ASTM E520-08(2023)e1
(Practice)Standard Practice for Describing Photomultiplier Detectors in Emission and Absorption Spectrometry
Standard Practice for Describing Photomultiplier Detectors in Emission and Absorption Spectrometry
ABSTRACT
This practice describes the photomultiplier properties that are essential to their judicious selection and use of in emission and absorption spectrometry. The properties covered here include structural features, electrical properties, and characteristics involved in precautions and problems. The structural features covered are envelope configurations, window materials, electrical connections, and housing for the external structure, and the photocathode, dynodes and anode, and rigidness of structural components for the internal structure. Electrical properties, on the other hand, incorporate the following: optical-electronic characteristics of the photocathode including spectral response; current amplification including gain per stage, overall gain, gain control (voltage-divider bridge), linearity of response, and anode saturation; signal nature; dark current including cathode size, internal aperture, and refrigeration effects; noise nature including additivity of noise power, signal-to-noise ratio, equivalent noise input; and photomultiplier properties as a component in an electrical circuit including output impedance, response time, and signal gating and integration possibilities. Finally, the characteristics involved in precautions and problems cover fatigue and hysteresis effects, illumination of photocathode, and gas leakage.
SCOPE
1.1 This practice covers photomultiplier properties that are essential to their judicious selection and use in emission and absorption spectrometry. Descriptions of these properties can be found in the following sections:
Section
Structural Features
4
General
4.1
External Structure
4.2
Internal Structure
4.3
Electrical Properties
5
General
5.1
Optical-Electronic Characteristics of the Photocathode
5.2
Current Amplification
5.3
Signal Nature
5.4
Dark Current
5.5
Noise Nature
5.6
Photomultiplier as a Component in an Electrical Circuit
5.7
Precautions and Problems
6
General
6.1
Fatigue and Hysteresis Effects
6.2
Illumination of Photocathode
6.3
Gas Leakage
6.4
Recommendations on Important Selection Criteria
7
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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Designation: E520 − 08 (Reapproved 2023)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Editorially corrected Sections 3 – 6 in March 2024.
1. Scope Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.1 This practice covers photomultiplier properties that are
Barriers to Trade (TBT) Committee.
essential to their judicious selection and use in emission and
absorption spectrometry. Descriptions of these properties can
2. Referenced Documents
be found in the following sections:
2.1 ASTM Standards:
Section
E135 Terminology Relating to Analytical Chemistry for
Structural Features 4
General 4.1
Metals, Ores, and Related Materials
External Structure 4.2
Internal Structure 4.3
3. Terminology
Electrical Properties 5
General 5.1
3.1 Definitions—For definitions of terms used in this
Optical-Electronic Characteristics of the Photocathode 5.2
standard, refer to Terminology E135.
Current Amplification 5.3
3.2 Definitions of Terms Specific to This Standard:
Signal Nature 5.4
Dark Current 5.5
3.2.1 solar blind, n—photocathode of photomultiplier tube
Noise Nature 5.6
does not respond to higher wavelengths.
Photomultiplier as a Component in an Electrical Circuit 5.7
3.2.1.1 Discussion—In general, solar blind photomultiplier
Precautions and Problems 6
General 6.1
tubes used in atomic emission spectrometry transmit radiation
Fatigue and Hysteresis Effects 6.2
below about 300 nm and do not transmit radiation above 300
Illumination of Photocathode 6.3
nm.
Gas Leakage 6.4
Recommendations on Important Selection Criteria 7
4. Structural Features
1.2 Radiation in the frequency range common to analytical
4.1 General—The external structure and dimensions, and
emission and absorption spectrometry is detected by photomul-
tipliers presently to the exclusion of most other transducers. the internal structure and electrical properties, can be signifi-
cant when selecting a photomultiplier.
Detection limits, analytical sensitivity, and accuracy depend on
the characteristics of these current-amplifying detectors as well
4.2 External Structure—The external structure consists of
as other factors in the system.
envelope configurations, window materials, electrical contacts
1.3 This standard does not purport to address all of the
through the glass-wall envelopes, and exterior housing.
safety concerns, if any, associated with its use. It is the
4.2.1 Envelope Configurations—Glass envelope shapes and
responsibility of the user of this standard to establish appro-
dimensions are available in an abundant variety. Two envelope
priate safety, health, and environmental practices and deter-
configurations are common, the end-on (or head-on) and
mine the applicability of regulatory limitations prior to use.
side-on types (see Fig. 1).
1.4 This international standard was developed in accor-
4.2.2 Window Materials—Various window materials, such
dance with internationally recognized principles on standard-
as glass, quartz and quartz-like materials, sapphire, magnesium
ization established in the Decision on Principles for the
fluoride, and cleaved lithium fluoride, cover the ranges of
spectral transmission essential to efficient detection in spectro-
metric applications. Window cross sections for the end-on type
This practice is under the jurisdiction of ASTM Committee E01 on Analytical
Chemistry for Metals, Ores, and Related Materials and is the direct responsibility of
Subcommittee E01.20 on Fundamental Practices. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved April 1, 2023. Published March 2024. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1998. Last previous edition approved in 2023 as E520 – 08 (2023). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E0520-08R23E01. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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E520 − 08 (2023)
4.3.2 Dynodes and Anode—Secondary-electron multiplica-
tion systems are designed so that the electrons strike a dynode
at a region where the electric field is directed away from the
surface and toward the next dynode. Six of these configurations
are shown in Fig. 2. Ordinarily a photomultiplier uses from 4
dynodes to 16 dynodes. There are several different configura-
tions of anodes including multianodes and cross wire anodes
for position sensitivity.
4.3.3 Rigidness of Structural Components—The standard
structural components generally will not endure exceptional
mechanical shocks. However, 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 Properties
5.1 General—The electrical properties of a photomultiplier
are a complex function of the cathode, dynodes, and the
voltage divider bridge used for gain control.
5.2 Optical-Electronic Characteristics of the
Photocathode—Electrons are ejected into a vacuum from the
conduction bands of semiconducting or conducting materials if
the surface of the material is exposed to electromagnetic
radiation having a photon energy higher than that required by
the photoelectric work-function threshold. The number of
electrons emitted per incident photon, that is, the quantum
efficiency, is likely to be less than unity and typically less than
0.3.
FIG. 1 Envelope Configurations
photomultipliers include plano-plano, plano-concave,
convexo-concave forms, and a hemispherical form for the
collection of 2-π radians of light flux.
4.2.3 Electrical Connections—Standard pin bases, flying-
leads, or potted pin bases are available to facilitate the location
of a photomultiplier, or for the use of a photomultiplier at low
temperatures. TFE-fluorocarbon receptacles for pin-base types
are recommended to minimize the current leakage between
pins.
4.2.4 Housing—The housing for a photomultiplier should
be “light tight.” Light leaks into a housing or monochromator
from fluorescent lamps are particularly unacceptable noise
sources which can be readily detected with an oscilloscope
adjusted for twice the power line frequency. A mu-metal
housing or shield is recommended to diminish stray magnetic
field interferences with the internal focus on electron trajecto-
ries between tube elements.
4.3 Internal Structure—The internal structure consists of
arrangements of cathode, dynodes, and anodes.
4.3.1 Photocathode—A typical photomultiplier of the
end-on configuration possesses a semitransparent to opaque
layer of photoemissive material that is deposited on the inner
surface of the window segment in an evacuated glass envelope.
In the side-on window types, the cathode layer is on a reflective
substrate within the evacuated tube or on the inner surface of
the window. FIG. 2 Electrostatic Dynode Structures
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E520 − 08 (2023)
5.2.1 Spectral Response—The spectral response of a photo- electron energy and the work function of the material used for
cathode is the relative rate of photoelectron production as a the dynode surface. Most often dynode surfaces are Cs-Sb or
function of the wavelength of the incident radiation of constant Be-O composites on Cu/Be or Ni substrates. The gain per
flux density and solid angle. Spectral response is measured at dynode stage generally is purposely limited.
the cathode with a simple anode or at the anode of a 5.3.2 Overall Gain—A series of dynodes, arranged so that a
secondary-electron photomultiplier. Usually, this wavelength- stepwise amplification of electrons from a photocathode
dependent response is expressed in amperes per watt at anode. occurs, constitutes a total secondary electron multiplication
5.2.1.1 Spectral response curves for several common stan- system. Ordinarily, the number of dynodes employed in a
dard cathode-types are shown in Fig. 3. The S-number is a photomultiplier ranges from 4 to 16. The overall gain for a
standard industrial reference number for a given cathode type system, G, is related to the mean gain per stage, g, and the
n
and spectral response. Some of the common cathode surface number of dynode stages, n, by the equation G=g . Overall
compositions are listed below. Semiconductive photocathodes, gains in the order of 10 can be achieved easily.
for example, GaAs(Cs) and InGaAs(Cs), and red-enhanced 5.3.3 Gain Control (Voltage-Divider Bridge)—Since, for a
multialkali photocathodes (S-25) are also available. A “solar given photomultiplier the cathode and dynode surface materi-
blind” response cathode of CsI, not shown in Fig. 3, provides als and arrangement are fixed, the only practical means to
a low-noise signal in the 160 nm to 300 nm region of the change the overall gain is to control the voltages applied to the
spectrum. Intensity measurements at wavelengths below 100 individual tube elements. This control is accomplished by
nm can be made with a windowless, gold-cathode photomul- adjusting the voltage that is furnished by a high-voltage supply
tiplier. and that is imposed across a voltage-divider bridge (see Fig. 4).
Selection of proper resistance values and the configuration for
Examples of Cathode Surfaces
Response Type Designation Window Cathode Surface
the voltage-divider bridge ultimately determine whether a
S-1? Lime Glass Ag-O-Cs
given photomultiplier will function with stability and linearity
(Reflection)
in a certain application. Operational stability is determined by
S-5? Ultraviolet Sb-Cs
Transmitting Glass (Reflection)
the stability of the high voltage supplied to the divider-bridge
S-11 Lime Glass Sb-Cs
by the relative anode and divider-bridge currents and by the
(Semitransparent)
stability of each dynode voltage as determined by the divider-
S-13 Fused Silica Sb-Cs
(Semitransparent)
bridge.
S-20 Lime Glass Sb-Na-K-Cs
5.3.3.1 To a first approximation, the error in the gain varies
(Semitransparent)
proportionately to the error in the applied high voltage multi-
5.3 Current Amplification—The feeble photoelectron cur-
plied by the number of stages. Therefore, for a ten-stage tube,
rent generated at the cathode is increased to a conveniently
a gain stability of 6 1 % is attained with a power-supply
measurable level by a secondary electron multiplication sys-
voltage stability of 6 0.1 %.
tem. The mechanism for electron multiplication simply de-
5.3.3.2 For a tube stability of 1 %, the current drawn from
pends on the principle that the collision of an energetic electron
the heaviest loaded stage must be less than 1 % of the total
with a low work-function surface (dynode) will cause the
current through the voltage divider bridge. For most spectro-
ejection of several secondary electrons. Thus, a primary
metric applications, a bridge current of about 0.5 mA to 1 mA
photoelectron that is directed by an electrostatic field and
is sufficient.
through an accelerating voltage to the first tube dynode will
5.3.3.3 The value of R (see Fig. 4) is set to give a voltage
effectively be amplified by a factor equal to the number of
between the cathode and the first dynode as recommended by
secondary electrons ejected from the single collision.
the manufacturer. Resistors R , R ···R , R , R , and R
2 3 n−2 n−1 n n+1
5.3.1 Gain per Stage—The amplification factor or gain
may be graded to give interstage voltages which are appropri-
produced at a dynode stage depends both on the primary
ate to the required peak current. With higher interstage voltages
at the output end of the tube, higher peak currents can be
drawn, but average currents above 1 mA are not normally
recommended. The value selected for decoupling-capacitors,
FIG. 3 Spectral Response Curves for Several Cathode Types FIG. 4 Voltage-Divider Bridge
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E520 − 08 (2023)
C, which serve to prevent sudden significant interstage voltage exists even when the cathode is not illuminated. This total
changes between the last few dynodes, is dependent on the current is referred to as dark current.
signal frequency. Typically, the capacitance, C, is about two
5.5.1 Spectral Response and Dark Current—In general,
nanofarads (nF). In Fig. 4, A can be a load resistor (1 MΩ to
those cathode surfaces which provide extended red response
10 MΩ) or the input impedance to a current-measuring device.
have both low photoelectric-work functions and low
5.3.3.4 The overall gain of a photomultiplier varies in a
thermionic-work functions. Therefore, higher dark currents can
nonlinear fashion with the overall voltage applied to the divider
be expected for tubes with red-sensitive cathodes. However,
bridge as shown in Fig. 5.
the S-20 surface, which has much better red response and
5.3.4 Linearity of Response—A photomultiplier is capable
higher quantum efficiency than the S-11 surface, has a thermi-
of providing a linear response to the radiant input signal over
onic emission level that is equal to or lower than that of the
several orders of magnitude. Usually, the dynamic range at the
S-11.
photomultiplier exceeds the range capability of the common
5.5.2 Cathode Size—The dark current from thermionic elec-
linear voltage amplifiers used in measuring circuits.
trons is directly proportional to the area of photocathode
5.3.5 Anode Saturation—As the light intensity impinging on
viewed by the first dynode.
a photocathode is increased, an intensity level is reached,
5.5.3 Internal Apertures—Some photomultipliers are pro-
above which the anode current will no longer increase. A
vided with a defining aperture plane (or plate) between the
current-density saturation at the anode, or anode saturation, is
photocathode and the first dynode. The target plate defines an
responsible for this effect. A photomultiplier should never be
aperture that limits the area of the cathode viewed by the first
operated at anode saturation conditions nor in the nonlinear
dynode and effectively reduces dark current.
response region approaching saturation because of possible
5.5.4 Refrigeration of Photocathodes—Dark current from
dam
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