Promises and Challenges of Two-Step Targets
for Production of Neutron-rich RIBs
W. L. Talbert, D. M. Drake, H.-H. Hsu and M. T. Wilson
TechSource, Inc., P. O. Box 31057, Santa Fe, NM 87594-1057
Abstract: Development of a prototype two-step target to produce neutron-rich RIBs is presented, with particular emphasis
on thermal analysis under high-power operation. The two-step target is an attractive concept for production of fissionproduct activities without interference by high-energy spallation reactions which occur in direct production targets. In this
concept, a high-energy production beam interacts with a primary target of refractory metal, depositing beam energy in the
primary target and producing low-energy neutrons that cause fissions in a surrounding secondary target of mixed UC2 and
excess C. Thermal analysis of the composite target presents challenges in cooling the primary target while maintaining the
secondary target at temperatures suitable for release of the fission products. The effects of fission energy deposition in the
secondary target are discussed, along with the complexities resulting from the thermally insulating character of the
secondary target material.
Separation of the two target components provides
advantages in thermal control. The primary target
requires cooling when irradiated by intense beams.
Such cooling can be effected to ensure independent
temperature control of the secondary target. The
secondary target requires operation at elevated
temperatures to efficiently release radio-nuclides, and,
therefore, it may have to be independently heated.
In the discussion below, a prototype two-step target
is described that is intended to be evaluated at the
ISAC facility [3] by irradiation with 500-MeV proton
beam at intensities up to 100 µA. Under such intense
irradiation conditions, the primary target must
withstand a heat load up to 50 kW and therefore, must
be cooled to avoid reaching the melting temperature of
Re. Efficient release of fission products requires that
the secondary target be operated at temperatures
between 1600 °C and 2100 °C.
INTRODUCTION
The two-step target concept for producing
radioactive ion beams (RIBs) has been suggested by
Nolen et al. [1] as an important component of the
target development program for the Rare Isotope
Accelerator (RIA) project [2]. The concept consists of
a primary target irradiated by an intense energetic light
ion beam for the purpose of neutron production,
surrounded by a secondary target in which the
neutrons interact to produce the desired radioactive
species.
In the present work, the primary target is a solid
cylinder made of the heavy refractory metal, Re. The
secondary target, a coaxial annular cylinder that
surrounds the primary target, consists of a mixture of
uranium carbide and excess carbon (UC2/C).
Interaction of neutrons, from the primary target,
produce fission products which can then be extracted
from the ion source as ions, mass separated, and
accelerated for use in nuclear physics, astrophysics and
materials science studies.
The two-step target concept offers several attractive
features. For example, the production of neutrons in a
heavy metal target results in a neutron multiplicity
significantly greater than one, and provides
enhancement of the secondary interaction rates,
compared to direct irradiation of the secondary target
material. Also, the absence of high-energy production
beam interactions in the secondary target material
ensures that for UC2/C, fission products are available
without interference from nuclear processes such as
spallation or fragmentation.
TARGET CONCEPT
The two-step target consists of two components, the
primary (neutron-producing) target that is irradiated by
an intense high-energy light ion beam, and the
secondary target containing the fissionable target
material in which the desired radioactivities are
produced.
Analysis of the two-step target was performed using
the MCNPX code [4] for energy deposition, neutron
tracking and fission rates. The ALGOR finite element
code [5] was used for thermal and stress analysis.
These computational approaches were also used to
predict the thermal performance of a test target at the
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
218
ISAC facility [3], in late 1999, which led to the
successful validation [6] of the design approach used
in the present work.
Primary Target
The target performance was first analyzed by
calculating the power distribution deposited in the
primary target by the production beam followed by
calculation of the neutron production rates in the
primary target. For the target described in this work,
the neutron production rate is approximately seven
neutrons per incident proton. The dimensions of the
target were then chosen to represent a close match to
the incident beam profile (diameter) and useful length
for neutron production (about 7 neutrons/proton),
while keeping the total energy deposition at a
reasonable value. Finally, the thermal behavior of the
primary target was evaluated. (The solid, heavy Re
target is subjected to very intense heating by the
intense production beam and therefore, care must be
taken not to reach the melting point of the primary
target.)
Although thermal evaluation of the Re primary target
is still underway, for reasonable total energy
deposition by the primary beam, the target was chosen
to have a radius of 0.75 cm and length of 6.5 cm, as
determined by neutron production rates. The energy
deposition rate turns out to be over 200 MeV/p for the
target dimensions chosen, corresponding to a power
level of over 20 kW at an incident proton beam current
of 100 µA.
Although cooling of intensely heated targets presents
severe challenges, these problems can be appropriately
addressed with contemporary engineering practices.
The present target is cooled by four H2O carrying
tubes, thermally connected to the outer surface of the
target, that run the length of the target. With this
cooling arrangement, temperature profiles in the center
of the target appear to be tolerable. The relatively poor
thermal conductivity of rhenium metal (~ 0.5 W/cm-K
at elevated temperatures (~ half that of tungsten))
imposes a limitation on the radius of the target, due to
the high thermal gradient required to transfer heat
generated by the beam to the cooling system.
The results derived from preliminary thermal
analysis of the temperature distribution within the
body of the target are shown in Fig.1.
FIGURE 1. Thermal analysis of the Re primary target,
showing temperature distributions within one quarter of the
target.
In the figure, the highest temperature (~ 2794 °C) is
located at the center of the target at the point of beam
entry into the target. Therefore, this concept meets the
design goal of the target of keeping the temperature
well below the melting point of Re (3180°C). Various
cooling approaches such as imbedding the cooling
tubes a short distance in the target have been
considered (as shown in Fig. 1) and optimization of the
approach is underway. However, the effect on the
neutrons (and fission rates) in the secondary target by
such modifications must also be evaluated.
The cooling tubes have been chosen to have inside
diameters of 0.25 cm and wall thicknesses of 0.05-cm.
Coolant conditions, extracted from thermal analyses of
the target, are ~ 20 liters/minute accompanied by a
temperature rise of about 60°C. Although, these
values are approximate due to the fact that a final
design has not been reached, they do, however,
illustrate the feasibility of using forced water cooling
of targets into which very high amounts of primary
beam energy have been deposited.
The results of thermal analyses are sensitive to the
details of the energy deposition profiles that require
fine zoning, especially at the incident end of the target.
This situation can be eased with the choice of a target
material with a higher thermal conductivity. However,
the choice of Re was made with attention to other
properties such as the ability to withstand thermal
stress, compactness, chemical inertness and neutron
production rates.
Secondary Target
The secondary target was modeled to determine
fission rates and energy deposition from the fission
events and scattered primary and secondary beam
particles, assuming a density of 2.5 g/cm3 for the
UC2/C mixture.
219
distribution. The ends of the target have one radiation
shield. In the figure, the radiation shields and primary
target have been removed to clearly show the
secondary temperature distribution.
The coaxial secondary target dimensions were chosen
to encompass the bulk of the fission events while
minimizing volume (to facilitate release of fission
products). This results in a length of 6.5 cm (same as
for the primary target) and an annular thickness of 2.1
cm with an inner radius of about 1.2 cm (chosen to
"clear" the primary target cooling system and allow
some radiation shielding for the inner surface).
The energy deposition rate in the secondary target is
much smaller than for the primary target, about 10
MeV per incident proton, or 5% of the primary target
deposition rate, largely attributed to fission events.
The energy deposition distribution for the secondary
target is concentrated in the inner and central regions.
Contrary to the thermal requirements for the primary
target, it is desirable to preserve as much heat as
possible within the secondary target to effect efficient
release of fission products. Results derived from
thermal analyses of the secondary target depend
crucially on the thermal conductivity of the target
material. The thermal conductivity for the UC2/C
target material utilized in these studies (as measured) is
quite low. It is hoped that UC2/C material with a
density higher than the 2.5 g/cm3 (used in the present
target analyses) can be utilized in the prototype target
that will be used for testing, and that the increase in
target material density will be accompanied by an
increase in thermal conductivity.
One goal of thermal analyses of the secondary target
is to maintain as large as possible volume of the
material in the temperature range of 1600°C to
2100°C. The release of fission products is believed to
be inefficient below the lower limit, while sustained
stability of the target material, including its
containment in a Re shell, is not believed to be assured
above the upper limit. Other refractory metals react
strongly with the carbon of the target material at much
lower temperatures.
The desired thermal behavior of the secondary target
is opposite to that of the primary target. Instead of
removing heat, to avoid catastrophic temperatures in
the primary target, fission fragment and radiation
transferred heat to the secondary target material must
be conserved and distributed to achieve high operating
temperatures over as large a volume as possible. Such
thermal behavior requires that heat be transferred
within the target, a difficult requirement given the low
inherent thermal conductivity of the secondary target
material (typically 0.01 W/cm-K) that leads to very
high thermal gradients within the secondary target
volume.
A simple application of two outer radiation shields
surrounding the secondary target (in addition to the
rhenium containment vessel) leads to the temperature
distribution shown in Fig. 2, where the temperature
profile is indicative of the energy deposition
FIGURE 2. Temperature distribution for the secondary
target with two radiation shields (not shown).
In the figure the maximum temperature of 1751°C
for the secondary target material, although above the
desired lower limit, does not approach the allowable
upper limit, and the thermal gradients are quite large,
nearly 600°C. The outer portions of the target material
may act as "cold sinks" for the fission product
activities released from the hotter portions of the
target.
A modification to the secondary-target design was
investigated as a possible means of achieving a better
temperature distribution within the target material.
Four radial fins of Re were inserted, extending the
angular width and length of the target, with the
intention that the larger thermal conductivity of Re
would assist in more uniform distribution of the heat
by means of radiation coupling and conductive
transfer.
Figure 3 illustrates the effect of this
modification. As noted, the temperature gradients have
been significantly improved (to less than ~400°C); the
maximum temperature (1734°C) is still well below the
allowable upper limit; and the "cold sink" regions of
the target material have been substantially reduced,
especially in the middle of the target.
FIGURE 3. Temperature distribution within the UC2/C
secondary target with two radiation shields (not shown) and
imbedded radial fins of Re (one shown on top face).
220
merely by reducing the distance required to distribute
the internal heat. Another benefit of reducing the
target material volume is the possibility of enhanced
release of the fission products, should the larger
density material have equivalent release properties,
simply by the reduction of target volume.
We are looking forward to receiving results from ongoing thermal conductivity measurements of higher
density target materials, with the expectation that
target thermal performance will improve. Without
such improvements, the two-step target concept
appears to have some basic limitations on the
conditions required for efficient release of fission
products.
A second modification has been suggested. By
inserting a 0.5-cm thick Re annulus behind the
secondary target, scattered protons and radiation can
be intercepted and their deposited heat transferred to
the secondary target.
Thermal analysis of this
modification, resulted in the temperature distribution
shown in Fig. 4. In this Figure, the effects of the added
heat are evident by the higher internal temperatures
(up to 1942°C) and larger volume that the target
reaches within the desired temperature range.
FISSION PRODUCTION RATES
The basic fission process in the secondary target is
induced by neutrons produced in the primary target.
An analysis of the neutron spectrum shows that it is
peaked at about 2 MeV, with the peak of the flux
located in the central part of the secondary target
annulus.
Because the effective fission threshold for 238U is
about 1 MeV, the neutron spectrum is on the "soft"
side for inducing high cross-section fissions.
However, the neutron spectrum has substantial
intensity above 10 MeV where the cross-section is at
least twice that of the peak of the neutron spectrum.
The use of natural uranium-carbide (UC2), with its
small component of 235U, results in a small, but
significant, increase in total fission rate. Scattered
high-energy protons entering the secondary target,
especially near the downstream end of the target,
provide another contribution to the total fission rate.
For the two-step target concept presented, the total
fission rate is 0.288 fissions per incident proton. The
production rates of typical fission products per proton
of interest are: 143Cs, 4.42E-04; 95Rb, 4.09E-04;
132Sn, 1.79E-04; 106Mo, 1.09E-04; 111Ru, 2.78E-04;
and 155Nd, 1.15E-04.
These fission rates are
substantial for an incident beam current of 100 µA,
corresponding to 6.24E+14 protons per second.
FIGURE 4. Temperature distribution for a secondary target
with two radiation shields and a posterior annulus for
capturing additional heat.
This modification to the concept has resulted in large
thermal gradients within the secondary target material,
because of larger heat transfer to the surface required
for effective radiation dissipation of the internal heat.
This approach is promising, but requires further
refinement.
Other approaches have been suggested for modifying
the temperature distribution within the secondary
target material, such as inserting radiation slots within
the target material or modifying the material with
thermal conductivity enhancing structures such as
graphite fibers. These alternative approaches have not
been considered in detail.
Analyses have been made of the effects from density
increases in the target material, from the nominal 2.5
g/cm3 by two and even three times. While the thermal
conductivity is intuitively expected to increase with
increasing target material density, such an effect
cannot be assured. Measurements are underway to
investigate
the
thermal
conductivities
and
hemispherical emissivities of samples with higher
densities [7].
An increase in target material density by a factor of
two leads to a much reduced secondary target volume
(e.g., from an annular thickness of 2.1 cm to 0.9 cm)
while preserving the same total fission rate. This
reduction in volume can be beneficial in transporting
heat in that the thermal gradients can be reduced
CONCLUSION
This project has the goal to determine the efficacy of
a two-step target for production of intense neutron-rich
radioactive ion beams. The design studies performed
to date indicate that (a) expected production rates of
activities of interest are substantial, and (b) the thermal
problems associated with cooling the primary target
while maintaining high temperatures in the secondary
target are tractable for the primary, and marginal for
the secondary target.
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The temperature profiles for the secondary target are
determined by the rather high thermal-conductivity
values available for the presently considered secondary
target materials.
Hopefully, more dense, higher
thermal conductivity secondary target materials will
reduce the secondary target volume while improving
their thermal behaviors.
ACKNOWLEDGMENTS
This work is supported by the U.S. Department of
Energy under SBIR Grant DE-FG03-01ER83314.
REFERENCES
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I. C., Nucl. Phys. A 701, 312c (2002).
2. Grunder, H. A., Nucl. Phys. A 701, 43c (2002).
3. Bricault, P. G., Dombsky, M., Schmor, P. W. and
Stanford, G., Nucl. Instrum. Meth. B 126, 231
(1997).
4. "MCNPX users manual: Monte Carlo N-particle
transport code system for multi-particle and highenergy applications," Los Alamos National
Laboratory report LA-UR-02-2607, L. S. Waters,
ed., v. 2.3.0 (2002).
5. The ALGOR code, v. 12, ALGOR Incorporated,
Pittsburgh, PA (2000).
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J. W., and Hsu, H.-H, Nucl. Phys. A 701, 303c
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