Attachment_A_-_NIST_Requirements_Document.docx

Attachment A NIST Requirements Document

I. BACKGROUND:

The Vacuum Double Crystal Spectrometer (VDCS) in the Dosimetry group of the Radiation Physics Division is devoted to perform high precision (down to 1 ppm) x-ray wavelength measurements in the low x-ray energy range in support of NIST’s X-ray Transition Energies Database. Due to high absorption in air, x-ray measurements in the few keV x-ray energy range must be performed in vacuum. The VDCS utilizes two high purity silicon crystals with the lattice spacing measured to sub ppm precision and tied to the definition of the meter in the international system of units. The spectrometer’s arrangement allows the determination of absolute x-ray wavelengths without any external reference by very precise angle difference measurements performed with calibrated precision angle encoders. A vacuum compatible photon counting X-ray area detector optimized for measuring low energy x-ray radiation in the 2 keV to 10 keV energy range is the subject of this procurement for the VDCS. This two-dimensional single-photon-counting detector is a very important prerequisite for making 1 ppm level measurements of x-ray calibration lines in support of NIST's standard reference x-ray data program. The large multipixel area detector is an essential tool to determine the systematic artifacts (error budget) of the measurements at the VDCS. Requirements for the photon counting x-ray area detector include a linear response over a high dynamic range, no background, low heat output, and high temporal stability.

II. PURPOSE:

The device to be acquired is a vacuum compatible photon counting position-sensitive X-ray area detector optimized for measuring low energy x-ray radiation in the 2 keV to 10 keV energy range. The most important requirements for this two-dimensional, vacuum compatible x-ray area detector include: linear response over a high dynamic range, no background, low heat output, and high-temporal stability. This detector will be used as the detection device of the Vacuum Double Crystal Spectrometer (VDCS) at the Dosimetry group of the Radiation Physics Division. The area detector is essential for reducing uncertainties and correct for systematics in our wavelength measurements on the VDCS in our X-ray metrology facility located in the Advanced Measurements Laboratory.

III. MINIMUM SPECIFICATIONS:

The Contractor shall provide one (1) vacuum compatible photon counting X-ray area detector that meets the following specifications:

1.1. Single-photon counting capable of discriminating individual photon events that will allow characterization of the alignment and correction for systematics; an integrating or analog detector is not acceptable for the Government purposes.

1.2. The discrimination of each event shall help suppress background signals by each detected event passing through a logic circuit that determines if the magnitude is above a lower limit threshold.

1.3. The dimensions (active area of detector) shall be > 25 mm by 75 mm. The x-ray beam is approximately 12 mm tall and 25 mm wide at the detection plane. The detector needs to be a minimum of two (2) times this size to determine beam center and divergence characteristics. Also in the horizontal dimension the beam will walk on the detector as a dependence of the wavelength.

1.4. The pixel dimensions (or more generally resolution elements) shall be < 250 μm by 250 μm for characterization of the axial or vertical divergence.

1.5. The detector point spread function shall be < 250 μm by 250 μm and be comparable to the pixel dimension, or smaller.

1.6. The system shall be fully vacuum compatible and shall perform measurements in a vacuum of approximately 10-6 mbar pressure within the VDCS vacuum chamber.

1.7. The volume of the vacuum chamber in which the detector will fit is constrained by the ability to pivot around the second crystal in a limited space. This volume is bounded by vacuum chamber dimensions < 90 mm (w) ; by < 150 mm (h); by < 85 mm (d).

1.8. The photon efficiency for the 2 keV to 10 keV energy range shall be > 50 %. Due to the use of multiple crystal reflections efficient detection is crucial for potentially dim x-ray sources.

1.9. Electronic background / pixel / s shall be < 0.001 counts/ pixel / s. For adequate uncertainty determination a 103 signal to noise ratio is required.

1.10. The detector linearity shall be better than 1% at >500,000 X-ray photons / pixel

1.11. The stable counting statistics shall be Poisson counting noise with a constant (or zero) detector background. In order to model the data from the detector, it will be necessary for the noise profile to be well characterized. A non-zero background can be caused by gamma rays or other high-energy events. Note that the detector background should NOT be a function of count rate.

1.12. The frame rate shall be 0.1 to 10 Hz. 1 frame per second will be a regular frame rate but the option of longer and shorter exposures should be available.

1.13. The data acquisition and control cable should be at least three (3) meters long, and the vacuum-side of the cable should be compatible in a vacuum of 10-6 mbar.

IV. ACCEPTANCE

The Government will accept the system within twenty-five (25) days of a system that meets all the required characteristics listed in the testing/inspection section above.