Commercial scintillating detectors are usuallu very expensive - hundreds, often thousands of dollars for a good size NaI(Tl) detector. Such detectors are not always affordable for amateurs, even on the secondhand market but for many applications they are the optimal, and sometimes the only solution.
There is a hidden, and often overlooked and underestimated gem though, made by a leading in the scintillator business Dutch company - the Scionix-Holland 38B57. This detector is nearly impossible to beat when it comes to value and it is pretty much "the best bang for the buck", delivering an incredible performance for its very low cost on the used parts market. The detector was manufactured as an OEM part about 10-15 years ago and not available for purchase as "new" but there are plenty of salvaged units out there, offered by various Internet sellers.
The 38B57 is a "classic" NaI(Tl) detector - the crystal is 38mm by 57mm (1.5" x 2.25"), surrounded by reflective powder, coupled with a 38mm Hamamatsu R980 10-stage head-on PMT and mounted in an Aluminum + Stainless Steel tube enclosure.
38B57 is employing an integrated design - the NaI(Tl) crystal is not encapsulated in its own aluminum canister, but it is directly interfaced (glued) to the PMT's Head-On photocathode window and then both, PMT's front part and the scintillating crystal are sealed together in an air-tight aluminum can. The assembly is wrapped with the Mu-metal magnetic / electrostatic shielding and together with the VD PCB is housed in a stainless-steel tube with an aluminum end-cap.
The integrated design keeps the cost down, but it means that this detector is not really serviceable beyond its voltage divider circuit / PCB - crystal and PMT cannot be decoupled from each-other and replaced without the complete destruction of the detector - this is one of the down sides of such design.
38B57 on the other hand, is a really a high-quality, spectroscopy grade detector and the lack of an additional glass window in front of the crystal improves the resolution by reducing photon refraction and/or reflection which would normally occur with the extra glass window of an encapsulated crystal.
These detectors will often show up on eBay for as little as $80/ a piece (at the time of this writing, but prices do change) and sometimes for even less, making "good size NaI(Tl) Scintillator for under $100" possible!
Obviously, if the PMT or crystal are damaged because the unit was mistreated or accidentally dropped, the detector goes in the garbage bin but if they were treated well and are in a good, working condition, the detector can provide excellent post-service life as a Gamma Scintillator (Counting or Gamma Spectroscopy probe) - I routinely measure the FWHM resolution to be better than 7% (@662keV) for the 38B57 detectors. This makes it an excellent choice for those who need a scintillator probe, are just beginning and want to try Gamma Spectroscopy and are on a budget.
I, actually started my Gamma Spectroscopy experiments years ago with such detector.
As the detectors are removed from the Exloranium GR-135 units and sold on eBay, they are not directly useable - they have a custom voltage divider circuit with transistors and diodes in the last stages, intended for use with the GR-135 hardware and must be modified with a "standard" voltage-divider circuit to get the best performance for both, linearity and resolution. Even the original connectors and the way they are powered is specific to the GR-135 unit. People have tried to use them without any modification, but the results are not great, and linearity is very poor.
The simple and easy to do modification brings it to a completely new level and one will be rewarded with a very capable detector once it is done.
The modification process consists of removing the original voltage divider, installing a "classic" VD circuit with appropriate impedance and mounting a coaxial connector on the housing.
This modification is not difficult but requires basic electronics, soldering and mechanical skills and one should be comfortable, working with SMD components in order to perform the procedure.
Modification
Funny enough, the most difficult part of the modification process is opening and removing the rear aluminum cap of the detector.
Enemy #3 of these inorganic scintillating crystals are rapid temperature changes which can cause the crystal to crack. (Enemy #1 is moisture and Enemy #2 is mechanical shock)
It requires quite a bit of heat for the adhesive to fail and let the cap go.
Inserting the blade of the utility knife between the edge of the tube and the cap while hot, allows for the cap to be pried off - this action must be carried out repeatedly at different spots around the perimeter of the cap until it comes off.
If the cap doesn't budge initially, just reheat quickly to a higher temperature, while monitoring the temperature of the crystal housing and once the cap is open, slowly cool down the top, heated edge, of the stainless-steel tube.
The aluminum cap can retain heat, so the process is as follows - heat up the cap, then quickly put down the heat-gun and try to pry it off with the utility knife, then repeat as necessary and change position around the perimeter of the cap.
Once the utility knife blade widens the gap enough to slip in a flat-head screwdriver between the edge and the rim of the cap, things become easy as twisting the screwdriver applies quite a bit of force to pry the cap open.
Some caps will come off quickly and easily, but others will have excessive amount of glue and can be "tough cookies".
Next step is to remove the silicone sealant, cables and the cable grommet.
DO NOT try to remove the stainless-steel tube from the bottom, aluminum part of the housing in order to gain better access to the PCB - it is not needed, and any such attempts could lead to the destruction of the detector!
All of the work is carried out through the back opening.
I cut the cables for this picture, but actually the wires should be de-soldered and completely removed. The picture shows the original Voltage Divider, with the transistors in the last stages. It is a tapered VD and the total impedance is fairly low - around 12MOhms.
Most components of the original VD must be removed, and some will be replaced with different values. The only components that stay are the 3 capacitors shown on the picture - everything else, marked with "X" in this picture, must be de-soldered.
I use 10 Mohm / 1/4W / 1206/ 0.1% tolerance resistors - Digikey part # 749-MCA1206MD1005BP500CT-ND - Vishay High Stability chip resistors.
Resistance tolerance is not super-critical as there are already differences in the PMT's Dynode stages to begin with, and 1% tolerance should work just as well.
All resistors should be installed just as shown on the picture.
(tip: buying these resistors in quantity of 100 pcs from Digikey is more cost-effective, especially if more than one detector is modified as each detector takes 12 resistors)
On the picture above:
A. Resistor is installed in the position of the removed capacitor.
B. Resistor is installed on top of the capacitor and in parallel.
C. Two resistors in series are installed between Dy1 and K. Single 2xR resistor (in this case 20M) can also be used but I found to be more convenient if I use 2 resistors as the distance between the pads allows for this, looks clean and helps if 2R is not a standard value.
I also seal the seam between the aluminum part of the housing and the stainless-steel tube with a strip of Kapton tape - just for "good measure".
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