U.S. patent number 3,898,456 [Application Number 05/491,988] was granted by the patent office on 1975-08-05 for electron multiplier-ion detector system.
This patent grant is currently assigned to The United States of America as represented by the United States Energy. Invention is credited to Leonard A. Dietz.
United States Patent |
3,898,456 |
Dietz |
August 5, 1975 |
Electron multiplier-ion detector system
Abstract
The invention relates to an improved ion detector for use in
mass spectrometers for pulse counting single ions which may have a
positive or a negative charge. The invention combines a novel
electron multiplier with a scintillator type of ion detector. It is
a high vacuum, high voltage device intended for use in ion
microprobe mass spectrometers.
Inventors: |
Dietz; Leonard A. (Schenectady,
NY) |
Assignee: |
The United States of America as
represented by the United States Energy (Washington,
DC)
|
Family
ID: |
23954494 |
Appl.
No.: |
05/491,988 |
Filed: |
July 25, 1974 |
Current U.S.
Class: |
250/299; 250/281;
250/283; 250/368 |
Current CPC
Class: |
H01J
49/025 (20130101); G01T 1/29 (20130101) |
Current International
Class: |
H01J
49/02 (20060101); G01T 1/00 (20060101); G01T
1/29 (20060101); B01D 059/44 (); G01T 001/20 ();
H01J 039/34 () |
Field of
Search: |
;250/368,369,281,282,283,298,299,300,213UT,489 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Grigsby; T. N.
Attorney, Agent or Firm: Carlson; Dean E. Freundel; Bernice
W.
Claims
What is claimed is:
1. A combined electron-multiplier ion-detector system for use in a
mass spectrometer comprising:
an ion beam source,
an electrostatic lens which receives, deflects and focuses the ion
beam into
an electron multiplier, having an extended dynode unit consisting
of a first conversion dynode and five additional electron
multiplying dynodes,
an accelerating lens means connecting said electron multiplier
to
a scintillator detecting surface,
a light pipe, optically coupled at one end with the scintillator,
and
a photomultiplier tube optically coupled to the second end of the
light pipe.
2. The combination defined in claim 1 in which the conversion
dynode consists of a nonmagnetic dynode holder supporting a
substrate covered by a thin amorphous oxide film surface.
3. The combination defined in claim 2 in which the conversion
dynode and the five additional dynodes act as a unit to convert a
single ion impact into a pulse of 500 or more secondary
electrons.
4. The combination defined in claim 2 in which the extended dynode
unit is floated at a voltage of .+-. 10KV from ground potential to
detect negative or positive ions respectively.
5. The combination of claim 3 wherein the accelerating lens means
consists of a cylindrical lens and a disc lens which act to
accelerate the pulse of secondary electrons and focus the pulse
onto the scintillator.
6. The combination of claim 5 in which the scintillator consists of
a flat plastic surface covered by a thin film of aluminum which
converts the secondary electrons to photons which in turn are
converted to photoelectrons in the photomultiplier tube, undergo
further charge amplification there, and are pulse counted in
associated electronic circuits.
Description
BACKGROUND OF THE INVENTION
In an ion microprobe mass spectrometer a finely collimated and
focused beam of energetic ions strikes a target surface. Secondary
ions sputtered from the surface are mass analyzed and pulse counted
in a particle detector which detects either positive or negative
ions, but not both kinds simultaneously. Prior art devices have had
rather high background counting noise of 20-40 counts per second
whereas the desired background is 1 count per minute or less. Few
mass spectroscopists have background counting rates less than 1
count per second even after many years of experience with the
operation. In an effort to improve prior art devices, an electron
multiplier was substituted for a scintillator detector in an ion
microprobe. The multiplier succeeded well in both the positive and
negative ion detection modes insofar as background counts were
concerned, i.e., a few counts per minute.
The novel electron multiplier-scintillation detection has been
developed for conventional magnetic sector mass spectrometers or
ion microprobes. It is designed for a beam of ions focused to a
small spot size or to a narrow line image at the focal plane of a
mass analyzer. However, the basic concept is not limited to focused
ion beams of kilovolts energy. With suitable modifications it
should work equally well with unfocussed ion beams or with low
energy ion beams.
SUMMARY OF THE INVENTION
The present invention relates to a novel electron
multiplier-scintillator ion detector for pulse counting positively
or negatively charged single ions in mass spectrometers, for
example, a 30-inch radius Dempster type mass spectrometer. A
special feature of the new detector is the concept of an extended
first dynode. More specifically, the conversion dynode and the five
additional stages of the electron multiplier act as a single unit
to convert each ion impact into a pulse of 500 or more electrons,
on the average.
The basic objective of this invention is to combine the best
features of an electron multiplier detector with the flexibility of
a scintillator detector so that either positively or negatively
charged ions can be detected with very high efficiency and be pulse
counted as single events. Very simply, what is desired is one pulse
detected and counted for each ion that strikes the conversion
dynode, while at the same time maintaining a low background
counting rate in the detector.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. I is a top plan view of the present ion detector system.
FIG. II is a more detailed view of the electron multiplier of FIG.
I.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. I, the ion detector system includes a source
of an ion beam, 1, which designates a series of mass spectrometer
defining slits. Grounded defining slits 2 and 3 terminate the ends
of an electrostatic analyzer 4 which functions to provide energy
filtering of the mass-analyzed source ion beam. The electrostatic
analyzer 4 also serves as a beam deflector to guide the ion beam
into an electron multiplier 5. The electron multiplier 5 consists
of a conversion dynode 7, and five stages of secondary electron
multiplying dynodes, a focus or deflector electrode 6 and a pair of
curved deflector electrodes 23 and 24 (shown more clearly in FIG.
2). A mica insulating slab 8 is positioned at each end of dynodes
20 and 25 and electrodes 6, 23, and 24. The ion beam enters the
multiplier section 5 through a rectangular aperture 9 to impinge on
the conversion dynode 7. The secondary electrons from the
multiplier exit through a circular aperture 10 through a
cylindrical accelerating lens 11 and a disc lens with cylindrical
symmetry 12 onto a flat plastic scintillator surface 13. The
scintillator 13 is covered by a thin film of aluminum, 500-1000 A
thick. The secondary electrons are converted into photons in the
scintillator 13 and then the photons are transmitted through a
quartz light pipe 14 which is optically coupled to the plastic
scintillator 13. The light pipe 14 is supported by an O-ring 15 in
a vacuum wall 16 and is in turn optically coupled to a conventional
photomultiplier tube 19 which is wired for pulse counting. A photon
of light, denoted at 17, is shown striking a photocathode 18 of the
photomultiplier tube 19.
A soft iron cylindrical magnetic shield (not shown) would surround
the electron multiplier and scintillator assembly. It would extend
from outside aperture 9 to beyond the scintillator 13. Vacuum
seals, mechanical mountings and high voltage insulation of the
light pipe and scintillator would follow conventional practice.
In FIG. II the electron multiplier 5 is shown in greater detail.
The conversion dynode 7 consists of a dynode holder 20 made of
nonmagnetic stainless steel supporting a glass or other suitable
substrate 22 covered by an oxide surface 21. The oxide surface 21
which converts ion impacts into secondary electrons is a thin
amorphous film of plasma-anodyzed aluminum oxide (Al.sub.2 O.sub.3)
of 100-200 A thickness, (1 A = 10.sup..sup.-8 cm). This film
surface 21 gives a higher yield and a more sharply peaked pulse
height distribution than that from an aluminum film or solid
aluminum metal surface which has been oxidized by exposure to pure
oxygen or air.
The focus or deflector electrode 6 may be made of Type 304
stainless steel. The five curved dynodes designated 25 may all be
made from CuBe alloy.
Resistors labeled 26, 27, 28, 29, 30, and 31 having the values set
forth, namely: 300K, 230K, 175K, 135K, 105K, 100K ohms form a
voltage divider connected serially across pairs of the dynodes
which supplies a geometric voltage division to the dynodes.
Alternatively, a linear voltage division, i.e. equal voltages
between dynodes, also works well. The electron multiplier is
designed to be floated at a nominal voltage such as .+-. 10KV from
ground potential, to detect negative or positive ions respectively.
Approximate electrode voltages are shown in FIG. 1 for both the
negative ion detection mode and the positive ion detection
mode.
Considering the operation of the electron multiplier-scintillating
ion detector system further, it will be seen that an ion beam
emerging from mass spectrometer slit 1 will be guided through the
electrostatic analyzer 4 or other suitable electrostatic lens which
acts as a deflector so that the beam impinges onto the conversion
dynode 7. The conversion dynode plus the five electron multiplying
stages 25, act as a unit or extended first dynode to provide on the
average more than 500 electrons per positively or negatively
charged ion impacting the surface of the conversion dynode. The
cost in high voltage to accomplish this is small, about 1KV out of
the .+-. 10KV in the electron multiplier. The electron multiplier
plays the role of an electronic preamplifier and provides virtually
noiseless amplification of each pulse of secondary electrons from
the conversion dynode. A current gain of several hundred is
sufficient to establish a favorable pulse-height distribution in
the electron multiplier section of the detector. These electrons
pass through aperture 10, are accelerated to about 9KV energy and
are focused onto the plastic scintillator 13. Conversion of these
secondary electrons into photons in the scintillator 13, then back
to photoelectrons in the photomultiplier tube 19 has little or no
effect on the shape of the pulse-height distribution detected at
the anode of the photomultiplier tube. This is true because the
statistical multiplication process is determined in the first few
stages of the electron multiplier. Any electron or photon
amplification after the first few stages increases the DC gain of
the system but does not noticeably change the shape of the
pulse-height distribution for a given ion species.
The surface film of the conversion dynode has been described as a
plasma-anodized metallic film. Thin oxide films of this type can
also be formed by RF sputtering of pure oxides. Because the escape
depth of ion-induced secondary electrons formed below an oxide
surface is only of the order of 10 A, an oxide film of 30-50 A
thickness produces as high a yield as one several hundred A
thickness. Therefore, the selection of the thickness of the film is
a matter of choice. A thin film of pure beryllium oxide may be an
alternate choice for the oxide film, since it may provide higher
secondary electron yields and more sharply peaked pulse-height
distributions than presently is realized from an Al.sub.2 O.sub.3
surface film.
Various modifications of the present detector system may be made.
For example, for unfocused ion beams or with low energy beams the
sensitive area of the conversion dynode may be increased and
venetian blind dynodes, transmission dynodes, mesh dynodes, or
other secondary multiplication arrangements following the
conversion dynode may be added. One obvious application for the new
detector principle is in quadrupole, monopole or similar rf mass
spectrometers. In these spectrometers the energy of the
mass-analyzed ions can be as low as a few tens of volts.
Post-acceleration of these low energy ions into the kilovolt range
is necessary for efficient detection by pulse counting or when
using a secondary electron detector in the DC mode to measure very
small ion currents. This detector could also be used to detect
energetic neutral atoms.
Tests with a 10-stage 56 AVP photomultiplier tube in the new
detector gave an output pulse width of less than 3 nanoseconds full
width at half maximum amplitude (FWHM). This suggests a resolution
time of 10 nanoseconds or less for the overall detector and
counting system.
* * * * *