ASTM E385-90(1996)
(Test Method)Standard Test Method for Oxygen Content Using a 14-MeV Neutron Activation and Direct-Counting Technique
Standard Test Method for Oxygen Content Using a 14-MeV Neutron Activation and Direct-Counting Technique
SCOPE
1.1 This test method covers the measurement of oxygen concentration in almost any matrix by using a 14-MeV neutron activation and direct-counting technique. Essentially, the same system may be used to determine oxygen concentrations ranging from over 50% to about 10 [mu]g/g, or less, depending on the sample size and available 14-MeV neutron fluence rates. Note 1-The range of analysis may be extended by using higher neutron fluence rates, larger samples, and higher counting efficiency detectors.
1.2 This test method may be used on either solid or liquid samples, provided that they can be made to conform in size, shape, and macroscopic density during irradiation and counting to a standard sample of known oxygen content. Several variants of this method have been described in the technical literature. A monograph is available which provides a comprehensive description of the principles of activation analysis using a neutron generator (1).
1.3 The values stated in either SI or inch-pound units are to be regarded separately as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety problems 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. Specific precautions are given in Section 8.
General Information
Relations
Standards Content (Sample)
Designation: E 385 – 90 (Reapproved 1996)
Standard Test Method for
Oxygen Content Using a 14-MeV Neutron Activation and
Direct-Counting Technique
This standard is issued under the fixed designation E 385; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope by Radioactivation Techniques
2.2 U.S. Government Document:
1.1 This test method covers the measurement of oxygen
Code of Federal Regulations, Title 10, Part 20
concentration in almost any matrix by using a 14-MeV neutron
activation and direct-counting technique. Essentially, the same
3. Terminology
system may be used to determine oxygen concentrations
3.1 Definitions (see also Terminology E 170):
ranging from over 50 % to about 10 μg/g, or less, depending on
3.1.1 accelerator, n—a machine that ionizes a gas and
the sample size and available 14-MeV neutron fluence rates.
electrically accelerates the ions onto a target. The accelerator
NOTE 1—The range of analysis may be extended by using higher
may be based on the Cockroft-Walton, Van de Graaff, or other
neutron fluence rates, larger samples, and higher counting efficiency
design types (1). Compact sealed-tube, mixed deuterium and
detectors.
tritium gas, Cockcroft-Walton neutron generators are most
1.2 This test method may be used on either solid or liquid
commonly used for 14-MeV neutron activation analysis. How-
samples, provided that they can be made to conform in size,
ever, “pumped” drift-tube accelerators that use replaceable
shape, and macroscopic density during irradiation and counting
tritium-containing targets are also still in use. A review of
to a standard sample of known oxygen content. Several
operational characteristics, descriptions of accessory instru-
variants of this method have been described in the technical
mentation, and applications of accelerators used as fast neutron
literature. A monograph is available which provides a compre-
generators is given in Ref (2).
hensive description of the principles of activation analysis
3.1.2 comparator standard, n—a reference standard of
using a neutron generator (1).
known oxygen content whose specific counting rate (counts
−1 −1
1.3 The values stated in either SI or inch-pound units are to
min [mg of oxygen] ) may be used to quantify the oxygen
be regarded separately as the standard. The values given in
content of a sample irradiated and counted under the same
parentheses are for information only.
conditions. Often, a comparator standard is selected to have a
1.4 This standard does not purport to address all of the
matrix composition, physical size, density and shape very
safety concerns, if any, associated with its use. It is the
similar to the corresponding parameters of the sample to be
responsibility of the user of this standard to establish appro-
analyzed. Comparative standards prepared in this way may be
priate safety and health practices and determine the applica-
used directly as “monitors” (see 3.1.4) in order to avoid the
bility of regulatory limitations prior to use. Specific precau-
need for monitor-sample calibration plots, in those cases where
tions are given in Section 8.
the usual monitor reference standard is physically or chemi-
cally dissimilar to the samples to be analyzed.
2. Referenced Documents
3.1.3 14-MeV neutron fluence rate, n—the areal density of
2.1 ASTM Standards:
neutrons passing through a sample, measured in terms of
−2 −1
E 170 Terminology Relating to Radiation Measurements
neutrons cm s , that is produced by the fusion reaction of
and Dosimetry
deuterium and tritium ions accelerated to energies of typically
E 181 Test Methods for Detector Calibration and Analysis
150 to 200 keV in a small accelerator. Fluence rate is also
of Radionuclides
commonly referred to as “flux density.” The total neutron
E 496 Test Method for Measuring Neutron Fluence Rate
fluence is the fluence rate integrated over time.
3 4
3 4
and Average Energy from H(d,n) He Neutron Generators
3.1.3.1 Discussion—The H(d,n) He reaction is used to pro-
duce approximately 14.7-MeV neutrons. This reaction has a
Q-value of + 17.586 MeV.
This test method is under the jurisdiction of ASTM Committee E-10 on Nuclear
Technology and Applicationsand is the direct responsibility of Subcommittee 3.1.4 monitor, n—any type of detector or comparison ref-
E10.05on Nuclear Radiation Metrology.
erence material that can be used to produce a response
Current edition approved Oct. 26, 1990. Published August 1991. Originally
published as E 385 – 69 T. Last previous edition E 385 – 80.
The boldface numbers in parentheses refer to a list of references at the end of
the text. Available from the Superintendent of Documents, U.S. Government Printing
Annual Book of ASTM Standards, Vol 12.02. Office, Washington, DC 20402.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 385
proportional to the 14-MeV neutron fluence rate in the irradia- 5. Significance and Use
tion position, or to the radionuclide decay events recorded by
5.1 The conventional determination of oxygen content in
the sample detector. A plastic pellet with a known oxygen
liquid or solid samples is a relatively difficult chemical
content is often used as a monitor reference standard in dual
procedure. It is slow and usually of limited sensitivity. The
sample transfer systems. It is never removed from the system
14-MeV neutron activation and direct counting technique
regardless of the characteristics of the sample to be analyzed.
provides a rapid, highly sensitive, nondestructive procedure for
In this case monitor-sample calibration plots are required.
oxygen determination in a wide range of matrices. This test
3.1.5 multichannel pulse-height analyzer, n—an instrument
method is independent of the chemical form of the oxygen.
that receives, counts, separates, and stores, as a function of
5.2 This test method can be used for quality and process
their energy, pulses from a scintillation or semi-conductor
control in the metals, coal, and petroleum industries, and for
gamma-ray detector and amplifier. In the 14-MeV INAA
research purposes in a broad spectrum of applications.
determination of oxygen, the multichannel analyzer may also
be used to receive and record both the BF neutron detector
6. Interferences
monitor counts and the sample gamma-ray detector counts as a
6.1 Because of the high energy of the gamma rays emitted
function of stepped time increments (3 and 4). In the latter
in the decay of N, there are very few elements that will
case, operation of the analyzer in the multichannel scaler
produce interfering radiations; nevertheless, caution should be
(MCS) mode, an electronic gating circuit is used to select only
exercised. F, for example, will undergo an (n,a) reaction to
gamma rays within the energy range of interest. 16
produce N, the same indicator radionuclide produced from
19 16
3.1.6 transfer system, n—a system, normally pneumatic,
oxygen. Because the cross section for the F(n,a) N reaction
16 16
used to transport the sample from an injection port (sometimes
is approximately one-half that of the O(n,p) N reaction, a
connected to an automatic sample changer) to the irradiation
correction must be made if fluorine is present in an amount
station, and then to the counting station where the activity of
comparable to the statistical uncertainty in the oxygen deter-
the sample is measured. The system may include components
mination. Another possible interfering reaction may arise from
to ensure uniform positioning of the sample at the irradiation
the presence of boron. B will undergo an (n,p) reaction to
and counting stations.
produce Be. This isotope decays with a half-life of 13.81 s,
and emits several high-energy gamma rays with energies in the
4. Summary of Test Method
range of 4.67 to 7.98 MeV. In addition, there is Bremsstrahlung
radiation produced by the high energy beta particles emitted
4.1 The weighed sample to be analyzed is placed in a
by Be. These radiations can interfere with the oxygen deter-
container for automatic transfer from a sample-loading port to
mination if the oxygen content does not exceed 1 % of the
the 14-MeV neutron irradiation position of a particle accelera-
boron present.
tor. After irradiation for a pre-selected time, the sample is
6.2 Another possible elemental interference can arise from
automatically returned to the counting area. A gamma-ray
the presence of fissionable materials such as thorium, uranium,
detector measures the high-energy gamma radiation from the
and plutonium. Many short-lived fission products emit high-
radioactive decay of the N produced by the (n,p) nuclear
energy gamma rays capable of interfering with those from N.
reaction on O. The number of counts in a pre-selected
40 40 40
counting interval is recorded by a gated scaler, or by a
NOTE 2—Argon produces an interferent, Cl, by the Ar(n,p) Cl re-
multichannel analyzer operating in either the pulse-height, or
action. Therefore, argon should not be used for the inert atmosphere
gated multiscaler modes. The number of events recorded for during sample preparation for oxygen analysis. Cl (t ⁄2 5 1.35 m) has
several high-energy gamma rays, including one at 5.88 MeV.
samples and monitor reference standard are corrected for
background and normalized to identical irradiation and count-
6.3 An important aspect of this analysis that must be
ing conditions. If the sample and monitor reference standard
controlled is the geometry during both irradiation and count-
sample are not irradiated simultaneously, the neutron dose
ing. The neutron source is usually a disk source. Hence, the
received during each irradiation must be recorded, typically by
fluence rate decreases as the inverse square at points distant
use of a BF neutron proportional counter. The amount of total
3 from the target, and less rapidly close to the target. Because of
oxygen (all chemical forms) in the sample is proportional to the
these fluence rate gradients, the irradiation geometry should be
corrected sample count and is quantified by use of the corrected
reproduced as accurately as possible. Similarly, the positioning
specific activity of the monitor, or comparator standard(s).
of the sample at the detector is critical and must be accurately
reproducible. For example, if the sample is considered to be a
4.1.1 N decays with a half-life of 7.13 s by b-emission,
thus returning to O. About 69 % of the decays are accompa- point source located 6 mm from a cylindrical sodium iodide
nied by 6.13-MeV gamma rays, 5 % by 7.12-MeV gamma (NaI) detector, a 1-mm change in position of the sample along
rays, and 1 % by 2.74-MeV gamma rays. Other lower intensity the detector axis will result in a 3.5 to 5 % change in detector
gamma rays are also observed. About 26 % of the beta efficiency (8). Since efficiency is defined as the fraction of
transitions are directly to the ground state of O. (All half-lives gamma rays emitted from the source that interact with the
and gamma-ray energies are taken from Ref (5) and decay detector, it is evident that a change in efficiency would result in
schemes are given in Ref (6). A useful elemental data base and an equal percentage change in measured activity and in
calculated sensitivities for 14-MeV instrumental neutron acti- apparent oxygen content. Positioning errors are normally
vation analysis (14-MeV INAA) are provided in Ref (7). (See minimized by rotating the sample around a single axis, or
also Test Methods E 181.) biaxially, during both irradiation and counting. Alternately,
E 385
dual detectors at 180° can be used to minimize positioning tivity due to recoil of N atoms produced in the air onto the
errors at the counting station. sample surface. The transfer system and data processing may
6.4 Since N emits high-energy gamma rays, determina- be controlled by PC-type microcomputers using programs
tions are less subject to effects of self-absorption than are written in BASIC (11), or by a minicomputer using programs
determinations based on the use of indicator radionuclides written in FORTRAN (4). Dual transfer systems transport the
emitting lower energy gamma rays. Corrections for gamma-ray sample and a monitor reference standard simultaneously. In
attenuation during counting are usually negligible, except in this case, two independent counting systems are often used.
the highest sensitivity determinations where sample sizes may Single sample transfer systems based on sequential irradiations
be large. of a sample and a monitor reference standard, or a comparator
6.5 The oxygen content of the transfer container (“rabbit”) standard, are also used.
must be kept as low as possible to avoid a large “blank”
NOTE 3—As mentioned previously in 6.2, argon should be avoided in
correction. Suggested materials that combine light weight and
the transfer gas, as well as in sample packaging, because of the
low oxygen content are polypropylene and high-density poly- 40
interferent Cl produced.
ethylene (molded under a nitrogen atmosphere), high purity
7.3 Monitor—The number of counts obtained from any
Cu, and high-purity nickel. A simple subtraction of the counts
given irradiation is dependent upon the oxygen content of the
from the blank vial in the absence of the sample is not adequate
sample, the length of irradiation, the neutron fluence rate, the
for oxygen determinations below 200 μg/g, since large sample
neutron energy spectrum, the delay time between irradiation
sizes may be required for these high-sensitivity measurements
and counting, and the length of the count. It is desirable to
and gamma-ray attenuation may be important when the sample
make a measurement in which the result obtained is a function
is present (9). If the total oxygen content of the sample is as
of only the oxygen content and independent of other variables.
low as that of the container (typically about 0.5 mg of oxygen),
This can be achieved by standardizing the experimental con-
the sample should be removed from the irradiation container
ditions and use of a monitor.
prior to counting. Statistical errors increase rapidly as true
7.3.1 In the dual sample transfer approach, the monitor is
sample activities decrease, while container contamination ac-
ordinarily a high-oxygen containing material that is irradiated
tivities remain constant. For certain shapable solids, it may be
with each sample in a position adjacent to the sample position,
possible to use no container at all. This “containerless”
transferred to an independent detector, and counted simulta-
approach provides optimum sensitivity for low-level dete
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