Characteristics of a RF-Driven Ion Source for a Neutron Generator Used For Associated Particle Imaging Page: 2 of 6

have a low efficiency because the ions produced are
mostly molecular [10]. For these Penning ion sources,
the life-time of the extraction aperture is also an issue.
These sources have high anode voltages which
accelerate the ions towards the cathode and cause
sputtering of the extraction aperture and produce an
increasingly larger beam spot as time goes on. Having
a small ion beam diameter is an important parameter in
API because it determines the x-y resolution of the
resultant image. Current API neutron generators can
only achieve an initial beam diameter of about 2-mm,
that will continue to increase as the operation of the
source progresses. For neutron generators where the
beam spot size does not matter, a low atomic ion
fraction in the ion source can be overcome by
increasing the ion current and beam energy. However,
since API requires a small beam spot, increasing the
beam current and beam energy will result in an
increase in the power density on the target surface.
The current API neutron generator using the Penning
source has a power of nearly 6 W, which would result
in a power density 700 W/cm2 for a 1-mm beam spot,
which is too high to be cooled effectively by air, and
the neutron yield would decrease because the
implanted deuterium/tritium ions would diffuse out of
the target surface. Thus, the only way to increase the
neutron yield while decreasing the beam spot size
simultaneously is to increase the atomic ion fraction,
and an increase from 10% to 90% of atomic ions while
keeping the beam current and energy constant would
increase the neutron yield by a factor of 4 [11] with the
operating conditions listed for API in Table 1. The
API neutron generator currently being developed will
utilize a RF-driven ion source which can provide over
90% atomic ions with a 1-mm beam spot diameter
with a power density of 200-400 W/cm2, which can be
cooled by air. Table 1 shows the operating parameters
of the RF-driven neutron generator and the operating
parameters of the commercially available API neutron
generator. The RF-driven neutron generator is being
developed specifically to increase the atomic ion
species so that the neutron yield can be made larger
and the beam spot made smaller without over heating
the target while also keeping the power consumption
and gas pressure low in the ion source.
TABLE 1. Comparison of RF and Penning ion sources
Ion Source: RF Penning
Power consumption: 100-125 W 50 W
Ion beam current: 20-50 gA 60 gA
Ion beam energy: 80 kV 95 kV
Beam spot diameter: 1-mm 2-3 mm
Atomic ion fraction: > 80% < 10%
Neutron yield: 108 n/s 107n/s
Power at target: 1.6-4 W 5.7 W

A prototype of the API neutron generator
schematic can be seen in Fig. 1. The RF-driven ion
source is mounted on the left and the beam is extracted
and accelerated towards the target on the right side.
The alpha and neutron trajectories are drawn on the
schematic to illustrate that this technique uses the
alpha detector to tag the neutrons produced that are
going in the opposite direction of the detector.
b0 aGnana
Plasma etector,
electrode energy and
(grounded) Shroud electrode (-80 time
kW) for electron
Multi-cusp suppression
magnets PVC Insulator,
m corrugated
RF D/T ion rings/donuts
D/T beam Ti trge -'78
Air cooling
Sa for target
Quartz source frtre
body Position sensitive alpha detector, gives start
time of alpha hit and x-y coordinates
FIGURE 1. The prototype API neutron generator schematic is shown
along with the trajectories of the neutrons and alphas to be used in
inspection. The rf-driven ion source is located on the left and the beam
is extracted and accelerated towards the target, which is negatively
The tagged neutrons typically make up about 4 to 10%
of the total neutrons produced. The power
consumption, operating and plasma ignition pressure,
atomic ion specie concentration, source cooling, size,
and source durability are all factors which must be
taken into consideration when constructing the ion
source. In commercial compact neutron generators,
the ion source used is a Penning ion source which
operates at low pressures and powers, but has very low
atomic ion species (<10%) [10]. The RF ion source
has been used extensively in high power, high yield
neutron generator situations with very high atomic ion
fraction and current density, but requires larger power
supplies and extensive water-cooling on the source and
target end. The if ion source used in this API neutron
generator is an improved compact RF-driven ion
source which can produce high atomic fraction at low
power consumption. It is 11 cm in diameter and 3.2
cm in length with 20 surrounding permanent magnet
columns. The source chamber is made of quartz with
an aluminum front plasma electrode. The RF power is

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Wu, Ying; Hurley, John P.; Ji, Qing; Kwan, Joe & Leung, Ka-Ngo. Characteristics of a RF-Driven Ion Source for a Neutron Generator Used For Associated Particle Imaging, article, August 8, 2008; Berkeley, California. ( accessed March 19, 2019), University of North Texas Libraries, Digital Library,; crediting UNT Libraries Government Documents Department.

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