GISAXS measurement time - Revision history

KevinYager: /* Factors affecting exposure time */

2015-01-27T16:18:23Z

‎Factors affecting exposure time

KevinYager: /* Sample damage */

2015-01-27T15:55:40Z

‎Sample damage

KevinYager: /* Detector saturation */

2015-01-12T21:38:41Z

‎Detector saturation

KevinYager at 21:00, 14 October 2014

2014-10-14T21:00:56Z

KevinYager at 13:29, 25 August 2014

2014-08-25T13:29:17Z

130.199.3.165 at 14:54, 20 June 2014

2014-06-20T14:54:49Z

KevinYager: /* Signal-to-noise ratio */

2014-06-18T14:36:47Z

‎Signal-to-noise ratio

KevinYager: /* Signal-to-noise ratio */

2014-06-18T14:36:26Z

‎Signal-to-noise ratio

KevinYager: /* Detector saturation */

2014-06-16T13:25:42Z

‎Detector saturation

KevinYager: /* Detector saturation */

2014-06-16T13:24:13Z

‎Detector saturation

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 * '''Sample volume''': Larger samples of course scatter more. Note that in transmission-mode, if the sample is too thick, it will instead attenuate the scattering (due to [[absorption]] or [[multiple scattering]]). In grazing-incidence experiments, [[GISAXS_sample_requirements#Dimensions|larger sample sizes]] increase scattering power and thus decrease measurement time. However, this is only useful up to a point: if the sample is large enough to fully-capture the incident beam (i.e. there is no spill-over from the [[beam projection]]), then increasing sample dimensions further will not change anything.
 * '''Ordering''': More highly ordered systems scatter more strongly.
-* '''Scattering contrast''': The higher the electron-density contrast between the structured materials, the strong the signal.
+* '''Scattering contrast''': The larger the electron-density difference between the structured phases, the stronger the signal.
 ==Signal-to-noise ratio==

@@ Line 43: / Line 43: @@
 ===Sample damage===
-Another concern with respect to counting time is sample damage. Synchrotron x-ray beams are extremely bright, and can easily destroy the structure one is attempting to measure. The effect is most pronounced for soft materials (polymers, etc.); and is even more rapid in the presence of oxygen, humidity, or solvent vapours. Performing measurements with the sample in vacuum can help forestall (but not eliminate) sample damage.
+Another concern with respect to counting time is [[radiation damage]]. Synchrotron x-ray beams are extremely bright, and can easily destroy the structure one is attempting to measure. The effect is most pronounced for soft materials (polymers, etc.); and is even more rapid in the presence of oxygen, humidity, or solvent vapours. Performing measurements with the sample in vacuum can help forestall (but not eliminate) sample damage.
 One should always be on the lookout for signs of sample damage in the GISAXS data itself. If the same spot is measured repeatedly, and the scattering pattern appears to be changing (in particular becoming broader and 'uglier'), then sample damage may be occurring.

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 ===Detector saturation===
-X-ray detectors can be saturated if the signal is too large. Every detector technology has a limit, with respect to the maximum instantaneous counting rate (which can only be mitigated by attenuating the beam), the maximum pixel count value, and (possibly) a maximum global (whole image) count rate/value.
+X-ray [[detector]]s can be saturated if the signal is too large. Every detector technology has a limit, with respect to the maximum instantaneous counting rate (which can only be mitigated by attenuating the beam), the maximum pixel count value, and (possibly) a maximum global (whole image) count rate/value.
 Fiber-coupled CCD detectors typically have a per-pixel dynamic range of, e.g., 16-bit = 65,536 counts. If a given pixel has more counts than this, the data file will store an erroneous value. The value may be pegged at the max-count, or may instead have a nonsensical value like -1, or the most-significant-bits may be lost, in which case the pixel will have an intermediate (possibly random-seeming) value. This saturation makes data within the saturated pixels useless. Worse still, CCD readout technology introduces cross-talk between pixels (and even between readout modules: i.e. between different regions of the detector image). The end result is that a saturated image may have significant artifacts: the pixels near the saturation region will have erroneous values, entire lines of pixels may become saturated, or 'ghosts' of the saturated data may appear throughout the image.

← Older revision		Revision as of 21:00, 14 October 2014
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−	The time required for a [[GISAXS]] measurement of course varies based on numerous factors. The most important factors are the flux of the x-ray source, and the inherent scattering power of the sample in question.	+	The time required for a [[GISAXS]] measurement of course varies based on numerous factors. The most important factors are the flux of the x-ray source, and the inherent [[scattering]] power of the sample in question.

	Generally, '''a single exposure (2D image) takes on the order of 1 s to 60 s''' at a [[synchrotron]] [[beamline]]. The same measurement on a [[labscale]] instrument will of course take considerably longer; typically 10 minutes to a couple hours for a single exposure.		Generally, '''a single exposure (2D image) takes on the order of 1 s to 60 s''' at a [[synchrotron]] [[beamline]]. The same measurement on a [[labscale]] instrument will of course take considerably longer; typically 10 minutes to a couple hours for a single exposure.

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 The time required for a [[GISAXS]] measurement of course varies based on numerous factors. The most important factors are the flux of the x-ray source, and the inherent scattering power of the sample in question.
-Generally, '''a single exposure (2D image) takes on the order of 1 s to 60 s''' at [[synchrotron]] [[beamline]]. The same measurement on a [[labscale]] instrument will of course take considerably longer; typically 10 minutes to a couple hours for a single exposure.
+Generally, '''a single exposure (2D image) takes on the order of 1 s to 60 s''' at a [[synchrotron]] [[beamline]]. The same measurement on a [[labscale]] instrument will of course take considerably longer; typically 10 minutes to a couple hours for a single exposure.
 A full measurement on a single sample typically involves [[GISAXS alignment|aligning the sample]] (which will take 2-5 minutes), and then exposures at a variety of incident angles. It is usually a good idea to collect data below the [[critical angle]], near the critical angle, and above the critical angle. This multi-angle data makes eventual data interpretation easier: the sub-critical-angle data provides a measure of the structure in the near-surface region. The data above the critical angle is complementary in that it probes the entire depth of the film. Measurements near the critical angle exhibit strong intensity enhancements, which is useful for weak signals; measurements well above the critical angle yield lower scattering intensity, but the data is less complicated by [[refraction distortion]] and [[dynamic scattering]] (see also [[GTSAXS]]).

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 ==Signal-to-noise ratio==
-The [http://en.wikipedia.org/wiki/Signal-to-noise_ratio signal-to-noise ratio] (SNR) is a measure of data quality, wherein one compares the strength of the signal of interest to the complicating [[background]] noise. In x-ray scattering measurements, various kinds of [[background]] (detector background, substrate scattering, instrument windows, air scattering, etc.) worsen the SNR. However, the largest source of noise is frequently simply [http://en.wikipedia.org/wiki/Shot_noise shot noise]: the inherent counting statistics arising from the small number of photons being detected. For integer counting, the SNR goes as:
+The [http://en.wikipedia.org/wiki/Signal-to-noise_ratio signal-to-noise ratio] (SNR) is a measure of data quality, wherein one compares the strength of the signal of interest to the complicating background noise. In x-ray scattering measurements, various kinds of [[background]] (detector background, substrate scattering, instrument windows, air scattering, etc.) worsen the SNR. However, the largest source of noise is frequently simply [http://en.wikipedia.org/wiki/Shot_noise shot noise]: the inherent counting statistics arising from the small number of photons being detected. For integer counting, the SNR goes as:
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 Fiber-coupled CCD detectors typically have a per-pixel dynamic range of, e.g., 16-bit = 65,536 counts. If a given pixel has more counts than this, the data file will store an erroneous value. The value may be pegged at the max-count, or may instead have a nonsensical value like -1, or the most-significant-bits may be lost, in which case the pixel will have an intermediate (possibly random-seeming) value. This saturation makes data within the saturated pixels useless. Worse still, CCD readout technology introduces cross-talk between pixels (and even between readout modules: i.e. between different regions of the detector image). The end result is that a saturated image may have significant artifacts: the pixels near the saturation region will have erroneous values, entire lines of pixels may become saturated, or 'ghosts' of the saturated data may appear throughout the image.
-[[Image:Rect3012.png|thumb|470px|left|Detector image of intense scattering that saturates some pixels near the beam center. Notice how "ghosts" of the saturated pixels appear in other parts of the image.]]
+[[Image:Rect3012.png|thumb|470px|left|Detector image of intense scattering that saturates some pixels near the beam center (during a [[GTSAXS]] measurement). Notice how "ghosts" of the saturated pixels appear in other parts of the image.]]
-[[Image:Saturation example02.png|thumb|250px|center|Detector image where an extremely bright reflection saturates pixels. Notice how, for this detector, the saturated pixels (which are along ''q<sub>z</sub>'' near the left edge of the detector) cause entire rows of pixels to yield erroneous (zero) values.]]
+[[Image:Saturation example02.png|thumb|250px|center|Detector image where an extremely bright reflection saturates pixels (during a [[GIWAXS]] measurement). Notice how, for this detector, the saturated pixels (which are along ''q<sub>z</sub>'' near the left edge of the detector) cause entire rows of pixels to yield erroneous (zero) values.]]
 Some detectors exhibit non-linear response even before saturation occurs. This non-linearity of course makes quantitative interpretation of data impossible. For any given detector, one must take care to avoid collecting data in the saturated or near-saturated (non-linear) regimes. (On the other hand, if all one wants is to get [[Tutorial:Qualitative_inspection|a qualitative sense]] of the sample structure, these artifacts may not matter, as long as one is aware of them.)