Monday, September 21, 2020

Lead Pig / Vault for Radioactive Isotope Sealed Disk Sources

 The Sealed Isotope Disk Sources by Spectrum Techniques are extremely useful as reference / calibration sources for Gamma Spectroscopy and for an incredible variety of experiments and research, involving radioactivity. 

These 1" Plexiglass (PMMA) disks are offered as a large assortment of isotopes at various activity levels, ranging from as low as 0.05 μCi to as high as 10 μCi for some isotopes. They afford safe handling of the radioactive material - the actual source isotope is completely sealed with epoxy resin, in the center of the disk, inside a 0.250" diameter hole (except for the Po-210 (Alpha) source disks which uses a special substrate and Mylar foil). The material is located 0.5 mm from the back surface (non-labeled side) of the disk.

I use a few of these disks as calibration sources for my Gamma Spectrometer setup. 
One does not need a special license to own them (the disks are "U.S. NRC and State Exempt Quantity") but keep in mind - they are still regulated by NRC! For example - it is illegal to sell / transfer them, lend them or ship the sources to a 3rd party. It is also illegal to stack disks in order to achieve higher activity and so on.

The problem I had, is with storing them safely - the disk sources are shipped in these simple acrylic storage cases with just some foam padding and virtually no shielding whatsoever. 

Imagine, during experiments that you need to shuffle a bunch of these 10 μCi isotope disks on your workbench - such radiation exposure is not only unnecessary but can easily be avoided with a proper storage solution.

I tried a number of off-the-shelf "lead pigs" but all of them were cylindrical, "pill-bottle" type containers, which in this case is just a huge waste of space & metal. The worst part was that every time, I needed a specific source, I had to dump all disks on my desk and fumble through them to get the one I need, then put the others back in the container. 

I decided to build my own custom DIY container for such sources - this is a super-simple, inexpensive, half-day project.

My design goals for the container were rather simple: a small footprint, easy to carry, at least 3/4" of lead shielding all around, fast and easy access to any source (no stacking) and a fast way to restore the shielded state after removing or replacing a disk.

For shielding, I cast 6 lead brick using "1kg Graphite ingot mold" (sourced from eBay for $15) and 99.9% pure lead melted from ingots. Each brick is over 0.750 kg of Lead and the size is approx. 3 1/2" x 1 1/2" x 3/4".

I built a custom-sized wooden box to house the lead brick assembly, featuring a carry handle (~11 Lbs. of Lead inside!), a hinged lid and a lid lock. The box was constructed using 3/4" thick pine stock and has external dimensions ~6.5" x 5" x 3 3/4". The internal dimensions are specific to my lead brick configuration - 5" x 3 1/2" x 2 1/2". The bottom is made of 1/2" thick plywood. The box was finished with a few coats of polyurethane varnish for protection and low-profile non-slip feet were mounted on the bottom.

The Lead bricks are an intended "tight fit" inside the box. 
The bricks forming the bottom and the top of the shielded volume were lined with a thin layer of closed-cell foam ("Neoprene") padding. 
The top cover metal brick has 4 countersink stainless steel screws mounted in each corner. These screws serve as adjustable "stand-offs" with the protruding portion of the screw-head forming a gap, adjusted to accommodate the thickness of the source disks (0.175").
The other 4 lead bricks form the sides of the shielded volume.
The combined thickness of the left, middle and right bricks dictate the exact internal width of the wooden box as the mold can be filled with different amounts of lead during casting. The goal is to build the external box to close tolerances so the lead bricks cant move around once inserted.

This configuration works perfectly well for up to 3 disks but can accommodate as many as 6 disks (in two layers) if necessary (the stand-off screws need to be readjusted for a larger gap in this case ~0.35"). 
The small volume left to the front of the brick assembly currently has a 1/2" plywood "gap filler" but can accommodate a small tube with tweezers for handling the disk sources or a container for less radioactive sources (K-40, Lu-176). Alternatively, one can cut / shorten the left and right bricks by 1/2" and build the wooden box accordingly.

The top (cover) brick in place, completely shielding the source disks.

 The little U-shaped handle on the top brick has a dual purpose - it sits flush with the top edge of the wooden box, thus preventing the brick from shifting when the wooden cover is closed and secured, essentially locking the brick into place.

For handling the source disks, instead of using tweezers, I realized that a SMD Vacuum tool, normally used for handling SMD components during PCB placement could work well. 

The vacuum tool allows you to grab individual disks and position them, while increasing the distance between one's hand and the source, just like a pair of tweezers will do but I find it more convenient to extract the disks from the tight space of the test chamber with this tool. 

I will probably post a laminated reference table on the inside of the wooden cover, listing the gamma energies of my calibration isotopes.

With 3 disks inside (each of them 1 μCi) of Eu-152, Co-60  and Cs-137, my LND7317-based Geiger Counter reads only 550-600 CPM (~0.16 mR/h) on the top surface of the wooden box. With the top shielding block removed, the top surface reads 2100 CPM (~0.6 mR/h)

If sources with "softer" gammas are stored the attenuation would be even better but Co-60 with it's energies over 1MeV is not attenuated as well by 3/4" thick lead.

As a bit of curiosity - the disposal instructions for these disk sources are rather bizarre if you are not an NRC Licensee : One should deface the source, removing / painting over / scratching off any "Radioactive" signs from the disk and then just throw the disk away in the regular household garbage. 

(I guess nobody cares about the guy at the waste disposal plant, where the disks might be pulverized going through the plastic recycling mill)