Lecture 3-Building a Detector (cont’d)

1. Lecture 3-Building a Detector (cont’d) George K. Parks Space Sciences Laboratory UC Berkeley, Berkeley, CA

2. Summary of Lecture 1 and 2 We learned • detecting particles and photons rely on the physics of how particles and photons interact with matter. • charged particles lose energy by ionization and radiation. • photons interact with matter by Photoelectric, Compton and Pair processes. • scintillators emit light when photons stop in them. Now we need a device that will detect this light from the scintillator so that we can process and obtain the original information. Today’s discussion: Scintillator+ Photomultiplier Tube SemiConductor Detectors (Solid State Detectors) ESA + Microchannel Plates

3. Photocathode • Photocathode is a primary element of PMTs. • Photocathode = alkali metal with low work function. • Photocathode material = semiconductor: Can be described by a band model. • hn incident on photocathode excites an e- from the valance band. • If e- has sufficient energy to cross the energy gap (forbidden region), e- are emitted as photoelectrons. • Photoelectrons are focused to dynodes for subsequent multiplication.

4. Photoelectron Emission Process • Absorption of the incident photon and transfer of energy to an electron within the photocathode. 2. Migration of the electron to the surface. Some energy is lost in e-e collision in migration 3. Escape of the electron from the surface of the photocathode. Sufficient energy must be left to overcome the potential barrier (work function) between material and vacuum. Work function typically 3-4 eV for most metals but ~1-2 eVfor semiconductors.

5. Photocathodes vs Spectral Emission of scintillators • Photocathodes have different Spectral Responses and Quantum Efficiencies. • Match scintillation spectrum with spectral response of PMT 1. CsI: 2. CsTe: 3. SbCs: 4. Bialkali (SbRbCs; SbKCs): 5. SbNaK (high T) 6. Multi-Alkali (SbNaKCs) 7. AgOCs 8. GaAsP (Cs) 9. GaAs (Cs) 10. InGAS (CS) 11. InP/InGaAsP(Cs)

6. Transmittance of window material of PMT • UV radiation absorbed by window material of PMT. • Short wavelength limit determined by UV transmittance of window material.

7. Dark Current • In total darkness, PMT still produces a small amount of current, called anode dark current. • Dark current consists of short pulses identical to pulses in Photoelectric current. • Sources of dark current: - Thermoionic - Ohmic leakage - Radioactivity - Field emissions 1 photo-e- <1 photo-e- 2 photo-e-

8. Dark current (cont’d) • Gain and Dark Current varies with applied HV • Can increase Gain by operating at higher Voltage, but Dark Current also increases. • Dark current comes from leakage current, thermoionic emissions, and fieldeffect.

9. Temperature Characteristics of Dark Current • Dark current for different photocathodes • As T increases, thermoionic emissionsincreases. • Affects spectral sensitivity. • Unit (Dark current=Amperes)

10. Linearity • Non-Linearity starts when anode current exceeds 10-5A.

11. Spatial Uniformity • Spatial non-uniformity = variation of sensitivity with position of the incident light on photocathode. • Variation comes from non-uniformity of photocathode and deviation of electron trajectory from photocathode to the first dynode.

12. Magnetic field effects • Magnetic field deflects electrons in PMT. • To reduce magnetic effect, shield PMT with m-metal. • Why peak not at 0? Unit (magnetic field mT)

13. Incident photons and PMT output

14. Pulse Height Distribution (Energy Spectra) • Energy resolutionof a detector is the amount of fluctuations recorded for each photon of the same energy measured by the detector. • Energy resolution NOTmeasurement errors. •Inherentrandomness of the photoelectric process • Pulse height distribution determined by combination of scintillatorConversion Efficiency and the QE of the PMT. ~5.9 keV ~662 keV

15. Energy Resolution • One of the main science goals is to make good measurements of energy distribution of incident radiation. • Energy resolutionis the amount of fluctuations recorded for each photon of the same energy measured by the detector. dN/dE E

16. Energy Resolution for Scintillation Detector • Resolution of energy measurement determined by the statistical variance of produced signal quanta. Energy resolution determined by the smallest number of quanta in chain, that is, the number of photoelectrons produced. Suppose a 5.9 keVphoton (Fe 55) stopped in NaI (Tl). Conversion Efficiency ~0.1 yields 590 eV. Light emitted at 420 nm ~4 eV. Thus ~148 photons produced and will arrive at the photocathode. Quantum Efficiency ~0.25. Produce ~37 photoelectrons. 1/(37)1/2x100~17% RMS = ~34% FWHM. Typical ~50% , non-uniform photocathode and gain variations.

17. Quantum Efficiency (Bi-Alkali) • QE = # PE emitted/ # incident hn • QE varies with incident l • QE = (Sx12400/l)x100% =100x10-3x12400x100/4000 ~ 30%@ 400 nm • Typical QE ~ 25%. Units QE % Sensitivity (mA/W)

18. Summary (Scintillation + PMT)

19. High energy charged particles Solid State Detectors • Total energy detectors • ~20 keV to few hundred MeV • Low end limited by dead layer • High end limited by thickness of the detector - Can stack detectors to increase energy range. - Practical issue of number of detectors to be stacked - New low dead layer detectors allow >2 keVe- and ~6 keV for p+ (CINEMA) • Super-high Particle (CR) measurements can also use semi-conductor detectors. - SSD can also measure photons (X- and g-rays)

20. Comparison NaI(Tl) vs Ge(Li) Pulse height Spectrum of Ag • Germanium detectors have excellent Energy Resolution. • Can Separate many closely spaced -ray energies not resolved NaI(Tl). Ge very expensive. NaI (Tl) is not. Ge Needs LN2. NaI(TL) operate Room T. Ge Size limited. NaI(Tl) can be very large.

21. Band Model • Solid state detectors are semiconductors. • Valence and conduction bands separated by a bandgap the size of which determines the material is insulator or semiconductor.

22. Solid state detectors • Silicon detectors PIPs: Produced by implanting p-type material into n-type Si substrate - Easy to produce - Very rugged Surface barrier Chemical process to create a diode surface Easily damaged, sensitive to solvents Not too common anymore • Typically both types run fully depleted (Electric field extending throughout the the bulk of the material) • Maximum thickness~1000 microns-defines maximum energy of particle that can be stopped in one detector • Particles can be incident on either side of the detector

23. Solid State Detectors (cont’d) • Li drifted Silicon - Can be made in thicknesses ~1 cm to stop very energetic ions (~100 MeV) - Reduced energy resolution for ion studies - Requires LN2 storage for stability • High purity Germanium • Large Z, high stopping power, • Very good energy resolution (X- and g-ray Astronomy) • Expensive! • Requires LN2 • CdZnTd (Cad Zinc Telluride) - High Z - Less $ than Ge but still expensive • New technology - Avalanche photodiodes - Delta-doped

24. Operation of semiconductor detector

25. Calibration of Detectors • The amount of energy required to produce electron-ion pair in Si detectors is 3.5 eV. Hence, if we know how many electron-ion pairs are produced, we obtain the energy of the particle. • Detectors are biased so electrons and ions are collected separately at anode and cathode. • A detector is calibrated with known beam energies. The size of the pulse measured is directly related to the original particle energy. • The distribution of pulse-height vs energy gives the differential energy spectrum

26. Detector resolution vs energy • Planar Ge(Li) detector (8mm x 4mm). • FWHM = 1 keV @ 1000 keV • Energy resolution = 0.1% at 1 MeV.

27. Energy Resolution of Semiconductor Detectors MaterialZr Bandgap Ion E (e-h) Energy Resolution Si (77oK) 14 2.33 1.12 eV 3.61 eV 400 eV @ 60 keV (77K) 1.16 3.76 550 eV @ 122 keV Ge (77K) 32 5.33 0.72 2.98 400 eV @ 122 keV (0.35%) 900 eV @ 662 keV 1300 eV @ 1332 keV CdTe(300K) ~50 6.06 1.52 4.43 1.7 keV @ 60 keV 3.5 keV @ 122 keV (2.8%) HgI2(300K) 6.4 2.13 4.3 3.2 keV@122 keV (2.6%) 5.96 keV@662 keV

28. ElectroStaticAnalyzers (ESA) • Most common detector used for low energy particles today is ESA + MCP • Consider an electric field across concentric cylindrical parallel plates with a mean radius R. The uniform E is radially directed so the electric force qE produces the centripetal acceleration to keep the beam in the circular path, qE = Mv2/R, where M is mass and q is charge. Hence, • ESAs are energy/charge detectors. • ESAs operate from a few eV (SC potential) to ~40 keV/e - Low end limited by SC potential - High end by practical issues of HV

29. Early design of quadrispheric analyzer • ESA designs include cylindrical, spherical and quadrispherical shaped plates. • ESAs are basically capacitors with voltage applied across the plates. • + and – charges are deflected in opposite directions. • Advantages of curved plates include reducing HV (analyzer constant) and UV rejection • Once E/q selected, particle is recorded by an electron detector CEM).

30. Low Energy Proton and Electron Differential Energy Analyzer (LEPEDEA) • Good energy resolution, mechanically and electrically simple, well-understood geometrical and energy responses, has ~180o polar FOV. • Polar angle response not uniform. • Azimuth angle response also not uniform and depends on polar angle (No cylindrical symmetry due to planar entrance aperture). • Top hat ESA design has uniform angular response (Carlson, Adv. Space Res. 1983)

31. Degradation and Noise • ESAs: No mechanical degradation other than failure at launch (vibration). • Ultraviolet (UV) photons: UV photons that scatter in the analyzer can reach the detector. Serrating (scalloping) outer plate and gold-blackening both surfaces can reduce scattered UV (Cluster ~50 cnts/s, FAST ~400 cnts/s). • Scattered Electrons: Electrons impinging at low angles have high probability of scattering with little energy loss and can result in anout of energy band response. Magnitude of the problem depends on ESA design but appears with high geometrical factor ESAs (like top hat). • Scattered ions: Similar to electrons, ions can strike the analyzer plate at glancing incidence. Serration helps to reduce this contribution. • Field Emission: Exposed insulating surface within ESA can acquire a surface charge. The E-field between charged surface and conducting metal become so large that electrons are emitted and counted. • HV: Noise counts with the highest energy stepsof top hat ESA have been observed in several instruments when deflection V~ 4 kV. Up to several hundred counts/s.

32. MicroChannel Plates (MCP) • MCPs consist of 2D array of millions of single CEMs. • Each channel is a continuous glass tube coated with high-resistance semiconductor that serves as an emitter of secondary electrons (dynodes). . • Incident radiation ejects an electron that accelerates down the channel, collide with wall, produce secondary electrons.

33. Operation of MCP (schematic) • 2 MCPs stacked (Chevron configuration) achieve more gain, ~106 • 3 MCPs stacked (Z-stack), Gain >107. • Angled channels reduce ion feedback currents. • A large pulse of output electrons is ejected from the positive end of the MCP into the anode.

34. Degradation of MCPs • MCPsage! (1) Aging decreases gain that depends on the total extracted chargeand results in decrease of pulse counting efficiency (2) This will decrease efficiency for an incident particle to produce secondary electrons at the entrance aperture. • Possible causes: Hydrocarbon contamination. Under UV and particle bombardment, hydrocarbon chains may form on the sensitive area changing the properties. • Strongly suggest that MCPs be scrubbed in 50-50% mixture of isopropanoland methanolfollowed by baking at 100o for ~30 minutes.

35. BURLE, Long-life MCP test • New MCPs show abrupt descrease in gain early in life. • Initial drop due to changes in the interior surface when contaminants are slowly scrubbed away by electron bombardment. • Typically, bias voltage must be raised to compensate (several times for SC). • Initial gain decrease may be avoided by scrubbing before launch! • Lifetime ~10-100 C/cm2

36. Background counts in detectors • Penetrating particles - Energetic radiation belt particles (inner radiation belt) - Cosmic rays (CR) • Solution: Reduce CR particle counts by including Anti-coincidence (AC) detector. - AC technique includes another detector system that views the same region of sky as the ESA, SST or Scintillator+PMT. - CR Penetrate through MCPs (for example) and AC and Coincidence circuits can be used to reject these counts. -AC can improve science quality by improving signal to noise ratio. • Rarely used in space plasma experiments with ESAs (WIND EESAH). • A/C generally not useful for inner radiation particles.

37. A scintillation detector • Incident photon hn stops in scintillator, generates scintillation photons. • Scintillation photons propagate to photocathode and produce photoelectrons • Photoelectrons multiplied at subsequent dynodes and collected at the anode • Measured current from Anode to ground directly proportional to photoelectron flux generated at photocathode

38. Schematic diagram of a working Balloon-borne X-ray Detector

39. Summary • Discussed briefly how to build detectors to measure Photons and Charged Particles • Basic principle is relatively simple. There are only a few components for particle and photon detection. However, there are many details that can affect the measurements. • Innovative ways to use these components to design one of a kind instrument to enhance science goals. • SSTs can be used to detect Energetic Neutral Atoms (ENA). • ESA + TOF + MCP can be used to determine M/q. • Pin hole, coded aperture, modulation collimators yield 2D information. • Stereoscopic view (2 SC) can yield 3D information.

40. Optimization • There are many parameters. Often the parameters work against each other. For example in imaging instruments, if we want good spatial resolution, we give up time resolutionbecause with smaller aperture, it takes more time to get the counts with the same statistics. • Similarly, to improve energy resolution, you also lose time resolution because there are less counts in narrower energy intervals. • Nature does not give you something for nothing!

41. Particle measurements (Reminder) • Most basic quantity that characterizes particles or photons is the distribution function or differential energy spectra. • What do particles detectors measure? - All particle and photon instruments measure fluxes of particles • How does one convert what the detector measures to the distribution function or energy spectra? - Need to perform pulse height analysis - Know Geometrical factor accurately. • Absolute calibration is most important for all detectors.

42. Issues to consider in instrument design - Energy resolutions: More science with high energy resolution. - Detector efficiency: Maintain as high as possible. - Pulse height defect: Difference between true and apparent energy - Channeling: effect of crystal orientation - Dead layer: low energy threshold - Radiation damage: degrades energy resolution and counting efficiency - UV rejection (MCPs): spurious counts contamination - Pulse pile up: Loss of true counts. - Leakage current: - Detector Noise: - Changes with bias voltage: Effect if bias voltage is low - Temperature sensitivity: loss of detector performance - Cleanliness: produce noise in system. - Micro-acoustic sensitivity:

43. The End

44. Temperature Characteristics of Photocathodes • Temperature change is large near the long wavelength cutoff. • Unit is in % /oC

45. Life Characteristics • Damage from the last dynode due to heavy electron bombardment

46. Summary of PMT • Decide photon counting or measure current • How to select photocathode material (eg, S-13, etc..) •Less than single photon spectrum; single and two photon spectra. Important for single photon counting. • Determine HV for operation (900, 1100, 1200, etc..) • Determine quantum efficiency of photocathode • Quantum efficiency • Collection efficiency • Window material • Photocathode material • Gain • Dark Current • Spatial Uniformity • Temperature Charactristics • Magnetic field effect • Life characteristics • Electronic noise • Dynamic range • Linearity • Time response

47. Channel Electron Multipliers • ESAs usually combine with CEMs or MCPs. • Straight tubes produce large ion feedback current. • Ion feedback can be reduced by curving the channeltrons Evans (1965)