ISO/TS 23878:2024

Technical
Specification
ISO/TS 23878
First edition
Nanotechnologies — Positron
2024-08
annihilation lifetime measurement
for nanopore evaluation in
materials
Nanotechnologies – Mesure d'annihilation de la durée de vie de
positrons pour l'évaluation de nanopores dans des matériaux
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 General .1
3.2 Experimental set-up .2
4 Symbols and abbreviated terms. 2
5 Principle . 3
6 Overview of the positron annihilation lifetime measurement . 4
7 Apparatus . 7
7.1 Specification of the detector .7
7.2 Discrimination of the detected signals .8
7.3 Measurement conditions .8
8 Preparation of the positron source. 9
9 Preparation of the measurement specimen . 9
10 Data analysis . 9
11 Reporting .10
11.1 Specific values .10
11.2 Pore dimension.11
12 Reference materials .11
Annex A (informative) Interlaboratory comparison .12
Annex B (informative) Configuration of the apparatus .15
Annex C (informative) List of parameters and measurement conditions . 19
Bibliography .21

iii
Foreword
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iv
Introduction
This document describes a method for measuring and reporting the lifetime of ortho-positronium utilizing
the positron annihilation lifetime technique. Some of the positrons introduced into insulating materials, like
oxides and organic polymers, can form the spin parallel positron-electron bound state ortho-positronium,
which tends to localize in voids. In its trapped state, ortho-positronium annihilates with a lifetime that is
less than its intrinsic lifetime of 142 ns in vacuum via a two-gamma annihilation process, where this lifetime
component is well correlated with the void dimension. Based on this principle, one can evaluate the average
porosity originating from nanometer size voids, such as free volumes in polymers. It is well documented that
the positron annihilation lifetime technique is a powerful tool for characterizing the nanopores of various
functional materials. Increased demands on the reliable evaluation of nanopores using this technique have
emerged for various industrial applications.
This document describes a method for performing positron annihilation lifetime measurements to analyse
the lifetime of the ortho-positronium ranging from 1 ns to 10 ns (ascribed to a pore size from approximately
0,3 nm to 1,3 nm in diameter), observed for polymeric materials. It also contains measurement procedures,
data analysis, and reporting sections. In the annexes, the results of an interlaboratory comparison using
two types of reference materials conducted by eight participating institutions, are described, followed by
details of measurement systems that are based on the available analogue and digital methods, and a list of
parameters and measurement conditions provided as a guide to the user.

v
Technical Specification ISO/TS 23878:2024(en)
Nanotechnologies — Positron annihilation lifetime
measurement for nanopore evaluation in materials
1 Scope
This document describes a method for performing positron annihilation lifetime measurements using a Na
+ +
positron source that decays with β emission. The β (positron) lifetime is determined from a measurement
of the lifetime of the ortho-positronium which ranges from 1 ns to 10 ns (ascribed to a pore size from
approximately 0,3 nm to 1,3 nm in diameter), as observed for polymeric materials in which the positronium
atoms mostly annihilate via a two-gamma annihilation process.
This document is not applicable to thin surface layers (that are less than several micrometers).
This document does not apply to measuring:
— non-positronium forming materials;
— positronium-forming materials that induce a spin conversion reaction;
— positronium-forming materials that contain chemicals influencing the annihilation process of ortho-
positronium by chemical reactions;
— positronium-forming materials that contain mesoporous silica gels with a large contribution from the
three-gamma annihilation process.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org
3.1 General
3.1.1
positronium
bound state of a positron and an electron
3.1.2
positron lifetime
component lifetime corresponding to the annihilation of a large number of positrons or positroniums, and
extracted from a measured lifetime spectrum

3.2 Experimental set-up
3.2.1
scintillator
material that luminesces when excited by radiation, wherein the luminescent energy is related to the energy
deposited by the injected radiation
3.2.2
photomultiplier tube
vacuum phototube that converts incident light to an electronic signal, the magnitude of which is based on the
energy and number of the incident photons, and subsequently amplifies that signal to provide an electrical output
3.2.3
counting rate
measured number of events per unit time where an event is the coincident detection of the photons generated
during the production and annihilation of positrons
3.2.4
positron source
+
emitter of positrons due to nuclear transmutation via β decay
22 22
Note 1 to entry: Sodium-22 ( Na) transmuting to Ne is a positron source, for example.
3.2.5
amplifier
module that increases the amplitude of a signal
3.2.6
gate and delay generator
module that generates a logic pulse of a desired duration (gate width) and with the desired delay relative to
a reference event
4 Symbols and abbreviated terms
ADC analogue-to-digital converter
CFDD constant fraction differential discriminator
CRM certified reference material
DCFD differential constant fraction discriminator
GDG gate and delay generator
I relative intensity of lifetime component
LLD lower level discriminator
MCA multichannel analyzer
NIM nuclear instrument modules
PMT photomultiplier tube
Ps positronium
RI radioisotope
TAC time-to-amplitude converter

ULD upper level discriminator
o-Ps ortho-positronium (triplet state)
p-Ps para-positronium (singlet state)
τ positron or positronium component lifetime extracted from the measured lifetime spectrum
Na sodium with an atomic mass number of 22
Ne neon with an atomic mass number of 22
Ne* excited state of neon with an atomic mass number of 22
5 Principle
A fraction of the total number of positrons injected into an insulating polymer, such as polyolefins, can form
positronium (Ps), the bound state of a positron and an electron. Ps forms either in:
— the singlet state, i.e. with antiparallel spins (called p-Ps);
— or in the triplet state with parallel spins (called o-Ps);
The positrons that do not form Ps will annihilate with the surrounding electrons with a mean lifetime of
several hundred ps. The annihilation of p-Ps and o-Ps in vacuum have intrinsic lifetimes of 125 ps and 142 ns,
respectively.
The o-Ps that is trapped in a pore will annihilate via the pick-off process with an electron at the pore
walls and will have its lifetime shortened from the intrinsic lifetime of 142 ns. The annihilation lifetime
of o-Ps decreases as the pore volume decreases, accordingly. An o-Ps lifetime of between 1 ns and 10 ns
is correlated with the size of the pores. Based on this principle, the pore sizes can be estimated from the
measured lifetime of o-Ps.
22 + 22
In this document, radioactive Na with a β decay is used for the positron source. The Na nucleus that
22 ∗
emits a positron transmutes to Ne within a short time, which subsequently relaxes by emitting a gamma
ray photon with an energy of 1,275 MeV (see Figure 1).
Figure 1 — Decay process of Na
The lifetime of a single positron annihilated in a target specimen, placed in close proximity to the Na
positron source, is determined by measuring the time difference between the detection of the 1,275 MeV
gamma ray photon and the 511 keV annihilation gamma ray photon in the specimen. The 1,275 MeV gamma
ray photon is emitted almost simultaneously with the β+ decay. A lifetime histogram (also known as
a positron annihilation lifetime spectrum) is obtained by accumulating the time differences over a large
number of annihilation events, so that the mean lifetime of o-Ps annihilated in the sub-nanometer and
nanometer-sized pores can be determined subsequently by data analysis.
The mean lifetime (τ = 1/λ, where λ is the annihilation rate) and the respective fraction (relative intensity I)
of each process can be determined by using a model function in the analysis and assuming a proper number
of decay functions ascribed to the number of expected positron and Ps annihilation processes.

The number of surviving (unannihilated) positrons in the specimen at a laboratory time frame t ≥ 0, is given by:
 
J
t
Nt() =−NI exp (1)
 
0∑ j
 
j=1
τ
j
 
where N is the initial number of the positrons, and J is the number of the annihilation components. Thus,
the time dependence of the observed annihilation events, that is the positron annihilation lifetime spectrum
[C(t)], is proportional to the rate of reduction of the positron number at a given time. It can be expressed as:
I
 
J
j t
Ct() =− exp (2)
 
∑
 
j=1
ττ
jj
 
The o-Ps lifetime is obtained as the long-lived component (with a lifetime greater than 1 ns) in the
experimental distribution (see Clause 10).
6 Overview of the positron annihilation lifetime measurement
A simplified schematic overview of a typical positron annihilation lifetime measurement system is shown in
Figure 2.
Figure 2 — Schematic overview of a positron lifetime measurement system
Start and stop signals, corresponding to the detection of the gamma ray photons from the production and
the annihilation of a positron, are generated by a set of gamma-ray photon detectors. Each detector consists
of a scintillator and a photomultiplier tube (PMT) that are placed on either side of the sample. Refer to
Clause B.1 for guidance on detector placement. The detected signals are analysed to measure the time delay
for each positron production and annihilation event.
The signals output from the PMT have amplitudes that range between a few mV to V, and are proportional
to the energy of each gamma-ray photon (see Figure 3) and the scintillation and detection efficiency of that
photon. The start and stop signals shall be selected by discrimination, that is, by processing only those
signals with pulse amplitudes greater than the lower level discriminator (LLD) but less than the upper level
discriminator (ULD), where the LLD and ULD are set differently for the start and stop signals.
The LLD and ULD assigned for each detector shall be set so that noise signals, as well as mismatch signals,
are excluded from acquisition (see Figure 4). The timing pulses, which act as timing surrogates for the
gamma-ray photons, can be produced by processing the PMT output signals according to several methods,
such as a constant fraction discrimination (see Clause B.2). These timing surrogates are often necessary
because:
— the amplitude of the signals from the PMTs is variable;
— stable timing fiducials are needed for subsequent computation of time delays.
The time delay, Δt, between the timing pulses from the start and stop signals is measured by a time-to-
amplitude converter (TAC) module in the analogue method (see Clause B.2) or by direct processing of
the digitized waveforms of the detected signals in the digital method (see Clause B.3). Consequently, by
accumulating the set of Δt, one for each positron production and annihilation event, a lifetime histogram
of the annihilated positrons is obtained (see Figure 5). The horizontal scale of the measurement system,
generally the bin width of the lifetime histogram, shall be calibrated to the unit of second.
Key
X channel
Y counts/channel
1 peak originating from the detection of 511 keV annihilation gamma ray photon
2 peak originating from the detection of 1,275 MeV gamma ray photon
NOTE 1 Figure 3 shows the typical amplitude distribution of the detected signals from a PMT with BaF scintillator.
NOTE 2 The values of the horizontal axis refer directly to the amplitude of the signals from the PMT and indirectly
to the energy of those gamma ray photons.
Figure 3 — Typical amplitude distribution with BaF scintillator
Key
X amplitude
Y counts/amplitude
1 for the start signals
2 for the stop signals
a
LLD.
b
ULD.
NOTE Figure 4 shows the typical setting of the ULD and the LLD for the threshold voltage of the DCFD.
Figure 4 — Typical setting of the ULD and the LLD

Key
X channel
Y counts/channel
NOTE The values of the horizontal axis correspond to the time-bins.
Figure 5 — Typical lifetime histogram of positron annihilations
7 Apparatus
7.1 Specification of the detector
The scintillation detector comprises a scintillator and a photon detector such as a PMT. A transparent crystal
or plastic containing aromatic compounds, which has a fast response as well as a short luminescence decay,
is typically used as the scintillating material. The time resolution of the measurement system, conventionally
given by the full width at half maximum of the prompt coincidence peak of the lifetime spectrum, is improved
by reducing the volume of the scintillator, but with a loss of detection efficiency. Typically, the scintillator is
in the form of a cylinder or frustum with a diameter and thickness from 1 cm to 3 cm and may be used in
combination with a suitable reflector. Table 1 lists examples of available scintillators.
Table 1 — Examples of scintillators
Material Max. luminescence Rise time Decay time Note
wavelength
BaF 220 nm ― 0,8 ns ―
a)
Plastic [EJ-228 and BC-418 ] 391 nm 0,5 ns 1,3 ns Base: Polyvinyl toluene
a)
EJ-228 and BC-418 are examples of suitable products available commercially. This information is given for the convenience of
users of this document and does not constitute an endorsement by ISO of these products. See References [1] and [2].
The PMT should have an output gain great enough to attain signals with appropriate amplitudes for
processing. The impulse response of the PMT should be of as short a duration as possible (typically, with a
transition duration of less than 1 ns). The window material of the PMT shall be selected in order to minimize
the absorption of the luminescent light from the scintillator. In the case of BaF , which emits luminescent light
in the UV range, fused silica is useful as the window material of the PMT. The diameter of the PMT window
(typically 25 mm to 50 mm) should be the correct size to enable optical coupling with the scintillator. This
coupling can be improved by using an appropriate optical coupling agent, such as silicone oil, grease, etc.
7.2 Discrimination of the detected signals
Typical pulse amplitude distributions using BaF for the scintillators are shown in Figure 3. The peaks in
the signal amplitude distribution shown in Figure 3 corresponding to the 1,275 MeV and 511 keV gamma ray
photons exhibit a broad distribution, which is due to the poor time and energy resolution of the detectors.
Generally, fluctuations in the amplitude of the detected signals lead to the lower time resolution of the
measurement system, as the difference between the LLD and ULD is increased.
When a plastic scintillator is used for the detector, no clear photoelectric peak is observed in the energy
distribution as shown in Figure 6. In this case, to determine the respective LLD and ULD for the start and
stop signals, energy levels for the Compton edges can be used.
Key
X channel
Y counts/channel
1 peak originating from the detection of 511 keV annihilation gamma ray photon
2 peak originating from the detection of 1,275 MeV gamma ray photon
NOTE 1 Figure 6 shows the typical amplitude distribution of the detected signals from a PMT with a plastic
scintillator.
NOTE 2 The values of the horizontal axis refer directly to the amplitude of the signals from the PMT and indirectly
to the energy of those gamma ray photons.
Figure 6 — Typical amplitude distribution with plastic scintillator
7.3 Measurement conditions
The time intervals (width per channel) in the lifetime histogram should be set in the range from 10 ps to
50 ps. The epoch (duration of observation period) for the histogram shall be
...