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Detective quantum efficiency of electron area detectors in electron microscopy.

McMullan G, Chen S, Henderson R, Faruqi AR - Ultramicroscopy (2009)

Bottom Line: Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology.In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated.We also show that part of the response of the emulsion in film comes from light generated in the plastic support.

View Article: PubMed Central - PubMed

Affiliation: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB20QH, UK. gm2@mrc-lmb.cam.ac.uk

ABSTRACT
Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology. We present MTF and DQE measurements for four types of detector: Kodak SO-163 film, TVIPS 224 charge coupled device (CCD) detector, the Medipix2 hybrid pixel detector, and an experimental direct electron monolithic active pixel sensor (MAPS) detector. Film and CCD performance was measured at 120 and 300 keV, while results are presented for the Medipix2 at 120 keV and for the MAPS detector at 300 keV. In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated. We also show that part of the response of the emulsion in film comes from light generated in the plastic support. Computer simulations of film and the MAPS detector have been carried out and show good agreement with experiment. The agreement enables us to conclude that the DQE of a backthinned direct electron MAPS detector is likely to be equal to, or better than, that of film at 300 keV.

No MeSH data available.


Related in: MedlinePlus

Calculated probability distributions obtained using the (a) CSDA and (b) FMC models for depositing energy  in a MAPS detector by an incident 300 keV electron. The results for backthinned (grey) and non-backthinned (black) detectors are shown. The dotted vertical lines indicate the position of the corresponding mean energy loss. The dashed line in the FMC calculation indicates the  energy dependence of the inelastic Rutherford cross-section.
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fig18: Calculated probability distributions obtained using the (a) CSDA and (b) FMC models for depositing energy in a MAPS detector by an incident 300 keV electron. The results for backthinned (grey) and non-backthinned (black) detectors are shown. The dotted vertical lines indicate the position of the corresponding mean energy loss. The dashed line in the FMC calculation indicates the energy dependence of the inelastic Rutherford cross-section.

Mentions: Naively the CSDA calculation would be expected to underestimate the variability in energy deposition and so overestimate the DQE. In fact the CSDA results agree remarkably well with the measured results while the corresponding FMC results are lower than the measured results. To understand this, the probability distribution, , for an incident electron to lose energy was calculated. As noted in Section 4, this distribution is variously known as the pulse height distribution, straggling function or Landau plot and with Eq. (16) can be used to calculate . Results for for both non-backthinned and backthinned detectors obtained using the CSDA approximation are presented in Fig. 18a. For a backthinned detector is very strongly peaked at the mean energy loss (corresponding approximately to the initial rate of energy loss multiplied by the thickness of the sensitive layer). As a result and the value of DQE(0) calculated using Eq. (16) can approach unity. For the calculation presented in Fig. 18a, and so that . Note that the low energy tails of in a CSDA calculation comes from electrons that only travel a short distance into the sensitive layer before being backscattered out of the detector while the high energy tail comes from electrons that are scattered by longer distances within the sensitive layer. The calculated obtained using the CSDA for a non-backthinned detector has two distinct peaks. The lower energy peak comes from electrons which only travel through the epilayer once, while the higher energy peak comes from electrons that are backscattered from the substrate. The fact that the higher energy peak is at more than twice the energy of the first peak reflects the greater rate of energy loss from the less energetic backscattered electrons. The presence of this backscatter leads to both an increase in the average energy and in the variance of the energy deposited per incident electron in the sensitive layer. The increase in variance will in general lead to a decrease in but the additional contribution to the average signal helps suppress the decrease as . As in the case of film, the presence of backscattering leads to a drop in with increasing . As the ratio of pixel size to range of the backscattering from the substrate is larger than in film, the drop in is spread over a wider range in . The strongly peaked shape of obtained using the CSDA approximation is not actually a good description of what is observed [8,26] and therefore the excellent agreement with experiment shown in Fig. 17 must be treated as fortuitous. The shape of obtained with the FMC simulations of a non-backthinned detector, as shown in Fig. 18b, gives a much better description of that observed experimentally. As the mean ionisation potential used in the CSDA, and corresponding value obtained from the inelastic cross-section used in the FMC calculation agree, i.e., 174 eV [35], it is not surprising that both calculations result in similar expected average energy loss. In the FMC calculation the average expected energy loss of occurs well above the most likely energy loss of .


Detective quantum efficiency of electron area detectors in electron microscopy.

McMullan G, Chen S, Henderson R, Faruqi AR - Ultramicroscopy (2009)

Calculated probability distributions obtained using the (a) CSDA and (b) FMC models for depositing energy  in a MAPS detector by an incident 300 keV electron. The results for backthinned (grey) and non-backthinned (black) detectors are shown. The dotted vertical lines indicate the position of the corresponding mean energy loss. The dashed line in the FMC calculation indicates the  energy dependence of the inelastic Rutherford cross-section.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC2864625&req=5

fig18: Calculated probability distributions obtained using the (a) CSDA and (b) FMC models for depositing energy in a MAPS detector by an incident 300 keV electron. The results for backthinned (grey) and non-backthinned (black) detectors are shown. The dotted vertical lines indicate the position of the corresponding mean energy loss. The dashed line in the FMC calculation indicates the energy dependence of the inelastic Rutherford cross-section.
Mentions: Naively the CSDA calculation would be expected to underestimate the variability in energy deposition and so overestimate the DQE. In fact the CSDA results agree remarkably well with the measured results while the corresponding FMC results are lower than the measured results. To understand this, the probability distribution, , for an incident electron to lose energy was calculated. As noted in Section 4, this distribution is variously known as the pulse height distribution, straggling function or Landau plot and with Eq. (16) can be used to calculate . Results for for both non-backthinned and backthinned detectors obtained using the CSDA approximation are presented in Fig. 18a. For a backthinned detector is very strongly peaked at the mean energy loss (corresponding approximately to the initial rate of energy loss multiplied by the thickness of the sensitive layer). As a result and the value of DQE(0) calculated using Eq. (16) can approach unity. For the calculation presented in Fig. 18a, and so that . Note that the low energy tails of in a CSDA calculation comes from electrons that only travel a short distance into the sensitive layer before being backscattered out of the detector while the high energy tail comes from electrons that are scattered by longer distances within the sensitive layer. The calculated obtained using the CSDA for a non-backthinned detector has two distinct peaks. The lower energy peak comes from electrons which only travel through the epilayer once, while the higher energy peak comes from electrons that are backscattered from the substrate. The fact that the higher energy peak is at more than twice the energy of the first peak reflects the greater rate of energy loss from the less energetic backscattered electrons. The presence of this backscatter leads to both an increase in the average energy and in the variance of the energy deposited per incident electron in the sensitive layer. The increase in variance will in general lead to a decrease in but the additional contribution to the average signal helps suppress the decrease as . As in the case of film, the presence of backscattering leads to a drop in with increasing . As the ratio of pixel size to range of the backscattering from the substrate is larger than in film, the drop in is spread over a wider range in . The strongly peaked shape of obtained using the CSDA approximation is not actually a good description of what is observed [8,26] and therefore the excellent agreement with experiment shown in Fig. 17 must be treated as fortuitous. The shape of obtained with the FMC simulations of a non-backthinned detector, as shown in Fig. 18b, gives a much better description of that observed experimentally. As the mean ionisation potential used in the CSDA, and corresponding value obtained from the inelastic cross-section used in the FMC calculation agree, i.e., 174 eV [35], it is not surprising that both calculations result in similar expected average energy loss. In the FMC calculation the average expected energy loss of occurs well above the most likely energy loss of .

Bottom Line: Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology.In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated.We also show that part of the response of the emulsion in film comes from light generated in the plastic support.

View Article: PubMed Central - PubMed

Affiliation: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB20QH, UK. gm2@mrc-lmb.cam.ac.uk

ABSTRACT
Recent progress in detector design has created the need for a careful side-by-side comparison of the modulation transfer function (MTF) and resolution-dependent detective quantum efficiency (DQE) of existing electron detectors with those of detectors based on new technology. We present MTF and DQE measurements for four types of detector: Kodak SO-163 film, TVIPS 224 charge coupled device (CCD) detector, the Medipix2 hybrid pixel detector, and an experimental direct electron monolithic active pixel sensor (MAPS) detector. Film and CCD performance was measured at 120 and 300 keV, while results are presented for the Medipix2 at 120 keV and for the MAPS detector at 300 keV. In the case of film, the effects of electron backscattering from both the holder and the plastic support have been investigated. We also show that part of the response of the emulsion in film comes from light generated in the plastic support. Computer simulations of film and the MAPS detector have been carried out and show good agreement with experiment. The agreement enables us to conclude that the DQE of a backthinned direct electron MAPS detector is likely to be equal to, or better than, that of film at 300 keV.

No MeSH data available.


Related in: MedlinePlus