High-spin isomers production in (α,n)-reactions

Results of the studies of the relative yield of high-spin and low-spin isomers in reactions 112Sn(α,n)115Te and 134Ba(α,n)137Ce in the energy range of the α-particles 15 – 31 MeV based on off-beam measurements of induced activity of members of the isomeric pair are presented. Extremely large relative yield for the first reaction is obtained. The features of the second one, which are promising for its use in fundamental studies, are revealed.


Introduction and measurement technique
Nuclear isomers i. e. excited metastable states of nuclei play an important role in a lot of areas of nuclear studies and fields of application of nuclear science.In a large number of papers and many monographs the phenomenon of nuclear isomerism is studied, analyzed and discussed.The isomers are widely used in various areas of applied science: radiochemistry, activation analysis, radiopharmacology, immunology etc. Isomers are high-capacity energy accumulators.Possibilities are being discussed and experiments are being performed to develop a method for using this energy.At the present time, further development of the investigations of the properties of nuclear isomers is expected in the context of unstable isomeric beams production.That is why studies of the properties of isomeric states, methods of producing them, and the processes involving such states are popular and promising.
Isomeric states population and production of them in the amount required for research and applied purposes is one of the basic problems and selection of a reaction suitable for obtaining a certain isomer in a significant amount with an acceptable percentage of impurities is a delicate task.Alpha-particle induced reactions are useful tool for these purposes because usually the maximal relative yield of highspin state in ~ 20 -50 MeV region of a compound nucleus excitation energy can be achieved by application of α-particle beam.That is confirmed by a lot of works devoted to the study of nuclear reactions such as (α,n), (α,2n), etc., see, for example, Refs.[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].In the measurements presented in this paper, both the low-spin (l.s.) and high-spin (h.s.) states of the isomeric pair are metastable, and for ease of presentation we prefer to define isomeric cross-section ratio (ICSR) as σh.s./σl.s.
In the present paper the results of the experimental study of the energy dependence of the ICSRs for reactions 112 Sn(α,n) 115 Te and 134 Ba(α,n) 137 Ce in the α-particle energy range E = 15 -31 MeV are demonstrated.The values of ICSRs are determined by means of off-beam measurements of the induced activity of members of an isomeric pair.The activation method is a reliable tool for identification of residual isotopes.This is its advantage over the method of direct in-beam measurements of the excitation functions by detecting of light reaction products.In addition, measuring the relative yields of isomers in one and the same experiment often makes it possible to exclude some sources of systematic errors (beam instability, target thickness, etc.) due to their direct reduction.All measurements of the yields were performed using the α-particle beam of the SINP MSU cyclotron (Eα = 31 MeV).The beam particles were slowed down using various aluminum foils.Thin enriched targets were used.The thickness of them does not exceed 5 mg/cm 2 .In contrast to most part of measurements of ICSRs presented in the literature in which scintillation γ-spectrometers were exploited, induced activity was measured using the ones based on high-resolution Ge(Li)-detectors.This increases the reliability of the results and expands the range of application of the activation method.The ICSR values for reaction 112 Sn(α,n) 115 Te were determined from the intensities of lines observed in the spectrum of daughter nucleus 115 Sb: 770 keV populated by the decay of low-spin state and the group of lines 724 keV, 1327 keV, and 1381 keV populated by the decay of high-spin state.The ICSR values for reaction 134 Ba(α,n) 137 Ce were determined from the intensity of line 254 keV (isomeric transition) and the aggregate intensity of two lines from the spectrum of the daughter nucleus 137 La: 447 keV and 437 keV.Given the presence of γ-transition between isomers of this pair, the ICSRs were determined using the formula from [16].The analysis of the excitation functions and the ICSRs is based on the calculations employing codes EMPIRE 3.1 [17] and TALYS [18].The default input parameters were used in this analysis.

Results and conclusion
The values of the relative yields of 115 Te isomers: 7/2 + (ground state), and excited state at E*=10 ±6 keV with spin (1/2 + ) (see, for example, [19]) σ7/2 /σ1/2 for the reaction 112 Sn(α,n) 115 Te measured by us in the The values of the relative yield of 137 Ce isomers: excited state at 254.3 keV with spin 11/2 -and ground state with spin 3/2 + σ11/2 /σ3/2 for the reaction 134 Ba(α,n) 137 Ce measured in the energy range 15 -29 MeV are shown in Fig. 2.These data are in a good agreement with the results of the measurements presented in Ref. [21].An exception is the point corresponding to an energy of 15 MeV where the value from Ref. [21] is approximately two times lower.The values obtained by us seems to be more reliable, since they were obtained using a more sensitive technique, namely, the mentioned above Ge(Li) detector was used.The discussed codes reproduce the ICSRs in low-energy region but greatly overestimate these values for E > 18 MeV.
So, in the present paper the results of the investigations of the yield of isomers in (α,n) reactions 112 Sn(α,n) 115 Te and 134 Ba (α,n) 137 Ce are presented.The results substantially supplement the data on the yield of isomers in reactions with α-particles.The 112 Sn(α,n) 115 Te reaction is characterized by extremely large, reaching 30, values of ICSR for (α,n)-reaction.The sole example characterized by approximately the same values of the ICSR was also found by our group in the reaction 86 Sr(α,n) 89 Zr [14].However, for both of these cases, the high-spin state lies below the low-spin state (Eh.s.<El.s.).At the same time, the most interesting for fundamental research are processes in which long-lived excited rather than ground high-spin states (Eh.s.>El.s) are populated with a high probability.It is these processes that make it possible to obtain high-purity beams of metastable high-spin isomers for studying, for example, superelastic processes, etc. From this point of view, the 134 Ba (α,n) 137 Ce reaction, in which the value of ICSR at the energy of 23 -25 MeV turns out to be large for this type of processes, reaching 5 -8, seems to be promising.Another promising process was presented in [9].Reaction 136 Ce(α,n) 139 Nd was considered, the value of ICSR reaches 10.The formation of beams of 11/2 -137 Ce or 11/2 -139 Nd isomers obtained on the primary beam of α-particles in the frame of the SPIRAL ISOLDE scheme, seems to be realistic because the lifetimes of these nuclides are 34.4 h. and 5.5 h., respectively.
Thus, the presented data and the results of other works used in the discussion demonstrate the importance and broad prospects of experimental studies of the isomeric yields in the α-induced reactions.

Figure 1 .
Figure 1.ICSRs for the reaction 112 Sn(α,n) 115 Te energy region 20-31 MeV are shown in Fig.1.Great values of the ICSR were found for this reaction.Both the EMPIRE 3.1 (green symbols) and TALYS (red symbols) codes greatly overestimate even these large values.Unfortunately, the aggregate absolute cross section of formation of two long-lived states of 115 Te is known only at E = 20 MeV [20] and equal to 40 mb in this point.The values of the relative yield of 137 Ce isomers: excited state at 254.3 keV with spin 11/2 -and ground state with spin 3/2 + σ11/2 /σ3/2 for the reaction 134 Ba(α,n)137 Ce measured in the energy range 15 -29 MeV are shown in Fig.2.These data are in a good agreement with the results of the measurements presented in Ref.[21].An exception is the point corresponding to an energy of 15 MeV where the value from Ref.[21] is approximately two times lower.The values obtained by us seems to be more reliable, since they were obtained using a more sensitive technique, namely, the mentioned above Ge(Li) detector was used.The discussed codes reproduce the ICSRs in low-energy region but greatly overestimate these values for E > 18 MeV.So, in the present paper the results of the investigations of the yield of isomers in (α,n) reactions

Figure 2 .
Figure 2. The same as in Fig. 1 for the reaction 134 Ba(α,n) 137 Ce