Cold neutron microprobe for materials analysis using tapered
capillary optics
V. A. Sharov,
a)
Q.-F. Xiao,
b)
and I. Yu. Ponomarev
X-Ray Optical Systems, Incorporated, 30 Corporate Circle, Albany, New York 12203
D. F. R. Mildner and H. H. Chen-Mayer
National Institute of Standards and Technology, Gaithersburg, Maryland 20899
͑Received 6 April 2000; accepted for publication 9 June 2000͒
A prototype monolithic capillary lens for focusing neutrons produced by thermally drawing straight
multicapillary bundles has been characterized with cold neutrons, and gives an intensity gain of a
factor of 25 at a focal distance of 8 mm, over the focal spot area of width 87
m. This is over an
order of magnitude smaller in area than for the multifiber capillary lens. The spatial resolution
available with the lens has been tested with prompt gamma measurements on slivers of dysprosium.
Background problems that can affect the spatial resolution of measurements taken at the focal
position of the lens are addressed. The boron glass of the tapered monolithic lens provides good
shielding from unfocused neutrons in the vicinity of the lens focus. © 2000 American Institute of
Physics. ͓S0034-6748͑00͒05109-1͔
I. INTRODUCTION
The use of cold neutron focusing enables higher spatial
resolution and considerable increase in the count rate for
neutron absorption techniques for materials analysis such as
prompt gamma activation analysis ͑PGAA͒ and neutron
depth profiling. A focusing lens
1
with thousands of indi-
vidual glass polycapillary fibers, each containing thousands
of hollow capillaries of diameter Ϸ9
m, placed at the end of
a neutron guide has given a current density gain of 80 at the
focus within an area defined by the full width at half maxi-
mum ͑FWHM͒ of 0.53 mm.
2
This lens has been installed on
a PGAA instrument.
3
The results
4
indicate an increase of
Ϸ60 in the gamma count rate for certain elements, and an
improvement in detection limit by a factor of 20. However,
the multifiber capillary lens has a large fraction of the lens
entrance area not occupied by the fibers so that the spatial
capture efficiency is low which leads to increases in the
background around the focus, despite the gain in neutron
current density.
Further increases in the neutron current density gain and
reduction in the focal size may be obtained by using a mono-
lithic tapered capillary lens rather than one with individual
fibers. Such a lens consists of a single piece of glass that
contains millions of small channels that are parallel at the
entrance and tapered towards a common focus at the exit. It
is obtained by thermally drawing bundles of straight multi-
capillary hollow glass fibers with initial diameters Ϸ10
m.
The advantage of the monolithic bundle of tapered capillar-
ies over the multifiber capillary lens is that a much larger
fraction ͑ϳ50%͒ of the lens entrance area is occupied by the
hollow capillaries. This leads to a higher spatial capture ef-
ficiency, and at the same time a considerable reduction in
background at the focus. Such a lens should enable even
further improvements in the capabilities for absorption mea-
surements.
Preliminary neutron transmission results
5
performed on
earlier prototype monolithic lenses fabricated for x-ray
focusing
6
show neutron current density gains only slightly
lower than for the multifiber lens, but with considerably nar-
rower focal spots (FWHMϷ0.16 mm), at a distance of Ϸ22
mm from the exit of the lens. Even though the entrance area
is much lower, the advantage of the monolithic lens is the
more efficient focusing of the incident neutrons than the mul-
tifiber lens. While the number of neutrons at the focus may
be much lower for these early prototypes, the focal area is
also much smaller so that the gains are comparable.
II. TAPERED CAPILLARY OPTICS
Monolithic lenses are produced by the thermal drawing
of straight multicapillary bundles to create the desired pro-
files and dimensions. Throughout this process it is important
to keep the profiles of the capillary channels as smooth as
possible because any step or kink resulting in the variation of
the local curvature reduces neutron transmission signifi-
cantly. Control of the optic profile during fabrication is de-
termined by many parameters, including the physical prop-
erties of the glass, the heating temperature and its
distribution, the temperature ramping, the pulling force and
speed, and air movement.
The optimum design of a polycapillary optic requires a
uniform bending of the fibers for the efficient transmission of
neutrons through the channels. Unlike the polycapillary op-
tic, the channel diameter of the monolithic focusing optic is
not constant along the length of the optic, but instead de-
creases towards the focusing end. We have used a ray tracing
program to simulate the performance of the optics to deter-
mine the best profile for the design of the prototype neutron
a͒
Present address: The Institute for Genomic Research, 9712 Medical Center
Drive, Rockville, MD 20850; electronic mail: vsharov@tigr.org
b͒
Present address: International Business Machines, CKP/028, 5600 Cottle
Road, San Jose, CA 95193.
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 71, NUMBER 9 SEPTEMBER 2000
32470034-6748/2000/71(9)/3247/7/$17.00 © 2000 American Institute of Physics