The Entrance Window

The entrance window is a relevant part of the sensor. Common glasses as well as most transparent plastics block UV radiation. Standard UV-transmitting materials like quartz and sapphire are expensive. For UVA and to some extent the UVB special glasses but also plastics like some PMMA-derivatives or some silicon gels are available with a reasonable transmittance down to about 330 nm. For UVC applications with the important 254 nm line of Hg, quartz remains the standard solution. 

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To verify the modelling of the data eolleetion process, calculations of SAT 4, in the entrance window of the XRII was compared to measurements of RNR p oj in stored data as function of tube potential. The images object was a steel cylinder 5-mm) with a glass rod 1-mm) as defect. X-ray spectra were filtered with 0.6-mm copper. Tube current and exposure time were varied so that the signal beside the object. So, was kept constant for all tube potentials. Figure 8 shows measured and simulated SNR oproj, where both point out 100 kV as the tube potential that gives a maximum. Due to overestimation of the noise in calculations the maximum in the simulated values are normalised to the maximum in the measured values. Once the model was verified it was used to calculate optimal choice of filter materials and tube potentials, see figure 9. 

This area is limited by the echo of the entrance window in front, the noise of the electronic equipment and the accuracy of the digitizer or the distance between the container walls. 

A commonly used detector is a Golay cell, in which there is a far-infrared absorbing material, such as aluminium deposited on collodion, inside the entrance window of the cell. 

It is often advantageous to place the window in the side of a proportional counter so that the x-ray beam passes perpendicularly to the central wire even though this necessarily shortens the path length.21 A second window may be placed opposite the entrance window to permit escape of the unabsorbed x-rays, which might otherwise excite characteristic lines upon being absorbed by the counter walls. 

A Q-switched Nd YAG laser (7 ns pulse duration, Quanta-Ray DRC-1A) operated at 10 Hz was used as a light source. The 1064 nm fundamental was frequency doubled to 532 nm for some experiments. In all experiments reported here a geometry was used which focused the laser beam in front of the entrance window of the sample cell such that the laser beam was diverging as it passed through the sample cell. In this geometry the laser beam was about 3 mm in diameter at the region viewed by the light detection system. 

Deviating from the setup discussed earlier, the y-ray beam can also be consistently collimated by structures other than the absorber holder. If this is the entrance window of the detector, the counter should have a lead shield, and the absorber must be sufficiently large to prevent radiation from passing by. For Mossbauer scattering experiments, the same arguments have to be considered. 

Frozen solution of deoxymyoglobin (Mb) has been the subject of an NFS investigation in the temperature range 3.2-230 K (Fig. 9.6) [15]. The synchrotron pulses were transmitted through the entrance window of the sample with a size of 12 mm 2 mm (width height). By scanning the sample area with a narrow beam (about 1 mm 0.3 mm), the homogeneity of the effective thickness was determined as 2.5%, which is more than ten times better than in the aforementioned case. 

For low-intensity applications, a concentrator lens can be used to focus the radiation onto the detector surface. The detector area can be kept small, which is advantageous for the use of expensive wide-band gap UV-photodiodes. The lens must be made of quartz or another UV-transmitting glass and is often used as the entrance window of a standard TO housing. A lens limits the angle of acceptance to usually less than 10 degrees. 

Figure 3.5-7 Shift of the focus A/ and reflectance / j and in % for different angles 0 of a ray relative to the normal of the entrance window of a rectangular cell, calculated for ,.(,// = 1.5. For a spherical cell A/ = 0 and all R are about 4 %.

It is a coaxial detector with a relative efficiency of 105% and was produced by CANBERRA Semiconductor n.v. in 2000. It is a so called extended range detector with a p-type crystal and a submicron deadlayer (0.5 Lim). It has also a high purity aluminium endcap. The entrance window thickness is 1.5 mm. The active volume of the crystal is 400 cm and the crystal diameter is 80 mm. The detector was first installed with a copper endcap with a carbon epoxy window glued to it. This paper describes the background reduction that was achieved by changing the endcap. 

The efficiency of detection for X-rays at very low energies, say at 4 eV, strongly depends on the thickness of the entrance windows at the top of the detector. This is... 

Flg. 4. Schematic representation of a Knudsen cell-quadiupole mass spectrometer system using broad-band photoionization (The entrance window, the optical system, and the lamp has to be rotated on the axis A by 90°)... 

Pinpointing the siting of cations in Linde Type A zeolites is crucial in the design and understanding of experiments aimed at fine tuning the dimensions of the entrance windows of the supercages. 

To avoid significant energy loss, the particles must go through as small a mass as possible when they move from the source to the detector. This is accomplished by making the source cover and the detector window as thin as possible. The entrance window of such detectors consists of a metallic layer, usually gold, with a thickness of 4 X 10 kg/m (40 /ag/cm ) or less. The measurements are performed in an evacuated chamber to avoid energy loss in... 

All these statements are taken into consideration in Fig. 1.2. A small volume element parallel to the entrance window is considered according to the given assumptions. This volume element has a front area of Fa and a path-length Az. It is assumed to be at a distance z [cm] from this entrance window. Its volume is given by... 

I z) [mol s cm" ] is the intensity of irradiation at a distance z from the entrance window per irradiated area. Therefore the amount of light absorbed by reactant Af within the volume element V is ( wavelength of irradiation)... 

This is a generalised form of Bouguer-Lambert-Beer s law. Inserting eq. (1.35) in eq. (1.34) gives the moles of light quanta which are absorbed by reactant A, at a distance z from the entrance window per second and per litre ... 

Specac) and the photoelastic modulator (PEM) (Hinds Instruments PM-90 with II/ZS50 (ZnSe 50 kHz) optical head). The beam, exiting the cell, is refocused by a ZnSe lens (f= 1 in, diameter 1.5 in, with an anti-reflective coating - Janos Technology) onto an MCT-A detector (TRS 50 MHz bandwidth, Nicolet). A bandpass optical filter (3-4 pm pass, Specac) is placed at the entrance window of the detector when the spectra of the hydrocarbon stretching modes are acquired. 

A collimator mounted in front of the entrance window limits the field of view of the instrument. 

The measurements have been performed positioning a collimated source at 2 meters from the entrance window in order to simulate a full area illumination. A filter on the radius at 8 cm. has been used, since for this measurement the calculated map has been used, and not the measured one that takes into account also the boarder effect. 

A further application of this algorithm could be thought in the X-ray Imaging detector field whenever a coded mask is plac on the entrance window of a position sensitive device. 

Hybrid detectors are different from dual MCPs in that they possess a single microchannel plate. The second amplification stage is often achieved using a scintillator which surmounts the entrance window of a photomultiplier. Sometimes a venetian-blind steel dynode is found in the initial part of a hybrid detector in order to perform ion-to-electron conversion. 

Transit time is the time interval between the arrival of the light pulse at the entrance window of the tube and the time at which the output pulse at the anode terminal reaches a peak. The transit time is usually not a factor in analytical spectroscopy, although it is important in scintillation counting. The transit time is a function of the geometry of the multiplier phototube as well as the voltage between the anode and cathode. Transit time will be of the order of 20-100 nsec in most multiplier phototubes at normal operating voltages. 

There are several types of photomultipliers, which differ in the nature of the entrance window, either crystal or fluoride, and in the nature of the sensitive layer on the photocathode. Some are only sensitive in the far-UV while others are more sensitive in the visible. The type of photomultiplier to be used is selected according to the wavelength of the line to be detected. A fatigue lamp (a small incandescent light source) is often used with photomultipliers to keep the temperature of the tube and its associated electronics constant. The fatigue lamp is switched on when the emission source is off and vice versa. 

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