Experimental determination of deuteron-induced

Radiochim. Acta 95, 187–192 (2007) / DOI 10.1524/ract.2007.95.4.187 © by Oldenbourg Wissenschaftsverlag, München

Experimental determination of deuteron-induced activation cross sections of yttrium By M. S. Uddin1 , ∗, M. Baba2 , M. Hagiwara2 , F. Tárkányi3 and F. Ditrói3 1 2 3

Institute of Nuclear Science and Technology, Atomic Energy Research Establishment, Savar, GPO Box No.3787, Dhaka 1000, Bangladesh Cyclotron and Radioisotope Center, Tohoku University, Aramaki, Aoba-ku, Sendai 980 8578, Japan Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen 4001, Hungary

(Received August 16, 2006; accepted in revised form November 20, 2006)

Yttrium / Deuteron / Activation cross section / Cyclotron / Excitation function Summary. Excitation functions were measured for the 89 Y(d, x)90m,88,87m,87m+g Y, 89 Y(d, x)88,89 Zr and 89 Y(d, x)85 Sr reactions in the energy range 9–40 MeV by the stacked-foil activation technique in combination with high resolution HPGe γ -ray spectroscopy. The data above 27 MeV have been measured for the first time. Yield calculations revealed that in the deuteron-induced activation of yttrium the direct production of 88 Y is about two times lower than that of 88 Zr. The maximum cross section for the production of the PET isotope 89 Zr by the Y + d process is higher than by Y + p. The yields and decay characteristics of the 88 Y and 88 Zr nuclides are suitable for thin layer activation (TLA) analysis.

and/or via generator system from 88 Zr. We considered it worthwhile to investigate deuteron induced reactions on yttrium. This study was done in the frame of our systematic investigations [7–9] of particles induced nuclear reactions on metals. In the literature a few works have been reported, mostly to test nuclear reaction theories [10–12] Production possibility and separation aspects of different Zr and Y radioisotopes were investigated by Degering et al. [13], Gomma and Nassiff [14] and Zweit et al. [5]. The energy range was limited up to 27 MeV and the overwhelming part of the literature data are related to the (d, 2n) reaction, which is somehow the most dominating process [15]. For other reactions the different data sets are rather discrepant. No data for production of 85 Sr have been reported.

1. Introduction Recently, accelerator utilization has been expanding rapidly, especially in technologically advanced countries. Production of radionuclides, which are used in many fields, especially in medicine, is one of the major areas of accelerator application. For efficient and proper production of a radionuclide, accurate nuclear data are needed [1, 2]. In recent years considerable efforts have been devoted to compilation and standardization of production data for both diagnostic and therapeutic radionuclides and the database is now fairly strong. Most of those data, however, relate to proton induced reactions. For other light charged particle induced reactions the database is still rather weak. This applies particularly to deuteron induced reactions. The radionuclides 88 Y, 87 Y, 88 Zr and 89 Zr have found some use in tumour diagnostics, tumour therapy and investigation of bio-kinetics [3–5]. Furthermore, the activation cross sections could be important for thin layer activation analysis (TLA) of yttrium alloys (especially of aluminum and magnesium) and yttrium oxide ceramics and as well as for dose estimation in accelerator technology. 88 Y is produced directly by particle-induced activation on nat Sr [6] *Author for correspondence (E-mail: [email protected]).

2. Experimental Excitation functions of the Y + d reactions were measured by the stacked-foil irradiation technique. Yttrium foils (thickness 0.049 g/cm2 ) were irradiated with 40 MeV deuterons at beam currents of 80 nA for 34 min using the k = 110 AVF cyclotron, Cyclotron and Radioisotope Center, Tohoku University, Sendai, Japan. Several stacks containing foils of elements like Y-Mo-Pd-Cu-Pb of accurately known thicknesses were prepared. Special care was taken in the determination of the energy degradation along the foils and the bombarding beam intensity. To minimize attenuation of the incident deuteron beam the length of each stack was adjusted. Several circular rings with sufficiently larger internal diameter than the beam were placed at the front of the stack in the aluminum holder to press the sample foils and to avoid inclination of the beam. Furthermore a number of Al foils (0.0067 g/cm2 ) were inserted into the stack as beam monitor and energy degrader to cover the broad energy range. Several extra foils also served as beam energy degrader. In this way the deuteron beam energy range of 9–40 MeV was covered. To avoid errors in the determination of the beam intensity and the energy, excitation functions of the monitor reactions were measured simultaneously with the reactions induced on yttrium.

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3. Data analysis

2003 [18]. The good consistency between the measured and the recommended excitation function of the monitor reaction confirmed the reliability of the beam energy degradation calculations. The data were corrected for the coincidence summing effect caused by the coincidental detection of two or more gamma rays in cascade by using the SUMECC code [19]. The combined uncertainty in each cross section was estimated by taking the square root of the quadratic sum of the following uncertainties: statistical uncertainty of gamma ray counting (0.29%–11%), uncertainty in the monitor flux (∼ 6%), and the uncertainty in the efficiency calibration of the detector (∼ 3%).

Cross sections for the independent and cumulative formation of several radionuclides in deuteron-induced activation of yttrium were measured. The activities of the residual nuclides were determined non-destructively using HPGe gamma-ray spectroscopy (CANBERRA, EURISYS MEASURES, EGPC50-195-R). Measurements were started about 20 hours after end of bombardment, due to the high radiation dose of the short half-life products in the simultaneously irradiated target and monitor foils. Measurements were done at 5 cm and 19 cm distances between the sample and the detector to reduce the coincidence summing, dead time, etc. The residual nuclei were identified by their characteristic gamma lines as well as by their half-lives. The decay data for the investigated radionuclides were taken from the NUDAT database [16] (National Nuclear Data Center, information extracted from the NuDat database, http://www.nndc.bnl.gov/nudat2.). They are given in Table 1. The complete decay of parent to daughter was followed to measure cumulative cross sections. For confirmation the gamma spectrum of each irradiated sample was measured three times, allowing long cooling time intervals (one to several months). The efficiency versus energy curve for the detector used was determined by using the standard point sources. The deuteron beam intensity was determined from the measured activity induced in the front foil of the stack and using the recommended data [17] of the monitor reaction, 27 Al(d, x)24 Na. The energy loss in sample foils as well as in degraders was calculated by the computer program SRIM-

4. Nuclear model calculations Model calculations were performed using the TALYS code [20] to make comparison with the experimental values. TALYS is a computer code system for the analysis and prediction of nuclear reactions. The basic objective behind its construction was the simulation of nuclear reactions that involve n, p, d, t, 3 He and alpha particles, in the 1–200 MeV energy range and for target nuclides of mass 12 and heavier. TALYS is new in the sense that it has been recently written completely from scratch (with the exception of one very essential module, the coupled channels code ECIS), using a consistent set of programming procedures. In this code, automatic reference is made to nuclear structure parameters such as masses, discrete levels, resonances, level density parameters, deformation parameters, fission barrier and gamma-ray parameters, etc. generally from the IAEA Reference Input Parameter Library. With TALYS, a complete set of cross sections of the Y + d processes have been obtained with minimum effort, through a four-line input file of type: Projectile: d; Target element: Y; Mass: 89; Energy: 40 MeV.

Table 1. Decay data of the investigated radionuclides. Nuclide

Half-life

E γ (keV)

Iγ (%)

3.27 d 83.4 d 3.24 h

909.1 392.9 202.5 479.5 898.0 1836.0 380.7 388.5 484.8 514.0

99.1 97.2 97.3 90.7 93.7 99.2 78.1 82.1 89.7 96.0

89

Zr Zr 90m Y 88

88

Y

106.65 d

87m

Y Y

13.37 h 3.33 d

Sr

64.84 d

87g

85

5. Results and discussion 5.1 Activation cross sections The cross sections for the formation of several radionuclides in the interactions of deuterons with yttrium, measured in this work, are presented in Table 2. The total uncertainties are also given. We discuss the various reactions below.

a: Taken from NUDAT database [16].

Table 2. Measured cross sections for the production of the investigated nuclides. Energy MeV 39.1 36.4 33.7 30.7 27.6 24.1 20.3 15.7 10.1

89

Zr

150 ± 11 169 ± 12 207 ± 13 248 ± 15 354 ± 15 575 ± 33 900 ± 56 892 ± 54 303 ± 18

88

Zr

523 ± 26 613 ± 30 718 ± 35 716 ± 36 633 ± 32 420 ± 22 79 ± 4.3 0.5 ± 0.04 0.25 ± 0.02

90m

Y

Cross sections (mb) 88 Y

8.1 ± 1.4 8.9 ± 1.5 10.7 ± 1.8 8.7 ± 1.5 10.8 ± 1.8 10.3 ± 1.8 11.8 ± 2.0 15.6 ± 2.7 15.0 ± 1.6

429 ± 25 428 ± 25 403 ± 24 311 ± 19 238 ± 15 126 ± 9 35 ± 2 4.7 ± 0.5

87m

Y

347 ± 28 184 ± 15 57 ± 5 7 ± 0.6 1.8 ± 0.16 0.55 ± 0.05

87m,g

Y

438 ± 22 226 ± 12 74 ± 5 4.7 ± 0.5

85

Sr

72 ± 4 63 ± 3 50 ± 4.5 11.7 ± 0.8


Experimental determination of deuteron-induced activation cross sections of yttrium

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reported earlier [10, 14]. The 88 Zr production cross section is about two times higher compared to the 88 Y production cross section (see below, Fig. 4). Therefore yttrium is of special interest as target to give large yield of 88 Zr via the 89 Y(d, 3n) reaction at a medium energy cyclotron. The present values are about 2–5 times lower than those of Gomma and Nassiff [14]. The present data are, however, in excellent agreement with Bissem et al. [10] in the overlapping energy range. A significant discrepancy in the energy scale is found between TALYS calculations and the experiment. 5.1.3 89Y(d, x)90m Y Fig. 1. Excitation function of the 89 Y(d, 2n)89 Zr reaction.

5.1.1 89Y(d, 2n)89Zr The measured excitation function for this reaction is shown in Fig. 1 together with the literature data [10, 13, 14]. The contribution from the decay of the 4.18 min isomeric state 89m Zr is included in the measured cross section. The cross section increases rapidly with energy, reaches a maximum and then shows a tail on the higher energy side. The 89 Zr production cross sections are large and the peak in the excitation function is found at about 18 MeV. The most of present measured values for the production of 89 Zr are more close to Bissem et al. [10] than Gomma and Nassiff [14]. A few data points collected from the figure reported by Degering et al. [13] are also shown in Fig. 1. Although the reaction is rather simple, no explanation was found for the discrepancy between our data and the literature data. In our reported data three points are available up to 16 MeV and these are consistent both with Gomma et al. (1973) and Degering et al. (1988). The data calculated by the TALYS code [20] show a shape similar to the experiment and are in good agreement with this work above 20 MeV.

The radionuclide 90 Y has two isomeric states, viz. 90m Y(t1/2 = 3.24 h) and 90g Y(2.67 d), and is produced by the 89 Y(d, p) reaction. The ground state 90g Y does not have any suitable gamma line; the cross section could not be measured. The measured cross sections for the production of 90m Y are shown in Fig. 3 together with the literature values [11]. The present data are in good agreement with the literature values in the overlapping energy range (up to 27 MeV). Corazza et al. (1971) reported data up to 27 MeV and our results are consistent with them. The cross sections are rather low, presumably the ground state is preferentially populated. Since this reaction entails mainly a stripping process, we did not perform the model calculation.

5.1.2 89Y(d, 3n)88Zr The direct formation cross section for 88 Zr through the Y(d, 3n) reaction is shown in Fig. 2. Two data sets were

89

Fig. 3. Excitation function of the 89 Y(d, p)90m Y reaction.

Fig. 2. Excitation function of the 89 Y(d, 3n)88 Zr reaction.

Fig. 4. Excitation function of the 89 Y(d, t + dn + p2n)88 Y reaction.


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5.1.4 89Y(d, t + dn + p2n)88Y The cross sections for the formation of 88 Y are shown in Fig. 4. The nuclide 88 Y is produced by the internal transition decay of the short-lived (t1/2 = 13.9 ms) 88m Y isomer and the decay of 88 Zr. The half-life of 88 Zr is 83.4 days. The spectra were measured one day after EOB to minimize the contribution from the decay of 88 Zr to the production of 88 Y. We separated the direct contribution and observed that the effect of the decay of 88 Zr on the 88 Y production cross section is very low and it can be neglected. Earlier authors [10, 14] reported the 88 Y production cross sections up to 25 MeV, out of which the results of Gomma and Nassiff [14] and these are not consistent with ourthe present work. both in shape and magnitudes. The results of Bissem et al. [10], on the other hand, are consistent with this work both in shape and magnitude. The results of TALYS calculations have similar shape as in this work, but there is considerable energy shift. 5.1.5 89Y(d, x)87m Y The isotope 87 Y has two isomeric states. The higher energy state 87m Y (t1/2 = 13.37 h) decays to the ground state 87g Y (t1/2 = 79.8 h). The measured 87m Y production cross sections shown in Fig. 5 depict the directly formed contribution and through the decay of 87 Zr. In view of Q-values of the possible reactions leading to 87 Zr, its contribution to the 87m Y production can be neglected below 30 MeV. The overlapping peak of 86 Y at 380.7 keV and the IT decay of 87m Y were separated by using other independent gamma rays of the contributing nuclides at two incident energies 36.4 MeV and 39 MeV. The decay of 86 Y does not contribute over the investigated energy range below 36 MeV. No other experimental data were found in the literature. Due to the long waiting time after EOB we could not measure the activation cross section of the parent 87 Zr. We found data sets measured by West et al. [12] for the production of 87 Zr. According to this and the decay scheme, in the cumulative cross section of 87m Y a significant contribution comes from the decay of 87 Zr. The TALYS calculation has again the same shape as the experiment, but there is disagreement in magnitude and energy scale.

Fig. 6. Excitation function of the 89 Y(d, x)87m,g Y reaction.

and by 98.43% internal transition decay of the metastable state were measured and are shown in Fig. 6. The present data are the first ones for this process. Attention was paid to the complete internal decay of the metastable state to be able to measure the cumulative production cross sections. The measured cross sections are mainly the sum of cross sections for the formation of the metastable state and the ground state. The results of the theoretical calculations using TALYS code are in agreement with this work in trend, but not in magnitude. 5.1.7 89Y(d, x)85Sr

The cross sections for the cumulative production of the ground state of 87 Y by the 89 Y(d, tn + d2n + p3n) process

The 85 Sr production cross section was measured by using the 514 keV gamma-ray. To separate this gamma line from the strong annihilation peak (511 keV), the measurement was done about 40 days after the end of irradiation when a separation of the peak could be obtained through highresolution gamma ray spectroscopy system. 85 Sr is expected to be produced via the processes 89 Y(d, 2nα) (Q = −12.03 MeV), 89 Y(d, 2t) (Q = −23.30 MeV), 89 Y(d, ndt) (Q = −29.62 MeV), 89 Y(d, 3n 3 He) (Q = −32.60 MeV) etc. The 85 Sr production cross sections shown in Fig. 7 sharply rise with the increasing incident beam energy due to several open particle-splitting channels. No earlier measurement was identified for this process. The TALYS calculation again agrees only partially with the experimental data.

Fig. 5. Excitation function of the 89 Y(d, x)87m Y reaction.

Fig. 7. Excitation function of the 89 Y(d, x)85 Sr reaction.

5.1.6 89Y(d, x)87m,g Y


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5.1.8 General discussion of experimental and theoretical data The experimental data measured in this work show that the cross sections for the (d, xn) reactions are rather high, followed by (d, pxn) reactions whose cross sections are about half of the (d, xn) reactions. The (d, p) process has a relatively low cross section, but since it involves the stripping mechanism, it is an exception. The formation of 85 Sr involves the emission of several neutrons and charged particles and is therefore rather complex in nature. All those observations are qualitatively in agreement with some older detailed experimental studies [21] and systematics [15] in the mass region 80 to 130. Our measurements now strengthen the database for deuterons on yttrium, especially in the energy region above 27 MeV. As far as the results of nuclear model calculations are concerned, it is obvious that the single phenomenalogical code TALYS describes the excitation functions of deuteron induced reactions only in approximate terms. For achieving better agreement, some change in input parameters is essential. But that treatment was beyond the scope of this study since the data here have primarily an application oriented character. It should be mentioned that another simple calculational code like ALICE-IPPE also does not reproduce well the deuteron induced reaction cross sections. More sophisticated codes like STAPRE and GNASH, on the other hand, give often better agreement.

5.2 Thick target integral yields From the measured excitation functions the thick target integral yields of all the radionuclides were deduced as a function of deuteron energy upto 40 MeV. The data were obtained by assuming the beam current as 1 µA and the irradiation time as 1 h. The results are shown in Figs. 8 and 9. The radionuclide 89 Zr is formed in the highest yield.

6. Conclusions The measured data above 27 MeV are new for all the investigated reactions. In the energy region below 27 MeV our data

Fig. 9. Integral yields of 88 Y, 88 Zr and a function of deuteron energy.

85

Sr from a yttrium target as

confirm some of the previous results but differ from some others. As discussed above, the results of TALYS calculations are only in partial agreement in disagreement with the experiment. The nuclides 88 Y and 88 Zr have significantly longer halflives than the other neutron deficient radioisotopes of these two elements. Therefore, the effects of the simultaneously produced radionuclides on the purity of 88 Y and 88 Zr are not critical. Significant amounts of these two radionuclides are produced by the 89 Y + d process at a medium energy accelerator. The direct production of 88 Y is low as compared to 88 Zr. But directly produced 88 Y is anyway not so interesting since it is not in a “no-carrier added form”. On the basis of the yield data it can be concluded that the Y + d route is much more efficient to produce 88 Zr than the nat Mo(d, x) process [9]. Significant amounts of 89 Zr can be produced at low energy accelerators using yttrium as target and the possibility to involve impurity level of gamma emitting radionuclides is very low at energies < 20 MeV. Considering the gamma rays, half-lives and yields, the radionuclides 88 Y and 88 Zr are the most suitable for TLA analysis. Particularly, the deuteron-induced activation of yttrium can be effectively used for wear studies in alloys and ceramics by TLA. Acknowledgment. The authors thank the cyclotron operation crew and colleagues at CYRIC, Sendai for their help in performing irradiations. The authors are highly grateful to Prof. Dr. S. M. Qaim, Juelich Germany, for his kind suggestions and corrections in the preparation of this paper. This work received financial support from the Japanese Society of Science and from the Hungarian Academy of Sciences. One of the authors of this paper (F. Ditrói) is a grantee of the Bolyai János Scholarship of the Hungarian Academy of Sciences.

References

Fig. 8. Integral yields of 87m,g Y, 87m Y, 89 Zr and target as a function of deuteron energy.

90m

Y from a yttrium

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