Thursday, January 19, 2023

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. This source will deliver a significantly higher X-Ray flux compared to the Am-241 source I've been using, thus cutting down on the integration time during XRF analysis.

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 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 the X-Ray tube controller (more on this later), just for testing I was driving the tube with my 3-channel power supply - 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 scaling factors in order to monitor the actual HV and Current. A TTL level signal controls the beam ON/OFF and there is a FILAMENT READY signal when 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.

Prototyping the X-Ray tube controller - using nRF52840 MCU with ARM Cortex M4F, 24LC32 EEPROM, 12-bit MCP4728 Quad DAC, large SHARP Memory display (400x240), bi-directional level shifter and a nifty I2C Rotary Encoder breakout.

On top is the Status display 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 10 to 50kV range (0.1kV steps) and Tube's current 0 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 parameters are persistent - stored in EEPROM and automatically loaded on startup.

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. 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 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 3 second blackout period while the filament is cooling. During this time the beam cannot be re-engaged. There is an INTERLOCK detection feature. 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 monitors the Low Voltage supply and disables the tube if under-voltage condition occurs.

There is a relay to control an external indicator, shutter or additional equipment, while the beam is ON. The nRF 52840 BLE will allow me to implement a Bluetooth connection to another device and control everything remotely. 

Overall, I am quite happy with the results and now I need to design the PCB.