Chapter-1 Photon nteracton wth matter and producton of fluorescent -rays 1.1 Introducton The study of nteracton of gamma rays wth matter has attaned a sgnfcant mportance n the feld of scence and technology. Precse knowledge of the mechansm by whch radatons nteract wth matter s requred for understandng dffuson and penetraton of radatons n the medum. Durng the last few decades, an advancement of technology, gamma ray and -ray spectroscopc technques fnd enormous applcatons n varous dverse felds such as n medcne (Computerzed Tomography (CT) magng, for the treatment of cancer, sterlzng medcal equpments, dagnostc studes etc.), n ndustry (for pasteurzng certan foods and spces, measurng and controllng the flow of lquds, non-destructve testng to gauge the thckness of dfferent materals, detecton of structural defects and others heterogenetes n objects), n agrculture (to nvestgate the propertes of sol and rradaton of seeds, nondestructve nspecton of deformatons on the structure of a sol sample. etc.), n botechnology etc. Therefore, accurate epermental data of varous -ray or gamma ray related spectrometrc parameters such as nteracton cross-sectons, photon attenuaton coeffcents, -ray fluorescence cross-sectons, absorpton jump factor and jump rato etc. are needed. 15
1.2 Bref ntroducton to fundamental processes and parameters governng the nteracton of photons wth matter 1.2.1 Mechansm of photon nteracton Photons are classfed accordng to ther mode of orgn, not ther energy. Thus, gamma-rays are the electromagnetc radatons accompanyng nuclear transtons. Bremsstrahlung or contnuous -rays are the result of the acceleraton of free electrons or other charged partcles. Characterstcs rays are emtted n atomc transtons of bound electrons between the K,, M... shells n atoms. Annhlaton radaton s emtted when a postron and negatron combne. The quantum energy of any of these radatons can be epressed as E=hν, where ν s the frequency and h s Planck's constant. Interactons of these photons wth matter are thought to be ndependent of the mode of orgn of the photon and dependent only upon ts quantum energy. Unlke charged partcles, a well-collmated beam of γ rays shows a truly eponental absorpton n matter. Ths s because photons are absorbed or scattered n a sngle event. That s, those collmated photons, whch penetrate the absorber, have had no nteracton, whle the ones absorbed have been elmnated from the beam n a sngle event. Ths can easly be shown to lead to a truly eponental attenuaton. When the photon nteracts, t mght be absorbed and dsappear or t mght be scattered, changng ts drecton of travel, wth or wthout loss of energy. Varous possble processes by whch the electromagnetc feld of the gamma-rays nteract wth matter are descrbed as follow: (Evans, 1955) Knds of nteracton Effects of nteracton 1. Interacton wth atomc electrons (a) complete absorpton 2. Interacton wth nucleons (b) elastc scatterng (coherent) 3. Interacton wth electrc feld surroundng (c) nelastc scatterng 16
nucle or (ncoherent) electrons 4. Interacton wth the meson feld surroundng nucleons There are 12 ways of combnng columns 1 and 2; thus n theory there are 12 dfferent processes by whch γ rays can be absorbed or scattered. Many of these processes are qute nfrequent and some have not yet been observed. However, n the energy doman met most frequently n nuclear transtons, say, 0.01 to 5 MeV, the promnent modes of nteracton are the Photoelectrc absorpton, Compton scatterng and Par producton. Bref descrpton of these three processes along wth some other processes whch are of small nterest n ths energy regon s gven below: 1.2.1.1 Photoelectrc absorpton The process of photoelectrc absorpton (photoonzaton) s one of the prncple mode of nteracton of photons wth matter. In ths process, an electron s ejected from an atom as a result of the absorpton of photon. The energy of the ejected photon s equal to the bndng energy of the photon mnus bndng energy of the electron of the atom. The law of conservaton of momentum requred that n addton to the ncdent photon and ejected electron, a thrd party (resdual atom) must take part n the nteracton. Consequently the photoonzaton s prohbted from the free electron and s epected to ncrease wth the tghtness of the electron and bndng energy. Generally the effect vares wth atomc number Z as Z 4-5 and ts varaton wth energy E, changng from about E -7/2 at low energes (E<m o C 2 ) to E -1 at hgh energes (E>m o C 2 ), where m o C 2 s the rest mass energy of the electron. Thus photoonzaton domnates at low energy and for hgh Z elements. Snce ejected electron usually comes from tghtly bounded nner shell, therefore, the photon energy must be at least or equal to bndng energy of the electron. The ejected electrons, called photoelectrons, have some angular 17
dstrbuton and the angle of mamum shfts towards zero degree wth ncrease n photon energy. 1.2.1.2 Compton scatterng In Compton scatterng (Compton, 1923), the ncomng photon nteracts wth the atomc electron and degraded photon along wth the electron s emtted. The energy and momentum of the ncdent photon s conversed between the scattered photon and struck electron whch s assumed to be free and at rest. In practce, above assumptons smply lmts the theory to those cases for whch the bndng energy of the struck electron s small as compared to the energy of the ncdent photon. For an ncdent photon of energy h, scattered at an angle by a free and statonary electron whch recols at an angle, the conservaton of energy and momentum yelds: ' h h (1.1) h 1 1 cos 2 m C 0 ' Where h s the energy of the scattered photon and m s the rest mass energy of C 2 0 the electron. The probablty of Compton scatterng per atom ( ) of the absorber vares lnearly wth atomc number Z and nversely wth energy E as: Z * E 1 (1.2) 1.2.1.3 Par producton Above the ncdent photon energes of 1.02 MeV, a thrd type of nteracton becomes ncreasngly mportant. In ths nteracton, known as par producton, the photon s completely absorbed and n ts place appears a postron-electron par whose 18
total energy s just equal to hυ, whch s gven as; h ν = (T e- + m o c 2 ) + (T e+ + m o c 2 ) (1.3) where T e- and T e+ are the knetc energy of the electron and postron, respectvely, and m o c 2 = 0.51 MeV s the electronc rest mass energy. The process occurs only n the feld of charged partcles, manly n the nuclear feld but also to some degree n the feld of an electron. The presence of ths partcle s necessary for momentum conservaton. The probablty of par producton κ per atom ncreases wth ncreasng photon energy and t usually ncreases more sgnfcantly wth atomc number appromately as: 2 Z * ln E (1.4) 1.2.1.4 Raylegh scatterng It s the case of elastc scatterng of gamma-ray from bound electrons. In ths case the electrons do not receve suffcent energy to eject themselves from the atom.e. bound electrons revert to ther ntal state after scatterng. For large hυ and small Z, Raylegh scatterng s neglgble n comparson wth Compton scatterng. 1.2.1.5 Thomson scatterng by the nucleus Ths process ncludes coherent scatterng of gamma rays by (a) free electrons and (b) nucleus as a whole (nuclear Thomson scatterng). 1.2.1.6 Delbruck scatterng Delbruck scatterng, or elastc "nuclear potental scatterng", s due to vrtual electron par formaton n the coulomb feld of the nucleus. It s also called elastc nuclear potental scatterng. The effect, f present, s etremely small and does not show 19
up clearly n eperments desgned to detect t. 1.2.1.7 Nuclear resonance scatterng Ths type of scatterng nvolves the ectaton of a nuclear level by an ncdent photon, wth subsequent re-emsson of the ectaton energy. 1.2.1.8 Photodsntegraton of nucle Photodsntegraton, or the "nuclear photo effect," s energetcally possble whenever the photon energy eceeds the separaton energy of a neutron or proton. Ecept for Be 9 (γ, n) and H 2 (γ, n), these effects are generally confned to the hghenergy regon above about 8 MeV. Even when photodsntegraton s energetcally allowed, the cross sectons are neglgble compared wth those of the Compton Effect and of absorpton by nuclear par producton. 1.2.1.9 Meson producton Mesons are produced f the γ rays energy s above 150 MeV. But cross-secton s very small (10-3 barn/atom) 1.2.2 Attenuaton of gamma ray photons As the radaton nteracts wth matter, ts ntensty wll decrease. It s mportant to know, how radaton ntensty decreases as t passes through a substance. The degree of attenuaton s dependent on the absorber materal and the energy of the radaton. For all the absorbng materals, the attenuaton of gamma radatons s eponental n character. Two mportant physcal spectroscopc parameters used for measurng the 20
etent of attenuaton of gamma ray as t passes through a gven absorber are lnear attenuaton coeffcent and mass attenuaton coeffcent. 1.2.2.1 near attenuaton coeffcent The probablty of a photon traversng a gven amount of absorber wthout any knd of nteracton s just the product of the probabltes of survval for each partcular type of nteracton. The probablty of traversng a thckness of absorber wthout a Compton collson s just e, where s the total lnear attenuaton coeffcent for the Compton process. Smlarly, the probablty of no Photoelectrc nteracton s e, where s the total lnear attenuaton coeffcent for the Photoelectrc process and of no par-producton collson s e, where s the total lnear attenuaton coeffcent for the par-producton process. Thus a collmated gamma-ray beam of ntal ntensty I o after traversng a thckness of absorber wll have a resdual ntensty I of unaffected prmary photons equal to I I o ( e e e ) I I I ( ) o e I (1.5) o e where the quantty ( ) s the total lnear attenuaton coeffcent. Ths attenuaton coeffcent s a measure of the number of prmary photons whch have nteractons. It s to be dstngushed sharply from the absorpton coeffcent, whch s always a smaller quantty, and whch measures the energy absorbed by the medum (Evans, 1955). 21
1.2.2.2 Mass attenuaton coeffcent The mass attenuaton coeffcent, m (cm 2 /g) of the materal can be calculated wth a smple relaton: m (1.6) where s the densty of the materal n g/cm 3. Epresson (1.5) can be epressed as: I m I (1.7) oe where the product of and s called mass thckness, defned as the mass per unt area. The lnear attenuaton coeffcent of the materal depends upon the energy of the ncdent photons and nature of the materal. Snce the attenuaton produced by a medum depends upon the dstrbuton of atoms present n that medum, also depends upon the densty of the medum. The mass attenuaton coeffcent s of more fundamental mportance than the lnear attenuaton coeffcent because m s ndependent of the densty and physcal state of the absorber as the densty has been factored out. It s convenent to measure the thckness of the absorber n g/cm 2, whle dealng wth mass attenuaton coeffcent. The advantage n usng unts of grams per centmeter square to measure absorber thcknesses s that equal amounts of varous absorbers measured n these unts gve roughly the attenuaton. The mass attenuaton coeffcent of a compound or a homogeneous mture can be obtaned form the weghted sum of the coeffcents for the elements usng the smple addtve rule as: 22
w ( ) (1.8) m where ( / ) s the mass attenuaton coeffcent for the th element and w s ts weght fracton. For a chemcal compound wth chemcal Formula ( A B C... 3 D ), the weght fracton for the th element s gven by: 1 2 n w n 1 A A (1.9) where A s the atomc weght of the th element. 1.2.3 Bref dscusson of epermental technques used for measurng lnear attenuaton coeffcent of rregular shaped samples 1.2.3.1 Two meda and Smplfed Two meda method Gamma ray transmsson geometry has been consdered to be the most accurate epermental technque for measurng lnear/mass attenuaton coeffcents of elements, chemcal compounds and composte materals. Varous workers have measured -ray and gamma ray attenuaton coeffcents for several elements, composte materals such as glasses, bologcal compounds, buldng materals and solutons etc usng ths geometry. Detal lterature survey of the epermental work done on the measurement of attenuaton coeffcent has been gven n Chapter-2. However, the measurement of attenuaton coeffcent by standard gamma ray transmsson technque depends manly on two factors; thckness of sample under nvestgaton and the sample must be of regular shape. Therefore for odd shaped samples of unknown thckness such as (such as rock fragments or constructon 23
materals) ths method fals to delvers accurate results. To overcome ths problem Slva and Appolon (2000) proposed a new method named Two meda method for measurng lnear attenuaton coeffcent of such rregular shaped sample. Ths method s based upon the applcaton of standard ambert-beer law for obtanng lnear/mass attenuaton coeffcent of odd shaped sample usng transmsson geometry. In ths method thckness of sample under study s not requred. Slva and Appolon (2000) n ther concluson also suggested that f two meda are the same, the method does not work at all. Thus, larger the dfference between attenuaton coeffcent values of two meda used; greater would be the accuracy of method. Ths condton could be best met f ar s chosen as one medum because attenuaton of ar s usually assumed as zero whle performng eperment wth standard gamma ray transmsson geometry (Elas, 2003). Accordng to another suggeston proposed by Slva and Appolon (2000), the medum under consderaton should be very homogenous. Ths condton could be best met f meda under consderaton would be n lqud form, snce lqud medum s n general more homogenous than medum n powder form. By ncorporatng these suggestons, Elas (2003) proposed modfcatons n two meda method used for measurng lnear attenuaton coeffcent of odd shaped samples by choosng ar as one of the medum. The modfed Two meda method s called Smplfed Two meda method. He theoretcally demonstrates that ths choce smplfes the equaton used, as well as the laboratory work. At the same tme, t also allows a greater number of repettons as well as ntroduces larger dfference n the values of attenuaton coeffcent of the par of meda used. Detal theoretcal formulaton of Smplfed Two meda method s gven Chapter-5. In present study Smplfed Two meda method has been used for measurng 24
lnear attenuaton coeffcent of rregular shaped Fa-G (flyash-lme-gypsum) samples. 1.3 Fluorescent -rays -ray fluorescence s the process n whch vacances are created n the target atom by photon bombardment. Characterstcs -rays emtted on decay of such vacances are known as fluorescent -rays. Varous processes nvolved n decay of nner shell vacances by electron emsson have acqured specfc name. The term Auger effect s used to descrbe those transtons n whch the decay of a vacancy n an atomc shell leads to two vacances n one or two dfferent prncpal shells. In Coster-Krong transtons (Coster and Krong, 1935) one of the two vacances produced n the non-radatve decay s n a dfferent subshell of the same prncpal shell that contaned the ntal vacancy. In addton, there ests the possblty n partcular cases that an ntal vacancy can lead to two vacances n the subshells of the same shell. Such transtons are called super Coster-Krong transtons. The onzaton of K//M shell followed by fllng of the K//M vacancy leads to producton of K//M seres of -ray. The strongest lnes n the gven seres s called lne and the weaker lnes are called, and and so on, although the relatve ntenstes of these lnes bear a lttle resemblance to the sequence of labelng. The normal transtons, also known are dagram lnes, are defned by smple atomc selecton rules,.e n 1 l 1 j 1or 0 Where n, l and j are the changes n the prncpal quantum number, the orbtal quantum number and total angular momentum of the electron undergong transton for 25
de-ectaton of state. The transtons nvolved n some typcal K and -ray seres has been shown n fgure 1.1 and 1.2 respectvely. 1.3.1 Fundamental processes and parameters governng the producton of fluorescent -rays 1.3.1.1 -ray producton cross sectons The fluorescent -ray producton cross-secton (Krause et al.,1978) j for an -ray j s the product of partal or subshell photo onzaton cross-secton P, fractonal radatve decay rates F j, fluorescence yeld : F (1.10) j P j Where refers to the subshell photoonzed, j to the fnal state of an -ray lne. Alternatvely, j could be consdered the desgnaton of characterstc -ray, for eample, K, etc. The partal fractonal emsson rate, F j, s gven by radatve rate, j, for the -ray relatve to the total radatve rate, R, for a vacancy n the th subshell: F j j (1.11) R Now, the cross sectons for the emsson of -rays under K α and K β peaks can be defned usng epresson by takng the fractonal ntensty of these -rays nto account as: K P F (1.12) K K K K P F (1.13) K K K In the shells havng more than one subshell (.e., M...) the effect of vacancy shftng due to Coster-Krong transtons s taken nto account, whle defnng the total or partal 26
-ray emsson cross sectons. The -ray emsson spectrum s more complcated than K -ray spectrum snce t contans the contrbuton from all the three subshells. Snce all the -ray emsson lnes are not resolved because of the lmted resoluton of the avalable spectrometers. The -ray emsson spectra of ntermedate Z elements taken wth the currently avalable PIPS S () detectors show dstnct peaks denoted by l, α, β and γ. Each peak covers a group of lnes of the -ray seres whch have very close energes and thus cannot be resolved due to lmted resoluton of the detector. The partal -ray emsson cross-sectons correspondng for,, and γ peak s gven as: [ F (1.14) 1 ( f12 f23 f13) 2 f23 3] 3 3 [ ( f f f ) f F (1.15) 1 12 23 13 2 23 3] 3 3 f 2 F 23 1 1 ] F 3 1 3 [ f 3 1 12 ] F 2 2 2 [ ( f 1 12 f 23 f 13 ) (1.16) 1 1F 1 ( 1 f12 2) 2F2 (1.17) Where σ 1, σ 2 and σ 3 and ω 1, ω 2 and ω 3 are I, II and III subshell photoonzaton crosssectons and subshell fluorescence yelds respectvely. F s the fracton of ntensty of 3 -rays orgnatng from III transtons whch contrbute to the peak of -ray spectrum. All other F s can be smlarly defned. f12s the Coster-Krong transton probablty of shftng of electron from I subshell to II subshell. All other f s can be smlarly defned. 27
1.3.1.2 Fluorescence yeld The fluorescent yeld (Bambynek et al., 1972) of an atomc shell or subshell s defned as the probablty that a vacancy n that shell or subshell s flled through a radatve transton. An atom wth a vacancy s n ected state; let s the total wdth of that state related to the mean lfe of the state by /. The wdth s the sum of the radatve wdth R, the radatonless wdth A and the Coster-Krong wdth CK fluorescent yeld s therefore gven by. The / (1.18) R Thus the fluorescence yeld of a shell s equal to the number of emtted photons when vacances n that shell s flled, dvded by the total number of vacances n that shell. The applcaton of ths defnton to the K shell of an atom, whch s normally contans two S 1/2 electrons s straght forward. In ths case, fluorescence yeld K s gven as: I K K (1.19) nk Where I K s the total number of characterstcs K shell -ray photons emtted and nk s the number of prmary K shell vacances. reasons. However, the stuaton becomes complcated for hgher atomc shells for the two () Shells above K shell consst of more than one subshell because electrons have dfferent angular momentum quantum numbers. Moreover, t s very dffcult to onze only one out of all subshell. All the subshells are onzed n specfc ratos dependng upon the subshell cross sectons of onzng process. The average fluorescence yeld, thus, depends n general on how the shells are onzed snce dfferent onzaton methods gve rse to dfferent prmary vacancy dstrbutons. 28
M V M IV M III M II M I K K III II I K K K K Fg. 1.1: Typcal K -ray emsson Spectrum. 29
O IV O III O II N VII N VI O I N V N IV N III N II N I M V M IV M III M II M I III II I Fg.1.2: Typcal -ray emsson Spectrum. 30
() Coster-Krong transtons whch are radaton less transtons among the subshell of an atomc shell havng the same prncpal quantum number make t possble for a prmary vacancy created n one of the subshells to shft to a hgher subshell before the vacancy s flled by another transton. Because Coster-Krong transtons change the prmary vacancy dstrbuton, great care s to be taken n formulatng the defntons of the quanttes. In the absence of Coster-Krong transtons, the vacancy dstrbuton n the subshell of a shell s proportonal to ther onzaton cross sectons only. Thus for hgher shells, n the absence of Coster-Krong transtons, the average fluorescence yeld for the shell can be defned as n 1 N (1.20) where n=1 for K shell n=2 for shell n=3 for M shell and so on where N refers to the relatve number of prmary vacances n the th subshell of th shell and s gven as: N n N 1 N (1.21) n and also N 1 (1.22) 1 The defnton of the average fluorescence yelds n the presence of Coster- Krong transtons are rather complcated (Fnk et al., 1966 and Bambynek et al., 1972) and nvolve Coster-Krong yelds f j. 31
1.3.1.3 Auger effect and yeld The ejecton of an electron from gven shell through photoelectrc nteracton, creates a vacancy n that shell and leaves the atom n an ected state. The atom reverts to the lower state when an electron from the hgher shells flls ths vacancy. As a consequence of ths transton, energy s released n the form of -ray photon. If the energy of emtted -ray s greater that the bndng energy of the hgher shell, then the electron can be ejected from the hgher shell along wth photoelectron. Ths etra electron s called Auger electron and the effect s called Auger effect. The process s schematcally shown n fgure 1.3. Auger effect s more common for low Z (.e. upto Z=20) element, because ther nner shell electrons are comparatvely loosely bound. For the same reason, ths effect s more promnent for and hgher shells than K-shell. Thus the term Auger effect s used to descrbe those transtons n whch the decay of a vacancy n an atomc shell leads to two vacances n one or two dfferent prncpal shells. The Auger yeld a s the probablty that a vacancy n the th subshell of th shell s flled through a non-radatve transton by an electron from a hgher shell. The Auger transtons are radaton less transtons and dffer from Coster-Krong transtons because the later occur among varous subshells of the major shell only whereas the former may occur from the hgher shell also. The average Auger yeld s defned n analogy to the defnton of average fluorescence yeld a as: a V 1 a (1.23) where the coeffcent k V s the modfed vacancy numbers n th subshell of the th shell due to Coster-Krong transtons. The sum of the average fluorescence yeld and average 32
Auger yeld of a shell for the same ntal vacancy dstrbuton s unty,.e. a 1 (1.24) k k 1.3.1.4 Coster-Krong transtons and yeld These are the non-radatve transtons whch occur n shells havng more than one subshell (.e, M, N and so on). In such shells, the vacances are shfted to hgher subshells from lower subshells as non-radatve transtons occurrng wthn a short tme of about 10-17 second. These transtons are called Coster-Krong transtons, whch were frst studed by Coster and Krong (1935). For eample, the vacances from low lyng I subshell may be shfted to II or III subshell and smlarly vacances from II subshell may be transferred to III subshell. For shell these transton probabltes are defned as; f 12, f 23 and f 13 ; where f 12 s the probablty of shftng vacancy from I subshell to II subshell and smlarly other s can be defned. The followng general relaton ests between the Auger yeld, the fluorescence yeld and the Coster-Krong yelds as: k a f 1 (1.25) j j1 For shell the above relaton gves followng three relatons 3 2 1 a a a 3 2 1 1 f f 23 12 1 f 13 1 (1.26) Then non-radatve Coster-Krong transtons occur wthn subshell s of great mportance n the measurement of fluorescence yeld and Auger yelds. These transtons alter the ntal vacancy dstrbuton of prmary vacances and must be taken 33
Atomc shells M K Incdent radaton Nucleus Vacancy Auger electron Characterstcs -rays Fg.1.3: The schematc dagram showng prncple of characterstcs -ray and Auger emsson. 34
nto account. 1.4 Absorpton edge jump rato and jump factor A plot of the total atomc cross secton versus ncdent photon energy s found to ehbt a characterstc saw tooth structure n whch the sharp dscontnutes, known as absorpton edges, arse whenever the ncdent energy concdes wth the onzaton energy of electrons n the K,, M shells. These sharp dscontnutes are due to the fact that photoelectrc nteracton becomes energetcally possble n the shell consdered. Rato of values of photoelectrc cross-secton on the hgher energy to that of lower energy sde of edge of a partcular shell/ sub-shell s called jump rato of that shell/ sub shell. Whereas, absorpton jump factor s defned as the fracton of the total absorpton that s assocated wth a gven shell rather than for any other shell. Typcal plot showng abrupt jumps at absorpton edges has been gven n fgure 1.4. 1.4.1 Bref descrpton of varous epermental technques used for measurng absorpton edge jump rato and jump factor Absorpton edge jump ratos and jump factors can be measured epermentally usng four dfferent methods gven as under: 1.4.1.1 Gamma-ray or -ray attenuaton method In ths method, mass attenuaton coeffcent of a gven target element are measured employng transmsson geometry at dfferent energes whch covers the energy regon lyng both above and below the partcular shell/ subshell absorpton edge of the target element under consderaton. Mass attenuaton coeffcents so obtaned are then plotted aganst photon energes and the resultant plot gves a saw tooth structure around edge of that partcular shell/ subshell of gven target element. By calculatng the 35
rato of mass attenuaton coeffcent on upper and lower energy branch of absorpton edge, ts correspondng jump rato s then determned. 1.4.1.2 Compton scattered photon method In ths method, the Compton scattered photons are made to fall on the absorber whose absorpton edge s to be studed. As Compton scattered photons are functon of scatterng angle, so by adjustng ths angle, the energy of the Compton scattered photon s vared. Mass attenuaton coeffcents are then measured as a functon of Compton scattered photon energy around that partcular absorpton edge of gven target. By plottng mass attenuaton coeffcents aganst photon energy, ts correspondng absorpton jump rato s determned. 1.4.1.3 Bremsstrahlung method In bremsstrahlung method, contnuous bremsstrahlung radaton from a weak beta source s allowed to fall on a thn target. Then the transmtted bremsstrahlung spectrum, so produced, shows a sudden drop n ntensty at the absorpton edge of the target element studed. From ths sudden drop, the absorpton edge jump factor can be measured for hgh Z elements. 1.4.1.4 Energy Dspersve -ray Fluorescence (EDRF) method In EDRF method, strong radoactve source (100 mc) s used to generate K and -ray photons n a gven target element. Then by notng the net counts fallng under K (= and and =l, and -ray peaks, ts correspondng K and -ray producton cross-sectons has been determned. Smlarly, by knowng the ncdent and transmtted -ray photon ntenstes, photoonzaton cross-secton of a 36
gven target of nterest has been measured. By makng use of these fluorescent parameters, jump ratos and jump factors of gven target element has been determned. In the present study, EDRF technque has been used for measurng K shell and III subshell absorpton edge jump rato and jump factor of some low and hgh Z elements. 37
Total atomc cross-secton(b/atom) Total atomc cross-secton(b/atom) 100000 50000 45000 40000 35000 30000 1 absorpton edge 25000 20000 2 absorpton edge 15000 10000 3 absorpton edge 10000 5000 0.015 0.02 0.025 0.03 0.035 0.04 0.045 Energy(MeV) 1000 K absorpton edge 100 0.01 0.1 1 Energy(MeV) Fg. 1.4: Typcal plot of Th showng varaton of atomc cross-secton (b/atom) wth energy (MeV). 38