A UNIVERSITY BASED COLD NEUTRON
SOURCE
D.V. Baxter, J.M. Cameron, D.L. Friesel, H. Nann, T.C. Rinckel, W.M. Snow, and J.W. Zwanziger
Indiana University, Bloomington, IN 47408.
Abstract.
A new concept for a cold neutron scattering facility is described. A low energy,
high intensity proton beam is used to produce the neutrons using (p,xn) reactions
The advantages of this approach and the neutron source capabilities are outlined.
for the ILL in France (presently the premier neutron
facility in the world), which states:1
INTRODUCTION
There is a general lack of publicity for the
importance of neutrons in the life sciences
community. The simple fact that neutron beams can
only be generated in central large scale facilities
such as the ILL is itself an obstacle.
Neutrons are a unique probe of the structure
and dynamics for a great variety of systems studied
today in many scientific disciplines. With the
construction of the Spallation Neutron Source (SNS) at
Oak Ridge, which will be the most intense short-pulsed
spallation neutron source (SPSS) in the world when
completed in 2006, the U.S. hopes to provide the first
state-of-the-art facility for neutron research in North
America in decades. However, the existence of the SNS
may not, of itself, be sufficient to ensure the expansion
of neutron use into new areas of science nor to establish
an American community of users comparable to that in
Europe (where they presently number some 5000
compared to some 1000 or so in this country).
Potential users in non-traditional fields such as
chemistry and biology are often unfamiliar with the
power of neutron techniques or find it difficult to
acquire the experience needed to perform critical
experiments of relevance to their fields. Similarly,
industrial researchers rarely are sufficiently familiar
with neutron techniques to identify the real-world
problems in which those techniques could provide
useful solutions. Major obstacles to expanding the
neutron community in this country, in our view, are this
existing general unfamiliarity of US researchers with
neutron techniques, a diminishing number of national
and local facilities where novice users may be
introduced to neutron scattering techniques, and the lack
of flexible facilities for engineering and technical
studies where new ideas for neutron science and
engineering can be pursued. As an example of the
situation in biology, we quote the most recent Roadmap
NEUTRON PRODUCTION
Overview
The flexibility and minimal cost of our design
for LENS (Low Energy Neutron Source) offer
significant advantages over existing neutron sources,
and even those under construction, for a number of
important research and educational projects. Preparing
the next generation of neutron scattering scientists will
require education in the use of pulsed sources as the
world’s flagship neutron facilities will soon be at pulsed
sources such as the SNS. Moreover, particularly in
areas where neutron science is presently emergent (such
as biomaterials and chemistry), long-pulsed spallation
sources may be the ideal sources for the future.2 It is
therefore important both to educate the next generation
of users on the relative merits of long- and short-pulsed
sources, and to provide facilities suitable for conducting
relevant research on the design of moderators and
instruments. The variable pulse width offered by our
source, its low cost, and its greatly reduced level of
background radiation, put it in a unique position to have
an impact on these issues.
Our target design closely resembles that
derived from extensive studies carried out at the Chalk
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
265
River Laboratory3, but the moderator is designed to
maximize the brightness for cold neutrons.
calculations5 using the same neutron scattering kernels
that were used in IPNS studies.
Low energy neutrons in the LENS target are
produced via the (p,n) reactions at 11 MeV incident
proton energy.
Accelerator
Indiana University already operates a
commercial 7 MeV proton linear accelerator consisting
of a 3 MeV radio frequency quadrupole accelerator
(RFQ) directly coupled to a 4 MeV drift tube linac
(DTL).4 The present RF amplifier system provides a
peak power of 600 kW at 0.2% duty factor. With the
existing ion source, the linac is capable of producing
peak currents on the order of 20 mA. The presence of
this accelerator at IUCF will make it possible to quickly
initiate the neutron source we propose. Below we also
outline straightforward modifications to this accelerator
that will increase greatly the intensity of the cold
neutron beams.
The present 425 MHz RF amplifiers limit the
duty factor of the linac to 0.2% at constant power. This
will limit the length of pulse that we can use at a given
frequency, but we will be able to vary the repetition rate
from 200Hz to 10Hz (with pulse widths varying from
10 µsec at the former frequency to 200 µsec at the
latter). Even shorter pulse widths are possible at reduced
intensity for moderator studies. This ability to easily
change the pulse frequency is another attractive feature
of our proposed source, as it will allow us to explore
instrumentation problems relevant to both short pulse
and long pulse spallation sources. This flexibility also
greatly enhances the facility’s instructional role. It is a
feature that would be impossible to realize at a large
national facility with multiple instruments.
A few relatively minor upgrades, replacing the
present RFQ vanes, and upgrading the ion source will
allow the peak current to be increased to 50 mA without
replacement of the present RF amplifiers. The front end
of the linac includes the ion source and the optics that
prepares the beam for injection into the RFQ.
Increasing the energy of the injected beam to more than
50 keV and upgrading to a high intensity microwave ion
source is necessary to achieve a peak proton current of
50 mA from the linac. Further enhancement is achieved
by replacing the present RF amplifier system with a
klystron that can handle a higher beam load; duty factor
would be increased to about 5%. This may also require
some additional cooling of the accelerator structure.
Finally, the proton energy would be increased to 11
MeV by addition of a second DTL section.
Low Energy Neutron Source
11 MeV Proton
beam
22 K CH4
moderator
Instrumentation
development
Education/Diffraction
Be target and
Be/C reflector
SANS
FIGURE 1 Schematic layout of the LENS cold neutron
facility.
In our MCNP model, the 9Be target is
surrounded by a vertical, cylindrical beryllium reflector
50 cm in diameter and 50 cm length. A ‘slab’
moderator, 12x12x4.5 cm in size, is filled with solid
methane at 22K. The moderator will contain low
density (10% by volume) aluminum foam for efficient
heat removal.
1013
Leth. Flux (n/sr/s)
1012
1011
1010
109
108
10-4
10-3
10-2
10-1
100
101
Energy (eV)
FIGURE 2 Monte Carlo simulations of the neutron flux
for the proposed LENS facility operated with a Be/Li
target and 2.5 mA of 11 MeV protons. The simulation
is for Cd-decoupled solid methane moderators in slab
geometry with dimensions 10 cm x 10 cm x 4.5 cm, and
operating at 30 K.
Target, Moderator, and Neutron
Reflector
With an 11 MeV proton beam of 50 mA peak
current energy and an accelerator duty factor of 5%, the
neutron yield from the 9Be target is 1014 s−1 with a γ-ray
yield of 3.4 × 1012 s−1. Based on detailed thermal
All of our simulations of the source
performance are based on MCNP Monte Carlo
266
analysis presented in the CANUTRON proposal for an
almost identical Li target operated at 25 kW,6 water
flowing at 30 l/min and 30oC through a Be base for our
composite target will keep its temperature below 400 oC.
We will spread out the beam to 5 cm in diameter,
thereby limiting the power density to below 1kW/cm2
with no significant impact on the low energy neutron
source brightness as the latter is fixed in scale by the
neutron mean free path in cold hydrogenous moderator
(a few cm).
The low γ-ray yield from these targets and low
incident proton energy relative to a spallation neutron
source are important features of LENS. They result in
low enough activity in the target/moderator system for
these materials to be handled without extensive remote
handling facilities (which are awkward to implement in
a university research environment). The heat load on the
moderator will be dominated by neutron capture
reactions not source background radiation; this means
that at LENS our moderators can be made somewhat
colder than at a spallation source. The severity of
radiation safety issues in general is also reduced, which
is important for the use of LENS as an educational
resource. Finally, there will be negligible contamination
of the neutron beam with gammas so that gamma filters,
supermirror benders, and other neutron optical devices
needed to deal with gammas at reactors and spallation
sources will be unnecessary, leading to a more efficient
use of the neutron flux (for cold neutrons these devices
typically lead to flux losses of about a factor of 2) and
simpler designs for elements such as neutron choppers.
Source Characteristic
Proton energy (MeV)
LENS
11
Time-averaged proton
current (microamps)
2500
Beam Power (kW)
27.5
Proton Current (protons/sec)
Neutron yield per incident proton
Total neutron yield (n/sec)
3 × 1016
4 × 10-3
1014
Neutron coupling (1 eV
neutrons/sr/source neutron)
1.8 × 10-3
Moderator brightness at 1 eV
(n/sr/sec/eV)
1.2× 1011
Moderator brightness at 4 meV
(n/sr/sec/eV)
1 × 1014
SUMMARY
Recent development of high-current lowenergy proton linear accelerators makes it possible to
produce useful neutron fluxes in a facility suitable for a
university or industrial setting. By keeping the proton
energy low, the activity in the source itself is reduced to
the point where it can be reconfigured easily to test new
ideas quickly and conveniently. The facility will also be
suitable for a significant program in neutron scattering
and for educating students at all levels from a variety of
disciplines. Our vision is to have this facility be the
prototype for a network of similar facilities at
universities across the nation. These facilities will form
a feeder network supporting the SNS user community in
a manner analogous with the highly successful network
of local reactors supporting the continent-scale ILL and
ISIS facilities in Europe.
REFERENCES
267
1.
The ILL Roadmap. 2001
2.
Mezei, F., Long Pulse Spallation Sources.
Physica B, 1997. 234-236: p. 1227-1232.
3.
Schreiber, S.O., M.A. Lone, B.G. Chidley,
M.S. deJong, S.A. Kushneriuk, and W.N.
Selander, IEEE Trans. On Nucl. Science, 1983.
NS-30: p. 1668.
4.
Friesel, D.L., and W. Hunt, Performance of an
AccSys Technology PL-7 Linac as an Injector
for the IUCF Cooler Injector Synchrotron,
XIX International Linac Conference, Chicago,
IL August 23-28, 1998, p. 61-63.
5.
Briesmeister, J.F., MCNP-A General Monte
Carlo N-Particle Transport Code, 2000, Los
Alamos National Laboratory.
6.
Lone, M.A., A.M. Ross, J.S. Fraser, S.O.
Schrieber, S.A. Kushneriuk, and W.N.
Selander, Low Energy 7Li(p,n)7Be Neutron
Source (CANUTRON) (PASS-18-5-R (AECL7413), Chalk River Nuclear Laboratories,
1982).
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