1、Designation: E 385 07Standard Test Method forOxygen Content Using a 14-MeV Neutron Activation andDirect-Counting Technique1This standard is issued under the fixed designation E 385; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, t
2、he year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the measurement of oxygenconcentration in almost any matrix by using a 14-MeV neut
3、ronactivation and direct-counting technique. Essentially, the samesystem may be used to determine oxygen concentrationsranging from over 50 % to about 10 g/g, or less, depending onthe sample size and available 14-MeV neutron fluence rates.NOTE 1The range of analysis may be extended by using higherne
4、utron fluence rates, larger samples, and higher counting efficiencydetectors.1.2 This test method may be used on either solid or liquidsamples, provided that they can be made to conform in size,shape, and macroscopic density during irradiation and countingto a standard sample of known oxygen content
5、. Severalvariants of this method have been described in the technicalliterature. A monograph is available which provides a compre-hensive description of the principles of activation analysisusing a neutron generator (1).21.3 The values stated in either SI or inch-pound units are tobe regarded separa
6、tely as the standard. The values given inparentheses are for information only.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and deter
7、mine the applica-bility of regulatory limitations prior to use. Specific precau-tions are given in Section 8.2. Referenced Documents2.1 ASTM Standards:3E 170 Terminology Relating to Radiation Measurementsand DosimetryE 181 Test Methods for Detector Calibration and Analysisof RadionuclidesE 496 Test
8、Method for Measuring Neutron Fluence andAverage Energy from3H(d,n)4He Neutron Generators byRadioactivation Techniques2.2 U.S. Government Document:Code of Federal Regulations, Title 10, Part 2043. Terminology3.1 Definitions (see also Terminology E 170):3.1.1 acceleratora machine that ionizes a gas an
9、d electri-cally accelerates the ions onto a target. The accelerator may bebased on the Cockroft-Walton, Van de Graaff, or other designtypes (1). Compact sealed-tube, mixed deuterium and tritiumgas, Cockcroft-Walton neutron generators are most commonlyused for 14-MeV neutron activation analysis. Howe
10、ver,“pumped” drift-tube accelerators that use replaceable tritium-containing targets are also still in use. A review of operationalcharacteristics, descriptions of accessory instrumentation, andapplications of accelerators used as fast neutron generators isgiven in Ref (2).3.1.2 comparator standarda
11、 reference standard of knownoxygen content whose specific counting rate (counts min1mgof oxygen1) may be used to quantify the oxygen content of asample irradiated and counted under the same conditions.Often, a comparator standard is selected to have a matrixcomposition, physical size, density and sh
12、ape very similar tothe corresponding parameters of the sample to be analyzed.Comparative standards prepared in this way may be useddirectly as “monitors” (see 3.1.4) in order to avoid the need formonitor-sample calibration plots, in those cases where theusual monitor reference standard is physically
13、 or chemicallydissimilar to the samples to be analyzed.3.1.3 14-MeV neutron fluence ratethe areal density ofneutrons passing through a sample, measured in terms ofneutrons cm2s1, that is produced by the fusion reaction ofdeuterium and tritium ions accelerated to energies of typically150 to 200 keV i
14、n a small accelerator. Fluence rate is alsocommonly referred to as “flux density.” The total neutronfluence is the fluence rate integrated over time.1This test method is under the jurisdiction ofASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of Subcommittee
15、E10.05 on Nuclear Radiation Metrology.Current edition approved June 1, 2007. Published July 2007. Originally approvedin 1969. Last previous edition approved in 2002 as E 385 90 (2002).2The boldface numbers in parentheses refer to a list of references at the end ofthe text.3For referenced ASTM standa
16、rds, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from the Superintendent of Documents, U.S. Government PrintingOffice, Washing
17、ton, DC 20402.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.3.1 DiscussionThe3H(d,n)4He reaction is used to pro-duce approximately 14.7-MeV neutrons. This reaction has aQ-value of + 17.586 MeV.3.1.4 monitorany type of detector
18、or comparison refer-ence material that can be used to produce a response propor-tional to the 14-MeV neutron fluence rate in the irradiationposition, or to the radionuclide decay events recorded by thesample detector. A plastic pellet with a known oxygen contentis often used as a monitor reference s
19、tandard in dual sampletransfer systems. It is never removed from the system regard-less of the characteristics of the sample to be analyzed. In thiscase monitor-sample calibration plots are required.3.1.5 multichannel pulse-height analyzeran instrumentthat receives, counts, separates, and stores, as
20、 a function oftheir energy, pulses from a scintillation or semi-conductorgamma-ray detector and amplifier. In the 14-MeV INAAinstrumental neutron activation analysis (INAA) determinationof oxygen, the multichannel analyzer may also be used toreceive and record both the BF3neutron detector monitorcou
21、nts and the sample gamma-ray detector counts as a functionof stepped time increments (3 and 4). In the latter case,operation of the analyzer in the multichannel scaler (MCS)mode, an electronic gating circuit is used to select only gammarays within the energy range of interest.3.1.6 transfer systema
22、system, normally pneumatic, usedto transport the sample from an injection port (sometimesconnected to an automatic sample changer) to the irradiationstation, and then to the counting station where the activity ofthe sample is measured. The system may include componentsto ensure uniform positioning o
23、f the sample at the irradiationand counting stations.4. Summary of Test Method4.1 The weighed sample to be analyzed is placed in acontainer for automatic transfer from a sample-loading port tothe 14-MeV neutron irradiation position of a particle accelera-tor. After irradiation for a pre-selected tim
24、e, the sample isautomatically returned to the counting area. A gamma-raydetector measures the high-energy gamma radiation from theradioactive decay of the16N produced by the (n,p) nuclearreaction on16O. The number of counts in a pre-selectedcounting interval is recorded by a gated scaler, or by amul
25、tichannel analyzer operating in either the pulse-height, orgated multiscaler modes. The number of events recorded forsamples and monitor reference standard are corrected forbackground and normalized to identical irradiation and count-ing conditions. If the sample and monitor reference standardsample
26、 are not irradiated simultaneously, the neutron dosereceived during each irradiation must be recorded, typically byuse of a BF3neutron proportional counter. The amount of totaloxygen (all chemical forms) in the sample is proportional to thecorrected sample count and is quantified by use of the corre
27、ctedspecific activity of the monitor, or comparator standard(s).4.1.116N decays with a half-life of 7.13 s by b-emission,thus returning to16O. About 69 % of the decays are accompa-nied by 6.13-MeV gamma rays, 5 % by 7.12-MeV gammarays, and 1 % by 2.74-MeV gamma rays. Other lower intensitygamma rays
28、are also observed. About 26 % of the betatransitions are directly to the ground state of16O. (All half-livesand gamma-ray energies are taken from Ref (5) and decayschemes are given in Ref (6). A useful elemental data base andcalculated sensitivities for (14-MeV INAA) are provided inRef (7). (See als
29、o Test Methods E 181.)5. Significance and Use5.1 The conventional determination of oxygen content inliquid or solid samples is a relatively difficult chemicalprocedure. It is slow and usually of limited sensitivity. The14-MeV neutron activation and direct counting techniqueprovides a rapid, highly s
30、ensitive, nondestructive procedure foroxygen determination in a wide range of matrices. This testmethod is independent of the chemical form of the oxygen.5.2 This test method can be used for quality and processcontrol in the metals, coal, and petroleum industries, and forresearch purposes in a broad
31、 spectrum of applications.6. Interferences6.1 Because of the high energy of the gamma rays emittedin the decay of16N, there are very few elements that willproduce interfering radiations; nevertheless, caution should beexercised.19F, for example, will undergo an (n,a) reaction toproduce16N, the same
32、indicator radionuclide produced fromoxygen. Because the cross section for the19F(n,a)16N reactionis approximately one-half that of the16O(n,p)16N reaction, acorrection must be made if fluorine is present in an amountcomparable to the statistical uncertainty in the oxygen deter-mination. Another poss
33、ible interfering reaction may arise fromthe presence of boron.11B will undergo an (n,p) reaction toproduce11Be. This isotope decays with a half-life of 13.81 s,and emits several high-energy gamma rays with energies in therange of 4.67 to 7.98 MeV. In addition, there is Bremsstrahlungradiation produc
34、ed by the high energy beta particles emittedby11Be. These radiations can interfere with the oxygen deter-mination if the oxygen content does not exceed 1 % of theboron present.6.2 Another possible elemental interference can arise fromthe presence of fissionable materials such as thorium, uranium,and
35、 plutonium. Many short-lived fission products emit high-energy gamma rays capable of interfering with those from16N.NOTE 2Argon produces an interferent,40Cl, by the40Ar(n,p)40Cl re-action. Therefore, argon should not be used for the inert atmosphereduring sample preparation for oxygen analysis.40Cl
36、(t12 = 1.35 m) hasseveral high-energy gamma rays, including one at 5.88 MeV.6.3 An important aspect of this analysis that must becontrolled is the geometry during both irradiation and count-ing. The neutron source is usually a disk source. Hence, thefluence rate decreases as the inverse square at po
37、ints distantfrom the target, and less rapidly close to the target. Because ofthese fluence rate gradients, the irradiation geometry should bereproduced as accurately as possible. Similarly, the positioningof the sample at the detector is critical and must be accuratelyreproducible. For example, if t
38、he sample is considered to be apoint source located 6 mm from a cylindrical sodium iodide(NaI) detector, a 1-mm change in position of the sample alongthe detector axis will result in a 3.5 to 5 % change in detectorefficiency (8). Since efficiency is defined as the fraction ofgamma rays emitted from
39、the source that interact with theE385072detector, it is evident that a change in efficiency would result inan equal percentage change in measured activity and inapparent oxygen content. Positioning errors are normallyminimized by rotating the sample around a single axis, orbiaxially, during both irr
40、adiation and counting. Alternately,dual detectors at 180 can be used to minimize positioningerrors at the counting station.6.4 Since16N emits high-energy gamma rays, determina-tions are less subject to effects of self-absorption than aredeterminations based on the use of indicator radionuclidesemitt
41、ing lower energy gamma rays. Corrections for gamma-rayattenuation during counting are usually negligible, except inthe highest sensitivity determinations where sample sizes maybe large.6.5 The oxygen content of the transfer container (“rabbit”)must be kept as low as possible to avoid a large “blank”
42、correction. Suggested materials that combine light weight andlow oxygen content are polypropylene and high-density poly-ethylene (molded under a nitrogen atmosphere), high purityCu, and high-purity nickel. A simple subtraction of the countsfrom the blank vial in the absence of the sample is not adeq
43、uatefor oxygen determinations below 200 g/g, since large samplesizes may be required for these high-sensitivity measurementsand gamma-ray attenuation may be important when the sampleis present (9). If the total oxygen content of the sample is aslow as that of the container (typically about 0.5 mg of
44、 oxygen),the sample should be removed from the irradiation containerprior to counting. Statistical errors increase rapidly as truesample activities decrease, while container contamination ac-tivities remain constant. For certain shapable solids, it may bepossible to use no container at all. This “co
45、ntainerless”approach provides optimum sensitivity for low-level determi-nations, but care must be taken to avoid contamination of thetransfer system.7. Apparatus7.1 14-MeV Neutron GeneratorTypically, this is a high-voltage sealed-tube machine to accelerate both deuterium andtritium ions onto a targe
46、t to produce 14-MeV neutrons bythe3H(d,n)4He reaction. In the older “pumped” drift-tubeaccelerators, and also in some of the newer sealed-tube neutrongenerators, deuterium ions are accelerated into copper targetscontaining a deposit of titanium into which tritium is absorbed.Detailed descriptions of
47、 both sealed-tube and drift-tube ma-chines have been published (1, 2).7.1.1 Other nuclear reactions may be used, but the neutronenergy must exceed 10.22 MeV (10) for the16O(n,p)16Nreaction to take place. The 14-MeV neutron output of thegenerator should be 109to 1012neutrons s1, with a usablefluence
48、rate at the sample of 107to 109neutrons cm2s1. The14-MeV fluence rate may be measured as described in TestMethod E 496.7.1.2 The neutron output from targets in drift-tube machinesdecreases quite rapidly during use because of depletion of thetritium content of the target in the pumped system. Conse-q
49、uently, the target must be replaced frequently. The use of asealed-tube-type neutron generator obviates the need to handletritium targets and provides for longer stable operation.7.2 Sample Transfer SystemThe short half-life (7.13 s) ofthe16N requires that the sample be transferred rapidly betweenthe irradiation position and the counting station by a pneumaticsystem to minimize decay of the16N. If the oxygen content inthe sample is low, it is desirable to use dry nitrogen, rather thanair, in the pneumatic system to avoid an increase in radioac-tivity due to rec
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