RickFEST -16/06/2008

Andy Sunyar - Brookhaven National Lab. - 1967

The Neutron Capture reaction

1. Classic Compound Nucleus Reaction

2. Not subject to the Coulomb barrier

3. 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 types4.E(exc.) = SN +EN –Eγ + ER obtained from the primaries

Primary Gamma ray

Secondary Gamma ray

1. Typically the neutron capture cross-section falls off as 1/v with increasing energy2. However superimposed on this we have resonances when the total energy coincides with the energy of a discrete state in the compound nucleus

Neutron capture cross-sectionas a function of neutronenergy for a typical mediumor heavy nucleus

Note: logarithmic scales

Neutron Capture Cross-section

Porter-Thomas Distribution

Distribution of PRIMARY gamma-ray energies from 1, 3 and 10 resonances.For 1 resonance most probable reduced intensity is zeroAs 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)

Basic interest was the nature of the Gamma band in deformed nuclei

4+

5+

3+

2+

Attempt No.1:- Measure intensities of “stopover” and crossover transitions and use ALAGA rules to deduce E2 intensities

Dumond’s exact focussing bent crystal diffractionSpectrometer.

GAMS 1, II and III areAll spectrometers of this type.

GAMS ii and III operated togetherWith diffraction in opposite sensesSo that zero error in diffraction angle cancelled.

GAMS I – 5.8m radiusGams II/III – 24m radius

Through Tube

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

Spectrum in 3rd Order from GAMS1.( 4+ - 2+) is 104 times more intense than its neighbour

167Er(n,γ)168Er

(gk – gR)

Q0(2)is constant within the band

Conclusion

Main point for our story – we had seen how rich the spectrum was!!

Attempt No.2:- InternalConversion using the L-Subshell ratios.

74.63 keV, 3+ - 2+

122.83 keV, 5+-4+

Transitions in the gamma-band of 168Er.

The BILL Spectrometer – Double-focussing π√2 spectrometer

The Results – gamma band transitions

Conclusion:- M1 admixturesremarkably constant in these nuclei.

Best explanation – they are due to slightly different deformations for neutrons and protons

GAMS 1, II and III Dumond type spectrometersBILL – Double focussing beta spectrometerAverage Resonance Capture – 2 and 24 keV

Complete level scheme up to ~ 2.5 MeV

Porter-Thomas Distribution

Distribution of PRIMARY gamma-ray energies from 1, 3 and 10 resonances.For 1 resonance most probable reduced intensity is zeroAs 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)

Average Resonance Capture at the HFBR

Neutron spectrum

In practice we create a beam of neutrons with abroad band of energies, which spans many resonances. In this situation the reduced primarygamma-ray intensities[I/Eγ

5] vary only a littleand reflect the multipolarity of the transition.

Sc filter at HFBR

At HFBR 2 and 24.3 keV filters with a Ge pair spectrometer to detect primary gamma rays.

GAMS 1, II and III Dumond type spectrometersBILL – Double focussing beta spectrometerAverage Resonance Capture – 2 and 24 keV

Complete level scheme up to ~ 2.5 MeV

2 keV

I(2 keV)

I(24 keV)

This comprehensive level scheme is anIdeal tool for testing nuclear models andArima and Iachello had just introduced the Interacting Boson model, where thenucleus is regarded as an inert core plusthe 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 Davidsontook this model and applied it to 168Erwith 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

Comparison of the level scheme with calculations in IBA -1

Warner,Casten and Davidson, PRC24 (1981) 1713

Gamma-Ray Induced Doppler Broadening - GRID

New opportunities aroseat ILL with GAMS 4 andGAMS 5 – diffraction devicesbut now flat crystal Spectrometers.

The nucleus recoils when it emits a gamma ray so when aSecondary gamma is emitted It will be Doppler broadened by an amount dependent on thelifetime of the state.

The example shown is the 1112keV transition in the decay of the 31+ stateIn 152Sm.

Rick and Hans Borner also applied it to 168Er.

Measurements at ILL of theL- and M-subshell ratios of the pureE2 (2+ - 0+) transition in 168Er

RawSpectrum

The Reactor Antineutrino Spectrum and Non-Proliferation

Meanwhile

Interest in measuring (anti)neutrino masseswas high. To establish the initial spectrum forsuch measurements we used BILL to determinethe cumulated beta spectrum from 235U, 239Puand 241Pu – the main sources of antineutrinos.

p n + e- + ν So we can convert this to theAntineutrino spectrum.

Same thing for 241Pu

As a reactor runs it breeds Pu. Since The neutrino spectra are different forthe three isotopes we can monitorState of the fuel by continuouslymeasuring the spectrum. If every reactorhad a suitable detector close by and thesignals went directly to the IAEA in Vienna then this could become an established part of non-proliferationTreaties.

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

Approximate focussing arrangementof the Cauchois Spectrograph.

Cauchois Geometry

Could one combine our active stopper with a diffraction device – in CauchoisGeometry?

Active stopper = extended sourceBent crystal

Multi-strip detector Fitted to focal circle.

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 planewith a continuous pixellated detector.

Main point – We should not blindly follow the same approach that was successful for twenty years.

Cocoyoc - 2000

Behind every good man there is a better woman

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