An Ion Guide for the Production of a Low Energy Ion Beam of Daughter Products of $\alpha$-Emitters

Abstract

A new ion guide has been modeled and tested for the production of a low energy ($\approx$ 40 kV) ion beam of daughter products of alpha-emitting isotopes. The guide is designed to evacuate daughter recoils originating from the $\alpha$-decay of a $^{233}$U source. The source is electroplated onto stainless steel strips and mounted along the inner walls of an ion guide chamber. A combination of electric fields and helium gas flow transport the ions through an exit hole for injection into a mass separator. Ion guide efficiencies for the extraction of $^{229}$Th$^{+}$ (0.06%), $^{221}$Fr$^{+}$ (6%), and $^{217}$At$^{+}$ (6%) beams have been measured. A detailed study of the electric field and gas flow influence on the ion guide efficiency is described for two differing electric field configurations. Comment: 13 pages, 11 figures, 4 tables. Submitted to Nucl. Instr. Meth. B

Full text

Page 1

arXiv:nucl-ex/0605027v1 22 May 2006
AnIonGuidefortheProductionofaLowEnergy
Ion Beam of DaughterProductsofα-Emitters
B. Tordoffa,∗,T. Eronenb,V.V. Elomaab,S. Gulickc,U. Hagerb,
P. Karvonenb, T. Kesslerb, J. Leec, I. Mooreb, A. Popovd,
S. Rahamanb, S. Rinta-Antilab, T. Sonodab, J.¨Ayst¨ ob,
aNuclear Physics Group, Schuster Laboratory, Brunswick Street, University of
Manchester, Manchester, M13 9PL, UK
bDepartment of Physics, University of Jyv¨ askyl¨ a, P.O. Box 35, 40014, Jyv¨ askyl¨ a,
Finland
cErnest Rutherford Physics Building, McGill University, 3600 rue University,
Montr´ eal, QC, H3A 2T8 Canada
dPetersburg Nuclear Physics Institute, Gatchina, St-Petersburg, 188350, Russia
Abstract
A new ion guide has been modeled and tested for the production of a low energy
(≈ 40 kV) ion beam of daughter products of alpha-emitting isotopes. The guide is
designed to evacuate daughter recoils originating from the α-decay of a233U source.
The source is electroplated onto stainless steel strips and mounted along the inner
walls of an ion guide chamber. A combination of electric fields and helium gas flow
transport the ions through an exit hole for injection into a mass separator. Ion guide
efficiencies for theextraction of229Th+(0.06%),221Fr+(6%), and217At+(6%)beams
have been measured. A detailed study of the electric field and gas flow influence on
the ion guide efficiency is described for two differing electric field configurations.
Key words: Gas cell,229Th, Electric field guidance, Ion guide
PACS: 29.25.Rm; 23.60.+e; 41.85.Ar
1Introduction
In order to probe the structure of low
energy nuclear isomeric states whose
∗Tel.: +358-14-260-2440; fax:+358-14-
260-2351
Email address: bwt@phys.jyu.fi
(B. Tordoff ).
surroundings can substantially influ-
ence the properties of the state [1],
it has become necessary to develop a
means of performing measurements
in a medium-free environment. The
method described here implements
the Ion-Guide Isotope Separator
On-line technique conceived [2], de-
veloped [3,4,5,6] and successfully
Preprint submitted to Elsevier Science8 February 2008

Page 2

used at the University of Jyv¨ askyl¨ a
for over 20 years. In the technique,
nuclear reaction products are ther-
malisedinfastflowingheliumgasand
are extracted as ions through an exit
hole for injection into a mass separa-
tor. The Jyv¨ askyl¨ a IGISOL system
has recently been upgraded, both in
general design [7] in order to handle
higher primary beam currents and in
post-gascell ionguidance. This latter
upgrade has resulted in the installa-
tion of a radio-frequency sextupole
ion guide which replaces the skim-
mer system based on previous tests
in Jyv¨ askyl¨ a [8] and elsewhere [9,10].
The rf-sextupole has the advantage
of a higher overall mass resolution
and better transmission efficiency.
A laser ion source has also recently
been developed [11] based upon res-
onance ionisation in a gas cell [12,13]
and variations thereof [14]. Devel-
opments in ion guide technology [8]
have shown that the addition of in-
guide electric fields improves the ex-
traction efficiency and reduces the
evacuation time of radioactive ions.
In this work two electric field ar-
rangements have been designed and
tested for the evacuation of daugh-
ter recoils originating from a
source inside a large volume gas cell.
Firstly, a standard electrostatic field
has been modeled and tested em-
ploying charged stainless steel plates
close to the guide exit hole based
upon an earlier design [15]. Secondly,
an electron emitter design [8] has
been tested, whereby a potential dif-
ference is created in the guide via a
source of electrons close to the exit
hole.
233U
2 Ion Guide Design
The ion guide dimensions are specif-
ically designed to maximise the pro-
duction of a radioactive ion beam of
229Th dictated by the shape and α-
recoil energy of the source. Moreover,
the guide is adaptable to any α-recoil
source and is fully compatible with
both the new laser ion source and rf-
sextupole ion guide.
2.1The229Th+source
The dimensions of the ion guide were
chosen so that the maximum range
of 84 keV α-recoils in 50 mbar He gas
(10 mm) was smaller than the guide
radius (57.5 mm). The source of
229Th was produced (Eq. 1) through
neutron capture and subsequent β−
decay of unstable nuclei from a seed
nucleus of
supplied by Isotope Products Labo-
ratory, Bubank, USA.
232Th. This source was
232Th(n,γ)233Th
β−
→
233Pa
β−
→
233U
(1)
The
isopropanol and chemically electro-
plated onto 12 stainless steel sheets
with dimensions 76.2 mm × 25.4
mm. These strips were then mounted
on the inside surface of the gas cell
defining its diameter to be 115 mm.
233U parent was dissolved in
The strength of the source supplied
was measured through analysis of its
α-spectrum (Fig. 1). The dominant
α-decay peak arising from the decay
2

Page 3

Fig. 1. Direct alpha spectrum from one
233U foil taken over a 10 minute collec-
tion time.
of233U to229Th was fitted yielding a
source strength of 106Bq. The inset
of Fig. 1 shows a magnified portion
of the spectrum where alpha peaks
relating to decays lower in the de-
cay chain have been identified. Table
1 lists the alpha energy and corre-
sponding isotope of these peaks. The
presence of decays from the
chain indicates a population of im-
purities not fully suppressed in the
extraction of233U from the breeder
reactor.
232Th
2.2 Gas Flow Ion Guidance
The helium gas flow was numeri-
cally modeled [16] for the ion guide
and results can be seen in Fig. 2.
The injection of He buffer gas on the
symmetry axis of the guide creates a
localised fast flowing region, whose
velocity stagnates at larger guide
radii. Increasing the pressure inside
the guide to 50 mbar confines the
region to a smaller volume and in-
creases the flow speed to 50 ms−1on
the symmetry axis. To take advan-
tage of this flow, electric field config-
urations have been designed which
IsotopeSourceAlpha Energy(MeV)
229Th
233U
4.845, 4.814, 4.839
224Ra
232Th
5.678
225Ac
233U
5.817
212Bi
232Th
6.089, 6.050
221Fr
233U
6.260
220Rn
232Th
6.278
216Po
232Th
6.778
217At
233U
7.066
213Po
233U
8.375
212Po
Table 1
Alpha peak energies and origin in the
source α-spectrum.
232Th
8.784
Fig. 2. Helium gas flow velocity distri-
bution for an ion guide pressure of 50
mbar. Each isovelocity contour denotes
0.4 ms−1of gas flow. Gas enters from
the left and exits through the hole on
the right.
preferentially direct charged recoils
from the walls of the chamber to the
central fast flowing gas region.
The evacuation time of the entire
guide due to gas flow alone can be
estimated from the cell volume and
exit hole conductance. The cell evac-
uation time, T [s], is empirically ap-
proximated for a helium filled cylin-
3

Page 4

der by [17]
T =V
C≈
πr2z
4.51 × 105φ2
exit
= 690 ms
(2)
where V is the cell volume, C [m3s−1]
is the hole conductance, φexitis the
exit hole diameter (2 mm) and r
(57.5 mm) and z (120 mm) are guide
dimensions. The constant in the de-
nominator (4.51×105) is in units of
[mm s−1] and is dependent upon the
elemental composition of the buffer
gas. One can estimate the mean life-
time, τ [s], of an ion diffusing from
the symmetry axis of an infinitely
long cylinder of radius r0[mm] and
pressure p [mbar] at room tempera-
ture as [17]
τ =
p
500
?
r0
24.05
?2
. (3)
Since this corresponds to 570 ms at
50 mbar with a cylinder radius of
57.5 mm, diffusion to the walls oc-
curs on a comparable timescale to
the total guide evacuation time if gas
flow alone is considered.
2.3 Modeling of Electrostatic Field
Guidance
The extraction end of the gas cell
is adaptable to two electric field
arrangements. Firstly, a series of
charged stainless steel plates of de-
creasing inner diameter (Table 2)
create an electrostatic field which
penetrates into thegascell anddraws
Fig. 3. Schematic drawing showing the
positions of the α- recoil source for de-
termining the ion guide efficiency.
charged α-recoils to the exit hole
region (Fig. 3).
Electrode
Number
Inner
φ
(mm)
Outer
φ
(mm)
Resistance
(kΩ)
1 50 1300
2 26 955
3 1495 30
4695 90
52115270
Table 2
Electrostatic ring dimensions. The elec-
trodes are numbered sequentially from
the gas cell toward the exit hole.
The
equipotential lines within the guide
are shown in Fig. 4 for an applied
voltage of -150 V placed on the out-
ermost electrode (electrode 5) in the
resistor chain. The field gradient is
variable through interchanging resis-
tance values connecting each plate.
Optimum values for the resistance
ratios were deduced through mod-
eling of the ion guide by the pro-
gram SIMION [18]. Care was taken
to maximise the penetration depth
of the electric field and to provide a
shapeanddistributionof
4

Page 5

Fig. 4. Equipotential lines arising from
charged stainless steel plates between
the ion guide exit hole and gas cell.
Fig. 5. Potential as a function of ra-
dius for various distances from the exit
hole of the electrostatic plate guide. In-
creasing the voltage on the electrostatic
plates only provides a significant poten-
tial close to the guide exit hole.
symmetric field which would guide
the ions to the exit hole of the guide.
The potential as a function of radius
for different distances from the exit
hole was modeled (Fig. 5) indicat-
ing that distances further than 2 cm
from the the hole are effectively re-
dundant due to the weak potential
difference present in this region. Ions
close to the exit hole will be success-
fully extracted, however some frac-
tion of ions will be lost due to colli-
sions with the guide front wall. This
problem has been addressed success-
fully elsewhere [19,20] through the
use of rf electric fields to prevent ions
hitting the guide walls.
Fig. 6. Cut through view of the electron
emitter end plate design. The gas cell is
on the right of the picture and the mass
separator is to the left.
2.4 Modeling of Electron Emitter
Field Guidance
A second variation of the ion guide
extraction region includes a circu-
lar focussing electrode followed by
a loop of thoriated tungsten W(Th)
wire (Fig. 6). The wire is doped
with 1%
the work function. By passing an
electric current through the wire,
it is Joule heated to ≈ 2000 K and
electron emission occurs. This cre-
ates a potential difference inside the
guide due to the spatial distribution
of electrons which draws positively
charged α-recoils through the exit
hole. By solving Poisson’s equation
iteratively using the particle-in-cell
technique, the resulting electric field
has been modeled for various emitter
currents, an example of which can be
seen in Fig. 7.
232Th in order to reduce
The potential as a function of ion
5