J. Mass Spectrom. Soc. Jpn.

Vol. 54, No. 1, 2006

Development of a Miniature Double Focusing Mass
Spectrograph Using a Focal Plane Detector
Masaru N>H=><J8=>,1῎ Michisato TDND96,1 Morio IH=>=6G6,1 Makiko O=I6@:,2Takamitsu SJ<>=6G6,3 and Itsuo K6I6@JH:1
1

Department of Physics, Graduate School of Science, Osaka University, Toyonaka, JAPAN

2

Research Division for Planetary Science, Institute of Space and Astronautical Science, Japan
Aerospace Exploration Agency, Sagamihara, JAPAN

3

Center for Deep Earth Exploration, Japan Agency for Marine-Earth Science and Technology,
Yokohama, JAPANA miniature mass spectrograph was newly designed and constructed as a prototype model for future lunar
or planetary explorations. The ion optical system was newly designed based on Mattauch῍Herzog geometry. The
mass spectrograph employs a focal plane detector consisting of a microchannel plate (MCP), a phosphor layer, aﬁber-optic plate (FOP) and a charge-coupled device (CCD). The 2D and 1D spectra of the residual gases, krypton
and neon were observed in preliminary experiments. The mass resolution of 130 was achieved experimentally,
and this was in good agreement with the simulation based on the transfer matrix method. The experimental
value of the detectable m/z range was also consistent with the calculated version. Moreover, stable isotopes ofKr and Ne were observed without saturation of the detector. A dynamic range of 300 was achieved.
(Received September 5, 2005; Accepted October 19, 2005)

1.

Introduction

The analysis of the atmosphere and surface substances in lunar and planetary explorations is of interest for space science. In particular, measurements ofthe elemental composition and the isotope ratio of
surface substances provide basic information on theorigin and evolution of the planets. Mass spectrometry
is obviously the most suitable method for these purposes, so mass spectrometers have often been employed in past and present space missions. For example,
two mass spectrometers were on board in the Viking
mission: one was a double focusing mass spectrometerusing Mattauch῍Herzog geometry for analysis of theMartian atmosphere1) and the other was a gas chromatograph(GC)῍mass spectrometer employing a double
focusing analyzer using Nier῍Johnson geometry forthe measurement of organic compounds in the surface
soil.2) Quadrupole mass spectrometers were also carriedon several space probes to analyze the atmosphere: theHuygens Probe in the Cassini῍Huygens mission3) andothers.4), 5) An instrument for space explorations mustfeature reduced size, weight and electric power consumption. Magnetic sector, quadrupole or time-of-ﬂight(TOF) mass spectrometers are generally employed forspace exploration. A magnetic sector analyzer is not
amenable to weight reduction due to its heavy magnet.
On the other hand, quadrupole and TOF analyzers
have good potential for weight reduction because they
῎ Correspondence to: Masaru N>H=><J8=>, Department of
Physics, Graduate School of Science, Osaka University, 1῍1Machikaneyama, Toyonaka, Osaka 560῍0043, JAPAN,
e-mail: nishiguchi@mass.phys.sci.osaka-u.ac.jp

consist of only electrodes. Magnetic analyzers, on theother hand, provide a large dynamic range. If the exploration target is near the Earth, e.g., lunar or Mars
exploration, the weight of the instrument is less crucial. In that case, a magnetic sector analyzer may provide the advantage of good quantitative performance.
When mass spectra are obtained by scanning themagnetic ﬁeld strength or the accelerating voltage ofions in a magnetic sector analyzer, most of the ionscannot pass through the collector slit and thus are notdetected. If all the ions that pass through the analyzer
are detected simultaneously, the detection e$ciency
can be drastically improved. Quantitative performance,
furthermore, can also be improved by simultaneous
detection because time-dependent ﬂuctuations can beeliminated. For these reasons, a mass spectrograph that
employs a magnetic sector analyzer and a focal plane
detector6) is ideally suited for trace and quantitative
analysis. For the same reasons, a mass spectrometer
employing multiple collectors7), 8) is also e#ective. Several mass spectrographs equipped with focal plane detectors have been developed since the 1970.9)῍13) For
example, Gi$n et al.9) applied a focal plane detector
comprising a microchannel plate (MCP), a phosphor
screen, a ﬁber-optic image dissector and a vidicon
camera system to a mass spectrograph with Mattauch῍
Herzog geometry.14), 15) Murphy and Mauersberger11)
developed an MCP detector system that consists of an
MCP, a phosphor layer on a ﬁber-optic bundle and aphotodiode array, and operated the detector in ion
counting mode for a mass spectrograph with
Mattauch῍Herzog geometry. Burgoyne et al.12) also
employed an MCP with an active phosphor for a mass
spectrograph with Mattauch῍Herzog geometry. Theyῌ 1 ῌ

M. Nishiguchi et al.

detected the phosphor images using a 1.32-cm chargecoupled device (CCD). Yurimoto et al. employed astacked CMOS-type active pixel sensor for twodimensional isotope ratio imaging in secondary ion
mass spectrometry.16)
Thus, several types of devices are used as focal plane
detectors.17) An MCP is the most often used device as a
focal plane detector.6) Although an MCP can provide a
cloud of electrons, a system, which can detect theseelectrons and retain the spatial information, is necessary for a mass spectrograph. For this reason, an MCP
is generally combined with two types of electrondetecting systems: one is a photon detector, e.g., a CCD
or a photodiode array with a phosphor screen, and theother is a multi-anode detector. In recent years, thanks
to progress in semiconductor technology, large CCDswith high spatial resolution have become available atcomparatively low cost.
As described above, a double focusing mass spectrograph is suited for space exploration near the Earthdue to its high sensitivity and good quantitative performance. We have therefore developed a miniature
double focusing mass spectrograph for future lunar
explorations.18) We newly designed an ion opticalsystem for use in the double focusing mass spectrograph equipped with a focal plane detector. The focal
plane detector is the most important part of the mass
spectrograph since the performance of the instrument
depends on the detector’s performance. The prototype
instrument employed an MCP-based position sensitive
detector. In this focal plane detector, the MCP is combined with a phosphor layer and a CCD. The size of theactive area of the focal plane detector is about 5 cmῌ
1 cm. Our target is to achieve a highly sensitive andgood quantitative performance by the simultaneous
detection of ions over a wide mass range.
In this paper, the ion optical system and details of the
instrument are described. We also report on the preliminary experiments for the evaluation of the instrument.
2.

Experimental, Results, and Discussion

2.1 Ion optics
The typical ion optical system for the double focusing mass analyzers uses either Mattauch῍Herzog
geometry14), 15) or Nier῍Johnson geometry.19) Nier῍Johnson geometry provides only one double focusing point,
i.e. the energy focal plane and the angular focal planeintersect at one point. On the other hand, Mattauch῍
Herzog geometry provides the double focusing on astraight line at the exit of the magnet. Double focusing
points arranged in a straight line are ideal for the use ofa focal plane detector. We have newly designed an ion
optical system based on Mattauch῍Herzog geometry.
As already stated, instruments for space explorationneed to be small and light. We have assumed lunar
surface exploration with the mass spectrograph loaded
on a rover. The measuring objects are the ice in thepoles and noble gases in the low mass range from He to
Ar. The required speciﬁcations of the mass spectrograph for loading on a rover are as follows18): the size ofthe mass analyzer must be less than 200 mmῌ150 mm
ῌ100 mm; the weight must be less than 3 kg; the mass
range of the measurements is from 1 u to 50 u. And, thetarget of the dynamic range is 104 required for themeasurement of the isotope ratios of Ar.
The magnet is the most important part of the instrument, since it is the heaviest part, and the detectable
range of mass to charge ratio (m/z) depends on the sizeof the magnet. Here, the detectable m/z range is deﬁned
as the ratio of the maximal to the minimal observablem/z, and is expressed in terms of the maximal radiusrmax and the minimal radius rmin of the trajectories in
the magnetic ﬁeld as (rmax/rmin)2; the maximal radius
should be increased, or the minimal radius should bedecreased to achieve a wide range of detectable m/z.
The maximal radius, however, directly depends on themagnet size; thus, it is limited. On the other hand, theminimal radius is also limited. Because the distributionof the magnetic ﬁeld strength would not be homogeneous in the region of the magnet fringe, the minimal
radius must be su$ciently larger than the gap width
between the pole pieces. In the prototype instrument,
the maximal radius and the minimal radius are 75 mm
and 25 mm, respectively. In this design, the detectable
m/z range of 7 is su$ciently achieved. If it is required
to detect from 1 u to 49 u, the observed mass region canbe altered by switching the accelerating voltage andthe ﬁeld strength of the electrostatic sector. When low
mass ions from 1 u to 7 u are observed at a highaccelerating voltage Vh, the high mass ions from 7 u to
49 u can be observed at a low accelerating voltage Vl῍Vh/7.
We employed a spherical electrostatic sector in theion optical system. The spherical electric ﬁeld exerts
focusing actions in both the vertical and horizontal
directions and contributes to achieving high transmission in the magnetic sector. The mean radius of thespherical electrostatic sector is 50 mm.
We considered the following two conditions in thedesign of the ion optical parameters: the ion beams
must be parallel in the drift space between the electrostatic sector and the magnetic sector, and both theenergy and angular focal planes must be placed at a
distance from the exit of the magnet. The ﬁrst condition is theoretically required to realize a straight line
for double focusing.15) This condition can be easily
fulﬁlled by adjusting the length of the ﬁrst drift space
between the ion source and the electrostatic sector. The
second condition is needed to mount a focal plane
detector on the mass analyzer. In the Mattauch῍Herzog
geometry, the focal planes are placed at the exit of themagnet. However, to install an MCP-based position
sensitive detector, it is desirable that the focal planes
be at a distance from the exit of the magnet. The
fringing ﬁeld of the magnet will exert a defocusing
action in the horizontal direction if the inclination
angle between the incident normal of the ﬁeld boundary and the optical axis is positive.20) Therefore, thesecond condition can be fulﬁlled with a positive inclination angle at the magnet entrance.
The design of the ion optical system was carried out
with the computer programs “TRIO”21) and “TRIODRAW”22) based on the transfer matrix method.23)-25)
The schematic drawing of the ion optical system isshown in Fig. 1. The deﬂection angle of the electrostatic sector is 60῎ and the gap between the electrodes is 10

ῌ 2 ῌ

Development of a Miniature Double Focusing Mass Spectrograph

Table 1.

The Ion Optical Parameters and the Elements of the Transfer Matrix

Drift space
Toroidal ESA entrance
Toroidal ESA
Toroidal ESA exit
Drift space
Deﬂection is in reverse sense
Sector magnet entrance
Sector magnet
Sector magnet exit
Deﬂection is in reverse sense
Drift space

DL1῏0.0345
RO1῏0.0000, GAP῏0.0050, NE1῏2
AE῏0.0500, WE῏60.00, C1῏1.0000, C2῏῎1.0000
RO2῏0.0000, GAP῏0.0050, NE2῏2
DL2῏0.0450
EP1῏48.71, RO1῏0.0000, GAP῏0.0050, NM1῏2
AM῏0.0500, WM῏74.73, N1῏0.0000, N2῏0.0000
EP2῏῎52.63, RO2῏0.0000, GAP῏0.0050, NM2῏2
DL3῏0.0269

Transfer matrix relating to the region from the ion source to the ESA exit:

x
a

x

a

g

d

0.5525
῎16.000.06250.00000.00000.00000.02500.8937

Transfer matrix of the whole system:

x
a

x
a

Table 2.

x

a

g

d

xx

xa

xg

xd

aa

῎0.326727.60

0.0000῎3.061

῎0.01825
0.006890.0000῎4.271

῎38.42
1677

5.313
269.7

῎1.211῎6.0808.554
῎121.9῎0.3034
0.7392

ag

ad

gg

gd

dd

yy

yb

bb

0.11601.552

῎0.6745
῎23.110.00435
῎0.00359

0.16191.727

῎0.4602
῎3.706

17.09
῎1378

῎0.04869
῎205.3

῎0.00011
0.7713The Elements of (xῌa) and (xῌd) for Several Mean
Radii in the Magnetic Fieldrm (mm)DL3 (m)

(xῌa)

(xῌd)

25
30
35
40
45
50
55
60
65
70
75

0.01220.01530.01830.02120.02410.02690.02980.03260.03550.03830.0411

῎0.0002
῎0.0002
῎0.0001
῎0.0001
0.00000.00000.00000.00000.00010.00010.00010.00000.00000.00000.00000.00000.00000.00000.00000.00000.00000.0000Fig. 1.

mm. The inclination angle at the magnet entrance is
48.71ῐ. The deﬂection angle of the magnetic sector is
74.73ῐ and the gap between the pole pieces is 4 mm.
The required size of the focal plane detector depends
on the radius and the deﬂection angle of the magnet. If
the maximal radius and the minimal radius are set at
66 mm and 25 mm, respectively, to achieve the detectable m/z range of 7, the required detector size is 70 mm.
The ion optical parameters, the elements of transfer
matrix relating to the region from the ion source to theexit of the electrostatic sector, and those of the wholesystem are shown in Table 1. The deﬁnitions of the ion
optical parameters are presented in ref. 21. Here x anda denote the positional and angular deviations of thearbitrary trajectories in the horizontal direction, and y
and b denote the same ones in the vertical direction.
They are deﬁned with respect to the optical axis. The
unit of x and y is meter, and the unit of a and b is radian.

Schematic drawing of the newly designed ion
optical system.

The mass and energy deviations, g and d, are deﬁned as(1)
m/e῏(m0/e0)(1῍g), U/e῏(U0/e0)(1῍g)
where m, e, U are the mass, charge, and energy of anarbitrary ion and m0, e0, U0 are those of a reference ion.
As expressed in Eq. (1), g and d are dimensionlessparameters. Hereinafter, each element of the transfer
matrix is denoted by row and column parameters, e.g.,
the element in row x and in column a is denoted as(xῌa). The element of (aῌa) in the transfer matrix relating to the region from the ion source to the electrostatic sector is equal to zero. This indicates that the angleof an ion beam is independent of its initial angle, i.e.,
the ion beams run parallel in the drift space between
the electrostatic sector and the magnetic sector. The
elements of (xῌa) and (xῌd) in the transfer matrix of thewhole system are equal to zero. This indicates that theῌ 3 ῌ

M. Nishiguchi et al.

double focusing is satisﬁed in this ion optical system.
The elements of (xῌa) and (xῌd) for several mean radii of
ion trajectories from 25 mm to 75 mm at 5 mm intervals in the magnetic ﬁeld are shown in Table 2. The
second column of the table shows the length of the lastdrift space between the magnetic sector and the detector. Double focusing is satisfactorily achieved for all
mean radii in the magnetic ﬁeld, and the double focusing points are in a straight line, as shown in Fig. 1. The
ion trajectories with the mean radii of 50 mm in themagnetic ﬁeld were simulated by “TRIO-DRAW,” asshown in Fig. 2. Figure 2(a) shows the ion trajectories
in the x direction and Fig. 2(b) shows those in the y
direction. We can conﬁrm the double focusing is achieved in the x direction in Fig. 2(a). And in Fig. 2(b),ion trajectories are compressed in the y direction before
entering the magnetic sector. This indicates the spherical electrostatic sector e#ectively contributes to ahigh transmission in the magnetic sector.
As mentioned above, this ion optical system has thefollowing features: the double focusing for the use of a
focal plane detector is fulﬁlled in a straight line placed
at a distance from the exit of the magnet; a hightransmission can be achieved by employing the spherical electrostatic sector.
2.2 Instrument
A prototype model employing the newly designed
ion optical system was constructed. The instrument isequipped with an EI ion source, a spherical electrostatic sector, a homogeneous magnetic sector and a focal
plane detector using a CCD. This instrument can detect
ions over a wide mass range simultaneously by employing the new ion optical system and equipping the
focal plane detector whose size is about 5 cm. The

Fig. 2.

details of the focal plane detector are presented insubsection 2.2.2. The technical drawing and a photograph of this instrument are shown in Figs. 3 and 4.
The instrument is set in a circular vacuum chamber
whose diameter is 40 cm, and the mass analyzer is ﬁxed
on a base plate whose size is about 20 cm῎20 cm. The
focal plane detector is ﬁxed to a movable stage. The
vacuum chamber is evacuated with a turbomolecularpump (TMP 280G, SHIMADZU CORPORATION,
Kyoto, Japan). The vacuum pressure is measured with
an ionization gauge (GI-TL2, ULVAC, Inc., Kanagawa,Japan) attached at the side of the vacuum chamber.
The base pressure is maintained at about 4῎10ῌ4 Pa.
2.2.1 Ion source and mass analyzer The EI ion
source is the same one as used for the JMS-HX110(JEOL, Tokyo, Japan). The development of a small andlight ion source is also important in space exploration.
A reliable ion source is, however, needed to evaluate
the ion optical properties. Thus, the commerciallyavailable ion source was employed in the prototype instrument.
The mean radius, the gap width of the electrodes, thedeﬂection angle and the height of the spherical electrostatic sector are 50 mm, 10 mm, 60῏ and 40 mm, respectively. If the accelerating voltage of ions is Va, thetheoretical voltage applied to the electrodes is ῍0.1980
Va. The a-slit is placed at the entrance of the electrostatic sector.
To save power consumption, the magnetic ﬁeld isgenerated by a permanent magnet. A schematic drawing of the magnetic circuit is shown in Fig. 5. The polepieces with the thickness of 8 mm were designed toachieve a su$cient uniformity of magnetic ﬁeld. The
thickness of the permanent magnet materials is 4 mm.
The gap width between the pole faces is 4 mm. Lowcarbon steel SS400 (JIS G3101) is used for the polepiece and the yoke. The permanent magnet is a sintered Nd῍Fe῍B magnet (NEOMAX 39SH, NEOMAX Co.,
Ltd., Osaka, Japan). Since the magnetic material isporous, the surface of the magnet is coated with TiN.
This coating prevents adsorbed gases ﬂowing out from

Ion trajectories simulated by “TRIO-DRAW.” (a)x-direction; (b) y-direction.

Fig. 3.
ῌ 4 ῌTechnical drawing of the prototype instrument.

Development of a Miniature Double Focusing Mass SpectrographFig. 4.

Fig. 5.

Prototype instrument in the vacuum chamber.

Schematic drawing of the magnetic circuit.

the magnet. The magnetic ﬂux density is 0.576 T.
When ions of m/z 4 are accelerated to 10 keV, the mean
radius of the ion trajectory in the magnetic ﬁeld is 50mm.
2.2.2 Focal plane detector The instrument employs an MCP-based position sensitive detector. The
detector consists of an MCP, a phosphor layer on aﬁber-optic plate (FOP) and a CCD. The schematic drawing of the focal plane detector is shown in Fig. 6. Ion
beams entering the channels of the MCP are converted
and ampliﬁed to electron clouds by the MCP. The MCP
also retains the spatial information of the proﬁle of theion beam. These electrons are accelerated to the phos-

Fig. 6.

phor layer and converted into photons. These photons
then pass through the FOP and are detected by the
CCD.
An assembly (F4301῍04, Hamamatsu Photonics, Shizuoka, Japan) comprising an MCP, a phosphor layer
and a FOP is used for the detector. The active area ofthe MCP is 55 mmῌ8 mm and the channel diameter is
12 mm. The MCP is composed of two stages and thetypical gain is over 106 if the voltage applied to the
MCP is 2.0 kV. The front surface of the MCP isgrounded. The type of phosphor is P46, and thetypical wavelength and the decay time are 530 nm and
300 ns, respectively. The CCD is a front-illuminatedtype CCD (S7175F, Hamamatsu Photonics, Shizuoka,
Japan). The CCD is a full frame transfer type (FFT).
The number of active pixels is 1024 (horizontal)ῌ128(vertical), each pixel is 48 mmῌ48 mm, and the active
area is 49.152 mmῌ6.144 mm.
The FFT-CCD is normally used in conjunction withan external shutter mechanism to ensure that no lightenters the CCD while the signal charges are beingtransferred from the imaging section. A pulsed voltage
is applied to the deﬂection electrode in the ion source to

Schematic drawing of the focal plane detector.
ῌ 5 ῌ

M. Nishiguchi et al.

prevent the ion beam entering the detector while thesignal charges are being transferred inside the CCD.
The CCD can be operated in two modes: line binningand pixel binning. In line binning, the signal charges inthe vertical shift register are accumulated in the horizontal direction and the output data comprises a onedimensional stream of 1024 channels. In the pixel binning operation, the signal charges of all pixels are sent
to the output section one after another and the output data comprises two-dimensional data of 1024῎128
channels. The schematic drawing of the data acquisition system is shown in Fig. 7. The timing signals forcontrolling the CCD and the shutter mechanism aresupplied by a digital I/O board (PCI-2472C, Interface,
Hiroshima, Japan). The clock frequency of the signal
charge transfer is 100 kHz. Thus, the frame rate in line
binning is about 100 frames per second (fps), and thatin pixel binning is about 1 fps. The analog signal fromthe CCD is ﬁrst ampliﬁed by the preampliﬁer installednear the CCD. The block diagram of the CCD control
system and the shutter mechanism in pixel binning isshown in Fig. 8. The output signal of the preampliﬁer isthen ampliﬁed by the main ampliﬁer, and the output of
the main ampliﬁer is acquired as a digital signal via a
12-bit A/D board (PCI-3163, Interface, Hiroshima,
Japan).

Fig. 7.

2.3 2D and 1D spectra
For the investigation of the ion optical system andthe detecting system, the 2D and 1D spectra of theresidual gases were observed. The 2D and 1D spectra of
the residual gases are shown in Figs. 9 and 10, respectively. The experimental conditions were as follows:
the width and the height of the main slit were 0.25 mm
and 2.0 mm; the width and the height of the a-slit were
0.5 mm and 3.0 mm; the accelerating voltage of the ionswas 2200 V; the voltage applied to the electrostatic
sector was ῍435.6 V; the electron energy and the electron current for the ionization were 70 eV and 250 mA.
The voltage applied to the MCP and the phosphor layer
was 0.7 kV and 3.7 kV. The operation mode of the CCD
was pixel binning, and the exposure time of the CCD
was 1.0 s. The 1D spectrum was obtained by accumulating the lines from 10 to 120 of the 2D spectrum
in the vertical direction. The abscissa axis of the 1D
spectrum shows the CCD channel. A mass resolution of
130 (10ῐ valley) was achieved at the peak of m/z 18.
The expanded 2D spectrum at m/z 18 of Fig. 8 is also
shown in Fig. 11(a).
For evaluation of the experimental results, we simulated the 2D spectrum using the transfer matrix
method in the second order approximation. The initial
position x0, y0 and the initial angle a0, b0 were set tocoincide with the experimental conditions of the main
slit; they were chosen in the assumption that ionsleaving via the main slit would pass through the a-slit.
The initial energy spreads of the ions were set from d῏ῌ0.001 to d῏0.001. These parameters were generated
by random numbers using the above initial conditions.
The simulated 2D spectrum is shown in Fig. 11(b). The
transfer matrix is the one in which the radius of themagnetic ﬁeld is 50 mm, corresponding to the experimental data shown in Fig. 11(a). The mass resolution of
the simulated spectrum is also 130 (10ῐ valley). The
experimental result is in good agreement with thissimulation.

Data acquisition and CCD control system.

Fig. 8.

Fig. 9.

2D spectrum of residual gases.

Timing chart of the CCD control system and shutter mechanism in pixel binning.
ῌ 6 ῌ

Development of a Miniature Double Focusing Mass SpectrographFig. 10.

Fig. 11.

1D spectrum of residual gases obtained from the 2D spectrum.

(a) 2D spectrum expanded around m/z 18 in Fig. 7. (b) Simulated 2D spectrum.

The detectable m/z range depends on the detector
size and the detector position. Here, the detector size isabout 50 mm; the detectable range varies from 2.5 to
4.4 according to the detector position. In the 1D spectrum of Fig. 10, the detectable m/z range was calculated to be 2.8. This was derived as follows. First, thechannel numbers of the abscissa axis were calibrated
to m/z. Since m/z is proportional to the square of themean radius of the ion trajectory in the magnetic
sector, the relation between m/z and the channel numbers of the abscissa axis can be expressed as m/z῏ax2
ῌbxῌc. Here, a, b, and c denote the calibration coe$cients, and x denotes the channel numbers. These coe$cients were calculated by applying the least-squares
ﬁtting to the channels numbers corresponding to thepeak centroids of m/z 14, 18, 28, and 32. In consequence, the 0 and 1024 channels correspond to m/z
12.4 and 34.1, respectively. Next, the detectable rangeof 34.1/12.4 ῏ 2.8 was obtained. On the other hand, themean radius of m/z 34.1 in the magnet is calculated as
68.5 mm when the accelerating voltage and the magnetic ﬂux density are 2200 V and 0.576 T, respectively.
When the maximal mean radius in the magnetic ﬁeld is68.5 mm, the minimal mean radius is calculated as 41.0
mm by assuming a detector size of 50 mm. The detectable m/z range is then calculated as (68.5/41.0)2῏2.8.
This value is consistent with the experimental result.
The 2D spectra of Kr and Ne were observed to investigate the dynamic range of the detector. The 2D and 1

D spectrum of Kr is shown in Fig. 12. The experimental
conditions were as follows: the background pressure
was 4.0῎10ῌ4 Pa in the vacuum chamber and thepressure was increased to 1.3῎10ῌ3 Pa when Kr gas
was introduced; the accelerating voltage of ions was
800 V; the voltage applied to the electrostatic sectorwas ῍158.4 V; the electron energy and the electron
current for ionization were 70 eV and 250 mA. The
voltage applied to the MCP and the phosphor layer was
1.0 kV and 4.0 kV. The operation mode of the CCD was
pixel binning and the exposure time of the CCD was 1.0
s. The 2D spectrum shown in Fig. 12(a) is expandedaround the spectrum of Kr. The 1D spectrum shown in
Fig. 12(b) was obtained by accumulating the lines from30 to 120 of the 2D spectrum in the vertical direction.
All the stable isotopes of Kr are observed simultaneously without saturation of the CCD. Here, the ratiosof peak areas were calculated; their peak areas areshown in Table 3. The errors show mean square errorsof ﬁve consecutive measurements. Each peak area
ratio (the roughly estimated isotope ratio) agrees withthe standard isotope ratio26) within a deviation of30ῐ. A dynamic range of 200 was obtained fromthe 78Kr/84Kr ratio.
The 1D spectrum of Ne is shown in Fig. 13. The
experimental conditions are as follows: the backgroundpressure was 4.0῎10ῌ4 Pa in the vacuum chamber, andthe pressure was increased to 1.3῎10ῌ3 Pa when Ne
gas was introduced; the accelerating voltage of ionsῌ 7 ῌ

M. Nishiguchi et al.

Table 3.

Peak Area Ratios of Kr and Ne
Peak area ratio

Standard isotope ratio

Kr/84Kr
Kr/84Kr
82
Kr/84Kr83
Kr/84Kr86
Kr/84Kr

0.0041 ῌ 0.0002
0.0336 ῌ 0.0002
0.173 ῌ 0.001
0.156 ῌ 0.001
0.276 ῌ 0.002

0.00620.04010.20340.20180.3032210.0031 ῌ 0.0003
0.1087 ῌ 0.0006

0.00300.102278
80

22

Ne/20Ne
Ne/20Ne

CCD, although the signal-to-noise ratio of the 21Ne peak
was not su$cient. The ratios of peak areas were calculated. These ratios of peak areas are also shown in
Table 3. The errors show mean square errors of ﬁve
consecutive measurements. These roughly estimatedisotope ratios agree with the standard values26) withina deviation of 6῍. In addition, a dynamic range of 300
was achieved with the 21Ne/20Ne ratio.
3.

Fig. 12.

Conclusion

We newly designed and constructed a miniature
double focusing mass spectrograph using a focal plane
detector as a prototype model for lunar and planetary
explorations. We employed the focal plane detector
consisting of an MCP, a phosphor layer, a FOP and a
CCD. We observed the 2D and 1D spectra of the residual gases, krypton and neon. The experimental value ofthe mass resolution was in good agreement with thesimulation. The experimental value of the detectable
m/z range was also consistent with the calculated
value. We conﬁrmed that stable isotopes of Kr and Ne
could be observed without saturation of the detector. A
dynamic range of 300 was achieved experimentally.

(a) 2D spectrum of Kr and (b) 1D spectrum of
Kr obtained from the 2D spectrum.

References
1)

2)

3)

4)

5)
Fig. 13.

1D spectrum of Ne obtained from the 2D
spectrum.

was 3050 V; the voltage applied to the electrostatic
sector was ῌ609.3 V; the electron energy and the electron current for the ionization were 70 eV and 250 mA.
The voltages applied to the MCP and the phosphor
layer were 0.9 kV and 4.0 kV. The operation mode of
the CCD was pixel binning and the exposure time of
the CCD was 1.0 s. The 1D spectrum was obtained byaccumulating lines 30 to 120 of the 2D spectrum in thevertical direction. All the stable isotopes of Ne wereobserved simultaneously without saturation of the

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ῌ 8 ῌ

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Development of a Miniature Double Focusing Mass Spectrograph

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Keywords: Double focusing mass spectrograph, Focal plane
detector, Ion optics

ῌ 9 ῌ