1、Designation: E385 11E385 16Standard Test Method forOxygen Content Using a 14-MeV Neutron Activation andDirect-Counting Technique1This standard is issued under the fixed designation E385; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, 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.1. Scope1.1 This test method covers the measurement of oxygen concentration in almost any matrix by using a 14-MeV
3、 neutronactivation and direct-counting technique. Essentially, the same system may be used to determine oxygen concentrations rangingfrom under 10 g/g to over 500 mg/g, depending on the sample size and available 14-MeV neutron fluence rates.NOTE 1The range of analysis may be extended by using higher
4、 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 c
5、ontent. Several variants of thismethod have been described in the technical literature. A monograph is available which provides a comprehensive description ofthe principles of activation analysis using a neutron generator (1).21.3 The values stated in SI units are to be regarded as standard. No othe
6、r units of measurement are included in this standard.1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of
7、 regulatorylimitations prior to use. Specific precautions are given in Section 8.2. Referenced Documents2.1 ASTM Standards:3E170 Terminology Relating to Radiation Measurements and DosimetryE181 Test Methods for Detector Calibration and Analysis of RadionuclidesE496 Test Method for Measuring Neutron
8、Fluence andAverage Energy from 3H(d,n)4He Neutron Generators by RadioactivationTechniques2.2 U.S. Government Document:Code of Federal Regulations, Title 10, Part 2043. Terminology3.1 Definitions (see also Terminology E170):3.1.1 acceleratora machine that ionizes a gas and electrically accelerates th
9、e ions onto a target. The accelerator may be basedon the Cockroft-Walton, Van de Graaff, or other design types (1). Compact sealed-tube, mixed deuterium and tritium gas,Cockcroft-Walton neutron generators are most commonly used for 14-MeV neutron activation analysis. However, “pumped”drift-tube acce
10、lerators that use replaceable tritium-containing targets are also still in use. Reviews of operational characteristics,descriptions of accessory instrumentation, and applications of accelerators used as fast neutron generators for activation analysisare available (2,3).3.1.2 comparator standarda ref
11、erence standard of known oxygen content whose specific counting rate (counts min1 mg ofoxygen1) may be used to quantify the oxygen content of a sample irradiated and counted under the same conditions. Often, acomparator standard is selected to have a matrix composition, physical size, density and sh
12、ape very similar to the correspondingparameters of the sample to be analyzed.1 This test method is under the jurisdiction of ASTM Committee E10 on Nuclear Technology and Applications and is the direct responsibility of Subcommittee E10.05on Nuclear Radiation Metrology.Current edition approved Nov. 1
13、, 2011Jan. 1, 2016. Published November 2011February 2016. Originally approved in 1969. Last previous edition approved in 20072011as E385 07.E385 11. DOI: 10.1520/E0385-11.10.1520/E0385-16.2 The boldface numbers in parentheses refer to a list of references at the end of the text.3 For referencedASTM
14、standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.4 Available from the Superintendent of Documents, U.S. Government Printing Office,
15、 Washington, DC 20402.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that u
16、sers consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.3 14-MeV neutron fluen
17、ce ratethe areal density of neutrons passing through a sample, measured in terms of neutrons cm2s1, that is produced by the fusion reaction of deuterium and tritium ions accelerated to energies of typically 150 to 200 keV ina small accelerator. Fluence rate has been commonly referred to as “flux den
18、sity.” The total neutron fluence is the fluence rateintegrated over time.3.1.3.1 DiscussionThe 3H(d,n)4He reaction is used to produce approximately 14.7-MeV neutrons. This reaction has a Q-value of + 17.586 MeV.3.1.4 monitorany type of detector or comparison reference material that can be used to pr
19、oduce a response proportional tothe 14-MeV neutron fluence rate in the irradiation position, or to the radionuclide decay events recorded by the sample detector.A plastic pellet with a relatively high oxygen content is often used as a monitor reference in dual sample transfer systems. It isnever rem
20、oved from the system regardless of the characteristics of the sample to be analyzed. It is important to distinguish thatthe monitor, whether an independent detector or an activated reference material, is not a standard used to scale the oxygen contentof the samples to be measured, but rather is used
21、 to normalize the analysis system among successive analytial passes within theprocedure.3.1.5 multichannel pulse-height analyzeran instrument that receives, counts, separates, and stores, as a function of theirenergy, pulses from a scintillation or semi-conductor gamma-ray detector and amplifier. In
22、 the 14-MeV instrumental neutronactivation analysis (INAA) determination of oxygen, the multichannel analyzer may also be used to receive and record both theBF3 neutron detector monitor counts and the sample gamma-ray detector counts as a function of stepped time increments (4-6).In the latter case,
23、 operation of the analyzer in the multichannel scaler (MCS) mode, an electronic gating circuit is used to selectonly gamma rays within the energy range of interest.3.1.6 transfer systema system, normally pneumatic, used to transport the sample from an injection port (sometimes connectedto an automat
24、ic sample changer) to the irradiation station, and then to the counting station where the activity of the sample ismeasured. The system may include components to ensure uniform positioning of the sample at the irradiation and countingstations.4. Summary of Test Method4.1 The weighed sample to be ana
25、lyzed is placed in a container for automatic transfer from a sample-loading port to the 14-MeVneutron irradiation position of a particle accelerator. After irradiation for a pre-selected time, the sample is automatically returnedto the counting area. A gamma-ray detector measures the high-energy gam
26、ma radiation from the radioactive decay of the 16Nproduced by the (n,p) nuclear reaction on 16O. The number of counts in a pre-selected counting interval is recorded by a gatedscaler, or by a multichannel analyzer operating in either the pulse-height, or gated multiscaler modes. The number of events
27、recorded for samples and monitor reference standard are corrected for background and normalized to identical irradiation andcounting conditions. If the sample and a monitor reference sample are not irradiated simultaneously, the neutron dose receivedduring each irradiation must be recorded, typicall
28、y by use of a BF3 neutron proportional counter. The amount of total oxygen (allchemical forms) in the sample is proportional to the corrected and normalized sample count and is quantified by use of thecorrected and normalized specific activity of the comparator standard(s).4.1.1 16N decays with a ha
29、lf-life of 7.13 s by -emission (7), thus returning to 16O. Sixty From Ref (8), sixty seven percent ofthe decays are accompanied by 6.12863-MeV gamma rays, 4.9 % by 7.11515-MeV gamma rays, and 0.82 % by 2.7415-MeVgamma rays. Other lower intensity gamma rays are also observed. About 28 % of the beta t
30、ransitions are directly to the groundstate of 16O. (All gamma-ray energies and decay schemes are given in Ref (8).Useful elemental data including calculatedsensitivities and reaction cross-sections for (14-MeV INAA) are provided in Refs (3) and (9). (See also Test Methods E181.)5. Significance and U
31、se5.1 The conventional determination of oxygen content in liquid or solid samples is a relatively difficult chemical procedure. Itis slow and usually of limited sensitivity. The 14-MeV neutron activation and direct counting technique provides a rapid, highlysensitive, nondestructive procedure for ox
32、ygen determination in a wide range of matrices. This test method is independent of thechemical form of the oxygen.5.2 This test method can be used for quality and process control in the metals, coal, and petroleum industries, and for researchpurposes in a broad spectrum of applications.6. Interferen
33、ces6.1 Because of the high energy of the gamma rays emitted in the decay of 16N, there are very few elements that will produceinterfering radiations; nevertheless, caution should be exercised. 19F, for example, will undergo an (n,) reaction to produce 16N,the same indicator radionuclide produced fro
34、m oxygen. Because the cross section for the 19F(n,)16N reaction is approximatelyone-half that of the 16O(n,p)16N reaction, a correction must be made if fluorine is present in an amount comparable to the statisticaluncertainty in the oxygen determination. Another possible interfering reaction may ari
35、se from the presence of boron. 11B willE385 162undergo an (n,p ) reaction to produce 11Be. This isotope decays with a half-life of 13.81 s, and emits several high-energy gammarays with energies in the range of 4.67 to 7.98 MeV. In addition, there is Bremsstrahlung radiation produced by the high ener
36、gybeta particles emitted by 11Be. These radiations can interfere with the oxygen determination if the oxygen content does not exceed1 % of the boron present.6.2 Another possible elemental interference can arise from the presence of fissionable materials such as thorium, uranium, andplutonium. Many s
37、hort-lived fission products emit high-energy gamma rays capable of interfering with those from 16N.NOTE 2Argon produces an interferent, 40Cl, by the 40Ar(n,p)40Cl reaction.Therefore, argon should not be used for the inert atmosphere during samplepreparation for oxygen analysis. 40Cl (t12 = 1.35 m) h
38、as several high-energy gamma rays, including one at 5.8796 MeV with a yield of 4.1 %.6.3 An important aspect of this analysis that must be controlled is the geometry during both irradiation and counting. Theneutron source is usually a disk source. Hence, the fluence rate decreases as the inverse squ
39、are at points distant from the target,and less rapidly close to the target. Because of these fluence rate gradients, the irradiation geometry should be reproduced asaccurately as possible. Similarly, the positioning of the sample at the detector is critical and must be accurately reproducible. Forex
40、ample, if the sample is considered to be a point source located 6 mm from a cylindrical sodium iodide (NaI) detector, a 1-mmchange in position of the sample along the detector axis was found to result in a 3.5 to 5 % change in detector efficiency (10). Sinceefficiency is defined as the fraction of g
41、amma rays emitted from the source that interact with the detector, it is evident that a changein efficiency would result in an equal percentage change in measured activity and in apparent oxygen content. The sample andmonitor (if present) may be rotated during exposure or counting, or both, to ensur
42、e exposure and counting uniformity. See, forexample, Ref (11). For counting, dual detectors at 180 can be used as an alternative to rotation to minimize positioning errors atthe counting station.6.4 Since 16N emits high-energy gamma rays, determinations are less subject to effects of self-absorption
43、 than aredeterminations based on the use of indicator radionuclides emitting lower energy gamma rays. Corrections for gamma-rayattenuation during counting are usually negligible, except for large samples as may be needed in the highest sensitivitydeterminations.6.5 The oxygen content of the transfer
44、 container (“rabbit”) must be kept as low as possible to avoid a large “blank” correction.Suggested materials that combine light weight and low oxygen content are polypropylene and high-density polyethylene (moldedunder a nitrogen atmosphere), high purity Cu, and high-purity nickel.Asimple subtracti
45、on of the counts from the blank vial in theabsence of the sample is not adequate for oxygen determinations below 200 g/g, since large sample sizes may be required forthese high-sensitivity measurements and gamma-ray attenuation may be important when the sample is present (12). If the totaloxygen con
46、tent of the sample is as low as that of the container (typically about 0.5 mg of oxygen), the sample should be removedfrom the irradiation container prior to counting. Statistical errors increase rapidly as true sample activities decrease, while containercontamination activities remain constant. For
47、 certain shapable solids, it may be possible to use no container at all (13). This“containerless” approach provides optimum sensitivity for low-level determinations, but care must be taken to avoid contaminationof the transfer system.6.6 Although the discriminator is used to eliminate the signal ori
48、ginating from gamma rays of energy less than 4.5 MeV, it ispossible when analyzing certain materials that very high matrix activities can result in multiple gammas of lower energy beingsummed, thereby generating a signal in the energy window. This effect can be minimized by reducing the specific act
49、ivity of theinterfering radionuclide or by altering the counting geometry to reduce the solid angle. Since the decay of these “coincidence”events are subject to the half-life of the radionuclide from which they are emitted, it may be possible to differentiate the interferingsignal from oxygen counts by decay rate if using an MCS-based sytem (14).7. Apparatus7.1 14-MeV Neutron GeneratorTypically, this is a high-voltage sealed-tube machine to accelerate both deuterium and tritiumions onto a target to produce 14-MeV neutrons by the 3H(d,n)4He reaction. In the
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