5 th ASRC International Workshop, 14 th -16 th March 2012

Transcription

1 Fission Fragment Fragment Spectroscopy with Large Arrays and STEFF A.G. Smith, J. Dare, A. Pollitt, E. Murray The University of Manchester W. Urban, T. Soldner ILL Grenoble I. Tsekhanovich, J. Marrantz CENBG Bordeaux UWS (Paisley) ; LPSC (Grenoble); ANL (USA) 5 th ASRC International Workshop, 14 th -16 th March 2012

2 Fission-fragment Spectroscopy as we know it

3 I, M I ~ I, z M I gi N

4 Perturbation to the 2 + Decay to G.S. in 104Mo: +340(20) mrad g factor of this rotational state measured 0 +

5 Collective g factors are low Where deformation is highest h 11/2 neutrons couple to collective 2 + state with g = -0.35

6 How is the fragment angular momentum generated? Macroscopic modes of motion? I Prime candidates from early A.C measurements P I P I Or something else? Particles in a tube (QM uncertainty) P

7 C P Angle GAMMAPSHERE + MWPC Angular Distribution Measurements Complementary e Fragment gamma rays produce strong systematic effects on distribution. Need clean spectra, good angular resolution and careful separation of components.. Stopped Moving Primary Complementary (P) (C)

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9 Statistical Model Code. Initial Spin Distribution (z-axis is fission axis) B ~ (r.m.s) J+1/2 Spread in M sub-statesstates Decay: Neutron transmission factors, C.G coupling, number of neutrons and E1 rays. Predicts: Feeding intensities of Yrast states and a 2, a 4

10 Points to only partial alignment of spin vectors at scission. i e.g. for I=2, 100 Zr: P(0)=100, P(1)=36, P(2)=2. Possible evidence for twisting mode? Needs Theoretical support.

11 How to go further? Tools: Euroball / Gammasphere : Actinide SF sources Nucleus identification by Limited by the array resolving power Limited by the fission yield Can we improve the fission yield? Stronger sources? Other fissioning systems? Can we improve the resolving power? Use Isomers Bigger arrays? -> AGATA Direct fragment identification -> STEFF

12 Ge Array Neutron Beam Channel Plate TOF FG Anode Fragment 1 Fragment 2 Target Catcher Foil FIFI at the ILL Isomer Search Proposal: G.S. Simpson

13 FiFi Mass Resolution 8%

14 Level Scheme below and above 107 ns Isomeric State in 94 Rb Data Combined from several experiments

15 SpecTrometer for Exotic Fission Fragments (STEFF) Design Goals Moderate solid angle for fission fragments Longest flight path possible at PF1B Direct measurement of many fission parameters (E,A,Z,Direction) Mass resolution ~ 2% Z resolution ~ 1/40 for light fragments. Allow for upgrades for ancillary detectors.

16 STEFF a 2v-2E 2E Spectrometer 6.7% 60 mstr

17 Targets up to ~5cm diameter STEFF Main Modules Fission Chamber Ionisation chamber Pos.-sens. MWPC MWPC electrostatic mirror

18 Mass Measurement 2E TOF : Time & Position -> Velocity A v 2 IC : Kinetic energy Flight times ns 2 2 A E T 2 A E T 2.2% 1% 1% 2T or 2E methods: A1 E2 T1 A E T A1 A2 A Ignore neutron evaporation P Sensitive to target effects.

19 Mass Spectrum 252 Cf Mass Resolution: Mass Resolution: 4.1 u FWHM (Old MWPC)

20 Electronics and DAQ Gamma Ray Tracking-4 card specification : - Trigger Inputs/Outputs (Fast NIM, SMA) Trigger Input Trigger out Gate In (as trigger or timestamps synchronisation) - 4 Inputs, 0 ± 550 mv into 50 Ohm - ADCs with 80 MHz (or 12.5 ns) - Event length 512 samples (6.5 us) - 32 ADCs and 32 TDCs VME Bus Limit : 4 Mbytes/sec DL GRT4 cards 0.5k/chan D.L. ALL WORKS!!

21 Digitised Pulse Shape: Extracted parameters

22 Amplit tude Digital Pulse Processing: High-pass filter Ballistic Def. Correction Integration Low-pass filter: noise reduction Currently Noise ~ 0.2 percent run 12: raw pulse traces 100 full energy hits 6000 central anode Time

23 Range R A Z a v b For a fixed v: a ~ 2/3 (SRIM: 0.62)

24 Range R A Z a v b v v-dependence Heavy FF: b = 0.93 (SRIM: 1.0) Light FF: Light FF: b = 1.05 (SRIM: 1.1).

25 STEFF at the ILL : Current PF1B Experiment PF1B neutron beamline Fission rate ~ 10 4 s -1 Flux (current experiment) = 5 x 10 7 cm -2 s U target : 1 cm g cm -2 Setup December day run + 25 Days April/May Aim: NEA High priority request for gamma data

26 STEFF-PF1B Mass Spectra Uneven triggering H vs. L Ev Fold GEANT4 Monte Carlo 2v Multiplicity Corrections to Bragg signals STEFF-PF1B Tails FG Screening Lohengrin Data100 MeV peak A decreasing

27 STEFF with ILL (EXILL) Exogam detectors + Shields Isomer Configuration would use one arm of STEFF only and STOPPED fragments 10 EXOGAM Clovers in CONFIG B Photopeak Efficiency ~ 7.5 %

28 STEFF at the ILL Mass resolution 3% (now limited by window/target effects) Fragment Energy resolution ~ 1 MeV Z resolution ~ 1/40 Currently using NaI/NE213 Scintillator Array Proposal to use 10 EXOGAM Ge detectors Prompt gamma-ray spectroscopy py excited levels in new nuclei Gamma-ray multiplicities and energy spectra (<5MeV) Isomer spectroscopy with lifetimes < 1 s NEA H.P request for gamma data energies and multiplicities Fission dynamics: excitation energy and spin.