Lab Safety for Particle Experimentalists SLAC Course 110

June 2006 Lab Safety for Experimentalists M. Breidenbach 1

Lab Safety for Particle ExperimentalistsSLAC Course 110

• Martin Breidenbach• June 2006

• December 2008• January 2009


Lab Safety for Particle Experimentalists• This is aimed at physics grad students and postdocs.• The lab is the R&D lab – and not the accelerator or big

detectors.– No accelerator hazards e.g. Radiation or magnets

• The lab is not particularly dangerous – you are more likely to be hurt in traffic at the entrance – but there are hazards that many of us have learned about through close experience. The points here are offered to jump you over these. There is not, and cannot, be a prescription to avoid all risk. Thinking is required…Risk can be intelligently managed!

• Many accidents are the result of a chain of errors or misjudgments. Reasonable precautions can make a single error inconsequential.

• I will avoid graphic illustrations of various accidents. However, the word horrible means exactly that. You would not want to see the pictures. When horrible is used, I mean it.


Disclaimers

• This talk is not a substitute for any required training. However, it does substitute for SLAC Course 251

• This talk is relatively dense and assumes you know basic physics.

• You and your line management are responsible for your safety. This talk is meant to expose you to some experience in a concentrated dose in the hope that you wont do again what many of us have learned by experience.


Topics

• Electrical• Electronic• Explosions• Implosions• O2 deficiency• Lasers• Chemistry• Falls• Radioactive Sources


Electrical Hazards

• Three routes to trouble:– Electroshock

– Arc Flash

– Reflex

Electroshock>~10 ma 60 Hz through the body is bad.“let-go” threshold 10-17 maChest paralysis (suffocation) ~30 maCardiac fibrillation 75-100 ma

• Cardiac fibrillation – need defibrillator in <5 minutes, preferably < 2minutes. This level of shock unlikely but plausible in the lab. If there is an AED within running distance, its good to know where…(There is an AED in Central Lab Annex, 2nd floor, center corridor)

• Body resistance not really prdictable – dry skin, <50 V, resistance high

• Damaged or wet skin, 600 V, significantly lower resistance• Neural damage, internal heating – very bad. But this is not

really a non-accelerator lab hazard at SLAC. Requires gross violation of electrical safety.

• If you or someone else gets shocked more than trivially, get professional help or call 911.



Arc Flash• : Electrical distribution system – including 480 VAC panels in

labs – can deliver enough current to make a horrible, continuous arc releasing substantial energy and radiation (i.e. an explosion) until some slow breaker opens. At 480, the arc will ionize air and Cu and just keep going. So: Stay out of 480 circuits hot or cold. The required training, experience, and PPE to work safely is unlikely for almost all physicists.

• Example: Consider a 480 V phase to phase fault with a current of 10,000 amps. This is 5 MW. If the upstream protection takes 10 cycles to open, this is 0.8 MJ. This is equivalent to 200 g TNT.

• Arc Flash hazard is divided into categories between 0 and 4, each with appropriate PPE. The PPE is rated by the incident energy it can take before the onset of 2nd degree burns.

• Arc Flash Hazard 1 PPE is rated at 4 Cal/cm2 (17 J/cm2). Note that this is much less than sunlight for an hour – because of the large UV component of the arc spectrum.

• Above Arc Flash Hazard 0 is for professionals!

Reflex

• Small shocks will cause an often large, more or less involuntary startle reaction. It can make you fall – as in off a ladder or platform.

• This kind of shock is classic when debugging proportional chambers, drift chambers, etc. The current and stored energy are usually too low to be an electroshock hazard. Think about where a startle reaction might take you!



Reasonable Electrical Practice

• Stay out of 480 circuits. ( note the period. Just stay out!!!) (breaker operation exception described next slide)

• Stay out of 208/120 3φ panels-(breaker operation exception next slide)

• Wiring a (disconnected!) chassis:– Know the standard electrical color code:

• Black (or possibly red or blue) is “hot”• White is “neutral”• Green is ground.• If, for some bizarre reason, you are forced to use cable that

does not conform, use properly colored shrink tube or colored tape to identify the wires.

• Insulate the chassis connections so you cannot touch them when you debug your work. RTV glob is the minimal acceptable insulation. Fiberglass covers are better.


Breaker and Disconnect Switch Operation

• Occasionally it may be necessary to open or close circuit breakers or switched disconnects.

• It is reasonable for you to operate these devices at SLAC if all of the following conditions are satisfied:– The NFPA70E Arc Flash Hazard Rating is 0 or -1.– The voltage is 480 V or less.– You are wearing appropriate PPE.– If you are closing a breaker, you reasonably understand why it was

open.– No other jurisdiction forbids it (e.g. you are not at SLAC).

• More details:– NFPA70E is the counterpart document to the National Electrical

Code that deals with operations as opposed to construction standards. It specifies hazards associated with various equipment configurations.

– In this context, operation of a circuit breaker or switch with covers on is permissible if Arc Flash Hazard 0 (and in probably rare cases of <10 KA short circuit current, Arc Flash Hazard -1).

– Panels at SLAC should be labeled with their Arc Flash rating for covers on and off.


• Most 115-208 V panels are Arc Flash Hazard 0. In rare cases, they may be Class 1.

• Personal Protective Equipment:– For Arc Flash Hazard 0:

• You wear safety glasses.• You wear cotton (or non-synthetic) long sleeved shirt.• You wear cotton (or non-synthetic) long pants.

• Technique: when operating a breaker or switch, stand to the side close to the wall and look away. If there should be an arc, you wont get it in the face.

• Reasonableness: If a 15 or 20 Ampere 115 or 208 V breaker tripped because of an overload that is understood and corrected, resetting is reasonable. If a 480 V breaker tripped mysteriously, leave it to an electrician.

• Some equipment, such as welders, connect with plugs and sockets that are mechanically interlocked so that the plug can not be inserted or removed with the switch on. Ensure that the equipment is off before operating the switch. (Some older sockets do not have this interlock, again ensure that the equipment is off before inserting or removing the plug.)


Clean Room Issues

• There are many clean rooms in use – e.g. EXO, Si Lab, and GLAST. Clean room clothing may be a problem for some activities: Electrical switching and TIG welding are the two prime examples.

• PPA has tested the Tyvek clean room suits, and they are reasonably ok. (The nylon zipper will burn, but the Tyvek does not)

• Nitrile gloves burn sufficiently to be a clear hazard.• We have not found an elegant solution for a clean,

reasonably non-flammable glove.• The working solution is a deerskin welding glove under

nitrile gloves.• The search continues for something better.• Remember that welding requires a fire watch person

with a CO2 extinguisher.


Sidebar – SLAC Electrical Power Distribution• The overhead transmission lines coming down from

Skyline are 230 KV 3 φ with a capacity of ~100 MW. • The Master Sub has transformers taking the 230 KV to

12.6 KV for distribution around the site.– 60 KV lines come in from campus and are used when the

230 KV line is unavailable.

• Substations usually reduce the distribution voltage to 480 V to panels in buildings.

• Smaller transformers take the 480 V to 208 V.

• Breaker panels are used to distribute these voltages. • Again: Stay out of the lab power distribution system!!!

In a 3 φ system, the stated voltage refers to the line to line voltage. The line to neutral voltage is down 1/√3. So the standard 120 V is line to neutral of a 208 system. The system is often referred to as 208/120 Volts.


Electrical Issues

• It requires a lot of paperwork to work hot on a chassis with exposed 120 VAC and it is an unnecessary risk, so insulate those connections to the power supply.

• Grounding limits the potential of a (conductive) device with a fault to (assumed grounded) you. A ground is effective only if it can carry enough current to trip a breaker and/or reduce the potential to non-hazardous levels.– It in a worst case fault, the impedance of the ground

connection should keep anything you might be touching below 50V to ground!) Assume you can get 100 amps out of a wall panel before a small breaker opens. Then you need < 0.5 Ώ to a solid ground.


Sidebar - GFI

• A Ground Fault Interrupter (GFI) compares the current on the hot and neutral by running both through a toroid transformer and amplifying the difference. An imbalance of 4 to 6 ma triggers the spring mechanism of the breaker (or outlet). A GFI breaker has no ground connection! (A GFI outlet has a ground connection for the 3 wire cord.)

• Note that 100V x 10 amp x 5 mS = 5J, which will hurt. However, 10 amp through you is unlikely (at 115 V), unless you are in salt water.)


GroundingTypical SLAC configuration:

480 V Building distribution system feeds 480 V to 208/115 V transformer to breaker panel.

(Typical residential configuration:

230 volt single phase center tapped pole transformer feeds a breaker panel)

The center tap connects to the neutral bus, and is grounded at the panel. There is usually a separate ground bus.

Note that the motor frame, or in general, any accessible conducting parts of a device are connected to the ground wire.

Double insulated devices are considered safe enough not to have a ground connection.

Breaker Panel

Neutral Bar

Ground Bar

Outlet

Normal Motor

480 V, 3φ, Delta

208/ 115 V, 3φ, Wye

Residential Transf ormerSecondary 220 V, Center tapped neutral

Primary: Distribution Voltage – 4-12 KV


Grounding – Current flow during a short

Note the short from the hot side of the motor winding to the frame.

If the short impedance is low enough, enough current (dashed lines) will flow as shown to trip the breaker.

In any case, the impedance of the ground wire should be low enough to keep the potential difference between the frame and building ground below dangerous levels (50V).

The same strategy applies to most laboratory equipment, where the ground conductor system must be adequate to keep the frame potential below 50 V.

Breaker Panel

Neutral Bar

Ground Bar

Outlet

Abnormal Motor

480 V, 3φ, Delta

208/ 115 V, 3φ, Wye

Person Touching Frame

Short to motor f rame


Wire Sizes

AWG Wire Size (solid)

Diameter (inches)

Resistance (Ohms/Km)

Nominal Current Capacity (Amperes), Insulated Cable

18 0.0403 20.9 5

16 0.0508 13.2 10

14 0.0640 8.28 15

12 0.0808 5.21 20

10 0.1019 3.28 30

Wire must be sized to prevent excessive heating and voltage drop. Reasonable practice with insulated wire is in table. Voltage drop ~5% usually considered ok – but note 20 amperes in #12 wire is ~30 meters (out and back).

Use proper extensions! The rule of thumb is #12 for 20 amperes, #14 for 15 amperes, and #16 for 10 amperes. However long extensions may need heavier wire. Calculate for a 5% voltage drop or less.

Do not daisy chain extensions.


Non Contact Voltage Detector

• These inexpensive devices ($10-20) capacitively sense AC voltages. All do 115V, 60 Hz AC, and some do 24 VAC. Very useful for homework!

• The sensors do not detect DC, and may not be used at SLAC as verification of the zero voltage state.


Capacitors store Energy

• “Doorknob” capacitors• Ceramic dielectrics such as Strontium Titanate – give

2.7 nF @30 KV or 1 J – (Nasty) in a device 2 3/8” x 7/8”

Pulse Capacitors

100 μF @ 2KV or 200 J – Totally deadly

Device size ~3 ¾ x 4 ½ x 7 3/4

Large Capacitors should be shorted when not in use

Bleeder resistors should not be trusted to discharge a capacitor.

You need more training to work with large pulsers.


Electrical Limits

• There is a variety of advice on what is dangerous: – SLAC EHS Manual Chapter 8– NFPA 70E – National Fire Protection Agency– DOE Electrical Safety Orders

• Below 50 V is ok.– There is serious burn hazard with high current supplies at

even a few volts. Car batteries can deliver 400 Amps without blinking. So that neat ring can dissipate 4 KW and amputate a finger painfully and quickly. Experienced people remove jewelry around car batteries and VME (or Fastbus) power supplies.

– Some sources claim stored (capacitive) energy > 10 J (below 50 V) is a hazard. Most of us are more excited about the burn possibilities of supplies that can deliver more than ~10 Amp…


Electrical Limits. continued

• Above 50 V– Below 5 ma power supply capability – you can’t fry

yourself. But you can jump. – The 10 J limit is becoming very real!

• Below ~ 250 V it’s hard to electrically puncture the skin, so the body impedance makes it hard to deliver 10 J. Note that this is for a dry, intact body! By 500 V, its only what the circuit can deliver. Believe!

– At high voltage, you will probably survive 10 J, but you will remember it until Alzheimer’s takes over. Be real careful when there’s more than 1 J.


Sidebar – Control of Hazardous Energy

• Formerly known as LOTO – Lock Out – Tag Out• Work on de-energized equipment connected to wall (no

plug to pull) by COHE procedures. This is hardly ever for physicists. But its good to understand the issues:– The idea is that upstream disconnects (breakers, knife

switches, fuses) must be open. And locked open. And tagged with locker’s name etc. There’s a course on this.

– But how do you know you got the right breaker? Sometimes its obvious, but often the panel is far away, and there is no visible conduit from the load to the breaker. So you measure the voltage. That’s hot work. And that requires PPE for anything that might go wrong – like using a bad meter that presents a low impedance to the bus (and a horrible arc). And you need a Hot Work Permit. Which PPA has never granted – at least in the last several years. So Fugeddahaboutit.


Electronics• Most modern signal processing electronics – from charge amplifiers

through computing – are harmless. You can diagnose the circuit being reasonably confident that you are far more likely to blow up the FET through Electrostatic Discharge (ESD) than it is to tickle you.

– When you change boards in your computer, turn it off but leave the AC connected. If you are not using an ESD wrist strap, hold the board in one hand and touch the cabinet with the other before inserting board. Etc.

• Silicon detectors may have bias supplies up to ~500 V (for high radiation damage environments). Its rare that the supply can deliver 5 mA. Its almost weird that the stored energy could approach 10 J. Special precautions are needed, including paperwork, if the supply can deliver more than 5 mA.

• Photomultipliers operate at 2-3KV. There are many older “bulk” supplies designed to power many PMT’s that can deliver 20 mA or more. These are serious supplies. Almost always, the HV is delivered in co-ax (RG-59), and the co-ax is terminated with modern HV connectors (MHV, Reynolds) that make it exceedingly difficult to accidentally contact the HV. However: If you need to debug a PMT base, use a NIM bin Power Supply that has a max current of 1 mA or less.

• Avoid adapter cables that change HV connectors into non-HV connectors, and especially forget HV cables that have alligator clips on the ends. They are called suicide cords for a reason!


Electronics Technique, basic!

• Remember to turn off the HV before sticking your hands inside. (Remove plug or lock off the equipment if not cord-connected*).

• Remember to discharge capacitors• Use proper grounding. The grounding of a PMT base or

LST or most other detectors than Si are usually through the HV co-ax.

• Be particularly careful if you have to work in the dark – e.g. searching for light leaks for PMT or APD based counters.

• *This comment sounds simple, but it isn’t. EXO refrigerators and compressors are connected with very expensive plugs and sockets to avoid locking issues!!


Sidebar – RG59

• Note that the capacitance of most random co-ax is ~ 100pF/foot. You can easily destroy electronics with a disconnected cable that charges up to a few KV. For higher HV, a charged cable is dangerous. (100 ft @100 KV ~ 50 J)

• The dielectric in most co-ax is polyethylene. Poly is a good dielectric, but it’s chemically close to napalm. A few cables in the lab are no problem, but a rack full is a serious issue. SLAC has had two(!!) serious fires that started from minor arcing in co-ax. The SPEAR 1 SLAC-LBL Magnetic Detector (aka Mark I) had spark chambers. In the 70’s, the pulsers ignited a very exciting fire. Aluminum racks melted. Months of work to rebuild. In the 90’s, ion pump HV cables started a cable fire in the SLC e- damping ring. Again, a major mess and months of expensive recovery. Bromated polyethylene or teflon dielectric is a little more expensive, but much safer…


Electronics, continued

• Laser supplies are serious. The flashlamp supplies often break the 10 J limit. If you open laser enclosures, you need laser safety training, but remember the HV basics: – The power supply should be disconnected from the wall– The energy storage capacitor should be grounded with a

ground hook.• A SLAC laser will (most likely??) have stored energy far below

1 KJ.• A simple ground hook without a series resistor is acceptable.• Two ground hooks are needed to discharge “floating”

capacitors.

• Supplies for Pockel’s Cells can be hefty.• There are occasionally high voltage low impedance

operational amplifiers (Trek) that are lethal. No hot work on these guys, and make sure the load is enclosed.


Electronics Techniques

• On occasion, it is necessary to debug a circuit that can hurt. If it has > 50 V and (5 mA or 10 J), there are hot work (energized circuit )requirements – permits and non-routine JHAM’s. But there are seatbelts for this car:– Make sure you are floating at high impedance to ground. A

dielectric mat on the floor, or dry wood for a ~few 100V is good. ESD wrist straps are relatively high impedance, so delicate components are protected but you are not grounded.

– Use one hand. If you touch something, make sure it will be finger to wrist or less. Pull your hand out of the chassis when adjusting the scope. Eliminate the potential of a hand to hand or hand to foot shock. And what is the chair made of?

– Think about what will happen if you are (very) startled by a shock.


Reynolds 10 KV

SHV 5 KV BNC 500 V

Note non-recessed pin. Use for low level signals only!

High Voltage Connectors


Miniature Connectors

Lemo HV 1500 V

Standard Lemo Signal only

Note recessed pin


More Electronics Advice

• If its >50 V, make sure somebody else is around. This is particularly true if you are debugging a drift chamber in the detector!

• If you get zapped, get checked out by medical. Too bad that you are embarrassed, don’t make it worse.

• Might be a good idea to take that CPR course, and know where the AED is.


Explosions• Explosions are all about stored energy… In the lab the prime

suspect is the gas bottle. The standard K bottle is 200 SCF at a pressure of 2200 PSI. Or V=42 liters and P= 147 Bar =15x106 Pascal

• U~PV/(γ-1) (for expansion to 1 bar) where γ=Cp/Cv

• U~0.9 MJ for a monatomic gas (e.g. helium or argon), U greater for nitrogen, oxygen, CO2.

• Scale: 1 gram of TNT ~ 4. 2 KJ• So gas bottle is ~ ¼ Kg TNT!!!!

– (For reference, a jelly donut is ~200 Calories (note those food calories are Kcal = 0.8 MJ, but at least jelly donuts don’t explode rapidly)

• Breaking the valve stem of a gas bottle is a big deal. It’s a deadly rocket. Handle bottles carefully! – They must have their valve cover on when not in use.– They must be strapped to a solid support at two heights to prevent

tipping.– They must have a proper regulator to control the output pressure.– Wear safety glasses if there is any possibility of a gas jet to your

face!!


Gas Bottles, continued.

• Gas bottles valves often have different threads to prevent inappropriate regulator use. For example, O2 regulators and pressure gauges and plumbing must be oil free. Be sure the regulator is correct, and don’t force the threads! Flammable gas bottles usually have left-handed threads.

• Occasionally there are even higher pressure bottles. EXO uses 6000 PSI argon for Joule-Thompson refrigeration. These bottles have their own special regulators.

• Some gases burn or explode. There is a very strong trend to use non-flammable gases for bulk applications – such as the Babar LST’s. However, isobutane often is a component of these gases, and might be used when developing a mixture. Hazardous gas detectors are used where there is a chance of a leak. These detectors are installed and maintained by EFD. They warn at a modest fraction of the Lower Explosive Limit (LEL) and must be heeded. Horrible accidents have happened to HEP experimentalists with flammable chamber gases.


Cryogenic Fluids• Cryogenic liquids expand when they warm up.

– Argon at STP is x860 liquid volume– Xenon at STP is x550 liquid volume– Nitrogen at STP is x710 liquid volume

• If a cryogenic liquid warms up, the pressure will increase. In a properly designed system, the volume of liquid is limited so that the warm system can handle the pressure. In addition, there should be relief valves and/or burst disks on any plumbing segment which can be isolated by the valves. Very few pressure systems can handle more than 2000 PSI, most much less. Weak links are usually windows, bellows, and feedthroughs.

• Simple safety principle for cryogenically recovering a gas into a pressure bottle – e.g. recovering xenon: Never dunk a recovery cylinder in liquid nitrogen for more than half its length.

• Wear safety glasses.• Use cryogenic rated gloves when pouring LN. Don’t spill LN into your

shoes! Cryogenic “burns” are serious.• Cryogenic fluids can cause oxygen deficiency hazards as they

vaporize. Ensure good lab ventilation when using LN for cooling. Be aware of the potential for nanoclimates – e.g. your head under a light blocking cloth.


Implosions• PMT’s - Work in the lab may involve large

photomultiplier tubes. The tubes are made of relatively thin glass, and are evacuated. Breaking them causes glass to be projected with high velocity. In certain situations, the shock wave from an imploding tube can trigger adjacent tubes. Always use goggles or safety glasses when handling these tubes. And these PMT’s are typically quite expensive!

• Thin Windows – Large thin windows for vacuum systems are rare in the R&D lab but common around accelerators. If the window was aggressively designed to limit multiple scattering, it can be quite delicate – and the shock wave from an implosion is serious. On most lab scale apparatus, breaking a window will “only” take out equipment…


O2 Deficiency

Altitude Effects on O2

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0 10000 20000 30000 40000 50000 60000

Altitude (ft)

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Barometric Pressure (mm Hg)

P02 in air

Po2 in Alveoli

Arterial O2 Saturation (%)

As the partial pressure of O2 drops, so does arterial O2 saturation. Judgment may be impaired first, but loss of consciousness occurs without warning.


O2 Deficiency

• Normal air is ~21% O2. Most O2 deficiency monitors alarm at ~19%.

• When working with cryogenic fluids, O2 deficiency can be a hazard if there is a spill. Remember that volume change of ~700. – Example: The SLD calorimeter had 50,000 liters of liquid argon. In a

worst case (but inconceivable) spill, the heavier than air argon expands by x 860 and produces enough gas to fill the full CEH pit almost twice!

• If there is a large spill – get out! A 200 liter LN dewar holds a quite serious amount of gas.

• Labs where there is a potential for a leak or a spill should be equipped with O2 deficiency monitors. These should give early warning and summon the Fire Department.

• In a small space, it is easy to displace enough air to be dangerous. O2 deficiency is perhaps the major hazard of Confined Spaces, and a special permit and training is required to enter a Confined Space.


Lasers• Some of the lasers in the lab have sufficient power to

permanently damage the retina.• Further, UV and IR lasers can’t be seen and can do

damage.• The primary level of control is containment – there

should be no laser light scattering around the lab from Class IIIb or IV lasers. Some lasers contain the beam in optical fibers with proper light tight terminations at both ends. Never operate these lasers with the fiber removed (at either end!) in a non laser-safe lab.

• Advanced training is required for work with these lasers. Laser goggles must be selected for the particular laser – “one size does not fit all”.


Sidebar - Laser Classifications (Loosely)

• Class I– Cannot cause eye damage either because <0.4 μW CW visible; or

completely enclosed. Note that if the enclosure is breached, controls for the native laser power class are required. CD players, laser printers, etc

• Class II– Cannot cause eye damage during the aversion response (0.25 sec) (aka

blinking). Only visible (400-700 nm); 0.4 μW < P < 1 mW (CW). Usually He-Ne lasers, laser pointers, range finders, etc

• Class IIIa– Cannot cause eye damage during aversion response. Injury possible with

optics or staring into beam. Visible, 1 mW < P < 5mW CW. Laser pointers, laser scanners, etc

• Class IIIb– Can cause injuries from viewing direct beam or specular reflection. 5 mW <

P < 500 mW CW. Diffuse reflection will not cause injury unless light collected by optics. Spectrometry sources, etc. Eye protection required.

• Class IV– Primary beam, specular and diffuse reflections can injure eyes and skin.

Also can ignite flammable material. All wavelengths with P > 500 mW. All pulsed lasers that the eye can focus (400 nm – 1400 nm). Significant controls and eye protection required.


Chemistry• This is not a general chemical hazards review, but a few special cases

that come up often. Use appropriate precautions and PPE. Material Safety Data Sheets (MSDS’s) should be first order check.

• Cleaning:– Ethanol and acetone are often used for cleaning UHV and other

components. Both are serious inhalation, transpiration, and fire hazards. Ensure good ventilation. A vapor hood is required if the quantities approach a liter.

– Make sure you know a fire extinguisher location.– For quantities more than a squeeze bottle squirt, wear appropriate

gloves.• Epoxies:

– The unreacted components of many epoxies are quite irritating. Wear nitrile gloves.

• Scintillation Phosphors – Occasional use is made of organic scintillators in their raw form. These chemicals may be toxic. Check the MSDS!

• Some chemicals (perhaps unlikely that you will encounter them) absolutely need special training and facilities:– Dangerous liquids: e.g. Be solutions, HF– Dangerous gases: e.g. Arsine, Chlorine, Bromine– Forget about it: e.g. Methyl Mercury


Vacuum Systems

• Laboratory UHV systems may be pumped with turbopumps or ionpumps.

• Turbopumps are somewhat delicate, but present few personnel hazards.

• Ion pumps are reliable, moderate cost devices but operate at substantial voltage levels. (The controller shown here will put out 3 KV at 7 ma.)

• The HV cables of modern pumps are reliable and safe, and some modern controllers shut down when the cable is disconnected. In general, there should be an independent ground connection between the supply and the vacuum plumbing, and the supply should be turned off before disconnecting the cable.


Vacuum System Baking

Metal vacuum systems often need to be baked to drive off water and other contaminants. Temperatures may be as low as 75 C for delicate internals, and up to 400 C for a “serious” bake.

•Heating is often done with “Heating Tapes”, glass insulated resistance wire.

•Do not exceed the tape temperature rating.

•Make sure the controller includes a GFI.

•Ground the vacuum system. Variacs are often used to control the voltage to the heaters. Note that Variacs are not transformers, and do not isolate the line.–Make sure the stainless is substantially (openly obvious to a casual observer) grounded.

•Check for fire hazards.

•Be aware of burn hazards!


Falls• Strangely enough, slips, trips and falls are the most likely

accidents. Falls can be quite serious.• In the lab, there may well be A ladders, but extension ladders

and scaffolding are unlikely. (Not the case near a detector) • The classic ladder accidents:

– Going on or above the penultimate step of an A ladder.– Using the top half of an extension ladder by itself.

• Try to tie extension ladders so they can’t slip.• Think about the surface supporting the ladder!• Don’t over-reach. Bad things happen when the Center of

Gravity is not over the base. Get down and move the ladder instead.

• Fall protection or barriers are required on elevated work surfaces.

• Be particularly careful of situations where the involuntary reaction to a (small) shock can initiate a fall.


Radioactive Sources• It is assumed that you have some knowledge of nuclear

physics…and that we will not talk here about accelerators or accelerator induced radioactivity.

• Types of sources:– α particles have ~no range and are stopped by the skin (unless

they get inside)– β’s ionize immediately, but usually do not have the range to do

damage.– γ’s go some distance before Compton scattering or photoelectric

effect kicks out an e- which ionizes internally.• Most lab sources are modest hazards if they are not ingested

or inhaled, usually meaning they are sealed:– Nanocuries to microcuries should not be carried in your pocket.– 100 microcuries is a serious source, but still can be handled in the

lab.– Millicuries and above need help from Operation Health Physics.

• Occasionally an unsealed source is needed when the recoil nuclei are of interest, or a liquid solution is needed to, for example, electroplate a source. Special handling procedures are required, and OHP must be brought in.

• For approximate point sources, dose will go ~ 1/r2. Even smaller sources can cause unwanted doses as r gets small…


Sidebar – Units1

• SI units are recommended, but not yet in common use.• Unit of Activity: Bequerel; 1 Bq = 1 disintegration/sec

– The Curie (Ci) = 3.7x1010 Bq• Unit of absorbed dose: Gray; 1 Gy = 1 joule/Kg

– 1 Gy = 100 rad (There are lots of survey meters around calibrated in rad’s, and occasionally even the (obsolete) Roentgen.

• {The Roentgen (R) measures the charge produced by γ’s showering in air. 1 R = 2.58x10-4 coul/Kg}

• Unit of equivalent dose: Sievert; 1 Sv = 1 Gy x wR

– wR = radiation weighting factor (was Q = quality factor in oldspeak)

• wR = 1 X and γ rays, all energies• wR = 1 electrons and muons, all energies• wR = 20 alphas

– The old unit is the REM; 1 Sv = 100 REM• 1 Mainly taken from Review of Particle Physics (2004)


Radiation Scales1

• Recommended limits for Radiation Workers:– CERN: 15 mSv/year– U.S. 50 mSv/year– SLAC 15 mSv/year

• Lethal dose: (LD50, no medical treatment) 2.5 – 3.0 Gy• Natural background: 0.4 – 4 mSv/year • Flux to deliver 1 Gy ~ 6.24x109/(dE/dX) charged

particles/cm2

• So it should be obvious now why a Ci is a big source.

• It is assumed that you have GERT (General Employee Radiation Training) . It is possible but unlikely that you will need RWT1 training. RWT2 training is for contaminated locations – not our labs!

• 1 Mainly taken from Review of Particle Physics (2004)


Coda

• SLAC’s PPA Safety Officers are Frank O’Neill, Joe Kenny and Sandy Pierson

• They may not know the answer to all your safety questions, but they usually can provide good pointers. Talk to them!

• Think!• If there is a problem requiring emergency help – call

911 from a SLAC phone, or 911 from a cell phone (assuming there is a signal). You will need to describe your location - obvious, but do you know the Building Number?

Documents

Lab Safety for Particle Experimentalists SLAC Course 110