Isomeric yield ratios for the natSb(γ,xn)120m,g,122m,gSb reactions measured at 40-, 45-, 50-, 55-, and 60-MeV bremsstrahlung energies

Nuclear Instruments and Methods in Physics Research B 283 (2012) 40–45

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Isomeric yield ratios for the natSb(c,xn)120m,g,122m,gSb reactions measured at 40-, 45-, 50-, 55-, and 60-MeV bremsstrahlung energies Nguyen Van Do a, Pham Duc Khue a, Kim Tien Thanh a, Guinyun Kim b,⇑, Man-Woo Lee b, Kyung-Sook Kim b, Sung-Chul Yang b,c, Eunae Kim d, Moo-Hyun Cho d,e, Won Namkung e a

Institute of Physics, Vietnam Academy of Science and Technology, 10 Dao Tan, Hanoi, Viet Nam Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea Nuclear Data Center, Korea Atomic Energy Research Institute, Daejeon 305-353, Republic of Korea d Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea e Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 25 July 2011 Received in revised form 11 April 2012 Available online 21 April 2012 Keywords: Isomeric yield ratio Photonuclear reaction nat Sb(c,xn)120m,g,122m,gSb Activation method 40-, 45-, 50-, 55-, 55-, and 60-MeV bremsstrahlung HPGe detector

a b s t r a c t We measured the isomeric yield ratios for the 120m,gSb and 122m,gSb isomeric pairs produced by nat Sb(c,xn)120m,g,122m,gSb photonuclear reactions in the bremsstrahlung energy region from 40 to 60 MeV with a step size of DE = 5 MeV by the activation method. The induced c-activities of the irradiated samples were measured by a coaxial high purity germanium detector coupled to a PC-based multichannel analyzer. The necessary corrections were made to improve the accuracy of the experimental results. The experimental results at bremsstrahlung energies of 40, 45, 50, 55, and 60 MeV were 0.045 ± 0.003, 0.046 ± 0.003, 0.048 ± 0.003, 0.050 ± 0.003, and 0.049 ± 0.003 for the 120m,gSb isomeric pair, and 0.341 ± 0.022, 0.362 ± 0.020, 0.374 ± 0.021, 0.371 ± 0.021, and 0.358 ± 0.022 for the 122m,gSb isomeric pair, respectively. The present results are the first measurements at bremsstrahlung energies just above the giant dipole resonance region. The obtained results confirm the dependence of the isomeric yield ratios on the incident energy and the reaction channel effect observed in some earlier experiments. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Nuclei with a metastable state (isomeric state) and an unstable ground state formed in nuclear reactions can be used to measure the relative populations of these two states, defining the so-called isomeric cross-section ratio. Studies of these isomeric ratios are considerably important for basic nuclear physics research and applications [1–4]. The isomeric ratios are mainly measured in nuclear reactions induced by neutrons [3–20], charged particles [21–28], and bremsstrahlung photons [29–32]. The investigations indicate that the isomeric ratio depends on the energy of the incoming projectile [10,11,21,22,25,27,29] and on the types of nuclear reactions or reaction channels [10,22,23]. Among the projectiles used, photons carry a relatively small momentum and do not introduce a large angular momentum into the compound nucleus; however, they are a good tool for investigation of the dependence of isomeric yield ratios on the incident photon energies and the mass difference between target and product nuclei [33]. In the present work, we chose the natSb(c,xn)120m,gSb and 123Sb (c,n)122m,gSb reactions for investigation. So far, the natSb ⇑ Corresponding author. Tel.: +82 539505320; fax: +82 539555356. E-mail address: [email protected] (G. Kim). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.04.015

(c,xn)120m,g Sb and 123Sb(c,n)122m,gSb nuclear reactions have been studied in the low energy region from the threshold to approximately 22–24 MeV [29–32]. At low energies, only simple photonuclear reactions such as (c,n) or (c,p) can be produced, and the characteristics of these reactions have been thoroughly studied. At higher photon energies, multiparticle photonuclear reactions can occur. Until now, little has been known about the mechanisms of those nuclear reactions. Some isomeric ratio measurements have been performed for the 120m,gSb and the 122m,g Sb isomeric pairs formed by different nuclear reactions, such as (n,c) [13–15], (n,2n) [16–20], (p,n) [24–26], (a,pn) [27,28], and (c,n) or (c,xn) [29–32]. The reported results depend on the type of the incoming projectiles in addition to the energy. The aim of the present work is to extend our incident bremsstrahlung photon measurements to higher energies and to reactions of varying degrees of complexity. These measurements were carried out at bremsstrahlung energies of 40, 45, 50, 55, and 60 MeV. Along with the existing reference data, the obtained results may improve our understanding of the energy dependence of isomeric yield ratios in the energy region from the threshold to just above the giant dipole resonance (GDR) region, as well as the reaction channel effect for the formation of the 120m,gSb and 122m,g Sb isomeric pairs.

N. Van Do et al. / Nuclear Instruments and Methods in Physics Research B 283 (2012) 40–45

2. Basic equations for the determination of the isomeric yield ratio The isomeric cross-section ratio, IR, is expressed by IR = rm/rg, where rm and rg denote the cross-sections for the formation of a metastable-state and an unstable ground-state, respectively. Because the isomeric-state and the ground-state have significantly different spin values, the isomeric ratio can be represented as the ratio of the cross-sections for the production of high- and low-spin states; specifically, IR = r(high-spin)/r(low-spin) [34–36]. In our case, the activation was performed by non-monoenergetic bremsstrahlung photons, and the isomeric ratio is expressed through the yields of the two states instead of the two cross-sections (i.e., IR = Yhigh-spin/Ylow-spin) [36–38], and the yield of the bremsstrahlung-induced reaction is expressed as follows:

Yk ¼

Z

Ecmax

rk ðEÞ/k ðEÞdE

ð1Þ

Eth

where k(=m,g) represents the isomeric state (m) or the ground state (g) of an isomeric pair, rk(E) is the energy dependent reaction crosssection, /k(E) is the number of c-quanta in the bremsstrahlung spectrum with energy E, and Ecmax and Eth are the maximum bremsstrahlung end-point energy and the reaction threshold, respectively. By considering that a pulsed bremsstrahlung beam was used for the sample irradiation, the relationship between the number of detected c-rays (Sk) and the reaction yield (Yk) can be expressed as follows:

Sk ¼

N 0 Ik ek ð1  ekk s Þð1  ekk ti Þekk tw ð1  ekk tc ÞY k kk ð1  ekk T Þ

ð2Þ

where N0 is the number of target nuclei, Ik is the intensity of the measured c-ray, ek is the detection efficiency for the c-ray of interest, kk is the decay constant of the k(=m,g) state, s is the pulse width, T is the cycle period, ti is the irradiation time, tw is the waiting time, and tc is the counting time. By considering the production of nuclides in the isomeric state and the ground state at the same time of irradiation and the decay of the isomeric-state to the ground-state, we can derive the isomeric yield ratio as follows:

IR 

   1 Ym kg F m Sg em Im Pkg Pkm þ ¼    ; Yg km F g Sm eg Ig kg  km kg  km

ð3Þ

where Sm and Sg are the photo-peak areas for the detected c-rays of the isomeric and ground states, P is the branching ratio for the decay of the isomeric state to the ground state, and the factor Fk is defined as follows:

F k¼m;g ¼

ð1  ekk s Þ  ð1  ekk ti Þ  ekk tw  ð1  ekk tc Þ kk ðTsÞ e 1  ekk T

ð4Þ

After the measurements, photopeak area analysis and necessary corrections were performed (such as the c-ray interference, counting losses due to the c-ray self-absorption, coincidence summing of cascade c-rays and the variation of bremsstrahlung intensity during the irradiation [39–44]), the isomeric yield ratio for the 120m,g Sb and 122m,gSb isomeric pairs was then calculated using Eq. (3). 3. Experimental procedure 3.1. Sample irradiation and measurement The experiments were carried out at the 100 MeV electron linac of the Pohang Accelerator Laboratory (PAL), the details of which are described elsewhere [45,46]. Bremsstrahlung photons were

41

produced when pulsed electrons of 40, 45, 50, 55, and 60 MeV from the 100-MeV electron linac hit a thin W target with a size of 100 mm  100 mm and a thickness of 0.1 mm. The details of the bremsstrahlung production are described elsewhere [42,44,47,48]. The experimental samples were prepared from high-purity (99.999%) natural antimony in powder form (200 mesh), made by the Alfa Aesar, a Johnson Mathey Company (Ward Hill, MA, USA). The antimony powders were encapsulated into identical polyethylene capsules with diameters of 12 mm. The characteristics of the antimony samples are given in Table 1. The irradiation sample was placed in air at a distance of 12 cm from the thin W target, and it was positioned at 0° to the direction of the electron beam. Five samples were irradiated separately with 40-, 45-, 50-, 55-, and 60MeV bremsstrahlung with corresponding beam currents of 28 ± 4, 31 ± 3, 35 ± 3, 40 ± 3, and 50 ± 3 mA, respectively. The irradiation time for each sample was 20 min. During irradiation of the samples, the electron linac was operated at a repetition rate of 15 Hz and a pulse width of 2.0 ls. Antimony has two naturally occurring isotopes, 121Sb and 123Sb, which have abundances of 57.36% and 42.64%, respectively. When the antimony sample was irradiated by the high energy bremsstrahlung, the 120m,g,122m,gSb isomeric pairs were produced from the 121Sb(c,n)120m,gSb, 123Sb(c,3n)120m,g Sb, and 123Sb(c,n)122m,gSb photonuclear reactions. The characteristics of the nuclear reactions are listed in Table 2 [49]. The induced activities of the activated samples were measured by a c spectrometer without any chemical purification. The c spectrometer consisted of a coaxial high purity germanium (HPGe) detector (ORTEC) with an energy resolution of 1.75 keV and a relative efficiency of 10% at the 1332.5 keV c peak of 60Co. The detector was coupled to a computer-based multichannel analyzer card system, which could determine the photopeak area of the c spectrum with the Gamma Vision software, version 5.10, EG&G ORTEC. The detector efficiency was determined experimentally using a set of standard gamma sources. The details of the HPGe detector efficiency calibration are given in our previous report [43,44]. After measurement and analysis of the c spectra, the radioactive isotopes under consideration were identified based on their characteristic c-ray energies and half-lives. Because the half-lives of the 120gSb (T1/2 = 15.89 m) and 122mSb (T1/2 = 4.191 m) isotopes are fairly short, the measurements were started as soon as possible after the end of the irradiation in order to obtain good counting statistics. In contrast, the half-lives of the 120m Sb (T1/2 = 5.76 d) and 122gSb (T1/2 = 2.7238 d) are long enough that their activity measurements were started some hours after the irradiation. Typical c-spectra of the antimony samples irradiated by 50-MeV bremsstrahlung are shown in Fig. 1(a) and (b). To measure the activities of the reaction products, we chose their characteristic c-rays, which are high intensity and well separated. To minimize the uncertainties caused by random coincidence and pile-up effects, we chose an appropriate distance from the sample to the detector for each measurement. Generally, the dead-time was kept below 0.5% during measurements. For this purpose, the activated sample was attached to a plastic sample holder and could be set at a distance from 5 to 105 mm from the surface of the HPGe detector. 3.2. Determination of isomeric yield ratio for the 120m,gSb isomeric pair The isomeric yield ratios for the natSb(c,xn)120m,gSb reactions induced by bremsstrahlung were determined from the measured c activity of the isomeric-state 120mSb (high-spin state 8) and the ground-state 120gSb (low-spin state 1+). The threshold energies required for the 121Sb(c,n)120m,gSb and 123Sb(c,3n)120m,gSb reactions are 9.242 and 25.016 MeV, respectively. The ground-state 120gSb (T1/2 = 15.89 m) nucleus decays directly to the ground-state of 120 Sn (0+) by electron capture (EC) and b+ reactions with a 98.3%

42

N. Van Do et al. / Nuclear Instruments and Methods in Physics Research B 283 (2012) 40–45

Table 1 Characteristics of the antimony samples. Sample

Diameter (mm)

Thickness (mm)

Weight (g)

Purity (%)

Emax.Bremss. (MeV)

Sb40 Sb45 Sb50 Sb55 Sb60

12 12 12 12 12

1.78 1.67 1.90 1.77 1.82

0.1350 0.1261 0.1440 0.1340 0.1378

99.999 99.999 99.999 99.999 99.999

40 45 50 55 60

Table 2 Nuclear reactions investigated and decay data of produced isomeric pairs Nuclear reaction 121

Sb(c,n)120mSb

123

Sb(c,3n)120mSb

121

120g

Sb(c,n) Sb Sb(c,3n)120gSb 123 Sb(c,n)122mSb 123

123

Sb(c,n)122gSb

Threshold energy, Eth (MeV) 9.242

120m,g

Sb and

122m,g

Sb [49].

Half-life, T1/2

Spin parity, Jp

c-energy, Ec (keV)

c-ray intensity, Ic (%)

5.76 d

8

89.9 197.3 1023.1 1171.3 703.8 1171.3 61.41 76.06 564.12 692.79 1140.55

79.5 87.0 99.4 100 0.149 1.7 55 23 71 3.85 0.76

25.016 9.242 25.016 9.103

15.89 m

1+

4.191 m

8

8.966

2.7238 d

2

Fig. 1. Typical c-ray spectra from the natSb sample irradiated with 50 MeV bremsstrahlung for 20 min: (a) spectrum recorded at 3 min after the end of the irradiation, with a counting time of 5 min and (b) spectrum recorded at 70 min after the end of the irradiation, with a counting time of 60 min.

branching ratio. Therefore, the ground-state nucleus emits two crays with low intensities; specifically, 703.8 keV (0.149%) and 1171.3 keV (1.7%) c-rays. For the activity measurement, the 1171.3 keV c-ray is preferable because its intensity is high. The isomeric-state 120mSb nucleus decays directly to the 2481.6 keV state of 120Sn (7) by an EC process with a branching ratio of 100%, and it emits several c-rays with high intensity; specifically, 89.9 keV (79.5%), 197.3 keV (87.0%), 1023.1 keV (99.4%), and 1171.3 keV (100%) c-rays. The 120mSb was also identified with the 1171.3 keV c-ray. The 1171.3 keV photopeak is a mixture of the 120mSb (T1/ 120g Sb (T1/2 = 15.89 m) isomeric pairs, as shown in 2 = 5.76 d) and Fig. 1. Therefore, an interference correction is required in the activity measurements. For this purpose, the activity measurement for the ground-state of 120gSb was started 2 min after the end of the irradiation, and the measurement for the isomeric-state 120mS was started after approximately 3 h. After the activity measurements and data analysis were completed, the isomeric yield ratios for the 120m,g Sb isomeric pair were calculated from Eq. (3) under a condition of P = 0. This experimental procedure has the advantage that the detection efficiency is not involved in the isomeric yield ratio calculation because the activities of both the 120mSb and 120gSb isotopes were measured by c-rays with the same energy of 1171.3 keV.

3.3. Determination of isomeric yield ratio for the 122m,gSb isomeric pair The isomeric-state 122mSb (high-spin state, 8) decays to the unstable ground-state nuclide 122gSb (low-spin state, 2) by emitting the 61.41 keV (55%) and the 76.06 keV (23%) c-rays. The activity of the isomeric-state nuclide was measured with the strongest c-ray, at 61.41 keV. Because the half-life of the isomeric-state 122m Sb is short (T1/2 = 4.191 min), the measurement was started as soon as possible after the end of the irradiation. The unstable ground-state 122gSb (T1/2 = 2.7238 d) nucleus decays by both an EC and a b process to form the stable isotopes 122 Sn and 122Te. The 564.12 keV (71%) c-ray was used for the activity measurement because it is interference-free. The measurement was started several hours after the end of the irradiation. After the measurements and appropriate corrections were made, the isomeric yield ratios for the 122m,gSb isomeric pair were calculated with Eq. (3) under a condition of P = 1.

4. Results and discussion The present isomeric yield ratios for the natSb(c,xn)120m,gSb reactions measured in the energy region from 40- to 60-MeV

43

N. Van Do et al. / Nuclear Instruments and Methods in Physics Research B 283 (2012) 40–45 Table 3 Isomeric yield ratios for the energies. Nuclear reaction

121

nat

Sb(c,xn)120m,gSb reaction at different bremsstrahlung

EBrem. (MeV)

IR = Yhigh/Ylow This work

Refs. 0.009 ± 0.003 [29] 0.01 ± 0.003 [29] 0.014 ± 0.003 [29] 0.027 ± 0.002 [29] 0.0179 ± 0.00107 [32] 0.026 ± 0.002 [29] 0.071 ± 0.007 [30] 0.026 ± 0.001 [29] 0.029 ± 0.001 [29] 0.0516 ± 0.007 [31] 0.034 ± 0.001 [30] 0.035 ± 0.001 [29] 0.0616 ± 0.009 [31] 0.036 ± 0.001 [29] 0.038 ± 0.001 [29] 0.0612 ± 0.005 [31] 0.037 ± 0.001 [31] 0.041 ± 0.001 [31] – – – – –

Sb(c,n)120m,gSb 121 Sb(c,n)120m,gSb 121 Sb(c,n)120m,gSb 121 Sb(c,n)120m,gSb

12 13 14 15

– – – –

121

16

–

121 121

121 121

121 121

121

Sb(c,n)120m,gSb 120m,g

Sb(c,n) Sb Sb(c,n)120m,gSb

17 18

– –

Sb(c,n)120m,gSb Sb(c,n)120m,gSb

19 20

– –

Sb(c,n)120m,gSb Sb(c,n)120m,gSb

21 22

– –

23 24 40 45 50 55 60

– – 0.045 ± 0.003 0.046 ± 0.003 0.048 ± 0.003 0.050 ± 0.003 0.049 ± 0.003

Sb(c,n)120m,gSb Sb(c,n)120m,gSb nat Sb(c,xn)120m,gSb nat Sb(c,xn)120m,gSb nat Sb(c,xn)120m,gSb nat Sb(c,xn)120m,gSb nat Sb(c,xn)120m,gSb 121

Table 4 Isomeric yield ratios for the energies.

123

Sb(c,n)122m,gSb reaction at different bremsstrahlung

Nuclear reaction

EBrem.(MeV)

IR = Yhigh/Ylow This work

Refs.

123

Sb(c,n)122m,gSb 123 Sb(c,n)122m,gSb 123 Sb(c,n)122m,gSb 123 Sb(c,n)122m,gSb

12 13 14 15

– – – –

123

16

–

17 18

– -

19 20

– –

21 22

– –

40 45 50 55 60

0.341 ± 0.022 0.362 ± 0.020 0.374 ± 0.021 0.371 ± 0.021 0.358 ± 0.022

0.092 ± 0.01 [29] 0.118 ± 0.015 [29] 0.12 ± 0.015 [29] 0.129 ± 0.021 [30] 0.0136 ± 0.00082 [32] 0.138 ± 0.026 [29] 0.15 ± 0.01 [30] 0.162 ± 0.03 [29] 0.154 ± 0.02 [29] 0.0154 ± 0.0013 [31] 0.182 ± 0.02 [29] 0.16 ± 0.014 [29] 0.0187 ± 0.0043 [31] 0.191 ± 0.012 [29] 0.213 ± 0.036 [29] 0.0193 ± 0.0015 [31] – – – – –

Sb(c,n)122m,gSb

123

122m,g

Sb(c,n) Sb Sb(c,n)122m,gSb

123

123

Sb(c,n)122m,gSb Sb(c,n)122m,gSb

123

123

Sb(c,n)122m,gSb Sb(c,n)122m,gSb

123

123

Sb(c,n)122m,gSb Sb(c,n)122m,gSb 123 Sb(c,n)122m,gSb 123 Sb(c,n)122m,gSb 123 Sb(c,n)122m,gSb 123

bremsstrahlung together with the reference data measured at lower bremsstrahlung energies from 12 to 24 MeV [29–32] are given in Table 3 and illustrated graphically in Fig. 2. Those for the 123 Sb(c,n)122m,gSb reactions are also given in Table 4 and illustrated in Fig. 3, respectively. The main sources of uncertainty for the present results are given in Table 5. Tables 3 and 4 show that the isomeric yield ratios of the 122m,gSb isomeric pair are much larger than those of the 120m,gSb isomeric pair. This observation is mainly a product of the spin states of the isomeric pairs and the target nuclei. The high spin state (Jhigh = 8) for the 122m,gSb and 120m,gSb isomeric pairs is the same, but the low spin state for those isomeric pairs is different (Jlow = 1+ for the 120gSb and 2 for the 122gSb). Furthermore, the spin difference between the high spin state of the isomeric nucleus and the spin of the target nucleus is 11/2 for the 121 Sb(c,n)120m,gSb reaction, but it is only 9/2 for the 123Sb(c,n) 122m,g Sb reaction. The isomeric yield ratio for the 123Sb(c,n)122m,gSb reaction is much larger than that of the 121Sb(c,n)120m,gSb reaction because of the smaller difference between the two spin states of 122m Sb and 122gSb, as well as the smaller difference between the high spin state of the 122mSb and the target spin of 123Sb.

Figs. 2 and 3 show the trend of energy dependence of the isomeric yield ratios for both the natSb(c,xn)120m,gSb and 123Sb (c,n)122m,gSb reactions over a wide range of energies, from the reaction threshold to 60 MeV. The transition from the previous data measured in the low energy region (from 12 to 24 MeV) to the present data measured at higher energies (from 40 to 60 MeV) is relatively smooth. There are no reference data measured in the 40– 60 MeV energy interval for direct comparison, but the present data appear to be in good trend agreement with those measured by Davydov et al. [29]; however, the present data differ considerably from those measured at 16, 18, 20, and 22 MeV by Thiep et al. [31,32]. The isomeric yield ratios for both the natSb(c,xn)120m,gSb and 123 Sb(c,n)122m,gSb reactions increase rapidly as the bremsstrahlung energies increase from the reaction threshold up to the GDR region at approximately 25–30 MeV. Then, they appear to increase only slightly over the entire investigated energy range. The rapidly increasing isomeric yield ratios can be explained on the basis of a compound nuclear reaction mechanism in which the increased momentum was transferred to the compound nuclei. However, the direct channel of the (c,n) reaction also occurs at higher incident energies. The directly emitted particles carry away a

Fig. 2. The isomeric yield ratios for the 120m,gSb nuclei produced from different nuclear reactions as a function of the incident beam energy.

Fig. 3. The isomeric yield ratios for the 122m,gSb nuclei produced from different nuclear reactions as a function of the incident beam energy.

44

N. Van Do et al. / Nuclear Instruments and Methods in Physics Research B 283 (2012) 40–45 123

Table 5 Uncertainty sources in the isomeric yield ratio measurements. Source of uncertainty

Uncertainties (%) 120m,g

Statistical error Detection efficiency Half-life Gamma intensity (%) Coincidence summing effect Net peak area fitting Gamma ray self-absorption Others Total uncertainty

Sb

1.5–3.0 – 0.4 1.0–2.0 2.0–3.0 2.0–3.0 0.5 3.5 4.9–6.6

122m,g

Sb

1.0–2.0 2.0–3.0 0.07 3.0–4.0 2.0–3.0 1.0–2.0 0.5–1.0 2.0 4.8–6.8

relatively large angular momentum, and only a fraction of the energy and angular momentum of the incident bremsstrahlung photon are transformed to the target nucleus. Above the energy of 25–30 MeV, the direct reaction channel becomes the dominant process. The direct reactions largely suppress the population of states with higher spins; consequently, the yield ratio of high to low spin isomers might not continue its rapidly increasing trend. Instead, it would level off or cease to change, as seen in Figs. 2 and 3. By neglecting the significantly scattered data in Refs. [31,32], the isomeric yield ratios for both the natSb(c,xn)120m,gSb and 123 Sb(c,n)122m,gSb reactions are well fitted to a function of the following form:

y ¼ A½1  expfkðEc max  E0 Þg

ð5Þ

where A, k and E0 are fitting parameters and Ecmax is the maximum end-point energy of the bremsstrahlung. With the least squares fitting method, we obtain the best values of the parameters in case of the 120m,gSb isomeric pair, as follows: A = 0.048 ± 0.001; k = (0.137 ± 0.013) (MeV1) and E0 = (10.82 ± 0.037) (MeV). The goodness of fit is 0.967. In the case of the 122m,gSb isomeric pair, we obtain A = 0.421 ± 0.023; k = (0.045 ± 0.008) (MeV1) and E0 = (6.799 ± 1.065) (MeV). The goodness of fit is 0.983. The solid curves in Figs. 2 and 3 provide a good demonstration of the saturating trend of the energy-dependence of the isomeric yield ratios for the natSb(c,xn)120m,gSb and 123Sb(c,n)122m,gSb reactions in the investigated energy range. We also plotted the isomeric yield ratios for the 120m,gSb and 122m,g Sb isomeric pairs produced by different types of nuclear reactions, such as (n,c) [13–15], (n,2n) [16–20], (p,n) [24–26] and (a,pn) [27–28], as shown in Figs. 2 and 3 for investigation of the reaction channel effect. Fig. 2 shows that the isomeric yield ratios for the 120m,gSb pair produced from the (n,2n) and (a,pn) reactions are approximately one order higher than those from the (c,xn) reaction. However, Fig. 3 shows that the isomeric yield ratios for the 122m,gSb pair produced from the (n,2n) and (a,pn) reactions are approximately 3–4 times higher than those from (c,n) reactions. From the observed trends, it can be concluded that the isomeric yield ratios for the 120m,gSb and the 122m,gSb isomeric pairs not only depend on the energies of the incident projectiles but also on the types of nuclear reactions. These results provide an independent confirmation of the reaction channel effect, as indicated previously [10,22,23]. 5. Conclusion We have measured the isomeric yield ratios for the Sb(c,xn)120m,gSb and 123Sb(c,n)122m,gSb reactions with bremsstrahlung end point energies of 40, 45, 50, 55, and 60 MeV. The present results have been measured for the first time. We observed that the isomeric yield ratios for the natSb(c,xn)120m,gSb and

nat

Sb(c,n)122m,gSb reactions increased gradually as the incident bremsstrahlung energies increased from the reaction threshold to approximately 60 MeV. In addition, the effect of reaction channels on the production of the 120m,gSb and 122m,gSb isomeric pairs was also observed. The values provided by the experimental data for the photonuclear reactions are relatively small compared with those measured by other reaction channels, such as (n,2n), (p,n) and (a,pn). Acknowledgements The authors would like to express their sincere thanks to the staff of the Pohang Accelerator Laboratory for their excellent operation of the electron linac and for their support. This work was partly supported by the National Research Foundation of Korea (NRF) through a grant provided by the Korean Ministry of Education, Science & Technology (MEST) in 2011 (Project No. 2011-0006306 and 2011-0025762); by the World Class University (WCU) program (R31-30005); by the Institutional Activity Program of Korea Atomic Energy Research Institute; and by the Vietnam National Foundation for Science and Technology Development (NAFOSTED). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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