Radiometry using synchrotron radiation at PTB

Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 1013–1018

Radiometry using synchrotron radiation at PTB M. Richter*, G. Ulm Physikalisch-Technische Bundesanstalt, Abbestrasse 2 -12, D-10587 Berlin, Germany

Abstract The facilities for radiometry using synchrotron radiation at the Physikalisch-Technische Bundesanstalt are presented and the methods for absolute calibration of radiation sources and detectors are described. Most of the measurements are based on the use of two different types of primary standards, storage rings as primary radiation source standards and cryogenic electrical substitution radiometers as primary detector standards. The facilities allow us to offer radiometric calibrations in the spectral range from the UV to x-rays with relative uncertainties down to 0.4%.  1999 Elsevier Science B.V. All rights reserved. Keywords: Synchrotron radiation; Radiometry; Detector calibration; Source calibration; Reflectometry

1. Introduction The Physikalisch-Technische Bundesanstalt (PTB) is a national institute for science and technology and the highest technical authority of the Federal Republic of Germany for the field of metrology and physical safety engineering. PTB provides scientific and technical services to industries, universities and other high-technology organizations. For more than 15 years, PTB has used synchrotron radiation of the electron storage ring BESSY I in Berlin for radiometry [1]. To absolutely calibrate radiation sources and energy dispersive x-ray detectors, BESSY I is used as a primary radiation source standard, whose emitted spectral radiant intensity can be calculated within the framework of classical electrodynamics (source-based radiometry) [2]. Moreover, to calibrate semiconductor photodiodes, photoemission diodes, as well as different types of *Corresponding author. Tel.: 149-30-82004-230; fax: 149-3082004-238. E-mail address: [email protected] (M. Richter)

photon counting detectors, absolute scales for spectral responsivity have been established between 2 and 1500 eV photon energy, which are based on the cryogenic electrical substitution radiometer, SYRES, as primary radiation detector standard (detectorbased radiometry) [3]. Finally, two reflectometer systems can be used to characterize optical components [4]. Table 1 gives a summary of the tasks of PTB at BESSY I. Many activities actually refer to radiometric calibrations for solar and astronomical missions of the European Space Agency (ESA) [5–9] and of NASA [10–12], as well as of the characterization and first absolute calibration of superconducting tunnel junction (STJ) detectors [13,14]. This work is completed by fundamental research into the physics of plasma, surfaces and semiconductors [5,15–18]. At present, PTB operates six experimental stations at BESSY I that are listed in Table 2 [1,3,19–22]. Three of them (nos. 1, 2 and 6) are scheduled in a first step to be transferred to the new storage ring BESSY II in Berlin-Adlershof after 1999 when BESSY I is expected to shut down. However,

0368-2048 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00383-1

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Table 1 Tasks for radiometry using synchrotron radiation Primary standard

Tasks

Source-based radiometry

Storage ring (BESSY I, II)

?Calibration of radiation sources: –deuterium lamps –hollow cathode sources –plasma sources ?Calibration of energy dispersive x-ray detectors ?Calibration of spectrometers

Detector-based radiometry

Cryogenic electrical substitution radiometer (SYRES)

Reflectometry

None

?Calibration of radiation detectors: –semiconductor photodiodes –photoemission diodes –photon counting detectors ?Characterization of optical components: –mirrors –gratings –filters

BESSY II is an electron storage ring of the third generation, optimized for the soft x-ray spectral range, and the main activities are extended to higher photon energies and to the use of highly brilliant undulator radiation. Hence, five new beamlines are under construction and, altogether, PTB will operate eight experimental stations at BESSY II (Table 3) [23–26]. The tasks are proposed to be extended to interferometry, spectroscopy, high-flux experiments and x-ray fluorescence analysis. Due to PTB’s activities also in the field of classical radiometry, all current radiometric primary standards are available in Berlin, i.e. storage ring, black-body radiator, cryogenic radiometer operated with laser as well as with synchrotron radiation, providing radiometric scales from the infrared to the x-ray range.

2. Source-based radiometry The use of an electron storage ring as a primary radiation source standard is based on Schwinger’s equation, which is derived from classical electrodynamics and expresses the spectral radiant intensity of synchrotron radiation emitted from a charged particle that is bent in a dipole magnetic field [27]. Hence, the radiant power of synchrotron radiation through an aperture can be calculated by appropriate integration when measuring the corresponding storage ring parameters and geometric quantities, i.e. particle energy, particle current, magnetic field, aperture size and aperture position with respect to the source [2]. Two different methods are used to measure the

Table 2 Experimental stations of PTB at BESSY I No.

Monochromator

Spectral range

Type of radiometry

Reference

1 2 3

Normal incidence Normal incidence None, undispersed synchrotron radiation Toroidal grating Rowland circle Plane grating (SX-700)

3 to 40 eV 3 to 40 eV 1 to 15 keV

Source-based Detector-based, reflectometry Source-based

[21] [3] 2

30 to 250 eV 250 to 1.8 keV 50 to 1.8 keV

Source-based Source-based Detector-based, reflectometry

[20] [19] [22]

4 5 6

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Table 3 Experimental stations of PTB at BESSY II No.

Monochromator

Spectral range

Type of radiometry

Status

Ref.

1

None, undispersed bending magnet radiation None, undispersed undulator radiation (1. harmonic) Plane grating (undulator) Normal incidence (undulator) 4 crystal (bending magnet) Plane grating (SX-700) (bending magnet) normal incidence (bending magnet) normal incidence (bending magnet)

6 to 50 keV

Source-based

Available in 1998

2

5 to 150 eV

Source-based

Available in 1998

[24,25]

20 to 1.9 keV

Detector-based, reflectometry

Available in 1998

[25]

3 to 40 eV

Detector-based reflectometry

Not yet available

2

1.75 to 10 keV

Detector-based, reflectometry

Available in 1998

[26]

50 to 1.5 keV

Detector-based, reflectometry

Available in 2000

2

3 to 40 eV

Detector based, reflectometry

Available in 2000

2

3 to 40 eV

Source based

Available in 2000

2

2 3 4 5 6

7 8

energy of stored electrons [28–31]. The first method is based on the spin polarization of the electrons due to their motion in the bending magnetic fields [28]. By a high-frequency field, resonant depolarization can be achieved, the resonance frequency determining the electron energy. Depolarization is indicated by measuring the signal of spin-dependent electron– electron scattering within the orbit. For the second method [30,31], infrared radiation of a cw CO 2 laser of known frequency is merged with the electron beam. The Compton back-scattered x-ray photons are detected by an energy-calibrated germanium detector. Measuring the cut-off of the energy distribution, i.e. the Compton edge, yields the electron energy. To determine the electron current above 1 mA, two toroidal dc beam current transformers are used that are based on the induction principle [32]. With a few electrons stored in the ring, i.e. below 1 nA electron current, stepwise loss of electrons leads to a stepwise decrease of the intensity of the emitted synchrotron radiation measured by liquid N 2 -cooled photodiodes. Thus, the electrons can be counted and, together with the known period of the electrons and the elementary charge, the electron current is obtained. In between, interpolation is realized over six orders of magnitude by measuring the photodiode current calibrated against the stored electron current

at the high and low end of the dynamic range that, in total, covers more than twelve orders of magnitude, from 0.8 pA to 1 A electron current. The magnetic field within a bending magnet is measured by a nuclear magnetic resonance probe. Moreover, the extension of a real synchrotron radiation source is taken into account by evaluating the polarization characteristics of the synchrotron radiation [33]. Fig. 1 shows the calculated radiant intensity in the storage ring plane of BESSY I and BESSY II operated at their nominal electron energies, respectively. With BESSY I at 800 MeV electron energy, a relative uncertainty in the determination of radiant power or photon flux of below 0.4% is achieved in the spectral range between 1 eV and 15 keV photon energy [2,34]. To absolutely calibrate a secondary radiation source standard, direct comparison to the storage ring as the primary standard is realized with the help of a turnable monochromator-detector system that is alternately exposed to radiation from both sources under analogous geometric conditions [19–21], as illustrated in Fig. 2 (a). As an example, PTB’s radiant intensity scales displayed in Fig. 1 are realized by deuterium and hollow cathode lamps, the latter being a source of numerous spectral lines. The relative uncertainties of these scales vary between 1u55% and 1u510%. Calibration of an energy

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Fig. 1. Radiant intensity of various radiation sources: electron storage rings BESSY I and BESSY II in the storage ring plane into a band pass of 1% of the photon energy, deuterium lamp into a band pass of 1% of the photon energy, and hollow cathode spectral line source, with the storage rings operated at 1 mA electron current and at their nominal electron energy (BESSY I: 800 MeV, BESSY II: 1700 MeV).

dispersive x-ray detector is performed by direct measurement of the undispersed synchrotron radiation [Fig. 2(b)] [7,35]. To correct for the energy resolution of the detector, its response function must be determined in addition by relative measurements behind a suitable monochromator. Relative uncertainties for the determination of detector efficiencies down to 1u51% are achieved.

3. Detector-based radiometry and reflectometry

Fig. 2. Calibration principles using the electron storage ring BESSY as the primary source standard (a, b) and using the cryogenic electrical substitution radiometer, SYRES, as the primary detector standard (c). Reflectometry (d) is a relative method.

To absolutely determine the radiant power of spectrally dispersed synchrotron radiation behind a monochromator, PTB uses the cryogenic electrical substitution radiometer, SYRES, as the primary radiation detector standard [3]. SYRES is a thermal detector based on the equivalence of radiant and electrical heating. As illustrated in Fig. 3, radiation is absorbed by a radiation cavity absorber. Design and coating of the cavity are optimized for 100% photoabsorptance. The cavity is thermally linked to a heat sink with a constant temperature of about 4.2 K. In the first step of a radiant power measurement, the synchrotron radiation is turned off by a beamshutter

and the cavity temperature is slightly increased by electrical heating and is controlled. Turning on the synchrotron radiation, the heater power decreases by the amount of the incident radiant power to maintain the absorber temperature. Thus, the radiant power is given by the heater power difference that results from synchrotron radiation turning off and on. The ambient temperature of the cavity as well of the last section of the beamline close to the radiometer amounts to about 4.2 K, which is maintained by a liquid He cryostat, to reduce the background of thermal radiation and to achieve superconductivity of the temperature sensor and heater leads. The relative

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relative uncertainties of the scales vary between 1u50.4% and 1u52%. Measurements to the reflectance and transmittance of optical components, like filters, mirrors and gratings, are performed at the beamlines for detector calibration too [Fig. 2(d)]. However, reflectometry is a relative method. The reflected or transmitted beam from a sample, as well as the direct beam, is measured by the same detector within a reflectometer system. The ratio yields the sample reflectance. Two reflectometer systems are available [4]. The angle of incidence to the surface-normal direction of the samples can be varied about two perpendicular axes to take advantage of the polarization of the synchrotron radiation. Fig. 3. Scheme of the cryogenic electrical substitution radiometer SYRES as a primary detector standard.

References uncertainties for the measurement of radiant power by SYRES are below 0.2%. To absolutely calibrate a secondary detector standard, direct comparison to SYRES as the primary standard is realized by turning both detectors alternately into the beam, as illustrated in Fig. 2(c). In this way, PTB has established absolute spectral responsivity scales of semiconductor photodiodes that cover the spectral range between 2 and 1500 eV photon energy. They are displayed in Fig. 4. The

Fig. 4. Absolute spectral responsivity of a Si n–p junction type photodiode and of a PtSi–n–Si Schottky-type photodiode. The relative uncertainties vary between 0.4 and 2%.

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