1、Designation: C1726/C1726M 10Standard 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 year of last revisio
2、n. 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 physical principles
3、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 standards are not practi
4、cal. In these situations, mod-eling may provide a cost-effective means of meeting usersdata quality objectives.1.2 A scientific knowledge of radiation sources and detec-tors, calibration procedures, geometry and error analysis isneeded for users of this standard. This guide assumes that theuser has,
5、 at a minimum, a basic understanding of theseprinciples and good NDA practices (see Guide C1592), asdefined for an NDA professional in Guide C1490. The user ofthis standard must have at least a basic understanding of thesoftware used for modeling. Instructions or further training onthe use of such s
6、oftware is beyond the scope of this standard.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 detectors withdifferent resol
7、utions, 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 including contami-nated field
8、s, 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 equipment for in situmeasure
9、ments 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 shall be used independen
10、tly 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 information onlyand are not con
11、sidered standard.2. Referenced Documents2.1 ASTM Standards:2C1490 Guide for the Selection, Training and Qualificationof Nondestructive Assay (NDA) PersonnelC1592 Guide for Nondestructive Assay MeasurementsC1673 Terminology of C26.10 NondestructiveAssay Meth-ods2.2 Other Standard:3ANSI N42.28 Perform
12、ance Standard for the Calibration ofGermanium Detectors for In Situ Gamma-Ray Measure-ments3. Terminology3.1 See Terminology C1673.4. Summary of Guide4.1 Passive gamma-ray measurements are applied in con-junction with modeling to nondestructively quantify radioac-tivity.4.1.1 Modeling may be used to
13、 (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 (6) evaluatethe effectiveness of shielding.4.1.2 Various models commonly use analytical, numericalintegrat
14、ion and radiation transport approaches. This guide1This practice is under the jurisdiction of ASTM Committee C26 on NuclearFuel Cycle and is the direct responsibility of Subcommittee C26.10 on NonDestructive Assay.Current edition approved March 1, 2010. Published April 2010.2For referenced ASTM stan
15、dards, 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.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor
16、, New York, NY 10036, http:/www.ansi.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.provides a brief review of several approaches to help the userselect a suitable method and apply that method appropriately.4.1.3 Modeling makes
17、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 Proces
18、s 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)
19、 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 param
20、eters, (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
21、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 d
22、isposal, 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 l
23、aboratory 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 rays
24、being 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
25、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 radioacti
26、vity.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 matric
27、es.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 det
28、ector. 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 accurat
29、ely modeled by attenuation calculations.7. Method DescriptionsFive commonly used methods are described. These include:(1) Generalized Geometry Holdup, (2) Far-field Approxima-tion, (3) Voxel Intrinsic Efficiency, (4) Radiation TransportCode, and (5) Hybrid Monte Carlo.7.1 Generalized Geometry Holdup
30、The method representsitems as a point, line, or area (1).4Three 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. Addi
31、tional 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.7.1.1 Advantages of this method are:
32、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 Typically, only point sources are used.4The boldface numbers in parentheses refer to a list of references at the en
33、d ofthis standard.C1726/C1726M 1027.1.1.4 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.5However, theerrors introduced by
34、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 judicial mea-
35、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 CalibrationPoin
36、t sources, representative of the ma-terial, mo, being measured, are positioned in off-axis positionsand the peak count rate is determined at each location. Theactivity of each location can be used to represent the activity/unit area of the area within the concentric ring, ai. See Fig. 1.This informa
37、tion is integrated to obtain calibration constantsfor 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 estimate
38、d 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 corre
39、ction 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 and ge
40、ometry 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 within th
41、e 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 of th
42、is method are:7.2.2.1 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-up geom-etries, where the far-field approximation for matrix attenuationdoes not apply, o
43、r very large items (for example, infiniteplanes).5In 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 li
44、nes, several pipes in the galley 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), pull
45、ing back far enough to makethem 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
46、 for CalibrationC1726/C1726M 1037.2.2.3 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
47、.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 efficiencies are then obtained as a func
48、tionof energy at the distance used for calibration. Typical calibra-tion distances range from 20 to 40 cm 7.9 to 15.7 in.Calibrations are performed with the source on the detector axisso that photons enter only the circular face of the detector.7.3 Voxel-Intrinsic EffciencyThe model (3) is typically
49、calibrated with a point source or sources as the far-fieldmethod, but the far-field algorithm for matrix attenuation is notused. Instead, the attenuation of each voxel is computed andthe overall activity is computed accordingly. The detector ischaracterized by using information for the detector dead layer,detector can thickness, crystal diameter, crystal length, and sidethickness. The intrinsic detector efficiency is computed by notonly measuring activity entering the top of the detector but alsothe side of the detector.7.3.1 Advantages of this method are:7.3.1.1 The dete
