E Ravisankar

Milind M Gulhane

Rajender K Bhatia

S K Gupta

T K Saha

V Nataraju

Varun K Yadav

Vilas M Mahadeshwar

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113
BARC  NEWSLETTER
Founder’s Day Special Issue   October 2013
DEVELOPMENT OF A NOVEL VARIABLE DISPERSION
ZOOM OPTICS FOR MAGNETIC SECTOR
MASS SPECTROMETER
Rajender K. Bhatia, Varun K. Yadav, Vilas M. Mahadeshwar, Milind M. Gulhane,
E. Ravisankar, T.K. Saha, V. Nataraju and S.K. Gupta
Technical Physics Division
Shri Rajender K. Bhatia is the recipient of the DAE Young Applied Scientist /
Technologist Award for the year 2011
Introduction
Magnetic Sector mass spectrometers are used in the
precise isotopic ratio measurement of the inorganic
samples in gaseous or solid form. TPD, BARC has been
developing these instruments [1] for various
applications in DAE. The instruments developed so far
are based on conventional design of collector system
whereby each collector is mounted on a mechanical
motion feed-through [2]. This set-up suffers from the
following limitations: a) In case of higher number of
faraday cups (5 or more) mechanical assembly is very
bulky and unwieldy, b) limited peak flatness (500 – 600
ppm) due to limited aperture size (~ 1
mm), c) difficulty in angular adjustment
of the collector cups affecting the peak
shape, d) the physical dimensions of the
collector viz., 2.5 mm, restricting the
placement of collectors at desired
locations which limits the mass range  for
multi-collection ratio measurement to
around 250 amu, e) prominent surface
bulging effects during the fabrication of
collector f) microphonic pick-ups in the
collector mounting mechanism leading
to higher noise in the amplifier and thus
the internal precision.
To circumvent these problems, a new
collector set up with variable dispersion
zoom optics is developed. The main
advantages of this set-up are elimination
of mechanical motion feed-throughs and improved
peak flatness leading to better precision and accuracy.
This paper discusses the design aspects, experimental
study and evaluation using Strontium standard for
isotopic ratio measurement.
Design Details
The setup consists of DC quadrupole [3], deflection
optics and five fixed collectors with wider input
apertures. DC quadrupole adjusts the dispersion of ion
beams by suitably varying the potentials such that these
are well aligned with the respective apertures on the
Fig. 1: Schematic of Variable dispersion zoom optics; five masses
dispersed by magnetic analyzer adjusted for optimized dispersion by
VDZO and collected by respective faraday collectors.
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BARC  NEWSLETTER
Founder’s Day Special Issue   October 2013
deflection optics. The deflection optics consists of thin
metallic plates stacked together but electrically insulted
from each other and hence forming different apertures
for the passage of ion beams. It deflects the ion beam
passing through respective apertures in such a way that
the distance between the beams increases when they
reach the respective collectors as shown in Fig. 1. The
increased distance allows using collectors with wider
apertures which leads to more peak flatness. For the
collection of ion beam, five faraday collectors with 3
mm aperture are used. These are mounted on a single
stainless steel plate enabling simpler and sturdier
mechanical assembly.
Computer Simulation
The entire geometry was simulated using SIMION 7.0
software [4] and a study was carried out for the
optimum set of potentials for all the ion optical
elements that will give the maximum transmission for
the isotopes of a given element. Two different potential
arrays-one with geometry of DCQ, deflection optics and
faraday cups and the other consisting of the 30 cm,
90º deflection stigmatic magnet geometry, were
coupled together in the work bench. The ion flight
paths through above optics were generated using ion
groups having an energy of 7 keV, originating from a
source slit of 0.3 mm (placed at 600 mm from the
magnet boundary) with a divergence of ±0.5º. The
masses of interest for these simulations correspond to
the isotopes of Sr, Nd and U. The ions were allowed to
pass through the given geometry of VDZO to be
collected on the faraday cups after the dispersion by
the magnet.  A comparison study was conducted for a
variety of design features of each component and the
final design was worked out for maximum transmission
of the ion beam for all required elements. Simulated
optimum potentials for different elements are
summarized in Table 1. The effect of DCQ potentials
on the beam width as well beam height was also
studied and the dimensions of the Faraday cup were
accordingly fixed.
Experimental
A working model based on the outcome of the
simulation studies was fabricated for experimentation.
The experiments were carried out in three different
parts. In the first part a micro-channel plate phosphor
screen (MCP-SP) imaging device was used to capture
the images of the ion beams to study the effect of
variation in voltages on DCQs and deflection optics on
dispersion of ion beams. The second part involves
placement of final collectors and obtaining flat top
peaks by optimizing deflection optics potentials. Third
part consists of carrying out isotopic ratio
measurements on these collectors for desired elements
by optimizing DCQ potentials and small adjustment of
deflection optics potentials.
To check the effect of VDZO on performance of the
instrument, DCQ potentials were optimized and isotopic
ratio measurement for strontium was carried out. The
Strontium standard sample (SRM 0978) was used for
the measurement of isotopic ratio 87Sr/86Sr using multi-
dynamic mode. The sample was deposited on the
rhenium side filaments of triple filament assembly. The
data was collected in 10 blocks of 10 ratios each, for
the same assembly to estimate the internal precision
(calculated as the relative standard error over all 100
ratios). The same sample was analyzed on ten separate
filament assemblies on a turret/sample magazine for
external precision measurement (relative standard
deviation).
Element Potentials on Deflection optics and DCQ in volts
L2 / L1 / C / R1 / R2 (-ve in volts) ; DCQ(+/- in volts)
Simulations Experimental
Sr 1000 / 0 / 0 / 0/  1000 ; 140 1030 / 0 / 0 / 0 / 1100;  120
Nd 1100 / 100/ 0 / 100 / 1100 ; 30 1200 / 150 / 0 / 70 / 1150;  20
U 1000 / 0 / 0 / 0/  1000 ; 40 1030 / 0 / 0 / 0 / 0 / 1100;  20
Table 1: Optimized values of potentials on VDZO for collecting various ion beams on faraday cups
115
BARC  NEWSLETTER
Founder’s Day Special Issue   October 2013
Results and Discussion
From the imaging studies involving MCP-PS, it was
observed that the dispersions for all the elements were
closely matching with the simulated values. For example
the dispersions obtained for Sr isotopes before and after
the application of DCQ potentials of ±120 volts
(Fig. 2) are almost same as the simulated values of 6.9
mm and 8 mm respectively. Similarly the VDZO
parameters were verified for Nd taking their images on
MCP-PS.
tabulated in  Table 2. It is seen that internal precision is
nearly same in both the systems (conventional and
VDZO) but the external precision (8 ppm) is much better
with VDZO as compared to that obtained with
conventional setup (22 ppm). The accuracies in case of
conventional and VDZO systems were found to be 50
ppm and 10 ppm respectively for a true value of
0.710244.
Fig. 2: Images of Sr isotopes on MCP
The peak shapes (for 88Sr isotope) without DCQ
potentials (but using deflection optics) were
sequentially studied on all the collectors by
suitably varying magnetic field. A peak flatness
of ~3000 ppm of mass and a beam width of
0.8 mm was observed on each of the collectors.
The isotopic ratio measurement of Sr,
corresponding to masses 86, 87 and 88, on
collectors LM2, C and HM2, were carried out
at a DCQ potential of ± 120 V. This resulted in
an increase in beam width to 1 mm and a
decrease in peak flatness to 2500 ppm which
is in full agreement with computer simulations.
Fig. 3 shows the simultaneous peaks of
strontium isotopes on LM2, C and HM2
collectors indicating ability of VDZO in matching
the peaks. Similarly, the peak matching for Nd
and U isotopes was also carried out.
The results for isotopic ratio measurement of
Sr on conventional and the VDZO setup are
Table 2: Comparison of zoom optics with conventional collector
setup for isotopic ratio measurement of standard Sr sample (SRM
987). Average 87/86 ratio with relative standard error (internal
precision; in parentheses)# has been given for each sample.
Sample Ratio (87Sr/86Sr)
Number On conventional setup On setup with
( without zoom optics) zoom optics
1. 0.710194 (6) 0.710246 (4)
2. 0.710210 (4) 0.710241 (8)
3. 0.710219  (4) 0.710239 (6)
4. 0.710199 (4) 0.710230 (7)
5. 0.710198  (4) 0.710242 (7)
6. 0.710239 (4) 0.710231 (8)
7. 0.710190 (5) 0.710234 (5)
8. 0.710222  (4) 0.710241 (8)
9. 0.710190  (4) 0.710230 (6)
10. 0.710220  (5) 0.710233 (5)
Average Ratio 0.710208 0.710237
RSD (external 23 ppm 8 ppm
precision)
(# the value in parenthesis is the variation in last significant digit of the
ratio value)
Fig. 3: Peak shape for Sr isotopes (86, 87, 88)
Magnet Curren (mA)
A
m
p
lif
ie
r 
Si
g
n
al
 (
V
)
116
BARC  NEWSLETTER
Founder’s Day Special Issue   October 2013
Conclusions
A new Faraday collector system (with five cups) for
thermal ionization mass spectrometer has been
developed based on variable dispersion zoom optics.
The system enables fixed collector cups with wider
apertures that facilitate analysis of different elements
with a simple adjustment of suitable potentials to
deflection optics rather than the mechanical movement
of collectors. Further, wider collectors help in
improvement of peak flatness from 500 ppm to 3000
ppm for Sr88. The system design was optimized for
analysis of U, Sr and Nd isotopic ratio analysis by
simulation. The system enables improvement in both
precision and accuracy of analysis. This has been
demonstrated by analysis of 87Sr/86Sr ratio where
precision and accuracy have been improved from 22
ppm to 8 ppm and 25 ppm to 10 ppm respectively.
Acknowledgements
Authors wish to thank Dr. S. Kailas Director, Physics
Group, BARC for his constant support throughout the
execution of this work. The contributions from all the
team members from mechanical and electronics group
of Technical Physics Division are highly appreciated.
References
1. Rajender K. Bhatia, Yogesh Kumar, K. Prathap Reddy,
V.K. Yadav, E. Ravisankar, T.K. Saha, V.Nataraju, V.K.
Handu, International Journal of Mass Spectrometry
313 (2012) 36
2. Rajender K. Bhatia*, R. Datta, Rekhu Chandak, M.
M. Gulhane, Pratap R. Kasina, A. M. Kasbekar, N.
Ved, M. Gopalakrishna,    E. Ravisankar, T. K. Saha,
V. Nataraju,  and V. K. Handu Proceedings of 11th
ISMAS symposium ‘TRICON’ 2009, Hyderabad,
India
3. Peter H. Dawson, Elsevier Scientific Publishing
Company, Amsterdam (Netherland)
4. D.A.Dahl, “SIMION version 7.0”, Idaho National
Engineering Laboratory, 2000


DEVELOPMENT OF A NOVEL VARIABLE DISPERSION ZOOM OPTICS FOR MAGNETIC SECTOR MASS SPECTROMETER

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DEVELOPMENT OF A NOVEL VARIABLE DISPERSION ZOOM OPTICS FOR MAGNETIC SECTOR MASS SPECTROMETER

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