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Equipment Operating Manual for X-ray Framing Camera
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Last Revision by: Peter Susalla / 1 January, 2004
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Table of Contents
Part 1: General Introduction x
Part 2: Operating Procedures
Part 3: Vacuum System x
Part 4: X-ray Source x
Part 5: X-ray Framing Camera x
Part 6: Miscellany x
Part 7: Appendix x
Part 8: Glossary x
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Part 1 – General Introduction
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This manual is designed to be a step-by-step guide to operation of the Drake Lab vacuum
system and associated x-ray framing camera instrumentation, as well a general reference
for the above. The reader will find a complete guide for all facets of the experiment,
including pumping down and repressurizing the vacuum chamber, changing out
instrumentation, manipulating and using the x-ray source, and taking pictures with the
framing camera. This guide will also contain other information useful to the
understanding of the operation of the system as pertinent. Fundamental concepts relative
to these procedures will be discussed briefly to provide a basic introduction to the user,
and will largely be qualitative in nature. A bibliography will be provided of cited works
and for further reading on these topics. At the end of this guide the user will also find a
glossary of terms and an appendix for formulae and acronyms. There is also an
accompanying binder of manufacturer’s instruction manuals for all of the appropriate
equipment.
A Checklist (in green font) with primary instructional steps will be found in Part 2.
Before operating any of the equipment read the safety warning on the following page.
Throughout the text additional safety warnings will be bold and red, while operation
caveats will be bold on yellow. It is of the utmost importance, for the safety of the user
and others using the lab that these warnings be fully read and understood.
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WARNING: The use of this equipment risks exposure to high
voltages, high temperatures and some hazardous chemicals.
All users of the system are expected to take proper cautions,
wear protective equipment when necessary, have completed
basic OSEH laboratory training and system checkout training
administered by Dr. Drake, Eric Harding or Peter Susalla.
For specific safety warnings on the equipment being used,
consult this manual and the manufacturer’s user guide. This
lab is compliant with OSEH regulations and maintains a
chemical inventory and Standard Operating Procedures for
dangerous substances. Emergency contact information can be
found by the lab phone, in the ‘Chemical Hygiene Plan’ binder
(the ‘Big Red Book’), and on the door to the lab. Call one of
the emergency contact personnel if there is ever any doubt
about a procedure or a problem arises. Contact numbers are
posted near the phone for the Department of Public Safety and
OSEH. Call 911 immediately in the event of a medical
emergency or fire.
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Part 2 – Operating Procedures
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2.1 – Introduction
This section provides a convenient and thorough checklist for operating all of the main
pieces of equipment necessary to produce data from the framing camera. It is worth
reiterating that the user must follow all operational and safety warnings listed below.
Failure to do so could cause damage to the instruments or injury to the operator.
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2.2 – Vacuum System Startup and Shutdown Procedures:
2.2.1 – Startup:
This section is a checklist for pumping the system down from atmospheric to operating
pressure.
1. See that all external valves are closed, and that all flanges are properly bolted
or clamped together.
2. Verify that the Electro-Pneumatic Gate Valve (EPGV) is closed (switch to
‘off’) and that the main system nitrogen valve is open (the small ball valve
under the flow hood, not the two needle purge valves).
3. Make sure both manual gate valves are open all of the way (turn counterclockwise).
4. Turn on the scroll pump by plugging it into the power strip on the floor of
the vacuum rack. The scroll pump will produce a loud, ‘gurgling’ sounds for
a 20-30 seconds while it starts up; this is usual.
5. Once the scroll pump has been operating for 2-3 minutes, open the EPGV,
which will expose the vacuum chamber to the pump. There will be an
audible change in the scroll pump as air is pumped out. After a few minutes
the system should reach a rough vacuum; confirm this with the convection
gauge (which should read less than 100mtorr)
6. Plug in the turbo pump fans (the cord with the large, yellow plug)
7. Once the system is at 100mtorr or lower, turn on the turbo pump (lower
right-hand button)
8. After a few minutes, turn on the ion gauge controller and the ion gauge
filament.
9. The system should reach a pressure in the 10-5 torr range within 30 minutes.
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2.2.2 – Instrument Change-out:
This section is a checklist for removing and replacing either the x-ray source or the
Maxmodule while the main system is left under pressure.
10. Close the manual gate valve seals off the appropriate volume.
11. Open the needle valve to allow a slow flow of nitrogen into the airlock. If the
Maxmodule is being removed, loosen the four retaining bolts (and hold onto
it!!), as there is no pressure release valve. The airlock should reach
atmospheric pressure in a few seconds.
12. Remove the instrument and seal the exposed end with aluminum foil or a
flange cap, purging with nitrogen to keep the airlock as clean as possible.
13. Return the instrument after modifications have been made and bolt it
securely back into place.
14. Turn off the ion gauge filament.
15. Close the EPGV. This will cause pressure to rise in the turbo pump foreline,
so complete the following steps as quickly as possible.
16. Open the ball airlock ball valve, exposing the volume to the scroll pump.
17. Wait at least 30 seconds, and then close the airlock ball valve.
18. Open the airlock gate valve. At this point an audible change will be heard in
the turbo pump as it works to bring the newly exposed volume down to a
high vacuum.
19. Open the EPGV.
20. Turn on the ion gauge filament.
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2.2.3 – Shutdown:
This section is a checklist for returning the system to atmospheric pressure from the
operating high vacuum.
21. Make sure that all instrumentation and power supplies are turned off before
beginning the shutdown. Failure to do so could result in damage to the
instrument or a possible electrical hazard.
22. Make sure that the x-ray-side airlock is at high vacuum, and that the manual
gate valve is open all of the way.
23. Turn off the turbo pump. The pump’s blades will spin down on their own
due to friction, so allow at least 15 minutes before completing the next step.
24. Close the EPGV.
25. Unplug the scroll pump from the power strip.
26. Slowly open the x-ray-side needle valve and start a slow flow of nitrogen into
the system. Watch the convection gauge and see that the pressure rises by no
more than a few torr per seconde. When the system reaches atmospheric
pressure nitrogen will flow out of the pressure release valve; turn off the
nitrogen flow.
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2.3 – X-ray Source Startup and Shutdown Procedures:
2.3.1 – Startup:
1. Follow the Vacuum System Startup Procedure as described above.
NOTE: Never run the x-ray source unless the system pressure is 10-5 torr or
lower. Continually monitor the pressure throughout the use of the source,
and if pressure rises to 10-5 torr, turn it off immediately.
2. Connect the anode feedthrough to the power supply labeled “Anode Voltage”
with an SHV cable, and connect the two cathode feedthroughs to the back of
the X-ray Source Control.
3. Connect the photodiode to the Keithley multimeter with a coaxial cable, turn
it on, and set the multimeter to ‘DCI’. Allowing the multimeter at least 30
minutes to warm up will allow it zero itself properly.
4. Turn the anode power supply on and make sure it is set to 0V. Check that
the Cathode Heating Current and emission control switches are all set to 0,
then power the X-ray source control on.
5. Set the Cathode Heating Current dial to around ‘50’ then wait 5 minutes.
The heating current should rise to around 5A.
6. Slowly increase the Cathode Heating Current to 6A. The heating current
dial should read ~80-90
7. Set the emission control ‘Coarse’ switch to 8, set the anode power supply to
100V, and turn the high voltage switch on the power supply ON.
8. Slowly increase the anode voltage in increments of 500V, noting the current
readout on the power supply and the analog ‘electron beam current’ meter
on the X-ray Source Control: they should roughly be equal. Also note the
photodiode current. It should also start to increase when anode voltage gets
to around 3000V.
9. Increase the anode voltage to 4750V. The x-ray source is now at full
operating power.
10. With the anode voltage at 4750kV, turn the x-ray emission ‘Coarse’ switch to
10; then increase the ‘Fine’ switch slowly. A notable increase in x-rays
should be detected by the photodiode. Set the electron current to 0.5mA.
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2.3.2 – Shutdown:
11. Turn both emission control switches to zero.
12. Set the anode power supply to 0V and turn it off.
13. If you plan to remove the x-ray source from the system, set the cathode
heating power dial to zero and turn off the Source Control. If not removing
the source, set the heating power such that ~3A of current flows through the
cathode. This will allow ultraviolet light and heat produced by the cathode
to free water vapor trapped in the metal surrounding the source head.
14. Disconnect all cables.
15. Turn off the multimeter.
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2.4 – RGA Startup and Shutdown Procedure
1. Pump the system down to high vacuum. The system pressure must be below
10-4 for the RGA to function properly. Failure to pump the system below this
pressure will cause damage to the RGA filament and other components.
2. Power on the RGA head (the switch on the back of the unit). A green LED
on the RGA head should indicate that it is receiving adequate power.
3. Start the RGA software control (icon is on the desktop of the control PC)
4. Open a connection with the RGA head: click on the small ‘plug’ icon on the
toolbar and select ‘connect.’
5. Turn on the RGA filament: click the filament icon on the toolbar. A greed
LED on the RGA head should indicate that the filament is operating
correctly.
6. If desired, turn on the Electron Multiplier by click the button on the toolbar
(another LED should light on the RGA head).
7. From the menus select the type and range of RGA scan desired. Generally,
the RGA can either run in ‘analog’ or ‘histogram’ mode, scanning from 1 to
200 amu. See the RGA200 Manual for more information about these modes.
8. Select (total pressure mode) from the (menu) to have the RGA display the
total pressure. This will require a new scan.
9. RGA scans can be printed to any printer from the ‘File’ menu.
10. To turn off the RGA: Turn off the electron multiplier and the filament. The
LEDs should turn off.
11. Disconnect the software link between the RGA and the computer.
12. Close the RGA program.
13. Power off the RGA head.
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2.4 – Framing Camera Procedure:
1. Connect the Maxmodule to the back of the system with four 10-32 machine
screws and washers.
2. Follow vacuum startup procedure.
3. Connect the phosphor and MCP to the appropriate current meters with SHV
cables; the current meters should already be connected to power supplies.
4. Connect the CCD camera to the computer with a 15-pin VGA cable and a
1/8” audio plug that powers the cooling fan.
5. Shroud the camera assembly with the large vinyl sleeve and use other strips
of cloth to block out any potential light leaks.
6. Take a long (10 minute) exposure to check the background light level.
Adjust the light shroud accordingly. Save the background image.
7. Turn on the CCD by starting the “SpectraSource CCD” program. The
camera is thermoelectrically cooled, so allow at least 30 minutes before
exposing.
8. Follow x-ray source startup procedure
9. Make sure the RGA filament is turned off; cross-talk between RGA ions and
the framing camera will occur.
10. Take a few test exposures between 1-5 minutes long (depends on type of
MCP) to make sure all parts are functioning properly.
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Part 3 – The Vacuum System
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3.1 – Introduction:
(pic of vac sys)
The Drake Lab primary vacuum system uses a two stage pumping system to
achieve pressures in the ‘high vacuum’ regime inside of a stainless steel chamber.
This chamber has an approximate volume of _____ These low pressures (~10-5
torr) are necessary for proper operation of the x-ray source and the x-ray framing
camera. The first stage utilizes a roughing/backing pump which removes the bulk
of the air from the vacuum chamber and provides foreline pressure for the high
vacuum pump. System pressure is monitored through two separate vacuum
gauges that supplement each pump. The vacuum system also uses two ‘airlocks’
for the primary instrumentation, which allows the instruments to be removed and
replaced while the vacuum pumps continue to operate, keeping the system cleaner
and reducing the time necessary to change out the instrumentation.
This part will contain information on basic vacuum concepts, proper handling
habits and detailed information on the vacuum pumps and components. A section
on vacuum system bakeout and the residual gas analyzer is included at the end of
this section.
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3.2 – Basic Vacuum Concepts
3.2.1 – Units
The unit for pressure, in the SI unit system, the standard for all scientific
applications, is the Pascal (Pa), defined as 1 kg/m/s2 , or 1 N/m2. Derived from
the Pascal are the bar (105 Pa) and the millibar (100 Pa). The standard atmosphere
(atm) is also defined in terms of Pa (101,325 Pa)
Units not derived from SI units are the mmHg and the torr, which are equivalent
and are equal to 1/760 atm. A conversion gives 1 Torr = 133.3 Pa, or 102 if doing
back-of-the-envelope type magnitude calculations.
Due to convention and tradition, the unit of pressure most widely used by the
vacuum science industry in the United States is the torr. Professional journal
articles, especially those published outside of the U.S., will give pressures in terms
of Pascals. Our lab and the vacuum hardware we utilize, as well as all of the
pressures defined in this manual use torr, to comply with this convention.
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3.2.2 – Gas Flow
The subject of gas flow in a vacuum system is a very lengthy and detailed subject,
far too broad to even scratch the surface in this section. Here we will discuss a
couple of the primary concepts that will serve as important stepping stones to the
world of Vacuum Science. For an exhaustive look at gas theory please consult
J.M. Lafferty, Foundations of Vacuum Science and Technology, pp. 1-149; a copy
of which is in the lab.
The flow of gas in a vacuum system can be divided into two regimes: the first
called the continuum or viscous flow, and the second called molecular flow.
There is a third regime, called transitional flow, that, like the name implies,
governs the transition from continuum to molecular flow.
The theory of continuum flow governs the behavior of gas when the interactions
between the gas molecules is the main factor (i.e. the mean free path of each
molecule is relatively short). During continuum flow gas behaves like a fluid, and
can thus be treated hydrodynamically. Continuum flow can be thought of as the
pressure region between atmospheric pressure and a rough vacuum.
Molecular flow is the name given to the regime where the behavior of the gas is
determined through interactions with the walls of the vacuum system, and not
between molecules (the mean free path is thus relatively long). In this flow
regime the gas must be thought of as individual particles, and not as a fluid in the
case of continuum flow. Molecular flow is the dominant regime when the system
is at a ‘high’ vacuum, which is the operating pressure for the experiments.
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Conductance is the term given to the resistance of the flow of a gas to the size and
orientation of vacuum components. Like electrical conductance (or resistance, the
reciprocal), a low gas conductance means it is harder for gas to flow, and the
opposite for high conductance. Conductance will have a significant effect on the
pumping speed of a vacuum chamber, and though the longer a system pumps
down the less effect conductance has, it is necessary to factor in when designing or
operating a vacuum system. For some basic conductance equations, see Appendix
??
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3.2.3 – Outgassing
One of the largest sources of gas inside a vacuum system (the other, of course,
being a leak) comes from gas trapped in the walls or some other component inside
the vacuum chamber. As the bulk of the gas is removed from the chamber, these
trapped pockets of gas behave as virtual leaks and prevent the system from
attaining its minimum pressure. As pumping time increases, these pockets of gas
eventually pump out (“outgas”), but can take several hours or days for this process
to be complete.
There are two methods that can be used, other than long pumping times, to
decrease the amount of outgassing in a system. The first is to follow the proper
vacuum habits as outlined in the next section. The second method is to follow a
vacuum bake-out procedure, as outline in section XX below.
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3.3 – Good Vacuum Habits
In working with a high vacuum system and handling vacuum components,
cleanliness is an overriding concern. Particulates and water vapor from the
environment and the air, as well as oils from fingerprints are a major problem for a
high vacuum system, and can result in outgassing, as described above.
It is therefore very important to always wear powder-free latex gloves when
working on the system or any components. If gloves become soiled from use
around the lab, change gloves frequently or clean with methanol.
Before mounting a piece of vacuum hardware on the system, it is a good idea to
rinse it first with methanol and wipe it clean with an Accuwipe. When reusing
using a viton gasket or o-ring, it should be first thoroughly cleaned with methanol
to remove old vacuum grease and contaminants, and should have a new coat of
Apezion ‘M’ grease reapplied: get a small on your index finger, rub together with
you thumb, and spread over all surfaces of the gasket. After the gasket is in place,
either clean your gloves with methanol or replace them.
(pic)
Copper gaskets require no vacuum grease, but should be thoroughly cleaned with
methanol and/or steel wool if they are beginning to oxidize. If a component is
particularly greasy, acetone can be used, but the component should receive a final
wash of methanol before it is returned to the system.
Note that rubber o-rings do not require any vacuum grease, but only need to be
thoroughly cleaned with methanol.
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When using graphite grease on flanges with blind threaded holes, try to use as
little as possible, for it is not only very bad for the vacuum system, but has a
tendency to get everywhere and is next to impossible to remove.
When a vacuum component is removed from the system, the exposed flanges
should be either covered with aluminum foil or a flange cap, and the vacuum
system should be purged with a small flow of nitrogen, if possible. Purging with
nitrogen will keep air, which contains water vapor, out of the system.
Also, to decrease the amount of outgassing in a system it is important to use
vacuum-rate components and materials at all times. Greases and adhesives can be
a significant source of outgassing, so be certain to check and make sure they
comply with high vacuum standards. There is a NASA-administered outgassing
test with which many greases and adhesives are compliant.
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3.4 – Vacuum Pumps
3.4.1 – Introduction
As mentioned in the introduction to this part, our vacuum system uses a two-stage
pumping system to bring the chamber from atmospheric pressure down to a high
vacuum. The roughing or backing pump, in our case a scroll pump, removes the
bulk of the air in the chamber and reduces the system pressure to a few mtorr.
Once the system reaches this pressure the high vacuum pump, a turbomolecular or
‘turbo’ pump is engaged, and brings the system pressure down to a pressure where
the instruments can safely operate, approximately 10-5 torr. This section will
provide a brief description of how these two pumps work, and will also describe
another type of roughing pump, the rotary-vane pump, that is also used in our lab.
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3.4.2 – Scroll Pumps
(pic)
A scroll pump is an oil-free mechanical (or ‘positive displacement’) pump that we
use to bring the system from atmosphere to a pressure that allows the turbo pump
to operate. The scroll pump also provides a backing pressure necessary for the
operation of the turbo pump. The pump operates through a combination of two
metal ‘scrolls,’ one stationary and the other rotating which traps then forces gas
entering from the inlet (the vacuum chamber) to the outlet (out into the
atmosphere).
(pic)
Because the scroll pump uses no oil to maintain an internal seal (see the rotaryvane pump, below), it is extremely clean, and because of it high pumping speed it
can ‘rough’ the system in less than one minute. See Lafferty, p.167 for more
information regarding scroll pumps. Because the scroll will stop almost
immediately after a loss of power, it is important that the inlet to the pump be
sealed off before shutdown. If the inlet it not sealed, gas from the atmosphere will
quickly back-stream into the system, which could contaminate the chamber and
the instrumentation. An electro-pneumatic gate valve is installed before the scroll
pump inlet in the vacuum line, and in controlled by a simple on/off switch. Our
scroll pump has a pumping speed of _____
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3.4.3 – Turbomolecular Pumps
(pic)
A turbomolecular (or simply ‘turbo’) pump is a kinetic mechanical vacuum pump
that is used to bring the system from a rough vacuum to a high vacuum necessary
for the operation of the instrumentation. A turbo pump operates in the molecular
flow regime by imparting kinetic energy to individual gas molecules through a
series of fan blades rotating at a high speed (50-60,000 RPM). Like the scroll
pump the turbo pump uses no oil within the vacuum assembly, but the rotor shaft
is lubricated externally, and this oil must be changed regularly.
To produce the best pump efficiency and the highest pumping speed a
compression between the inlet and the outlet of the turbo pump must be created.
This is done with a backing pump, in our case the same scroll pump used to rough
out the system. Most turbo pumps have a required backing pump speed which
needs to be met in order to achieve the proper compression ratio and to spin up to
full speed. Out turbo pump has a pumping speed of ____ and requires a backing
speed of ______
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3.4.4 – Rotary-vane pumps
(pic)
A rotary-vane pump, which is one of the oldest styles of vacuum pumps still in
use, uses an oil-sealed vane which rotates; trapping and expelling gas in a similar
method to the scroll pump.
(pic)
Because a rotary-vane pump uses oil internally, it is necessary to have one or more
traps to catch the oil as it is expelled when the pump is first turned on. One
usually uses a combination of a foreline trap consisting of a thick copper mesh and
a micromaze trap which acts as an oil baffle and can be baked out to remove oil
contamination. Despite careful use of traps, we have discovered that it is nearly
impossible to avoid oil contamination, thus the scroll is to be only used for
roughing/backing the main vacuum chamber. Our lab has two rotary-vane pumps
that are used for smaller containment systems where a light amount of oil will not
be harmful, or for primary system backup in case of emergencies.
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3.5 – Vacuum Components
3.5.1 – Introduction
This section will cover the basic components that make up the vacuum system
excluding the pumps, which were covered in the previous section. This section
also outlines some basic vacuum concepts and proper methods that should be used
when working with the system.
Our vacuum chamber and components, like most high vacuum systems, are
constructed out of type 304 stainless steel, which is favored for its strength and
cleanliness. We generally use two types of vacuum connections, a ConFlat-type
using a knife edged flange for most of the hardware, and a Quick Flange-type with
self-centering o-rings for the pump fittings. We also use Swagelok brand fittings
to connect gas lines and other vacuum connections
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3.5.2 – Flanges and Fittings:
3.5.2a ConFlat (CF) Flanges:
ConFlat flanges, or ‘CF-type’ as there are a wide variety of manufacturers, are the
standard type of vacuum flange used in our lab. A CF flange compresses a copper
or viton (rubber) gasket between two knife edges to make a seal (see diagram),
and the two flanges are bolted together with standard hex, Allen, or 12-point head
machine screws.
The viton gaskets can be reused several times, but a copper gasket is permanently
compressed after use, and is therefore not reusable. Copper gaskets tend to be
favored where precision and permanency is required, whereas viton gaskets are
used on components that are removed frequently. There is also a difference in
cost: viton gaskets tend to be several times as expensive as a comparable copper
one. CF flanges come in a variety of standard, English unit sizes (2.75”, 4.5”, 6”,
8”, etc.) and shapes, from simple blank flanges to complicated cross, cubes and
spheres. We generally use 2.75”, 6”, and 8” CF flanges in our lab.
3.5.2b Quick Flange (QF/KF) Flanges:
Quick Flange (or QF, Kwik Flange/KF) flange utilize a centering ring-mounted oring and an external, circular clamp to hold the two flanges together.
QF flanges are used, as the name implies, when a quick connection or
disconnection needs to be made. QF flanges also come in standard sizes, but
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generally use metric units: i.e. a QF40 flange uses a 40mm diameter connection.
In our lab QF flanges are used to connect the pumps together and to the system.
Very little else needs to be said, as they are a very simple flange to use, but it is
important to tighten the circular clamp as tight as possibly to ensure a good
connection.
3.5.2c Swagelok Fittings
Swagelok brand fittings are used to connect gas lines and additional vacuum lines
to the system. Swagelok fittings work by compressing a two-piece ferrule
together and around the tubing with a threaded nut. Once the initial compression
has been made, the tube can be disconnected and reconnected quickly without
leaking. To make a Swagelok fitting: slip a nut, followed by the two-piece ferrule
on a length of tubing such that it will fit into the Swagelok body piece (such as a
valve or a regulator). Use a 9/16” wrench
3.5.3 – Vacuum Valves and Gas Fittings:
3.5.3a Gate Valves
3.5.3b Needle/Ball Valves
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3.5.4 – Vacuum Gauges:
3.5.4a Convection Gauge
3.5.4b Ion Gauge
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3.6 – Vacuum Bakeout Procedure
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3.7 – Residual Gas Analyzer
3.7.1 – Introduction
A residual gas analyzer (RGA) is a device used to characterize and quantify the
types of gas remaining in a vacuum system after pump-down. An RGA can also
be used as a leak detector or run in several other modes as the situation may
require.
Because of the ambiguous nature of RGA results, a good background in gases and
partial pressure analysis, along with thorough knowledge of the vacuum system
and surrounding environment is necessary. For more information on RGAs and
partial pressure analysis see Appendix ?
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3.7.2 – RGA: Basic Theory of operation
A Residual Gas Analyzer of the type used in our lab (Stanford RGA200) is a
combination of three separate components. The firs component, called the ionizer,
ionizes the gas in the vacuum chamber with electrons emitted from a heated
filament. These gas ions are then focused down into the body of the RGA, where
they enter a mass spectrometer.
A quadrupole mass spectrometer, which is composed of four parallel bars arranged
in a cross sorts the incoming ions by mass by the use of sinusoidal radio-frequency
voltages. The mass spectrometer is run at a specific voltage and frequency to
allow a specific range of ion masses to pass through on a stable trajectory. All
masses not in the specified range are sent on unstable trajectories and become
neutralized by striking the quadrupole itself. The spectrometer can be set to scan
across an assigned range or can monitor one mass at a time.
Ions of a specific mass that are allowed to pass through the mass spectrometer are
then sent into a collector, called a Faraday cup, where their charge is measured by
absorbing some electrons from the metal collector and thus inducing a current.
This charge is then coupled with the mass known from the mass spectrometer, and
is read out by a computer and displayed and stored in memory.
Our RGA also has a secondary ion detection device called an Electron Multiplier.
This collector can be used simultaneously with the Faraday cup. The Electron
Multiplier is designed to work at the ultra-high vacuum regime (~10-9 torr), thus
for our applications is of little use.
Because the RGA gives a mass-to-charge ratio (M/C), and not simply a mass, a
certain amount of analysis needs to go into interpreting an RGA scan. For
33
example: a detection of an ion at a mass-to-charge of 18 could either be a singlycharged ion of water vapor, or a doubly-charged ion of argon isotope 36. Thus
knowing how to read a scan, as well as knowing what types of molecules might be
present in the vacuum system is very important. The RGA software has a built-in
simple analysis program, which will estimate the composition of the residual gas
based on a linear regression procedure (see below for details on running this scan).
This analysis is fine for most applications in our experiments.
The RGA will plot M/C ratios as a function of partial pressure in torr. Remember
that a partial pressure is the pressure an individual species of gas if it is measured
by itself (i.e. not in an environment with a mixture of gases). The sum of all the
partial pressures in a system will give you the total pressure. Thus, we can use the
RGA as a second, and possibly more accurate, high vacuum gauge (see below).
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Part 3 – The X-ray Source
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3.2 - Theory of Operation:
Our lab uses a soft x-ray source that based on a simple spark-gap procedure. A
moderate current (~6A) is run through a cathode composed of a hairpin tungsten
filament to heat it and release a large number of electrons through thermionic
emission. These electrons are then accelerated through a high potential difference
(~5000V) across a vacuum gap to strike an anode material. X-rays can be
produced at lower potential differences (above 2500V), though the flux increases
significantly with voltage.
X-ray production is governed by the Bremstrahlung process, where electrons are
accelerated in the presence of a nucleus of the anode material and in the process
emit an x-ray photon. The wavelength of the x-rays is determined by the anode
material, e.g. an aluminum anode will produce strong x-rays at 1.5 keV, silicon at
1.8 keV. X-rays of other energies, representing different transitions, are also
possible. The x-ray flux can be controlled by limiting electron emission from the
cathode through a feedback resistor in the control circuit, which in turn is
dependant on the cathode heating current.
See literature provided by the J.E. Manson Co. for further information regarding
the theoretical operation of the source.
36
3.3 – X-ray Source Description of Components
Note: Our x-ray source is very similar to a widely-used model produced by the
J.E. Manson company, thus may often be referred to as the ‘Manson’ source.
The source is housed in three separate vacuum flanges that are designed to mate
and mount to the vacuum system. All three pieces connect with standard 2.75”
ConFlat flanges and are sealed by Viton gaskets.
(pic)
The first enclosure is the anode feedthrough, where a silver SHV plug connects to
a copper rod inside the source, with the anode material mounted at the other end.
The anode feedthrough is mated to the cathode feedthrough flange via three small
bolts that can be tightened with a ¼” 12-point box wrench.
(pic)
The cathode feedthrough flange has two SHV plugs to connect the cathode to the
X-ray source control. The cathode feedthrough flange has three standard bored
holes and three threaded holes. The threaded holes are used to mate the three
components together.
(pic)
A standard 4.5”, rotatable ConFlat nipple is mated to the cathode feedthrough
flange via three 1”, fine-thread bolts. The nipple then mates to the vacuum system
using six 1.5” ¼-20 bolts and nuts.
37
(pic)
The anode is connected by an Allen-head machine screw into a tapped end of the
copper feedthrough rod. It is important that the anode is mounted to the
feedthrough as parallel as possible; to assure the anode face is orthogonal to the
electron beam from the cathode. It may prove useful to insert a flat copper washer
between the anode and the feedthrough to assure correct mating. The anode
feedthrough passes through the steel cathode support tube (the ‘body’ of the
source), where it is insulated from the system via a ceramic ring.
(pic)
The cathode is connected to the two copper leads, which in turn are connected to
SHV plugs inside the cathode feedthrough flange with barrel connectors. These
copper leads can be easily replaced by removing the cathode feedthrough flange
from the anode feedthrough, and then disconnecting the leads with a small
flathead screwdriver.
(pic)
It is important that the copper cathode leads are insulated with ceramic beads
along their entire length, from where they connect to the SHV plugs to where they
mount to the cathode.
The leads run alongside the source body and pass through the source head. To
connect the cathode to the leads, take two barrel connectors and two short pieces
of copper wire. Connect the wires to the cathode itself with the barrel connectors,
then set the cathode in its mounting ring and place it at the correct position inside
the source head. Once the cathode is firmly secured, connect the two pairs of
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leads together with another set of barrel connectors. Make sure that all leads are
insulated with ceramic beads. Different sizes of beads are available; smaller ones
are more useful for the portion of the lead that passes through any of the mounting.
It is also important that the barrel connectors are isolated from each other and from
the system.
(pic)
A 25μm Beryllium window is mounted over one of the two x-ray beam line
outlets. This window should be pointed down the chamber towards the framing
camera, as is designed to block visible light that is produced by the cathode
filament.
WARNING: Beryllium can be very dangerous if inhaled in a powder form.
Handle the x-ray source with extreme care, especially when mounting into the
system. If the Beryllium looks cracked or is breaking off leave the area,
consult the Beryllium Standard Operating Procedure (which can be found in
the Chemical Hygiene Plan Book) and get or call an emergency contact
person.
39
3.4 – Photodiode
(pic)
A photodiode is mounted in one of the x-ray beam lines, and is employed as an
absolute reference to measure x-ray flux (see Appendix ? for flux formula). The
photodiode is treated as an ideal detector, i.e. its quantum efficiency is very close
to 100%. The photodiode has two small leads that run through a vacuum flange
and into a plug, which can be connected with a banana plugs into the multimeter.
Normally the housing around the photodiode (a 4.75” CF nipple) need not be
removed, but if the wiring is damage it can be easily removed and the wiring
replace. Aluminum foil is wrapped around the entire photodiode mount to help
block stray light from the cathode polluting the x-ray signal. This foil wrap will
need to be replaced if the photodiode housing is removed.
A 25μm Beryllium window is mounted between two vacuum flanges inside the
assembly directly above the photodiode. This window is employed to block
visible light from reaching the photodiode (see Beryllium warning above).
When the source is mounted, the photodiode mount needs to be removed to
correctly align the x-ray source. Since the photodiode mount includes a bare
beryllium shield, once the photodiode is removed inspect the beryllium, looking
for large cracks or flakes (it can be seen at the center of its small mounting cage
inside the mount), make sure it is intact, then IMMEDIATELY cap the end of the
photodiode mount and set the mount in a safe place. When the x-ray source is
properly aligned, remove the cap, inspect the beryllium again, then re-mount the
photodiode to the system.
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3.5 – Alignment
Since the production and collection of x-rays is dependant on geometry, the source
is designed so that most components are adjustable. With the source mounted on
the vacuum system, insert a steel dowel into one of the line-of-sight (LOS) holes
in the cathode mounting tube. The dowel should fit snugly in the LOS hole, and
should appear to be directly in the center of the flange where the photodiode
would mount. If the dowel is too high or two low, the problem can solved by
slightly adjusting the rotatable nipple. If the dowel is too far to the left (it is
unlikely to be too far to the right), the problem must be corrected from within the
source. The cathode mounting tube is secured to the cathode support tube by four
small set screws. There is ½” of material on which the cathode mounting tube can
slide and be set in the correct position. Once the correct position is found, mark
the location on the cathode support tube.
Now that the orientation of the cathode mounting tube is found, one needs to set
the correct position of the cathode and the anode. The anode needs to be set so
that its center can be seen through the LOS hole. The anode position can be
adjusted by moving its feedthrough, with is secured by a small set screw near the
cathode feedthrough flange. Once the anode is set in the correct position, the
cathode can be set. Assume for now that the correct anode-cathode separation
distance is 13mm, which will place the center of the anode face in the focal point
of the electron beam. If this is indeed the focal point, it will produce a mark
~1mm in diameter at the center of the anode. If the mark is larger than 1mm, it is
likely that the cathode is too close to the anode. The cathode position is set by a
stack of aluminum spacer rings, all of which are secured by small set screws.
Now that the geometry is correct, the source is ready for operation.
41
Part 4 – The X-ray Framing Camera
42
4.2 – Introduction:
The framing camera is a combination of amplifiers, converters and recorders that
produced an image of the of the photon flux produced by the x-ray source. The
heart of the camera is the microchannel plate, which performs the amplification
and is the primary focus of our experiments. This part will cover the instruments
that make up the x-ray framing camera and will describe their function and use.
Also included is a step-by-step tutorial (with accompanying images) to
dismantling and replacing the microchannel and phosphor plates.
4.2.1 – Introduction to Microchannel Plates (MCPs):
As the microchannel plate is the most important component of the whole
experimental system, it is necessary to discuss here . A microchannel plate is a
constructed from stacked glass tubes, which are heated and stretched, then sliced
into thin wafers, producing glass plates full of thousands of small channels (hence
the name). The channels run clear through the plate, from the front to the back,
and are usually angled by a small amount with respect to the face of the plate
(more about this below). A transparent conducting material (usually Indium Tin
Oxide) is coated on the front and back of the plate, and when under test conditions
a large negative potential difference is applied from the front to the back of the
plate.
The principle of microchannel plate operation is thus: a photon created at a
distance enters a pore of the plate and strikes the wall of the channel. An electron
absorbs the photon and is emitted from the side of the channel. Because of the
negative potential difference the electron is then accelerated down the channel
from where it entered. This one electron will likely strike the side of the channel
itself and free more electrons through secondary collisions, and these will in turn
43
produce more electrons, eventually exiting out the rear of the channel in a spray of
V11 times as many as entered (where V is the voltage across the plate; this is called
the gain of the plate).
The gain of a microchannel plate is not only dependent on the accelerating
voltage, but is also dependent on the quantum efficiency (QE) of the material used
to make the plate. Improving the quantum efficiency of MCPs will be a primary
focus of our long-term experiments, as it can improve the gain by one or two
factors of 10. A ‘bare’ MCP, usually created out of a lead oxide glass has a QE of
1-5% (i.e. 1-5 photons in 100 will produce an electron in pore of the plate).
Coating the surface of an MCP with a photosensitive material can easily improve
the QE by a power of 10. Our lab has both uncoated plates and coated, with the
coated plates having Cesium-Iodide, Potassium-Bromide and Magnesium-Fluoride
photocathodes.
Another way to improve the QE of an MCP is by considering the geometry of the
plate itself. It is worth pointing out that photons that strike the surface of the plate
and not in one of the pores will very likely not produce any amplification. One
would therefore like to make the pores of the plate as large as possible, but this
weakens the structural integrity of the plate. A rough ratio of channel size and
center-to-center spacing is therefore adopted e.g. ______ from our set of plates.
Another way geometry affects gain is in the orientation of the channels
themselves. If the channels are orthogonal to the surface of the plate, a photon
normal to the plate might pass straight through without colliding with the walls of
a channel. To increase the number of collisions the channels are angled slightly
(5-10°) with respect to the normal; this is called the bias angle. Out lab has MCPs
of 5° and 10° bias angles.
44
Microchannel plates are extremely delicate, and must therefore be handled
with the utmost care.
For more and detailed information on Microchannel plates please see the
Appendix for included papers and references.
4.2.2 – Phosphor Plate
The phosphor plate is utilized to convert the electron signal from the MCP into
visible light that can be recorded with a camera. Once the electrons exit the rear
of the MCP they are accelerated by a positive potential difference toward the
phosphor plate. Electrons striking the phosphor cause it to glow at a visible
wavelenght (a process call scintillation), and this light is passed through a fiber
optic plate and out the rear of the Max Module. Because of the stacked fiber
nature of the phosphor plate, it preserves the two-dimensional structure of the
original MCP image. The phosphor plate also forms the transition between the
vacuum chamber and the atmosphere.
Because the phosphor material is lightly held on the fiber optic plate (by static
electricity) special care needs to be taken when handling the plate. The phosphor
material can be recoated onto the plate, but this requires us to send the plate out
for repair and is expensive.
45
4.3 – Max Module:
4.3.1 - Introduction
The Max Module is a commercially produced device that converts the x-ray flux
to visible light via a combination Microchannel and phosphor-coated plate.
The Max Module holds the two plates at a fixed distance apart and contains of the
electrical connections necessary to produce the required plate voltages. The
phosphor plate connection is a single SHV connector that feeds into a copper strip
on which the phosphor plate sits. The MCP can be connected to up to eight
different source lines, if a gated or stripped MCP is being used (see Appendix XX
for more information on these kinds of MCPs). The current configuration of the
Max Module connects all eight of the MCP signal lines together, and any can be
used to connect a voltage to the MCP (see above image).
The original configuration of the Max Module allows it to accept 2” diameter
MCPs only. We have made a special insert that allows the Max Module to accept
46
either 2” or 1” MCPs as well. Special care must be taken when working with the
1” insert to prevent damage to the phosphor plate (it is very easy to scrape off
some of the phosphor material).
4.3.2 – Microchannel Plate (MCP) and Phosphor Plate change-out procedure.
This section will cover the operating procedure for adding/removing the MCP or
phosphor plate in the Max Module. A step-by-step guide with pictures is included
below. A duplicate of this section can be found in the binder labeled “LLNL
Maxmodule ID#100.001-4-LLNL-UOFM,” and is the operating procedure
outlined by our colleagues at the Lawrence Livermore National Laboratory
(LLNL).
MCP Proceedure:
Phosphor Plate Proceedure:
47
4.4 – CCD Camera
4.4.1 – Introduction
The CCD, or charge-coupled device camera is the standard for nearly all digital
imaging applications, from commercial digital cameras to multi-million dollar
devices used in the most advanced astrophysical instruments.
(pic)
In our lab, the CCD is the last instrument in the chain of producing a usable image
that originated at the x-ray source. The CCD simply images the rear of the
phosphor plate (with the use of a lens to change the image size) and saves it on a
computer. This image can be view and manipulated as the experiment warrants.
The scope of this section is not broad enough to cover the theory of CCD
operation. Instead, the operation of the camera and computer software as it
pertains to the experiment will be discussed. For detailed information on CCD
theory, please see Howell, Handbook of CCD Astronomy, pp. 1-74, two copies of
which are in the lab. Further reading on CCDs can be found in Appendix XX.
Our CCD camera is mounted on an optical rail on a series of linear and rotary
stages designed to allow precise alignment and focusing options. The camera lens
is also mounted on stages to allow the camera/lens system to be moved and
achieve different magnifications for different applications. Because of the openair nature of the Max Module and CCD camera setup, it was necessary to
construct a miniature ‘darkroom’ around the whole assembly. Many overlapping
layers of cloth are necessary to block light which would otherwise pollute the
48
camera. When closing up the darkroom it may take a few attempts to find a
configuration that allows the least amount of light to pass.
49