Available online at www.sciencedirect.com
NIM B
Beam Interactions
with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 829–833
www.elsevier.com/locate/nimb
A D–D neutron generator using a titanium drive-in target
I.J. Kim *, N.S. Jung, H.D. Jung, Y.S. Hwang, H.D. Choi
Department of Nuclear Engineering, Seoul National University, Seoul 151-744, South Korea
Received 30 November 2007; received in revised form 7 January 2008
Available online 18 January 2008
Abstract
A D–D neutron generator was developed with an intensity of 108 n/s. A helicon plasma ion source was used to produce a large current
deuteron beam, and neutrons were generated by irradiating the deuteron beam on a titanium drive-in target made of commercial pure
titanium. The neutron generator was test-run for several hundred hours, and the performances were investigated. The available range of
the deuteron beam current was 0.8–8 mA and the beam could be accelerated up to 97.5 keV. The maximum neutron generation rate in
the test-runs was 1.9 108 n/s, which was achieved by irradiating a 7.6 mA deuteron beam at 94.0 keV on a 0.5 mm-thick target. The
operation of the neutron generator was fairly stable, such that the neutron generation rate was not altered by high voltage breakdowns
during the test-runs. Neutron generation efficiency was rated as low as 10% when compared to an ideal case of irradiating a 100% monatomic deuteron beam on a perfect TiD2 target. Factors causing the low efficiency were suggested and discussed.
Ó 2008 Elsevier B.V. All rights reserved.
PACS: 29.25.Dz; 29.25.t
Keywords: D–D neutron generator; Helicon plasma ion source; Titanium drive-in target
1. Introduction
A neutron source using the D–D or D–T reaction offers
an excellent technique because it can supply a high-flux
neutron beam from a small source and can supply a pulsed
neutron beam or an electronically collimated neutron beam
[1] for special applications. Hence, neutron generators
using these reactions can be used as powerful tools in various fields where a small neutron beam facility is required.
Various types of research has been performed to develop
reliable and intense neutron generators [2–6]. As early as
the 1960s, sealed-tube neutron generators were commercially offered, followed by the high intensity, compact D–
T neutron generators with an intensity exceeding 1012 n/s
in the 1970’s [5,6]. However, neutron generators have been
used only in limited areas, such as basic science research or
oil well logging, because of a short life-time and high main-
*
Corresponding author. Tel.: +82 2 880 7214; fax: +82 2 3285 8214.
E-mail address: [email protected] (I.J. Kim).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.nimb.2008.01.012
tenance cost. Recently new research has been performed to
develop more reliable neutron generators having an
improved life-time [7–9]. Demands for more reliable neutron generators are rising, especially for applications in
homeland security and nonproliferation. Additionally, it
is expected that new neutron generators may be used in
areas that have previously been restricted by economic
problems from using such generators.
In this study, a D–D neutron generator using the drivein target technique [6,10] was developed. In spite of a smaller cross-section, the D–D reaction has some advantages
over the D–T reaction. Tritium contamination is a major
source of radioactive waste in a D–T neutron generator,
which requires very careful handling [11]. Alternatively, a
D–D neutron generator is nearly free from tritium contamination, since only a small amount of tritium is produced
by the D(d, p)T reaction. The reaction by product would
be considered less serious when the intensity of the source
does not exceed 1010 n/s, because the production rate of tritium is very low. In addition, the energy of a D–D neutron
is about 1/6 of that of a D–T neutron, which allows a D–D
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I.J. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 829–833
neutron generator to supply a greater thermal neutron flux
with less radiation protection shields. And the drive-in target in a D–D neutron generator is continuously loaded
with the incident deuterons even though the concentration
is balanced by a loss mechanism during the beam exposure.
Hence the effort of preparing for a target pre-loaded with
deuterium can be saved. The present design of neutron generator based on a RF ion source and drive-in target could
allow a compact device to operate economically.
2. Neutron generator design
The neutron generator was designed after considering a
previous study [12]. It has the features of continuous pumping, an ion source based on a RF driven helicon plasma [13]
and a titanium drive-in target. The deuteron beam is accelerated by a double-stage electric field created between the
ion source and the target by applying a positive and negative bias voltage to the ion source and target, respectively.
A schematic structure of the neutron generator is shown
in Fig. 1 and the overall features are similar to the prototype [12]. Each component was modified to enhance the
neutron generation rate and the operational stability. A
new, large current ion source of helicon plasma type was
used to increase the beam current and improve the operational stability. The target system was modified to improve
the performance of cooling and high voltage insulation,
and the target was drawn closer to the ion source to reduce
the loss of the available beam current. Additionally, the
other components were modified to improve the operational stability, the details of which are omitted from
Fig. 1 for simplicity.
As shown in Fig. 1, a positively charged deuteron beam
was produced from the ion source. The beam had an
energy of 10–30 keV when it was extracted from the ion
source and gained an additional energy of several tens
keV as it moved to the target. An ion beam collimator,
Faraday cup and secondary electron suppression electrode
were placed in the beam path. The ion beam collimator
defined the beam area to restrict the irradiation within
the target area. The removable Faraday cup measured
the beam current, and the suppression electrode prevented
the secondary electrons escaping from the target according
to a mechanism described in [12].
A new helicon plasma ion source was developed to supply a large current deuteron beam [13]. A detailed drawing
of the ion source is shown in Fig. 2. A half-helical antenna
was used to deliver a 13.56 MHz RF wave to the source.
The beam extraction position was drawn into the denser
plasma region to increase the beam current and a threeelectrode extraction system was used to secure the reliability of the source operation by blocking the back-streaming
electrons. The plasma electrode was made of copper while
the beam extraction hole was especially made of molybdenum for its low sputtering property. The screening electrode and ground electrode were made of stainless steel
and copper, respectively. The plasma discharge tube was
made of quartz and the insulation components were made
of TeflonÒ. In a previous study, the ion source was tested
using hydrogen gas, which achieved a large current proton
beam of 50 mA at 34 kV and an additional beam with a
high monatomic fraction of 94% [13].
Fig. 3 shows a detailed drawing of the titanium drive-in
target and assembly that was designed such that the target
was directly cooled by water and could be easily replaced
after use. Jet impingement cooling was applied to simplify
the coolant channel, and a cone-shaped projection was
devised on the reverse side of the target to avoid formation
of a stagnation point in the cooling water. The titanium
drive-in target was made of pure, commercial grade-II titanium and fabricated by lathe cutting. The surface was
ground with diamond paste (6 lm) and rinsed with de-ion-
Fig. 1. Neutron generator schematic.
I.J. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 829–833
831
impinging deuteron beam with an energy range of 10–
300 keV. Hence, it would take longer than 14,000 h for
the 0.5 mm-thick target to be completely sputtered, supposing that the current density of the deuteron beam was
1.6 mA/cm2 (8 mA on 5 cm2).
3. Experimental results
Fig. 2. Cross-sectional view of the ion source.
Fig. 3. Detailed drawing of the target assembly. Titanium drive-in target
(a) and the assembly (b) (t: target thickness, units are in mm).
ized water to remove the oxide layer. The targets were prepared with two thicknesses, 0.5 and 1.0 mm. The target
holder was made of copper. A set of coaxial copper tubes
was brazed to the target holder, through which bias voltage
and cooling water was supplied to the target. De-ionized
water was used to cool the target, whose temperature and
specific resistivity were controlled by a set of chiller and
ion-exchange-resin column while it was circulating in a
closed loop. Here the pipe sections of the loop between
the chiller and the target assembly were made of plastic
tube. Life times of the targets were expected to be much
longer than 5000 h unless the cooling of the target would
fail. According to Bay et al. [14] and Karamanis [15], the
sputtering yield of the target would be within 2 103–
6 103 atoms/ion when bombarded with a normally
The neutron generator was assembled and tested. In the
test-runs, the performance was investigated by increasing
the current and energy of the beam stepwise. The neutron
generation rate was measured by counting protons from
the D(d, p)T reaction using a Si detector. The detector
was placed at 118° to the beam direction, and the rate
was corrected for the anisotropic emission of protons and
the cross-sectional difference from the D(d, n)3He reaction.
The detailed about the measurement is reported in a separate paper [16].
The vacuum system was composed of a turbo molecular
pump (1000 lps) and a rotary pump (600 lpm), and the typical base and operation pressure was 4 107 Torr and
0.2 mTorr, respectively. For the neutron generation run,
generally about 30 ml (at standard temperature and pressure) of pure D2 gas (99.7%) was spent per minute. About
40 l of water was used per minute to cool down the ion
source and the target, however it was recycled by refrigerating it using chillers. The total amount of electric power
consumption was approximately 12 kW, which was mostly
spent for cooling (7.5 kW), ion beam generation and
acceleration (2.5 kW) and vacuum pumping (1.5 kW).
The ion source supplied deuteron beams to the target
with a current of 0.8–8 mA. The operation parameters of
the ion source and resultant beam current supplied to the
target are shown in Table 1. Within this current range,
the operation of the ion source was very stable and the fluctuation of the current was only ±1.5% for several hours.
Driving the ion source towards a higher beam current
was only achieved at the cost of unstable operation. In
order to produce a larger deuteron beam current, the gas
pressure was lowered at the discharge tube. However,
under this condition, the plasma stability was seriously
affected by arcing discharges at the beam extraction electrodes. Even though the discharge did not occur frequently,
once it occurred, the beam current immediately increased
or decreased by about 5% and was not recovered in tens
of minutes. The ion source was run for about 200 h and
the aging appeared as two ways. One was the coating of
the plasma discharge tube with a metallic layer, a common
problem among RF type ion sources [17], which decreased
the ion beam current by about 10%. Another was the
deformation of the insulator supporting the screening electrode in Fig. 2. Because the insulator was made of TeflonÒ,
it was thought that it might be attributed to the thermal
stress and radiation of the electrons, ions and soft X-rays
[18]. Hence it should be replaced by something like Al2O3
or BN. However, the other insulating components in the
Fig. 2 were rarely altered, although they were also made
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I.J. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 829–833
Table 1
Ion source operation parameters and resultant deuteron beam current
Gas flow
rate (sccm)
28
28
24
23
23
a
Pressure at down
stream (Torr)
-4
1.9 10
1.9 10-4
1.6 10-4
1.6 10-4
1.6 10-4
RF-power
(W)
1242
1210
900
800
730
Electromagneta
P/S current (A)
(B-field strength)
24.0
22.5
22.0
21.9
22.0
(293 G)
(275 G)
(268 G)
(267 G)
(268 G)
Ion beam extraction bias power
supply
Voltage (kV)
Current (mA)
30
26
22
16
8.3
14.7
11.2
7.3
4.4
1.7
Screening electrode
bias (kV)
Beam
current (mA)
3.5
3.0
2.5
2.2
0.9
8.1
6.4
4.0
2.3
0.76
Magnetic field: nonuniform magnetic field [13].
of TeflonÒ, and it was expected their life-time might be
much more than 500 h.
The target held a negative bias up to 75 kV, above
which it was limited by a spark discharge. Though the
spark discharge was not serious when the beam current
was less than a few mA, it worsened as the current
increased. Hence, the range of available deuteron beam
current was 0.8–8 mA, and the maximum energy of the
deuteron beam was 97.5 keV. The performance of the cooling target was tested for the 1 mm-thick target by a computational method using the CFX-5 code [19], the result of
which is reported separately [20]. The cooling performance
was still poor. It was expected that the hot spot temperature at the target would increase from 400 to 800 °C with
increasing beam power of 500–1000 W. Hence it exceeded
the critical temperature of 250 °C at which outgassing of
hydrogen from a titanium hydride target takes place [5].
The operation of the neutron generator was fairly stable
during test-runs over several hundred hours. A result of a
typical neutron generation run is shown in Fig. 4. The
energy of the beam was increased from 55.0 to 96.0 keV
stepwise, and the neutron generation rate increased up to
1.5 108 n/s. The neutron generation rate was continuously monitored every minute, while no serious high voltage breakdowns were recorded that could have caused
drops in the neutron generation rate. After a change in
the beam energy, a slow drift of the neutron generation rate
was observed, as shown in Fig. 4. The formation and
behavior of the deuterons in the target was of concern
but are less known. The concentration profile of deuterons
is mainly determined by the range distribution and transport of the deuterons in the target. The two phenomena
were closely related to the beam energy. The change of
the beam energy altered the concentration profile of the
deuterons which are stopped in the target and also the target temperature. To this end, the neutron generation rate
slowly drifted with changes of the beam energy and became
saturated as the concentration profile of the target nuclei
reached equilibrium. It took less than tens of minutes for
the drifting generation rate to become saturated, after
which the rate was nearly constant. The slow variation of
the neutron generation rate after changes of the beam
energy was also observed by Reijonen et al. [21].
The neutron generation efficiency was evaluated by
comparing the measured rate to the theoretical maximum
rate expected in the idealistic case of a 100% monatomic
deuteron beam irradiating a target which has been maximally deuterated (TiD2). The generation efficiency, however, rarely exceeded 10% during the test-runs. Two
factors were considered to attribute to the low neutron generation efficiency. The first factor was the monatomic fraction of the deuteron beam. In a previous study on the ion
source, a proton beam with a high monatomic fraction of
94% was produced [13]. On the other hand, the monatomic
fraction of the deuteron beam was degraded during the
test-runs because the operation of the ion source was not
feasible in the same high RF-power region due to the instability of plasma. Considering the main atomic reactions in
the plasma, it was estimated that the monatomic fraction of
the beams must not exceed 60% [13]. According to the
work by Kim [6], the neutron generation rate would be
reduced by 30% if a deuteron beam composed of 60% Dþ
1
and 40% Dþ
2 ions instead of a 100% monatomic deuteron
beam were irradiated on a uniform titanium deuteride target. The second factor was the loaded amount of the deuteron target nuclei and the shape of the concentration
profile, because most of the neutrons were generated at
the surface layer and hence the generation rate was proportional to the amount of target nuclei in the surface region.
An average concentration of target nuclei was estimated
based on the measured rate of 1.6 108 n/s as shown in
Fig. 4. Measured neutron generation rate during a neutron generator run
(deuteron beam current: 7.8 mA, target thickness: 1 mm).
I.J. Kim et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 829–833
Fig. 4. Here it was assumed that the concentration profile
of the target nuclei would be uniform and the deuteron
þ
beam would be composed of 60% Dþ
1 and 40% D2 ions.
The resulting concentration was 0.23 deuterons per titanium atom. Hence, it was suspected that the concentration
of the target nuclei did not fully lead to titanium dideuteride and also that the concentration at the surface layer
was depleted. Additionally, the overheated and oxidized
target surface has caused such a low concentration. Since
a maximum temperature of 600 °C was calculated at the
target at the rate of 1.6 108 n/s [20] and a thick oxide
layer has been observed on the sample target via AES
(Auger Electron Spectroscopy) after the run. The high temperature dilutes the concentration of the target nuclei by
accelerating the diffusion and degassing processes. Oxidation lowers the concentration of the target nuclei at the surface by detrapping the deuterons [22] or by retarding the
migration of the target nuclei into the surface region
[23,24]. The target examined with AES had been run at a
low ion beam power range of 120–170 W for 7 h. On the
target surface, carbon and silicon were also observed but
the amounts were negligible. Here carbon originated from
the residual gases in the vacuum system [25], and Si was
suspected that it came from the quartz plasma discharge
tube. The neutron generation rate was slightly enhanced
by 5–30% by using the 0.5 mm-thick target. A maximum
neutron generation rate of 1.9 108 n/s was achieved using
this target when the beam current and the beam energy
were 7.6 mA and 94.0 keV, respectively.
4. Conclusions
A D–D neutron generator was developed using a helicon plasma ion source and titanium drive-in target. The
range of the deuteron beam current was 0.8–8 mA, and
the beam could be accelerated up to 97.5 keV. The operation of the neutron generator was fairly stable and not
altered by high voltage breakdowns during test-runs. A
neutron generation rate of 1.9 108 n/s was achieved by
irradiating a 7.8 mA deuteron beam on a titanium drivein target at 94.0 keV.
A neutron generation rate exceeding 108 n/s is comparable to that obtained by commercial D–T neutron tubes.
However, the neutron generation efficiency rarely exceeded
10%, compared to the idealistic case of a 100% monatomic
deuteron beam irradiating on a target deuterated maximally (TiD2). To further enhance the neutron generation
rate, more work is required to stably operate the ion source
in the high power region and to improve the cooling of the
target.
The slow drift of the neutron generation rate at a given
energy was qualitatively understood as a result of re-equil-
833
ibration of the concentration of the target nuclei following
the change of beam energy by the transport mechanism. A
quantitative study on the transport mechanism of deuterium inside titanium is under way with slow progress
because many factors are involved in the governing transport equation.
Acknowledgement
This research was performed under a program of the Basic Atomic Energy Research Institute (BAERI), which is a
part of the Nuclear R&D Programs funded by the Ministry
of Science & Technology (MOST) of Korea.
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