The n_TOF facility at CERN: a scientific succes! Daniel Cano Ott on behalf of the n_TOF collaboration daniel.cano@ciemat.es

Physics motivation

Nuclear data for nuclear technologies Neutron capture and fission cross sections for the transmutation of nuclear waste in Accelerator Driven Systems (NTOF-ND-ADS EU FP-5 project) and the design of new and future nuclear reactors. Nuclear data for astrophysics Neutron capture cross sections relevant to nuclear astrophysics, cosmochronometry, stellar nucleosynthesis… Basic neutron reaction and nuclear structure data with neutron beams. Fundamental cross sections. Photon strength functions and nuclear level densities.

A brief history of the n_TOF facility 1997 - Concept by C.Rubbia validated by TARC exp. [CERN/EET/Int. Note 97-19] May ’98 - Further development of the initial idea towards a working facility [CERN/LHC/98-02+Add] Aug ’98 – CERN-GELINA joint Letter of Intent [CERN/SPSC/98-15, I220] 1999 – Construction started Oct 2000 – First proton on the spallation target 2001 to 2004 – Experimental program of n_TOF-Ph1 (NTOF-ND-ADS 5th European Union Framework Programme). 2005 to 2007 – Shutdown due to the LHC startup + modifications of the spallation target June 2007/January 2008 – n_TOF Review Pannels. Green light for a new target! December 2007 / January 2008 – Inspection, removal and analysis of the old target. March 2008. MoU signed by CERN. November 2008 – Start of the n_TOF-Ph2 commissioning.

n_TOF has done key contributions to the neutron data community

The n_TOF facility itself!

Available facilities worldwide and uniqueness of n_TOF Some of the existing neutron time of flight facilities (GELINA – EU, ORELA, RPI – USA) for cross section measurements are based on electron LINACS (photoproduction). They are and will be excellent tools for investigating stable or slightly radioactive materials: Good energy resolution. Low intensity per accelerator pulse / high repetition rate -> low duty cycle. Need of “massive” samples (several 100 mg) However, cross section data for radioactive isotopes available in small amounts (1 - 10 mg) need high intensity sources: pulsed spallation sources like LANSCE-LANL (USA) and n_TOF @ CERN (Europe): Intrinsically worse energy resolution (partially at n_TOF @ its185 m flight path!) High intensity at even low repetition rates -> high duty cycle. Measurements with samples of a few mg and high intrinsic activities (1 GBq).

The n_TOF Collaboration 40 Research Institutions 120 researchers Spokesperson: E. González (CIEMAT)

The n_TOF facility

(A Google-view of) The n_TOF facility at CERN n_TOF 185 m flight path Proton Beam 20GeV/c 7x1012 ppp Pb Spallation Target Neutron Beam 10o prod. angle Booster 1.4 GeV Linac 50 MeV PS 20GeV

n_TOF beam characteristics The n_TOF facility was built and commissioned in a period of 2 years and provides unique features for measuring capture and fission cross sections of unstable (and also stable) isotpes. White neutron beam: 0.1 eV up to 1 GeV DE/E ~ 1·10-4 in the RRR Capture and fission beams: F=4 cm beam F=8 cm beam Performance Report, CERN-INTC-2002-037, January 2003,CERN-SL-2002-053 ECT

A fully digital DAQ!

2. The n_TOF data acquisition system n_TOF has been the first neutron beam line world-wide proposing, building and operating a fully digital DAQ. Nowadays, it is becoming a standard at every laboratory. The n_TOF DAQ consists of ~50 flash ADC channels with 8 bit amplitude resolution and sampling of 500 MSample/s. The full history of EVERY detector is digitised during a period of 16 ms (0.7 eV < En < 20 GeV) and recorded permanently on tape. The system has nearly zero dead time. Simple electronics but everything needs to be done by software.

Pulse shape fitting, particle discrimination and pileup reconstruction for C6D6 detectors. C. Guerrero et al, submitted to NIM-A. Pulse shape analysis and pileup reconstruction for the BaF2 detectors. E. Berthomiueux et al. To be submitted to NIM-A.

Advanced neutron detectors and monitors

A low mass neutron beam monitor Small foil with 6Li deposit in the beam. Si detectors outside the beam for α,t detection. Carbon fiber scattering chamber (low neutron sensitivity). 6LiF α,t n

n_TOF fission detectors Position sensitive Parallel Plate Avalanche Chambers. Allow to reconstruct the trajectories of the fission fragments. Tassan-Got et al. To be submitted to NIM-A Fast induction chamber (FIC). Large number of samples inside a compact detector. P. Cennini et al. Submitted to NIM-A

The low neutron sensitivity C6D6 detectors Neutron beam Sample changer The low neutron sensitivity C6D6 detectors Low neutron sensitivity C6D6 detectors with carbon fibre housing. R. Plag et al. Published in NIM-A

10B doped carbon fibre capsules The n_TOF Total Absorption Calorimeter (TAC) for (n,g) measurements 40 BaF2 crystals covering 95% of 4p. 98% detection efficiency for capture g-ray cascades. C12H20O4(6Li)2 Neutron absorber 10B doped carbon fibre capsules n beam 237Np sample

Cross section analysis techniques

High accuracy Pulse Height weighting technique The efficiency of the C6D6 detectors for detecting capture cascades depends on the de-excitation pattern of the capturing nucleus. It can be made proportional to the capture cascade by using the Pulse Height Weighting technique: n_TOF has demonstrated that it is necessary to obtain the Weighting Functions for each specific setup and that it is possible to do it with Monte Carlo simulations. Taín et al. Published in NIM-A

The experimental programme

n_TOF experiments 2002-4 Capture Fission 151Sm 204,206,207,208Pb, 209Bi 232Th 24,25,26Mg 90,91,92,94,96Zr, 93Zr 139La 186,187,188Os 233,234U 237Np,240Pu,243Am Fission 233,234,235,236,238U 209Bi 237Np 241,243Am, 245Cm n_TOF experiments 2002-4 Measurements of neutron cross sections relevant for Nuclear Waste Transmutation and related Nuclear Technologies Th/U fuel cycle (capture & fission) Transmutation of MA (capture & fission) Transmutation of FP (capture) Cross sections relevant for Nuclear Astrophysics s-process: branchings s-process: presolar grains Neutrons as probes for fundamental Nuclear Physics Nuclear level density & n-nucleus interaction High energy part is based on Simulated flux… since the g-flash was generating an oscillation for a few 2-3 us

(n,f) cross sections Measurements with PPACs. L. Tassan-Got et al. In preparation. C. Paradela et al. In preparation

(n,f) cross sections with FIC Targets interesting for the Th/U cycle and transmutation purposes: 235U, 233U, 245Cm, 241Am, 243Am M. Calviani et al. In preparation. 233U 235U: some discrepancies in the resonance valleys with evaluated libraries and exp’ts Discrepancies in the epithermal region

Ratio with FIC2 fission can be extracted up to 300 MeV Fission measurements with the Fission Ionization Chamber (FIC) – 2/2 245Cm: poor previous experimental results Statistical error bars Sistematic errors ~8% 238U/235U: both isotopes are fission standards up to 200 MeV. PRELIMINARY Ratio with FIC2 fission can be extracted up to 300 MeV PRELIMINARY 241Am 241Am has 239Pu contaminant which significantly contributes to the total σ.

(n,g) cross section measurements with C6D6 detectors Resolved Resonance Region Unresolved Resonance Region 232Th level spacing reduced neutron width distribution mass (232Th) = 2.8037 g diameter = 1.5 cm purity = 99.5% F.Gunsing, Nuc. Data. Conf.-2007 G. Aerts et al. (n_TOF Collaboration), Phys. Rev. C 73, 054610 (2006)

24,25,26Mg(n,γ) measurements first known Impurities Mg resonance Motivation: Neutron poison for s-process Strength of 22Ne(α,n)25Mg neutron source Magnesium anomalies in presolar grains first known Mg resonance at 20 keV Impurities In, Sb M.Heil (FZK), Nuclei in the Cosmos IX, Geneva 2005

90,91,92,93,94,96Zr(n,γ) measurements 90Zr is neutron magic Zr abundances are sensitive to the neutron flux in AGB models. s-process branching at 95Zr

Cosmochronometer: tu = 15 ± 2 Gy 186,187,188Os (n,γ) measurement Cosmochronometer: tu = 15 ± 2 Gy M. Mosconi et al., Int. Conf. Nuc. Phys. in Astrophysics 2007, Dresden K. Fujii et al., Nuc. Data. 2007, Nice

204,206,207Pb, 209Bi (n,γ) measurement 204Pb 206Pb C.Domingo-Pardo et al. (n_TOF Collaboration), Phys. Rev. C 74/75, 2006/7

151Sm(n,γ) measurement U. Abbondanno et al. (n_TOF Collaboration), Phys. Rev. Lett. 93 (2004), 161103 S. Marrone et al. (n_TOF Collaboration), Phys. Rev. C 73 (2006) 03604

151Sm(n,γ) measurement U Abbondanno et al. Phys. Rev. Lett. 93 (2004), 161103 S. Marrone et al. Phys. Rev. C 73 (2006) 03604 n_TOF MACS-30 = 3100 ± 160 mb <D0> = 1.48 ± 0.04 eV, S0 = (3.87 ± 0.20)×10-4

233,234U (n,γ),233,234,235U(n,f) measurement Neutron induced capture Neutron induced fission ENDF/B-VI n_TOF (PPACS) W.Dridi, PhD-Thesis, 2006 C.Paradela, PhD-Thesis, 2005

237Np (n,γ) measurement with the TAC 43.3 mg, 1.29 MBq - En=2keV upper limit due to the Ti-canning resonances contaminations - Total systematic uncertainty of 6% - Large number of overlaping resonances (about half eV level spacing) - Simultaneous fit transmission+capture-> total CS unchanged, relative ratio capture/scatt changes C. Guerrero et al. (n_TOF Collaboration), Proc. Int. Conf. Nuc. Data for Sci. and Tech. 2007, Nice.

240Pu (n,γ) measurement 51.2 mg, 458 MBq ENDF/B-VII n_TOF Bouland et al. ‹D0› (eV) 12.1 12.1±0.1 12.06 ‹Gg›(meV) 30.6 32.4±0.8 31.9 S0(10-4) 1.13 1.04±0.08 1.07 - Huge activity of the sample, measurable at n_TOF with the TAC C. Guerrero et al. (n_TOF Collaboration), Proc. Int. Conf. Nuc. Data for Sci. and Tech. 2007, Nice.

243Am (n,γ) measurement with the TAC -g-ray contaminations from the Am-243 sample have a very large E (comp. to Pu, Np), this makes high pileup effects, need more work. First (n,γ) measurement EVER. 10 mg, 75 MBq D. Cano-Ott et al. (n_TOF Collaboration), Proc. Int. Conf. Capture Gamma-Ray Spec. 2005, Santa Fe.

geometry used for the MC simulations with GEANT4 Nuclear structure: TAC as a γ-ray spectrometer Analysis of the calorimeter data and comparison to realistic Monte Carlo simulations of its response to the EM cascades. geometry used for the MC simulations with GEANT4 Cascade Generator model by J.L Tain (IFIC-Spain) Sn+En Statistical region Known region A+1Z

TAC response 197Au(n,g): Esum and mg Adjusted PSF + Egidy’s NLD good reproduction of the 197Au(n,g) TAC data not only for deposited energy (with and without conditions in mg) but also for mg (Esum>1.5 MeV). mg>0 mg>2

TAC response 197Au(n,g): Ecr vs. mg

TAC response 197Au(n,g): Esum vs. mg

237Np TAC data Experimental MC simulation Experimental MC simulation Fits on actinides are not as accurate as for 198Au, it can be attributed to : Level scheme not as well know as for 198Au Tunning of the GammaRayStrengrth function (only 2 weeks while 1 month for 198Au) The sahpe is irrelevan to some extend, important is the effcieincy.

240Pu TAC data

240Pu TAC data The precision in the estimation of edet is better than 2% even if the shape of the Esum spectrum is not perfectly reproduced.

The n_TOF publications All details can be found in the n_TOF web server http://www.cern.ch/n_TOF Summary of the papers published since 2002 7 Phys. Rev. C 11 Nucl. Instr. Meth. A 3 Nucl. Phys. A 1 Phys. Lett. 2 Phys. Rev. Lett. 3 international publications ~50 international conference proceedings + 10 publications in preparation to be published in 2008

Future plans

n_TOF experiments 2008-… Capture Fission Stable Isotopes: Mo,Bi, Ru: r-process residuals Fe, Ni, Zn, Se: s-process and structural materials Radioactive Isotopes: 234,236U, 231,233Pa: Th/U fuel cycle 239,240,242Pu,241,243Am, 245Cm: transmutation of minor actinides Fission 231Pa,234,235,236,238U 241Pu,245Cm,241,243Am, 244,245Cm 234U: study of vibrational resonances below the bareer

Better experimental conditions Photon time distribution (E>1MeV)

New experimental area at 20 m The (future) second n_TOF experimental area New experimental area at 20 m n_TOF target Experimental area at 185 m Flight-path length : ~20 m at 90° respect to p-beam direction 100 times more intense!

New target design (ready in 2008) Pb target with enhanced water cooling and corrosion resistant: Moderator is de-coupled from the cooling circuit (10B or other liquids) Compatible to the vertical flight path Chemical monitoring of the coolant protons

Summary and conclusions n_TOF @ CERN is a first class neutron Time Of Flight facility coupled to a spallation neutron source: 185 m flight path, favorable duty cycle and excellent energy resolution. It allows to measure 1 mg mass samples of highly radioactive materials . Excellent facility for measuring neutron capture and fission cross sections of Minor Actinides: Np, Pu, Am, Cm… High performance detectors for fission (PPAC and FIC) and capture (TAC) cross section measurements Fully digital Data Acquisition System. Accurate cross section measurements. The n_TOF operation was interrupted in 2004 for the LHC startup and will restart in 2008 FINALLY!