Mini-X2 X-Ray Tube System for XRF

Figure 2. Mini-X2 block diagram.

Mini-X2 X-ray Tube

Anode material:  The Mini-X2 X-ray Tube is available with one of four anode materials:  silver (Ag), gold (Au), rhodium (Rh), and tungsten (W).

Maximum Power:  The Mini-X2 X-ray Tube is available with maximum power of 4 W or 10 W.

Maximum HV:  Mini-X2 X-ray Tube is available with an HV range of either 10 to 50 kV or 35 to 70 kV.

• Note that the mechanical dimensions depend on the HV and power options.
• Note that the higher power and/or HV variants require more radiation shielding than the lower power and/or HV variants.

X-ray tube interface:  The Mini-X2 interfaces with X-ray tubes supplied by NSI (Newton Scientific).  Their standard tubes use an analog interface while their new UltraMini tubes use a digital (I2C) interface.  Both are supported by the Mini-X2 Controller.

Mini-X2 Controller

There is a single Mini-X2 Controller which interfaces with all the Mini-X2 X-ray Tube modules.  Each Controller is programmed, at Amptek, for a specific X-ray Tube module, with its P/N, S/N, maximum kV, maximum power, etc.  Amptek’s Firmware Manager software can be used to reconfigure the Controller for a different tube.

Amptek’s standard Mini-X2 Controller supports only USB communication and only the standard X-ray tubes.  Contact Amptek for Controllers which interface with the UltraMini and/or which support an RS232 interface.

Specifications

Specifications, terms and pricing subject to change.

Figure 2. Mini-X2 Isopower. The current and voltage must be set in accordance with this curve or the Mini-X2 may be severely damaged.  Damage of this kind is not covered under warranty.  Amptek’s control software limits the power to this curve.  If one commands the system to a power exceeding the power limit, the software will use the commanded HV and rollback the current to meet the power limit.  Note that the curves differ based on the tube.

Figure 3. Mini-X2 Angular Response, 120° cone.

Figure 4. Mini-X2 120° cone.
NOTE: When using the 2 mm collimator, the X-ray cone is 5°.

Figure 5. Mini-X2 Controller connectors.

Figure 6. Mini-X2 software control panel.

Vacuum Application

The Mini-X2 can be configured with a standard conflat for vacuum applications.

Figure 7. Mini-X2 and X-123SDD with vacuum couplings.

Additional Information

Use of Mn secondary target to mimic ⁵⁵Fe source

How to chose the anode material for an X-Ray tube

X-ray tube

The heart of the Mini-X2 is a compact X-ray tube which uses a transmission target.  The high voltage power supply (HVPS) produces a bias voltage between the target (which is grounded) and the filament.  This voltage accelerates electrons produced at the filament into the target.  When these electrons decelerate in the target material, they produce bremsstrahlung radiation, X-rays with a continuous energy spectrum.  They also produce X-rays at the characteristic energy of the target material.  Many of these X-rays are directed towards the window, made of Be (beryllium), where they can be collimated into the sample.  The X-ray tube contains shielding which stops X-rays outside of the 120^o cone.

High Voltage Power Supply

The HVPS takes the 12 VDC input and steps it up to the commanded bias (10kV to 50kV).  It is a switch mode regulator with a conventional Cockcroft-Walton multiplier, operating between 40 and 100 kHz.

There are three inputs to the HVPS:  an analog voltage which sets the HV, an analog voltage which sets the current, and an ON/OFF logic signal.  The HV and current signals have a range of 0 to 4V, which correspond to the HPVS settings: a 2V input to the HV results in a half-scale output of 25 kV.  There are three outputs from the HVPS: an analog voltage reading the HV, an analog voltage reading the current, and a STATUS logic signal.  These have the same scale factors as the inputs.

The Mini-X2 X-ray tube modules are supplied by NSI.  NSI’s standard X-ray tube modules have analog inputs and outputs, which are the Mini-X2 Controller’s standard inputs and outputs.  NSI has recently released a smaller tube, called the “UltraMini”, which uses a digital I2C interface.  Amptek’s Mini-X2 controller can also support the I2C interface to the UltraMini.

Mini-X2 Controller

The Controller takes the control values commanded via USB and uses these to set the proper control voltages and to send the ON/OFF command.  It also reads the outputs.  There are a few key details to the control and interface module:

• The interlock circuit is very important, both for radiation safety and for successful operation of the Mini-X2. Please refer to sections 4.2 and 8.2 for details.
• The software compares the commanded values for HV and current to the outputs. If the inputs and outputs differ (outside of tolerance limits and sustained for a certain time), the HVPS is disabled.
• The Mini-X2 Controller is based on a Silicon Labs 8051 microcontroller, running the same communications software as Amptek’s digital pulse processors (DP5, X123, PX5, etc). This software interface is called FW6 and is described in the Amptek Digital Products Programmer’s Guide.

Safety Interlock

The AUX connector on the Mini-X2 Controller contains a safety interlock,
designed for use with a failsafe warning lamp. A typical application
circuit is sketched below. The controller applies a configurable voltage
across the external interlock circuit and monitors the current; the tube is
only enabled if the current is within a programmable range. It turns off
if the switch is open or if the lamp fails.

Warning lamp and beeper

The Mini-X2 controller includes an LED and a beeper which indicate that the tube’s HV and current are enabled. They flash/beep at about 1 Hz. The safety interlock drives a lamp with a failsafe circuit. See the Mini-X2 User Manual for warning lamp technical specifications.

Figure 7. Mini-X2 Gold (Au) Output Spectrum at 10, 20, 30, 40, 50 kV.

Figure 8. Mini-X2 Silver (Ag) Output Spectrum at 10, 20, 30, 40, 50 kV.

Figure 9. Relative Output Spectra: Ag and Au Targets. Spectra taken at 50 kV/20 µA for 10 minutes (no collimator).  Note that the Au tube gives 3 times more counts for the same current and time. This is because the bremsstrahlung increases for increasing Z.

Mini-X2 Silver (Ag) Output Spectra with Various Filters

Figure 10. Mini-X2 Ag Output Spectrum with 3 mil Cu Filter.	Figure 11. Mini-X2 Ag Output Spectrum with 2 mil Mo Filter.
Figure 12. Mini-X2 Ag Output Spectrum with 1 mil Ag Filter.	Figure 13. Mini-X2 Ag Output Spectrum with 1 mil W + 10 mil Al Filter.
Figure 14. Mini-X2 Ag Output Spectrum with 80 mil Al Filter.

Mini-X2 Gold (Au) Output Spectra with Various Filters

Figure 15. Mini-X2 Au Output Spectrum with 3 mil Cu Filter.	Figure 16. Mini-X2 Au Output Spectrum with 2 mil Mo Filter.
Figure 17. Mini-X2 Au Output Spectrum with 1 mil Ag Filter.	Figure 18. Mini-X2 Au Output Spectrum with 1 mil W + 10 mil Al Filter.
Figure 19. Mini-X2 Au Output Spectrum with 80 mil Al Filter.

The above spectra have not been normalized (i.e. not taken at the same current for the same time etc.). They have been provided only to show the shape of the spectrum and demonstrate how a filter can be used to obtain a beam specific to an application. Keep in mind that when any filter is used it reduces the flux coming out of the tube. An Al filter reduces the flux much less than a W or Ag filter. The higher the Z of the filter or the thicker the filter, the less flux will be available. It is therefore necessary to raise the current of the x-ray tube to compensate.

The above spectra were taken with the Amptek XR-100T-CdTe detector, a PX5 digital processor and power supply, and the silver (Ag) and gold (Au) versions of the Mini-X. In the spectra above there are notches observed at 26.7 and 31.8 keV. These are the Cd and Te K absorption edges. The XRS-FP software was used to correct the escape peaks generated in the detector. Please see application note ANCDTE1 for more information on the effects of the CdTe detector on the output spectrum.

The following filters are provided with the Mini-X2:

Why should I use a filter on the X-ray tube?

• The primary beam filter, the filter on the X-ray tube, improves the signal to background ratio for the characteristic X-ray peaks one wants to measure. The tube produces X-rays over a wide range of energies, including many low energy brehmstrahlung X-rays and characteristic X-rays from the target. These scatter from the target, obscuring the peaks to be measured. The filter absorbs the lower energy X-rays, which do not excite the X-rays one wants to measure anyway, thus improving the signal to background ratio for the peaks of interest.
• They can filter the characteristic lines of the tube’s target. For example, in figures 20 and 21 a filter is used which cleanly cuts low energy photons up to about 12-15 keV. This removes the gold (Au) L lines from the tube at 9.71 and 11.44 keV. Figure 15 above shows a Cu filter which cleans the low energy photons but also creates a clean region above 10 keV.

Figure 20. This figure shows the output spectrum of the Mini-X2-Ag at 40 kV unfiltered, and filtered with 80 mils (2 mm) of aluminum (Al). In the unfiltered spectrum, a large fraction of brehmstrahlung counts are at low energy (between 5 and 15 keV), which are removed in the filtered spectrum.

Figure 21. Shows spectra measured from a lead target using the unfiltered and filtered tube. With no filter, the lead characteristic X-rays are superimposed on a large background of scattered X-rays. With a filter, the signal to background ratio is significantly improved. This will reduce the measurement uncertainty in these lines. Notice how the L-gamma line is much more visible in the filtered spectrum than in the un-filtered.

Filters on an X-Ray Tube Application Note in PDF

Collimator and Safety Plug

The Mini-X2 is provided with two collimators to facilitate its use in XRF applications. They consist of a brass collimator with aluminum (Al) inserts and a cover that screws into the Mini-X2. The collimators have 1 and 2 mm diameter holes. A brass safety plug is also provided which, when installed, reduces the flux from an operating tube to less than 25 µSv/h (2.5 mrem/hr) at 5 cm away in accordance with requirement 5.2.2.2.2 of the NBS Handbook for Radiation Safety for X-Ray Diffraction and Fluorescence Analysis Equipment.

Figure 22. Mini-X2 collimator and safety plug.

Figure 23. Mini-X2 with safety plug installed.

Figure 24. Mini-X2 with collimator installed.

Caution

This device produces X-Rays when energized. To be operated only by qualified personnel.

Radiation Precautions

The Mini-X Is intended to generate x-ray radiation during normal operation. The Mini-X has been designed to focus radiation in the designated output direction, however radiation in other directions is possible and should be addressed with shielding and/or monitoring in the final application.

Radiation Levels

Radiation levels external to the X-ray tube housing with the brass safety plug ON do not exceed 25 µSv/h (2.5 mrem/h) measured 5 cm from the surface of the housing in accordance with Requirement 5.2.2.2.2 of the National Bureau of Standards (NBS) Handbook for Radiation Safety for X-Ray Diffraction and Fluorescence Analysis Equipment.

For more information please see the NBS Handbook.

Examples of Shielding (that comply with the above standard) for a 50 kV tube running at 80 µA (4W).

Note that 10W or 70 kV tubes will need more shielding.

• 1 mm (0.040 in of Pb will result in radiation levels of 0.5 mrem/h.
• 6.35 mm (0.250 in) of Fe will result in radiation levels of 0.5 mrem/h.
• 3.18 mm (0.125 in of Brass will result in radiation levels of 2.5 mrem/h.

The inside of the housing can also be lined with 3.18 mm (0.125 in) of aluminum (Al) in order to absorb the XRF from the shielding material.

The shielding needs to fully enclose the X-ray cone and sample.  X-rays are scattered in all directions, 4π steradians.  In many cases, the most intense radiation hazard is from backscattered radiation (often where the operator is located).  Any gaps in the shielding, e.g. for the tube or a spectrometer or sensor wires, will pass radiation.  Where possible, use a jog in such gaps rather than a direct path.

Figure 26. Mini-X2 mechanical dimensions (in [mm]).

Figure 27. Nickel plated brass collimator cover mechanical dimensions (mils/mm).

Mini-X2 STP files