Sunday, July 12, 2020

Scintillation Gamma Spectroscopy: Shield

Since I was working with and using a lot of lead for my "Lead Pigs", I decided to prepare the shield of my Gamma Spectroscopy (GS) setup.
Shielding the GS detector from the environmental radiation is extremely important for low activity samples and detection of trace isotopes. Some weak peaks will be otherwise masked by the background radiation noise. The scintillating detector is a supper-sensitive device producing hundreds of pulses per second just from the background so the more Lead for shielding, the better! You cant really over do it with the shielding. There is a parameter called "halving thickness" for Pb and shielding can be easily 50lb or 150lb of Pb - based on what are the limitations - space, weight, cost, portability etc. 
Generally speaking, the weaker and smaller the sample is, the more shielding is needed. For example - energy calibration with 1uCi of Cs-137 requires no shielding, while looking for traces of Uranium or Thorium in a small mineral sample or soil could require hundreds of pounds of lead to bring the peaks out of the background. Traces of Co-60, producing high energy gamma in 1330 keV range could "hide" under the broad 1460 keV K-40 peak and at these energies a lot of lead is required.
Shielding for Spectroscopy, on the other hand is not that simple - "just pile up lots of lead" and you are done! Besides the environmental background, one should worry about what is taking place inside the shielded volume itself - samples emitting beta particles or strong gamma will cause secondary X-rays. This is also the case of low-energy X-Rays induced in the shield by the primary gamma rays as part of the Pb X-Ray fluorescence (XRF). 
Geometry and distance between sample and the detector crystal is also a factor in the efficiency.r Backscatter will produce additional peaks as well.
Additional shielding from different materials is needed to slow down the beta particles before they hit the lead in the main shielding and also to absorb / suppress the secondary x-rays - tin, bismuth, cadmium and copper are normally used.
My detector shield is constructed from 3 layers - outer shielding, which has a total thickness of 12.7 mm rolled lead sheet on a 4" form, a 22mm Lead cast main shield around the detector (made of two parts) and inner shield inserts made of thick copper foil and tin / pewter foil.
A final consideration is the mechanical constructions and geometry - the best ratio between shielding efficiency and amount of lead used (read: weight and cost) is achieved when the sample chamber is kept small and most lead is placed as close as possible around the detector.  As the radius increases, more lead is needed to achieve the same attenuation as when the lead is right around the sample volume and detector..

These are the tools I used for the casting process in addition to about 20+ kg of "soft" Lead ingots

The key component is this "5kg Casting Clay Graphite Crucible", purchased on eBay for $30.

The Lead ingots were melted down using a large propane "Weed torch". 
I constructed a very simple furnace out of concrete blocks. After all of the lead liquified, I scooped out the floating oxides and impurities with a steel spoon and turned off the flame. The crucible keeps the lead molten for at least 7-8 min with no flame going so there was a plenty of time to work with it. There is a little value in getting the molten lead very hot - it just oxidizes faster and one must wait longer for it to cool down.

To make the cavity /hole for the detector, I used an 8.4 oz can of Red Bull, tightly packed with fine sand and clay. I also added some cement just to harden a bit the contents - as the lead cools down it exerts uneven pressure on the walls of the can and it will wrinkle them if the walls are not supported. 
The can was inserted and centered into the molten lead and held in place until the Lead cooled and solidified. 
A word of caution (aside from the dangers of working with a large volume of molten metal) - nearly everything floats in molten lead so inserting the aluminum can all the way down to the bottom of the crucible, displacing the liquid lead,  centering it and holding it in place until the lead cools down - all this, while fighting with the can's buoyancy was a bit of a chore - thick casting gloves and a steel rod, inserted in the can as an improvised "handle" helped a lot. (it took 2 attempts until I got it right in terms of procedure and exact lead quantity). The bottom of the aluminum can had to be submerged,  initially, at an angle to allow the air to escaped from the concave cavity on the bottom of the can. This picture is from my first attempt - l kept this first cast and turned it into a "lead pig"/ sample holder-chamber.

 The cast shield, after emptying the can of sand and peeling it off the internal wall of the cast. A few taps with a small hammer punched the hole where the bottom of the can was - the lead is very thin in this area due to the concave bottom of the can and its sharp edge. It worked perfectly, leaving a nice round edge hole.
The thickness of the shield wall where the crystal of the detector is situated is more than 22mm and it tapers off  gradually in a "bullet shape" (after approx. 60mm height) to 6mm wall thickness at the very top.
As the lead cools down, it shrinks before solidifying completely, leaving an uneven void on the top of the cast. I used a propane torch to melt another 500g ingot and "top it off" filling all of the gaps and smoothing it out while the cast was fairly hot.
The inner wall will be covered with 3-layer "sandwich" of the inner shield insert - rolled tin pewter, 2" copper pipe and aluminum foil on the inside.

 All it took was just a few gentle taps for the cast to come out of the graphite crucible and the result is nearly perfect! The crucible's shape and material are optimal for an easy release of the cast.
The inside diameter is 55mm and overall height is 110mm (the dimensions of the crucible would have probably allowed me to add yet another 10mm of height)

 Here is the shield, in place on top of the sample holder.
The outside diameter of the shield matches perfectly the aluminum flange of my sample holder.

A strip of lead sheet 1/8" thickness, 6" wide and 60" long (~23 lbs of Lead) was rolled onto a 4" ID Schedule 40 PVC pipe form as a 12.7 mm  (1/2") thick Lead "sleeve" (4+ complete turns). Before rolling the lead, I also added 2 turns of copper foil over the PVC. The sleeve (pvc form + copper + lead) is held together with duct tape in a nice, tight, easy to handle package.

The entire sample holder is to be placed in a small lead castle made out of 1" thick lead bricks in a couple of layers - all around, overlapping a bit the bottom of the middle shield.
Start to finish it took about an hour to complete the cast.
There is a plenty of space between the shield and detector for the copper, aluminum and tin insert around the probe and lining also the inner lower volume, holding the samples.

I used the short cast from my first attempt and filled the bottom of the opening with about 1 lbs. of molten lead, creating an improvised lead "bowl" / chamber. This is my "large sample holder". 
I also cast a small 1 cm thick lid for when it is used as a stand-alone "Lead Pig" for temporary desktop storage of hot samples.

This is what the main shield looks when both pieces are put together. The bottom of the detector crystal sits just above the seam. 
With small samples, it is possible for the detector to be completely inserted, moving the crystal into the lower part. In this case, the top cap of the detector sits flush with the top of the shield and the detector is completely recessed.

I had to build a wooden box / cradle so I can keep everything together and transport the entire shield assembly. The shield is pretty heavy - over 35 kg of pure Lead. 1" Lead bricks create a base for the bottom portion of the main shield  (placed in a polyethylene foam spacer - later I replaced the polyethylene with wood). 
Also, visible is the copper foil lining the inside of the two parts of the main lead shield - self-adhesive copper tape was used to cover the inner walls. Inserts of much thicker rolled copper foil, as well as 2" copper pipe section are placed in the detector shield and the sample holder. All this is surrounded with tin-pewter sheets. The bottom of the sample holder is lined with a stack of 6 copper disks and 6 tin-pewter disks.

Some of the components of the modular lead shield.

Four Lead bricks are placed around the base of the sample holder . These bricks serve as a raiser for the outer sleeve.
There are 4 more bricks, laying flat, lining the bottom of the cradle, right underneath the sample holder.

The top of the outer shield sleeve sits flush with the top of the inner cast shield.

Using 4" PVC pipe, I made a "collar" shield for the top portion of the detector. 
8 Lead Ingots were placed on the inside of the form. I had to carefully select the 8 ingots from a pool of  about 20, until I found the combination that will fit in - still, some force and the use of a mallet was required to jam them up on the inside.

The two outer shield components.

The collar, placed around the top portion of the protruding detector. 
A lead cap with a hole for the coaxial cable is then placed on top.

The complete "Large sample" version of the shield in all of it's glory - almost 70 lbs of pure Lead.
This version is not as convenient to use as the tray sample holder and it requires partial disassembly / assembly to access the sample chamber but shielding is much better.
The shield is modular, easy to transport, it takes only a couple of minutes to put it together and can be used in different configurations.

The 1"+ thick lead cap on top of the shielding. The lead cap was spray-painted with "Plasti Dip" rubberized coating to protect from lead exposure as it is handled often. It was casted in the same manner as the main shield but using a steel rod to displace the lead and make the hole in the center so no drilling was needed.

The counts per second rate of the background without and with the shield, The unshielded probe is reading an average of around 110-130 cps. After inserting it in the shield, the rate drops down to less than 5 counts/ sec. This is a factor of over 10 times reduction in the natural background.

Histogram of the shielded detector background count rate - average 3.2cps is not bad at all!

As I mentioned before, you cant really over-do the shielding part when dealing with weak activity - the more lead, the better it is for reducing the radiation background noise and bringing out the weak peaks. 
I found my amount of shielding to be adequate for most of my experiments as I don't usually do "super-low" activity testing as in food, water or soil samples. 
The current setup IMHO is a good compromise between amount of lead shielding and weight / cost / portability. The whole shield can be dismantled and transported quite easily and it doesn't require a special, sturdy support table as it will be with 200-300 lbs lead castle. 

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