The C Series Windows are Impermeable to Moisture!

Unlike Polymer Windows, the C-Series Windows are Impermeable to Moisture

Amptek “C-Series” X-ray windows utilize silicon nitride (Si3N4) with an aluminum coating to extend the low energy response of our silicon drift detectors (SDDs) down to carbon (C).

Amptek provided an alternative to liquid nitrogen when it introduced the first thermoelectrically cooled detector. Now with the “C-Series,” we offer an alternative to the Beryllium (Be) window for general XRF analysis, and superior performance as compared to polymer windows for soft X-ray analysis.

Amptek low energy "C Series" X-ray windows

Amptek low energy “C-Series” X-ray windows

Applications

C1 Windows: Laboratory, benchtop and handheld instruments.  This window is light-tight so it can be used in normal ambient room light.

C2 Windows: Vacuum applications and EDS (EDX) in scanning electron microscopes (SEMs).  The C2 window has much better efficiency at the lowest energies but is not light-tight and therefore must be used in dark environments.

ElementC1C28 µm Be
(0.3 mil)
12 µm Be
(0.5 mil)
Li0%29%0%0%
Be0%13%0%0%
B0.06%19.7%0%0%
C4.3%43.9%0%0%
N20.2%59.2%0%0%
O29.4%62%0%0%
F46.1%69%4%1%
Ne58.1%72.9%20%9%
Na65.4%75.1%40%27%
Mg70.6%77.3%59%47%
Al75.4%80.3%73%64%
Si64.7%81.8%82%75%
 C1C2
Thickness (Si3N4)90 nm40 nm
Aluminum Coating (Grounded)250 nm30 nm
Window Diameter6.3 mm5 mm
Window Area30 mm²20 mm²
Grid TypeHexagonal Si, 15 µm thick
Open Area Grid78%78%
Helium Leak Rate<1 x 10-10 mbar l/s
Operating Temperature-55°C to +150°C (0 bar pressure differential)
-40°C to +85°C (1 bar front pressure differential)
Pressure Testing for C1 and C2 Windows:1.6 bar front pressure differential for 10 seconds
10 cycles of 1 second duration with 1.6 bar front differential pressure

Amptek C Series Windows Frequently Asked Questions (FAQ)

What are the advantages of the C-Series windows?

The C-Series windows offer less attenuation (higher intrinsic efficiency) than the standard Be windows for X-ray energies below 1.5 keV (for elements below Al). The C windows are about a factor of 2 more efficient for Na Ka X-rays (at 1.04 keV) than the thinnest Be window (8 µm). With Be windows it is virtually impossible to measure elements below Na in the periodic table, and even Na and Mg are marginal. C-Series windows make it practical to measure light elements.

One should measure in vacuum to take full advantage of the C series windows. Attenuation in air can be more important than attenuation in the windows for low energies. For Na Ka X-rays at 1.04 keV the attenuation length in air is only 2.2 mm: an air path of 2.2 mm will attenuate these X-rays as much as a Be window. This not only reduces the sensitivity at Na, but path length variations become very important: an additional 0.2 mm of air changes the Na Ka intensity by 10%, making accurate analysis difficult.

Are there any disadvantages to the C-Series windows?

Above 1.5 keV the C series windows offer slightly more attenuation (lower efficiency) than the Be windows. This difference is small for most elements. For example the C windows offer 10-15% lower efficiency than Be around 3 keV. For X-rays above 6 keV the efficiency for the two windows is nearly identical.

The C-Series windows are made from silicon nitride and coated with aluminum. The characteristic X-rays of Al, Si, and N are all observed in the spectrum. All Amptek detectors contain aluminum (e.g. the multilayer collimator is coated with aluminum) so this interference is always present. The Si interference may be a disadvantage in some applications.

The C2 windows are not light-tight. They are intended only for use in a dark system. Operating a C2 detector in light will not damage the detector, but the photocurrent through the detector will make it impossible to detect X-rays. The C1 windows can be operated in normal ambient light.

Are the C series windows strong enough to hold vacuum?

Yes, the C-Series windows will hold vacuum. The detector hybrid has a vacuum inside, so the C series windows usually hold 1 atmosphere of pressure. Used in a vacuum chamber, the pressure across the window is actually reduced.

One must be careful when using these detectors in vacuum systems. It is common for debris from samples or from handling to exist inside the chamber of an XRF system. When one begins to pump or breaks vacuum, the pieces of debris can fly around inside the chamber and penetrate C1 and C2 windows. Cleanliness is vital, but one must not touch the C series windows when cleaning the system.

We recommend that you allow at least two minutes to vent from vacuum to atmosphere, and that you vent to dry nitrogen (preferable) or air which has passed through desiccants and filters.

Can the C-Series windows be used for high energies as well as low energies?

Yes, the C-Series windows are fine for high energies. Above roughly 6 keV, they offer essentially the same sensitivity as Be windows.

For what applications do you recommend the C-Series windows?

The C1 windows are very well suited to the ED-XRF of samples containing Na, Mg, and heavier elements, particularly when a dark environment is not possible (e.g. handheld and portable instruments).

The ideal application for the C2 windows is SEM-EDS, also known as EDX i.e. the excitation of characteristic X-rays by electrons in an SEM. Because these electrons have a short range in the sample, they excite the X-rays very close to the surface, yielding many X-rays from all the elements down to carbon. The high efficiency of these windows at the low energies makes these detectors very useful.

The C2 windows can certainly be used for ED-XRF from light elements, i.e. measuring characteristic X-rays produced by X-ray excitation. But the X-ray yield for light elements is low and their attenuation length in most samples is low. A powerful low energy X-ray source is required for ED-XRF of light elements.

How accurate are the published attenuation curves?

These curves represent nominal material parameters and normal incidence. Manufacturing variations will occur, so some variation is expected. In many applications the beam of incident X-rays may not be at normal incidence. Even more commonly a range of angles are incident on the window. This will cause the path length to be longer, hence greater attenuation will occur. It is good practice to calibrate each system with known standards.

Do you have spectra from real samples illustrating the differences?

The plots below show spectra measured by an Amptek 25 mm2 SDD with a C1 window, a C2 window, and a 13 µm (1/2 mil) Be window in vacuum. The samples were 99% pure Mg (left) and baking soda, NaHCO3 (right). The tables compare the measured ratio of photopeak count rates with that calculated from the nominal attenuation curves. The increase in efficiency is clear and is in good agreement with the published values. Also shown is a spectrum measured with the same Be window in air, with a 2 cm path length. Note that the air attenuation is far more important than the attenuation in Be.

Performance Gallery

Sodium (Na) Spectrum: C1 vs. 0.5 mil Be Window

Carbon (C) spectrum taken in vacuum with an Amptek SDD equipped with a C2 window

Amptek C-Series X-ray Windows compared with polymer windows

Amptek C2 Window compared with polymer window (0 to 2 keV)

Spectra of Amptek SDD with C1, C2 and 13 µm (0.5 mil) Be window

Spectra of Amptek SDD with C2 window and 13 µm (0.5 mil) Be window in vacuum.

Spectra of Amptek SDD with C2 window and 13 µm (0.5 mil) Be window in vacuum