1、Designation: E520 08 (Reapproved 2015)1Standard Practice forDescribing Photomultiplier Detectors in Emission andAbsorption Spectrometry1This standard is issued under the fixed designation E520; the number immediately following the designation indicates the year oforiginal adoption or, in the case of
2、 revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEEditorial corrections were made to 1.1, 3.2.1, and 4.2.1 in February 2016.1. Scope1.1 This practice co
3、vers photomultiplier properties that areessential to their judicious selection and use in emission andabsorption spectrometry. Descriptions of these properties canbe found in the following sections:SectionStructural Features 4General 4.1External Structure 4.2Internal Structure 4.3Electrical Properti
4、es 5General 5.1Optical-Electronic Characteristics of the 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 Photoca
5、thode 6.3Gas Leakage 6.4Recommendations on Important Selection 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 a
6、ccuracy depend onthe characteristics of these current-amplifying 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 sa
7、fety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E135 Terminology Relating to Analytical Chemistry forMetals, Ores, and Related Materials3. Terminology3.1 DefinitionsFor terminology relating to detectors refe
8、rto Terminology E135.3.2 Definitions of Terms Specific to This Standard:3.2.1 solar blind, nphotocathode of photomultiplier tubedoes not respond to higher wavelengths.3.2.1.1 DiscussionIn general, solar blind photomultipliertubes used in atomic emission spectrometry transmit radiationbelow about 300
9、 nm and do not transmit wavelengths above300 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
10、 configurations, window materials, electrical contactsthrough the glass-wall envelopes, and exterior housing.4.2.1 Envelope ConfigurationsGlass envelope shapes anddimensions are available in an abundant variety. Two envelopeconfigurations are common, the end-on (or head-on) andside-on types (see Fig
11、. 1).4.2.2 Window MaterialsVarious 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-
12、on typephotomultipliers include plano-plano, plano-concave,1This practice is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores, and Related Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition approved Dec. 15, 2
13、015. Published February 2016. Originallyapproved in 1998. Last previous edition approved in 2008 as E520 08. DOI:10.1520/E0520-08R15E01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume in
14、formation, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1convexo-concave forms, and a hemispherical form for thecollection of 2- radians of light flux.4.2.3 Electrical
15、ConnectionsStandard pin bases, flying-leads, or potted pin bases are available to facilitate the locationof a photomultiplier, or for the use of a photomultiplier at lowtemperatures. TFE-fluorocarbon receptacles for pin-base typesare recommended to minimize the current leakage betweenpins.4.2.4 Hous
16、ingThe 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. A mu-metal housing or shieldis recommended t
17、o 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 photomultiplier of theend-on configuration possesses a s
18、emitransparent 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 the inner surface ofthe window.4.3.2 Dynodes a
19、nd 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. Ordinarily a photomultiplier uses from 4dynodes to 16
20、 dynodes. There are several different configura-tions of anodes including multianodes and cross wire anodesfor position sensitivity.4.3.3 Rigidness of Structural ComponentsThe standardstructural components generally will not endure exceptionalmechanical shocks. However, specifically constructed phot
21、o-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 complex function of the cathode, dy
22、nodes, 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 electromagneticradiation having a pho
23、ton 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.FIG. 1 Envelope ConfigurationsFIG. 2 Electrostatic Dynode StructuresE520 08
24、 (2015)125.2.1 Spectral ResponseThe spectral response of a photo-cathode 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
25、of asecondary-electron photomultiplier. Usually, this 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 typ
26、eand spectral response. Some of the common cathode surfacecompositions 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 of CsI, not shown in Fig. 3,provide
27、s a low-noise signal in the 160-nm to 300-nm region ofthe spectrum. Intensity measurements at wavelengths below100 nm can be made with a windowless, gold-cathode photo-multiplier.Examples of Cathode SurfacesResponse Type Designation Window Cathode SurfaceS-1? Lime Glass Ag-O-Cs(Reflection)S-5? Ultra
28、violetTransmitting 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 convenientlymeasurable level by a se
29、condary 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 primaryphotoelectron that is directe
30、d 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 dynode stage depends both on
31、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, arranged so that astepwise amp
32、lification of electrons from a photocathodeoccurs, 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, g, and thenumber of dynode stage
33、s, 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 overall gain is to control the
34、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 forthe voltage-divider bridge u
35、ltimately 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 currents and by thestability of each d
36、ynode 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 attained with a power-supplyvoltag
37、e stability of 6 0.1?%.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 mA to 1 mAis sufficient.5.3.3.3 The value of R
38、1(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 voltagesat the output end of the
39、 tube, higher peak currents can bedrawn, but average currents above 1 mA are not normallyrecommended. The value selected for decoupling-capacitors,FIG. 3 Spectral Response Curves for Several Cathode Types FIG. 4 Voltage-Divider BridgeE520 08 (2015)13C, which serve to prevent sudden significant inter
40、stage voltagechanges between the last few dynodes, is dependent on thesignal frequency. Typically, the capacitance, C, is about twonanofarads (nF). In Fig. 4, A can be a load resistor (1 M to 10M) or the input impedance to a current-measuring device.5.3.3.4 The overall gain of a photomultiplier vari
41、es in anonlinear fashion with the overall voltage applied to the dividerbridge as shown in Fig. 5.5.3.4 Linearity of ResponseA photomultiplier is capableof providing a linear response to the radiant input signal overseveral orders of magnitude. Usually, the dynamic range at thephotomultiplier exceed
42、s the range capability of the commonlinear voltage amplifiers used in measuring circuits.5.3.5 Anode SaturationAs the light intensity impinging ona photocathode is increased, an intensity level is reached,above which the anode current will no longer increase. Acurrent-density saturation at the anode
43、, or anode saturation, 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 discret
44、e charge carriers. 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 packet of electrons is produced, andthis process c
45、ontinues repetitively 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 distributedan
46、d follow Poisson 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 il
47、luminated. This 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 tub
48、es with red-sensitive 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 thermionic elec-trons is directly p
49、roportional to 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 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
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