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Integral cross section of the 99Tc(γ, γ′)99mTc reaction in the 15–50 MeV energy region
Appl. Radial. Isoi. Vol. 42, No. 2. pp. 149-153, 1991 1n1. J. Radiar. Appl. Insfrum. Parr A Printed in Great Britain. All rights reserved
0883-2889/91 $3.00 + 0.00 Copyright ep 1991 Pergamon Press plc
Integral Cross Section of the 99Tc(y, Y’)~~“Tc Reaction in the 15-50 MeV Energy Region TSUTOMU
SEKINE,’ KENJI YOSHIHARA,’ ZSOLT NfiMETH* and ARPAD
LbSZLd
LAKOSI,’
VERES2
‘Department of Chemistry, Faculty of Science, Tohoku University, Aoba-izu Sendai 980, Japan and *Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.0. Box 77, Hungary (Received
1990; in revised,form
5 March
15 June
1990)
Photoexcitation of radioactive 99Tc was studied by irradiation with 15-50 MeV bremsstrahiung. The integral cross sections of the 99TcQj, ‘J’)~~“Tc reaction and the reference “‘In(~, ;“)“‘“In reaction were found to be nearly constant as (5.8 + 1.0) x lo-” cm* MeV and (9.3 f 0.4) x IO-” cm’ MeV, respectively, in the given energy range. The large second increase of the cross section described in earlier reports was observed neither for ‘151n nor for 99Tc.
strahlung of a linear electron accelerator. A second peak in the higher energy region (> 15 MeV) was also sought. Analytical applications of the 99Tc(y, Y’)~~“Tc reaction have been studied (Sekine et al., 1989) in the framework of a research project established between Japan and Hungary.
Introduction The
long-lived
important nuclide been
9YT~ nuclide
fission
product.
accompanying cited
as a waste
nuclear
(t,:,
2.13
x 10’ y) is an
Accumulation reactor
management
of
this
operation problem
has to
be
solved in the future. Also, environmental pollution due to nuclear explosion-produced 9yTc has been detected. In spite of the importance of this nuclide, however, its nuclear data are not always satisfactory. This study provides some new information on nuclear excitation of y9Tc. The (y, 7’) reaction is a useful tool for studying nuclear excitation phenomena. Such experiments have been performed using accelerators and radioisotopes (Yoshihara et al., 1978; Veres, 1980). In the low-energy region, the process is described in terms of transitions between individual nuclear levels. The excitation mechanism is characterized by resonance absorption of photons. Non-resonant activation processes proposed by LjubiEic ef al. (198 1) and KrEmar et al. (1986) were not justified (Yoshihara er al., 1986; Bikit et al., 1987; Collins er a/., 1988). On the other hand, some experimental results reported on the cross section of the (y, y’) reaction in the giant resonance region exhibited a broad peak around the (y, n) threshold, and then either a second peak or a gradual rise at higher energies (Silva and Goldemberg, 1958; Bogdankevich et al., 1956, 1961; Meyer-Schiitzmeister and Telegdi, 1956). The second increase beyond 15 MeV in the excitation function of (;,. y’) reaction of “51n was found to be much more prominent than the first one (Bogdankevich et al.,
Experimental 99Tc, in the chemical form of ammonium pertechnetate, was purchased from Amersham International, and was deposited on filter paper or silica powders as supporting materials for irradiation. In some irradiations, 99Tc in the chemical form of tetrabutylammoniumtetrachlorooxotechnetate(V) was used. The amounts of technetium were determined by a liquid scintillation counter which was calibrated for spectrophotometrically determined quantities of technetium (Hashimoto, 1987). Metallic copper and gold foils were used as flux monitors just in front of or behind the samples. Indium metal or indium oxide was used in reference (y, y’) experiments performed in the same manner as in the technetium experiments. All the target materials were of high purity (> 99.5%). Electrons (15-50 MeV) from the linear electron accelerator at the Laboratory of Nuclear Science, were converted to bremsTohoku University, strahlung by means of a platinum plate (thickness: 1 or 2 mm). The targets were irradiated at various distances in both in-beam and off-beam directions [Fig. I(a)]. In some cases the targets were irradiated in another configuration [Fig. I(b)] and were cooled with nitrogen gas flow chilled with liquid nitrogen (Sekine et al., 1982). In these cases the unconverted
1956).
In this paper we report the integral cross section of the 99Tc(y, y’)y9mTc reaction induced by brems4RI
42 2-D
149
150
Tsu~ow~
SEKINE
et
al
We preferred the use of copper and gold flux monitors, using their reported (y, n) excitation functions for calculations. Adopting the cross section data of Fultz et al. (1962) for gold, we obtained consistent results with the radioactivity production of copper (Fultz et al., 1964) within 5%. Experimental radioactivity of yttrium (““Y) produced by the *‘Y(y, tQE8Y reaction could be explained within an error of 10% when calculation was carried out using the excitation function of yttrium (Lepretre et al., 1971) on the basis of our copper-gold monitor method. Almost all the radioactivity production data in this paper were reproduced systematically using the flux distribution based on Schiff’s formula, and the excitation functions, carefully inspected, as stated above.
beam duct
Results
(b) Fig. 1. The arrangements used in our experiments (a) Stacked targets in a quartz tube were placed along the beam course (in-beam) and perpendicular to it (off-beam). A 2mm thick Pt converter was used under water cooling. (b) Arrangement with a special cooling apparatus. The nitrogen gas evaporated in the 50-L reservoir was cooled again and was blown to the targets during the irradiation. The 1 mm thick platinum converter was placed just in front of the sweep-magnet.
electrons were eliminated by a sweep magnet placed between the platinum converter and the targets. The induced activities were measured repeatedly with pure Ge or Ge(Li) detectors connected to multichannel pulse-height analyzers. The counting efficiencies of the detectors were determined using standard “*Eu and ‘**Ta sources.
Determination The saturated formula
of the Bremsstrahlung activity produced
P A = N a(E)f(E) J
Flux
(A) is given by the
dE
where N is the number of atoms, u(E) is the reaction cross section, andf(E) is the differential flux of the incident photons defined by f(E)
dE = F
(2)
s where F is the total flux. Actually, f(E) was calculated by the formula of Schiff (1951). If the radioactivity is known in the flux monitors, the height of the function f(E) can be normalized so that la(E)f(E) dE satisfies equation (1). A compilation of the photon-induced reaction cross sections is available for this purpose (Dietrich and Berman, 1988).
A typical y-spectrum obtained by 30 MeV irradiation of a Tc sample is shown in Fig. 2. Activities of y9mTc (l,,* 6.02 h) and 96T~ (4.3 d) were detected. The latter nuclide was not observable when the irradiation energy fell below 20 MeV. The production of 96T~ is attributed to the (7, 3n) reaction. The yield of this reaction fits the general tendency for isotope production rates against the atomic number of target material reported by Kato et al. (1976). Typical examples of production rates of indium, technetium, gold and copper isotopes are shown in Fig. 3, for both in-beam and off-beam directions. This figure corresponds to the results obtained when 20 MeV electrons were converted to bremsstrahlung using the apparatus shown in Fig. l(a). The yields of the (y, n) reaction products ly6Au and it4”In dominate those of the (n, y) reaction products 19*Au and “6mIn, by l-2 orders of magnitude for the in-beam direction. In contrast, one can see a reverse tendency for the off-beam direction: the (n, y) products are more abundant than the (p, n) products, reflecting the influence of neutron generation in the platinum converter. The (n, y) products were synthesized in almost the same amounts in both directions, at short distances. This indicates a nearly isotropic distribution of photoneutrons generated from the converter. This observation is consistent with earlier experimental results (Masumoto et al., 1978). 99mTc and l’SmIn in the beam course were produced predominantly by the (y, y’) reaction, as indicated by their high in-beam/off-beam activity ratio. The contribution of the (n, n’) reaction becomes considerable in the perpendicular direction. Its magnitude will be estimated below. We note that the ‘i5”In activity is slightly larger than that of 9ymTc in the in-beam course, but the two activities are almost the same in the off-beam course. Similar results were obtained with 15 MeV electrons. In order to check the fast neutron-induced reactions in the present configuration, the 65Cu(n, p)65Ni reaction with a threshold of 1.38 MeV was also studied. The reaction product 65Ni was chemically
Cross section of the Tc(y, y’)%“Tc
reaction
1 100
I
1000 Channel
500
0 Fig. 2. Typical
-._._._---.. -._--.--- . ..__ .,_._. ...__..._. . __- _.._._
I
y-spectrum
recorded
16 h after irradiation
separated from copper by using Dowex 1 X 8 at 8 N HCl. 65Ni was detected at 50 MeV, but it was under the detection limit at 15 MeV irradiations. Since the lack of “Ni produced in the ‘*Ni(y, n) reaction indicates that the copper samples were free of Ni contamination, no contribution to the 65Ni activity from the @Ni(n, y) reaction is expected. This shows that
in-course
%u
1
I
2000
1500 of Tc samples
with 30 MeV bremsstrahlung.
the harder neutron components which may favor distribution in the forward direction, are very poor at 15 MeV irradiations. A rough estimate of the neutron flux was possible at thermal and higher energies using reported cross sections a(therma1) for “-?n(n, y)“‘?n or 19’Au(n, y)19*Au, G (slow, reactor spectrum) for “SIn(n, n’) “smIn and a(14 MeV) for @Cu(n, p)65Ni. The flux thus estimated could only explain 1 or 2% produced in the in-beam of the 99mTc radioactivity direction, even if the 99Tc(n, n’)99mTc reaction cross section (2.4 x 10~25cm2) for the reactor neutron energy spectrum is adopted (Ikeda et al., 1988).
‘%I” Discussion
%
L->10
lo'o
20 Distmx
30
40
I mm
Fig. 3. The activities produced vs the distance from top of the stacked targets. The maximum energy bremsstrahlung is 20 MeV.
the of
We estimated the (n, n’) contribution to isomer production more exactly by assuming that neutron flux is nearly isotropic in all directions. This was also proved experimentally, as shown in Fig. 3, where “6mIn and 19*Au yields are identical in both courses. It follows that 99mTcactivities A (99mTc),.“, from (n, n’) reaction are also identical; consequently the observed 99mTc activities in the two courses differ from each other by the (y, y’) contributions. On the other hand, by making a second assumption that bremsstrahlung energy spectra are identical in all directions, the activity ratios A(99mTc),7. to A(‘96A~)r,n come to be identical in the two courses. By combining these results, the (y, y’) and (n, n’) contributions can be separated. The same method is applied to “5mIn. The results at 15 and 20 MeV irradiations are listed in Table 1. Under our present experimental conditions, the shape of the excitation function of the 99Tc(y, Y’)~~~Tc reaction cannot be determined, because its main part is considered to lie below 15 MeV. However, an integral cross section can be estimated by comparing
TSUTOM~ SEKINE et al
152 Table I. Estnnated
Produced nuclide “irn,”
Y9mTC
contribution of the (y, y’) and (n, n’) processes to the isomer “reduction
Incident electron energy (MeV)
In-beam
_._~_
-~p/. 7’)
(“, n’)
Integralcross (x 10~17crn~
Off- beam
(;,, i’ ’ )
(n,n’)
IS 20
98 7% 98.3
1.3% 17
25% 20
15% 80
I5 20
98.2 91.5
I.8 2.5
20 I5
80 85
the production rate of q9mTc with that of “““In produced by the (v, ?;I) reaction. The excitation function of the “%(~, y’)“SmIn reaction was studied by many authors (Goldemberg and Katz, 1953; Bogdankevich et al., 1956; Burkhardt et al., 1955; Huizenga and Vanderbosch, 1962; Kruger et al., 1965; Varhue and Williamson, 1986). A broad resonance peak was found around 9 MeV. An integral cross section cr,“, is defined as
o,nt= When the differential uniformly distributed activity will be:
s a(E)
dE.
(3)
flux of incident photonsf(E) is between 0 and E, the produced
A IN = o,,l f(E). However, f(E) is not uniform duce an equivalent flux Feq:
(4)
in general.
We intro-
A IN = g,,,Feq. When a,(E)
a reference is adopted,
Feq=
reaction
J
with a cross
s
section
= _KO
oo(E) dE
Table 3. Integral cross section of the “SI”(7. ;“)“““‘In reaction UD to I2 MeV
of
(6)
‘=“”
where R, is the radioactivity production rate of the reference reaction. That is, Feq is a weighted mean of the flux with a weighting factor n,(E). We calculated go,., from the most reasonable fit of the “%r~~. y’) ‘15”In cross section, which satisfies the existing cross section data (Bogdankevich et a/., 1956; Burkhardt et al., 1955; Goldemberg and Katz, 1953; Varhue and Williamson, 1986). Then R, was calculated from the fitted cross section curve multiplied by the actual f(E) determined as described above. In
Goldemberg and Katz (1953) Burkhardt CI al. (1955) Bogdankevich rl al. (1956) Varhue and Williamson (I 986) This work
section MeV)
9.5 3.5 6.4 7.2 9.3
this way, we obtained the integral cross section cln, of the “51n(y, ;“)“5mIn reaction from our experimental data as R R 0 I”,= - oo,,, = Ro Feq where R is the observed radioactivity production rate. We note that all the values were obtained using the same o,,,~ value in the 4-12 MeV energy range. The values at 15, 20 and 50 MeV are listed in Table 2. They are consistent with a constant integral cross section value above I5 MeV. The average value is (9.3 f 0.4) x IO-” cm’ MeV. This value is compared with those obtained by other investigators in Table 3. The agreement is reasonable especially in the cases of Goldemberg and Katz (1953) and of Varhue and Williamson (1986). The data obtained by Bogdankevich et al. (1956) are smaller than ours, and those obtained by Burkhardt are too small if our value finally contains about 20% error. Although the 99Tc(y, Y’)~‘“Tc excitation function is not known yet, there are reasons. such as nearly equal atomic numbers and first neutron separation energies, to suppose it to be quite similar to that of the “51~(y, y’)lj5m In reaction. Therefore, we can apply the same procedure as in the case of ‘151n to determine the integral cross section of 99Tc. The results for ‘151n are quite different from those obtained by Bogdankevich et al. (1956) in the higher energy region. According to their results, the cross section exhibits a steep increase above 15 MeV, as
701 0 1151~ Boodankewch
-
Table 2. Integral cross section for “‘I”(7, Y’) “Sml” and “9Tc(y, Y’)~‘~Tc reactions
50 MeV 30 MeV 20 MeV ISMeV Average
9.7 * 0.2 9.2 i 0.2 9.2 f 0.3 9.3 + 0.4
5.5 5.7 6.9 5.5
* * i +
I
I.5 0.4 0.4 0.6
5.8 + 1.0
0
I
10
I
I
20
30
Energy
I
I
40
50
I 60
I MeV
Fig. 4. The integral cross sections for 99mTcand “‘“In productions
against
the maximum
energy
of bremsstrahlung.
Cross
section of the 99Tc(y,Y’)~~“‘Tcreaction
shown in Fig. 4. A possible explanation for the discrepancy is the increasing production of “3mIn from the “‘In(jr, 7-n) reaction. Since the energies of the two isomeric transitions are close to each other, it may not have been possible to distinguish them with a poor resolution NaI detector of Bogdankevich et al. (1956). Our results are not necessarily opposite to the presence of a faint peak of the excitation function in the higher energy region, but it should be small even if it exists. We should emphasize that the data measured before the advent of semiconductor detectors were inaccurate and sometimes erroneous.
Conclusion (1) The integral cross section of the 99Tc(y, Y’)~~“Tc reaction was determined for the first time in the 15-50 MeV energy range. (2) The integral cross section of the 99Tc(y, y’)99mTc reaction was nearly constant in the energy range examined, indicating that the second peak of nuclear excitation is small if it exists at all. This experimental observation could apply to the case of the “5mIn reaction. “‘In(y, 7’) Acknowledgements-The authors thank Professor M. Yagi and Dr K. Masumoto for their help in LINAC irradiations. They also thank Professor Shoda for his careful reading of this paper. This work was partly supported by the Cooperative Research Project of the Hungarian Academy of Sciences and the Japan Society for Promotion of Science.
References Bikit I., Slivka J., Anicin I. V., Marinkov L., Rudic A. and Hamilton W. D. (1987) Photoactivation of “‘Cd” without a “nonresonant” contribution. Phys. Rev. C. 35, 1943. Bogdankevich 0. V., Lazareva L. E. and Nikolaev F. A. (1956) Inelastic scattering of photons by indium-115 nuclei. J. Exp. Theor. Phys. (USSR) 31, 405. Bogdankevich 0. V., Lazareva L. E. and Moiseev A. M. (1961) Inelastic scattering of photons on Rh”’ nuclei. J. E.up. Theor. Phys. (USSR) 12, 853. Burkhardt J. L., Winhold E. J. and Dupree T. H. (1955) Photoexcitation of the isomeric state of indium-115’. Phq’“. Rec. 100, 199. Collins C. B.. Anderson J. A., Paiss Y., Eberhard C. D., Peterson R. J. and Hodge W. L. (1988) Activation of “51n’” by single pulses of intense bremsstrahlung. Phys. Rev. C 38, 1852. Dietrich S. S. and Berman B. L. (1988) Atlas of photoneutron cross sections obtained with monoenergetic photons. At. Data Nucl. Data Tables 38, 199. Fultz S. C., Bramblett R. L.. Caldwell J. T. and Kerr N. A. (1962) Photoneutron cross-section measurements on gold
using nearly monochromatic
153
photons. Phys. Rev. 127, 1273. Fultz S. C., Bramblett R. L., Caldwell J. I. and Henry R. R. (1964) Photoneutron cross sections for natural Cu. Cu6’ and Cu6’. Phys. Rev. B 133, 1149. Goldemberg J. and Katz L. (1953) High energy gamma-gamma cross section of In’15. Phys. Rev. 90, 308. Hashimoto K. (1987) Syntheses and properties of Tc(II1) fi-diketonates. PhD thesis, Tohoku University. Huizenga J. R. and Vanderbosch R. (1962) Photoexcitation of the isomeric states of Cd”’ and In’l5 with monoenergetic gamma rays. Nucl. Phys. 34, 457. Ikeda N., Seki R.. Kamemoto M. and Otsuji M. (1988) A possibility of (n, n’) activation analysis for technetium-99. Radioisotopes 37, 414. Kato T., Masumoto K., Sato N. and Suzuki N. (1976) The yields of photonuclear reactions for multielement photonactivation analysis. J. Radioanal. Chem. 32, 51. KrEmar M., LjubiEiC A., Logan B. A. and Bristovii- M. (1986) Photoactivation of isomeric levels in “‘In and “Sr. Phys. Rev. C 33, 293. Kruger P., Crawford T. M., Goldemberg J. and Barber W. C. (1965) Excitation of isomeric states in Rhlo3 and In”’ by electrons. Nucl. Phys. 62, 584. Leoretre A.. Beil H.. Bereere R.. Carlos P.. Vevssiere A. and Sugawara M. (1971) The giant dipole states-in the A = 90 mass region. Nucl. Phys. A175, 605. Ljubi?iC A., Pisk K. and Logan B. A. (1981) Photoactivation investigation with I’%. Phys. Rev. C 23, 2238. Masumoto K., Kato T. and Suzuki N. (1978) Activation yield curves of photonuclear reactions for multielement photon activation analysis. Nucl. Instrum. Methods 157, 567. Meyer-Schiitzmeister L. and Telegdi V. L. (1956) Inelastic nuclear scattering of photons by Au’~‘. Phys. Rev. 104, 185. Schiff L. I. (1951) Energy-angle distribution of thin target bremsstrahlung. Phys. Rev. 83, 252. Sekine T., Yoshihara K., Masumoto K. and Yagi M. (1982) Recoil implantation using a low temperature irradiation facility at LINAC. Res. Rep. Lab. Nucl. Sci. Tohoku Univ. 15, 219. Sekine T., Yoshihara K., Ntmeth Zs., Lakosi L. and Veres br. (1989) A new detection method of %Tc by nuclear excitation. Radioanal. Nucl. Chem. 130, 269. Silva E. and Goldemberg J. (1958) Inelastic scattering of photons in YES. Phys. Rev. 110, 1102. V&hue W. J. and Williamson T. G. (1986) Inelastic photon scattering bv “‘In. Ao~l. Radiat. Isot. 37. 155. Veres A. (lb80) Gamma’activation of nuclear isomers and its applications. At. Energy Rev. 18, ?71. Yoshihara K., Ikeda N. and Veres A. (1978) Nuclear chemistry of indium and related compounds. J. Radioanal. Chem. 45, 293. Yoshihara K., Ntmeth Zs., Lakosi L., Pavlicsek I. and Veres A. (1986) Major mechanism of photoactivation for the “iIn(;j, )?‘)“‘In” process by “Co ;I-ray irradiation. Phys. Rec. C 33, 728.
0883-2889/91 $3.00 + 0.00 Copyright ep 1991 Pergamon Press plc
Integral Cross Section of the 99Tc(y, Y’)~~“Tc Reaction in the 15-50 MeV Energy Region TSUTOMU
SEKINE,’ KENJI YOSHIHARA,’ ZSOLT NfiMETH* and ARPAD
LbSZLd
LAKOSI,’
VERES2
‘Department of Chemistry, Faculty of Science, Tohoku University, Aoba-izu Sendai 980, Japan and *Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.0. Box 77, Hungary (Received
1990; in revised,form
5 March
15 June
1990)
Photoexcitation of radioactive 99Tc was studied by irradiation with 15-50 MeV bremsstrahiung. The integral cross sections of the 99TcQj, ‘J’)~~“Tc reaction and the reference “‘In(~, ;“)“‘“In reaction were found to be nearly constant as (5.8 + 1.0) x lo-” cm* MeV and (9.3 f 0.4) x IO-” cm’ MeV, respectively, in the given energy range. The large second increase of the cross section described in earlier reports was observed neither for ‘151n nor for 99Tc.
strahlung of a linear electron accelerator. A second peak in the higher energy region (> 15 MeV) was also sought. Analytical applications of the 99Tc(y, Y’)~~“Tc reaction have been studied (Sekine et al., 1989) in the framework of a research project established between Japan and Hungary.
Introduction The
long-lived
important nuclide been
9YT~ nuclide
fission
product.
accompanying cited
as a waste
nuclear
(t,:,
2.13
x 10’ y) is an
Accumulation reactor
management
of
this
operation problem
has to
be
solved in the future. Also, environmental pollution due to nuclear explosion-produced 9yTc has been detected. In spite of the importance of this nuclide, however, its nuclear data are not always satisfactory. This study provides some new information on nuclear excitation of y9Tc. The (y, 7’) reaction is a useful tool for studying nuclear excitation phenomena. Such experiments have been performed using accelerators and radioisotopes (Yoshihara et al., 1978; Veres, 1980). In the low-energy region, the process is described in terms of transitions between individual nuclear levels. The excitation mechanism is characterized by resonance absorption of photons. Non-resonant activation processes proposed by LjubiEic ef al. (198 1) and KrEmar et al. (1986) were not justified (Yoshihara er al., 1986; Bikit et al., 1987; Collins er a/., 1988). On the other hand, some experimental results reported on the cross section of the (y, y’) reaction in the giant resonance region exhibited a broad peak around the (y, n) threshold, and then either a second peak or a gradual rise at higher energies (Silva and Goldemberg, 1958; Bogdankevich et al., 1956, 1961; Meyer-Schiitzmeister and Telegdi, 1956). The second increase beyond 15 MeV in the excitation function of (;,. y’) reaction of “51n was found to be much more prominent than the first one (Bogdankevich et al.,
Experimental 99Tc, in the chemical form of ammonium pertechnetate, was purchased from Amersham International, and was deposited on filter paper or silica powders as supporting materials for irradiation. In some irradiations, 99Tc in the chemical form of tetrabutylammoniumtetrachlorooxotechnetate(V) was used. The amounts of technetium were determined by a liquid scintillation counter which was calibrated for spectrophotometrically determined quantities of technetium (Hashimoto, 1987). Metallic copper and gold foils were used as flux monitors just in front of or behind the samples. Indium metal or indium oxide was used in reference (y, y’) experiments performed in the same manner as in the technetium experiments. All the target materials were of high purity (> 99.5%). Electrons (15-50 MeV) from the linear electron accelerator at the Laboratory of Nuclear Science, were converted to bremsTohoku University, strahlung by means of a platinum plate (thickness: 1 or 2 mm). The targets were irradiated at various distances in both in-beam and off-beam directions [Fig. I(a)]. In some cases the targets were irradiated in another configuration [Fig. I(b)] and were cooled with nitrogen gas flow chilled with liquid nitrogen (Sekine et al., 1982). In these cases the unconverted
1956).
In this paper we report the integral cross section of the 99Tc(y, y’)y9mTc reaction induced by brems4RI
42 2-D
149
150
Tsu~ow~
SEKINE
et
al
We preferred the use of copper and gold flux monitors, using their reported (y, n) excitation functions for calculations. Adopting the cross section data of Fultz et al. (1962) for gold, we obtained consistent results with the radioactivity production of copper (Fultz et al., 1964) within 5%. Experimental radioactivity of yttrium (““Y) produced by the *‘Y(y, tQE8Y reaction could be explained within an error of 10% when calculation was carried out using the excitation function of yttrium (Lepretre et al., 1971) on the basis of our copper-gold monitor method. Almost all the radioactivity production data in this paper were reproduced systematically using the flux distribution based on Schiff’s formula, and the excitation functions, carefully inspected, as stated above.
beam duct
Results
(b) Fig. 1. The arrangements used in our experiments (a) Stacked targets in a quartz tube were placed along the beam course (in-beam) and perpendicular to it (off-beam). A 2mm thick Pt converter was used under water cooling. (b) Arrangement with a special cooling apparatus. The nitrogen gas evaporated in the 50-L reservoir was cooled again and was blown to the targets during the irradiation. The 1 mm thick platinum converter was placed just in front of the sweep-magnet.
electrons were eliminated by a sweep magnet placed between the platinum converter and the targets. The induced activities were measured repeatedly with pure Ge or Ge(Li) detectors connected to multichannel pulse-height analyzers. The counting efficiencies of the detectors were determined using standard “*Eu and ‘**Ta sources.
Determination The saturated formula
of the Bremsstrahlung activity produced
P A = N a(E)f(E) J
Flux
(A) is given by the
dE
where N is the number of atoms, u(E) is the reaction cross section, andf(E) is the differential flux of the incident photons defined by f(E)
dE = F
(2)
s where F is the total flux. Actually, f(E) was calculated by the formula of Schiff (1951). If the radioactivity is known in the flux monitors, the height of the function f(E) can be normalized so that la(E)f(E) dE satisfies equation (1). A compilation of the photon-induced reaction cross sections is available for this purpose (Dietrich and Berman, 1988).
A typical y-spectrum obtained by 30 MeV irradiation of a Tc sample is shown in Fig. 2. Activities of y9mTc (l,,* 6.02 h) and 96T~ (4.3 d) were detected. The latter nuclide was not observable when the irradiation energy fell below 20 MeV. The production of 96T~ is attributed to the (7, 3n) reaction. The yield of this reaction fits the general tendency for isotope production rates against the atomic number of target material reported by Kato et al. (1976). Typical examples of production rates of indium, technetium, gold and copper isotopes are shown in Fig. 3, for both in-beam and off-beam directions. This figure corresponds to the results obtained when 20 MeV electrons were converted to bremsstrahlung using the apparatus shown in Fig. l(a). The yields of the (y, n) reaction products ly6Au and it4”In dominate those of the (n, y) reaction products 19*Au and “6mIn, by l-2 orders of magnitude for the in-beam direction. In contrast, one can see a reverse tendency for the off-beam direction: the (n, y) products are more abundant than the (p, n) products, reflecting the influence of neutron generation in the platinum converter. The (n, y) products were synthesized in almost the same amounts in both directions, at short distances. This indicates a nearly isotropic distribution of photoneutrons generated from the converter. This observation is consistent with earlier experimental results (Masumoto et al., 1978). 99mTc and l’SmIn in the beam course were produced predominantly by the (y, y’) reaction, as indicated by their high in-beam/off-beam activity ratio. The contribution of the (n, n’) reaction becomes considerable in the perpendicular direction. Its magnitude will be estimated below. We note that the ‘i5”In activity is slightly larger than that of 9ymTc in the in-beam course, but the two activities are almost the same in the off-beam course. Similar results were obtained with 15 MeV electrons. In order to check the fast neutron-induced reactions in the present configuration, the 65Cu(n, p)65Ni reaction with a threshold of 1.38 MeV was also studied. The reaction product 65Ni was chemically
Cross section of the Tc(y, y’)%“Tc
reaction
1 100
I
1000 Channel
500
0 Fig. 2. Typical
-._._._---.. -._--.--- . ..__ .,_._. ...__..._. . __- _.._._
I
y-spectrum
recorded
16 h after irradiation
separated from copper by using Dowex 1 X 8 at 8 N HCl. 65Ni was detected at 50 MeV, but it was under the detection limit at 15 MeV irradiations. Since the lack of “Ni produced in the ‘*Ni(y, n) reaction indicates that the copper samples were free of Ni contamination, no contribution to the 65Ni activity from the @Ni(n, y) reaction is expected. This shows that
in-course
%u
1
I
2000
1500 of Tc samples
with 30 MeV bremsstrahlung.
the harder neutron components which may favor distribution in the forward direction, are very poor at 15 MeV irradiations. A rough estimate of the neutron flux was possible at thermal and higher energies using reported cross sections a(therma1) for “-?n(n, y)“‘?n or 19’Au(n, y)19*Au, G (slow, reactor spectrum) for “SIn(n, n’) “smIn and a(14 MeV) for @Cu(n, p)65Ni. The flux thus estimated could only explain 1 or 2% produced in the in-beam of the 99mTc radioactivity direction, even if the 99Tc(n, n’)99mTc reaction cross section (2.4 x 10~25cm2) for the reactor neutron energy spectrum is adopted (Ikeda et al., 1988).
‘%I” Discussion
%
L->10
lo'o
20 Distmx
30
40
I mm
Fig. 3. The activities produced vs the distance from top of the stacked targets. The maximum energy bremsstrahlung is 20 MeV.
the of
We estimated the (n, n’) contribution to isomer production more exactly by assuming that neutron flux is nearly isotropic in all directions. This was also proved experimentally, as shown in Fig. 3, where “6mIn and 19*Au yields are identical in both courses. It follows that 99mTcactivities A (99mTc),.“, from (n, n’) reaction are also identical; consequently the observed 99mTc activities in the two courses differ from each other by the (y, y’) contributions. On the other hand, by making a second assumption that bremsstrahlung energy spectra are identical in all directions, the activity ratios A(99mTc),7. to A(‘96A~)r,n come to be identical in the two courses. By combining these results, the (y, y’) and (n, n’) contributions can be separated. The same method is applied to “5mIn. The results at 15 and 20 MeV irradiations are listed in Table 1. Under our present experimental conditions, the shape of the excitation function of the 99Tc(y, Y’)~~~Tc reaction cannot be determined, because its main part is considered to lie below 15 MeV. However, an integral cross section can be estimated by comparing
TSUTOM~ SEKINE et al
152 Table I. Estnnated
Produced nuclide “irn,”
Y9mTC
contribution of the (y, y’) and (n, n’) processes to the isomer “reduction
Incident electron energy (MeV)
In-beam
_._~_
-~p/. 7’)
(“, n’)
Integralcross (x 10~17crn~
Off- beam
(;,, i’ ’ )
(n,n’)
IS 20
98 7% 98.3
1.3% 17
25% 20
15% 80
I5 20
98.2 91.5
I.8 2.5
20 I5
80 85
the production rate of q9mTc with that of “““In produced by the (v, ?;I) reaction. The excitation function of the “%(~, y’)“SmIn reaction was studied by many authors (Goldemberg and Katz, 1953; Bogdankevich et al., 1956; Burkhardt et al., 1955; Huizenga and Vanderbosch, 1962; Kruger et al., 1965; Varhue and Williamson, 1986). A broad resonance peak was found around 9 MeV. An integral cross section cr,“, is defined as
o,nt= When the differential uniformly distributed activity will be:
s a(E)
dE.
(3)
flux of incident photonsf(E) is between 0 and E, the produced
A IN = o,,l f(E). However, f(E) is not uniform duce an equivalent flux Feq:
(4)
in general.
We intro-
A IN = g,,,Feq. When a,(E)
a reference is adopted,
Feq=
reaction
J
with a cross
s
section
= _KO
oo(E) dE
Table 3. Integral cross section of the “SI”(7. ;“)“““‘In reaction UD to I2 MeV
of
(6)
‘=“”
where R, is the radioactivity production rate of the reference reaction. That is, Feq is a weighted mean of the flux with a weighting factor n,(E). We calculated go,., from the most reasonable fit of the “%r~~. y’) ‘15”In cross section, which satisfies the existing cross section data (Bogdankevich et a/., 1956; Burkhardt et al., 1955; Goldemberg and Katz, 1953; Varhue and Williamson, 1986). Then R, was calculated from the fitted cross section curve multiplied by the actual f(E) determined as described above. In
Goldemberg and Katz (1953) Burkhardt CI al. (1955) Bogdankevich rl al. (1956) Varhue and Williamson (I 986) This work
section MeV)
9.5 3.5 6.4 7.2 9.3
this way, we obtained the integral cross section cln, of the “51n(y, ;“)“5mIn reaction from our experimental data as R R 0 I”,= - oo,,, = Ro Feq where R is the observed radioactivity production rate. We note that all the values were obtained using the same o,,,~ value in the 4-12 MeV energy range. The values at 15, 20 and 50 MeV are listed in Table 2. They are consistent with a constant integral cross section value above I5 MeV. The average value is (9.3 f 0.4) x IO-” cm’ MeV. This value is compared with those obtained by other investigators in Table 3. The agreement is reasonable especially in the cases of Goldemberg and Katz (1953) and of Varhue and Williamson (1986). The data obtained by Bogdankevich et al. (1956) are smaller than ours, and those obtained by Burkhardt are too small if our value finally contains about 20% error. Although the 99Tc(y, Y’)~‘“Tc excitation function is not known yet, there are reasons. such as nearly equal atomic numbers and first neutron separation energies, to suppose it to be quite similar to that of the “51~(y, y’)lj5m In reaction. Therefore, we can apply the same procedure as in the case of ‘151n to determine the integral cross section of 99Tc. The results for ‘151n are quite different from those obtained by Bogdankevich et al. (1956) in the higher energy region. According to their results, the cross section exhibits a steep increase above 15 MeV, as
701 0 1151~ Boodankewch
-
Table 2. Integral cross section for “‘I”(7, Y’) “Sml” and “9Tc(y, Y’)~‘~Tc reactions
50 MeV 30 MeV 20 MeV ISMeV Average
9.7 * 0.2 9.2 i 0.2 9.2 f 0.3 9.3 + 0.4
5.5 5.7 6.9 5.5
* * i +
I
I.5 0.4 0.4 0.6
5.8 + 1.0
0
I
10
I
I
20
30
Energy
I
I
40
50
I 60
I MeV
Fig. 4. The integral cross sections for 99mTcand “‘“In productions
against
the maximum
energy
of bremsstrahlung.
Cross
section of the 99Tc(y,Y’)~~“‘Tcreaction
shown in Fig. 4. A possible explanation for the discrepancy is the increasing production of “3mIn from the “‘In(jr, 7-n) reaction. Since the energies of the two isomeric transitions are close to each other, it may not have been possible to distinguish them with a poor resolution NaI detector of Bogdankevich et al. (1956). Our results are not necessarily opposite to the presence of a faint peak of the excitation function in the higher energy region, but it should be small even if it exists. We should emphasize that the data measured before the advent of semiconductor detectors were inaccurate and sometimes erroneous.
Conclusion (1) The integral cross section of the 99Tc(y, Y’)~~“Tc reaction was determined for the first time in the 15-50 MeV energy range. (2) The integral cross section of the 99Tc(y, y’)99mTc reaction was nearly constant in the energy range examined, indicating that the second peak of nuclear excitation is small if it exists at all. This experimental observation could apply to the case of the “5mIn reaction. “‘In(y, 7’) Acknowledgements-The authors thank Professor M. Yagi and Dr K. Masumoto for their help in LINAC irradiations. They also thank Professor Shoda for his careful reading of this paper. This work was partly supported by the Cooperative Research Project of the Hungarian Academy of Sciences and the Japan Society for Promotion of Science.
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