Characteristics of a RF-Driven Ion Source for a Neutron Generator Used For Associated Particle Imaging Page: 1 of 6
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Characteristics of a RF-Driven Ion Source for a Neutron
Generator Used For Associated Particle Imaging
Ying Wu",2, Paul Hurley3, Qing Ji', Joe Kwan', Ka-Ngo Leung",2
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720
Department of Nuclear Engineering, University of California-Berkeley, Berkeley, CA 94720
3Special Technologies Laboratory, Santa Barbara, CA 93111
Abstract. We present recent work on a prototype compact neutron generator for associated particle imaging (API). API
uses alpha particles that are produced simultaneously with neutrons in the deuterium-tritium (2D(3T,n)4a) fusion reaction
to determine the direction of the neutrons upon exiting the reaction. This method determines the spatial position of each
neutron interaction and requires the neutrons to be generated from a small spot in order to achieve high spatial
resolution. The ion source for API is designed to produce a focused ion beam with a beam spot diameter of 1-mm or
less on the target. We use an axial type neutron generator with a predicted neutron yield of 108 n/s for a 50 A D/T ion
beam current accelerated to 80 kV. The generator utilizes a RF planar spiral antenna at 13.56 MHz to create a highly
efficient inductively-coupled plasma at the ion source. Experimental results show that beams with an atomic ion
fraction of over 80% can be obtained while utilizing only 100 watts of RF power in the ion source. A single acceleration
gap with a secondary electron suppression electrode is used in the tube. Experimental results, such as the current
density, atomic ion fraction, electron temperature, and electron density, from ion source testing will be discussed.
Keywords: Neutron generator, RF plasma ion source, Associated particle imaging
PACS: 29.25.Dz, 52.77.-j, 52.50.Dg, 52.50.Qt, 52.59.Ye, 29.25.Lg
Neutron generators that utilize if-driven plasma ion
sources have been developed at the Lawrence
Berkeley National Laboratory (LBNL) [1-6]. These
neutron generators are useful in the interrogation of
cargo or luggage containers, which may contain
explosives, contraband, or special nuclear materials.
Here, the neutrons are typically produced by the
deuterium-deuterium (Eqn. 1) or the deuterium-tritium
(Eqn. 2) fusion reactions with a fairly large ion source
that produces up to tens of mA of D/T ion current.
The ions are accelerated to energies of 100 keV or
higher towards a metal hydride target which can
produce neutron yields of over 109 n/s for D-D and
1011 n/s for D-T.
2D+2D-*1n+3He (E =2.45MeV, EH =0.82 MeV) (1)
2D+ 3T-> 'He +'n, (En =14.1MeV, E, =3.5MeV) (2)
These neutron generators require large power
supplies as well as active water-cooling that makes
transportation difficult. Therefore, the development of
a compact neutron generator with a moderate yield
that is easily portable would provide greater flexibility
in the detection of contraband materials. Associated
Particle Imaging (API) is a special type of active
neutron interrogation that uses the deuterium-tritium
neutron production reaction and takes advantage of the
alpha particle that is produced in the reaction to track
the neutron trajectory . Since the alpha particle
travels in the opposite direction of the neutron,due to
conservation of linear momentum, the use of a position
sensitive alpha detector can provide the neutron
direction and D-T reaction time. This limits the
interrogation neutrons to those that are tagged by the
alpha detector traveling in the opposite direction of the
alpha particles and acts as a virtual collimation for the
isotropically produced neutrons . Once the neutron
reaches the target material, it can be inelastically
scattered and produces characteristic gamma rays,
which are then detected. The time between the
detection of an alpha particle and the detection of a
gamma ray provides the neutron flight time which can
be used to locate the neutron interaction. Using this
method, the location of the target material and its
identity can be determined . Currently, commercial
API neutron generators utilize Penning ion sources
that are compact and low on power consumption, but
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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. (https://digital.library.unt.edu/ark:/67531/metadc1013252/m1/1/?rotate=270: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.