Charged particles detectors Arrays (1)

Transcription

1 Charged particles detectors Arrays (1) Basic concepts of particle detection: scintillators & semiconductors Light charged particles (p, α, e) Arrays: DIAMANT, ISIS, EUCLIDES, MiniOrange Large Arrays: CHIMERA, INDRA (MULTICS, RingCOUNTER (Garfield)) Heavy fragments detection: RFD (recoil filter detector), Saphir

2 Charged Particles identifications Organic scintillators energy levels stilbene C 14 H 12 singlet triplet absorption fluorescence phosphorescence the slow component (τ ~ ms) due to delayed phosporescence (from triplet state) is larger for particles with large de/dx prompt fluorescence (from singlet state): ~ few ns light yield S = scintillator efficiency kb = fitting constant

3 Inorganic Scintillators: CsI(Tl), BaF 2, Light output: L( t) h t exp τ f h + s τ s t exp τ = f τ f s Sum of two exponential functions: fast & slow components CsI(Tl) α particle E α =95 MeV τ f = 800 ns τ s = 4000 ns 1. τ s independent of particle nature 2. R = h s /(h f +h s ) increases with decreasing ionisation density 3. τ f increases with decreasing ionisation density L slow it is possible to identify different particles N.B. CsI have been used at first for particle studies: -lessfragile thannai - good particle discrimination L fast

4 Optimization of particle discrimination: - ballistic deficit effect: amplitude degradation at the output of pulse shaping circuit, due to finite shaping time constant [N.B. the preamplifier rise-time corresponds to the charge collection time full charge collection is obtained only with shaping time constant] B = 1 F = 1 Q p /Q t Q p partial charge collection Q t full charge collection particle-type information is carried by preamplifier rise-time constant ballistic deficit B depends on type of particle Very good method for particle discrimination with CsI(Tl) + PIN photodiode detectors instead of PM tube: usefull in compact geometry PIN diode semiconductor Device (photodiode)

5 J. Gal et al., NIMA366(1995)120 CsI(Tl) + PIN photodiode detectors mm 3 pulse generator simulted α and p signals measured signals ballistic deficit Figure of merit M xy = P w quality of particle identification x x P y + w y particle separation down to M ~ 0.6 separation energy threshold E α ~ 4 MeV E p ~ 2.5 MeV

6 Also used for particle discrimination with CsI(Tl) + PIN photodiode detectors - zero-crossing method: the zero-crossing point of a bipolar signal depends on the pulse-shape of the original unipolar signal (input rise time) 1. different particles give different unipolar signals (different rise time) particle-type information is carried by the preamplifier rise-time constant zero-crossing depends on type of particle CR-RC-CR double differentiating circuit 2. bipolar signal with zero-crossing depending on input rise time the time interval measured by TAC is proportional to the decay-time of the detector pulse

7 DIAMANT: EUROBALL ancillary 84 CsI(Tl) with PIN diode detectors 4π geometry inner radius R = 32mm to 49mm light charged particle detector array α p 4π array Particle identification: - ballistic deficit; - zero-crossover; - mixed method. energy time p α target Advantages: - high efficiency: ε proton 70%, ε α 50% - low energy threshold: 2 MeV for p, 4 MeV for α Limitation: -largedead time (long decay time of light pulse τ ~ 1000 ns) limitation in count rate - large absorption & scattering of γ s limitation in resolving power of Ge array Operating mode: DIAMANT alone: particle-xn channels DIAMANT + Ge : particle-xn + xn channels

8 Charged Particles identifications by Solid State Detectors (Si, ) - smaller dead time (τ ~ ns, compare to 1000 ns for CsI) - good energy resolution (~ kev) - limited γ-absorption - possibility of large solid angle coverage (~ 4π) - large sizes, up to 20 cm 2 Detection method: 1. E energy loss 2. E- E telecopes 3. Pulse Shape Analysis Si detectors are preferred to Ge: - simplicity of operation (room temperature, ) -lowerγ-absorption (Z Si =14, Z Ge = 32, ρ Si = 2.33 g/cm3 ρ Ge = 5.32 g/cm3, ) -

9 - Si detectors, E method de dx mz E 2 28 Si i (@95 MeV) + 58 N p and α de/dx spectra p α - usefull to discriminate between few types of particles: p, α - detailed simulation studies of de/dx in Si for various particles different de/dx for different particles Si detector n-type 2 segments 170 µm overlapping dstribution

10 Si-Ball: NORDBALL ancillary 17 E Si detectors n-type, pentagon shape p α energy register unit α p -thickness170 µm - shielding with absorber foils to optimize p and α penetration (optimum thickness: MonteCarlo simulation) - 4π geometry: Ω ~ 90% 4π - Inner radius R = 5 cm - Housing sphere radius R = 10 cm Multiplicity identifier particle identifier spectrum target

11 T. Kuronyanagi et al., NIMA316(1992)289 γ-spectra particle gated 3pα analysis of contaminant shows 12 % α interpreted as p 0.9 % p interpreted as α 2pα 4p as a consequence of the overlapping energy distributions of p and α 3p 2p

12 - Si detectors, E-E method de dx E mz E 2 E = mz MeV/A t ~ µm t ~ 1000 µm E T = E + E Very usefull method to separate ions up to A = 25-30

13 ISIS/EUCLIDES: GASP/EUROBALL ancillary 40 E- E Si n-type telescopes 130 µm µm γ-spectra particle gated increased selectivity Ge array - capacitance C = 850 pf, 130 pf - shielding with Al foil (~10-20 µm) to reduce radiation damage from scattered beam ions (optimum thickness: MonteCarlo simulation) 4π geometry: Ω E ~ 72% 4π, Ω E ~ 65% 4π Inner radius R = 6.7 cm E 32 S(@140 MeV) + 40 Ca ISIS efficiency: ε proton 50%, ε α 40% E Operating mode: Stand-alone + InnerBall + Innerball + Hector + Neutron-Wall + Recoil Filter Detector

14 -Si detectors, PSA method zero-crossover method Si-Ball Berlin (EUROBALL ancillary) 162 Si (p + -n-n + structure) - thicknes ~ 500 µm - Active area ~ 750 mm 2 p α Time ZCO [ns] S N Energy [MeV] Energy [MeV] α Light particles E=8-8.2 MeV proton p heavy-ions Heavy particles E= MeV C peak G. Pausch et al., NIMA365(1995)176 Time ZCO [ns]

15 Dedicated Arrays: CHIMERA Multifragmentation, transition liquid-gas Heavy Ion Reactions 10 MeV/A 1 GeV/A E*~ MeV, 4 T 8 MeV - large number of particles - several Z and A - wide dinamic energy range J. Pochodzalla et al., Phys. Rev. Lett. 75(1995)1040 Caloric curve of the nucleus Caloric curve of the water Multifragmentation: T 5 MeV, E* 4-5/A MeV Vaporization: T>6 MeV, E*>10/A MeV

16 CHIMERA (GANIL, LNS, GSI(?)) 1192 E-E Si-CsI(Tl) telescopes Si thickness ~ 300 µm CsI(Tl) thickness ~ 10 cm 9 rings in forward direction sphere Inner radius R = 40 cm Large scattering chamber 1 1m TARGET 30 Beam detectors 504 detectors Si Si-CsI(Tl) telescope 300µm (Pd)18x18mm 2 10cm CsI(Tl) unique device for: - granularity (1192 mod.) - low energy threshold ( 250 kev/a for H.I.) - efficiency (~ 95% ) good event reconstruction even with high multiplicity E-E, E-TOF, PSD in CsI(Tl)

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18 Fast PSD H.I. CsI(Tl) p d Li 3He t 6He 4He light particles Slow Identification method Si 300µm (Pd)18x18mm 2 10cm CsI(Tl) HF Cyclotron TOF HF Start 100 ns E-E Z,E (dε/dx AZ 2 /E) E-TOF M,E Si- E (high gain) CsI-Fast C Si- E (low gain) CsI-Fast Z =50 Sn Si- E TOF A=16

19 Other Dedicated Arrays for charged particles: MULTICS (+MEDEA, LNS), Ring Counter (+GARFIELD, LNL) Heavy Ion Reactions at intermediate energy 10 MeV/A 100 MeV/A low energy threshold + high energy resolution MULTICS 48 modules 3 stages telescope E IC [MeV] E Si [MeV] E IC+Si [MeV] E CsI [MeV]