Cosmic spikes

After identification of a cosmic spike, another approach to remove the spike is to use a median filter on several related spectra. The spectra can be related in time, for time-series data as with data taken from a process on-line analyzer, or related in space, in the case of a Raman image dataset in which spectra are taken from spatially segregated locations. 

Pitfalls of PCA analysis mostly arise from systematic errors, which makes any spectrum appear different from the other spectra even if no chemical difference is present. These artificial differences will appear in the PCA results usually as at least one additional factor. As each additional factor decreases the ability to distinguish between useful factors with Raman information and those with noise, clustering could be lost or artificial clustering could be created. Some typical problems are as follows drifts in either axis (wave-number or intensity) during the collection of the data inconsistent background removal unfiltered cosmic spikes. Because of the exceptionally high intensity of a cosmic spike and the fact that they are completely random in position and occurrence, these anomalies create an additional factor for each occurrence. Transformation of factors in FA is another troublesome point. Mathematical methods which take into account bandwidths and constraints that intensity be real (non-negative) provide some aid [23,24]. 

Consider you have forgotten to switch on multi-read 28 with your CCD detector and the raw data are full of cosmic-ray spikes. How do you remove them without spoiling the image ... 

In similar manner, spikes in the image from cosmic rays are extinguished by simple application of the median filter with a small submatrix size (3 or 5). 

C reaches the Earth s surface at the rate of 2.3 atoms/cm2/s after production by cosmic ray interaction in the atmosphere, corresponding to a total production of 1.4 x 1015Bq/y. 14C is also formed by the 14N(n, p) reaction by atmospheric tests of nuclear weapons. About 2.2 x 1017 Bq were made in the atmospheric test spike of the 1950s and 1960s that has been primarily transferred to the oceans and the biosphere. This means that 14C is the most significant fallout nuclide from the point of view of population dose. Nuclear power plants also release 14C as part of their normal operation contributing 0.1 x 1015 Bq/y. 

Cosmic rays hit the CCD array at random times with arbitrary intensity, resulting in spikes at individual pixels. When the array is summed and processed, sharp spectral features of arbitrary intensities may appear in the Raman spectra. These artifacts are typically removed before multivariate calibration. 

Figure 8.31. Dark spectra from a CCD for 10 msec (A), 60 sec (B), and 180 sec (C) integrations, without bias correction. A is the bias spectrum, while B and C show additional signal due to thermally generated electrons. As explained in the text, the spikes on curve C result from cosmics.

Figure 8.32. Temperature dependence of dark spectrum for an EEV 15-11 deep depletion CCD. Integration time was 60 sec in all cases. Positive spikes are due to cosmics negative spike at 1450 cm is due to a column in this CCD with weak response.

Figure 8.35. Spectra of sucrose in water, with water and cuvette spectrum subtracted. Spectrum A is a single 10 sec integration, containing cosmics at 800 and 1700 cm . Spectrum B is an average of ten 10 sec spectra, with the spikes filtered by comparing successive spectra. See text for details.

For some radionuclides, only the direct measurement is needed, based on calibration with a radionuclide standard and reference to the measured sample mass. For other radionuclides, the isotope ratio to its stable element is needed. An example of a more complex situation is measurement of the 14C/12C isotope ratio of an environmental or archeological sample in comparison to the modern atmospheric CO2 value. This value must be adjusted for anthropogenic 14C produced by atmospheric testing of nuclear weapons and by other nuclear operations, and also for changes in atmospheric CO2 with cosmic-ray flux fluctuations over time. For an element such as Pu, which has no stable isotope, the total quantity is measured by isotope dilution mass spectrometry in which the sample is traced (or spiked) with 242Pu or 244Pu (see Section 17.3.3). 

The choice of detectors now appears to have swung back to Ge. The use of a PIN Ge detector operated at 77K and biased at 250V gives a slightly better performance than InGaAs and increases the spectral range out to a 3500 cm Raman shift. These detectors are, however, sensitive to cosmic rays, and efforts must be made to ensure that these spikes do not contaminate the interferogram [41],... 

Asteroids (and comets), however, can be mixed by coalescence with stony and icy bodies. If large enough, they may transport volatilizable material into earth s atmosphere (the Tunguska event is probably associated with an ammonia spike in the Greenland ice core record). The stony body can fall to the earth s surface but evaporates (in the Tunguska event there is speculation of cosmic carbon input on the other hand, nitrate increase as would be expected from NO formation by heating the atmosphere (see Chapters 2.6A.4 and 5.4.1) has not been found (Rasmussen et al. 1984, 1999, 2008). 

Although it has yet to be modelled in FllnS, cosmic ray strikes on the detector are relatively common and create a spike in an interferogram, and it can take several time constants for the detector to return to normal operation. 

It is important to remove cosmic ray spikes from the interferograms before the data processing, because Fourier transforming an interferogram containing a cosmic ray spike will result in a high frequency sinusoidal modulation added to the recovered spectra. 

Cosmic rays passing through the photosensitive region can produce thousands of photoelectrons. This effect results in a very strong sharp signal in the Raman spectrum. Quantification of spike noise is difficult due to the random nature of its occurrence. However, it is usually quite obvious to the observer when a spectrum of biological tissue contains a contribution from spike noise. These spikes can be erased from the spectrum or the whole spectrum can be discarded. They can also be circumvented by averaging several scans. 

Hit quality indices calculated in this way are independent of the normalization of the spectra, preventing the baseline of a noisy spectrum from being shifted by negative spikes caused either by noise or (for Raman spectra) a cosmic ray event. 

Cosmic rays In the observation of Raman spectra, cosmic ray interference may occur with charged coupled device (CCD) detectors. These detectors are sensitive to high energy photons and particles. The interference shows up as very sharp, intense spikes in the Raman spectra and so can easily be distinguished from true bands. There are programs available to remove these spikes. 

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