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subshell photoionization cross section measurements using a gamma excited variable energy X-ray source
Nuclear Instruments and Methods 193 (1982) 391-394 North-Holland Publishing Company
391
INNER.SHELL/SUBSHELL PHOTOIONIZATION CROSS SECTION MEASUREMENTS USING A GAMMA EXCITED VARIABLE ENERGY X-RAY SOURCE B.S. SOOD, K.L. ALLAWADHI and S.K. ARORA Nuclear Science Laboratories, Department o[ Physics, Punjabi University, Patiala-147002, India
The method developed for the determination of K/L shell photoionization cross sections in various elements, 39 -< Z --<92, in the characteristic X-ray energy region using a gamma excited variable energy X-ray source has been used for the measurement of Lnl subshell photoionization cross sections in Pb, Th and U. The measurements are made at the K X-ray energies of Rb, Nb and Mo, since these are able to excite selectively the Lm subshells of Pb, Th and U, respectively. The results, when compared with theoretical calculations of Scofield, are found to agree within the uncertainties of determination.
The method [1, 2] developed for the determination of K/L shell photoionization cross sections in the characteristic X-ray energy region using a gamma excited variable energy X-ray source has been applied for the measurement of Lm subshell photoionization cross sections in Pb, Th and U at 13.596, 16.896 and 17.781 keV, respectively since no such measurement has been reported so far. The method involves irradiation of suitably selected primary targets with 59.5 keV gamma rays from 24~Am for producing K fluorescent X-rays that are made to interact with the secondary targets. The choice of primary target materials is determined by the requirement that the K shell fluorescent X-rays of the primary target are able to excite selectively Lin subshells of the secondary target. The experimental arrangement used in the present measurements is shown in fig. 1. 59.5 keV gamma rays from a 241Am radioactive source (RS) were allowed to fall, in turn, on experimental primary targets (EFT) of Rb, Nb and Mo whose external conversion X-rays were allowed to fall on the experimental secondary targets (EST) of Pb, Th and U, respectively. Since the energies [3] of all the components K~I, K~2, Kal, etc. of the K X-rays of Rb, Nb and Mo were lower than Ln but higher than the LIII edge energies of Pb, Th and U, respectively, only Lm and higher shell X-rays of EST were produced by the interaction of K conversion X-rays of EFT with the atoms of EST. The Lm X-rays were counted with a proportional counter spec-
trometer (PCS) with a resolution of 12% at 14.4 keV from 57Co. 26 keV gamma rays and Np L X-rays which were also emitted [4], along with 59.5 keV gamma rays from 241Am, were almost completely absorbed with a graded filter of absorption efficiency better than 99.9% at 26 keV, so that their scattering from EPT did not interfere with the production of Lux vacancies in EST. The background spectrum, recorded with an equivalent aluminium primary target which gave almost the same scattering of 59.5 keV gamma rays as EFT, was subtracted in each case to take I.s
Fig. 1. Experimental set-up used for the measurement of Lm subshell photoionization cross sections; LIH subshell photoionization cross sections; RS-24~Am disc source; G F graded filter to absorb 26 keV gamma rays and Np L X-rays; EPT-experimental primary target; EST-experimental secondary target; PCS-proportional counter spectrometer; [.S-lead shielding; GA-graded absorbers of Fe and AI.
0029-554X/82/0000-0000/$02.75 O 1982 North-Holland
IX. X-RAY ABSORPTION
392
B.S. S o o d et al. / Photoionization cross section m e a s u r e m e n t s
into account any possible contribution to L-shell vacancy production in EST by the scattering of 59.5 keV gamma rays from EPT, to ensure that L~II X-rays from the EST were produced only by the interaction of K conversion X-rays from the respective EPT with the Lui electrons in EST. The geometrical arrangements of RS, EPT, EST and PCS were adjusted to get the best possible peak-to-background ratio. The intensity NEsT(LIxI) of Lm fluorescent Xrays emitted from EST when K shell fluorescent X-rays from EPT interact with Lm electrons in EST, as measured with the spectrometer under the photopeak, is given by
Nesa-(K) = NEzr(K)(N/Mcsv)tesrflesT(K,K)crcsv(K) WCST(K)~
~ (K),
(2)
where the various terms have the same meaning as explained above, but correspond to CST. Since the Lni X-ray energy from EST is very nearly equal to the K X-ray energy of CST e(Ln0-~ ~(K). Thus eqs. (1) and (2) lead to
NEsT(Lm) MEsT tcsr /3esT(K,K) o'Esr(Lm) = NesT(K)Mesa- test/3EsT(K,Lm)
wesa-(K)
NEsa-(Lm) = NEvr(K)
× WEsT(LIII) O-csa-(K).
(3)
x (N/MEsT)t~TflmsT(K,Lm)crEsT(Lm) O9
× WEsT(LIII)~
~(LIlI) ,
(1)
where NEzr(K) is the number of K X-rays of EPT falling per second on EST; N is Avogadro's number; MasT is the atomic weight and tESTis the thickness of EST. /3EsT(K, Lin) is a factor that takes into account the absorption of incident EPT K X-rays and emerging EST Lni X-rays in the EST; O'EsT(LnI) is the Lm photoionization cross section in the element of EST at the weighted mean energy of K X-rays from the EPT. WEsT(Lm) is the Lm subshell fluorescence yield of EST element; o) is the EST to PCS solid angle; E(Lm) is the effective detector efficiency including absorption of Liu X-rays in the air column between EST and PCS. In order to avoid the need for measurement of the absolute intensity of primary K X-rays falling on EST, counter efficiency and EST-PCS solid angle etc., the EST was replaced by a CST (comparison secondary target) of the same dimensions as that of EST, and chosen in such a way that (1) its K edge lies below the energies of all the K X-ray components from the EPT, and (2) the weighted mean energy of K X-rays emitted from it lies very close to the weighted mean energy of Lni X-rays from EST. As, Rb and Sr were found to satisfy the above conditions, and thus they qualify to act as CST for the EST of Pb, Th and U, respectively. NesT, the number of K X-rays emerging from CST as counted per second by the counter is given by
Obviously the determination of O'EsT(LIn)reduces to the measurement of the ratio of the counting rate NEsT(LnI)/NcsT(K) and computation of /3esT(K,K), /3~sT(K, Lin) and O'csT(K). The ratio N~sT(LIn)/NEsT(K) was determined by measuring the areas under the Lni X-ray photopeak of EST and K X-ray photopeak of CST when the targets were irradiated, in turn, with the K X-rays of EPT. A X e + CO2 filled proportional counter, manufactured by Reuter Stokes, U.S.A., coupled with an Ortec 142 PC preamplifier, an Ortec 451 spectroscopy amplifier, and a ND-600 multichannel analyser was used for the measurement of X-rays. A sufficient number of runs ranging from 1000 s to 8000 s were taken so as to achieve a statistical accuracy of - 1 - 2 % . Typical spectra of Lm X-rays, and K X-rays from EST and CST are shown in figs. 2 and 3, respectively along with the background recorded with an equivalent A1 target. The actual minus background spectrum gives the intensity of Lm/K X-rays which are produced by the interaction of K shell primary X-rays with Lm/K subshetl/shell electrons in EST/CST. The scattering from EST/CST is seen to be the same as that from equivalent AI target. The contribution in the spectrum arising due to natural radioactivity of Th and U EST was cancelled out while subtracting the background with an equivalent A! target /3esT(K,K)/13Esr(K,Lm) is determined [1, 2, 5] from the thickness of the target, the absorption coefficients for incident K X-rays of EPT, and emitted K/Lm X-rays of CST/EST in the materials of CST/EST. The absorption
B.S. Sood et al. / Photoionization cross section measurements
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Fig. 3. Comparison secondary target spectra recorded w i t h proportional counter spectrometer. A-Mo EPT and Sr CST. B-Eq. Al primary target and Sr CST. C-K X-ray spectra of Sr ( A - B).
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Fig. 2. Experimental secondary target spectra recorded w i t h proportional counter spectrometer; (a) lead; (b) uranium. A-Rb/Mo EPT and pb/U EST. B--Eq. Al primary target and Pb/U EST. C-LnI X-ray spectra of Pb/U ( A - B).
coefficients were taken from the tables of Veigele [6]. The values of Wcsx(K) and WEsT(L.I) were taken from Bambynek et al. [7], as described in an earlier communication [8]. The values of O'csx(K) were calculated by finding the index to the power of energy with which the cross sections vary using various values of the cross section from Scofield's tables [9]. The targets of Mo, Pb, Th and U used in the present experiment were cut to the required size (4 cm diameter) from thin metallic foils obtained from Reactors Experiments Inc., U.S.A., while the targets of As, Sr, Nb and Rb were prepared from the metallic powders/salts by the method reported earlier [2]. The contribution [5] of the scattering of the primary K shell X-rays from the EST/CST in the region of the photopeak of the secondary Lm/K
Table 1 Measured values of Lm photoionization cross sections in Pb, Th and U compared w i t h t h e t h e o r e t i c a l values of Scofield [9]. LIII photoionization cross s e c t i o n (b/atom) Sample
Element
Energy of
Present
measurements
measurements
Theoretical values
29000 _ 3200 22000 +-2200 23000 _+2400
29111 22889 21580
(keV) 1 2 3
Pb Th U
13.596 16.896 17.781
IX. X-RAY ABSORPTION
394
B.S. Sood et al. / Photoionization cross section measurements
X-ray spectrum, the Compton scattering of primary K shell X-ray spectrum, the Compton scattering of primary K shell X-rays from LnI/K electrons of EST/CST, and secondary ionization by photo- and Compton electrons are negligibly small compared to the contribution of the photoelectric absorption in the Lm/K shell, and have, therefore, been neglected [1, 5]. The experimental results for Lm photoionization cross section are compared in table 1 with the theoretical calculations of Scofield [9], as no other experimental data are available for comparison. The total uncertainty in the results is 10-11% and is mainly due to uncertainty in the fluorescence yields. The errors in the values of WEsT(LIII) and Wcsr(K) were - 8 % and 1-5%, respectively. The uncertainty in flEsT(K,Lm) and flcsT(K,L) has been estimated to be 2-4%. Within the quoted errors the results agree well with Scofield's calculations.
Financial assistance from U.G.C., India, is gratefully acknowledged. References [1] K.L. Allawadhi and B.S. Sood, Phys. Rev. AI3 (1976) 688. [2] S.K. Arora, K.L. Allawadhi and B.S. Sood, J. Phys. B; At. Molec. Phys. 13 (1980) 3157. [3] E. Storm and I. Israel, Nucl. Data Tables A7 (1970) 565. [4] Radiochemical Centre Amersham, Radiation Sources, Industrial Laboratory, Nuclide Index (1977). [5] S.K, Arora, K.L. Allawadhi and B.S. Sood, J. Phys. B; At. Molec. Phys. (in press). [6] Wm. J. Veigele, Atom. Data Tables 5 (1973) 51. [7] W. Bambynek, B. Crasemann, R.W. Fink, H.U. Freund, H. Mark, C.D. Swift, R.E. Price and P.V. Rao. Rev. Mod. Phys. 44 (1972) 716. [8] K.L. Allawadhi, B.S. Ghumann and B.S. Sood, Paramana 8 (1977) 433. [9] J.H. Scofield, Lawrence Livermore Lab. rep. no. 51326 (1973) unpublished.
391
INNER.SHELL/SUBSHELL PHOTOIONIZATION CROSS SECTION MEASUREMENTS USING A GAMMA EXCITED VARIABLE ENERGY X-RAY SOURCE B.S. SOOD, K.L. ALLAWADHI and S.K. ARORA Nuclear Science Laboratories, Department o[ Physics, Punjabi University, Patiala-147002, India
The method developed for the determination of K/L shell photoionization cross sections in various elements, 39 -< Z --<92, in the characteristic X-ray energy region using a gamma excited variable energy X-ray source has been used for the measurement of Lnl subshell photoionization cross sections in Pb, Th and U. The measurements are made at the K X-ray energies of Rb, Nb and Mo, since these are able to excite selectively the Lm subshells of Pb, Th and U, respectively. The results, when compared with theoretical calculations of Scofield, are found to agree within the uncertainties of determination.
The method [1, 2] developed for the determination of K/L shell photoionization cross sections in the characteristic X-ray energy region using a gamma excited variable energy X-ray source has been applied for the measurement of Lm subshell photoionization cross sections in Pb, Th and U at 13.596, 16.896 and 17.781 keV, respectively since no such measurement has been reported so far. The method involves irradiation of suitably selected primary targets with 59.5 keV gamma rays from 24~Am for producing K fluorescent X-rays that are made to interact with the secondary targets. The choice of primary target materials is determined by the requirement that the K shell fluorescent X-rays of the primary target are able to excite selectively Lin subshells of the secondary target. The experimental arrangement used in the present measurements is shown in fig. 1. 59.5 keV gamma rays from a 241Am radioactive source (RS) were allowed to fall, in turn, on experimental primary targets (EFT) of Rb, Nb and Mo whose external conversion X-rays were allowed to fall on the experimental secondary targets (EST) of Pb, Th and U, respectively. Since the energies [3] of all the components K~I, K~2, Kal, etc. of the K X-rays of Rb, Nb and Mo were lower than Ln but higher than the LIII edge energies of Pb, Th and U, respectively, only Lm and higher shell X-rays of EST were produced by the interaction of K conversion X-rays of EFT with the atoms of EST. The Lm X-rays were counted with a proportional counter spec-
trometer (PCS) with a resolution of 12% at 14.4 keV from 57Co. 26 keV gamma rays and Np L X-rays which were also emitted [4], along with 59.5 keV gamma rays from 241Am, were almost completely absorbed with a graded filter of absorption efficiency better than 99.9% at 26 keV, so that their scattering from EPT did not interfere with the production of Lux vacancies in EST. The background spectrum, recorded with an equivalent aluminium primary target which gave almost the same scattering of 59.5 keV gamma rays as EFT, was subtracted in each case to take I.s
Fig. 1. Experimental set-up used for the measurement of Lm subshell photoionization cross sections; LIH subshell photoionization cross sections; RS-24~Am disc source; G F graded filter to absorb 26 keV gamma rays and Np L X-rays; EPT-experimental primary target; EST-experimental secondary target; PCS-proportional counter spectrometer; [.S-lead shielding; GA-graded absorbers of Fe and AI.
0029-554X/82/0000-0000/$02.75 O 1982 North-Holland
IX. X-RAY ABSORPTION
392
B.S. S o o d et al. / Photoionization cross section m e a s u r e m e n t s
into account any possible contribution to L-shell vacancy production in EST by the scattering of 59.5 keV gamma rays from EPT, to ensure that L~II X-rays from the EST were produced only by the interaction of K conversion X-rays from the respective EPT with the Lui electrons in EST. The geometrical arrangements of RS, EPT, EST and PCS were adjusted to get the best possible peak-to-background ratio. The intensity NEsT(LIxI) of Lm fluorescent Xrays emitted from EST when K shell fluorescent X-rays from EPT interact with Lm electrons in EST, as measured with the spectrometer under the photopeak, is given by
Nesa-(K) = NEzr(K)(N/Mcsv)tesrflesT(K,K)crcsv(K) WCST(K)~
~ (K),
(2)
where the various terms have the same meaning as explained above, but correspond to CST. Since the Lni X-ray energy from EST is very nearly equal to the K X-ray energy of CST e(Ln0-~ ~(K). Thus eqs. (1) and (2) lead to
NEsT(Lm) MEsT tcsr /3esT(K,K) o'Esr(Lm) = NesT(K)Mesa- test/3EsT(K,Lm)
wesa-(K)
NEsa-(Lm) = NEvr(K)
× WEsT(LIII) O-csa-(K).
(3)
x (N/MEsT)t~TflmsT(K,Lm)crEsT(Lm) O9
× WEsT(LIII)~
~(LIlI) ,
(1)
where NEzr(K) is the number of K X-rays of EPT falling per second on EST; N is Avogadro's number; MasT is the atomic weight and tESTis the thickness of EST. /3EsT(K, Lin) is a factor that takes into account the absorption of incident EPT K X-rays and emerging EST Lni X-rays in the EST; O'EsT(LnI) is the Lm photoionization cross section in the element of EST at the weighted mean energy of K X-rays from the EPT. WEsT(Lm) is the Lm subshell fluorescence yield of EST element; o) is the EST to PCS solid angle; E(Lm) is the effective detector efficiency including absorption of Liu X-rays in the air column between EST and PCS. In order to avoid the need for measurement of the absolute intensity of primary K X-rays falling on EST, counter efficiency and EST-PCS solid angle etc., the EST was replaced by a CST (comparison secondary target) of the same dimensions as that of EST, and chosen in such a way that (1) its K edge lies below the energies of all the K X-ray components from the EPT, and (2) the weighted mean energy of K X-rays emitted from it lies very close to the weighted mean energy of Lni X-rays from EST. As, Rb and Sr were found to satisfy the above conditions, and thus they qualify to act as CST for the EST of Pb, Th and U, respectively. NesT, the number of K X-rays emerging from CST as counted per second by the counter is given by
Obviously the determination of O'EsT(LIn)reduces to the measurement of the ratio of the counting rate NEsT(LnI)/NcsT(K) and computation of /3esT(K,K), /3~sT(K, Lin) and O'csT(K). The ratio N~sT(LIn)/NEsT(K) was determined by measuring the areas under the Lni X-ray photopeak of EST and K X-ray photopeak of CST when the targets were irradiated, in turn, with the K X-rays of EPT. A X e + CO2 filled proportional counter, manufactured by Reuter Stokes, U.S.A., coupled with an Ortec 142 PC preamplifier, an Ortec 451 spectroscopy amplifier, and a ND-600 multichannel analyser was used for the measurement of X-rays. A sufficient number of runs ranging from 1000 s to 8000 s were taken so as to achieve a statistical accuracy of - 1 - 2 % . Typical spectra of Lm X-rays, and K X-rays from EST and CST are shown in figs. 2 and 3, respectively along with the background recorded with an equivalent A1 target. The actual minus background spectrum gives the intensity of Lm/K X-rays which are produced by the interaction of K shell primary X-rays with Lm/K subshetl/shell electrons in EST/CST. The scattering from EST/CST is seen to be the same as that from equivalent AI target. The contribution in the spectrum arising due to natural radioactivity of Th and U EST was cancelled out while subtracting the background with an equivalent A! target /3esT(K,K)/13Esr(K,Lm) is determined [1, 2, 5] from the thickness of the target, the absorption coefficients for incident K X-rays of EPT, and emitted K/Lm X-rays of CST/EST in the materials of CST/EST. The absorption
B.S. Sood et al. / Photoionization cross section measurements
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Fig. 3. Comparison secondary target spectra recorded w i t h proportional counter spectrometer. A-Mo EPT and Sr CST. B-Eq. Al primary target and Sr CST. C-K X-ray spectra of Sr ( A - B).
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Fig. 2. Experimental secondary target spectra recorded w i t h proportional counter spectrometer; (a) lead; (b) uranium. A-Rb/Mo EPT and pb/U EST. B--Eq. Al primary target and Pb/U EST. C-LnI X-ray spectra of Pb/U ( A - B).
coefficients were taken from the tables of Veigele [6]. The values of Wcsx(K) and WEsT(L.I) were taken from Bambynek et al. [7], as described in an earlier communication [8]. The values of O'csx(K) were calculated by finding the index to the power of energy with which the cross sections vary using various values of the cross section from Scofield's tables [9]. The targets of Mo, Pb, Th and U used in the present experiment were cut to the required size (4 cm diameter) from thin metallic foils obtained from Reactors Experiments Inc., U.S.A., while the targets of As, Sr, Nb and Rb were prepared from the metallic powders/salts by the method reported earlier [2]. The contribution [5] of the scattering of the primary K shell X-rays from the EST/CST in the region of the photopeak of the secondary Lm/K
Table 1 Measured values of Lm photoionization cross sections in Pb, Th and U compared w i t h t h e t h e o r e t i c a l values of Scofield [9]. LIII photoionization cross s e c t i o n (b/atom) Sample
Element
Energy of
Present
measurements
measurements
Theoretical values
29000 _ 3200 22000 +-2200 23000 _+2400
29111 22889 21580
(keV) 1 2 3
Pb Th U
13.596 16.896 17.781
IX. X-RAY ABSORPTION
394
B.S. Sood et al. / Photoionization cross section measurements
X-ray spectrum, the Compton scattering of primary K shell X-ray spectrum, the Compton scattering of primary K shell X-rays from LnI/K electrons of EST/CST, and secondary ionization by photo- and Compton electrons are negligibly small compared to the contribution of the photoelectric absorption in the Lm/K shell, and have, therefore, been neglected [1, 5]. The experimental results for Lm photoionization cross section are compared in table 1 with the theoretical calculations of Scofield [9], as no other experimental data are available for comparison. The total uncertainty in the results is 10-11% and is mainly due to uncertainty in the fluorescence yields. The errors in the values of WEsT(LIII) and Wcsr(K) were - 8 % and 1-5%, respectively. The uncertainty in flEsT(K,Lm) and flcsT(K,L) has been estimated to be 2-4%. Within the quoted errors the results agree well with Scofield's calculations.
Financial assistance from U.G.C., India, is gratefully acknowledged. References [1] K.L. Allawadhi and B.S. Sood, Phys. Rev. AI3 (1976) 688. [2] S.K. Arora, K.L. Allawadhi and B.S. Sood, J. Phys. B; At. Molec. Phys. 13 (1980) 3157. [3] E. Storm and I. Israel, Nucl. Data Tables A7 (1970) 565. [4] Radiochemical Centre Amersham, Radiation Sources, Industrial Laboratory, Nuclide Index (1977). [5] S.K, Arora, K.L. Allawadhi and B.S. Sood, J. Phys. B; At. Molec. Phys. (in press). [6] Wm. J. Veigele, Atom. Data Tables 5 (1973) 51. [7] W. Bambynek, B. Crasemann, R.W. Fink, H.U. Freund, H. Mark, C.D. Swift, R.E. Price and P.V. Rao. Rev. Mod. Phys. 44 (1972) 716. [8] K.L. Allawadhi, B.S. Ghumann and B.S. Sood, Paramana 8 (1977) 433. [9] J.H. Scofield, Lawrence Livermore Lab. rep. no. 51326 (1973) unpublished.