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Measurement Science and Technology
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Characterisation of the incident beam and current
diffraction capabilities on the VESUVIO
To cite this article: G Romanelli et al 2017 Meas. Sci. Technol. 28 095501
- The VESUVIO Spectrometer Now and
A G Seel, M Krzystyniak and F FernandezAlonso
- Radiative neutron capture as a counting
technique at pulsed spallation neutron
sources: a review of current progress
E M Schooneveld, A Pietropaolo, C
Andreani et al.
- The VESUVIO electron volt neutron
J Mayers and G Reiter
View the article online for updates and enhancements.
This content was downloaded from IP address on 26/10/2017 at 13:28
Measurement Science and Technology
Meas. Sci. Technol. 28 (2017) 095501 (6pp)
Characterisation of the incident beam
and current diffraction capabilities
on the VESUVIO spectrometer
G Romanelli1,2, M Krzystyniak1,3, R Senesi2,4,5, D Raspino1, J Boxall1,
D Pooley1, S Moorby1, E Schooneveld1, N J Rhodes1, C Andreani2,4,5
and F Fernandez-Alonso1,6
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX,
United Kingdom
Università degli Studi di Roma ‘Tor Vergata’, Dipartimento di Fisica and NAST Centre,
Via della Ricerca Scientifica 1, 00133, Roma, Italy
School of Science and Technology, Nottingham Trent University, Clifton Campus, Nottingham,
NG11 8NS, United Kingdom
CNR-IPCF Sezione di Messina, viale F. Stagno D’Alcontres 37, 98158, Messina, Italy
Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi, Piazza del Viminale 1, Italy
Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT,
United Kingdom
Received 21 February 2017, revised 23 June 2017
Accepted for publication 28 June 2017
Published 16 August 2017
The VESUVIO spectrometer at the ISIS pulsed neutron and muon source is a unique
instrument amongst those available at neutron facilities. This is the only inverted-geometry
neutron spectrometer accessing values of energy and wavevector transfer above tens of eV
and Å , respectively, and where deep inelastic neutron scattering experiments are routinely
performed. As such, the procedure at the base of the technique has been previously described
in an article published by this journal (Mayers and Reiter 2012 Meas. Sci. Technol. 23
045902). The instrument has recently witnessed an upsurge of interest due to a new trend to
accommodate, within a single experiment, neutron diffraction and transmission measurements
in addition to deep inelastic neutron scattering. This work presents a broader description of
the instrument following these recent developments. In particular, we assess the absolute
intensity and two-dimensional profile of the incident neutron beam and the capabilities of the
backscattering diffraction banks. All results are discussed in the light of recent changes to the
moderator viewed by the instrument. We find that VESUVIO has to be considered a highresolution diffractometer as much as other diffractometers at ISIS, with a resolution as high
as 2 × 10−3 in backscattering. Also, we describe the extension of the wavelength range of the
instrument to include lower neutron energies for diffraction measurements, an upgrade that
could be readily applied to other neutron instruments as well.
Keywords: deep inelastic neutron scattering, transmission, diffraction
(Some figures may appear in colour only in the online journal)
© 2017 Crown copyright. Reproduced with the permission of the
Controller of Her Majesty’s Stationery Office Printed in the UK
G Romanelli et al
Meas. Sci. Technol. 28 (2017) 095501
1. Introduction
The VESUVIO spectrometer [1, 2] at the ISIS pulsed neutron and muon source [3] is a unique instrument with a broad
incident neutron energy spectrum, from cold to epithermal
neutrons. It is the only inverted-geometry spectrometer [4, 5]
in the world where electron-volt neutrons are used to investigate a broad science programme embracing a variety of topics
[6] ranging between fundamental systems such as bulk [7–9]
and confined water [10, 11] and proton conductors [12] all
the way to real-life materials such as dental cements [13].
The main goal of experiments on VESUVIO is the measurement of atomic momentum distributions and nuclear quantum
effects in condensed matter systems. This is possible using
deep inelastic neutron scattering (DINS) [14, 15], a technique
available at pulsed spallation neutron sources [16] and based
on values of energy and wavevector transfers above tens of
eV and Å , respectively. VESUVIO is an indirect-geometry
spectrometer where the energy selection of scattered neutrons
is performed using nuclear resonances in thin Au foils moving
in and out of the secondary neutron path [17, 18]. We note
that DINS measurements on 4 He [19] and H2O [20] have been
reported on direct-geometry instruments, where values of the
incident neutron energy up to few eV can be achieved using
The resonant filters used on VESUVIO have a high capture
cross section in the region of the nuclear resonances but are
otherwise relatively transparent to thermal neutrons and other
particles, such as photons generated within the sample. As
incident, transmitted, and scattered beams are polychromatic
over a broad energy range, data acquired during a measurement contain information from several techniques, such as
diffraction [21, 22], transmission [23], gamma-­dopplerimetry
[24] and neutron prompt-gamma activation analysis [25].
Moreover, DINS measurements of moderate-weight elements such as lithium [26, 27], fluorine [28] and oxygen [29]
continue to attract increasing attention. As several techniques
based on the use of neutrons with different energies are
applied concurrently during a measurement on VESUVIO,
a complete characterisation of the beam intensity and profile
is needed.
A recent and detailed description of the VESUVIO set-up
and operations was published in 2012 in this journal [1] and
was mainly centred on the detection of epithermal neutrons and
the analysis of DINS observables. In recent experiments, concurrent DINS and neuron diffraction were used to monitor and
augment the reliability of experimental data, as for example
in [21, 22, 30]. Similarly, a combined use of DINS and neutron transmission is exemplified in [23] in the framework of
the development of scattering kernels. Moreover, the interest
in transmission and diffraction measurements has grown as
the result of a change to the water moderator (CWM) in the
target station 1 (TS-1) in February 2016, bringing an increase
in the thermal neutron flux [31]. The change consisted in the
removal of one of the two poisoning Gd foils within the moderator, increasing the effective volume where neutrons are
moderated. Motivated by this development, we present (1)
Figure 1. Schematic diagram of the VESUVIO spectrometer. See
text for details.
an absolute beam profile measurement at the sample position
before and after the CWM; (2) the beam profile perpendicular
to the direction of incident neutrons as a function of incident
neutron energy; (3) a characterisation of the diffraction capabilities of the instrument after the CWM; and (4) an extension
of the instrument wavelength range for diffraction measurements, a procedure that can be readily implemented on other
neutron instruments as well.
2. Experimental
At present, the VESUVIO spectrometer (figure 1) is equipped
with (a) a 6 Li doped glass incident beam monitor at 8.57 m
from the moderator; (b) a similar monitor for the transmitted
beam at 13.45 m from the moderator; (c) a detector bank in
backscattering consisting of 132 6 Li doped GS20 scintillatingglass detectors covering the angular region 130◦ θ 166◦
at L1 0.70 m from the sample position; (d) 64 γ-ray YAP
detectors [32] covering the angular range 37◦ θ 67◦ at an
average distance L1 0.50 m from the sample; (e) moving
Au foils used for the foil cycling [17, 33] and double difference [18] techniques. The sample area is composed of a tank
of 0.4 m in diameter and 0.6 m in height and is generally evacuated down to 10−6 mbar. The sample is placed at the centre of
the tank at L0 = 11.00 m from the moderator. Finally, a U foil
cycled in and out of the incident beam can be used to select
neutron energies in the epithermal region.
All measurements are performed using the time-of-flight
(ToF) technique following the relation
√ +√
ToF =
with mn the neutron mass, E0 and E1 the initial and final energy
of the neutron, respectively. The secondary path for measurements at the sample position is set to L1 = 0 and the neutron
wavelength for elastic scattering, E0 = E1, is defined as
h ToF
m L0 + L1
G Romanelli et al
Meas. Sci. Technol. 28 (2017) 095501
Figure 2. Incident neutron flux at the sample position as measured
by the calibrated monitor before the CWM (blue line) and measured
by the nGEM after the CWM (black line). The red dashed line is
proportional to E0−0.9 and the region shadowed in red represents the
energy range used in DINS. The insert shows the spectrum of the
incident neutron beam after the CWM normalised to the spectrum
before the change. The orange spectrum has been obtained from
the VESUVIO incident monitor while the green spectrum from the
nGEM detector.
3. Results and discussion
3.1. Absolute calibration of the beam profile
Figure 3. Colour contour plots of the incident neutron beam at the
sample position measured using an nGEM camera. The beam flux
has been integrated between 100 eV and 101 eV (a) and 100 meV
and 101 meV (b).
The incident beam profile at the sample position was measured with a 6 Li monitor with absolute count-rate calibration
available from the ISIS detector group. Measurements were
done in April 2015, before the CWM, and are reported as a
function of incident neutron energy in figure 2 as a blue line.
Additional measurements with a commercial neutron gas
electron multiplier (nGEM) [34] were performed in December
2015, before the change, and repeated in February 2016 after
the change. Spectra registered by the camera highlighted a
gain in the neutron flux shown in the inset of figure 2 (green
line) as a function of neutron wavelength. This ratio is compared to and agrees with the one obtained from the VESUVIO
incident monitor signals recorded during the same period
(orange line). The black line in the main figure 2 corresponds
to the absolute flux after the CWM. It is easy to notice that the
flux for thermal and cold neutrons is increased by a factor of
2.5 as a result of the CWM, while the flux in the epithermal
neutron region remains mostly unchanged. The gain becomes
a loss as the neutron energy exceeds the MeV threshold with a
small decrease in the number of fast neutrons. In the analysis
of DINS data, ToF spectra are corrected by the neutron beam
profile assuming an energy dependence of the form E0−α with
α = 0.9 [1]. It is important to note that this assumption still
holds accurately after the CWM, as shown by the red line in
figure 2.
3.2. Two-dimensional shape of the incident beam
A two-dimensional beam profile measurement was performed
using the nGEM camera. The result of the measurement is
shown in figure 3 after integration between 100 eV and 101 eV
(a) and between 100 meV and 101 meV (b). As one can appreciate, the beam shape is approximately circular. The shape of
the beam has been investigated over the whole energy range
and did not show a measurable change in dimensions, as
shown by figure 3. Moreover, we observed no change in the
shape of the beam before and after the CWM. The beam profile can be described as a 3 cm diameter umbra and a 5 cm
diameter penumbra [35]. We note that an assessment of the
beam profile in the MeV region (1.5 m after the sample position) was presented in [36] using a single-crystal diamond
detector. A horizontal scan of the beam showed a Gaussianlike shape characterised by a full-width-at-half-maximum
G Romanelli et al
Meas. Sci. Technol. 28 (2017) 095501
Figure 5. Bragg peaks from Na2Ca3Al2F14 measured by a single
detector at a scattering angle ∼161°. Data correspond to an
integrated proton current of 1.6 mAh.
beam; a Si powder from the National Bureau of Standards
[44]; a CeO2 powder diffraction standard; and a Na2Ca3Al2F14
powder. The powder samples were all contained in cylindrical Vanadium containers with a diameter smaller than the
beam size, as opposed to the Pb sample. The Pb sample was
measured both before and after the CWM, the Si sample was
measured only before it, and CeO2 and Na2Ca3Al2F14 were
measured only after it. Figure 4 shows the ratio of Braggpeak width (∆ToF ) to centre (ToF) obtained by a Gaussian fit
for a variety of scattering angles and incident neutron wavelengths, with λ the wavelength associated to the ToF at the
peak maximum.
Results from Si and Pb measured before the CWM showed
a small dependence on λ, and the relative resolution at about
160° (figure 4, bottom panel) was about 2 × 10−3 slightly
worsening to 3 × 10−3 at lower scattering angles (middle
panel). No discrepancy is observed between large slab or
small cylindrical samples, suggesting that any contribution to
the resolution from the finite size of the sample is negligible
in this geometry. On the other hand, after the CWM, a strong
dependence of the resolution on λ is evident, with a peak just
below 2 Å . Comparing figure 4 to the insert in figure 2, one
can notice that the lower resolution at 2 Å is accompanied
by an increased flux at the same incident neutron wavelength.
After the CWM, a larger effective size of the moderator has
increased the average time required by thermal neutrons to
be emitted, therefore broadening the time-pulse width. Yet, a
larger number of neutrons can be moderated by the thicker
moderator, providing an increase in the average brightness.
A mild dependence on the scattering angle is shown in figure 4
(top panel). One should note that the resolution is as high as
2 × 10−3, a value similar to other diffractometers at ISIS. A
recent study on formic acid combining DINS and diffraction
Figure 4. Resolution in diffraction mode as the ratio of the fitted
width of a Gaussian line shape to its centre for: Si powder in a
cylinder before the CWM (black empty diamonds); the standard
Pb slab before the CWM (black empty squares); the same Pb
standard after the upgrade (red full squares); and a CeO2 powder in
a cylinder after the upgrade (full circles). The middle and bottom
panels correspond to detectors 3 (∼131◦) and detectors
44 (∼161◦), respectively.
between 3.7 cm and 4.5 cm, depending upon the energy of
detected neutrons.
3.3. VESUVIO as a diffractometer
Because of the polychromatic nature of incident and scattered
beams, VESUVIO now works routinely as both a spectro­
meter and a diffractometer. Importantly, this feature holds for
other inverted geometry spectrometers at ISIS such as IRIS
[37], OSIRIS [38] and TOSCA [39], all equipped with diffraction detectors in backscattering. Diffraction is generally
used to calibrate the spectrometer and routines for the data
analysis are available in MANTID [40–43]. Here, we investigate the change in diffraction resolution as a consequence of
the CWM.
We analysed a collection of diffraction standards: a Pb slab
with a thickness of 2 mm completely covering the neutron
G Romanelli et al
Meas. Sci. Technol. 28 (2017) 095501
spectra measured on VESUVIO has reported lattice param­
eters with a relative error of ca 10−4 [30].
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3.4. Extension of the spectral range
Finally, we introduce a recent improvement in the VESUVIO
data acquisition procedure. Since the inauguration of the
second target station (TS-2) at ISIS, for every 5 proton pulses
from the synchrotron there are 4 heading to TS-1 and one to
TS-2. The fifth of these pulses is considered an empty frame
for TS-1 and has been traditionally vetoed on VESUVIO. Yet,
the frame is not empty but populated with colder neutrons
from the previous pulse. The detection of cold neutrons from
the fifth pulse had been implemented earlier on TOSCA by
the use of a disk chopper [45]. Figure 5 shows a spectrum
from a backscattering detector with the green counts below
20 ms from the first four proton pulses heading to TS-1 and the
counts in red corresponding to the time when the fifth pulse
is directed to TS-2. Raw ToF spectra have a discontinuity at
20 000 μs corresponding to the expected 4:1 ratio of detected
pulses. Figure 5 shows a spectrum from Na2Ca3Al2F14 in the
region of Bragg peaks normalised by the incident monitor.
It is easy to notice how two of these peaks, previously non
detected, can now be measured owing to the extension of the
spectral range up to d-spacing d ∼ 7 Å . We note that, as monitors and detectors are at different positions, the sole normalisation of the spectra by the incident monitor does not remove
the discontinuity in the diffraction spectra, and an additional
correction is needed.
4. Conclusions and outlook
The VESUVIO spectrometer has been described as a versatile instrument where several techniques have recently
become available in a synchronous manner. This feature is
due to the polychromatic nature of incident, transmitted and
scattered beams. The neutron beam shape and its intensity
have been characterised before and after the CWM on TS-1.
No changes were found in the shape of the incident beam,
circular with constant size from cold to epithermal neutrons.
An increase in beam intensity as the result of the CWM
has been found mainly in the thermal region. We have also
shown that VESUVIO is a good diffractometer as well as a
unique spectro­meter, with a resolution in diffraction as high
as 2 × 10−3.
This work was partially supported within the CNR-STFC
Agreement (2014–2020) concerning collaboration in scientific research at the ISIS pulsed neutron and muon source.
We would like to thank Dr T Minniti, Dr F Orlandi and Dr G
Skoro for useful and constructive discussions. We would also
like to thank D Nixon, E Oram, L McCann and M Gigg for
their precious help to make the analysis of these data possible
in the MANTID framework.
G Romanelli et al
Meas. Sci. Technol. 28 (2017) 095501
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