Radiation transducer ** Radiation transducer (photon detector) Any device that converts an amount of radiation into some other measurable phenomenon. electric signals. - External photoelectric (photomultiplier), internal photoelectric (photovoltaic and photoconductive types) - Thermal detectors. - ** Ideal transducer would have a high sensitivity, a high signal-to-noise ratio, a constant response over a considerable range of wavelengths, a wide dynamic range, a fast response time, and a zero output in the absence of illumination. - Dynamic range: the electric signal would be directly proportional to the radiant power P (linearity). - Human eyes dynamic range: 10 10 10 14, resolution up to ~7 10 7 different color (~5.7 10-5 nm) but poor response to quantity. ** Modern electronic detectors: 10 3 10 5. - Taking the dark current into account, S = kp + bkgnd over the dynamic range.
Radiation transducer - Photosensors
Radiation transducer Type of radiation transducers: (1) Photoelectric (quantum) detectors: A: Photomultiplier (PMT) B: CdS photoconductivity C: GaAs photovoltaic cell D: CdSe photoconductivity E: Se/SO photovoltaic cell F: Silicon photodiode G: PbS photoconductivity (a) emission of electrons photocurrent (b) electron promotion to conduction bands enhanced conductivity (photoconduction) ** The electric signal results from a series of individual events (absorption of single photons). The signal behavior described by statistics (shot noise). (2) Thermal detectors: H: Thermocouple
From Hamamatsu PMT handbook ** Photomultiplier (PMT): A vacuum tube consisting of an input window, a photocathode, focusing electrodes, and electron multipliers, and an anode sealed into an evacuated glass tube.
(1) Window materials (2) Photocathodes MgF 2 115 nm Al 2 O 3 (sapphire) 150 nm Synthetic silica 160 nm UV glass 185 nm Borosilicate glass 300 nm
Detector Photomultiplier (2) Photocathodes Solar-blind vacuum UV PMT -- InP/InGaAsP, or InP/InGaAs on an InP substrate -- Field-assisted photocathodes, applying a bias voltage extending sensitivity to 1.4 or 1.7 μm. (Detection of light in this region was previously not possible.) -- Cooled to -60 C to -80 C to an acceptable level of dark current.
(3) Electron multipliers (Dynodes) Dynode materials: Alkali antimonide Beryllium oxide (BeO) Magnesium oxide (MgO) Gallium phosphide (GaP) Gallium Arsine Phosphide (GaAsP) Coated on a substrate electrode (Ni, SS, CuBe alloy)
(3) Electron multipliers (Dynodes)
(3) Electron multipliers (Dynodes) Accelerating voltage
(3) Electron multipliers (Dynodes) Nonliearity, 2% deviation
Analog and digital (pulse counting) modes
Setting the PMT tube supply voltage This figure implies that the PMT should be operated in the range between the voltage (Vo) at which the plateau region begins and the maximum supply voltage.
Count rate linearity Correction formula: N = M/(1-Mt) N: true count rate M: measured count rate T: pulse pair resolution (in this case 18 nm) Correction factor 1 1- M t
Step-by-step PMT selection Step 1 - Do you want to detect at wavelengths below 300nm? -- Borosilcate glass The standard, low cost, window material for wavelengths greater than 300 nm. -- UV glass Extending sensitivity down to 185 nm. -- quartz made from fused silica, this material transmits down to 160 nm. -- Magnesium fluoride (MgF 2 ) UV transmission down to 110 nm. -- Sapphire For metal ceramic photomultiplier windows http://www.et-enterprises.com/photomultipliers/step-by-step-photomultiplier-selection-guide
Step-by-step PMT selection Step 2 - What is the wavelength of your source? (The quoted wavelength ranges refer to standard borosilicate windows.) -- Bialkali (K - Cs - Sb), 280-630nm high blue and good green response with low dark current. -- Rubidium bialkali (Rb - Cs - Sb), 280-680nm high blue and enhanced green response but twice the dark current of the bialkali. -- Multialkali S20 (Na - K - Cs - Sb), 280-850nm From the uv to the infrared but may require cooling to reduce dark curent. -- High temperature bialkali (Na - K - Sb), 280-630nm For high temperature operation at temperatures above 60 C. -- solar blind + MgF 2 window (KBr, Csl, RbTe, CsTe), 110-360nm when sensitivity in the uv and vuv only is required.
Step-by-step PMT selection Step 3 - Is low background glass an advantage? Photomultipliers with minimal levels of naturally occuring K, Th and U, are recommended for low background scintillation counting and most photomultipliers from Electron Tubes are made with low background windows. Radionuclide activities are given in the table below:
Step-by-step PMT selection Step 4 - What detection area and geometry do you require?
Step-by-step PMT selection Step 5 - Are high light levels or low temperature a consideration? ** If photocathode current is greater than 1 na then light levels are high. ** The current carrying capacity of a photocathode is dependent on the operating temperature, rankings at 20 ºC. All become less conductive with decreasing temperature.
Step-by-step PMT selection Step 6 - Have you considered signal / background? ** Dark current or dark count is always a consideration in low light level applications or where the dynamic range exceeds 10 5 (dynamic range is simply the ratio of the highest to the lowest light level measured). -- dark current and count rate increase with pmt diameter. -- dark current and count rate increase with pmt temperature. -- dark current increases approximately linearly with gain. -- dark count rate is essentially independent of gain.
Step-by-step PMT selection Step 7 - What is the optimum photomultiplier gain? ** Typical photomultipliers gain: 10 3 to 10 8. ** The more dynode stages in the PMT, the higher the gain capability. -- Operating a high gain pmt at high gain: best photoelectron collection, high gain for low light levels, best time response, signal/background optimized. operating a high gain pmt at low gain: Fast, BeCu, 52 mm photomultipliers poor photoelectron collection, poor gain linearity, slower time response, restricted dynamic range. -- Operating a low gain pmt at low gain: extended life time, good gain linearity, best signal/background performance operating a low gain pmt at high gain: unstable if maximum ratings are exceeded
Step-by-step PMT selection Step 8- Which dynode structure best meets your performance needs? ** decide: the gain required ** decide: the dynode structure ** decide: the number of stages to best meet the performance demands of your application -- Photomultipliers with plano-concave windows and circular focused multipliers give the best timing performance. -- The number and type of dynodes, the overall voltage, and the diameter of the photocathode.