The Study and Analysis of Neutron Activation with the Application of Gamma Ray Emission

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1 Cumhuryet Ünverstes Fen Fakültes Fen Blmler Dergs (CFD), Clt:36, No: 3 Özel Sayı (2015) ISSN: Cumhuryet Unversty Faculty of Scence Scence Journal (CSJ), Vol. 36, No: 3 Specal Issue (2015) ISSN: The Study and Analyss of Neutron Actvaton wth the Applcaton of Gamma Ray Emsson Hamed MORADİ 1,*, Tayeb NAMDARAN 2, Mohammad Rasool EYVAZİ 3, Iman POURDAD 3 1 Researcher and lecturer at Iran Techncal and Vocatonal Tranng Organzaton (TVTO) 2 Researcher and student at Educatonal Organzaton 3 Electroncs Student Receved: ; Accepted: Abstract. In the neutron actvaton method, gamma ray s used to trgger and actvate neutron. In ths method, sotropc neutron sources, ts generators, and thermal trggers are studed and analyzed. In ths paper, a number of computatonal and analytcal methods, ncludng Monte-Carlo smulaton method and Maxwell-Boltzmann method, have been used to enable us to perform large-sample prompt gamma neutron-actvaton analyss (LS-PGNAA) and to use ths large-sample n smulaton and laboratory samples. Keywords: Neutron, densty, LS-PGNAA, gamma, atomc mass 1. INTRODUCTION In ths paper, we have made an effort, wth the help of smulaton and computatonal methods, to produce a large emsson of Gamma ray n order to actvate and trgger neutron; therefore, to reach ths am, we have utlzed sotropc samples, generators sources, and a number of practcal laboratory and non-ntrusve methods to determne neutron densty dstrbuton. Ths would enable us to perform the actvaton practcally. 2. EXPLANATION When the nteracton between neutron and matter takes place, there s a possblty for dfferent sorts of scatterng, absorpton, and gap to occur. Photoelectrc effect s descrbed as the transmsson of whole gamma energy to an electron n the nner shell electrons. After the energy absorpton, the electron leaves the atom wth an energy equal to the ncdent gamma ray energy mnus the electron bndng energy. The gap n the nner shells s flled wth an electron from the hgher shells, and the X-ray and auger electrons are fnally radated, whch are due to low energy levels easly absorbed nto the matter. As result, they are often not able to escape the matter. *Correspondng author. Emal address: Hamedlee@yahoo.com Specal Issue: The Second Natonal Conference on Appled Research n Scence and Technology Faculty of Scence, Cumhuryet Unversty

2 MORADİ, NAMDARAN, EYVAZİ, POURDAD Compton scatterng s defned as the partal transmsson of gamma-ray energy to the electron at the tme of collson; when no energy s absorbed, only a part of the energy s transmtted to the detector. The part of the transmtted energy to the electron from the ncdent gamma ray falls n the contnuous Compton, that s approxmately n the area between zero and maxmum energy. When after the collson, the remander of the energy, e.g. Compton gamma, are fully absorbed n the detector matter, the Compton electron energy and the absorbed energy or the consecutve absorbed energes are recorded at the maxmum energy. The remanng part and the energy varable, whch are the result of the part of the Compton dstrbuton n a contnuous spectrum and the absorbed collecton of other levels of maxmum gamma n equal energy, do not provde us wth any useful nformaton n the analyss of neutron actvaton. The maxmum Sngle Escape (SE) and Double Escape (DE) Peak are also advantageous n the analyss of actvaton n comparson wth ther dscrete values of energy. In the neutron actvaton method based on the small-sample prompt gamma emsson, the self-sheldng of neutrons can be gnored. When the neutron beams are used wth the smallsamples n the neutron actvaton method from the prompt gamma, the approprate temperature s lower than the heat temperature. As a result, more effectve neutrons are selected. Snce the cross-secton s n reverse proporton wth the speed of capturng the neutrons, the colder neutrons n component analyss n a sample matter n a constant flux, result n a more effectve and detecton and hgher senstvty, and n areas dstant from neutron source, they result n colder neutrons and gamma wth lower energy n the sample area. In the nstrumental method of neutron actvaton, delayed gamma spectrum s recorded n a poston other than when a sample s under radaton. Two better postons, therefore, can be selected: one poston s when neutron flux s the hghest possble, and other poston s when background gamma s the lowest possble, and also when the doman of the detector s the largest, that s the measurement doman s not so small that t causes dstorton n the spectrum, but t s large enough for the measurement of the mnmum detecton over a specfc perod of tme. In the prompt gamma neutron actvaton method, to mprove the neutron flux, exposure and measurement postons are the same. The sample can placed close to the reactor core. But the background gamma s slghtly hgh there. Therefore, the sample s placed at the end of the neutron gude system, where the neutron flux s slghtly lower n comparson wth the locaton of the nstrumental analyss of neutron actvaton. The detector s placed n a way that the measurement doman s optmum for the half-lfe. Snce after each neutron absorpton one or more prompt gamma rays are often emtted, theoretcally a counter can measure almost all of the capturng reactons of the neutrons. However, ths s not the case n the nstrumental method of neutron actvaton, because n ths case, the deployed atoms whch are n the capturng perod are recorded. 166

3 The Study and Analyss of Neutron Actvaton wth the Applcaton of Gamma Ray Emsson Practcally, blue gamma rays energy s of the order of several mega ev. Most gammas are termnated not at the maxmum energy but n the Compton spectrum, and n the prompt gamma neutron actvaton method, the more energy the gamma ray has, the less chance of absorpton of all gamma energy by the detector matter. Therefore, the less energy the gammas have, there s a hgher probablty of detecton at maxmum energy of the nstrumental method of neutron actvaton. In fgure 1, the change curve of varous cases n the applcaton of neutron actvaton over tme are demonstrated. The advantage of usng neutron actvaton n large-sample s that ansotropc samples can be analyzed and there s no need for samplng. On the other hand, problems such as the self-sheldng of neutron and weakenng of gamma may occur. In the nstrumental method of neutron actvaton wth small samples, neutron self-sheldng and gamma weakenng are neglgble, but when larger samples are analyzed, both these factors should be consdered. In the neutron actvaton through prompt gamma emsson from large sample n practcal and standard status, we can use the Suk method, n whch the operaton s performed on a clay pot wth the dameter around 15 cm, wdth of 10 cm, and thckness of 5 cm. The dmensons of the neutron beam are 2 cm3,.e. smaller than the pot s thckness. Fgure 1. The graph demonstrates the energy for some cases of neutron actvaton. The ssue of neutron self-sheldng and gamma weakenng are solved through an nternal sngle-norm method. By ths method, the maxmum rato of lght levels for gamma detecton from sample component (x) and comparator component (y) can be descrbed as follows. To determne the absolute fracton of the components mass n Suk s clay devce, we assume that all the components are oxdzed and that the oxdes make up the 100% of the sample matter. If all the components can be determned the same as oxdes, the assumpton can be generally true n the clay devce. The nternal sngle-norm method cannot be used as a general method n neutron actvaton method wth prompt gamma from a large sample, snce t cannot be assumed 167

4 MORADİ, NAMDARAN, EYVAZİ, POURDAD that the components of the sample matter make up a 100%, because all components cannot be determned through the combnaton of the both methods of PGNAA and NAA, nor can all the components be oxdzed. The followng equaton shows the amount of neutron actvaton n ths method: Ax Nx.bx(Ey.x) φ(en,r)σa,x(en)ε(eγ,x,r)dendr = Ay Nx.bx(Ey.x) φ(en,r)σa,y(en)ε(eγ,y,r)dendr (1) In the frst-order approxmaton, the equaton s smplfed by defnng 1 σ a as the mcroscopc absorpton cross-secton, and t s expected from ths rato to be the same for any devce under radaton. The vector space of the result s w x (r), whch s equal to the normal densty dstrbuton. When the result of σφ s ntegrated ndvdually on energy spectrum, equaton 2 s yelded. The relatve maxmum doman of energy,.e. w x (r) (E γ,x,r)dr, should be calculated for each of the sample emsson stuaton accordng to the ncdent neutron beams. The ' ' σ ax, b x/σ ay, by rato should be determned for all the components. In ths case, the components relatve matter fracton (N x /N y ) can be calculated through equaton 2. Ax = Ay Nxσa, xbx(eγ, x) wx(r)ε(eγ, x, r)dr Nyσa, yby(eγ, y) wy(r)ε(eγ, y, r)dr (2) In gamma and neutron transport, we can beneft from Monte-Carlo calculatons and Maxwell-Boltzmann velocty method. Varous knds of thermal neutron beams are used n a research. When a beam s formed, we mght decde to defne the neutron beam cross-secton as the unform level surface, and the source as neutron emtters wth a thermal velocty dstrbuton n a specfc drecton. Ths approxmaton saves a lot of tme for a complete nuclear reactor modellng whch ncludes moderator and core geometry. In calculaton method of thermal neutron beam smulaton, we have used Maxwell-Boltzmann velocty dstrbuton functon. Neutron-capture rate densty n an atom s shown n formula 3: 1 R= 0 (r)φ(r, v)σa(v)dvdr V (3) In equaton 3, f the atom densty ϕ ( r) s constant and the rato of volume (V) to neutron flux can be consdered lnear, the small sample of neutron sheldng can be gnored, and we can then use ths equaton. Equaton 4 calculates the complete thermal flux, and equaton 5 can be used to calculate neutron absorpton densty: 168

5 The Study and Analyss of Neutron Actvaton wth the Applcaton of Gamma Ray Emsson 2KT < φ > th =< φ > MB º φ(v)dv = nvm(v)dv = 2n (4) 0 0 πm N 0 π T N R= <φ> 0 σ = 0 nv σa,0 (5) MB a,0 0 V 2 T V If we want to calculate the densty of the smulated samples wth each other, we can use Monte-Carlo equatons. Smulatons have been carred out wth the Monte-Carlo MCNP4b nstructons. Partcles wth the volume of 10*0 *10 mm 2 are used as the countng sdes for the nteractve neutrons (n.y) n volume elements. Ths nteracton 1/v s counted to determne the neutron densty dstrbuton and to compare ths wth the practcal neutron densty dstrbuton whch s determned by the actvty of the copper peces. Sdes dmensons are chosen to match the peces of copper wre. Energy dstrbuton used n smulaton s Maxwell- Boltzmann velocty dstrbuton n the heat room. These smulated neutrons approxmate the sample n a onedmensonal fashon, from a crcular desgn wth a dameter of 254 cm whch s used as the underlyng surface and s drected toward the sample perpendcularly. The neutron beam s smulated unformly n cross-secton. In Monte-Carlo smulaton, countng s done between a 0.25 cm transport along y axs from the poston where the copper wres are located. Ths means that countng s performed from 0.5 cm along y axs 250 cm on each sde of the copper wre, and the standard devaton n the mddle of the sample for the total of 10 sdes s equal to 0.3. Through the followng equaton, we can calculate the length of the wre for neutron densty: ns, ne, χ = F r D f 2 σs, 2 σ e, +( ) F 2 The ch-square s the reduced χ 2 r, the degrees of dfferentaton D f, 3 densty nm ( ), couplng coeffcent of equaton F, neutron densty standard devaton e ndex for the operator, s ndex for the smulator, and for the pure copper wre. ne, F = n s, σ (6) neutron 3 ( m ) (6-1), 2 2 (σe, ) (σs, ) σf = F ( ) + ( ) χ r ne, ns, (7) In equaton 7, σ F can be calculated. 169

6 MORADİ, NAMDARAN, EYVAZİ, POURDAD Fgure 2. Neutron Beam Schematc n Pure Copper Wre. In fgure 2, a schematc vew of the neutron beam n copper wre sample has been demonstrated. Rotaton around ts axes has also been llustrated. The am of ths expermental method s to compare the neutron densty dstrbuton yelded from experments ( ne ( r )) and the smulated amount ( ne ( r )). For the comparson, whch s made wth a measured pece of copper n neutron relatve densty experment, smulaton of the mean value ( ne ( r)) s used n the postons near a pece of copper. The reducton comparson n 2 2 ( χr ) λ facltates the reverse degree of the stable balance between the experment and the smulaton. Sometmes we are faced wth ths problem that n the smulaton no neutron s counted n one of the correspondng sdes wth the copper pece. In ths case, uncertanty n MCNP s equal to zero, whch occurs n less than % of the cases. To make sure that all these sdes have a correct uncertanty, uncertanty of these sdes s consdered equal to the uncertanty that s resulted when a neutron s counted nstead of zero. Snce ns, and n, e mght be devated due to the erroneous estmaton of the detector s mpact, F s used for the couplng of these dstrbutons. It s supposed that exposure and measurement are equal for all samples. In calculatng the amount of F n the sample contanng sand and the sample contanng graphte and lthum sulfate n smulaton, neutrons are counted n all drectons. Ths condton does not exst for the ar sample based on an amount of F wth a standard devaton larger than ( σ F ). Snce F should be equal for all the samples under the same exposure condtons for determnng the actvty, the amounts for samples contanng sand and samples contanng graphte and lthum sulfate should be added to the sample contanng ar. Table 1 ncludes the amounts couplng factor, transports, and rotatons n relaton wth the standard devaton and the error changes resultng from matchng. 170

7 The Study and Analyss of Neutron Actvaton wth the Applcaton of Gamma Ray Emsson Table 1. In terms of the standard devaton and the amount of rotaton and the error to the due For achevng better matchng between the results of the experments and the smulatons, t s necessary to calculate the error related to mprecson n determnng the poston of the PTFE preservatve n relaton to the neutron beams. The center of mass for each PTFE sheath n the experment n the most deal condton possble s placed n relaton to the center of the neutron beam, wth the copper wres perpendcular to the neutron beams drecton. Therefore, t s allowed for the manual postonng n whch the sample small devatons along y-axs ( Δ y ) and x-axs equvalent to rotatons around Y ( Φ ) and axsθ. By usng equaton 7, we can calculate neutron actvaton method around the axes. In the process of comparng, a mnmum amount 2 r x wth a network of varables s defned: for all the parameters θϕδ,, x, Δ y, F, mnmum amount 2 x r s defned by usng non-lnear least squares method. For the amount of F, n the comparng process wth the use of ar contanng sample, there s not much room for maneuver. In the next part, the dfference between ns( r), ne( r ) as a fracton of neutron densty dstrbuton n a poston of the sample wll be mentoned, where neutron beam enters n ( r )). ( n Ths shows a more tangble mpact than mprecson n smulaton accordng to the rate experment n e (r) especally farther from neutron beams where amount of n e (r) and n e (r) are nfntesmal. In both of the researches carred out n ths chapter between experments and smulatons, consderng the neutron densty dstrbuton n a large sample under radaton wth a thermal neutron beam wth a dameter of 2.54 cm, a good equvalence has been obtaned. Frstly, neutron densty dstrbuton wth %2 n r ) s n equlbrum for all samples. Secondly, e ( n F s n stable equlbrum for both of the determned amounts. If we want to acheve the average dstance between the atomc mass and d drecton, we have: 171

8 MORADİ, NAMDARAN, EYVAZİ, POURDAD dcm = nad na v (8) Δd = na d - dcm v na v (9) The amount of d center of mass (d cm ) s calculated n equaton 8, and the average dstance between neutron and center of mass n the drecton of (Δd)d s resulted from equaton 9. The amount of Δ r can be acheved by substtutng r and d. 1 f( s, t) 1-h( s, t )+ ( s) 1-h( s, t) P( s, t) = d + exp - ln - g( s, t) -1 1cm (10) In equaton 10, four functons (h.d.f.g) have been used. For correcton the unt problem, n the natural logarthm of -1, 1 cm s nserted. The d functon n the mnmum s s the horzontal asymptote whch shows the probablty of neutron absorpton from the ncdent beam or scattered neutron by the PTFE bottle. Ths probablty s larger than the probablty of neutron absorpton asymptote n copper slug places at 180 degree angle, because PTFE weakens the neutron beam. The f functon demonstrates the probablty of scattered neutron absorpton wth the sample matter when s s nfnte. All functon except d are dependent on the sample matter. Functons g and h are n lnear proporton wth the rato s / t. By changng the functons n equaton 10, t s observed that the frst part (d) demonstrates a weakenng n the shell. The second part shows the probablty of scattered neutron absorpton n the sample matter or the shell. When scatterng s nfnte, the thrd part defnes the neutron relatve densty curve, whch s measured outsde the sample, based on the scatterng n the sample matter or the shell. 3. CONCLUSION In the mentoned cases of laboratory research, neutron densty dstrbuton s performed n a large sample under radaton to a thermal neutron beam wth a defnte dameter accordng to a good procedure. The obtaned sample has a stable equlbrum. Ths sample s equal to the rato s / t and transport coeffcent, but the effectve atomc mass, except for the number of all absorbed neutrons n the sample matter and the center of mass for neutron densty dstrbuton, s practcally equal. If a change occurs n the dstance, t s dependent on the amount of atomc mass. When neutron densty dstrbuton n the smulated matter s performed wth absorpton 172

9 The Study and Analyss of Neutron Actvaton wth the Applcaton of Gamma Ray Emsson cross-secton and macroscopc scatterng, the resulted errors n the number of neutrons absorbed and the assumed poston of center of mass ( Δd, Δr ) are all dependent on the effectve atomc mass. In concluson, n the experment, no correcton should be appled to the effectve atomc mass n the sample matter n neutron actvaton method based on large sample emtted prompt gamma. The free gas method can be used from the methodology wth the help of M e equal to 1000 for the smulated sample matters. Also, n LS-PGNAA, no range s drectly calculated, expect for the rato Ω x /Ω ref for calculatng gamma weakenng factor and neutron selfsheldng. Therefore, no equvalence s necessary between the measurement range and the calculated effectve spatal angle. To analyze the components of the sample matter wth an effect lne dstrbuton of the heterogeneous component, only the methodology should be compatble. When the spectrum of the actvaton method based on neutron from prompt gamma from ths sample should be constant, the sample mght move lke a screw wth both rotaton and transport. The advantage of the rotaton method s that neutron densty dstrbuton n the sample matter s more homogenous than the tme when the sample does not rotate. In the exstng state n gamma actvaton method based on prompt gamma emsson n large sample, neutron densty dstrbuton s more homogenous when the sample matter s radated wth a synchronc exposure. REFERENCES [1] Alfass, Z.B., Chung, C.Prompt gama neutron actvaton analyss, CRC Press, Boca Raton, Florda, USA (1995) [2] Ln, X., henkelmann, R., Instrumental neutron actvaton analyss of large samles: A polt experment, J. Radoanal. Nucl. Chern. 251, (2002) [3] Overwater, R. M.W., Hoogenboom, J. E., Accountng for the thermal flux depresson n Volumnous sample for the nstrumental neutron actvaton analyss, Nucl. Sc. Eng. 117, (1994) [4] Oura, Y., Ebhara, M., Yoneda, S., Nakamura, N., Chemcal composton of the kobe Meteorte; Neutron-nduced prompt gamma ray analyss study, Geochem. J.36, (2002) [5] Bresmester, J., MCNP a general Monte Carlo N-partcal transport code, verson 4B Los Alamos natonal laboratory, Los Alamos, USA (1997) [6] Mackey, E. A., Personal Communcaton, (1998) [7] Lndstrom, R. M., Frestone, R. B., Pavott-Corcuera, R., Summary report of the thrd research.coordnaton meetng Development of a database for prompt gamma-ray neutron actvaton analyss., Internatonal nuclear data commttee, IAEA, Venna, Austra(2003) 173