1、Designation: E 803 91 (Reapproved 2002)Standard Test Method forDetermining the L/D Ratio of Neutron Radiography Beams1This standard is issued under the fixed designation E 803; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar 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 defines an empirical technique for themeasurement of the effective collimation ratio, L/D, of
3、neutronradiography 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 measuremen
4、ts,particularly for poorly defined apertures.2. Referenced Documents2.1 ASTM Standards:E 748 Practices for Thermal Neutron Radiography of Ma-terials2E 1316 Terminology for Nondestructive Examinations23. Summary of Test Method3.1 Determination of neutron beam L/D ratio using the NU(no umbra) techniqu
5、e3is accomplished 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 th
6、e NU technique is that L/D canbe determined accurately without the need for expensiveinstrumentation. Neutron radiography practices are discussedin Practices E 748 and the terms are defined in TerminologyE 1316.4. Significance and Use4.1 The quality of a neutron radiographic image is depen-dent upon
7、 many factors. 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 particularelemen
8、t of interest. Use of this test method allows theradiographer and the user to determine and periodically checkthe effective collimation ratio.5. Apparatus5.1 NU Device (see Figs. 2(a), 2(b), and 3) employs neutronabsorbing rods positioned at various distances from the imageplane. In practice this de
9、vice consists of cadmium and nylonrods located in V-grooves accurately machined in the surface ofan aluminum channel section set at a 45 614 angle to the sidesupport plate. Near the image plane end the V-grooves aremachined on 0.283-cm centers. After 21 V grooves, countingone on the end, the grooves
10、 are machined on 0.707-cm centersto the source end. The 0.64-mm diameter cadmium and nylonrods are laid into the V-grooves and secured with neutrontransparent adhesive tape. The aluminum channel is supportedby side plates to maintain the 45 614 angle relative to theimage plane. While cadmium rods wi
11、th diameters other than0.64 mm may be used, the exact rod diameter must be knownand the depth of the V grooves must be 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 single A unitused with appropriate spacers may be u
12、sed to 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
13、 will 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 Commi
14、ttee E07 on NondestructiveTesting and is the direct responsibility of Subcommittee E07.05 on Radiology(Neutron) Method.Current edition approved May 15, 1991. Published July 1991. Originallypublished as E 803 86. Last previous edition E 803 86.2Annual Book of ASTM Standards, Vol 03.03.3Newacheck, R.
15、L., and Underhill, P. E., “The NU Method for Determining L/DRatio Of Neutron Radiography Facilities,” Aerotest Operations, Inc., Report A.O.77-27, June 1977.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.2(a) Support Channel Subasse
16、mbly with Rod SpacingFIG. 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
17、from base line to reduce 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 laye
18、r of transparent tapeE 803 91 (2002)22(b) L/D Apparatus Assembly7. Data Analysis7.1 Visual AnalysisA visual determination of the L/D ratiocan be made directly from the neutron radiograph. Whenobserving the individual rod images, the umbral image can berecognized as the “white” line along the center
19、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 disappear. Beyond this point a less intensewhite line will appear and increase in width with increasing roddistance. Use ofa5to10-power magnifier will
20、aid indetermining the point at which the “white” line disappears andthen increases in width with a decreased intensity. Based on thevisual observation, determine the rod with zero umbral widthand then determine its distance (b) from the cassette. The L/Dratio is as follows:L/D 5b/rod diameter!7.2 Mi
21、crodensitometric AnalysisThe second data analysismethod is based on a microdensitometric scan across thecadmium rod images beginning with the “0” position rodnearest the film. A typical scan is shown in Fig. 4. Adensitometer aperture of 20 3 300 m and no horizontalexpansion is suggested for this met
22、hod. 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 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
23、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 AnalysisThis methodalso uses scanning microdensitometric traces for L/D ratiodeterminations and is applicable for both high and low L/Dratios. For t
24、his method the recommended microdensitometerE 803 91 (2002)3settings are: 20 3 300-m aperture and 503 (or more) chartrecording expansion. These settings will produce individualwave forms as shown in Fig. 5. At least two wave forms mustbe scanned, one near the film plane and one other near the pointw
25、here 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.For L/D ratios above 100, the “0” centimetre rod image shouldnot be used because the unsharpness due to the film/conversionscreen
26、combination overrides the unsharpness due to the L/Dratio. For the lower L/D ratios (under ;100), the simplifiedequation 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. This
27、dimension is measured along a horizontal line (parallel tobackground) through the average of the low-density 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-men
28、t may be centimetres or inches and need not be convertedto the unmagnified value.7.3.2 Using this dimension, determine the value of b asfollows (see Fig. 6):b 5 U1X1!/U12 U2! 1 X0where:U1= umbral width of a rod near the image plane,U2= umbral image width of a rod near the distance wherethe umbra dis
29、appears,X0= distance from the film to the rod chosen for U1, cm,andX1= distance between the two rods chosen for analysis,cm.Since L/D =brod diameter , it is possible to determine L/Ddirectly as follows:L/D 5FU1X1U12 U21 X0G/ rod diameterFor low L/D ratios (2 cm), and becausematerials with very high
30、neutron attenuation coefficients areavailable, a unique approach to L/D ratio determination ispossible. If an opaque rod with a diameter much smaller thanthe source diameter is placed near the image plane, an umbralshadow will be cast as shown in Fig. X1.1.X1.2 For a given source diameter (D) and a
31、given roddiameter (d), there will be a rod to image plane distance (b)where the width of the umbral shadow on the image plane willequal zero. For this particular distance a simple formula can bedeveloped to determine L/D ratio:DXYZ is similar to DSTZTherefore:L/D 5 b/dwhere:L = source to film distan
32、ce (Note X1.1),D = source size = XY in Fig. X1.1,b = object to film distance, andd = object size = ST in Fig. 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 determi
33、nedfrom a neutron radiograph of a system of rods.X2. 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) th
34、e variations of conversion screen 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 microdens
35、ito-meter scan of all rods.X2.2 Use 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 rodFIG. 6 Diagrammatic Math ModelFIG. X1.1 Zero Umbra GeometryE 803 91 (2002)6diameter is accurately k
36、nown. Any of the analysis techniquesutilizing microdensitometer scans should provide an accuracyof ;5 % for L/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 exam
37、ple, if the umbral image is observedat 4 cm but is not visible in the 4.5-cm rod image, one can onlysay that 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
38、 visual accuracy is limited to 16.7 % (between18.75 and 21.87 L/D).X2.4 The accuracies noted above have been experimentallyverified by analysis of neutron radiographic images producedwith facilities having well defined geometrical configurations.The most significant point to be considered in the use
39、 of theNU method for determining L/D ratios is that the image is atrue indicator. If the values of L/D determined 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 are
40、more accurate. One should proceed to analyze the sourceconfiguration with pinhole techniques to locate source 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 oro
41、val shaped aperture, the NU device will indicate the L/D rationormal to the rod direction. Two measurements are 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
42、of this standard are expressly advised that determination of the validity of any such patent rights, and the 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 fi
43、ve years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical
44、committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake 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).E 803 91 (2002)7
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