1. Radiation Sensors Zachariadou K. | TEI of Piraeus

2. Part-V Semiconductor Sensors Radiation Sensors

3. Part-V Semiconductor Sensors The course is largely based on :  G. F. Knoll, “Radiation detection and measurement” ; 3rd ed., New York, Wiley, 2000  Gordon Gilmore & John D. Hemingway, “ Practical Gamma-Ray Spectrometry”; Willey, 21008

4. Gas detectors Gas detectors Gas-filled detectors consist of a volume of gas between two electrodesScintillators the interaction of ionizing radiation produces UV and/or visible light Solid state detectors crystals of silicon, germanium, or other materials to which trace amounts of impurity atoms have been added so that they act as diodes Other, Cerenkov etc… Types of detectors

5. Semiconductor Detectors

6. They work on same principle as gas-filled detectors: Gas-filled detectors: production of ion pair Semiconductors: production of electron-hole pairs AdvantagesDrawbacks  Only 3eV are required for ionization compared to about 30eV required to create an ion pair in typical gas filled detectors  Good stability  Thin entrance windows  Simplicity in operation  High Energy resolution  Compact size  Fast timing characteristics  Variable effective thickness to match the requirements of the application  Limitation to small sizes  Radiation-induced damage  Need for cooling (thermal noise)

7. Band structure-Carriers in an Electric Field Conduction Band Valence Band + + + - - - 0 EoEo E o + E g EFEF Insulators: Eg>5eV Semiconductors: E g ~1eV Air: >35eV Scintillators~15eV Thermal excitation: A valence electron gains sufficient thermal energy to be elevated to the conduction band Probability per unit time for thermal excitation: Depends on the ratio of E g over the absolute temperature Materials with large E g have low thermal excitation probability

8. Migration in an electric field At low to moderate values of the electric field intensity: At low to moderate values of the electric field intensity: The drift velocity is proportional to the electric field In Gases: mobility of free electrons >> mobility of positive ion In Semiconductors: mobility of electrons ~ mobility of holes At higher electric field: At higher electric field: the drift velocity rises slowly with the field and reaches a saturation velocity: Saturation velocity : Time to collect the carriers over typical dimensions (0.1cm) : Semiconductors are among the fastest- responding radiation detectors

9. Semiconductors basics

10. Ionization energy

11. Unbiased p-n junction If it functions as a detector This detector would very poor performance Charge carriers migrate across the junction Conduction electrons in the p- side will combine with holes vice versa Accumulated space charges create an electric field that opposites the conduction carriers migration The space charges do not contribute to conductivity. The depletion region has very high resistivity  electron-holes pairs created by the passage of radiation will be swept out  their motion is an electrical signal. + p n e- o - -+ Depletion region

12. ρ(x) E(x) V(x) 0-ab V Contact potential :~1V The thickness of the depletion region is small The electric field is not enough to make the charge carriers to move fast  incomplete charge collection + p n e- o - -+

13. biased p-n junction

14. Forward bias + e-o - -+ p n + o - -+ p n The contact potential is reduced by bias V Large currents are conducted Reverse bias The contact potential is increased by bias V The minority carriers are attracted across the junction. The reverse current is very low

15. + e- o - -+ ρ(x) E(x) V(x) 0 -a b V Reverse bias V

16. + e- o - -+ For x=0: if N= dopant concentration Resistivity : where μ is the mobility of the majority carrier High resistivity(ρ)  large depletion region (d) (detecting region)

17. + e- o - -+ Higher reverse bias Thicker depletion region The capacitance per unit area decreases Small capacitance means less electronic noise resulting to better energy resolution We use largest possible voltage up to fully deplete the junction

18. Semiconductor Radiation sensors  The module is reverse-biased-->a depletion region is set up with an electric-field that sweeps charge-carriers to the electrodes.  When a charged particle passes across the silicon strip electron-hole pairs are created.  The electric field in the depletion region sweeps the new electron-hole pairs to the electrodes where they are collected  The time taken for collection decreases as the bias voltage is increased. In a silicon detector 300 m thick, electrons are collected in about 10 ns and holes in about 25 ns.

19.  Germanium  need for cryogenics Energy to create +- pair = 2.9 eV  Silicon can be used at room temperature. Energy needed to create +- pair= 3.6 eV Less performance for energetic radiation such as  rays (it s a light material : atomic number 14)  CdTe is the most often used because it combines heavy materials (atomic numbers 48 and 52) with relatively high bandgap energies. Why Ge over Si ? Z Ge > Z Si (32 vs 14)  photo-electric effect x 60  Compton scattering x 2 Semiconductor Radiation sensors

20. Semiconductor detectors Operational characteristics  Leakage current  Noise and Energy Resolution  Bias voltage  Pulse rise time  Radiation damage  Channeling  Entrance window  Energy calibration  Pulse height defect

21. Leakage current Bulk leakage current Surface leakage current Minority carrier current – Mostly small thermal generation of electron-hole pairs in the depletion region– Need for cooling Contamination of the surfaces  clean techniques Silicon resistivity : 50,000Ωcm For bulk 1cm 2 R=5000Ω If V=500V Leakage current I=0.1A The current is of the order of 10 -6 A by a pulse of 10 5 radiation induced carriers The Leakage current must not exceed 10 -9 A Deteriorates energy resolution

22. Detector noise  Fluctuations in the bulk generated leakage current  Fluctuations in the surface leakage current Parallel noise  Poor electrical contactsseries noise For silicon diode detectors 3 contributions to noise are most significant:

23. Detector Bias Voltage  Sufficiently high Bias Voltage for complete charge collection  saturation region Incomplete charge collection. The pulse height rises with applied voltage  Low Bias Voltage & electric field: The electrons liberated by the incident radiation gain enough energy from the electric field to create further electron-hole pairs. Basis of the operation of silicon avalanche detectors`  Higher Bias Voltage  multiplication region Corresponds to the region of ion saturation in a gas-filled ion detector

24. Pulse rise time Semiconductors are among the fastest radiation detectors. Pulse rise time of the order of 10ns or less The rise time of the output pulse limited by the time required for complete migration of the electrons-holes created by the incident radiation from their point of formation to the opposite extremes of the depletion region The time is minimized with  High electric field  Small depletion width

25. Dead layer Energy loss before the particle reaches the active volume of the detector The dead layer =metalic electrode + thickness of silicon beneath the electrode in which charge collection is inefficient. The dead layer can be a function of the applied voltage

26. Radiation damage BUT the Non-ionizing energy transferred to the atoms cause irreversible changes Loss in energy resolution of the detector Increase in leakage current The energy that goes to the creation of electron-hole pairs leads to fully reversible processes

27. Channeling In crystalline materials: The rate of energy loss of a charged particle may depend on the orientation of its path with respect to the crystal axes. Particles traveling parallel to crystal plane show lower energy loss Channeled particles penetrate farther in the crystal The energy deposition depends on the crystal orientation To minimize the channeling, detectors are fabricated from silicon cut so that the (111) orientation is perpendicular to the wafer surface

28. Energy calibration The response of semiconductor diode radiation detectors when applied on the measurement of fast electrons, protons, alphas: is Linear The energy calibration obtained for one particle type is very close to that obtained using a different radiation type Most common calibration source: 241-Am

29. Pulse Height defect Response of semiconductor detectors to very heavy ions (fission fragments) Pulse height defect: is the difference between the true energy of the heavy ion and its apparent energy (as determined from an energy calibration of the detector obtained using alpha particles) The pulse height observed is substantially less than that observed for a light ion of the same energy

30. Applications of Semiconductor sensors Charged particle spectroscopy Charged particle spectroscopy Heavy ion and Fission Fragment Particle identification (Energy loss) (For particle identification through dE/dx we choose detectors thin compared with the particle range) X ray spectroscopy with silicon p-i-n diode

31. Germanium detectors for Gamma-ray spectroscopy For gamma ray detection large depletion region is needed Using Silicon or Germanium depletion beyond 2-3 mm are difficult to achieve. A. Reduce impurity concentration to achieve large depletion regions For depletion voltage V<1000V and N=10 10 atoms/cm 3 d~10mm High Purity Germanium (HPGe) or intrinsic germanium : ultra pure Germanium B. Reduce impurity concentration by Lithium ion drifting Ge(Li) detectors HPGe type is now in favor because they don’t need permanent cooling as Ge(Li) while detection efficiency and energy resolution are essentially identical

32. Ge-detectors CONFIGURATIONS n p+ n+ Ultra-pure Ge (HPGe) impurity concentration: ~10 9 -10 10 cm -3 ! (Ge concentration ~ 10 22 cm -3 )  Planar Configuration  Coaxial Configuration  maximization of the sensitive volume ( (diameter ~8 cm, length ~7-8 cm)  larger depleted volume  more efficient detection signal -HV np+n+ -HV signal n p+ n+

33. Ge Energy resolution Excellent energy resolution in gamma ray spectroscopy The energy resolution: combination of 3 factors: Inherent statistical spread in the number of charge carriers Contributions of electronic noise Variations in the charge collection efficiency Most significant in detectors with large volumes and low average electric field FWHM: F=Fano Factor value ε=value necessary to create an electron-hole pair E=incident gamma-ray energy For F=0.08 E=1333MeV ε=2.96eV W D =1.32KeV

34. Ge Energy resolution Comparative pulse height spectra recorded by NaI (Tl) and a Ge(Li) detector

35. Silicon detectors and the CMS experimentmore by Caio Laganá (http://www.academia.edu/1680843/Silicon_detectors_and_the_CMS_experimentCaio Laganá