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Basic Detection Techniques 2009-2010 http://www.astro.rug.nl/~peletier/DetectionTechniques.html Detection of energetic particles and gamma rays Semiconductor detectors Peter Dendooven KVI [email protected] 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 Note: also known as solid-state detectors 2 Semiconductor band structure energy conduction band electrons >6 eV band gap at T=0 K ~1 eV empty holes occupied valence band insulator semiconductor at T>0 K ! kT at 300 K = 0.025 eV ni = pi = N c N v e metal " Eg 2kT = AT 3/ 2 e " Eg 2kT ni: intrinsic density of electrons in conduction band pi: intrinsic density of hole in valence band Nc,v: number of states in conduction, valence band Eg: band gap at 0 K A: temperature-independent constant 3 Relevant properties of intrinsic Si and Ge Si Ge atomic number 14 32 density (g/cm3) 2.33 5.32 4.96 x 1022 4.41 x 1022 12 16 1.115 1.165 0.665 0.746 intrinsic carrier density at 300 K (/cm3) 1.5 x 1010 2.4 x 1013 mobility (cm2/V/s) at 300 K: electrons holes 1350 480 3900 1900 mobility (cm2/V/s) at 77 K: electrons holes 2.1 x 104 1.1 x 104 3.6 x 104 4.2 x 104 3.62 3.76 (*) 2.96 atomic density (atoms/cm3) dielectric constant (relative to vacuum) band gap (eV) 300 K 0K ionisation energy (eV) 300 K 77 K (*) due to the small band gap, Ge needs to be cooled to reduce the leakage current to an acceptable level (usually LN2 cooling, 77 K is used) 4 How to get a signal ? r E e I h ! Si!wafer 1 cm 1 cm 300 µm I = I e + I h = A n i e (v e + v h ) = A n i e E (µ e + µ h ) A: surface area v: velocity µ: mobility • mimimum ionizing particle deposits ~400 eV/µm creates ~3.3 x 104 e-h pairs • free charge carriers in the same volume 4.5 x 109 ! particle signal is drowned solution: reduce number of free charge carriers, i.e. deplete the semiconductor ! ! doping ! blocking contact 5 Doped semiconductors: n-type and p-type donor level acceptor level • pentavalent elements (group V/15, e.g. P, As, Sb) have one electron too much to fit in: “donor impurities” • extra electrons are lightly bound - energy level close to the conduction band - thermally excited into the conduction band - recombination with holes: ne >> nh ! n-type semiconductors - electrons are the majority charge carriers - holes are the minority charge carriers • trivalent elements (group III/13, e.g. Ga, B, In) have one electron too little to fit in: “acceptor impurities” • electrons in missing bond slightly less bound - energy level close to the valence band - thermally excited electrons fill the acceptor level, creating holes - holes recombine with conduction band electrons: nh >> ne ! p-type semiconductors - holes are the majority charge carriers - electrons are the minority charge carriers 6 7 Conductivity of doped semiconductors • • typical doping levels for detector silicon: 1012 atoms/cm3 heavily doped semiconductors (n+, p+): 1020 atoms/cm3 results in very high conductivity (good for contacts) 8 Action of radiation electrons ~1 eV holes • • e-h pairs are produced along track (Ne=Nh) energy needed per e-h pair is largely independent on – energy of radiation – type of radiation – temperature ! average energy needed = ionisation energy (") semiconductor • " is small: Si, 300 K: 3.62 eV Ge, 77 K: 2.96 eV ! good energy resolution advantages of semiconductor detectors: • good energy resolution • high density, relatively high atomic number (Ge): - good stopping power - good foton interaction probability 9 The Fano factor statistical variation in the number of e-h pairs ": ionisation energy E – # of e-h pairs N = E: energy deposited by radiation quantum " – if excitations are independent: Poisson statistics ! deviation standard Fano factor: F" ! for Si, Ge: F ~ 0.1-0.15 ! "N = N = E # variance " 2N = E # observed statistical variance ! E # "E stat FWHM ~ 3 times better than Poisson statistics not fully understood phenomenon ! 10 Blocking contact: the p-n junction n-type p-type mobile electrons mobile holes diffusion: holes to n-region, electrons to p-region uncompensated fixed charges build up emerging “contact” potential stops diffusion depletion region 11 The depletion region detects ionising radiation depletion region • thermally generated charge carriers are quickly swept away due to the contact potential ! highly suppressed charge carrier density ! relatively small amount of charge carriers created by an ionising particle is easily detected • poor performance because: - small contact potential (~1 V): slow-moving charges can be trapped, resulting in incomplete charge collection - depletion layer is thin: - high capacitance ! large electronic noise - small sensitive volume cannot detect high-energy radiation 12 Reverse biasing – + increased depletion region • • • bias: 100 - 1000 V/cm V >> contact potential depletion region thickness increases – smaller capacitance, smaller electronic noise – quick and complete charge collection • very large electric field: multiplication ! silicon avalanche detector 13 Maximizing the depletion region # 2 " V &1/ 2 d=% ( $ eN ' d: depletion region thickness V: reverse bias voltage ": dielectric constant e: electronic charge N: net impurity concentration (atoms/cm3) •! normal semiconductor purity: depletion 2-3 mm maximum • problem for “long-range” radiation (e.g. high-energy #-rays) • increasing d only by decreasing N – further refining techniques • Si: not (yet) possible • Ge: high-purity germanium (HPGe) – N ~1010 atoms/cm3 (relative impurity 10-12 !) – depletion up to a few cm – compensated material by lithium ion drifting • Si(Li), Ge(Li): up to ~2 cm (so-called p-i-n structure) 14 Germanium detector configurations planar $, %: ultrapure n, p type coaxial 15 Some examples of Si detectors 16 Some examples of Si detectors 17 Anatomy of a Ge detector 18 Some examples of Ge detectors 19 Some examples of Ge detectors 20 Other semiconductor materials • Why ? – higher atomic number for higher #-ray interaction probability – room-temperature operation (&Ge) • Commercially available detectors – CdTe – Cd1-xZnxTe (CZT) – HgI2 • Large crystal growth problems cause slow development – impurities – defects ! small volumes only 21 Properties high-Z semiconductor detectors material Si atomic number density bandgap (g/cm3) (eV) ionisation energy (eV) 14 2.33 1.12 3.61 32 5.32 0.72 2.98 48/52 6.06 1.52 4.43 Cd0.8Zn0.2Te 48/30/52 6 1.64 5.0 HgI2 80/53 6.4 2.13 4.3 (300 K) Ge (77 K) CdTe (300 K) (300 K) (300 K) 22