Sunday, June 7, 2020

DIY: Making a large "Lead Pig" container for Radioactive Minerals and Materials

  As my collection of Uranium and Thorium Minerals grew, I was in a need for properly shielded, large storage containers - I, certainly don't keep under my bed samples of torbernite, autunite or vials of Uranyl Nitrate but shielding is important factor in addition to distance.
Small "Lead Pigs" are readily available from a variety of sources, but these offer generally very limited space and are good enough to store small samples / vials with fairly low activity - mainly used in Nuclear Medicine. I normally use them to store vials with Uranium, Thorium or Radium compounds and very small mineral samples (unmounted, thumbnail size) or just as temporary desktop storage during experiments.
Any larger than an inch mineral specimens would not fit and often the thin lead walls simply would not provide sufficient gamma shielding - some rich minerals exhibit activity, well in the excess of 600K CPM. Furthermore, this is an extremely inefficient way (in terms of space and cost) to store large number of specimens even if they can fit in - not to mention that minerals are usually mounted and displayed in "Perky" boxes, which will never fit in these small containers. 
Large containers on the other hand get very costly to procure.

These small Nuclear Medicine "Lead Pigs" are fairly inexpensive and are available on eBay or from other online sources but the wall thickness can be as thin as 3/32" for the small ones although, most of them are with 1/8" lead wall and some of the better ones exceed 1" wall thickness. A nice feature of these containers is the secondary outer plastic shell container or epoxy paint which prevents you from handling bare, toxic lead metal. They are fairly light and easy to move around but the main issue I have, is the relatively small volume - they are really designed to contain small vials.

Thanks to eBay, I picked up a number of Lead sheets (99.9% pure Lead Metal) with size 12" x 12" x 1/8" and 8" x 15" x 1/8". Lead itself is not very expensive metal but shipping, due to it's density can run up the cost quite high - I've seen large, commercial lead-lined containers where the shipping cost is sometimes two times the cost of the actual container due to the enormous weight. Buying lead as sheets seems to be a good deal as often the shipping is just a flat rate regardless of the number of sheets, since the sellers use "USPS Flat Rate" boxes and can fold the lead sheets to fit.

Constructing the box is fairly easy - just like making a box out of cardboard in kindergarten - 4 cuts/slots, each approx. 3" long into the sheet on two opposing sides and 3" from each edge, then forming flaps by bending and folding the sheet of lead into a box. 
Using a sheet of 12" x 12" results in a box 6" x 6" x 3" and zero leftover. Pure lead is an extremely soft metal and easy to cut with a pair of  "aviation snips" - especially thickness of 1/8" or less. An added benefit of this size box, is that after folding it, two of the side walls will become actually 1/4" thick because of the folded flaps on the inside are doubling the wall thickness (later, I had to put 2 small inserts to fill the gaps because the internal flaps will get a bit shorter due to the bend radius)
To make the folds, I used two thick aluminum plates and a few clamps, sandwiching the lead sheet in the middle and using the edge of the aluminum plate for a nice, sharp bend, folding it by hand and tapping it with a rubber mallet.
The lid was made out of another 12" x 12" lead sheet with an overlapping lip of 3/4" and size of approx. 6" 1/4 x 6" 1/4. I just flipped the box over the stock sheet and traced the outline, then added the dimensions of the lip overlap. The folding process for the lid is almost identical to the one for the box but it was a bit more challenging, using bare (gloved, really)  hands, due to the narrow strip of thick material to fold.. Gentle taps with a rubber mallet along the sides did the job quite nicely. One can even do a Telescope-style box where the cover extends all the way down, essentially doubling  the side walls but I was going to use a larger hard shell and needed to be able to center the box inside.

Once the lid was cut and roughly formed, I folded the lid flaps (this time, on the outside) and using a hammer and series of gentle taps, I did the final fitting, so the lid closes nice and smooth. The Lead metal is so soft that hand forming it with a little bit of tool help (small rubber mallet and hammer) and one can achieve absolutely perfect and smooth fit. The total weight of this inner container when empty is about 12 Lb. and provides a volume of 108 cubic inches. 
I didn't use any sealant on the inside seams (corners / flaps) as I don't want it airtight and to trap any Radon gas generated by the Radium as a daughter product.

Such lead container can really benefit from a hard shell - the soft lead is fairly easy to accidentally deform and damage if the box is dropped, handled too roughly or just stacked.

I constructed a wooden box using 9" x 3"1/2 x 3/4" ("4 x 1 lumber") pine for the sides, 9" 3/4 x 9" 3/4 plywood for the lid and bottom and coated decking screws. 
The handles were made of strong Dacron rope and pieces of vinyl tubing for added comfort. The extra long handles afford more distance from your body when carrying the container. 

I made the outer wooden box slightly larger (8" x 8" on the inside) and lined the walls and bottom with the same 1/8" thickness lead sheet material. The top is covered with yet another, secondary lead lid - a removable 8" x 8" x 1/8" plate placed over the inner box lid (this plate can be setup as a portable shield, once the container is open).

Each of the side plates for the lining are sized 1/8" shorter the inside space and they lock each-other in place by being offset. The bottom square plate has dimensions with 1/4" shorter than the inside space and it is installed after the side plates, locking them into place so no additional fasteners are needed for the lining.
The holes for the rope handles were drilled thru the wood sides and the lead lining.

 
The height of the side plate lining is 1/8 shorter than the depth of the box, so when the top plate / cover is placed, it  sits flush with the to wooden box edge. The top lead plate (cover) is with the exact dimensions of the inside space and it rests on the edge of the side plates.

 I can add more plates or bricks of lead around the walls as more scraps become available from other projects or just fill the gap between the lining and the inner box with lead shot. Currently the total lead thickness of the inner box + lining varies between 1/4" (6.3mm) (top and bottom) up to 3/8" (9.5mm) (sides)
The whole thing is quite heavy in it's final configuration - 30 lb /13.6 kg. of lead.

My ultimate goal is to have at least 1/2" or more Lead metal all around. Lead sheets can even be nailed on the outside of the wooden box if need be - when it comes to gamma ray shielding there is nothing wrong with over-doing it (except for the weight). 
1/2" Lead provides over 50% attenuation of Gamma Rays (also known as "halving-thickness") at up to 2 MeV (this is the equivalent of 1" steel or 7.2" water shielding). There is a point of a diminishing returns though - the amount of lead and the weight would not justify the amount of attenuation for low amounts of activity - you'll need for an example 3 cm of Lead (~1"  3/16) to attenuate the intensity to 1/8th of the original intensity. 
I also added HDPE (High-Density Polyethylene) plates (polyethylene is a very hydrogen-carbon rich plastic, which works well as a neutron shield / moderator) to fill the gap between the inner lead container and the outer lead sheets, lining the inside walls of the wooden box. This acts also as "padding" and keeps the inner box centered so the lid can be easily removed. 
Natural Uranium contains 0.7% of  235U, known for it's spontaneous fission so some neutron shielding would not hurt. Solid blocks of paraffin (candle wax) would work even better for those planning to store strong neutron emitters such as 252Cf 😀
This HDPE filler between the inner and outer box can also be substituted with small lead bricks, if one is willing to deal with the extra weight.
Future plans include to line the inside of the inner container with a "sandwich" liner of aluminum and copper foil - this will slow down the beta particles before they reach the lead shield and are abruptly stopped, thus reducing the generation of secondary low-energy X-Rays (the Bremsstrahlung effect caused by the rapid deceleration of a beta particle).

The Lead Box and my Infab Revolution Maxi-Flex X-Ray Lead Gloves (0.50mm Pb equivalent) for handling extra "hot" samples or just digging thru the contents of the lead containers.

I intentionally left the lead surface as bare metal and didn't paint it or cover it with duct tape, self-adhesive vinyl or rubber spray-on coating (all, good options btw.) for two reasons - one day I might need to re-use it for a different project (and don't want to deal with sticky residue) and also the bare lead metal lid (as weird as it might sound) is a good reminder for me: *GLOVES ON* when handling the contents of box (and honestly I do like the bare metal look).
All mineral samples inside, are organized by activity - the ones with highest activity are positioned towards the center of the volume and the lower activity rocks are placed around the perimeter as a natural shield.
I also installed a plywood cover with two small hinges to complete the outer box - all from Home Depot.
The wood surface was later covered with clear polyurethane varnish after applying the graphics. 

I built two of these "lead pig" containers.

I thought, at this point the containers were worthy of some stenciled graphics! 😀 
This way, it is easier to identify them as well.
After painting on the graphics, I applied 3 coats of clear polyurethane varnish. 

Yet another option is a Lead lined Ammunition steel can - in this case I used a .50 BMG Ammo Can which provides an ample space for large specimens - volume is approx. 11" x 6.5" x 5.5"
 The main drawback is the container's shape - "tall and narrow" so things has to be piled on top of each-other - not as optimal as the shallow boxes I made previously. In addition, a single handle is not the most convenient way to transport it if the lead lining is more than 1/4" - it gets awfully heavy to carry with one hand. 
On the positive side - lining with lead is very easy and straightforward - 1 x U-shaped piece of lead sheet for the large side walls and the bottom combined, 2 pieces of lead sheet for the front and back walls and yet one more for the lid. The front and back lining is the exact size of the walls and it is placed first. The U-shaped large piece is placed second and locks the other two pieces of lining in place.
The steel container is also very sturdy and air-tight due to the rubber gasket.

Stencil and Airbrush and the final look is pretty cool!

A probably unnecessary safety warning: use gloves when working with Lead, don't eat / drink while doing while dealing with lead metal and wash hands very well afterwards. Methods of cutting (like a saw or Dremel tool) which create lead dust are bad idea and will require respirator and cleanup.

Actual Lead Pig 😀