1、AReference numberISO 11537:1998(E)INTERNATIONALSTANDARDISO11537First edition1998-07-15Non-destructive testing Thermal neutronradiographic testing General principlesand basic rulesEssais non destructifs Essai de neutronographie thermique Principesgnraux et rgles de baseCopyright International Organiz
2、ation for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO 11537:1998(E) ISO 1998All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by an
3、y means, electronicor mechanical, including photocopying and microfilm, without permission in writing from the publisher.International Organization for StandardizationCase postale 56 CH-1211 Genve 20 SwitzerlandInternet isoiso.chPrinted in SwitzerlandiiContents Page1 Scope . 12 Background material .
4、 13 Neutron radiography method . 14 Facilities . 35 Neutron sources . 36 Neutron collimators 47 Imaging methods and conversion screens 68 Film 79 Cassettes . 710 Applications for thermal neutron radiography 811 Improved contrast 912 Image quality indicators . 913 Neutron activation 9AnnexesA (inform
5、ative) Glossary of terms related to neutron radiography 10B (informative) Thermal neutron linear attenuation coefficientsusing average scattering and thermal absorption crosssections for the naturally occurring elements 12C (informative) Bibliography 15Copyright International Organization for Standa
6、rdization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO ISO 11537:1998(E)iiiForewordISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO memberbodies). The w
7、ork of preparing International Standards is normally carried out through ISO Technical Committees. Eachmember body interested in a subject for which a Technical Committee has been established has the right to be reprensentedon that committee. International organizations, governmental and non-governm
8、ental, in liaison with ISO, also take part in thework. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnicalstandardization.Draft International Standards adopted by the Technical Committees are circulated to the member bodies for voting.
9、Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.International Standard ISO 11537 was prepared by Technical Committee ISO/TC 135, Non-destructive testing,Subcommittee SC 5, Radiation methods.Annexes A, B and C of this International Stand
10、ard are for information only.Copyright International Organization for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO 11537:1998(E)ISOivIntroductionThis International Standard is intended to provide guidelin
11、es for the production of neutron radiographs that possessconsistent quality characteristics, and as an aid to the user in determining the suitability of thermal neutron radiographicinspection for a particular application.Recommended practices are stated without detailed discussion of the technical b
12、ackground for the preference.Copyright International Organization for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-INTERNATIONAL STANDARD ISO ISO 11537:1998(E)1Non-destructive testing Thermal neutron radiogra
13、phictesting General principles and basic rulesRadiation protection - Health warningExposure of any part of the human body to neutrons, X- or gamma rays can be injurious to health. It is thereforeessential that whenever neutron radiographic equipment or radioactive sources are used, adequate precauti
14、onsshould be taken to protect the radiographer and any other person in the vicinity.Limits for the safe levels of neutron, X- or gamma radiation as well as the recommended practice for radiationprotection are those valid in different countries. If there are no official regulations or recommendations
15、 in a country,the latest recommendations of the International Commission on Radiological Protection should be applied.1 ScopeThis International Standard specifies the basic practices and conditions that are to be observed for thermal neutronradiography of materials and components for flaw detection.
16、 It is concerned with techniques using photosensitive film as arecording medium. However, it recognizes that alternative methods of imaging may be used more widely in the future. Thescope includes neutron production and collimation methods, converter screen selection, radiographic film, neutronradio
17、graphic inspection techniques and the type of material to be inspected. This practice is generally applicable to specificmaterial combinations, processes and techniques.2 Background materialA glossary of terms relating to neutron radiography is presented in annex A. Attenuation of neutrons in matter
18、 is presented inannex B.3 Neutron radiography methodNeutron radiography and X-radiography share some similarities but produce different results when applied to the sameobject. Neutrons replace X-rays as the penetrating beam of radiation whose intensity is modulated by an object, resulting ina film i
19、mage of the features of the object. Since the absorption characteristics of materials for X-rays and neutrons are verydifferent, the two techniques generally tend to complement one another. Neutron and X-ray attenuation coefficients,presented in figure 1 as a function of atomic number, are a measure
20、 of this difference.Copyright International Organization for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO 11537:1998(E)ISO2Figure 1 A comparison of mass attenuation coefficients for thermal neutrons and X
21、raysCopyright International Organization for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO ISO 11537:1998(E)34 FacilitiesA neutron radiography facility typically includes a source of thermal neutrons, a n
22、eutron beam collimator, a conversionscreen, film and an exposure cassette. A schematic diagram of a representative neutron radiography facility is shown infigure 2.Key1 Shielding 7 Divergent neutron beam2 Neutron source 8 Object3 Moderator 9 Film4 Gamma filter 10 Emulsion5 Aperture - diameter (D) 11
23、 Conversion screen6 Collimator 12 Length (L)Figure 2 Typical neutron radiography facility with divergent collimator5 Neutron sourcesNeutron sources suitable for thermal neutron radiography can be classified into three general categories: radioactive isotopes; sealed tubes and accelerators of particl
24、es; and nuclear reactors.Each of these sources produces high-energy neutrons that require moderation (slowing down) to thermal energies. This canbe accomplished by surrounding the neutron source with beryllium, graphite, water, oil, plastic or some other moderatormaterial.Copyright International Org
25、anization for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO 11537:1998(E)ISO45.1 Isotopic sourcesIsotopic sources have the advantage of being small and portable but because of their relatively low neutron
26、yield require longexposure times to achieve a given radiographic quality. Many isotopic sources have been used for neutron radiography andthe most common of these are shown in table 1. Californium (252Cf) is one of the most popular isotopic sources used forthermal neutron radiography because of its
27、low neutron energy and small physical size which permit efficient moderation andhigh total neutron yield.Table 1 Radioactive sources for neutron radiographyRadiation source Reaction Half-life Remarks241Am-242 Cm-Be (, n) 163 days high neutron yield but short half-life241Am-Be (, n) 458 years gamma e
28、asily shielded, very long half-life210Po-Be (, n) 138 days low gamma background, short half-life124Sb-Be (g, n) 60 days high gamma background, short half-life, highneutron yield, easily moderated252Cf spontaneous fission 2,65 years small size, high neutron yield, long half-life, andeasy moderation o
29、f neutron energies make thisan attractive portable source5.2 Accelerator sourcesSealed tubes and low-voltage accelerators utilizing the 3H(d,n)4He reaction, high-energy X-ray machines utilizing the (x, n)reaction, and Van de Graaff accelerators and cyclotrons using charged-particle neutron reactions
30、 have been used asneutron sources for thermal neutron radiography. The targets of these accelerators are surrounded by materials that willmoderate the neutrons to thermal energies. The thermal neutron fluence rate of accelerator sources before collimation canbe as high as 109neutrons cm-2sec-1.5.3 N
31、uclear reactorsNuclear reactors are a preferred neutron source for thermal neutron radiography because of their high neutron yield. Thehigh neutron intensity makes it possible to provide a tightly collimated beam and high-resolution radiographs can beproduced with a relatively short exposure time. S
32、ome of the disadvantages of using nuclear reactors for neutron radiographyare the high cost of installation and operation, lack of portability, and vulnerability to strict and complex regulation.6 Neutron collimatorsNeutrons are emitted in all directions from a source and are further scattered rando
33、mly by the moderator. A means ofcollimating thermal neutrons into a beam is provided to produce a high quality neutron radiograph. A well collimated thermalneutron beam coupled with the ability to place the object being inspected close to the imaging system will provide the bestradiographic resoluti
34、on.6.1 General collimator design considerationsNeutron collimators utilize materials having a high cross section for the absorption of thermal neutrons such as boron orcadmium. These materials should be applied to maximize the number of neutrons reaching the imaging system directly formthe source an
35、d minimize the number of neutrons scattered back into the beam. Often, a combination of materials is used toachieve the best neutron collimation and to mask out unwanted secondary radiation from the beam. It is sometimesnecessary to use materials, such as lithium carbonate, that produce neutron capt
36、ure decay products that will not result infogging of the imaging film. Materials that have a high neutron scattering cross section, such as hydrogeneous materials, ormaterials that emit radiation during neutron capture that may fog the imaging film should not be used indiscriminately nearCopyright I
37、nternational Organization for Standardization Provided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO ISO 11537:1998(E)5the imaging system. Examples of this latter class of materials include indium, dysprosium and cadmium. Film fog
38、ging mayalso result from the 470 keV gamma ray produced by neutron capture in boron.The spatial resolution of a neutron radiography system is influenced by the length (L) of the collimator, the diameter, orlongest dimension of the inlet aperture (D) and aperture shape. The ratio of L /D is used gene
39、rally to describe the effectivecollimation of the system. For example, if the L /D ratio of a system is several hundred, one can expect the system to becapable of producing radiographs of higher resolution than a system having an L /D of 10. Although L /D is an importantmeasure of system capability,
40、 other factors, such as object size and scattering characteristics, obviously affect the ultimateradiographic quality that can be achieved. In addition to the L /D value, the L value itself is important. Because the L value isfinite, a separation, l, between the object and imaging device results in
41、an unsharpness in the image described bymagnification factor L /(L-l).6.2 Collimator typesAlthough there have been many different collimator designs that have been used experimentally, few of these have survivedfor use in commercial neutron radiograph facilities. The need to radiography relatively l
42、arge objects has led to thewidespread use of the divergent collimator design shown in figure 2. The divergent collimator consists of a tapered channelmade of neutron-absorbing material and originating at its small end (inlet aperture) at the source of highest neutron flux in themoderator.Another col
43、limator type worthy of note is the pinhole collimator shown in figure 3. High resolution neutron radiography can beproduced with systems utilizing pinhole collimators. The pinhole is fabricated from material such as cadmium, gadolinium orboron which have a very high attenuation for thermal neutrons.
44、Key1 Shielding 7 Object2 Neutron source 8 Film3 Moderator 9 Emulsion4 Gamma filter 10 Conversion screen5 Pinhole aperture diameter (D) 11 Length (L)6 Diverging neutron beamFigure 3 Typical neutron radiography facility with pinhole collimatorCopyright International Organization for Standardization Pr
45、ovided by IHS under license with ISONot for ResaleNo reproduction or networking permitted without license from IHS-,-,-ISO 11537:1998(E)ISO66.3 Beam filtersIt is often desirable to minimize the gamma radiation that contaminates the neutron beam. This gamma radiation comes fromthe neutron source and
46、can cause film fogging and reduced image contrast. Filters made of lead or bismuth may be installednear the collimator inlet to reduce this unwanted gamma radiation in the beam. When using a bismuth filter it is advisable toencase the filter in a sealed aluminium can to prevent the spread of alpha c
47、ontamination due to the 210Po producted by theneutron capture reaction in 209Bi.6.4 Neutron scatteringBack-scattered radiation from the walls or equipment can be reduced by masking the neutron beam to the smallest practicalexposure area and by the careful use of neutron absorber and gamma shielding
48、materials. Back-scattered radiation can bedetected by placing a marker made of neutron absorbing material such as gadolinium and a marker made of gammashielding material such as lead, on the back of the film cassette during neutron exposure. If back-scattered radiation is aproblem, one or both of th
49、e markers will appear on the film. If backscattering is present, one should minimize materials thatscatter or emit radiation in the exposure area (see 7.1). Gadolinium or some other suitable neutron absorber can be placedbehind the detector to effectively minimize the influence of back-scattered neutrons on the image.7 Imaging methods and conversion screensNeutrons,
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