Quantum detector measurement

The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

Quantum detectors are based on semiconductors. The absorption of a photon excites an electron from the valence band into the conduction band. This can be measured either through a change in resistance (photoconductive... 

The name quantum detector is given to a radiometric detector having equal or near equal response over the entire range to be measured, UV or VIS and calibrated in quantum units. As is the case for all meters, they may be calibrated for any... 

The ideal quantum detector response curve is a square wave, i.e., equally sensitive over the entire wavelength range of measurement. These types of detectors should be used for all measurements of incident intensity, UV and VIS, if a spec-troradiometer is not available. While spectrally blind they will at least give a truer reading of the total amount of radiation actually being received by the samples. Figure 7 shows the response curves of two of these detectors. 

These units when coupled with near linear (quantum) detector sensors are useful for monitoring processes for total incident intensity on a sample. One should remember, however, that because they are spectrally blind, if change is noted it will be difficult to know just where in the spectrum band this is being measured, if it occurred. 

A particular kind of quantum detector (or photodetector) used in laboratories and industry is known as the thermopile detector. In an experiment, it is desired to measure the radiation from a black flat plate extending to infinity, A detector has been placed parallel to the plate. The catalog of the detector indicates that the maximum allowable incident radiation is 200 mW/oii5. Determine the highest tolerable plate temperature which will not damage the thermopile detector. Neglect the radiation emitted by the detector itself. 

A.L. Migdall, R.U. Datla, A. Sergienko, J.S. Orszak, Y.H. Shih, Absolute detector quantum-efficiency measurements using correlated photons, Metro-logia32, 479-483 (1995)... 

Although the emphasis in this section has been on linearly polarized incident radiation, considerable enhancement of the k-quantum photocurrent may occur for circularly (or elliptically) polarized radiation, as recently discussed by a number of authors [7.46]. We note that information relating to the intermediate-state lifetime of the detector (t,) can be obtained by measuring the two-quantum detector output for various values of t. ... 

When we measure photons emitted by a single molecule using a quantum detector such as a photomultiplier tube, we interpret /(/) to be the photon counting rate (fluorescence intensity at time i) and we can deduce the fluorescence intensity autocorrelation function by counting the number of photon pairs separated by... 

Recently, Baumann et al.(43) have measured time-resolved photoconductivity in PDA-TS-6 crystals as well as polyacetylene excited by 25 ps pulses of a Nd YAG laser (ftU) = 2.3 eV). The response time of the detector was 200 ps. The transient signal shown in fig.5 reveals a fast initial peak with instrument-limited pulse-shape followed by a slower decaying tail. The field dependence of the peak height (fig.6) parallels that of the carrier generation process and is in accord with what Donovan and Wilson have found on a 20 ns time resolution. The quantum efficiency associated with the fast photocurrent peak is 1.5x10 times the dc-quantum efficiency measured at hu) = 2.7 eV. 

Unfortunately, this simple treatment is rarely valid in practice, since the D value of most detectors varies with the modulation frequency, /, of the radiation being measured (see Chapter 6). With pyroelectric detectors, D varies approximately thus if the speed of the moving mirror is halved while keeping the measurement time constant, the SNR will increase. Conversely, D of most quantum detectors usually increases as / increases up to some maximum value ( 1 kHz for photoconductive MCT detectors). It then remains approximately constant as the modulation frequency is increased, and Anally drops off at frequencies above 1 MHz. Since it is rare that interferometers are operated at scan speeds that modulate the incident radiation at frequencies greater than 1 MHz, operating at high scan speeds when an MCT detector is employed is usually beneficial. 

The response time of the detector should also be borne in mind when the time resolution is to be reduced well below 1 s. If the firequency of the HeNe interferogram is raised much above 10 kHz, the response of the DTGS detector is too slow and a faster detector must be used. In the mid-infrared, this is not a major problem, since MCT detectors operate optimally for modulation frequencies above 1 kHz. For near-infrared measurements, however, while InSb has a very fast response time, other quantum detectors, such as InGaAs, cannot be used at data acquisition speeds much above 5 kHz (see Section 18.2.5). 

It was found that that in the case of soft beta and X-ray radiation the IPs behave as an ideal gas counter with the 100% absorption efficiency if they are exposed in the middle of exposure range ( 10 to 10 photons/ pixel area) and that the relative uncertainty in measured intensity is determined primarily by the quantum fluctuations of the incident radiation (1). The thermal neutron absorption efficiency of the present available Gd doped IP-Neutron Detectors (IP-NDs) was found to be 53% and 69%, depending on the thicknes of the doped phosphor layer ( 85pm and 135 pm respectively). No substantial deviation in the IP response with the spatial variation over the surface of the IP was found, when irradiated by the homogeneous field of X-rays or neutrons and deviations were dominated by the incident radiation statistics (1). 

Because NEP is roughly proportional to D is more useful for comparing detectors of differing sizes. D depends on the wavelength distribution striking the detector (if it is quantum) and the frequency at which the radiation is modulated, so these measurement parameters need to be included for a D value to have meaning. Often detectivity is written as where Tis the temperature of the blackbody source of radiation or the wavelength of the... 

Three important detectors make use of the ionization, called here the initial ionization, that follows the absorption of x-rays by a gas and the ejection ol photoelectrons from the molecules involved. These photoelectrons subsequently ionize other molecules. The relatively large energy of the x-ray quantum thus leads to the production of a number of ion pairs, each consisting of an electron and a relatively immobile positive ion. if these ion pairs do not recombine, the extent of this initial ionization is determined by (and measures) the energy of the x-ray quantum. 

Intensity measurements are simplified when a detector always gives one electrical pulse for each x-ray quantum absorbed the detector remains linear so long as this is true. For low intensities, when the rates of incidence upon the detector are low, the Geiger counter fulfills this condition. As this rate increases above (about) 500 counts per second, the number of pulses per second decreases progressively below the number of quanta absorbed per second. This decrease occurs even with electronic circuits that can handle higher counting rates without appreciable losses. 

The main goal of the Planck instrument is to improve the accuracy of the measurement of the cosmic microwave background (CMB), in order to extract cosmological parameters that remain poorly constrained after the results of WMAP (Wilkinson microwave anisotropy probe) and of the best ground-based experiments. The basic idea of HFI-Planck is to use all the information contained in the CMB radiation, i.e. to perform a radiometric measurement limited by the quantum fluctuations of the CMB radiation itself. In these conditions, the accuracy is only limited by the number of detectors and by the duration of the observation. 

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