Scintillation detectors

Transcription

1 Basic Detection Techniques Detection of energetic particles and gamma rays Scintillation detectors Peter Dendooven KVI

2 Contents Interaction of radiation with matter high-energy photons charged particles heavy charged particles electrons neutral particles neutrons neutrinos General radiation detection concepts pulse mode operation energy spectrum detector efficiency timing Radiation detectors semiconductor detectors operation principle examples (silicon, germanium) other materials scintillation detectors principle organic scintillators inorganic scintillators photosensors gas detectors ionisation proportional Geiger 2

3 A generic scintillation detector radiation scintillation material photosensor e electric pulse scintillation light photon! electron radioluminescence/fluorescence scintillation (fast) phosphorescence (slow) scintillators may be organic and inorganic solids, liquids and gases 3

4 Organic scintillators: energy levels weak interactions between molecules! molecular levels are relevant electronic levels ("E ~ few ev) vibrational levels ("E ~ 0.1 ev) room temperature (kt = ev): all in ground state! works as gas, liquid, solid 4

5 Organic scintillators: scintillation mechanism ionising radiation: excitation & de-excitation to first electronic excited state, lowest vibrational level in a short time (few 100 ps) decay to ground-state vibrational levels: few ns decay time (fast!) no self-absorption, because: # emission > # absorption (Stokes shift) scintillation final states are not populated phosphorescence (up to ms decay time) 5

6 Organic scintillators: Stokes shift 6

7 Types of organic scintillators pure organic crystals anthracene (C 14 H 10 ) highest efficiency amongst organic scintillators stilbene (C 14 H 12 ) liquid organic solutions organic scintillator dissolved in a solvent possible 3 rd component as wavelength shifter plastic scintillators organic scintillator dissolved in a solvent and polymerized 7

8 Pure inorganic scintillators: mechanism absorption emission radiation creates electron-hole pairs electron-hole recombination via photon emission inefficient photon energy > visible light emission wavelength = absorption wavelength! self-absorption 8

9 Activated inorganic scintillators: mechanism absorption action of radiation: creation of electron-hole pairs holes ionise activation sites electrons migrate until they drop into an ionised activated site, leaving the activator in an excited state excited activator state decay time (~ ns) >> electron, hole migration time! activators act as luminescence centers, recombination centers scintillation photons of longer wavelength than pure scintillators no self-absorption phosphorescence can occur (long-lived excited state) 9

10 Types of inorganic scintillators unactivated fast unactivated slow Tl-activated Ce-activated glass 10

11 Unactivated fast inorganic scintillators fast component with low light yield BaF 2 only high-z scintillator with decay time < 1 ns $ = 0.6 ns at # ~ 220 nm, 15% of light yield CsI slow component $ = 630 ns at # ~ 220 nm, 85% of light yield fast component $ ~ 10 ns slow component $ up to several µs related to impurities 11

12 Unactivated slow inorganic scintillators BGO (bismuth germanate, Bi 4 Ge 3 O 16 ) very high Z CdWO 4 (cadmium tungstate) PbWO 4 (lead tungstate) very high Z very poor light yield (OK if high energy) detector PANDA experiment electromagnetic calorimeter PbWO 4 crystals www-panda.gsi.de 12

13 Tl-activated inorganic scintillators slow and bright NaI(Tl) most widely used scintillator high light yield ( photons/mev) $ = 230 ns CsI(Tl) photons/mev $ = 0.68 (64%), 3.34 (36%) µs 13

14 Ce-activated inorganic scintillators relatively fast ($ ~ ns) and bright examples: GSO(Ce) (gadolinium silicate, Gd 2 SiO 5 ) LSO(Ce) (lutetium oxyorthosilicate, Lu 2 SiO 5 ) LaCl 3 (Ce), LaBr 3 (Ce) 14

15 Glass scintillators containing Li or B and activated with Ce for neutron detection enriched to ~95% 6 Li: 6 Li(n,%) 3 H with %, 3 H being detected Q = 4.78 MeV! detected energy = neutron energy MeV 15

16 Scintillators: light yield (output) / efficiency mostly: ~3 x band gap for 1 electron-hole pair quenching : non-radiative de-excitation modes e.g. lattice vibrations, heat, impurity-related effects phosphorescence (in pulse mode operation) 16

17 Scintillation efficiency vs. particle type, energy scintillation efficiency decreases with increasing ionisation density along particle track! scintillation efficiency depends on particle type energy! non-linear response typically worse in organic scintillators 17

18 Scintillators: time response normally, decay time ($ D ) is often adequate for the fastest scintillators, rise time is important 2 parametrizations of rising edge: exponential (time constant $ R ) gaussian (standard deviation & R ) more pratical: FWHM I = I 0 e "t # D I = I ( 0 e "t # D " e "t # ) R I = I 0 g(t) " e #t $ D $ R, & R $ D Properties of NE111 (a.k.a. Pilot U) $ D 1.7 ns FWHM $ R & R FWHM 0.2 ns 0.2 ns 1.54 ns 18

19 Pulse shape discrimination organic scintillators fast and slow component slow component intensity depends on particle type (for same energy) basis for pulse shape discrimination fast slow 19

20 Temperature dependence many scintillators show temperature dependence of scintillation efficiency (light output) decay constant 20

21 Properties of some organic scintillators 21

22 Properties of some inorganic scintillators 22

23 Photosensors purpose: transform scintillation light to an electric signal two steps: 1. photon! electron 2. electron multiplication parameters efficiency (wavelength dependent) (step 1.) gain (step 2.) time response (step 2.) types photomultiplier tube (PMT) conventional MCP-based photodiode (PD) conventional avalanche photodiode (APD) Gieger-mode APD (silicon photomultiplier) 23

24 The photomultiplier tube: principle 24

25 The photoemission process example: GaP undoped Zn-doped Ce covered 1. photon excites an electron to the conduction band photon energy (typically 2-3 ev) > band gap 2. electron migrates to the surface energy loss determines escape depth (up to ~25 nm) semi-transparent to photons 3. electron escapes from the surface electron needs to overcome the electron affinity (surface barrier) suitable semiconductor: ev high thermoionic noise ( dark current ) low-energy (long-wavelength) cut-off for incoming photons (usually red or near-infrared) negative electron affinity (NEA) materials are superior 25

26 The photoemission process example: GaP undoped Zn-doped Ce covered 1. photon excites an electron to the conduction band photon energy (typically 2-3 ev) > band gap 2. electron migrates to the surface energy loss determines escape depth (up to ~25 nm) semi-transparent to photons 3. electron escapes from the surface electron needs to overcome the electron affinity (surface barrier) suitable semiconductor: ev high thermoionic noise ( dark current ) low-energy (long-wavelength) cut-off for incoming photons (usually red or near-infrared) negative electron affinity (NEA) materials are superior 26

27 The quantum efficiency sensitivity of photocathodes: 1. (radiant) sensitivity, responsivity current per unit light flux [Amperes/Watt, Amperes/lumen] 2. quantum efficiency (QE) QE = number of photoelectrons emitted number of incident photons strong function of wavelength window transmittance usually included in sensitivity 27

28 PMT window transmittance 28

29 Photocathodes spectral response 29

30 Secondary electron emission similar process to photoemission mutiple electrons emitted per incident electron secondary emission ratio ' multiple (N) dynodes: overall gain = ' N in practice overall gain up to ~

31 PMT gain vs. voltage 31

32 The MCP-PMT hollow glass tube inner surface is secondary electron emitter voltage across tube pushes electrons through microchannel plate (MCP) channel diameter: µm excellent timing: TTS ~100 ps 32

33 PMT pulse timing properties timing performance depends on transit time spread (TTS) 33

34 PMTs come in all shapes and sizes 34

35 PMTs come in all shapes and sizes 35

36 The conventional photodiode Si wafer transparent light absorbed in contact a.k.a. silicon photodiode absorption of photons in Si: window ~350 < # < ~1000 nm no internal amplification: small signal to noise ratio requires a low-noise amplifier leakage current and noise limit the area to ~1 cm 2 cooling reduces leakage current exponentially 36

37 Photodiode quantum efficiency no electron escape needed (as in conventional photocathode) quantum efficiency up to 80% broader spectral response than PMT 37

38 The avalanche photodiode (APD) in the high-field region, additional e-h pairs are created avalanche process leads to a gain of up to few 100 gain very sensitive to applied voltage and temperature (few %/K) 38

39 APD breakdown operation voltage above a threshold: breakdown Geiger discharge output pulse size is independent of light intensity no energy determination 39

40 The Geiger-mode APD (silicon photomultiplier) discharge needs to be quickly quenched many small pixels ( /mm 2 ) recovers dynamic range high-gain: excellent timing low noise (single photon detection) breakdown voltage operating voltage 40

41 The Geiger-mode APD (silicon photomultiplier) 41

42 SiPM performance: single photon counting phe 1 phe 2 phe 3 phe 4 42

43 SiPM performance: timing coincidence timing resolution for 511 kev gamma s: 100 ps 43

44 Comparison photosensors PMT APD SiPM photo-detection efficiency [%] gain dynamic range 10 6 large 10 3 /mm 2 bias voltage [V] phe resolution [%] 50 poor 4 time jitter (1&) [ps] 200 > magnetic field compatibility no yes yes (up to 15 T) 44

45 Conclusion scintillators are versatile but complicated 45