Measurement of reaction cross-sections for 89 Y at average neutron energies of 7 . 24-24 . 83 MeV

We measured neutron-induced reaction cross-sections for Y(n,γ)Y and Y(n,α)Rb reactions with the average neutron energy region from 7.45 to 24.83 MeV by an activation and off-line γ-ray spectrometric technique using the MC-50 Cyclotron at Korea Institute of Radiological and Medical Sciences. The neutron-induced reaction cross-sections of Y as a function of neutron energy were taken from the TENDL-2013 library. The flux-weighted average cross-sections for Y(n,γ)Y and Y(n,α)Rb reactions were calculated from the TENDL-2013 values based on mono-energetic neutron and by using the neutron energy spectrum from MCNPX 2.6.0 code. The present results are compared with the flux-weighted values of TENDL-2013 and are found to be in good agreement


Introduction
Photon and neutron induced reaction cross-sections of various materials in a wide range of energies are important for different applications such as the design of radiation shielding, the calculation of absorbed dose in the human body during radiotherapy, the activation analysis, and the physics and technology of fusion and fission reactors [1].Yttrium is naturally mono-isotopic and nontoxic material and has good physical properties [2].Neutron, photon, and proton-induced reaction crosssections of yttrium are important for its application in making X-ray intensifying screen and laser.Yttrium is also used as alloys with other materials as the control rod for power reactors [3].
The interest in high-energy neutrons is rapidly growing because of a number of potential applications in nuclear science and technology concerning fast neutrons.In the high-energy region above ~20 MeV, a truly monoenergetic neutron beam is not feasible in a strict sense.For certain nuclear reactions, there is a strong dominance of neutrons in a narrow energy range and are called quasi-mono energetic neutron sources [4].Quasi-mono energetic neutrons based on the 7 Li(p,n) 7 Be reaction with flexibility in energy and shape of neutron field is controllable in the energy range 11-175 MeV [5][6][7][8].
The 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reaction cross-sections were reported with various mono-energetic neutron beam in the energy range from threshold to 15.8 MeV [9].There is no experimental data exist for higher neutron energies.In the present work, we have determined the neutron-induced reaction cross-sections of yttrium in the average neutron energies of 7.45 to 24.83 MeV from 9 Be(p,n) reaction and by an activation and off-line γ-spectrometry technique using proton beam from the MC-50 Cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS).The theoretical reaction cross-section values based on mono-energetic neutron beam were taken from the TENDL-2013 library [10].The neutron spectrum for 9 Be(p,n) reaction was calculated with the MCNPX 2.6.0 code [11].

Experimental set-up
The experiment was carried out by using the MC-50 cyclotron at KIRAMS.In the present work proton beam of energy 25, 35, and 45 MeV were impinging on 5 mm Be target producing neutrons from the 9 Be(p,n) reaction.The experimental details and sample characteristics are given in our previous work [12].
The neutron energy spectra for three different proton energies were estimated by using the MCNPX 2.6.0 code [12].The flux-weighted neutron energies for the 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions from their threshold energy (E th ) to the maximum neutron energy (E max ) based on the neutron spectrum in Fig. 1 were estimated for three proton energies as follows:

Data analysis
The aluminium wrapper were used as neutron flux monitor.The photo-peak area of 1368.6 keV observed in the γ-ray spectrum was due to the formation of 24 Na from the 27 Al(n, ) 24 where n is the number of target atom, I γ is the branching intensity of the analysed γ-ray, ε the detection efficiency, λ is the decay constant (=ln2/T The nuclear spectroscopic data used for determining the cross-sections values were taken from refs.[14,15].

Results and discussion
The flux-weighted average cross-sections of the 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reaction were determined in the present work and listed in Table 1.The uncertainties in the measured cross-sections are briefly discussed in our earlier work [12] and are in between 16 and 20% based on a statistical error of 10-13% and a systematic error of 12-15%.
The cross-sections of 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reaction with mono-energetic neutrons are available in the literature [9].However, we could not compare directly the present flux-weighted average cross sections with the literature data measured with mono-energetic neutrons.The cross-sections of 89 Y as a function of neutron energy were taken from TENDL-2013 nuclear data library [10], based on TALYS-1.6 code.The fluxweighted average cross-sections were obtained by using the following equation:  4) are given in Table 1.It can be seen from Table 1 that the fluxweighted average cross-sections of 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions obtained from TENDL-2013 based on the TALYS 1.6 agree in general with the present experimental results.The 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reaction crosssections for mono-energy neutron from TENDL-2013 are plotted in Fig. 2 and Fig. 3. and compared with experimental data from literature [9].Our experimental data and flux weighted average value of TENDL-2013 are also plotted in Figs. 2 and 3. Our experimental data are found to be in good agreement, which indicates the correctness of the present approach.
flux as a function of neutron energy, E n .The flux-weighted neutron energies for 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions with a 45 MeV proton beams are 18.05 and 24.83 MeV, respectively.Those for a 35 MeV proton beam are 13.41 and 18.22 MeV for 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions.For a 25 DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2015 MeV proton beam the flux-weighted neutron energy is 7.45 and 12.95 MeV.

Figure 1 .
Figure 1.The calculated neutron spectra produced by the 9 Be(p,n) reaction using MCNPX code.The irradiated 89 Y sample along with Al wrapper was taken out from the irradiated assembly after 30 minutes and mounted on Perspex plates.The γ-ray counting of the reaction products from the 89 Y sample and 27 Al wrapper was done by using an energy-and efficiency-calibrated HPGe detector coupled to a PC-based 4 K-channel analyzer.The γ-ray counting was done for at least three half-lives in live time mode.The high energy neutrons flux causes different reactions in 89 Y target.
Na reaction was used to determine the neutron flux.The net photo-peak areas of the different γrays of reaction products of interest were calculated by subtracting the Compton background from the gross peak area.The numbers of observed γ-rays (N obs ) under 1368.6 keV photo-peak are related to the cross section ) ( n R E V for the 27 Al(n, ) 24 Na reaction and the integrated neutron flux

1 / 2 )
of the isotope of interest, T I , T C , CL and LT are the irradiation time, the cooling time, the real time, and the counting time, respectively.The integrated neutron flux n ) for each irradiation condition was obtained from the reaction threshold (3.245 MeV) of the 27 Al(n, ) 24 Na reaction to the maximum neutron energies of 23, 33, and 43 MeV as shown in Fig. 1.However the threshold energies for the 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions are zero and 6.6 MeV, respectively.Thus the integrated neutron flux obtained from the 27 Al(n, ) 24 Na reaction has to change based on threshold energies of the 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions.Then the integrated neutron flux ratios for the 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions were obtained from the threshold to the maximum neutron energy divided by the flux from the reaction threshold (3.245 MeV) of the 27 Al(n, ) 24 Na reaction to the maximum neutron energies obtained by using the MCNPX 2.6.0 code as shown in Fig. 1.The detailed calculations of flux factors are given in our earlier work [12].Using the factors, the integrated neutron fluxes for the individual reactions were obtained.Then the fluxweighted average cross-sections for 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions were calculated as follows:

V
is the cross-section for the 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reaction with a mono-energetic neutron, taken from TENDL-2013.The neutron flux ) ( n E I is estimated by the MCNPX 2.6.0 code as shown in Fig. 1.The flux-weighted average cross-sections of 89 Y(n,γ) 90m Y and 89 Y(n,α) 86 Rb reactions based on Eq. (

Figure 3 .
Figure 3.The 89 Y(n,D) 86 Rb reaction cross-sections from literature [9] and those obtained from the TENDL-2013 [10] as a function of mono-energetic neutron energy.