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PAPER • OPEN ACCESS
Production of negatively charged radioactive ion
beams
To cite this article: Y Liu et al 2017 New J. Phys. 19 085005
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This content was downloaded from IP address 168.151.130.120 on 27/11/2017 at 15:50
New J. Phys. 19 (2017)085005 https://doi.org/10.1088/1367-2630/aa609c
PAPER
Production of negatively charged radioactive ion beams
*
Y Liu
1,3
, D W Stracener
1
and T Stora
2
1
Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States of America
2
ISOLDE, CERN, CH-1211 Geneva 23, Switzerland
3
Author to whom any correspondence should be addressed.
E-mail: liuy@ornl.gov
Keywords: radioactive ion beam, ISOL production, negative ion source, beam purification, HRIBF, ISOLDE
Abstract
Beams of short-lived radioactive nuclei are needed for frontier experimental research in nuclear
structure, reactions, and astrophysics. Negatively charged radioactive ion beams have unique
advantages and allow for the use of a tandem accelerator for post-acceleration, which can provide the
highest beam quality and continuously variable energies. Negative ion beams can be obtained with
high intensity and some unique beam purification techniques based on differences in electronegativity
and chemical reactivity can be used to provide beams with high purity. This article describes the
production of negative radioactive ion beams at the former holifield radioactive ion beam facility at
Oak Ridge National Laboratory and at the CERN ISOLDE facility with emphasis on the development
of the negative ion sources employed at these two facilities.
1. Introduction
The study of radioactive nuclei far from stability is at the very frontier of nuclear science [1,2]. Research with
these exotic nuclei addresses a wide range of important topics in fundamental nuclear science, including the
underlying nature and stability of atomic nuclei, the origin of elements, the evolution of the cosmos, and tests of
current models for the fundamental interactions that are basic to the structure of matter. It also provides the
scientific foundation for innovative applications for society such as discovering new radionuclides for cancer
diagnosis and treatment, detecting nuclear contraband at ports of entry, and assessing the safety of our nuclear
stockpile. The experimental research in this frontier relies heavily on the range and intensity of radioactive ion
beams (RIBs)available in the laboratory, which has provided the impetus for worldwide construction of RIB
facilities dedicated to providing beams of short-lived nuclei for fundamental and applied research. A number of
reviews summarize the recent progress in upgrading the capabilities of existing RIB facilities and constructing
next-generation RIB facilities throughout the world [3–6].
Two complementary methods of RIB production are commonly used: in-flight fragmentation [6,7]and
isotope separator on-line (ISOL)[8,9]. The in-flight method is based on a heavy ion beam, accelerated to high
energies to bombard a relatively thin production target. Interaction of the beam particles with the target nuclei
results in a wide range of radioactive nuclei via projectile fragmentation and in-flight fission. A system of large-
acceptance dipole magnets then separate out the rare isotopes of interest for use in experiments. In the ISOL
approach, a light ion beam (typically protons, deuterons, or helium isotopes)from a driver accelerator strikes a
thick production target producing radioactive nuclei via nuclear reactions. The target material is kept at high
temperature, enabling the radioactive species to diffuse out of the target and subsequently effuse to an ion source
to be ionized, extracted, and accelerated to a mass separator where the beam of interest is selected. A post-
accelerator is often needed to further accelerate the radioactive ions to desired energies for experiments. The in-
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REVISED
20 January 2017
ACCEPTED FOR PUBLICATION
15 February 2017
PUBLISHED
24 August 2017
Original content from this
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and DOI.
*
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the US Department of Energy.
The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States
Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this
manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these
results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-
access-plan).
flight method requires one high-energy, heavy-ion accelerator and, since the process is independent of
chemistry, it is universal and fast. The fragment ions are produced and separated at velocities similar to that of
the primary beam, so rare isotopes with half-lives down to microseconds can be obtained. However, if the
radioactive ions need to be slowed down for the particular experiment, the resulting beams have large emittance
and large energy spread. In contrast, the ISOL method relies on diffusion and effusion processes in the target and
ion source, and the associated delay times make beams of short-lived isotopes (<1s)much more difficult to
produce, especially for refractory and chemically reactive elements. However, with post-extraction acceleration,
the ISOL technique can be used to provide intense RIBs with excellent beam quality and well defined and
variable energies that are required for detailed studies in nuclear structure and nuclear astrophysics. Because of
these unique research opportunities, a number of RIB facilities are based on the ISOL technique [4,10–13].
The ion sources used at ISOL facilities are compact and are designed for high efficiency for specific elements,
with the ability to ionize trace quantities of radioisotopes in a high background of other elements [14,15]. While
ion sources for stable elements generally focus on production of high ion currents, ion sources for RIB
generation focus on selectivity and efficiency due to the often extremely low production rates of the radioactive
isotopes of interest. Furthermore, compact and rugged designs capable of stable operation over a few weeks in
harsh radiological and thermal environments are required. In most facilities, positive-ion beams are used either
at low energies directly from the ion source or injected into an accelerator such as a linac or a cyclotron. Negative
ion generation is of particular importance for RIB facilities employing tandem electrostatic accelerators for post-
acceleration, owing to the fact that these accelerators require negative-ion injection. Beams from a tandem
accelerator have precisely controlled energies, good energy resolution, and excellent beam quality (low
emittance and small beam size)and, hence, are ideally suited to study reactions of fundamental importance in
nuclear physics and nuclear astrophysics. A particular example is the capability to search for resonances in
reaction cross-sections relevant to explosive nucleosynthesis in astrophysical systems such as novae, supernovae,
and x-ray bursts [16]. This unique capability has led to the construction of a few ISOL facilities designed with a
tandem as the post-accelerator [8]including the former HRIBF at ORNL [12].
Venezia and Amiel [17]first demonstrated the production and separation of negative ions of radioactive
iodine isotopes from fission products by surface ionization and electromagnetic separation at the Soreq on-line
isotope separator. In their initial work, the graphite catcher for fission fragments acted as the surface ionizer and
the yields of negative iodine ions were very low. One of the reasons for the low ion yields was the relatively high
work function of the graphite. A negative surface ionization source with low work function material LaB
6
as the
ionizer was later developed [18]and used to study the fission yields of short-lived bromine and iodine isotopes
[19]. Low yields of negative radioactive ions of Br and I were also obtained from a UO
2
-graphite surface [20]for
delayed neutron emission studies at the SOLAR on-line mass spectrometer facility [21].
The first on-line production of intense negative RIBs of halogens [14,22]was obtained with an efficient
negative surface ionization source at the ISOLDE RIB facility for research in solid state and nuclear physics
[23,24]. The ion source was equipped with a planar sintered porous LaB
6
ionizer and demonstrated 10%–50%
ionization efficiencies for Cl, Br, I and At. A similar LaB
6
ionizer configuration was employed in a source to
provide halogen isotopes for nuclear spectroscopy at the on-line mass separator OSTIS [25]. The early
experiments at ISOLDE revealed that the LaB
6
ionizer was susceptible to poisoning when it was used with
inappropriate target materials decomposing at high temperatures. Since then, some progress has been made in
mitigating the drawbacks and improving the performance of the ISOLDE negative surface ionization source by
various means, including new ionizer geometries, new low work function ionizer materials, and proper choice of
target materials [24,26]. Recently at ORNL (in early 2016)an ion source with a LaB
6
ionizer was used for several
weeks to provide pure beams of radioactive bromine and iodine for studies of total decay heat in fission products
and for measurements of the production rate of antineutrinos in the decay of fission fragments.
The former HRIBF used a 25 MV tandem for post-acceleration with the unique capability to accelerate RIBs
to energies above the Coulomb barrier for nuclear physics experiments [12]. Both positive-ion and negative-ion
sources were developed or adapted at HRIBF for this research program. Although negative RIBs were commonly
produced by charge exchange in a Cs-vapor cell to convert initially positive ion beams to negative ion beams
prior to injection into the tandem accelerator, negative ion generation with a negative ion source was preferred
for some RIBs. In spite of the limitation in ion-species capability, negative ion sources offer distinct advantages
to perform research activities with RIBs [14]. For example, direct production of negative ions eliminates the
charge exchange process to transform positive ions into negative ions, which is generally inefficient and often
results in a rather large, undesired energy spread in the resulting negative beams due to multiple scattering of the
ions in the charge exchange cell volume (see section 2). Since, in many cases, the purity of the RIB is crucial to the
success of the experiment, selectivity in the ion source can be very useful. Negative surface ionization sources are
highly selective—only the elements with high electron affinities can be efficiently ionized. This property has
been used to great advantage to obtain high purity negative RIBs for certain elements, such as the halogens, for
which the positively charged counterparts are often contaminated by isobars from neighboring elements and
2
New J. Phys. 19 (2017)085005 Y Liu et al
molecular sidebands. For these reasons, extensive research effort has been devoted to the development of
negative ion sources for specific RIBs of interest [12,14].
The present paper describes the research and development that has been conducted at the former HRIBF
and the ISOLDE facility for the production and purification of negative RIBs for fundamental and applied
research. Section 2provides brief descriptions of the methods of negative ion formation with emphasis on the
specific ion source techniques employed for negative RIB production at these two facilities. Section 3provides a
review of negative RIB production at the former HRIBF. Section 4presents the specific negative ion sources
developed at HRIBF and ISOLDE as well as the latest results of negative RIB production at ISOLDE. Section 5
discusses beam purification methods based on negative ions. Finally, future developments and new concepts for
the production of negative RIBs are discussed in section 6.
2. Methods of negative ion formation
Negative ions can be produced by double charge-exchange of positive ions produced with a positive ion source
or by direct extraction from a negative ion source. For the production of negative RIBs, charge-exchange has
been the most frequently used method. The charge-exchange process is considered to be a two-step process in
which positive ions sequentially capture two electrons as they interact with a vapor target of low ionization
potential (IP)metal, such as Cs or Na. The yield of negative ions depends on the collision energy (beam energy),
the electron affinity (EA)of the projectile element, the IP of the target element, and the density of the exchange
vapor. In general, highest yields are expected for the projectile-target combinations with minimum energy
defects in both of the electron-capture processes [27–31]. Table 1lists the charge-exchange efficiencies measured
at the HRIBF for some elements with a Cs-vapor charge-exchange cell (CEC). As noted, the highest efficiencies
for producing As
−
(EA =0.81 eV),Sn
−
(EA =1.22 eV), and Se
−
(EA =2.02 eV)are respectively, 42%, 41%, and
22%, with optimum projectile energies ranging from 20 keV to 50 keV, while the best efficiency for Br
−
(EA =3.37 eV)beams was only 1.2%. Extensive studies of negative ion formation by charge transfer in metal
vapors have been reported [30]. These studies show optimal conversion efficiencies of ∼0.5 to >90% for various
elements considered. In particular, Heinemeier et al reported 2%–6% efficiencies for elements with very low EA
values, such as Be
−
(EA =0.24 eV, metastable),B
−
(EA =0.28 eV)and Fe
−
(EA =0.25 eV)using Na or Mg
vapors and a fixed projectile energy of 20 keV. These results indicate that useful intensities of negative RIBs can
be obtained for many elements with a proper selection of the charge exchange vapor and the ion energy.
Although positive ion sources in combination with a CEC is a versatile means of generating negative RIBs, as
pointed out earlier, negative ion sources are desired for certain radioactive species for higher beam intensities
and better beam purity. The major challenge associated with radioactive beam production at an ISOL facility
concerns the required fast release and efficient ionization of radioactive trace products from the target-and-ion-
source unit within a much larger vapor load of impurities from the target and structural materials.
Consequently, the ion source should ideally exhibit high efficiency, high temperature operation, simple design,
radiation resistance, and long lifetime. Although a variety of negative ion sources are available, a limited number
of them can meet these criteria. The most successful and widely employed for RIB applications has been
negative-ion sources based on surface ionization. Sputter-type negative ion sources have also been used for some
Table 1. Measured charge exchange efficiencies with Cs-vapor
(unpublished data from HRIBF)for selected elements.
Element EA (eV)
Conversion effi-
ciency (%)
Beam
energy (keV)
As 0.81 42 20
30.7 40
Se 2.02 22 20
16.6 40
Sn 1.22 41 50
Br 3.37 0.6 20
0.8 40
1.2 50
Sr 0.05 0.6 20
0.9 40
Rb 0.498 0.5 50
Note. Column 2 lists the electron affinity (EA)of the element. Column 3
gives the measured efficiency for conversion into negative ions for the
specific positive-ion beam energy shown in Column 4.
3
New J. Phys. 19 (2017)085005 Y Liu et al
specific beams such as fluorine and for isotopes with relatively long lifetimes. The basic principles of these
negative ion sources are discussed below, while the detailed descriptions of the specific sources are given in
section 4.
2.1. Negative surface ionization
When neutral atoms or molecules come into the proximity of a hot metal surface, a certain fraction of them will
be thermally desorbed as positively or negatively charged ions, depending on the electropositive or
electronegative nature of the neutral species. The underlying physics and theory of this surface ionization
process have been extensively studied and reviewed [32–34]and will not be described in detail here. Briefly, for
the formation of negative ions, under thermodynamic equilibrium conditions, the degree αof ionization for
neutral particles of EA E
a
evaporating from an ideal surface with a work function fat temperature Tis
quantitatively given by the Saha–Langmuir equation:
exp , 1
n
n
g
g
E
kT
iia
00B
a==
f-
()
()
where n
i
and n
0
are the numbers of the ions and neutrals respectively leaving the surface in unit time, g
i
and g
0
are
the statistical weights of the ionic and neutral states, respectively, and k
B
is the Boltzmann constant. The
ionization efficiency βis defined as the ratio of the negative ion flux leaving the surface to the incident neutral
flux and is given by
1exp . 2
g
g
E
kT1
1
i
a
0
B
b==+
a
a
f
+
--
⎡
⎣
⎢⎤
⎦
⎥
()
()
These equations show that for high ionization efficiencies E
a
>fis required. If E
a
−f>k
B
T, the negative
ion formation process is exothermic and β∼1. This requirement could be met for high EA elements such as
halogens. In fact, negative surface ionization has been a preferred method for halogen RIBs because of the high
efficiency and high purity due to low ionization efficiency for neighboring elements, such as noble gases and
alkali metals.
LaB
6
has a low work function in the range of 2.4–3.0 eV [35,36]and has been most frequently used as the
ionizer material for generating negative halogen beams. Since the majority of the elements in the periodic table
have electron affinities less than the work function of LaB
6
, it cannot provide good negative ion yields for most
elements. In addition, LaB
6
is known to dissociate and evaporate at temperatures of about 1400 °C under
vacuum and is prone to poisoning from impurities outgassing from the target material. Therefore, it is extremely
desirable to find materials with lower work functions that can maintain their properties at high temperature with
variable impurity loads. New ionizer materials have been investigated [26]at ISOLDE, including gadolinium
hexaborides (f=1.5 eV), alloys of iridium and cerium (f=2.6 eV), and tungsten impregnated with low work
function oxides such as BaO and SrO (f=1.0 eV). It is well known that the work function of a metal surface can
be lowered by adsorption of minute amounts of low work function materials such as the alkali metals. This
approach was also explored [37]in a source equipped with a permeable Ir ionizer and controlled feeding of a
highly electropositive vapor such as Cs through the Ir matrix to decrease the surface work function for negative
ion generation.
Equation (2)gives the surface ionization efficiency for a planar surface. For a tubular (cavity)geometry,
provided that the neutral isotopes have the possibility to interact several times with the cavity material and have a
high probability to be captured in the plasma well before extraction, significantly higher efficiencies for all
halogens are predicted. The enhancement is expressed as Nin a modified Saha–Langmuir expression in
equation (3)[38]:
N1exp . 3
N
N
g
g
E
kT1
1
i
a
0
B
b==+
a
a
f
+
--
⎡
⎣
⎢⎤
⎦
⎥
()
()
The theoretical Saha–Langmuir ionization efficiencies for planar and tubular LaB
6
ionizer configurations are
reported for some representative parameters in figure 1,reflecting the two types of ion sources that have been
operated online at ISOLDE with pulsed proton beam from the PSBooster [26]. The variation in the effective
work function of LaB
6
surface, from a nominal value of 2.6–2.9, and 3.4 eV, reflects the possible variations of the
material properties and poisoning by impurities in the target-ion source unit and has a strong influence on the
predicted efficiencies. As shown for Astatine this variation can lead up to a 500-fold change in the ionization
efficiency. In contrast, a variation of 200 K of the LaB
6
surface temperature will minimally affect the efficiencies if
the work function is constant. For a planar ion source (MK4), the maximum achievable ionization efficiency of
50% comes from a geometrical ‘view’factor of the ion source configuration-about 50% probability for the
isotopes to escape the target-ion source unit without interaction with the LaB
6
ionizer. A high efficiency is
predicted for cavities with work functions of 2.6 eV, e.g., approaching 100% for halogens, and reasonable
efficiencies are obtained even when poisoning is observed to degrade the work function of the LaB
6
ionizer up to
4
New J. Phys. 19 (2017)085005 Y Liu et al
3.4 eV. Iodine and astatine are shown to be the most sensitive and challenging elements to ionize owing to their
low electron affinities with respect to the ionizer materials used. It is noted that these calculations are for
ionization efficiency only, independent of other properties such as diffusion from the target matrix, chemical
reactivity, sticking times to interfaces, and molecule formation.
2.2. Sputter generation of negative ions
In sputter-type negative ion sources, negative ions are also formed on a low work function solid surface.
However, the ion formation process is most commonly accepted as a two-step process that involves first the
ejection of atomic and/or molecular particles from the surface via knock-on sputtering and then electron
transfer between the surface and the ejected particles. The ionization of the sputtered particles is complicated by
the dependence on the characteristic properties of the outgoing particles, the emitting surface, and the projectile
ions as well as the mechanism of the ejection process. Various models have been proposed [41]for a theoretical
description of the ionization process, including an electron tunneling model, a bond-breaking model, and a
substrate-excitation model. However, there is still no theoretical model that can predict the secondary ion
probability with sufficient accuracy for practical applications. An exponential relationship scaling of the
ionization probability P
−
of sputtered negative ions with the work function of the surface is predicted by the
theoretical treatments and observed experimentally [42]
Pexp , 4
E
a
0
µ-
f
e
--
()
()
where ε
0
is a characteristic parameter of the ion emission process and is reported to depend on the normal
component of the ion emission velocity [43,44]. Similar to negative surface ionization, P
-
can be greatly
enhanced by lowering the work function of the sputter site, which can be achieved with the addition of a sub-
monolayer of alkali atoms such as cesium to the surface [45].
Cs-sputter negative ion sources are more versatile than negative surface ionization sources, but are
unsuitable for ISOL generation of short-lived radioactive beams. The sputter target is also the production target
for the radioactive species, so the area to be sputtered should match with the area irradiated by the production
ion beam. Consequently, the primary production beam must overlap with the Cs-sputter ions and in turn with
the resulting negative radioactive beam. This would adversely affect negative ion generation and extraction. On
the other hand, Cs-sputter sources can be used for batch-mode generation of relatively long-lived RIBs where
production and ionization are performed independently (see section 4.3). In fact, the batch-mode technique is
one of the early approaches for producing negative ion beams of isotopes with lifetimes longer than a few hours
[46–51]. A multi-sample Cs-sputter ion source was developed at HRIBF for batch-mode generation of RIBs of
long-lived isotopes [52,53]. Chemically active radioactive species are often released from target materials in a
variety of molecular forms, for example, fluorine isotopes were found to be released from Al
2
O
3
production
targets primarily as AlF. Since the species of interest may be distributed in several mass channels in the form of
these molecular sidebands, beam intensities of the desired radioactive species are diluted. The sputter method
Figure 1. Theoretical ionization efficiencies (solid and dashed lines)calculated with equations (2)and (3)for the planar and tubular
configurations noted ‘MK4’and ‘tub’in the legend. The symbols represent measured data from the indicated ion sources. The work
functions and temperatures are displayed in the legend. The value of g
i
/g
0
is approximated at 0.5 for these simulations. The poorly
known electron affinity of Astatine is taken at 2.31 eV ([39]and Rothe, private communication). For a tubular configuration, N=20
based on previous studies with positive tubular surface ion sources [40].
5
New J. Phys. 19 (2017)085005 Y Liu et al
for negative ion formation is found to be an effective means for simultaneously dissociating molecular carriers
and efficiently ionizing highly electronegative atomic constituents present in the molecule. A unique Cs-sputter
negative-ion source was conceived [54,55]at HRIBF that could solve the molecular sideband problem for
17,18
F
beams. Details of these sputter-type ion sources are reported in section 4.
3. Production of negative RIBs at HRIBF
The HRIBF [12]at ORNL was a Department of Energy User Facility with a mission to deliver high-quality RIBs
using the ISOL technique. The ISOL technique was implemented at HRIBF (figure 2)with the following major
components: the Oak Ridge Isochronous Cyclotron (ORIC)driver accelerator, two RIB production systems—
Injectors for Radioactive Ion Species (IRIS1 and IRIS2), and the 25 MV tandem electrostatic accelerator for RIB
post-acceleration. The ORIC provided a light-ion beam (proton, deuteron, or alpha)which was directed onto a
thick target mounted in a target-ion source (TIS)assembly located on IRIS1 or IRIS2. Radioactive atoms or
molecules that diffused from the target material were transported to the ion source where they were ionized,
extracted, and formed into a beam. Typically, the RIB (either positive or negative ions)was accelerated from the
ion source at an energy of 40 keV and passed through the first-stage mass analyzer which selected beams of a
single mass with resolving power of ∼600:1 (FWHM). The mass-selected beam could then be sent through an
optional Cs-vapor charge exchange cell before being further accelerated to 200 keV energy from the high voltage
production platform and passed through the second-stage mass analyzer (isobar separator)with a mass
resolving power MMD
(
)up to 100 00/1 depending on the emittance of the incoming beam. Next, the beam of
negatively charged ions was injected into the 25 MV tandem for post acceleration. Alternatively, beams of either
charge could be transported to the low-energy RIB spectroscopy station [56]where the decay properties of the
isotopes were studied.
Negative ions at 200 keV were injected into the tandem electrostatic accelerator, which has a folded-
geometry and has been operated with beam at terminal potentials up to 25.5 MV (highest in the world), and as
low as 1 MV with excellent reliability. At the terminal, electrons were stripped away from the beam particles
using thin carbon foils or a dilute gas. The resulting positive ions were selected based on energy, mass, and charge
state by a 180°dipole magnet and subsequently accelerated down the high-energy side of the tandem. For light
nuclei (A<80), single-stripping allowed for acceleration up to at least 5 MeV per nucleon. For heavy ions
(A>80), a second foil stripper could be inserted at about a third of the way down the high energy column to strip
to higher charge states to achieve higher energies. This capability enabled HRIBF to be the first RIB facility to
accelerate A=130 neutron-rich fission fragments above the Coulomb barrier. A 90°energy-analyzing dipole
magnet mounted on a rotating platform at the base of the tandem high-energy tube selected the beam of interest
and directed it into one of several beam lines for nuclear physics experiments.
Based on the capabilities of the driver cyclotron, ORIC, several types of RIB production targets were
developed for the HRIBF [57]. ORIC provided proton beams up to 54 MeV, deuteron beams up to 49 MeV, and
4
He beams up to 85 MeV with intensities up to 20 μA for proton, 15 μA for deuteron, and 5 μA for
4
He on target.
For neutron-rich RIBs, a uranium carbide target was developed to produce fission fragments via proton-
Figure 2. HRIBF layout showing the major ISOL components and RIB experiment stations.
6
New J. Phys. 19 (2017)085005 Y Liu et al
induced fission. While several uranium carbide target geometries and formats were developed, this was
essentially a single target for many radioactive beams. On the other hand, radioactive beams on the proton-rich
side of the nuclide chart required the development of several targets. Due to the low energy of the driver beam,
the radioactive nuclei were produced with Aand Zvery close to the target nucleus, so unique production targets
were developed for each element, including hafnium oxide targets for fluorine beams, silicon carbide targets for
aluminum beams, and liquid germanium targets for arsenic beams. In all cases, the production targets and the
ion sources developed at the HRIBF were designed such that any production target could be used with any ion
source, so the best combination for a particular experiment could be used. Negative-ion sources were used
extensively with oxide targets for fluorine beams and with uranium carbide targets to provide pure beams of
bromine and iodine isotopes from fission.
The ISOL production technique was used at HRIBF to produce beams of more than 200 radioactive isotopes,
including more than 175 neutron-rich species from fission of uranium (see figure 3). Many of these were post-
accelerated to energies required for nuclear physics experiments (several MeV per nucleon)while others were
used at low energies (40–200 keV)for decay studies (figure 4). More than 50 of those RIBs, including doubly
magic
132
Sn, were available at intensities of 10
6
ions per second or greater. A careful comparison of figures 3and
4show a number of isotopes that are available as low-energy beams but are not available as post-accelerated
beams. This is due to the fact that it is not possible to make a negative ion for some elements, including noble
gases and specific elements such as Zn and Cd. The large gap in available beams from Zr to Pd is a general short-
coming of the ISOL technique—these elements have very low vapor pressures even at high temperatures and
thus will not diffuse out of the production target.
Most of the negative-ion beams at HRIBF were produced using a standard charge-exchange method in
which radioactive species were first ionized as positive ions and then passed through a cell filled with Cs vapor to
form a negatively charged ion beam. The most commonly used positive-ion source was the electron beam
Figure 3. Post-accelerated radioactive and stable ion beams that were available at the HRIBF for a variety of nuclear reactions and
spectroscopic studies [12]. About 80 stable isotopes from 39 elements were also produced as negative ions with a Cs-sputter ion source
and accelerated in the tandem for nuclear physics experiments. Reproduced from [12]. © IOP Publishing Ltd. All rights reserved..
Figure 4. Low-energy RIBs produced at the HRIBF. Depending on the desired intensity and purity, these RIBs were extracted from the
ion source as either positive-ion or negative-ion beams.
7
New J. Phys. 19 (2017)085005 Y Liu et al
plasma ion source (EBPIS)[58,59]—a universal and efficient ion source that can ionize essentially any atomic or
molecular species transported into the plasma volume. Nevertheless, negative-ion sources were needed to
provide higher beam intensities and better beam purity for some radioisotopes. For example, the EBPIS is not
selective and often generated beams with strong isobar contaminants that could not be readily separated by the
magnetic isobar separator. In addition, the charge-exchange processes often resulted in rather large energy
spreads in the negative-ion beams which in turn limited the isobar suppression capability of the electromagnetic
isobar separator. In these cases, additional isobar suppression methods or steps were necessary. Considerable
effort was put forth to investigate potential negative-ion sources based on different ionization methods,
including surface ionization [37,60,61], plasma-sputtering [62], Cs-sputtering [54,63], and electron-beam
plasma [64]. Three negative-ion sources that have been successfully used online for RIB production are
described in the next section.
4. Negative-ion sources for the production of RIBs
4.1. Kinetic-ejection negative-ion sources (KENIS)
The kinetic-ejection negative-ion source (KENIS)[54]was first developed at HRIBF for the production of
17
F
and
18
F beams. Radioactive
17,18
F beams are of considerable interest for studying nuclear astrophysical reactions
responsible for element synthesis in the Universe. They were produced at HRIBF through the respective
reactions
16
O(d, n)
17
For
18
O(α,pn)
18
F using thin fibrous oxide materials such as A1
2
O
3
,Y
2
O
3
, ZrO
2
, or HfO
2
[57]. Although negative surface ionization sources with LaB
6
ionizers have been shown to be highly efficient for
halogens such as Cl, Br, and I, their efficiencies for negative F ions are extremely poor [22,55]. This could be
explained by the observation [55,65]that fluorine isotopes were released to the ion source primarily in
molecular forms, such as AlF, which have relatively low electron affinities and thus small probabilities for
negative surface ionization. The charge-exchange method with the EBPIS has been used for generating negative
F beams, but ionization efficiencies for the fluorine compounds were found to be on the order of 1% and the
desired F ions were distributed in several molecular sidebands. Moreover, the charge-exchange efficiencies from
the positive molecular ions to negative atomic fluorine ions, e.g., AlF
+
→F
−
, were also very low. Therefore, an
efficient negative-ion source was needed to dissociate the molecules and ionize the atomic F simultaneously.
This requirement has led to the development of KENIS, which is a unique Cs-sputter negative-ion source for
direct production of negative
17,18
F beams.
A schematic view of the HRIBF KENIS is shown in figure 5. The ion source and the target assembly are
designed with the following principal components. A Ta target-material reservoir is attached to a Ta vapor-
transport tube. A 50% porosity W-ionizer for formation of intense Cs
+
beams via surface ionization, is co-
axially located in the center of and fixed to the exit end of the vapor-transport tube. Cs vapor is transported to the
porous W-ionizer from an externally located Cs-oven through an independent tube, co-axially located within
the vapor-transport tube and weld attached to the W-ionizer. A negatively biased acceleration grid accelerates
the Cs
+
beams to energies of 150–300 eV to bombard the inner surface of a negatively biased, conical geometry
Ta cathode. Annular apertures, located between the vapor-transport tube and the W-ionizer allow the
radioactive species emerging from the target reservoir to flow through the acceleration grid and deposit on the
Figure 5. Schematic view of the KENIS developed for the production of
17,18
F beams at HRIBF.
8
New J. Phys. 19 (2017)085005 Y Liu et al
conical cathode surface where they are sputtered by positive Cs ions and a fraction of them are re-emitted as
negative ions. The Cs
+
beam not only serves to eject particles from the cathode surface, but also lowers the work
function of the surface, thereby greatly enhancing the yield of negative ions. Negative-ion beams are extracted
through a circular aperture (diameter f=2mm)in the apex of the cathode cone.
In offline studies, the performance of this source was evaluated by injecting SF
6
molecules into the high-
temperature target reservoir filled with fibrous Al
2
O
3
materials. SF
6
decomposes and reacts with Al
2
O
3
at the
target operating temperatures to form aluminum fluoride molecules, which can be effusively transported to the
ion source. The mass spectrum of the negative ions extracted from the KENIS was clean and simple, with
essentially 100% of the F-ions appearing in the mass 19 channel. As a reference, the positive ions generated using
the EBPIS contained various fluoride molecular ions, with only about 13% of the total fluorine in the atomic F
+
channel [54]. To measure the ionization efficiency of the KENIS, SF
6
was introduced into the source through a
calibrated leak at a precisely controlled rate. The ratio of the F
−
ions detected to the number of neutral fluorine
atoms injected was the overall ionization efficiency and was measured to be 5%–7%.
In low intensity, online tests, mass-analyzed beams of
17
F
−
,upto10
7
ions s
–1
μA
–1
of deuteron beam
(figure 6)were obtained using fibrous ZrO
2
and HfO
2
target-materials through the
16
O(d, n)
17
F reaction. In
general, ion yields increased with increasing target temperature until the target material started to dissociate. At
∼1800 °C, ZrO
2
and HfO
2
started to dissociate and the high vapor pressure of oxygen and oxide molecules
reduced the efficiency of the ion source. This effect occurred at much lower temperatures for Al
2
O
3
(around
1400 °C)when the alumina was in contact with tantalum or other refractory metals. During online operations,
with the KENIS installed on the IRIS1 RIB production platform,
17,18
F isotopes were produced using HfO
2
cloth
targets. This HfO
2
cloth consisted of thin fibers of about 5 μm diameter woven together to form a low-density,
rugged material that was free from volatile impurities. However, the fastest and most efficient transport
mechanism from the target to the ion source was the AlF molecule, so a layer of Al
2
O
3
foam was wrapped around
the HfO
2
target to provide the required Al atoms. The AlF molecule was transported to the ion source where it
was collected on the cone and negative fluorine ions were subsequently sputtered off the cone by the Cs
+
ions
generated in the W ionizer and accelerated to about 300 eV by the negative potential on the cone.
The KENIS was the primary ion source for generation of negative
17,18
F beams at the HRIBF from
1998–2012, with post-accelerated
17
F beam intensities reaching 10
7
particles/second and
18
F beam intensities on
the order of 2 ×10
5
particles/second. The
18
F beam was mixed with the stable isobar
18
O at a ratio of
18
O/
18
F∼
8. Pure
18
F beam could be obtained by fully stripping the beam to 9
+
charge state, with some loss in intensity.
The average TIS lifetime during online operation was about 1200 h (3000 μAh)with the main failure mode being
low fluorine yields due to beam-induced target damage. Availability of these fluorine beams enabled many
experiments, including direct measurements of astrophysically important proton capture cross sections, e.g. the
18
F(p, α)
15
O and
17
F(p, γ)
18
Ne reaction cross sections [66], and measurements of elastic scattering and breakup
of
17
Fona
208
Pb target and simultaneous two-proton emission from an excited state in
18
Ne [67].
4.2. Negative surface ionization sources
Negative surface ionization sources are generally characterized by a high degree of ion beam purity (chemical
selectivity)and high efficiencies for highly electronegative species. These advantages have been utilized for
negative RIB generation of high-EA elements such as the Group VIIA halogens (Cl, Br, I and At).
Figure 6.
17
F
−
beam yields from low-intensity online tests with Al
2
O
3
, ZrO
2
, and HfO
2
target-materials.
9
New J. Phys. 19 (2017)085005 Y Liu et al
4.2.1. Negative surface ionization source at HRIBF
At the HRIBF, positive RIBs of the group VIIA elements, e.g., bromine and iodine, were available with the EBPIS
and uranium carbide targets, but they were strongly mixed with isobars of the neighboring elements. For
example, the mass-88 positive ion beam consists of 7% Br, 33% Kr, and 60% Rb. Even after the Cs-vapor CEC,
this negative-ion beam was still a mixture of 26% Br and 74% Rb. A negative surface ionization source with a
spherical-sector LaB
6
ionizer [60,61]was developed to address this problem. A schematic view of the ion source
and target assembly is given in figure 7.
Similar to the KENIS, the ion source is connected to the target reservoir via a Ta vapor-transport tube
(8.5 mm inner diameter)and the spherical-sector LaB
6
ionizer is co-axially located at the exit end of the
transport tube. The ionizer (spherical radius: 2.5 mm; diameter: 4.3 mm)is machined from a solid LaB
6
rod and
pressed into a 6 mm diameter Ta holder. Atomic or molecular species released from the target reservoir enter the
ionization region through the annular slots surrounding the ionizer holder. The spherical-geometry, extraction
optics are designed to focus negative-ion beams through a very small extraction aperture (0.41 mm diameter)
through which all charged and neutral particles must pass before leaving the ionization volume of the source.
The neutral species will strike the hot extraction electrode bounce back and forth between the hot surfaces of the
ionization volume, eventually striking the hot LaB
6
surface. Thus, this geometry enhances the probability for
ionization of highly electronegative species that enter the volume. The negatively ionized particles are then
accelerated and focused through the small aperture by the electric field produced between the spherical
geometry ionizer and the extraction electrode that is maintained at positive potential. This ion source also
generates large currents of electrons, which are deflected away from the extraction electrode to a grounded plate
by a magnetic field perpendicular to the beam axis, which is produced by an electromagnet mounted outside the
vacuum chamber along with internal pole pieces near the extraction aperture. In offline studies, the transport
tube was heated resistively to 1400 °C–2000 °C by passing an electric current through the tubular structure,
while the temperature of the LaB
6
ionizer was typically operated around 1650 °C, below the onset (1730 °C)for
thermal dissociation of LaB
6
, as calculated using the software HSC Chemistry [68]. The performance of the ion
source for generating negative beams of Cl and Br was investigated by feeding AlCl
3
and AlBr
3
vapors at low-feed
rates into the source and ionization efficiencies up to about 28% and 15% for Cl
−
and Br
−
, respectively, were
obtained [61,69].
In online operations, the LaB
6
source has shown to be able to provide isobarically pure beams of Br and I
isotopes. Table 2lists the production rate of Br and I isotopes from proton-induced fission of
238
U in a uranium
carbide target irradiated with a 54 MeV proton beam. Also, listed are the measured negative ion rates, the total
TIS efficiency, and the negative-ion beam intensities injected into the post-accelerator when a 5 μA proton
driver beam was used. As noted, an overall TIS efficiency of 7% was obtained for the relatively long-lived
83
Br, in
close agreement with the offline ionization efficiency of 15% for Br. Despite the somewhat low ionization
efficiencies for Br and I isotopes, the negative-ion yields for Br isotopes were about 25 times greater than the
positive-ion yields from the EBPIS followed by charge exchange and the beam was pure, no contamination with
Figure 7. Schematic view of the negative surface ionization source used at HRIBF. This ion source utilizes a LaB
6
ionizer with a
spherical-sector geometry.
10
New J. Phys. 19 (2017)085005 Y Liu et al
isobars from neighboring elements. For iodine isotopes, the gain in yield from the LaB
6
ion source was about a
factor of 10 greater than achieved from the EBPIS followed by charge exchange and again, the beam was pure.
4.2.2. Negative surface ionization sources at ISOLDE
Negative surface ionization sources with planar and hollow-tube ionizers have been developed at ISOLDE. As
discussed in section 2.1, the overall ionization efficiency can be significantly enhanced with the tubular geometry
as given by equation (3). Figure 8shows the cross-section view of the TIS assemblies with a planar-ionizer source
(MK4)and a tubular-ionizer source. The important functional elements for the ion source operation are
indicated, namely a magnet to create a perpendicular field of about 0.08 T to deflect the emitted electrons, an
electrostatic deflector/collector typically polarized at 1 kV (0–3 kV range), made of copper, to collect the
electrons. For the planar MK4 ion source, the LaB
6
sintered pellet is held in place with a molybdenum fitting in
the tantalum cavity, while for the tubular ion source, two Molybdenum rings are used to hold the tubular ionizer
in the tantalum transfer line. A capillary is used to inject Cs vapor to provide positive charge compensation of the
emitted electrons with counter-propagating Cs
+
ions, aiming at creating a positive plasma well in the tube to
Table 2. Measured ion yields from a UC target coupled to the LaB
6
surface ionization source at HRIBF.
Isotope Half-life
In-target production rate ions s
–1
μA
–1
1
H
+
Negative-ion TIS yield ions s
–1
μA
–1
1
H
+
TIS effi-
ciency (%)
Beam injected
into the tan-
dem with
5μA, 54 MeV
1
H
+
83
Br 2.40 h 9.04E+07 6.4E+06 7.1 3.2E+07
84g
Br 31.8 m 1.12E+08 5.2E+06 4.6 2.6E+07
85
Br 2.90 m 1.34E+08 3.2E+06 2.4 1.6E+07
86
Br 55.1 s 1.32E+08 8.5E+05 0.6 4.3E+06
87
Br 55.6 s 1.11E+08 1.5E+06 1.3 7.5E+06
88
Br 16.3 s 6.82E+07 2.3E+05 0.3 1.2E+06
132m
I 1.39 h 1.35E+08 8.2E+0 0.61 4.1E+06
132g
I 2.3 h 1.35E+08 7.3E+05 0.54 3.7E+06
134m
I 3.5 m 3.60E+08 1.5E+06 0.42 7.5E+06
134g
I 52.5 m 4.13E+08 2.7E+06 0.65 1.4E+07
136m
I 46.9 s 2.20E+08 1.9E+06 0.87 9.5E+06
136g
I 83.4 s 2.26E+08 1.2E+06 0.53 6.0E+06
137
I 24.5 s 2.25E+08 2.2E+05 0.10 1.1E+06
138
I 6.2 s 7.65E+07 2.3E+04 0.03 1.2E+05
Figure 8. Cross-section view of the TIS unit at ISOLDE with a negative surface ionization source of a planar ionizer (left)and a tubular
ionizer (right).
11
New J. Phys. 19 (2017)085005 Y Liu et al
trap the negative ions. The tubular negative-ion source has been developed with different low work function
materials including a GdB
6
ceramic tube, a BaO/SrO impregnated W tube, and Ir
5
Ce metallic alloys. Ionization
efficiencies for Br
−
and I
−
of 10% have been measured when operating the ion source at 1700 °C.
The MK4 ISOLDE negative-ion source equipped with a planar ionizer is the traditional ISOLDE negative-
ion source that was developed in the eighties by Vosicki and collaborators and used with a number of different
target materials to produce halogen beams—with the exception of fluorine beams. At that time, the
radioisotopes were produced with a CW 600 MeV proton beam from the synchro-cyclotron impinging target
materials such as niobium and mixed niobium thorium powders, tantalum and mixed tantalum thorium foils,
and uranium carbide [22]. This type of ion source was again used to produce RIBs after the ISOLDE facility was
moved to the proton synchrotron booster to use its pulsed 1.4 GeV proton beam [4]to produce radioactive
nuclei. In table 3, we present the beam intensities for isotopes of Cl, Br, and I from the TIS unit UC263A-MK4
operated online in 2005, which combined the MK4 with a uranium carbide target. The unit was operated at
target temperatures of 1800 °C and 2000 °C while the LaB
6
pellet was at 1300 °C.
As seen in table 3, the reported TIS efficiency, which takes into account the efficiencies for all of the processes
going from in-target production to ion beam extraction, decreases with the half-lives of the isotopes of a given
chemical element. This trend was further analyzed by measuring the temporal isotope release function of the TIS
unit, collecting and analyzing the so-called release curves [70], exploiting the pulsed nature of the proton beam
of the PSB and time-dependent isotope activities collected with a tape station [71]. The release function p(t)takes
the form:
pt A 1 exp exp 1 exp . 5
t
t
t
t
t
t
ln 2 ln 2 ln 2
rf s
aa=- +-
-- -
⎜⎟
⎛
⎝
⎞
⎠
()()
() ()
() ( ) ()
() () ()
The extracted parameters from the fits to the measured release profiles for Cl, Br, and I from a LaB
6
ion
source are reported in table 4. Figure 9shows that the ionized fractions for the bromine isotope series as a
function of their half-lives (solid line), computed from the release curve for Br and the ion source efficiency,
compare well with the released ionized fractions computed using the in-target production and measured beam
intensities of the Br isotopes (full diamonds). The ion source efficiency was initially 8.4%, measured with the
long-lived
83
Br beam. It however rapidly dropped to 0.1%, which was attributed to the poisoning of the LaB
6
with the outgassing of the carbide target. The systematic comparison in figure 9was done after the poisoning had
taken place and no further decrease in source efficiency was observed. The results thus indicate that the drop in
efficiencies for exotic isotopes is related to the release characteristics of the TIS unit, and not to the ionization
process itself.
The LaB
6
MK4 ion source was operated again in 2016 with a mixed Th/Ta foil target, for fission fragment
and spallation products, in particular Astatine beams. The reported ion yields are
38
Cl: 1.10
5
/μC,
85
Br:
1.10
4
/μC,
128
I: 9.10
5
/μC, and
204
At: 9.10
3
/μC, respectively. The unit was operated at target temperature of
Table 3. Measured ion yields from UC target using the MK4 LaB
6
surface ionization source at
ISOLDE.
Isotope Half-life Yield/Production rate (ions s
–1
μA
–11
H
+
)TIS efficiency (%)
38
Cl 37.2 m 1.6 10
5
39
Cl 56 m 1.1 10
5
40
Cl 81 s 4.3 10
4
41
Cl 38.4 s 1.4 10
4
42
Cl 6.9 s 1.1 10
3
43
Cl 3.3 s 3.0 10
2
82
Br 6.13 m 7.3 10
6
0.55
83
Br 2.37 h 1.810
6
8.4
85
Br 2.90 m 9.6 10
5
87
Br 55.6 s 2.1 10
5
1.0
88
Br 16.3 s 2.7 10
5
1.5
89
Br 4.4 s 8.4 10
4
90
Br 1.9 s 1.5 10
3
0.13
91
Br 0.54 s 2.5 10
3
92
Br 0.34 s 1.4 10
3
0.056
122
I 3.6 m 4.4 10
4
137
I 24.2 s 1.3 10
5
138
I 6.4 s 7.3 10
5
139
I 2.3 s 1.7 10
4
140
I 0.86 s 3.3 10
3
141
I 0.43 s 3.5 10
3
12
New J. Phys. 19 (2017)085005 Y Liu et al
1900 °C and ion source temperature of 1200 °C and displayed slower release characteristics when compared to
the UC
x
target coupled with positive ion sources at 2000 °C. Although halogens are volatile elements, they are
reactive elements and prone to strong adsorption and reactions with various structural materials. Data on
surface adsorption and desorption properties have been documented for transition metal interfaces such as
tantalum and molybdenum [72]. The absolute desorption rates are found in the 1–8×10
14
s
−1
range and
enthalpies in 3.7–5.1 eV ranges for F, Cl, and Br. This provides desorption times in the 0.5–4.0 ms range. When
folding this with an estimated number of interactions with a foil target (100 000)or with the structural material
(500), this provides an overall effusion delay time of the order of 0.25–400 s for TIS temperatures up to 1300 °C,
well in the range to account for the slow release properties and reduced efficiencies for exotic isotopes as shown
in figure 9and table 4[73].
The electron current measured on the collector plate can be used to assess the thermionic emission of the
LaB
6
pellet, from which its effective work function can be assessed. The electron current saturated above 2 kV
applied on the deflector plate. A similar value of 2 kV was also used by Pelletier et al [35]to determine the work
function of LaB
6
sintered pellets. Figure 10 shows the dependence of emitted electron current I on LaB
6
ionizer
temperature, expressed as log(I/T
2
)versus e/kT (so-called Richardson plot [74]), for the TaThO576-MK4 unit
operated in 2016 at ISOLDE, together with the electron emission currents measured with various MK4 units
previously operated at ISOLDE-SC. It can be seen that the historical MK4 ion sources were operated with a LaB
6
whose work function was close to 2.9 eV, somewhat larger than the 2.66 eV found for a nominal LaB
6
material.
4.3. Batch-mode Cs-sputter negative-ion sources
Negative-ion beams of relatively long-lived radioactive isotopes can be generated using Cs-sputter ion sources in
batch mode. In this mode, long-lived radioactive isotopes are either produced via direct irradiation of the sputter
target and then transferred to a Cs-sputter source or the radioisotope may be produced in a thick target,
chemically separated from the target matrix, mixed with metal powder, formed into a sputter target, and
inserted into a Cs-sputter ion source. Either way, this technique avoids the high target temperatures required for
fast diffusion and fast effusive-flow to the ion source. Two Cs-sputter ion sources have been designed and
evaluated at HRIBF for this application: a batch-mode Cs-sputter source and a multi-sample Cs-sputter source
[52,53]. As illustrated in figure 11, both are based on the previously developed single-sample Cs-sputter sources
[75]with a conical geometry, W-surface ionizer for surface ionization of Cs atoms and similar ion optics for
accelerating Cs
+
ions and extracting negative ions. The batch-mode source allows for sequential (without
opening the vacuum system)on-line production and ionization of moderately long-lived species. That is, the
species of interest are produced by irradiating the target materials with light-ion beams from the ORIC for an
optimum time based on the half-life and the target is then transferred to the ion source position to be sputtered
Table 4. Release time characteristics for the unit
UC264A-LaB
6
.
Isotope t
r
(ms)αt
f
(ms)t
s
(ms)
Cl 85 000 0.25 79 7180
Br 305 0.87 3865 116 000
I 83 0.65 587 9550
Figure 9. TIS efficiency expressed as released fraction for Br radioisotopes from the UC264A target unit. ─calculated with the release
curve and the ion source efficiency for
83
Br, #calculated from in-target production rates and measured ion intensities (Δcalculated
before poisoning of the LaB
6
ionizer).
13
New J. Phys. 19 (2017)085005 Y Liu et al
with a 1–5 keV Cs
+
beam. A second target can be irradiated with ORIC beams while generating negative-ion
beams from the initial target. This source was conceived and tested for the generation of
56
Ni
−
(T
1/2
=6.077 d)
beams, but could be employed for the generation of several other species with lifetimes in excess of a few hours,
such as
18
F(T
1/2
=109.77 min).
The multi-sample source was designed for batch-mode generation of long-lived species and was used to
deliver beams of
7
Be (T
1/2
=53.22 d),
10
Be (T
1/2
=1.51 ×10
6
yr),
26g
Al (T
1/2
=7.17 ×10
5
yr), and
82
Sr (T
1/2
=25.55 d), which can be produced offline, processed chemically, and placed into the Cs-sputter ion source. It
Figure 10. Left: Richardson plot of log(I/T
2
)versus e/kT of the MK4 LaB
6
ionizer in the TaThO576 unit. Right: electron current
measured at the collector plate as a function of temperature; lines are the theoretical thermionic emission curves from LaB
6
with
quoted work functions. Diamonds are measured electron currents from the MK4 ion sources operated at ISOLDE-SC.
Figure 11. Schematic view of the Cs-sputter negative-ion sources for generating negative-ion beams of long-lived radioactive species
at HRIBF: (a)batch-mode source and (b)multi-sample source.
14
New J. Phys. 19 (2017)085005 Y Liu et al
employs a small (30 mm diameter)sample wheel made of copper that can also hold up to eight, 6.4 mm diameter
sputter samples and can be remotely indexed under vacuum to select the desired sample. This source was
specifically designed for providing radioactive
7
Be
−
beams for the nuclear astrophysics experiment at HRIBF to
directly measure the
7
Be(p, γ)
8
B reaction cross section in inverse kinematics,
1
H(
7
Be,
8
B)γ, with a windowless
hydrogen target and the Daresbury recoil separator [76]. The
7
Be(p, γ)
8
B reaction is important for
understanding the observed flux of solar neutrinos from the Sun. Since Be does not form stable atomic negative
ions, molecular ions such as BeO
−
are commonly used for injection into tandem accelerators in which the
oxygen atoms are removed by a stripper foil at the terminal. The sputter samples for
7
Be beams were pressed
copper powder containing a small amount of
7
BeO. The production of the molecular negative ions was
evaluated in offline tests [53]with MgO instead of BeO because MgO is chemically similar to BeO and posed no
health hazards. Two types of pressed powder sputter targets were tested: Cu and Ag mixed with 3% MgO by
weight. Similar or higher MgO
−
currents were obtained with the Cu targets. The ionization efficiency for MgO
−
was estimated based on the measured MgO
−
and Cs
+
ion currents and the sputter yields reported in literature. A
typical efficiency obtained in such a way was near 0.5%, as shown in figure 12. It has been suggested [77,78]that
the ionization efficiency for BeO
−
could be a factor of 3–5 larger than that of MgO
−
. Therefore, the estimated
ionization efficiency for BeO
−
was about 2% with this multi-sample source.
The production of
7
Be beams was a collaboration between the University of North Carolina, the Colorado
School of Mines, and the HRIBF. The radioactive
7
Be was produced at the Triangle Universities Nuclear
Laboratory (TUNL). Samples of lithium metal were activated at TUNL using a 10 MeV beam of protons from the
FN tandem accelerator, producing
7
Be via the
7
Li(p, n)
7
Be reaction. Up to 50 mCi of
7
Be per day were produced
using proton beam intensities of about 10 μA. The activated lithium slugs containing about 300 mCi of
7
Be were
shipped to HRIBF, where a simple wet chemical process was performed to separate the
7
Be from the lithium. The
separated
7
Be was added to a copper powder matrix, converted to an oxide (
7
BeO), and pressed into a sputter
sample designed for use with the multi-sample sputter source. The influence of the cathode geometry, chemical
composition and ion source parameters on the production of
7
BeO
−
beam was studied in a series of tests at the
online test facility at HRIBF. Figure 13 shows the
7
BeO
−
beam currents measured as a function of time from two
independent samples, each containing 1 mCi of
7
Be (2.5 ×10
14
atoms)within a copper metal matrix. Average
beam intensities of about 1–2 million
7
BeO
−
/second were obtained under good operating conditions, and a
single sample produced useable beams for about a week with a total efficiency of about 0.5%–1.0% for
production of
7
BeO
−
beam.
For the
1
H(
7
Be,
8
B)γmeasurement [76],aCu/
7
BeO sample with 120 mCi of
7
Be (3×10
16
atoms)was used.
The
7
BeO
−
beam from the multi-sample source was typically 20 pA, monitored with a movable tape collector
and a HPGe detector system. During the 5 d experiment, the total
7
Be
−
output (measured at the tape system)was
about 5.2 ×10
13
ions, corresponding to 0.21% of the total sample activity. During this experiment, the post-
accelerated beam intensity peaked at about 2 ×10
77
Be/second on target. Similarly, beams of
10
Be
−
,
26g
Al
−
, and
82
Sr [79]were obtained with the multi-sample source and delivered to experiments with peak intensities more
than 10
7
ions s
–1
.
Figure 12. Measured MgO
−
ion currents with a pressed Cu/MgO powder target and the estimated ionization efficiencies for MgO
−
.
15
New J. Phys. 19 (2017)085005 Y Liu et al
5. Beam purification with negative ions
Beam purity is critical for most experiments with exotic RIBs. Often the beams extracted from an ISOL-type
target are contaminated with isobars that have very small mass differences and are usually in quantities
exceeding the beams of interest by orders of magnitudes. Consequently, isobar suppression is one of the main
challenges for forefront nuclear research on exotic nuclei. A variety of beam purification techniques have been
developed including development of ion sources that have high selectivity, such as surface ionization and
resonance laser ionization, and using chemical properties to select a particular element. One of the more
effective beam purification techniques is based on molecular ion extraction [80]which takes advantage of
differences in the chemistry of neighboring elements resulting in different efficiencies to form molecular ions at
high temperatures. At the HRIBF, this technique was successfully utilized by extracting the positively charged
molecular ions, selecting the mass in the first-stage mass separator, and passing the beam through the Cs-vapor
charge exchange cell to break up the molecule and to generate the negatively charged atomic ions, which were
then injected into the tandem for post-acceleration. A good example of this technique used to greatly enhance
the quality of the RIB is the purification of Sn and Ge beams [81]. The atomic A=132 beam from proton-
induced fission in a UC
x
target consisted of 87% Te, 12% Sb, and 1% Sn, as shown in figure 14, and the Sn
isotope was the nucleus of interest. It was observed that if the Sn isotopes were extracted from the ion source as
SnS
+
molecular beams, the intensity of the TeS
+
and SbS
+
beams were at least four orders of magnitude lower.
The SnS
+
beam was converted to a Sn
−
beam in the CEC with an efficiency of about 40%. The resulting
negative-ion beams at mass 132 consisted of more than 95%
132
Sn
−
. Beams of
132
Sn, purified to ∼95% were
accelerated to a few MeV/nucleon and delivered to experiments at rates up to 10
6
ions s
–1
.
Figure 13.
7
BeO
−
currents measured at OLTF from two different samples. Each sample contained about 1 mCi of
7
Be.
Figure 14. In-target cumulative production rates of Sn, Sb, Te, and I isotopes from proton-induced fission on
238
U with 40 MeV
protons.
16
New J. Phys. 19 (2017)085005 Y Liu et al
Similarly, the negative-ion beams of Ge isotopes were strongly contaminated with isotopes of Ga, As, and Se
when they were extracted from the ion source as atomic ions but were about 95% pure when extracted as GeS
+
ions, as illustrated in figure 15.
In general, the molecular transport and breakup method resulted in a net relative enhancement of the
negative ion of group IVA species (Sn
−
or Ge
−
)by a factor of ∼10
4
relative to neighboring isobaric contaminant
species (Sb
−
and Te
−
,orAs
−
and Se
−
). Table 5presents the post-accelerated neutron-rich Ge and Sn RIBs at
HRIBF with enhanced purity using sulfide extraction. The production of sulfide molecules was achieved by
introducing H
2
S gas into the target enclosure during bombardment in a controlled manner using a variable leak.
Other molecules used to enhance beam purity include chlorides for Ga and In beams and fluorides for Sr and Ba
beams [82].
6. Future
We have reported the present status of the development and operation of negative-ion sources used to produce
RIBs. At both HRIBF and ISOLDE, these developments lead to the production of a range of specific, yet very
important, negatively charged RIBs. The purity of these beams was unmatched. The specificities of the elements
Figure 15. Beam purity with and without sulfur transport for
80
Ge beams. The spectra have been normalized to the same number of
counts in the Ge region.
Table 5. Accelerated neutron-rich Ge and Sn RIBs at HRIBF, which were purified using sulfide molecular transport followed by molecular
breakup in the Cs-vapor charge exchange cell.
RIB species Half-life
Energy
range (MeV)
Highest intensity (pps on
target)Purity (%)Comments
75g
Ge 1.38 h 2 ×10
5
Purified Ge beams are extracted from the
ion source as GeS
+
77g
Ge 11.2 h 8 ×10
5
78
Ge 1.47 h 175 2 ×10
6
67
79g
Ge 19 s 2 ×10
5
80
Ge 29.5 s 179 2 ×10
5
95
81
Ge 7.6 s 8 ×10
4
82
Ge 4.55 s 183–350 3 ×10
4
22
83
Ge 1.85 s 220–327 1500 43
84
Ge 0.947 s 220–327 95 12
123m
Sn 40.1 m 1 ×10
7
18
125m
Sn 9.5 m 5 ×10
6
0.8
126
Sn 2.3 ×105 a 378 1 ×10
7
50 Purified Sn beams are extracted from the
ion source as SnS
+
127g
Sn 2.12 h 1 ×10
7
128g
Sn 59.1 m 384 3 ×10
6
>99
129m
Sn 6.9 m 5 ×10
5
130
Sn 3.72 m 391 3 ×10
6
>99
131
Sn 58.4 s 2 ×10
6
132
Sn 39.7 s 316–560 9 ×10
5
96
133
Sn 1.44 s 316 2 ×10
4
33
134
Sn 1.04 s 316–560 3 ×10
3
38
17
New J. Phys. 19 (2017)085005 Y Liu et al
for which they are best suited, namely halogens and a few other possible beams, calls for further developments to
further improve the results, for instance on the compatibility of the sources with different production targets or
to improve the release characteristics to reach more exotic beams. This could possibly be achieved using new
types of low work function materials, new materials to reduce the adsorption times of the halogens, as well as
more favorable geometries to further increase the ionization efficiency for the most challenging of the halogens,
Astatine.
Another future development will need to be focused on the post-acceleration of negative RIBs at facilities
such as ISOLDE or TRIUMF, where the negative ions need to be converted to positive-ions before re-
acceleration. Charge-breeding using an electron beam ion source or an electron cyclotron resonance ion source
has been widely used [83]to convert singly charged positive ions into highly charged positive ions for subsequent
acceleration. It may be possible to utilize such charge-breeding techniques to convert negatively charged atomic
or molecular RIBs for post-acceleration, but to our knowledge, no attempts of charge breeding starting from
negative-ion beams have yet been undertaken. The challenge will be to efficiently capture the negative ions and at
the same time effectively extract the highly charged positive ions. New and novel injection schemes will be
needed.
Acknowledgments
This material is based upon work supported by the DOE, Office of Science, Office of Nuclear Physics and this
research used resources of the HRIBF of ORNL, ORNL which was a DOE Office of Science User Facility.
ORCID iDs
Y Liu https://orcid.org/0000-0001-5903-3112
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