 Z = 3 in the TW vs Tof 40 Z = 2 20 1 Z = 1 0 25 30 35 40 45 ToF [ns] 8
Global reconstruction strategy • Fragments are reconstructed using an iterative procedure that matches VTX tracks and TW hits • A minimization algorithm determines pc/Z and the track trajectory L • Fragment velocity from ToF: L β = ToF · c • Fragments mass: pc Mc 2 = A scoring function based on both β · γ Y and global charge (from VTX and • Mass resolution: TW) is used to rate the combinations of VTX / TW tracks s✓ ∆ p ◆ 2 ◆ 2 ∆ M ✓ γ 2 ∆ t and to select the best track = + M p t candidate 9
MC simulation • A detailed MC simulation of a 12 C beam @ 400 MeV/nucleon impinging on a 0.5 mm gold target, has been developed with FLUKA for the evaluation of the efficiencies, the resolutions and the background PDF modeling / cross feed subtraction • The comparison of E loss , ToF and Y coordinates measured from the TW detector for DATA and MC events has been obtained for events in which a fragmentation occurred (number of tracks associated to a reconstructed vertex greater than 1) 3 3 10 10 × × 6 100 10 140 MC MC MC ToF Y E loss 120 DATA DATA DATA 80 5 10 100 60 4 80 10 60 40 3 10 40 20 20 2 10 0 0 25 30 35 40 45 -40 -20 0 20 40 0 20 40 60 80 100 120 140 160 180 200 E [MeV] ToF [ns] y [cm] loss 10
FIRST performances: resolution • The global reconstruction algorithm has been benchmarked against the MC simulation • Angular and kinetic energy resolutions have been measured to evaluate possible bias introduced by the reconstruction algorithm and to optimize the binning choice for differential cross section measurements 200 0.2 σ (E true -E rec ) [MeV/nucleon] ) [MeV/nucleon] σ ( θ true - θ rec ) [°] ] ° ) [ • The E kin resolution rec σ ( θ ) σ (E kin ) H 180 0.18 H θ - increases as a He true He 160 0.16 θ ( Li Li σ function of kin,rec 140 0.14 Be Be fragment E kin as - E B B 120 0.12 kin,true expected C C 100 0.1 (E σ • We need to unfold 80 0.08 the spectrum (Used 60 0.06 40 the RooUnfold Tool 0.04 20 with Bayesian 0.02 0 approach) 0 0 100 200 300 400 500 600 700 800 0 1 2 3 4 5 6 E [MeV/nucleon] [ ] θ ° kin E kin [MeV/nucleon] θ [°] 11
FIRST performances: efficiencies • Tracking efficiencies are evaluated using a MC simulation for each fragment produced in the interaction of the 12 C beam with the gold target ε trk ( θ ) 1 ε ε trk = n REC / n PROD 0.9 0.8 0.7 • n PROD : fragments emerging from 0.6 the target in the magnet 0.5 acceptance 0.4 1 6 H Li • n REC : reconstructed tracks built 2 7 H Li 0.3 using the true VTX and TW hits 3 8 H Li 0.2 belonging to the true MC tracks 3 7 He Be 8 B 9 4 He Be 10 B under study 0.1 6 10 11 He Be B 0 0 2 4 6 [ ] θ ° θ [°] 12
Cross Section Measurements • Differential cross section, with respect to kinetic energy and angle, for the i-th isotope ZA X with charge Z and mass number A • N TG : number of atoms in the target per unit surface ( ρ × d × N A / A) • N C : number of total 12 C impinging on the target from Start Counter • ε trk ( θ ) / ε trk (E kin ): tracking reconstruction efficiency per angular/energy bin for each isotope (as defined in previous slide) • Y i ( θ ) / Y i (E kin ): fragment yields for a given isotope ZA X in an angular / energy bin ΔΩ / Δ E kin , measured from mass fits 13
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