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VERDI - High Resolution Fission-Fragment Time-of-Flight Spectrometer

Learn about the concept and setup of the particle spectrometer VERDI, its scientific program, and its applications in nuclear physics research.

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VERDI - High Resolution Fission-Fragment Time-of-Flight Spectrometer

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  1. VERDI – a high resolution fission-fragment time-of-flight spectrometerS. Oberstedt

  2. Motivation Concept of the particle spectrometer VERDI Experiment set-up Diamonds for fission-fragment timing Scientific programme Conclusion & Outlook

  3. Motivation • 7 MV Van-de-Graaff accelerator • 7LiF(p,n)7Be, TiT(p,n)3He, D2(d,n)3He, TiT(d,n)4He • DC (Ip,d < 50 A), pulsed beam available • 4 + 1 non-T beam line • n < 109 /s/sr • NEPTUNE isomer spectrometer • ionisation chambers, NE213 neutron/gamma-ray detectors, BF3 counters, HPGe detectors • Bonner spheres • fast rabbit systems (T1/2 > 1s) for activation studies GELINA neutron TOF spectrometer Mono-energetic neutron source • 70 - 140 MeV electron accelerator • repetition frequency: 40 - 800 Hz • neutron pulse: 2 s - 1 ns @ FWHM • n = 3.4  1013/s @ 800 Hz • 12 different flight paths with a length between 8 and 400 m • ionisation chambers, C6D6 detectors • high-resolution -ray detectors • fission chambers for flux monitoring

  4. Motivation • Neutron Data for Waste Transmutation and Safety of Different Reactor Systems (ADS, Generation IV, ...) • Basic Research in Nuclear Physics and Neutron Data Standards • 10B(n, ) 7Li, 16O(n, ) 13C • Neutron-induced fission cross-sections • Fission-fragment characteristics as Y(A, TKE), p, E() • LCP emission, ... • Improvement of nuclear cross-section data files • Precise input data for improved modelling of neutron-induced reactions

  5. Nuclear Energy Agency Nucl. Sci. Committee NEA Databank IAEA - INDC BROND CENDL JEFF ENDF JENDL WPEC Motivation WPEC: Working Party for Evaluation Co-operation JEFF: Joint European Fission + Fusion datafile

  6. Motivation • Reliable predictions on fission product yields relevant in modern nuclear applications (GEN-IV, ADS…) • Radio-toxicity of the nuclear waste • Decay heat calculations • Delayed neutron yields relevant during reactor operation • Prediction of fission-fragment mass and kinetic energy distributions • Prompt -ray emission spectrum and multiplicity (as a function of fragment mass) • Delayed neutron emission / pre-cursor yields

  7. Delayed neutron emission • Resolution of discrepancies in existing delayed-neutron data pointed out in the JEFF/DOC-922 by A. D’Angelo and J. Rowlands, which may lead to an underestimation of the reactivity even for 235U(fast) of about 16% (details and further examples in JEF/DOC-909) • Scarce experimental data about energy dependence of total delayed neutrons • Reduce the uncertainties in the delayed neutron data for the major actinides 235U, 238U (, and 241Pu) • Improve the situation for the standard reaction 252Cf (SF)

  8. Delayed neutron emission • neutron-rich nuclei • typical half lives between hundreds of microseconds and tens of seconds • neutron-induced fission is the most efficient production mechanism for delayed neutron decay pre-cursors • delayed neutron are released by delayed neutron pre-cursor nuclei, immediately after β- decay into a level above the neutron binding energy

  9. Delayed neutron emission • Delayed neutron yield may show a variation of up to 3.5% • Need for measuring fission fragment emission yields with a high precision, i. e. high mass resolution

  10. Fission-fragment characteristics

  11. Fission-fragment characteristics Are quantitative predictions of fission fragment yields possible ?

  12. 235U(n, f) @ 10 eV En= 0.01MeV En= 0.25MeV 237Np(n, f) 237Np(n, f) Fission-fragment characteristics

  13. 238U(n, f) Fission-fragment characteristics

  14. E*  5.8 MeV E*  6.1 MeV Fission-fragment characteristics • 238U (n, f) @ En = 0.9 – 2 MeV E. Birgersson et al., Nucl. Phys. A817 (2009) 1-34

  15. Fission-fragment characteristics • Why does this concept fail? • Is prompt neutron emission well under control? • microscopic neutron emission data do not fit to results from integral experiments (even for 235U !!!) • although average emission energy () differs by only 50 keV • Is the dependence on excitation energy incorrectly treated? • Extra/interpolation of prompt neutron data from neighbouring nuclei not correct? • Uncertainty due to iterative neutron correction in a 2E-experiment... • ... • Is the multi-modal fission model not correct?

  16. VElocity foR Direct particleIdentification

  17. E ~ A v2A ~ E t2/L2 unambiguous identification of the particle mass precise fission-fragment decay studies VERDI – the concept • Double time-of-flight spectrometer to measure the velocity of each individual fission fragment (FF)  pre neutron-emission masses • Subsequent measurement of the FF kinetic energy  post neutron-emission masses

  18. VERDI – the concept • Simultaneous measurement of kinetic energy and velocity of both fission fragments • 2v  pre-neutron masses, Ai* (i = l, h), TKE • v,E  post-neutron masses, Ai, Ek,i (i = l, h) • i(Ai*) from the difference Ai* - Ai  TXE(Ai) • delayed decay modes of fission fragments

  19. VERDI – the requirements fragment mass (arbit. units) fragment mass (amu) • Set-up of the time-of-flight tube with an energy-sensitive detector device • energy resolution : E/E = 0.005 for fission fragment kinetic energies timing resolution : t = 50 ps flight path length : L = 40 cm  fragment mass resolution : A/A > 130

  20. VERDI – basic ingredients • High resolution energy detector • High precision (transmission) time pick-up • radiation hard • spectrometer efficiency   0.005 – 0.01 • for a mass resolution of A/A  100

  21. VERDI – the energy side • Axial ionisation chamber: • Simple to construct and to use • Splitted electrodes allow element identification (cf. LOHENGRIN) • No radiation damage • Very good intrinsic energy resolution • Timing characteristics??? • Difficult to get a large area detector • Energy loss in the entrance window • Large area silicon detectors: • Relatively cheap • Easy to use • Excellent pulse height stability • Excellent energy resolution • Promising timing characteristics • Subject to radiation damage

  22. VERDI – the energy side PIPS (area: 900 mm2) 239Pu, 241Am, 244Cm • Energy resolution for α-particle kinetic energy: • E = 0.006 → close to our design specifications for fission-fragments

  23. VERDI – the timing side • -channel plate detectors: • Very good intrinsic timing characteristics • Difficult to handle • Requires excellent vacuum p < 10-6 mbar • Subject to radiation damage (especially in an intense neutron field)??? • Difficult to build • Diamond detectors: • New detector material • Relatively few experimental results • Difficult to produce single-crystal diamonds • Pulse height stability difficult to predict • Promising timing characteristics (with Ni-ion @ 30 MeV/u t  30 ps) • Never tested with fission fragments (0.5 MeV/u < vFF < 2 MeV/u) • Radiation hard

  24. L = 50 cm VERDI - the design • 2 x 19 PIPS detectors • CVD (or MCP) ultra-fast • time pick-up detectors • Can be handled with NIM • electronics

  25. CVDD - material properties • CVD synthesis is based on the decomposition of gaseous hydrocarbon molecules, usually CH4 • neutron-dose meter (ITER) • first applied to beam diagnostic applications, e. g. intensity and profile • start detectors for fixed-target heavy-ion experiments

  26. CVD diamonds • chemical vapour deposited (CVD) diamond • poly-crystalline (pc) and as single crystals (sc) available • ultra-fast timing characteristics • No pulse-height resolution for pcCVDDs • scCVDD provide excellent energy resolution comparable with PIPS detectors (tested @ GSI) • limited in size and expensive

  27. pcCVDD - material properties triple -source irradiated with a 90Sr/90Y -source (3MBq, 72h)

  28. 252Cf PIPS, 300 m pcCVD, 100 m 225 mm Experimental set-up • pcCVDD material • size: 1  1 cm2 • thickness: 100 m

  29. CVD pulse height spectrum PIPS pulse height spectrum  no coincidence  coincidence with 7 PIPS  no coincidence  coincidence with 7 PIPS Pulse-height “analysis”

  30. 252Cf pcCVD, 100 m pcCVD, 100 m 95 mm pcCVDDD - Intrinsic timing resolution • pcCVDD material • size: 1  1 cm2 • thickness: 100 m

  31. Determination of the timing resolution • By means of a Monte-Carlo simulation • Experimental fission-fragment distribution • Post-neutron fragment yield • Post-neutron fragment kinetic energy • Geometry of the detector set-up • Variation of the time-resolution parameter until reproduction of the measured time distribution

  32. pcCVDDD - Intrinsic timing resolution

  33. pcCVDDD – pulse-height stability

  34. 252Cf PIPS, 300 m pcCVD, 100 m 330 mm VERDI - the timing resolution • Up to 7 PIPS detectors • ORTEC 900 mm2 • CANBERRA (same specs.) • CANBERRA 450 mm2) • Eurysis (40 mm2) • pcCVDD material • size: 1  1 cm2 • thickness: 100 m 

  35. Eurysis 40 mm2 t  0.6 ns VERDI - the timing resolution Ortec PIPS 900 mm2 t  1.0 ns

  36. VERDI – data analysis • Energy calibration from reference distributions published in “The Nuclear Fission Process” • Channel-to-time conversion using a systematic trend obtained from 233,235U and 239Pu • No pulse-height defect correction applied

  37. VERDI – FF distributions Ortec 900mm2 Eurisys 40mm2

  38. VERDI – challenges • Timing properties of the PIPS detectors • CANBERRA offers “specially treated” detectors • Standard detectors useless • … • CVDD detectors not yet at their intrinsic limit: • Present limits due to the high capacitance ? • Signal rise time ? • Limited by the available pre-amplifier modules ?

  39. Scientific programme • single (v, E) measurements: • thermal-neutron induced fission (233,235U, 239(,241)Pu) and 252Cf(SF) • KFKI, SCKCEN, ILL, ... • with a 11cm2 CVDD and n,th > 107/s/cm2: cth > 2.5 /s or 106 /(120 h) • thermal flux (10 - 100) • sample or CVDD size 4 • double (v, E) measurements: • same as for the single (v, E) set-up • ternary particle emission

  40. Scientific programme • single (v, E) measurements: • post-neutron mass and kinetic energy distributions • delayed-neutron pre-cursor yields • investigation of sub-sequent decay processes • double (v, E) measurements: • post-neutron mass- and energy-distributions Y(A, Ek) • pre-neutron mass- and energy-distributions Y(A*, Ek*) • p as a function of fragment mass • total kinetic energy distribution TKE • p as a function of the total excitation energy TXE

  41. Scientific programme • Measurements at accelerator-driven neutron sources: • fission cross-sections are about 200 – 500  smaller • neutron fluxes in general not much higher than 107/s/cm2 • charged-particle induced (fission) reactions possible • Central VERDI flange can be easily adapted

  42. Fundamental aspects • Spectroscopy of neutron-rich isotopes: • relevant for the nucleosynthesis • astrophysical r-process)

  43. Summary • Fission fragment timing resolution  < 250 ps possible • Optimization of detectors possible to reduce timing resolution • VERDI with mass resolution A/A  100 possible • To reach at  < 100 ps seems to be very challenging • radiation hardness of the CVDD start trigger proven • spectrometer efficiency   0.5% • Next improvement: • single crystal diamond as event tirgger • energy resolution (v, 2E) • 100% trigger efficiency

  44. Outlook • First experiment @ KFKI beginning 2010 • EFNUDAT programme • Proposal submission • CVDD transmission detector • under investigation • thickness around 5 m • first -particle signals extracted • Test with fission fragments soon  • Construction of a -channel plate detector…

  45. R. Borcea F.-J. Hambsch Van de Graaff technical team A. Oberstedt Örebro University Nothing without a good team!

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