1、Designation: E803 91 (Reapproved 2013)Standard Test Method forDetermining the L/D Ratio of Neutron Radiography Beams1This standard is issued under the fixed designation E803; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、 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 defines an empirical technique for themeasurement of the effective collimation ratio, L/D, of neu
3、tronradiography beams. The technique is based upon analysis of aneutron radiographic image and is independent of measure-ments and calculations based on physical dimensions of thecollimator system. The values derived by this technique shouldbe more accurate than those based on physical measurements,
4、particularly for poorly defined apertures.2. Referenced Documents2.1 ASTM Standards:2E748 Practices for Thermal Neutron Radiography of Mate-rialsE1316 Terminology for Nondestructive Examinations3. Summary of Test Method3.1 Determination of neutron beam L/D ratio using the NU(no umbra) technique3is a
5、ccomplished by radiographing theNU device with the neutron beam to be measured andsubsequently analyzing the radiograph by one of three meth-ods. Each of the three methods is based upon the determinationof that point at which the umbral shadow width reaches zero.See Fig. 1. A key feature of the NU t
6、echnique is that L/D canbe determined accurately without the need for expensiveinstrumentation. Neutron radiography practices are discussedin Practices E748 and the terms are defined in TerminologyE1316.4. Significance and Use4.1 The quality of a neutron radiographic image is depen-dent upon many fa
7、ctors. The L/D ratio is one of those factorsand constitutes a numerical definition of the geometry of theneutron beam. The L/D ratio required for a specific neutronradiographic examination is dependent upon the thickness ofthe specimen and the physical characteristics of the particularelement of int
8、erest. Use of this test method allows theradiographer and the user to determine and periodically checkthe effective collimation ratio.5. Apparatus5.1 NU Device (see Fig. 2(a) and (b), and Fig. 3) employsneutron absorbing rods positioned at various distances from theimage plane. In practice this devi
9、ce consists of cadmium andnylon rods located in V-grooves accurately machined in thesurface of an aluminum channel section set at a 45 614 angleto the side support plate. Near the image plane end theV-grooves are machined on 0.283-cm centers. After 21 Vgrooves, counting one on the end, the grooves a
10、re machined on0.707-cm centers to the source end. The 0.64-mm diametercadmium and nylon rods are laid into the V-grooves andsecured with neutron transparent adhesive tape. The aluminumchannel is supported by side plates to maintain the 45 614 angle relative to the image plane. While cadmium rods wit
11、hdiameters other than 0.64 mm may be used, the exact roddiameter must be known and the depth of the V grooves mustbe adjusted accordingly.5.2 A single A unit as shown in Fig. 2(b) is used for L/Dvalues expected to be less than 150.Alternately, a singleAunitused with appropriate spacers may be used t
12、o accommodate awide range of L/D values.6. Procedure6.1 Place the NU device against the cassette with the finelyspaced rods nearest the cassette.6.2 Align the plane of the cassette perpendicular to the axisof the neutron beam.6.3 Expose the single-emulsion film and NU device for atime span that will
13、 produce a nominal background film densityof 2.5 6 0.4.6.4 Process the exposed film in accordance with the manu-facturers recommendations.6.5 Analyze the resultant image in accordance with one ormore of the three methods outlined in Section 7.1This test method is under the jurisdiction of Committee
14、E07 on NondestructiveTesting and is the direct responsibility of Subcommittee E07.05 on Radiology(Neutron) Method.Current edition approved June 1, 2013. Published June 2013. Originallyapproved in 1986. Last previous edition approved in 2008 as E803 91 (2008).DOI: 10.1520/E0803-91R13.2For referenced
15、ASTM standards, 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.3Newacheck, R. L., and Underhill, P. E., “The NU Method for Determining L/DRa
16、tio Of Neutron Radiography Facilities,” Aerotest Operations, Inc., Report A.O.77-27, June 1977.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States17. Data Analysis7.1 Visual AnalysisAvisual determination of the L/D ratiocan be made direct
17、ly from the neutron radiograph. Whenobserving the individual rod images, the umbral image can berecognized as the “white” line along the center of the rodimage. This “white” line will decrease in width for the rodslocated farther and farther from the film. At some point theumbral images will disappe
18、ar. Beyond this point a less intensewhite line will appear and increase in width with increasing roddistance. Use of a 5 to 10-power magnifier will aid indetermining the point at which the “white” line disappears andthen increases in width with a decreased intensity. Based on thevisual observation,
19、determine the rod with zero umbral widthand then determine its distance (b) from the cassette. The L/Dratio is as follows:L/D 5 b/rod diameter!7.2 Microdensitometric AnalysisThe second data analysismethod is based on a microdensitometric scan across thecadmium rod images beginning with the “0” posit
20、ion rodnearest the film. A typical scan is shown in Fig. 4.Adensitometer aperture of 20 300 m and no horizontalexpansion is suggested for this method. The value of b isobtained from the intersection of a straight line originatingfrom the tip (low film density) of the scan of the “0” rod anda curved
21、line through the tips of the remaining wave forms asshown in Fig. 4. This method gives the best results for L/Dratios up to a few hundred. Higher L/D ratios cannot bedetermined by this method due to the inability to obtain a stablewave form for large values of b.7.3 Alternative Microdensitometric An
22、alysisThis methodalso uses scanning microdensitometric traces for L/D ratiodeterminations and is applicable for both high and low L/Dratios. For this method the recommended microdensitometersettings are: 20 300-m aperture and 50 (or more) chartrecording expansion. These settings will produce individ
23、ualwave forms as shown in Fig. 5. At least two wave forms mustbe scanned, one near the film plane and one other near the pointwhere the umbra disappears. Care must be taken not to gobeyond the point where the umbral image disappears. Mi-crodensitometer settings must remain the same for all scans.FIG
24、. 1 Diagram of Zero Umbra Image ConfigurationNOTE 1Rods at “A” positions are 1 cm each side of center line (22 ea.)NOTE 2Rods at “B” positions are 2 cm each side of center line (9 ea.)NOTE 3Rods at “C” positions are 2.5 cm each side of center line (1 ea.)NOTE 4All dimensions from base line to reduce
25、 accumulative errorsNOTE 5Rod arrangement shown for single system device. For an add-on device, to form a double system, extend the 11 spaces for 7.78 cm to 19spaces for 13.43 cm and eliminate the close spacing (20 for 5.65 cm)NOTE 6Rods held tightly in position with one layer of transparent tapeFIG
26、. 2 (a) Support Channel Subassembly with Rod SpacingE803 91 (2013)2For L/D ratios above 100, the “0” centimetre rod image shouldnot be used because the unsharpness due to the film/conversionscreen combination overrides the unsharpness due to the L/Dratio. For the lower L/D ratios (under ;100), the s
27、implifiedequation using X2and U0for the “0” rod image may be usedwith good results.7.3.1 To determine the value of b it is necessary to measurethe umbral image width for the two rods selected. Thisdimension is measured along a horizontal line (parallel tobackground) through the average of the low-de
28、nsity scan of theindividual wave form. The desired dimension is the distancebetween the intersections of this horizontal line with linesdrawn through the two sides of the wave form. The measure-ment may be centimetres or inches and need not be convertedto the unmagnified value.7.3.2 Using this dimen
29、sion, determine the value of b asfollows (see Fig. 6):b 5 U1X1!/U12 U2!1X0where:U1= umbral width of a rod near the image plane,U2= umbral image width of a rod near the distance wherethe umbra disappears,X0= distance from the film to the rod chosen for U1, cm, andX1= distance between the two rods cho
30、sen for analysis, cm.Since L/D =brod diameter, it is possible to determine L/Ddirectly as follows:L/D 5FU1X1U12 U21X0G/rod diameterFor low L/D ratios (2 cm), and because materialswith very high neutron attenuation coefficients are available, aunique approach to L/D ratio determination is possible. I
31、f anopaque rod with a diameter much smaller than the sourcediameter is placed near the image plane, an umbral shadow willbe cast as shown in Fig. X1.1.X1.2 For a given source diameter (D) and a given roddiameter (d), there will be a rod to image plane distance (b)where the width of the umbral shadow
32、 on the image plane willequal zero. For this particular distance a simple formula can bedeveloped to determine L/D ratio:XYZ is similar to STZTherefore:L/D 5 b/dwhere:L = source to film distance (Note X1.1),D = source size = XY in Fig. X1.1,b = object to film distance, andd = object size = ST in Fig
33、. X1.1.NOTE X1.1When b LB. Therefore L may also beconsidered Source to Object Distance. Thus, if the rod diameter is known,the L/D ratio can be calculated because the value of b can be determinedfrom a neutron radiograph of a system of rods.FIG. 6 Diagrammatic Math ModelFIG. X1.1 Zero Umbra Geometry
34、E803 91 (2013)6X2. ACCURACYX2.1 The NU method for determining L/D ratios is particu-larly accurate in the normal range of L/D ratios used forneutron radiography, that is, 20 to 250. Major sources ofinaccuracy are (1) the variations in the cadmium rod diameter;(2) the variations of conversion screen
35、to centerline of first roddistances; (3) the inherent unsharpness of the film/conversionscreen system; and (4) the effect of conversion screen gammaon the film density of the cadmium rods. (4) applies primarilyto the method of L/D determination using a 1:1 microdensi-tometer scan of all rods.X2.2 Us
36、e of the linear regression analysis of individual rodumbral image measurements should provide accuracies of ;2to 3 % for L/D ratios up to 1000 assuming the cadmium roddiameter is accurately known. Any of the analysis techniquesutilizing microdensitometer scans should provide an accuracyof ;5 % for L
37、/D ratios up to 250 and the visual observation isequally accurate when interpreted by a trained film reader.X2.3 The visual determination has certain limitations fixedby rod spacing. For example, if the umbral image is observedat 4 cm but is not visible in the 4.5-cm rod image, one can onlysay that
38、the L/D lies between (4/0.064) and (4.5/0.064) or 62.5and 70.3. The accuracy is therefore limited to 12.5 %. Simi-larly at an L/D of 20, because the cadmium rods are spaced at0.2 cm, the best visual accuracy is limited to 16.7 % (between18.75 and 21.87 L/D).X2.4 The accuracies noted above have been
39、experimentallyverified by analysis of neutron radiographic images producedwith facilities having well defined geometrical configurations.The most significant point to be considered in the use of theNU method for determining L/D ratios is that the image is atrue indicator. If the values of L/D determ
40、ined by the NUmethod disagree with the values determined by geometricalcalculations based on alleged source size and source to filmdistances, it is most probable that the NU method values aremore accurate. One should proceed to analyze the sourceconfiguration with pinhole techniques to locate source
41、 leakageor other problems should the values differ widely. The tech-nique has been found to be equally accurate for circular orsquare aperture configurations. In the case of a rectangular oroval shaped aperture, the NU device will indicate the L/D rationormal to the rod direction. Two measurements a
42、re necessaryto characterize the source.ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the
43、riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of
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45、ke your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org). Permission rights to photocopy the standard may also be secured from the ASTM website (www.astm.org/COPYRIGHT/).E803 91 (2013)7
