Study of prompt neutron emission spectra in fast neutron induced fission of 238 U and 232 Th and 30 kev neutron induced fission on 235 U

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1 Study of prompt neutron emission spectra in fast neutron induced fission of 238 U and 232 Th and 30 kev neutron induced fission on 235 U S. Appannababu 1, J. Praena 2, P.F. Mastinu 1, T. Marchi 1, V.V. Desai 3, S.V. Suryanarayana 3, B.K. Nayak 3, A. Saxena 3, L. Morelli 4, S. Barlini 5, G. Collazuol 6, M. Cinausero 1, L. Sajo-Bohus 7, G. Martin Hernandez 8, M. Degerlier 9, R. Capote Noy 10 1 INFN Laboratori Nazionali di Legnaro, Italy 2 University of Sevilla, Spain 3 BARC Mumbai, India 4 INFN and Università di Bologna, Italy 5 INFN and Università di firenze, Italy 6 INFN and Università di Padova, Italy Nevsehir University, Turkey 7 Universidad Simon Bolivar, Caracas, Venezuela 8 CEADEN, Cuba 9 Nevsehir University, Turkey 10 IAEA, Vienna, Austria Abstract We propose here to measure the prompt neutron emission spectrum in fast neutron induced fission of 238 U, 232 Th and in 30 kev neutron induced fission on 235 U targets at CN Van de Graaff accelerator facility at LNL. Fast quasi-mono energetic neutrons up to 3.2 MeV will be produced using the 7 Li(p,n) reaction up to a maximum proton bombarding energy of 5.0 MeV. The 30 kev neutrons will be produced by the same reaction near threshold, corresponding to a proton energy of 1.88 MeV. Three small ionization chambers mounted with the fissile targets in the cathode plane will be placed one after another just behind the 7 Li target at the end of the beam pipe in order to work both as target and to tag neutron induced fission events. Liquid scintillator detector (BC501) with pulse shape discrimination properties will be used to detect prompt neutrons emitted in coincidence with fission events with Time-of-Flight (ToF) measurements. Another detector developed for the BELINA project will also be used, thanks to the very high efficiency in the energy range below 1 MeV. A long counter will be used as a monitor and laced at high angle. We aim to carry out these measurements at 1.88, 3.0, 4.0 and 5.0 MeV proton beam energy respectively. For feasibility study of the experiment a beam time of minimum 7 days is required.

2 Study of prompt neutron emission spectra (PFNS) in fast neutron induced fission reaction is of topical interest because of its importance in engineering and design of new reactors for nuclear energy production, based on fast neutron induced fission [1]. The uncertainties affect the design parameters of thermal, fast, fusion-fission hybrids and accelerator driven systems reactor designs. For instance, the uncertainty in criticality due to uncertainties in PFNS alone can affect criticality by about 50 pcm to a few hundred pcm (1 pcm = 10-5 _k/k) depending upon the reactor spectra. The PFNS data have larger uncertainties in the lower (less than about 0.1 MeV) and higher (greater than about 4.0 MeV) energy regions. The uncertainty in PFNS (as the initial fission neutron source) influences considerably the uncertainty in the estimate of the high energy tail of the reactor spectra above the (n, 2n) threshold, and in this way, influences the uncertainty in the estimated production rate, for instance, of 232 U, in thorium fuel cycle. The data available on PFNS in EXFOR data needs clean-up and improvements. Most investigations concerning the prompt neutron spectrum in nuclear fission have been concentrated in the fission induced by thermal neutrons and in spontaneous fission, and only 3 measurements has been carried out with TOF technique below 500 kev (see table 1). TOF technique is used only to measure the PFNS neutron spectrum, while thermal neutrons has been used to induce fission. This technique has many advantages, but a drawback is the high background. Using a pulsed 30 kev neutron beam, the background is limited in the region below 30 kev. Moreover, large discrepancy are also present comparing exitation function and integral

3 measurements. For 235 U, a huge discrepancy is still present mainly in the energy range below 1 MeV, as one can notice from error bars in Fig Due to the large discrepancy the IAEA has not been able to perform a new evaluation and a CRP has been made in order to measure accurately the PFNS on transuranic elements. Moreover, limited data are also available on prompt neutron emission spectra for fast neutron induced fission of actinide targets. Therefore, a systematic study of prompt neutron energy spectrum and their multiplicities in neutron induced fission of actinides over an energy range of neutron from near threshold to several MeV is strongly recommended by the most important agencies, as IAEA and NEA.

4 Fig. 2: Example of Fission Chamber tested at BARC for prompt neutron spectrum measurement of 252 Cf spontaneous fission. In the present proposal the BELINA experimental set-up will be used to perform a feasibility study for this kind of measurement. The 7 Li(p,n) reaction from near threshold proton energy up to 5 MeV proton will be used to produce neutron from 30 kev up to 3.2 MeV to induce fission in 233,238 U and 232 Th target deposited in a Fission Chamber like the one reported in Fig. 2 and recently tested at BARC (Mumbai) using a 252 Cf deposited onto the cathode plane. In the present study we plan to use three of such chambers mounted one after the other in order to increase the statistics. Prompt neutron spectra from the fission of the targets will be detected in one BC501 detector placed at 80.0 cm from the fission source by using time of flight (ToF) technique. The start signal for TOF measurement will be given by the fission detector. Pulse shape analysis will be used to further discriminate the neutron signals from the gamma background. In order to detect prompt neutrons below 1 MeV with high efficiency we plan to use a neutron detector developed within the BELINA project. The detectros is a BaF 2 crystal covered with a Boron 10 enriched disk and the neutron is detected by means of the 480 kev gamma of the capture. The large amount of boron used allow the detector to have high efficiency, even in the energy range where the cross section of 10 B(n,gamma) drop down.this will allow to measure neutrons well below the usual detection threshold that we can achieve with standard BC501 liquid scintillators and with higher efficiency of usual Li-glass doped detectors. A long counter will be also used to monitor the neutron beam. In the following the beam time schedule proposed for this feasibility study: DAY 1: mounting and setting up the neutron beam line DAY 2: mounting and setting up the detectors and DAQ DAY 3: beam preparation at 1.88 MeV and beam calibration using the 7 Li(p,n) reaction near treshold

5 DAY 4 to 5: measurement of 235 U PFNS Day 6 to 9: measurement of fast neutron induced PFNS on 238 U and 232 Th using 3.0, 5.0 and 5.0 MeV proton beam. References [1] S.Noda, R.C.Height, R.G.Nelson, M. Devlin and J.M.O Donnel. Phys. Rev.C (2011). [2] D.G.Madland and J.R.Nix, Nucl. Sci. and Eng. 81,213 (1982). [3] IAEA INDC(NDS)-0583 [4]EPJ web of conferences 27,00001(2002)