Complete DIY XRF Setup

My DIY XRF setup is finally complete - it is comprised of an Amptek X-123 detector using the proprietary Amptek 25 mm2 / 500 μm Si-PIN X-Ray detector element (model FSJ32MD-G3SP), Amptek Pre-Amplifier and DP5 DPP /MCA.

Details about the detector are in THIS post.

On the exciter side, in the past, I have used X-Rays from an Am-241 source (59.54keV). Unfortunately, there is no exempt quantity or a way to obtain high activity, pure Am-241. The ones used in modern household smoke detectors are only 0.9 uCi (unless obtained from the old Pyrotronics industrial smoke detectors with up to 80uCi) but still have the Am-241 mixed and pressed into Gold and Silver foil which has parasitic emissions of the said metals in addition to the Am-241 decay product - usually a fairly strong Np La line emission (from Np-237 decay product). The Neptunium La-line is observed even with recently created Am-241) La at 13.95keV.

I decided to switch over to an X-Xay Tube where the emission can be controlled, the spectrum is uniform with the exception of the XRF emissions from the target material but this is normally a single metal.

 I am using Moxtek Magnum series transmission X-Ray tube with Tungsten target - 10W total electrical power (-50kV / 200 uA) controlled by an X-ray source controller of my own design.

The X-Ray tube is placed inside a Lead shield. The aluminum box on the right-top in this picture is the detector enclosure, housing the detector element, preamp, power supply and the Amptek DP5 Digital Pulse Processor.

This is my test setup while doing XRF on a piece of copper foil for initial Energy Calibration.

The aluminum box in the center of the picture is the X-Ray tube's high-voltage power supply module. It is a high-frequency switching supply with a very easy to work interface - 3 output channels (2 analog and 1 digital) and 3 input channels (2 analog and 1 digital) + 12V main power. The supply is very efficient and the current draw is around 1.5A at maximum power.

The X-Ray source aperture and detector at almost 90 degrees so the X-Rays are skimming the surface of the specimen.

This is the first XRF of Copper foil with the new source - the Ka line is at 8.05keV and to the right the Kb-line is at 8.90keV. 

I still need to adjust the detector Peaking Time and Flat Top Time as well as the Slow and Fast detector thresholds - currently I have over 99% of Dead time due to the current settings intended for low X-Ray flux source.

My X-Ray Source Controller works flawlessly, and I am really happy with the end-result.

X-Ray Source Controller for MOXTEK and AMPTEK Mini X-Ray tubes

 I needed controller for the Magnum series 50kV / 10W MOXTEK X-Ray source. As it turns out the controller, once sold by MOXTEK is no long available (discontinued) and the Moxtek salesperson told me - "We expect customers to develop their own controllers". This is not a big deal - their controller has very basic functionality anyways, so I went ahead and developed my own design to control the tube.

One of my design goals is to have a stand-alone unit - no computer required. I don't want to fumble with numerous applications while doing XRF and prefer to have a piece of hardware with actual buttons I can press when it comes to X-Ray tube control.

I was pleasantly surprised to find out that the AMPTEK Mini-X2 tube uses the same electrical interface as MOXTEK so my controller will work for AMPTEK Mini-X2 just as well.

My design is based on the nRF52840 System-on-a-Chip (SoC) using Cortex M4F processor and employs 6 control channels as required by the Tube's interface  - 4 Analog and 2 digital channels. Of these 6, there are 2 Analog outputs (12-bit DACs), used for setting up tube's HV and Emission Current parameters, 1 digital output (5V TTL signal) to turn the source ON / OFF. There are also 2 Analog inputs (12-bit ADCs) to monitor the Tube's working parameters as they are returned by the Moxtek HV PS module and a digital input (5V TTL signal) to report when the beam is on and stable (Filament fully heated). 

I had to employ Level shifters as the nRF52840 is a 3.3V chip and the MOXTEK module has 5V TTL levels for the digital signals. Furthermore, the set ranges on the Analog output channels are 0-to-4V so I had to use the 4.096V internal DAC reference voltage which means the DAC must be powered with 5V Vdd.

For the Analog inputs, I used voltage dividers to bring down the Monitor channel voltages in the range of 0-to-3VDC and used the built-in ADC reference of 0.6V with gain of x5.

There is also 5th Analog Input channel, internal to the controller, with its own voltage divider, used to monitor the Low-Voltage Main Input Power and to inhibit controller operation if the input power is not nominal.

OSH Park delivered again beautiful, high-quality PCBs. The ordering process is very simple and a pure joy - I almost feel sorry I don't have more PCB projects to order. The PCB design was done with Eagle but I am not big fan of what Autodesk is doing with Eagle and very likely to switch over to KiCad in the near future.

The assembled and ready X-Ray Source controller - XTC-2000 (a.k.a. "X-Ray Tube Commander 2000" :-) 

(Chat GPT suggestion :-)

Using a rotary encoder with a pushbutton makes the UI really quick and intuitive. The button (knob-press) is used to enter adjustment mode and the user can dial the whole number and the tenths after the decimal point for each parameter separately. After entering Set mode, the encoder push-button scrolls through different cursor positions. 

The encoder is equipped with its own microcontroller (Atmel SAM D09) which takes care of all of the quadrature input stuff - counts, phase-detection, timing, etc. and just reports the actual tick count over I2C bus to the MCU. This makes it really fast and easy to use and I can reset the tick counter with a command if needed.

The complete and working controller during bench-testing and DAC/ADC non-linearity compensation and alignment. The white (unpopulated in this picture) 4-pin JST connector near the encoder is AUX I2C expansion connector, used for the remote temperature sensor.

Currently, the code is fairly small (~1500 lines only) yet it is pretty complete and mature and the core functionality is all done and bug-free thus "Version 1.0".

In the unlikely event of some commercial interest I might write a menu system for setting up various internal parameters and calibration values, but even at this stage, XTC-2000 has more features, better functionality and better ergonomics than both, the discontinued Moxtek controller and the entirely software-controlled Amptek controller. 

As part of the safety and tube health features, I added temperature sensing and monitoring using MCP9809 chip.

While designing the PCB, I foresaw that having an extra I2C bus connector for future expansion might come handy so I added one to the board layout.

The sensor I am using is MCP9808 - a very accurate and precise chip with I2C interface. Resolution is actually much better than 0.25°C but for my purpose 1° Celsius is completely sufficient.

The controller constantly monitors the temperature of the tube and it will shut it off if temperature exceeds +60°C and until cools down below +55°C. The sensor presence is auto-detected on startup - if the cable is not plugged in, the controller will work normally, without any limitations.

The critical part here is the connection cable between the controller and the MCP9808 breakout board - the cable must be fully shielded due to the proximity to the tube's HV cables and must be of very low capacitance as the I2C bus does not allow for high capacitance on the signal lines or the useable bus speed will begin to drop.  At 70cm cable length the sensor works just fine. The cable I used was foil-shielded 4-conductor USB cable.

The last thing currently pending on my development list is a suitable enclosure.

XRF Exciter source using a Moxtek Miniature X-Ray Tube

 I am working on a new XRF Exciter source, employing a pretty cool miniature, ceramic, 10W X-Ray tube with Tungsten target by Moxtek (MAGNUM series). This source will deliver an immensely higher X-Ray flux compared to the Am-241 source I've been using, thus cutting down on the integration time during XRF analysis and bringing out peaks hiding in the noise.

The Moxtek X-Ray tube comes with a High-Voltage Power Supply module which allows for control of both, the tube voltage (-10kV to -50kV range) and the tube's emission current (0 to 200 uA). The maximum electrical power is 10W into the tube - here are the specs.

The brass housing of the X-Ray tube with the beryllium window aperture and the two high-voltage supply cables. The tube is in a Grounded Anode configuration and the two cables deliver both, High-Voltage to the Cathode and power to the filament. The brass housing is massive enough to dissipate plenty of heat. In its final configuration, the tube will mounted inside of 1+" thick lead shielding as some X-rays are generated in all directions besides the collimated main beam. These X-Rays are much attenuated but still a radiation hazard so proper shielding is mandatory.

My test setup. The HV PS has a very neat and straight-forward interface for controlling the tube's operational parameters of the tube. It is also very efficient when it comes to power - it requires 9V to 12V DC and about 1A of current. The efficiency is a little over 80% - around 12W input power which is fantastic.

While I am designing and prototyping the X-Ray tube controller (more on this later), just for a quick testing I was driving the tube with my 3-channel power supply in a rather "manual fashion" - Ch.1 is the main power, Ch.2 controls the HV - 0.8V to 4V are scaled to -10kV to -50kV range and Ch.3 sets the beam's current - 0 to 4V are scaled to 0-200uA range.

The tube's module returns monitor signals - voltages with the same exact scaling factors as the control voltages in order to monitor the actual HV and Current. A TTL level signal controls the beam ON/OFF and there is a FILAMENT READY return signal from the HV PS when the filament is heated, and the beam is ON and stable.

One important requirement is that the beam should not be turned ON sooner than 2 seconds after it has been turned OFF to prevent damage to the tube's filament. For the same reason the tube should not be turned ON for a minimum of 1 second and no less than that - all these will be part of my design goals for the controller.

Prototyping the X-Ray tube controller - using nRF52840 MCU with ARM Cortex M4F, 24LC32 EEPROM for storing configurations, 12-bit MCP4728 Quad DAC for Tube control, large SHARP Memory display (400x240), bi-directional level shifter, Non-Latching Relay, MCP 9808 temperature sensor and a nifty I2C Rotary Encoder breakout.

There are various other components - voltage dividers, voltage regulator, power conditioning, buzzer, pull-up and pull-down resistors, etc. located on the controller board - I designed the board with some thru-hole components so I can easily continue the development once I have the boards in hand and swap components as needed.

Controller's User Interface

On top is the Status display, temperature reading (when sensor is plugged in) and the current timer display. Second section, below, is the Mode selector and Timer Selector display - it also shows the Last Run Log and calculated X-Ray tube power. Third section is the X-Ray tube's Parameter (S) Set configuration where the user can dial in voltage in -5kV to 50kV range (0.1kV steps) and Tube's current 5uA to 200uA (0.1uA steps). Bottom band is the (M) monitor display showing the return signals from the tube's power supply - digitized with 12-bit ADCs. 

All set parameters are persistent - stored in EEPROM and automatically loaded on startup. There are also two user-configurable Presets with Tube parameters for quick switching between different set of values for different experiments. I might increase the number of presets to 3 eventually.

There are 3 operational modes - MOMENTARY when beam is ON while the OPERATE button is pressed and turned off when released. The second mode is TOGGLE - pressing the OPERATE button turns ON the beam and starts a count up timer. Second press of the OPERATE button turns OFF the beam. Sequential ON/OFF will integrate the beam time in the "Last Run time" until a RESET action. The third mode is COUNTDOWN timer - user can dial desired beam time and OPERATE button STARTS/PAUSES the countdown. The X-Ray beam is turned OFF when the timer expires but it can be PAUSED, STOPED or canceled at any time. The user can also use the rotary encoder to add or remove time from the timer while the beam is ON.

I have added many safety features!

 When the beam is turned OFF there is a 2 seconds blackout period while the filament is cooling. During this time the beam cannot be re-engaged. It is not possible also to run the tube for less than 1 second - if any such attempt is made, the controller will automatically "pad" the time for a total of 1 second. There is an INTERLOCK detection feature which inhibits any operation unless the interlock switch of door/ enclosure is closed or overridden with a key. In COUNTDOWN and TOGGLE mode, pressing on the TIMER RESET/MODE button or the Rotary Encoder button acts as an EMERGENCY SHUT-OFF. In TOGGLE mode there is also Timeout feature which will turn off the tube after a period of time if left unattended. The controller also constantly monitors the Low Voltage supply and disables the tube if under-voltage condition occurs. Tube temperature is monitored with an external sensor attached to the tube housing and the tube is turned OFF if temperature reaches 60C.

Control voltages for the HV and the Emission Current are always kept at 0VDC when the tube is OFF to prevent the tube from firing up due to a transient on the TTL "tube enable" signal during controller power-up and shut-down. These control voltages go up to the programmed levels just before the X-ray tube is turned ON and are dropped again to 0VDC shortly after the tube is turned OFF.

Another safety feature is a "parameter watchdog" - 2 seconds after the x-ray tube is on the beam is stable the controller will be actively monitoring for a difference between the set control voltages and the return monitor voltages - if a specified tolerance is exceeded, the controller will turn OFF the tube.

There is a relay with NO/NC contacts used for control of external equipment - X-Ray ON indicator, shutter, acquisition system, etc

The nRF52840 BLE will allow me to implement a Bluetooth connection to another host device (Smart phone for example) and control everything remotely. 

Overall, I am quite happy with the results. This is the PCB design for the controller board. Critical modules are socketed and can be replaced easily. There is a terminal block and a DB-9 connector directly compatible with the Moxtek DB-9 on the Magnum series tubes and auxiliary 2-pin power connector used for tubes with higher than 4W power.

For testing, I looped the DAC outputs used to Set the x-ray tube control parameters (the "S" display line on the display) to the ADC inputs for monitoring the tube's return (the bottom, "M" display line) and whatever is programmed as SET tracks perfectly on the MONITOR. The ADCs exhibit a small non-linearity up to about 2.4V (they run with 3.00V reference) - I plotted voltage set vs. voltage read and created a curve to correct it, which improved the accuracy quite a bit.

The internal ADCs of nRF52840 are configured for 12-bit resolution and using the built-in 3.0V (0.6V x5 Gain) voltage reference. The ADC noise is very typical for these chips - around ~3mV swing as seen on the plot. I might employ a ring-buffer and toss out the Min and Max values over a couple of seconds samples while averaging the rest in an attempt to get a more stable readout if I get bored - now the least significant digit exhibits some ADC noise from time to time - it is fairly stable due to the over-sampling conversation I am doing so no real motivation to address this more or less non-issue.

Gamma Dog - finishing touches (Volume Control)

One thing that was really bugging me with the Gamma Dog circuit was the original analog audio volume adjustment. 

The MCU swings a digital output pin, driving the amplifier input between 0V and 3.3V. This output is connected directly to a linear (class D) 1W audio amplifier which employs a tiny trimmer-potentiometer for gain adjustment.

To adjust the audio level in the field, first I have to remove the screw plugging the adjustment access hole (everything on the front panel is dust-proof) and then use a small flathead screwdriver to turn the internal trimmer-pot - hardly a convenient thing to do every time I needed to change the volume.

I normally keep the audio level pretty loud (3/4 of full power) as it helps during windy conditions but when it is quiet and one is kneeling right in front of the instrument, digging a hole or trying to pinpoint specimens, the loud tone in your face can get a bit annoying. Sometimes there will be a couple of us using Gamma Dogs near each-other and becomes a pretty loud "concert" so quick volume control is a "plus". Not to mention the "dirty looks" I was getting at indoor mineral shows from ladies looking for "healing crystals" who didn't like the loud, variable pitch tone produced by the instrument. (I guess it wasn't "resonant to their aura" :-)

Solving the issue entirely on the software side without re-wiring the audio circuit and using push-pull between two PWM digital outputs was going to be pretty intrusive, and also I didn't want to use additional timers and CPU cycles just to control the volume, so I decided to implement a simple hardware solution - digital potentiometer control.

The two main candidates - Analog Devices AD5171 (6-bit resolution) and Analog Devices AD5243 (8-bit resolution) digital potentiometers - 10K resistance. Most digi-pots out there, unfortunately are using SPI interface for control.

I, on the other hand, have a number of I2C devices in my Gamma Dog, so I wanted to stick with the microcontroller's I2C interface bus - this saves me the use of an additional digital output for Chip Select (CS) signal needed with the SPI interface.

I mounted the chips on SMD-to-thru-hole adapter boards - Mouser #535-LCQT-MSOP10 from Aries Electronics for the MSOP-10 component (AD5243) and Mouser #485-121210 from Adafruit for the SOT-23-8 packaged (this one was a bit tricky to install as the MSOP-8 footprint on the board is larger and the leads were not overlapping the solder pads but rather just reaching the very edge of each pad)

At the end, I focused on the AD5243 chip - it is an 8-bit potentiometer - this means 256 positions, and the chip has two independent, addressable potentiometers / channels which gives me a lot more flexibility for future expansion. This IC is also using very low power - drawing about 6uA. 

Here is the Analog Devices Datasheet.

 Prototyping the solution. Added benefit was that I didn't have to write an Arduino library "from scratch" to control it - Rob Tillaart already did this. His library was written for AD5241 and AD5242 but works just as well with the AD5243 as it uses the same command set - only the I2C device address of the chip had to be changed to 0x2F. The I2C bus signals SDA and SCL require pull-up resistors.

The two SMD 10K pull-up resistors and a ceramic 100nF filter capacitor mounted on the converter PCB.

On the board I connected Ch.1 pot's B1 pin to ground and added also 10uF / 16V tantalum capacitor across the power supply rail to minimize any transient disturbance and low frequency ripple.

I designed the digital potentiometer volume control as an add-on board which is inserted inline between the speaker / amp board and the main board's speaker connector. This connector provides signal and power to both, the digi-pot and the audio amplifier. The I2C bus is connected to the I2C bus on the Latching Relay FeatherWing shield with a separate line. 

JST connectors allow for an easy, no-soldering install in the units.

The complete add-on volume control module. The board was placed inside a heat-shrink tube for electrical and mechanical protection. On the left side are the output to the amplifier board pigtail and the input connector. On the right side is the I2C bus connector. The 3-pin input/output JST connectors deliver power to the board and to the audio amplifier.

The add-on board installed in the Gamma Dog. Visible on the right is the I2C bus line going to the FeatherWing Relay pins which are basically a signal feed-through to the MCU board.

 Adding these volume control modules to my fleet of Gamma Dogs was effortless and looks as elegant as an inline add-on board can be. I made 3 such modules.

Crude schematics I sketched while developing the volume control. 

The digital potentiometer acts as an adjustable voltage divider for the output signal. The "wiper" is connected to the input of the audio amplifier. 

Ch. 2 potentiometer is currently unused. 

Writing the firmware support, I implemented two ways to adjust the Audio Volume using the Gamma Dog's user-interface. 

A menu item in the Configuration Menu can select one of 6 volume levels - from a very soft sound suitable for a quiet room to maximum loudness for windy conditions. 6 Volume Levels seems to cover the entire range nicely and I see no need for the "classic" 0 to 10 volume range. This adjustment sets the default volume level of the instrument, and it is persistent (saved in the EEPROM and then loaded on startup).

If the user needs only a quick, temporary adjustment - double-click on the BLUE/UP button will switch the instrument to Constantly Open (Latched) Squelch Mode and the current volume level will be displayed right next to "#" symbol. 

The Constantly Open squelch Mode will produce naturally a continuous tone that can be used as audio level feedback. The user then can press the GREEN/DOWN button to cycle thru the available 6 volume levels. Once a selection is made, same BLUE/UP Double-Click action will turn off the Constantly Open Squelch Mode and return the instrument to normal squelched mode with the new audio volume level. 

The volume adjustment in this case is not persistent, and the instrument will revert to the default volume level set thru the Config Menu System on the next restart.

The add-on volume control board bumped up the firmware code version to 4.0.  The board is backwards compatible with older firmware versions - if the AD5243 code is missing, the pot will automatically set the "wiper" to the "middle position" (127) during power-on (which actually equates to MAX volume when 10K chip is used). The useful adjustment range with a 10K potentiometer chip is approx. 0 to ~64-70 (1/4 of the 8-bit range) in my circuit - anything above 70 and all the way to 255 is really MAX volume.

Ideally, I should have probably picked a lower resistance value (2.5K?) but I don't need such fine adjustments with only 6 volume steps to spread across an 8-bit range.

Another future possibility is to use a digi-pot control for the HV bias power supply for the detector - currently this is done with another trimmer-pot on the HV board and requires the removal of 3 screws to access it.

I am quite happy with the volume control implementation, and it works just as expected!

Firmware version 4.3 marks the end of the current development efforts for my branch of Gamma Dog - the instrument at this stage is mature and polished, performance is excellent, firmware is clear of bugs.

Hardware version 5 is already developed as a new PCB layout and adds small improvements and a couple of features - "Charge Complete" LED indicator and circuit for measuring and displaying the HV Bias for the PMT (up to 1.1kV). This can be used for diagnostics and voltage adjustments in the field, when switching to different detectors. If I decide to fabricate more PCBs these will be ver. 5 and firmware will go to ver. 5 as well.

In my free time I'll work on documentation, schematics and source code, pending a decision whether to open it and make the project public.

Unfortunately, given the niche application of this instrument, the general interest has been rather sparse, but I would love to hear feedback.

Gamma Dog - Performance

 So how does the Gamma Dog perform? 

The short answer is - fantastic! 

I am really happy with the performance. Charles and I have been testing our instruments inside and out and they work great - sensitivity is excellent so as the ease of use. The operation is a complete joy and a fun user experience just to go out in the field and hunt for radioactive minerals with the Gamma Dog. We seem to have nailed all of our design goals and there isn't anything I can think of to improve the instrument further.

Here is just a small fraction of the specimens I have collected with my Gamma Dog.

This little Euxenite crystal (on top of a US Dime for scale) I found under 2-3 inches of silt and sand in a wash near the pegmatite at White Signal, NM

This is the haul of Euxenite crystals over a couple of days Charles and I found in a single "honey" pot at White Signal, NM. 

I found this gigantic Euxenite crystal, one of the biggest I've seen - it wasn't difficult to find though, with an with activity of almost 8000 CPS, the Gamma Dog was howling. Charles found an unusually high-activity area and started digging a hole but then he attributed the activity to the "mass-effect". I decided to give the hole a second chance and it produced this fantastic crystal.

Another interesting Euxenite - the "corner" void was caused by a weathered quartz crystal which crumbled and disintegrated when I pulled it out. I guess the radiation onslaught over millions of years was too much for it.

A very interesting crystal habit of Euxenite (White Signal, NM) - I found the one on the left with my Gamma Dog and Charles found the one on the right. Both crystal are in the collection of Charles now. 

Picture by Charles David Young.

Allanite-(Ce) crystals I found over a couple of hourd with my GD at the Kingsman Feldspar Mine, Kingsman, AZ

Closeup of one of the Allanite-(Ce) specimens from Kingsman, AZ

Quartz with Copper ore and black Uraninite inclusions from the Green Monster Mine south of Las Vegas, NV.

Uraninite (the two on the left) crusted with secondary minerals (Gummite) and Zircon? (on the right) from the Biermann Quarry, Bethel, Fairfield County, Connecticut

Collection of Samarskite-(Y) crystals, I managed to find within a few hours at Dollar Bill claims, Little Rincon Mountains, Pima County, Arizona 

I found this nice doubly terminated and fairly large for the locality (2.5cm x 1.5cm x 1cm) Euxenite-(Y) crystal at the Dollar Bill claims, Little Rincon Mountains, Pima County, Arizona

(picture credit: Charles David Young)

Charles Young did a quick XRF of the crystal above, showing that the Uranium peak is smaller than the Yttrium peak - characteristic signature of Euxenite-(Y).

A quick overview of the Gamma Dog.

Gamma Dog EXP - External Probe

The idea behind Gamma Dog EXP is to have a self-contained electronics module  which I can connect to various external probes. This allows a certain level of flexibility - detectors using smaller crystal can be attached to a wand and used in the "Metal Detector" style setup, sweeping larger areas much faster or an alpha scintillator can be connected to check for contamination. I also use this setup very effectively during mineral shows (sans the wand) and I can quickly sweep across the tables with specimens.

The large internal detector unit is just too big and heavy to be mounted on a pole and the range of scanning around a person with it is limited to an arm's reach.

 This is the entire setup mounted on a "Metal Detector" style armrest and pole using a low-profile detector using 40x80 mm NaI(Tl) crystal

The self-contained electronics module came out pretty compact. It can be placed in a backpack or even hanging from a belt.

Over-the-shoulder strap is another, very convenient way to carry the unit.

`BNC connector for attaching the scintillating probe. Bias voltage is set internally (up to 1000V) and VD impedance of the PMT must be above 60MOhms.

Gamma Dog - skinned :-) the electronics only. 

The black cylinder is a 63x63 NaI(Tl) scintillator connected with a short coaxial cable to the Gamma Dog's main board. The 6600mAh LiPo battery is on the right-hand side, speaker + amp, the 3 buttons and the display module can be seen as well.

The electronics modules for my Gamma Dogs - 3 are based on Version 3 Hardware and one unit (used as a Spare/Backup) - is a prototype based on the older Version 2 hardware.

My Gamma Dog pack - ready to hunt for radioactive rocks.

An inverse color scheme option is available in firmware 3.9

(Menu Selectable)

The mount for the detector was designed with TinkerCad and 3D printed with 100% in-fill for maximum mechanical strength.

Mount is comprised of a lower cup and a top cap. A threaded feed-thru is used to secure the cup to the bottom of the pole. I used a feed-thru to reduce weight and possibly add an elastic strap to pull-down the top cap.

The cap is designed with a slit for the BNC connector and cable. Cap slides over the pole and a pair of 0-rings - one on each side provide friction resistance. This facilitates easy installation or removal of the detector in just a few seconds. The friction from the o-rings is strong enough to lock the detector firmly in place.

If I need to remove the detector, all I need is to pull the top cap up and slide the bottom of the detector into the cup. To lock the detector in place I push the cap down and move the 0-ring right behind it.

The bottom cup provides additional mechanical protection to the detector housing.

This type of mount is fully adjustable for various detector lengths and diameters up to 2". Ill probably design another set for detectors with diameter between 2" and 2.5"

My 40x80mm NaI(Tl) compact detector in carbon-fiber housing, mounted at the end of the pole.

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).