Activation Cross Sections for Reactions Induced by 14 Mev Neutrons On

Home , Isotopes of indium, Isotopes of tin

Radiochim. Acta 93, 311–326 (2005)  by Oldenbourg Wissenschaftsverlag, München

Activation cross sections for reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets with reference to 111In production via radioisotope generator 112Sn(n,2n)111Sn → 111In

By Emil Betˇ ak´ 1,2,∗, Renata Mikołajczak3 , Joanna Staniszewska3, Stefan Mikołajewski4 and Edward Rurarz4

1 Institute of Physics, Slovak Academy of Sciences, 84511 Bratislava, Slovakia 2 Faculty of Philosophy and Sciences, Silesian University, 74601 Opava, Czech Rep. 3 Radioisotope Centre, 05-400 Otwock-Swierk,´ Poland 4 Sołtan Institute for Nuclear Studies, 05-400 Otwock-Swierk,´ Poland

(Received March 20, 2004; accepted in ﬁnal form January 10, 2005)

Activation cross sections / Ge detector / medicine. Due to the short half-life of 113mIn its application Neutron-induced reactions / γ-ray spectroscopy / is limited to rapid physiological studies that can be scanned Tin region / Neutron generator within a maximum of 4 to 8 hours. It is still in use, because it also allows repeated investigations within few hours. The preparation of 113Sn → 113mIn generator requires the use of γ Summary. We measured activation cross sections via -ray high-ﬂux reactor and a strongly enriched 112Sn target in the spectroscopy using high-purity germanium detectors for 16 reaction 112Sn(n,γ)113Sn. reactions induced by (14.4 ±0.2) MeV neutrons on isotopes of tin. The cross sections are: An alternative is the use of the longer-lived (carrier- free) 111In. Medical investigators have shown that 111In is σ(112 ( , )111 ) = ( ± ) Sn n 2n Sn 1104 43 mb, an important radionuclide for locating and imaging cer- σ(112Sn(n, p)112mIn) = (33.6 ±2.1) mb, tain tumors, visualization of the lymphatic system etc., σ(112Sn(n, p)112gIn) = (42.7±3.1) mb, σ(114Sn(n, 2n)113Sn) = (1270 ±115) mb, and is applied in myriad of labeling. For several in-vivo σ(114Sn(n, p)114m2In) = (20.5 ±1.1) mb, studies (e.g. the study of slow biological processes, for σ(115Sn(n, p)115mIn) = (35.2 ±2.6) mb, which the observation periods of 1 to 3 days after the σ(116Sn(n, p)116m2In) = (11.1 ±0.5) mb, administration are necessary), 111In has very favourable σ(117Sn(n, np)116m2In) = (1.35 ±0.11) mb, decay characteristics [1–3]. Its half-life is 2.8 days. The σ(117Sn(n, p)117mIn) = (4.5 ±0.4) mb, 100% electron-capture decay leads to an excited state σ(117Sn(n, p)117gIn) = (12.8±0.7) mb, of 111Cd. This state is deexcited by the cascade of the σ(117 ( , )117m ) = ( ± ) Sn n n Sn 246 21 mb, 171.3 keV and 245.4 keV gamma-rays having intensities σ(118 ( , )117m ) = ( ± ) Sn n 2n Sn 816 70 mb, of 90.3% and 94%, respectively. The relevant average en- σ(118Sn(n,α)115gCd) = (1.26 ±0.16) mb, σ(120 ( ,α)117m ) = ( . ± . ) ergies (and intensities) of conversion electrons are low, Sn n Cd 0 27 0 04 mb, . . σ(120Sn(n,α)117gCd) = (0.29 ±0.06) mb and namely 144 6 keV (10%) and 218 6 keV (6%). The energy σ(124Sn(n, 2n)123mSn) = (590 ±26) mb. of the Auger electrons is only about 20 keV. These gamma- ray energies are in the optimum range of the photo-peak Two 112Sn targets enriched to 62.5% and 84%, respectively, were used for these measurements in addition to the natural efﬁciency for commercially available gamma cameras or tin. The cross sections were compared with experimental data scanners. They can be imaged either separately or jointly found in the literature, with published empirical formulae and (184 photons per 100 EC disintegrations). In the latter case with model calculations including also the pre-equilibrium an improved statistics or more rapid data accumulation is contribution. For reactions which do not involve protons from achieved. below the closed Z = 50 shell, the agreement to the data is As summarized in Table 11, the isotope 111In can be pro- reasonable, it is somewhat weaker for the (n, p) reactions duced either directly or indirectly via precursors: 111Sb and and still worse in the case of (n,α), where, however, the 111Sn. The list of reactions given in this table is not exhaus- pre-equilibrium component is not described properly by the tive; other reactions, may come into use in the future. models included so far. The possibility of production of The isotope 111In is usually produced by 4He bombard- 111Sn → 111In generator system is considered. ment of a silver target via the 109Ag(4He, 2n)111In reaction, by deuteron bombardment of a cadmium target (natural or 1. Introduction enriched), the nuclear reactions are there 110Cd(d, n)111In 111 ( , )111 113m and Cd d 2n In, or by proton bombardment of cad- Carrier-free In, which is commercially available from mium (natural and enriched), the nuclear reactions in this 113 = → the radioisotope generator system Sn (T1/2 115 d) 111,112,113,114 ( , )111 = 113m case are Cd p xn In (x 1–4) [6, 7]. In (T1/2 = 100 min), is being used extensively in nuclear *Author for correspondence (E-mail: [email protected]). 1 Table 1 has been constructed using the data of Refs. [4, 5]. 312 E. Betˇ ak´ et al.

Table 1. Thresholds and Coulomb barri- ers for reactions leading to production of Reaction Nat. abund. Q-value Threshold Coulomb 111In. of target energy barrier (per cent) (MeV) (MeV) (MeV)

111Cd(p, n)111In 12.75 −1.644 1.66 8.49 112Cd(p, 2n)111In 24.07 −11.039 11.14 8.47 113Cd(p, 3n)111In 12.26 −17.579 17.73 8.45 114Cd(p, 4n)111In 28.86 −26.622 26.86 8.43 110Cd(d, n)111In 12.39 3.107 0 8.15 111Cd(d, 2n)111In 12.75 −3.868 3.94 8.13 109Ag(3He, n)111In 48.65 6.533 0 15.52 109Ag(4He, 2n)111In 48.65 −14.044 14.56 15.17 + + EC,β EC,β 112Sn(p, 2n)111Sb −→ 111Sn −→ 111In 0.95 −16.627 16.78 8.82 75 s 35 m 110Cd(3He, 2n)111Sn → 111In 12.39 −5.619 5.77 15.82 112Sn(γ, n)111Sn → 111In 0.95 −10.788 10.79 − 112Sn(n, 2n)111Sn → 111In 0.95 −10.788 10.89 −

The yields of 111In from nuclear reactions induced However, 109Ag(3He, n)111In reaction is unsuitable for the by 4He on Ag are much lower than from reactions in production of 111In because its yield is too low [about which Cd (irrespective of whether enriched or natural) is 7.4 × 104 Bq (2 µCi)/µA h at EOB] for 40 MeV bombard- bombarded by protons [2.22 × 106 Bq (60 µCi)/µAh for ing 3He particles [6]. Comparative study on the above ways 109 4 111 111 Ag( He, 2n) In reaction at E4He = 24 MeV compared leading to In for a medium size medical cyclotrons shows to 2.22 × 108 Bq (6000 µCi)/µAh for 112Cd(p, 2n)111In that the highest yield is obtained in the two reactions on reaction with cadmium target enriched to 97% and to highly enriched 112,113Cd targets, namely 112Cd(p, 2n)111In 7 113 111 3.7 × 10 Bq (1000 µCi)/µA h for natural cadmium target (Ep ≤ 30 MeV) and Cd(p, 3n) In (Ep ≤ 60 MeV) [6]. at Ep = 22 MeV] [6]. The reason for our choice of an en- Due to the lack of relevant nuclear data, not much can be riched 113Cd target is the advantage of a larger yield, namely said about indirect methods of the 111In production (the last 6.1 × 108 Bq (16 500 µCi)/µAh forthe(p, 3n) reaction at four reactions in Table 1). 65 MeV compared to the yield of the (p, 2n) reaction on the The radionuclide 111In can be produced by 112Sn(γ, n) enriched 112Cd at 26 MeV and to the yield of the (p, n) reac- 111Sn → 111In using medium-energy electron accelerators. tion on an enriched 111Cd target using 16 MeV protons [6, 7]. The achieved production scale of tens of MBq of no- When preparation of 111In is carried out by bombard- carrier-added (NCA) 111In per several hours run [8 × 107 Bq ing the cadmium target with protons (deuterons), the 111In (2.2mCi)/µA h] is sufficient for many types of applica- activity at the end of bombardment (EOB) contains undesir- tions [9]. Small scale production of 111In (tens of kBq) has able contaminations of other indium radionuclides, namely been reported [10] for reactor irradiations utilizing the fast 109 110m 114m 112 111 In (T1/2 = 4.3h), In (T1/2 = 4.9h),and In (T1/2 = component of the neutron spectrum in the Sn(n, 2n) Sn 49.5 d). The first two of them have relatively short half-lives → 111In reaction. Another potential possibility of 111In pro- and a suitable waiting period is required after EOB to let di- duction is to make use of fast neutrons produced by neutron minish drastically their activities. The chemical separation generators, which are commonly used in many laboratories. of 111In from irradiated cadmium target cannot be performed The aim of this work is to obtain information on 111In before at least 99% of these radioisotopes has decayed. activities induced by 14 MeV neutrons bombarding 112Sn 114m The third mentioned nuclide In (with major Eγ in keV targets. Production rates and achievable radionuclidic pu- and Iγ in per cent: 191.6 (16.7), 558 (3.6), 725.2 (3.5)) has rity after chemical separation were determined. Produc- a significant meaning due to the radiation dose to patient tion of no-carrier-added 111In has been carried out using receiving 111In radiopharmaceutical containing as contam- highly enriched 112Sn targets. No attempt has been made ination this long lived radionuclide. The toxicity (damage so far to produce 111In via the radioisotope generator that can be caused by chemicals and radioactivity adminis- 112Sn(n, 2n)111 Sn → 111In by using 14 MeV neutrons. In this tered with the radiopharmaceutics) of 111In is 80 times lower paper we present this method of 111In production in detail. than that of 114mIn [8]. The 114mIn contamination should be Having so strongly enriched samples of 112Sn, it is interest- reduced to the lowest level accepted by nuclear medicine. ing to determine the cross sections poorly known for 14 MeV Undesirable 114mIn is always present in the final indium frac- neutron induced reactions. Some of them are feasible only tion derived from a cadmium target irradiated by protons. Its with samples of different isotopic enrichment, where we use level changes from 0.003% to 3% of the 111In activity [6]. two reaction channels leading to the same activity. This contamination level depends upon the isotopic composition of the enriched target material and the type of nuclear reaction employed. The same situation is observed with deuterons, as par- 2. Experimental ticles bombarding the cadmium target but the 114mIn contam- 2.1 Neutron source ination level is intolerable, about 6%. The bombardment of silver with 3He and 4He particles Neutrons of 14.4 MeV [11] have been produced for our provides 111In, which contains no trace of long-lived 114mIn. studies in the 3H(d, n)4He reaction by bombarding thick Reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets 313 tritium-titanium targets with a deuteron beam from a small Cockroft–Walton accelerator of 100 kV installed at the An- drzej Sołtan Institute for Nuclear Studies at Swierk,´ Poland.

2.2 Neutron detectors Two different types of neutron detectors have been used in this experiment. The first one was a multisphere Bonner- type neutron detector [12]. We also used a second detector (as frequently done in neutron laboratories) in the form of 64 copper disks. The activity of Cu (T1/2 = 12.8 h) induced in the reaction 65Cu(n, 2n)64Cu (Q-value 9.9MeV),where the reaction cross section is well known, has been used as a measure of the neutron flux. This reaction is insensi- tive to low-energy neutrons because of its high-threshold energy. The 64Cu activity was identified and measured by de- tecting gamma rays of 1345.8 keV with a known intensity = . ± . Fig. 1. Scheme of the experimental arrangement. The 14 MeV neutron (Iγ 0 471 0 011% per decay) [13]. beam passes through the enriched 112Sn sample and then through the The choice of the neutron monitor has been a subject natural tin and copper monitor samples. of several criteria for the threshold detector: i) high threshold; ii) high cross section; iii) one or two stable isotopes; iv) available in high purity; v) inexpensive and easy to han- Table 2. Abundances of tin isotopes in natural and enriched 112Sn tar- dle; vi) form a residual nucleus that emits a gamma-ray with gets. a convenient energy and half-life. The Cu monitor fulfills these requirements and it seems to be well suited for this Tin isotope Abundance Abundance γ in natural in enriched application. The disadvantage of 1346 keV ray line emit- target targets 64 ted by Cu (very small self-absorption) is its low emission (%) (%) probability [14]. More threshold detector material was used in order to obtain good counting statistics. Sample No. 1 Sample No. 2 The tritium target head employed in the present work uses only 1.7 mm layer of water cooling. The process of 112 0.971 62.5 84.0 114 0.651 2.1 13 thermalization by this thin layer of water is excluded but 115 0.341 0.7 0.75 it disturbs the 14 MeV neutron spectrum. The contribution 116 14.531 9.6 2.0 from the low-energy secondary neutrons produced in the tar- 117 7.687 3.4 0.25 get or its immediate vicinity in the (n, 2n), (n, pn), (n, n) 118 24.230 7.9 − 119 8.594 2.9 − etc. reactions is difficult to analyze quantitatively. These − 64 120 32.591 7.6 effects could significantly affect the result of the Cu forma- 122 4.633 1.4 − tion from the 63Cu(n,γ)64Cu process only in the case where 124 5.795 1.9 − the capture cross section is very large. The 63Cu nucleus has small cross sections (few mb only) for the radioactive capture of fast neutrons (En > 1 MeV) excluding thermal energy better than 99%. The copper monitor foil 44 mm diameter neutrons, where it reaches 4.3b. 1mmthick(mass13.25 g) was 99.9% pure. All three samples (112Sn, natSn and Cu) were placed in that order in a stick-like arrangement along the deuteron 2.3 Sample preparation beam axis (see Fig. 1) and were irradiated by the same in- Samples of enriched 112Sn, natural tin and copper (neutron cident neutron flux. These disks (triad) were weighed and monitor) have been used in the form of metallic disks. Sam- grouped into three stacks. ples of tin enriched in 112Sn were supplied by Techsnab- The loss of neutrons and the change in their energy in the export, Moscow, Russia. The abundance of isotopes in the samples were negligible. 112Sn targets is listed in Table 2. Two targets of 112Sn enriched to 62.5% with masses 72.5 mg and 98.1mgwere 2.4 Irradiations cut to disks of diameters 7 mm and 10.9 mm and thick- nesses 0.3mm and 0.16 mm, respectively. The third en- After passing through a 3.4 cm diameter aperture formed by riched sample with 84% enrichment was also shaped to a system of collimators, a continuous, stable, and homoge- disk of 5.1 mm diameter, 0.15 mm thickness and of mass neous 100-keV deuteron beam bombarded a water-cooled 20.6mg.The5.6 g sample of natural tin (diameter 22 mm, T-Ti target on a copper backing mounted in a tin-wall target thickness 2.2 mm) was prepared from spectrographically chamber made of copper. The enriched sample was placed at pure tin (Johnson/Mathey and Company, London, United the back side of the chamber (the smallest possible distance). Kingdom). The second 1.88 g sample of natural tin foil The distance from the surface of the T-Ti target to this sam- (Goodfellow Cambridge Limited, Cambridge, United King- ple is 6 mm (6.5 mm to the natural tin sample). The solid dom) 44 mm in diameter and 0.17 mm thick had purity angles subtended by the circular samples (0.7 mm diameter 314 E. Betˇ ak´ et al.

Table 3. Nuclear data pertaining to the product nuclides studied in this experi- Reaction Q-value Product Properties of product nuclide ment [23]. (MeV) nuclide T1/2 Eγ (keV) Iγ (%)

112Sn(n, 2n)111Sn −10.788 111Sn 35.3 min 457.6 0.38 762.0 1.48 954.1 0.51 1153.0 2.7 1542.7 0.74 1610.5 1.31 1914.7 2.00 2179.5 0.28 2212.1 0.23 2323.3 0.33

114Sn(n, 2n)113Sn −10.299 113Sn 115.09 d 391.7 64.97

118Sn(n, 2n)117mSn −9.327 117mSn 13.60 d 158.6 86.4

124Sn(n, 2n)123mSn −8.488 123Sn 40.06 min 160.3 85.7

112Sn(n, p)112mgIn 0.116 112mIn 20.56 min 156.4 13.2 112gIn 14.97 min 606.4 1.11 617.1 4.6

114Sn(n, p)114m2In −1.206 114m2 In 49.51 d 190.3 15.05

115Sn(n, p)115mIn 0.283 115mIn 4.49 h 336.2 45.8

116Sn(n, p)116m2In −2.496 116m2 In 54.29 min 818.7 11.5 1097.3 56.2 1293.5 84.4 1507.4 10.0 1753.8 2.46

117Sn(n, p)117mIn −0.673 117mIn 116.2 min 315.3 19.1 117gIn 43.2 min 552.9 10.0

118Sn(n,α)115gCd 2.081 115gCd 53.46 h 492.4 8.03 527.9 27.5

120Sn(n,α)117m,gCd 0.967 117mCd 3.36 h 564.4 14.7 1066.0 23.1 1234.6 39.8 a 1997.3 26.2 117gCd 2.49 h 273.3 27.9 1303.3 18.4 1576.6 11.2

a: Here, we have not used the value of 11.0% stated in [23], but we preferred the value of 39.8 given by Gusiev and Dmitriev [24] (see Sect. 3.2.14 for details). for enriched 11 and 22 mm for natural tin) viewing a thin 64Cu activities, the cross section of the 65Cu(n, 2n)64Cu reac- circular neutron source were calculated using the tables of tion at 14.4 MeV neutron energy was taken as (964±68) mb Ref. [15]. from Ref. [18, 19]. Prolonged irradiation of the T-Ti tar- The neutron energies (and their errors) were determined get by the deuteron beam can produce a 2H target through from irradiation geometries of all samples including the implantation of deuterium into titanium. Parasitic neutrons 2 3 consideration of loss of 18 keV deuteron energy in the tita- (En ≈ 3 MeV) produced in the subsequent H(d, n) He re- nium oxide layer on the titanium tritide surface (the tritium action cannot induce the 64Cu activity because the threshold concentration in this layer is small). Taking into account of the 65Cu(n, 2n) reaction is 9.9 MeV. The process of ra- that our tritium targets are thick (stop completely 100 keV diative capture at 14-MeV neutron energy on the 69.17% deuterons) and the data of [11, 16], we estimated the neu- abundant 63Cu leads to 64Cu activity with the cross section tron energy range as 14.2–14.6 MeV. The overall weighted equal to 2.6 mb [20]. The error introduced to the neutron average of the neutron energy was therefore equal to (14.4± 0.2) MeV2. In the calculations of neutron ﬂuxes from the made from aluminium and the layer of water was about 5 times thicker. The irradiation geometry and tritium target head are similar. Irrespective of such small differences, the method of cal- 2 The effective neutron spectra have been calculated according to culation of neutron spectra was identical and it states that about Ricci [17]. Our tritium head was made of copper and the layer 84% of neutrons that hit the sample (excluding lower energy sec- of cooling water was 1.7 mm thick. In [17], the target head was ondary neutrons) have energy of (14.4± 0.2) MeV. Reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets 315

flux calculations involved by this process lies in the error these discrepancies exceeded the quoted experimental un- range of a standard cross section for the 65Cu(n, 2n)64Cu certainties. Also the published values are usually given for reaction and can by ignored. To induce mainly the 111Sn ac- one neutron energy with a rather large spread, so that no tivity (T1/2 = 35.3 min), the duration of bombardment of the study of cross section variation with energy is possible. tin samples was kept at 40 min and 1 h (about one and two Hence, it was considered desirable to remeasure the cross half-lives of 111Sn). sections of observed reactions on tin isotopes in order to have our own data basis. γ 2.5 Activity measurements Mostofthepeaksseeninthe spectra from the enriched 112Sn sample (measured shortly after the irradiation, after 2 We needed two Ge(Li) spectrometers to perform our ex- and after 6 hours) result from 111Sn. The 111Sn nucleus has periments. Small, enriched irradiated samples of 112Sn were a large number of gamma transitions, which make it easy to measured on a 72 cm3 Ge(Li) spectrometer (Canberra GC detect and identify. The 123mSn and 112,116In activities are also 1520, of the Metrological Laboratory of Radioactive Materi- formed in the 124Sn(n, 2n)123Sn and 112,116Sn(n, p)112,116In re- als, Swierk)´ with perfectly determined efficiencies for solid actions on the tin isotopes existing in the enriched sample and liquid (in penicillin vials) samples for 1, 5, 10 and 15 cm during neutron irradiation of the 112Sn sample. distances between the sample and the detector and the energy When natural tin is irradiated, many activities are induced resolution 1.8 keV at 1332 keV. We performed the measure- in its isotopes. The radioactive nuclides produced by the ments at 5 and 10 cms. The cascade summing corrections are (n, 2n) reaction from natural tin target are 111,113,117m,123m Sn. fortunately small (about 1%) at these distances. The natural Many indium activities are induced by the (n, p), (n, pn), tin samples and Cu monitors with higher masses and dimen- (n, np)and(n,α) reactions on the various tin isotopes. sions were measured using another spectrometer with large A few of them are suitable for detection, for example 177 cm3 Ge(Li) detector (ORTEC, model GMX, berylium 112,116m2,117m,g In. Because the isotopes of tin have adjacent window), which is mainly used for environmental samples. mass numbers, the same radioactive products are produced Due to low activities of these samples, we chose the geometry by different reactions. For example, 116m2In can be pro- condition (0.3 cm distance source–detector), which improves duced in the 116Sn(n, p)116m2In and 117Sn(n, np)116m2In reac- the detection. However, another error-coincidence summing tions. Similarly, 117mSn can be formed in the two reactions, appears in such close geometry. This detector has been cal- 118Sn(n, 2n)117mSn and 117Sn(n, n)117mSn.Insuchcases,we ibrated at close (0.3 cm) geometry as well as at far (29 cm) could isolate the individual cross sections because we used source–detector distances with 2 inch multigamma source an enriched 112Sn target with another composition of tin iso- (109Cd, 57Co, 123mTe, 203Hg, 113Sn, 85Sr, 137Cs, 88Yand60Co) topes as is in the natural sample. Du Pont standard and extended sources of long-lived 133Ba Similar considerations apply also to the cadmium ra- and 152Eu. In the calculations of intensities measured with dioisotopes formed in (n,α) reactions. Two of them are this detector at close geometry, the contribution of summing suitable for the cross section determination, namely correction was included in accord with [21, 22]. The detector 118Sn(n,α)115gCd and 120Sn(n,α)117m,gCd. was connected to a computerized spectrometric system. The The activities of 111,123mSn and of 112,116m2 In in the enriched data were processed using the Genie-2000 program (CAN- 112Sn sample, and those of 111,113,117m,123m Sn, 112,116,117 In and BERRA Industries) at a personal computer. The γ lines used 115g,117m,g Cd in the natural tin sample were easily identified in the calculations of the reaction cross sections, taken from via observation of their gamma lines with known half-lives. Ref. [23], are given in Table 33. The relevant decay properties [1, 5, 24] are listed in Table 3.

3. Results 3.2 Comparison with previous measurements 3.1 Cross sections The cross sections measured in this work are shown and The three most pronounced reactions in the neutron energy compared with previous measurements [25–49] in Table 4 , , range of interest are (n, 2n), (n, p)and(n,α). In general, the ((n 2n) reactions), Table 5 ((n p) reactions) and Table 6 cross sections of reactions with neutron emission [(n, 2n) (other ones). for example] have higher values for medium mass nuclei (as tin) than those with charged particle emission [(n, p), (n, d), 3.2.1 112Sn(n,2n)111Sn reaction (n, t), (n,τ)and(n,α)]4. 112 As this reaction is very important for the estimation of the When the enriched Sn target is irradiated with 14 MeV 111 111 induced Sn activity (mother activity) in radioisotope gen- neutrons, the predominant radionuclide produced is Sn erator, accurate reaction cross section data are required. The (parent isotope for radioisotope generator) formed in the contribution from the 112Sn(n, np)111In reaction in compar- 112 , 111 Sn(n 2n) Sn reaction. A survey of the published cross ison with the main (n, 2n) reaction is unknown. This cross sections for this reaction and some tin isotopes showed dis- section, as estimated from semi-empirical formula [50] at crepancies between values given by different authors, and 14.4 MeV neutron energy, is about 170 mb. The data reported in the literature scatter considerably. 3 All γ lines presented in Table 3 have been used to determine the The results of Bormann et al. [26] are in agreement with cross sections. They were assigned different weights correspond- ing to their intensities. the present data. The attenuation of the incident neutrons 4 We use the symbols p, d, t, τ and α, for outgoing protons, and self absorption of the emitted gamma-rays represented deuterons, tritons, 3He and 4He, respectively. a very small correction. 316 E. Betˇ ak´ et al.

Table 4. The (n, 2n) cross sections on some tin isotopes between 13.5 and Neutron energy a Cross section (mb) 15.5MeV. (MeV) 112Sn(n, 2n)111Sn 114Sn(n, 2n)113Sn 118Sn(n, 2n)117mSn 124Sn(n, 2n)123mSn

15.4 ± 0.2 890 ± 98 [35] 15.1 ± 0.2 805 ± 87 [35] 14.95 1650 ± 280 [25] 1275 ± 49 [25] 823 ± 441 [25] 545 ± 28 [25] 14.9 ± 0.5 b 1230 ± 340 [36] 14.87 ± 0.17 1217 ± 138 [26] 14.76 1222 ± 47 [25] 14.68 1580 ± 280 [25] 807 ± 45 [25] 551 ± 30 [25] 14.6 1326 ± 112 [27] 14.5 794 ± 109 [35] 14.44 1480 ± 250 [25] 1084 ± 42 [25] 787 ± 43 [25] 544 ± 28 [25] 14.4 ± 0.2 1104 ± 43† 1270 ± 115† 816 ± 70† 590 ± 26† 14.4 ± 0.3 1508 ± 116 [28] 14.4 1300 ± 150 [29] 1800 ± 100 [29] 976 ± 120 c [29] 547 ± 23 [29] 14.4 ± 0.3 1275 ± 100 [30] 1239 ± 30 [30] 957 ± 100 [30] 14.22 790 ± 44 [25] 570 ± 30 [25] 14.11 ± 0.15 1110 ± 127 [26] 14.1 1610 ± 322 [31] 14.1 1530 ± 230 [32] 14.1 ± 0.5 900 ± 100 [33] 1090 ± 130 [33] 880 ± 100 [33] 540 ± 50 [33] 13.99 1310 ± 220 [25] 1152 ± 44 [25] 747 ± 40 [25] 552 ± 28 [25] 13.9 808 ± 87 [35] 13.85 ± 0.2 b 726 ± 145 [34] 13.75 744 ± 41 [25] 564 ± 29 [25] 13.57 1310 ± 220 [25] 1036 ± 40 [25] 725 ± 38 [25] 573 ± 30 [25]

†: The results of this work are printed in bold face; a: Uncertainties less than 0.1 MeV or not given by the original work are not stated here, and the energies differing by less than 0.05 MeV are grouped into the same line; b: D–D neutron source; c: Data on 118Sn include a contribution from 118 Sn(n, 2n)117mIn+117 Sn(n, n)117mSn reaction.

Table 5. The (n, p) cross sections on some tin isotopes between 13 and 15 MeV.

Neutron energy a Cross section (mb)

(MeV) 112 Sn(n, p)112gIn 112 Sn(n, p)112mIn 114Sn(n, p)114m2In 116Sn(n, p)126m2In 117Sn(n, p)117mIn 117Sn(n, p)117gIn

14.95 11.52 ± 0.86 [25] 3.38 ± 0.62 [25] 13.30 ± 0.71 [25] 14.9 ± 0.5 5.1 ± 1.6 [36] 10.7 ± 2.6 [36] 14.87 39.7 ± 7.4 [37] 28.0 ± 2.8 [37] 13.1 ± 2.5 [25] 14.8 ± 0.5 14 ± 2 [45] 16.5 ± 2 [45] 14.65 17.7 ± 3.4 [25] 10.68 ± 0.83 [25] 3.20 ± 0.61 [25] 10.81 ± 0.57 [25] 14.6 10.8 ± 0.7 [41] 14.6 ± 0.237.7 ± 7 [38] 33 ± 6 [38] 23 ± 15 [38] 10.9 ± 1.3 [42] 3.92 ± 0.55 [42] 9.8 ± 1.2 [42] 14.58 42.3 ± 7.3 [37] 30.5 ± 3.3 [37] . +1.2 . ± . . ± . 14 510−2.6 [39] 5 4 1 5 [39] 9 6 2 1 [35] 14.43 19.0 ± 4.7 [25] 9.46 ± 0.87 [25] 3.04 ± 0.56 [25] 10.61 ± 0.56 [25] 14.4 ± 0.2 42.7 ± 3.1† 33.6 ± 2.1† 20.5 ± 1.1† 11.1 ± 0.5† 4.5 ± 0.6† 12.8 ± 0.7† 14.4 ± 0.468± 20 [40] 49 ± 15 [40] 7.9 ± 0.8 [30] 5.7 ± 1.5 [46] 9.8 ± 1.6 [46] 14.4 13 ± 0.7 [29] 8.7 ± 0.6 [29] 8.0 ± 0.7 [29] 3.7 ± 0.5 [29] 5.9 ± 0.4 [29] 14.28 42.0 ± 7.5 [37] 29.0 ± 2.8 [37] 14.23 9.21 ± 0.84 [25] 2.98 ± 0.55 [25] 9.27 ± 0.49 [25] 14.2 ± 0.211± 4 [36, 43] 2.8 ± 0.8 [43] 9.2 ± 2.7 [43] 14.1 ± 0.59.5 ± 1.1 [33] 2.2 ± 0.4 [45] 14.0 2.75 ± 1.25 [44] 13.99 9.37 ± 0.78 [25] 2.79 ± 0.54 [25] 8.55 ± 0.46 [25] 13.88 41.2 ± 7.7 [37] 29.6 ± 2.8 [37] 9.5 ± 2.1 [35] 13.76 8.71 ± 0.73 [25] 2.49 ± 0.46 [25] 8.35 ± 0.44 [25] 13.65 41.0 ± 7.8 [37] 30.1 ± 3.0 [37] 13.57 8.30 ± 0.67 [25] 2.40 ± 0.44 [25] 7.74 ± 0.40 [25] 13.40 42.5 ± 7.8 [37] 30.9 ± 2.9 [37] 2.5 ± 1.0 [35] 10.0 ± 2.1 [35] 13.35 21.1 ± 5.2 [25] 7.29 ± 0.62 [25] 2.15 ± 0.39 [25] 7.14 ± 0.37 [25] 13.0 20.1 ± 4.6 [25] 3.2 ± 1.3 [35] 10.2 ± 2.9 [35]

†: The results of this work are printed in bold face; a: Uncertainties less than 0.1 MeV or not given by the original work are not stated here, and the energies differing by less than 0.05 MeV are grouped into the same line. Reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets 317

Table 6. The (n,α), (n, n)and(n, np) cross sections on some tin isotopes between 13 and 15 MeV.

Neutron energy a Cross section (mb)

(MeV) 118Sn(n,α)115gCd 120Sn(n,α)117mCd 120Sn(n,α)117gCd 117Sn(n, n)117mSn 117Sn(n, np)116mIn

15.00 0.205 ± 0.050 [25] 0.319 ± 0.057 [25] 2.09 ± 0.29 [25] 14.81 ± 0.31 1.23 ± 0.09 [47] 14.8 1.1 ± 0.1 [49] 14.7 1.13 ± 0.08 [47] 0.183 ± 0.072 [25] 0.227 ± 0.079 [25] 1.38 ± 0.17 [25] 14.6 0.9 ± 0.08 [41] 0.21 ± 0.09 [41] 0.26 ± 0.03 [41] 14.6 ± 0.20.15 ± 0.04 [38] 0.21 ± 0.05 [38] 14.5 1.14 ± 0.08 [47] 284 ± 32 [35] 1.26 ± 0.17 [48] 14.45 0.197 ± 0.080 [25] 0.196 ± 0.051 [25] 1.11 ± 0.17 [25] 14.4 ± 0.2 1.26 ± 0.16† 0.27 ± 0.04† 0.29 ± 0.06† 246 ± 21† 1.35 ± 0.4† 14.31 ± 0.13 0.94 ± 0.06 [47] 14.23 0.135 ± 0.056 [25] 0.166 ± 0.049 [25] 14.1 0.93 ± 0.06 [47] 1.23 ± 0.07 [48] 13.99 0.103 ± 0.045 [25] 0.168 ± 0.038 [25] 0.53 ± 0.13 [25] 13.9 0.76 ± 0.05 [47] 272 ± 35 [35] 1.17 ± 0.11 [48] 13.76 0.150 ± 0.055 [25] 0.148 ± 0.037 [25] 0.40 ± 0.10 [25] 13.69 0.73 ± 0.05 [47] 13.58 0.114 ± 0.045 [25] 0.111 ± 0.025 [25] 0.353 ± 0.073 [25] 13.52 ± 0.15 0.53 ± 0.04 [47] 13.4 0.55 ± 0.04 [47] 0.076 ± 0.024 [25] 287 ± 44 [35] 13.33 0.58 ± 0.04 [47] 0.123 ± 0.045 [25] 0.295 ± 0.068 [25] 13.0 416 ± 99 [35]

†: The results of this work are printed in bold face; a: Uncertainties less than 0.1 MeV or not given by the original work are not stated here, and the energies differing by less than; 0.05 MeV are grouped into the same line.

The 112Sn(n, 2n)111Sn reaction cross sections measured the energy range of interest. The present data were compared by Ikeda et al. [25] are higher than ours in the 14–15 MeV with that of Sakane et al. [37]asshowninTable5andthe energy range. This reaction cross section was deduced by two other works [38, 40]. Our results are in very good agree- measuring the 762 keV gamma-ray emitted in the decay ment taking into account the neutron energy range 14.4 ± of 111Sn. The probable reason for this discrepancy is that 0.2MeV. they used lower branching ratio (0.66%) for the 762 keV gamma-ray than the values reported in Refs. [1, 23, 24] 3.2.4 114Sn(n,2n)113Sn reaction (1.4%, 1.48% and 1.15%, respectively). The substitution of The 391.7 keV gamma-ray emitted in the decay of 113Sn was those branching ratios into the activation formula strongly used to deduce the value of the 114Sn(n, 2n)113Sn reaction decreases the values of cross sections published by Ikeda cross section. As shown in Table 4, our value is consistent and they get closer to our results. To eliminate possible ex- with the earlier values, except for that reported in [29]. perimental errors in the branching ratio term of activation formula, the cross section values in the present work were 3.2.5 114Sn(n, p)114m2In reaction deduced using different gamma-ray transitions (see Table 3). Remaining results are more or less isolated results with The cross section of this reaction is very important from the no overlapping of error bars with other measurements within point of view of the 111In contamination by this long-lived their uncertainties. radionuclide. It has been measured using the 191.6keV γ 114 ray (Iγ = 16.7%) emitted from the metastable state of In 112 112g 3.2.2 Sn(n, p) In reaction to its ground state (T1/2 = 71.9 s). Two photo-peaks with energies of 558 keV and 725 keV but of lower intensities due The cross section of the 112Sn(n, p)112gIn reaction was meas- to the 114m2In decay to 114Cd helped us in the identiﬁcation ured using the 606.4 keV and 617.1 keV gamma-rays, emit- of 114m2In activity. Two samples of metallic tin with masses ted in the decay of 112gIn. The results of these measurements of 72.5 mg and 98.1 mg but of the same enrichment (62.5%) are shown in Table 5 together with the results of previous ex- were irradiated by 14 MeV neutron beam to produce 111Sn, periments. The present data are consistent with results from 111In and as by product 114m2In. Thereafter the lighter sam- the literature, except for the values reported by Chursin [39] ple was dissolved in 1 cm3 concentrated HCl as the ﬁrst step and Lulic´ [29], which are four times lower than the present of the radiochemical operations. Activities of the second results. sample were measured all time in the metallic form. The radionuclidic purity of the 111In was ascertained by examining 3.2.3 112Sn(n, p)112mIn reaction its γ-ray spectra with Ge(Li) spectrometers. The extent of This measurement was carried out using the 155.5keV 114m2In contamination in both samples was determined from gamma-ray emitted from the metastable state of 112In. There the spectra taken during six weeks after the end of bombard- is only a limited number of previous data for this reaction in ment when the 191 keV peak of 114m2In was easily resolved. 318 E. Betˇ ak´ et al.

In the first γ ray spectra (after EOB) the effect of sum-up of values measured in Refs. [25, 35, 36, 42, 43, 46], while the K X-rays with 171 keV γ line of 111In masked its existence. result of Ref. [29] is two times lower and that of Ref. [45] by Such a long interval allowed us to determine carefully the about 28% higher. 114m2In contamination level. Experimental cross section data concerning the excitation of the long lived isomeric state 3.2.11 118Sn(n,2n)117mSn reaction in 114In with 14 MeV neutrons are scarce. Our results agree well with the data reported by Ikeda et al. [25] as shown in The isomeric activity in 117Sn is produced by both reac- Table 5. tions, namely 118Sn(n, 2n)117mSn and 117Sn(n, n )117mSn. En- riched isotope of 112Sn and natural tin, which have different 117 118 3.2.6 115Sn(n, p)115mIn reaction amounts of Sn and Sn isotopes were used in order to determine the contributions of each reaction. During neu- The 115Sn(n, p)115mIn reaction cross section was deduced by tron irradiation of the tin samples, the 112m,117m,gIn, 117mSn measuring the 336.2keVγ-ray emitted in the decay of iso- and 123mSn are also formed with emission of gamma lines meric state 115In. Experimental data for this reaction are centered around 158 keV. Since the half-life of 117mSn ac- scarce due to the very small abundance (0.651%) of 115Sn. tivity (14 days) is much longer than those of the remaining Only Lulic´ et al. [29] have measured this reaction cross sec- ones, its activity was measured after decay of interfering tion previously and the present value (35.2 ± 2.6) mb is one activities by providing a suitable cooling period. The con- order of magnitude higher than their result. tributions of the 117Sn(n, n)117mSn and 118Sn(n, 2n)117mSn activities were separated and cross sections deduced. The 3.2.7 116Sn(n, p)116m2In reaction present data (Table 4) are consistent with experimental The cross section of the 116Sn(n, p) 116m2In reaction has been values in the literature [25, 35] in which the contribution of 117 , measured using five γ transitions (see Table 3) emitted in the Sn(n n ) reaction was subtracted and they are lower 117 , 117m the decay of 116m2In. The results of these measurements are than the results with contribution of the Sn(n n ) Sn shown in Table 5 together with the results of previous ex- reaction. periments. The present results agree within experimental errors with those obtained in Refs. [25, 33, 36, 41–43]. How- 3.2.12 117Sn(n, n )117mSn reaction ever, the data of Refs. [39] and [44], which are based on the Experimental data for the 117Sn(n, n)117mSn reaction are incorrect spectroscopic data, are few times smaller than the scarce. Only one excitation function measured in the present value. 13–15 MeV energy range by Grochulski et al. [35] is available (see Table 6). The present result agrees with their data 117 116m2 3.2.8 Sn(n, np) In reaction very well. In addition to the 117Sn(n, p)116m2In reaction, 116m2In is produced in the second reaction channel on 117Sn isotope: 3.2.13 118Sn(n, α)115gCd reaction 117Sn(n, np)116m1In. We are aware of only two earlier meas- 118 ,α 115g urements [25, 48] and these results are shown in Table 6. Our The cross section of the Sn(n ) Cd reaction was measured using the 492.4 keV and 527.9 keV gamma-lines cross section value is about 50% higher than those given in 115g the cited references. emitted in the decay of Cd. It may be noted from Table 6 The contributions of both reaction channels leading to the that the value of the cross section obtained by us is in general same activity were possible to separate using samples with agreement with the literature values. different enrichment of 116Sn and 117Sn. In this method, the final cross section error is quite large because of subtraction 3.2.14 120Sn(n, α)117mCd reaction of almost equal values. Strictly speaking, the cross section The four gamma lines (see Table 3) emitted in the decay of obtained is the sum of the cross sections for three reac- 117mCd have been used to determine the 120Sn(n,α)117mCd re- tion channels leading to the same residual nucleus, namely, action cross section. Branching ratios for all of them were 117 , 117 , 117 , Sn(n np), Sn(n pn)and Sn(n d). taken from the tables [23]. The value of branching ratio (11%) for the 1234.6 keV gamma line taken from these Ta- 117 117m 3.2.9 Sn(n, p) In reaction bles is probably incorrect, as it gives several times higher The 117Sn(n, p)117mIn reaction cross section has been de- value of cross section than the other three lines. So, this duced by measuring the 315.3 keV gamma-ray emitted in the value was adopted from [24] to be 39.8%, what yields con- decay of 117mIn. The present result is compared to those of sistent cross sections. As shown in Table 6, our value is in previous works in Table 5. Our value is smaller than those good agreement within the limits of the quoted uncertainties. reported in Ref. [45] and it is two times higher than those of Ref. [43] and another value of [45]. All other results are 3.2.15 120Sn(n, α)117gCd reaction consistent within the limits of the quoted uncertainties. The presence of 117gCd activity in the tin sample was identified via observation of its known 273.3 keV, 1303.3keV 3.2.10 117Sn(n, p)117gIn reaction and 1576.6 keV gamma lines. The current result is com- The 552.9 keV gamma-ray emitted in the 117gIn decay was pared with previous values in Table 6. Our value is in good used to deduce the value of the 117Sn(n, p)117gIn reaction agreement with those reported in the literature within experi- cross section. As shown in Table 5, our value agrees with the mental errors. Reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets 319

3.2.16 124Sn(n,2n)123mSn reaction induced reactions is of great importance in the estimation of unknown data and in the adoption of cross sections among The 160.3 keV gamma ray emitted in the decay of 123mSn discrepant experimental values. The systematics do not re- was used to determine the value of the 124Sn(n, 2n)123mSn re- flect, however, the fact that a considerable portion of the action cross section. As shown in Table 4 our value deduced (n, p) reaction cross section (and similarly in other chan- from the activities in the enriched as well as in the natural tin nels) at energies 14 to 15 MeV is due to nonequilibrium samples agrees well with the other measurements. processes. Some of the authors (e.g. [66, 75, 79]) try to obtain semi-empirical formulae which include pre-equilibrium 3.3 Uncertainty estimation component of the reaction cross section. Most of the experimental data are taken at energies near Uncertainties in the cross section values reported in this 14-MeV neutron energy. There are several formulae describ- work are composed of uncertainties from various sources ing the isotopic dependence of cross sections for different entering into well known activation formula. The dominant reactions at neutron energy of 14.5 MeV [19, 26, 41, 50, 53, sources are gamma-ray detector efficiency (5%), photo-peak 56, 58, 60–67, 69, 72–76, 79]. counting statistics (1 to 18%), neutron flux determination Our measured reaction cross sections have been obtained [standard cross section of the 65Cu(n, 2n)64Cu for the neu- at 14.4 ± 0.2 MeV, whereas the results of about 200 meas- tron flux monitor with error ±3%5], error of the half-lives urements of cross sections for the (n, p), (n, np), (n,α), and (1 to 18%), the gamma-ray branching ratios (1 to 14%), cas- (n, 2n) reactions were studied and useful formulae were pro- cade (coincidence) summing (1 to 33%) for close geometry posed for each of the aforementioned reactions at 14.9MeV (source-to-detector distance 0.3 cm) and self-absorption of in Refs. [25, 78]. Recently, Habbani and Osman [74] have gamma-rays in the sample (1%). Other uncertaities, such as compared several published formulae of the type given by sample weight, isotopic abundance, irradiation, cooling and Eq. (1) for 14.5 MeV neutrons, but they also proposed a new measuring time were of the order of 0.1% each. The individ- ones for (n, p), (n,α)and(n, 2n) reactions. We compared ual uncertainties are independent and vary from isotope to our experimental results with estimations obtained using isotope. The total error in the cross section was obtained by semi-empirical formulae presented in some of relatively re- taking square roots of the sums of the squares of all the indi- cent references [69–71]. vidual errors. The resulting overall error is partly systematic and amounts to 7 to 30%. 3.4.1 112Sn(n,2n)111Sn reaction A comparison between our experimental results and semi- 3.4 Comparison with systematics empirical predictions 6 for the 112Sn(n, 2n)111Sn reaction Until now, a large number of experimental data has been cross section is shown in Table 7. published on the (n, 2n)and(n, charged particle) reaction As mentioned earlier, the 112Sn(n, 2n)111Sn and cross sections induced by 14- to 15-MeV neutrons (see, e.g., 112Sn[(n, pn) + (n, d) + (n, np)]111In contributions could CINDA [51]). It has been known for a long time that these not be separated accurately. Therefore, their sum is re- cross sections vary rather smoothly with the mass num- ported in the Table 7. Estimated cross section of the ber A, neutron number N, and proton number Z of the target [(n, pn) + (n, d) + (n, np)] reaction obtained using formula nucleus. of Ref. [26] is ca. 170 mb. After subtraction of this value, it The experimental cross section of reactions induced by is seen that the agreement between measured and the calcu- fast neutrons can be approximated by lated values is good. A disagreement by factor of 3 is seen with respect to the predictions of Ref. [54]. σ(n, x) = Cσne exp(as), (1) 3.4.2 114Sn(n,2n)113Sn reaction where σne is the neutron nonelastic cross section, and C and a are fitting parameters for different reactions. Accord- Similar comparison as in the preceding section, but this time ing to Ref. [76], the proton emission probability increases between our experimental result and semi-empirical predic- with increasing relative proton number. Relation of the same tions for the 114Sn(n, 2n)113Sn reaction cross sections is pre- form is expected to hold for d, t, τ, α, and neutron emis- sented in Table 7. sion. Eq. (1) represents the product of two factors, each of The present experimental value is in agreement with the which may be assigned to a stage of nuclear reaction within values given by five formulae with the exception of that of the framework of the statistical model of nuclear reactions. Ref. [54]. The pre-exponential term models compound-nuclear formation by the incoming neutron product from the compound 3.4.3 118Sn(n,2n)117Sn reaction ( − )/ nucleus. The existence of a strong N Z A dependence The total cross section (σ ) is a sum of cross sections for implied by Eq. (1) exists for the neutron-induced reac- tot the formation of isomeric state (σm) and ground state (σg). tion cross sections, as has already been shown by several Although we measured only cross section for excited iso- authors [26, 41, 53, 55–57, 59–67, 69, 72–76, 78, 79]. The precise knowledge of the systematics for different neutron- 6 We do not include in the Table 7 the systematics of Lu and Fink [50], as it gives the cross sections normalized to the 5 As shown by Mannhart and Schmidt [14], weak γ ray counting full reaction (capture) cross sections calculated within the op- leads to larger errors. Anyway, the 65Cu(n, 2n)64Cu reaction was tical model, and is thus not directly comparable to the other used in combination with the Bonner-type detector in our case. systematics. 320 E. Betˇ ak´ et al.

Table 7. Comparison of our data with semi-empirical Cross sections (mb) predictions at 14.5MeV. (n, 2n)† (n, p)† (n,α)†

Ref. 111Sn 113Sn 123m Sn 112In 117In 117Cd 115Cd

[26] 1187 1344 1616 [53] 1276 1324 1565 [74] 1091 1168 1592 37.5 9.9 3.7 5.2 [73] 1126 1321 1549 74.9 13.4 3.2 7.6 [55, 58] 1107 1343 1581 [54] 365 562 1202 [52] 1070 1320 1830 [56] 45.5 12.8 3.2 4.9 [60] 72.9 14.9 4.9 6.3 [61, 62] 56.1 15.6 3.6 5.6 [63] 75.8 14.9 3.6 2.8 [65] 79.0 15.4 [67] 70.6 15.0 [72] 63.0 14.6 [75] a 28.6 10.6 [75] b 33.5 14.0 [58] 89.4 16.0 [68] 73.3 17.4 3.7 5.3 [70] 59.9 24.1 [66] 2.4 4.0 [71] 7.2 10.2

This work 1104 ± 43 1270 ± 115 1460 ± 181 75.2 ± 3.117.3 ± 0.80.56 ± 0.07 1.26 ± 0.27

†: Columns for individual reactions are marked by the residual nucleus; a: The “global” systematics by Tel et al.; b: The systematics by Tel et al. distinguishing odd- and even-N targets.

117 mer in Sn, it is possible to evaluate the value of σg cross (1460 ± 181) mb. The mean value of the total cross section section for this reaction because semi-empirical formulae from semiempirical formulae is (1562 ± 185) mb. predict total (n, 2n) cross sections (σg = σtot − σm). The pre- 118 117 dictions of σ for the Sn(n, 2n) Sn reaction are (except , , , , tot 3.4.5 112 115 117Sn(n, p)112 115 117In reactions that of [54]) all between 1330 and 1630 mb [26, 52, 53, 55, 58, 73, 74]. The mean value of total cross section is centered All isotopes of indium formed in the (n, p) reactions with around (1464 ± 104) mb excluding the result of Ref. [54]. 14 MeV neutrons on the tin isotopes exhibit isomerism. In

After subtraction of σm = 816mb from total cross section we the present experiment we determined both σm and σg for 112,117 112,117 arrive at σg = (648 ± 125) mb. the Sn(n, p) In reactions. We compare our experimental values of total cross sections for the 112Sn(n, p)112In and 117Sn(n, p)117In reactions with semi-empirical predic- 3.4.4 124Sn(n,2n)123Sn reaction tions existing in literature in Table 7. 124 112 Rather little experimental information on the Sn(n, 2n) Table 5 presents the values of σm and σg for the Sn 123g 112m,g Sn (T1/2 = 129 d) reaction is available in the litera- (n, p) In reaction. Taking σm value from excitation func- ture. A single (very old) measurement was described by tion at 14.58 MeV equal to (30.5 ± 3.3) mb from Ref. [37], Klyucharev et al. [31] in 1964 who give at 14.1 MeV neu- values of Fink and Wen-Deh Lu [40], Gopych et al. [38] tron energy the cross section value of (900 ±180) mb. In the and ours, it was possible to calculate the weighted mean energy region around 14 MeV there are many (see Table 4) value to be (32.9 ± 1.7) mb. After performing the same pro- 124 123m measurements of the Sn(n, 2n) Sn reaction cross sec- cedure taking the values of σg around 14.5 MeV neutron tion including our data. From these two groups of measure- energy, it was possible to determine the weighted mean 124 123 ments a total cross section of the Sn(n, 2n) Sn reaction value of (42.3 ± 2.6) mb. Then the total cross section σt for 112 112 can be deduced. In Table 7 we compare experimental σtot the Sn(n, p) In reaction is equal to (75.2 ± 3.1) mb. We with predictions of semi-empirical formulae for the reaction compare our experimental total cross section with the semi- of interest. empirical predictions in Table 7.

A comparison of experimental σm for the formation of The mean value of calculated total (n, p) cross section for 123mSn in the 124Sn(n, 2n)123mSn reaction is given in Table 4. the 112Sn(n, p)112In reaction is (61.4 ± 19.0) mb. If we re-

If we take into account σm(14.4MeV) = (544 ± 28) mb ject the result of Refs. [74, 75] from the mean, as they are from the excitation function [25], σm(14.4MeV) = (547 ± 2 times lower than the others, we obtain (69.1 ± 12.1) mb. 23) mb [29] and our result σm((14.4 ± 0.2) MeV) = (590 ± The weighted mean of total cross section is in good agree- 26) mb, we can determine the weighted value (560 ±15) mb ment with our experimental value. We compare our ex- at 14.5 MeV neutron energy. Then the total experimen- perimental total cross section for the 117Sn(n, p)117In reac- tal cross section σtot =[(900 ± 180) + (560 ± 16)] mb = tion with semi-empirical predictions for existing formulae Reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets 321 in Table 7, which are concentrated around 15.5 mb. Badikov and Pashchenko [65] estimated the error of their calculations to be 0.39 mb. In the same reference, the estimated experimental value of the 117Sn(n, p)117In reaction cross section is given as equal to (16 ±4) mb. Our experimental value of the total cross section agrees well with the estimated value and with semi-empirical predictions. As regards the 115Sn(n, p)115mIn reaction, we measured the cross section for excitation of the isomeric state of 115In. The contribution from the 115Sn(n, p)115gIn reaction to the total (n, p) cross section cannot be measured directly using the activation method, as the ground state of 115In has very long half-life 4.1 × 1014 y and it is practically treated as a stable nucleus. We have estimated the (n, p) reaction leading to the ground state using the EMPIRE-II code (ver. 2.18) [77] with default optical model parameters for par- Fig. 2. The (n, p) cross sections for tin isotopes. Evaluated data (stars) ticles but with a provision for inclusion of a pre-equilibrium taken from Refs. [65, 67, 72] and the circles are the weighted averages mechanism. This calculation yields 11.5 mb, and the influ- of our data and other experiments. The dashed line represents the cross section trend as described by the formula of Belgaid and Asghar [72] ence of uncertainty of the choice of the parameters on the and the full one by Konobeev and Korovin [73]. The broken dotted- cross section can be estimated as 30%. Thus, the total cross dashed line corresponds to the new simple suggestion by Tel et al. [75], section obtained by combination of activation measurement which reflects odd or even numbers of neutrons in the target. to the isomeric state and the calculations of the ground state formation is (46.7 ± 4.3) mb. Only Lulic´ et al. [29] measured this cross section previously, but the present value is about one order of magnitude higher than theirs. Some authors of semi-empirical formulae [56, 58, 65, 67, 72] added a list of evaluated empirical cross sections for ca. 160 isotopes for comparison of their predictive force. Eval- uations for the 116,117,118,119,120 Sn(n, p) reactions exist in this group of data. If we add our data to the five evaluated ones, we can try to compare them with predictions of the formulae. Several simple semi-empirical formulae have been proposed based on the Levkovski formula which contains only two fitted parameters. In several cases (e.g., [74, 75]), also phenomenological formulae yielding different values for odd and even N (and similarly, odd or even Z) are given in addition to the simple ones, though still based on the Levkovski-type two-parameter formula. Some other papers suggest to use relatively complicated expressions with more parameters (e.g. [67, 73] for some reactions). We have employed here the four-parameter version of Konobeev and Korovin [73] and the five-parameter one by Belgaid and Asghar [72] in Fig. 3. Separation energies (left scale, dashed lines) for neutrons the hope that they are capable to describe reasonably the (squares) and protons (stars) and the Q values (right scale, solid lines) , experimentally observed smooth decrease of σ(n, p) with for the (n p) reactions separately on the odd–odd (crosses) and the even–even (circles) tin isotopes as functions of the mass number. increasing mass number for given element than simple formulae. In Fig. 2 the (n, p) cross sections on tin isotopes are plotted against mass number A using the formulae of justment [4]. Apparently good linearity of the (n, p)cross Refs. [72, 73]. For a comparison, we have drawn here also sections on tin isotopes may be useful in quick predictions the recent odd-even-dependent prediction by Tel et al. [75]. with relatively good accuracy for the (n, p) cross sections As can be seen in Fig. 2, the (n, p) values as a function of not measured yet. It can be also used to determine which of A show linear dependence in semilog plot and they decrease the experimental data are probably incorrect and thus need with increasing A. The cross sections for the (n, p) reactions to be measured more accurately in the future. on 112,115,117 Sn measured by us as well as evaluated (n, p) cross sections are close to a straight line. This trend is caused , , 3.4.6 118 120Sn(n, α)115 117Cd reactions by the systematic variation of the neutron (Sn) and proton (Sp) separation energies (already emphasized by Qaim [59]). The systematic behaviour of the (n,α) reaction induced by Equivalently, the Qnp value (see Fig. 3) is the principal phys- 14 MeV neutrons is similar to that of the (n, p) reaction. ical quantity responsible for the isotopic trend. The Sn, Sp After examining many experimental data, several authors and Qnp values (as well as other Q values) were obtained have proposed semi-empirical formulae which are collected from the ground-state masses given in the latest mass ad- in Ref. [74]. We added some other formulae to this collection. 322 E. Betˇ ak´ et al.

120 Sn(n,α)117 Cd reaction the formalism of Nishioka et al. [81], and for some as- pects the master-equation approach to the pre-equilibrium Cross sections for both excited isomeric and ground states exciton-model [82] is employed (see also [83]). The op- in 117Cd were measured in the present work. The calculated tical model parameters of Wilmore and Hodgson [84] for values for the 120Sn(n,α)117Cd total reaction cross sections neutrons, Becchetti–Greenlees [85] for protons and Mc- are presented in Table 7. They are centered around 4 mb and Fadden and Satchler [86] for α’s, respectively, have been disagree with our experimental value which is about eight used. Though the code enables (in principle) to calculate times lower. also the α emission, only the compound-nucleus descrip- In Refs. [50, 78] the results of about 200 measurements tionofitisusedtothataim8, so that the calculated cross of cross sections under the same experimental conditions for sections of the (n,α) reactions at 14 MeV are necessarily the (n, p), (n, np), (n,α)and(n, 2n) reactions were studied underestimated. and formulae were proposed for each of the aforemen- The reactions near 14 MeV show a great sensitivity to the tioned reaction type but at neutron energy of 14.9MeV.The details of the level densities used. This effect is very remark- calculated value of cross section for the 120Sn(n, p)117Cd able near the closed shells [87, 88], what is – unfortunately reaction using the formula for (n,α) reaction is equal – also our case. Therefore all results originating from the to 2.5mb. model calculations near the doubly-closed nucleus 132Sn, but also for all lighter Sn isotopes as well, must be taken with 118 Sn(n,α)115 Cd reaction extreme care. The results of the cross sections calculated using the During short irradiations of tin samples we induced only the EMPIRE code [77] compared to the experimental results are = . 115 ground state activity (T1/2 2 23 d) of Cd. In order to ob- in Table 8. As already mentioned, the statistical calculations ,α tain the total cross section for the (n ) reaction, we add in the vicinity of the closed shells are not sufficiently reli- = . cross section for induced isomeric activity (T1/2 44 6d) able, because the level density formulae lose their validity . ( . ± measured by Levkovski et al. [49] at 14 8MeVas 1 1 in that region. One has to keep in mind that the tin iso- . ) σ 0 1 mb to our measured value of g cross section. We ex- topes are of Z = 50, what is exactly the closed proton magic pect that this value does not change strongly between 14.4 shell. It is not therefore surprising that the overall agreement . σ = and 14 8 MeV. We compare the total cross section tot is not as good as one might expect. Relatively good fit to ( . ± . ) 1 26 0 16 mb obtained in this way with the calculated the data is obtained when no protons are emitted, i.e. when ones from semi-empirical formulae in similar way as for the one does not touch the proton closed shell. For these reac- 120 ,α 117 Sn(n ) Cd reaction presented earlier in Table 7, which tions, the agreement is acceptable. Comparably worse are also presents such comparisons. the results for the (n, p) reactions, where – up to one ex- . The calculated cross section for this reaction at 14 9MeV ception – the calculated cross sections are visibly lower than neutron energy using formula from Ref. [78] is equal to the measured ones. The large discrepancy in the case of the . 4 1 mb. The mean value from nine predicted values is equal 115Sn(n, p) reaction is probably partially due to the presence ( . ± . ) to 5 5 2 1 mb. After rejecting result of Ref. [71] this of some other effect. On the other hand, the fact that the cal- ( . ± . ) value is centered about 5 2 1 4 mb, i.e. 3 times higher culated (n,α) reactions are three- to four-times lower than than experimental value. the measured ones is a straight indication of strong presence ,α In both (n ) reactions, the cross sections from system- of pre-equilibrium emission mechanism for α’s and the low 7 atics are much higher than the experimental points .Obvi- theoretical values are easily explainable by insufficient de- ously, phenomenological systematics fail to describe well scription of pre-equilibrium component of cluster emission = the disturbance due to the closed Z 50 magic shell by ex- in the code (see the note above). α traction of two protons constituting the particle. In fact, Some of our measured reactions have been calculated such process is strongly handicapped, what is not contained recently by Han [89] using rather sophisticated mixture of in simple formulae used for systematics. the optical model, multistep reactions and DWBA. His results for the (n, 2n) reactions on 112Sn and 114Sn are 1200 mb and 1220 mb, respectively, in rather good agreement with 3.5 Theoretical calculations our data. He also presents the 117Sn(n, p) cross section of Even though 14 MeV incident neutron energy is a very mod- 12.9 mb, to be compared with our sum of the reactions lead- est one, for proper treatment of the reactions we have to ing to the ground and the isomeric states. Finally, he gives use codes which allow for some presence of pre-equilibrium the cross section for the 120Sn(n,α) reaction (again as a sum admixtures, in addition to the gross effect well described over the ground and the isomer states) as 0.4 mb. Other by the compound nucleus theory. Probably the most sophis- reactions, which we have measured, are not given in his ticated and developed code to this aim – if we consider work. only those which are available to the physical community – is EMPIRE-II (version 2.18 Mondovi) by Herman [77]. 4. Radiochemistry Therein, the initial stages of the reaction are described within the Tamura–Udagawa–Lenske multi-step direct for- Various separation techniques have been described for the malism [80], subsequent multi-step compound process uses separation of 111In from macroamounts of tin targets [9,

7 We remind here that our data correspond well to those obtained 8 Extension of the EMPIRE code as to include also the cluster for these reactions by other groups. emission is in progress. Reactions induced by 14 MeV neutrons on natural tin and enriched 112Sn targets 323

Table 8. Comparison of the calculated and experimental cross sections. Reaction Final state Cross section (mb) J Π Energy (keV) Calculated Exper. (this work)

117Sn (n, n) 117m Sn (11/2)− 315 180 246 ± 21

112Sn (n, p) 112gIn 1+ 03342.7 ± 3.1 112Sn (n, p) 112mIn 4+ 156 40 33.6 ± 2.1 114Sn (n, p) 114m2In 5+ 190 12.7 20.5 ± 1.1 115Sn (n, p) 115mIn (1/2)− 336 7.0 35.2 ± 2.6 116Sn (n, p) 116m2In 5+ 127 6.0 11.1 ± 0.5 117Sn (n, p) 117gIn (9/2)+ 08.012.8 ± 0.7 117Sn (n, p) 117mIn (1/2)− 315 2.8 4.5 ± 0.4

117Sn (n, np) 116m2In 5+ 127 0.55 1.35 ± 0.11

112Sn (n, 2n) 111Sn 835 1104 ± 43 114Sn (n, 2n) 113Sn 990 1270 ± 115 118Sn (n, 2n) 117m Sn (11/2)− 315 745 816 ± 70

118Sn (n,α) 115g Cd (1/2)+ 00.351.26 ± 0.16 120Sn (n,α) 117g Cd (1/2)+ 0 0.095 0.29 ± 0.06 120Sn (n,α) 117m Cd (11/2)− 136 0.068 0.27 ± 0.04

10, 47, 90–92]. Each of them should be tested and adapted where A = A0 at t = 0. The maximum activity of daughter for laboratory conditions before being used, but the main occurs at the time 114m2 question is the In content in the final preparation. That (λ /λ ) is why we focused our attention on low cost, simple and = ln B A . tm λ − λ (3) reliable production method of 113mIn from 113Sn (radioiso- B A tope generator) due to identical chemistry. The method is For the 111Sn → 111In decay the value of t is equal to 4 well established at the Radioisotope Centre POLATOM in m hours. The amounts of 111In and 114m2In impurity were calcu- Swierk.´ It was clear that this method of the 111In sepa- lated for t = 4h. ration is rather slow (many hours), target recovery of en- m Indium-111 produced by the bombardment of 62.5% enriched 112Sn is impossible and chemical separation yield riched 112Sn with 14 MeV neutrons contains 1.4 per cent is about 80%. With this in mind, we chose this technique of 114m2In. That isotope with its half-life of 50 days gives since it gives the information on 114mIn content without large approximately 80 times the dose per milicurie (37 MBq) expenses. At each step of the separation of radio-indium of 111In required for patient examination [8]9.Takinginto from macroamounts of the enriched tin, γ-ray spectromet- account the 1.4% contamination of 111In by 114m2In, the ric evaluation was performed. The irradiated metallic tin activity ratio is A(114m2In)/A(111In) = 0.014 and the dose was dissolved in 1 ml of concentrated (32%) HCl. During ratio is D(114m2 In)/D(111In) = 0.014×80 = 1.12. Therefore, the dissolving time, the induced activities were measured the total dose ratio is D(114m2In)/[D(111In) + D(114m2 In)]= as a function of time. After the tin had dissolved, the so- 1.12/(1.12 + 1) = 0.528. So, in our case with 1.4% con- lution was transferred quantitatively to the beaker, wash- tamination, the 114m2In contributes by 53% of the total dose ing with four portions of concentrated HCl, each of them (medical requirements of 0.2% imply this value to be only of 0.5 ml volume. The solution was evaporated to dryness 14%). The ratio of contamination is 7 times higher than that and redissolved in 5 ml of 5 M HCl. In order to oxidize + + accepted by the European Pharmacopoeia [94]. Therefore, Sn2 to Sn4 solution of ceric sulphate was added (0.5g the (n, 2n) reaction path using 14 MeV neutrons is not suit- of Ce(SO ) ·H O for tin mass of about 80 mg). The ob- 4 2 2 able for production of high-purity 111In. The situation may tained solution was again evaporated to dryness, dissolved be different if 112Sn of much higher enrichment than 62.5% in 5 ml 0.4 M HCl and adsorbed on a zirconium oxide col- is used. umn with a flow rate of 1 ml/hour. The 111In was then eluted from the column with 6 ml of 0.1 M HNO3.Thera- dionuclidic purity of the effluent containing radio-indium solution (1 ml) was checked by γ-ray spectrometry just 5. Conclusions after the separation and several times during the next six We have measured the activation cross sections for sixteen weeks. reactions induced by 14.4 MeV neutrons on different iso- In the radionuclide generator concept [a radionuclide A topes of tin with special attention to the 112Sn(n, 2n)111Sn (parent) decays to form another radionuclide B (daughter)] reaction as a potential source of the 111In. The 14 MeV neu- the number of atoms of each type which are parent at any tron irradiation of 112Sn was a first step in generator-like time t is given by the expression [93] production of 111In from decay of 111Sn.

λA −λ −λ = ( A t − B t ), 9 Ref. [8] is already somewhat obsolete. We use it only as a rough B A0 λ − λ e e (2) B A estimate. 324 E. Betˇ ak´ et al.

We have compared our results with those measured pre- 7. Qaim, S. M.: Nuclear data relevant to cyclotron produced short- viously and also with predictions based on semi-empirical lived medical radioisotopes. Radiochim. Acta 30, 147 (1982). formulae. The agreement varies from case to case. Our 8. Dahl, J. R., Tilbury, R. S.: The use of a compact, multiparticle cyclotron for the production of 52Fe, 67Ga, 111In and 123I for medical measured cross sections for all the reactions were compared purposes. Int. J. Appl. Rad. Isot. 23, 431 (1972). to cross sections calculated using the pre-equilibrium plus 9. Malinin, A., Kurenkov, N., Kozlova, M., Sevastianova, A.: Pro- compound nucleus code EMPIRE-II [77]. As we are just duction of radionuclides by photonuclear reactions. Production at the closed proton shell, the statistical model calculations of carrier free indium-111. Radiochem. Radioanal. Lett. 59, 213 necessarily do no fit perfectly the measured data. However, (1983). 10. Arif, M., Zaidi, J. H., Qureshi, I. H.: Fission neutron spectrum av- the overall agreement between theory and experiment is not eraged cross sections of some threshold reactions on tin: small so bad for reactions with nucleon emission, but the calcula- scale production of 111Sn in a nuclear reactor. Radiochim. Acta tions are insufficient for the case of the α emission, which is 75, 175 (1996). 11. Seagrave, J. D., Graves, E. R., Higwood, S. J., Mc Dole, C. J.: not fully handled by the code at all reaction stages. 3 4 111 D(d,n) He and T(d,n) He Neutron Source Handbook. Report There are two routes for the formation of In: indirectly LAMS-2162, Los Alamos National Laboratory, Los Alamos, NM, 111 112 111 through the decay of Sn formed via the Sn(n, 2n) Sn USA (1958). reaction and directly via the 112Sn(n, n p), 112Sn(n, pn)and 12. Bramblett, R. L., Ewing, R. I., Bonner, T. W.: A new type of neu- 112Sn(n, d) reactions. The yield of 111In formation in the sec- tron spectrometer. Nucl. Instrum. Methods 9, 1 (1960). 13. Christmas, P., Judge, S. M., Ryves, T. B., Smith, D., Winkler, G.: ond case is low, mainly due to low cross section equal to 64 ∼ The decay scheme of Cu. Nucl. Instrum. Methods 235, 397 10 mb (this estimation is based on our experimental re- (1983). sults and formulae for cross section calculations in radioac- 14. Mannhart, W., Schmidt, D.: Measuremen of neutron reaction cross tive decay series given in Ref. [95]). An irradiation of 100 sections between 8 and 14 MeV. In: Internat. Conf. on Nuclear 111 Data for Sci. and Technol., Sept. 26 – Oct. 1, 2004, Santa Fe. minutes (optimum time since Sn precursor with T1/2 = . Abstracts. (Haight R. C., Chadwick M. B., Kawano, T., Talou, P., 35 min reaches the saturation) of 160 mg of 62 5% enriched eds.) Report LA-UR-04-5900, Los Alamos, NM, USA (2004), 112 112 Sn (equivalent of 100 mg of Sn) with commercial neu- abstract No. 239, p. 48. tron generator giving 109 n/cm2/s flux of 14 MeV neutrons 15. Konijn, J., Tollander, B.: Solid Angle Computations for a Circular would lead to about 1 MBq (∼ 30 µCi) of 111In. Using in- Radiator and a Circular Detector. Report AE-101, Aktiebolaget tense neutron generator with neutron flux 1010 n/cm2/s, this Atomenergi, Studsvik, Sweden (1963). µ 16. Liskien, H., Paulsen A.: Neutron production cross sections and en- activity can be increased to about 10 MBq (ca. 300 Ci). ergies for the reactions T(p, n)3He, D(d, n)3He, and T(d, n)4He. These yields are not encouraging. In addition, the contami- Nucl. Data Tab. 11, 569 (1973). nation level of 111In by about 1.4% of the longlived 114m2In 17. Ricci, E.: Output spectrum from 14 MeV neutron generator. J. In- absolutely eliminates this method as one leading to high- org. Nucl. Chem. 27, 41 (1965). 111 114m2 18. 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