Tuesday, October 26, 2021

Gamma Dog - The Ultimate Radioactive Rock / Mineral Finder!

It was time to put my newly built large scintillator to work. 

A friend of mine and an avid REE mineral collector Charles Young designed sometime ago an instrument intended specifically for finding radioactive rocks. The design evolved from his initial concept to become a pretty clever and viable solution. Charles and I have been talking for awhile, exchanging thoughts and ideas what such instrument should be and what features are needed. Finally, Charles managed to do something I've been thinking about for years but never really had the time to dive into! His work was truly an inspiration for me and eventually precipitated as the "Gamma Dog project".

With the core R&D work done by Charles, we decide to collaborate in the design efforts and improve both, the hardware and software side in order for this instrument to become the ultimate tool for the Radioactive Rockhound.

Currently, there are no such specialized prospecting tools on the market. Geiger Counters are more or less useless due to their lack of gamma sensitivity and commercial scintillators require that your eyes are "glued" to the meter's scale or listen to a divided click-rate and trying to figure out if the rate changes "by ear" which is a pretty strenuous activity by itself. 

We wanted something which is easy to use, very sensitive and allows the user to focus on the environment / terrain and not on the instrument.

Meet the γDog! 

The instrument's housing is a 14.5" long 4" diameter ABS plastic tube with two end-caps, a few handles and control panel on the back cap. Weight is ~5.5 Lbs with 63 x 63mm NaI(Tl) crystal. 

I think the outside look is a bit ominous and I'll probably need to dress it a bit with some decals for the sake of the public. After all, you press a red button on a big, black cylindrical object - it beeps and the first thing you see on the display is the radiation trefoil - I can see how an ignorant or less intelligent person could hyperventilate seeing this.

The 3 major internal components are scintillating detector, electronics module and battery along with an interconnect (coaxial cable).
The scintillating detector is heavily padded with closed-cell foam ("Neoprene") to protect the delicate crystal and PMT from any mechanical shocks and vibrations. The unit can accommodate different LiPo battery sizes - usually 2x18650 cells in parallel (4400 mAh) or 3x18650 cells in parallel (6600 mAh). Bottom cap is glued and it is water-tight. The top cap is removable (3xSS Screws) and hosts the entire Electronics module and control panel.

Currently, I am prototyping the latest version (v3) of the Gamma Dog. I designed the PCB with AutoCAD Eagle and the fabrication was done through the OSHPark service - the PCBs came out absolutely superb! Highly recommend OSHPark for small projects - quality, pricing and ordering process are fantastic!
The MCU board and HV latching relay board are stacked on the motherboard headers. Visible between the headers are the DAC and EEPROM boards.

The new hardware version expands v2 with a second button for an improved UI, 4K EEPROM for storing settings and internal data and a 12-bit DAC for digitally adjusting the minimum pulse height threshold.

There is an RF can for shielding the HV circuit which is attached to the HV board (not installed yet in the picture).
There are 6 connectors on the motherboard - one female BNC for connecting the scintillator and 5 for power, buttons, display, speaker and LEDs (4 are on the top side) - not counting the IPX to SMA coaxial connection.
Not pictured is the latching relay shield which plugs into the top MCU headers. 

A quick demonstration of the "γDog v2 PLUS" and the UI.
(Youtube link)

Hardware Features:

Most of the instrument's housing is occupied by the padded scintillator (see the diagram above). There is also a battery compartment and the entire electronics assembly is located in the top end-cap. A fixed metal handle with a soft nylon strap handle and a a couple of special finger pulls / attachment points facilitate easy handling and maneuvering of the instrument while searching and digging. The enclosure is sealed and water-proof up to the control panel so it can be partly submersed if needed. The control panel is water-resistant and can withstand occasional splash of mud, water or light rain and it is easily protected with a transparent plastic "shower cap" and a rubber band when weather conditions deteriorate severely. There is 1/4" thick Plexiglass bezel protecting the display.

  •  Large (63x63mm NaI(Tl) crystal) Scintillating detector - this is the detector I previously built, but almost any NaI(Tl) / CsI (Tl) scintillating detector can be used or even Bicron BC412 Plastic scintillators. Requirements are a voltage divider impedance of 50M-120M, common signal/PMT HV bias line and voltage up 1000V - Ludlum 44-2 probe for example is a possible "off-the-shelf" solution.
My large 63 x 63 mm NaI(Tl) detector is the "heart" of the γDog. The larger the crystal is, the more sensitive the instrument is going to be - this is critical when looking for radioactive specimens "in the wild". Smaller size detectors or scintillating plastic detectors on the other hand are better when searching in tailings piles, mine dumps or inside mines, where the radioactivity is abundant to begin with.
  •  Adjustable regulated and filtered High-Voltage Power Supply for the PMT Bias, positive polarity, 600V-1000V range, controlled with a latching relay for a reduced power consumption.

γDog's High-Voltage Power Supply and Pulse Shaping Amplifier.
  •  Pre-amplifier with adjustable gain ratio and adjustable pulse shape - rise/decay time constants as well as decoupling capacitor and load resistor are hosted on the HV board
  •  Adjustable minimum pulse level detection threshold using a comparator on the input of the MCU
The scintillator pulses as they are amplified (yellow trace) are "digitized" by the comparator (blue trace) when they exceed the set threshold (white cursor line on the Y axis).

Nordic nRF52840 System on a Chip is the "brains" of γDog.

  •  Nordic nRF52840 SoC (installed on the Adafruit Feather Sense Board) - ARM Cortex M4F processor, 1MB Flash and 256K SRAM, 21 GPIO, 6 x 12-bit ADCs, up to 12 PWM outputs and an array of environmental sensors - Accelerometer, Temperature, Humidity, Magnetometer, Barometric pressure, etc. as well as Bluetooth connectivity.
  •  Amplified (Class D amplifier) panel-mounted 1W speaker 
  •  2 backlit buttons (Power and Squelch) with indicator LEDs
  •  High-contrast, ultra-low power SHARP Memory eInk graphics display (144 x 168 pixels)
  •  Rugged Micro-USB port (/w protective cap) for battery charging and firmware updates. Use while charging with an external battery pack is also possible. 
  •  Built-in LiPo battery charge controller with 200mA charge current
  •  6600 mAh / 3.7V LiPo Battery (or 3x 18650 cells in parallel) which provides up to 24 hours of continuous run time. Smaller battery can also be used - 2x18650 (4400 mAh) will still provide a full day of continous operation.

Software Features:

There are a couple of novel features which make this instrument different than anything else out there - the count-rate based variable frequency tone feedback coupled with an adjustable level squelch control. Both features are aimed at easy detection and location of radioactive sources. 

Listening to slow clicks while trying to adjust to the background count rate in order to detect any changes when radiation is detected is way too strenuous on the brain - changes in the pitch of a tone on the other hand are picked up immediately and very easy to follow. The squelch control on the other hand makes the instrument "vocal" only when needed, near a radioactive source above background level.

  •  Audio is main method of feedback via the variable frequency tone and alert beeps.
  •  Super-simple and responsive 1 button UI for Squelch level adjustments with click, double-click and long-press actions (all with audible feedback) to execute various actions such as Sql Level Adjustment, Auto-Set, etc. 
  •  2 user-selectable display modes - Rate and Histogram. Modes can be selected at startup or changed later, during normal operation, with 2 consecutive long-press actions within 3 seconds 
  • In the Rate mode, single-click of the Squelch button will increase the squelch level and double-click will decrease the level. Audio frequency will increase 10Hz for each 10 CPS and displays shows numerical count rate.
  •  In the Histogram mode - display shows 140 seconds scrolling count-rate histogram and double-click will toggle between constantly open Squelch and Normal Squelch Control. In addition, in this mode the audio is generated with smaller (5Hz) steps per 10 CPS. This mode can be useful when searching in places with many high-activity sources like mine dumps / tailings. etc. as very high rates will keep the audio frequency half of the rate - i.e. 4000 CPS will result in ~2kHz tone when squelch level is set really low.
  •   2 indicator LEDs - Green Squelch button LED flashes when the number of counts reaches 2x the squelch setting counts and also indicates the startup sequence point for Histogram mode change, Red Power button LED indicates sleep status / HV power off (when blinking) and normal operation (solid glow)
  •   Graphics display shows Firmware version, Internal Diagnostic results and alerts, Current Count Rate in CPS (refreshed every second, resolution 10 CPS) or a count-rate rolling histogram, battery level as voltage and percentage of capacity (refreshed every minute), Current Squelch Level Setting, Count Overflow and Low Battery Alerts.
One of my contributions to the project was the development of the display interface and the firmware code for it, including the UI. γDog PLUS uses 1.3" Ultra-Low Power SHARP Memory display (144 x 168 pixels, same kind as the one used in the Pebble Watch). The display is a very high-contrast eInk type, super-easy to read in direct sunlight, recessed in the front panel for mechanical protection and it shows useful information improving on the ergonomics and the interaction with the instrument.
Data on the display is refreshed every second for the Count Rate readout an every minute for Battery-related information.

  •   Internal diagnostic routine reports over-temperature, water intrusion (by measuring the internal air humidity level in the enclosure) and various levels of the battery condition - both visually, on the display and with a series of beeps.
  •   Audible and on-screen "critical battery level" reminder - every minute when voltage drops bellow 10%
  •   On-screen Charging / Charge Complete Status Indicator
  •   An Advanced Accelerometer-based PMT / Battery saver will shut down the High-Voltage Power Supply and the pulse pre-amplifier after 10 minutes of inactivity (no instrument movement) during normal operation or after 1 minute while charging the battery overnight. The normal operation resumes within 1 second of the instrument detecting a physical movement (for an example - picking it up). During normal operation the red power LED glows solid and blinks when the instrument is in Sleep Mode. I designed this feature with the use of a latching relay to further save power. Going in and out of sleep mode is accompanied by an audible feedback (descending or ascending tone chirp). Current draw drops from 70 mA to 10 mA when the HV supply is powered down. The DAC is also powered down during sleep. There is also selectable "No Sleep" mode.
  •   Auto-Squelch Set will sample and establish the squelch trigger level at the current sampled background radiation level during startup or when it is activated by the user with a long-press of the Squelch button.
  •   Tone frequency generation is set always to begin at the current squelch level and frequency always starts low when the squelch opens, regardless of the current rate or range of operation - this allows for the instrument to operate in a comfortable for the human ear frequency range regardless of the current measured count-rate. In other words, the lower end of the audio tone frequency range is dynamically adjusted to begin at Squelch level. 
  •   Histogram Mode displays scrolling and auto-ranging relative count-rate histogram

The scrolling count-rate histogram displays the count-rate history over the last 140 seconds. The histogram is dynamically normalized to the count-rate range over this period of time with the minimum count-rate overt the past 140 seconds on the bottom.

Typical background rate histogram (130 CPS to 200 CPS). 
The histogram is constantly updated in background at 1 sec intervals and always available for the last 140 sec. It displays also Min, Max and Average Rate.

High-rate event (2560 CPS) (specimen of Uraninite), followed by drop to background level. 
The displayed histogram is scaled to show the maximum rate peak which on the other hand "pushes down" the background level.
  •   Response to count rate changes with a pre-set hysteresis which prevents the squelch from opening during short rate fluctuations.
  •   Excessive Count Rate / Overflow alert above 10 000 CPS (600K CPM) - the maximum rate is not really limited but the count accuracy will drop when excessively high rates are detected.
  •   Compensated count accuracy for overall error in counting of 0.1% (electronics only)
Test of the hardware and the software counting algorithm. Feeding the input with pulses from a function generator shows that the counting accuracy is spot-on! Maximum error is 10 CPS.

Test Setup for counting accuracy and pre-amp alignment.
 Maximum count rate is up to 17K CPS (~1M CPM) for short pulses (~50 uS). Average pulse length of 100uS will allow count rate of 10K CPS (600K CPM).

   Instrument Usage

Sensitivity comparison between γDog and a Geiger Counter - there is no contest!

(Youtube link)

Charles Young showing his field use of the standard model Gamma Dog.
(no display and indicator LEDs but otherwise almost identical)
(Youtube link)

Prospecting for Radioactives in New Mexico - Charles Young, August 2021

(Youtube link)

Prospecting for Radioactives in Southern New Mexico - Charles Young, November 2021
(Youtube link)

UPDATE: development of Version 3 hardware and firmware has been completed. The UI is now using 2 buttons - it was specifically designed to improve the user-interactions and ergonomics if display output is also present. 

Additionally, the UI can now adjust the Minimum Pulse Height Threshold. This parameter along with the last used display mode are saved in the unit's EEPROM. Adjustments of the Pulse Height Threshold are done using a 12-bit Digital-To-Analog Converter (DAC) and displayed in millivolts (accurate to less than 1mV). There are UI coarse and fine steps when changing this value as well as a way to reset it to the default settings.

The version 3 hardware installed in the front panel cap.
Only 2 connections are needed to the body of the instrument - coaxial cable with BNC connectors to the detector and a connection to the LiPo Battery pack.

Current draw for v3 is 68mA during the normal operation mode with Squelched audio and only 10 mA during Sleep mode.

The blue button  is Squelch Control (UP/+), the green button is  Display Mode and Squelch Control (DOWN/-) and the yellow button is the ON/OFF switch. The user now can toggle the squelch control open/close by a double-click of the green button. Squelch Auto-Set is as before - long-press of the blue button.

P.S. This is old but here is an article in Atlas Obscura on radioactive mineral collecting (shameless self-promotion:  I am quoted a couple of times and there are pictures of minerals currently in my own collection).

No comments: