Undispersed Synchrotron Radiation Research Articles

Calibration and characterization of semiconductor X-ray detectors with synchrotron radiation

Quantitative measurements of X-rays require absolutely calibrated detectors. In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II in Berlin, X-ray detectors can be calibrated with low uncertainties. Two complementary approaches are realized for energy-dispersive detectors. One approach uses the storage ring as primary source standard and the detector response to the calculable spectral photon flux is measured in the undispersed synchrotron radiation. This requires the operation of the storage ring with only a few stored electrons. The other approach is the comparison to a primary detector standard, mainly a cryogenic electrical substitution radiometer. Here, monochromatized radiation of very high spectral purity has to be used which is available at different beamlines in the laboratory in the photon energy range from the UV up to about 120 keV. This method can also be applied for the calibration of non-energy-dispersive detectors. With monochromatized radiation, the response function of energy-dispersive detectors is recorded and compared to Monte Carlo simulations. A photon-energy-dependent parameterization of the response function allows deconvoluting the experimental spectra. The responsivity of Si photodiodes and the detection efficiency of Si(Li), HPGe and CdTe detectors are determined with low uncertainties in a broad spectral range.

Detector Calibration and Measurement of Fundamental Parameters for X-Ray Spectrometry

Energy-dispersive X-ray spectrometry offers the opportunity for a fast and reliable determination of the composition of a specimen. For fundamental parameter based quantification, detailed knowledge of the X-ray generation cross sections and of the detection system’s efficiency is required. The detection efficiency is determined comparing the measured and calculated spectrum of undispersed synchrotron radiation (SR) from the electron storage ring BESSY II in the spectral range from 0.1 keV to 100 keV. Alternatively, monochromatized SR in the spectral range from 0.1 keV to 60 keV is used to determine the detection efficiency with a typical relative uncertainty of 1% to 2% by direct comparison with a reference detector. Employing well-calibrated detectors and monochromatized SR of well-known radiant power and high spectral purity, fluorescence yields have been investigated and resonant Raman scattering was studied as an example of a basic effect creating spectral background.

Calibration of energy-dispersive X-ray detectors at BESSY I and BESSY II

The undispersed synchrotron radiation from bending magnets at the primary standard source BESSY I has been used for many years for the calibration of energy-dispersive X-ray detectors. It has the advantage of continuously covering a broad spectral range from a few electronvolts up to a few kiloelectronvolts with lower uncertainty of the calculated spectral photon flux than any other source in that spectral range. With the recent establishment of BESSY II as a primary source standard, the range of radiometry is further extended into the X-ray range up to about 50 keV. In this paper we present results on the calibration of a Si(Li) detector at BESSY I and II. BESSY I was operated at a nominal electron energy of 800 MeV and BESSY II at 900 MeV and 1700 MeV. The comparison of the primary source standards BESSY I and II yielded perfect agreement. The relative uncertainty of the comparison of 0.007 is dominated by the measurement at B1. It is demonstrated that BESSY II operated at 1700 MeV electron energy is much more suitable for the calibration of energy-dispersive X-ray detectors.

Measurement of detection efficiency and response functions for an Si(Li) x‐ray spectrometer in the range 0.1–5 keV

AbstractEfficiency and response functions were determined for a modern commercial energy‐dispersive x‐ray spectrometer in the photon energy range 0.1–5 keV using dispersed and undispersed synchrotron radiation. The spectrometer was equipped with an Si(Li) detector crystal, a thin‐film window and a digital pulse processor. Monochromatized synchrotron radiation from the PTB VUV radiometry beamline at the electron storage ring BESSY was used for the absolute determination of the detection efficiency with a typical relative uncertainty of 1–2% in the photon energy range 0.1–1.5 keV by direct comparison with calibrated photodiodes. At higher photon energies, the efficiency was found by comparison of the measured and calculated undispersed synchrotron radiation spectrum. The absolute intensity of the synchrotron radiation was known with a relative uncertainty of less than 1%. In the overlapping region, the results from the two independent experiments are in full agreement. The energy dependence of the measured efficiency can be explained only with a detector model assuming that there is no dead layer. A simple model for the effect of incomplete charge collection (ICC) was applied to describe the measured response functions. Consequences of the ICC such as broadening and shift of low‐energy peaks and redistribution of counts around 1.84 keV are explained with the model, in accordance with the experimental results. Copyright © 2001 John Wiley & Sons, Ltd.

Kinetic energy study of the ion fragments produced by dissociative photoionization of nitric oxide

Abstract The kinetic energy distribution spectra of the ion fragments N+ and O+ produced by dissociative photoionization of NO and the dissociative photoionization efficiency curve for the ion fragments with specific kinetic energies are observed at photon energies from 19 to 60 eV. Three groups of N+ fragments are observed with their kinetic energies centered at 0, 0.32 and 1.1 eV, respectively. The two groups of energetic N+ fragments correspond to the predissociation of the c 3π and B′ 1∑+ states of NO+, respectively. These two states are found to predissociate primarily into N + ( 3 P) + O( 3 P) . Shape resonances are found for the c 3π and B′ 1∑+ states centered at about 28.7 and 29 eV respectively. The O+ fragments are found to have a continuous kinetic energy distribution concentrated in the region from 0 to 0.6 eV. The dissociations proceeded mainly through the vibrational levels of the ionic states of NO leading to N + ( 3 P) + O( 3 P) . By use of the undispersed synchrotron radiation, the O− fragment is found to have a continuous kinetic energy distribution concentrated from 0 to 2 eV.

Determination of photon emission probabilities of radionuclides using a Si(Li) detector calibrated by the primary standard source, BESSY

BESSY is an electron storage ring which is optimized for use as a primary radiometric standard source for the infrared to X-ray region. The relative uncertainty of the spectral photon flux Φ E is less than 0.45% (1σ) in the spectral range 5 < E < 15 keV. Measurements in undispersed synchrotron radiation produced a quantum detection efficiency for a Si(Li) detector with relative uncertainties of less than 0.5%. The calibrated detector was used to measure the photon flux of radioactive standards of known activity ( 57Co, 65Zn and 88Y) in the photon energy range from 5 to 15 keV. From these measurements, the K α X-ray and gamma-ray emission probabilities p( E) of the radionuclides were deduced. Our results for the K α X-ray lines of 57Co, p(6.4 keV)=0.501(5) and 65Zn, p(8.0 keV)= 0.346(3) and for the ratio p( 57 Co; 14.4 keV)/p( 88 Y; 14.1 keV)=0.179(4) have small uncertainties and are not affected by uncertainties in the fluorescence yields. They confirm recommended data.

Self-calibration of semiconductor photodiodes in the soft x-ray region

A self-calibration procedure is presented for semiconductor photodiodes to be used as detectors in the soft x-ray region. In this procedure the spectral responsivity is calculated according to a model from experimentally accessible parameters of the detector. The thicknesses of the dead layer and the space charge region as well as the diffusion length have been determined in monochromatic radiation by investigating the angular dependence of the photocurrent. The mean energy for electron-hole pair creation has been determined in calculable undispersed synchrotron radiation of the primary standard source BESSY. The obtained uncertainties of the spectral responsivity in the photon energy region between 150 and 2500 eV are ≤4.2% for newly developed Si n on p diodes and ≤6% for GaAsP/Au diodes. The calibrated photodiodes were used to determine the quantum efficiency of photoemissive gold diodes which is up to four orders of magnitude lower than that of semiconductor photodiodes.

Excited ionic and neutral fragments produced by dissociation of the N+2* H band

NI and NII fluorescent radiation has been observed when N2 was irradiated by undispersed synchrotron radiation with an upper energy limit of approximately 200 eV. The excited fragments originate from dissociation of a band of excited ionic states of N2 lying between 34.7 and 44 eV.

Calibration of semiconductor photodiodes as soft x-ray detectors

A new procedure for the calibration of photon detectors is presented, based on the electron storage ring BESSY as a calculable radiometric standard source and an energy dispersive Si(Li) detector. The quantum detection efficiency of the Si(Li) detector is determined in the undispersed synchrotron radiation of BESSY, and the detector to be calibrated is compared to the Si(Li) detector in monochromatic radiation. The procedure is applied in the photon energy range from 800 to 3500 eV to calibrate semiconductor photodiodes which are promising as transfer standard detectors due to their high quantum efficiency, excellent stability in intense radiation, and insensitiveness to surface contaminations.

Complete characterization of a Si(Li) detector in the photon energy range 0.9–5 keV

The response function of a Si(Li) detector has been measured in the photon energy range 0.9–5 keV using monochromatized synchrotron radiation at a double crystal monochromator. The response is described by a hypermet function and the spectral dependence of its parameters is presented. The quantum detection efficiency of the detector is determined from measurements in the undispersed synchrotron radiation of the electron storage ring BESSY, which is a primary radiometric standard.

Ar 3s, 3p satellite lines studied by fluorescence spectroscopy

The fluorescent radiation emitted by the Ar 3s, 3p satellite states (observed by photoelectron spectroscopy) has been recorded in the wavelength range between 400 and 780 Å with an energy resolution ranging from 50 to 100 mV. The fluorescence was stimulated by use of undispersed synchrotron radiation.

Kinetic energies of fragment ions produced by dissociative photoionization of NO

The kinetic energies of ions produced by dissociative photoionization of NO have been measured at the discrete resonance lines of He (584 Å) and Ne (736 Å), and with undispersed synchrotron radiation. O + ions were identified with energies from 0 to approximately 0.5 eV and two groups of N + ions one with energy of 0.36 eV and another with energies between 0.9 and 1.5 eV, apparently produced by predissociation of the c 3Π and B′ 1Σ states, respectively.