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Radiation Safety Primer

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LutzH

Thu Nov 12 2009, 10:41PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

I just wanted to post this seperate as a reminder to be safe. Sometimes we can all get so caught up in the equipment, and in the building, to the point where we forget some of the basics. Yes many will say I have heard it all before, and I know this already. My response to this is that it never hurts to review when your life is involved :)

The basic principal used in medicine reference radiation dose is "ALARA" it stands for: (As Low as Reasonably Achievable). This principal applies also for hobby use, yes officialy I should be tellling everyone not to mess with X-ray's period, and that they are very hazardous, and that X-ray's and ANY equipment capable of producing them, requires licensing just about everywhere on the planet. After telling folks this they will of course disassemble their equipment, and take it to the dump. Ya sure they will, just like a kid will throw away his ice cream bar when you tell him that it has sugar, and that it could be bad for your teeth!

Sooooo back to reality, please folks lets observe basic safety measures, and lets not get so caught up in the design of our equipment that we forget this. Here are some very basic concepts. These represent a very basic starting point. I would strongly suggest that you go a little further, and do some research on your own. If you can manage even a basic understanding of the science involved, then all the better for your safety, and for the safety of others like your neighbors who live below in your appartment! If you are not careful then they could become unwilling participants in an experiment on the effects of ionizing radiation. This reasearch has already been done, and we do not need to repeat these tragic findings.

Starting Point:

Minimum Shielding (Based on Voltage to X-ray tube): 15-70KV use about 2 mm or 1/16"min.of lead. Use 3-4mm or about 1/8"min. for 70-120KV. For 120-250KV use about 6mm or 1/4" of lead. For 250KV+ find a large uninhabited field with a handy rock wall, and use a 25 meter or 100 foot extension chord, you get the idea here :) Use layers of lead for thicker shielding, to make a 4mm thick shield for example: 4 x a 1mm sheets is better than a single 4mm sheet. Concrete can be substituted but testing is essential due to density variations in concretes. Remember X-ray tubes produce off axis X-rays, so you need 360 shielding. Also high energy X-rays can be scattered by thick objects etc. so secondary radiation needs to be considered also.

Distance: The intensity of the radiation goes down with distance via the inverse square law, thus twice the distance from the source, and you have about 1/4 of the radiation.

Time: Restrict the tube on time to only what is needed for the experiment.

System Speed: The faster your receptor the lower the hazard.

X-rays can be a lot of fun, but they demand respect just like the high voltage, which helps create them. One last note of warning reference X-ray tubes with Beryllium windows:

These tubes emit massive amounts of soft X-rays, a second of close range exposure to a diffraction X-ray tube for example at full power, can deliver a dose high enough to permanently remove the exposed skin. Skin grafts are very painful, in addition to the cancer which will show up later. So if you have to use a Beryllium window tube, permanently mount a .5mm Aluminum filter on the tube port. This will remove only the soft X-rays which do not contribute to your picture anyway. So have fun, but be safe :) We want to oogle your results, and not grieve your passing. Take care and good luck....Lutz :)

plazmatron

Fri Nov 13 2009, 12:34AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I second that!

I also strongly recommend that anyone experimenting with x-rays acquires a suitable dosimeter.

DON'T buy the "cold war" dosimeters though, as they are only suitable for high dose rates.
Buy low range dosimeters only. Ex lab or hospital.

I have two dosimeters that I wear. One has a fsd of 2mSv, and the other a fsd of 2000mSv.
The first is quite sensitive, and will in fact respond to active Uranium ore, and the x-rays from an old TV line output tube. The second is considerably less sensitive, and is worn in case of an accident (i.e. if the first one should go off scale).
In practice, the dose rate whilst taking a radiograph with my gear is close to zero, since everything is on a timer, and is operated remotely.

In comparison, a CDV-746 dosimeter has an fsd of 600 r (71 Sv!), this is around 3550 times the maximum allowable yearly dose of 20mSv!!!

Les

Bjørn

Fri Nov 13 2009, 03:08AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I started a wiki article based on this thread

If someone can make a nice high resolution graph of the attenuation factor for different thickness of different materials suitable for shielding it would be nice.

LutzH

Fri Nov 13 2009, 09:33PM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

Thanks, I think this will reach a lot of folks this way, and maybe save a few lives, good show!!!

The problem with a single graph is that in the lower KV range the lead thickness becomes very thin. Maybe a log graph would work. The unit used is HVL (Half Value Layer) the thickness required to reduce the beam intensity in half. Then it is further expressed in terms of how many HVL's are required to reduce the dose to acceptable levels. Sorry but I do not have any handy to post, but I will look for some. Thanks

TheMerovingian

Mon Nov 16 2009, 08:56AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Good guide, i'm sure that now everyone will approach more carefully to X-rays. Maybe some guide would be necessary also for HERF, high voltage and electromagnetic projectile accelerators, both about safety and law regulations.

Bjørn

Mon Nov 16 2009, 11:20AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The problem with a single graph is that in the lower KV range the lead thickness becomes very thin.

Maybe a simple table would be better with columns for the most relevant materials.

Proud Mary

Tue Nov 17 2009, 08:44PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

My own view in X-Ray Underground is to be skeptical of individuals who ask only about PSUs and sources, and seem to have no interest in, or budget for, shielding and dosimetry. .

My view may seem uncharitable, but if someone finds it problematic to generate 50kvp, and needs advice about it, then they would be better doing something else.

A cheap plastic GM counter may give some indication of the presence of rays greater than, say, 15kv - 20kv, but it cannot measure them. It cannot detect the most dangerous rays below ~20keV at all.

Remember that if you have 60kVp on the anode, then roughly two-thirds of your output will be below 20keV

In addition to 360 deg shielding, our best prophylactic against the rays is the simple timer. Make sure you are not around when the anode voltage goes on.

If you live in a flat/apartment, you must make sure that the rays cannot injure your neighbours.

I don't entirely agree with Les/Plasmatron's view that quartz fibre dosimeters are of no use. An array of six, for example, over a period of one or more hours, can give a very fair estimation of the beam shape, after which more accurate dosimetry can be brought into play. I assume Les meant that a single QFD is not much use on its own, to which I will agree to the extent that it is much better than nowt.

To a friend, I would advise them to have three completely independent dosimetres, for example: a calibrated ionization chamber pair,
a BGO/PMT detector, and a PIN diode detector. I also use use a 60cm Phillips X-ray photometer (basically a very big quartz-glass GM tube) as a 'sniffer' 12keV plus.

Once the properties of a particular source and its shielding and cooling are known, then one shan't be forever measuring shielding leakage etc, but it would be negligent to set forth without this basic dosimetry.

Just a few Grays and you will be done for, but this does not represent that many minutes even with a dental X-ray tube.

I have ten sources altogether, all but two of them working at their best below 30kV. For example, a silver anode is best excited by ~23kV (silver peak) and is of course the best possible voltage for exposing silver halide film. (I use ISO 400 5" x 4" Ilford sheet film from China in an old fashioned dark slide, which is easily push-processed to ISO 1600 at the expense of a little grainyness. This costs about a dollar a pop, including processing chemicals)

I also have a fluoroscope, kindly given to me by Plazmatron, which I am building into a low power Vickers metallurgical microscope stand.

My message here is not to discourage, but to raise the bar. The incontinent generation of X-rays is something to be discouraged,
whilst those who go into the arena with the right armour, training, precautions, and openness to hazard, will always have such small help as I am able to afford.

One does not need the brains of an Archbishop to design and build the anode and heater/filament supplies of an X-ray tube, so long as attention is paid to detail, so I would say the chief problem for the amateur machine was mechanical. To raise, lower and focus the tube with good accuracy in respect of the film. I have solved this by mounting my tubes - which are all of the earthed-anode end-fire XRD/XRF types - on a medium-format enlarger stand, which is very cheap on ebay now chemical photography has sailed over the horizon never to return, but is robust enough to take the weight of lead for the tube screen without significant vibration.

Serious X-ray players are always welcome to get in touch, but to others, the idly curious, I am not able to offer any sort of opinion over the flickering image of the Minotaur, its tastes, habits, or what it might be like in its labyryinthine Den,

Stella X

plazmatron

Fri Dec 04 2009, 03:55AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Proud Mary wrote ...

I don't entirely agree with Les/Plasmatron's view that quartz fibre dosimeters are of no use. An array of six, for example, over a period of one or more hours, can give a very fair estimation of the beam shape, after which more accurate dosimetry can be brought into play. I assume Les meant that a single QFD is not much use on its own, to which I will agree to the extent that it is much better than nowt.

My comment was just intended to point out, that high range Dosimeters should not be relied upon for personal dosimetry, and that suitable low range dosimeters should be acquired if possible.
I forked out a good Â£80 for my lowest range dosimeter, and even with my very limited finances, I consider it an excellent investment.

Of course cold war dosimeters are indeed excellent for directly measuring your x-ray beams

Les

Fabio

Fri Dec 04 2009, 11:22AM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

Hi all!

I think that this site can be extremely useful for all ionizing radiations amateurs:

i also think that a cheap radio remote control can be extremely useful too and if you collect radioactive items, you need some lead lined boxes and a safe (or at least a container with key latches)

Proud Mary

Fri Dec 04 2009, 01:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi Fabio,

I've always thought RadPro calculator an excellent piece of work.

It's X-ray calculator section is good of its kind, but goes no lower than 50kVp - while all my experiments use XRD tubes operating well below this figure. My favourite set up is to use a silver anode with 24kVp - just enough to excite the Ag peaks around 23keV.

Stella

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