The neutron time of flight facility CERN

Transcription

1 The n_tof Collaboration, The neutron time of flight facility CERN Giuseppe Tagliente Istituto Nazionale Fisica Nucleare, Sezione di Bari

2 The n_tof Collaboration (~100 Researchers from 30 Institutes) CERN Technische Universitat Wien Univ. of Camberra Joint Research Center, Geel Charles Univ. (Prague) IN2P3-Orsay, CEA-Saclay Univ. of Athens, Ioannina, Demokrito Greece INFN Bari, Bologna, LNL, LNS, Trieste, ENEA Bologna, CNR Bari BARC Univ. of Tokio Univ. of Lodz ITN Lisbon IFIN Bucarest JINR Dubna Austria Australia Belgium Czech Republic France Italy India Japan Poland Portugal Rumania Russia CIEMAT, Univ. of Valencia, Santiago de Compostela, University of Cataluna, Sevilla Spain Univ. of Manchester, Univ. of York, Hearthfordshire UK

3 n_tof Scientific Motivations Neutron cross sections relevant for Nuclear Astrophysics Measurements of neutron cross sections relevant for Nuclear Waste Transmutation and related Nuclear Technologies (ADS) Neutrons as probes for fundamental Nuclear Physics

4 Neutron cross sections Nuclear energy (fission products & Structural material) Nuclear Astrophysics (stellar nucleosynthesis) Astrophysics Advanced nuclear reactors (actinides)

5 Abundances beyond Fe ashes of stellar burning number fraction Gap B,Be,Li a-nuclei 12 C, 16 O, 20 Ne, 24 Mg,. 40 Ca Fe peak Elements heavier than Fe are the result of neutron capture processes NEUTRONS r-process peaks (nuclear shell closures) s-process peaks (nuclear shell closures) Au Pb H C 10 Fe 1 Au Th, U mass number

6 Nucleosynthesis s-process lifetime 10 4 years n n neutron/cm 3 r-process lifetime ms n n neutron/cm 3 b-decay lifetime: few hours to few years The canonical s-process Cu 62 Cu 9.74 m 63 Cu Cu 12.7 h neutrons Ni 60 Ni Ni Ni Ni 100 a 64 Ni Co 58 Co d 59 Co Co a 61 Co 1.65 h Fe 56 Fe Fe Fe Fe d 60 Fe a 61 Fe 6 m

7 Stellar Models

8 s-process stellar sites Low mass Asympotic Giant Branch (AGB) M M 13 C(a,n) 16 O T ~ 8 kev N n < 10 7 n/cm 3 22 Ne(a,n) 25 Mg T ~ 23 kev N n ~ n/cm 3 Massive stars M M 22 Ne(a,n) 25 Mg In core He-burning T ~ 26 kev N n ~ 10 6 n/cm 3 In shell C-burnig T ~ 90 kev N n ~10 11 n/cm 3

9 n_tof Goal *** cross section uncertainties <5% *** safe control of systematic uncertainties

10 n_tof Goal

11 The nuclear waste problem Figura Nucleosintesi (frecce che si muovono) Foto FIC 244, 245 Cm 1.5 Kg/yr 241 Am:11.6 Kg/yr 243 Am: 4.8 Kg/yr 239 Pu: 125 Kg/yr 237 Np: 16 Kg/yr LLFP Quantities refer to yearly production in 1 GW e LW reactor LLFP 76.2 Kg/yr

12 Nuclear energy and the need of nuclear data EU programs Objectives of the nuclear industry/research in Europe: improve safety and efficiency of current reactors (LWR) develop a new generation of reactors (Gen IV fast reactors and Accelerator Driven Systems) Strategic Research Agenda of the European Sustainable Nuclear Energy Technology Platform (SNETP) Availability of accurate nuclear data (cross sections, decay constants, branching ratios, etc.) is the basis for precise calculations both for current (increased burn-up efficiency, plant life extension) and new generation reactors. Need to Additional measure experimental fission cross measurements section of and actinides their detailed analysis and interpretation are required (from Th in a to broad Cm) range with of half-life neutron from energies a few and years materials. up. This is particularly true for fuels containing minor actinides for their transmutation in fast spectra. NEA/WPEC-26 (ISBN ) The overall list of requirements is rather long: capture cross sections of 235,238 U, 237 Np, Pu, 241,242m,243 Am, 244 Cm fission cross sections of 234 U, 237 Np, 238, Pu, 241,242m,243 Am, Cm

13 Response (counts / ns) n_tof Time line Commissioning 10 0 n-tof 232 Th ( at/b) Neutron Energy / ev 208 Pb GELINA 232 Th ( at/b) 208 Pb Commissioning 63 Ni 62 Ni Phase I Measurement campaign Phase II Measurement campaign Phase III Concept by C.Rubbia CERN/ET/Int. Note Construction started New Target installed Upgrades 10 B-water Class-A area 2 nd Exp. area

14 The n_tof facility at CERN 185 m flight path Linac 50 MeV Booster 1.4 GeV PS 20 GeV n_tof is a spallation neutron source based on 20 GeV/c protons from the CERN PS hitting a Pb block (~360 neutrons per proton). Two experimental areas, one at 185 m (EAR1) and the second one at 20 m (EAR2)

15 n_tof EAR1 EAR1 20 GeV/c protons Spallation Target 185 m

16 n_tof EAR1 & EAR2 EAR2 20 m EAR1 20 GeV/c protons Spallation Target 185 m

17 The spallation target(s) Phase 1: Phase 2: 2008 today

18 The new spallation target The cooling and the moderator systems in the target are separated, so to optimize neutron spectrum or minimize background Vessel Moderator (4 cm) protons Retention vessel Pb Ø = 60 cm L = 40 cm Cooling water (1 cm)

19 The facility

20 h [m] 24.5 Beam Dump EAR-2 Exp. Hall Service Gallery ISR Tunnel EAR1 2 nd Collimator Shielding Filter Box Magnet 1 st Collimator Pit Shielding Target

21 The n_tof facility Advantages of the PS proton beam: high energy, high peak current, low duty cycle. Main feature: high instantaneous neutron flux (10 6 n/pulse). very convenient for measurements of radioactive isotopes (maximizes signal-tobackground ratio) ideal facility for actinides (nuclear technology) Other features of the neutron beam: high resolution in energy in EAR1 (DE/E = 10-4 ).. study resonances wide energy range (25 mev<e n <1GeV). measure fission from thermal to GeV low repetition rate (< 0.8 Hz) no wrap-around

22 Ratio evaluated flux EAR2 vs EAR1 EAR2 (2014/2015) EAR1 (2014) 40 Courtesy of M. Sabate Gilarte

23 Detectors for capture reactions Capture reactions are measured by detecting g-rays emitted in the de-excitation process. At n_tof, two detection systems are used, for different purposes. C 6 D 6 (deuterated liquid scintillators) low neutron sensitivity device used for low cross-section samples Total Absorption Calorimeter (TAC) C 12 H 20 O 4 ( 6 Li) 2 High-efficiency 4p detector (40 BaF 2 scintillators with neutron shielding) mostly used for fissile isotopes (actinides) 23

24 and for fission Several systems have been used for detecting fission fragments. The main problem in fission measurements is the background due to a-decay. Micromegas Fission Ionization chamber Chamber (FIC) low-noise, standard high-gain, detector, with radiation-hard fast gas and electronics detector Neutron beam Neutron beam Parallel Plate Avalanche Counters (PPAC) Fission fragments detected in coincidence Very good rejection of a-background

25 The n_tof measurements Phase 1 ( ) Capture 151 Sm 232 Th 204,206,207,208 Pb, 209 Bi 24,25,26 Mg 90,91,92,94,96 Zr, 93 Zr 186,187,188 Os 233,234 U 237 Np, 240 Pu, 243 Am Fission 233,234,235,236,238 U 232 Th, 209 Bi, 237 Np 241,243 Am, 245 Cm Phase 2 ( ) Capture 25 Mg, 88 Sr 58,60,62 Ni, 63 Ni 54,56,57 Fe 236,238 U, 241 Am Fission 240,242 Pu 235 U(n,g/f) 232 Th, 234 U 237 Np (FF ang.distr.) (n,a) 33 S, 59 Ni Phase 3 ( ) Capture 70,72,74,76 Ge, 88 Sr 242 Pu 237 Np 171 Tm, 204 Tl, 147 Pm Fission 240 Pu, 235 U (n, p) (n,a) 7 Be 25

26

27 The experimental results: Zr isotopes Courtesy of R. Gallino and S. Bisterzio Nucleus N ʘ Normalized to N(Si)=10 6 atoms N s / N ʘ % Old N s / N ʘ % n_tof 90 Zr Zr Zr Zr Zr Solar abundances, N, from Lodders 2009, accuracy 10% The s-abundances, N s, are calculated using the TP stellar model for low mass AGB star (1.5-3 M ).

28 The experimental results: 186,187 Os 187 Os c 187 Os (186) (187) 186 Os (186) (187)

29 The experimental results: 186,187 Os Cosmological way Astronomical way Gyr 14 2 Gyr Nuclear way: Re/Os clock Gyr(*) Th/U clock Gyr (*) 0.4 Gyr uncertainty due to cross-sections

30 The experimental results: 151 Sm Laboratory t 1/2 = 93 yr reduced to t 1/2 = 3 s-process site 152 Gd 154 Gd 151 Eu 152 Eu 153 Eu 154 Eu s-process 150 Sm 152 Sm 151 Sm 153 Sm The branching ratio for 151 Sm depends on: Termodynamical condition of the stellar site (temperature, neutron density, etc ) Cross-section of 151 Sm(n,g) 151 Sm used as stellar thermometer!!

31 The experimental results: 151 Sm background Measured for the first time at a time-of-flight facility Resonance analysis with SAMMY code. Maxwellian averaged cross-section experimentally determined for the first time Maxwellian averaged (n,γ) cross section of the 151 Sm and previous calculation (symbol) NO PREVIOUS MEASUREMENTS! s-process in AGB stars produces 77% of 152 Gd, 23% from p process

32 The experimental results: 63 Ni Scenarios in massive stars

33 The experimental results: 63 Ni 63 Ni (t 1/2 =100 y) represents the first branching point in the s-process, and determines the abundance of 63,65 Cu 62 Ni sample (1g) irradiated in thermal reactor (1984 and 1992), leading to enrichment in 63 Ni of ~13 % (131 mg) In 2011 ~15.4 mg 63 Cu in the sample (from 63 Ni decay). After chemical separation at PSI, 63 Cu contamination <0.01 mg First high-resolution measurement of 63 Ni(n,g) in the astrophysical energy range.

34 The experimental results: 171 Tm(n, γ) kev (mb) Isotope 171 Tm: 170 Er(n, γ) 171 Er (β -, 7.5h) 171 Tm (enrichment 1.8%) 3.6 mg of 171 Tm (1.9 y) [1.3x10 19 atoms] KADoNiS = Chemical separation and sample 171 Tm (97.9%) Tm (2.1%) Tm(0.07%) YEAR 171 Tm deposit (20 mm diameter) Aluminum (7 mm) backing Frame (50 mm diameter) Mylar (5 mm)

35 The experimental results: 171 Tm(n, γ) First experimental measurement

36 The experimental results: Ge(n, γ) The neutron capture cross section on Ge affects the abundances for a number of heavier isotopes up to a mass number of A = 90.

37 The experimental results: 73 Ge(n, γ) Courtesy of C. Lederer ENDF/B-VII n_tof ENDF/B-VII n_tof

38 Counts The experimental results: 70 Ge(n, γ) ENDF/B-VII n_tof Courtesy of C. Lederer

39 26 Al(n, p), (n, α) Observation of the cosmic ray emitter 26 Al is proof that nucleosynthesis is ongoing in our galaxy. The neutron destruction reactions 26 Al(n, p) and 26 Al(n, α) are the main uncertainties to predict the galactic 26 Al abundance. There are only few experimental data on these reactions and they exhibit severe discrepancies.

40 26 Al(n, p), (n, α) The sample was produced by IRMM in collaboration with LANCSE The p and a will be detected by double sided silicon strip detectors arranged as E DE telescope The neutron fluence will be monitored by a 10 B sample

41 7 Be(n, p), (n, α) BBN successfully predicts the abundances of primordial elements such as 4 He, D and 3 He * A serious discrepancy (factor 2-4) between the predicted abundance of 7 Li and the value inferred by measurements Cosmological Lithium Problem

42 7 Be(n, p), (n, α) Approximately 95% of primordial 7 Li is produced from the electron capture decay of 7 Be (T 1/2 =53.2 d) 7 Be is destroyed via (n,p) ( 97%) and (n, α) ( 2.5%) reactions A higher destruction rate of 7 Be can solve or at least partially explain the Cosmological Lithium Problem Only one direct measurement (P. Bassi et al., ev) 7 Be(n, p) 7 Be(n, α)

43 n_tof program on 7 Be(n,cp) EAR2 Two different measurements at n_tof 20 m i) n+ 7 Be -> a+a - Coincidences technique 4 mg, PSI (2015) ii) n+ 7 Be -> p+ 7 Li - Telescope technique 100 ng, PSI+ISOLDE (2016) 20 GeV/c protons Spallation Target The higher flux in EAR2 allows to: measure short-lived radioisotopes (down to a few weeks) collect data on a much shorter time measure (n,charged particle) reactions with thin samples measure samples of very small mass (<<1 mg)

44 7 Be(n,a) measurement Silicon detectors directly inserted in the beam (3x3 cm 2 active area, 140 mm thickness) Detection of high energy a-particles Strong rejection of background (sample preparation) Electrodeposited sample Droplet sample

45 7 Be(n,a) cross-section n_tof measurement reveals: Courtesy of M. Barbagallo 1/v behaviour of the 7 Be(n,a) 4 He reaction cross-section. The only previous measurement (@0.025 ev) underestimates the reaction crosssection.

46 Measured fission reactions Figura Nucleosintesi (frecce che si muovono) Foto FIC In the two experimental campaigns, measured I capture and fission cross sections for most long-lived actinides (432 y and above). For some of them, measured FF anysotropy as well.

47 More to be measured Figura Nucleosintesi (frecce che si muovono) Foto FIC Second experimental area at n_tof allows to measure also some short-lived actinides. 230 Th

48 The 233 U(n,f) cross section Half-life: 1.59x10 5 y Sample: 29 mg (/4) Activity: 2.6 MBq (each sample) Fission cross-section on 233 U measured in a single measurement from thermal to 20 MeV, with 5 % accuracy, and high resolution.

49 The fission cross-section of 236 U JEFF and ENDF/B-VII.0 Half-life: 2.34x10 7 y Sample: 21.4 mg (/4) Activity: 13 kbq (each sample) Contamination of 235 U 0.05% JENDL-4 n_tof data confirm results from GELINA (C. Wagemans et al.). Below a few kev, ENDF and JENDL overestimate cross section (x100). Resonances in ENDF and JEFF are from 235 U!! JENDL-4 is (mostly) correct

50 The 237 Np(n,f) reaction Half-life: 2.14x10 6 y Sample: 63 mg (/4) Activity: 0.41 MBq (each sample) ENDF/B-VII JEFF Below threshold, some corrections on current libraries are needed (based on n_tof and LANL recent data). 50

51 The 241 Am(n,f) at n_tof Half-life: 432 y Sample: 2.26 mg (/8) Activity: 35 MBq (each sample) Contamination (undeclared): 239 Pu and 242m Am Efficiency corrections not very accurate (due to high threshold). Normalized to Dabbs (1983), 3 rd resonance. Small (but important) undeclared contamination of 239 Pu and 242m Am, affects region E n <1 ev JEFF-3.1 needs a major revision M. Mastromarco et al., in preparation 51

52 The 243 Am(n,f) at low energy Half-life: 7370 y Sample: 4.8 mg (/8) Activity: 4.4 MBq (each sample) Contamination (declared): 241 Am 2.5% Contamination (undeclared): 239 Pu, 242m Am M. Mastromarco et al., in preparation 52

53 The 245 Cm(n,f) reaction Half-life: 8500 y (18.1 y) Sample: 1.71 mg (/4) Activity: 87 MBq (each sample) Contamination (declared): 244 Cm 6.6% Very large a background (0.1 GBq) High thresholds necessary (large uncertainty in efficiency corrections). Only cross section shape with good accuracy (3%). For absolute cross section, need to normalize to recommended value at thermal energy. However, large uncertainty (30%) on thermal data. Used two recent measurements of the thermal cross section (ILL and SCK-Mol) that agree within 5%. New data 53

54 The fission cross section of 245 Cm Half-life: 8500 y Sample: 1.71 mg Activity: 87 MBq (each sample) Contamination (declared): 244 Cm 6.6% (18.1 y) Nuclear explosion neutron source Below 30 ev, two (very old) measurements exist, showing large discrepancies. Above 30 MeV, only one measurement with neutrons from a nuclear test. n_tof can provide similar results as a nuclear explosion (but with fewer side effects ) From thermal energy to 30 ev a revision of the evaluations is needed. Above 30 ev, n_tof confirm previous data and evaluations.

55 235 U(n,f) between 10 and 30 kev ( (n,f) X )/ (n,f) IRMM Weston ENDF/B-VII.1 The flux calculated on the basis of the 235 U(n,f) cross section found systematically lower than expected in the kev range (M. Barbagallo et al., Eur. Phys. J A 49 (2013) 156). The (n,f) cross section in this range potentially overstimated by 6-8%. Courtesy of P. Schillebeecks Several evidences of a problem 1.05 in the 235 U(n,f) cross section between 10 and 30 kev Need to investigate it further (a new measurement is planned at n_tof) Neutron energy / ev

56 238 U/ 235 U(n,f) ratio up to 1 GeV The 238 U/ 235 U cross section ratio has been measured at n_tof up to 1 GeV. Four datasets have been collected and compared (different detectors and techniques). Agreement between them is within 3%. Perfect agreement between n_tof ratio and ENDF-B/VII.1 (up to 200 MeV) All n_tof datasets have been combined in order to obtain an evaluated n_tof 238 U/ 235 U cross section ratio.

57 Angular distribution of FF Fission Fragment angular distribution important to: obtain information on the state of the nucleus at saddle point (spin, parity, ) and on the fission dynamics calculate more reliable detection efficiency, thus improving accuracy of cross sections. The effect on the cross section is particularly important for coincidence technique (PPAC, due to backing the angular acceptance is limited to 65º) Neutron beam 57

58 Angular anisotropy in 232 Th fission reaction Measured anisotropy from fission threshold to 1 GeV!!!

59 Conclusions There is need of accurate new data on neutron cross-section both for astrophysics and advanced nuclear technology. Since 2001, has provided an important contribution to the field, with an intense activity on capture and fission measurements. Several results of interest for stellar nucleosynthesis (Sm, Os, Zr, Ni, Fe, etc ). Important data on actinides, of interest for nuclear waste transmutation. Two experimental areas EAR1 and EAR2 The EAR1 at 185 m allows to perform high resolution measurements in optimal conditions (borated water moderator, Class-A experimental area, etc ). The EAR2 at 20 m has opened new perspectives for frontier measurements on short-lived radionuclides. 59

60 Thank you

61 neutron density neutron density time 10-4 time - C proton diffusion 13 C(a,n) 16 O 22 Ne(a,n) 25 Mg

62 7 Be(n, p), (n, α) Sandwich of silicon detectors directly inserted in the beam Detection of both alpha particles (E 9 MeV) Coincidence technique: Strong rejection of background Sample: Silicon 1 neutrons sample Silicon mg of 7 Be from water cooling of SINQ spallation target. (activity of 478 kev g-rays 1 GBq/mg) Isotopic composition: 1:1 7 Be- 10 Be 1:5 7 Be- 9 Be