1、Designation: E 520 98 (Reapproved 2003)Standard Practice forDescribing Photomultiplier Detectors in Emission andAbsorption Spectrometry1This standard is issued under the fixed designation E 520; the number immediately following the designation indicates the year oforiginal adoption or, in the case o
2、f revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers photomultiplier properties that areessential to their judicious selectio
3、n and use of photomultipli-ers in emission and absorption spectrometry. Descriptions ofthese properties can be found in the following sections:SectionStructural Features 4General 4.1External Structure 4.2Internal Structure 4.3Electrical Properties 5General 5.1Optical-Electronic Characteristics of th
4、e Photocathode 5.2Current Amplification 5.3Signal Nature 5.4Dark Current 5.5Noise Nature 5.6Photomultiplier as a Component in an Electrical Circuit 5.7Precautions and Problems 6General 6.1Fatigue and Hysteresis Effects 6.2Illumination of Photocathode 6.3Gas Leakage 6.4Recommendations on Important Se
5、lection Criteria 71.2 Radiation in the frequency range common to analyticalemission and absorption spectrometry is detected by photomul-tipliers presently to the exclusion of most other transducers.Detection limits, analytical sensitivity, and accuracy depend onthe characteristics of these current-a
6、mplifying detectors as wellas other factors in the system.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bil
7、ity of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related Materials23. Terminology3.1 DefinitionsFor terminology relating to detectors referto Terminology E 135.3.2 Definitions of Terms Specifi
8、c to This Standard:3.2.1 solar blind, nphotocathode of photomultiplier tubedoes not respond to wavelengths on the high side.3.2.1.1 DiscussionIn general, solar blind photomultipliertubes used in optical emission spectroscopy transmit radiationbelow about 300 nm and do not transmit wavelengths above3
9、00 nm.4. Structural Features4.1 GeneralThe external structure and dimensions, aswell as the internal structure and electrical properties, can besignificant in the selection of a photomultiplier.4.2 External StructureThe external structure consists ofenvelope configurations, window materials, electri
10、cal contactsthrough the glass-wall envelopes, and exterior housing.4.2.1 Envelope ConfigurationsGlass envelope shapes anddimensions are available in an abundant variety. At present,two envelope configurations are common, the end-on (orhead-on) and side-on types (see Fig. 1).4.2.2 Window MaterialsVar
11、ious window materials, suchas glass, quartz and quartz-like materials, sapphire, magnesiumfluoride, and cleaved lithium fluoride, cover the ranges ofspectral transmission essential to efficient detection in spectro-metric applications. Window cross sections for the end-on typephotomultipliers includ
12、e plano-plano, plano-concave,convexo-concave forms, and a hemispherical form for thecollection of 2-p radians of light flux.4.2.3 Electrical ConnectionsStandard pin bases, flying-leads, or potted pin bases are available to facilitate the locationof a photomultiplier, or for the use of a photomultipl
13、ier at lowtemperatures. TFE-fluorocarbon receptacles for pin-base typesare recommended to minimize the current leakage betweenpins.1This practice is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores, and Related Materials and is the direct responsibility ofSubcommi
14、ttee E01.20 on Fundamental Practices.Current edition approved June 10, 2003. Published July 2003. Originallyapproved in 1998. Last previous edition approved in 1998 as E 520 98.2Annual Book of ASTM Standards, Vol 03.05.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
15、ken, PA 19428-2959, United States.4.2.4 HousingThe housing for a photomultiplier shouldbe “light tight.” Light leaks into a housing or monochromatorfrom fluorescent lamps are particularly bad noise sourceswhich can be readily detected with an oscilloscope adjusted fortwice the power line frequency.
16、A mu-metal housing or shieldis recommended to diminish stray magnetic field interferenceswith the internal focus on electron trajectories between tubeelements.4.3 Internal StructureThe internal structure consists ofarrangements of cathode, dynodes, and anodes.4.3.1 PhotocathodeA typical photomultipl
17、ier of theend-on configuration possesses a semitransparent to opaquelayer of photoemissive material that is deposited on the innersurface of the window segment in an evacuated glass envelope.In the side-on window types, the cathode layer is on a reflectivesubstrate within the evacuated tube or on th
18、e inner surface ofthe window.4.3.2 Dynodes and AnodeSecondary-electron multiplica-tion systems are designed so that the electrons strike a dynodeat a region where the electric field is directed away from thesurface and toward the next dynode. Six of these configurationsare shown in Fig. 2. Ordinaril
19、y a photomultiplier uses from 4to 16 dynodes. There are several different configurations ofanodes including multianodes and cross wire anodes forposition sensitivity.4.3.3 Rigidness of Structural ComponentsThe standardstructural components generally will not endure exceptionalmechanical shocks. Howe
20、ver, specifically constructed photo-multipliers (ruggedized) that are resistant to damage by me-chanical shock and stress are available for special applications,such as geophysical uses or in mobile laboratories.5. Electrical Properties5.1 GeneralThe electrical properties of a photomultiplierare a c
21、omplex function of the cathode, dynodes, and thevoltage divider bridge used for gain control.5.2 Optical-Electronic Characteristics of thePhotocathodeElectrons are ejected into a vacuum from theconduction bands of semiconducting or conducting materials ifthe surface of the material is exposed to ele
22、ctromagneticradiation having a photon energy higher than that required bythe photoelectric work-function threshold. The number ofelectrons emitted per incident photon, that is, the quantumefficiency, is likely to be less than unity and typically less than0.3.5.2.1 Spectral ResponseThe spectral respo
23、nse of a pho-tocathode is the relative rate of photoelectron production as afunction of the wavelength of the incident radiation of constantflux density and solid angle. Spectral response is measured atthe cathode with a simple anode or at the anode of asecondary-electron photomultiplier. Usually, t
24、his wavelength-dependent response is expressed in amperes per watt at anode.5.2.1.1 Spectral response curves for several common stan-dard cathode-types are shown in Fig. 3. The S-number is astandard industrial reference number for a given cathode typeand spectral response. Some of the common cathode
25、 surfaceFIG. 1 Envelope ConfigurationsFIG. 2 Electrostatic Dynode StructuresE 520 98 (2003)2compositions are listed below. Semiconductive photocathodes,for example, GaAs(Cs) and InGaAs(Cs), as well as red-enhanced multialkali photocathodes (S-25) are also available.A “solar blind” response cathode o
26、f CsI, not shown in Fig. 3,provides a low-noise signal in the 160- to 300-nm region of thespectrum. Intensity measurements at wavelengths below 100nm can be made with a windowless, gold-cathode photomul-tiplier.Examples of Cathode SurfacesResponse Type Designation Window Cathode SurfaceS-1 Lime Glas
27、s Ag-O-Cs(Reflection)S-5 UltravioletTransmitting GlassSb-Cs(Reflection)S-11 Lime Glass Sb-Cs(Semitransparent)S-13 Fused Silica Sb-Cs(Semitransparent)S-20 Lime Glass Sb-Na-K-Cs(Semitransparent)5.3 Current AmplificationThe feeble photoelectron cur-rent generated at the cathode is increased to a conven
28、ientlymeasurable level by a secondary electron multiplication sys-tem. The mechanism for electron multiplication simply de-pends on the principle that the collision of an energetic electronwith a low work-function surface (dynode) will cause theejection of several secondary electrons. Thus, a primar
29、yphotoelectron that is directed by an electrostatic field andthrough an accelerating voltage to the first tube dynode willeffectively be amplified by a factor equal to the number ofsecondary electrons ejected from the single collision.5.3.1 Gain per StageThe amplification factor or gainproduced at a
30、 dynode stage depends both on the primaryelectron energy and the work function of the material used forthe dynode surface. Most often dynode surfaces are Cs-Sb orBe-O composites on Cu/Be or Ni substrates. The gain perdynode stage generally is purposely limited.5.3.2 Overall GainA series of dynodes,
31、arranged so that astepwise amplification of electrons from a photocathode oc-curs, constitutes a total secondary electron multiplicationsystem. Ordinarily, the number of dynodes employed in aphotomultiplier ranges from 4 to 16. The overall gain for asystem, G, is related to the mean gain per stage,
32、g, and thenumber of dynode stages, n, by the equation G = gn. Overallgains in the order of 106can be achieved easily.5.3.3 Gain Control (Voltage-Divider Bridge)Since, for agiven photomultiplier the cathode and dynode surface materi-als and arrangement are fixed, the only practical means tochange the
33、 overall gain is to control the voltages applied to theindividual tube elements. This control is accomplished byadjusting the voltage that is furnished by a high-voltage supplyand that is imposed across a voltage-divider bridge (see Fig. 4).Selection of proper resistance values and the configuration
34、 forthe voltage-divider bridge ultimately determine whether agiven photomultiplier will function with stability and linearityin a certain application. Operational stability is determined bythe stability of the high voltage supplied to the divider-bridgeby the relative anode and divider-bridge curren
35、ts and by thestability of each dynode voltage as determined by the divider-bridge.5.3.3.1 To a first approximation, the error in the gain variesproportionately to the error in the applied high voltage multi-plied by the number of stages. Therefore, for a ten-stage tube,a gain stability of 61 % is at
36、tained with a power-supplyvoltage stability of 60.1 %.5.3.3.2 For a tube stability of 1 %, the current drawn fromthe heaviest loaded stage must be less than 1 % of the totalcurrent through the voltage divider bridge. For most spectro-scopic applications, a bridge current of about 0.5 to 1 mA issuffi
37、cient.5.3.3.3 The value of R1(see Fig. 4) is set to give a voltagebetween the cathode and the first dynode as recommended bythe manufacturer. Resistors R2, R3Rn2,Rn1, Rn, and Rn+1may be graded to give interstage voltages which are appropri-ate to the required peak current. With higher interstage vol
38、tagesat the output end of the tube, higher peak currents can bedrawn, but average currents above 1 mA are not normallyrecommended. The value selected for decoupling-capacitors,C, which serve to prevent sudden significant interstage voltagechanges between the last few dynodes, is dependent on thesign
39、al frequency. Typically, C = 2 nF. In Fig. 4, A can be a loadresistor (1 to 10 MV) or the input impedance to a current-measuring device.5.3.3.4 The overall gain of a photomultiplier varies in anonlinear fashion with the overall voltage applied to the dividerbridge as shown in Fig. 5.5.3.4 Linearity
40、of ResponseA photomultiplier is capableof providing a linear response to the radiant input signal overFIG. 3 Spectral Response Curves for Several Cathode TypesFIG. 4 Voltage-Divider BridgeE 520 98 (2003)3several orders of magnitude. Usually, the dynamic range at thephotomultiplier exceeds the range
41、capability of the commonlinear voltage amplifiers used in measuring circuits.5.3.5 Anode SaturationAs the light intensity impingingon a photocathode is increased, an intensity level is reached,above which the anode current will no longer increase. Acurrent-density saturation at the anode, or anode s
42、aturation, isresponsible for this effect. A photomultiplier should never beoperated at anode saturation conditions nor in the nonlinearresponse region approaching saturation because of possibledamage to the tube.5.4 Signal NatureThe current through a photomultiplier iscomposed of discrete charge car
43、riers. Each effective photoelec-tron is randomly emitted from the cathode and travels adistance to the first dynode where a small packet of electronsis generated. This packet of electrons then travels to the nextdynode where yet a larger bunch of electrons is produced, andthis process continues repe
44、titively until a final large packet ofelectrons reaches the anode to produce a measurable electricalimpulse. Therefore, the true signal output of a multiplier is atrain of pulses that occur during an interval of photocathodeillumination. These pulse amplitudes are randomly distributedand follow Pois
45、son statistics. This is a characteristic ofso-called “shot-effect” noise.5.5 Dark CurrentThermal emission of electrons from thecathode and dynodes, ion feed-back, and field emission, alongwith internal leakage currents, furnish an anode current thatexists even when the cathode is not illuminated. Th
46、is totalcurrent is referred to as dark current.5.5.1 Spectral Response and Dark CurrentIn general,those cathode surfaces which provide extended red responsehave both low photoelectric-work functions and lowthermionic-work functions. Therefore, higher dark currents canbe expected for tubes with red-s
47、ensitive cathodes. However,the S-20 surface, which has much better red response andhigher quantum efficiency than the S-11 surface, has a thermi-onic emission level that is equal to or lower than that of theS-11.5.5.2 Cathode SizeThe dark current from thermionicelectrons is directly proportional to
48、the area of photocathodeviewed by the first dynode.5.5.3 Internal AperturesSome photomultipliers are pro-vided with a defining aperture plane (or plate) between thephotocathode and the first dynode. The target plate defines anaperture that limits the area of the cathode viewed by the firstdynode and
49、 effectively reduces dark current.5.5.4 Refrigeration of PhotocathodesDark current fromS-1-type photomultipliers can be reduced considerably bycooling the photocathode. The S-1 dark current is reduced byan approximate factor of ten for each 20 K temperaturedecrease.5.6 Noise NatureSince noise power is an additive circuitproperty, a consideration of the major sources of noise in aphotomultiplier is important. The four principal noise sourcesof concern are shot noise, thermionic emission noise, fieldemission noise, and leakage-current noise. Johnson no
