ASTM E520-98(2003)
(Practice)Standard Practice for Describing Photomultiplier Detectors in Emission and Absorption Spectrometry
Standard Practice for Describing Photomultiplier Detectors in Emission and Absorption Spectrometry
SCOPE
1.1 This practice covers photomultiplier properties that are essential to their judicious selection and use of photomultipliers in emission and absorption spectrometry. Descriptions of these properties can be found in the following sections: SectionStructural Features4General4.1External Structure4.2Internal Structure4.3Electrical Properties5General5.1Optical-Electronic Characteristics of the Photocathode5.2Current Amplification5.3Signal Nature5.4Dark Current5.5Noise Nature5.6Photomultiplier as a Component in an Electrical Circuit5.7Precautions and Problems6General6.1Fatigue and Hysteresis Effects6.2Illumination of Photocathode6.3Gas Leakage6.4Recommendations on Important Selection Criteria7
1.2 Radiation in the frequency range common to analytical emission and absorption spectrometry is detected by photomultipliers presently to the exclusion of most other transducers. Detection limits, analytical sensitivity, and accuracy depend on the characteristics of these current-amplifying detectors as well as other factors in the system.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E 520 – 98 (Reapproved 2003)
Standard Practice for
Describing Photomultiplier Detectors in Emission and
Absorption Spectrometry
This standard is issued under the fixed designation E520; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope Metals, Ores, and Related Materials
1.1 This practice covers photomultiplier properties that are
3. Terminology
essential to their judicious selection and use of photomultipli-
3.1 Definitions—For terminology relating to detectors refer
ers in emission and absorption spectrometry. Descriptions of
to Terminology E135.
these properties can be found in the following sections:
3.2 Definitions of Terms Specific to This Standard:
Section
3.2.1 solar blind, n—photocathode of photomultiplier tube
Structural Features 4
General 4.1
does not respond to wavelengths on the high side.
External Structure 4.2
3.2.1.1 Discussion—In general, solar blind photomultiplier
Internal Structure 4.3
tubes used in optical emission spectroscopy transmit radiation
Electrical Properties 5
General 5.1
below about 300 nm and do not transmit wavelengths above
Optical-Electronic Characteristics of the Photocathode 5.2
300 nm.
Current Amplification 5.3
Signal Nature 5.4
4. Structural Features
Dark Current 5.5
Noise Nature 5.6
4.1 General—The external structure and dimensions, as
Photomultiplier as a Component in an Electrical Circuit 5.7
well as the internal structure and electrical properties, can be
Precautions and Problems 6
General 6.1
significant in the selection of a photomultiplier.
Fatigue and Hysteresis Effects 6.2
4.2 External Structure—The external structure consists of
Illumination of Photocathode 6.3
envelope configurations, window materials, electrical contacts
Gas Leakage 6.4
Recommendations on Important Selection Criteria 7
through the glass-wall envelopes, and exterior housing.
4.2.1 Envelope Configurations—Glass envelope shapes and
1.2 Radiation in the frequency range common to analytical
dimensions are available in an abundant variety. At present,
emissionandabsorptionspectrometryisdetectedbyphotomul-
two envelope configurations are common, the end-on (or
tipliers presently to the exclusion of most other transducers.
head-on) and side-on types (see Fig. 1).
Detectionlimits,analyticalsensitivity,andaccuracydependon
4.2.2 Window Materials—Various window materials, such
thecharacteristicsofthesecurrent-amplifyingdetectorsaswell
asglass,quartzandquartz-likematerials,sapphire,magnesium
as other factors in the system.
fluoride, and cleaved lithium fluoride, cover the ranges of
1.3 This standard does not purport to address all of the
spectral transmission essential to efficient detection in spectro-
safety concerns, if any, associated with its use. It is the
metricapplications.Windowcrosssectionsfortheend-ontype
responsibility of the user of this standard to establish appro-
photomultipliers include plano-plano, plano-concave,
priate safety and health practices and determine the applica-
convexo-concave forms, and a hemispherical form for the
bility of regulatory limitations prior to use.
collection of 2-p radians of light flux.
2. Referenced Documents
4.2.3 Electrical Connections—Standard pin bases, flying-
leads,orpottedpinbasesareavailabletofacilitatethelocation
2.1 ASTM Standards:
of a photomultiplier, or for the use of a photomultiplier at low
E135 Terminology Relating to Analytical Chemistry for
temperatures. TFE-fluorocarbon receptacles for pin-base types
are recommended to minimize the current leakage between
This practice is under the jurisdiction ofASTM Committee E01 onAnalytical pins.
ChemistryforMetals,Ores,andRelatedMaterialsandisthedirectresponsibilityof
Subcommittee E01.20 on Fundamental Practices.
Current edition approved June 10, 2003. Published July 2003. Originally
approved in 1998. Last previous edition approved in 1998 as E520–98.
Annual Book of ASTM Standards, Vol 03.05.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 520 – 98 (2003)
FIG. 2 Electrostatic Dynode Structures
FIG. 1 Envelope Configurations
4.2.4 Housing—The housing for a photomultiplier should multipliers (ruggedized) that are resistant to damage by me-
chanicalshockandstressareavailableforspecialapplications,
be “light tight.” Light leaks into a housing or monochromator
from fluorescent lamps are particularly bad noise sources such as geophysical uses or in mobile laboratories.
whichcanbereadilydetectedwithanoscilloscopeadjustedfor
5. Electrical Properties
twice the power line frequency.Amu-metal housing or shield
is recommended to diminish stray magnetic field interferences 5.1 General—The electrical properties of a photomultiplier
with the internal focus on electron trajectories between tube are a complex function of the cathode, dynodes, and the
elements. voltage divider bridge used for gain control.
4.3 Internal Structure—The internal structure consists of 5.2 Optical-Electronic Characteristics of the
arrangements of cathode, dynodes, and anodes. Photocathode—Electrons are ejected into a vacuum from the
4.3.1 Photocathode—A typical photomultiplier of the conductionbandsofsemiconductingorconductingmaterialsif
end-on configuration possesses a semitransparent to opaque the surface of the material is exposed to electromagnetic
layer of photoemissive material that is deposited on the inner radiation having a photon energy higher than that required by
surfaceofthewindowsegmentinanevacuatedglassenvelope. the photoelectric work-function threshold. The number of
Intheside-onwindowtypes,thecathodelayerisonareflective electrons emitted per incident photon, that is, the quantum
substrate within the evacuated tube or on the inner surface of efficiency, is likely to be less than unity and typically less than
the window. 0.3.
4.3.2 Dynodes and Anode—Secondary-electron multiplica- 5.2.1 Spectral Response—The spectral response of a pho-
tion systems are designed so that the electrons strike a dynode tocathode is the relative rate of photoelectron production as a
at a region where the electric field is directed away from the functionofthewavelengthoftheincidentradiationofconstant
surfaceandtowardthenextdynode.Sixoftheseconfigurations flux density and solid angle. Spectral response is measured at
are shown in Fig. 2. Ordinarily a photomultiplier uses from 4 the cathode with a simple anode or at the anode of a
to 16 dynodes. There are several different configurations of secondary-electron photomultiplier. Usually, this wavelength-
anodes including multianodes and cross wire anodes for dependent response is expressed in amperes per watt at anode.
position sensitivity. 5.2.1.1 Spectral response curves for several common stan-
4.3.3 Rigidness of Structural Components—The standard dard cathode-types are shown in Fig. 3. The S-number is a
structural components generally will not endure exceptional standard industrial reference number for a given cathode type
mechanical shocks. However, specifically constructed photo- and spectral response. Some of the common cathode surface
E 520 – 98 (2003)
n
number of dynode stages, n, by the equation G= g . Overall
gains in the order of 10 can be achieved easily.
5.3.3 Gain Control (Voltage-Divider Bridge)—Since, for a
given photomultiplier the cathode and dynode surface materi-
als and arrangement are fixed, the only practical means to
change the overall gain is to control the voltages applied to the
individual tube elements. This control is accomplished by
adjustingthevoltagethatisfurnishedbyahigh-voltagesupply
andthatisimposedacrossavoltage-dividerbridge(seeFig.4).
Selection of proper resistance values and the configuration for
the voltage-divider bridge ultimately determine whether a
given photomultiplier will function with stability and linearity
in a certain application. Operational stability is determined by
the stability of the high voltage supplied to the divider-bridge
by the relative anode and divider-bridge currents and by the
FIG. 3 Spectral Response Curves for Several Cathode Types
stability of each dynode voltage as determined by the divider-
bridge.
5.3.3.1 To a first approximation, the error in the gain varies
compositions are listed below. Semiconductive photocathodes,
proportionately to the error in the applied high voltage multi-
for example, GaAs(Cs) and InGaAs(Cs), as well as red-
plied by the number of stages. Therefore, for a ten-stage tube,
enhanced multialkali photocathodes (S-25) are also available.
a gain stability of 61% is attained with a power-supply
A “solar blind” response cathode of CsI, not shown in Fig. 3,
voltage stability of 60.1%.
providesalow-noisesignalinthe160-to300-nmregionofthe
5.3.3.2 For a tube stability of 1%, the current drawn from
spectrum. Intensity measurements at wavelengths below 100
the heaviest loaded stage must be less than 1% of the total
nm can be made with a windowless, gold-cathode photomul-
current through the voltage divider bridge. For most spectro-
tiplier.
scopic applications, a bridge current of about 0.5 to 1 mA is
Examples of Cathode Surfaces sufficient.
Response Type Designation Window Cathode Surface
5.3.3.3 The value of R (see Fig. 4) is set to give a voltage
S-1 Lime Glass Ag-O-Cs
between the cathode and the first dynode as recommended by
(Reflection)
S-5 Ultraviolet Sb-Cs the manufacturer. Resistors R , R ···R ,R , R , and R
2 3 n−2 n−1 n n+1
Transmitting Glass (Reflection)
may be graded to give interstage voltages which are appropri-
S-11 Lime Glass Sb-Cs
atetotherequiredpeakcurrent.Withhigherinterstagevoltages
(Semitransparent)
S-13 Fused Silica Sb-Cs
at the output end of the tube, higher peak currents can be
(Semitransparent)
drawn, but average currents above 1 mA are not normally
S-20 Lime Glass Sb-Na-K-Cs
recommended. The value selected for decoupling-capacitors,
(Semitransparent)
C, which serve to prevent sudden significant interstage voltage
5.3 Current Amplification—The feeble photoelectron cur-
changes between the last few dynodes, is dependent on the
rent generated at the cathode is increased to a conveniently
signalfrequency.Typically, C=2nF.InFig.4, Acanbeaload
measurable level by a secondary electron multiplication sys-
resistor (1 to 10 MV) or the input impedance to a current-
tem. The mechanism for electron multiplication simply de-
measuring device.
pendsontheprinciplethatthecollisionofanenergeticelectron
5.3.3.4 The overall gain of a photomultiplier varies in a
with a low work-function surface (dynode) will cause the
nonlinearfashionwiththeoverallvoltageappliedtothedivider
ejection of several secondary electrons. Thus, a primary
bridge as shown in Fig. 5.
photoelectron that is directed by an electrostatic field and
5.3.4 Linearity of Response—A photomultiplier is capable
through an accelerating voltage to the first tube dynode will
of providing a linear response to the radiant input signal over
effectively be amplified by a factor equal to the number of
secondary electrons ejected from the single collision.
5.3.1 Gain per Stage—The amplification factor or gain
produced at a dynode stage depends both on the primary
electron energy and the work function of the material used for
the dynode surface. Most often dynode surfaces are Cs-Sb or
Be-O composites on Cu/Be or Ni substrates. The gain per
dynode stage generally is purposely limited.
5.3.2 Overall Gain—Aseriesofdynodes,arrangedsothata
stepwise amplification of electrons from a photocathode oc-
curs, constitutes a total secondary electron multiplication
system. Ordinarily, the number of dynodes employed in a
photomultiplier ranges from 4 to 16. The overall gain for a
system, G, is related to the mean gain per stage, g, and the FIG. 4 Voltage-Divider Bridge
E 520 – 98 (2003)
5.5.2 Cathode Size—The dark current from thermionic
electrons is directly proportional to the area of photocathode
viewed by the first dynode.
5.5.3 Internal Apertures—Some photomultipliers are pro-
vided with a defining aperture plane (or plate) between the
photocathode and the first dynode. The target plate defines an
aperture that limits the area of the cathode viewed by the first
dynode and effectively reduces dark current.
5.5.4 Refrigeration of Photocathodes—Dark current from
S-1-type photomultipliers can be reduced considerably by
cooling the photocathode. The S-1 dark current is reduced by
an approximate factor of ten for each 20 K temperature
decrease.
A. Venetian Blind-15 Dynodes
5.6 Noise Nature—Since noise power is an additive circuit
B. Box and Grid-11 DynodesC
property, a consideration of the major sources of noise in a
C. Venetian Blind-11 Dynodes
photomultiplier is important. The four principal noise sources
FIG. 5 Overall Gain Dependence on Applied Voltage (SbCs
of concern are shot noise, thermionic emission noise, field
Cathode)
emission noise, and leakage-current noise. Johnson noise is a
property of the anode load resistor in a measuring circuit and
will not be treated here.
several orders of magnitude. Usually, the dynamic range at the
(1) The shot-noise equation describes the maximum shot-
photomultiplier exceeds the range capability of the common
effect noise as follows:
linear voltage amplifiers used in measuring circuits.
1/2
i 5 ~2qIDf! (1)
5.3.5 Anode Saturation—As the light intensity impinging
rms
on a photocathode is increased, an intensity level is reached,
above which the anode current will no longer increase. A
where:
current-density saturation at the anode, or anode saturation, is
i = root-mean-square (quadratic) noise current,
rms
responsible for this effect. A photomultiplier should never be
q = charge on each carrier, C,
operated at anode saturation conditions nor in the nonlinear
I = total current through tube, A, and
response region approaching saturation because of possible
Df = band pass, Hz.
damage to the tube.
The shot-noise component is inversely proportional to the
5.4 Signal Nature—Thecurrentthroughaphotomultiplieris cathode radiant sensitivity.
composedofdiscretechargecarriers.Eacheffectivephotoelec-
(2)The Nyquist equation describes the thermal noise as
tron is randomly emitted from the cathode and travels a
follows:
distance to the first dynode where a small packet of electrons
1/2
i 5 @~4kTDf/R!# (2)
rms
is generated. This packet of electrons then travels to the next
dynode where yet a larger bunch of electrons is produced, and
this process continues repetitively until a final large packet of
where:
electro
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.