Study of (n,2n) reaction on 191,193Ir isotopes and isomeric cross section ratios

The cross section of 191Ir(n,2n)190Irg+m1 and 191Ir(n,2n)190Irm2 reactions has been measured at 17.1 and 20.9 MeV neutron energies at the 5.5 MV tandem T11/25 Accelerator Laboratory of NCSR “Demokritos”, using the activation method. The neutron beams were produced by means of the 3H(d,n)4He reaction at a flux of the order of 2 × 105 n/cm2s. The neutron flux has been deduced implementing the 27Al(n,α) reaction, while the flux variation of the neutron beam was monitored by using a BF3 detector. The 193Ir(n,2n)192Ir reaction cross section has also been determined, taking into account the contribution from the contaminant 191Ir(n,γ )192Ir reaction. The correction method is based on the existing data in ENDF for the contaminant reaction, convoluted with the neutron spectra which have been extensively studied by means of simulations using the NeusDesc and MCNP codes. Statistical model calculations using the code EMPIRE 3.2.2 and taking into account pre-equilibrium emission, have been performed on the data measured in this work as well as on data reported in literature.


Introduction
Studies of neutron induced reactions are of considerable significance, both for their importance to fundamental research in Nuclear Physics and Astrophysics and for practical applications.These tasks require improved nuclear data and high precision cross sections for neutron induced reactions.Furthermore, the formation of a high spin isomeric state in the residual nucleus of a reaction is of considerable importance for testing nuclear models, as it is governed by the spin distribution of the level densities and the level scheme of the nuclei involved [1-3].The 191 Ir(n,2n) reaction presents an interesting case since the high spin value 11 − of the second isomeric state (m2) of 190 Ir relative to the corresponding value 4 − of the ground state (g), offers great sensitivity for such studies.
The 191 Ir(n,2n) 190 Ir g+m1 reaction has been investigated in the past by many groups, including our group [4], from ∼ 8 to ∼ 24 MeV, with the data differing however, by as much as 20%.The differences in the case of 191 Ir(n,2n) 190 Ir m2 reaction though, can reach up to 50% at some energies and in the high energy region there are only few data points in literature.
As for the 193 Ir(n,2n) reaction, there are many data sets in literature concentrated in the region of ∼ 15 MeV, with many discrepancies among them, while only few data points exist in the lower and higher energy regions.
The purpose of the present work was to experimentally deduce the cross section of the 191 Ir(n,2n) 190 Ir g+m1 , 191 Ir(n,2n) 190 Ir m2 and 193 Ir(n,2n) 192 Ir reactions at 17.1 ± a e-mail: vlastou@central.ntua.gr0.3 and 20.9 ± 0.2 MeV, implementing the activation technique.Furthermore, theoretical statistical model calculations were performed using the code EMPIRE 3.2.2 and compared to all available experimental data.

Irradiations
The 191,193 Ir(n,2n) reaction cross sections have been measured at the 5.5 MV tandem T11/25 Accelerator Laboratory of NCSR "Demokritos".The neutron beam was produced via the the 3 H(d,n) 4 He reaction.A new Ti-tritiated target of 373 GBq activity has been used, consisting of a 2.1 mg/cm2 Ti-T layer on a 1 mm thick Cu backing.The flange with the tritium target assembly was air cooled during the deuteron irradiation.Two collimators of 5 and 6 mm in diameter were used and the deuteron beam current was measured both at the collimators and the target and was kept at ∼ 1 µA.During the irradiations, the flux variation of the neutron beam was monitored by a BF3 detector placed at a distance of 3 m from the neutron production.The spectra of the BF3 monitor were stored at regular time intervals (∼ 200 sec) in a separate ADC during the irradiation process.The absolute flux of the beam was obtained with respect to the cross section of the 27 Al(n,α) reference reaction.A Au foil was also used to cross check the experimental neutron flux, as well as the simulated one, via the 197 Au(n,γ ) reaction.
High purity Ir and Al natural samples of 1.3 cm in diameter, having a thickness of ∼ 0.5 mm, were placed at a distance of ∼ 2 cm from the tritium target and were ND2016 irradiated for up to 96 h.The induced activity of product radionuclides was measured with two HPGe detectors of 56% and 100% relative efficiency, properly shielded with lead blocks to reduce the contribution of the natural radioactivity.The efficiency of the detectors at the position of the activity measurements (10 cm) was determined via a calibrated 152 Eu point source.Corrections for selfabsorption of the sample, coincidence summing effects of cascading gamma rays and counting geometry were taken into account along with the decay of product nuclides over the whole time range, as well as the fluctuation of the neutron beam flux over the irradiation time.

Neutron beam characterization
A comprehensive understanding of the energy dependence of the neutron flux is of major importance for the reliability of neutron induced reaction cross section measurements.For the investigation of the quasi-monoenergetic neutron beams produced via the 3 H(d,n) reaction at the tandem Laboratory of NCSR "Demokritos", the multiple foil activation method has been applied along with Monte Carlo simulations implementing the NeuSDesc and MCNP codes.

Monte Carlo simulations
In the absence of time-of-flight capabilities, the investigation of the neutron fluence energy dependence has been carried out using the Monte Carlo simulation codes NeuSDesc [5] and MCNP [6].The NeuSDesc software (developed at IRMM) estimates the neutron energy distribution at any distance, taking into account the details of the tritiated target.The output can then be used as input for MCNP simulations in order to include all the other details of the experimental setup (Al flange, Cu backing, target foils etc).The results of the MCNP simulations for the neutron flux at 20.9 MeV, on the Al foil at the front of the multiple foil stack is shown in Fig. 1.The main origin of the long tail of parasitic neutrons arises from the Ti and Cu backing of the Tritium target, along with the Al flange, and is 2-3 orders of magnitude lower than the main neutron peak at 20.9 MeV.In the case of 17.1 MeV, the tail of parasitic neutrons is much lower.

Multiple foil activation
For the experimental investigation of the neutron beam flux, the multiple foil activation technique has been applied, which is widely used for the determination of the neutron flux density around the irradiated samples along with unfolding techniques [7-9].As a trial case of the facility, the deuterons were accelerated to 2.0 MeV and passed though two 5 µm Mo foils in order to degrade their energy to 0.8 MeV, where the cross section of the 3 H(d,n) 4 He reaction is high enough to produce a neutron beam at 15.3 MeV at a flux of the order of ∼ 10 6 n/s • cm 2 .
High purity foils of natural Au, Ti, Fe, Al, Nb, and Co were placed in close contact, at a distance of 1.7 cm from the neutron beam production and were irradiated for several hours.The neutron induced reactions on these foils, namely 58 Ni(n,p) 58 Co, 93 Nb(n,2n) 92m Nb, 197 Au(n,γ ) 198 Au, 56 Fe(n,p) 56 Mn, 59 Co(n,α) 56 Mn, 115 In (n,n ) 115m In, 46 Ti(n,p) 46m+g Sc, 47 Ti(n,p) 47 Sc, 48 Ti(n,p) 48 Sc, 64 Zn(n,p) 64 Cu and 27 Al(n,α) 24 Na, have different threshold  The experimental reaction rate R i was deduced from the analysis of the experimental spectra for each of the above reactions i, according to the following expression: with N i (t B ) being the number of residual nuclei, with decay probability λ i , produced during the neutron activation time t B and N τ i the number of target nuclei for each reaction i.
In order to test the reliability of the simulations, the neutron spectral distribution of Fig. 1 has been used to calculate the simulated reaction rates (R.R)of the above reactions, using the expression: where σ i (E) are the excitation functions of the reactions taken from the ENDF/B-VII.1 library and (E) the function of the neutron fluence normalized to the experimental fluence on each foil.In fact, the normalized neutron spectral distribution has been cut in energy slices E starting from the threshold of each reaction up to the maximum neutron energy of 20.9 MeV and the sum has been deduced.The resulting simulated reaction rates have been compared with the experimental ones and seem to agree well, thus verifying the reliability of the simulations.

Measurements and results
Natural Ir consists of two isotopes 191 Ir and 193 Ir having 37.3% and 62.7% abundances, respectively.Thus, the 193 Ir(n,2n) 192 Ir threshold reaction is contaminated by the 191 Ir(n,γ ) 192 Ir reaction, which is affected by both high energy and mainly low energy parasitic neutrons.The experimental details for the appropriate corrections and the determination of (n,2n) reaction cross section for both Ir isotopes are described below.

The 191 Ir(n,2n) 190 Ir reaction
The 191 Ir(n,2n) reaction leads to the formation of 190 Ir in its ground 4 − state (T 1/2 = 11.78 d), as well as its metastable m1 1 − state (T 1/2 = 1.12 h) and m2 11 − state (T 1/2 = 3.087 h), which decay to 190 Os.Due to the short half life of m1, the sum of the metastable m1 and the ground state cross sections was determined via the most intensive 518.5 keV transition of 190 Os.The γspectroscopy measurements started 30 h at 17.1 MeV and 67 h at 20.9 MeV, after the end of the irradiation to ensure full decay of the m1 isomeric state to the ground state of 190 Ir and lasted for 50 h.The population of the second isomeric state m2 can be measured independently through the 616.5 keV transition of 190 Os [4], while the intensity measurements started 5 h at 17.1 MeV and 1 h at 20.9 MeV, after the end of the irradiation and lasted for 2 and 15 h, respectively.Figure 2 shows the γ -ray spectra emitted by the Ir sample after the irradiation at 20.9 MeV.The experimental cross sections for the population of the m2 ND2016 and g+m1 states at 17.1 and 20.9 MeV are presented in Fig. 3 along with data from literature and theoretical predictions.

The 193 Ir(n,2n) 192 Ir reaction
The residual nucleus 192 Ir decays to 192 Pt with a half life of 74.2 d.The characteristic γ -ray transitions 308.5, 316.5 and 468.1 keV from the de-excitation of 192 Pt can be used for the determination of the cross section of the 193 Ir(n,2n) 192 Ir reaction.In this work, the most intensive one, namely the 316.5 keV (82.8%) has been used.The 192 Ir nucleus however, can also be produced by the 191 Ir(n,γ ) reaction channel, which is always present and open to low energy parasitic neutrons.Thus, its contribution to the production of 192 Ir should be taken into account.
The contamination from the 191 Ir(n,γ ) reaction has been deduced using the σ (E) excitation function from the ENDF-VII library and the simulated (E), normalized to the experimental fluence on the first Al foil, following the same procedure implemented in the multiple foil activation technique, as described in 2.2.2.The fine tuning of the low energy fluence (below 1 MeV), was accomplished through the normalization via the 411 keV γ -ray originating from the 197 Au(n,γ ) reaction.This low energy part was subsequently used for the determination of the 191 Ir(n,γ ) contribution to the yield of the 316.5 keV γ -ray.At 20.9 MeV this correction was of the order of 20%, while at 17.1 MeV the contamination was reduced to 2%.Naturally, this correction affects the overall uncertainty of the cross section for the 193 Ir(n,2n) reaction, shown in Fig. 3 along with data from literature and theoretical predictions.

Figure 1 .
Figure 1.The simulated neutron energy distribution at 20.9 MeV, using the NeuSDesc and MCNP codes.
energies ranging from ∼ 0 to ∼ 9 MeV.The induced activities of product radionuclides were measured off-line by HPGe detector systems.

Figure 2 .
Figure 2. Experimental spectra from the decay of the g+m1 states (upper panel), m2 state (middle panel) of 191 Ir(n,2n) reaction and g+m1 states (bottom panel) of 193 Ir(n,2n) reaction after irradiation at 20.9 MeV.The acquisition time is 15 h for the middle panel and 50 h for the other two.The characteristic γrays used for the determination of the cross sections, are indicated with their energy values.