- Email: [email protected]
A study of pre-equilibrium emission in some proton- and alpha-induced reactions
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720 www.elsevier.com/locate/nima
A study of pre-equilibrium emission in some proton- and alpha-induced reactions B.P. Singha,, Manoj K. Sharmaa, M.M. Musthafab, H.D. Bhardwajc, R. Prasada a
Department of Physics, A.M. University, Aligarh, India Department of Physics, Calicut University, Calicut, India c Department of Physics, DSN College, Unnao, India
b
Available online 28 February 2006
Abstract Excitation functions (EFs) for a large number of proton- and alpha-induced reactions in the energy range E8–60 MeV have been measured employing stacked foil activation technique. The theoretical calculations of the EFs have been carried out using the semiclassical code, which includes compound nucleus (CN) and pre-equilibrium (PE) emission into consideration. In general, the theoretical calculations agree well with the experimental data. The PE fraction has also been calculated and found to depend strongly on the energy of the incident particle. r 2006 Elsevier B.V. All rights reserved. PACS: 25.70.Gh Keywords: Pre-equilibrium emission; Proton- and alpha-induced reactions; Exciton model; Pre-equilibrium fraction
1. Introduction In order to have the optimum irradiation conditions for the production yield of various radioisotopes, more and more experimental nuclear reaction cross-sections are required. The cross-sections for the production of residual nuclei by medium energy nuclear reactions are of importance not only for the development of reactor technology but also for many fields of basic and applied sciences including astrophysics, environmental sciences, medicine, accelerator technology, space sciences etc. More recently, these cross-sections are also in demand in order to know the transmutation probabilities required for the proposed accelerator driven systems popularly known as energy amplifiers [1]. The nuclear data requirements for the accelerator driven technologies are extreme with respect to both the target element coverage and type of reaction data. As such, more detailed and accurate measurements are needed to provide these data. Efforts have been made to obtain the estimates of basic nuclear reaction cross-sections Corresponding author. Tel./fax: +91 571 2701001.
E-mail address: [email protected] (B.P. Singh). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.030
for a number of elements both experimentally as well as theoretically. Though, considerable data is available in literature on nucleon and light ion-induced reactions, but the cross-section values measured by different groups of workers for the same reaction, generally, do not agree. Further, these measurements were done mostly by using detectors and electronics of low resolution and poor efficiency. As such, it has been realized that there is a great need of new, reliable and self-consistent cross-section data taken by detectors of high resolution and better efficiency. Further, it is needless to say that the value of cross-section for a specific reaction depends on the reaction mechanism. For example, at moderate excitation energies, reactions induced by nucleons and light-heavy ions are found to proceed through compound nucleus (CN) as well as pre-equilibrium (PE) emission [2]. As such, precise measurement of excitation functions (Efs) for such cases may be used to find out the relative contribution of equilibrium and PE reaction components. In order to understand the mechanism of PE emission, a programme of precise measurement and analysis of cross-sections for proton- and alpha-induced reactions in several elements [3–7] has been undertaken. These measurements may also
ARTICLE IN PRESS 718
B.P. Singh et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720
provide a reliable database for testing the capability of codes with respect to calculating residual nuclei production. Further, an attempt has been made to parameterize the model used in the present code. Several excitation functions exist in literature [8–10] including that of our work [3–7,11] for neutron-, proton- and a-induced reactions. As a matter of fact, it will not be possible to measure the EFs for all the reactions required for various applications. Therefore, one will have to rely on theoretical predictions of the codes. In the present work, calculations have been done using the statistical model code ACT [11]. The experimental details are given in Section 2, while the details of theoretical calculations are given in Section 3, of the paper.
(a)
(b)
(c)
(d)
(e)
(f)
2. Experimental details
Fig. 1. Experimentally measured and theoretically calculated EFs for proton-induced reactions. 104
65
102 101 100
5
5
(b)
104
σ(mb)
102 101 100
12
15
(c)
18 21 24 Eα(MeV)
27
PRESENT WORK PRE+CN CN
10
20
(d)
30 40 50 Eα(MeV)
104 123
103
102
σ(mb)
σ(mb)
101
10-2
30
Au(α,n)200mTI
10
101
70
Sb(α,n)126I
101 PRESENTWORK PRE+CN CN
10-1
10-1
60
102
100
PRESENT WORK PRE+CN CN
100
10-2 12
(e)
102
10-1
197 3
Sb(α,n)124I
100
PRESENT WORK Phys Rev 139 (1965) B886 PRE+CN CN
10-1
121
103
104
The excitation functions for (p, n) reactions in several target nuclei viz., 51V, 58Ni, 60Ni, 61Ni, 62Ni, 63Cu, 65Cu, 89 Y, 93Nb, 119In, 121Sb, 123Sb, 130Te, and 197Au have been measured. As a representative case some of the EFs are shown in Figs. 1(a)–(f). The EFs for (a, n) reactions in 55 Mn, 63Cu, 65Cu, 93Nb, 121Sb, 123Sb, 141Pr, 165Ho, and 197 Au have also been measured. As a typical example some of the EFs are shown in Figs. 2(a)–(f). Literature data, if
10 15 20 25 30 35 40 45 Eα(MeV)
104
Nb(α,n)96mI
93
103
3. Results and discussion
PRESENT WORK PRE+CN CN
100
10 15 20 25 30 35 40 45 Eα(MeV)
(a)
102 101
PRESENT WORK PRE+CN CN
10-1
10-2
Cu(α,n)68Ga
103 σ(mb)
σ(mb)
104
Cu(α,n)66Ga
63
103
σ(mb)
In the present work, stacked foil activation technique has been used. Samples were either self-supporting or deposited on Al backing by vacuum evaporation technique. Stacks of samples were irradiated, using the proton and alpha particle beams at the Variable Energy Cyclotron Centre (VECC), Kolkata, India. In order to achieve wide energy variation, in some cases, the Al degraders of suitable thickness sandwiched between the successive samples were used. The beam energy degradation along the stack was determined using the Stopping power tables [12]. The beam currents of E100–200 nA were used depending on the individual case. Keeping in view the half-lives of interest, the stacks were irradiated for the optimum times ranging from about 30 min to few hours. After the irradiation, the activities of the radioactive products in the targets were measured non-destructively using 100 cm3. HPGe detector (resolution E2 keV for 1.33 MeV g-ray of 60Co), coupled to a multi-channel analyzer. The efficiency versus energy curves of the detector were determined using various standard gamma sources including 152Eu source of known strength at various source-detector separations. The residual nuclei of interest were identified by their characteristic g-lines as well as by their measured half-lives. The measured intensities of the identified gamma rays from the residual nuclei of interest were used to compute the crosssections [3]. The details of the experimental technique employed in the present work, the method of the data reduction and the details of error analysis are given elsewhere [5].
18
24 30 36 Eα(MeV)
42
10
48
(f)
15
20
25 30 35 Eα(MeV)
40
45
Fig. 2. Experimentally measured and theoretically calculated EFs for a-induced reactions.
ARTICLE IN PRESS B.P. Singh et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720 1.0 123 55
0.8
Mn 93
0.6
63
65
Sb
Cu
121
Sb
Nb 141
Cu
Pr
FR
available, is also shown in these figures. Several theoretical model-based computer codes, i.e., CASCADE [13], ACT [11], ALICE [14], HIVAP [15], etc., are available in literature to compute the reaction cross-sections. The code CASCADE is based on Hauser–Feshbach (HF) [16] formalism and calculates the cross-section for evaporation residues (ER) based on compound nucleus model. The code HIVAP has mainly been used for heavy ion (HI) reactions for calculating production cross-sections as well as fission contribution. Further, in code ALICE, Weisskopf-Ewing model has been used for CN calculations and hybrid model is used for PE-emission. In the present work, the analysis of the data has been done using the code ACT [11], which is based on the lines of code STAPRE [17]. In code ACT the HF theory is used for CN calculations, while PE emission is simulated using exciton model [18]. In HF model (ACT) the conservation of angular momentum is taken into account at each step of de-excitation, which is not considered in Weisskopf-Ewing model (code ALICE). Further, in code ACT, it is possible to calculate separately the production cross-sections for both ground as well as metastable states. However, the code ALICE does not give the production cross-section for metastable state of the residue. It may, however, be mentioned that in some of our earlier publications, we have used the code ALICE as well, and comparison of the calculations done using code ACT and ALICE are presented [5–7]. In code ACT, the level density parameter ‘a’, initial exciton number no, and the strength parameter FM of the square of two-body residual interaction matrix element are some of the important parameters. The level density parameters a’s are taken consistently from the tables of Dilg et al. [19]. In exciton model, the intermediate states of the system are characterized by the excitation energy E and the number np of the excited particles and nh of the holes. Initial configuration of the compound system defined by its exciton number no ð¼ np þ nh Þ is an important parameter of PE formalism. It is, therefore, of particular interest to look for the initial number and the assumed division of excitons into particles and holes required to reproduce the data. In literature, values of no ¼ 123 are used for proton induced reactions and no ¼ 426 are used for alpha induced reactions. In the present analysis, choice of no ¼ 3 for proton induced reactions and no ¼ 6, for alpha induced reactions are found to give satisfactory reproduction of the experimental data. These values of no ( ¼ 3 for proton and 6 for alpha induced reactions) may be justified, since the first projectile–target interaction may give rise to the excitation of one particle above the Fermi energy, leaving behind a hole in excited state, i.e., in all 3(2p+1h) and 6(5p+1h) initial exciton states for proton- and alpha-induced reactions, respectively. In order to calculate the internal transition rates it is necessary to calculate the value of matrix element |M| for two-body residual interaction. However, in the absence of any microscopic calculations for |M|, the expression |M|2 ¼ FM A3 U1 (where, A and U are the mass number and excitation energy of the compound system respec-
719
0.4
197
0.2
165
Au
Ho
0.0 0
10
20
30 40 E (MeV)
50
60
70
Fig. 3. Variation of PE fraction as a function of incident a-energy.
tively) given by Kalbach-Cline [20] has been used, generally, which relates the square of the average value of the matrix element by a strength parameter FM of twobody residual interactions, which may be treated as an adjustable parameter. In literature, values of FM having large variations have been used [21]. In the present calculations a value of F M ¼ 430 MeV3 has been found to give satisfactory reproduction of the data. Further, as can be seen from Figs. 1(a)–(f) and 2(a)–(f) that the measured cross-sections are well reproduced when a combination of CN and PE emission are considered in the calculations. Further, the total pre-equilibrium fraction ‘FR’, which is a measure of relative strength of PE component needed to reproduce the experimental EFs have also been calculated. The total FR at a given excitation energy, may be defined as the ratio of the sum of the cross-sections for PE emission of all emitted particles (restricted to n, p, d and alpha) to the total cross-section. As a typical example, the computed FR for the alpha induced reactions, as a function of incident particle energy is shown in Fig. 3. As can be seen from this figure that the FR increases with the increase in incident energy, as expected. It may, however, be mentioned that the trends in FR discussed above may not be conclusive in view of the limited number of cases studied in the present work. The cross-sections obtained in the present work would be useful to upgrade theoretical codes, for the estimation of the activity for future accelerator development, radiation safety problems and for isotope production.
Acknowledgements The authors thank the VECC personnel and to the IUCDAEF, Calcutta Centre, for all their help and cooperation during the experiments. Authors are also thankful to the Chairman, Department of Physics, AMU for providing necessary facilities to carryout this work. One of the authors (MKS) thanks to DST, New Delhi, India, for
ARTICLE IN PRESS 720
B.P. Singh et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720
providing financial support vide the project no SR/FTP/ PS-46/2003. References [1] C. Rubbia, et al., Conceptual design of a fast neutron operated high power energy amplifier, Report CERN/AT/95-94(ET). [2] M. Blann, Ann. Rev. Nucl. Sci. NS-25 (1975) 123. [3] B.P. Singh, Ph.D. Thesis, AMU, Aligarh, India, 1991. [4] M.M. Musthafa, Ph.D. Thesis, AMU, Aligarh, India, 1996. [5] M.M. Musthafa, M.K. Sharma, B.P. Singh, R. Prasad, Appl. Radiat. Isot. 62 (2005) 419. [6] B.P. Singh, M.G.V. Sankracharyulu, M. Afzal Ansari, H.D. Bhardwaj, R. Prasad, Phys. Rev. C 47 (1993) 2055. [7] M.M. Muthafa, B.P. Singh, M.G.V. Sankaracharyulu, H.D. Bhardwaj, R. Prasad, Phy. Rev. C 52 (1995) 3174. [8] I.-G. Brin, B. Strohmaier, H. Freiesleben, S.M. Qaim, Phy. Rev. C 52 (1995) 2546.
[9] S. Mukherjee, N.L. Singh, G. Kiran Kumar, L. Chaturvedi, Phy. Rev. C 72 (2005) 014609. [10] J. Pal, S. Saha, C.C. Dey, P. Banerjee, S. Bose, B.K. Sinha, M.B. Chatterjee, S.K. Basu, Phy. Rev. C 71 (2005) 034605. [11] H.D. Bhardwaj, Ph.D. Thesis, AMU, Aligarh, India, 1985. [12] L.C. North Cliffe, R.F. Schilling, Nucl. Data Tables A 7 (1970) 256. [13] F. Puhlhofer, Nucl. Phys. A 280 (1977) 267. [14] M. Blann, Report N0. PSR- (1991) 146,.NEA Data Bank, Gif-surYveette, France. [15] G. Audi, A.H. Wapstra, Nucl. Phys. A 295 (1995) 409. [16] W. Hauser, H. Feshbach, Phys. Rev. 87 (1952) 366. [17] M. Uhl, B. Stromaier, Report IRK 76/01 (1981), NEA Data Bank Cedex, France. [18] J.J. Griffin, Phys. Rev. Lett. 17 (1966) 478. [19] W. Dilg, W. Schantl, H.K. Vonach, Nucl. Phys. A 217 (1973) 269. [20] C. Kalbach-Cline, Nucl. Phys. A 210 (1973) 590. [21] K.K. Gudima, S.G. Mashnik, V.D. Toneev, Nucl. Phys. A 401 (1983) 329.
Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720 www.elsevier.com/locate/nima
A study of pre-equilibrium emission in some proton- and alpha-induced reactions B.P. Singha,, Manoj K. Sharmaa, M.M. Musthafab, H.D. Bhardwajc, R. Prasada a
Department of Physics, A.M. University, Aligarh, India Department of Physics, Calicut University, Calicut, India c Department of Physics, DSN College, Unnao, India
b
Available online 28 February 2006
Abstract Excitation functions (EFs) for a large number of proton- and alpha-induced reactions in the energy range E8–60 MeV have been measured employing stacked foil activation technique. The theoretical calculations of the EFs have been carried out using the semiclassical code, which includes compound nucleus (CN) and pre-equilibrium (PE) emission into consideration. In general, the theoretical calculations agree well with the experimental data. The PE fraction has also been calculated and found to depend strongly on the energy of the incident particle. r 2006 Elsevier B.V. All rights reserved. PACS: 25.70.Gh Keywords: Pre-equilibrium emission; Proton- and alpha-induced reactions; Exciton model; Pre-equilibrium fraction
1. Introduction In order to have the optimum irradiation conditions for the production yield of various radioisotopes, more and more experimental nuclear reaction cross-sections are required. The cross-sections for the production of residual nuclei by medium energy nuclear reactions are of importance not only for the development of reactor technology but also for many fields of basic and applied sciences including astrophysics, environmental sciences, medicine, accelerator technology, space sciences etc. More recently, these cross-sections are also in demand in order to know the transmutation probabilities required for the proposed accelerator driven systems popularly known as energy amplifiers [1]. The nuclear data requirements for the accelerator driven technologies are extreme with respect to both the target element coverage and type of reaction data. As such, more detailed and accurate measurements are needed to provide these data. Efforts have been made to obtain the estimates of basic nuclear reaction cross-sections Corresponding author. Tel./fax: +91 571 2701001.
E-mail address: [email protected] (B.P. Singh). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.030
for a number of elements both experimentally as well as theoretically. Though, considerable data is available in literature on nucleon and light ion-induced reactions, but the cross-section values measured by different groups of workers for the same reaction, generally, do not agree. Further, these measurements were done mostly by using detectors and electronics of low resolution and poor efficiency. As such, it has been realized that there is a great need of new, reliable and self-consistent cross-section data taken by detectors of high resolution and better efficiency. Further, it is needless to say that the value of cross-section for a specific reaction depends on the reaction mechanism. For example, at moderate excitation energies, reactions induced by nucleons and light-heavy ions are found to proceed through compound nucleus (CN) as well as pre-equilibrium (PE) emission [2]. As such, precise measurement of excitation functions (Efs) for such cases may be used to find out the relative contribution of equilibrium and PE reaction components. In order to understand the mechanism of PE emission, a programme of precise measurement and analysis of cross-sections for proton- and alpha-induced reactions in several elements [3–7] has been undertaken. These measurements may also
ARTICLE IN PRESS 718
B.P. Singh et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720
provide a reliable database for testing the capability of codes with respect to calculating residual nuclei production. Further, an attempt has been made to parameterize the model used in the present code. Several excitation functions exist in literature [8–10] including that of our work [3–7,11] for neutron-, proton- and a-induced reactions. As a matter of fact, it will not be possible to measure the EFs for all the reactions required for various applications. Therefore, one will have to rely on theoretical predictions of the codes. In the present work, calculations have been done using the statistical model code ACT [11]. The experimental details are given in Section 2, while the details of theoretical calculations are given in Section 3, of the paper.
(a)
(b)
(c)
(d)
(e)
(f)
2. Experimental details
Fig. 1. Experimentally measured and theoretically calculated EFs for proton-induced reactions. 104
65
102 101 100
5
5
(b)
104
σ(mb)
102 101 100
12
15
(c)
18 21 24 Eα(MeV)
27
PRESENT WORK PRE+CN CN
10
20
(d)
30 40 50 Eα(MeV)
104 123
103
102
σ(mb)
σ(mb)
101
10-2
30
Au(α,n)200mTI
10
101
70
Sb(α,n)126I
101 PRESENTWORK PRE+CN CN
10-1
10-1
60
102
100
PRESENT WORK PRE+CN CN
100
10-2 12
(e)
102
10-1
197 3
Sb(α,n)124I
100
PRESENT WORK Phys Rev 139 (1965) B886 PRE+CN CN
10-1
121
103
104
The excitation functions for (p, n) reactions in several target nuclei viz., 51V, 58Ni, 60Ni, 61Ni, 62Ni, 63Cu, 65Cu, 89 Y, 93Nb, 119In, 121Sb, 123Sb, 130Te, and 197Au have been measured. As a representative case some of the EFs are shown in Figs. 1(a)–(f). The EFs for (a, n) reactions in 55 Mn, 63Cu, 65Cu, 93Nb, 121Sb, 123Sb, 141Pr, 165Ho, and 197 Au have also been measured. As a typical example some of the EFs are shown in Figs. 2(a)–(f). Literature data, if
10 15 20 25 30 35 40 45 Eα(MeV)
104
Nb(α,n)96mI
93
103
3. Results and discussion
PRESENT WORK PRE+CN CN
100
10 15 20 25 30 35 40 45 Eα(MeV)
(a)
102 101
PRESENT WORK PRE+CN CN
10-1
10-2
Cu(α,n)68Ga
103 σ(mb)
σ(mb)
104
Cu(α,n)66Ga
63
103
σ(mb)
In the present work, stacked foil activation technique has been used. Samples were either self-supporting or deposited on Al backing by vacuum evaporation technique. Stacks of samples were irradiated, using the proton and alpha particle beams at the Variable Energy Cyclotron Centre (VECC), Kolkata, India. In order to achieve wide energy variation, in some cases, the Al degraders of suitable thickness sandwiched between the successive samples were used. The beam energy degradation along the stack was determined using the Stopping power tables [12]. The beam currents of E100–200 nA were used depending on the individual case. Keeping in view the half-lives of interest, the stacks were irradiated for the optimum times ranging from about 30 min to few hours. After the irradiation, the activities of the radioactive products in the targets were measured non-destructively using 100 cm3. HPGe detector (resolution E2 keV for 1.33 MeV g-ray of 60Co), coupled to a multi-channel analyzer. The efficiency versus energy curves of the detector were determined using various standard gamma sources including 152Eu source of known strength at various source-detector separations. The residual nuclei of interest were identified by their characteristic g-lines as well as by their measured half-lives. The measured intensities of the identified gamma rays from the residual nuclei of interest were used to compute the crosssections [3]. The details of the experimental technique employed in the present work, the method of the data reduction and the details of error analysis are given elsewhere [5].
18
24 30 36 Eα(MeV)
42
10
48
(f)
15
20
25 30 35 Eα(MeV)
40
45
Fig. 2. Experimentally measured and theoretically calculated EFs for a-induced reactions.
ARTICLE IN PRESS B.P. Singh et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720 1.0 123 55
0.8
Mn 93
0.6
63
65
Sb
Cu
121
Sb
Nb 141
Cu
Pr
FR
available, is also shown in these figures. Several theoretical model-based computer codes, i.e., CASCADE [13], ACT [11], ALICE [14], HIVAP [15], etc., are available in literature to compute the reaction cross-sections. The code CASCADE is based on Hauser–Feshbach (HF) [16] formalism and calculates the cross-section for evaporation residues (ER) based on compound nucleus model. The code HIVAP has mainly been used for heavy ion (HI) reactions for calculating production cross-sections as well as fission contribution. Further, in code ALICE, Weisskopf-Ewing model has been used for CN calculations and hybrid model is used for PE-emission. In the present work, the analysis of the data has been done using the code ACT [11], which is based on the lines of code STAPRE [17]. In code ACT the HF theory is used for CN calculations, while PE emission is simulated using exciton model [18]. In HF model (ACT) the conservation of angular momentum is taken into account at each step of de-excitation, which is not considered in Weisskopf-Ewing model (code ALICE). Further, in code ACT, it is possible to calculate separately the production cross-sections for both ground as well as metastable states. However, the code ALICE does not give the production cross-section for metastable state of the residue. It may, however, be mentioned that in some of our earlier publications, we have used the code ALICE as well, and comparison of the calculations done using code ACT and ALICE are presented [5–7]. In code ACT, the level density parameter ‘a’, initial exciton number no, and the strength parameter FM of the square of two-body residual interaction matrix element are some of the important parameters. The level density parameters a’s are taken consistently from the tables of Dilg et al. [19]. In exciton model, the intermediate states of the system are characterized by the excitation energy E and the number np of the excited particles and nh of the holes. Initial configuration of the compound system defined by its exciton number no ð¼ np þ nh Þ is an important parameter of PE formalism. It is, therefore, of particular interest to look for the initial number and the assumed division of excitons into particles and holes required to reproduce the data. In literature, values of no ¼ 123 are used for proton induced reactions and no ¼ 426 are used for alpha induced reactions. In the present analysis, choice of no ¼ 3 for proton induced reactions and no ¼ 6, for alpha induced reactions are found to give satisfactory reproduction of the experimental data. These values of no ( ¼ 3 for proton and 6 for alpha induced reactions) may be justified, since the first projectile–target interaction may give rise to the excitation of one particle above the Fermi energy, leaving behind a hole in excited state, i.e., in all 3(2p+1h) and 6(5p+1h) initial exciton states for proton- and alpha-induced reactions, respectively. In order to calculate the internal transition rates it is necessary to calculate the value of matrix element |M| for two-body residual interaction. However, in the absence of any microscopic calculations for |M|, the expression |M|2 ¼ FM A3 U1 (where, A and U are the mass number and excitation energy of the compound system respec-
719
0.4
197
0.2
165
Au
Ho
0.0 0
10
20
30 40 E (MeV)
50
60
70
Fig. 3. Variation of PE fraction as a function of incident a-energy.
tively) given by Kalbach-Cline [20] has been used, generally, which relates the square of the average value of the matrix element by a strength parameter FM of twobody residual interactions, which may be treated as an adjustable parameter. In literature, values of FM having large variations have been used [21]. In the present calculations a value of F M ¼ 430 MeV3 has been found to give satisfactory reproduction of the data. Further, as can be seen from Figs. 1(a)–(f) and 2(a)–(f) that the measured cross-sections are well reproduced when a combination of CN and PE emission are considered in the calculations. Further, the total pre-equilibrium fraction ‘FR’, which is a measure of relative strength of PE component needed to reproduce the experimental EFs have also been calculated. The total FR at a given excitation energy, may be defined as the ratio of the sum of the cross-sections for PE emission of all emitted particles (restricted to n, p, d and alpha) to the total cross-section. As a typical example, the computed FR for the alpha induced reactions, as a function of incident particle energy is shown in Fig. 3. As can be seen from this figure that the FR increases with the increase in incident energy, as expected. It may, however, be mentioned that the trends in FR discussed above may not be conclusive in view of the limited number of cases studied in the present work. The cross-sections obtained in the present work would be useful to upgrade theoretical codes, for the estimation of the activity for future accelerator development, radiation safety problems and for isotope production.
Acknowledgements The authors thank the VECC personnel and to the IUCDAEF, Calcutta Centre, for all their help and cooperation during the experiments. Authors are also thankful to the Chairman, Department of Physics, AMU for providing necessary facilities to carryout this work. One of the authors (MKS) thanks to DST, New Delhi, India, for
ARTICLE IN PRESS 720
B.P. Singh et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 717–720
providing financial support vide the project no SR/FTP/ PS-46/2003. References [1] C. Rubbia, et al., Conceptual design of a fast neutron operated high power energy amplifier, Report CERN/AT/95-94(ET). [2] M. Blann, Ann. Rev. Nucl. Sci. NS-25 (1975) 123. [3] B.P. Singh, Ph.D. Thesis, AMU, Aligarh, India, 1991. [4] M.M. Musthafa, Ph.D. Thesis, AMU, Aligarh, India, 1996. [5] M.M. Musthafa, M.K. Sharma, B.P. Singh, R. Prasad, Appl. Radiat. Isot. 62 (2005) 419. [6] B.P. Singh, M.G.V. Sankracharyulu, M. Afzal Ansari, H.D. Bhardwaj, R. Prasad, Phys. Rev. C 47 (1993) 2055. [7] M.M. Muthafa, B.P. Singh, M.G.V. Sankaracharyulu, H.D. Bhardwaj, R. Prasad, Phy. Rev. C 52 (1995) 3174. [8] I.-G. Brin, B. Strohmaier, H. Freiesleben, S.M. Qaim, Phy. Rev. C 52 (1995) 2546.
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