|
Detech offers a complete set of technical documentation
for our current product line. As such, we've provided a sampling of that
information available here. To learn more about a specific topic, simply
click on a link below. Whether it's step-by-step instructions or component
specs, we have the information you need. Copies of technical papers are
available by contacting our sales department at 413.284.9975.
Want a glimpse into the DeTech multiplier process?
We've created a page to give you an overview of
the 6 step process of converting glass stock into a fully functional
electron multiplier. From extrusion to final testing, see how DeTech manufactures
electron multipliers that are second to none!
Cleanliness | Collection
Efficiency | Conversion Dynodes | Dark
Noise | Detection Schematics | Full
Width Half Maximum | Gain | High
Pressure Operation | Historieses | Internal
Bias Resistor | Lifetime | Multiplier
Operation | Multiplier Resistance | Multiplier
Terminology | Non-Conductive Zone | Pre-Conditioning
| Pulse Height Distribution | Rise
Time | Shelf Life | Signal
Connection | Vacuum Baking
Cleanliness
– In the event that a multiplier becomes contaminated with
lint, dust, or other particulate the multiplier should be flushed with
either isopropyl or methanol. The unit can then be blown with dry nitrogen
and baked at 150°C until dry. Multipliers should not be cleaned
in any acid, this would have a detrimental effect on its lifetime and
performance.
back to top
 Collection
Efficiency - Collection efficiency is the ability to detect an
input event and convert that event into output signal. Ion optics play
a major role in attaining good collection efficiency. If signal is lost
during the first strike stage results will be inaccurate. When an input
event strikes the surface of the cone secondary electrons are created.
These electrons should travel down the channel and strike the emissive
surface creating more electrons. If the first strike electrons exit the
cone then the detector will have poor collection efficiency. There are
two solutions to this problem. A grid can be placed on the cone. The grid
acts as a barrier between any outside potentials that may attract the
secondary electrons. It forces the electrons to travel down the channel.
The second option is to add focusing plates to the input ion stream. If
the plates are placed properly the first strike event will occur at the
apex of the cone. The secondary electrons will then be created deep enough
in the voltage gradient that they will travel down the channel. By improving
collection efficiency signal to noise will increase and data analysis
will be more accurate.
back to top
Conversion Dynodes - The
primary advantage for utilizing a conversion dynode is to increase the
secondary emission characteristics of heavier ions. Since conversion dynodes
can operate anywhere between 5 and 50 KV, the attraction of the heavier
ion is greater thus an increase in the secondary emission ratio occurs.
In addition, the mass discrimination in the multiplier is reduced. If
the conversion dynode is set for positive ion detection, there is a negative
high voltage placed on the dynode. When the ion strikes the dynode, secondary
electrons are created, thus being attracted to the multiplier. For negative
ion detection, a positive ion is produced from the surface and detected
by the electron multiplier. The positive ions are produced primarily by
secondary ion emissions (sputtering) either from the metal (of which the
conversion dynode is constructed) or the gasses absorbed on its surface.
DeTech manufactures several types of dynodes, created from several different
materials. We have full capability of testing dynode multiplier configurations
in positive and negative ion mode at voltages up to +/- 30KV. Please contact
a DeTech engineer for more information or design ideas.
back to top
Dark Noise - Noise is considered
to be any unwanted signal seen. Generally noise can be broken down into
two categories, dark counts and dark current.
Dark counts occur when an input event causes the multiplier
to create a pulse of electrons. A properly functioning multiplier will
not create electron pulses without an input event. When this type of noise
occurs, it is typically caused by the environment the unit is in. Dirty
source, high pressures, and neutrals are a few things that may cause this
type of noise. Detector Technology tests 100% of its product to ensure
that the multipliers are working properly and are not creating dark counts.
Dark current is generally caused by microphonics. This
occurs when electronic signals are placed in close proximity. Crosstalk
and voltage arcing are two of the major causes of dark current. The best
method to decrease microphonic noise is to increase shielding around the
signal lead of the electron multiplier. Detector Technology builds its
product with this in mind. Many of our models include shielding to decrease
any possible microphonic noise.
back to top
Detection Schematics - There
are many ways to run an electron multiplier. Below is a sample of various
ways to run a unit based on either positive or negative ions in various
modes. Please consult with a DeTech engineer for any installation questions.
back to top
Full Width Half Maximum (FWHM) -
Full width half maximum is a measurement describing the width of
the pulse height distribution in pulse counting applications. It is mathematically
expressed as:
| FWHM= |
Peak Gain Value |
X 100 |
 |
| Gain Value at half peak height |
When using a channel electron multiplier in pulse counting
applications it is advised to use a multiplier with a FWHM of 150% or
less. In pulse counting applications it is important to have a multiplier
capable of producing pulses with similar amplitudes.
back to top
Gain – The gain of
a multiplier is defined as the ratio of the output to input. This can
be measured in terms of counts or current. For counting, each input event
creates one charged pulse, which is treated as one count. The intensity
of the pulse is the gain of the multiplier. For current mode, an average
input and output current is measured. The ratio of the output to input
current determines the gain. The method chosen is based on the application
and on the style of multiplier used. Usually when the multiplier gain
is less than 106 then current measurements are preferred. If the gain
is higher than 106 then either counting or current measurements may be
taken.
back to top
 High
Pressure Operation – In several detection applications it
is necessary to operate at elevated pressures such as 10-4 – 10-2
Torr. Traditionally channel electron multipliers are not able to function
efficiently at these pressures because of ion feedback. Detector Technology
has developed an unconventional style of channel electron multipliers
that has the capability of operating in this high pressure range.
The Series 2000 is a multi-channel detector that has internally
twisted channels. The tightly twisted channels prevent ion feedback. Ion
feedback occurs when residual gases travel down the channel and strike
the emissive surface. Secondary electrons can be released causing noise
and decreased lifetime. At higher pressures more residual gases reside
in the channel, by tightly twisting the channels the residual gases are
not able to gain enough kinetic energy to create secondary electrons.
Therefore, noise is eliminated and lifetime is increased.
back to top
Historieses - When an electron
multiplier is subjected to a large input it has the potential to go through
a period of dead time before the next pulse can be detected. This phenomena
is called Historieses.
When introducing a large input into the detector a high level of electron
bombardments occur on the back end of the detector. This causes a large
amount of heat to be generated on the output end which directly effects
the resistance of the detector. When the heat increases in a detector
the resistance decreases. With Historieses the back end of the detector
becomes hotter than the input end causing an imbalance in the resistance
of the detector. Since the resistance on the font end is higher the voltage
shifts to the front end causing a temporary loss of gain. The detector
requires time to recover and balance the resistance off before it can
accept the second pulse. The time that it needs to recover is related
to the temperature of the detector. The faster it can cool down the better
the recovery time.
One way to improve upon Historieses is to increase the
resistance in the detector. This will allow the detector to operate at
a cooler temperature and be less susceptible to voltage shifts within
the multiplier during high gain amplification. The tradeoff with increasing
the resistance is the reduction in the dynamic range of the multiplier.
back to top
Internal Bias Resistor - The
channel electron multipliers manufactured by Detector Technology include
a self biasing resistor. This is most useful when using the multiplier
in EIC applications. The internal bias resistor is located towards the
back end of the channel and can be seen on the outside of the tube. It
appears to be black rather than silver. This resistor is manufactured
to be 3 – 5% of the overall resistance of the detector. It is used
to bias the back of the channel so exiting electrons are attracted to
the collector plate.
back to top
Lifetime – Multiplier
lifetime is determined by two classic failure mechanisms: failure through
electron depletion or through ion contamination. The mode of failure that
occurs is related primarily to the vacuum pressure during operation. The
magic pressure that separates the two modes of failure is 5x10-7 torr.
Deeper vacuum pressures will see electron depletion failure and any higher
pressure environments will see ion contamination.
Electron Depletion - In
a contamination-free environment the lifetime of an electron multiplier
has been found to depend upon the number of electron impacts per unit
surface area. Therefore the lifetime of a multiplier is determined by
the cumulative charge output. Experiments performed at Detector Technology
have confirmed that the total charge of a multiplier is typically 4
coulombs before the lifetime is exhausted. This mode of failure is under
ideal conditions, of vacuum pressure on the order of 5x10-7 torr or
deeper. The typical area to fail is the back end of the multiplier where
the most secondary electrons are released.
 Ion
Contamination - Since most mass spectrometers and residual gas
analyzers do not operate under ideal vacuum pressures the mode of failure
is not from electron depletion, but from ion contamination. In a non-ideal
operational setting, electron irradiation can modify or otherwise damage
the surface through a variety of mechanisms. The net effect can be one
or more of the following:
- Changes in surface composition
- Structural modifications or damage
- Electron charging and heating
The main concern is the possible effects of electron
stimulated desorption (ESD), electron stimulated adsorption (ESA), and
electro migration. All three phenomena can oxide the surface, deposit
carbonaceous layers on the surface, or influence the concentration and
depth distribution of alkali ions in the reduced layer. Once this occurs
and the surface composition is affected the surface work function can
be dramatically decreased, thus causing a loss in gain and concluding
in a loss of lifetime.
The input end of the multiplier can also be damaged through
the impact of high molecular weight ions at high energies. These impacts
can result in chemical changes in the glass surface and even physical
sputtering which can reduce the effective sensitivity of the device.
Once a change in surface composition is created, then
cleaning of the multiplier is very difficult. To prevent this effect,
proper conditioning of the multiplier is critical to remove any loosely
absorbed gasses on the surface prior to full operation. Also, operating
at the best possible vacuum pressure will also increase lifetime tremendously.
back to top
 Multiplier
Operation – DeTech multipliers are manufactured from a lead
silicate glass that is processed to have secondary emissive and resistive
properties. When the interior surface is struck by an ion, electron or
photon secondary electrons are released. The voltage applied to the multiplier
causes the secondary electrons to travel down the channel. Eventually,
the secondary electrons will strike the emissive surface again, causing
the release of subsequent electrons. The electrons are eventually collected
at the output end of the multiplier. This signal is fed into the appropriate
electronics.
back to top
Multiplier Resistance - Electron
multiplier resistance plays a significant role in the multiplier’s
performance when dynamic range is of concern. DeTech has the capabilities
of manufacturing multipliers with various resistance ranges based on the
glass compositions selected.
Dynamic Range is the ratio of the maximum to minimum detectable signal.
In pulse counting mode it is the measure of maximum count rate capability
and in the analog mode it is the maximum linear output current. Both of
these measures are directly dependent on the CEM bias current (strip current)
at the operating voltage. Linear output is approximately 10% of the bias
current. For example:
I (linear output) = (E / R) * .10
Where: E = Applied voltage (volts)
I = Linear output current (uA)
R = Resistance (MW)
The resistance decreases when the operating temperature
increases. Therefore, the bias current increases for a given applied voltage.
This results in a proportionate increase in dynamic range.
There are limits for how low a channel electron multiplier can drop before
effecting performance. If the resistance drops below 10MW, thermal runaway
may occur. Thermal runaway is a non-recoverable rapid loss in gain. Maintaining
a resistance above 10MW is considered safe.
back to top
Multiplier Terminology -
Below is a schematic of a typical electron multiplier and terms that are
often used to refer to features and components of their assemblies.
back to top
 Non-Conductive
Zone - On the outside of the detector there is a section of the
multiplier surface that appears to be rough in texture. This is the non-conductive
zone. The multiplier surface is resistive because of the special processing
that it goes through during manufacturing. A section of the resistive
layer is removed so that high voltage can be applied to the multiplier
and the current flows down the internal channel. If this area was left
resistive then the multiplier would not work efficiently. The multiplier
would act as a current divider and the performance of the channel would
be effected.
back to top
Pre-Conditioning – In
order to maximize the lifetime of a multiplier it should be pre-conditioned.
During this process loosely bonded water molecules are released from
the surface. To perform preconditioning a multiplier should be placed
under vacuum. It is recommended that a vacuum of 10-6 Torr or better
be used. Once the appropriate vacuum is reached a small input should
be applied to the multiplier. In counting mode approximately 15,000
counts/second is desirable. In current mode approximately .1uAmp is
needed. The multiplier voltage should be slowly raised to reach an appropriate
gain level. In counting mode this should be approximately 107. In current
mode this should be approximately 105. The multiplier should be run
in this state for several hours. This should be done in order to avoid
a rapid release of water molecules. If the water molecules release rapidly
then the multiplier surface can be permanently damaged, thus shortening
the life of the detector.
back to top
Pulse Height Distribution (PHD)
- Pulse height distribution is the graphical representation of
the distribution of the amplitude of the pulses generated by an electron
multiplier at a particular voltage. In pulse counting applications the
graph appears to be quasi-Gaussian, or bell shaped. In analog applications
the graph appears to be negative exponential.
back to top
Rise
Time - The rise time is defined as the length of time it takes
for the leading edge of the produced pulse to go from 10% to 90% of its
maximum amplitude. Channel electron multipliers usually have a rise time
of approximately 3-5nsec. The rise time is determined by the length of
the multiplier channel.
back to top
Shelf Life – It is
ideal to store the multiplier in vacuum, but not required. DeTech lead
glass multipliers are air stable devices that can be stored indefinitely,
as long as the units are kept out of direct sunlight, dry, and kept in
their original sealed bags prior to use. DeTech fully warranties all of
its multipliers for shelf life.
back to top
Signal Connection - There
are two methods for collection of signals for an electron multiplier:
either a closed cap collector or an isolated collector. Below are the
electronic schematics for both methods.
back to top
Vacuum Baking - Vacuum baking
is used as a method of decreasing water molecule adsorption on the emissive
surface of channel electron multipliers. When water molecules are on the
emissive surface they are released during multiplier use. If the molecules
are released too quickly permanent damage may occur to the emissive surface.
When vacuum baking Detector Technology recommends baking at no hotter
than 250°C for 12-15 hours. During this time period water molecules
are slowly released from the emissive surface in a safe manner. It is
important to vacuum bake in a clean environment. If other materials in
the vacuum bake outgas the channel electron multiplier may adsorb these
gases and the emissive surface could be adversely effected. Detector Technology
recommends that all units be vacuum baked. This helps to precondition
the unit and increase lifetime.
back to top
|
