Experimental Validation of Build-Up Factor Predictions of Numerical Simulation Codes

Transcription

1 Inernaional Symposium on Digial Indusrial Radiology and Compued Tomography - Tu.. Experimenal Validaion of Build-Up Facor Predicions of Numerical Simulaion Codes Andreas SCHUMM *, Chrisophe BENTO *, David ROUE **, Manuel TESSIER ** * EDF R&D, avenue du general de Gaulle, 94 Clamar ** Insiu de Soudure Indusrie, 90 rue des Vanesses, 9594 Roissy Absrac. The accepabiliy of resuls obained wih modeling codes relies on a horough code validaion. Validaing scaered radiaion calculaions in radiographic modeling is paricularly difficul, especially for high wall hicknesses. One means o validae resuls is a cross-code comparison wih oher modeling codes, and has been carried ou for he MERATO code hroughou wo inernaional benchmarks. We presen here an original experimenal approach o complemen he validaion : As opposed o he radiional experimenal seup, using a lead blocker o access he scaered radiaion, we propose a srong collimaor o separae direc and scaered radiaion, applied o wall hicknesses up o 50mm for a Co60 source. Inroducion Validaion of numerical simulaion codes is a key requiremen for heir indusrial applicaion, and mus define he domain of validiy of a code. In general, a validaion relies on hree ypes of argumens: A heoreical validaion, which compares he simulaion wih analyical resuls, obained mahemaically on canonical cases. This ype of validaion is evidenly limied o simple configuraions, and in radiography migh be applied on he hickness of he half-aenuaion layer for he primary radiaion of a monochromaic source. A cross-code comparison, ideally beween codes developed by differen eams wih differen echniques, and applied o an analyical benchmark problem (sressing a single phenomenon) or an inegral problem (simulaing he enire inspecion chain). Experimenal validaion, wih he obvious drawback ha measuremen errors limi he precision wih which he comparison can be carried ou. Definiion of he build-up facor Scaered radiaion is produced hroughou he propagaion of he primary radiaion hrough he inspecion par by elasic (Rayleigh) or inelasic (Compon) collisions of phoons wih maer. Upon such a collision, he phoon s direcion is modified, and in he case of inelasic collisions, i ransmis par of is energy o he ejeced elecron. Since he phoon s direcion afer a collision is he resul of a saisical process, he scaered radiaion generaes an addiional exposure on he film which reduces he conras. As he quaniy of scaered radiaion received on he film mus be appreciaed wih respec o he direc radiaion, he noion of a build-up facor is ofen used. The build-up facor is defined as +(scaered radiaion / direc radiaion). This definiion, which originally sems from he Licence: hp://creaivecommons.org/licenses/by-nd/3.0

2 field of radiaion shielding, allows o deermine he oal radiaion a a given poin from he direc radiaion (which can easily be calculaed) by muliplicaion wih he build-up facor. Validaion by Cross-code comparison A firs aemp o validae he code has been made in 008 and 009 in he frame of an inernaional benchmark, organized by BAM []. For his series of numerical experimens, he objecive was o simulae he profile of direc and scaered radiaion (easily accouned for individually by he paricipaing codes) on seel plaes of consan hickness in he range of 0 o 70mm, and up o 50mm for an exension of he iniial programme, for monochromaic radiaion in he energy range beween 00keV and MeV. The energies were chosen o bring o evidence differences inroduced by he underlying simplifying hypoheses, and are less likely o wipe ou differences by averaging over several energies, as would be he case for realisic specra. The obvious drawback of his approach, however, is he impossibiliy o compare he simulaions wih acual experimens, as monochromaic radiaion in his range is unavailable. Experimenal validaion of he build-up facor: lead blocker Obaining experimenal values for build-up facors is known o be difficul. In his book Indusrial Radiology [], he auhor R. Helmshaw esimaes ha is unlikely o obain a precision beer han ±0%. The darkening of a radiographic film is a measure of he oal radiaion inciden on he film, and hus accouns for scaered and direc radiaion simulaneously. The experimenal deerminaion of a build-up facor herefore requires he isolaion of eiher he direc or he scaered radiaion from he oal radiaion, in order o calculae he build-up. The radiional seup for build-up facor experimens consiss in placing a lead blocker on he par. The objecive here is o obain opical densiy measuremens on he par of he film behind he blocker, and a region of he film ouside he zone affeced by he blocker. The raio of boh measuremens hen allows o calculae he build-up facor, supposing ha he blocker effecively sopped all direc radiaion. This experimen was carried ou in our faciliies in 003 in a firs experimenal validaion campaign. The build-up is calculaed from wo measuremens Od oal and s s S D Toal S This experimen is problemaic for hree reasons: I is difficul o obain wihin one exposure usable opical densiy values for boh he scaered radiaion (underneah he blocker) and he oal radiaion. If we consider an opical densiy range of.5 o 4.5 usable, he experimen does no allow o obain buildup values beyond 3, and is herefore limied o small wall hicknesses. A work-around soluion would be o carry ou wo exposures wih differen exposure imes, while aiming o obain idenical or close opical densiy values in boh exposures. Supposing a linear film response, he build-up could hen be deermined from he exposure imes. While a sufficienly hick blocker effecively inhibis radiaion o pass, is presence

3 reduces he scaered radiaion normally observed wihou blocker, if is diameer is oo large. This effec can easily be demonsraed by simulaion (see figure ), and leads o an underesimaion of he build-up. Boh opical densiy values also conain he film fog, which mus be known and subraced o calculae he build-up correcly. If wo exposures are carried ou, i is imporan o mainain idenical developmen procedures o reproduce he fog. scaered (lef) and direc (cener) radiaion of he configuraion plae + lead blocker for a par of 70mm hickness and a 0mm diameer blocker Experimenal validaion of he build-up facor: srong collimaion An alernaive seup consiss in using a srong collimaion using drilled plaes, in order o isolae direc radiaion in he zone underneah he collimaion unnel. A second exposure is hen necessary o obain he oal radiaion. Like he previous configuraion, his seup is no wihou problems: Two exposures (wih and wihou collimaor) are required, wih differen exposure imes. For he build-up calculaion, he opical densiies obained mus be scaled by aking ino accoun exposure ime, supposing a linear film response. An addiional complexiy is caused by he varying source-film disance wih and wihou blocker, which mus also be aken ino accoun in he calculaions. Like in he previous seup, he separaion of direc and scaered radiaion is no perfec. A Mone Carlo simulaion indicaes a small conribuion of scaered radiaion in he zone of he film under he collimaion unnel. As an example, we obained a build-up of.8 (insead of ) for a 50mm hick seel par wih 50mm collimaion behind and 50mm before he par, for a unnel diameer of 3mm. The specrum indicaes ha he scaered radiaion is essenially of firs order, which means i was mos likely generaed wihin he collimaion unnel. As far as he fog is concerned, he same precauions as wih he blocker seup apply, o reproduce idenical and well known fog values. As saed, he exposure wihou collimaor is carried ou wih he film siuaed righ behind he par, which inroduces a modificaion of he source-film disance, which mus be accouned for in he exposure imes o obain usable opical densiy values in boh exposures. The collimaion lengh is in fac a compromise beween he isolaion capabiliy for direc radiaion, and he increase in exposure ime due o he increased source-film disance. If we suppose a linear relaion beween opical densiy and exposure ime, he calculaion of he build-up facor is as follows: 3

4 Wih : exposure ime wih collimaion : exposure ime wihou collimaion r : residue build-up wih collimaion (no exacly ) : build-up wihou collimaion I : radiaion inensiy (consan) c : a facor inegraing geomerical divergence and aenuaion (consan for our experimen) The firs exposure wih collimaion (index ) provides a measure of he direc radiaion. r wih fog I. c. The second exposure wihou collimaion accouns for he oal radiaion. I. c. We can hen calculae he exposure ime which would have obained he same opical densiy as in he experimen wihou collimaion Ic Wih Ic Ic r we obain r or expressed in erms of opical densiy values read on he film fog fog r This resul clearly underlines he imporance of a precise knowledge of he fog value. 4

5 Alernaive approach: Disan film An ineresing hough experimen for an alernaive experimenal seup would be o increase he par-film disance sufficienly o obain only direc radiaion on he film, due o he deviaion of he primary radiaion upon a scaering even. One migh hink of his seup as a naural collimaion, and he effec is indeed used in micro-focus omography, where he enlargemen facor obained by increasing he deecor disance also comes wih he addiional benefi of reducing scaered radiaion. However, for indusrial radiography, his seup is impracical: Increasing he source/film disance increases he opical divergence, which mus hen be compensaed for by increasing he exposure ime When he film is siuaed righ behind he par, i is much less subjec o ambien scaered radiaion, since i is sufficien o proec he back side of he film by a blocker. If he film is locaed furher away from he par, ambien radiaion can reach he film, and addiional proecive measures become necessary. The experimen mus be carried ou far from walls, ground or ceiling of he room, and he source mus be collimaed. A simulaion sudy o evaluae he feasibiliy showed ha a build-up of.8 can be obained for a disance of 50cm, and. for m. Compared o he value of.8 obained for he collimaion unnel, which appeared simpler o realize, we oped for he srong collimaion experimen. Experimenal seup To carry ou he acual experimen, he french Insiu de Soudure (insiue of welding) developed a mechanical device as shown in he picure. The collimaion unnel is formed by a number of drilled lead plaes which can be sacked o increase he unnel lengh. The unnel diameer was 0mm, which is a compromise beween separaion capabiliy and mechanical feasibiliy. The mechanical device allows o keep he plaes aligned. The experimenal program consised of a number of exposures wih collimaions of 0mm before he par and 0, 40 and 00mm behind, o sudy he effec of he collimaion lengh on he precision. As indicaed before, an increase of collimaion lengh requires an increase of exposure ime. 5,5 mm Plaque en fer Plaque en plomb (filraion anérieure) e e fe e Plaque en plomb (filraion posérieure) Source 00 mm 0 mm 50 mm 300 mm Figure : Experimenal seup 5

6 Resuls Knowing source-film disance (DSF), exposure imes and opical densiies, he radio of he build-up values of he experimens wih () and wihou (r) collimaion can be calculaed from he following equaion, where index corresponds o he non-collimaed experimen. r DSF DSF fog fog Using his raio allows o reason in erms of experimenal daa, knowing ha he calculaion of he build-up value requires he use of a simulaion resul, he heoreical build-up for he experimen wih collimaion r: exp r r héo exp Reference Ind (r/) exp r heo exp heo Error Bu (%) A 0,86,06, A 0,776,03, B 0,788,03,6 7, A 0,646,53, B 0,639,53, A 0,656,, A 0,765,5 3, A 0,66,74, A 0,559,8, A 0,707 3,34 4,7 7 4, A 0,48, 4,4 0 Comparison of experimenal and heoreical (simulaed) build-up values The able shows he following: For collimaion lengh behind wih 40mm, he build-up values are coheren beween experimens and simulaion for all hree plae hicknesses of 70, 00 and 50mm For a collimaion lengh of 00mm, an error of 4% is observed beween simulaion and experimen for he 70mm and 00mm plaes (due o exposure ime consrains, he 50mm plae could no be characerized a 00mm collimaion lengh) 6

7 Conclusions The correlaion of build-up values beween experimenal and simulaion resuls is globally saisfying, and indicaes ha he collimaion experimen is in principle feasible, alhough no wihou problems. The bes resuls are obained wih a 40mm collimaion unnel. On boh experimens wih 00mm collimaion lengh, differences of up o 4% have been observed. These differences migh be due o alignmen problems of he source and he sack of collimaion plaes. Anoher facor migh be he raher imporan modificaion of he exposure condiions beween boh experimens due o he increase of he source-film disance and he necessary exposure ime increase. References [] G.-R. Jaenisch, C. Bellon, A. Schumm, J. Tabary, and Ph. Duvauchelle, A proposed benchmark problem for scaer calculaions in radiographic modelling in Review of Progress in QNDE. 8. edied by D. O. Thompson and D. E. Chimeni. AIP Conference Proceedings vol American Insiue of Physics. Melville. NY (009). pp [] R. Halmshaw, «Indusrial Radiology», e ediion, Chapman & Hall, 995 7