Monday, September 20, 2021

Bigger is better or building a larger NaI(Tl)-based scintillating Gamma Spectroscopy detector

In the world of scintillators, larger crystals means more sensitivity - when you have a bigger volume of scintillating material, it can capture more photon interactions and will produce more pulses. High-energy photons sometime can pass thru a crystal without interacting with it and more material in the way means higher chance for interaction.

I've been doing some radioactive rockhounding and sensitivity is everything when it comes to finding that one specimen a foot or so deep in the ground, under rocks, dirt and mud (which btw act as a pretty good shielding). A larger scintillating detector was in order and having excellent results with my first build, I just went right ahead. This new detector is intended for both, Gamma Spectroscopy and Counting mode when prospecting.

The Photomultiplier Tube 

For PMT I wanted a good quality 3" device and Tom from Irad Inc came thru with a very nice brand new 8-stage Hamamatsu R6233-05 PMT. This device is intended for use in medical equipment (-05 designation) and came with a factory installed VD board which Tom removed before shipping as it didn't serve the GS purpose.

The device is fairly short (only 123mm) and compact for its 3" diameter size. It has 8 dynode stages which are nested tightly in the "neck" portion. A plastic cap on the back insulates the pins. 
The gain (2.7 x105) is less than R980 but this is to be expected with 2 dynode stages less - still it is impressive to see that the 10-stage R980's gain is 3.7 x105

The Bialkali photocathode has an optimal response at 420nm wavelength of the NaI(Tl) crystal. The Photocathode is a tad more sensitive than R980. Effective diameter is 70mm which works perfect for me since the crystal I was going to use is 63mm and larger photocathode than the crystal is optimal - no photons will be lost leaking.

The factory label - P.H.R. (Peak to Height Ratio) is very good at 7.8%

The PMT came with the factory VD board removed and the 3 stand-offs cut off. I marked the Cathode pin (K) with a silver dot and Anode pin (P) with a beige dot, To the left of the Anode pin is the Grid pin and to the right of the Anode is D8. On the left side of the Cathode pin is D1 and then clockwise all the way to D8.
The identification of K and P is easy - the Cathode and Grid are the only two pins separated by a single(!) blank space (missing pin)

All VD components - 10 resistors, a cap, lead wires and BNC connector were provided as a kit by iRad Inc.
A 20M resistor is connected between the Grid and the Cathode and another 20M between the Grid and D1. Eight 10M resistors are connected between each Dynode (D1 to D8)  and between D8 and P.
The resistors are with precision of 1% tolerance and were all checked with an LCR bridge.
Total impedance of the VD is 120M which is excellent for battery operated HV supplies.

10nF / 2kV filter capacitor is installed between D8 and GND (Cathode effectively)

Schematic diagram of the 8-stage Voltage Divider used.

The machined plastic cap came from iRad Inc. as well. 
A BNC connector was installed on the cap and connected with 2 silver-plated Teflon insulated wires. I drilled a small hole on the side of the cap for the grounding lead for the Mu Shield. The hole and wire were sealed on the inside with black RTV sealant after the wire was soldered to the BNC's ground lug.

The NaI(Tl) scintillating crystal 

Larger crystals have better stopping power resulting in overall better efficiency and this affect mostly the high-energy gamma response.

My NaI(Tl) crystal was purchased on eBay and came from Ukraine. The 63mm x 63mm crystals out of Russia and Ukraine seem to be currently very popular on eBay and they tend to produce excellent results.

Date code is Dec 1987, most likely made in USSR.

The crystal is hermetically sealed in an aluminum canister with an optical glass window and surrounded by reflecting material. The size of the crystal is 63mm x 63mm. The outside diameter of the aluminum can is 71.1mm.

The crystal looks pristine!
There is a lot of junk sold on eBay - cracked crystals, failed seals, yellowing or cloudy crystals etc.
One need to very carefully choose what they are buying - some of the garbage crystals I've seen are not even good for counting yet sellers ask hundreds of dollars. If you see any blemishes or imperfections these could be signs of trouble and better to stay away! A failed seal will let moisture in and will destroy the crystal over time - NaI is an extremely hygroscopic substance.
Fortunately, my crystal looks perfect as if it was made yesterday - not 30 years ago.  
Even when the crystal visually looks great it doesn't mean one will get the best resolution tho!

The Size-adapter / Centering Collar

The outer size of the R6233 PMT is listed as 76 +/-0.8 mm and the outer size of the crystal canister is 71.1mm so there is a net difference of ~5mm.
I wanted to center the crystal to the photocathode and have some reliable way to keep it centered while taping the assembly together so I decided to design a centering collar.

The centering / size-adapter was designed with Autodesk TinkerCAD.

The adapter was printed with conventional 3D printing technology. 
The bottom portion for the crystal was originally very tight - there was a small "shrinkage" of the print in this area - about 0.5 mm total so I had to sand it until the adapter can slide freely around the crystal.

"Dry fit" of the three components.

The purpose of the "fingers" on the adaptor is to be able to tape it or glue it to the PMT and secure it reliably.

During the assembly, the adapter should fit tightly around the PMT but it should allow the crystal canister to move freely ("float")

The inside area where the transition from one ID size to the other ID takes place is tapered for the glass edge of the PMT. The adapter will center the optical window of the crystal canister exactly to the Photocathode area, while providing both, mechanical stability and light-proofing of the interface area.

Optical Assembly

An edge reflector was applied around the front edge of the glass. On this PMT, the photocathode wraps around, to the side for a few millimeters and a reflector in this area will bring back photons otherwise escaping thru the glass edge. I used pure white PVC tape around the edge and then the whole PMT was light-proofed with the cap taped securely as well.

For optical interface medium, I used crystal-clear silicone fluid with viscosity of 100K cSt. It is sold as Differential fluid for high-end RC cars. Consistency is much thicker than honey but flows and spreads nicely and it is absolutely transparent. 
I started by applying an air-bubble-free drop of the fluid in the center of the window and pressing the PMT firmly on top.
I worked the PMT with large circular movements while pressing downwards for a couple of minutes, overlapping the edge as much as 1/3 the diameter of the window, until any excess fluid squeezes out from the edge and only a super-thin smooth layer is left. This process shouldn't be rushed and excessive force can damage the PMT and/or crystal.
I did notice about 0.2% improvement in FWHM resolution vs. the usual optical silicone grease, probably due to eliminating any possible air pockets / bubbles. 
I also used a 60K cSt fluid in the past with excellent results.
The purpose of the fluid is to eliminate glass-to-air and air-to-glass transitions and reduce the refraction and reflection from the surface of the glass - this makes for an optimal interface between the crystal and the photocathode.
Every time a photon reaches a glass-to-air surface it can be either reflected back or refracted in some direction. The optical fluid substitutes the air with almost the same refractive index medium as the glass, thus reducing light scattering and loss.

I placed the collar around the crystal before I applied the silicone fluid and mated the two glass surfaces. When I was satisfied with the quality of the interface, I simply pulled up the collar while adjusting the PMT's position and pressing it down to the crystal canister.

The entire optical assembly was taped together using electrical tape (3M 88 tape). I started with 8 pieces of tape stretched as tight as possible along the length of the assembly from the crystal's cylinder to the top of the PMT. This "pulls down" the PMT to the crystal with a slight pressure. The assembly was then taped all around starting from bottom to top while still applying pressure between the canister and the PMT.

Mu-Metal Shielding

Electrons just emitted by the photocathode can easily be deflected by an external magnetic field and miss the grid and the dynode cascade. The effect of magnetic field deflection of the electron's trajectory is worst when the field is parallel to the photocathode. Earth's own magnetic field, while weak, it is fully sufficient to cause a deflection for these "freshly emitted" low-energy electrons. Earth's magnetic field is even enough to cause deflections in the dynode stage so good magnetic shielding must be provided for the PMT.

The Mu shielding was constructed in two parts. The sheets of Mu-Metal were again sourced from iRad Inc. but they can be obtained from a variety of sources.
The top portion of the Mu shield was wrapped around the neck of the PMT in 2 turns and the bottom "fingers" were folded following the contour of the larger photocathode area.
The grounding lead from the cap was folded and soldered across the Mu-metal collar.

The bottom part of the Mu-shield. About 1.5 turns of mu-metal with "fingers" folded down over the top part "fingers", overlapping and covering all gaps.
This part of the shield extends a bit beyond the photocathode and overlaps the crystal - it is the more important part of the shielding as it covers the area between the Photocathode and the Grid.
If the detector will be exposed to stronger magnetic fields, more shielding should be added especially in this area.

The two pieces of metal are grounded together with a few small spot-soldering joints. I was careful to minimize the heating in order to avoid melting the electrical tape or changing the magnetic properties of the shield.

Final Assembly

A couple of layers of electrical tape were applied over the Mu-metal shielding in order to make it a tight fit around the PMT.
I fabricated 2 sleeves out of closed-cell foam ("Neoprene") - one for the neck of the PMT and another for the crystal canister. 
The foam acts both as mechanical and thermal insulation - it protects from mechanical shocks and rapid temperature changes of the crystal. It is also a good idea to have it for protecting the crystal from a thermal shock when the Heatshrink wrap is applied. Another layer of electrical tape covered the foam sleeves.

Applying the large size heatshrink wrap is a tricky procedure - it has to be heated evenly from all sides so it shrinks equally around the assembly while being careful not to overheat the crystal - a thermal shock can cause it to crack if heating is happening too rapidly. 
I did the entire process with little breaks to allow the temperature to slowly equalize as I was heating it with the heat gun.

The bottom closed-cell foam sleeve was closed off with a thin sheet of the same type foam, protecting the bottom of the aluminum canister.

The completed detector came out quite nice and solid. 
I still might try to fabricate an outer shell aluminum casing for it, just to make it "bulletproof"
The "Fat Man" codename was changed to Da Gamma Bee II / DGB-2525

Conclusion

After waiting sufficient time for the Photocathode to "calm down" from its excited state due to being exposed to daylight during the assembly I started testing the detector first in counting mode just to make sure the VD circuit is all good!

(!) A word of caution - PMT should never be powered up when exposed to ambient light - this will absolutely destroy the photocathode in a matter of a second.

For reference - this is the background count measured with my VD-modified Scionix-Holland 38B57 - 1.5" x 2.25" crystal and R980 PMT (counter is set to x100K range)

The newly built detector shows more than 2 times increase in the background radiation count. The counter is set to the x100K range so the reading is approx. 20K CPM.
The background measured later with PRA was ~270 CPS or 16.2K CPM but this is normal as PRA rejects malformed pulses, not matching the sampled pulse shape so count will be a bit lower as only pulses good for GS are sorted.
Testing was done at 650V

A quick Gamma Spectroscopy plot of Cs-137 and the results are fantastic - a really nice and clean plot with resolution of 6.53% FWHM at 662keV!
Such resolution is more or less as good as it gets for these larger NaI(Tl) crystals.
I used Gamma Spectacular GS-USB-PRO PMT driver set at 650V.
The linearity at this voltage seems very good too - I didn't have to use any Audio Gain factor and all peaks showed up in the right bins indicating that 650V is nearly optimal. In Theremino MCA the Energy equalizer stayed in the same state as before, adjusted after linearity testing and optimization of my other DIY detector. Only the Energy trim had to be adjusted a bit. This is very good sign!

"Fat Man" and "Little Boy" in their carry case.
I built these detectors for the fraction of the cost of a commercial detector, getting the same performance. Interestingly enough the cost of the two is very similar regardless of the size difference.

A new member joins my family of scintillating detectors!
 (L-R) Da Gamma Bee I / DGB-1531 with 40 mm x 80 mm crystal and Hamamatsu R980, Da Gamma Bee II / DGB-2525 with 63 mm x 63 mm crystal and Hamamatsu R6233, modified Scionix-Holland 38B57 with 1.5" x 2.25" crystal and R980 PMT and Gamma Spectacular GS-1525 with 1.5 x 2.5" crystal and Adit PMT.