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RickFEST -16/06/2008

"We can be smarter than them!". W.Gelletly. Physics Department,University of Surrey. RickFEST -16/06/2008. "We can be smarter than them!". Andy Sunyar - Brookhaven National Lab. - 1967. The Neutron Capture reaction. Primary Gamma ray.

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RickFEST -16/06/2008

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  1. "We can be smarter than them!" W.Gelletly Physics Department,University of Surrey RickFEST -16/06/2008

  2. "We can be smarter than them!" Andy Sunyar - Brookhaven National Lab. - 1967

  3. The Neutron Capture reaction Primary Gamma ray Secondary Gamma ray • Classic Compound Nucleus Reaction • Not subject to the Coulomb barrier • Very high fluxes of neutrons available • - HFBR or ILL Reactor. • - At lLL targets in 5 x 1014 ncm-2s-1 • -HFBR produced external beams of various types • 4.E(exc.) = SN +EN –Eγ + ER obtained from the primaries

  4. Neutron Capture Cross-section Neutron capture cross-section as a function of neutron energy for a typical medium or heavy nucleus Note: logarithmic scales • Typically the neutron capture cross-section falls off as 1/v with increasing energy • However superimposed on this we have resonances when the total energy coincides • with the energy of a discrete state in the compound nucleus

  5. Porter-Thomas Distribution • Distribution of PRIMARY gamma-ray energies from 1, 3 and 10 resonances. • For 1 resonance most probable reduced intensity is zero • As we approach an infinite number of resonances the distribution becomes a Gaussian of width [2/(no.of resonances)]. In this situation we should see all primary gamma rays of low multipolarity. Failure to do so would be significant (Average Resonance Capture = ARC at HFBR)

  6. 5+ 4+ 3+ 2+ Basic interest was the nature of the Gamma band in deformed nuclei Attempt No.1:- Measure intensities of “stopover” and crossover transitions and use ALAGA rules to deduce E2 intensities

  7. Dumond’s exact focussing bent crystal diffraction Spectrometer. GAMS 1, II and III are All spectrometers of this type. GAMS ii and III operated together With diffraction in opposite senses So that zero error in diffraction angle cancelled. GAMS I – 5.8m radius Gams II/III – 24m radius

  8. Through Tube

  9. 167Er(n,γ)168Er GAMS 1 Diffraction spectrometer – spectrum in 2nd,3rd,4th and 5th order Note:-In 3rd order we are looking at ~ 14 keV

  10. 167Er(n,γ)168Er Spectrum in 3rd Order from GAMS1.( 4+ - 2+) is 104 times more intense than its neighbour

  11. Conclusion (gk – gR) is constant within the band Q0(2) Main point for our story – we had seen how rich the spectrum was!!

  12. The BILL Spectrometer – Double-focussing π√2 spectrometer Attempt No.2:- Internal Conversion using the L- Subshell ratios. 74.63 keV, 3+ - 2+ 122.83 keV, 5+-4+ Transitions in the gamma- band of 168Er.

  13. The Results – gamma band transitions Conclusion:- M1 admixtures remarkably constant in these nuclei. Best explanation – they are due to slightly different deformations for neutrons and protons

  14. GAMS 1, II and III Dumond type spectrometers BILL – Double focussing beta spectrometer Average Resonance Capture – 2 and 24 keV Complete level scheme up to ~ 2.5 MeV

  15. Porter-Thomas Distribution • Distribution of PRIMARY gamma-ray energies from 1, 3 and 10 resonances. • For 1 resonance most probable reduced intensity is zero • As we approach an infinite number of resonances the distribution becomes a Gaussian of width [2/(no.of resonances)]. In this situation we should see all primary gamma rays of low multipolarity. Failure to do so would be significant (Average Resonance Capture = ARC at HFBR)

  16. Average Resonance Capture at the HFBR Neutron spectrum Sc filter at HFBR In practice we create a beam of neutrons with a broad band of energies, which spans many resonances. In this situation the reduced primary gamma-ray intensities[I/Eγ5] vary only a little and reflect the multipolarity of the transition.  At HFBR 2 and 24.3 keV filters with a Ge pair spectrometer to detect primary gamma rays.

  17. GAMS 1, II and III Dumond type spectrometers BILL – Double focussing beta spectrometer Average Resonance Capture – 2 and 24 keV Complete level scheme up to ~ 2.5 MeV 2 keV I(2 keV) I(24 keV)

  18. This comprehensive level scheme is an Ideal tool for testing nuclear models and Arima and Iachello had just introduced the Interacting Boson model, where the nucleus is regarded as an inert core plus the valence particles regarded as bosons. Initially only s (L =0) and d (L=2) bosons were considered. H = – kQ.Q –k/L.L + k//P.P Rick, Dave Warner and Walter Davidson took this model and applied it to 168Er with great success. This exploration of algebraic models became Rick’s preoccupation over the next decade or more and we will hear more about that from Alison in the next talk. See Warner,Casten and Davidson,PRC24(1981)1713

  19. Warner,Casten and Davidson, PRC24 (1981) 1713 Comparison of the level scheme with calculations in IBA -1

  20. Gamma-Ray Induced Doppler Broadening - GRID New opportunities arose at ILL with GAMS 4 and GAMS 5 – diffraction devices but now flat crystal Spectrometers. The nucleus recoils when it emits a gamma ray so when a Secondary gamma is emitted It will be Doppler broadened by an amount dependent on the lifetime of the state. The example shown is the 1112keV transition in the decay of the 31+ state In 152Sm. Rick and Hans Borner also applied it to 168Er.

  21. Raw Spectrum Measurements at ILL of the L- and M-subshell ratios of the pure E2 (2+ - 0+) transition in 168Er

  22. The Reactor Antineutrino Spectrum and Non-Proliferation Meanwhile Interest in measuring (anti)neutrino masses was high. To establish the initial spectrum for such measurements we used BILL to determine the cumulated beta spectrum from 235U, 239Pu and 241Pu – the main sources of antineutrinos. p n + e- + ν So we can convert this to the Antineutrino spectrum.

  23. Same thing for 241Pu As a reactor runs it breeds Pu. Since The neutrino spectra are different for the three isotopes we can monitor State of the fuel by continuously measuring the spectrum. If every reactor had a suitable detector close by and the signals went directly to the IAEA in Vienna then this could become an established part of non-proliferation Treaties.

  24. Search for Mono-energetic positrons from 152mEu • Nuclear transition takes place whilst an inner shell vacancy is present. If E(tr) exceeds 2m0c2 – B(e-) then electron can go directly into the vacancy and we are left with a Mono-energetic positron. For K shell E(e+) = Eγ - 2m0c2 + B(e-) Probability depends on relative lifetimes of K vacancy and nuclear state.  Τ > 3.5 x 10-15 s for 1511 keV level Colvin, Schreckenbach and Gelletly, J.Phys.G11(1985)L227

  25. Cauchois Geometry Approximate focussing arrangement of the Cauchois Spectrograph.

  26. Active stopper = extended source Bent crystal Multi-strip detector Fitted to focal circle. Could one combine our active stopper with a diffraction device – in Cauchois Geometry?

  27. Could we use a thin active stopper and use it as the source for a beta spectrometer? Below is one I prepared earlier. Second step was to fill the focal plane with a continuous pixellated detector. Main point – We should not blindly follow the same approach that was successful for twenty years.

  28. Cocoyoc - 2000

  29. Behind every good man there is a better woman

  30. Preparing the next Generation-----and the next talk.

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