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INSTRUMENTATION

36 NIST CENTER FOR NEUTRON RESEARCH

The very small-angle neutron 

scattering (VSANS) instrument

J. G. Barker,1 J. J. Moyer,1 D. Adler,1 L. Wroten,1 C. Wrenn,1 D. Pierce,1 G. Jensen,1,2 C. Gagnon,1,3

S. R. Kline,1 J. Chabot,1 T. Thai,1 A. Malone,1 B. Dickerson,1 N. Maliszewskyj,1 C. Parikh,1,2 N. Shmunis,1,2

S. Pheiffer,1 D. Ogg,1 D. W. Ogg,1 M. Rinehart,1 S. Mullendore,1 D. Johnson,1 and C. J. Glinka1,2

T

he new 48-m VSANS (Very Small Angle Neutron Scattering) 

instrument is now a fully subscribed member of the CHRNS 

instrument suite. New design features of the instrument 

expand the capabilities of our two 30 m SANS instruments. 

The minimum momentum transfer q is reduced by a factor 

of up to five by utilizing a higher resolution (1 mm) detector. 

Meanwhile, new optics increase beam intensity by a factor 

of 100-fold to 4,000-fold over standard pinhole collimation 

when using these collimation conditions. Expanding from one 

to nine 2D detectors placed on three carriages instead of one 

greatly expands the range of scattering angles from a single 

measurement. Typical experiments now measure a larger range 

of scattering angles. The ratio of the maximum to minimum 

angles is now 130, a factor of seven larger than that obtained 

on 30 m SANS instruments.

VSANS also has the ability to adjust the wavelength resolution 

to better match the needs of the experiment. For typical 

SANS instruments a neutron velocity selector (NVS) is used to 

choose the range of wavelengths. For VSANS the full-widthhalf-maximum (fwhm) of the NVS spectrum is 12.5 % of the 

mean wavelength. As an option to provide higher resolution, 

two reflections from highly oriented pyrolytic graphite (HOPG) 

crystals reduce the fwhm to 1 %. Alternatively, the wavelength 

resolution can be relaxed to increase the beam intensity 

through the use of filters to limit the white beam produced 

by the cold source to a range of 4 Å to 8 Å. A polycrystalline 

Be filter removes wavelengths shorter than 4 Å, and an “X” 

shaped supermirror insert removes wavelengths longer than 

8 Å. This filtered white beam has four times the intensity 

produced using the NVS of the same mean wavelength of 

5.3 Å, as shown in Figure 1.

The front and middle carriages use four 2D detectors composed 

of arrays of 8 mm diameter linear position-sensitive tube 

detectors. Each tube can measure count rates similar to that 

allowed on the entire 2D detector on the 30 m SANS. Thus, 

VSANS count rates now can be 100 times higher. The positions 

of the detector arrays are adjustable such that a center square 

opening allows the smaller scattering angles to pass downstream 

to the next carriage. Typically, the front carriage is placed at a 

distance one fifth of the middle carriage, thus increasing the 

maximum scattering angle by a factor of five. The four panels 

generally cover 100 cm x 100 cm area, which is 50 % wider than 

the 65 cm x 65 cm detector used on the 30 m SANS instruments. 

This expanded angular range allows measurements of samples 

that are changing with time without having to measure with 

more than one set of detector distances.

By simultaneously increasing the sample-to-detector distance, 

sample aperture and beam stop sizes, the beam intensity can be 

increased by up to a factor of six over current practice on the 

30 m SANS instruments. To take advantage of this possibility, a new 

version of our demountable liquid cells has been implemented 

with an increased sample size but with the same outer profile 

as the cells currently used in our existing temperature control 

blocks. Then, increasing the beam stop size from 50 mm to 

100 mm, the sample aperture size from 13 mm to 22 mm 

diameter, and doubling the detector distance, the beam current 

for the same qmin can be tripled. The larger beam stop size also 

doubles the contribution of gravity to the q-resolution, but the 

insertion of prisms eliminates the effect of gravity.

The rear carriage uses a 6LiF-ZnS(Ag) scintillation screen to 

convert the neutron to a burst of light that is then intensified 

and its position detected via three charge coupled detectors 

(CCD). The position of the neutron is detected to an accuracy 

of 0.9 mm fwhm using the rear detector. This rear carriage 

has three small beam stops: 12 mm diameter for converging 

beam collimation, and 6 mm and 12 mm wide by 300 mm tall 

rectangular beam stops used with narrow slit collimation. 

The converging beam refractive lens optics used on VSANS 

achieves a minimum q of 2.6 x10-4 Å-1, a factor of three 

smaller than can be realized using lenses on the 30 m SANS 

instruments, thus enabling measuring objects three times larger. 

The new optics has nearly equal pathlengths of 24 m before and 

after the sample using a 6.7 Å wavelength beam that is blocked 

by a 12 mm diameter beam stop. Using standard pinhole 

collimation, the source and sample aperture diameters would 

be 6 mm and 3 mm, respectively. By using refractive lenses to 

focus the beam, the sample aperture is enlarged to 10.4 mm, 

increasing the beam intensity by a factor of (10.4/3.0)2 = 12. 

By inserting a series of intermediate masks, twelve separate 

beams are defined. Inserting refractive prisms after the 

lenses corrects the parabolic trajectory of the neutron beam, 

1 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899

2 University of Delaware, Newark, DE 19716

3 University of Maryland, College Park, MD 20742
R

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iiNIST CENTER FOR NEUTRON RESEARCH FOREWORD aAf

t oncs r ncNmovmeey upptreosahn smuReree nsetts ao arpcnhrd e. sTOehnps t ptoyo ertar yuonwu ite tehs ces fleoyr ebatrhra’e ts eNd SthT e C5e0ntth er the rearcstaorr y hoaf s thbee eren avcetory r. gTohoodu, gwh e athre e hstsrtovring cal to re mabpilrotvy e of tmehxae nraegaectmoer’ns t oppreorgartaiomn. al Ars epaabrt ilotf y tvha a s efrfoobrut, swt ae ghng ad an reactor shuapf

tgeertnay sddeehid s m a outo ttrahm ge e dreoraf vcetthso) ae r rpned aancwtt oe (r pbtro rmopuaegrry hfor ct oom on sng nee veptrhual e mpmnes a awjo, nr d inD2igcestohad ellras-tcoioaupn rcaoe cf tn y thcte hoe d qeusaord Duy rc2e 02re0fsr. gerator n ant cipation of tChe tases ctoiof n thoen se farecilaty bildtey veenohpamnecnemts en ntts hcs an rebpe orft. ound n deovnesodpemraebne t pprrooggrram ess wwaits ah thso e cmoammde sn sioour ning nsotrf uVmSeAnNt S and cMoantaining VSANS data has been subm tted and we expect jor assemb y work on CANDOR s underway and we are anAdow s dwite ioencnao, bmthpng s e ethemurmal tanneeouutrs oXn -rmay aagnng d nsteauttioron wn raads ougprgaprahdye. d, fnor the next sett e oVf SnAsNtrS aumned nt CAdNeaDs dORs, cduesssiged at n wothrk e c2o0n1t4 nues nesutrturomn etnrt pdee-vaexos psmpeecntt rommeeettenr gto . Trhee p daece sigSn P Nof a S on neNw Gc-5 o d As us wnde erowoak y. tnheat u we makto e tmhe prloonvegm-teernm ts ftuhtat ure asosuf rte he thNe CaNvaR, at bils tey ssoef nt al by tron measurement capabilit es and a re ab e source of Nattiohe n,A” mreecriocman mPehnydsecd al nScorceaesty ng tit ned ve“stNmeeuntrtos nn s fnoer utthroe n Mwne a sotrure mpenatnani tiong n dseevveeroal pfmuteunrt. e n mapcrocoverdamenncte s wto ith thte hNe CreNpRo. rt, co d st somuprcoe rtwanitth yon, we e bwailsel d reopn ace quour cu d deurrteenr t um quthd erheybdy rogen logmung w adge ng faovr ethee ngStPhNs. S Wree pwailcl etmakee ntt, hue pogprapdoe rtouur n ty coto d nnestuatrl otn he gaα−γn station, and rep ace the NG-7 gu de and re ocate the for th s nstrument. tnouchoee nutsttbraeonsnt dst. ng oTohs e as tro eesvpedearenrfch orcem pd rorbedy suetcahere ch d hbigwy h qtthh a e uuarseety ras obof f e stcshoene uNrcce Ce rNoeR wf su tas s repotart nceod ntn aini thng a s repcorort. I am ss-sectioden igohf ttehd e to s mphreasre s vwe itrh esyeoau rch th s from the past year

Foreword

INSTRUMENTATION

2019 ACCOMPLISHMENTS AND OPPORTUNITIES 37

allowing all 12 beams to converge to the same spot on the 

detector. To minimize attenuation of the beam from phonon 

scattering, the prisms are cooled to 155 K. The observed gain 

in beam intensity using 12 beams versus pinhole collimation 

is 120 after correcting for lens and prism attenuation. A new 

larger demountable liquid cell and temperature-controlled 

sample blocks are now being designed to take full advantage 

of this option.

Narrow slit collimation is obtained using two motorized XY 

slits where the width and height of a rectangular aperture 

is adjustable over the full width and height of the incoming 

neutron guide which has a width of 60 mm and a height 

of 150 mm. These two slits can be adjusted to produce a 

narrow beam at the rear detector with a width smaller than 

the smallest 6 mm wide beam stop. Using 6 Å wavelength, 

a source aperture that is 2.5 mm wide by 150 mm tall and 

sample aperture that is 1.25 mm wide by 75 mm tall, the 

measured beam intensity is 31,000 s-1. An additional factor 

of four increase in beam intensity is achieved by using the 

filtered white beam wavelength option. The measured parasitic 

background outside the beam stop was 91 s-1. Test scattering 

measurements have been made to qmin = 1.5x10-4 Å-1. 

Commonly available quartz capillaries of 2 mm diameter can be 

used for liquid cell holders.

Polarized beam experiments are also possible at VSANS. In 

fact, several improvements have been made to the beam 

polarization compared to the NG7 30 m SANS instrument 

providing flipping ratios of up to 300. Mathematical simulations 

of beam depolarization caused by instrument magnetization 

profile led to redesigning of the permanent magnet and soft 

steel structures surrounding the double V shaped polarizing 

super mirror insert before the source aperture and the RF 

coil located upstream of the sample. The redesign minimized 

nonuniformity of magnetic field lines in the beam area which 

was causing beam depolarization in the simulations. The beam 

polarization at the sample position is now measured to be 

99.3 % with a measured flipping efficiency of the RF flipper 

approaching 100 % within our measurement capability of 

+/- 0.0003. The large 2 m long sample area allows room for a 

3

He spin filter downstream of the sample. Several experiments 

have been performed at VSANS, measuring all four polarization 

scattering cross-sections.

As an example of the capabilities of VSANS, the structural 

evolution of a sheared carbon black sample was measured 

using four converging beams and the high-resolution detector 

on VSANS [1]. The structure of carbon black suspensions 

play a pivotal role in the design and processing of electrode 

formulations used in electrochemical energy storage 

applications. Unfortunately, this structural evolution occurs at 

length scales too large for typical SANS experiments and time 

scales too short for USANS experiments. However, VSANS, 

with its lower attainable q-range and high neutron flux, 

provides a unique measurement of these structural changes 

under shear. VSANS data (Figure 2) shows the transient 

densification of carbon black agglomerates that occurs at low 

shear rates, as evidenced by a decrease in intensity at low q. 

This densification is related to a time-dependent decrease in 

both the viscosity and electrical conductivity of the suspension. 

Therefore, the combination of this time-dependent VSANS 

data and steady-state USANS data provides useful information 

for understanding the microstructural origin of the transient 

properties of carbon black suspensions. 

Nearly all of the experiments in the last year utilized at least one 

of the new capabilities provided by the new instrument. Further 

developments in the next year will concentrate on constructing 

and modifying sample environment equipment to permit 

routine user experiments that take full advantage of the higher 

beam intensity collimation options available on VSANS.

Reference

[1] J. B. Hipp, J. J. Richards, N. J. Wagner, 

in preparation, (2019).

0

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Counts (A.U.)

Wavelength (Å)

White

Beam

Velocity

Selector

Graphite

Monochromator

FIGURE 1: Time of flight (TOF) spectrum for the three wavelength resolution 

options. The blue curve is obtained using a NVS with rotation speed chosen 

to produce a mean wavelength of 5.3 Å. The black curve is the spectrum from 

the filtered white beam. The red curve is a calculated spectrum for the HOPG 

monochromator. The beam intensity is proportional to the area under the curve.

FIGURE 2: Transient structural evolution measured for a sheared sample of 

Vulcan XC-72 carbon black suspended in propylene carbonate.