Electron Volt neutron spectroscopy at high- and low-q - - PowerPoint PPT Presentation

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electron volt neutron spectroscopy at high and low q
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Electron Volt neutron spectroscopy at high- and low-q - - PowerPoint PPT Presentation

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Electron Volt neutron spectroscopy at high- and low-q investigation of high energy excitations in condensed matter and detector development Antonino Pietropaolo 06-05-2004 OUTLINE Physical context Kinematical constraints

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Electron Volt neutron spectroscopy at high-ω and low-q

investigation of high energy excitations in condensed matter and detector development

Antonino Pietropaolo 06-05-2004

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OUTLINE

  • Physical context
  • Kinematical constraints
  • Instrumentation development
  • Some preliminary results
  • Future perspectives
  • Conclusions
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Physical context

  • High energy excitations in condensed matter

– Electronic structures in rare earths systems – Band structures of insulators and semiconductors – Magnetic properties of superconductors – High energy vibrational states in molecular and H-metal systems

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Electronic structures in atoms and compounds

  • Electronic structure of many electron atoms is described by different

models:

– Central field approximation-CFA (periodicity of the chemical properties) – Thomas Fermi (statistical-semi-classsical calculations) – Harthree-Fock (variational method) – Corrections to the CFA (L-S coupling) – Fine structure and multiplet splitting – Crystal field effects

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Rare earths systems

High energy excitation in Praseodymium

A.D.Taylor, et al, PRL 61/11 (1988), 1309.

Seraching for the 1.2 eV transition Density of states of δ-Plutonium

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Insulators and semiconductors

investigation of the diamond band structure One important example

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Superconductors

High-energy magnetic excitations in URu2Si2 some magnetic properties

  • f the SC (e.g. magnetic

phase transitions) can be understood in terms of electronic configuration

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the neutron probe

  • Neutron has a large magnetic dipole moment
  • It interacts with the unpaired electron spin
  • Inelastic magnetic scattering with q > 0

27

1.913 5.051 10

n N N

µ µ µ

−

= − = ×

JT-1

s r

Nuclear magneton Magnetic dipole moment

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cross sections

( ) (

)

1 2 2

; ,

f f i

E d r G Q E E dE d E

µ ν

σ µ ν δ ω   = − −   Ω   h

0.29 b (classical electron radius)2

Ei-Ef

(energy transfer) wave vector transfer Magnetic form factor final energy initial energy

energy eigenvalues of the electronic quantum states µ >, ν>

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Sm Pr

Form factors

Investigating the Q- dependence of G(Q)

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Molecular systems

  • High-lying vibrational states in molecular systems

(e.g. O-H stretching mode in H2O)

  • Extrapolation of the vibrational density of states

2 2

( , ) ( , ) E G Q E S Q E Q =

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Further examples

H in metal matrix

Higher harmonics in the vibrational spectrum give information on the possible ahnarmonicity of the H single particle potential in a metal (H-metal force)

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Kinematical constraints

Q [Å-1]

ħω = 1.5 eV

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Kinematical constraints

Ef = 6.671 eV (Uranium) Ef = 72.0 eV (Lantanum)

HIGH ENERGY NEUTRONS HAVE TO BE DETECTED AT SMALL ANGLES

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The ISIS pulsed source

LINAC Sinchrotron Target station Experimental halls ISIS II

800 MeV protons

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Instrumentation development

  • Effective neutron detection systems for eV neutrons

– Standard 6Li- or 3He-based neutron counters show a 1/v behaviour for neutron absorption efficiency – Not effective above 20 eV

  • Dedicated instrumentation

– Spectrometers

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  • FSD BANK: proton dynamics

RFS or RDS ?

  • VLAD: high energy excitations

RDS

Vesuvio:

V L A D:

n

The e.VERDI instrument

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Two possible configurations

RFS RDS

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A closer look at the RDS set up

X

γ

sample Analyser foil (n,γ) reactions γ detector

n’ n DAE

“stop” “start” target

p

TOF spectra

Fast Electronic chain

Solid state (CZT)

Scintillator (YAP)

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exploring unexplored regions of the (Q,ω) space

VLAD

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Some preliminary results

  • DINS measurements on

water with RDS: Measuring <Ek> and n(p) 1 eV ω 10 eV 35 Å-1 q 70 Å-1 good agreement with standard RFS measurements

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VLAD results

  • HINS measurements on a ice sample
  • n VLAD prototype

2 1

( ) 9

E E g

d ω ω =

∫

± 2 atoms/cell

Previous measurements on HRMECS at IPNS Argonne National Laboratory

2 1

( ) 8

E E g

d ω ω =

∫

± 0.1 atoms/cell

Andreani et al. J. Chem Phys. 83, 750 (1985) 2.5 Å-1 q 5.5 Å-1 Andreani et al. Appl. Phys. Lett submitted for publication

q 7 Å-1

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Future perspectives

  • Physics:

– Measurements on rare earth systems (e.g. Praseodymium) – Measurements on diamond (band structure investigation)

  • Technology:

– Improvement of ω resolution – Improvement of q resolution – R&D on neutron detectors

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Conclusions

The investigation of electronics structures of complex systems (rare earths metals and compounds, semicondctors, insulators, superconductors) and the high-lying vibrational states in molecules and metal-H systems are interesting physical topics. High-ω coupled to low-q measurements are needed due to the kinematical constraints Neutrons well above 1 eV have to be detected at small angles, so…. New and effective detection techniques/instrumentation have to be developed/designed Test measurements on both VESUVIO and VLAD show that RDS configuration is an effective configuration both for DINS and HINS e.VERDI instrument: FSD and VLAD bank for DINS and HINS…

There is a lot of interesting work to be done !