AE1S Science and Engineering Blog: May 2023

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 are part of the Exploranium GR-135 RIID device and hundreds such units are being decommissioned all the time by various US Government agencies (Border Patrol, Cost Guard, etc. ) and sold to equipment recyclers / salvagers.

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!

The 38B57 detector located in its black, shock-proof rubber protector, inside an Exploranium GR-135 Radioisotope Identifier unit. The white connectors on top are how the detector is connected to the electronics - the left one is the temperature sensor for compensation and the right one is for power to the detector.

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.

Unmodified, freshly decommissioned detectors.

I cut off the connectors in order to remove the detector without damaging the protective rubber booth.

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.

The cap is glued very well, with 2 different types of adhesives (including a special conductive adhesive) and one must use a heat-gun, an utility knife, flathead screwdriver and some patience to take the cap off.

Fortunately, there are no heat sensitive components in the very back of the housing, but heating must be done quickly before the heat creeps down the housing, towards the NaI(Tl) crystal.

TIP: Holding the detector with a moist paper towel can provide additional cooling and heatsinking effect to the crystal housing while performing this procedure.

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, hot edge, of the stainless-steel tube.

The aluminum cap can retain heat, so the process is as follows - heat up the cap and the edge of the stainles-steel tube, then quickly put down the heat-gun and try to pry it off with the utility knife, then repeat it as many times as needed while changing the 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 of the stainless-steel tube and the rim of the cap, things will become easy - twisting the screwdriver applies quite a bit of force to pry the cap open.

Most caps will come off quickly and easily, but some might have an excessive amount of glue and can be "tough cookies" to crack.

Once the aluminum rear cap is removed, this is how the detector looks on the inside.

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 on the PCB 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 red "X" in this picture, must be de-soldered.

The best and fastest way to remove these SMD resistors is using 2 soldering irons equipped with fine tips (I use ETP tips for this task with my Weller stations). This method also carries less chance for PCB damage. Each resistor is heated simultaneously on both sides and picked up by the two soldering iron tips as if tweezers are used. It takes me just a few minutes to remove all of the unnecessary components. Solder wick is used to clean the pads and prepare them for the new resistors. The old soldering flux can be cleaned off with alcohol pads or alcohol-soaked Q-tips.

This picture shows how the PCB should look like after de-soldering the original voltage divider. 3 out of the 4 SMD capacitors (10nF/200V) are left in place. The 4th capacitor on the very left is removed and later a resistor will be installed in this position.

PMT Check (optional)

After removing all of the necessary components, it is a good time to conduct a check of the PMT integrity.

The PMT is sealed and glued inside the stainless-steel tube and it is difficult to tell if the detector has been dropped and the glass envelope of the PMT is broken.

Fortunately, one can do a quick electrical check - all that is needed is an ohm-meter with high resistance range (preferably 100M) and a flashlight. The positive (red) lead of the ohm-meter is connected to the first dynode (Dy1) and the negative (black) lead to K (Cathode).

On the PCB these two test pads are where originally the red wire was connected (K) and the white wire (Dy1).

Normally, in dim light, there will be very high resistance and the ohm-meter will show "open circuit".

Shining briefly with a flashlight in the back of the PMT, thru the PCB's center hole, where the glass evacuation port is located should show a lower resistance on the ohm-meter - around 50M or less (depending on the flashlight power).

This is an indication that the PMT is good and under vacuum.

The light knock out electrons from the Photocathode and they cross thru the vacuum to Dy1 acting as Anode.

If the resistance remains high (infinite), there is a possibility that the PMT is broken.

Next step is to install the SMD resistors for the new, "classic" voltage divider.

I have discussed choosing resistors for PMT VD in other posts but for general purpose (counting with Eberline or Ludlum meters or other battery-operated meters for example) 10 MOhm resistors are normally used. The Hamamatsu R980 PMT used in the detector is a 10-stage PMT with 2R (20M) between K and Dy1 and R(10M) for all other resistor positions (between the rest of the Dynodes).

The footprint on the PCB requires 1206 package resistors.

The total impendence of the VD will be 120M, which results in minimal voltage drop even with very low current HV power supplies.

(For Spectroscopy applications, a lower value for R/2R should be used - 1M or 2M is generally a good choice.)

I used 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 small factory differences in the PMT's Dynode stages to begin with. Thick film resistors with 1% tolerance, should work just as well and they will be more economical.

All resistors should be installed as shown on the picture.

(tip: buying these resistors in quantity of 100 or 500 pcs from Digikey is more cost-effective, especially if more than one detector is modified - each detector needs 12 resistors)

Key points on the picture above:

A. Resistor is installed on the pads of the previously removed capacitor.

B. Resistor is installed on top of the capacitor and in parallel, soldering the resistor terminals to the capacitor's terminals.

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 one of the standard values.

This is how the PCB should look like after all of the resistors are installed. The yellow wire is connected to the PMT's Anode (P) pad and supplies both, HV Bias to the PMT and return signal - it returns back the positive pulses generated by the PMT to the external circuit.

The grounding lead wire, attached to the detector's housing (circled in red) must be connected to the K pad (PMT's Cathode). The stainless-steel part of the housing acts as electrostatic shield for the PMT and must have solid ground connection.

After the K lead (black wire) is installed, the grounding wire to the housing is connected at the junction of K-2R.

If the original wire is not long enough to reach the K pad, it can be extended with a piece of bus wire, as shown.

The two pads circled on the picture must be bridged with a short piece of jump wire.

This is an important step and should not be omitted!

If this jumper is not installed the detector will not work.

Here are the schematics of how the modified detector should be wired.

The aluminum cap is drilled in the center and a female BNC connector is installed - I recommend using a good quality connector with Teflon center conductor, like Amphenol UG-625/U.

Alternatively, a MHV or even SHV connector can be used but there is not much clearance on the inside and fitting a standard SHV bulkhead connector will be rather difficult and will require modification to the back side of the connector.

Standard UG-625/U BNC connectors will also require some trimming of the center terminal to prevent it from touching the PCB or the PMT's evacuation tube in the center. Half of the solder cup can be removed with wire cutters, leaving enough for a reliable solder joint.

The drill diameter for the hole in the cap is 3/8" for a round connector and if the connector barrel is D-shaped (to prevent rotation), then 11/32" drill bit is used, and the rest is shaped with a set of small round and small flat files, until the connector can just fit through the hole without being able to rotate. Using connectors with D-shaped barrels is the better choice - it locks the barrel in place and prevents the connector from loosening itself when operated.

The yellow and black wires (silver-plated stranded wire with Teflon insulation) are soldered to the BNC connector. These wires are about 1" long (actually, they can be a bit shorter than this) and are carefully bent in a spiral fashion and routed in a way not to touch the board or components when the cap is closed.

The original cable opening is sealed with a piece of self-adhesive copper tape and a length of Kapton tape on top. The housing must be completely light-proof (!) and air-tight. Black RTV sealant around the BNC connector (on the inside) can be applied before the connector's lock-washer is placed and nut is tightened.

I highly recommend that the BNC connector's lock washer is used to prevent it from loosening.

The aluminum rear cap is glued back with hot-melt glue to the stainless-steel housing - I apply small amount around the perimeter, near the top edge of the cap with a glue-gun, heat up the cap and press it flush.

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".

The modified 38B57 detectors - completed and tested, ready to be installed in Gamma Dogs. After the modification, these detectors can be directly connected to counters such as Eberline ASP-1 or most Ludlum counters. They will certainly outperform Ludlum 44-2 probe for example.

Here is a Gamma Spectroscopy plot done with one of the modified detectors.

The FWHM resolution for 662keV (1uCi of Cs-137 source disk) is 6.9%. The detector was running on 575V and connected to a Gamma Spectacular GS-USB-PRO. (The second peak from the left is XRF coming off the lead castle - the peak is suppressed due to the graded shielding).

These detectors output ~110 CPS (6600 CPM) for the Natural radiation background at my location when unshielded (~0.1 uSv/h).

"Gamma Spectroscopy Only" Use / 12MOhm Total Impedance VD

If the detector is to be used for Gamma Spectroscopy only, with GS-USB-Pro or a Lab Grade PS driver providing "stiff" HV Bias, lower impedance VD will result in better stability, better SNR and even faster response.

The 12MOhm VD on the other hand pulls more current and it is too low for portable, battery operated, meters - it will cause a significant voltage drop and an increased battery usage.

The original VD can be modified by removing the active components and only some of the resistors while keeping all of the existing 1MOhm resistors - this is really simple and logical, but I decided to provide the pictures anyways in case somebody wants to follow this guide as a step-by-step.