EXPERIMENTAL STUDIES OF PROTON INDUCED REACTION CROSS-SECTIONS ON

Transcription

1 EXPERIMENTAL STUDIES OF PROTON INDUCED REACTION CROSS-SECTIONS ON NAT MO Mayeen Uddin KHANDAKER, G.uinyun KIM 1, Kwangsoo KIM, and Dongchul SON Department of Physics, Kyungpook National University, Daegu Young Seok LEE Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang Abstract Proton induced reaction cross-sections were measured by the stacked-foil technique for the reactions of nat Mo(p, xn) 99m, 96(m+g), 95g, 95m, 94g Tc, from their respective thresholds up to 30MeV. The radioactivity of the activation products was determined by high-resolution γ-ray spectrometry. Excitation functions for the production of 99m Tc, 96(m+g) Tc, 95m Tc, 95g Tc, and 94g Tc radioisotopes were measured, and compared with the earlier reported experimental data. The present results showed a good agreement with all other reported experimental data, and also with the theoretical data MENDL-2P in the investigated energy region. Keywords: Activation cross-section; stacked-foil technique; Mo+ p reactions; MC-50 Cyclotron. 1 gnkim@knu.ac.kr; Tel.:

2 Introduction Beside the general interest of basic nuclear physics, intermediate energy activation cross-section data are of increasing importance for a wide variety of applications. The remarkable applications are in the field of space and environmental sciences, medical sciences (radioisotope production, dosimetry application, radiation therapy etc.), spallation neutron sources, radiation and shielding effects in space, activation technology to accelerator-based nuclear waste transmutation and energy production. We have introduced a systematic study of medium energy proton induced reactions on some important structural and instrumental materials. Present study of proton induced activation reactions on natural molybdenum is a part of the above systematic measurements due to its importance as structural materials, and the rigorous use of activation cross-section of molybdenum for proton beam over a wide energy range. It is also very useful as a refractory and corrosion resistant material in accelerator applications. Moreover, the activation data of molybdenum are of interest for wide area; the thin layer activation technique (TLA) to determine the ratio of wear, corrosion and erosion processes of molybdenum, radiation safety, and estimation of radioactive wastes. We have used natural Molybdenum as target material for the production of medically important radioisotopes; such as production of 99m Tc, 96(m+g) Tc, 95m Tc, 95g Tc, and 94g Tc. These isotope production via proton induced reaction on natural molybdenum is very advantageous because of its metallic form, favorable physiochemical characteristics (good thermal and electrical conductivity, and its very high melting point: 2896 K) and of its low buying cost [1]. These isotopes can be produced commercially by nuclear reactors. But the facility is not available around the world and also expensive. These radioisotopes can also be produced by accelerator, though currently, there is no supply of acceleratorproduced isotopes of these kinds in anywhere in the world. But, it is possible to produce the medically important radioisotopes such as; 99m Tc and 99 Mo by proton bombardment on natural molybdenum via one proton-one neutron and two neutron emission reactions, respectively. Several authors [2-4] have reported a variety sets of data for proton induced activation cross-sections on molybdenum in the medium energy range. But large discrepancies are found among these sets of data. Therefore, the available data are not sufficient and reliable for the application in production of isotopes from molybdenum target in the medium energy range. This work was performed to find reliable excitation functions for proton induced reactions on natural molybdenum in the energy range 5 ~30MeV using the azimuthally varying field-type (AVF) MC-50 cyclotron at Korea Institute of Radiological and Medical Sciences (KIRAMS). The main purposes of this experiment was two-fold: first to report measurement of the excitation functions for nat Mo(p, xn) 99m, 96(m+g), 95g, 95m, 94g Tc nuclear reactions in the

3 energy region up to 30MeV where the predominant mechanism is usually changeable; and second to compare this experimental data with the previous published values. Experimental In the present work, the excitation functions of nat Mo(p, xn) 99m, 96(m+g), 95g, 95m, 94g Tc reactions were measured by using the activation technique combined with high resolution gamma-ray spectrometry. Special care was taken in preparation of uniform targets with known thickness, determination of the proton energy degradation and the intensity of the proton beam along the stacked target, and also in determination of the activities of the samples. High purity (> 99.99%) molybdenum foil (100 µm thick) with natural isotopic composition ( 92 Mo 14.84%, 94 Mo 9.25%, 95 Mo 15.92%, 96 Mo 16.68%, 97 Mo 9.55%, 98 Mo 24.13% and 100 Mo 9.63%) was used as target element for irradiation. Also, copper and aluminum foils with known cross sections were used as monitor and energy degrader foils respectively. All of the foils were sandwiched together following the order Al-Cu-Mo, repeatedly in the stacks. The stacked samples were then placed in an aluminum holder for irradiation. These stacked samples were then irradiated by an external beam line of cyclotron with 35MeV nominal proton energy. The irradiation of the stacked samples was done (with a 0.1 cm diameter beam and 45~50nA intense current) for 30 minutes. The beam intensity was kept constant during irradiation. It was necessary to ensure that equal areas of the monitor and target foils intercepted the beam. The irradiation geometry was kept in a position so that the foils were exposed by maximum beam line. Data Evaluation The activity of the produced radioisotopes of the target and monitor foils were measured continuously on the basis of their gamma radiation energy. In this experiment, high purity germanium detector (HPGe) was used for gamma ray measurement. The source to detector distances were kept long enough (10~80 cm) to assure low dead time and point like geometry. The HPGe-detector was coupled to a 4096 multi-channel analyzer (MCA) with the associated electronics to determine the photo-peak area of gamma ray spectra by using gamma vision (EG&G ORTEC) computer program. Considering the cases of long-lived radio-nuclides, the activity measurements were done after taking sufficient cooling time. This was done due to the complete decay of most of the undesired short-lived nuclides so that we can identify and separate the complex gamma lines easily. The efficiency versus energy curve for the HPGe-detector was drawn using the standard gamma-ray point sources ( 133 Ba, 57 Co, 60 Co, 137 Cs, 54 Mn and 109 Cd) with known strengths. The data of these standard sources were

4 taken for different source to detector distances (10~80cms). The molybdenum (Mo) target foils, copper (Cu) monitor foils, and aluminum (Al) energy degrader foils were measured in the same counting geometry with the same detector (HPGe) calibrated by the above mentioned standard gamma-ray sources. The use of the multiple monitor foils decreases the probability of introducing unknown systematic errors in activity determination. The beam energy degradation along the stack was determined using the computer program SRIM-2003 [5] assuming the incident proton beam energy was 35MeV. The cross-sections of the investigated reactions were deduced in the proton energy range 10~30MeV by using the well-known activation formula: λtd CPS exp σ = ε λtirr N ϕ, (1) I (1 exp ) γ where CPS is a net counts per second under a photo peak, λ ( / T 1/2 ) is the disintegration constant, I γ is gamma ray intensity, ε is the efficiency of the detector, t irr is the irradiation time, ϕ is the proton flux, t d is the decay time, and N is the total number of target nuclide. The decay data (monitor and molybdenum) used in the calculation was taken from Browne and Firestone [6] and is furnished in Table 1. The threshold energies in table 1 were calculated using the Los Alamos National Laboratory T-2 Nuclear Information Service on the internet [7]. The standard cross-section data for the monitors was taken from internet service [8]. Table 1: Decay data of the produced radioisotopes Produ ced Nuclei Halflife Decay mode (%) Gamma-ray Energy, E γ (kev) Intensity, I γ (%) 99m Tc 6.02 h IT+β - (100) m Tc 51.5 m IT (98) EC (2) g Tc 4.28 d EC (100) m Tc 61.0 d IT+ EC(100) Tc 20.0 h EC (100) Contributing reactions 100 Mo(p,2n) 100 Mo(p,pn) 99 Mo 99m Tc 96 Mo(p, n) 97 Mo(p,2n) 98 Mo(p,3n) 96 Mo(p, n) 97 Mo(p,2n) 98 Mo(p,3n) 96m Tc decay 95 Mo(p, n) 96 Mo(p, 2n) 97 Mo(p, 3n) 98 Mo(p, 4n) 95 Mo(p, n) 96 Mo(p, 2n) 97 Mo(p, 3n) 98 Mo(p, 4n) Threshol d Energy, E th (MeV)

5 94 Tc 293 m EC (100) Zn h EC+β + (100) Zn d EC+β + (100) Al h β + (99.97) Al h EC (100) Mo(p, n) 95 Mo(p, 2n) 96 Mo(p, 3n) 97 Mo(p, 4n) 63 Cu(p, 2n) 65 Cu(p, 4n) Cu(p, n) Al (p, x) 24 Na Al (p, x) 22 Na The total uncertainty of the measured experimental cross-section data is mainly depend on the quality of the monitor data used, the uncertainty of the number of target nuclei (uniformity and thickness of the used target foils), counting statistics and peak area determination, separation of complex peaks, nuclear data and gamma-ray abundances, and detector efficiencies that used in the data evaluation. The total uncertainties of the measured cross-sections were calculated by considering the statistical uncertainties and other uncertainties. The following errors were considered to derive the total uncertainty of cross-section values; statistical error (1-13%), error of the monitor flux (2~ 4%), error due to the beam flux energy (2~ 4%), error due to the detector efficiency (0.5~2%), and error due to the gamma ray intensity (1~2%). The overall uncertainty of the cross-section results is around 10%. Results and Discussions The measured excitation functions of the proton-induced reactions on molybdenum [ nat Mo(p, xn) 99m, 96(m+g), 95g, 95m, 94g Tc] are graphically represented in the Fig. 1-6 respectively. The nat Mo(p, xn) 99m, 96(m+g), 95g, 95m, 94g Tc processes. The production cross-section of 99m Tc is calculated through the analysis of keV gamma peak. Basically, this radionuclide can be produced in two processes. One is direct process through the reaction 100 Mo(p,2n) 99m Tc and the other is indirect process via 100 Mo(p,pn) 99 Mo 99m Tc reaction. Although theoretically, the reaction 98 Mo(p,γ) 99m Tc has little contribution in the production cross section of 99m Tc, but we neglected this contribution in the present case. This is because, the process has no practical relevance for the production of 99m Tc and the cross section of the 98 Mo(p,γ) 99m Tc reaction is very negligible amount [4], and besides this, we are considering the formation of an isomeric state radionuclide. The present result of 99m Tc radionuclide formation is shown in Fig.1. We found a good agreement with evaluated data MENDL-2P [13]. The data reported by Scholten et al. [4] is quite higher than our data but the trend of peak formation agrees to each other. In their experiment, they

6 worked with the 97.4% and 99.5% enriched 100 Mo and 98 Mo sample, respectively but we performed the present experiment using natural molybdenum sample. Moreover, it should be emphasized that the 100 Mo(p,2n) 99m Tc reaction cannot be compared to a normal (p,2n) reaction, since the product activity is an isomeric state. Actually, the systematic are generally valid for total (p,xn) cross-sections but not for the formation of higher spin isomers. Even detailed statistical model calculations, incorporating pre-compound model and nuclear structural effects are often incapable of reproducing the isomeric cross section [9]. An accurate experimental data base is thus crucial to consider the feasibility of this reaction for a possible production of 99m Tc at a cyclotron. A limiting factor in this regard would be the level of co-produced long-lived 99g Tc impurity. Experimentally, this is very hard to determine and was outside the scope of the present work. Cross Section [mb] Present Work B. Scholten et al. [4] MENDL-2P [13] Proton Energy [MeV] Fig. 1: Excitation function of nat Mo(p,xn) 99m Tc The radioisotope 96m Tc decays into 96g Tc through the internal conversion process (98%) and make the contribution of low energy and very weak gamma-line which is not suitable for quantitative assessment. 96m Tc also decays into 96 Mo by electron capture and positron emission but the gamma lines emitted during the decay are also very weak and not independent. Moreover, as the half-life of 96m Tc is only 51 minutes, so it decays completely during greater than few hours cooling time. However, 96 Nb also have very little (because the production cross section of 96 Nb nuclide is in the order of 1~2mb) contribution in keV gamma line. As 96m Tc emits very weak gamma line which is not suitable for analysis and also the production cross-section of 96 Nb is small, that is why; improper cooling time or an incomplete separation of its contribution can cause only minor error in the resulting

7 cross-section of the 96g Tc production. As in the present experiment, we considered a short cooling time, that is why through the analyzing of keV gamma ray peak, we have calculated the production cross-section of 96(m+g) Tc radionuclide through the (p,xn) reaction. Basically, to obtain the reliable results, we measured the excitation function of 96(m+g) Tc radionuclide but not 96g Tc radionuclide because for short cooling time both ( 96m Tc and 96g Tc) the radionuclide has contribution in the keV gamma ray peak. We confirmed consistency between the results of 812.5keV and 778.2keV, and we found agreement between these. However, the measured excitation function of 96(m+g) Tc production is shown in Fig. 2. The contributing reactions have shown in Table-1. Our results showed very good agreement with the recent data reported by S. Takacs et al, M.S. Uddin et al. [10-11] and evaluated data MENDL-2P [13], and this fact confirms the reliability of the measured crosssection values of 96(m+g) Tc production. Cross Section [mb] Present Work J. J. Hogan [14] M. Bonardi et al. [12] S. Takacs et al. [10] M. S. Uddin et al. [11] MENDL-2P [13] Proton Energy [MeV] Fig. 2: Excitation function of nat Mo(p,xn) 96(m+g) Tc The production cross-section of 95m Tc is calculated with the analysis of keV gamma peak. The contributing reactions for this calculation have shown in the Table-1. The present result of 95m Tc radionuclide formation is shown in Fig. 3. The availability of the reported cross section values for this radionuclide formation is not enough more. However, we compared our data with the recent published values reported by M. Bonardi et al., M.S. Uddin et al [12, 11], and evaluated data MENDL-2P [13], and we got a good agreement among these values.

8 Cross Section [mb] Present Work M. Bonardi et al. [12] M. S. Uddin et al. [11] MENDL-2P [13] Proton Energy [MeV] Fig. 3: Excitation function of nat Mo(p,xn) 95m Tc In order to calculate the production cross-section of 95g Tc radionuclide, we considered the gamma line keV, and confirmed the result with the keV gamma line. The present result of 95g Tc radionuclide formation is shown in Fig. 4. The contributing reactions for this radionuclide production have shown in the Table-1. In this case, our results showed good agreement with the latest data reported by M. Bonardi et al. and M.S. Uddin et al. [12, 11], and evaluated data MENDL-2P [13]. 200 Cross Section [mb] Present Work M. Bonardi et. al. [17] MENDL-2P [13] Proton Energy [MeV]

9 Fig. 4: Excitation function of nat Mo(p,xn) 95g Tc The present result of 94g Tc radionuclide formation is shown in Fig. 5. This result shows very good agreement with the data reported by the authors M. Bonardi et al., and M.S. Uddin et al. [12, 11]. Due to the short half-life and the existence of numerous reaction channels, this radionuclide was produced abundantly in the investigated energy region studied in the present work. Cross-Section [mb] Present Work M. Bonardi et al. [12] M.S. Uddin et al. [11] MENDL-2P [13] Proton Energy [MeV] Fig. 5: Excitation function of nat Mo(p,xn) 94g Tc Conclusions In this experiment, we have measured the excitation functions for the production of 99m Tc, 96(m+g) Tc, 95m Tc, 95g Tc, and 94g Tc radio-nuclides through the proton-induced activation on natural molybdenum in the energy range 10-30MeV using the stacked-foil technique. We found agreement between the proton fluxes obtained by using Cu and Al monitor foils. It is very important to measure the beam flux accurately because; the accuracy of the cross-section depends largely on the beam flux values. However, we have calculated the proton flux values for every copper monitor foils in the stack, and used these values in determining the cross-sections of respective molybdenum foils following the order of sample in the stack. We found that the proton beam energy degradation was in fewer scale. In the present work, we have compared our measured values mostly with the data reported by the following authors; S. Takacs et al., M.S. Uddin et al., M. Bonardi et al. [10-12], and evaluated data MENDL-2P [13]. The authors did their experiments in recent time and our results showed a general

10 good agreement with their reported values for all of the investigated radio-nuclides and also the evaluated value MENDL-2P [13]. Acknowledgments The author would like to express their sincere thanks to the staffs of the MC-50 Cyclotron Laboratories for their cordial help in performing the irradiations of the samples. This work is partly supported through the Project number M of the Ministry of Science and Technology (MOST) and through the Science Research Center (SRC) program of the Institute of High Energy Physics, Kyungpook National University. References [1] R.B. Firestone, C.M. Baglin and F.S.Y. Chu, Table of Isotopes, 8 th Edition. Update on CD-ROM. Wiley, New York. (1998). [2] S.M. Kormali, D.L. Swindle and E.A. Schweikert, J. Radiat. Chem. 31, 437 (1976). [3] P. M. Grant, B. R. Erdal and H. A. O Brien, Appl. Radiat. Isot. 34, 1631 (1983). [4] B. Scholten, R.M. Lambrecht, H. Vera Ruiz and S.M. Qaim, Appl. Radiat. Isot. 51, 69 (1999). [5] J. F. Ziegler, J. P. Biersack and U. Littmark, SRIM 2003 code, Version 96.xx. The stopping and range of ions in solids. (Pergamon, New York, 2003). [6] E. Browne and R.B. Firestone, Table of Radioactive Isotopes, edited by V.S. Shirley, (Wiley, New York, 1986). [7] Reaction Q-values and thresholds, Los Alamos National Laboratory, T-2 Nuclear Information Service. Available from< [8] Monitor cross-section data Available at < [9] S.M. Qaim, in Proceedings of International Conference on Nuclear Data for Science and Technology, edited by J. K. Dickens (Gatlinburg, May., 1994) p 186. [10] S. Takacs, F. Tarkanyi, M. Sonck and A. Hermanne, Nucl. Instr. Meth. B 198, 183 (2002). [11] M.S. Uddin, M. Hagiwara, F. Tarkanyi, F. Ditroi and M. Baba, Appl. Radiat. Isot. 60, 911(2004). [12] M. Bonardi, C. Birattari, F. Groppi and E. Sabbioni, Appl. Radiat. Isot. 57, 617 (2002). [13] Medium Energy Activation Cross Sections data: MENDL-2P, Available at: < [14] J. J. Hogan, Phys, Rev. C6, 810 (1972).