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Isomeric cross-section ratios for some nuclides using 14.5 MeV neutrons
Applied Radiation and Isotopes PERGAMON
Applied Radiation and Isotopes 51 (1999) 81±84
Isomeric cross-section ratios for some nuclides using 14.5 MeV neutrons F.I. Habbani *, I.A. Ahmed Department of Physics, Faculty of Science, University of Khartoum, P.O. Box 321, Khartoum, Sudan Received 15 May 1998; accepted 4 August 1998
Abstract The activation technique has been used for measurements of isomeric cross-section ratios for the following neutron-induced reactions using 14.5 MeV neutrons: 81 Br(n, 2n)80m,g Br, 113 In(n, 2n)112m,g In, 197 Au(n, 2n)196m,g Au, 90 Zr(n, 2n)89m,g Zr, 198 Pt(n, 2n)197m,g Pt, 92 Mo(n, 2n)91m,g Mo and 82 Se(n, 2n)81m,g Se. The results obtained have been discussed and compared with some corresponding values found in the literature. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Neutrons; Isomeric cross-sections; Radionuclides
1. Introduction Isomeric cross-section ratios are useful for various studies related to nuclear reactions and nuclear structure, such as transfer of angular momentum, spindependence of nuclear level density, re®nements in gamma transition theories and testing of theoretical nuclear models. Determinations of isomeric cross-section ratios around 14 MeV neutron energy have been carried out by various investigators (Mangal and Gill, 1963; Grimeland et al., 1965; Mangal and Khurana, 1965; Winiwarter, 1970; Rao et al., 1971; Qaim, 1972; Kao and Alford, 1975). However, some discrepancies are observed among the literature values which might be attributed to variations in experimental methods and/or the nuclear constants used. In the present work measurements have been carried out for some isomeric cross-section ratios around 14 MeV neutrons using two methods: one based on standard activation crosssection measurement and the other one on application of least squares ®tting to the time behaviour of gamma-ray activity resulting from the ground state
* Corresponding author. Tel.: +249-11-780-539; fax: +24911-780-539; e-mail: [email protected].
decay of the isomeric pair. The results obtained by the latter method would be independent of several sensitive factors, such as photo-peak eciency, branching ratio, sample mass and self-absorption. However, there are some limitations on the applicability of the method. Seven product nuclides produced via the (n, 2n) reaction at 14.5 MeV neutron energy, with genetically-related isomeric states, were selected for this study: 80m,g Br, 112m,g In, 196m,g Au, 89m,g Zr, 197m,g Pt, 91m,g Mo and 81m,g Se. The results obtained are discussed and compared with some corresponding literature values.
2. Experimental methods Two methods have been used in this study: a standard method and a relative method. 2.1. Standard method The standard activation method has been used for the determination of the separate cross-section of the ground state sg and the metastable state sm for the particular target nuclide, using the nuclear reaction 27 Al(n, a)24 Na of the well known cross-section as standard and ¯ux monitor. For this method it is required
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 5 6 - 0
82
F.I. Habbani, I.A. Ahmed / Applied Radiation and Isotopes 51 (1999) 81±84
to measure the gamma spectrometer photo-peak eciency as a function of energy in the geometry of the experiment, as well as to use various nuclear constants such as half-lives, branching ratios, etc. The isomeric cross-section ratio sm/sg is then found.
Z Z0 exp ÿlg t
The total disintegration rate of Z given by R = lgZ may then be written as: R gD mC
C
When fast neutrons strike a target nuclide X, the product nucleus may be formed as the nuclide Z in the ground state which decays with the decay constant lg or as the nuclide Zm in the metastable state which decays to the ground state with the decay constant lm. The reaction X(n, 2n)Z will then have the cross-section sg and the reaction X(n, 2n)Zm the cross-section sm. The ground state nuclide Z may be formed in two ways: directly from the target nuclide X or indirectly through the decay of the metastable state nuclide Zm. During an irradiation period ti the number of metastable state nuclides will be: sm NF 1 ÿ exp ÿlm ti lm
lg exp ÿlm t1 ÿ exp ÿlm ti lg ÿ lm lg 1ÿ 1 ÿ exp ÿlg ti exp ÿlg t lg ÿ lm
By observing the activity of the sample as a function of time and applying the method of least squares ®tting Eq. (4) the ratio of m to g can be found, which gives the isomeric cross-section ratio sm/sg = m/gb. 3. Experimental measurements The samples of In, Au, Zr, Pt and Mo were pure metal foils of thickness 0.15 mm and diameter 25 mm. Se was in pure oxide form compressed into a pellet of mass 1 g and diameter 25 mm. Br was in pure liquid form sealed in a thin glass tube of size 2.5 ml. The nuclear reactions and associated nuclear constants used are given in Table 1. The samples were irradiated using the neutron generator based on the T(d, n)4 He reaction, operated at a high voltage of 120 KV and a beam current of 500 mA. The neutron energy in the forward direction was determined to be 14.5 2 0.2 MeV (Shaddad, 1995). The samples were sandwiched between Al foils, which acted as standard and ¯ux monitors, and placed at the back of the TiT target for irradiation. The irradiation times ranged between 10 min for Se and Mo, 15 min for Pt
1
where N is the number of target nuclei and F is the neutron ¯ux. The number of ground state nuclides produced during the same irradiation period will be: Z0
4
where g = sgNF, m = smNFb, D = [1 ÿ exp(ÿlgti)] exp(ÿlgt) and
2.2. Relative method
Zm 0
lm bZ m 0 exp ÿlm t ÿ exp ÿlg t 3 lg ÿ lm
sg NF sm NFb 1 ÿ exp ÿlg ti 1 ÿ exp ÿlg ti lg lg sm NFb exp ÿlg ti ÿ exp ÿlm ti 2 lg ÿ lm
where b is the fraction of the disintegrations of the metastable state that produces ground state nuclides (branching ratio). The number of ground state nuclides present after the passage of time `t' following irradiation will then be (Csikai et al., 1991):
Table 1 The nuclear reactions and associated nuclear constants (Lederer and Shirley, 1978) Reaction
81
Br(n, 2n)80m,g Br In (n, 2n)112m,g In 197 Au(n, 2n)196m,g Au 90 Zr(n, 2n)89m,g Zr 198 Pt(n, 2n)197m,g Pt 92 Mo(n, 2n)91m,g Mo 82 Se(n, 2n)81m,g Se 27 Al(n, a)24 Na 113
a
Target nuclide abundance (%)
49.4 4.3 100 51.4 7.2 14.8 9.19 100
g = ground state, m = metastable state.
T1/2
Eg (keV)
Branching ratio (%)
ga
ma
g
m
g
m
17.4 m 14.4 m 6.18 d 78.5 h 18.3 h 15.5 m 18.5 m 15.02 h
4.42 h 20.9 m 9.7 h 4.18 m 1.57 h 65 s 57.3 m
± 511 355.8 909.2 191.4 1637 275.9 1368
616.2 155.5 188 1507 279 652.9 103
± 44 8.69 99 3.68 23 ± 100
6.7 12.8 37.4 6 2.3 48.3 9.79
F.I. Habbani, I.A. Ahmed / Applied Radiation and Isotopes 51 (1999) 81±84
83
Table 2 Measured isomeric cross-section ratios Reaction
sm/s g standard
sm/sg relative
Literature values
81
1.9520.15
1.742 0.05
1.63 20.16 (Grimeland et al., 1965), 2.02 20.11 (Kao and Alford, 1975), 1.882 0.19 (Rao et al., 1971) 4.5 20.1 (Csikai, private communication), 4.4 20.4 (Shakir, 1978), 4.162 0.2 (IAEA TR, 1974) 0.07620.041 (IAEA TR, 1987), 0.05920.005 (IAEA TR, 1974), 0.13320.065 (Mangal and Khurana, 1965) 0.12120.025 (IAEA TR, 1974), 0.13020.020 (Shakir, 1978) 0.82 20.26 (IAEA TR, 1987), 0.76 20.48 (Winiwarter, 1970), 0.492 0.21 (Mangal and Khurana, 1965) 0.07820.032 (IAEA TR, 1974), 0.08120.019 (Winiwarter, 1970) 2.53 20.15 (Kao and Alford, 1975), 2.79 20.08 (Hasan et al., 1972), 4.020.8 (Rao and Fink, 1967)
Br(n, 2n)80m,g Br
113
In(n, 2n)112m,g In
4.120.3
4.82 0.1
197
Au(n, 2n)196m,g Au
0.05820.019
0.05720.006
0.12920.05
0.003420.0002
1.0220.37
0.53720.121
0.05420.032 ±
± 3.642 0.01
90
Zr(n, 2n)89m,g Zr
198 92 82
Pt(n, 2n)197m,g Pt
Mo(n, 2n)91m,g Mo Se(n, 2n)81m,g Se
and Au, 20 min for In and 30 min for Br and Zr. The samples were measured using a HPGe gamma spectrometer system. The experimental setup and the method for activation cross-section measurements were described elsewhere (Habbani and Paic, 1988; Shaddad, 1995; Osman and Habbani, 1996). The standard cross-section for the reaction 27 Al(n, a)24 Na used was 116.046 mb (IAEA TR, 1983). 4. Results and discussion The results for the measured isomeric cross-section ratios together with some recent literature values are given in Table 2. The results for Br, In and Au show good agreement for the isomeric cross-section ratios measured by the two methods and corresponding literature values. The results for Zr and Pt show discrepancy between the two methods. In both cases the metastable half-life is much smaller than the ground state one (this also applies to Au and Mo). In such cases the application of the relative method has low reliability in view of the large dierence in the half-lives. This is in contrast to the standard method which is suitable across this group of isomers, since the total and metastable crosssections can easily be determined directly without the need for further curve analysis. Au has the advantage over the other target nuclides in this group of much higher abundance (100%). In the case of Mo the count rates from the ground state were also very low so that the relative method could not be used. In the case of Se the (n, 2n) reaction on 82 Se leads to two isomeric states: the metastable state of T1/2 = 57.3 m and Eg = 103 keV above the ground state with 99% IT and the ground state of T1/2 = 18.5 m decaying by beta to the excited state of 81 Br which decays with a
gamma emission of 275.9 keV to the ground state. The peak at 275.9 keV was used to obtain the isomeric cross-section ratio by the relative method. However, it was not possible to determine the total cross-section in this case because the half-life of the metastable state is longer than that of the ground state and so the standard method could not be used. A similar situation >Tg1/2 and it was exists for Br, but in this case Tm 1/2> possible to solve the problem by appropriate subtraction of the metastable part of the composite decay curve. It should be noted that the use of the ground state activity, if a metastable state is also present, should be handled with care or preferably be avoided whenever a proper substitution is possible. So the two methods can complement each other for the evaluation of the ground state cross-section, incorporating the high reliability of the isomeric cross-section ratio by the relative method with the easy and direct measurement of the metastable state cross-section using the standard method.
Acknowledgements The authors wish to thank the IAEA for technical assistence and support, Professor J. Csikai for his advice and encouragement and the technician Bashir Badawi for running the machine.
References Csikai, J., Buczko, Cs.M., Pepelnik, R., Agrawal, H.M., 1991. Activation cross-sections related to nuclear heating of high T c superconductors. Ann. Nucl. Energy 18 (1), 1. Grimeland, B., Kjellsby, E., Vines, J., 1965. Cross-sections of some reactions induced in nitrogen, phosphorus, copper
84
F.I. Habbani, I.A. Ahmed / Applied Radiation and Isotopes 51 (1999) 81±84
and bromine with neutrons of energy 14.8 MeV. Phys. Rev. 137, 878. Habbani, F., Paic, G., 1988. Measurement of (n, p) and (n, np) cross-sections for some Fe isotopes at 14.7 MeV. Int. J. Appl. Rad. Isotopes 39 (10), 1041. Hasan, S.S., Parasad, R., Seghal, M.L., 1972. The 14.8 MeV neutron cross-sections for enriched isotopes of 82 Se, 92 Mo, 117 Sn, 128 Te and 130 Te. Nucl. Phys. A 181, 101. IAEA TR, 1974. Handbook on Nuclear Activation CrossSections. Tech. Rep. 156. IAEA TR, 1983. Nuclear Data Standards for Nuclear Measurements. Tech. Rep. 227. IAEA TR, 1987. Handbook on Nuclear Activation Data. Tech. Rep. 273. Kao, T.H., Alford, W.L., 1975. Isomeric cross-sections for the (n, 2n) reaction on 82 Se, 81 Br and 45 Sc at 15.1 MeV. Nucl. Phys. A 237, 11. Lederer, C.M., Shirley, V.S., 1978. Table of Isotopes. J. Wiley and Sons. Mangal, S.K., Gill, P.S., 1963. Isomeric cross-section ratios for (n, 2n) reactions at 14 MeV. Nucl. Phys. 49, 510. Mangal, S.K., Khurana, C.S., 1965. Isomeric (n, 2n) crosssection ratios at 14.8 MeV. Nucl. Phys. 69, 158.
Osman, K.T., Habbani, F.I., 1996. Measurement and Study of (n, p) Reaction Cross-Sections for Cr, Ti, Ni, Co, Zr and Mo Isotopes Using 14.7 MeV Neutrons. INDC(SUD)001, NDS, IAEA. Qaim, S.M., 1972. Activation cross-sections, isomeric crosssection ratios and systematics for (n, 2n) reactions at 14±15 MeV. Nucl. Phys. A 185, 614. Rao, P.V., Fink, R.W., 1967. Neutron reaction cross-sections for selenium and iron at 14.4 MeV. Phys. Rev. 154 (4), 1023. Rao, P., Wood, R.E., Palms, J.M., Fink, R.W., 1971. Neutron activation cross-sections for As, Br, Rb and Sr Isotopes at 14.4 MeV. Phys. Rev. C 3, 629. Shaddad, I.A., 1995. (n, p) and (n, a) Reactions cross-sections measurements and systematics around 14 MeV neutron energy. Ph.D. Thesis, University of Khartoum. Shakir, N.S., 1978. Measurement of isomeric cross-section ratios by induced neutron reactions. M. Phil. Thesis, Aston University. Winiwarter, P., 1970. Isomeric cross-section ratios for the reactions 192 Os(n, 2n)191 Os and 198 Pt(n, 2n)197 Pt at 14 MeV. Nucl. Phys. A 158, 77.
Applied Radiation and Isotopes 51 (1999) 81±84
Isomeric cross-section ratios for some nuclides using 14.5 MeV neutrons F.I. Habbani *, I.A. Ahmed Department of Physics, Faculty of Science, University of Khartoum, P.O. Box 321, Khartoum, Sudan Received 15 May 1998; accepted 4 August 1998
Abstract The activation technique has been used for measurements of isomeric cross-section ratios for the following neutron-induced reactions using 14.5 MeV neutrons: 81 Br(n, 2n)80m,g Br, 113 In(n, 2n)112m,g In, 197 Au(n, 2n)196m,g Au, 90 Zr(n, 2n)89m,g Zr, 198 Pt(n, 2n)197m,g Pt, 92 Mo(n, 2n)91m,g Mo and 82 Se(n, 2n)81m,g Se. The results obtained have been discussed and compared with some corresponding values found in the literature. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Neutrons; Isomeric cross-sections; Radionuclides
1. Introduction Isomeric cross-section ratios are useful for various studies related to nuclear reactions and nuclear structure, such as transfer of angular momentum, spindependence of nuclear level density, re®nements in gamma transition theories and testing of theoretical nuclear models. Determinations of isomeric cross-section ratios around 14 MeV neutron energy have been carried out by various investigators (Mangal and Gill, 1963; Grimeland et al., 1965; Mangal and Khurana, 1965; Winiwarter, 1970; Rao et al., 1971; Qaim, 1972; Kao and Alford, 1975). However, some discrepancies are observed among the literature values which might be attributed to variations in experimental methods and/or the nuclear constants used. In the present work measurements have been carried out for some isomeric cross-section ratios around 14 MeV neutrons using two methods: one based on standard activation crosssection measurement and the other one on application of least squares ®tting to the time behaviour of gamma-ray activity resulting from the ground state
* Corresponding author. Tel.: +249-11-780-539; fax: +24911-780-539; e-mail: [email protected].
decay of the isomeric pair. The results obtained by the latter method would be independent of several sensitive factors, such as photo-peak eciency, branching ratio, sample mass and self-absorption. However, there are some limitations on the applicability of the method. Seven product nuclides produced via the (n, 2n) reaction at 14.5 MeV neutron energy, with genetically-related isomeric states, were selected for this study: 80m,g Br, 112m,g In, 196m,g Au, 89m,g Zr, 197m,g Pt, 91m,g Mo and 81m,g Se. The results obtained are discussed and compared with some corresponding literature values.
2. Experimental methods Two methods have been used in this study: a standard method and a relative method. 2.1. Standard method The standard activation method has been used for the determination of the separate cross-section of the ground state sg and the metastable state sm for the particular target nuclide, using the nuclear reaction 27 Al(n, a)24 Na of the well known cross-section as standard and ¯ux monitor. For this method it is required
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 5 6 - 0
82
F.I. Habbani, I.A. Ahmed / Applied Radiation and Isotopes 51 (1999) 81±84
to measure the gamma spectrometer photo-peak eciency as a function of energy in the geometry of the experiment, as well as to use various nuclear constants such as half-lives, branching ratios, etc. The isomeric cross-section ratio sm/sg is then found.
Z Z0 exp ÿlg t
The total disintegration rate of Z given by R = lgZ may then be written as: R gD mC
C
When fast neutrons strike a target nuclide X, the product nucleus may be formed as the nuclide Z in the ground state which decays with the decay constant lg or as the nuclide Zm in the metastable state which decays to the ground state with the decay constant lm. The reaction X(n, 2n)Z will then have the cross-section sg and the reaction X(n, 2n)Zm the cross-section sm. The ground state nuclide Z may be formed in two ways: directly from the target nuclide X or indirectly through the decay of the metastable state nuclide Zm. During an irradiation period ti the number of metastable state nuclides will be: sm NF 1 ÿ exp ÿlm ti lm
lg exp ÿlm t1 ÿ exp ÿlm ti lg ÿ lm lg 1ÿ 1 ÿ exp ÿlg ti exp ÿlg t lg ÿ lm
By observing the activity of the sample as a function of time and applying the method of least squares ®tting Eq. (4) the ratio of m to g can be found, which gives the isomeric cross-section ratio sm/sg = m/gb. 3. Experimental measurements The samples of In, Au, Zr, Pt and Mo were pure metal foils of thickness 0.15 mm and diameter 25 mm. Se was in pure oxide form compressed into a pellet of mass 1 g and diameter 25 mm. Br was in pure liquid form sealed in a thin glass tube of size 2.5 ml. The nuclear reactions and associated nuclear constants used are given in Table 1. The samples were irradiated using the neutron generator based on the T(d, n)4 He reaction, operated at a high voltage of 120 KV and a beam current of 500 mA. The neutron energy in the forward direction was determined to be 14.5 2 0.2 MeV (Shaddad, 1995). The samples were sandwiched between Al foils, which acted as standard and ¯ux monitors, and placed at the back of the TiT target for irradiation. The irradiation times ranged between 10 min for Se and Mo, 15 min for Pt
1
where N is the number of target nuclei and F is the neutron ¯ux. The number of ground state nuclides produced during the same irradiation period will be: Z0
4
where g = sgNF, m = smNFb, D = [1 ÿ exp(ÿlgti)] exp(ÿlgt) and
2.2. Relative method
Zm 0
lm bZ m 0 exp ÿlm t ÿ exp ÿlg t 3 lg ÿ lm
sg NF sm NFb 1 ÿ exp ÿlg ti 1 ÿ exp ÿlg ti lg lg sm NFb exp ÿlg ti ÿ exp ÿlm ti 2 lg ÿ lm
where b is the fraction of the disintegrations of the metastable state that produces ground state nuclides (branching ratio). The number of ground state nuclides present after the passage of time `t' following irradiation will then be (Csikai et al., 1991):
Table 1 The nuclear reactions and associated nuclear constants (Lederer and Shirley, 1978) Reaction
81
Br(n, 2n)80m,g Br In (n, 2n)112m,g In 197 Au(n, 2n)196m,g Au 90 Zr(n, 2n)89m,g Zr 198 Pt(n, 2n)197m,g Pt 92 Mo(n, 2n)91m,g Mo 82 Se(n, 2n)81m,g Se 27 Al(n, a)24 Na 113
a
Target nuclide abundance (%)
49.4 4.3 100 51.4 7.2 14.8 9.19 100
g = ground state, m = metastable state.
T1/2
Eg (keV)
Branching ratio (%)
ga
ma
g
m
g
m
17.4 m 14.4 m 6.18 d 78.5 h 18.3 h 15.5 m 18.5 m 15.02 h
4.42 h 20.9 m 9.7 h 4.18 m 1.57 h 65 s 57.3 m
± 511 355.8 909.2 191.4 1637 275.9 1368
616.2 155.5 188 1507 279 652.9 103
± 44 8.69 99 3.68 23 ± 100
6.7 12.8 37.4 6 2.3 48.3 9.79
F.I. Habbani, I.A. Ahmed / Applied Radiation and Isotopes 51 (1999) 81±84
83
Table 2 Measured isomeric cross-section ratios Reaction
sm/s g standard
sm/sg relative
Literature values
81
1.9520.15
1.742 0.05
1.63 20.16 (Grimeland et al., 1965), 2.02 20.11 (Kao and Alford, 1975), 1.882 0.19 (Rao et al., 1971) 4.5 20.1 (Csikai, private communication), 4.4 20.4 (Shakir, 1978), 4.162 0.2 (IAEA TR, 1974) 0.07620.041 (IAEA TR, 1987), 0.05920.005 (IAEA TR, 1974), 0.13320.065 (Mangal and Khurana, 1965) 0.12120.025 (IAEA TR, 1974), 0.13020.020 (Shakir, 1978) 0.82 20.26 (IAEA TR, 1987), 0.76 20.48 (Winiwarter, 1970), 0.492 0.21 (Mangal and Khurana, 1965) 0.07820.032 (IAEA TR, 1974), 0.08120.019 (Winiwarter, 1970) 2.53 20.15 (Kao and Alford, 1975), 2.79 20.08 (Hasan et al., 1972), 4.020.8 (Rao and Fink, 1967)
Br(n, 2n)80m,g Br
113
In(n, 2n)112m,g In
4.120.3
4.82 0.1
197
Au(n, 2n)196m,g Au
0.05820.019
0.05720.006
0.12920.05
0.003420.0002
1.0220.37
0.53720.121
0.05420.032 ±
± 3.642 0.01
90
Zr(n, 2n)89m,g Zr
198 92 82
Pt(n, 2n)197m,g Pt
Mo(n, 2n)91m,g Mo Se(n, 2n)81m,g Se
and Au, 20 min for In and 30 min for Br and Zr. The samples were measured using a HPGe gamma spectrometer system. The experimental setup and the method for activation cross-section measurements were described elsewhere (Habbani and Paic, 1988; Shaddad, 1995; Osman and Habbani, 1996). The standard cross-section for the reaction 27 Al(n, a)24 Na used was 116.046 mb (IAEA TR, 1983). 4. Results and discussion The results for the measured isomeric cross-section ratios together with some recent literature values are given in Table 2. The results for Br, In and Au show good agreement for the isomeric cross-section ratios measured by the two methods and corresponding literature values. The results for Zr and Pt show discrepancy between the two methods. In both cases the metastable half-life is much smaller than the ground state one (this also applies to Au and Mo). In such cases the application of the relative method has low reliability in view of the large dierence in the half-lives. This is in contrast to the standard method which is suitable across this group of isomers, since the total and metastable crosssections can easily be determined directly without the need for further curve analysis. Au has the advantage over the other target nuclides in this group of much higher abundance (100%). In the case of Mo the count rates from the ground state were also very low so that the relative method could not be used. In the case of Se the (n, 2n) reaction on 82 Se leads to two isomeric states: the metastable state of T1/2 = 57.3 m and Eg = 103 keV above the ground state with 99% IT and the ground state of T1/2 = 18.5 m decaying by beta to the excited state of 81 Br which decays with a
gamma emission of 275.9 keV to the ground state. The peak at 275.9 keV was used to obtain the isomeric cross-section ratio by the relative method. However, it was not possible to determine the total cross-section in this case because the half-life of the metastable state is longer than that of the ground state and so the standard method could not be used. A similar situation >Tg1/2 and it was exists for Br, but in this case Tm 1/2> possible to solve the problem by appropriate subtraction of the metastable part of the composite decay curve. It should be noted that the use of the ground state activity, if a metastable state is also present, should be handled with care or preferably be avoided whenever a proper substitution is possible. So the two methods can complement each other for the evaluation of the ground state cross-section, incorporating the high reliability of the isomeric cross-section ratio by the relative method with the easy and direct measurement of the metastable state cross-section using the standard method.
Acknowledgements The authors wish to thank the IAEA for technical assistence and support, Professor J. Csikai for his advice and encouragement and the technician Bashir Badawi for running the machine.
References Csikai, J., Buczko, Cs.M., Pepelnik, R., Agrawal, H.M., 1991. Activation cross-sections related to nuclear heating of high T c superconductors. Ann. Nucl. Energy 18 (1), 1. Grimeland, B., Kjellsby, E., Vines, J., 1965. Cross-sections of some reactions induced in nitrogen, phosphorus, copper
84
F.I. Habbani, I.A. Ahmed / Applied Radiation and Isotopes 51 (1999) 81±84
and bromine with neutrons of energy 14.8 MeV. Phys. Rev. 137, 878. Habbani, F., Paic, G., 1988. Measurement of (n, p) and (n, np) cross-sections for some Fe isotopes at 14.7 MeV. Int. J. Appl. Rad. Isotopes 39 (10), 1041. Hasan, S.S., Parasad, R., Seghal, M.L., 1972. The 14.8 MeV neutron cross-sections for enriched isotopes of 82 Se, 92 Mo, 117 Sn, 128 Te and 130 Te. Nucl. Phys. A 181, 101. IAEA TR, 1974. Handbook on Nuclear Activation CrossSections. Tech. Rep. 156. IAEA TR, 1983. Nuclear Data Standards for Nuclear Measurements. Tech. Rep. 227. IAEA TR, 1987. Handbook on Nuclear Activation Data. Tech. Rep. 273. Kao, T.H., Alford, W.L., 1975. Isomeric cross-sections for the (n, 2n) reaction on 82 Se, 81 Br and 45 Sc at 15.1 MeV. Nucl. Phys. A 237, 11. Lederer, C.M., Shirley, V.S., 1978. Table of Isotopes. J. Wiley and Sons. Mangal, S.K., Gill, P.S., 1963. Isomeric cross-section ratios for (n, 2n) reactions at 14 MeV. Nucl. Phys. 49, 510. Mangal, S.K., Khurana, C.S., 1965. Isomeric (n, 2n) crosssection ratios at 14.8 MeV. Nucl. Phys. 69, 158.
Osman, K.T., Habbani, F.I., 1996. Measurement and Study of (n, p) Reaction Cross-Sections for Cr, Ti, Ni, Co, Zr and Mo Isotopes Using 14.7 MeV Neutrons. INDC(SUD)001, NDS, IAEA. Qaim, S.M., 1972. Activation cross-sections, isomeric crosssection ratios and systematics for (n, 2n) reactions at 14±15 MeV. Nucl. Phys. A 185, 614. Rao, P.V., Fink, R.W., 1967. Neutron reaction cross-sections for selenium and iron at 14.4 MeV. Phys. Rev. 154 (4), 1023. Rao, P., Wood, R.E., Palms, J.M., Fink, R.W., 1971. Neutron activation cross-sections for As, Br, Rb and Sr Isotopes at 14.4 MeV. Phys. Rev. C 3, 629. Shaddad, I.A., 1995. (n, p) and (n, a) Reactions cross-sections measurements and systematics around 14 MeV neutron energy. Ph.D. Thesis, University of Khartoum. Shakir, N.S., 1978. Measurement of isomeric cross-section ratios by induced neutron reactions. M. Phil. Thesis, Aston University. Winiwarter, P., 1970. Isomeric cross-section ratios for the reactions 192 Os(n, 2n)191 Os and 198 Pt(n, 2n)197 Pt at 14 MeV. Nucl. Phys. A 158, 77.