Double-neutron capture reaction and natural abundance of 183W, 195Pt and 199Hg isotopes

There are much data on neutron cross sections over the chart of nuclides for stable isotopes and not as much for the radioactive ones. Double neutron capture experiments could be fruitful to provide more data. Time-integrated mean flux of slow neutrons reaches the value of 2.3-1012 n/cm2 s at the irradiation port near the active zone of the IBR-2 pulsed reactor of JINR. This is enough to detect the double neutron capture products by the activation method. A high capture cross section is obtained in the present experiment for intermediate radioactive 182Ta and 194Ir target nuclides. Together with the known data for 198Au, these values may prove an essential role of double neutron capture process for nucleosynthesis of 183W, 195Pt and 199Hg isotopes at stellar conditions.


Introduction
In a double neutron capture process the relatively short-lived 194 Ir (19.28 h) is present as an intermediate target nucleus. The 195 Pt could be found among the final products of activation due to the following process: 194 Ir (n, γ) 195m Ir β -→ 195m Pt (13/2 + ). The latter nuclide is convenient for detection being 4.01 d-lived and the main channel (40%) in the decay of 3.67 h-lived 195m Ir (11/2 -) isomer. The ground state of 195 Ir (3/2 + ) (2.29 h) is produced with a large yield in the reaction, but it decays directly to the ground state of the stable 195 Pt, and not to the isomer [1]. Such properties provide an advantageous option for production of 195m Pt through the double neutron capture reaction. In the case of nat Ta target, the consequent two-neutron capture leads to 183 Ta (T1/2 = 5.1 d) via the intermediate 114.4 d-lived 182 Ta isotope. In the present work the 195m Pt and 183 Ta activities were detected by the γspectroscopy method after irradiation of 193 Ir (98.5% enriched) and nat Ta targets at the IBR-2 reactor. The method of 195m Pt production by double neutron capture reaction was stressed in [2] as useful for clinical applications. In fact, there are recommended neutron cross section values given in [3 -6]. However, there are not enough data available to describe the formation of 195m Pt, which puts the most significant restriction on calculation of the yield. Only two of six important values are known for thermal cross section plus one value for the resonance integral. It would be impossible to evaluate the yield of 195m Pt from such initial data, even in the case when the well-developed computer program is available. The results of [2] were obviously obtained using theoretically estimated cross sections, though it is known that the cross sections, the resonance integral values and isomer-toground state ratios cannot not be predicted in theory. One may hope for fast progress in neutron data, but the experiment on observation of 195m Pt in neutron irradiations seems more direct and fruitful. The flux supplied by the IBR-2 reactor is enough to detect the activation of double neutron capture products. Initially, a test experiment [7] has been performed using relatively low neutron flux generated by the MT-25 electron accelerator at FLNR, JINR. The flux about 108 n/cm 2 s was not enough to observe the products of double neutron capture, but major products of Ir isotopes activation were successfully detected and the corresponding σth and Iγ values were measured. The results are given in Table 1. One of the important conclusions is that the most efficient way to obtain 195m Pt is through the process 194g Ir (n, γ) 195m Ir β -→ 195m Pt. The alternative branch with the population of 194m Ir (171 d) at the first step of neutron capture is inefficient because of the low cross section measured for this high-spin product (see Table 1). A large spin-difference Δ I ≈ 9 between the initial 193 Ir (3/2+) and the final 194m Ir (10 or 11) nuclides suppresses the yield of the product in agreement with the systematics [8] and the results obtained for the Hf isomers [9]. At the second step of neutron capture, the  [3 -6] is given in [7].

The 195m Pt yield due to double neutron capture (experiment)
The numbers of radioactive atoms accumulated after irradiation time t as a result of single and double neutron capture N1 and N2, respectively may be obtained by the solution of linear differential equations taking into account the accumulation and exponential decay law. The following notations are used below: the decay constants λ1 and λ2 for the products with mass numbers (At + 1) and (At + 2), where λ = ln2/T1/2, At is the mass number of the stable target. Obviously, the yield of products is proportional to the number of target atoms N0 and is determined by the flux F of neutrons per cm 2 s. Let us assume that only thermal neutrons are involved and their cross sections are σ1 and σ2. At moderate neutron flux we may neglect the target material exhausting as well as burning-up of the (At + 2) product due to the capture of the third neutron.
If necessary (at high fluxes), a factor of burning-up for the (At + 1) product may be introduced replacing λ1 with (λ1 + σ2·F). The resonance neutron contribution is described by similar equations substituting the resonance integral Iγ instead of σ. However, the resonance neutron flux Fr and the Westscott parameter for the activation product must be specified.
As mentioned above, the accumulation of 195m Pt (4.01 d) proceeds through the radioactive βdecay of 3.67 hlived 195m Ir. This means that the longer-lived product is formed after the decay of the short-lived predecessor. The half-life 3.67 h of 195m Ir is much shorter than a typical irradiation time comparable to the half-life of a product. Therefore, it is logical to assume that 195m Ir is transformed to 195m Pt with no time delay and the parameter λ2 corresponds to the decay of 195m Pt (4.01 d). Definitely, the population efficiency k = 0.40 for the final product must also be introduced into Eq. (2) as a reducing factor. The cross section of 194g Ir activation at the first step is known, but the branch leading to the 195mIr isomer at the second step remained uncertain until now. In the present experiment, the corresponding cross section and the resonance integral were successfully determined at IBR-2 using the fluxes about 2.3·10 12 and 2.0·10 11 n/cm 2 s for thermal and resonance neutrons, respectively. The method of Cd-difference was applied when two enriched 193Ir(98.5%) targets of 20 mg weight each were exposed at the vertical channel of the IBR-2 reactor, FLNP, JINR. The targets with and without Cd shielding were irradiated during the 17 -d reactor run. Metal foils of Ta served as spectators. The Ir samples were dissolved by electrochemical method for consequent isolation of the Pt fraction applying the chromatography. Gamma spectroscopy with HP Ge detector was used for the activity measurements. The dissolving yield was calibrated by the 192Ir activity (present due to the 191Ir admixture), while the Pt isolation method was tested elsewhere. Finally, the gamma lines of 195m Pt decay were measured with a good statistical accuracy, and the production process 194 Ir(n, γ) 195m Ir→ 195m Pt is characterized by the following values: σth = 5150 b and Iγ =295 b including the reduction factor due to the β-decay branch. The cross sections determined now are enough to evaluate the activity yield at the high neutron flux about 2.5·10 15 n/cm 2 s, as at the Oak Ridge reactor. In calculations one must take into account that the N1 intermediate product is partially exhausted due to the second neutron capture. Then, the equilibrium activity of 195m Pt may reach 1.0 Ci per mg of the 193 Ir target material (more details are given below). This value of activity satisfies the requirements for production of great specific-activity solutions necessary for radiotherapy applications. Of course, proper technical tools and methods must be developed for the chemical processing of intense β,γ-ray sources.

Nuclear and astrophysical consequences
A high cross section is obtained for the double-neutron capture process 193 Ir (n,γ) 194 Ir (n, γ) 195m Ir (3.67 h)→ 195m Pt. Taking into account the efficiency of the βdecay branch leading to 195m Pt, one immediately deduces the values of σth = 12900 barns and Iγ = 740 b characterizing the constituent 194 Ir(n, γ) 195m Ir(11/2 -) reaction. The decay branch of low-spin 195g Ir(3/2 + ) to 195m Pt(13/2 + ) was not observed being negligible [1]. In reaction the cross section for the 2.29 h-lived products must exceed by an order of magnitude the observed one for the population of isomeric 195m Ir(11/2 -) state as follows from the typical isomer-to-ground state ratios depending on the spin for ( n, γ) products [9]. Therefore, a total capture cross section for the short-lived 194 Ir(19.28 h) target nuclide must be extremely high (105 barns) unlike the cross section assumed in [10]. This is a surprising result in itself.   In general, the double neutron capture way differs from the standard s-and r-processes. The second neutron capture occurs prior the β-decay of the first capture product (unlike the s-process), while the capture of the third and further neutrons is improbable (unlike the r-process). Table 2. Parameters of the thermal and resonance-neutron capture reactions measured in the present work for radioactive odd-odd isotopes of 182 Ta,194g Ir and the known data [3] for 198 Au. Experimentally observed neutron cross section deduced here for radioactive 194 Ir target is comparable to the highest thermal cross sections known over the nuclide chart and it requires an appropriate interpretation, probably, due to a strong compound resonance exactly near the neutron binding energy in 195 Ir. Both m and g products of 195 Ir reach the 195 Pt ground state after decay, and the known abundance of stable 195 Pt isotope comprises the production through the double-neutron capture by 193 Ir.
In the same experiment the spectator nat Ta targets were also irradiated and the second-step 182 Ta(n, γ) 183 Ta reaction demonstrated values of σth= 25300 b and Iγ= 16600 b substantially exceeding the tabular data [3]. Meanwhile, the cross section 47000 b was reported for 182 Ta in the publication [11] not specifying σth and Iγ. The numerical values given here (except the estimate 10 5 b for 194 Ir) were obtained with the standard deviation about 10% including the errors due to the calibration and recalculations. A high value of σ th= 25100 b was obtained in [3] for the neutron capture by radioactive 198 Au with production of 199 Au and then 199 Hg after βdecay. Measured cross sections for radioactive odd-odd nuclides, such as 182 Ta,194 Ir,and 198 Au are given in

Summary
Production and chemical isolation of 195m Pt isomeric activity is of interest for radiotherapy of patients. The efficiency of double neutron capture reaction for accumulation of 195m Pt is now proved by the experiment on irradiation of the 193 Ir enriched target at the IBR-2 reactor. A high cross section is obtained for the neutron capture reaction by radioactive 194g Ir nuclide. The nat Ta targets were also irradiated over the experiment as spectators and they demonstrated a high cross section for the second neutron capture by radioactive 182 Ta. The cross sections of neutron capture by the odd-odd radioactive targets, such as 194gI r, 182 Ta, and 198 Au, exceeding 10 4 b are of importance for understanding within the nuclear reaction theory. On the other hand, the natural isotope abundances are influenced due to the observed high probability of the double-neutron capture process, in particular, for the production of 183 W, 195 Pt, and 199 Hg isotopes.