Wednesday, December 6, 2023

13 Years later ...

 Today, 13 years after this post, FCC issued my son's first callsign - KC1TVB. Congratulations to Vichren for passing the exam and joining the lines of the Amateur Radio Operators!

Update: Vichren applied for and got a vanity callsign - N1VAS. His very first DX on HF was a KH6 station in Hawaii on 10 meters using only 10W from an Elecraft KX3 into a portable Buddipole dipole, slicing thru a pileup with a 5/9 on his first call!

Inside the W1VCM Club Station at the Vintage Radio and Communications Museum, Windsor, Connecticut.

Vichren, N1VAS discussing radios and repeater operations with Bob Allison, WB1GCM who was the Chief ARRL Test Engineer. 

Friday, November 3, 2023

Gamma Dog - Assisted Squelch Auto-Set (ASAS) / "Smart Squelch" System

One of the main features of the Gamma Dog project is the unique Squelch System controlling the audio output.
Anyone familiar with Amateur Radio will be familiar with the Squelch System used in FM Hand-Held Transceivers.
In a nutshell, the Squelch System mutes the audio output of the instrument if the currently detected rate is at or around the natural radiation background level. If the rate goes up, above the natural radiation background level (in the presence of a radioactive specimen for example), the Squelch System enables the audio system of the instrument and a tone with variable pitch is produced. Once the rate drops to background level or below, the audio is again silenced.

This post will delve deeper into the inner workings of the Squelch algorithm and the Assisted Squelch Auto-Set (ASAS) System.

I felt like I should include something more amusing than the boring rate plots :-)
Here is AI's idea of what "Gamma Dog" is.

The Gamma Dog's Squelch System works by detecting a differential between the Natural Radiation Background (NRB) and the current detected rate coming from the scintillating detector. The Squelch System evaluates the Natural Background, mutes the audio and sets a threshold level at almost the same rate as the NRB. When the currently detected rate exceeds this threshold level, the audio output is enabled.

In other words, the current rate is treated as a relative rate to a pre-set reference point (the Squelch Level) and the absolute value is insignificant for the Squelch System.

I said "almost the same as the NRB" because establishing the optimal rate for squelch system at any given moment is a bit tricky due to the randomness of decay and the constantly changing conditions during prospecting.

Gamma Dog will sample the NRB rate on startup or any time when the Assisted  Squelch System or the user activates an "Auto-Set" action (by pressing and holding the blue Squelch button or using one the Gesture Control triggers).

The term "Auto-Set" comes from the fact that the Squelch Level (rate) is established automatically, based on sampling of the current rate. This is always done on startup, but later a manual Set can be done by the user by overriding the established level and setting it to a different value.
When "Auto-Set" is activated, Gamma Dog will use one of 3 available methods to determine the Squelch Level (the method used is selected by Menu Item #20):

    1. Single Sample - Squelch Level will be set to the currently detected rate, using a single sample at the moment when Auto-Set action is triggered.

    2. SD5 method (default) - The Gamma Dog will average the rate over the last 5 seconds, calculate the Standard Deviation in the rate and then use the upper bound of the 1-sigma threshold to compute and set the Squelch Level.

    3. SD10 - same exact method as SD5 but the rate is evaluated over the last 10 seconds before Standard Deviation is calculated.
The 1-Sigma Upper Threshold is calculated simply by summing the mean rate over 5 or 10 seconds and the Standard Deviation of all samples taken in the data set. 

The horizontal purple line is the Squelch Level as result of a Single Sample (S-S Mode).
The orange line shows the actual current rate, and the blue line shows the smoothed rate.
It is obvious that the Squelch level is set too close to the mean rate and the current rate deviates too often above it.
 Another problem that can arise from this method - in some cases (purely random), it can cause the Squelch to be set too low or too high as it relies on a single rate sample and if this sample happens to be a peak or a deep valley the squelch level will be off.

In this mode (SD5), the Squelch Level is set to the upper bound of 1-Sigma of the Standard Deviation, evaluated for the past 5 seconds (data set includes 1 sample per second).
 This is the Gamma Dog's default method. It does a better job at finding the correct Squelch Level while the system responds better to quick rate changes due to the short rate history being evaluated.

The last available option (SD10) is the same as SD5 but the evaluation of the Standard Deviation is done for the rate over the last 10 seconds. 
It is the most precise way to find the correct Squelch level and results in very minimal number of excursions of the Current Rate above the Squelch Level but due to the longer history of rates in the data set, it is less "dynamic."

Note: The plots above were made using the Gamma Dog's Bluetooth functionality - all of the data displayed on these plots was sent in real-time via BT to a companion Android App and plotted there. 
The yellow line at the bottom of each plot shows the current battery capacity in percent.

As discussed above, critical aspect to the operation of the instrument is a properly set Squelch Level - the audio is used to alert the user of an anomaly in the detected rate.  When the level is correctly set, it will make the squelch system responsive to small fluctuations detected just above the background level. The level should exclude or rather - "minimize" any fluctuations of the rate due to the randomness of radioactive decay.

Why I calculate standard deviation and not just "pad" the mean rate? 
The answer is simple - "padding" or offsetting the level from the mean rate can cause inaccuracies because the deviation from the mean rate varies with the isotopic content, distribution of radioactive material and detector sensitivity.

For U and Th isotopes with extremely long half-lives as well as other decay-chain products, which makeup good part of the Natural Background, the distribution of decay events over a very short period of time (5-10 sec) can be reasonably approximated by a normal distribution, although the actual distribution is still governed by the exponential decay law.

In my testing, assuming normal distribution of the decay events and using 1-sigma of the Standard deviation works very well for figuring out a good Squelch Level.

The "sweet spot" is when the current natural background rate (NRB) only occasionally opens up the Squelch System but not too often - up to 3-4 times per minute is a good level. It indicates that the squelch is very close to the optimal level by occasional breaks in the silence.

The Squelch level can be Auto-Set (by sampling the Natural Radiation Background rate) or adjusted Manually, when the user is modifying the already set level with the GD Squelch Level +/- controls.
In either case, if the Squelch Level is incorrectly set against the NRB level, this could result in missing possible specimen finds or less-than-optimal Squelch functionality. 

One of the problems Charles and I encountered in the field while surveying and prospecting different areas is the Squelch response to highly localized Natural Radiation Background Levels.

For example - when surveying a REE deposit in Southern New Mexico we realized that the NRB is extremely localized - there are very well-defined areas of the pegmatite exhibiting very high Natural Radiation Background ("hot zones") and just a few yards away, other areas with comparatively very low NRB ("cold zones"). Some of these "cold zones" exhibit natural background even lower than what is considered "normal" for the area! 
Such contrast and localization posed a unique challenge - it meant that we had to constantly sample the NRB rate and activate "Auto-Set" of the Squelch system manually, every time we crossed over from one zone type to another. On the other hand, to know that you have crossed over to a different zone you had to monitor constantly the display - something I didn't want to do. With Charles's version of the Gamma Dog it was even more difficult due to the lack of display.
It is also worth noting that both zones in this pegmatite produced very nice Euxenite crystals which could have been easily missed if the squelch level was not correctly set. 

In this situation, if we focus only on moving through the terrain without paying attention at the current squelch level, we can enter an area where the NRB rate is much lower than Squelch Level and any small peaks or fluctuations, possibly indicating the presence of a specimen, will be masked and hidden by the rate gap in the squelch system (that is if the rate is not promptly re-adjusted). 
It could be some time, before one realizes that the instrument has been quiet for too long due to a Squelch rate set too high for the current NRB Level.
The opposite is also true - crossing from an area with low background rate to an area with high NRB will cause the Squelch system to stay constantly open which defeats the purpose of having it in first place and it will require the user to activate the "Auto-Set".

To resolve this issue, we came up with the "Smart Squelch" or Assisted Squelch Auto-Set (ASAS) System.

Assisted Squelch Auto-Set System

The purpose of ASAS is to constantly monitor the current rate, compare it to the state of the Squelch System and detect conditions which can indicate that the Squelch rate might be incorrectly set for the current background. 
Once ASAS determines that the Squelch Level needs an adjustment, it will trigger an Auto-Set action, same as if the user pressed the Auto-Set button and it will re-sample and evaluate the NRB.
This allows for very easy, smooth and worry-free operation - ASAS System does all of the necessary adjustments when they are needed, and the user can focus on the terrain and not on the instrument. 

Note: The implementation of ASAS is different between the one I designed, and the one Charles is using in his GD version. The main difference is the algorithm and the fact that in my version, due to the availability of display, menu system and persistent configuration parameters, I can configure many internal aspects of the ASAS behavior and make it more flexible and more customizable for a particular situation.

These plots show how ASAS monitors the state of the Squelch System.
Out of the 20 Configuration Menu Items, there are 4 menu items dedicated to the ASAS system.

In my design, there are 2 separate timers configured with a single menu item, setting the period for a squelch condition to be present continuously, before an Auto-Set is triggered.
One timer is reset every time the Squelch opens, while the other timer is reset every time the Squelch closes. 
Basically, I monitor the state of the Squelch System over time while constantly cross-reference the state with the currently detected rate and if a discrepancy is present for longer than time "window", an action is taken.
The timers can be set for 15, 30, 45 or 60 seconds.

One additional functionality I implemented in ASAS is the ability to "decide" if Squelch Auto-Set is needed while the squelch has been open for a long time.
When the user needs to pinpoint the location of a specimen, it helps for the squelch to stay open so variations in the pitch of the audio tone can be closely monitored. In such case Auto-Set should not be triggered.

In my implementation of ASAS, I have a menu item which specifies a second, virtual threshold level which, when exceeded, it will prevent the Auto-Set from activating due to continuously open squelch by resetting the Open Squelch Timer.

The logic behind this is that if a specimen is found and the user is now trying to localize it, the detected rate will be vastly higher than the NRB / Squelch Rate and not just slightly above it as expected - the ASAS system will identify this large difference in rates and it will not trigger an Auto-Set, thus allowing the user to listen to the audio tone without an interruption.
The "ASAS Reset Lvl" Menu Item, determines the height of the virtual "localization rate" as a percent of the current rate above the squelch rate. This value can be configured as 37%, 50%, 62%, 75%, 100% and 150%.

For example - if ASAS Reset Lvl is set to 100%, the Squelch Rate is set to 200 CPS and the detector reports a current rate of >400 CPS (or exceeding the Squelch rate by more than 100%), ASAS will not trigger an Auto-Set assuming the user has found a specimen and just tries to pinpoint it. If the rate drops below 400CPS then the continuously open squelch condition will trigger an Auto-Set when the timer for it expires.

Another functionality in my ASAS System is the ability to add a "padding" (called ASAS Tolerance) between Squelch level and Current Detected Rate, while the squelch is monitored for continuously closed condition. The amount of padding or tolerance is adjustable with a menu item ("ASql Toler") and specifies in CPS how low the current detected rate must drop below the Squelch Level before the timer of the continuously closed squelch activates an Auto-Set action.
This differential can be set from just below the current Squelch Level to a few hundred CPS lower and it is intended to add "adjustable damper" for the response to constantly closed squelch.
The options are "Minimum" , 100 CPS, 150 CPS, 200 CPS, 250 CPS and 500 CPS

By default, the ASAS System is deactivated and can be toggled ON/OFF by double-clicking the GREEN Button.
The last menu item of the ASAS System configures whether the system is activated automatically at the startup of the instrument, or the user will activate it at a later time.

The ASAS System in action. 
In this video I simulate "wrong squelch setting" by manually overriding the established Squelch Level while ASAS is active and setting it very low and then very high.

When the ASAS is active, the "Sql" indicator above the Squelch Level is replaced with "(A)".
The ASAS timeout is set to 15 seconds in this demonstration.

The two separator bars on the display (above and below the rate readout) shrink, showing when one of the timers is about to expire. The top bar is for Open Squelch conditions and the bottom bar is for Closed Squelch conditions. 
A full width bar represents the maximum duration the timer is set to, and the bar will shrink to a dot (which is the trigger point if conditions for adjustments are still present). At the moment of the Auto-Set, a short beep is produced to indicate that ASAS has taken an action.
If an event (a change in the detected rate) changes the state of the Squelch, the bar resets to full width. 

Charles searching for REE in Petaca, NM while using the "Smart Squelch" System.
The Gamma Dog produces distinctive beeps every time when the Automatic Squelch System is triggered to re-sample the Squelch level ("Auto-Set").

A more extreme rockhounding in Petaca, NM by Charles in March, 2024

Tuesday, September 5, 2023

Pulse Height Analyzer (PHA) option for Eberline ASP-1

When it comes to those big, beige, analog metering systems, heavy metal and clunky radiation detection istruments from the 80s... I am a huge fan of ... not Ludlum Model X but the Eberline ASP-1.  I have 3 such units and they are really cool and versatile "blast from the past"! 

For anyone interested I made a brief review and comparison between Ludlum Model 3 and ASP-1.

Eberline ASP-1 was and still is a fantastic meter for its time! In the name "Analog Smart Portable" (ASP), the "Smart" stands for "Microcontroller" - an 8-bit Intel 80C31 is in the heart of the meter, driving an 8-bit DAC (AD7524) which on the other hand drives the Analog Metering System. Two old-school 74HC157 multiplexers read the state of various config DIP-switches and the main rotary Range switch.

Reviewing the Service Manual for the meter years ago, I noticed that Eberline designed a Pulse Height Analyzer (PHA) module for it but for all these years I was never able to find, or even see a picture of one in existence. I am not even sure if this option was ever manufactured / sold for ASP-1 but I have seen dozens of ASP-1 units and not a single one had this module installed.

The Pulse Height Analyzer allows the meter to count pulses only when they have a specific energy range (of course, when the meter is equipped with an energy discriminating detector (scintillator)).

The PHA creates a "window" which has an adjustable width and can also be moved up and down the energy range so the counter can count only pulses produced due to gamma rays from a specific isotope - for example it can count only Cs-137 gamma at 662keV while ignoring any other energies, making it very selective at what activity is measured.

From a practical point, I can set it up for a specific U or Th decay chain isotope and being able to differentiate in the field if I am dealing with U or Th for example (that is if I don't have my Raysid with me)

The PHA module is interfaced via a "PHA Option" socket on the main board of ASP-1. This socket must be populated with a special shorting connector when the PHA Option is not installed and so was the case with all of my 3 ASP-1 units, until now...

After I was unable to procure the ASP-1 PHA Option for years, I got tired of looking for it and decided to make my own modules.

The PHA Option socket with the Shorting Connector removed.

The same PHA Option but for a different meter - the Eberline ESP-2. 
The schematics are virtually identical - only board outline and component topology appear to be different from the one, specific to ASP-1.

Fortunately, the Service Manual includes full documentation and schematics for this board.

This is the schematics for the PHA board published in the Eberline ASP-1 technical manual.
There are no exotic components and everything for this circuit is readily available on DigiKey. 
The circuit itself is fairly simple, using comparators and voltage dividers for reference to determine the energy "window". There is also a Dual BCD up-counter alongside a bunch of other passive components. 
The board outline is very specific as this board needs to fit between existing connectors and components located on the top side of the main board. 
The R32 Trimmer-potentiometer controls the width of the energy "window", and the slide switch allows to turn off PHA counting mode and to GROSS count all pulses. 

I started by re-creating the schematics in AutoCAD Eagle.

 I decided to keep the old-school DIP-14/DIP-16 versions for the ICs but replaced all other components with their SMD versions, using 1206 footprint for quick and easy soldering. 

Since it is a fixed size board there was no gain of going SMD for the ICs and their SMD versions were actually more expensive. 

I kept the original component names as listed in the technical manual - this makes component installation and troubleshooting very easy.

The PCB Layout created in AutoCAD Eagle.
Because of the location of the board, right behind the analog metering system, I realized that this board can also conveniently host LED backlighting for the metering system (the stock backlighting uses 2 incandescent light bulbs). 
I utilized the unused pin 2 on the Option header to provide the switched ground to the LED lighting and a small modification of the main board ties Pin 2 to the light switch. The LED Anodes are connected by jumper wire to the +VBAT

Top Layer of the completed PHA Option Board. Some additional milling of the board outline was needed to make it fit between the components.

Bottom PCB layer with the male DIP-16 header for interface with ASP-1

The PHA Board installed in the Option socket. The Blue Pot is the "Window" trimmer, and the red slide switch changes the mode between "GROSS" and "PHA" counting.

The PHA Option board sits nicely behind the Analog Metering System.

I have 3 completed PHA Option for ASP-1 modules left, available for purchase at $50/each + shipping.

Saturday, July 15, 2023

Gamma Dog hardware - PCB v4.0

I finished the design of PCB v4.0 for the Gamma Dog and OSH Park service came through again with excellent manufacturing quality boards.

The improvements in the schematics and layout are not huge but this version incorporates the progress in the development, which continued after PCB v3.0.

Gamma Dog Main PCB v4.0

Bottom layer with a Marie Curie quote.

Changes from PCB version 3.0:

  • Changed the placement of the DAC board and EEPROM board connectors. The new version of the 12-bit DAC daughterboard is slightly larger due to additional QT connectors and it wouldn't fit in the old footprint. 
  • Added HV voltmeter circuitry for measuring and adjusting the HV PMT Bias in the field - the measurement range is 0 - 1255V (max). The High-Voltage reading is displayed when entering the menu system's diagnostic screen and will allow for field adjustments when changing different detectors. The circuit provides also HV PS status data for the Self-Diagnostics / Health-Check during startup. The accuracy is better than the on-screen resolution of 1V.
  • Added the I2C digital potentiometer for the Digital Audio Volume Control to the main board (MSOP-10 package) - with the previous version it was an in-line add-on board as this feature was developed after fabrication of PCB v3.0
  • Added support for the LED in the GREEN/DOWN button - this LED now serves as "Charging" (flashing) and "Charge complete!" (solid) indicator during the internal battery charging.
  • Added an SMD jumper allowing the user to configure the power source for the Audio Power Amplifier module - options now are either the regulated 3.3V from the MCU board or the direct battery power (4.1V max) (affording the highest possible audio volume).
  • Re-organized (separated) and moved the DOWN Button and Audio Amplifier connectors - the GREEN/DOWN button now has a dedicated 3-pin connector (previously, it was shared with the Audio Power Amplifier), and the connector is placed right next to the BLUE/UP button connector, closer to the button's location.
  • Removed all of the unnecessary and unused component footprints, related to the old analog circuit for setting Minimum Pulse Height Threshold with a trimmer-pot.
  • Improved the matching between the motherboard's pads and the corresponding HV PS module Output and GND pads for easier installation of the piggybacked PS module.
  • Various small changes in component placement, trace routing and overall layout optimizations.

The Motherboard PCB with SMDs, sockets, headers and connectors installed. Visible, on the top-right is the HV voltmeter's VD string of precision resistors. 
The input impedance of the voltmeter is >1 GOhm in order to reduce any voltage drop caused by the measuring circuit.

Second Level of boards.
DAC, EEPROM and HV Power Supply + Pulse Amplifier.

Third level. 
The nRF52840 MCU module along with various sensors and RFI Shielding Can installed around the HV circuit.

Fourth Level completes the board stack.
Latching Relay module and RFI shielding cover installed.

Calibrated and tested V4 boards ready for the housings.

The official Firmware version is now v4.5, supporting the new hardware features and changes the way how unit-specific data is handled. 
The unique "Identity data" of each Gamma Dog unit, such as calibration offsets for various voltage dividers, pulse amplifier DC bias value, exact DAC reference voltage, serial number, etc is now stored in a dedicated "Read-Only" area of the EEPROM and not hard-coded in the firmware as it was before. 
Each unit is flashed with its "identity" during manufacturing, removing the need for unit-specific firmware code and simplifying development and future firmware updates.

New feature in the V4 hardware - High-Voltage Digital Meter - 0V to 1255V.
The PMT Bias reading is shown when entering the menu system. 
The value is constantly measured and displayed at a rate of 4Hz until a button is pressed. 
Adjusting the detector voltage in the field only requires a small screwdriver.

"Behind the scenes" - the machining work of the front panel. 
A 1/4" thick Plexiglas bezel for the display is already glued with black RTV sealant inside the display opening. The edge of the bezel is painted black to reduce internal reflections of the edge.
The display board is attached with self-tapping screws in blind holes. The blind holes for the audio boards are threaded and the board is secured with 5mm nylon stand-offs and 2.5mm nylon screws.

This picture shows all of the UI components and interconnects installed on the Control Panel. 
The wiring is done with JST connectors for a quick, easy, and clean assembly / disassembly.

The main board is mounted on brass stand-offs and connected to the Control Panel's UI components.
 There are only two connectors beyond this point - the white LIPO battery pack connector seen in the lower-right corner and the female BNC connector for the scintillating detector, seen just to the left of the battery connector cable.

The Complete electronics package of Gamma Dog V4. 
As a finishing touch, a 3D-printed bezel for the speaker opening retains a piece of open-cell foam to protect the speaker from dust and debris. Under the foam there is a second, fine metal speaker grill for mechanical protection.
Another cosmetic change in the V4 enclosure is a change in the way, the display and speaker board are mounted on the control panel. I used blind screw-holes from the back side to avoid exposed screw-heads on the front and the look now is nice and clean. The only screw-heads visible are the ones holding the main board. 
A self-diagnostics routine is executed on startup - the very first stage (pictured) is the check of all UI components.

Menu System in firmware version 4.5. In this video the Bluetooth connectivity is disabled, and the menu list skips the two BT-related items. Total of 20 Menu Items.

 At this point, I consider the firmware to be "fully matured" and bug-free.

Thursday, May 11, 2023

Modifying Scionix-Holland 38B57/1.5M-E1 Scintillating Detector

 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.


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.

Only the marked with "X" components should be removed, keeping all existing 1Mohm resistors in place.

This is how the PCB should look like after de-soldering the unnecessary components.

Additional 5x 1MOhm resistors are needed (I used the ones salvaged from other units) to complete the 12MOhm Voltage Divider.

The rest of the modification is just as outlined above.