New perspectives for electron Volt neutron spectroscopy on inverse - - PowerPoint PPT Presentation

Düsseldorf-Neuss, Germany May 12 2003

Antonino Pietropaolo

Università degli studi di Roma “Tor Vergata” and Istituto Nazionale per la Fisica della Materia ( I NFM) UdR Tor Vergata

ICANS-XVI

16th Meeting of the International Collaboration on Advanced Neutron Sources

New perspectives for electron Volt neutron spectroscopy on inverse geometry instruments at pulsed sources

Düsseldorf-Neuss, Germany May 12 2003

THE EXPERIMENTAL TEAM

• University of Rome Tor Vergata and INFM.

– Carla Andreani, Antonino Pietropaolo, Roberto Senesi

• University of Rome Tor Vergata and INFN:

– Annalisa D’Angelo

• University of Roma Tre and INFM:

– Silvia Imberti

• University of Milano-Bicocca and INFM:

– Giuseppe Gorini, Marco Tardocchi

• ISIS Facility

– Nigel J. Rhodes, Erik M. Schooneveld

Düsseldorf-Neuss, Germany May 12 2003

OVERVIEW

• Applications of epithermal neutrons;
• The inverse geometry configuration:
• The Resonance Filter Spectrometer (RFS) configuration
• The Resonance Detector Spectrometer (RDS) configuration
• Some experimental results on VESUVIO at ISIS with

RDS set up and comparison with RFS

• Possible applications of RDS on VESUVIO
• Conclusions

Düsseldorf-Neuss, Germany May 12 2003

Epithermal Neutron Spectroscopy

• Deep Inelastic Neutron Scattering (DINS)

Investigation of the single particle properties of both quantum systems and molecular fluids and solids

– Momentum distribution n(p) – Mean kinetic energy <Ek> – Impulse Approximation: 1 eV , q Å-1

• Inelastic Neutron Scattering

High energy excitations in a variety of systems

–

1 eV , q < 10 Å-1 small angle scattering and Ei and Ef well above 1 eV

ω ≥ h

20 ≥

ω ≥ h

Düsseldorf-Neuss, Germany May 12 2003

The inverse geometry configuration

sample moderator

L0

velocity selector neutron counter acquisition

L1

1 D 1

L L m t = t+ t = + k k       h

Düsseldorf-Neuss, Germany May 12 2003

How to select the final neutron energy in the 1-100 eV range

• No choppers

– Neutrons are too fast

• No crystal analysers

– Neutrons have too short λ

Then …

• Nuclear resonances

– Two different experimental

configurations ………

Düsseldorf-Neuss, Germany May 12 2003

The Resonance Filter Spectrometer

sample filter Li-glass detector

n’ n DAE

stop start target p bunch TOF spectra

P M T

electronic chain

Düsseldorf-Neuss, Germany May 12 2003

Filter Difference Method

λn

n α t DAE

6Li-glass (Ce)

PHOTONS

ηLi-glass ! 1/v

For VESUVIO’s detector above 20 eV heavy detection efficiency loss

electronic chain

P M T

Düsseldorf-Neuss, Germany May 12 2003

The Resonance Detector Spectrometer

X

γ

sample Analyser foil (n,γ) reactions γ detector

n’ n DAE

stop start target p bunch TOF spectra Fast electronic chain

Düsseldorf-Neuss, Germany May 12 2003

DINS measurements on a Pb sample

238U analyser copuled to a CZT detector

thermal region: diffraction peaks

epithermal region: recoil peaks Er [eV]: 6.67 20.80 36.60 66.00

Düsseldorf-Neuss, Germany May 12 2003

A comparison with Li-glass detectors

• 50 µm thick Uranium foil (15 cm x 15

cm).

• Detector at 90° at wall at 30 cm from

the sample

• Lower Level Discriminator at 40

keV.

• Peak to Background (P/B) ratio much

better for CZT rather than for Li-glass detector.

P/B " 1 P/B " 0.6 P/B " 0.3 P/B " 0.1 P/B " 0.3 P/B " 0.2 P/B " 0.1

Düsseldorf-Neuss, Germany May 12 2003

Extracting n(p) and < Ek>

It has been possible to measure the recoil up to final neutron energy of about 70 eV. In the RFS configuration

• nly the first two peaks can

be analysed

Düsseldorf-Neuss, Germany May 12 2003

The CZT solid state detector

~10 cm

crystal size: 5 x 5 x 5 mm3

Düsseldorf-Neuss, Germany May 12 2003

600 keV LLD (short run) 40 keV LLD

DINS measurements on a Pb sample

238U coupled to a YAP scintillator

P/B " 6 P/B " 0.8 P/B " 0.6 P/B " 10 P/B " 5.5 P/B " 2.7 P/B " 0.4 P/B " 0.1

Düsseldorf-Neuss, Germany May 12 2003

• Position : 1 m from sample at 25

degree, next to Li-glass detector.

• U-foil : 50 µm thick, size 3.5 cm *

2.5 cm.

• LLD : 600 keV.
• YAP has much better Peak to

Background (P/B) ratio as compared to Li-glass.

A comparison with Li-glass

P/B " 0.2 P/B " 0.6 P/B " 0.10 P/B " 0.04 P/B " 0.6 P/B " 0.35

Düsseldorf-Neuss, Germany May 12 2003

RDS for VLAD on VESUVIO

• Construction of a

detector bank in forward scattering (1°< 2θ < 5°) VLAD: an array of γ detectors with good spatial resolution

• Fast electronics for a

finer time sampling at higher final energies

Filter changer n CZT and YAP are good candidates

Düsseldorf-Neuss, Germany May 12 2003

Accessing unexplored kinematical regions

Ef = 6.671 eV (uranium) Ef = 72.0 eV (lantanum)

Düsseldorf-Neuss, Germany May 12 2003

Conclusions

• It is interesting accessing unexplored kinematical regions for neutron

spectroscopy such as high ħω coupled to low q (small angle scattering): investigation of high energy excitations in several systems ranging from magnetic materials to semiconductors

• High energy neutrons have to be detected and….
• A proper detection approach is needed: RDS

– Effective up to 70 eV final neutron energy – Efficiency independent from En – Beam time not shared between foil-in and foil-out as in RFS

Düsseldorf-Neuss, Germany May 12 2003

• CZT solid state detectors and YAP scintillators can be

considered good candidates for VLAD on VESUVIO at ISIS

• Beyond the specific objectives presented here, it is important to

improve the RDS performances (testing new detectors) so to provide an effective instrumentation for inverse geometry epithermal neutron spectrometers operating at new generation pulsed sources.