SANS 11537-2004 Non-destructive testing - Thermal neutron radiographic testing - General principles and basic rules《无损检测 热中子射线照像检测 一般原则及基本规则》.pdf

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1、 Collection of SANS standards in electronic format (PDF) 1. Copyright This standard is available to staff members of companies that have subscribed to the complete collection of SANS standards in accordance with a formal copyright agreement. This document may reside on a CENTRAL FILE SERVER or INTRA

2、NET SYSTEM only. Unless specific permission has been granted, this document MAY NOT be sent or given to staff members from other companies or organizations. Doing so would constitute a VIOLATION of SABS copyright rules. 2. Indemnity The South African Bureau of Standards accepts no liability for any

3、damage whatsoever than may result from the use of this material or the information contain therein, irrespective of the cause and quantum thereof. ICS 19.100 ISBN 0-626-16183-5 SANS 11537:2004Edition 1ISO 11537:1998Edition 1 SOUTH AFRICAN NATIONAL STANDARD Non-destructive testing Thermal neutron rad

4、iographic testing General principles and basic rules This national standard is the identical implementation of ISO 11537:1998 and is adopted with the permission of the International Organization for Standardization. Published by Standards South Africa 1 dr lategan road groenkloof private bag x191 pr

5、etoria 0001 tel: 012 428 7911 fax: 012 344 1568 international code + 27 12 www.stansa.co.za Standards South Africa SANS 11537:2004 Edition 1 ISO 11537:1998 Edition 1 Table of changes Change No. Date Scope Abstract Specifies basic practices and conditions that are to be observed for thermal neutron r

6、adiography of materials and components for flaw detection. Keywords industrial neutron radiography, industrial radiography, non-destructive tests, procedures. National foreword This South African standard was approved by National Committee StanSA SC 5120.20A, Engineering materials Non-destructive te

7、sting, in accordance with procedures of Standards South Africa, in compliance with annex 3 of the WTO/TBT agreement. AReference numberISO 11537:1998(E)INTERNATIONALSTANDARDISO11537First edition1998-07-15Non-destructive testing Thermal neutronradiographic testing General principlesand basic rulesEssa

8、is non destructifs Essai de neutronographie thermique Principesgnraux et rgles de baseISO 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 any means, electronicor mechanical, including photocopying and

9、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 . 13 Neutron radiography method . 14 Facilities . 35 Neutron s

10、ources . 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 (informative) Glossary of terms related to neutron radiography 10B (

11、informative) Thermal neutron linear attenuation coefficientsusing average scattering and thermal absorption crosssections for the naturally occurring elements 12C (informative) Bibliography 15ISO ISO 11537:1998(E)iiiForewordISO (the International Organization for Standardization) is a worldwide fede

12、ration of national standards bodies (ISO memberbodies). The work 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 committ

13、ee. International organizations, governmental and non-governmental, 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 Technic

14、al Committees are circulated to the member bodies for voting.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, Rad

15、iation methods.Annexes A, B and C of this International Standard are for information only.ISO 11537:1998(E)ISOivIntroductionThis International Standard is intended to provide guidelines for the production of neutron radiographs that possessconsistent quality characteristics, and as an aid to the use

16、r in determining the suitability of thermal neutron radiographicinspection for a particular application.Recommended practices are stated without detailed discussion of the technical background for the preference.INTERNATIONAL STANDARD ISO ISO 11537:1998(E)1Non-destructive testing Thermal neutron rad

17、iographictesting 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 pre

18、cautionsshould 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 recommenda

19、tions 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 detec

20、tion. 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, neutron

21、radiographic 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 m

22、atter 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 f

23、ilm image 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 me

24、asure of this difference.ISO 11537:1998(E)ISO2Figure 1 A comparison of mass attenuation coefficients for thermal neutrons and X-raysISO ISO 11537:1998(E)34 FacilitiesA neutron radiography facility typically includes a source of thermal neutrons, a neutron beam collimator, a conversionscreen, film an

25、d 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 Conversion screen6 Collimator 12 Length (L)Figure

26、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 particles; and nuclear reactors.Each of these sources prod

27、uces 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.ISO 11537:1998(E)ISO45.1 Isotopic sourcesIsotopic sources have the advantage o

28、f being small and portable but because of their relatively low neutron 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 popul

29、ar isotopic sources used forthermal neutron radiography because of its 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

30、 high neutron yield but short half-life241Am-Be (, n) 458 years gamma easily 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 yea

31、rs small size, high neutron yield, long half-life, andeasy moderation of 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 Graa

32、ff accelerators and cyclotrons using charged-particle neutron reactions 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

33、sources before collimation canbe as high as 109neutrons cm-2sec-1.5.3 Nuclear 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-resolut

34、ion radiographs can beproduced with a relatively short exposure time. Some 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

35、emitted in all directions from a source and are further scattered randomly 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

36、 close to the imaging system will provide the bestradiographic resolution.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 nu

37、mber of neutrons reaching the imaging system directly formthe source and 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

38、 to use materials, such as lithium carbonate, that produce neutron capture 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 f

39、og the imaging film should not be used indiscriminately nearISO ISO 11537:1998(E)5the imaging system. Examples of this latter class of materials include indium, dysprosium and cadmium. Film fogging mayalso result from the 470 keV gamma ray produced by neutron capture in boron.The spatial resolution

40、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 generally to describe the effectivecollimation of the system. For example, if the L /D ratio of a system is sev

41、eral 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, other factors, such as object size and scattering characteristics, obviously affect the ultimateradiograph

42、ic 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 an unsharpness in the image described bymagnification factor L /(L-l).6.2 Collimator typesAlthough there ha

43、ve 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 large objects has led to thewidespread use of the divergent collimator design shown in figure 2. The diverge

44、nt 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 collimator type worthy of note is the pinhole collimator shown in figure 3. High resolution neutron radiograph

45、y 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.Key1 Shielding 7 Object2 Neutron source 8 Film3 Moderator 9 Emulsion4 Gamma filter 10 Conversion screen5 Pi

46、nhole aperture diameter (D) 11 Length (L)6 Diverging neutron beamFigure 3 Typical neutron radiography facility with pinhole collimatorISO 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 neut

47、ron source and 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 sp

48、read of alpha contamination 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 g

49、amma shielding 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 the 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 placedbehi