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Terms used in the production of radioisotopes with electron accelerators
Internalional Journal ~f Applied Radiation & l.sotopes, Vol. 31. pp. 254 to 255
in which N is the n u m b e r of target a t o m s per cm 2, nit the number of photons of energy k per electron, ak the cross section at energy k and k0 the maximal energy of the bremsstrahlung beams. Reactions with monoenergetic photons are studied by several m e t h o d s such as tagging, positron annihilation in flight and photon difference. However, these at values are, with a few exceptions, only known for (y, n) and (3', 2n) reactions below 30 meV. 17' s)
Pergamon Press Ltd 1980. Printed in Great Britain
Terms Used in the Production of Radioisotopes with Electron Accelerators G. A. B R I N K M A N and J. VISSER Institute for Nuclear Physics Research, P.O. Box 4305, 1009 AJ Amsterdam, The Netherlands
(Receired 30 August 1979) Isotope production with bremsstrahlung beams can be expressed in terms of ak, aq, monitor response units and in dis ,,oj-,R t or dis. g - t C i - ~. The different expressions are discussed, but none of them seems to be convenient if thick converters are used and therefore threshold detectors are recommended.
B a ~ l oa % To circumvent the calculation of the intergral over the product of the complex bremsstrahlung spectrum nt and the cross section a k a special formalism has been developed. The total energy content U of the photon beam in the target is given by:
U =
:?
nkk dk.
A number of equivalent quanta ~ is defined by
1
Introduction
Q = k'o U(ko)
THE LARGER part of radioisotopes for use in applied research (medical, biomedical, industrial) is produced by (n, 7) reactions with thermal neutrons in a nuclear reactor. The total yield of a radioactive product may be calculated by means of very simple, well-known formulae.(t) At the m o m e n t a broad scala of short-lived, often ~+-emitting radioisotopes produced with charged particle accelerators are in use for biomedical applications. (2) The energy of the bombarding particle decreases very rapidly on passing through the target and the formulae become more complicated. The saturation activity , Yp produced by a beam of I gA charged particles with energy E, in a target of mass number A, and a thickness of X g c m -2 is given by:
i.e. Q is the n u m b e r of quanta with energy ko that have the same energy content as the bremsstrahlung beam. The cross section per equivalent q u a n t u m aq is defined by:
0.1021 r::
YP - -
~4~- J0 a(x)dx =
0.1021 r E,'
Ar
dx
--dE(Ci) JE, alE)dE l
in which a (x) is the cross section (mb) at depth x, Eo is the energy of the outgoing particles. °) The most accurate method to measure the beam intensity is the use of a Faraday cup. but such a device asks for special arrangements. In practice, use is made of monitor foils for which cross sections are very well known. (`*) Comm o n monitors for proton beams are the reactions: 12C(p, pn) t IC, I 2C(p. 3p3n) VBe, 27Al(p, 3pn)2'*Na, 27Al(p, 3p3n)22Na. 27Al(p. 5p5n)lSF up to 30 GeV. whereas the ~SNi(p, pn) ~VNi reaction has also been recommended) ~ In this article we will discuss the special terms that are in use for the production of radioisotopes with electron accelerators.
Based on a t If the shape of the bremsstrahlung spectrum and the cross sections are known, the saturation activity ~ Yb per electron can be calculated from. (6' 7)
Yb = N f0I° nkak dk
~" ak nk dk aq
.Q The saturation activity can then be calculated from:
In general, U is measured with a P2 tg) or a Wilson "°) ionization chamber (quantameter)` consisting of an array of AI or Cu plates. The current measured, through the collection of electrons generated in the air (or Ar/CO2) between the plates, is proportional to U and almost independent of k. These quantameters can be calibrated with a calorimeter,"~) a scintillating crystal "2) or with an electron beam whose intensity is calibrated with a Faraday cup." 31 If ~ Yb in an irradiated target is measured, aq can be calculated with a simple formula given in Ref. (3). For (7, n) reactions below 30 MeV ere can be roughly calculated from aq ~-0.15 At`*~3 rob. For spallation reactions above 200 MeV aq can be calculated by a formula given by RUDSTAM,"" which formula has proved to be correct within a factor of two.
Based on Monitor Reactions A Simple c o m m o n l y used method to express radioisotope yields, is the use of monitor foils. With two identical monitor foils, one in front and the other behind the target (sandwich), a correction can be made for the degradation of the photon beam intensity in the target. The monitors that are in use are not very m u c h different from those mentioned before for proton accelerators. At low electron • energies, the following reactions have been studied: --SSMn(y, n)54Mn at 20 MeV 17' t 51 --t97Au(y, n)196Au at 20 MeV its' 16) --t2C()', n) t tC at 30-60 MeV It7) --65Cu()', n)64Cu at 3 0 ~ 0 MeV "7,t8)
254
Technical Note As discussed before, the SSNi(~,,n)STNi reaction can be of interest.~. ~9~ At higher energies: --~C(3', n) ~IC up to 1 OeV ~2°'~ --27A10,,2pn-)24Na, for which reaction the cross section has been measured by many investigations between 0.1 and 5 GeV ~22~ - - t h e production of ~sF, 22Na, 24Na from low Z-targets (11 ~< z ~< 20)( ~ can be very convenient for monitoring purposes, because these three isotopes have very different half-lives and can therefore be used for short as well as for long irradiation times - - ~ V can become very useful as a monitor at high energies. Whereas (y, n), (y, 2n), (~,,p) and (y, d) reactions do not produce a measurable amount of radioisotopes, many spallation products can easily be measured ~z4' TM KATO~s~ has used the yield of the (y, n) reaction as an internal standard for other reactions induced in the same element. Based on other quantities It is sometimes practical to express radioisotope production in terms of exposure (disintegrations per Rfintgen per m o l e = 0 . 3 8 8 × 104 B q s C - l k g m o l - t ) . The exposure can be measured with a Victoreen thimble ionization chamber.~7. ~6~ Targets and the Victoreen chamber must have the same area and must be placed in a similar box. ~ The yield--us electron energy--curves go through a maximum, because at higher energies the yields do not change considerably whereas the exposure increases almost linear with Eo . ~ For a standard experimental setup the yield can also be expressed in disintegrations per gram per Coulomb (or pAh) of the integrated electron beam. ~ " ~7~ The integrated electron beam current can be measured with a non-intercepting ferrite beam monitor, calibrated with a Faraday cup.~s~ Thick Converters The above mentioned methods can only be used with thin converters, because with thicker converters the number of low energy photons is enhanced in comparison with the number of high energy photons, t19~ This effect is mainly due to the production of bremsstrahlung by electrons which are degraded in energy in the converter and partly due to an increase in the total absorption coefficient with increasing photoenergy. For a particular accelerator and a particular experimental setup (regarding the converter thickness, the dimensions of the target and its distance to the converter) the forementioned methods are very well sited. For intercomparison between laboratories we would therefore recommend the use of threshold detectors for measuring the shape of the bremsstrahlung spectrum. At low energies (y, n) reactions can be very suitable: 19~Au (threshold 8.1 MeV), 6~Cu (9.9 MeV), 19F (10.4 MeV), SSNi (12.2 MeV), 160 (15.7 MeV) and 12C (18.7 MeV). At higher energies (~,xn) reactions on tg~Au°°~ with threshold energies for x = i,2,3,5,7 of 8.1, 19.7, 23.2, 38.8 and 55.1 MeV can be used. In a 59Co foil irradiated at 140 MeV at Saclay ° ~ we could easily measurei ssco (threshold 10.5 MeV), S4Mn (17.2 MeV), 57Co (19.0 MeV), S6Mn (28.0 MeV), S2Mn (38.2 MeV) and SSCo (40.5 MeV) and an irradiated 5IV foil at 140 and 409 MeV: 47Sc (18.2 MeV), 4~Sc (28.9 MeV), '*sV (31.9 MeV), 49Cr (47.0 MeV) and ,L4 Sc (49.0 MeV). ~1' 24.25~ The yields of these reactions should then be measured as a function of the converter thickness and of the electron energy. There is no problem with thick converters in activation analysis, because standards of reference samples
255
(pure or in a mixture) of the elements that are analysed can be irradiated simultaneously with the target compound, serving as in internal standard.
Acknowledgements--This work is part of the research program of the Institute for Nuclear Physics Research (IKO), made possible by financial support from the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organisation for the Advancement of Pure Research (ZWO). References 1. SVOBODA K. The Uses of Cyclotrons in Chemistry, Metallurgy and Biology. p. 383. Oxford (1969). 2. CLARK J. C. and BUCKINGHAMP. D. Short-lived Radioactive Gases for Clinical use. Butterworth, London (1975). 3. BRINKMANG. A. Int. J. appl. Radiat. Isotopes 31, 85-90 (1980). 4. CUMMINGJ. B. Ann. Rev. nucl. Soc. 13, 261 (1963). 5. BRtNKMAN G. A., HELMER J. and LINDNER L. Radiochem. radioanalyt. Lett. 28, 9 (1977). 6. LUTZ G. J. Analyt. Chem. 41,424 (1969). 7. BOL'ow B. and FORKMAN B., Tech. Rept Series IAEA 156, 483 (1974). 8. BERMANB. L. Lawrence Livermore Lab. UCRL-78482 0976). 9. PRUIT'r J. S. and DOMEN S. R. NBS Monogr. 48, (1962). 10. WILSON R. R. Nucl. lnstrum. Meth. I, 101 (1957). 11. H. BURCKHART, DIenL R. and ZIEGLER B. Nucl. Instrum. Meth. 159, 1 (1979). 12. KOCH H. W., LElSS J. E. and PRUtYr J. S. Bull. APS II, 199 (1956). 13. GOMEZ R., PINE J. and SILVERMANNA, Nucl. lnstrum. Meth. 24, 429 (1963). 14. RUDSrAM G., JANSSON G. G. and LINOGRENS K. Z. Naturf 21a, 1027 (19.66). Physica Scripta 7, 49 (1973); 15, 308 (1977). 15. OKAY., KATO T., NAMURAK. and SAITO T. Bull. chem. Soc. Japan 40, 575 (1967). 16. OKAY., KATO.T., NOMURA K. and SAITO T. J. nucl. Soc. Technol. 4, 346 (1967), 17. KATO T., MASUMOTO K., SATO N, and SuzuKI N. J. radwanalyt. Chem. 32, 51 (1976). 18. KATO T. J. radioanalyt. Chem. 16, 307 (1973). 19. ViSSERJ., BAKKER C. N. M. and BRINKMAN G. A. Int. Jl. appl. Radiat. Isotopes 30, 749 (1979). 20. MASAIKEA. J. phys. Soc. Japan 19, 427 (1964). 21. HVLTI~NJ. Nucl. Phys. Soc. A158, 225 (1970). 22. JOttNSSON B., JgRUND A. and FORKMAN B. Z. Phys. A273, 97 (1975) and "Erratum". Z. Phys. A276, 410 (1976). 23. Ol NAPOLI g., ROSA G., SALVETTIF., TERRANOVA M. L., CARVALHO V~ H. G , MARTINSJ. B. and TAVARESO. A. P. J. inorg. Nucl. Chem. 37, II01 (1975). 24. BOLOW B., JOHNSSON B., NILSSON M. and FORKMA~ B. Z. Phys. A278, 89 (1976). 25. NAPOLI DI V., SALWTTI F., TERRANOVA M. "L., CARVALHO DE H. G. and TAVARES0. A. P. J. inorg, nucl. Chem. 40, 175 (1978). 26. JOHNS H. E., KATZ L., DOUGLASR. A. and HARLAMR. N. H. Phys. Rev. 80, 1062 (1950). 27. Research Rept Laboratory of Nuclear Science, Tokohu University 9, 2, p. 281 (1976). 28. YAO N. and KONDO K. Radiochem. radioanalyt. Lett. 30, 173 (1977). 29. NCRP-Rept no. 31, NBS Handbook 97, (1964). 30. LINDGREN K. and JONSSONG. G. Nucl. Phys. AI66, 643 (1971). 31. BRINKMAN G. A., LINDNER L.,POLAK P: and VISSER T. Unpublished results.
in which N is the n u m b e r of target a t o m s per cm 2, nit the number of photons of energy k per electron, ak the cross section at energy k and k0 the maximal energy of the bremsstrahlung beams. Reactions with monoenergetic photons are studied by several m e t h o d s such as tagging, positron annihilation in flight and photon difference. However, these at values are, with a few exceptions, only known for (y, n) and (3', 2n) reactions below 30 meV. 17' s)
Pergamon Press Ltd 1980. Printed in Great Britain
Terms Used in the Production of Radioisotopes with Electron Accelerators G. A. B R I N K M A N and J. VISSER Institute for Nuclear Physics Research, P.O. Box 4305, 1009 AJ Amsterdam, The Netherlands
(Receired 30 August 1979) Isotope production with bremsstrahlung beams can be expressed in terms of ak, aq, monitor response units and in dis ,,oj-,R t or dis. g - t C i - ~. The different expressions are discussed, but none of them seems to be convenient if thick converters are used and therefore threshold detectors are recommended.
B a ~ l oa % To circumvent the calculation of the intergral over the product of the complex bremsstrahlung spectrum nt and the cross section a k a special formalism has been developed. The total energy content U of the photon beam in the target is given by:
U =
:?
nkk dk.
A number of equivalent quanta ~ is defined by
1
Introduction
Q = k'o U(ko)
THE LARGER part of radioisotopes for use in applied research (medical, biomedical, industrial) is produced by (n, 7) reactions with thermal neutrons in a nuclear reactor. The total yield of a radioactive product may be calculated by means of very simple, well-known formulae.(t) At the m o m e n t a broad scala of short-lived, often ~+-emitting radioisotopes produced with charged particle accelerators are in use for biomedical applications. (2) The energy of the bombarding particle decreases very rapidly on passing through the target and the formulae become more complicated. The saturation activity , Yp produced by a beam of I gA charged particles with energy E, in a target of mass number A, and a thickness of X g c m -2 is given by:
i.e. Q is the n u m b e r of quanta with energy ko that have the same energy content as the bremsstrahlung beam. The cross section per equivalent q u a n t u m aq is defined by:
0.1021 r::
YP - -
~4~- J0 a(x)dx =
0.1021 r E,'
Ar
dx
--dE(Ci) JE, alE)dE l
in which a (x) is the cross section (mb) at depth x, Eo is the energy of the outgoing particles. °) The most accurate method to measure the beam intensity is the use of a Faraday cup. but such a device asks for special arrangements. In practice, use is made of monitor foils for which cross sections are very well known. (`*) Comm o n monitors for proton beams are the reactions: 12C(p, pn) t IC, I 2C(p. 3p3n) VBe, 27Al(p, 3pn)2'*Na, 27Al(p, 3p3n)22Na. 27Al(p. 5p5n)lSF up to 30 GeV. whereas the ~SNi(p, pn) ~VNi reaction has also been recommended) ~ In this article we will discuss the special terms that are in use for the production of radioisotopes with electron accelerators.
Based on a t If the shape of the bremsstrahlung spectrum and the cross sections are known, the saturation activity ~ Yb per electron can be calculated from. (6' 7)
Yb = N f0I° nkak dk
~" ak nk dk aq
.Q The saturation activity can then be calculated from:
In general, U is measured with a P2 tg) or a Wilson "°) ionization chamber (quantameter)` consisting of an array of AI or Cu plates. The current measured, through the collection of electrons generated in the air (or Ar/CO2) between the plates, is proportional to U and almost independent of k. These quantameters can be calibrated with a calorimeter,"~) a scintillating crystal "2) or with an electron beam whose intensity is calibrated with a Faraday cup." 31 If ~ Yb in an irradiated target is measured, aq can be calculated with a simple formula given in Ref. (3). For (7, n) reactions below 30 MeV ere can be roughly calculated from aq ~-0.15 At`*~3 rob. For spallation reactions above 200 MeV aq can be calculated by a formula given by RUDSTAM,"" which formula has proved to be correct within a factor of two.
Based on Monitor Reactions A Simple c o m m o n l y used method to express radioisotope yields, is the use of monitor foils. With two identical monitor foils, one in front and the other behind the target (sandwich), a correction can be made for the degradation of the photon beam intensity in the target. The monitors that are in use are not very m u c h different from those mentioned before for proton accelerators. At low electron • energies, the following reactions have been studied: --SSMn(y, n)54Mn at 20 MeV 17' t 51 --t97Au(y, n)196Au at 20 MeV its' 16) --t2C()', n) t tC at 30-60 MeV It7) --65Cu()', n)64Cu at 3 0 ~ 0 MeV "7,t8)
254
Technical Note As discussed before, the SSNi(~,,n)STNi reaction can be of interest.~. ~9~ At higher energies: --~C(3', n) ~IC up to 1 OeV ~2°'~ --27A10,,2pn-)24Na, for which reaction the cross section has been measured by many investigations between 0.1 and 5 GeV ~22~ - - t h e production of ~sF, 22Na, 24Na from low Z-targets (11 ~< z ~< 20)( ~ can be very convenient for monitoring purposes, because these three isotopes have very different half-lives and can therefore be used for short as well as for long irradiation times - - ~ V can become very useful as a monitor at high energies. Whereas (y, n), (y, 2n), (~,,p) and (y, d) reactions do not produce a measurable amount of radioisotopes, many spallation products can easily be measured ~z4' TM KATO~s~ has used the yield of the (y, n) reaction as an internal standard for other reactions induced in the same element. Based on other quantities It is sometimes practical to express radioisotope production in terms of exposure (disintegrations per Rfintgen per m o l e = 0 . 3 8 8 × 104 B q s C - l k g m o l - t ) . The exposure can be measured with a Victoreen thimble ionization chamber.~7. ~6~ Targets and the Victoreen chamber must have the same area and must be placed in a similar box. ~ The yield--us electron energy--curves go through a maximum, because at higher energies the yields do not change considerably whereas the exposure increases almost linear with Eo . ~ For a standard experimental setup the yield can also be expressed in disintegrations per gram per Coulomb (or pAh) of the integrated electron beam. ~ " ~7~ The integrated electron beam current can be measured with a non-intercepting ferrite beam monitor, calibrated with a Faraday cup.~s~ Thick Converters The above mentioned methods can only be used with thin converters, because with thicker converters the number of low energy photons is enhanced in comparison with the number of high energy photons, t19~ This effect is mainly due to the production of bremsstrahlung by electrons which are degraded in energy in the converter and partly due to an increase in the total absorption coefficient with increasing photoenergy. For a particular accelerator and a particular experimental setup (regarding the converter thickness, the dimensions of the target and its distance to the converter) the forementioned methods are very well sited. For intercomparison between laboratories we would therefore recommend the use of threshold detectors for measuring the shape of the bremsstrahlung spectrum. At low energies (y, n) reactions can be very suitable: 19~Au (threshold 8.1 MeV), 6~Cu (9.9 MeV), 19F (10.4 MeV), SSNi (12.2 MeV), 160 (15.7 MeV) and 12C (18.7 MeV). At higher energies (~,xn) reactions on tg~Au°°~ with threshold energies for x = i,2,3,5,7 of 8.1, 19.7, 23.2, 38.8 and 55.1 MeV can be used. In a 59Co foil irradiated at 140 MeV at Saclay ° ~ we could easily measurei ssco (threshold 10.5 MeV), S4Mn (17.2 MeV), 57Co (19.0 MeV), S6Mn (28.0 MeV), S2Mn (38.2 MeV) and SSCo (40.5 MeV) and an irradiated 5IV foil at 140 and 409 MeV: 47Sc (18.2 MeV), 4~Sc (28.9 MeV), '*sV (31.9 MeV), 49Cr (47.0 MeV) and ,L4 Sc (49.0 MeV). ~1' 24.25~ The yields of these reactions should then be measured as a function of the converter thickness and of the electron energy. There is no problem with thick converters in activation analysis, because standards of reference samples
255
(pure or in a mixture) of the elements that are analysed can be irradiated simultaneously with the target compound, serving as in internal standard.
Acknowledgements--This work is part of the research program of the Institute for Nuclear Physics Research (IKO), made possible by financial support from the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organisation for the Advancement of Pure Research (ZWO). References 1. SVOBODA K. The Uses of Cyclotrons in Chemistry, Metallurgy and Biology. p. 383. Oxford (1969). 2. CLARK J. C. and BUCKINGHAMP. D. Short-lived Radioactive Gases for Clinical use. Butterworth, London (1975). 3. BRINKMANG. A. Int. J. appl. Radiat. Isotopes 31, 85-90 (1980). 4. CUMMINGJ. B. Ann. Rev. nucl. Soc. 13, 261 (1963). 5. BRtNKMAN G. A., HELMER J. and LINDNER L. Radiochem. radioanalyt. Lett. 28, 9 (1977). 6. LUTZ G. J. Analyt. Chem. 41,424 (1969). 7. BOL'ow B. and FORKMAN B., Tech. Rept Series IAEA 156, 483 (1974). 8. BERMANB. L. Lawrence Livermore Lab. UCRL-78482 0976). 9. PRUIT'r J. S. and DOMEN S. R. NBS Monogr. 48, (1962). 10. WILSON R. R. Nucl. lnstrum. Meth. I, 101 (1957). 11. H. BURCKHART, DIenL R. and ZIEGLER B. Nucl. Instrum. Meth. 159, 1 (1979). 12. KOCH H. W., LElSS J. E. and PRUtYr J. S. Bull. APS II, 199 (1956). 13. GOMEZ R., PINE J. and SILVERMANNA, Nucl. lnstrum. Meth. 24, 429 (1963). 14. RUDSrAM G., JANSSON G. G. and LINOGRENS K. Z. Naturf 21a, 1027 (19.66). Physica Scripta 7, 49 (1973); 15, 308 (1977). 15. OKAY., KATO T., NAMURAK. and SAITO T. Bull. chem. Soc. Japan 40, 575 (1967). 16. OKAY., KATO.T., NOMURA K. and SAITO T. J. nucl. Soc. Technol. 4, 346 (1967), 17. KATO T., MASUMOTO K., SATO N, and SuzuKI N. J. radwanalyt. Chem. 32, 51 (1976). 18. KATO T. J. radioanalyt. Chem. 16, 307 (1973). 19. ViSSERJ., BAKKER C. N. M. and BRINKMAN G. A. Int. Jl. appl. Radiat. Isotopes 30, 749 (1979). 20. MASAIKEA. J. phys. Soc. Japan 19, 427 (1964). 21. HVLTI~NJ. Nucl. Phys. Soc. A158, 225 (1970). 22. JOttNSSON B., JgRUND A. and FORKMAN B. Z. Phys. A273, 97 (1975) and "Erratum". Z. Phys. A276, 410 (1976). 23. Ol NAPOLI g., ROSA G., SALVETTIF., TERRANOVA M. L., CARVALHO V~ H. G , MARTINSJ. B. and TAVARESO. A. P. J. inorg. Nucl. Chem. 37, II01 (1975). 24. BOLOW B., JOHNSSON B., NILSSON M. and FORKMA~ B. Z. Phys. A278, 89 (1976). 25. NAPOLI DI V., SALWTTI F., TERRANOVA M. "L., CARVALHO DE H. G. and TAVARES0. A. P. J. inorg, nucl. Chem. 40, 175 (1978). 26. JOHNS H. E., KATZ L., DOUGLASR. A. and HARLAMR. N. H. Phys. Rev. 80, 1062 (1950). 27. Research Rept Laboratory of Nuclear Science, Tokohu University 9, 2, p. 281 (1976). 28. YAO N. and KONDO K. Radiochem. radioanalyt. Lett. 30, 173 (1977). 29. NCRP-Rept no. 31, NBS Handbook 97, (1964). 30. LINDGREN K. and JONSSONG. G. Nucl. Phys. AI66, 643 (1971). 31. BRINKMAN G. A., LINDNER L.,POLAK P: and VISSER T. Unpublished results.