1、Designation: C1726/C1726M 10 (Reapproved 2018)Standard Guide forUse of Modeling for Passive Gamma Measurements1This standard is issued under the fixed designation C1726/C1726M; the number immediately following the designation indicates theyear of original adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide addresses the use of models with passivegamma-ray measurement systems. Mathematical models basedon ph
3、ysical principles can be used to assist in calibration ofgamma-ray measurement systems and in analysis of measure-ment data. Some nondestructive assay (NDA) measurementprograms involve the assay of a wide variety of item geom-etries and matrix combinations for which the development ofphysical standa
4、rds are not practical. In these situations, mod-eling may provide a cost-effective means of meeting usersdata quality objectives.1.2 A scientific knowledge of radiation sources anddetectors, calibration procedures, geometry and error analysisis needed for users of this standard. This guide assumes t
5、hat theuser has, at a minimum, a basic understanding of theseprinciples and good NDA practices (see Guide C1592/C1592M), as defined for an NDAprofessional in Guide C1490.The user of this standard must have at least a basic understand-ing of the software used for modeling. Instructions or furthertrai
6、ning on the use of such software is beyond the scope of thisstandard.1.3 The focus of this guide is the use of response models forhigh-purity germanium (HPGe) detector systems for the pas-sive gamma-ray assay of items. Many of the models describedin this guide may also be applied to the use of detec
7、tors withdifferent resolutions, such as sodium iodide or lanthanumhalide. In such cases, an NDA professional should determinethe applicability of sections of this guide to the specificapplication.1.4 Techniques discussed in this guide are applicable tomodeling a variety of radioactive material inclu
8、ding contami-nated fields, walls, containers and process equipment.1.5 This guide does not purport to discuss modeling for“infinite plane” in situ measurements. This discussion is bestcovered in ANSI N42.28.1.6 This guide does not purport to address the physicalconcerns of how to make or set up equi
9、pment for in situmeasurements but only how to select the model for which thein situ measurement data is analyzed.1.7 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem
10、shall be used independently of the other. Combiningvalues from the two systems may result in non-conformancewith the standard.1.8 The values stated in inch-pound units are to be regardedas standard. The values given in parentheses are mathematicalconversions to SI units that are provided for informa
11、tion onlyand are not considered standard.1.9 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the Worl
12、d Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2C1490 Guide for the Selection, Training and Qualification ofNondestructive Assay (NDA) PersonnelC1592/C1592M Guide for Making Quality NondestructiveAssay Measurements (Withdrawn 2018)3C1673 Ter
13、minology of C26.10 Nondestructive Assay Meth-ods2.2 Other Standard:4ANSI N42.28 Performance Standard for the Calibration ofGermanium Detectors for In Situ Gamma-Ray Measure-ments3. Terminology3.1 See Terminology C1673.1This guide is under the jurisdiction of ASTM Committee C26 on Nuclear FuelCycle a
14、nd is the direct responsibility of Subcommittee C26.10 on Non DestructiveAssay.Current edition approved April 1, 2018. Published May 2018. Originallyapproved in 2010. Last previous edition approved in 2010 as C1726/C1726M 10.DOI: 10.1520/C1726_C1726M-10R18.2For referenced ASTM standards, visit the A
15、STM 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.3The last approved version of this historical standard is referenced onwww.astm.org.4Available from Amer
16、ican National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally rec
17、ognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.14. Summary of Guide4.1 Passive gamma-ray measurements are appl
18、ied in con-junction with modeling to nondestructively quantify radioac-tivity.4.1.1 Modeling may be used to (1) design and plan themeasurements, (2) establish instrument calibration, (3) inter-pret the data acquired, (4) quantify contributions to themeasurement uncertainty, (5) simulate spectra, and
19、 (6) evaluatethe effectiveness of shielding.4.1.2 Various models commonly use analytical, numericalintegration and radiation transport approaches. This guideprovides a brief review of several approaches to help the userselect a suitable method and apply that method appropriately.4.1.3 Modeling makes
20、 use of knowledge of the measure-ment configuration including the shape, dimensions and mate-rials of the detector, collimator, and measurement item content.4.1.4 The exact geometry may be approximated in themodel. The degree of approximation acceptable is assessed ona case by case basis.4.1.5 Proce
21、ss knowledge may be required to provide infor-mation about inner containers, intervening absorbers, matrixmaterials or which radionuclides are present.4.1.6 The models make use of basic physical interactioncoefficients. Libraries and data sets must be available.4.1.7 Models are typically used to: (1
22、) account for field ofview and geometry effects, (2) account for matrix attenuation,(3) account for container wall and other absorbers, (4) modeldetectors, (5) transfer calibrations from one configuration toanother, (6) bound the range of assay values due to variationsin modeling representation para
23、meters, (7) iteratively refineassessments and decision making based on comparisons withobservations.4.1.8 Scans may be performed using low-resolution, por-table gamma-ray detectors (for example, NaI) to identify thelocation of activity and assist with the modeling.4.1.9 Measurement uncertainties are
24、 estimated based onuncertainties of the assumptions of the model.5. Significance and Use5.1 The following methods assist in demonstrating regula-tory compliance in such areas as safeguards (Special NuclearMaterial), inventory control, criticality control, decontamina-tion and decommissioning, waste
25、disposal, holdup and ship-ping.5.2 This guide can apply to the assay of radionuclides incontainers, whose gamma-ray absorption properties can bemeasured or estimated, for which representative certifiedstandards are not available. It can be applied to in situmeasurements, measurement stations, or to
26、laboratory mea-surements.5.3 Some of the modeling techniques described in the guideare suitable for the measurement of fall-out or natural radio-activity homogenously distributed in soil.5.4 Source-based efficiency calibrations for laboratory ge-ometries may suffer from inaccuracies due to gamma ray
27、sbeing detected in true coincidence. Modeling can be anadvantage since it is unaffected by true coincidence summingeffects.6. Procedure6.1 Modeling may lead to a bias if any of the measurementparameters do not match the physical characteristics of theitem. Uncertainties in the item parameters of the
28、 following maylead to a bias:6.1.1 Matrix distribution is homogenous throughout thecontainer,6.1.2 Hidden containers,6.1.3 Matrix identification,6.1.4 Container fill heights,6.1.5 Mass attenuation coefficients,6.1.6 Matrix density,6.1.7 Detector parameters, and6.1.8 Physical distribution of radioact
29、ivity.6.2 If the quantity of nuclear material is “infinitely thick” tothe emitted gamma rays, measurement results will be biased.This hazard is common when measuring items containing largequantities of heavy elements (for example, thorium, uranium,or plutonium) or items with highly attenuating matri
30、ces.Alternate NDA assay methods are recommended if this condi-tion exists.6.3 Self attenuation, commonly present in lumps of actinidematerial, will bias results low unless lump corrections arecomputed.6.4 The Generalized Geometry Holdup Method must becalibrated with the collimator attached to the de
31、tector. If thedetector recess changes from the calibration position, theresults will be biased.6.5 Absorber foils that are used to reduce count rate must beincluded in the model.6.6 Attenuation corrections for very thick items may besomewhat compromised by coherent scattering, which may notbe accura
32、tely modeled by attenuation calculations.7. Method DescriptionsFive commonly used methods are described. These include:(1) Generalized Geometry Holdup, (2) Far-fieldApproximation, (3) Voxel Intrinsic Efficiency, (4) RadiationTransport Code, and (5) Hybrid Monte Carlo.7.1 Generalized Geometry HoldupT
33、he method representsitems as a point, line, or area (1).5Three method calibrationsare obtained from one set of calibration measurements. Pointsources of the same material as that to be measured are oftenused for the calibration. Measurements and calibrations aremade with a collimator attached. Addit
34、ional attenuation cor-rection factors are needed for a complete analysis. The detectorcalibrations remain the same for all measurements, but attenu-ation correction factors will vary with the specific measure-ment. Results are typically reported in units of mass.5The boldface numbers in parentheses
35、refer to a list of references at the end ofthis standard.C1726/C1726M 10 (2018)27.1.1 Advantages of this method are:7.1.1.1 The detector efficiency is easily determined; threedifferent types of geometry calibrations are performed concur-rently.7.1.1.2 Any cylindrical collimator could be used.7.1.1.3
36、 Typically, only point sources are used.7.1.1.4 Additional geometry corrections do not require useof half-life or gamma ray yields.7.1.2 Disadvantages of this method are:7.1.2.1 Some holdup items being measured may not havegeometries that simulate points, lines, or areas.6However, theerrors introduc
37、ed by these assumptions are often small com-pared to other errors.7.1.2.2 The model assumes uniform concentration and dis-tribution of radioactive material. The uncertainties due to theseassumptions can be mitigated by taking multiple overlappingmeasurements (subject to time constraints) and judicia
38、l mea-surement placement.7.1.2.3 The calibration applies only to the exact detector-collimator configuration used during the calibration.7.1.2.4 Special nuclear material licenses may be requiredfor the calibration sources.7.1.3 Typical applications include uranium and plutoniumholdup.7.1.4 Calibrati
39、onPoint sources, representative of thematerial, mo, being measured, are positioned in off-axis posi-tions and the peak count rate is determined at each location.The activity of each location can be used to represent theactivity/unit area of the area within the concentric ring, ai. SeeFig. 1. This in
40、formation is integrated to obtain calibrationconstants for point, line, and area configurations.7.2 Far-field ApproximationThis method is used for thecalculation of activity in well-defined geometries (2). Themethod assumes that the matrix attenuation correction for theitem being measured can be est
41、imated using a far-field matrixcorrection approximation. Additional correction factors areneeded for other types of attenuation and geometry. Templatesmay be prepared that match parameters of the items beingmeasured and the positioning of the detector during themeasurement. Geometry and attenuation
42、correction factors arecomputed from the information supplied by the templates. Thismodel can be used for many shapes. Usually measurements aremade with a collimator to provide detector shielding anddirectional response. The detector calibration remains the samefor all measurements, but attenuation a
43、nd geometry correctionfactors will vary with the specific measurement. Results arereported in activity, concentration, or mass units.7.2.1 Advantages of this method are:7.2.1.1 The detector efficiency is easily determined.7.2.1.2 The calibration can be applied to any gamma-emitting radionuclide with
44、in the energy range of the calibrationsource and the validity of the correction factors.7.2.1.3 Models can be constructed for cylinders, boxes,point sources, and disc geometries.7.2.1.4 Detector collimation is incorporated in the modeland does not affect the detector calibration.7.2.2 Disadvantages
45、of this method are:6In a gaseous diffusion plant there are many items that contain holdup andcannot be measured as points, lines or areas. Two examples are converters and pipesin pipe galleys. In order to have a large enough standoff for pipes to meet the criteriafor lines, several pipes in the gall
46、ey are usually within the field-of-view. Convertersare typically measured from outside cell housings, which places the detector severalfeet away. Because the converters have a large diameter (from 1.2 m to 2.7 m for thesizes that can be reliably measured by gamma), pulling back far enough to makethe
47、m line sources would place several converters into the field-of-view, and thenthey would not be long enough to meet the line source definition. In addition, theinternal structure of converters is too complex to model them as point, line, or area.FIG. 1 Detector Position for CalibrationC1726/C1726M 1
48、0 (2018)37.2.2.1 The model does not apply to the analysis of activityin a non-uniform condition (for example, activity in soil in anexponential distribution).7.2.2.2 The calibration does not apply to close-upgeometries, where the far-field approximation for matrix at-tenuation does not apply, or ver
49、y large items (for example,infinite planes).7.2.2.3 Correction factors assume incoming gamma rays areparallel to the detector axis and, therefore, have reducedaccuracy for the off-axis portion of activity.7.2.3 Typical applications include modeling of cylinders,boxes, points and discs with specific dimensions.7.2.4 CalibrationTypically, a radionuclide point source,with activity traceable to national standards, is positioned at afixed distance from the detector. This source needs to encom-pass the energy range of gamma-rays that may be used for theanalysis. Detector efficienc
