Borcea, C

Buono, S

Cennini, P

Dahlfors, M

Dangendorf, V

Ferrari, A

García-Muñoz, G

Kadi, Y

Lacoste, V

Nolte, R

Radermacher, E

Rubbia, Carlo

Saldaña, F

Vlachoudis, V

Weierganz, M

Zanini, L

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN  ¾  SL DIVISION Geneva, Switzerland December 2001   CERN SL-2001-070 EET       First results from the neutron (nTOF) facilty at CERN   C. Borcea1, S Buono2, P Cennini1, M. Dahlfors1, V Dangendorf3, A Ferrari1, G. Garcia-Munoz1, Y Kadi1, V. Lacoste1, R. Nolte3, E Radermacher1, C Rubbia4, F Saldana1, V Vlachoudis1 M Weierganz3 and L Zanini1  1 CERN, Geneva, Switzerland 2  CRS4, Cagliari, Italy 3 Physikalisch Technische Bundesanstalt, Braunschweig, Germany 4 Universita di Pavia e Sezione dell’INFN, Pavia, Italy     Abstract A new high-flux spallation neutron source, the neutron Time-Of-Flight facility (nTOF), recently became operational at CERN. The neutron energy ranges from the thermal GeV range. Performance results from the commissioning are reported.       Applied Physics A manuscript No.(will be inserted by the editor)First results from the neutron (nTOF) facility at CERNC. Borcea1?, S. Buono2, P. Cennini1, M. Dahlfors1??, V. Dangendorf3, A. Ferrari1, G. Garcia-Munoz1???, Y. Kadi1, V. Lacoste1, R. Nolte3, E. Radermacher1, C. Rubbia4, F. Saldana1,V. Vlachoudis1, T. Weierganz3, L. Zanini11 CERN, Geneva, Switzerland2 CRS4, Cagliari, Italy3 Physikalisch Technische Bundesanstalt, Braunschweig, Germany4 Universita` di Pavia e Sezione dell’INFN, Pavia, ItalyReceived: .... / Revised version: ....Abstract A new high-flux spallation neutron source,the neutron Time-Of-Flight facility (nTOF), recently be-came operational at CERN. The neutron energy rangesfrom the thermal to the GeV range. Performance resultsfrom the commissioning are reported.1 IntroductionThe aim of the neutron Time-Of-Flight (nTOF) facilityat CERN is the measurement of cross-sections neededfor the design of innovative Accelerator Driven System(ADS) applications such as incineration of nuclear waste,energy production, radioisotope production for medicalapplications and for many other basic science subjects,in particular astrophysics. A complete overview of thephysics potential is given in Refs. [1,2]. Therefore, anTOF facility has been built at the CERN PS, deliveringa maximum intensity of 3 × 1013 protons in four pulseswithin a 14.4 s supercyle at a momentum of 20 GeV/c.The source is followed by a 182 m flight basis. This al-lows neutron cross-sections of almost any element to bestudied systematically and with excellent resolution us-ing targets of very modest mass — necessary for unstableor otherwise expensive materials — in the interval from1 eV to 250 MeV.2 The nTOF facilityThe simulation of the detailed geometry of the lead tar-get has been performed to estimate the neutron flux atSend offprint requests to:? On leave from IFIN-HH, PO Box MG-6, Bucharest-Magurele, Romania?? On leave from Uppsala University, Uppsala, Sweden??? Now at Universidad Politecnica, Madrid, Spain200 m. Two Monte Carlo codes were used successively:FLUKA [3] and the EA-MC Monte Carlo code [4].Following an overall optimisation between neutronflux and ∆λ resolution (λ = effective neutron path), thespallation target was chosen to be a lead block of 80 ×80 × 40 cm3, followed by a water moderator of 5 cmthickness [2]. In the final design [5] the neutron emissiontakes place at an angle of 10◦ with respect to the protonbeam direction and the target is made of pure lead blocksalready used in the TARC experiment [6]. A thin singlemetallic window (aluminium alloy) of 1.6 mm thicknessis the interface between the moderator and the vacuumin the nTOF tube [5].The time-of-flight tube (Fig. 1) starts directly behindthe window and ends where the sloped floor of the TT2Atunnel (1.18% gradient) touches the tube, thus allowinga length of 200 m. The pressure in the vacuum tube is≈ 1 mbar. The tube is made up of four different sectors,the first one (Ø = 80 cm), closest to the target, is madeof aluminium alloy whereas the others (Ø = 80, 60 and40 cm) are made of stainless steel [5].Two collimators were installed to reduce the radiusof the neutron beam. The first one, 2 m in length (beamshaping collimator), is located at 136.7 m and the secondone is placed at 178 m.A 2 m long dipole magnet, locatedat 145 m, is used to sweep away charged particles.3 The commissioningTwo commissioning phases took place, one in November2000 and another one at the beginning of April 2001,aimed at checking the performance of the facility andat comparing the data with the simulation. The inci-dent proton beam delivered by the PS was monitored inintensity by current transformers and in shape by digiti-sation of CCD cameras. Additional measurements wererelated to safety aspects, namely the temperature of the2 C. Borcea et al.target, the temperature of the cooling water and the ac-tivity of the resin filters, and different positions insidethe tunnel.We restrict ourselves to the second commissioningperiod where two parallel-plate ionisation chambers withfissile deposits, one with 235U and another with 238U,were used. These detectors are inter-comparison instru-ments and were provided by PTB Braunschweig. A fulldescription of these fission chambers can be found inRef. [7]. The detection efficiency of these detectors ispractically constant and equal to 95% up to neutron en-ergies of 10 MeV. In both commissioning phases pairsof gold foils were irradiated, followed by their gammaactivation measurement.The raw time-of-flight spectrum was corrected runby run for small shifts in the t0 value, since the PS tim-ing signal was sometimes not constant. Next the con-tributions from α particles and other charged particlesproduced by high-energy neutrons were eliminated bycutting in the flash ADC spectrum where a very clearseparation was observed from the fission products. Anadditional correction had to be applied owing to thepresence of tantalum, on which the 235U is evaporated,and of platinum, which is the material of the electrodes.After all these corrections, the neutron fluence was ob-tained by dividing the data with the expected valuesfrom the ENDF [8] database. The preliminary result isshown in Fig. 2 for the 235U and 238U fission detector.In the case of the 235U detector a clear disagreementwith the expectation is observed at the highest ener-gies whereas for the 238U detector, which is only sen-sitive to high-energy neutrons, the agreement is good.Recent measurements in the experimental zone revealedthe presence of a γ background, presumably coming fromthe slowing down of fast neutrons with subsequent cap-ture and for which the origin is not yet known. Thesebackground neutrons might be responsible for the ex-cess of data at high energies.4 ConclusionThe preliminary results from the commissioning mea-surements show a good agreement between the data andexpectation. More measurements and calculations, mainlyconcerning the background, are now being performed be-fore the facility is ready to make precise physics measure-ments.References[1] Proposal for a Neutron Time Of Flight Facility,CERN/SPSC 99-8, SPSC/P 310, 17 March 1999.[2] C. Rubbia et al., ‘A High Resolution Spallation DrivenFacility at the CERN–PS to Measure Neutron CrossSections in the Interval from 1 eV to 250 MeV’,CERN/LHC/98-02 (EET), Geneva, 30 May 1998.C. Rubbia et al., ‘A High Resolution Spallation DrivenFacility at the CERN–PS to Measure Neutron CrossSections in the Interval from 1 eV to 250 MeV: aRelative Performance Assessment’, CERN/LHC/98-02(EET)-Add. 1, Geneva, 15 June 1998.[3] A. Fasso et al., in ‘Intermediate Energy Nuclear Data:Models and Codes’, Proceedings of a Specialists Meet-ing, Issy les Moulineaux (France) 30 May – 1 June 1994,p.271, published by OECD, 1994 and references therein.[4] F. Carminati et al., ‘TARC General Purpose MonteCarlo’, CERN Internal Report CERN/LHC/EET 96-011, Geneva, 17 April 1996.[5] Neutron TOF Facility (PS 213) Technical Design Report,CERN/INTC/2000-004, 11 February 2000.[6] H. Arnould et al., ‘Neutron-Driven Nuclear Transmuta-tion by Adiabatic Resonance Crossing’, CERN-SL-99-036 EET, also as final report to the European Com-mission, Contract No F141-CT96-0009, EUR 19117 EN,ISBN 92-828-7759-0.[7] D. B. Gayther, Metrologia 27 (1990) 221.[8] ENDF/B-VI, OECD NEA Data bank (1996).First results from the neutron (nTOF) facility at CERN 3Φ=0.8¿=6000.05 mΦ=0.4Φ=0.6SweepingMagnetFirstCollimator(fixed part)FirstCollimator(removable part)FilterTubeCollimator69.8570.5 72.50133.3 135.6 137.6133.9134.9 139.1 141.1 172.145.1Φ=0.4 Φ=0.4MeasuringStationForward ShieldingMeasuringStationBack ShieldingReservedfor theMeasuring StationΦ=0.2Al-Alloy Al-AlloyEscape Lane175 177.5 182.4 189.9179 182.2 190.1191.7ValveΦ=0.40 mInterface FlangesΦ=0.20 mShutterSecond CollimatorValveΦ=0.20 m200 mFirst sectionΦ = 80 cm4 m aluminum66 m steel1 mbarSecond sectionΦ = 60 cm68 m steelThird sectionΦ = 40 cm62 m steel Experimental area@ 185 m tube touching the floor @ 200 m(1.18 % slope)First collimator @ 136.7 m(beam shaping collimator)Φint = 11 cm1 m iron + 1 m concreteSweeping magnet @ 145 m Second collimator @ 178 m(source screening collimator)  Φint = 2 cm (σcapture)Φint = 8 cm (σfission)0.5 m borated polyethylen + 1.25 m iron + 0.75 m bor. polyeth.Fig. 1 Time-of-flight tube sections up to the end of the TT2A tunnel (200 m).10 410 510 -2 10 -1 1 10 102 103 104 105 106 107 108Neutron Energy (eV)Simulation (Scaling Factor=0.853)PTB 235U ChamberPTB 238U ChamberIntegral Neutron Flux (dn / dlnE / 7 1012 pr)Fig. 2 Preliminary integrated neutron fluence from the 235U (upper data curve) and 238U (lower data curve) fission detectortogether with the expectation.

First Results from the Neutron (nTOF) facility at CERN

First Results from the Neutron (nTOF) facility at CERN

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